domain: chim.mate chim phys

text:
Changan Wang
email: changan.wang@hzdr.de

Hongbin Zhang
email: hzhang@tmm.tu-darmstadt.de

Kumar Deepak

Chao Chen

Arnaud Fouchet

Juanmei Duan

Donovan Hilliard

Ulrich Kentsch

Deyang Chen

Min Zeng

Xingsen Gao

Yu-Jia Zeng

Manfred Helm

Wilfrid Prellier

Shengqiang Zhou

Tuning the metal-insulator transition in epitaxial SrV O 3 films by uniaxial strain

Keywords: Oxide thin film, Strain engineering, Metal-insulator transition, Lattice distortion, Correlated electrons

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INTRODUCTION

Correlated transition-metal oxides host a vast landscape of novel phenomena in condensed matter physics, such as multiferroicity, metal-insulator transitions (MIT), quantum magnetism and colossal magnetoresistance [START_REF] Georges | Dynamical Mean-field Theory of Strongly Correlated Fermion Systems and the Limit of Infinite Dimensions[END_REF][START_REF] Copie | Chemical Strain Engineering of Magnetism in Oxide Thin Films[END_REF][START_REF] Cheong | Transition Metal Oxides: the Exciting World of Orbitals[END_REF][START_REF] Wang | Device Performance of the Mott Insulator LaVO 3 as A Photovoltaic Material[END_REF][START_REF] King | Atomic-scale Control of Competing Electronic Phases in Ultrathin LaNiO 3[END_REF]. Among these materials, perovskite SrVO 3 (SVO) with a 3d 1 electronic configuration is one of such typical strongly correlated systems used to study the MIT. Although SVO bulk and films have been extensively studied in the past, exhibiting a robust metallic character and an enhanced Pauli paramagnetic behavior, theoretical studies have predicted that a MIT in SVO thin films can appear as the SVO thickness is reduced to a few unit cells [START_REF] Zhong | Electronics with Correlated Oxides: SrVO 3 /SrTiO 3 as A Mott Transistor[END_REF][START_REF] Pardo | Electron Confinement, Orbital Ordering, and Orbital Moments in d 0 -d 1 Oxide Heterostructures[END_REF]. Fortunately, experimental results in ultrathin films seem to verify this prediction: a dimensional-crossover-induced MIT was observed in single SVO ultrathin films [START_REF] Yoshimatsu | Dimensional-crossover-driven Metal-insulator Transition in SrVO 3 Ultrathin Films[END_REF][START_REF] Gu | Two-Dimensional Mott Insulators in SrVO 3 Ultrathin Films[END_REF][START_REF] Fouchet | Study of the Electronic Phase Transition with Low Dimensionality in SrVO 3 Thin Films[END_REF][START_REF] Fouchet | Interface Chemical and Electronic Properties of LaAlO 3 /SrVO 3 Heterostructures[END_REF]. Similar results have also been obtained in other materials, including LaNiO 3 , La 2/3 Ca 1/3 MnO 3 and CaVO 3 [START_REF] Scherwitzl | Metalinsulator Transition in Ultrathin LaNiO 3 Films[END_REF][START_REF] Liao | Origin of the Metal-insulator Transition in Ultrathin Films of La 2/3 Sr 1/3 MnO 3[END_REF][START_REF] Gu | Metal-insulator Transition Induced in CaVO 3 Thin Films[END_REF]. Based on the Mott-Hubbard theory, MIT can be controlled by the competition between the on-site Coulomb repulsion U and the electronic bandwidth W [START_REF] Georges | Dynamical Mean-field Theory of Strongly Correlated Fermion Systems and the Limit of Infinite Dimensions[END_REF]. The aforementioned MIT in these films is attributed to a reduced effective W with decreasing film thickness, which facilitates a Mott insulating state and forms a pseudogap at the Fermi level [START_REF] Yoshimatsu | Dimensional-crossover-driven Metal-insulator Transition in SrVO 3 Ultrathin Films[END_REF]. On the other hand, chemical substitution can also be used to study the MIT. It has been reported that a MIT is induced in SVO alloyed with La or Ca (La 1- x Sr x VO 3 or Ca 1-x Sr x VO 3 ), in which W is influenced by the resultant changes in the bond angle and band length of oxygen ions with transition metal ions. Thus, the electron correlation U/W is enhanced in La 1-x Sr x VO 3 or Ca 1-x Sr x VO 3 [START_REF] Dao | Metal-insulator Transition in Low Dimensional La 0.75 Sr 0.25 VO 3 Thin Films[END_REF][START_REF] Inoue | Bandwidth Control in a Perovskite-type 3d 1 -correlated Metal Ca 1-x Sr x VO 3 . I. Evolution of the Electronic Properties and Effective Mass[END_REF]. For the ultrathin films, however, one should notice that the dimensional-crossover-induced MIT experiences a thickness-dependent relaxation of strain due to the lattice mismatch with substrates, which is inevitable and is not considered in these studies, although the strain relaxation may have a great effect on the MIT of these ultrathin films. In addition, chemical substitution always induces chemical disorders in strongly correlated systems that may also influence the MIT [START_REF] Sarma | Contrasting Behavior of Homovalent-substituted and Hole-doped Systems: O K-edge Spectra From LaNi 1-x M x O 3 (M= Mn, Fe, and Co) and La 1-x Sr x MnO 3[END_REF]. Therefore, in-depth understanding of the origin of MIT in correlated transition-metal oxides remains challenging and is essential from both fundamental and practical points of views [START_REF] Mannhart | Oxide Interfaces-an Opportunity for Electronics[END_REF].

Heteroepitaxial strain engineering has emerged as a powerful tool to study strongly correlated systems and to explore new functionalities [START_REF] Tokura | Orbital Physics in Transition-metal Oxides[END_REF]. Effective control over lattice strain in the films has revealed many important properties, including ferromagnetism, superconductivity and ferroelectricity [START_REF] Choi | Enhancement of Ferroelectricity in Strained BaTiO 3 Thin Films[END_REF][START_REF] Locquet | Doubling the Critical Temperature of La 1.9 Sr 0.1 CuO 4 Using Epitaxial Strain[END_REF][START_REF] White | Strain-induced Ferromagnetism in Antiferromagnetic LuMnO 3 Thin Films[END_REF]. Here, we use helium (He) ion irradiation to induce "strain doping" in SVO films. Strain doping by irradiation of noble He atoms results in an uniaxial lattice expansion along the out-of-plane direction without changing the in-plane lattice constants [START_REF] Guo | Strain doping: Reversible Single-axis Control of A Complex Oxide Lattice via Helium Implantation[END_REF][START_REF] Wang | Defect-induced Exchange Bias in A Single SrRuO 3 layer[END_REF][START_REF] Herklotz | Controlling Octahedral Rotations in A Perovskite via Strain Doping[END_REF]. For instance, Gao et al. reported that the out-of-plane lattice parameter in epitaxial La 0.7 Sr 0.3 MnO 3 thin films is continuously and independently manipulated by using He ion irradiation [START_REF] Guo | Strain doping: Reversible Single-axis Control of A Complex Oxide Lattice via Helium Implantation[END_REF]. On the other hand, this approach bypasses the Poisson effect that always changes the lattice constant in all three directions and can clarify the related effects between order parameters and single degrees of freedom [START_REF] Guo | Strain doping: Reversible Single-axis Control of A Complex Oxide Lattice via Helium Implantation[END_REF][START_REF] Herklotz | Controlling Octahedral Rotations in A Perovskite via Strain Doping[END_REF]. Thus, this approach may provide more underlying information on MIT in correlated transition-metal oxides.

In this work, we have applied He ion irradiation to effectively modify the out-of-plane lattice parameter of the SVO films that were epitaxially grown on the STO substrates. Transport measurements revealed the emergence of the MIT with a crossover from metallic to insulating state in SVO films as the irradiation fluence is increased. Further studies of transport and magnetoresistance of the SVO films at low temperatures verified that the observed MIT is mainly attributed to electron-electron interactions rather than disorder-induced weak localization. Moreover, the combination of density functional theory and dynamical mean field theory (DFT+DMFT) calculations also indicated the enhanced electron-electron interaction due to the decrease of bandwidth W as the out-of-plane lattice is gradually expanded. These results therefore show the prospect of strain engineering by using He ion irradiation, providing a platform for modifying and exploring correlated behavior in complex oxides.

EXPERIMENTAL SECTION

SVO thin films (thickness: 50nm) were epitaxially grown on SrTiO 3 (001) single crystal substrate (a sub = 3.905 Å) by pulsed laser deposition (PLD). The deposition temperature was 650 ºC, the oxygen pressure was 1×10 -6 mbar, the KrF laser energy was 200 mJ, and the rate of 2 Hz was applied. After the deposition, these SVO films were then irradiated by He ions using an energy of 5 keV with different fluence values of 1×10 15 , 1.75×10 15 , 2.5×10 15 , 3×10 15 , and 3.5×10 15 He/cm 2 . The sample area was scanned over by the ion beam to ensure a lateral uniform irradiation. For convenient description in article, the samples were referred to as 1×10 15 , 1.75×10 15 , 2.5×10 15 , 3×10 15 , and 3.5×10 15 , respectively. Besides, the Stopping and Range of Ions in Matter (SRIM) simulations confirmed that He-ions give rise to a relatively homogeneous lattice displacement into SVO thin films [START_REF] Ziegler | SRIM-The Stopping and Range of Ions in Matter[END_REF]. The crystal structures were characterized by X-ray diffraction (XRD) (PANalytical X'Pert PRO diffractometer) using Cu Kα radiation. The electrical transport measurements were carried out using the van der Pauw configuration in a Lakeshore Hall measurement system. It can be seen that the out-of-plane lattice of the SVO films increases in a continuous manner with increasing the fluence. The increase percentages with respect to the pristine one are 0.78%, 2.04%, 2.99%, 4.11%, and 4.58%, corresponding to 1×10 15 , 1.75×10 15 , 2.5×10 15 , 3×10 15 , and 3.5×10 15 , respectively. Figure 1(c) presents the reciprocal space mappings (RSMs) for all samples around (103) asymmetric reflections. One clearly sees that the in-plane lattice parameter of the pristine SVO film is the same as that of the STO substrate, further demonstrating that the SVO films are epitaxially grown on the substrate. When the fluence increases, H values of the SVO diffraction peaks are found to remain the same as that of the STO substrate due to the constraint from the substrate itself, while L shifts towards lower values due to the out-of-plane lattice expansion. Even after the irradiation with the highest fluence of 3.5×10 15 He/cm 2 , the in-plane lattice is still epitaxially locked to the substrate. The strain induced by the irradiation is relieved along the out-of-plane direction and therefore only expands the c-axis parameter in SVO films [START_REF] Guo | Strain doping: Reversible Single-axis Control of A Complex Oxide Lattice via Helium Implantation[END_REF]. Different from other methods that often involve changes in all three lattice directions, these results demonstrate that a single out-of-plane lattice parameter is independently controlled by the strain arising from He ion irradiation. In order to explore transport properties of the SVO films, Figure 2(a) shows the evolution of resistivity versus temperature with increasing fluence of He ions. The pristine film exhibits metallic behavior (dρ/dT > 0) in the investigated temperature range, being consistent with previous reports [START_REF] Fouchet | Study of the Electronic Phase Transition with Low Dimensionality in SrVO 3 Thin Films[END_REF][START_REF] Gu | Metal-insulator Transition Induced in CaVO 3 Thin Films[END_REF]. With increasing the fluence of irradiation, the resistivity increases compared with the pristine sample, which is attributed to the increased out-of-plane lattice and disorder induced by irradiation. In particular, the films with intermediate fluence display a MIT characterized by an upturn in resistivity . The characteristic MIT temperatures (T MI ) are ~40 K and ~68 K for fluences 1.75×10 15 and 2.5×10 15 He/cm 2 , respectively, and are indicated by arrows in Figure 2(a). T MI shifts to a higher temperature upon increasing the fluence.

RESULTS AND DISCUSSION

Finally, insulating behavior (dρ/dT < 0) is found at all temperatures for the films with the fluence larger than 3×10 15 He/cm 2 . All these results therefore obviously indicate that the SVO films undergo a MIT with a crossover from a metallic to insulating state, accompanying the expansion of the out-of-plane lattice parameter due to the irradiation.

To further explore the effect of irradiation on electrical transport behavior, we first describe the metallic state. Vanadate SVO is considered to be a Fermi liquid (FL) [START_REF] Gu | Two-Dimensional Mott Insulators in SrVO 3 Ultrathin Films[END_REF], for which the temperature dependence (T) of resistivity follows the equation with an exponent n = 2

ρ ideal (T )=ρ 0 + A T 2 (1)
where the residual resistivity ρ 0 is due to the electron-impurity scattering caused by defects, and A is a measure of the strength of electron-electron interactions. However, Equation 1does not fully describe experimental data [START_REF] Hussey | Phenomenology of the Normal State In-plane Transport Properties of High-Tc Cuprates[END_REF]. ρ(T) can only be well described when we introduce the resistivity saturation (ρ SAT ) as follows [START_REF] Mikheev | Tuning Bad Metal and Non-Fermi Liquid Behavior in A Mott Material: Rare-earth Nickelate Thin Films[END_REF][START_REF] Gurvitch | Ioffe-Regel Criterion and Resistivity of Metals[END_REF]

ρ -1 (T )= ρ ideal -1 + ρ SAT -1 (2)
where ( ρ SAT ) is a parallel resistor, that is applied to a wide range of materials exhibiting the resistance saturation [START_REF] Mikheev | Tuning Bad Metal and Non-Fermi Liquid Behavior in A Mott Material: Rare-earth Nickelate Thin Films[END_REF][START_REF] Gunnarsson | Colloquium: Saturation of Electrical Resistivity[END_REF]. Here, the dashed lines in Figure 2(a) are the fitting results by Equation 1 and 2, which can well describe all SVO films in the metallic state. The fitting parameters of the SVO films are listed in Table 1. ρ 0 continuously increases, which is attributed to the increased defects induced by the irradiation, while ρ SAT is essentially fluence-independent, which is often related to the maximum resistivity [START_REF] Mikheev | Tuning Bad Metal and Non-Fermi Liquid Behavior in A Mott Material: Rare-earth Nickelate Thin Films[END_REF][START_REF] Wang | Localization-driven Metal-insulator Transition in Epitaxial Hole-doped Nd 1-x Sr x NiO 3 Ultrathin Films[END_REF]. More remarkable, when the fluence is increased, the coefficient A that quantifies the electronelectron interaction is enhanced by approximately 8 times, from 5.53×10 -10 Ω•cm/K 2 for the pristine to 4.29×10 -9 Ω•cm/K 2 for sample 2.5×10 15 , which is believed to account for the appearance of the MIT. films, which shows the resistivity upturns below T MI in both samples. This phenomenon is attempted to be interpreted by quantum corrections of the conductivity (QCC) [START_REF] Lee | Disordered Electronic Systems[END_REF]. In this three-dimensional system (3D), the temperature (T) dependence of the resistivity (ρ) can be described by [START_REF] Fouchet | Study of the Electronic Phase Transition with Low Dimensionality in SrVO 3 Thin Films[END_REF][START_REF] Lee | Disordered Electronic Systems[END_REF]:

ρ(T )= 1 σ 0 +a 1 T P/ 2 + a 2 T 1 /2 + b T 2 (3)
where

a 1 T P /2
is the 3D weak localization (WL) contribution (P = 2 or 3 is the electronelectron interactions or electron-phone scattering, respectively) [START_REF] Herranz | Magnetic Field Effect on Quantum Corrections to the Lowtemperature Conductivity in Metallic Perovskite Oxides[END_REF], while consistent with previous observations in SVO films [START_REF] Fouchet | Study of the Electronic Phase Transition with Low Dimensionality in SrVO 3 Thin Films[END_REF]. Although QCC successfully describes the insulating region below T MIT, it is not clear whether WL or REEI is the main contribution to the resistivity upturn (or QCC) [START_REF] Herranz | Magnetic Field Effect on Quantum Corrections to the Lowtemperature Conductivity in Metallic Perovskite Oxides[END_REF]. Since both effects have similar temperature-dependent behaviors in the 3D case, Δσ~T 1/2 or Δσ~T P/2 with P=2-3 for REEI or WL, respectively. To clarify the driving force for this phenomenon in our SVO systems, the field dependence of the resistivity at low temperature needs to be considered [START_REF] Herranz | Magnetic Field Effect on Quantum Corrections to the Lowtemperature Conductivity in Metallic Perovskite Oxides[END_REF]. MR is observed which decreases with increasing temperature. Moreover, the MR is found to be roughly proportional to B 2 for each temperature, and linear fits are also shown in Figure 3(b). These observations rule out that the WL is the main contribution to the insulating behavior at low temperatures [START_REF] Gu | Two-Dimensional Mott Insulators in SrVO 3 Ultrathin Films[END_REF]. This is because the effect of the WL leads to a negative MR originating from the suppression of self-interference effects by applying a magnetic field [START_REF] Gu | Two-Dimensional Mott Insulators in SrVO 3 Ultrathin Films[END_REF][START_REF] Herranz | Magnetic Field Effect on Quantum Corrections to the Lowtemperature Conductivity in Metallic Perovskite Oxides[END_REF]. Therefore, the observed MIT in our systems is mainly attributed to the electronelectron interactions rather than the WL. To shed light on the physical origin of the MIT in SVO, we performed DFT+DMFT calculations. Our calculations were carried out using the projection-embedding implementation based on the WIEN2k package [START_REF] Haule | Dynamical Mean-field Theory within the Full-potential Methods: Electronic Structure of CeIrIn 5 , CeCoIn 5 , and CeRhIn 5[END_REF][START_REF] Berger | Practical Experience from Porting and Executing the Wien2k Application on the EGEE Production Grid Infrastructure[END_REF], where the auxiliary impurity problem is solved using the continuous-time quantum Monte Carlo method [START_REF] Haule | Quantum Monte Carlo Impurity Solver for Cluster Dynamical Mean-field Theory and Electronic Structure Calculations with Adjustable Cluster Base[END_REF]. The local density approximation is applied for the exchange correlation potential as parameterized by Perdew-Wang [START_REF] Perdew | Accurate and Simple Analytic Representation of the Electrongas Correlation Energy[END_REF]. All the d-orbitals of V atoms are considered as correlated orbitals, while the hybridization and self-energies are computed in an energy window of ±10 eV around the . Thus, the abnormal temperature dependence of the resistivity in SVO films can be mainly attributed to electronic correlations, whose strength is tunable via He ion irradiation which is a matured technique. This study offers an effective method to explore and control MIT in correlated materials, allowing for the design of future electronic applications.

CONCLUSIONS

We have successfully manipulated the out-of-plane lattice parameter in SVO films without altering other degrees of freedom by using He ion irradiation. The transport study indicates a metal-insulator transition (MIT) with a crossover from the metallic to insulating state in SVO films as the fluence is increased. The observed MIT can be attributed to the decrease of the electronic bandwidth W due to the expansion of the out-of-plane lattice parameter induced by the irradiation, and then enhanced electron-electron interactions. The detailed investigations of transport, magnetoresistance and DFT+DMFT calculations in the insulating regime also reveal an obvious signature of strong electron-electron interactions, which is the dominant force in driving the observed MIT in SVO films. These results clearly pave a way to tune correlated behavior and develop novel functional devices based on transition-metal oxides.

Figure 1 .

 1 Figure 1. (a) XRD θ-2θ scans around the (002) reflection of SVO thin films on STO substrates under different helium fluences. (b)The out-of-plane lattice constant changes as a function of the helium fluence where percentages represent the change in the c-axis relative to the pristine state. (c) Reciprocal space maps around (103) reflections for different helium fluences.

Figure 1 (

 1 Figure 1(a) shows the XRD scans of the (002) reflection for SVO films without and with irradiation. The pristine SVO film is epitaxial to the STO substrate (a sub = 3.905 Å) and undergoes tensile strain from the substrate, which is in agreement with previous reports [9,10]. With the fluence of He ions increases, the position of the SVO (002) peak exhibits a systematic shift towards the low angle direction, indicating an expansion in the out-of-plane lattice parameter c. The c-axis values can be estimated from the XRD diffraction peaks and are summarized in Figure 1(b). It can be seen that the out-of-plane lattice of the SVO films

Figure 2 .

 2 Figure 2. (a) Electrical resistivity vs. temperature for SVO films with increasing fluence. Arrows indicate the minima of resistivity (or MITs). The dashed lines are the fitting of data in the metallic region using Equation 1 and Equation 2. (b) Increase of resistivity on the insulating side for SVO films irradiated with 1.75×10 15 and 2.5×10 15 He/cm 2 . The dashed lines are the fitting of data in the insulating region according to Equation 3.

a 2 T 1 / 2

 212 stands for the renormalized electron-electron interaction (REEI), representing the density of state correction at the Fermi energy. Another term bT 2 accounts for the classic low temperature dependence of the resistivity. Clearly, the resistivity at the insulating side of the MIT in both samples can be well fitted with Equation 3 (see dashed lines in Figure 2(b)), which is

Figure 3 .

 3 Figure 3. (a) The field dependence of the magnetoresistance MR=[R(B)-R(0)]/R(0) in the SVO sample irradiated with fluence of 1.75×10 15 He/cm 2 at different temperature. (b) The MR is plotted versus the square of the magnetic field. The linear fits to the data are also shown.

Figure 3 (

 3 Figure 3(a) shows magnetoresistance (MR, defined as [R(B)-R(0)/R(0)]) at different temperatures for the SVO sample irradiated with the fluence of 1.75×10 15 He/cm 2 . A positive

Figure 4 .

 4 Figure 4. (a): DFT band structure (a) of the pristine (black, c = 3.87 Å) and 3.5×10 15 (red, c = 4.05 Å) samples. (b): Imaginary part of the DFT+DMFT self energies for the degenerate d xz and d yz orbitals for the pristine (solid line) and 3.5×10 15 (dashed line) samples. The inset shows the self energy for the d xy orbitals. (c) and (d) show the DFT+DMFT spectral functions for the pristine and 3.5×10 15 samples, respectively. The horizontal dashed lines in (a), (c) and (d) indicate the Fermi energy.

Fermi

  energy, resulting in U = 8.0 eV and J = 0.8 eV. The DFT+DMFT calculations are all done at 50 K, where the in-plane lattice constants are fixed to 3.905 Å corresponding to that of SrTiO 3 substrates, with the out-of-plane lattice constant following the experimental XRD measurements.Following Figure4(a), the bandwidth W of the d xz and d yz bands is reduced upon irradiation, i.e., the band width W is inversely proportional to the lattice constant along the c-axis for the d xz and d yz orbitals, while that of the d xy bands remains almost a constant due to the fixed in-plane lattice constants. The reduced bandwidth leads to enhanced density of states (DOS) at the Fermi energy (not shown), leading to a possibly enhanced conductivity following the Drude model which would be in contradiction to the experimental observation. We found that the origin of the reduced conductivity upon irradiation can be attributed to enhanced scattering due to electronic correlations. Considering the renormalization factorZ = ( 1-dIm Σ dω | ω=0 ) -1, where ℑ Σ denotes the DMFT self-energy at Matsubara frequency ω n (as shown in Figure4(b)), it is reduced from 0.77 (pristine) to 0.56 (3.5×1015 ) for both d xy and d yz orbitals, whereas it remains a constant of 0.57 for the d xy orbital.Thus, the expansion of the c-axis lattice constant upon irradiation causes orbital-selective enhancement of the electronic correlations, resulting in the abnormal temperature dependence of the resistivity. However, there is no orbital-selective Mott transition[START_REF] Koga | Orbital-selective Mott Transitions in the Degenerate Hubbard Model[END_REF]. For instance, for the irradiated samples there exists an abnormal MIT-like dependence of resistivity with respect to the temperature (Figure2(a)), an interesting question is whether a finite band gap is opened. According to the spectral functions for the pristine (Figure 4(c)) and 3.5×10 15 (Figure 4(d)) samples obtained in our DFT+DMFT calculations, there is no gap opening, whereas the bandwidth for the d xy and d yz bands is renormalized by a factor of 0.25 compared to the DFT electronic structure (cf. Figure 4(a)). It is noted that for the 3.5×10 15 sample, the hybridization of the t 2g bands with the other bands has been significantly changed (Figure 4(d))

Table 1 .

 1 Transport properties of SVO films with different irradiation fluence ) presents the resistivity at the insulating side of the MIT for 1.75×10 15 and 2.5×1015

	Sample	T MI	ρ 0	A	ρ SAT
		(K)	(Ω•cm)	(Ω•cm/K 2 )	(Ω•cm)
	Pristine	-	1.80×10 -4	5.53×10 -10	5.56×10 -4
	1×10 15	-	4.10×10 -4	2.25×10 -9	4.86×10 -4
	1.75×10 15	40	5.62×10 -4	2.54×10 -9	7.75×10 -4
	2.5×10 15	68	8.29×10 -4	4.29×10 -9	5.60×10 -4
	3×10 15	>300	-	-	-
	3.5×10 15	>300	-	-	-

ACKNOWLEDGMENTS

Support by the Ion Beam Center at Helmholtz-Zentrum Dresden-Rossendorf is gratefully acknowledged. C. A. Wang thanks China Scholarship Council (File No. 201606750007) for financial supports. S. Zhou acknowledges financial support from the German Research Foundation (ZH 225/10-1). This work was also supported by the Shenzhen Science and Technology Project under Grant Nos. JCYJ20170412105400428 and KQJSCX20170727101208249. D. C. thanks the financial support from the National Natural Science Foundation of China (Grant Nos. 11704130 and U1832104).


Cédric Masante
email: cedric.masante@gmail.com

Julien Pernot

Aurélien Maréchal

Nicolas Rouger

High temperature operation of a monolithic bidirectional diamond switch

Keywords: Diamond, Electrical properties characterization, High power electronics, MOS-FET, Bidirectional

We report the 250 • C operation of a diamond-based monolithic bidirectional switch. A normally-ON double gate deep depletion MOSFET was fabricated with a 400 nm p-type channel with a boron doping of [N A -N D ]= 2.3×10 17 cm -3 and an Al 2 O 3 gate oxide thickness of 50 nm. The I st and III rd quadrants transistor characteristics are successfully measured by controlling the channel conductivity with both gates separately, with a clear ON and OFF state. A threshold voltage around 35 V is obtained with a low minimum gate leakage current of 1.00 × 10 -4 mA/mm at a gate-source bias V GS = 50 V. The bidirectional switch is then obtained by operating the MOSFET in the I st quadrant of each gate setup. This first proof of concept offers a reverse conducting and reverse blocking diamond MOSFET, with only one drift region layer.

Introduction

The use of wide bandgap semiconductor material is of high interest for power electronics applications, which requires devices able to withstand both large current densities and large electric fields. Diamond meets these requirements thanks to its attractive physical properties, particularly its high breakdown field (up to more than 10 MV/cm) and superior thermal conductivity (22 W/cm.K). A focus is made on high temperature and high blocking voltage applications in which it is considered that diamond has the greatest potential [START_REF] Donato | Diamond power devices: State of the art, modelling and figures of merit[END_REF]. In recent years diamond based power MOSFETs have been demonstrated [START_REF] Kawarada | Diamond MOSFETs using 2D hole gas with 1700V breakdown voltage[END_REF]- [START_REF] Oi | Vertical-type two-dimensional hole gas diamond metal oxide semiconductor field-effect transistors[END_REF], mostly relying on the surface transfer doping effect [START_REF] Strobel | Surface transfer doping of diamond[END_REF] (HFET) due to their relative ease of fabrication compared to the more standard MOSFET architectures (which require both p-type and n-type doping in the same device).

Complementary to these structures a bulk conduction diamond MOSFET was recently introduced, the diamond deep depletion MOSFETs (D3MOSFET), to take full advantage of the diamond bulk properties [START_REF] Pham | Deep-Depletion Mode Boron-Doped Monocrystalline Diamond Metal Oxide Semiconductor Field Effect Transistor[END_REF]- [START_REF] Pham | 200V, 4MV/cm lateral diamond MOSFET[END_REF]. Such diamond MOSFETs devices have been shown to exhibit a diode-like reverse conduction [START_REF] Masante | [END_REF], but no reverse blocking capability, similar to SiC JFETs or GaN HEMTs [START_REF] Liu | Surge Current Capability of GaN E-HEMTs in Reverse Conduction Mode[END_REF], [START_REF] Sheridan | Reverse conduction properties of vertical SiC trench JFETs[END_REF].

In several applications (e.g. matrix converters, multi-level converters, AC/AC converters), reverse conducting (RC) and reverse blocking (RB) power devices, also known as bidirectional power transistors, with the lowest on-state resistance are required. In this context, monolithic bidirectional power devices have been previously demonstrated [START_REF] Park | Diode Bridge Embedded AlGaN/GaN Bidirectional Switch[END_REF]- [START_REF] Dai | Characterization and Implementation of Hybrid Reverse-Voltage-Blocking and Bidirectional Switches using WBG Devices in Emerging Motor Drive Applications[END_REF], and we report here the first monolithic bidirectional diamond power FET (RC+RB) based on the D3MOSFET design.

In the view of limiting the on-state resistance of the bidirectional diamond MOSFET, only one drift region is proposed with two isolated gates. Its high temperature operation and low specific on-state resistance is demonstrated in this article.

Device structure, fabrication and experimental method

The sample consists of a 400 nm boron-doped p-type layer grown by microwave plasma assisted CVD, on a 4 × 4 mm 2 High Pressure High Temperature (HPHT) Nitrogen-doped electrically insulating diamond substrate (fig 1). This p-layer is used for the drift region of the bidirectional P-MOSFET, since boron doping is the best option for volume conduction in diamond (activation energy of boron is 0.38 eV, doping dependent, and a hole mobility up to 2000 cm 2 •V -1 •s -1 ).

The doping level of the p-type layer was measured to be [N A -N D ]= 2.3 × 10 17 ± 0.5 × 10 17 cm -3 by capacitance voltage measurements on MOS capacitors fabricated on the same sample. This was performed at temperatures ranging from room temperature to 250 • C, with an AC signal amplitude of 30 mV and a frequency carefully chosen to avoid frequency dependent effects. This high doping range for the drift region was chosen to demonstrate a high electric field and a high current capability with diamond FETs as previously demonstrated in [START_REF] Masante | [END_REF]. To lower the contact resistances, a 200 nm of highly boron-doped ([B]> 4 × 10 20 cm -3 ) diamond is selectively grown at the source and drain contacts. A Titanium/Platinum/Gold (30 nm/20 nm/30 nm) stack is then deposited by electron beam evaporation and annealed for 30 min at 500 • C in high vacuum condition (< 10 -7 mBar) to form the drain and source contacts. In order to limit the gate surface required, reducing the risk of critical failure and lowering the leakage current, a mesa structure has been designed; A reactive ion etching of O 2 plasma is used to etch down to the insulating diamond substrate around devices, as depicted in fig 1a). After an UV ozone treatment to saturate the diamond surface with oxygen bonds, the 50 nm Al 2 O 3 gate oxide layer was deposited following the process detailed by T. T. Pham et al. [START_REF] Pham | High quality Al 2 O 3 /(100) oxygen-terminated diamond interface for MOSFETs fabrication[END_REF] to decrease the interface state density. The Savannah 100 deposition system from Cambridge NanoTech was used with a deposition temperature of 380 • C, followed by a 30 min annealing at 500 • C. Finally, the Titanium/Platinum/Gold (30 nm/20 nm/30 nm) gate metal stack is deposited by electron beam evaporation. Elementary p-type D3MOSFETs with only one gate were also fabricated on the same diamond substrate, which showed a normally-on behavior, with a threshold voltage of +35V and a low gate leakage of 10nA/mm under a high electric field of more than 6MV/cm both in diamond and gate oxide [START_REF] Masante | [END_REF] .

The measured MOSFET dimensions are L G1D = 4 µm, L G1 = L G2 = 2 µm, L G1G2 = L G2S = 5 µm and a width of W = 60 µm. They can ideally be further reduced but were chosen towards a first proof of concept and related to lithography and fabrication limitations.

Considering the measured doping level of [N A -N D ]= 2.3 × 10 17 cm -3 , the drift lengths should ideally not exceed 2 µm [START_REF] Chicot | Optimal drift region for diamond power devices[END_REF]. A dual channel keithley 2612 sourcemeter was used to characterize the I st and III rd quadrant transistor characteristic of each setups, depicted in fig 2a ) andb).

To achieve a bidirectional switch, the device is always operated in the I st quadrant of the setup A or B depending of the bias sign applied between the ohmic contacts. In setup A, the gate 2 voltage controls the channel conductivity (transistor forward bias) while the drain and gate 1 are set at the same bias. The gate to drain distance is then L G1G2 +L G1 +L G1D and the depleted region below the gate 1 remains small as the drain to gate 1 potential difference is small. The setup B simply corresponds to the symmetrical case (transistor reverse bias). In this study, the switch between the two setups is performed manually to avoid the need of an external gate driver, the quasi-static bidirectional switch device characteristic is then reconstructed from both setups characteristics.

Results and discussion

Both setups transistor characteristics at 250 • C are shown in fig. 2d) and e). The on and off state are obtained in the I st quadrant for a negative drain-source bias V DS < 0 V (ptype MOSFET). The III rd quadrant characteristic has been successfully measured with the two gate setup, exhibiting the distinctive reopening of the channel in a diode-like behaviour. The bidirectional switch characteristic is shown in fig. 2f), obtained with the I st quadrant of each setups.

The minimum 250 • C on-state resistance is measured at 159 mΩ.cm 2 and 152 mΩ.cm 2 in setup A and B respectively, in the same order of magnitude than reported values on H-terminated MOSFETs, with a threshold of around 35 V (fig. 2c)). The on-state resistance is partially limited by the well known presence of a Fermi level pinning, at the oxygen-terminated diamond and oxide interface [START_REF] Pham | Deep-Depletion Mode Boron-Doped Monocrystalline Diamond Metal Oxide Semiconductor Field Effect Transistor[END_REF], [START_REF] Pham | High quality Al 2 O 3 /(100) oxygen-terminated diamond interface for MOSFETs fabrication[END_REF], [START_REF] Loto | Gate Oxide Electrical Stability of p-type Diamond MOS Capacitors[END_REF], [START_REF] Pham | Comprehensive electrical analysis of metal/Al 2 O 3 /O-terminated diamond capacitance[END_REF], accounting for an uncontrolled depleted layer at the diamond/oxide interface. This is also complemented by the probable existence of a depletion layer at the nitrogen-doped diamond substrate and the boron-doped layer. Both of these effects reduce the effective conductive channel thickness. Moreover as stated already, the device's drift and channel length are oversized due to fabrication limitations, resulting in a larger on-state resistance.

Leakage current, shown in fig. 2c), was found to be greatly varying from one gate to the other by orders of magnitude. In the case presented here, at 250 • C gate 1 and gate 2 exhibit at a gate-source voltage V GS = 50 V a leakage current of I G 1 = 0.217 mA/mm and I G 2 = 1.00×10 -4 mA/mm respectively. At least one of the gates in all the measured transistors exhibited a large leakage current. This prevented us from probing the off-state blocking capabilities of the device, a moderately high drain bias (|V DS | > 50 V) invariably caused the presence of a large gate current in the same order of magnitude than the on state drain current, eventually leading to a non-reversible breaking of the oxide. The leakage mechanism has previously been reported to be traps to traps hopping in bulk oxide with interface/valence band recombination [START_REF] Pham | Comprehensive electrical analysis of metal/Al 2 O 3 /O-terminated diamond capacitance[END_REF]. Applying a large gate bias, causing the presence of a large electric field in the oxide, may promote the bulk oxide trap formation, reducing the energy barrier between hopping sites as their average distance reduces. Controlling this trap density to a sufficiently low value even after several measurement cycles is of great importance to achieve a reliable high breakdown device. This could be achieved (i) by using a field plate architecture, reducing the well known electric field crowding effect, especially detrimental in 2D lateral devices. And (ii) by achieving a better control of the lithography and lift-off process, to reduce the roughness at the gate metal edge that is causing large local electric field crowding effects [START_REF] Umezawa | Defect and field-enhancement characterization through electron-beam-induced current analysis[END_REF]. MOSFETs with a corbino geometry have been tried to avoid the presence of corners. But the lift-off process was found to be more challenging and no devices could be obtained. Given that diamond MOSFET devices are yet not optimized, the breakdown is usually not limited by intrinsic diamond properties, but rather by imperfections of the process, the oxide layer or in the diamond crystalline structure. In a single gate design with very similar sample characteristics, it has been shown that a blocking voltage of 200 V [START_REF] Pham | Deep-Depletion Mode Boron-Doped Monocrystalline Diamond Metal Oxide Semiconductor Field Effect Transistor[END_REF] and a critical field greater than 5.4 MV/cm [START_REF] Masante | [END_REF] can be achieved. This is about twice larger than the critical field of SiC or GaN devices, without the use of electric field relaxation features to improve the breakdown. Optimized diamond devices could significantly improve the high power switch devices.

Perspectives

The fig. 3 shows the calculated 1D lateral static performances of a lateral bulk conduction diamond device with and without a recess gates design, in comparison to the measured device and reported values of bidirectional GaN HEMTs, bidirectional vertical 4H-SiC MOSFET and IGBT. A doping level dependant ideal breakdown and hole mobility with no compensation are assumed. The gate lengths and gate to source lengths are fixed at 1 µm. Thanks to its higher critical field, it is expected that diamond could reach higher breakdown voltages than GaN HEMT devices, in the order of a few kV, with low R ON , either with a bulk or surface conduction.

Lateral diamond devices could achieve similar performances than bidirectional vertical 4H-SiC devices at large blocking voltages, especially at high temperature. This is particularly valuable given the complex fabrication process of vertical bidirectional devices. Further reduction of the on state resistance could be achieved by operating in enhancement mode, either by accumulating holes at the MOS interface with a negative gate bias (p-type) or at zero gate bias thanks to the surface transfer doping effect. Similarly to what had been demonstrated with GaN and 4H-SiC, comparatively to the use of 2 devices, bidirectional switches offers a lower R ON , a smaller device area, no additional voltage drop in any operating quadrant (i.e. no diode-like voltage drop), smaller commutation loops thanks to monolithic integration and an optimal packaging with all power and driving electrodes on one side, and a thermal interface at the backside (for a lateral design).

Conclusion

High temperature operation of a diamond-based monolithic bidirectional power switch based on a double gate diamond deep depletion MOSFET is demonstrated in this work. The low resistivity of diamond at high temperature allows to reach a current density of 20 mA/mm at V DS = -20 V, despite the design being oversized on purpose to facilitate its fabrication. The high leakage current that may occur in one or both of the gates made impossible to measure the off state blocking voltage, but single gate breakdown measurements were previously reported with a blocking voltage up to -200 V and a critical field of at least 5.4 MV/cm. Lateral diamond bidirectional devices could be an attractive solution for high breakdown (> 1kV) and high temperature applications.

Figure 1 :

 1 Figure 1: a) Schematic representation of the double gate lateral p-type D3MOSFET at left, the 0.2 µm selectively grown heavily boron doped layer ([B] > 5×10 20 cm -3 ) is deposited on top of a boron doped layer ([N A -N D ]= 2.3×10 17 cm -3 ) with thickness of 0.4 µm. The metallic contacts all consist of a Titanium/Platinum/Gold (30 nm/20 nm/30 nm) stack. b) SEM top view of one of the fabricated device.

Figure 2 :

 2 Figure 2: Schematic representation of the double gate lateral D3MOSFET in a) the setup A and b) the setup B. When one of the gate is controlled, the other one is shorted with the closest ohmic contact. Because the experimental set-up is limited to 4 probes, the substrate is left floating. c) Measured transfer characteristics of setup A (red) and B (black), both the drain current I D and gate leakage current I G are plotted in full and open symbols respectively. d) Measured transistor characteristic at 250 • C in the setup A and e) the setup B. f) The I st quadrant of both setups are then use to reconstruct the transistor characteristic of the bidirectional double gate lateral D3MOSFET.

Figure 3 :

 3 Figure 3: Measured R ON as function of breakdown voltage of the fabricated diamond MOSFET and other reported wide band gap bidirectional devices from [15]-[17], [19]. The lines represent the calculated ideal characteristics at RT and 250 • C of the bidirectional lateral D3MOSFET device with gate lengths and gate to source lengths of 1 µm and optimized drift lengths. The dashed lines correspond to a more optimized recess gate design.

Acknowledgment

This research is partially funded by French ANR Research Agency under grant ANR-16-CE05-0023 #Diamond-HVDC and the French ministry of research. This work is supported by Laboratoire d'Excellence LANEF in Grenoble (ANR-10-LABX-51-01).