Jun-Seok Oh

Yolanda Aranda-Gonzalvo

James W Bradley
email: j.w.bradley@liv.ac.uk

Time-resolved mass spectroscopic studies of an atmospheric-pressure helium microplasma jet

Using molecular beam mass spectroscopy, time-resolved measurements of the ionic species in the plasma plume of an atmospheric-pressure helium microplasma jet have been made for a range of excitation frequencies (5, 10 and 25 kHz) and source-instruments orifice distances (1, 7 and 11 mm). Ionic species can only be observed in the visible plasma plume, with the main positive species being N 2 + (65.26%) and O 2 + (21.11%), and few percentages of N + , O + , NO + and He + . For the negative ions, the majority species are O 3 -(22.68 %), O 2 -(H 2 O) (10.49 %) and a large range of minority species observed namely, O n -, OH n -, (H 2 O) n -, CO n and clusters, O m -(H 2 O) n , (OH) m -(H 2 O) n , O n -(CO 3 ) and CO 3 -(H 2 O) n . The flux of ions created from air species such, NO + and O 3 -, are seen to be maximized a distance of several mm from the nozzle, whereas the He + concentration continually decreases with distance from the exit orifice. The time-resolved measurements (time-resolution down to 2 µs) show positive ions appear twice in one full period of the voltage waveform, correlated directly with positive and negative current peaks. The rise and fall times of the positive ions are typically tens of µs. In contrast, the appearance of negative ions is correlated only with the negative part of discharge current, with one main peak in the detected ionic flux seen per cycle. The rise time of the negative ions is about 10 µs, independent of mass, however we observed longer decay times from 100 to 150 µs increasing with mass. With increased driving frequency, the time

Introduction

Low-pressure plasma discharges are a useful tool in a number of surface treatment and polymerisation applications, since the plasma can act isotropically on irregular surfaces in large volumes whilst delivering good uniformity on substrates of all sizes [START_REF] Liston | Plasma surface modification of polymers: relevance to adhesion VSP BV Chapter[END_REF]. In recent years, however, there has been much interest in developing plasmas on the micro-scale which operate at, or close to atmospheric-pressure [START_REF] Tachibana | Current status of microplasma research[END_REF] to deliver cheap processing solutions with opportunities to functionalise surfaces on the small scale. One candidate plasma device is the atmospheric-pressure plasmas jet (APPJ) [START_REF] Koinuma | Development and application of a microbeam plasma generator[END_REF], which can be operated in different gases and is technologically simple, environmentally-friendly and very economical. The application areas of APPJs include material etching [START_REF] Jeong | Etching materials with an atmospheric-pressure plasma jet Plasma Sources[END_REF], deposition [START_REF] Ito | High speed deposition of SiO 2 films with plasma jet based on capillary dielectric barrier discharge at atmospheric pressure[END_REF], surface modification [START_REF] Szili | Controlling the spatial distribution of polymer surface treatment using atmosphericpressure microplasma jets Plasma Process[END_REF], sterilization [7][8] and in biomedicine such as a wound treatment [START_REF] Lee | Sterilization of bacteria, yeast, and bacterial endospores by atmospheric-pressure cold plasma using helium and oxygen[END_REF] taking advantage of the very low temperature plasma produced in(to) the ambient air. There are two well known configurations namely the plasma pencil [START_REF] Price | Cold air atmospheric pressure micro plasma jet application method and device United States[END_REF] and the plasma needle [START_REF] Stoffels | Plasma needle: a non-destructive atmospheric plasma source for fine surface treatment of (bio)materials[END_REF]. Recently, plasma jets, which produce a plasma plume emerging for a fine capillary have been scaled down to produce treatment on the nano-scale (down to tens of nm) [START_REF] Kakei | Production of Ultrafine Atmospheric Pressure Plasma Jet with Nano-capillary[END_REF].

To accompany the technological development, a number of researchers have undertaken advanced diagnostic studies to understand the internal physical and chemical phenomena in APPJs. These include the use of voltage-current probes [START_REF] Urabe | Investigation of discharge mechanisms in helium plasma jet at atmospheric pressure by laser spectroscopic measurements[END_REF], optical emission spectroscopy (OES) to get gas temperatures [START_REF] Walsh | Contrasting characteristics of pulsed and sinusoidal cold atmospheric plasma jets[END_REF][14] from emission profile and electron density from Stark broadening [START_REF] Qian | Stark broadening measurement of the electron density in an atmospheric pressure argon plasma jet with double-power electrodes[END_REF], 2-D imaging to observe plasma bullet development [START_REF] Teschke | Highspeed photographs of a dielectric barrier atmospheric pressure plasma jet[END_REF], laser absorption spectroscopy (LAS) [START_REF] Kakei | Production of Ultrafine Atmospheric Pressure Plasma Jet with Nano-capillary[END_REF] and two-photon absorption laser induced fluorescence (TALIF)

techniques [START_REF] Mazouffre | Tow-photon laser induced fluorescence spectroscopy performed on free nitrogen plasma jets Plasma Sources[END_REF] to determine the density of atoms, Schlieren photograph [START_REF] Oh | Imaging gas and plasma interaction in the surface chemical modification of polymers by micro-plasma jet[END_REF] to observe the invisible gas dynamics and recently molecular beam mass spectrometry [START_REF] Skalny | Mass spectrometric study of negative ions extracted from point to plane negative corona discharge in ambient air at atmospheric pressure Intl[END_REF][20][21] [START_REF] Rees | Mass and energy spectrometry of atmospheric pressure plasmas Plasma Process[END_REF] [23] [START_REF] Bruggeman | Mass spectrometry study of positive and negative ions in a capacitively coupled atmospheric pressure RF excited glow discharge in He-water mixtures[END_REF] to determine the ionic composition (both negative and positive ions) from APPJs.

Here we show for the first time the temporal evolution of the mass-selected ionic species in the plasma plume as it interacts with ambient air and correlate the rise and fall of the ionic flux of which the current carried by plasma bullets as they are transported down the plume. In a number of previous mass spectroscopic studies of plasma at atmospheric pressure using an array for different source configurations (not just the µ-APPJ), the emphasis has been on understanding the plasma composition in complex gases or gas mixtures, such air [START_REF] Skalny | Mass spectrometric study of negative ions extracted from point to plane negative corona discharge in ambient air at atmospheric pressure Intl[END_REF], N 2 -O 2 mixtures [START_REF] Malovi | Mass analysis of an atmospheric pressure plasma needle discharge Plasma Sources[END_REF], He-air [START_REF] Stoffels | Mass spectrometric detection of short-living radicals produced by a plasma needle Plasma Sources[END_REF][22], He-O 2 [START_REF] Waskoenig | Atomic oxygen formation in a radio-frequency driven micro-atmospheric pressure plasma jet Plasma Sources[END_REF] or He-water mixtures [START_REF] Bruggeman | Mass spectrometry study of positive and negative ions in a capacitively coupled atmospheric pressure RF excited glow discharge in He-water mixtures[END_REF] as a feed gas. However, here we concentrate on using a pure He jet and study its interaction with the ambient environment creating secondary ionic species, outside of the main discharge.

The experimental arrangement

µ-plasma jet setup

The µ-plasma jet used in this study was made using a 15 cm long quartz glass tube of 1 mm inner diameter (ID) and 3 mm outer diameter (OD) on which two copper ring electrodes of 8 mm length were attached as shown in figure 1. The electrode close to the outflow was powered by high ac voltages (6-9 kV, V p-p ) over a range of frequencies (5-25 kHz). The power supply consisted of a sine wave oscillator (Farnell, LF1) driving a commercial audio amplifier (HQ power, VPA2350MB) with a voltage step-up transformer (Express Transformers, UK) at the output stage to generate the required high voltages for discharge breakdown. The second electrode was electrically grounded. The jet struck in helium gas (99.996 % purity), was operated essentially as a dielectric barrier discharge (DBD), with gas flow rates (between 0.2 and 5.4 L min -1 ), and controlled using a manual rotameter (Omega Engineering). The flexible design allowed the separation and position of the ring electrodes to be changed on the glass tube to aid breakdown and optimise the visible jet length. After preliminary investigations, a gap separation of 25 mm was chosen for all subsequent experiments with a fixed He flow rate of 1.38 slm producing visible plumes of 12 mm in length. To obtain 2-D spatially resolved mass spectroscopic measurements, the µ-plasma jet was mounted on an x-y stage, having a travel range of 20 cm in the axial direction and 12 cm in the radial with 1 mm spatial resolution with the stage placed in front of the mass spectrometer entrance orifice.

Molecular beam mass spectroscopy

The molecular beam mass spectrometer (MBMS) used here was a quadrupole-based mass spectrometer (QMS) system, the HPR-60 MBMS (from Hiden Analytical Ltd.) having a three-stage differentially pumped inlet system separated by aligned skimmer cones and turbo molecular pumps. The pressure reduction stages, (P 1 )-(P 3 ) shown in figure 1(a), provide a pressure reduction from atmospheric-pressure to 10 -1 Torr at the (P 1 ), 10 -5 Torr at the (P 2 ) and 10 -7 Torr at the (P 3 ) stage. A molecular beam is formed which is directed into the QMS.

Positive and negative ions, generated by the jet (either between the electrodes or through interactions in ambient air) were readily detected, with a mass resolution of 0.01 amu and with an upper mass range of 300 amu. The instrument can also be used in residual gas analyzer (RGA) mode, having an internal ionisation source in the last pressure redactor stage.

The mass spectrometer was operated in secondary ion mass spectroscopy (SIMS) mode, with a time resolution of 10 µs down to 2 µs depending on plasma frequency, obtained by gating the detector using an internal gate signal synchronized to the driving voltage waveform.

The end of the jet capillary was aligned with the centre of the sampling orifice (100 µm diameter) and the (nozzle-orifice) distance d were varied from 1 to 15 mm. Preliminary investigations showed that ionic signals could only be detected with the plasma plume parallel to the axis of the instrument, that is, no ions were detected at radial distances greater than the plume width of 1 mm (close to the spatial resolution of the x-y stage). The sampling time of the detector was set at 1 s and ions, both positive and negative, of masses between 1 and 100 amu were collected.

To restrict the amount of data in subsequent experiments we chose just three measurement

Discharge and bullet current measurements

To interpret the temporal mass spectroscopic measurements, a correlation to the discharge current and voltage waveforms is necessary. The main discharge current I T (flowing between transformer and the driving electrode) was measured using a current probe 1 (Tektronix TCP0030) (see figure 1 The width of the current pulse (FWHM) is several micro-seconds. For the bullet current measurements in the outflow, a metal collector plate (copper), connected to the ground line was used and I b flowing in this circuit was monitored by the current probe 2. The presence of the metal collector perturbs the free-stream conditions of the plasma plume, however it gives the same electrical conditions as used in the MBMS measurements, where the sampling orifice of the mass spectrometer acted as an external grounded electrode.

2-D imaging

The visible plasma jet and plasma bullet investigations were carried out using both time- was taken at a delay time (= 35.6 µs) from which we can distinguish an intense emission head ('bullet head') and a long and faint tail ('bullet halo'). This feature will be discussed later in the context of the MBMS results. The time for the bullet head to pass a fixed point was observed to be few hundred ns but the bullet tail was seen to persist for significantly longer, almost the entire time to the emergence of the next main discharge event. From observation of the bullet head we have calculated the a propagation speed for the bullet of a few tens of kilometre per second, in very good agreement with previous reports [2][16][18]. This is much faster than the gas speed of a few tens metre per second. The current pulse (recorded in the driving circuit corresponding to this bullet) is shown in figure 2(c). , as well as small portions of N + , O + , N 2 H + , NO + and Ar + . CO 2 + is also observed but its concentration is less than 0.02 %. The general composition of ions is consistent with those in atmospheric air as shown in table 1.

Results and discussion

Positive ions

He + ions detected here are created inside the capillary (the primary discharge) via electronneutral ionization processes in the transverse electric field between electrodes, namely R1) e - +He 2e -+He + and R2) e -+He m 2e -+He + , where He m is the 2 3 S state metastable helium.

Both He + and He m species are transported outside the capillary by the mass flow action of the He gas flowing with a speed of 30 m s - ). The rate constants R5-R7 have been used in the modelling of both atmospheric pressure [START_REF] Waskoenig | Atomic oxygen formation in a radio-frequency driven micro-atmospheric pressure plasma jet Plasma Sources[END_REF] and low pressure plasmas [START_REF] Stafford | O 2 ( 1 ) production in He/O 2 mixtures in flowing low pressure plasmas[END_REF]. The reaction rate for the charge transfer from He + (or He 2 + ) and the ionisation by He m to both N 2 and O 2 are similar in order (~10 -11 to 10 -10 cm 3 s -1 ). Therefore, the positive ion yield should reflect the composition of the ambient air. It therefore stands to reason that N 2 + was the highest yield with O 2 + the second highest.

Negative ions

For the negative ions, we detected 34 species as shown in figure 4 [START_REF] Bruggeman | Mass spectrometry study of positive and negative ions in a capacitively coupled atmospheric pressure RF excited glow discharge in He-water mixtures[END_REF] but the product ions readily undergo further attachment processes (A -+M A -M), where M denotes molecules in air, to form heavier species.

The dissociative electron attachment of H 2 O and subsequently hydration has been reported

for negative ion formation in Ref. [START_REF] Bruggeman | Mass spectrometry study of positive and negative ions in a capacitively coupled atmospheric pressure RF excited glow discharge in He-water mixtures[END_REF] where a He-water mixture was used to produce moist gas, then fed into a glow discharge in an isolated space (not exposure to air). In our work, however, the most intense peaks of negative ions are oxygen molecules such as O ) as also found in [START_REF] Bruggeman | Mass spectrometry study of positive and negative ions in a capacitively coupled atmospheric pressure RF excited glow discharge in He-water mixtures[END_REF] is due to the presence of atmospheric oxygen (contained in moisture free) which become mixed or entrained in the He gas flow.

Since the positive and negative ions are formed through wholly different processes it is expected that each process may be separated spatially and temporally. To understand better the processes leading to ionic formation, time-resolved mass spectroscopy has been carried out at a number of locations along the visible glow and the results are discussed in section 3.4 below. we see the negative ions extend further past the end of the visible plume than the positive species, being still detectable at 15 mm.

Spatial distribution of ionic concentrations

Time-resolved SIMS, 2-D imaging and current measurements

Since the first work conducted in the group of Engemann [START_REF] Teschke | Highspeed photographs of a dielectric barrier atmospheric pressure plasma jet[END_REF] it has been shown many times that the visible plasma emission extending from the DBD micro-jet propagates at very fast speeds (up to ~10 2 km s -1 ) in the form of discrete packets referred to as plasma "bullets".

Recently, it is also reported that for ac driven APPJs, bullets have their origin when the driving voltage reaches a threshold in the positive and negative going parts of the voltage cycle [START_REF] Oh | Discharge and Plasma Bullet Formation in a Capillary DBD Atmospheric Pressure Micro-plasma Jet[END_REF]. In brief, the plasma bullet triggered from positive half period shows an intense and bright emission. While negative half period bullet is observed faint and broad. However, initiation of both is localised around few mm and can be clearly observed only within duration of a few microseconds after initiation.

The ionic content of the bullet (its head and following tail or halo) has not yet been determined. Here, through time-resolved measurements (He + and secondary positive and negative) ions we can correlate the time over which ionic species rise, persist and subsequently fall during the voltage cycle to the time the bullet of length ~12 mm (consisting of head and tail) passes a point in space.

For the time-resolved MBMS studies, the mass selected ion concentrations were measured at three distances 1, 7 and 11 mm from the exit orifice of the capillary over two whole periods of the driving voltage (e.g. this is 400 µs when operating at 5 kHz). The timeresolution of the ionic concentrations (both positive and negative) changed depending on the driving frequency, i.e. 10 µs at 5 kHz and 2 µs at 25 kHz. To aid interpretation, we have chosen to display a 6 positive species (O 2 + , O + , N 2 + , N + , He + and Ar + ) and 6 negative species

(O -, OH -, O 3 - , OH -(H 2 O
) and H 2 O -), having a behaviour and response representative of all the ions detected as listed in table 1 and shown in figure 4. For the case of the negative ions, their rise to correlated to the negative part of the voltage cycle, reaching their peak on fast time-scales (~10 µs) similar to the positive ions, but they persist for much longer sustaining times up to 100-150 µs, displaying much slow decays, and

Effect of distance

showing little response to the subsequent positive voltage half cycle. The correlation of negative ions with negative voltage swings on the powered electrode may be associated with the axial electric field which will act to accelerate electrons out into the plume, where they readily attach to molecules in dissociative reactions to form families of negative ions. In the positive half cycle the concentration of electrons in the plume may be reduced as the powered electrodes acts as an effective electron sink.

The negative ions, with decay time constants τ ~20 µs display a modulation once every cycle rather than a twice as with the positive ions. We may be able to understand that heavier species, such as negative ion clusters will have lower mobilities and longer lifetimes in air than the lighter positive ions [START_REF] Waskoenig | Atomic oxygen formation in a radio-frequency driven micro-atmospheric pressure plasma jet Plasma Sources[END_REF], however here O -displays decay times many times longer than O + (see figure 6 to 8). It is reported that ions such as OH -can have lifetime of 1 ms being stable in air [START_REF] Sekimoto | Negative ion formation and evolution in atmospheric pressure corona discharges between point-to-plane electrodes with arbitrary needle angle[END_REF] however primary ions in reactions such as O -have shorter lifetimes and short mean free paths of about 0.1 µm.

Close inspection of the figure 6 to 8 shows there is some small modulation of the measured intensities of the lighter ions O -, H -and OH -in the positive half cycle of the voltage. With increasing distances, it is clear that atomic O -flux is reduced. The fluxes of heavy molecules, O 3 -and OH -(H 2 O), are maximised at the 7 mm where a strong interaction between plasma jet and surrounding air is expected as mention above subsection 3.3.

The amount of mixing of the air with the He stream will be an important factor in determining the amount of negative ionic species (clusters) detected in the plume created through reactions R8 and R9, and then by further oxidisation and/or hydration processes.

It is clear in figure 6 to 8 that both the discharge current and the locally measured current in the plume (bullet current) consist of a rapid pulse of width of only several microseconds; however the peaks ionic spectra last much longer (~50 µs and ~100 µs for the positive and negative ions respectively).

Inspection of the bullet current waveforms (for example that in figure 2(c)), shows the head contains most of the total charge associated with bullet with about 25 % contained in the long tail. For positive currents the head contains positive ions and few negative ions (<10%) which are decaying in density from the previous pulse. The duration of tail which also consists mostly of positive ions passing any point coincide with the duration of the positive ionic signals in mass spectroscopic results. For negative going bullet currents the head contains a high proportion of negative ions and some positive ions (as shown in mass spectroscopic results in figure 6). In this case, the bullet tail contains negative ionic species.

Ionic life-times

For bullet formation outside the capillary, we see a bright head of length typically 2-3 mm followed by a visually weaker but attached tail. As the head advances (with velocities up to 23 km s -1 measured using the ICCD camera), the tail elongates, being essentially clamped to the end of the capillary for at least 10 µs observed before the next main discharge event, see also [START_REF] Tachibana | Current status of microplasma research[END_REF][16] [START_REF] Walsh | Frequency effects of plasma bullets in atmospheric glow discharges[END_REF][35] [START_REF] Laroussi | The plasma pencil: a source of hypersonic cold plasma bullets for biomedical applications[END_REF][37]. Even when the bullet is terminated at the external grounded electrode (or current collector), the bullet tail persists for some time. In free stream conditions, it has been observed that after about several µs the bullet head dissipates, as the mole fraction of He gas in ambient air falls below a critical value as turbulence sets in to the flow [START_REF] Oh | Imaging gas and plasma interaction in the surface chemical modification of polymers by micro-plasma jet[END_REF].

The results from the time-resolved mass spectrometric measurements here show remarkably long times for the existence of both positive and negative species, between ~50 and ~150 µs respectively. This time-scale is much longer that the time the bullet traverses between the capillary end and the mass spectrometer orifice, and longer than the time we observe the optical emission from the long bullet tail. However, although the tail is not observed some 50 µs after the bullet initiation we argue that the passing bullet has created a halo of secondary positive and negative ions which have significant life-times. The fast increase in ionic signal for both polarity ions (10 µs rise time), correlated with the local bullet current measurement is characteristic of the creation of species as electrons (or possibly He + ) ions rapidly pass through the gas. Once created, the negative ions (including heavy clusters) have longer lifetimes than the positive species, possibly due to higher ionic stabilities, more complex destruction reaction routes or lower ion mobilities in the ambient air [START_REF] Sekimoto | Negative ion formation and evolution in atmospheric pressure corona discharges between point-to-plane electrodes with arbitrary needle angle[END_REF][38]. The distance over which ions travel to the mass spectrometer from their point of creation can be determined, from knowledge of the lifetimes and the speed of the gas that transports them downstream. Here we have a He gas speed of 30 ms -1 and assuming 50-100 µs lifetimes it gives a distance of about 1.5 to 3 mm. It seems likely therefore that the detected species both positive and negative ions have been created close to the spectrometer orifice, however coming from distance much greater than reported elsewhere [38] and this we cannot explain with the limited data of this study.

Effect of driving frequency

The ionic intensities measured for pulse frequencies of 10 and 25 kHz are shown in figures Although we see almost a flat response in the negative ion signal at 25 kHz, inspection of the curves on a linear scale reveal the same rise and decay time constant as for 5 and 10 kHz.

At 25 kHz, half the period is 20 µs, close to the decay time constant for the negative ions;

hence it is possible that an equilibrium concentration along the plume of negative ions may develop with only a small modulation in intensities during the voltage cycle, as can be seen in figure 10.

Conclusions

Using time-resolved molecular beam mass spectrometry negative and positive ions have The temporal increase of the signal of both positive and negative ionic species in the mass spectra is correlated directly with the rise of the discharge and local bullet currents. The current from the head of individual bullets only exists for several microseconds, much shorter than the detected time of the ionic species (50 to 150 µs). As the bullet head passes through the neutral (non-turbulent) He stream, it produces a halo of secondary ions of both polarities.

Inspection of the bullet current waveforms together with time-resolved optical imaging and mass spectroscopic results indicates that the bright head of the bullet contains most of the net space charge, with the faint bullet tail (halo) of lower space charge persisting for long times over which the ionic species are detected. At highest frequency of 25 kHz, when half the driving period becomes close to the characteristic decay time of the negative ions a constant (almost equilibrium) concentration of negative ions can be maintained adjacent to the orifice of the mass spectrometer.

  positions in the visible plume, namely at d = 1, 7 and 11 mm, each position showing different interactions of the emerging He + and the ambient air. Three different driving frequencies (5, 10 and 25 kHz) were also used, providing temporal resolutions of 10, 5, and 2 µs. Since ions have a significant travel time in the instrument dependent on their mass, to correlate properly the time of their entrance into the instrument and the phase of the voltage cycle, the detection signals were temporally corrected using an algorithm developed at Hiden Analytical Ltd [25].

  (b)), with a 120 MHz bandwidth. The electrode voltage was measured with a high voltage probe (HVP-15HF). Both V and I were displayed on a digital oscilloscope (DPO3034). I T was averaged over 128 cycles, and the large and continuous sinusoidal displacement current I d observed in the waveform, (which can be measured without gas flow), was removed from the total signal to give the true discharge current (I D = I T -I d ). I D is composed of the internal discharge current and the free-stream "bullet" current I b .

Figure 2

 2 Figure 2 shows waveforms for (a) driving voltage, (b) currents, total I T and discharge currents I D , and (c) bullet current for a sinusoidal driving voltage 8 kV (V p-p ) of frequency 10 kHz. The bullet current (described below) was measured 7 mm downstream and the discharge and bullet current peaks coincide in this electrode arrangement because I D flow to two pathways, toward the upstream ground electrode and downstream towards the external ground, of roughly equal distance for the driving electrode. The current peaks are several milli-amperes or less, much smaller than the displacement current (few tens mA) as showing in figure 2(b). The width of the current pulse (FWHM) is several micro-seconds. For the

  figure3(a) an ICCD exposure time of 0.5 s was used. To resolve individual plasma bullets (known to be present in the plume and correlated to the positive and negative excursions of

Figure 4 (

 4 Figure 4(a) and (b) shows the time-averaged mass spectra of positive and negative ions obtained 7 mm from the exit of the capillary along the discharge axis. The excitation voltage was 8 kV at 10 kHz frequency and the flow rate was fixed at 1.38 L min -1 . The raw mass

Figure 5 (

 5 Figure 5(a) and (b) shows the relative concentration of selected positive and negative ions detected along the length of the plasma plume, from 1 mm from exit of the capillary to a

Figure 6

 6 Figure 6 to 8 shows the temporal evolution of the ionic species together in each figure with the driving voltage waveform, the discharge current I D and local bullet current I b measurements. The relative concentrations of the positive and negative species reflect the time-average mass spectra in figure 4 with the most intense species being N 2 + and O 2 + for the positive ions O 3 -and O -for the negative ions. The magnitude of the positive and negative local bullet currents I b follow those of the general ionic intensity, i.e. small currents are associated with small ionic intensities. So we can see the passing bullet delivers positive ionic species to the instrument in each voltage half cycle. All the three figures 6-8, show the same strong features. In the positive voltage half cycle with an associated positive discharge current pulse (and directly correlated with the start of

9 and 10 3 -

 103 respectively. It is clear that the peak ionic fluxes are reduced at higher frequencies. Also, we see only one current peak per half cycle of the discharge voltage. These observations are not understood but will be explored in further studies. At 25 kHz the negative ions respond less well the voltage modulation, however the positive ions intensities still show a clear time modulation, with 10 µs rise and decay times similar to those in figures 6-8 for a 5 kHz frequency. The N 2 + and O 2 + signals rise and fall as before at other frequencies but atomic ions, N + and O + , are delayed around 10 µs relative to these, this may be due to the stepwise process necessary to produce atomic ions from air species. For the negative ions, at 10 and 25 kHz the atomic O -and OH -molecule intensities are measured in advance of the heavier ions such as O and OH -(H 2 O), this may also be due to multi-step processes necessary for their creation and possibly longer travel times from the point creation to the mass spectrometer orifice.

3 -

 3 been detected in the output plume of an atmospheric pressure plasma micro-jet operating in He at 5-25kHz. The results show that positives ions (for instance He + N 2 + and O 2 + ) are created on the positive and negative voltage swings and persist for about 50 µs. The creation of negative ions (e.g. O ), OH -(H 2 O) and clusters) is correlated however only to the negative part of the voltage cycle, but these species have very long decay times (3 or 4 times longer for the full decay than the positive ions). The ionic components are only observed in the visible plasma plume and up to 1 to 2 mm distance beyond its end.

Figure captions FIG. 1 FIG. 2 FIG. 3 FIG. 4 AFIG. 5 4 .FIG. 6 FIG. 7 FIG. 8 FIG. 9 FIG. 10

 captions123454678910 Figure captions

Figure 1 (Figure 2 (Figure 3 (Figure 4 (Figure 5 (Figure 6 (Figure 7 (Figure 8 (Figure 9 (Figure 10 (

 12345678910 Figure 1 (Fig 1 R1.bmp)

Table 1

 1 The positive and negative species detected in the plasma plume with their relative 1 abundance by atomic percentage. 2

	amu	Standard atmosphere	Micro-plasma jet and ambient air
		Neutrals	Positive ions	Negative ions
			He + (1.79)
			N + (2.38)
			O + (2.48)	O -(1.89)
				OH -(0.76)
		H2O (0.4)	H2O + (0.37)
		N2 (78.08)	N2 + (65.26)
			N2H + (1.38)
			NO + (3.76)
		O2(20.94)	O2 + (21.11)	O2 -(5.43)
			(OH)O + (0.15)	(OH)O -(2.68)
			O(H2O) + (0.12)	O -(H2O) (1.19)
				OH -(H2O) (5.79)
		Ar (0.9337)	Ar + (0.56)
		CO2 -(0.039)
				NO2 -(0.79)
				O3 -(22.68)
				O2 -(H2O) (10.49)
				(OH)O -(H2O) (3.80)
				O -(H2O)2 (0.46)
				OH -(H2O)2 (8.19)
				CO3 -(3.60)
				HCO3 -(3.25)
				NO3 -(0.36)
				O3(OH) -

Acknowledgement

We thank Mr. Alan Roby for the construction of the microplasma jet rig. This work was supported by the Engineering and Physical Sciences Research Council (EPSRC) grant reference: EP/G048444/1.


Patrik Š Ajev
email: patrik.scajev@ff.vu.lt

Masashi Kato

K Stutis

Jaraši Nas

A diffraction-based technique for determination of interband absorption coefficients in bulk 3C-, 4H-, and 6H-SiC crystals

Keywords: absorption coefficient, free carriers, light-induced transient grating, SiC Determination of interband absorption coefficient PACS 78.47.+p -Time-resolved optical spectroscopies and other ultrafast optical measurements in condensed matter. 81.70.Fy -Nondestructive testing: optical methods. 78.40.-q -Absorption and reflection spectra: visible and ultraviolet

A diffraction-based technique for determination of interband absorption coefficients in bulk 3C-, 4H-and 6H-SiC crystals

Patrik Šcajev, Masashi Kato, Kestutis

Introduction

Progress in epitaxial growth technologies of SiC has expanded its applications for power devices, such as high-frequency power controllers and bipolar switching devices, operating under varying high injection conditions. Therefore, an appropriate attention must be paid to the peculiarities of recombination and transport processes at low and high carrier densities.

A deeper understanding of these processes in high-density carrier plasma can be obtained by applying time-resolved optical techniques, which allow the injection of an excess carriers by a short laser pulse and subsequent monitoring of the recombination and diffusion processes [START_REF] Gudelis | Fast and slow carrier recombination transients in highly excited 4H-and 3C-SiC crystals at room temperature[END_REF][START_REF] Jaraši Nas | Nonequilibrium carrier recombination in highly excited bulk SiC crystals[END_REF][START_REF] Klein | Carrier lifetime measurement in n -4H-SiC epilayers[END_REF][START_REF] Hassan | Comparative studies of carrier dynamics in 3C-SiC layers grown on Si and 4H-SiC substrates[END_REF][START_REF] Neimontas | The determination of high-density carrier plasma parameters in epitaxial layers, semiinsulating and heavily doped crystals of 4H-SiC by a picosecond four-wave mixing technique[END_REF][START_REF] Neimontas | Characterization of Electronic Properties of Different SiC Polytypes by All-optical Means[END_REF][START_REF] Malinauskas | Optical monitoring of nonequilibrium carrier lifetime in freestanding GaN by time-resolved four-wave mixing and photoluminescence techniques[END_REF]. Therefore knowledge of absorption coefficent at commonly used laser wavelengths is an important issue for more precise determination of injected carrier density profile and related plasma parameters in SiC, e.g. nonlinear and surface recombination rates, carrier diffusivity and band gap renormalization [START_REF] Gudelis | Fast and slow carrier recombination transients in highly excited 4H-and 3C-SiC crystals at room temperature[END_REF][START_REF] Jaraši Nas | Nonequilibrium carrier recombination in highly excited bulk SiC crystals[END_REF][START_REF] Galeckas | Auger recombination in 4H-SiC: Unusual temperature behavior[END_REF][START_REF] Galeckas | Optical characterization of excess carrier lifetime and surface recombination in 4H/6H-SiC[END_REF][START_REF] Klein | Recombination processes controlling the carrier lifetime in n -4H-SiC epilayers with low Z 1/2 concentrations[END_REF].

In this work we present a novel way for determination of absorption coefficient α in a wide temperature range. The proposed method is based on recording of a transient free carrier grating by strongly absorbed UV light (hν > E g ) and measurements of the probe beam diffraction efficiencies on the grating for two symmetric directions: the Bragg (BR) and anti-Bragg (ABR) one. The method was applied in 80 -800 K range for three commonly used SiC polytypes which are commercially available as bulk wafers. The determined absorption coeficient values were found in good agreement with the known data at room temperature [START_REF] Sridhara | Penetration depths in the ultraviolet for 4H, 6H and 3C silicon carbide at seven common laser pumping wavelengths[END_REF], thus confirming applicability of the proposed technique.

Samples and experimental techniques 2.1. The light induced transient grating technique

A standard experimental setup was used for recording light-induced transient gratings (LITG) in SiC [START_REF] Jarasiunas | Implementation of diffractive optical element in four-wave mixing scheme for ex situ characterisation of hydride vapor phase epitaxy-grown GaN layers[END_REF]. The experimental setup is provided on figure 1. The spatially modulated structure of non-equilibrium carriers was generated in a semiconductor by two coherent ~5.6 ps duration picosecond pulses, using the third harmonic (λ 3 = 351 nm) of a Nd:Yttrium Lithium Fluoride (Nd:YLF) laser. The grating recording beam passed a diffractive optical element -a permanent diffraction grating with a fixed period, and the two first order diffracted beams were selected by a spatial mask. These beams passed an optical telescope with two lenses of f 1 and f 2 focal lengths and created refractive index modulation in the sample, called a transient grating. The diffraction from this grating was monitored by an optically delayed probe beam at 1053 nm. The energies of excitation, transmitted and diffracted beams were measured by Si photodetectors. The excitation energy was varied using an attenuator. The reflection from a quartz plate provided the excitation energy calibration. For the background scattered light measurements at photodetectors, an electromechanical shutter was used. The diameters of excitation and probe beams were 650 µm and 180 µm, respectively.

The grating recording beams intersected at angle Θ in a sample plane, creating an interference pattern with a spatial period Λ:

( ) ( ) 2 / sin 2 / 3 Θ = Λ λ
.

(1) The spatially in-plane modulated light intensity was used to generate nonequilibrium carriers, as described by equation ( 2

): ( ) ( ) ( ) ( ) Λ + = Λ + = / 2 cos 1 / / 2 cos 1 ) ( 0 0 x N h x I x N π ν π α . ( 2
)
Here I 0 =(1-R)I inc is the total excitation fluence in the sample, α, R are the absorption and reflection coefficients at λ 3 with quantum energy hν, I inc is the incident excitation fluence and N 0 = αI 0 /hν is the nonequilibrium electron-hole concentration. The generated carriers induced a complex refractive index change (temporary modulation of a refractive index and an absorption coefficient), which is proportional to the generated carrier density:

k i n n FC ∆ + ∆ = ∆ ~∝ N 0 .
The change of the real part of refractive index was dominant at the used probe wavelength λ 1 [START_REF] Eichler | Laser-Induced Dynamic Gratings[END_REF] and described by the Drude-Lorentz formula [START_REF] Fisher | Optical phase conjugation[END_REF]:

0 N n n eh FC = ∆ , 2 2 2 2 * 0 1 2 2 ω ω ω ω ε - - = Γ Γ eh eh m n e n , + = 1 1 1 h e eh m m m , (3)
here ω=2π/λ 1 is the probe frequency, Γ ω is the frequency of the effective bandgap (direct, E Γ =7.3 eV for 4H-and 6H-SiC [START_REF] Zollner | Dielectric functions of bulk 4H and 6H SiC and spectroscopic ellipsometry studies of thin SiC films on Si[END_REF]), n eh is the refractive index change by one electron-hole pair, * eh m is the reduced electron-hole effective mass, and n 1 is the refractive index for probe wavelength. The recorded free carrier grating was probed by an optically delayed probe beam from the same laser at wavelength λ 1 =1053 nm. The diffraction efficiency η of the probe beam primarily depends on phase modulation Φ of the beam by the grating, η ∝ Φ 2 = (2π∆nd * /λ 1 ) 2 , and as well on a parameter ( )

2 1 * 1 / 2 Λ = n d Q πλ
, which describes the propagation and overlapping of the diffracted beams inside the photoexcited crystal. If the excited layer is thin (d* =1/α) and the distance between the grating peaks Λ is large (Λ >> d*), then Q << 1 and multiple symmetric diffraction orders are observed in the far field of diffraction (so called diffraction on a "thin grating" [START_REF] Eichler | Laser-Induced Dynamic Gratings[END_REF]). If the excited layer thickness increases and grating period Λ decreases, then the parameter Q may become high enough, Q > 10, that only one dominant diffraction order is observed. The latter case corresponds to probe beam diffraction on a "thick Bragg grating". Light diffraction in the intermediate range of 1 < Q < 10 corresponds to a transient regime between the thin and thick gratings, and will be further called as an "intermediate grating". Probe beam diffraction efficiency on the Bragg grating is described by equation (4) [START_REF] Eichler | Laser-Induced Dynamic Gratings[END_REF]:

( ) ( ) ( ) ( ) - - = ∆ = = = ϕ νλ α π ϕ λ π η cos exp 1 tan cos ) ( tan 1 0 2 1 2 0 h d I n dz z n I I eh T D t . ( 4
)
Assuming strong interband absorption αd >> 1, the carrier injection depth is small and the probe beam, incident at angle ϕ, integrates the refractive index change over the depth z (as well it integrates the injected carrier density N 0 (z)∝I 0 (z)/hν).

As we consider light diffraction on a transient free carrier grating, recorded in a relatively thin surface layer d*, its parameters are changing with time. Firstly, the thickness d* may increase due to carrier diffusion to the depth, therefore evolution of the grating in-depth profile N(z) must be taken into account. In addition, grating modulation ∆n(x,t) ∝ N(x,t) decays due to carrier recombination and in-plane diffusion. A solution of the continuity equation [START_REF] Neimontas | Characterization of Electronic Properties of Different SiC Polytypes by All-optical Means[END_REF][START_REF] Eichler | Laser-Induced Dynamic Gratings[END_REF] provides the transient grating dynamics N(x,z,t), seen in decay of its diffraction efficiency:

( ) ( ) . 4 1 , 1 1 1 , / 2
Λ = + = - = - - Λ - Λ + = = D t t z t Dt x N t z x N D D R G G t R π τ τ τ τ τ η η α τ π π (5)
Here τ G is the grating decay time, τ R is the carrier recombination lifetime, τ D is the grating diffusive erase time due to ambipolar diffusion with coefficient D. As the modulated carrier density N decays due to carrier recombination and diffusion, and the thickness d* increases with time, the diffraction efficiency must be measured shortly after excitation pulse until the grating temporal and spatial profile retains the features of photoexcitation (i.e. at t <τ R , τ D ; then d = d*).

Assuming different α values for SiC polytypes at used 351 nm wavelenght (α = 290 cm -1 for 4H-, 2260 cm -1 for 3C-, and 1060 cm -1 for 6H-SiC [START_REF] Sridhara | Penetration depths in the ultraviolet for 4H, 6H and 3C silicon carbide at seven common laser pumping wavelengths[END_REF]), the light penetration depth d* and the chosen period Λ = 2.85 µm resulted in Q = 10.84, 1.39, 2.97 values, correspondingly. Light diffraction features for these "thick" or "intermediate" surface gratings are given in part 2.3.

Samples

We investigated low doped 3C-(n 0 ~10 16 cm -3 ) and high resistivity 4H-and 6H-SiC samples of thickness d = 260 µm, 360 µm and 400 µm, respectively. At very low intrisic carrier density, the spectral and temperature dependences of absorption coefficient can be described solely by phonon assisted light absorption processes. The samples were mechanically polished to optical quality on both sides in order to minimize the scattering losses of the probe beam and render high signal to noise ratio. The nitrogen cryostat was used for varying the sample temperature in the 80-800 K range.

The used experimental conditions ensured the grating decay time τ G above 100 ps, i.e longer than the picosecond probe pulse duration. The τ G values were estimated on basis of the measured carrier lifetimes τ R in 0.5 ns -1 µs range [START_REF] Jaraši Nas | Nonequilibrium carrier recombination in highly excited bulk SiC crystals[END_REF][START_REF] Neimontas | Non-equilibrium carrier diffusion and recombination in semi-insulating PVT grown bulk 6H-SiC crystals[END_REF] and diffusive decay times τ D > 230 ps for used period Λ = 2.85 µm (the D values for 3C SiC varied in 1.5 -9 cm 2 /s interval with temperature [START_REF] Hassan | Comparative studies of carrier dynamics in 3C-SiC layers grown on Si and 4H-SiC substrates[END_REF], and similarly in the other polytypes). Therefore diffraction efficiency value if measured at the end of laser pulse (t ≈ 20 ps) was not affected by the grating decay and reflected the instantaneus values of ∆n. Similarly, the diffusion did not increase the initial thickness of the photoexcited layer d*. This allowed us to apply algorithms of grating diffraction efficiency on the intermediate gratings with fixed Q values for calculation of the relevant d* and α values.

Light diffraction on intermediate gratings

Let us consider laser beam absorption in the media and deduce formulas for the first diffraction orders on the intermediate grating (i.e. for the Bragg and anti-Bragg beams). Approaching the thin grating regime, intensities of both diffraction orders become equal, while the anti-Bragg beam intensity vanishes at approaching the Bragg diffraction condition. The Bragg difraction angle is given by Λ = / ) sin( 21 λ ϕ [START_REF] Eichler | Laser-Induced Dynamic Gratings[END_REF]. The angle 2ϕ is between the transmitted probe and the first order diffracted beams (in our case 2 / 3Θ = ϕ as λ 1 =3λ 3 ). In this work diffraction on intermediate gratings will be considered, thus the first order Bragg beam I BR is stronger than the symmetric anti-Bragg beam I ABR (see figure 2(a)). Due to the Snell's law, the incident beams inside the crystal propagate with a lower angle The Anti-Bragg beam electric vector can be obtained by integrating the electric vectors of diffracted beams on the depth elements [START_REF] Eichler | Laser-Induced Dynamic Gratings[END_REF]. The phase difference ( )

∆l ABR = OE -DE 2ϕ 1 ϕ 1 ϕ I BR I T I ABR ϕ 1 2ϕ 1 I I z O Λ Θ A B C D E ∆l BR = OC -BC = λ 1 /n 1
z ϕ ∆
of the beams, diffracted at the interface (z = 0) and at the depth z varies with z, and is expressed by their path difference ( )

z l ABR ∆ (see details in figure 2(b)): ( ) ( ) ( ) ( ) ( ) ( ) . 4 1 / 2 3 cos / 2 cos 1 , / 2 2 1 2 1 1 1 1 1 ϕ ϕ ϕ ϕ λ π ϕ - ≈ - = ∆ ∆ = ∆ z z z l l n z ABR ABR (6)
The maximum ϕ angles are deduced using the limit of internal reflection for the Bragg and anti-Bragg beams as 2ϕ = π/2 = arcsin(λ 1 /Λ), which leads to λ 1 = Λ. If using Λ = λ 1 = 1053 nm, we obtain ϕ 1 = π/4n 1 ≈ 0.3 rad, thus approximation by ( 6) leads to a negligible error of about 0.1 %. Therefore, combining the phase difference from [START_REF] Neimontas | Characterization of Electronic Properties of Different SiC Polytypes by All-optical Means[END_REF] and Bragg condition Λ ≈

1 1 1 2 / n λ ϕ , we get ( ) [ ] ( ) [ ] ( ) 2 1 2 1 1 2 1 2 1 1 4 1 / 2 ; 2 4 1 / ϕ πλ ϕ πλ ϕ - Λ = = - Λ = ∆ n q qz z n z . ( 7
)
Here q is the parameter of phase shift for the Anti-Bragg beam. The phase factor for complex electric vector becomes ( ) ( )

z i ϕ ∆ exp
, and the vector can be expressed as:

( ) ( ) z i z E E ϕ α ∆ - - = • 2 exp 0 .
(8) Complex electric vector after avaraging over the thickness and neglecting the probe beam intensity losses with depth due to diffraction is expressed as:

( ) ( ) ( ) - + - = • d d dz z dz z iq E E 0 0 0 2 exp 2 / 2 / exp α α . ( 9
)
This equation was integrated analytically. For Bragg beam, q = 0 (all electric vectors are phase matched as phase difference is multiple to wavelength, see figure 2(b)) and the electric vector is equal to E 0 . After calculations we got:

( ) ( ) ( ) ( ) ( ) ( ) 2 2 2 0 * 2 / exp 1 exp 2 / exp 2 / cos 2 1 1 1 d d d qd Q E E E I I BR ABR α α α - - - + - - + = = • • . ( 10
)
Here Q=q/α. At very low absorption coefficients, equation [START_REF] Klein | Recombination processes controlling the carrier lifetime in n -4H-SiC epilayers with low Z 1/2 concentrations[END_REF] leads to I ABR /I BR =16sin 2 (qd/4)/(q 2 d 2 ). A case of relatively weak interband absorption of the pump beam can be approximated by condition αd < 8, then oscillating function of cos(qd/2) can be eliminated by using a slightly slanted sample across the probe beam waist (slant of ∆d = 4π/q). For strong absorption (αd > 8), more simple formula from ( 10) is obtained, assuming that sample thickness d is infinite:

[ ] 2 2 2 1 2 1 1 2 0 * 1 1 4 1 2 1 1 Q n E E E I I BR ABR + = - Λ + = = • • ϕ α πλ . ( 11
)
The latter approximation gives an error ≤5% for α determination at αd > 8 condition. In our case the αd > 8 condition is satisfied.

The Q value, determined experimentally, enables one to determine the absorption coefficient by equation [START_REF] Sridhara | Penetration depths in the ultraviolet for 4H, 6H and 3C silicon carbide at seven common laser pumping wavelengths[END_REF], as the other parameters are given by experimental conditions. In figure 3 we present dependence of the calculated diffraction efficiency ratio I ABR /I BR versus absorption coefficent values for two grating periods. At very strong absorption, equation ( 11) is approximated by I ABR /I BR = 1 -q 2 /α 2 . The upper limit for determination of absorption coefficient is α max = 3q(Λ) and it corresonds to I ABR /I BR = 0.9. In our case, using Λ = 2.85 µm, λ 1 = 1053 nm, α max equals to 10 4 cm -1 , while for Λ = 1.3 µm period the α max value is 5×10 4 cm -1 . The lower limit for absorption coefficent is obtained from αd < 8 condition, at which the α min = 8/d ~200 cm -1 can be determined for 400 µm thick sample. The calculations have shown that the proposed method allows determination of high absorption coefficient using very thick wafers (αd > 8), when its value can not be determined by transmission measurements or ellipsometric techniques [START_REF] Zollner | Dielectric functions of bulk 4H and 6H SiC and spectroscopic ellipsometry studies of thin SiC films on Si[END_REF]. On the other hand, lower absorption coefficient (αd < 8) can be extracted either by the proposed method or by conventional UV-VIS spectrometry [START_REF] Zollner | Dielectric functions of bulk 4H and 6H SiC and spectroscopic ellipsometry studies of thin SiC films on Si[END_REF], as well by scanning the carrier density in-depth profile [START_REF] Grivickas | Fundamental band edge absorption in nominally undoped and doped 4H-SiC[END_REF]. The proposed method allows determination of the temperature-dependent absorption coefficient, fitted by the approximations, given in section 2.4.

Approximations of absorption coefficient and refaractive index temperature and quantum energy dependences

Absorption coefficient temperature and quantum energy dependence in SiC can be approximated using temperature dependent energy gap E g (T), absorption strength A, and phonon energy E ph [START_REF] Galeckas | Temperature dependence of absorption coefficient in 4H-and 6H-Silicon Carbide at 355 nm laser pumping wavelength[END_REF]:

( ) ( ) ( ) ( ) ( ) ( ) ( ) - + - + - - - - = 1 / exp / exp 1 2 2 kT E E T E h kT E E T E h A T ph ph g ph ph g ν ν α . ( 12
)
The bandgap temperature dependence can be desribed by Passler formula [START_REF] Grivickas | Fundamental band edge absorption in nominally undoped and doped 4H-SiC[END_REF]:

( ) ( ) ( ) ( ) - Θ + Θ - = 1 / 2 1 2 / 0 p p p p g P g T E T E ε . ( 13
)
Here ε = 3⋅10 -4 eV/K, Θ p = 450 K and p = 2.9 for both 4H-and 6H-SiC. The formula is valid in 77-450 K range. The same temperature dependence of bandgap can be used for all polytypes. The temperature dependence of SiC bandgap can be also precisely fitted by vibrational energy function E g D (T) [START_REF]Application of a time-resolved four-wave mixing technique for the determination of thermal properties of 4H-SiC crystals[END_REF]:

( ) ( ) ( ) ( ) ( ) dx x x T k T f T f a E T E T D B D D D D D g D g D 3 1 / 0 3 4 1 ) exp( / 18 / , / 0 - Θ - Θ = Θ Θ - = . ( 14
)
Here a D accounts for atomic oscillator interaction (if a D =1 there is no interaction), Θ D = 1000 ± 50 K is the Debye temperature and E g (0) is the bandgap at zero temperature, the same as in Passler formula [START_REF] Grivickas | Fundamental band edge absorption in nominally undoped and doped 4H-SiC[END_REF].

The spectral dependence of absorption coefficent is given by [START_REF] Grivickas | Fundamental band edge absorption in nominally undoped and doped 4H-SiC[END_REF]:

( ) [ ] [ ] 2 2 2 1 p g p g E E A E E A T + - + - - = ω ω ω ω α h h h h . ( 15
)
The given above formulas were used for approximations of bandgap and absorption coefficient temperature dependences. The E g (T) dependences were taken from literature and were approximated by E g P and E g D functions [START_REF] Eichler | Laser-Induced Dynamic Gratings[END_REF][START_REF] Fisher | Optical phase conjugation[END_REF]. The fitting curves are shown in figure 4. The Choyke [20] data seem to be deviating from Grivickas data [START_REF] Grivickas | Fundamental band edge absorption in nominally undoped and doped 4H-SiC[END_REF] in 4H-and 6H-SiC. In 3C the deviation is 60 meV, which is the phonon energy, as the bandgap was approximated disregarding phonon absorption. In 4H and 6H the gaps reduce much faster with temperature which may be the fact of high doping [START_REF] Galeckas | Temperature dependence of absorption coefficient in 4H-and 6H-Silicon Carbide at 355 nm laser pumping wavelength[END_REF]. Grivickas [START_REF] Grivickas | Fundamental band edge absorption in nominally undoped and doped 4H-SiC[END_REF] [20], approximated with E g D ( 14) function. The fitting parameters are shown on the plot. The fitting by E g P ( 13) function is shown for comparison, using E g (0 K) values, shifted accordingly to the determined E g (300 K) values (table 1) for relevant polytypes. Note very similar E g P and E g D dependencies in 3C polytype, which can be a result of lower 3C doping.

Sample absorption coefficient in the vicinity of E g (figure 5) was calculated using relationships from [START_REF] Pankove | Optical Processes in Semiconductors[END_REF] with photon energy dependent refractive index n and reflection coefficent R. Ordinary refractive index was used for hexagonal polytypes, as the absorption edge measurements were performed in the E⊥c geometry (the c axis is almost perpendicular to the surface of the wafers). In wide spectral range it can be described as:

( ) ( ) ( ) 2 2 1 49 . 5 1 Γ - + = E eV E n
according to [START_REF] Zollner | Dielectric functions of bulk 4H and 6H SiC and spectroscopic ellipsometry studies of thin SiC films on Si[END_REF], here E Γ = 7.3 eV is the energy of the effective direct gap. Refractive index of 3C has very similar value and spectral dependence [22]. Therefore ordinary refractive index is essentially the same for all polytypes [22] and weakly dependends on temperature: dn/dT = 3.6×10 -5 K -1 at RT [START_REF]Application of a time-resolved four-wave mixing technique for the determination of thermal properties of 4H-SiC crystals[END_REF].

Experimental results and discussion

UV-VIS absorption edges in SiC at RT

The absorption edges for the polytypes were measured by UV-VIS spectrometer in the E⊥c geometry. The data are shown in figure 5. The largest α values determined from these measurements are of about 300 cm -1 . It is clear that it is not possible to measure much higher α values at 351 nm in 6H-or 3C-SiC due to large sample thickness, neither determine their temperature dependences.

The spectral measurements provided values of bandgaps E g , phonon energies E ph and absorption strengths A, which were used for approximation of α(T) dependence (see Appendix I) . The obtained absorption edge fitting parameters are given in Table 1.

2.25 2.50 2.75 3.00 3.25 3.50 0 10 20 30 [START_REF] Jarasiunas | Implementation of diffractive optical element in four-wave mixing scheme for ex situ characterisation of hydride vapor phase epitaxy-grown GaN layers[END_REF][START_REF] Fisher | Optical phase conjugation[END_REF] and reference [START_REF] Sridhara | Penetration depths in the ultraviolet for 4H, 6H and 3C silicon carbide at seven common laser pumping wavelengths[END_REF] respectively. Subscrptus nukelt

A 2 E g =3.2 eV E g =2.94 eV 4H-SiC 6H-SiC (αhν) 1/2 , eV 1/2 cm -1/2 E, eV 3C-SiC 293 K E g =2.3 eV A 1
The phonon assisted optical transitions involve the lowest conduction bands of the polytypes in valleys 3C-: X, 4H-: M, 6H-: M-L [22]. Average phonon energy is in 65-70 meV range for 4H- [START_REF] Grivickas | Fundamental band edge absorption in nominally undoped and doped 4H-SiC[END_REF][START_REF] Galeckas | Temperature dependence of absorption coefficient in 4H-and 6H-Silicon Carbide at 355 nm laser pumping wavelength[END_REF] and 63 meV for 6H-SiC [START_REF] Huang | Room-Temperature Phonon Replica in Band-to-Band Transition of 6H-SiC Analyzed Using Transmission Spectrums[END_REF]. These values coincide quite well with our approximations (in table 1). It was suggested [START_REF] Galeckas | Temperature dependence of absorption coefficient in 4H-and 6H-Silicon Carbide at 355 nm laser pumping wavelength[END_REF] that LA phonons of 76-79 meV are determining the absorption. However, it was concluded in [START_REF] Grivickas | Fundamental band edge absorption in nominally undoped and doped 4H-SiC[END_REF] that both the acoustic and optical phonons participate in transitions, so the fitted (table 1) phonon energies stand for the average of those participating in the absorption. Our measurements provided phonon energies lower than LA, thus indicating a need to account contribution of low energy TA phonons (besides LA and optical ones). Assuming that only acoustic phonons participate in absorption in equal strength, the average phonon energy is obtained as E av =(E TA1 +E TA2 +E LA )/3 and leads to the E av values of 57 meV, 61 meV and 59 meV for 3C-, 4H-and 6H-SiC, respectively, using data from [22]. The shape of α(hν) dependence in different spectral ranges can be described by phonon-emission related slope A 1 (see table 1 and Appendix I), which has different value in the vicinity of the bandgap (A 1 abs , see figure 5) and highly above E g (A 1 ref , which stands for excitation wavelength λ 3 with hν = 3.53 eV) due to multiphonon absorption, excitonic effects [START_REF] Grivickas | Fundamental band edge absorption in nominally undoped and doped 4H-SiC[END_REF] and band nonparabolicities [START_REF] Persson | Relativistic band structure calculation of cubic and hexagonal SiC polytypes[END_REF]. This feature was strongly pronounced in 6H (table 1) and could be explained by strong conduction band nonparabolicity in 6H [START_REF] Persson | Relativistic band structure calculation of cubic and hexagonal SiC polytypes[END_REF]. The determined A 1 /A 2 ratio in 4H is few times larger than that calculated according to estimated phonon energy (table 1). This is probably caused by two different conduction bands with close energies (separated by ~100 meV) [START_REF] Sridhara | Penetration depths in the ultraviolet for 4H, 6H and 3C silicon carbide at seven common laser pumping wavelengths[END_REF][START_REF] Persson | Relativistic band structure calculation of cubic and hexagonal SiC polytypes[END_REF]. This may lead to A 1 value two times larger at higher energies than at lower ones. Close bandgaps (including few conduction and valence bands) are also present in 6H-(100-200 meV), while in 3C-SiC there are close in energy valence bands (separated by 10 meV << E ph ), which leads to one effective bandgap. Thus in 3C polytype the fitting of experimental data was found the best, showing only excitonic absorption impact near the bandgap. Under these assumptions, the approximation of absorption coefficient by equations [START_REF] Jarasiunas | Implementation of diffractive optical element in four-wave mixing scheme for ex situ characterisation of hydride vapor phase epitaxy-grown GaN layers[END_REF][START_REF] Zollner | Dielectric functions of bulk 4H and 6H SiC and spectroscopic ellipsometry studies of thin SiC films on Si[END_REF] reasonably well described the absorption edges for all polytypes.

Temperature dependences of interband absorption coefficients at 351 nm

The dependencies of Bragg diffraction efficiencies on Λ = 2.85 µm grating were measured at various excitation fluences, as shown in figure 6. The quadratic increase of diffraction efficiency with excitation was used for extraction of n eh values for the SiC polytypes, which are given on the plot (n eh error is about 0.2×10 -22 cm 3 ). The experimental values were found very similar to calculated ones by equation (3) (n eh = (8.4; 8.8; 7.3)×10 -22 cm 3 for 3C, 4H and 6H respectively, using reduced optical masses m eh = 0.24; 0.23; 0.27 [START_REF] Persson | Relativistic band structure calculation of cubic and hexagonal SiC polytypes[END_REF]). The measured ratios of I ABR /I BR diffraction efficiencies for different polytypes were used for extraction of absorption coefficient values and their temperature dependences (figure 7). The used excitation energy density for α determination were in the 0.1-2 mJ/cm 2 range (it corresponds to ∆N = 5×10 16 -7×10 18 cm -3 at 300 K), thus decrease of the probe beam intensity due to its diffraction was negligible (below 1%, see figure 6), and α values were calculated according to equation [START_REF] Sridhara | Penetration depths in the ultraviolet for 4H, 6H and 3C silicon carbide at seven common laser pumping wavelengths[END_REF]. Due to lower absorption coefficient in 4H, the α increase with temperature was found the largest, spanning over one order of magnitude. We note that the excitation induced sample heating, ∆T (due to injected carrier thermalisation) in the used injection range was negligible (it peaked up to ∆T < 10 K at 80 K and < 1 K at 300 K in 3C-SiC and was much smaller for other polytypes).

The fitting of α(T) dependences was performed under two different simulation models, based on E g (T) dependencies (see Appendix I and table 2). Using Passler approximation (solid lines in figure 7(b)), we used phonon energies and slope A values as given in table 2. This is consistent with the average acoustic phonon energies and slope A values, obtained from the fitting of absorption edges. Using Debye approximation, higher phonon energies (~70-80 meV) and slope A values (table 2) were used (dashed lines) with respect to those determined from the absorption edges. Thus the phonon energy of 60 meV for all studied SiC polytypes gives a reasonably good fit of the absorption edges and temperature dependences of interband absorption coefficients. 2. Fitting by Passler and Debye approximations for E g (T) are denoted by solid and dashed curves, respectively. Comparison with data at 351 nm [START_REF] Sridhara | Penetration depths in the ultraviolet for 4H, 6H and 3C silicon carbide at seven common laser pumping wavelengths[END_REF] and at 355 nm [START_REF] Galeckas | Temperature dependence of absorption coefficient in 4H-and 6H-Silicon Carbide at 355 nm laser pumping wavelength[END_REF] are given.

Table 2. SiC absorption coefficient approximation parameters at 3.53 eV.

Conclusions

A new method, based on the light diffraction on transient free carrier grating, was used for the determination of interband absorption coefficient at wavelenghts well above the bandgap. The absorption coefficient values in the range from 200 to 5000 cm -1 were measured for three bulk SiC polytypes in the 80-800 K range. The increase of indirect absorption coefficient by order of magnitude in 4H and by 3-5 times in 3C and 6H was approximated using ~60 meV phonon energy and the temperature dependent decrease of bandgaps. The temperature dependent interband absorption coefficients will ensure more correct analysis of the temperature and density dependent carrier recombination rates and diffusivity.

Accnowledgements. The research was sponsored by EUREKA project No. E!4473.

SiC polytype E g (0), eV

A ref , cm -1 /eV 2
A abs , cm -1 /eV 2 Passler E g (T)

Debye E g (T)

Α, cm -1 /eV 2 E ph ,meV Α, cm -1 /eV

Figure 1 .

 1 Figure 1. Light induced transient grating setup.

Figure 2 .

 2 Figure 2. Experimental scheme (a) and formation of diffracted Bragg and anti-Bragg beams (b). The sample is positioned normally to the probe beam in order to get equal reflection coefficients for both diffracted beams.

  2(b)).

1 Figure 3 .

 13 Figure 3. Dependence of diffraction efficiency ratio I ABR /I BR on absorption coefficient for two grating periods: calculations according equation (10) -dotted lines, and with averaged oscillating functionsolid lines. The dashed lines designate limits for determination of the absorption coefficient.

Figure 5 .

 5 Figure 5. Absorption edges (E⊥c) of the three SiC polytypes at 293 K, determined by conventional UV-VIS spectrometer. Solid lines -experimental data, dashed lines -linear fits (15). The A 1 and A 2 are the phonon-emission and phonon-absorption governed slopes of the absorption spectra. The arrows indicate determined bandgaps at 293 K.

3 Figure 6 .

 36 Figure 6. Dependence of Bragg diffraction efficiency on excitation fluence. γ are the slopes of the curves in the log-log plot. The γ~2.0 values verify theoretical approximation by formula (4).

Figure 7 .

 7 Figure 7. Temperature dependences of I ABR /I BR diffraction efficiency ratio (a) and the determined absorption coefficients (E⊥c) (b). The fitting parameters are given in Table2. Fitting by Passler and Debye approximations for E g (T) are denoted by solid and dashed curves, respectively. Comparison with data at 351 nm[START_REF] Sridhara | Penetration depths in the ultraviolet for 4H, 6H and 3C silicon carbide at seven common laser pumping wavelengths[END_REF] and at 355 nm[START_REF] Galeckas | Temperature dependence of absorption coefficient in 4H-and 6H-Silicon Carbide at 355 nm laser pumping wavelength[END_REF] are given.

  data, approximated with E g P , may give more consistent results as it was obtained in low doped sample.

		3.2
		3	4H: E g0 D =3.26, f=0.96	Θ=1000 K
		2.8	6H: E g0 D =3.02, f=0.85
	E	2.4 2.6	4H: 0.024 3C: 0.06 6H: 0.035 E g0 D -E g0 P =		This work E g D fit Choyke [20] E g P fit
		2.2	3C: E g0 D =2.335, f=0.75 3C: E g0 D =2.387, f=0.8
			0	300	600
				T, K

g , eV

Figure 4. The bandgap temperature dependences of SiC polytypes

Table 1 .

 1 Absorption coefficient approximation parameters at 293 K.

	Polytype	E g abs , eV	A 1 abs , cm -1 /eV	A 2 abs , cm -1 /eV	E ph abs , meV	A(3.53 eV) abs , cm -1 /eV 2	A 1 /A 2 abs	A 1 /A 2 calc	A 1 (3.53 eV) ref , cm -1 /eV
	3C-SiC	2.30	4275	330	63	1100	13.0	10	4800
	4H-SiC	3.20	12056	441	63	3100	27.3	11	12500
	6H-SiC	2.94	16640	1043	60	4300	16.0	11	10700
	"abs", "calc", "ref" subscripts indicate values deduced from absorption edges, calculations according

  2 E ph , meV

	3C-SiC	2.335	1200	1100	1250	63	1250	70
	4H-SiC	3.232	3200	3100	3400	59	4300	82
	6H-SiC	2.973	2700	4300	3100	63	3400	78


Maura Cesaria

Anna Paola Caricato

Gilberto Leggieri

Armando Luches

Maurizio Martino

Giuseppe Maruccio

Massimo Catalano

Maria Grazia Manera

Roberto Rella

Antonietta Taurino
email: antonietta.taurino@le.imm.cnr.it

Structural characterization of ultrathin Cr-doped ITO layers deposited by double target pulsed laser ablation

Keywords: 68.37.Lp, 81.15.Fg, 68.55.Ln, 68.60.Bs

In this paper we report on the growth and structural characterization of very thin (20nm) Crdoped ITO films, deposited at room temperature by double target pulsed laser ablation on amorphous silica substrates. The role of Cr atoms into the ITO matrix is carefully investigated at increasing doping content by transmission electron microscopy (TEM). Selected area electron diffraction, conventional bright field and dark field as well as high resolution TEM analyses, and energy dispersive x-ray spectroscopy demonstrate that (i) crystallization features occur even despite the low growth temperature and thin thickness, (ii) no chromium or chromium oxide secondary phases are detectable, regardless of the film doping levels, (iii) the films crystallize as crystalline flakes forming large angle grain boundaries; (iv) the observed flakes consist of crystalline planes with local bending of the crystal lattice. Thickness and compositional information about the films are obtained by Rutherford back-scattering spectrometry. Results are discussed by invoking the combined effects of growth temperature, smaller ionic radius of the Cr cation compared to the trivalent In ion, doping level, film thickness, double-target doping technique and peculiarities of the pulsed laser deposition method.

Introduction

Degenerate wide band gap oxide semiconductors recently attracted large interest as promising host compounds for diluted magnetic semiconductors (DMSs) [1,2] with high Curie temperature for room temperature spintronic applications, ranging from logic circuits to spin-LED devices allowing polarized emission [START_REF]Semiconductor Spintronics and Quantum Computation[END_REF][START_REF] Dietl | [END_REF][5][6][7][8][9][10]. In this context, indium tin oxide (briefly ITO) [11] is a very interesting material due to its good opto-electronic properties, good adaptability to be interfaced with known semiconductor/electronic materials, and indications suggesting RT carrier-mediated ferromagnetism in Cr-doped indium oxide films [12].

In the general framework of DMSs, a theoretical comprehensive model able to explain the basic mechanisms leading to ferromagnetism is unavailable and different results are reported even for the same composition and depending on the growth technique. A key point is the relationship between magnetic order and dopant uniformity and/or clustering above the solubility limit of the magnetic dopant in the host matrix [1,13]. In this respect, the drawback of low solubility of a magnetic element can be overcome by non-equilibrium growth techniques, such as Pulsed Laser Deposition (PLD) [1,[14][15][START_REF] Eason | Pulsed Laser Deposition of Thin Films[END_REF]. In addition, thin films grown by PLD exhibit higher performances and better structural characteristics compared to other equilibrium deposition techniques [START_REF] Zheng | [END_REF][18][19].

Generally, avoiding the formation of dopant oxides and dopant oxide segregation at the grain boundaries (GBs) within the growing film necessitates of carefully tuned deposition conditions. Therefore, the growth of Cr-doped ITO films is not a straightforward issue and a controlled growth method is required in order to obtain films with tunable magneto-opto-electronic properties. Moreover, since growth techniques and parameters strongly affect film properties through the involved structural features, the investigation of the resulting film microstructure is of paramount importance to tune growth evolution and to understand the correlation between film microstructure and its macroscopic opto-electronic properties. Therefore, a direct control of the morphological features of the films is required.

According to this, the present paper focuses on the investigation of the structural properties of very thin Cr-doped ITO films grown by the double target PLD approach. The characterization of ITO films is not considered in this paper because already widely investigated in the literature [20][21][22][23][24][25][26][27][28][29]. Samples as thin as 20 nm were grown on amorphous silica substrates at room temperature. Two different doping levels were considered and compared. A very low film thickness was chosen to investigate the effect of Cr insertion into ITO, devoting particular attention to growth kinetics, occurrence of different phases and structural evolution induced by the dopant content. In fact, PLD-induced growth hyperthermal-conditions and high density of nucleation sites, doping-promoted heterogeneous nucleation and changes of the film microstructure and morphology related to Cr-induced strain effects are expected to strongly influence the initial growth and the structural characteristics of the films. Indeed, the first few tens of nanometers act as seed coating, which critically determines the features of the growing and final film. In this respect, substrate nature and surface are important parameters in determining the initial film nucleation stage and the crystallization. The subsequent growth favors the surface coverage and transformation of the amorphous phase to the crystalline one, coalescence of the islands, increasing grain size and crystallinity as well as strain relaxing beyond the critical thickness.

The doping process by PLD can be accomplished by using several approaches such as ablation of a single target obtained by premixing of the materials to be deposited [30,31], ion implantation [32], dual beam method [33][34][35][36][37], dual target simultaneous PLD apparatus [38] and switching ablation among targets with different composition. It must be emphasized that the microstructure of the pellet material strongly affect the final film morphology and structural quality through the formation, distribution and density of particulates in the deposit [39][40][41][42]. In this respect, ablation by a single premixed multielemental pellet has significant limitations and drawbacks in target density, fixed concentration of dopant, inability to change in situ the doping conditions during the deposition process, differential scattering in the plume itself among species with very different masses that can induce incorrect film stoichiometry and non uniform dopant distribution as well as cluster or secondary phases of the dopant oxide in the oxygen background atmosphere.

The approach based on periodic switching of the laser beam between sintered ITO and metallic Cr targets was our choice due to the following reasons: i) a priori minimization of the formation of clusters and dopant oxide phases thanks to the high purity of the metallic dopant target compared to a single premixed multi-elemental sintered pellet, ii) independent ablation of the targets that avoids any interaction among the ablated species during the plasma plume expansion and possibly enables to ablate the dopant metallic target in oxygen-free background atmosphere, and iii) low costs and possibility to finely control in situ the dopant localization and amount by changing the ratio between the number of laser pulses on each target.

Experimental details

Cr-doped ITO (20 nm) films were deposited by PLD using a Lambda Physics 305i ArF excimer laser ( = 193nm , = 20ns) on amorphous silica substrates held at room temperature. No post-annealing treatment was carried out. Whereas the amorphous nature of the chosen substrate can delays the onset of the crystalline phase, it avoids the strain effects and driven growth commonly induced by mismatched and oriented substrates.

Figure 1 sketches the PLD experimental set-up composed of i) a stainless steel vacuum chamber, ii) a UV laser source with associated beam delivery optics, iii) a combined rotative-turbo pump system, iv) a mechanism to inject gas within the chamber during reactive PLD, v) a target holder equipped with a computer controlled multi-target carousel mechanism which enables the in situ interchange of the two targets and the rotation of each target around its own axis. The laser beam was incident at 45°w ith respect to the target surface. Before starting the film deposition, the chamber was evacuated down to a background pressure as low as ~10 -5 Pa. Then a dynamic flow of molecular oxygen was injected in the deposition chamber, resulting in a background pressure of 1 Pa for ITO deposition. The Cr ablation was performed in a background pressure of 10 -3 Pa. In order to clean the target surface before the deposition, a shutter was placed between the substrate and the target to avoid contamination of the substrate and a few thousands of laser pulses were focused on the target at 10Hz. In order to avoid any contamination problem both targets were encapsulated with a stainless steel cylindrical cup.

In such a way only one target at once was exposed to the laser beam and to the backscattered elements.

Commercially available sintered ITO, with weight composition of 90% In 2 O 3 and 10% SnO 2 , and metallic Cr pellets were used as ablation targets. During the laser-target interaction, the pellets were continuously rotated to avoid the drilling and damaging of the material to be ablated at the same target position. All depositions were carried out at a laser fluence of 4 J/cm 2 and at a target-substrate distance of 55 mm. The repetition rate of the laser pulses was set to 1 Hz for both ITO and metallic Cr. The Cr insertion within the ITO matrix was accomplished by periodically switching the ablating laser between the ITO and Cr target. Several switching cycles were needed to obtain the desired thickness of the doped films. Two dopant concentrations were obtained by changing the total ablation time on each target, which will be referred later on as low-doping (LD) and high-doping (HD), respectively. The ratio between the number of pulses on the Cr target and the number of pulses on the ITO target was 0.1 for the LD and 0.4 for the HD, respectively.

Concerning the used doping technique, the following remarks can be inferred.

The independent ablation of the two targets and the choice of the number of laser pulses allows to achieve finely tuned doping levels. Then, the very different and tunable pulse numbers on the two targets avoid the formation of dopant or dopant oxide multilayers.

The PLD technique favors the dopant migration and dilution thanks to the pulsed nature of the PLD deposition-flux and the hyper-thermal energetic distribution of the laser-produced plasma, which is tunable either by the laser fluence or by the background pressure and composition in the deposition chamber [15][START_REF] Eason | Pulsed Laser Deposition of Thin Films[END_REF][START_REF] Zheng | [END_REF][18], at fixed laser fluence. Moreover, the independent ablation of the targets avoids any gas-phase interaction between the different species composing the ITO and the Cr targets and allows ablation of the dopant metallic target in oxygen-free background atmosphere. This feature minimizes the formation of clusters and dopant oxide phases during the plasma plume expansion.

Finally, since target interchange, positioning and rotation are adjustable from outside the deposition chamber by a computer-controlled stepper motor, the internal conditions of the deposition chamber, namely clean atmosphere and background pressure, are fully preserved in each step and doping content is finely tunable in situ.

Based on the above discussion, the choice of the described deposition approach is expected to be the best-suited to dope ITO films with Cr even for room temperature growth.

The films were deposited on <100> Si substrates and on carbon coated Cu grids for Rutherford back scattering (RBS) and transmission electron microscopy (TEM) investigations, respectively. Film morphology, crystal structure, and chemical composition of the films deposited on Cu grids were examined by plan-view TEM analyses; in particular, a Leo 922 Transmission Electron Microscope, operating at an accelerating voltage of 200 kV was used for bright field/dark field as well as for selected area electron diffraction (SAED) investigations. High resolution analyses and chemical investigations by energy dispersive X-ray spectroscopy (EDS) were performed by using a Tecnai G2 instrument, operating at 300 kV.

The nominal film thickness was first controlled by means of the total numbers of laser pulses and the target-substrate distance. Rutherford backscattering spectrometry (RBS) with 2.2 MeV He + ions at 160 o scattering angle was used to infer the film composition and thickness. The RBS spectra were analyzed by the Rutherford backscattering manipulation program (RUMP) [43].

Results and discussion

In order to test the influence of the deposition parameters on the physical properties of each film, microstructural investigations are of paramount importance, also to obtain a deeper understanding of the film macroscopic properties. Electrical and optical analyses can give information about the film crystalline properties through the steepness of the transmittance spectrum near the fundamental absorption edge, transparency, reflectance, and tailing behavior of the absorption coefficient as well as changes in carrier concentration and mobility [44]. However, the investigation of the crystallographic features, such as texture, crystallite size and shape, lattice parameter, strain effects is the decisive test to correctly interpret important physical properties such as resistivity and optical data. A direct control of the film morphological and structural features is also a basic requirement in order to fully understand the magnetic properties since they depend on dopant uniformity or clustering in the resulting coating as well as the occurrence of localized secondary magnetic phases and dopant segregation at interfaces or GBs. In this respect, high resolution transmission electron microscopy (HRTEM) is a very powerful analysis tool, due to its high sensitivity to detect secondary phase agglomerates, even including few atomic units, at subnanometric resolution.

The RBS spectrum of the LD sample deposited on (100) Si substrate is shown in figure 2. From the RBS analysis it came out that i) the Cr content was sufficient to induce a detectable Cr signal, corresponding to a dopant concentration of 3% and 10%, for the LD and HD, respectively; ii) the thickness of the deposited film was 20 nm within 6%, and iii) the Cr signal indicated an almost uniform depth distribution of the dopant inside the film, for both the doping levels.

The crystalline features and the microstructural details of the deposited samples can be observed in figure 3. Figures 3(a formed by large and contiguous crystallites with different crystalline orientation, forming GBs at their intersection; ii) the crystallites have irregular shape and average size of few hundreds of nm; iii) each crystallite exhibits a typical bending-contour contrast, consisting in dark lines, sometimes intersecting and forming a star-like contrast, as shown in the high magnification images reported as inset of figures 3(a) and 3(b); here the details of high symmetry bending contour figures and of the grain boundaries are evidenced; iv) the contrast of these images demonstrates the absence in the films of secondary phases, such as inclusions of metallic chromium or chromium oxides. Concerning the bending-contour contrast, this kind of contrast is generated by a local bending of atomic planes with respect to the direction of the incident electron beam. Due to the crystal bending, the excitation error, i.e. the deviation from the Bragg condition of a given diffracted beam, changes across the crystallite, causing the contrast visible in the BF images. The local bending of the crystal lattice is related to enhanced internal strain and thus to a modification of the elasticity. Comparison between the BF images of the LD and HD samples (figures 3(a) and 3(b)) indicates a preferential alignment and torsion of the GBs as well as lengthening of the polycrystalline foils when dealing with the higher Cr content. As far as the diffraction analysis is concerned, the LD and HD samples exhibited the same features, which are represented by the SAED pattern reported in figure 3(c). The pattern shows spotty rings confirming the polycrystalline nature of the films. The interplanar distances obtained from such diffraction pattern, reported in table I, are in agreement with the corresponding values associated to the cubic bixbyite structure of In 2 O 3 (lattice constant of 1.0118 nm) and no trace of secondary phases can be detected. Therefore, both BF images and SAED patterns indicate that, regardless of the doping level, Cr atoms don't aggregate in the film to form secondary phases, but are included into the ITO matrix, possibly as substitutional atoms. Thus, the internal strain, resulting in the observed bending of the host crystal lattice, is most likely the result of the incorporation of the Cr atoms within the ITO lattice. In fact, i) the low growth temperature as well as the absence of post-deposition thermal annealing rule out strain effects due to film-substrate thermal expansion mismatch and ii) the amorphous substrate avoids the strain components usually induced by film-substrate structural difference and lattice constant mismatch. Since neither thermal strain nor substrate-induced strain component can explain the observed lattice bending, the dopant-induced growth stress is the only remaining strain component.

A careful analysis was devoted to the data derived from both the high resolution images and diffraction patterns in order to point out any possible change of the lattice parameter, since the substitution of the In 3+ (0.94 Å) cations by the smaller Cr 3+ ions (0.76 Å) [45] in the lattice sites is expected to decrease the final distances between Cr and their oxygen neighbors after relaxation. In this respect, it must be noticed that lattice contraction would result from relaxing of the dopant-induced strain and strain relaxing is a thickness-dependent effect. Namely, whenever strain effects are induced, a film becomes less and less stable during its growth evolution and the stored elastic density energy is fully released beyond a critical thickness. Since the deposited films are very thin (20 nm), no critical strain gradients and/or partial strain relaxation should occur. Therefore, no significant change of the ITO lattice constant should take place by Cr doping at the present growth stage. Indeed, reduction of the lattice constant could not be evidenced by the high resolution image and diffraction pattern analyses, within the resolution limit of the technique. Further investigations are in progress to ascertain strain-relaxation induced change of the lattice constant for thicker films, while preserving the considered doping levels.

Figures 4(a) and 4(b) report of the high resolution (HRTEM) images obtained from a GB region for the LD and HD samples, respectively. They show completely misoriented grains, as confirmed by the zone axis attributed to each grain, compatible with high angle GBs. Moreover, regardless of the doping level, the grains are in close contact with each other and a certain amount of overlap between contiguous grains can be observed (figure 4(b)). No secondary or amorphous phase is observed at the GB as well as in the crystallites forming the boundaries, within the detectability limits of the technique. It is worth noting that in the case of the HD sample a single GB between two slightly overlapping grains is visible confirming the preferential alignment and torsion of the GBs at high doping level.

As a further investigation, the high resolution image in figure 5 At last, chemical analyses were carried out by EDS technique. In this respect, the electron beam was focused on a suitably selected point of the sample and the X-ray signal was detected, collected and processed. The X-ray photon energy of the peaks detected in the spectra was correlated with the atomic species within the films. Figure 6 reports the EDS spectra obtained from the LD and HD samples; both spectra cover the energy range of interest, namely where the chromium related peaks are expected. It is worth noting that the Cr-Kα peak, at 5.41 keV, is scarcely visible in the spectrum of LD sample whereas the same peak results well evident in the spectrum of the HD sample. Its average intensity, obtained from a series of spectra recorded from different points of the sample, is (7.8±1.3)% the intensity of the main In-Lα peak, at 3.28 keV. Further EDS investigations were performed in order to show eventual chromium segregation at the GBs, but, in agreement with the BF and HRTEM analyses, no concentration change was detected, within the EDS detection capability.

From the obtained experimental results the following remarks can be inferred: i) taking into account both low-growth temperature and film thickness (only 20 nm), the degree of crystallinity and the complete substrate coverage observed in the films would not be expected. Basically, the transformation from amorphous state to crystalline phase requires the nucleation of critical-size clusters and their subsequent growth as well as improved lattice order. The PLD deposition peculiarities combined with growth parameters optimized as reported in ref. [25,26,28] were surely decisive in determining the onset of crystallization by ad-atom coalescence and attachment at the amorphous/crystalline interfaces. Therefore, the observed crystallinity can be attributed to the hyperthermal energy-distribution of the PLD-induced deposition flux and to the pulsed nature of the deposition. While the former feature improves the ad-atom superficial mobility, favoring collision events and the probability for more ad-atoms to aggregate even at low substrate temperatures, the latter effect allows that the deposited species spreading over the growing surface before the arrival of the next depositing pulse. The absence of Cr oxides, dopant clusters and segregation at the GBs can be related to the PLD-induced growth kinetics combined with the discussed advantages of the doubletarget ablation and choice of a metallic target for the dopant. Indeed, the very different doping levels, used for the LD and HD samples, rule out the possibility that just a too low dopant content could have favored dopant dilution. As a further confirmation, the internal strain, resulting in the bending of the host lattice, was detected even for the LD sample and it was observed to increase significantly turning in the HD sample. In this respect, polycrystallinity, size and shape distribution of the crystalline flakes composing the films and their evolution as a function of the doping level suggest combined effect between PLD-enhanced nucleation density and dopant-induced strain gradients. In fact, since the supersaturation of the PLD deposition flux increases the density of nucleation sites, the growth of small contiguous island and their coalescence is favored with respect to the growth of separated and large islands. Furthermore, the smaller Cr ions compared to the In cations introduce lattice-potential fluctuations capable of capturing ad-atoms. That is the increase of the doping level enhances dopant localization effects and dopant effectiveness as nucleation centers. On the other hand, the stored strain density energy drives the superficial diffusion of the ad-atoms depending on the distribution of the directions and regions of low/high strain. Therefore, the observed preferential alignment and torsion of the GBs as well as the lengthening of the polycrystalline flakes with the increase of the doping level, suggest that the growth is influenced by the Cr-induced strain gradients, namely the growth strain component introduced by defect/dopant. Indeed, both low growth temperature and very thin thickness of our films don't let the material to respond to the stored stress via reduction of the stored strain energy density.

Finally, as a further check of the Cr-induced modification of the host ITO matrix, the sample morphology was investigated by atomic force microscopy (AFM) in contact mode. The average roughness was found to increase from nearly 0.1nm to nearly 0.4-0.5nm.

Summary

In the present paper the structure and compositional features of Cr-doped ITO films grown at room temperature on amorphous silica substrates have been reported and discussed in relation to doping level, film thickness, substrate choice and features of the deposition method. The films crystallize as thin polycrystalline flakes, hundreds of nm wide, forming large angle GBs and exhibit local bending of the crystal lattice caused by Cr-induced internal strain. The lattice structure of the films coincides with the cubic bixbyite structure of ITO, no dopant segregation phenomena and no evidence of secondary phase related to metallic Cr or Cr oxides are found, suggesting the successful incorporation of Cr atoms in the host ITO lattice. The degree of crystallinity observed in the films, despite of the low growth temperature (RT) and their very low thickness (20 nm), is explained by the peculiar growth kinetics allowed and involved by the PLD technique combined with the used efficient doping method, namely the double-target ablation using a metallic pellet as dopant source.

  figure 3. Figures 3(a) and 3(b) show the bright field (BF) images of the LD and HD samples,

  (a) was obtained from the centre of a high symmetry bending-contour figure (see the detail evidenced by an arrow in figure 3(b) of the HD sample). It can be seen that 211 and 002 lattice fringes dominate the contrast of the image. Fast Fourier transform (FFT) was calculated on two different regions of the image (marked with dashed rectangles 1 and 2) obtaining the patterns reported as inset in figures 5(b) (region 1) and 5(c) (region 2). The spots correspond to the main periodicities present in the phase contrast image, and to their vectorial composition; the zone axis is the [120]. By comparing these patterns, it can be noted that the intensities of the spots change, the -211 and 2-1-1 being the most intense of pattern b, and the 004 and 00-4 prevailing in patterns c and d. In the case of the experimental image of figure 5(a), the only parameter which can affect the intensity of the spots is the orientation of the lattice planes with respect to the zone axis direction [120]. Therefore, the intensity change confirms the elastic deformation of the sample which produces a bending of the lattice planes, locally varying the diffraction conditions. The filtered inverse FFTs, reported in figures 5(b) and 5(c), enhance the different phase contrast generated by the crystal bending in the two different areas of the image.

Figure 1 :

 1 Figure 1: PLD experimental set-up using a double-target carousel

Figure 2 :

 2 Figure 2: RBS spectrum of the LD sample

Figure 3 :

 3 Figure 3: BF TEM images of the LD (a) and HD (b) samples and their typical SAED pattern (c). The

Figure 4 :Figure 5 :

 45 Figure 4: High-resolution TEM images of a typical grain boundary region of the LD (a) and HD (b)

Figure 6 :

 6 Figure 6: EDS spectra obtained from LD (a) and HD (b) samples

Figure 1 :Figure 2 .Figure 3 :

 123 Figure 1: PLD experimental set-up using a double-target carousel

Figure 4 :

 4 Figure 4: High-resolution TEM images of a typical grain boundary region of the LD (a) and HD (b) films

Figure 5 :

 5 Figure 5: High-resolution TEM image obtained from a high symmetry bending contour figure of the HD sample (a). FFTs (inset) and filtered inverse FFTs (b, c) obtained from the regions in the dashed rectangles 1 and 2, respectively

Figure 6 :

 6 Figure 6: EDS spectra obtained from LD (a) and HD (b) samples

Table Captions Table 1 :

 Captions1 Interplanar distances estimated from the diffraction pattern in figure 3(c) and the corresponding values of the cubic bixbyte structure of In 2 O 3 .

Table I

 I

	Experimental data	Theoretical data
	N	D exp (Å)	D theo (Å)	hkl
			In 2 O 3 cubic*
	1	7.17±0.03	7.155	110
	2	5.07±0.02	5.059	200
	3	4.13±0.01	4.131	211
	4	2.90±0.01	2.921	222
	5	2.54±0.01	2.530	400
	6	2.39±0.01	2.380	411
	7	2.16±0.01	2.157	332
	8	1.97±0.01	1.984	431
	9	1.79±0.01	1.788	440
	10 1.62±0.01	1.599	620
	11 1.51±0.01	1.491	631
	* ICDD PDF card No. 06-0416

Acknowledgements

The authors acknowledge the useful assistance and support of Dr. E. Piscopiello and Mr. M. Palmisano during the TEM analyses at the Tecnai G2 laboratory at the ENEA UTS Center in Brindisi.