Sato Takuma

EPR studies on defects in semiconducting barium disilicide

In this study, I focused on semiconducting barium disilicide BaSi2. Since BaSi2 is composed of less toxic and earth-abundant elements, this environmental-friendly material takes advantage over other materials in terms of elemental strategies. Our previous researches found its potentials for solar cell applications. For instance, BaSi2 shows a large absorption coefficient (3 × 10 4 cm -1 @ 1.5 eV, whose value larger than crystalline Si by 40 times) in spite of its indirect bandgap because of its characteristic band structure.

So far, we have successfully demonstrated Si-based hetero-and homo-junction solar cells. In order to design devices with higher conversion efficiencies, I investigated intrinsic defects in undoped-BaSi2 films and bulks in this study. This is because the defects promote photogenerated carriers to recombine each other during their travel to electrodes. As a result, photogenerated currents decrease. Therefore, defect control is indispensable to realize the devices with a high conversion efficiency. My previous researches found BaSi2 films grown by molecular beam epitaxy contained the defects while their origins are unknown yet. Electron paramagnetic resonance (EPR) enables me to access microscopic information such as identification of paramagnetic defects, determination of local structures, etc.

First, I performed EPR measurements on a-axis orientated BaSi2 epitaxial films. Assessing the observed g value carefully, I succeeded in detection of EPR active defects in this material for the first time. In addition, it was confirmed that the EPR line observed from the film (anisotropic g value: 2.003-2.010) decreased by introducing atomic hydrogens (H). In our previous study, H-passivated BaSi2 films showed significant improvements on photoresponsivity, indicating that such defects can affect optical properties.

Second, I revealed that some different types of defects formed depending on Ba/Si composition ratios through the EPR measurements of powder and single crystal of BaSi2. To accomplish this identification, I adopted a variety of EPR techniques from the multi-frequency cw-EPR to the advanced pulse-EPR. In Si-rich samples, I observed at least two different defects with averaged g values (gav) > 2. Each defect exhibited different behavior on temperature dependence and relaxation time. On the other hand, in Ba-rich samples, other EPR lines (gav < 2) appeared. One of the defects had a narrow linewidth and hyperfine structures which may stem from 29 Si (I = 1/2), 135 Ba (I = 3/2), or 137 Ba (I = 3/2). I performed further identification by adopting hyperfine spectroscopy such as

ELDOR-detected NMR (ED-NMR). ED-NMR is a technique that has attracted attention in recent years as a new

technique for measuring hyperfine coupling constants. The hyperfine coupling constant of |A| ~ 70 and 80 MHz was successfully obtained in ED-NMR spectra of Si-rich and Ba-rich, respectively. These values seemed to correspond to those observed in single crystals. The density functional theory (DFT) calculation was also carried out to get a picture of possible defect models. Through simulations of the EPR spectra using the obtained parameters, I suppose that at least and Si vacancies (VSi) were formed as a paramagnetic defect in the Ba-rich sample.

At last, I also conducted photoluminescence (PL) measurements, which is very powerful for determining defect levels. Four PL peaks were observed at almost the same photon energy in each thin film sample and the polycrystalline sample. These results suggest that similar defects are formed in the thin films and the polycrystalline samples. In addition, I compared these results with those obtained by other methods (the deep level transient spectroscopy and temperature dependence of electrical resistivity), enabling me to propose a radiative defect model Abstract 2 in BaSi2. Combining the EPR results with those of PL, I concluded that the definitive defect affected its photoresponsivity originated from VSi with 0.8 or 0.9 eV of the PL peak in the BaSi2 epitaxial films.

As a conclusion, I performed EPR and PL measurements on the BaSi2 films as well as polycrystalline and monocrystalline BaSi2 bulks in order to clarify the defect profiles. In the bulk samples, the EPR measurements revealed that at least VSi tend to be formed under Ba-rich condition. On the other hand, I found that there are other types of defects in Si-rich BaSi2, which relate to relatively deeper defect levels within the bandgap from viewpoints of PL measurements. The defects observed in the BaSi2 films grown by molecular beam are good agreements with those detected in the Si-rich bulk samples.

Chapter 1 Introduction

Application of photovoltaic devices

The demand for electricity, which has increased due to the recent development of IoT and AI technologies, has further increased the importance of renewable energy such as solar, wind, biomass, hydropower, tides, and geothermal energies, alternative to commonly used fossil fuels, is of great importance. 1 Fossil fuels not only constitute a limited energy source, but also the main reason of increased air pollution and global warming. Photovoltaic (PV) technology, which is known for converting solar energy into electricity through a semiconducting material, plays a key component of decarbonizing electricity.

The first solar cell, which was created by Charles Fritts in 1883, achieved a power conversion efficiency (η) of 1-2% by covering selenium with a thin layer of gold. In 1954, physicists at Bell Laboratories discovered that crystalline silicon c-Si is more efficient than selenium, and created the first practical solar cell with improved η of 6%. 2 Since then, the η has been improved by adopting technologies such as back-surface field (BSF), 3,4 surface passivation, 5,6 texture structure, 7,8 and back contact. 9,10 In recent years, a heterojunction with intrinsic thin-layer (HIT) type solar cell has an i-type amorphous Si (a-Si) layer sandwiched between heterojunction interfaces of p-type a-Si and n-type c-Si. Research on batteries is also advancing, with the same structure having Panasonic's conversion efficiency of 25.6% 11 in 2014 and Kaneka's 26.33% 12 in 2016. Figure 1.1-1 presents the global installed capacity of solar PV system from 2007 to 2017. In 2017, the total PV global capacity increased to over 400 GW, rose by almost 33% compared with the year 2016. The market expansion was mainly due to the increasing competitiveness of solar PV combined with the rising demand for electricity in developing countries, as well as to the increasing awareness of solar PV's potential to alleviate pollution, reduce CO2 emissions and provide energy access. [START_REF] Yamashita | Effects of molecular beam epitaxy growth conditions on grain size and lattice strain in BaSi2 epitaxial films[END_REF] At present, the η of c-Si single-junction solar cells, which share about 90% of the solar cell market, reaches around 26%, but the improvement in η has been only 1.5% in the last 20 years. The rest of the market are composed of the third generation PV technologies which use advanced thin-film solar cells modules including cadmiumtelluride (CdTe) modules, a-Si modules, and copper indium gallium selenide sulfide Cu(In,Ga)(S,Se)2 (CIGS) Fig. 1.1-1 Global renewable power capacity, 2007-2017. [START_REF] Yamashita | Effects of molecular beam epitaxy growth conditions on grain size and lattice strain in BaSi2 epitaxial films[END_REF] modules. 14 Since sunlight has a wide energy distribution from infrared to ultraviolet, the η of PV is limited by the heat loss (i) due to a loss of transmitted photons through materials with a wide bandgap (Eg), and (ii) due to a heated loss caused be a difference between energy of absorbed photons and Eg in materials with a narrow Eg. In other words, achieving a η of over 30% requires a drastic review of the current solar cell structures. In recent years, multi-junction solar cells have been highly evaluated for their potential as next-generation solar cells. Due to a combination of a material having a wide Eg and a material having a narrow Eg, the present structure, in which absorb sunlight more effectively, can dramatically improve the η. The η of the three-junction solar cell InGaP/GaAs/Ge that has been put into practical use today is as high as 32%. However, since the raw material contains rare and/or toxic elements, it is difficult to significantly expand its use for consumer use, and its use is limited. There is a need for the development of Si-based multi-junction solar cells composed of resource-rich elements.

Parameters of photovoltaic devices

Solar cell efficiency refers to the portion of energy in the form of sunlight that can be converted via PVs into electricity. The 𝜂 of a solar cell is considered the most important criterion when assessing a solar cell's quality.

The most efficient commercially available solar PV module on the market has an 𝜂 of 22.5%, whereas the majority are from 15% to 17% efficiency rating. Most relevant for solar energy conversion is the terrestrial solar spectral irradiance on the surface that differs from the extraterrestrial irradiation (AM0) due to the effect of filtering by the atmosphere. Air Mass 1.5 Global (AM1.5G) describes the radiation arriving at earth's surface after passing through 1.5 times a standard air mass, with the sun at 48.2° from zenith with integrated power density of 100 mW/cm 2 , including both direct and diffuse radiation. This is the power density that is usually referred to as "one sun". The AM also depends on the position of the sun and can be defined as follows:

AM = 1 cos(θ)
where θ is the elevation angle of the Sun as shown in Fig. 1.2-1. [START_REF] Al-Alwani | [END_REF] AM1.5G is used as the standard spectral distribution of light to measure a solar cell's efficiency. Generally, 99% of the light that reaches the Earth's surface is at wavelengths of less than 2500 nm in the AM1.5G spectrum, and 88% is less than 1350 nm. To be efficient, a solar cell should be able to absorb the largest number of photons possible. Figure 1.2-2 compares the AM0 and AM1.5G spectra. 16 Figure 1.2-4 shows recent best research-cell efficiencies in the world, measured under standard AM1.5G illumination, which is summarized by national renewable energy laboratory (NREL). Single gallium arsenide (GaAs) junction solar cells (𝜂 = 28.9%) are approaching the theoretical limiting power efficiency of 33.7%, noted as the Shockley-Queisser limit. 17 The maximum 𝜂 of single c-Si solar cells is 27.6% with concentrated area. As for thinfilm solar cells, CIGS solar cells reach a maximum 𝜂 of 23.3%, showing great potential compared with c-Si solar cells. Besides, organic-inorganic hybrid perovskite CH3NH3PbI3 solar cell is a rising star in solar cell family. Its efficiency increased dramatically from 3.8% to 23.3% in a few years, catching up with CIGS solar cells. I discuss current density-voltage (J-V) characteristics of pn junction solar cells during light irradiation. As a simple model, I consider the model of the pn junction formed by a p-type semiconductor having a thickness of d and a n-type semiconductor which has sufficiently thicker than its minority carrier (hole) diffusion length Lh. When the solar cell is irradiated with light, the following differential equation holds for the hole density distribution pn (x) in the n-type neutral region.

D n d 2 p n (x) dx 2 + G(x) - p n (x) -p n0 τ = 0 .
Dn:diffusion coefficient of hole, pn0:hole concentration at thermal equilibrium, τ:minority career lifetime

G(x):electron-hole pair generation rate per unit time and unit volume

Here, assuming that photons of monochromatic light enter the surface of this pn junction solar cell by Φ0

[s/cm -2 ]. The number of photons Φ(x) reaching the n-type neutral region is described below by using the reflectance at the surface R, and the light absorption coefficient α.

Φ(x) = (1 -R)Φ 0 e -α(x + d) . JSC flowing when the terminal is short-circuited is called a short-circuit current density. In the case of a pn homojunction, the magnitude of VOC is roughly determined by the built-in potential, that is, the difference of Fermi level between the p-type semiconductor and the n-type semiconductor. That is, since VOC is considered to be a value proportional to the magnitude of Eg,

V OC = E g q -ΔE
Therefore, VOC generally increases monotonically with Eg. On the other hand, JSC is determined by the sum of the number of photons having energy larger than Eg in the photon spectrum Φ (E) of AM1.5 that pours into the solar cell, so the maximum value of JSC can be expressed as follows.

J SC = q ∫ Φ(E) dE ∞ E g
Therefore, JSC monotonically decreases with respect to Eg. The conversion efficiency η of a solar cell is given by,

η = P m P in = V m ×J m P in = FF×V OC ×J SC P in
Pin : Incident energy, Pm : Power at optimal operating point, Vm : Voltage at optimal operating point, Jm : Current density at optimal operating point, FF : fill factor

Hence, η has a maximum point for Eg. Generally, the maximum value of η is obtained when Eg = 1.4 to 1.6 eV, but may shift due to the influence of the quality of the crystal, etc.

So far, we have used the equation of the J-V characteristic using the ideal equivalent circuit of the solar cell, but in actuality it is necessary to consider the series resistance Rs and the parallel resistance Rsh as shown in Fig. 1.2-6. Furthermore, in order to take into account the recombination current, it is necessary to introduce an ideal coefficient γ. The equation of the J-V characteristic considering Rs, Rsh, and γ is given by,

J = J 0 {exp ( q(V-SJR s ) γk B T ) -1} -J L + V-SJR s SR sh
. S : the area When Rs → 0 and Rsh → ∞ in the above equation, the ideal case is obtained.

∂n h ∂t = G -R h,d + G h,d , ∂n e,d ∂t = R e,d -R h,d -G e,d + G h,d .
Since charge neutrality of the semiconductor must prevail even when the different concentrationns are changing, we can rewrite the third formula as

e (n D + -n e -n A -+ n h -n e,d ) = 0,
where we assume that the defects are negatively charged when they are occupied by electrons, i.e. they are acceptortype defects.

Regarding the rate of recombination Re,d, whenever an electron that moves through the lattice at its thermal velocity 𝜈e comes within a capture cross section 𝜎e of an defect that is occupied by hole, it will be captured. So, the rate at which capture happens all electrons is

R e,d = σ e ν e n e n h,d ,
where nh,d (= ndne,d) is the density of defects not occupied by an electron and then occupied by a hole, which are the recombination partners for the electrons. The capture cross sections for electrons, 𝜎e, and for holes, 𝜎h, have values of the order in the range of 10 -18 -10 -15 cm 2 . Similarly, the recombination rate for holes is given by

R h,d = σ h ν h n h n e,d .
For the generation rates of electrons or holes from the defects into the CB or VB, it should be proportional

𝑁 𝑒,𝑑 = 𝑛 𝑑 1 β 𝑒 /(σ 𝑒 ν 𝑒 𝑛 𝑒 ) + 1 = 𝑛 𝑑 1 𝑒𝑥𝑝[(𝐸 𝑑 -𝐸 𝐹 )/𝑘 𝐵 𝑇] + 1 With ne = NC exp[-(EC -EF)/kBT],
we obtain the coefficient of emission of electrons,

β 𝑒 = σ 𝑒 ν 𝑒 𝑁 𝐶 exp (- 𝐸 𝐶 -𝐸 𝑑 𝑘 𝐵 𝑇 ) .
Similarly, for the coefficient of emission of holes,

β ℎ = σ ℎ ν ℎ 𝑁 𝑉 exp (- 𝐸 𝑑 -𝐸 𝑉 𝑘 𝐵 𝑇 ) .
When the defect levels are close to the CB or VB, the defects are less effective. Most of the time the levels are not occupied by electrons or holes because captured carriers are much more frequently emitted back to the CB or VB than annihilated by recombination with a hole or an electron captured from the VB or CB. On the other hand, captured electrons and holes by the defect levels located at the middle of Eg, recombination with each carrier and then annihilations occur for much less time. We have to, therefore, control the defects, especially, "deep" defects in order to extract photogenerated carriers as much as possible. respectively. 22 The characteristic crystal structure of BaSi2 is explained by the Zintl-Klemm concept. [START_REF] Kauzlarich | Chemistry, Structure and Bonding of Zintle Phase and Ions[END_REF] According to the concept, two valence electrons are supplied one by one from a Ba atom to two Si atoms.

Therefore each Si atom possesses five valence electrons following the Octet rule. [START_REF] Jensen | [END_REF] As a result, each Si atom with five valence electrons bonds with the other three Si atoms. These tetrahedra are found in the allotrope of phosphorus P, which has five valence electrons. 25 As reported by Imai et al., 26 2.1-3. Among them, Ba5Si3, BaSi, Ba3Si4, and BaSi2 follow the Zintl-Klemm concept, that is, electrons are from Ba atoms with low electronegativity to Si atoms with high electronegativity to become cation and anion atoms, respectively. In these materials, the anion atoms are bonded by covalent bonds (Table 2-1). spectrum resulting from its band structure and DOS 42,43 are important so as to find "good" candidates for thin-film photovoltaic devices. BaSi2 meets such fundamental requirements for an absorber desirable for thin-film photovoltaic techniques.

Another promising feature of BaSi2 is its excellent minority carrier properties. For the formation of solar cells, material should have not only suitable Eg and large , but also large minority carrier diffusion length (L) and lifetime (). As for L, a longer L is desirable for photovoltaic devices, which is measurable with electron beam induced current (EBIC) techniques. The L was roughly estimated to be approximately 10 μm, assuming that the EBIC profile varies as exp(-x/L), where

x is the distance from the Al edge (point A) along the dotted line, and L is the diffusion length of minority carrier for BaSi2. As can be seen in section 2.3, the L is much larger than a grain size of the BaSi2. 44,45 On the other hand,  of undoped BaSi2 was measured by means of microwave detected photoconductive decay (-PCD), where carriers were generated by a 5 ns laser pulse with a wavelength of 349 nm. Photoconductivity decay was monitored by the reflectivity of microwave with the frequency of 26 GHz. High-sensitivity measurement was realized by the differential detection of the reflected microwave intensity between the areas with and without laser irradiation. Then,  can be calculated by analyzing the photoconductivity decay curves. The decay curve was assigned three different regions (Fig. 2.2-4(a)). Comprehensive investigations on ex situ and in situ doping had been done. In Ref [48], possible dopants to obtain n-and p-type BaSi2 are well listed. As can be seen in Fig. 2.2-1(b), the Si p state contributes the VBM mainly.

Therefore, by the replacement of the Si atoms in BaSi2 with a group 13 (15) element, we can achieve a p-(n-) type BaSi2. 33,49 This is the same way as that widely used to control the electrical properties of semiconductors such as Si.

Growth of BaSi2 thin-film

There are three main methods to grow BaSi2 films thicker than 1 μm: molecular beam epitaxy (MBE), magnetron sputtering, and vacuum evaporation. In terms of photovoltaic device applications, the formation of highquality films is one of the most important points.

BaSi2 can be grown epitaxially on Si (111) and Si (001) substrates by MBE. [59][60][61][62][63] The growth processes will be described well in Section 4.1. The obtained films exhibit a three-fold symmetric domain rotated by 120 ° on Si (111), and it forms a twice symmetric domain rotated by 90 ° on Si (001) as shown in Figs. 2.3-1. These domains are randomly formed on the Si substrate (Figs. 2.3-2). [60][61][62][63] Noted that MBE is the best method to examine a potential of BaSi2 for photovoltaic devices because it is easier to control the atomic ratio of Ba and Si during the film growth.

The magnetron sputtering is a practical method to form large area devices. Matsuno et al. formed single phase polycrystalline BaSi2 films on Si(111) by helicon-wave excited plasma (HWP) sputtering, using a stoichiometric BaSi2 target. 64,65 As the HWP has a higher plasma density, uniform in a large volume, 66 and lower ion energy than those of usual capacitive-coupled plasmas, substrate damage can be greatly reduced. As a result, a typical value of n = 2 × 10 16 cm -3 was obtained at RT. The photoresponsivity rapidly increased for photon energies larger than the band gap of BaSi2, and reached 0.75 A/W at a photon energy of 2.0 eV and Vbias = -0.5V applied to the indium-tin-oxide (ITO) electrode with respect to the Al electrode. Recently, a marked enhancement of photoresponsivity was achieved in C-doped BaSi2 films due to the increase of minority carrier lifetime. 38 Vacuum evaporation is another feasible method. An UHV is not a prerequisite; therefore, the equipment can be simple and inexpensive. with n varies from 10 17 to 10 22 cm -3 at different substrate temperature. The maximum value of photocurrent was obtained at a photon energy of 1.9 eV, corresponding to an EQE of 22% at a reverse applied voltage of 2.0 V. 67 One problem of the evaporated films was cracking due to the mismatch in thermal expansion coefficient between BaSi2 and c-Si. Moreover, the grown films contained metallic phases such as Ba2Si and Ba5Si3. 68,69 Hara et al.,however, found out that the source premelting procedure suppressed the oxidation and made the films homogeneous. 70 Recently, a theoretical basis of thermal evaporation of BaSi2 had reported based on the calculation from relative partial molar enthalpies of Ba and Si. 71 111) and (b) Si(001). 61,63 current was large in the homojunction diode as expected. The IQE became pronounced at λ < 800 nm in Fig. 2.4-4(c), while the IQE was very small at λ > 800 nm because the photogenerated electrons in the p + -Si recombined with holes before reaching the built-in field region. On the other hand, the IQE exceeded 30% at λ = 500 nm. Considering that the absorption length (3/α) at λ = 500 nm is approximately 100 nm in BaSi2, 35 so the IQE spectrum in Fig. 2. 4-4 (c) was attributed to the photogenerated carriers originating from the p-BaSi2 layer and then they were separated by the built-in electric field in the BaSi2 pn junction diode. The IQE value distinctly increased as the p of the p-BaSi2 layer decreased from 1 × 10 17 to 1 × 10 16 cm -3 , while the JSC values were 1.3 and 3.6 mA/cm 2 , respectively. On the basis of these results, the operation of a BaSi2 homojunction solar cell was achieved for the first time. 80 η was as small as ∼0.1% because of large leakage currents caused by defects resulting from step bunching at the p + -BaSi2/p + -Si interface. 84 After the first operation, we improved the 𝜂 up to 0.28% by considering optimum carrier concentrations and thickness at each layers. The most largest 𝜂 was realized in n + -BaSi2 (20 nm, n ~ 10 19 cm -3 )/p-BaSi2 (500 nm, p ~ 10 16 cm -3 )/p + -BaSi2 (20 nm, p ~ 10 19 cm -3 ) diodes on the B implanted p + -Si/p-Si(111). 85 As described above, we are at the next stage to improve 𝜂 of homojunction solar cells. We had revealed much a lot of electric and optical characteristics and successfully demonstrated the potential of BaSi2 for photovoltaic device applications. Moreover, as the future perspective, the realization of BaSi2 homojunction solar cells enables us to design Si-based tandem PV devices which BaSi2 p-n or p-i-n structures work as top cells. According to Deng, we can achieve above 30% of 𝜂 by using BaCxSi2-x as its absorption layer. 86 To achieve such next generated PV devices, the remaining and unknown issue is "defect" in BaSi2, which plays the most important role in conversion efficiency. . 106 The initial and final charge states are labeled in parenthes.

Magnetism in BaSi2

Magnetic susceptibility Orthorhombic BaSi2 exhibit diamagnetic property. That was reported by Evers in 1978. 108 He measured the magnetic mass susceptibility χ g of BaSi2 by the Faraday balance under high vacuum conditions between 25 and 580 °C. He didn't report the measured value of orthorhombic BaSi2 at, for instance, RT because he focus on a metalsemiconductor transition 109 in the literature. He only mentioned that the results of susceptibility showed horizonal slope against reciprocal magnetic field and reported the susceptibility of -0.41--0.43×10 -6 emu/g above 415 °C, at which the phase transition from metal phase (trigonal BaSi2).

Nuclear magnetic resonance (NMR)

Regarding the magnetic resonance, there is a report by Goebel et al. 110 Their interest was chemical bonding in intermetallic compounds because the nature of idealized ionic and covalent bonding was relatively well understood whereas there was still a lack of knowledge about that of intermetallic one. They considered the Zintl phase as a model system, for the Zintl phase are polar intermetallic phases dominated by covalent and ionic bonding. 234). In the literature, they reported static NMR signals of cations while there was no data for Ba. Table 2-5 shows the quantum chemical caluculation results of electron counts of silicon atoms. According to the Zintl-Klemm concept, the charge transfer should be complete in the materials. On the other hand, they found the real material doens't follow the concept perfectly. Also, the electron count of Si3 site differs significantly from that of Si4 and Si5 site as seen in Table 2345. While there was no magnetic resonance study on BaSi2 except for the above mentioned one as long as I know, investigations on bare Zintl anion clusters such as Si4 4-tend to increase in the field of material science. 111,[113][114][115] Table 2-5 Electron counts of silicon atoms in BaSi2 (q in electrons per atom). 110

Aim of this thesis

As described above, BaSi2 has physical properties suitable for solar cell applications. In addition, we had already demonstrated operation of solar cells by using BaSi2, and the maximum conversion efficiency of silicidebased heterojunction solar cells with crystalline Si of ~ 10% has been achieved. More recently, a homojunction solar cell device that can be expected to further improvement on conversion efficiency has been successfully operated. On the other hand, investigation of defects that greatly affect conversion efficiency is still limited. It is clear that the defect exists, but its identity remains unknown. Therefore, the purpose of this study is to detect and clarify defects from a microscopic viewpoint using the electron paramagnetic resonance.

Chapter 3 Principle of electron paramagnetic resonance

Electron paramagnetic resonance (EPR) is a powerful experimental method for studying systems with unpaired electrons. In systems with unpaired electrons, irradiation with electromagnetic waves causes a dipole transition. As a result, an absorption spectrum, i.e., the EPR spectrum are obtained. Examples of EPR applications include free radicals in solids, liquids and gases, transition metal ions, photogenerated species, and point defects in solids. Especially, regarding point defects in solids, EPR provides us fruitful information on their microscopic structures as well as their dynamics as a faction of temperature. For instance, EPR has revealed the defect symmetry and its chemical identity in various semiconductors such as Si, SiC, etc.

A dipole magnetic field (B) created by the magnetic dipole (μ) is given by the following equation.

B = 1 r 3 [ 3(μ⋅r) r 2 -μ] .
Thus, all magnetic moment always accompanies the angular mementum G.

G = r×p

Where r is the radius vector and p is the momentum. The ratio of the magnetic moment to the angular momentum is called the magnetogyric ratio (γ = μ/G). If the magnetic moment is parallel or antiparallel to the angular momentum,

μ = 1 c IS, I = q 2πR v.
Here q is the charge, R is the loop radius, v is the velocity of the charge, and S = π𝑅 2 . Therefore, the magnitude of the magnetogyric ratio is given by,

γ = μ G = ( qvR 2c ) /(m q vR)= q 2m q c
.

In quantum mechanical descriptions, the angular momentum is written as G = ℏL, then the orbital magnetic moment of electron μ L is given by

μ L = γG = - eℏ 2m q c L= -βL.
Here β is called the Bohr magneton. In radio spectroscopy a dimensionless quantity named the g-factor is introduced instead of the magnetogyric ratio, then it can be written as μ L =-g L βL (g L = 1). Similarly, μ S = -g S βS (gS= 2.0023 ... 1). Now, the Hamiltonian when the magnetic moment is placed in a magnetic field is given by ℋ = -𝝁 ⋅ 𝑩, so the spin magnetic moment is spitted into following two energies.

E ± = -μ•B = ±(1/2)g S μ B B 0
This is called the Zeeman effect. EPR observes the microwave absorption spectrum corresponding to the energy difference due to Zeeman splitting (Zeeman energy). In other words, as resonance conditions hν = g S βB 0

In practice, if we extend the two-level model to N spins, the emission of photons will occur as well. The characteristic quantity is then the population difference of N, which follows a Boltzmann distribution in thermal equilibrium. In the high-temperature approximation E ≪ k B T , one obtains the population difference n = Ng S βB 0 /2k B T , where 𝑘 𝐵 denotes the Boltzmann's factor, and T the temperature. Therefore, the sensitivity of the spectrometer increases with the total number of spins, for larger magnetic fields and at lower temperatures. Due to the proportionality of the magnetic field and the frequency, we would now suspect that the sensitivity improves with larger frequencies as well.

But in this case, the corresponding waveguides become smaller, which means that the samples have to be smaller too.

Consequently, they also contain fewer detectable spins N. In a scheme of EPR, a spin polarization is bigger, resulting in expecting an increase of sensitivity. This parameter is usually the more important one in real materials. Besides that, several aspects restrict the used frequency range to the microwave region. For example, in the high-frequency regime, it is difficult to produce sufficiently homogeneous magnetic fields and it is expensive to shrink the microwave components. On the other hand, in a real material system, not all of the EPR-detectable mechanisms are fielddependent (i.e. frequency-dependent).

EPR parameters

We now turn to the formal description of the relevant processes which affect the EPR spectrum. First, for conventional EPR, it is sufficient to consider only the electronic ground state, since the energy difference between the ground and excited states of the defect system (on the order of eV) is significantly higher than the one between the dipole transitions (in the range of eV-meV). The effective spin Hamiltonian characterizing the defects are described below.

ℋ(𝑟

) ≡ ℋ = ℋ 𝒵 + ℋ ℱ𝒮 + ℋ ℋℱ + ℋ 𝒩𝒵 + ℋ 𝒬 ℋ 𝒵 = μ 𝐵𝑩 𝑇 • 𝒈 • 𝑺 ... electronZeeman interaction ℋ ℱ𝒮 = 𝑺 𝑇 • 𝑫 • 𝑺... fine-structure interaction ℋ ℋℱ = 𝑺 𝑇 • 𝑨 • 𝑰... hyperfine interaction ℋ 𝒩𝒵 = -μ 𝑛𝑩 𝑇 • 𝒈 𝑛 • 𝑰 ... nuclear Zeeman interaction ℋ 𝒬 = 𝑰 𝑇 • 𝑷 • 𝑰 ...

nuclear quadrupole interaction

where g, D, A, gn, and P are effective g tensor, fine structure tensor, hysymmetrical, three-dimensional matrices, which can be diagonalized and are characterized by their principal values and eigenvectors. In experiments, it is easier to obtain the relative signs for the matrix elements (e.g. 𝑔 𝑖𝑗 ) than the overall one (e.g. g) [31]. From the above equations, we see that ℋ Z depends on the magnetic field but not ℋ HF . In the presence of g-strain (broadening of the g-values in a frozen solution), it is better to obtain the hyperfine tensor at low magnetic fields and corresponding frequencies (S-and X-band), whereas the g-tensor is measured at high frequencies (Q-and W-band).

Electon Zeeman interaction

In this subsection, I focus on the term in the spin Hamiltonian that represents the interaction of the electron spin with the external magnetic field B, i.e., electronic Zeeman term;

ℋ 𝒵 = β𝑩 𝑇 • 𝒈 • 𝑺 . In the description of the spin Hamiltonian, all spatial information is aggregated in the g-tensor (3×3 matrix). In the principal axes system (X, Y, Z) of g-tensor, the spin Hamiltonian becomes,

ℋ 𝒵 = βB(𝑛 𝑋 𝑛 𝑌 𝑛 𝑍 ) ( 𝑔 𝑋 0 0 0 𝑔 𝑌 0 0 0 𝑔 𝑍 ) ( 𝑆 𝑋 𝑆 𝑌 𝑆 𝑍 )
Here, the spin operator is defined as S ≡ 1/2hσ using a Pauli matrix. Where each component of the Pauli matrix is

σ 𝑥 = ( 0 1 1 0 ) , σ 𝑦 = ( 0 -𝑖 -𝑖 0 ) , σ 𝑥 = ( 1 0 0 -1 ).
Therefore,

ℋ 𝒵 = 1 2 βB ( 𝑛 𝑍 𝑔 𝑍 𝑛 𝑋 𝑔 𝑋 -𝑖𝑛 𝑌 𝑔 𝑌 𝑛 𝑋 𝑔 𝑋 + 𝑖𝑛 𝑌 𝑔 𝑌 -𝑛 𝑍 𝑔 𝑍 )
by diagonalizing the matrix,

E(±) = ± 1 2 βB(∑ 𝑛 𝑖 2 𝑔 𝑖 2 𝑖 ) 1/2 = ± 1 2 β [B(n T g)(g T n)B] 1/2 ,
Thus, we can obtain eigen energies, E(+1/2) -E(-1/2) = (|𝑔 𝑇 𝑛|)βB ≡ 𝑔βB. We often use the principal axes of the g-tensor introduced on the right side of the equation. Single crystal makes possible to determine the complete gtensor, the principal values and direction of the principal axes. In contrast to single crystal, we can observe only these principal values of the g-tensor in non-orientated samples such as powders.

As seen in Fig. ,g-tensor is anisotropc in generall. This deviation from the value of free electron (ge) is ascribed to the current induced by moving around the spin, which is called the spin-orbit-coupling (ℋ 𝒮𝒪 = β𝑩 𝑇 • 𝒈 • 𝑺). As a result of the spin-orbit-coupling, in real system, g = 𝑔 𝑒𝟏 -2𝜆𝚲. The λ is spin-orbit-coupling constant and tend to larger in heaver nuclei.

Here,the elements of Λ are given by

Λ ij = ∑ < 0|𝐿 𝑖 |𝑛 > < 𝑛|𝐿 𝑖 |0 > 𝐸 𝑛 -𝐸0 (𝑖, 𝑗 = 𝑥, 𝑦, 𝑧) 𝑛≠0
where 0 and n are the atomic orbitals that are singly occupied in the ground and the n-th excited states. In aspect of spectroscopy, it is easier to observe the contribution of the g-tensor. From the respect of DFT calculations, it is much more difficult to calculate the g-tensor because we have to take all excited states into the accounts.

Overall, we can distinguish between,

• Δ𝑔 ≈ 10 -5 -10 -2 : typical for defects with a weak spin-orbit coupling (λ ≈ 1 -100c𝑚 -1 ) in a strong crystal field. This applies for example to light impurity atoms in ionic crystals and defects with an s-ground state, which in principle do not have a spin-orbit coupling at all. • Δ𝑔 ≈ 10 -2a few 10 -1 : typical for transition-metal ions with a 3𝑑 𝑛 -configuration (λ ≈ 10 2 -10 3 𝑐𝑚 -1 ) in a strong crystal field (e.g. ionic crystals). • Δ𝑔 ≈ 1: the concept of a g-shift is no longer applicable. This is for example the case for rare-earth ions with a 4𝑓 𝑛 -orbital configuration.

Hyperfine interaction

In this subsection, I focus on the term in the spin Hamiltonian that represents the interaction between electron spin, S, and nuclear spin, I, i.e., hyperfine coupling term;

ℋ ℋℱ = 𝑺 𝑇 • 𝑨 • 𝑰 The hyperfine tensor are divided int two parts; an isotropic part (aiso) and an anisotropic part (T). The A tensor, therefore, is given by, A = aiso1 + T Formally, the isotropic hyperfine interaction is defined by the Hamiltonian,

ℋ iso 𝐻𝐹 = 8π 3 𝑔 𝑛 gμ 𝑛 μ 𝐵 |ψ (𝑹 = 0)| 2 𝑺 𝑇 • 𝑰
whereψ(𝒓) stands for the one-particle wavefunction at the atomic site R. This means that it is proportional to the probability of presence at the position of the nucleus. Since only s-wavefunctions don't vanish at the position of the nucleus (|ψ (𝑹 = 0)| 2 ≠ 0). The anisotropic dipole interaction is described by the following Hamiltonian

ℋ 𝒹𝒾𝓅 ℋℱ (𝑟) = 𝑔 𝑛 gμ 𝑛 μ 𝐵 [ 3(𝐼 𝑇 •𝑟)(𝑆 𝑇 •𝑟) 𝑟 5 - 𝑆 𝑇 •𝐼 𝑟 3 ]
where r is the distance vector between the electron and nucleus, which is averaged over the wave function of unpaired electrons. In contrast to the Zeeman interaction, the hyperfine interactions is more local interactions (r < 1 nm). The most important characteristics from the aspect of spectroscopy is that the A-tensor doesn't depend on the applied field (or the applied frequency).

Nuclear quadrupole interaction

The nuclear quadrupole interaction is not directly observed in EPR because this is the results of the interaction of electric quadrupole moment of an atomic nucleus with the electric-field gradient at its lacation. The Hamiltonian of this interaction is given by

ℋ 𝒬 = 𝑰 𝑇 • 𝑷 • 𝑰
As it is clear from the Hamiltonian, electronic spin are not included. Hence, in standard EPR measurement, we don't take it into the acccount. This effect only shows up on the spectra when we measure the NMR transition containg I > 1/2 via the ESR such as ESSEM, ENDOR, and EDNMR etc. In its principal axes, the nuclear quadrupole tensor is defined as

P = ( 𝑃 𝑥 0 0 0 𝑃 𝑦 0 0 0 𝑃 𝑧 ) = ( 𝐾(-1 + η) 0 0 0 𝐾(-1 -η) 0 0 0 2𝐾 )
where K = e 2 Qq/4I(2I-1)h is the parameter directly related to the size of the observed spectral shifts and splittings.

The η represent its asymmetry of the electric-field-gradient. The nuclear quadrupole moment, Q, is related to the shape of charge distribution of atomic nuclei. The Q is propotional to the charge of Z of the nucleus. On the other hand, the electric-field inhomogeneity at the position of a nucleaus is describede as eq which stems from the second derivatives of electrostatic potential at the nucleus, V(R).

Others

The nuclear Zeeman term is the same concept as the electron Zeeman term. The only difference between two is the range of the resonances. In the case of nuclei, the resonance occurs in the region of radio frequency (~ MHz) due to the mass of the nuclei. The ratio of the nuclei to electron is around 2000, resulting in the magnetogyric ratio become small. This means that the energy splitting is smaller than that of the electron.

The absorption of the radio frequency corresponding to the nuclear Zeeman energy is called the nuclear magnetic resonance (NMR). In comparison with EPR, its sensitivity is low so that the spin polarization of the nuclei is small at thermal equilibrium. However, since each nuclei possesses specific magnetogyric ratio, the applicable field over almost whole materials as well as atomic selectivity. Their fine-structure term is explained in the same manner as the nuclear quadrupole term. This interaction should be taken into the account in the system with S > 1/2. One consequence of this contribution is the fine structures which are important to understand zero field splitting. Furthermore, in the coupled spins system, the exchange interaction term (ℋ 𝒥 = -2J𝑠 1 ⋅ 𝑠 2 ) is also needed to be paid careful attentions to understand the spectra.

Relaxation

EPR is caused by the difference in the occupation number of states with different spin numbers. Therefore, the continuous wave EPR, which is widely used at present, often observes the saturation of the EPR signal due to the strong microwave irradiation. In a spin system that has changed from a ground state to an excited state due to microwave absorption, the energy returns to the ground state by transferring energy to the lattice system. The relaxation time is defined as a physical quantity characterizing the transition from the excited state to the ground state.

The exchange of energy between the spin system and the lattice system is called spin-lattice relaxation time (T1), and is an important parameter that determines the microwave intensity dependence. The spin-spin relaxation time (T2)

that governs coherence and decoherence between unpaired electrons also affects microwave intensity dependence.

In EPR measurement using continuous microwave, the saturation factor s is defined by the following equation.

S = 1 1 + γ 2 𝐵 1 2 𝑇 1 𝑇 2
When s < 1, the amplitude of the cw EPR signal decreases and broadening of the signal may be observed.

In practice, by plotting the EPR amplitude as a function of square root of microwave power Pmw, one can confirmed whether the EPR line is saturated or not. Deviation from linearity means microwave saturation.

On the other hand, since T1 and T2 are unique physical quantities at each paramagnetic center, if T1 and/or T2 are significantly different at each paramagnetic center, one of them is measured in a saturated state, and the other is EPR. Helps you understand the outline of the line. Generally, T2 does not depend much on temperature, whereas T1 has a strong temperature dependency. Clarifying the physics governing matter through the temperature dependence of T1 is one of the real thrills of magnetic resonance. However, it is difficult to separate T1 and T2 by the currently widely used CW method. A promising method is EPR measurement using pulse microwave.

Chapter 4 EPR study on BaSi2 film

In this chapter, I show the results obtained on a-axes-orientated epitaxial BaSi2 films. The films are fabricated on Si substrates by molecular beam epitaxy (MBE). 59-63, 101,102 My final goal is to evaluate defects in BaSi2 thin-films, which are expected to act as light absorbing layers. Since this is the first EPR study on BaSi2, it was not known whether an EPR line was actually observed. However, if an EPR line could be observed in thin-films directly, that is the shortest and most efficient route. A point to be noted here is that it is extremely difficult to separate the EPR lines of BaSi2 from those of the Si substrate because a BaSi2 thin-film of about 500 nm is deposited on the 625μm-thick Si substrate at 625 μm. In order to suppress the EPR line derived from the Si substrate, Si substrates (Floating-Zone (FZ) Si substrate) having a high resistivity, i.e., low carrier density was used. Therefore, even if the origin of the paramagnetic center in the Si substrate is observed, it is highly likely that the origin of the paramagnetic center is an isotropic one having no crystal symmetry. On the other hand, the BaSi2 thin-film is an epitaxial film that is uniaxially oriented in a direction perpendicular to the plane. Hence, the origin of the anisotropic EPR line can be ascribed to the BaSi2 thin film. The above is the strategy in this chapter.

a-axis orientated epitaxial BaSi2 films

A crystal growth of BaSi2 was performed using a molecular beam epitaxy (MBE) chamber having a maximum ultrahigh vacuum of about 10 -8 Pa. The MBE chamber is equipped with a Knudsen cell (K-cell) for supplying Ba and an EB gun for supplying Si. Further, the deposition rate RSi of Si is controlled by a feedback system using an electron impact emission spectroscopy (EIES) sensor. The procedure for preparing a sample is described below.

In this study, Si (111) substrate was used to fabricate a BaSi2 epitaxial film. On the surface of the Si substrate, there is a natural oxide film of about 1 nm and organic pollutants, metal pollutants, and particles. Therefore, before growing BaSi2, it is necessary to remove them and obtain a clean surface. The substrate pre-pretreatment shown in Table 4-1 was performed. First, in order to remove organic contamination on the Si surface, organic cleaning with acetone / methanol / pure water was performed. Next, the native oxide layer was removed with a 5% aqueous hydrofluoric acid (HF) solution. Further, it was immersed in an aqueous hydrochloric acid (HCl) solution for 5 mins for the purpose of removing metal contamination. At this time, hydrogen peroxide solution (H2O2) was also simultaneously added to form a protective oxide layer on the Si surface. Then, the protective oxide layer was removed again with the 5% HF aqueous solution, and then immersed in an aqueous ammonia solution (NH4OH) to remove particles and organic substances. Note that the H2O2 was also added into this NH4OH aqueous solution to form a protective oxide film. Finally, the substrate was transferred into the ultrahigh vacuum chamber after removing moistures. The substrate was heated at 900 °C in the chamber for 30 min to remove the protective oxide film and obtain a clean surface of Si (thermal cleaning, TC).

After obtaining a clean surface of the Si substrate, a 5-nm-thick BaSi2 template layer was deposited on the Si substrate heated at 500 °C by supplying Ba atoms (reactive deposition epitaxy, RDE). This template layer serves as a seed crystal for a layer laminated thereon. 119 Next, a BaSi2 thin-film having a thickness of about 450 nm was grown by MBE in which Ba and Si were simultaneously supplied and reacted on the template layer heated to 580 °C.

During the MBE growth, the deposition rate of Si (RSi) was fixed at RSi = 0.9 nm/min and the deposition rate of Ba (RBa) was changed from 1.0 nm/min to 3.2 nm/min, resulting in the films with RBa/RSi = 1.1 to 3.6 were obtained. 51,97 Thereafter, a 5-nm-thick amorphous Si (a-Si) was deposited in situ as a surface passivation layer on undoped BaSi2 thin-films heated to 180 °C. 104,120,121 In this study, FZ undoped n-Si (111) substrates (ρ = 1000-10000 Ω cm) were used to suppress the EPR line from the Si substrate. RHEED, XRD, and Raman spectroscopy were performed for characterizations of the films. In the XRD measurement, out-of-plane -2 measurement was performed to determine an a-axis lattice constant, and in-plane φ-2θχ measurement was performed to determine the b-and c-axis lattice constants. An Nd: YAG laser (532 nm) was used for Raman spectroscopy. In all the samples, the Raman spectra due to Si4 tetrahedra constituting BaSi2 were observed in a region between 250-500 cm -1 with Th symmetry in the lattice of BaSi2. 96,[123][124][125][126][127] On the other hand, at RBa/RSi = 1.1 and 1.5, a Raman spectrum derived from c-Si precipitated in the film was observed at 520 cm -1 . The wavelength of the incident light used in this time was 532 nm, and considering the light absorption coefficient of BaSi2, it can be said that these samples contained c-Si precipitated within 3𝛼 ~ 100 nm from the surface. 35 The atomic ratio of Ba and Si in the films were not evaluated by the Rutherford backscattering method, etc. However, from the previous reports, the atomic ratio closest to its stoichiometry was considered to be RBa/RSi = 2.2. 51 The point to be noted for thin-film samples used in this study are summarized below. ➢ RBa/RSi = 1.1:

Crystalline Si precipitate were detected within 100 nm from the surface. XRD results also showed the peak derived from the precipitate. The crystallinity was significantly worse than the other samples, and it is presumed that the vicinity of the surface was amorphous.

➢ RBa/RSi = 1.5:

Crystalline Si precipitate were detected within 100 nm from the surface, but not as bad as RBa/RSi = 1.1. The crystallinity was considered to be relatively good from the RHEED pattern after MBE growth.

➢ RBa/RSi = 1.9-2.5:

Both surface condition and crystallinity were good. BaSi2 single phase.

➢ RBa/RSi = 3.0:

After MBE growth, a spot pattern was confirmed, while no precipitate was confirmed by neither XRD nor Raman spectroscopy, suggesting that it was a BaSi2 single phase.

➢ RBa/RSi = 3.6: After MBE growth, a spot pattern was confirmed, and a peak derived from the precipitate was confirmed in XRD.

EPR spectra of BaSi2 epitaxial films

For the measurements, the samples have been cut to pieces with typical dimensions of 20 mm × 2 mm × 0.0625 μ m and were transferred into EPR tubes. The details of their size are listed in Table 4-3. These tubes were sealed under Ar atmosphere. Raman spectra also show growth of BaSi2.

Summary

I observed different paramagnetic centers in a Si substrate and BaSi2 epitaxial films as I listed below.

The Defect A in BaSi2 film : 2.010-2.013

The Defect B in BaSi2 film : 2.003-2.010 (anisotropic)

Paramagnetic center from Si substrate on its cleavage planes. : isotropic

To characterize the paramagnetic centers in BaSi2, I performed multi-frequency EPR measurements. However, no obvious EPR signal for BaSi2 was obtained. This is probably because the amount of the paramagnetic center is too low to detect. In addition, judging from the microwave power and temperature dependencies, the paramagnetic centers in BaSi2 are considered to have longer relaxation times. As a result, the observations were difficult at Q-band and high field. On the other hand, the EPR measurements for w/ and w/o atomic H revealed that the Defect B is likely to affect photoresponsivity.

Chapter 5 EPR study on BaSi2 bulk

In chapter 4, I successfully observed the EPR lines from the BaSi2 thin-film for the first time, suggesting that the observed EPR lines may be closely related to the photoresponsivity based on the research on the effect of atomic hydrogens. In order to identify the origin of this EPR lines, it is strongly required to separate them from the EPR lines derived for the Si substrate. Therefore, I tried to separate each EPR line by means of various EPR techniques such as multi-frequency EPR measurements, etc. Unfortunately, all the trials failed. This was because BaSi2 is too sensitive toward moistures to perform wet etching for removals of the paramagnetic center of Si substrate. 147 The electrically detected EPR (EDMR) might be one of the solutions because the EDMR has much better sensitivities for the defect detections. [148][149][150][151][152] On the other hand, further complicating the study is the lack of EPR studies on BaSi2. Therefore, we decided to conduct EPR measurements on bulks to get insights on intrinsic defects of BaSi2 bulks, first of all.

Powder

First, we performed EPR on powder samples. Compared to single crystals, the powder samples can only provide principal values of EPR parameters, such as g-tensors and/or hyperfine tensors. However, because it is relatively easy to prepare and the volume can be increased, it is considered to be the best as a first step of EPR studies in BaSi2. In addition, by appropriately changing the composition ratio of Ba and Si during sample preparation, it is possible to systematically investigate the effects of the composition ratio on defect formation. The primary goal is to take a bird's-eye view of intrinsic defects in BaSi2.

Samples

Polycrystalline BaSi2 bulk samples were provided by Dr. Imai (NIMS) and Mr. Mesuda (Tosoh Corp.). All the samples were prepared by Ar-arc melting of 1 : xs (xs = 2.10, 1.90, 2.06, 2.00, 1.94, and 1.88) molar mixture of Ba (3N, Furuuchi Chem. Corp.) and Si (10N, Furuuchi Chem. Corp.). The chemical compositions of synthesized polycrystalline samples were determined by inductively coupled-plasma optical emission spectroscopy (ICP-OES).

Table 5-1 lists the Si-to-Ba ratio of the starting materials xs and that of synthesized samples (NSi/NBa). Group1: Si-rich (x > 2.00) Group2: Ba-rich (x < 2.00) I note that samples I and J and samples K-N were provided by different research groups. Table 5-1 shows the order different from NSi/NBa obtained by ICP-OES based on the results of EPR and photoluminescence described later.

Effect of composition ratio of Ba and Si

Polycrystalline BaSi2 were powdered by using an agate in a glove box in order to avoid the generation of new defects due to a reaction with moistures and oxygens in the air. Then, the powdered samples were put into Xband tube (inner diameter is 3 mm) and Q-band tube (Inner diameter is 2 mm) to a height of approximately 1 cm, and sealed in a glove box. the pulse shot reputation time of the sequence (srt) in msec, the accumulations, and the video gain for the pulse detection (V.G.) in dB.

cw-X-and Q-band spectra

Table 5-2 Experimental conditions of cw-X-band. The Q value of the cavity, the microwave frequency (νMW) in MHz, the microwave power (PMW) in mW, the modulation frequency (νm) in kHz, the modulation amplitude (Bm) in mT, the time constant (TC) in msec, the conversion time (CT) in msec, the accumulations, and the receiver gain (R.G.) are listed.

Table 5-3. Experimental conditions of cw-Q-band. The Q value of the cavity, the microwave frequency (νMW) in MHz, the microwave power (PMW) in mW, the modulation frequency (νm) in kHz, the modulation amplitude (Bm) in mT, the time constant (TC) in msec, the conversion time (CT) in msec, the accumulations (acc) , and the receiver gain (R.G.) in dB are listed.

General comparison

For ➢ Si-rich samples → gav > 2 (I-L)

➢ Ba-rich samples → gav < 2 (M and N) Threshold is stoichiometric ratio Table 5-5. Experimental conditions of pulse-Q-band. The microwave frequency (νMW) in MHz, the microwave pulse power (PMW) in mW, the details of the pulse sequence: tp [nsec] -τ [nsec] -2tp [nsec] -τ -echo, the detection window of the echo [nsec], the pulse shot reputation time of the sequence (srt) in msec, the accumulations, and the video gain for the pulse detection (V.G.) in dB.

Summary

The different paramagnetic defects are formed in BaSi2 depending on the composition ratio of Ba and Si (Table 56789).

Label

NSi/NBa

Defect1 Defect2 Defect3 Defect4 Defect5 I 2.404 ◎ 〇 - - - J 2.087 ◎ 〇 - - △ K 2.134 ◎ 〇 - - △ L 2.054 ◎ 〇 - - △ M 1.987 △ - △ ◎ 〇 N 1.916 △ - △ ◎ 〇
◎:Observable (dominant), 〇:Observable, △:Observable (weak), -:Unobservable Shape:Observed EPR lines at g ~ 2.000 and 1.995

• Defect 1 [Si-rich]
Relaxation:T2 short enough not to be observed in pulse-EPR Others:B Observed only in Ba-rich samples

• Defect 5 [Ba-rich]
Shape:Hyperfine structure centered on g ~ 1.982

Relaxation:Most saturable defect observed in Ba-rich sample

Others:EPR line dominant with Si-rich samples

Hyperfine spectroscopy

The selection rule of EPR is |Δ𝑚 𝑆 | = 1, |Δ𝑚 𝐼 | = 0 where ΔmS and ΔmI is the change in the magnetic quantum number of the electron and the nuclear spin. When there are some interactions such as hyperfine, quadrupole, spin-spin, spin exchange, … interactions, their states are mixed to an observable extent and the "forbidden" transition which doesn't obey the selection rule of EPR is observed. In a field of magnetic resonance, such conditions are often met. This is probably because the energy difference is not too large comparing with the interactions in contrast to, for instance, optical transition.

Hyperfine interaction is a local interaction between electron spin and nuclear spin. Therefore, by determining the hyperfine coupling tensor, it is possible to obtain extremely useful information about each paramagnetic center, for examples, the type of defect and the local structure. In this section, I aimed to measure the types and structures of paramagnetic centers observed in Si-rich and Ba-rich samples using various methods. In Hyperfine spectroscopies, it is mandatory to have relaxation time of each paramagnetic center to be investigated. For example, in ED-NMR, the detection pulses should be applied after 5 T2 in order to exclude the influence of T2 after HTA-pulse. On the other hand, the hole burning created by HTA-pulse recovers according to T1. In a general semiconductor, T2 has a weak temperature dependency and T1 has a strong one. Therefore, by changing the temperature, it is possible to search for the optimum measurement condition of each paramagnetic center. In section 5.1.3, I had already studied the relaxation times (T1 and T2). Based on those results, I optimized experimental conditions for each hyperfine spectroscopies.

HYSCORE, ENDOR, ED-NMR

Compared to g-tensor, it often requires sophisticated techniques to measure the hyperfine structures. Once the hyperfine structures are observed, we can compare the experimental results with those theoretically computed by DFT, etc. [145][146][147] This is because the hyperfine interaction basically considers only the ground state of unpaired electrons, while the theoretical g-tensor considers excited states as well as the ground state. The simplest measurements of the hyperfine structures are to perform multi-frequency EPR 137,160 by using continuous microwaves.

Since the hyperfine tensor is field independent, we can separate each component of the observed EPR spectra by comparing with g-tensor which is field-dependent. In complicated or disordered spin system like this study, the advanced EPR spectroscopies are needed to get a whole picture as I listed on the Table 5.10. These three pulse EPR sequences possess merits and demerits. Therefore, it is the best way to choose which sequence is appropriate for each study after trials of all sequences.

MW RF and MW MW

the viewpoint of solar cell devices (Fig. 5. 5-6(a)). Noted that although hyperfine tensor doesn't depends on the magnetic field, however, the regime of weak and strong coupling can change. This is because each scheme is defined for nuclear Lamor frequency, ν I (Fig. 5.2-6(b)).

Large hyperfine coupling is much more interesting in this study

Table 5-16. ED-NMR peak positons for Sample I-N at different magnetic field (Q-band).

This time, often asymmetric ED-NMR spectra were observed. In previous reports, ED-NMR spectra have been observed at symmetric positions. [168][169][170] Also, the observed ED-NMR signals in this study are very broad compared to previous reports. It may be considered that one of the factors of such asymmetry is that a plurality of ED-NMR signals are superimposed to form an apparently asymmetric shape. Regarding this point, improvement of resolution is expected by ED-NMR measurement in W-band (~ 94 GHz). 168 Since 135 Ba (or 137 Ba) has I > 1/2, it is also necessary to consider the influence of the nuclear quadrupolar interactions. Judging from the nuclear Zeeman frequencies of 29 Si, 135 Ba, and 137 Ba, almost all the ED-NMR peaks are consider to correspond to the value of |A/2|. However, since there is no information on quadrupole tensors of Ba nuclei in this material, I don't state that the ED-NMR peak positions are equal to |A/2|. I will discuss these results based on the DFT results later in section 5.6.

In this section, we succeeded in observing the hyperfine coupling constants in BaSi2 for the first time by applying ED-NMR. At the same time, there are many unclear points as described above. ED-NMR spectra indicate the large hyperfine couplings in BaSi2. Next, the narrow and less anisotropic EPR line at g ~ 1.998 has hyperfine coupling based on its temperature dependence and cw-and pulse-spectra (the Defect 5). Prior to the simulation, possible EPR lines were considered from the viewpoint of Si and Ba isotopes. Figures 5.4-5 shows how many 29 Si are likely to be present in a system with N Si atoms based on the natural abundance of 29 Si. As can be seen from the figure, when I focus on one Si4 tetrahedron constituting BaSi2, the probability that two or more 29 Si are included in one Si4 tetrahedron is about 1% .

Ba-rich sample

In an unit cell, the probability of two 29 S is increases to 14%. In the 2×2×2 supercell, it is most likely that there are 6 or 7 29 Si atoms in the system. What is important now is that by arranging the histogram symmetrically, I can imagine the outline of EPR lines which should be observed in real experiments. Since at least three hyperfine structures are observed in the cw-X-band EPR spectra, if the unpaired electrons have a charge distribution spanning at least about 16 Si atoms, I can interprit that the three hyperfine structures stem from the interactions with three different 29 Si.

However, considering that the hyperfine interaction is a local interaction between the electron spin and the nuclear spin, if the charge distribution covers 16 Si atoms, the hyperfine coupling constant itself should be small even in Xband. Therefore, it can be difficult to expect a value as large as those observed in experiments. Thus, I also have to consider the contributions of 135 Ba and/or 137 Ba as well as 29 Si.

Simulation of powder pattern of the Si-rich:

• Defect 1: g1 = 1.993-1.998, g2 = 2.012-2.014, g3 = 2.025-2.030

• Defect 2: g1 = 1.997, g2 = 2.004-2.005, g3 = 2.010-2.011 H strain (60, 60, 100) (60, 60, 100) Ratio of each defect (Defect 2 : Defect 3 : efect 5)

1.9 : 5 : 1 0.3 : 4 : 1

Spin concentrations

The spin concentrations of the powder samples (Samples I-N) were evaluated by using a sample having a known spin concentration. In this study, five TEMPO disolved in orthoterphenyl with different mol/L were used as standard data. Spin concentrations, NS, are given the following equation, 172,173

N S = DI⋅V √ P⋅B m ⋅Q⋅c⋅S(S+1)⋅n B ⋅f (B 1 ,B m )
.

where, DI, V, P, Bm, nB, Q, c, S, and f are double integrations, volume of sample, microwave power, modulation amplitude, Boltzmann factor, resonator factor, resonator calibraion factor, electron spin, and resonator filled profile, respectively. Also, TEMPO changes magnetization as function of temperatures according to the Curie's law.

M = C/T, C : Curie constant TEMPO dissolved in orthoterphenyl were introduced into an X-band EPR tube at the same height as powdered BaSi2, and measured at the center position of cavity where the spectrum intensity reaches its maximum. In order to correctly estimate the spin concentrations, the measurement were performed at a microwave power of 5 μW at which the EPR spectrum was unsaturated from. The obtained EPR spectra was subtracted the baselines, and then integrated twice to evaluate DI. The obtained from DI were normalized based on the experimental conditions, Simulation of powder pattern of the Ba-rich:

• Defect 3:g1 = 1.844, g2 = 1.910, g3 = (1.930)

• Defect 4:tentative, gav ~ 1.975 at X-band

• Defect 5:g1 = 1.977, g2 = 1.980, g3 = 1.983-1.984, large hyperfine coupling of 22, 45, 84 MHz Table 5-22 Calculated isotropic part of hyperfine-tensor for VSi (q = -1).

Size of cell

Hyperfine tensor [MHz] 1×1×1 Ba → -76, -50, -33, -22, -21

Si → +34×2 2×2×1 Ba → +92×2, +50×2, -46 Si → +23×2 2×2× 2 Ba → +109×2, +63×2, -50 Si → +22×2
It can be seen from this example that no great change occurs anymore between 2×2×1 and 2×2×2 models for DFT hyperfine data, showing that four cells are large enough to accomodate the defect. We therefore propose to harmonize all following results at the 2×2×1 (i.e. four cells) level.

The isotropic part of hyperfine tensor is given by

a iso = 8π 3 g n g eff μ n β|ψ(R = 0)| 2 S⋅I
where ψ (R =0) stands for the one-particle wavefunction at the atomic site R. The nuclear g-factor, the effective gfactor, the nuclear magneton, and the Bohr magneton are denoted as gn, geff, μn, and β, respectively. As is clear from the above equation, the wavefunction of the ground state is a factor which causes the differences. When the size of the cell is too small to be delocalized, unpaired electrons are localized at near the defects. Thus, in too small size of the cell, isotropic hyperfine tensor is overestimated. Furthermore, it is readily to imagine that, as shown later, anisotoropy of g-tensor became larger in a smaller sized cell because of a strong spin-orbit interaction. Therefore, I have to use a large enogh cell for unpaired electrons to delocalize. In subsequent calculations, I compared the hyperfine tensor calculated by 2×2×1 supercells. From Figs. 5.5-3 and 5.5-4, some atoms can have strong hyperfine interactions (for instance, Sii -has large aiso as seen in Fig. 5.5-3(c)) while almost all atoms show small ones less than 10 MHz. On the other hand, in practical, I have to take their anisotropic part into accounts in order to compare the calculated values with those by experiments. The whole hyperfine tensor is given by A = aiso + T , where aiso and T are isotropic and anisotropic parts of hyperfine tensor. In a system of principal axes, the principal values of the tensor are described as (A11, A22, A33) = (aiso -T11(1 + δ), aiso -T22(1δ), aiso + 2T33). I can observe only the principal values by means of the hyperfine spectroscopies because I used powder samples in this study. I show histgrams of the principal values including the contributions of the anisotropic parts for each defect (Figs. 5.5-5 and 5.5-6 ).

Comparing among the results, almost all defects such as VSi (q = -1) contain strong anisotropic hyperfine coupling with 29 Si as I marked by arrows in Figs. 5.5-5 and 5.5-6. This might be due to a contribution of Si-2p-orbital electrons.

On the other hand, large hyperfine couplings are often stemed from Ba atoms except for Sii (q = -1) and SiBa (q = -1).

g-tensor

Next, I computed g-tensors of each defects. I listeed an effect of the calculated crystal size on g-tensor on Table 5-23, which are results for VSi (q = -1) obtained after geometry-optimizations. It can be seen that the anisotropy decreases as the cell size increases. In subsequent calculations, I compared the gtensor calculated in 2×2×1 supercells for each defect (Table 5 -24).

Nuclear quadrupole tensor

In 135 Ba and 137 Ba atom which possess I = 3/2 (> 1/2), the quadrupole interaction has to be taken into account. The Electric field gradient at the position of nucleus causes energy splitting depending on the direction of nulear spin. Hence, I also evaluate the quadrupole coupling constant, K = e 2 Qq/12h (in the case of I = 3/2), where e, eq, Q, and h are the elementary charge, the largest magnitude component of electric field gradient at the nucleus, the electric quadrupole moment of the nuleus, the Plank's constant, respectively.

The principal values of the nuclear quadrupole tensor are given by,

Chapter 6 Photoluminescence spectroscopy

EPR measurements provided microscopic information on defects in BaSi2. However, it is difficult to determine their defect levels in the band gap by EPR. Therefore, I tried to determine the defect levels by photoluminescence (PL) spectroscopy, which is one of the most powerful methods for determination of the defect levels. A radiative defect model is also established by comparing the PL measurement results of samples having different atomic ratios with the defect levels previously reported. Furthermore, I paid special attention to the difference in formation of defects between the polycrystalline samples and thin-films by PL measurement.

Sample

I performed the PL measurements basically on the same samples as those used for the EPR measurements.

The preparation and characterization of the polycrystalline sample are as summarized in

PL spectra of polycrystalline samples

I show the PL spectra of the polycrystalline samples measured at 8 K in Fig. 6.2-1. Among all the samples, relatively broad spectra were observed. The band gap of BaSi2 is equal to 1.3 eV at RT. Also, there is a report showing that the band gap increased up to 1.4 eV at 8 K with decreasing temperature. 91 Therefore, all the observed peaks are

Chapter 7 Total summary

My thesis focused on the investigation of intrinsic defects in BaSi2 for defect control which is indispensable to design higher-η photovoltaic devices. The obtained results and the most significant contributions can be summarized as follows:

EPR study for BaSi2 epitaxial films I performed the electron paramagnetic resonance measurements on a-axis-orientated BaSi2 epitaxial films grown on the Si substrate by molecular beam epitaxy. Through the EPR experiments, I successfully detect paramagnetic centers in the films (the Defect A: an isotropic center with g = 2.00 , the Defect B: an anisotropic center with g = 2.003-2.010), which is pioneering results in BaSi2. Also, I found that atomic hydrogens deactivate the centers.

In other words, I considered that the Defect B can affect optical property significantly. Hence, aiming for further identification of the defects, I made full use of the conventional EPR techniques as well as multi-frequency EPR measurements. The only barrier to overcome was a paramagnetic center from the Si substrate. Since BaSi2 is sensitive to moistures, it was impossible to apply a conventional technique to remove the center originating from the substrate.

Therefore, I changed a strategy from the investigation of BaSi2 films to BaSi2 bulk samples.

EPR study for BaSi2 bulks

I revealed that a deviation from stoichiometry produces five different paramagnetic defects in BaSi2. First, I measured relaxation times (T1 and T2) by pulse EPR measurements and showed that paramagnetic centers possess different time orders of relaxation times. Based on these results, temperature dependence, microwave intensity dependence and multifrequency EPR measurements revealed the presence of at least two paramagnetic defects in Sirich samples and at least three in Ba-rich samples.

In addition, I set up on further identification by evaluating hyperfine couplings of each defect by hyperfine spectroscopy using advanced EPR pulse sequence. HYSCORE revealed that all defects had a small hyperfine coupling of 2-3 MHz with 29 Si. Then, I continued to work on the hyperfine spectroscopy to obtain larger hyperfine coupling constants which correspond to deep defects that were interesting from the viewpoint of solar cell application. I have also started EPR studies using single crystal samples to determine all the components of g-tensor and hyperfine tensor. Although it is necessary to determine the crystal axes by XRD in the future, I have already succeeded in obtaining an interesting angular dependence of the EPR spectra. In a rather short future (2-3 months), I will be able to have a complete list of all simple defects for BaSi2. The EPR measurements on single crystal have such an awesome and tremendous potential.

Based on the above experimental results, each paramagnetic center was separated and simulated using Easyspin. In addition, the spin density of each paramagnetic center was evaluated using TEMPO as a standard sample.

As mentioned earlier, the experimental results could be explained by assuming the presence of two different paramagnetic centers in the Si-rich sample and three in the Ba-rich sample.

Finally, I also tried to obtain EPR parameters by DFT calculation. The g-tensor, hyperfine tensor, and nuclear quadrupole tensor were calculated for a total of 12 defect models with different charge states for 6 different possible defects in BaSi2. Now, the computations are going on very well with 2×2×1 supercell within the limited computational resources. Furthermore, the calculated results showed that each defect model possesses compatible gtensors and hyperfine tensor, suggesting that I can establish defect models in BaSi2 in near the future.

Combining all the above results, it was shown that a negatively charged Si vacancy (VSi -) can be identified in a Ba-rich sample as a specific defect model for the Defect 5. On the other hand, it was revealed that the EPR spectrum observed in the BaSi2 thin-film (the Defect B) could correspond to the paramagnetic center observed in the Si-rich sample (the Defect 2). As for the origin of the Defect 2 is still under discussion.

Photoluminescence spectra of BaSi2

Through photoluminescence (PL) spectroscopy, I tried to obtain information on defect levels that cannot be obtained by the conventional EPR measurements. Four corresponding PL peaks were observed for thin and bulk samples, suggesting that similar defects are present in both samples.

By combining EPR and PL, I also confirmed that the decrease in the EPR line originating from the Defect B, which was observed in the thin film sample into which atomic hydrogen was introduced, corresponded to the decrease in the PL peak of 0.8-0.9 eV. From the viewpoint of solar cell application, the significant improvement in optical properties of the sample with atomic hydrogen was due to the fact that defects located at relatively deep positions in the band gap were inactivated by hydrogen. not accomplished yet. However, now that this study has clarified the paramagnetic centers in BaSi2, for example, EPR measurements using single crystal BaSi2 will further clarify the nature of the defects. The most potential application of BaSi2 is to solar cells, but recently its low thermal conductivity has raised expectations as a thermoelectric conversion material, too. I hope that the defect information identified through this research will be useful in that respect as well.

In the future, rapid scan EPR, 191 which has recently been remarkably developing, will provide a new perspective in addition to electrically-detected EPR as a means of directly observing defects in thin film samples.

From a relation between defects and optical properties, EPR measurements on powder or single crystal BaSi2 with atomic hydrogen introduced will provide more direct information on the effect of atomic hydrogen toward the defects.

In addition, the present study confirmed the correlation between PL and EPR spectra to some extent, indicating that there is plenty of room for studying optically-detected-EPR experiments. 192,193 Focusing on EPR spectroscopy, I hope that the ED-NMR used here could show its amazing potential for investigating hyperfine coupling in disorder systems such as powder samples. The development and improvement of a two-dimensional ED-NMR sequence using Chirp pulses reported recently by Jeschke et al. 194 will show further benefits of ED-NMR, as well as W-band ED-NMR. A ED-NMR spectra simulation combining with DFT calculation will also help us reveal the full picture of the defects in BaSi2.

At the end

In my personal opinion, I have successfully demonstrated that EPR is very good spectroscopy to analyze the defects of BaSi2. At the beginning of the EPR study of the film, it seemed difficult to characterize the defects in BaSi2 by EPR. However, after the investigation on the bulk, I could obtain plenty of results. Moreover, the observed EPR line in the bulk agreed well with that observed in the films. I strongly recommend the "next" challenger to perform electrically-detected EPR on the films and also to continue ED-NMR measurements at the higher field, for instance, W-band.

spectrometer and interpriate spectra. I appreciate it to Dr. Paul NÖEL in Eidgenössische Technische Hochschule Zürich that he helped me prepare the samples and gave me scientific advice.

comments and helpful suggestions.

Special thanks

I am now really satisfied with my Ph. D course in Tsukuba and Grenoble. The three years I spent there were and will be the most precious in my life. I did not imagine such wonderful experiences. I express great gratitude to all the people I worked with and met.

SUEMASU-sensei has given me a lot of opportunities since I started my Ph. D course. First of all, thank you very much for accepting me to your amazing laboratory. I am really glad that I was one of the members of my laboratory. I appreciate it to you that you gave me not only a good environment for researches but also a lot of advice.

Serge is one of the best benefactors in my French student life. Without you, I wouldn't decide to continue researches in a field of EPR. You are always working with a lot of enthusiasm. I respect you and want to become a researcher like you. I am honor to be one of your students. Thank you very much, Serge.

I also express my gratitude to Anne-Laure. The experiences of high-field EPR measurements were very impressive. Also, thank you very much for telling me about the beautiful mountains to visit. I have not visited all the mountains yet, but I will in the future.

TOKO-sensei taught me the attitude of a young researcher and also gave me a very nice opportunity for my next carrier. Thank you so much for all your advice and help.

As senior students in Japan, Dr. Ryota TAKABE and Dr. Tianguo DENG, and Dr. Toshiki Owing to all members of the Suemasu-Toko laboratory, I could enjoy my student life in Tsukuba. Thank you very much.

In France, thanks to all my friends in the laboratory. Also, I especially thank Mr. Damien DEJEAN and Mr. Jérôme NENICH. Without you guys, I couldn't spend such a great daily life in Grenoble. Thanks a lot.

Finally, thank you very much, my parents, for watching me warmly so far.

February, 2020

Takuma SATO

A.3.1. Method

Two intrinsic point defects, Si vacancy (VSi) and Ba substituted for Si antisites (BaSi), were introduced into one of the four Si(3) sites as previously reported. 51,106 I adopted a previously reported interstitial site, the fractional coordinate of which is (0.5841, 0.25, 0.2251), as an initial configuration for Si interstitials (Sii). 174 The vibrational frequency calculation at Γ point was performed with QUANTUMN ESPRESSO code 175 within the framework of ab initio pseudopotential density functional perturbation theory. 204 I employed the norm-conserving pseudopotentials with generalized gradient approximation in Troullier-Martins type 205 and the cut-off energy of the plane-wave basis sets was 45 Ry. For k points in the Brillouin zone, a 3×4×2 Monkhorst-Pack mesh was used for a perfect crystal (with an orthorhombic cell). The estimated energy error in self-consistency was less than 10 -14 a.u. Relaxation was performed until the total energy changed by less than 10 -5 a.u. and the components of forces were smaller than 10 -4 a.u. during geometry optimization. 79 回応用物理学会秋季学術講演会 19p-436-16, 名古屋, Sept. 19 (2018).

Fig. 1 . 2 - 1 16 Fig. 1 . 2 - 3

 12116123 Fig. 1.2-1 Illustration of the air mass concept. 15 Fig. 1.2-2 AM0 and AM1.5G solar irradiance spectra.16

Fig. 1 .

 1 2-7 shows the effect of Rs and Rsh on the J-V characteristics. In the J-V characteristics under light irradiation, the slope near VOC indicates 1/Rs, and the slope near Isc indicates 1/Rsh. That is, the slope of the current near VOC becomes gentler as the Rs increases, and the slope of the current near JSC becomes sharper as Rsh decreases. Therefore, since Rs and Rsh greatly affect the FF of for the changes in the concentrations of electrons in the CB ne, of holes in valence band (VB) nh, and of trapped in defects ne,d as a function of the generation and recombination rates are given by ∂n e ∂t = G -R e,d + G e,d ,

  to the concentration of ne,d or nh,d. Therefore, the generation rates of electrons and holes are G e,d = β e n e,d and G h,d = β h n h,d . Here βe and βh are derived from the consideration of detailed balance in the dark. In the absence of external excitation (G = 0) and in a steady state, ∂𝑛 𝑒 ∂𝑡 = G-𝑅 𝑒,𝑑 + 𝐺 𝑒,𝑑 = -σ e ν e n e (𝑛 d -𝑛 e,d ) + β 𝑒 𝑛 𝑒,𝑑 , Therefore,

  At room temperature (RT) and under atmospheric pressure, BaSi2 exhibits orthorhombic structure (space group Pnma (no. 62)) as shown in Fig. 2.1-1. This material contains 16 Si atoms and 8 barium atoms in its unit cell (Z = 8), and every 4 Si atoms form to a tetrahedral structure. Its lattice constants are 8.9326 Å, 6.7262 Å, 11.5335 Å,

  the molecular orbital (MO) diagram of BaSi2 is interpreted to consist of a combination of the density of states distributions (DOS) of Ba atoms and 4Si4, and the part of Si4 4-is similar to the occupied MOs of the P4 tetrahedron.

Figure 2 . 1 -

 21 Figure 2.1-2 shows the phase diagram of the binary system of Ba and Si. 27 Each compound has a crystal structure as shown in the Figs.

  Figures 2.2-3(a)(b) and 2.2-3(c)(d) show secondary-electron (SE) and electron EBIC results, respectively, with acceleration voltage Vac = 5 kV. Front-side Schottky contacts were formed with Al on the BaSi2 surface via wire bonding, and the back-side ohmic contact was made with sputtered Al. In the EBIC method, carriers generated within the diffusion length in the n-type BaSi2 are collected by the electric field under the Al contact and sensed as a current in the external circuit. In Figs. 2.2-3(c) and 2.2-3(d), the brighter regions show higher collection of electron-beam-induced carriers in the BaSi2.

  Figure 2.2-3(e) shows the EBIC line-scan data along dotted line AA' in Fig.2.2-3 (c). The EBIC profile shows an exponential dependence of the distance from the Al contact.

Fig. 2 .

 2 Fig. 2.2-1 (a) Band structure 33 and (b) atomical resolved and orbital projected DOS of BaSi2. 34

Figure 2 .

 2 Fig. 2.2-3 (a), (b) SE and (c), (d) EBIC images around Al contact. (e) EBIC line-scan data along dotted line AA'. 44

Fig. 2 . 3 - 1 62 Fig. 2 .

 231622 Fig. 2.3-1 Epitaxial relationship of BaSi2 on (a) Si(111) and (b) Si(001).61,62

Fig. 2 .

 2 Fig. 2.4-3 (a) Band alignment; (b) J-V characteristics under AM1.5G illumination and (c) IQE spectrum for n + -BaSi2/p-BaSi2/p + -BaSi2/p + -Si diodes. 80

Fig. 2 . 5 - 8

 258 Fig. 2.5-8 Calculated formation energies as a function of the Fermi energy of the intrinsic point defects in BaSi2 under (a) Sirich and (b) Ba-rich conditions. 106

  111 Since 29 Si is contained with low natural abundance (~ 4.68 %)112 , they prepared the 29 Si enriched samples for the measurements. Figure 2.5-10 shows the 29 Si Magic angle spinning (MAS) NMR signals. The MAS NMR signal of BaSi2 seen at the bottom of Fig. 2.5-10 corresponded to three crystallographically different Si sites in BaSi2. The isotropic shift was negative and between -50.2 and -365.5 ppm, which corresponded to those of being diamagnetic or non-metallic materials (in the range of -50.2 --365.5 ppm). Furthermore, they also confirmed the temperature dependence of the shift, concluding that the dominant interaction was chemical shielding. Besides, based on quantum mechanical calculations, the intensities of the signals from Si3 and Si4 site (in Fig. 2.1-1) were equal because of equal multiplicity of the occupied sites. Since chemical shielding is the dominant mechanism, they evaluate the anisotropic shift (δ aniso ) and asymmetric parameter to get a picture of the electron distribution in the vicinity of the silicon atoms (Table

Fig. 2 .

 2 Fig. 2.1-12. X-band EPR spectrum at 150 K (g = 2.004(2)) of the sample containg [Si2.4Ge6.6] 3-. 116

Figure 4 . 1 -

 41 1 shows the RHEED pattern after Thermal Cleaning (TC), RDE, and MBE growth. After TC in all samples, clear RHEED patterns along with Si [112 ̅ ] were confirmed. It can also be seen that the BaSi2 template layer was formed in all samples by the RDE method. After MBE growth, a streak pattern indicating that the film was grown epitaxially when RBa/RSi = 1.5-2.5. On the other hand, a halo pattern was observed in RBa/RSi = 1.1, which Next, Raman spectra of the films are shown in Fig.4.1-4.

Figures 4 . 2 -

 42 Figures 4.2-1 shows the EPR line results for the sample with RBa/RSi = 2.2, together with the result for the Si substrate measured as a reference. In EPR measurements, sample arrangements where the static magnetic field and the normal of the substrate surface (corresponding to the a-axis of BaSi2 thin-film) are parallel is defined as 0 deg. The measurement were carried out by changing the angles at every 15 deg up to 90 deg where the static magnetic field and the a-axis are perpendicular (Fig. 4.2-1 (a)).

Figures 5 . 1 -

 51 Figures 5.1-1 shows XRD powder patterns of samples I-N. Focusing on the region between 2𝜃 = 40-60 deg (Fig. 5.1-1(b)), the observed patterns are in well agreements with previous reports. 88,90 Thereby, I confirmed that BaSi2 was obtained in all samples. However, in Sample I, which is the most Si-rich, extra peaks derived from crystalline Si (c-Si) were observed at 2𝜃 = 47.5 deg and 2𝜃 = 56 deg. It is inferred that the composition ratio BaSi2 of Sample I is smaller than the value obtained by ICP-OES due to the amount of Si precipitated. Similarly, in sample N, which is the most Ba-rich sample, a peak of a precipitate (judging from the phase diagram, 27 probably Ba3Si4 31 ) was observed at 2𝜃 ~ 48 deg as shown by a triangle (▼) in Fig. 5.1-1 (b).

Figure 5 .

 5 Figure 5.1-2(a) shows the powder cw-EPR spectrum measured at 𝜈 ~ 9.7 GHz (X-band), PMW = 0.005-0.02 mW, and T = 10 K. Table5-2 shows more detailed measurement conditions. Figure 5.1-2(b) shows the powder cw-EPR spectrum measured at ν ~ 34 GHz (Q-band), PMW = 5×10 -2 mW, and T = 15 K. More detailed measurement

Fig. 5 .

 5 Fig. 5.1-1 (a) Powder XRD pattern of sample I-N. (b) Enlarged the results of (a) between 2𝜃 = 40-60 deg. In sample I, at 2𝜃 = 47.5 and 56 deg, the peaks of crystalized Si were observed. In sample N, the peak of precipitation (probably Ba3Si4 31 ) was confirmed at 2𝜃 = 48 deg (as shown as a triangle, ▼).

  Figs. 5.1-2 and 5.1-3, it can be confirmed that a slight amount of those observed in the Ba-rich samples were formed even in the Si-rich sample. Figure 5.1-4 (a) shows the superimposed cw-X-band spectra of samples M and N recorded at 50 K. It can be seen that the two spectra agree well at g < 2 or less. On the other hand, the spectrum is slightly different at g > 2.

Figure 5 .

 5 Figure 5.1-4(b) shows the difference between samples M and N. From Fig.5.1-4(b), it is inferred that the sample M has a small amount of a different paramagnetic center having gav > 2, which seemed to correspond to the dominant center in the Si-rich group from the comparison of the g values. Thus, I concluded that the defects from different origins were likely predominant in the Si-rich and the Ba-rich samples, whose threshhold seemed to be at their stoichiometric ratio (i.e., x = 2).

Fig. 5 .

 5 Fig. 5.1-4 (a) EPR spectra of Sample M and N recorded with X-band frequency at 50 K. (b) A Subtracted spectrum (Sample N -Sample M). I speculated that a line placed at blue arrow corresponded to that observed in Si-rich samples.

•

  Shape:Broad and anisotropic EPR line (g ~ 2.04-1.98) Relaxation:Inferred to have a long T1 due to insensitivity toward temperature rise Others:EPR line dominant with Si-rich samples • Defect 2 [Si-rich] Shape:Relatively narrow line centered at g ~ 2.005 Relaxation:Relative intensity of the spectra decreases with increasing temperature Others:-Defect 3 [Ba-rich] Shape:Broad and anisotropic EPR line(g1 = 1.930, g2 = 1.905, g3 = 1.855) Relaxation:Observed at low temperature (T < 25 K) and under strong PMW Others:Observed only in Ba-rich samples • Defect 4 [Ba-rich]

First, I determined

  the initial values of the Defect 3 as g1 = 1.930, g2 = 1.910, g3 = 1.844 from the what is observed in the cw-Xband and cw-Q-band spectra. The line width was set to initial values in the same manner as in the procedure used for the Si-rich samples.

I

  performed the Davies ENDOR measurements, which are widely used for large hyperfine coupling. Although I successfully observed a broad spectrum with a maximum value of |A/2| ~ 40 MHz in the Ba-rich samples, a poor signal to noise ratio made detection of large hyperfine couplings difficult, especially, in the Si-rich samples. Therefore, I performed the ED-NMR measurements, which has been attracting attention as a new hyperfine coupling method in recent years. As a result, I succeeded in getting ED-NMR spectra with peaks at about 35 MHz for the Si-rich sample and about 35, 100, and 165 MHz for the Ba-rich sample.

  GUSHI taught me how to manage research life in Suemasu-Toko laboratory. Especially, Gushi-san helped me live a life in Grenoble, too. Thank you very much. I also thank Julia and Timothée for your kind teaching for EPR measurements in France. I look forward to seeing you at future EPR meetings in the future. As junior students, Mr. Zhinhao XU, Mr. Mr, Suguru Yachi, Mr. Miftahullatif EMHA Bayu, Mr. Yudai YAMASHITA, Ms. Komomo KODAMA, Mr. Rhota SUGIYAMA, Mr. Shu SUGIYAMA, Mr. Taira NEMOTO, Mr. Sho AONUKI, Ms. Loise BENINCASA, Ms. Yurika HAKU, Mr. Ryuichi YOSHIDA, and Mr. Ryota KOITABASHI helped my experiments and lots of discussion and activated me by their own attitude for research.Thank you very much. Xu-san, I am sure that you can obtain interesting results in Canada. Yamashita-kun, your MBE techniques were outstanding. I wish that you all obtain nice results in the future.

7 .

 7 L. Benincasa, H. Hoshida, T. Deng, T. Sato, K. Toko, Y. Terai, T. Suemasu "Investigation of defect levels in undoped-BaSi2 epitaxial films by PL measurement" 第 79 回応用物理学会秋季学術講演会 20a-PA4-2, 名 古屋, Sept. 20 (2018).8. L. Benincasa, H. Hoshida, T. Deng, T. Sato, K. Toko, Y. Terai and T. Suemasu "Study of the origin of defect levels in undoped-BaSi2 epitaxial films by PL measurement" 第 66 回応用物理学会春季学術講演会 E 10a-PB2-10, 東京, March 10 (2019). 9. 山下 雄大, 佐藤 拓磨, 倉持 豪人, 都甲 薫, 末益 崇 "三段階成長法による BaSi2 光吸収層の高品位 エピタキシャル成長" 第 66 回応用物理学会春季学術講演会 11p-W834-7, 東京, March 11 (2019). 10. 佐藤 拓磨, G. Serge, A. L. Barra, 末益 崇 "電子スピン共鳴によるバルクおよび薄膜 BaSi2 内の欠陥の 調査" 第 66 回応用物理学会春季学術講演会 11p-W834-18, 東京, March 11 (2019). 11. Z.Xu, T. Sato, J. Nakamura, A. Koda, K. Shimomura, T, Suemasu "Investigation of hydrogen states in semiconducting BaSi2 by muon spin rotation" 第 80 回応用物理学会秋季学術講演会 20P-E303-5, 札幌, Sept. 20 (2019).12. 根本 泰良, 佐藤拓磨, 召田 雅実, 倉持豪人, 都甲 薫, 末益 崇 "スパッタリング法による C-doped BaSi2 の作製と特性評価" 第 80 回応用物理学会秋季学術講演会 20P-E303-1, 札幌, Sept. 20 (2019).

13 .

 13 伯 ゆりか, 佐藤 拓磨, 山下 雄大, 都甲 薫, 末益 崇 "BaSi2 におけるポストアニールが光学特性に 及ぼす影響 " 第 80 回応用物理学会秋季学術講演会 21a-PA3-5, 札幌, Sept.21 (2019).

Table 2 -

 2

					Fig. 2.1-1 Crystal structure of orthorhombic
					BaSi2. 23
		Ba2Si	Ba5Si3	BaSi	Ba3Si4	BaSi2	Ba24Si100
	Ba/Si	2	1.67	1	0.75	0.5	0.24
	Crystal System	Orthorombic	Tetragonal	Orthorhombic	Tetragonal	Orthorhombic	Cubic
	Space Group	Pnma	P4/ncc	Cmcm	P42/mnm	Pnma	P4132
	Si average charge	-4	-3.33	-2	-1.5	-1	-0.48
	Si-Si distance	2.6	2.74	2.502	2.424	2.404	2.415
	Si sublattice	1 D	Si2 dangbel	1D	Si4 6-:	Si4 4-:
				zigzag-chain	butterfly-shaped	isolated tetrahedron

1. Compounds of Ba-Si binary system.

Table 2

 2

	-2 lists the conduction types and carrier concentrations by different dopants in BaSi2. In particular, B-doped
	BaSi2 shows p-type conductivity, and the hole concentration p can be continuously controlled in the range of
	10 16 ~10 20 cm -3 . Therefore, B-doped BaSi2 films are often adopted to design solar cells structures. On the other hand,
	as for n-type BaSi2, various dopants are still being studied. While Sb-doped BaSi2 shows n-type conductivity with n
	controlled between 10 16 and 10 20 cm -3 , Sb atoms have a large diffusion coefficient in BaSi2. 48 Thereby, a distribution
	of the Sb atom into undoped BaSi2 after fabricating p-i-n device structure is considered to hinder a realization of a
	steep junction. Therefore, other atoms with small diffusion coefficient in BaSi2 such as As has been under
	investigations. 50 Thus, we can control the conduction type of BaSi2 while there is room for improvement. The bipolar
	property is also a good advantage for design high-η BaSi2 homojunction solar cells composed of p-n or p-i-n device
	structures.

Table 2 -

 2 2 Conduction types and carrier concentrations by different dopants in BaSi2.

	Dopant

Conduction type Carrier concentration [cm -3 ] Notes Reference undoped B Al Ga In N P As Sb

	n or p	10 16	Strongly depend on RBa/RSi.	51
	p	10 16 ~10 20	Continuously control up to p + .	52
	p	10 16 ~10 17	Diffusion is too strong.	53
	n	10 15 , 10 20	Can't be controlled properly.	54
	p	10 16 ~10 17	Can't be up to p + .	54
	p	10 16 ~10 17	Plasma source.	55
	n	10 16 ~10 18	GaP source.	56
	n	~10 19	As ion implantation.	57
	n	10 16 ~10 20	Continuously control up to n + .	58

  Trinh et al. used BaSi2 granules to form BaSi2 films by vacuum evaporation, and achieved a large  of 4.8 μs in the films grown at 500 °C. Evaporated undoped-BaSi2 films show n-type conductivity

Table 2 -

 2

4 Experimental and quantum mechanically calculatied NMR spectroscopic parameters of BaSi2. 110

Table 4

 4

-2 shows the details of the sample growth conditions.

Table 4 -

 4 1 Pretreatment method for Si substrate

		Process	Time Purpose
	1 2	Ultrasonic cleaning with acetone Ultrasonic cleaning with methanol	10 min 10 min	Removal of organic materials and particles
	3	Ultrasonic cleaning with ultrapure water	5 min
	4	5% HF	30 s	Removal of natural oxide film
	5	HCl : H2O2 : H2O = 1 : 1 : 4	5 min	Removal of metal contaminations Formation of protective oxide film
	6	5% HF	30 s	Removal of protective oxide film
				Removal of organic materials and
	7	NH4OH : H2O2 : H2O = 1 : 1 : 4	5 min	particles
				Removal of protective oxide film
	8	N2 blowing	-	Removal of moistures
	9	Thermal cleaning in ultrahigh chamber 900 °C	30 min	Removal of protective oxide film

Table 4 -

 4 2 The details of MBE growth conditions

		MBE
	Label	RSi (nm/min)	RBa (nm/min)	RBa/RSi	Time (h)	Thickness (nm)
	A		1.0	1.1	8	450
	B		1.4	1.5	8	430
	C		1.7	1.9	8	380
	D	0.9	2.0	2.2	8	450
	E		2.3	2.5	8	440
	F		2.7	3.0	8	450
	G		3.2	3.6	8	480

Table 4 -

 4

3

The size of samples for EPR studies.

Table 5 -

 5 1 Atomic ratio of powder samples. Actual values were determined by ICP-OES.

	Label	I	J	K	L	M	N
	Starting material Ba : Si = 1 : xs	2.10	1.90	2.06	2.00	1.94	1.88
	x = NSi/NBa	2.404	2.087	2.134	2.054	1.987	1.916
	Hereafter, from the viewpoint of composition ratio of Ba and Si, I divide 6 different samples into 2 following groups.

Table 5 -

 5 2 Experimental conditions of cw-X-band. The Q value of the cavity, the microwave frequency (νMW) in MHz, the microwave power (PMW) in mW, the modulation frequency (νm) in kHz, the modulation amplitude (Bm) in mT, the time constant (TC) in msec, the conversion time (CT) in msec, the accumulations, and the receiver gain (R.G.) are listed.

	Label	Q	νMW	PMW	νm	Bm	TC	CT	acc	R.G.
	I	2100	9.6526	2×10 -2					74
	J K L	4000 4300	9.6556 9.6571 9.6554	5×1 0 -3	100	0.2	81.92	163.84	82 60 100	5×10 5
	M N	2500	9.6554 9.6536			0.05			51 51

Table 5 -

 5

	Label	Q	νMW	PMW	νm	Bm	TC	CT	acc	R.G.
	I		34.1310						79
	J		33.9209						20
	K L	N/A	33.9310 34.0177	5×10 -2	100	0.25	83.89	81.92	81 25	60
	M		33.9129						9
	N		34.0560						24

3. Experimental conditions of cw-Q-band. The Q value of the cavity, the microwave frequency (νMW) in MHz, the microwave power (PMW) in mW, the modulation frequency (νm) in kHz, the modulation amplitude (Bm) in mT, the time constant (TC) in msec, the conversion time (CT) in msec, the accumulations (acc) , and the receiver gain (R.G.) in dB are listed.

Table 5 -

 5

	Label	νMW	PMW	tp	τ	window srt	acc	V.G.
	I	9.6970	6.69×10 -1						51
	J	9.6971	6.76×10 -1					130	45
	K L	9.6970 9.7012	6.70×10 -1 2.12×10 -4	16	200	200	20	135	51 48
	M	9.6957	2.12×10 -4					45	27
	N	9.7014	2.11×10 -4					20	24

4

. Experimental conditions of pulse-X-band. The microwave frequency (νMW) in MHz, the microwave pulse power (PMW) in mW, the details of the pulse sequence: tp [nsec]τ [nsec] -2tp [nsec]τecho, the detection window of the echo [nsec], the pulse shot reputation time of the sequence (srt) in msec, the accumulations, and the video gain for the pulse detection (V.G.) in dB.

Table 5 -

 5

4

. Experimental conditions of pulse-X-band. The microwave frequency (νMW) in MHz, the microwave pulse power (PMW) in mW, the details of the pulse sequence: tp [nsec]τ [nsec] -2tp [nsec]τecho, the detection window of the echo [nsec],

Table 5 -

 5 5. Experimental conditions of pulse-Q-band. The microwave frequency (νMW) in MHz, the microwave pulse power (PMW) in mW, the details of the pulse sequence: tp [nsec]τ [nsec] -2tp [nsec]τecho, the detection window of the echo [nsec], the pulse shot reputation time of the sequence (srt) in msec, the accumulations, and the video gain for the pulse detection (V.G.) in dB.

	Label	νMW	PMW	tp	τ	window srt	acc	V.G.
	I	33.8247	6.69×10 -1					75	33
	J	33.4878	6.76×10 -1					20	24
	K L	33.6412 33.5439	6.70×10 -1 2.12×10 -4	16	200	200	20	25 20	27 24
	M	33.5304	2.12×10 -4					5	30
	N	33.5280	2.11×10 -4					20	6

Table 5 -

 5 9 Defects in BaSi2.

Table 5 -

 5 10. Three different hyperfine spectroscopies.

	HYSCORE (ESEEM) 161,162	ENDOR 163	ED-NMR 164
	Electron spin echo envelope	Electron-nuclear double	Electron-electron double
	modulation	resonance	resonance detected NMR
	One frequency	Two frequency	Two frequency
	.

  It is necessary to carry out additional experiments and further research based on simulations.

	Sample	2.02	2.01	ED-NMR peak position [MHz] 2.00 1.98
	I	-	-25, +20	-25, +20	-	-	-
	J	-39, +35	-39, +35	-39, +35	low S/N	-	-
	K	-	-	-28 +35	-	-	-
	L	-	-33, +34	-	-	-	-
	M	-	-22, +35	-	-98, -24, +24, +38	-	-
	N	-	-	-	-35, +29	-89, -14, +29, +92, +162	-165, -88, -16, +29, +95, +164

Table 5 -

 5 17 Fitted values for simulation of Si-rich samples.

	Label	I	J	K	L
			Defect 1
	g	(2.0249, 2.0121, 1.9975) (2.0251, 2.0121, 1.9975) (2.0297, 2.0138, 1.9929) (2.0271, 2.0126, 1.9935)
	g strain	(0.0075, 0.0065, 0.0100)	(0.0075, 0.0075, 3.05×10 -6 )	(0.0075, 0.0066, 0.0060) (0.0100, 0.0070, 0.0062)
	H strain [MHz]	(61, 10, 87)	(55, 15, 60)	(70, 27, 100)	(55, 16, 100)
			Defect 2
	g	(2.0098, 2.0046, 1.9966) (2.0098, 2.0046, 1.9966) (2.0111, 2.0049, 1.9966) (2.0103, 2.0043, 1.9966)
	g strain	(0.0035, 0.001, 0.0005)	(0.0035, 0.0015, 5.83×10 -6 )	(0.0045, 0.0028, 0.0047) (0.0045, 0.0023, 0.0027)
	H strain [MHz]	(25, 22, 34)	(25, 29, 24)	(25, 40, 40)	(25, 34, 37)
			Ratio of the Defect 1 to the Defect 2
		1.14	2.50	2.83	2.86

Table 5 -

 5 18 Initial values for simulation of Ba-rich samples.

	Label	M	N
		Defect 1
	g	(2.0271, 2.0126, 1.9935)	(2.0271, 2.0126, 1.9935)
	g strain (0.01, 0.007, 0.006243)	(0.01, 0.007, 0.006243)
	H strain (55, 16, 100)	(55, 16, 100)
		Defect 3
	g	(1.930, 1.910, 1.844)	(1.930, 1.910, 1.844)
	g strain (0.0125, 0.025, 0.02)	(0.0125, 0.025, 0.02)
	HF	ASi = 22, ABa = 45, ABa = 84	ASi = 22, ABa = 45, ABa = 84
		Defect 5
	g	(1.9836, 1.9802, 1.9773)	(1.9832, 1.99798, 1.9768)
	g strain (0.002, 0.0015, 0.002)	(0.002, 0.0012, 0.002)

Table 5 -

 5 23 Calculated g-tensor VSi (q = -1).

	Size of cell	g1	g2	g3	gav
	1×1×1	1.835	1.952	1.996	1.928
	2×2×1	1.974	1.985	2.001	1.987

Table 5 -

 5 24 Calculated principal values of g-tensor and their average, gav.

	g1	g2	g3	gav
	Si-rich
	VBa(q = -3)	1.878	2.023	2.047	1.983
	VBa(q = -1)	1.676	1.969	2.042	1.896
	Sii (q = -1)	1.934	1.989	2.005	1.976
	Sii (q = +1)	1.933	2.030	2.078	2.014
	SiBa (q = -3)	1.957	2.003	2.057	2.006
	SiBa (q = -1)	1.936	2.004	2.134	2.025
	Ba-rich
	VSi(q = -1)	1.974	1.985	2.001	1.987
	VSi(q = +1)	2.013	2.042	2.047	2.034
	Bai (q = +1)	1.951	1.992	1.996	1.980
	Bai (q = +3)	1.958	2.027	2.051	2.012
	BaSi (q = +1)	1.958	1.983	2.023	1.994
	BaSi (q = +3)	1.995	2.031	2.051	2.026

Table 2 -

 2 1 (the polycrystalline samples, sample I-N). Note that the polycrystalline samples before powdering were used. As thin film samples, undoped samples grown at different Si substrate temperatures during MBE growth were prepared. Samples O-AA with different deposition rate ratios (RBa/RSi) were prepared (Table6-1).

Table 6 -

 6

			1 Details of MBE growth conditions
			MBE
	Label	TS [°C]	RSi (nm/min)	RBa (nm/min)	RBa/RSi	Thickness (nm)
	O			0.9	1.0
	P			1.8	2.0
	Q R	580	0.9	1.8 2.0	2.2 2.6	500
	S			2.8	3.1
	T			3.6	4.0
	U			0.4	0.4
	V			0.8	0.9
	W			1.1	1.2
	X	650	0.9	1.4	1.5	500
	Y			2.1	2.3
	Z			2.6	2.9
	AA			3.3	3.7

Table 7 -

 7 1 summarize the defects in BaSi2 revealed by this studies.

		Defect 1	Defect 2	Defect 3	Defect 4	Defect 5
		Si-rich		Ba-rich
	g-tensor	g1 = 1.993-8,	g1 = 1.997,	g1 = 1.844,	gav ~ 1.975	g1 = 1.977,
		g2 = 2.012-4,	g2 = 2.004-5,	g2 = 1.910,	at X-band	g2 = 1.980,
		g3 = 2.025-30	g3 = 2.010-1	g3 = 1.930		g3 = 1.983-4
	Nspin	2.27-9.28	2.22-	4.22-7.56	6.43 × 10 17 -	8.97 × 10 15 -
	[spins/cm -3 ]	× 10 16	× 10 16	× 10 16	1.11 × 10 17	3.02 × 10 17
	Origin	(Sii + ?, BaSi 3+ ??)	?	?	?	VSi -
	PL peak	P1 (0.8 eV) or P2 (0.9 eV)	N/A		Nonradiative?

Table A -

 A 1. Calculated Raman active optical phonon frequencies [cm-1 ] of BaSi2 at Γ point. The modes are categorized to external modes (ext) including the translation of Si cluster (t) and rotation of that (r), internal modes of Si cluster (int), and the modes of Ba. Their relative magnitudes of displacement are also shown as vs = very strong, s = strong, m = medium, and w = weak. 佐藤 拓磨, 髙部涼太, 小玉 小桃, 都甲 薫, 末益 崇 "DLTS 法による BaSi2 エピタキシャル膜内の欠 陥評価" 第 78 回応用物理学会秋季学術講演会 7p-PB4-9, 福岡, September 8 (2017). 2. 佐藤 拓磨, 山下 雄大, 都甲 薫, 末益 崇 "BaSi2 エピタキシャル膜ラマンスペクトルの膜厚および Ba/Si 体積レート比依存性" 第 65 回応用物理学会春季学術講演会 19a-F202-8, 東京, March 19 (2018).3. T. Deng, T. Sato, Z. Xu, R. Takabe, S. Yachi, Y. Yamashita, K. Toko, and T. Suemasu "Investigation on p-BaSi2/n-Si heterojunction solar cells using a Si(001) substrate" 第 65 回応用物理学会春季学術講演会 19a-F202-1, 東京, March 19 (2018).4. 山下 雄大, 佐藤 拓磨, 都甲 薫, 末益 崇, "DLTS 法を用いた undoped BaSi2 光吸収層の欠陥評価" 第 15 回「次世代の太陽光発電システム」シンポジウム PB-39, 札幌, July 12 (2018). 5. 山下 雄大, 佐藤 拓磨, 都甲 薫、上殿 明良、末益 崇 "MBE 法により成長した BaSi2 光吸収層中の 点欠陥評価" 第79 回応用物理学会秋季学術講演会 19p-436-15, 名古屋, Sept. 19 (2018).6. 佐藤 拓磨, Gambarelli Serge, 山下 雄大, 末益 崇 "電子スピン共鳴による BaSi2 薄膜の欠陥評価" 第

	Dec. 13, 2019.
	Domestic conferences
	1.
	Freq. [cm-1]		Si	Ba	Freq. [cm-1]		Si	Ba
	51.2	ext	m	s	134.7	ext(t,r)	s	-
	52.7	ext	s	vs	141	ext(r)	s	w
	63.4	ext(t)	s	vs	145.5	ext(t,r)	vs	w
	64.1	ext(t,r)	m	s	149.8	ext(r)	vs	w
	66.2	ext(t,r)	m	vs	154.8	ext(r)	vs	w
	71	ext(r)	m	s	156.7	ext(r)	vs	m
	71.7	ext(t,r)	m	s	267.9	int	s	-
	73.3	ext(t,r)	w	vs	268.9	int	s	-
	75.8	ext(t)	s	vs	282.3	int	vs	-
	75.9	ext(r)	m	vs	286.7	int	vs	-
	82	ext(t,r)	s	vs	361	int	vs	-
	93.9	ext(r)	m	vs	362	int	vs	-
	103.3	ext(r)	vs	w	364.6	int	vs	-
	113.2	ext(r)	vs	-	365.1	int	vs	-
	118.9	ext(t,r)	s	w	384.9	int	vs	-
	124.1	ext(r)	s	w	387.6	int	vs	-
	127.8	ext(t,r)	vs	w	499.8	int	vs	-
	130.6	ext(t,r)	vs	w	502.5	int	vs	-

Acknowledgement

This thesis and all experimental results have been completed thanks to a lot of supports. Here, I acknowledge all of them such as supervision of my Ph. D course, technical advice, research funds and equipment for experiments.

Submitted to the Graduate School of

Pure and Applied Sciences

Here, the nuclear quadrupole tensor is traceless (Q1 + Q2 + Q3 = 0) and 𝜂 are described as,

If the value of K is close to the value of 𝜈eff (-) ( = |𝜈n -|A|/2|| ) in a strong hyperfine situation, the quadrupole resonance frequencies are broadened as νeff/K deviates from 0, which the ENDOR and ED-NMR spectra make broaden.

In practical, the most strongest hyperfine interaction in Bai (q = +1), (A11, A22, A33) = (-43, -42, -38), I obtained the value K = 84 MHz and η = 0.04. By using these calculated values, I can evaluate principal values Q1 = -81, Q2 = -87, and Q3 = +168 MHz. In the same way, the most strongest hyperfine nteraction in VSi (q = -1), (A11, 96, 96, 107), I obtained the value K = -12 MHz and 𝜂 = 0.24. I can evaluate the principal values, (Q1, Q2, Q3) = (-9.1, -14.9, +24) in MHz.

Summary

As a conclusion, the following summarizes the characteristics of defects that can be considered in the case of Si-rich and Ba-rich BaSi2 bulk samples.In each supercell, the defect is dipicted by a red color. Highlighted atoms have hyperfine interactions greater than 20 MHz. Also, the largest hyperfine couplings are listed with their principal axes in the supercell for Ba and Si, respectively. In a term of quadrupole tensor, I simply categorized them into three regimes: high field regime (|νeff/K| >> 1), low field regime (|νeff/K| << 1), and intermediate regime (|νeff/K| ~ 1). In case that the values of |νeff/K| depends on its site, I denoted that "depends on sites"and the regime for the largest hyperfine coupling, |ν eff/K|, is shown in parentheses.The information such as g-tensor, hyperfine tensor, and quadrupole tensor are listed for each defect in Tables 5-25 and 5-26. In the tables, the Aii (i = 1, 2, 3) greater than 20

MHz are only listed.

Appendix

A. Raman spectroscopy and DFT calculations

Raman spectroscopy enables us to access to the atomic structure of defects in BaSi2 with the help of firstprinciples calculation because vibrational frequencies are specific to a moleclue's chemical bonding and symmetry.

There have been several defect studies using Raman spectroscopy such as Se vacancies in Cu(In, Ga)Se2 and Si precipitations in β-FeSi2 films. 195,196 However, there has been no such discussion about defects in BaSi2 although there have been serveral rerpots on Raman spectra of BaSi2 from both experimental and theoretical viewpoints so far. 69,[123][124][125] A.1. Sample I carried out the Raman spectroscopy on the same film samples (denoted as Sample O-T in Table 6-

A.2. Lattice constants and residual stress

The peak shift of Raman spectrum Δω is proportional to the magnitude of the residual stress σres in the film. 197,198 Hence I estimate σres by using lattice constants of sample . The lattice constants as a function of RBa/RSi

where d hkl and d0 hkl is the measured lattice spacing and that of unstrained BaSi2, respectively. As the lattice constants of relaxed sample, those of powdered XRD are used. 22 Following the Hooke's law, the residual stress in the film is obtained from the evaluated strain as,

where sijkl are the compliance constants of single crystal BaSi2, and the values calculated by Peng et al. are used for them. 125 Evaluation of the residual stress in the film by XRD is carried out assuming an in-plane stress condition (σ11 = σ12 = σ13 = 0) as applied to, for example, <111>-oriented Al thin films on a Si(001) substrate. 199 However, in this work, it was difficult to obtain diffractions of sufficient intensity from the asymmetric planes of BaSi2. Hence, diffractions such as (0k0) and (00l) obtained in the in-plane XRD measurement were used. The stress σ22( ∥ σb) and σ33( ∥ σc) in the plane were roughly estimated by assuming that the principal axes of stress coincided with the crystal axes.

A.3.2. Computed results

First, I confirm the reproducibility of this calculation. Table A-1 shows the calculation results of the optical modes at the Γ point (q = (0,0,0)) in a BaSi2 perfect crystal. The relative magnitudes of displacement of Si4 cluster and Ba for each mode are also shown. As reported previously by Peng et al.,125 lattice vibrations of Ba atoms is hardly observed at wavenumbers higher than 200 cm -1 , and all the calculated modes are internal vibrations of Si4 clusters. On the other hand, the modes observed below 200 cm -1 are the external vibrations (rotation, translation) of the Si4 clusters and/or the lattice vibrations of Ba. For comparison, Table A-2 summaries reported Raman peak positions due to a Si4 cluster in BaSi2. 69,[123][124][125] Terai et al. also pointed out such feature of modes in the measurement of the angle dependencies of polarized Raman spectra of a-axis-oriented undoped BaSi2 epitaxial films on Si(001). 127 Comparing the present calculated peak positions of Raman active modes to the experimental result in powdered BaSi2, I confirm that the peak positions are almost reproducible (Table A-2). 124 In particular, with respect to the internal vibrations of Si4 cluster, the reproductivity is within ± 2-3%. In Raman scattering experiments, the Ag modes at around 500 cm -1 as shown in

List of publications

Paper

Thesis summary

In this study, I focused on semiconductor barium disilicide BaSi2. Since BaSi2 is constituted of less toxic and earth-abundant elements, this environmental-friendly material takes advantage in terms of elemental strategies compared to other materials. Our previous researches found its potentials for solar cell applications. For instance, BaSi2 shows a large absorption coefficient (3 × 10 4 cm -1 @ 1.5 eV, whose value larger than crystalized Si by 40 times) because of its characteristic band structure.

So far, we have successfully demonstrated Si-based hetero-and homo-junction solar cells. In order to design further better devices, I investigated intrinsic defects in undoped-BaSi2 in this study. For, the defects promote for photogenerated carriers to recombine each other during travel to electrodes. As a result, photogenerated currents decrease. Therefore, defect control is indispensable to realize the devices with high conversion efficiency. My previous researches found such films grown by molecular beam epitaxy method contained the defects while their origins are unknown yet. EPR enables me to access microscopic information such as identification of paramagnetic defects, determination of local structures, etc.

First, I performed electron paramagnetic resonance (EPR) measurements on a-axis orientated BaSi2 epitaxial films. Assessing the observed g value carefully, I succeeded in detection of EPR active defects for the first time in this material. In addition, it was confirmed that the EPR line observed from the film (anisotropic g value: 2.003-2.010) decreased by introduction of atomic hydrogens (H). In our previous study, the BaSi2 film contained atomic H showed significant improvements on photoresponsivity, indicating that this defect can affect optical properties.

Second, I revealed some different type of defects were formed depending on composition ratios through the EPR measurements of powder and single crystal of BaSi2. To accomplish this identification, I adopted