domain: phys spi

text:
T Chommaux

P O Renault

D Thiaudière

P Godard

F Paumier

T Girardeau

S Hurand

Ph Goudeau

In situ electrical and mechanical study of Indium Tin Oxide films deposited on polyimide substrate by Xe ion beam sputtering

Keywords: In situ deformation, Synchrotron x-ray diffraction, Electrical properties, Mechanical properties, Indium tin oxide, Thin films, Elastic anisotropy

Four series of Indium Tin Oxide (ITO) thin films 600 nm thick were deposited on polyimide substrates by Xe ion beam sputtering under different deposition conditions: with or without O2 flow, and at room temperature or 100°C heated substrates. Both electrical and mechanical properties of the four different films were investigated in situ by electrical resistance measurements and synchrotron x-ray diffraction during equibiaxial deformation tests. The ITO films are found to have a low elastically anisotropy, i.e. an elastic anisotropy index slightly smaller than 1. The elastic regime domain is quite small and is associated to a small effective negative gauge factor. During biaxial straining, electrical measurements show that all films are very brittle with crack onset strains corresponding to applied strains ranging from 0.15 to 0.3%. The introduction of oxygen flow during deposition delays the crack onset as the decrease of deposition temperature does.

Introduction

Indium tin oxides (ITO) have been used as transparent conductive electrodes in many different optoelectronic devices because of the very good combination of high optical transmittance in the visible range and low electrical resistivity. The combination of optical and electrical properties of ITO can be understood in detail from free electron theory [START_REF] Hamberg | Evaporated Sn-doped In2O3 films -basic opticalproperties and applications to energy-efficient windows[END_REF]. For most of the current applications, ITO films are deposited on rigid substrates but deposition of ITO films on stretchable and foldable polymeric supports is desired for many new applications. Polymeric substrates indeed present the advantages of light weight, low cost, mechanical stretchability and bendability.

Deposition of thin films onto flexible substrates obviously introduces risks of cracking and accompanying loss of electrical conductivity. Many types of mechanical deformation tests are used to study film failure. The two most commons are the uniaxial tensile test [START_REF] Leterrier | Mechanical integrity of transparent conductive oxide films for flexible polymer-based displays[END_REF][START_REF] Cairns | Strain-dependent electrical resistance of tin-doped indium oxide on polymer substrates[END_REF][START_REF] Peng | In situ electro-mechanical experiments and mechanics modelling of tensile cracking in indium tin oxide thin films on polyimide substrates[END_REF] and the bending test [START_REF] Suo | Mechanics of rollable and foldable film-on-foil electronics[END_REF][START_REF] Park | Mechanical stability of externally deformed Indium-Tin-Oxide Films on polymer substrates[END_REF]. In situ microscopy (optical, Scanning Electron Microscopy,…) can be performed to track crack onset and monitor the density of cracks [START_REF] Leterrier | Mechanical integrity of transparent conductive oxide films for flexible polymer-based displays[END_REF][START_REF] Souza | Structure and properties of low temperature plasma carburized austenitic stainless steels[END_REF][START_REF] Marx | The influence of a brittle Cr interlayer on the deformation behavior of thin Cu films on flexible substrates: Experiment and model[END_REF]. According to literature, the fracture behaviour is dependent on film thickness [START_REF] Leterrier | Adhesion of silicon oxide layers on poly(ethylene terephthalate)[END_REF][START_REF] Mohanty | Thickness-dependent fracture behavior of flexible ZnO:Al thin films[END_REF], film adhesion, substrate and film elastic moduli [START_REF] Beuth | Cracking of thin bonded films in residual tension[END_REF][START_REF] Hutchinson | Mixed-Mode Cracking in layered materials[END_REF], film architecture [START_REF] Cordill | Role of layer order on the equi-biaxial behavior of Al/Mo bilayers[END_REF], deformation mode [START_REF] Faurie | In situ x-ray diffraction analysis of 2D crack patterning in thin films[END_REF] among other things. Several groups [START_REF] Leterrier | Mechanical integrity of transparent conductive oxide films for flexible polymer-based displays[END_REF][START_REF] Cairns | Strain-dependent electrical resistance of tin-doped indium oxide on polymer substrates[END_REF][START_REF] Peng | In situ electro-mechanical experiments and mechanics modelling of tensile cracking in indium tin oxide thin films on polyimide substrates[END_REF] have investigated the mechanical properties of ITO films on polymer substrates focusing on the cracking mechanisms and the effect on electrical properties. A recent study of Kim et al. demonstrates that the degree of crystallinity and crystalline quality are certainly the primary factors for determining the mechanical strength of ITO on polymer substrates [START_REF] Kim | The crystallinity and mechanical properties of indium tin oxide coatings on polymer substrates[END_REF].

The present experimental study focuses on the effect of the elaboration process of ITO films on the electrical and mechanical properties. Four point electrical resistance and synchrotron x-ray diffraction (XRD) measurements were performed in situ during biaxial loadingunloading of ITO films -Polyimide composites. In particular, we investigate the differences induced by changing the temperature of elaboration, and the O2 gas flow during ITO deposition by Xe ion beam sputtering on polyimide substrate. The magnitude of biaxial applied deformation imposed to ITO films is small (less than 1%) so that the onset of cracking can be investigated in depth. Up to a tensile deformation of about 0.3% (corresponding to the elastic domain limit), XRD data show no significant difference between the four samples and reveal the slight anisotropic elastic mechanical behaviour of ITO.

Moreover, in the elastic domain, results demonstrate that the electrical resistance is decreasing as the ITO films elongate. This feature reveals a negative effective strain gauge associated with piezoresistive properties. For larger applied strains, the film failure attributed to crack formation is captured first by the electrical measurements prior to strain measurements by x-ray diffraction during mechanical testing. The resistance versus true strain curves plotted for the four ITO films reveal the crack onset strain differences related to the different deposition conditions and exhibit a hysteresis that is induced by a significant closure of cracks during unloading.

Experimental

Sample elaboration and as-deposited characterization

Four series of ITO thin films were prepared by ion beam sputtering on polyimide substrates.

A ceramic target of In2O3/SnO2 (90:10) was used. Fig. 1 presents a schematic illustration of the deposition chamber (Nordiko 2000 system). Crystallinity of ITO which is a critical physical parameter for physical properties is obviously strongly dependent of physical vapor deposition conditions. Here, the target to substrate distance is 55 cm. When the pressure had been reduced to about 7x10 -5 Pa, prior to deposition, the ITO ceramic target was presputtered according to a procedure divided into three steps of 10 min each (consisting of increasing the Xe beam current step by step) in order to increase slowly the temperature of target and to remove surface impurities. The deposits of ITO films 600 nm thick are performed with a Xe+ ion gun sputtering beam at 1.2 keV at a working pressure of approximately 10 -4 Pa with and without introduction of small quantity of oxygen (O2 flow at 5 sccm). The optional O2 reactive gas is inserted to the deposition chamber through a second ion gun without acceleration voltage (Fig. 1). For another set of samples, the substrate was raised up to a temperature of 100°C for 1 hour. Then the temperature was maintained at 100° C during deposition. Let us notice that the substrate temperature has been monitored during a deposition test; for a setpoint temperature of 100°C, the actual temperature measured at the surface of polyimide substrate increases up to about 80°C and remains roughly constant during the deposit which lasts for about 40 minutes. Deposition parameters (temperature and oxygen flow) are listed in Table 1. Growth rate measurement of 0.15 nm/s was performed by determining the film thickness using ex situ x-ray reflectometry measurement. The polyimide (PI) substrates 125 µm thick (Kapton HN®) have a cruciform shape with arms of 200 mm long and 20 mm wide (Fig. 2). The ITO deposition onto PI is performed through a circular mask of 20 mm diameter that is positioned at the centre of PI cruciform shape sample.

Interestingly, even if surface activation by ion-assisted reactions can be useful for improving the adherence of a transparent conductive oxide film to a polymer substrate [START_REF] Cho | Surface modification of polymers by ion-assisted reaction[END_REF], no specific surface treatment nor adhesion layer has been used in the present work, and the adhesion is very good though, as will be shown in subsection 3.2.

The electrical properties were characterized by Hall effect measurements in a HMS 5500 setup. These measurements that were carried under direct current (d.c.) condition probe the mobility of free carrier on macroscopic scale including therefore all scattering mechanisms. All the reported measurements were done at room temperature and room pressure. The magnetic field was provided by a permanent magnet of intensity B=0.58 Tesla. We prepared and tested several square-shaped PI samples, which were elaborated at the same time as cruciform PI specimens for in situ tensile tests. In some cases, we repeated the measurement several times in order to check for reproducibility, and found variations of about 2% for resistance, and of 7% for mobility and concentration.

Tensile loading with in situ resistance and x-ray diffraction measurements

We have employed the DIFFABS-SOLEIL biaxial tensile tester working in the synchrotron environment for in situ diffraction measurements of thin crystalline films mechanical response [START_REF] Geandier | Development of a synchrotron biaxial tensile device for in situ characterization of thin films mechanical response[END_REF]. The cruciform substrates were gripped by a cam rotating in a cylindrical fixation. The equibiaxial loading -unloading of the cruciform sample was controlled by displacement of the grips in the x and y axes where x and y correspond to the in-plane reference frame (z is the out of plane axis) related to the branches of the cruciform substrate (Fig. 2). The displacement grip speed corresponds to a deformation rate at the centre of about 2x10 -6 s -1 . The in-plane macroscopic strains (εxx and εyy) of the polyimide substrate were measured by Digital Image Correlation (DIC) [START_REF] Besnard | Finite-element' displacement fields analysis from digital images : application to Portevin-Le Châtelier bands[END_REF][START_REF] Djaziri | Combined synchrotron X-ray and image-correlation analyses of biaxially deformed W/Cu nanocomposite thin films on Kapton[END_REF] at the centre of the cruciform shape substrate by using images of about 6 x 9 mm². The incident x-ray beam was monochromatized using a double-crystal monochromator to a wavelength of λ = 0.06888 nm (i.e., an energy of 18 keV). The x-ray beam was of 295 x 260 μm² in size (horizontal and vertical full width at half maximum, FWHM, respectively). A hybrid pixel area detector (XPAD detector) [START_REF] Pangaud | XPAD3-S: A fast hybrid pixel readout chip for X-ray synchrotron facilities[END_REF] was used at a sample-to-detector distance of 621 mm. This XPAD 2D detector contains 240 x 560 pixels, with a pixel size of 130 x 130 µm². At this distance, the 2θ angular acceptance covers 7.3°. This set-up enables the measurement of a diffractogram every 4 seconds during a continuous loading. Thanks to a homemade python code, the raw data were re-calculated to get usual 2θ angles. Initial diffractograms are shown in Fig. 3 for the four different samples. The diffractograms show two diffraction peaks related to ITO films and two diffraction peaks related to TiO2 powder (rutile phase); these latter allow performing calibration and correction of sample vertical movement during loading tests. It is indeed important to distinguish various causes producing diffraction peak shifts from strain variations. For an incident ω angle fixed at 7°, the ITO(222) and ITO(400) Bragg peaks, associated with 2theta angles of about 13.4° and 15.5° probe planes almost parallel to the surface, i.e. the out-of-plane elastic strain εzz.

Concerning electrical resistance measurements, four electrical contacts are made at the periphery of circular ITO films with silver paste, and thin copper wires were used as connecting leads to the electrometers. During continuous loading -unloading, a constant current (I= 1 mA) from a current source (Keithley 6221 programmable current source) was allowed to pass through two terminal leads (A and B) of the four-point probe, and the voltage (V) across the two other leads (C and D) was measured using a multimeter (Keithley 2182 nanovoltmeter) (Fig. 2). Automated data acquisition was achieved thanks to a home-made LabVIEW program.

Results and discussion

Electrical properties and microstructure of the as-deposited ITO films

Typical x-ray diffraction profiles acquired during in situ tensile tests are shown in Fig. 3 for the four selected deposition conditions. All profiles indicated the presence of a single crystallographic cubic phase of In2O3 bixbyite and can be compared with referenced phase of the JCPDS database #6-0416. The presence of a given amount of amorphous phase of ITO reported by Legeay and Castex [START_REF] Legeay | A gradual annealing of amorphous sputtered indium tin oxide: Crystalline structure and electrical characteristics[END_REF] is not excluded, the related hump being impossible to decorrelate here from the high background signal due to PI substrate scattering. The amount of TiO2 powder that is sprayed on the PI backside is not the same for the four samples while the ITO film thicknesses are similar. Hence, the intensities of TiO2 diffraction peaks cannot be compared from one sample to another. On the other hand, since the films have the same thickness, the integrated intensities of ITO films could be quantitatively related to the diffraction volume and hence to the amount of crystallinity if no crystallographic texture was present. However, preferential grain orientation between the samples elaborated with O2 flow or without can be observed. For samples without O2 flow, the relative ratios of {004}/{222} integrated intensities are relatively close to the one of JCPDS database while the samples with O2 flow show a {004} excess (Table 2). Interestingly, if the deposition temperature does not change the crystallographic texture for films elaborated without O2 flow, it induces opposite behaviours for the degree of crystallinity. As expected, the increase of substrate temperature induces an increase of degree of crystallinity for samples deposited without O2 flow while it is the opposite for samples deposited with O2 flow. At low substrate temperatures (RT), the mobility of atoms at the surface growing film surface is very low due to limited diffusion during the film formation. This can lead to amorphous structure of ITO.

But in the present case of sputtering technique, there is a significant energy transfer due to the high energy of particles (ions and neutrals) impinging into the film [START_REF] Thornton | The microstructure of sputter-deposited coatings[END_REF] which can explain the nucleation of crystallite of ITO at RT. Following the same reasoning, this should also be the case for the samples elaborated with O2 flow. But, in the latter case, the degree of crystallinity at 100°C is more than two times smaller than the one at RT. The two sets of samples have different crystallographic texture that clearly depends on the O2 flow.

Noteworthy, the relative ratios of {004}/{222} integrated intensities are constant versus temperature and hence texture can be considered as similar for samples elaborated at RT and 100°C for samples deposited without O2 flow. On the contrary, the crystallographic texture changes with temperature for samples deposited with O2 flow (Table 2). Song et al. have reported that the <001> texture is caused by high energy ion bombardment of the growing film surface [START_REF] Song | Study on crystallinity of tin-doped indium oxide films deposited by DC magnetron sputtering[END_REF], but, as the working pressure is the same in our case, the energy of ion bombardment should be similar if the influence of the target-substrate distance is neglected.

For sputtering at 200°C and at higher oxygen flow, the <111> orientation was dominant while the <100> orientation was mainly built up at lower oxygen pressure [START_REF] Betz | Thin films engineering of indium tin oxide: Large area flat panel displays application[END_REF]. On the contrary we observed that the increase of oxygen flow favoured the <100> orientation. This could be due to the fact that the main parameter is the sputtering pressure that does not change in our deposition procedure and is about two orders of magnitude lower than the one of Song et al. [START_REF] Song | Study on crystallinity of tin-doped indium oxide films deposited by DC magnetron sputtering[END_REF]. This result implies that the main parameter of texture growth in Betz et al.

[24] could be the sputtering pressure rather than the oxygen flow. Moreover, crystallinity of ITO is not improved with increase in oxygen flow contrary to the observation of Song et al. [START_REF] Song | Study on crystallinity of tin-doped indium oxide films deposited by DC magnetron sputtering[END_REF]. This inconsistency is certainly due to the influence of several deposition parameters that are not in the same range, namely O2 pressure, working pressure, target-substrate distance among others. They argued that enough oxygen supplement should be needed to form stoichiometric films and this could favour crystallization. Our observations may suggest that O2 molecules adsorbed on the growing film surface could result in suppressing the surface migration of In atoms during the crystal growth and hence increasing the structural disorder in growing films. Electrical characteristics of the four selected as-deposited ITO films obtained from Hall measurements are listed in Table 1 and presented in Fig. 4. We notice first that as-deposited ITO films have a high negative carrier concentration (∼10 21 cm -3 ). The ITO films with O2 flow were found to have seven times smaller carrier concentration. This phenomenon can be attributed to free carriers that are provided from two different kinds of electron donor sites: substitutional doping of Sn on In sites in the In2O3 crystal structure and divalent oxygen vacancies [START_REF] Hamberg | Evaporated Sn-doped In2O3 films -basic opticalproperties and applications to energy-efficient windows[END_REF][START_REF] Taga | Electrical properties of heteroepitaxial grown tin-doped indium oxide films[END_REF][START_REF] Kim | Hall measurements of treated indium tin oxides surfaces[END_REF]. The densities of oxygen vacancies in the bixbyite crystal structure and of the crystallographic defects are likely to reduce the conductivity through a substantial decrease of mobility [START_REF] Kim | Hall measurements of treated indium tin oxides surfaces[END_REF]. In another study, an increase of the conductivity of five orders of magnitude has been reported upon thermal annealing [START_REF] Bardos | Effect of the oxygen absorption on properties of ITO layers[END_REF]. The main objective of the thermal treatment was to increase the crystallinity of ITO films, but the authors observed a hightemperature out-diffusion of interstitial oxygen. Hence the thermal annealing induced a decrease of resistivity that could be explained by a reduced number of traps and increased mobility. In the present study, the resistivity was found to be around 1 x 10 -3 Ω cm, a lowest value of 7.2 x 10 -4 Ω cm was achieved for Xe sputtered ITO film at 100°C (Xe_100°C). The mobility increases with O2 flow and temperature from about 1.4 for Xe at RT (Xe_RT) to achieve only 10 cm 2 .V -1 .s -1 for Xe+O2 sputtered ITO film at 100°C (Xe+O2_100°C). ITO films sputtered at higher temperature show an increase of their mobility while the carrier concentrations remain similar (or slightly decrease). As a result, ITO films deposited at higher temperature show a lower resistivity that is only related to the larger mobility. These observations are qualitatively consistent with the observations of Hamouda et al. for Xe magnetron sputtering [START_REF] Hamouda | Electrical and optical properties of sputtered ultra-thin indium tin oxide films using xenon/argon gas[END_REF]. But a lower resistivity can also be induced by creation of oxygen vacancies that increases electron carrier density. In sputtered ITO films, these oxygen vacancies can be controlled by post-annealing treatment or different sputtering conditions [START_REF] Hamberg | Evaporated Sn-doped In2O3 films -basic opticalproperties and applications to energy-efficient windows[END_REF][START_REF] Kim | The crystallinity and mechanical properties of indium tin oxide coatings on polymer substrates[END_REF][START_REF] Cho | Surface modification of polymers by ion-assisted reaction[END_REF][START_REF] Legeay | A gradual annealing of amorphous sputtered indium tin oxide: Crystalline structure and electrical characteristics[END_REF][START_REF] Song | Study on crystallinity of tin-doped indium oxide films deposited by DC magnetron sputtering[END_REF][START_REF] Betz | Thin films engineering of indium tin oxide: Large area flat panel displays application[END_REF][START_REF] Taga | Electrical properties of heteroepitaxial grown tin-doped indium oxide films[END_REF][START_REF] Kim | Hall measurements of treated indium tin oxides surfaces[END_REF][START_REF] Bardos | Effect of the oxygen absorption on properties of ITO layers[END_REF][START_REF] Hamouda | Electrical and optical properties of sputtered ultra-thin indium tin oxide films using xenon/argon gas[END_REF][START_REF] Luo | Effective creation of oxygen vacancies as an electron carrier source in tin-doped indium oxide films by plasma sputtering[END_REF][START_REF] Vaufrey | ITO-on-top organic light-emitting devices: a correlated study of opto-electronic and structural characteristics[END_REF][START_REF] Shigesato | A microstructural study of low-resistivity tin-doped Indium oxide prepared by dc magnetron sputtering[END_REF][START_REF] Park | Effects of the cathode magnetic field strength on the properties of sputtered ultrathin ITO films[END_REF] among which the impact of the ion energy bombardment [START_REF] Vaufrey | ITO-on-top organic light-emitting devices: a correlated study of opto-electronic and structural characteristics[END_REF] or the reduction reaction of hydrogen radicals [START_REF] Luo | Effective creation of oxygen vacancies as an electron carrier source in tin-doped indium oxide films by plasma sputtering[END_REF]. In the present study, the introduction of oxygen flow during sputtering increases resistivity by about a factor of two by greatly decreasing carrier density (by a factor of seven) while increasing mobility (by a factor of three to four).

To sum up, the four selected deposition conditions made it possible to fabricate four ITO films on polyimide substrates with different microstructure and electrical properties. The four samples are polycrystalline (with single bixbyite phase) certainly with a given amount of amorphous ITO phase that has not been quantified. The samples elaborated without oxygen exhibit no significant preferential orientation while the samples prepared with O2 flow show a slight <001> texture that increases with deposition temperature (Table 2). The full widths at half maximum that reflect the sizes of the crystallites and the strain heterogeneities, reveal that adding the oxygen flow induces better crystallinity but the integrated intensities reveal that the total amount of crystallized ITO is lower. This observation can also be correlated with the increase of electrical carrier mobility that is significantly improved when adding the oxygen flow. Moreover, samples with (resp. without) O2 flow are transparent (resp. opaque); the residual stress states that have been estimated by the Stoney method on ITO films deposited on silicon single crystals during the same deposition run are -250 MPa +/-70 MPa for Xe_RT and Xe_100°C, -550 MPa +/-100 MPa for Xe+O2_RT and -400 +/-100 MPa for Xe+O2_100°C. All the films are in a compressive stress state explained by the high energetic deposition process. The deposition temperature is not high enough to induce a significant thermal stress contribution.

Evolution of mechanical and electrical properties of ITO films during in situ deformation tests

As mentioned in the introduction, the main objective of this study is to investigate the mechanical properties as a function of the microstructure. Figs. 5 present a typical set of in situ data acquired during the loading-unloading tests (namely DIC and XRD measured strains and electrical resistance); here is the example of the ITO film elaborated without O2 flow at 100°C (Xe_100°C). The mechanical deformation test lasts for about one hour. The initial loading of 17N for the cruciform ITO-polyimide samples that ensures a stable position of sample during loading is used as a reference state for all parameters: macroscopic strains, elastic XRD strains, and electrical resistance. This initial loading corresponds to an equibiaxial tensile strain of 0.12% that can be related to an applied tensile biaxial stress of 215 MPa (with a Young's modulus E=116 GPa and a Poisson's ratio ν=0.35 [START_REF] Neerinck | Depth profiling of thin ITO films by grazing incidence X-ray diffraction[END_REF]).

Firstly, the macroscopic or true strain measurements ( and components) show the anisotropic behaviour of Kapton® substrates that leads to a non-perfect equibiaxial deformation test. During loading, the applied true strains increased linearly with time up to about 0.7%. Once unloaded, the applied true strain components are equal to approximately 0.1%. In other words, the unloaded strain state is different from the initial one while the loading force is going back to the initial value. This residual strain at the centre of the substrate is due to the geometry of the cruciform shape substrate. Indeed, if the maximum applied strain of about 0.7% that is measured at the centre of the cruciform shape substrate is lower than the yield strain of polyimide, the strains of the sample are much larger than that in the toe weld region. So, considering the distribution of strains in the whole sample, the deformation test can be over the yield point in some regions and this is why the applied strain at the center of the cruciform shape substrate is not going back to zero when the sample is unloaded (see Fig. 5a).

Secondly, the elastic strains, or in other words the lattice strains deduced from the two for c11, c12, and c44 respectively [START_REF] Guo | Gigahertz Acoustic Vibrations of Elastically Anisotropic Indium-Tin-Oxide Nanorod Arrays[END_REF]. These values lead to a strong elastic anisotropy with a Zener anisotropy index of 0.39. This value corroborates qualitatively the fact that the [400] direction is stiffer than the [222] direction as observed by Veith et al. [START_REF] Veith | Investigation of morphological and electrical characteristics of tin doped indium oxide layers produced by a quasi single source precursor system[END_REF] and ourselves; but the anisotropy of 0.39 is far too high, and does not reflect quantitatively our results.

Another important feature of the change of elastic strains as a function of time is the departure from linearity at = -0.35% which is shown in Fig. 5a at t~1200 s and, even more clearly in Fig. 6 for the two samples that have been submitted to larger applied strains, i.e. Xe+O2_RT and Xe_100°C. For these two latter samples, the loading part can indeed be clearly separated into two domains: elastic domain and crack domain. As the applied macroscopic strain is continuously increasing, the change of XRD strain slopes can be related to crack propagation in brittle films [START_REF] Cordill | Role of layer order on the equi-biaxial behavior of Al/Mo bilayers[END_REF]. Indeed, when a failure occurs in the ITO thin film, stress is relaxed locally and transferred to the substrate. This phenomenon leads to a decrease in the average stress supported by the film that is captured by x-ray diffraction. In the first domain that is related to the elastic domain, the elastic strains measured in ITO crystalline part can be compared to DIC macroscopic strains of substrate. Using Hooke's law = + and ν=0. [START_REF] Wittkowski | Elastic properties of indium tin oxide films[END_REF] we obtained values that are similar to meaning that the strain is transmitted unchanged through the film-substrate interface. As mentioned in section 2.1, the ion beam sputtering technique at the low working pressure produces high energetic ions and neutrals impinging on the film; these deposition conditions allow to create highly dense films which are very well adherent to the polyimide substrates. Noteworthy, no delamination occurs when the ITO-Polyimide composite was bent or even folded.

Thirdly, Figs. 5 also present the electrical resistance evolution versus time. Fig. 5b (which is a zoom of Fig. 5a) helps to uncover the negative slope of relative resistance evolution ∆R/R which is hidden by the large magnitude of ∆R/R. The relative electrical resistance has to be plotted on a different scale (on the right axis) as the relative evolution is quite huge compared to the applied strain: 250% versus 0.7%. This huge increase of resistance is related to crack nucleation and propagation [START_REF] Leterrier | Mechanical integrity of transparent conductive oxide films for flexible polymer-based displays[END_REF]. The relative resistance that is drastically decreasing during unloading reveals a partial closure of cracks. This typical electrical resistance behaviour of thin films on polymeric substrates has been reported many times for uniaxial tensile tests [2-4, 37, 38] or bending tests [START_REF] Shen | Study of indium tin oxide films deposited on colorless polyimide film by magnetron sputtering[END_REF][START_REF] Park | The effect of film microstructures on cracking of transparent conductive oxide (TCO) coatings on polymer substrates[END_REF]. The difference in mechanical properties between ITO films and polyimide substrate is an issue, and tensile deformation may lead to crack patterns and thereby increased resistivity [START_REF] Leterrier | Mechanical integrity of transparent conductive oxide films for flexible polymer-based displays[END_REF][START_REF] Cairns | Strain-dependent electrical resistance of tin-doped indium oxide on polymer substrates[END_REF]. As expected, electrical resistance measurements are more sensitive than XRD measurements to capture the crack onset. Indeed, as soon as there is one crack or a few microcracks, the electrical flow is modified and hence induces substantial increase of the film electrical resistance. The advantage of this in-situ electrical probe technique is that the exact crack onset can be determined for brittle films [START_REF] Cordill | Measuring electromechanical properties of thin films on polymer substrates[END_REF]. On the contrary, a sufficient density of cracks is required before the stress relaxation can be captured by XRD measurements as clearly shown in Fig. 6. Monitoring of the changes in slopes can give information about processes occurring in the ITO film. As expected, the curves superimposed on each other rather well in the first domain related to the elastic regime. Then for brittle films such as ITO, the changes of slope as applied deformation increases reveal the relaxation of strain related to cracks. The crack features are well captured for Xe_100°C and Xe+O2_RT samples at applied deformation of about 0.36% and 0.42% respectively. During unloading, the slopes of these last two samples are different from the slopes in the elastic regime. This behaviour reveals the process of stress redistribution in the film-substrate composite. At the beginning of unloading, the relaxation of the applied load starts in the softer material, namely the polymer substrate, and hence is not captured by XRD measurements. Then at the end of unloading, the slopes become again similar to the one at the beginning because of crack closure. All these deformation processes are confirmed by electrical measurements. Fig. 7 focuses on the early stages of the tensile deformation; this figure highlights both the negative slope of relative resistance evolution versus tensile strain, and the departure from linearity that reveals the crack onset strain. The electrical resistance increases sharply at some critical strain which depends on film deposition conditions. The resistance increase is indeed explained by the appearance of the cracks. For the four samples, the crack onset strain is at very small applied strains, ranging from 0.1% up to 0.3%. These small strain values reveal the brittleness of our ITO films that is much lower than the crack onset reported by many authors. Their films failed under applied uniaxial tensile strains typically in the range 0.8-1.2% [START_REF] Park | Mechanical stability of externally deformed Indium-Tin-Oxide Films on polymer substrates[END_REF], 1.59% [START_REF] Peng | In situ electro-mechanical experiments and mechanics modelling of tensile cracking in indium tin oxide thin films on polyimide substrates[END_REF] 1.4-2.2% [START_REF] Leterrier | Mechanical integrity of transparent conductive oxide films for flexible polymer-based displays[END_REF], or even 2.5-3% [START_REF] Cairns | Strain-dependent electrical resistance of tin-doped indium oxide on polymer substrates[END_REF]. Alike brittle materials, the cracking properties of ITO strongly depend on the layer thickness, crystallinity, interface quality, density of defect or roughness [START_REF] Leterrier | Mechanical integrity of transparent conductive oxide films for flexible polymer-based displays[END_REF][START_REF] Cairns | Strain-dependent electrical resistance of tin-doped indium oxide on polymer substrates[END_REF][START_REF] Peng | In situ electro-mechanical experiments and mechanics modelling of tensile cracking in indium tin oxide thin films on polyimide substrates[END_REF][START_REF] Suo | Mechanics of rollable and foldable film-on-foil electronics[END_REF][START_REF] Park | Mechanical stability of externally deformed Indium-Tin-Oxide Films on polymer substrates[END_REF][START_REF] Park | The effect of film microstructures on cracking of transparent conductive oxide (TCO) coatings on polymer substrates[END_REF]. Among these parameters, only the microstructure (crystallite size, amount of crystallinity, crystallographic texture) changes between the four samples. Independently of these microstructural considerations, Fig. 7 shows that the introduction of O2 during deposition seems to delay the crack onset strain as the decrease of deposition temperature does. This observation cannot be directly correlated to the amount of crystallisation due to crystallographic texture changes. We can however notice that the crack onset strain is at about 0.12% for Xe_100°C versus 0.24% for Xe_RT as the amount of crystallinity increases with temperature. This last result is indeed associated to an integrated intensity that increased by a factor of 2.5 and to a similar crystallographic texture (similar integrated area ratios of {400} and {222} diffraction peaks). Hence, the brittleness increases as the crystallinity increases contrary to the observations reported by different groups [START_REF] Park | The effect of film microstructures on cracking of transparent conductive oxide (TCO) coatings on polymer substrates[END_REF][START_REF] Park | Processing of indium tin oxide thin film to enhance electrical conductivity and flexibility[END_REF]. The other interesting point of Fig. 7 is that it revealed the piezoresistive nature of the four ITO films. The influence of strain on electrical properties of ITO has been reported and can be exploited in the fabrication of strain gauges [START_REF] Dyer | Preparation and piezoresistive properties of reactively sputtered indium tin oxide thin films[END_REF]. If the samples were only submitted to geometrical deformation (without piezoresistivity nor defect nucleation), the electrical measurement should mainly reveal the thickness change with a perfect four point probe method as underlined by the dashed line. All ITO films show a negative effective strain gauge factor of about -2 (Table 3). If such negative piezoresistive responses have already been reported [START_REF] Dyer | Preparation and piezoresistive properties of reactively sputtered indium tin oxide thin films[END_REF][START_REF] Fang | ρ, n and µ are the resistivity, carrier concentration and the mobility, respectively. Table 2: X-ray diffraction peak characteristics obtained from fitting: full width at half maximum (FWHM) and integrated[END_REF], most of the time no change of resistance is reported as a function of deformation in the elastic domain. Our gauge factors agree reasonably well with the values reported by Fang et al. [START_REF] Fang | ρ, n and µ are the resistivity, carrier concentration and the mobility, respectively. Table 2: X-ray diffraction peak characteristics obtained from fitting: full width at half maximum (FWHM) and integrated[END_REF]. These authors observed that the oxygen pressure during ITO film deposition affects the resistivity and gauge factor. They measured gauge factors of about -0.5 or -1 for ITO films deposited at low oxygen pressure (around 10 Pa), and these films show a resistivity of ~10 4 Ω cm that is similar to the one of our films.

Conclusions

We report in situ mechanical and electrical properties evolution of ITO sputtered films during an equibiaxial loading-unloading cycle. The effect of deposition parameters on in situ mechanical and electrical properties of 600 nm thick ITO films deposited on polyimide substrates was investigated. Four different microstructures were fabricated by changing deposition parameters including oxygen flow and deposition temperature. The resistivity was found to be around 1 x 10 -3 Ω cm, a lowest value of 7.24 x 10 -4 Ω cm was achieved for Xe sputtered ITO film at 100°C. During loading, development of microcracks generated by mechanical stress in ITO is evidenced by a substantial increase of film electrical resistance at very small applied deformations. The introduction of O2 flow during deposition delays the crack onset strain as the decrease of temperature does from an applied equibiaxial strain of 0.15% up to 0.3%. In the elastic domain, in situ XRD tensile tests allow demonstrating that the ITO polycrystalline films exhibit a very low elastic anisotropy as observed by Veith et al.

[34] and contrary to what has been recently reported in literature by Guo et al. [START_REF] Guo | Gigahertz Acoustic Vibrations of Elastically Anisotropic Indium-Tin-Oxide Nanorod Arrays[END_REF]. For all Xe sputtered ITO films, the Zener elastic anisotropy index was estimated to be 0.95+/-0.05.

In addition, a negative effective gauge factor of about -2 was measured for all ITO films.

Table 3: Strain gauge factor of the four films obtained from the evolution of resistance as a function of applied strains in the elastic regime (slopes of the linear parts of Fig. 6).

Tables: List of figures captions:

  ITO(222) and ITO(400) diffraction peaks as a function of time are shown in Figs.[START_REF] Suo | Mechanics of rollable and foldable film-on-foil electronics[END_REF]. Due to the measurement configuration, these strains are related to diffracting planes parallel to the sample surface; the measured strain values are then negative due to Poisson's effect. Indeed, an in-plane elongation of the film induces a contraction in the out of plane or perpendicular direction. The two curves corresponding to the two diffracting planes are strikingly very well superimposed. This result demonstrates that the ITO film Xe_100°C is elastically perfectly isotropic, i.e. exhibits an elastic anisotropy index, defined for cubic material as A=2c44/(c11-c12), equals to 1. This is indeed an interesting finding of the presented results: the ITO films shows anisotropic index equal to 0.95+/-0.05. The literature related to elastic properties of ITO is very scarce. From a residual stress analysis study on {004} and {222} diffraction peaks of ITO films, Veith et al. reported a 10% difference in values of residual stresses analysed from {004} and {222} diffraction peaks of ITO films[START_REF] Veith | Investigation of morphological and electrical characteristics of tin doped indium oxide layers produced by a quasi single source precursor system[END_REF]; this difference can be attributed to an elastic anisotropy of ITO with [100] direction of ITO slightly stiffer than the[111] direction. This observation is qualitatively and quantitatively in agreement with our result. From Brillouin scattering, Wittkowski et al. also reported that their ITO films were elastically anisotropic[START_REF] Wittkowski | Elastic properties of indium tin oxide films[END_REF]. More recently, by combining the frequency signatures of the vibrational modes of ITO nanorods with finite-element simulations, Guo et al. determined the three elastic constants for single-crystalline cubic ITO, which were 277.5, 107, and 33.8 GPa,

Figure 1 :

 1 Figure 1: Schematic of deposition chamber. The ion sputtering beam is composed of Xe. The ITO target -substrate distance is of about 55 cm. Base working pressure is of about 10 -4 Pa.

Figure 2 :

 2 Figure 2: Schematic of the cruciform shape substrate specimen. The ITO film is deposited at

Figure 3 :

 3 Figure 3: Typical x-ray diffractograms obtained for the four samples from XPAD detector

Figure 4 :

 4 Figure 4: Evolution of Hall electrical properties as function of the partial pressure of O2 (O2

Figure 5 :

 5 Figure 5: In situ XRD strain obtained from ITO (222) and (400) diffraction peaks, true strain

Figure 6 :

 6 Figure 6: XRD strains of the ITO (222) diffraction peak measured versus macroscopic

Figure 7 :

 7 Figure 7: Relative resistance measurements versus macroscopic applied strains. Only the

Table 1 :

 1

	Sample set	O2 flow [sccm]	Deposition T° [°C]	ρ [Ω.cm]	n [cm -3 ]	µ [cm 2 .V -1 .s -1 ]
	Xe_RT	0	20	1.6 x 10 -3	2.8 x 10 21	1.4
	Xe_100°C	0	100	7.2 x 10 -4	2.4 x 10 21	3.5
	Xe+02_RT	5	20	2.6 x 10 -3	3.7 x 10 20	6
	Xe+02_100°C	5	100	1.9 x 10 -3	3.3 x 10 20	10

Table 2 :Table 3 :

 23

		Xe_RT Xe_100°C Xe+02_RT Xe+02_100°C
	Effective Gauge factor	-2.0 ± 0.1	-0.7 ± 0.1 -1.7 ± 0.3	-2.6 ± 0.2

Acknowledgements

This work partially pertains to the French Government program "investissements d'avenir" (LABEX INTERACTIFS, reference ANR-11-LABX-0017-01) and (EUR INTREE, reference ANR-18-EURE-0010). The region Nouvelle-Aquitaine and the French synchrotron SOLEIL via the NACRES project supported this work.


Smiri Badreddine

R S Joshya

Demir Ilkay

Saidi Faouzi

Altuntas Ismail

Delphine Lagarde

Cedric Rober

Marie Xavier

Maaref Hassen

Systematic optical study of high-x InxGa1-xAs/InP structures for infrared photodetector applications

Keywords: High-x InxGa1-xAs, dislocation density, S-shaped, FKOs, LSE, localized state, SWIR detector

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

High-x (x>0.53) InxGa1-xAs/InP material systems have achieved great success in optoelectronics and received enormous research interest. One of the most important applications is short-wave (1-3.6 um) infrared (SWIR) detectors [START_REF] Jurczak | 2.5-µm InGaAs photodiodes grown on GaAs substrates by interfacial misfit array technique[END_REF] [START_REF] Zhang | Advances in InGaAs/InP single-photon detector systems for quantum communication[END_REF], which is required for the various field, such as spectroscopy [START_REF] Demir | Comprehensive growth and characterization study on highly n-doped InGaAs as a contact layer for quantum cascade laser applications[END_REF], astronomy [START_REF] Tan | Emerging technologies for high performance infrared detectors[END_REF] and medicine [START_REF] Ji | Deep-level traps induced dark currents in extended wavelength In x Ga 1-x As/InP photodetector[END_REF]. Yet, in order to extend the response of the detector to longer wavelengths, it is necessary to grow the InxGa1-xAs alloy with a high In content (x> 0.53) [START_REF] Zhao | Surface-interface analysis of In x Ga 1-x As/InP heterostructure in positive and negative mismatch system[END_REF]. Epitaxial growth requires suitable substrate materials, but lattice mismatch always limits the heteroepitaxy of such materials.

However, the composition of the InxGa1-xAs layer is very important because it affects the electronic properties of these devices. In order to achieve this goal, the x value in InxGa1-xAs must be increased to 0.65 to 0.85, which will cause a significant lattice mismatch (+0.55% to +2.2%) between the InxGa1-xAs layer and the InP substrate [START_REF] Jurczak | 2.5-µm InGaAs photodiodes grown on GaAs substrates by interfacial misfit array technique[END_REF]. The growth of high-x In is accompanied by the generation of dislocations and other defects caused by lattice mismatch, which will lead to poor surface quality and no uniformity in the alloy composition [START_REF] Smiri | Optical and structural properties of In-rich InxGa1-xAs epitaxial layers on (1 0 0) InP for SWIR detectors[END_REF] [START_REF] Asar | Structural and electrical characterizations of In x Ga 1-x As/InP structures for infrared photodetector applications[END_REF]. For the reasons mentioned above, carriers are captured and redistributed by spatial potential fluctuation and localized energy states, which is referred to as carrier location [START_REF] Del Alamo | Nanometre-scale electronics with III-V compound semiconductors[END_REF] [START_REF] Smiri | Carriers' localization and thermal redistribution in InAlAs/InP grown by MOCVD on (311)A-and (311)B-InP substrates[END_REF].

Moreover, carrier's localization in semiconductor systems has been extensively studied [START_REF] Wei | Carrier Localization Effects in InGaN/GaN Multiple-Quantum-Wells LED Nanowires: Luminescence Quantum Efficiency Improvement and "Negative" Thermal Activation Energy[END_REF].

Such carrier localization phenomenon usually increases the dark current of the devices [START_REF] Du | Improved performance of high indium InGaAs photodetectors with InAlAs barrier[END_REF].

In this framework, Xiaoli et al. [START_REF] Zhao | Surface-interface analysis of In x Ga 1-x As/InP heterostructure in positive and negative mismatch system[END_REF] have shown that this high dark current is still one of the critical technology issues for the development of extended-wavelength InxGa1-xAs/InP (x>0.53) photodetectors (PDs).

The influence of carrier localization is also manifested in the temperature abnormality « Sshape » of the peak PL energy and the form of the low-temperature PL decay curve [START_REF] Smiri | Effect of piezoelectric field on type II transition in InAlAs/InP (311) alloys with different substrate polarity[END_REF] [START_REF] Smiri | Power-and temperature-dependent photoluminescence investigation of carrier localization at inverted interface transitions in InAlAs/InP structures[END_REF].

In order to fit the curve, several methods have been used, such as double exponential model, triple exponential model, stretched model, etc [START_REF] Varshni | Temperature dependence of the energy gap in semiconductors[END_REF] [17] [START_REF] Pässler | Basic Model Relations for Temperature Dependencies of Fundamental Energy Gaps in Semiconductors[END_REF]. Nevertheless, these methods only provide mathematical fitting without detailed physical explanation. Hence, the exact nature of the carrier localization and carrier dynamics are key issues for understanding and improving InGaAs devices. This type of high x InxGa1-xAs anomalous behavior requires a thorough study to fully explain the physics behind this phenomenon, especially the dark current mechanism. Furthermore, more research is required on both device structure and dark current mechanism of InGaAs photodetectors.

In this letter, we used a luminescence model for localized state ensemble (LSE) developed by Li et al. [START_REF] Viña | Temperature dependence of the dielectric function of germanium[END_REF], taking into account generation, thermal activation, and recapturing of excitons by using Pässler model to follow the energy evolution. Moreover, we used the photoreflectance (PR) spectroscopy to obtain important properties of the samples. By analyzing the Franz-Keldysh oscillations, we estimated the built-in electric fields. In addition, timeresolved PL measurements (TRPL) were carried out at 10 K. The investigation of the electronic properties presented in this paper could yield on improvement of the efficiency of the high-x InxGa1-xAs/InP structures (x>0.65) and make them useful for many types of devices used in optoelectronics and microelectronics.

Experimental details

The three samples, 190 nm-thick InxGa1-xAs investigated in this paper, were fabricated by Metalorganic Vapor Phase Epitaxy (MOVPE) at 560°C on InP (100) substrate. For the InxGa1-xAs samples called S1, S2 and S3, it corresponds to x = 0.650, x= 0.661 and x = 0.667 and are described in detail in Ref [START_REF] Smiri | Optical and structural properties of In-rich InxGa1-xAs epitaxial layers on (1 0 0) InP for SWIR detectors[END_REF].

The samples were characterized by HR-XRD, PR, PL and TRPL spectroscopy. Then, the samples were characterized by HR-XRD and PL. The HR-XRD measurements were performed using Rigaku SmartLab diffractometer, equipped with a rotating Cu anode (wavelength Kα = 1.54056 A) which provides 9 kW X-ray power and four bounced Ge (220) monochromator allowing a 2theta precision of 0.0004°. We used a tungsten lamp in PR, and the modulating source an Ar + laser (532 nm) chopped at 250 Hz. An InGaAs photodiode detected the PR signal. With PL spectroscopy, we studied the samples between 10 and 300 K while keeping the samples in a closed-cycle helium circulation cryostat. The samples are excited using 514.5 nm line of the continuous-wave Ar + laser. The excitation density was fixed at 80 W/cm 2 . The luminescence was dispersed using a JOBIN YVON HRD1 monochromator and detected by a cooled Ge diode detector with a built-in amplifier. The TRPL spectra were measured at a temperature of 8 K using a closed cycle cryostat. In this case, the PL dynamics was excited by a frequency-doubled Ti: Sa laser at λexc = 375 nm, the laser-pulse duration was 2 ps pulse-repetition frequency was 4MHz. A super conducting nanowire single photon detector (single quantum) detected the PL signal from the InGaAs samples using a time-correlated single-photon counting system (picottarp from picoquant). The instrument response function is at the order 150 ps. The excitation power is 50 mW, and the laser spot size is about 10 µm 2 ). Table I: InxGa1-xAs samples characteristics and calculated parameters using HR-XRD.

Results and discussions 1. HR-XRD results

Photoreflectance results

The photoreflectance technique (PR) is an important and useful tool to investigate and qualify the interband transitions and electric field distributions in semiconductor and multilayer structures because of its derivatives-like nature and sharp spectral features. The main advantage of PR is its high sensitivity, even at room temperature. However, the existence of overlapping spectral characteristics of different regions of multilayer structures can complicate PR analysis. Figure 2 shows the PR spectra of the three high-x InxGa1-xAs samples at room temperature.

PR is a differential form of reflectance; therefore, its sensitivity is very high, as shown in figure 2. In this case, transitions originating from the InP substrate and InGaAs layer are visible [21] [22]. In addition, the PR spectra reveal many oscillations above the high-x InxGa1-xAs layer and InP substrate for the three samples [23] [24]. These oscillations are related to the Franz-Keldysh oscillations (FKO), resulting from an electric field present in the different samples. FKO has a regular periodicity and presents only the energies above the critical point [START_REF] Chen | Ultrafast probes at the interfaces of solar energy conversion materials[END_REF]. There appear to be two sets of FKOs, one related to the InP gamma-point of 1.39 (± 0.02) eV and the other linked with the InGaAs bandgap. Note that the shape and position of the PR spectrum in all samples are not identical. The shift in the extrema of the FKOs is observed. This is due to the variation of lattice mismatch between the InGaAs layer and the InP substrate, increasing the indium content in InxGa1-xAs. The FKOs have been frequently used to define the built-in electric field inside the semiconductor device. In this article, we will only be interested in this FKO related to the InGaAs layer. Now, to estimate the value of electric fields in the high-x InxGa1-xAs layer, the FKO period is analyzed using an asymptotic expression for the PR line shape given by Aspnes and Studna [START_REF] Aspnes | Schottky-Barrier Electroreflectance: Application to GaAs[END_REF]. In the region 0.7-0.9 eV, the oscillatory R/R signal can be described by:

(

) 1
where Γ is the broadening parameter, is an arbitrary phase factor and Ω is the electro-optic energy expressed as follows:

(2)

Note that q is the electron charge, is the reduced inter-band effective mass in the electric field direction and is the internal electric field.

In order to extract PR parameters ( , , etc.), the energy of FKO extrema (En) is given by [START_REF] Smiri | Effect of V/III ratio on the optical properties of (3 1 1)A and (3 1 1) B oriented InAlAs/InP heterostructures[END_REF]:

(3) and ( 4) Thus, by plotting as a function of (see figure 3), from linear fitting, we can determine and E0 from the slope and the intersection with the ordinate at the origin, respectively.

Recalling equation 2, Fint can be extracted by:

(5) is important to note that sample S3 with the highest residual strain presents the highest internal electric field of 76.5 kV/m. According to D. Yan et al. [START_REF] Yan | Photoreflectance characterization of an InP/InGaAs heterojunction bipolar transistor structure[END_REF], the value of the internal electric field in the ternary alloy semiconductor (In0.53Ga0.47As), lattice-matched to InP substrate is 30 kV/cm. Yet, for the sample S1, S2 and S3, the residual strain in epitaxial layers increases with the increase of indium concentration. As all the structures examined in this paper have the same geometric structure, we believe that the significant difference in the electric fields is due to different amounts of dislocation density. A high concentration of dislocation density will reduce the quality of the structure, form electron dispersion centers, and generate a strong built-in electric field in the sample.

Sample xIn E0

(eV) Table II: Summary of PR results for InxGa1-xAs layers. Bandgap energy E0, broadening , internal electric field Fint, and electro-optical energy ђΩ. Some value from the literature are added. Also, as shown in figure 5, the PL peak energy as a function of lattice temperature shows a "S-shape" (red/blue/red-shift) behavior which is the foot-print of the presence of localized carriers [START_REF] Smiri | Optical and structural properties of In-rich InxGa1-xAs epitaxial layers on (1 0 0) InP for SWIR detectors[END_REF]. The S-shape behavior becomes more pronounced at high indium concentration.

 (

Photoluminescence results:

The phenomenon of the S-shape behavior at low temperature can be explained as follows : at cryogenic 10 K (party I in figure 6), the excitons are strongly located in levels whose energy distribution extends within the band gap of the material. As lattice temperature to  60 K, excitons thermalize and relax to the local minima (alloy disorder) so that the PL peak energy decreases and we observed the first red shift. With further increase of temperature from 60 K to 100 K (party II in figure 6), the localized carriers gain enough thermal energy to be transferred to higher energy levels in the band-tails until they reaches maximum of band continuum, thus leading to blue-shift of the PL peak energy. Finally, when the lattice temperature increased above 100K (party III in figure 6),, the carriers are thermally activated and prevented from localization (free carrier recombination). A schematic diagram is shown in figure 6. The classic models such as Varshni [START_REF] Varshni | Temperature dependence of the energy gap in semiconductors[END_REF], Vina [START_REF] Viña | Temperature dependence of the dielectric function of germanium[END_REF], and Pässler [START_REF] Pässler | Basic Model Relations for Temperature Dependencies of Fundamental Energy Gaps in Semiconductors[END_REF] explain well the behavior at high temperature but unable to reproduce it at low temperature. The localized effects at low temperature can be explained using the Localized State Exciton Model or modified Pässler model with the impact of thermal redistribution of localized carriers [START_REF] Li | A model for steady-state luminescence of localized-state ensemble[END_REF] [34] [START_REF] Li | Thermal redistribution of localized excitons and its effect on the luminescence band in InGaN ternary alloys[END_REF]:

(6)
The second term describes the shrinking of band according to the classic Pässler equation with α and Θ are Varshni parameter and the Debye temperature, respectively. The dimensionless coefficient x (T) can be obtained by numerically solving the equation [START_REF] Su | Electroluminescence probe of internal processes of carriers in GaInP single junction solar cell[END_REF]: [START_REF] Smiri | Optical and structural properties of In-rich InxGa1-xAs epitaxial layers on (1 0 0) InP for SWIR detectors[END_REF] where σ is the broadening parameter for the distribution of the localized states at central energy E0. τr, τtr, Ea and KB are the radiative lifetime, non-radiative lifetime, activation energy, and the Boltzmann constant. At high temperature, the solution of equation 7 can be approximated to be:

Equation 6 can be rewritten as [START_REF] Smiri | Carriers' localization and thermal redistribution in InAlAs/InP grown by MOCVD on (311)A-and (311)B-InP substrates[END_REF]: Using the LSE model, the temperature dependence of the position of the PL peaks of the three S-shaped structures can be well reconstructed, as shown in figure 5. From the calculation, we obtained the same E0 value as obtained by PR results. However, ΔE=Ea-E0 is a necessary parameter to understand the abnormal temperature dependence of temperature better. From a physical point of view, there are two situations. ΔE can be positive (+) or negative (-), but ΔE is usually positive. In our case, it is positive and it increases by increasing the indium concentration. For samples S3, S2, and S1, the values are 16.5 meV, 11.0 meV, and 4.8 meV, respectively, that is, Ea is located above E0. In this case, the localized carriers are thermally activated to a higher energy state. This is why the S-shape is more evident in the S3 sample.

E (9)
This explains the larger occupied localized states and the proximity of empty states located at 10 K. In fact, the excitons gain enough thermal energy to overcome the small barrier and excite to a higher localized state above Ea with larger potential. The depth of potential fluctuation assigned by σ is the result of fluctuations in the indium composition and the presence of the electric field in our structures [START_REF] Fraj | Carrier localization in In0.21Ga0.79As/GaAs multiple quantum wells: A modified Pässler model for the S-shaped temperature dependence of photoluminescence energy[END_REF]. The potential depth is found to be 10.5;

15.0 and 16.8 meV. It is larger with increasing concentration indium and electric-filed. As a result, we can that the increase of potential depth increases the dark current of the devices, which could be the crucial factor in controlling the performance of high indium content InxGa1-xAs SWIR detector [START_REF] Wang | Direct correlation of defects and dark currents of InGaAs/ InP photodetectors[END_REF]. increase in the radiation lifetime [START_REF] Deveaud | Enhanced radiative recombination of free excitons in GaAs quantum wells[END_REF]. This could explain that the decay time in sample S3 is longer than the one in sample S1 as the excitons could be strongly localized. The larger electric-filed in the high-In structures could also lead to a spatial separation of electrons and holes, yielding an increase in recombination time [START_REF] Fraj | optical investigation of In0.21Ga0.79 As multiple quantum wells grown on (001) and (113) A GaAs substrates[END_REF]. Figure 8 shows the measured PL lifetime τInGaAs vs. the pump power for an excitation wavelength of 375 nm. The PL lifetime of the InxGa1-xAs first increases and then saturates for pump powers beyond 60 μW. This could to due to the filling and saturation of localized states. Then of high density of carriers, the free excitons could be more sensitive to the local electric field, yielding an increase of the recombination time. This hypothesis should be confirmed by further investigations [START_REF] Shi | Time-Resolved Photoluminescence Characterization of InGaAs/GaAs Nano-Ridges Monolithically Grown on 300 mm Si Substrates[END_REF].

TRPL results

Conclusion

We have studied high-x InxGa1-xAs layers on InP substrates by HR-XRD, PL, PR and TRPL with varying indium concentration. PR measurement revealed oscillations above the InGaAs layer, which are related to the Franz-Keldysh oscillations (FKO). From an analysis of the FKO, we have estimated that the electric field increases with indium concentration. The temperature-dependent PL measurement demonstrates the critical role played by carrier localization. The S-shape behavior is the result of a competitive process between localized and non-localized carriers. For increasing indium concentration in InGaAs, the density of localization increases. Luminescence measurements have been successfully modeled and interpreted using the developed LSE model. These theoretical studies have quantitatively explained the temperature-dependent spectra observed. LT-TRPL measurements showed longer decay times in the high-x samples, which is likely due to increased localization and electric field effects.

Our works shows that the high defect densities leads the carrier's localization, thereby increasing the dark current of the device. In addition, it's imparts a deep level understanding of the behavior of carriers and optoelectronic parameters in high-x InxGa1-xAs structures. In the next work, we will study the compensation of the strain layer by adding the buffer layer in our high-x InxGa1-xAs structures for minimizing the dislocation density and the residual strain (ɛ) in the InxGa1-xAs epitaxial layer.

Figure 1

 1 Figure 1 shows the HR-XRD patterns of InxGa1-xAs epilayer on InP substrate for different indium concentrations. The peaks at 63.5 • and 62.2 • corresponds to InP and InxGa1-xAs respectively. Increase of indium content in InxGa1-xAs epilayer shows a decrease in angle due to the increase of lattice constant [7]. The FWHM of an epilayer examined by HR-XRD is an important parameter to characterize the crystalline quality and dislocation. The values of lattice parameter and dislocation density are calculated and summarized in the Table I. Increase of indium content from x = 0.650 to 0.667 shows an increase of the dislocation density from 2.1*10 9 m -2 to 3.1*10 9 cm -2 . However, these values only reflect the average dislocation density of the epilayer. The lattice mismatch has also been determined for different indium content in InxGa1-xAs/InP. The addition of indium increases the lattice mismatch, but this is significantly less compared to GaAs substrate [19] [20].

Figure 1 :

 1 Figure 1: 2θ/ω HR-XRD spectra of the InxGa1-xAs layers having different In alloys grown on InP by MOVPE [7] .

Figure 2 :

 2 Figure 2: Room temperature PR spectra (dots) obtained from samples S1, S2 and S3 at different indium concentration and fits (green lines) performed using Eq. ~1.

Figure 3 : 2 .

 32 Figure 3: Plot of the energy extremes En vs. index number Hn for each sample. The straight lines are fittings to the FKO model using eq.3. The lines slope and ordinates to origin relate to the electric field strength Fint and the InGaAs bandgap energy E0.

Figure 4 :

 4 Figure 4: Comparison of the PL spectra of the three InGaAs samples with different In concentrations at 10K.

Figure 4

 4 Figure4compares the PL spectra of the InxGa1-xAs epilayer with various indium concentrations (x = 0.650, 0.661, 0.667) at 10 K under the same excitation power density of 80 W/cm 2 of 514 nm. As indium concentration increases to a small amount of 1.7%, the PL peak shows a large red-shift of about 170 nm, which is most suitable for short-wave infrared applications. The FWHM of the PL emission increases significantly with increasing indium concentration in InxGa1-xAs: incorporating high indium in InxGa1-xAs induces the structural in-homogeneities that cause localized potential fluctuations in its band structure[START_REF] Tongbram | Impact of an InxGa1-xAs Capping Layer in Impeding Indium Desorption from Vertically Coupled InAs/GaAs Quantum Dot Interfaces[END_REF] [START_REF] Fraj | Carrier localization in In0.21Ga0.79As/GaAs multiple quantum wells: A modified Pässler model for the S-shaped temperature dependence of photoluminescence energy[END_REF].

Figure 5 :

 5 Figure 5: The PL peak energy of InxGa1-xAs epilayer as a function of lattice temperature. green and red solid lines de-scribes the classical and modified Pässler model fit to the experimental data respectively. The parameters yielding to the best fit are tabulated in TableIII.

Figure 5

 5 Figure 5 shows compares the experimental data with the classical and modified Pässler Model. The modified Pässler model gives a good agreement with experimental data. The fitting parameters are summarized in table III.

Figure. 6 :

 6 Figure. 6: Schematic diagram for describing the hopping of carriers in localized levels. Dotted lines I , II and III indicate the transition process of carriers between different localization states with increasing temperatures.

Figure 7 : 7 .

 77 Figure 7: Normalized TRPL decay curves for InxGa1-xAs/InP samples for x= 0.65 to 0.667 at 10K with excitation power 50 µW. A single exponential decay (solid line) fitted with experimental data (symbols). Time-resolved photoluminescence measurement was carried out to understand the influence of indium concentration on the carrier dynamics in the high-x InxGa1-xAs layers. The PL lifetime profile of InxGa1-xAs at 10K for different indium concentrations are shown in figure 7. To obtain the PL lifetime, we fitted with a single-exponential decay, [38] where and τ are amplitude and decay time-constant, respectively. The lifetime is determined to be 0.33 ns, 0.49 ns, and 0.59 ns for the sample S1, S2, and S3, respectively [39]. Surprisingly, the low PL lifetime of 0.33 ns is obtained in sample S1, which has the best crystalline quality compared to the others. The addition of only 1.7% of indium concentration results in a strong enhancement of PL lifetime by ~80%. Regarding this increase of lifetime with increasing the indium concentration, two factors should be considered: the localization of excitons and the local electric field. It is well known that the localization of excitons yields an

Figure. 8 :

 8 Figure. 8: PL lifetime as a function of excitation power (µW) of the InxGa1-xAs/InP samples for x= 0.65 to 0.667 at 10K

Table III :

 III The best-fit parameters of the experimental data of InxGa1-xAs for different In content, obtained by the modified Pässler model.

								T (k)
	S1	0.650	0.76	16.70	4.8	2000	2.70	200
	S2	0.661	0.71	24.26	11.0	1500	2.73	185
	S3	0.667	0.68	29.30	16.5	535	2.60	243

Acknowledgments

This work was supported by the University of Monastir, Laboratory of Micro-Optoelectronics and Nanostructures (LMON), Faculté des Sciences, 5019, Monastir, Tunisia and by the University of Toulouse, INSA-CNRS-UPS, LPCNO, Toulouse, France.