Synchrotron Radiation X-Ray Absorption Spectroscopy and Spectroscopic Ellipsometry Studies of InSb Thin Films on GaAs Grown by Metalorganic Chemical Vapor Deposition

College of Physical Science and Technology, Guangxi University, Nanning 530004, China Laboratory of Optoelectronic Materials and Detection Technology, Guangxi Key Laboratory for the Relativistic Astrophysics, Guangxi University, Nanning 530004, China Graduate Institute of Electronics and Department of Electrical Engineering, National Taiwan University, Taipei 106-17, Taiwan Department of Physics, Indiana University of Pennsylvania, Indiana, PA 15705-1087, USA Department of Physics, University of North Florida, Jacksonville, FL 32224, USA National Synchrotron Radiation Research Center, Hsinchu 300-76, Taiwan Department of Electrical and Computer Engineering, Missouri University of Science and Technology, Rolla, MO 65409, USA


Introduction
As a member of the III-V compound semiconductors family, the growth of ultrathin films of indium antimonide has attracted a great deal of attention for its use in midwavelength infrared detectors (viz., thermal imaging cameras and forward looking infrared systems), magnetic sensors, magnetoresistors, field-effect transistors, photoconductors, and high-speed electronic devices .Among other semiconductors, the intrinsic binary InSb possesses the highest electron mobility (∼7.8 × 10 4 cm 2 /V•s), higher breakdown field (∼10 3 V/cm), high saturated electron drift velocity (∼5 × 10 7 cm/s), small effective mass (m * e ∼0.013 m o ), lowest energy band gap E g ∼0.18 eV (at 300 K), and ballistic length up to ∼0.7 μm (at 300 K). e InSb material can be used as a lattice-matched substrate for epitaxial growth of CdTe [34] and other relevant heterostructures and superlattices [35].e InSb-based ternary as well as quaternary alloys are equally valuable for realizing midwavelength infrared detectors [36][37][38][39].More recently, there has been a growing challenge to explore the possibility of developing InSb-based quantum dots and nanostructured laser diodes [40][41][42] for optoelectronic as well as photonic device applications in the 3-5 μm wavelength range for room temperature operations.
However, the incompatibility of the lattice constants between the two materials strongly limits the quality of InSb epilayers grown on GaAs [23].In general, the epifilms suffer from high density of dislocations or defects near the film/substrate interface.ese defects propagating throughout the entire material create antiphase domains and cause autodoping effects.e layers with intrinsic defects may also affect the carrier mobility and leakage current in both the electronic and photonic devices.To circumvent these difficulties and improve the crystallinity of InSb films, several strategies are pursued-especially in optimizing the growth conditions by controlling the V/III ratios, pressure, growth temperature, growth rate, and film thickness [20-22, 31, 47, 48].
Accurate estimation of InSb epifilm thickness is of paramount importance using them for device engineering.Traditionally, the thin semiconducting films are characterized by using Hall measurements, XRD, ultraviolet-visible spectrophotometry, cross-sectional scanning electron microscopy [20,21,23,49,50], and so on.Being destructive, most of these methods are not convenient for assessing material quality required in electronic industry for device productions.Due to the narrow band gap of InSb, no reflectance interference fringes appear in the ultraviolet-visible wavelength range from the InSb/GaAs heterostructure.erefore, a variety of nondestructive and penetrative tools have been exploited, providing nanoscale resolution for evaluating film thickness with greater degree of accuracy.Some of these techniques used in characterizing large area wafers include the atomic force microscopy, energy-dispersive X-ray spectroscopy, secondary-ion-mass spectrometry, and SE.RSS is another valuable and nondestructive tool to offer useful information on the crystalline quality and other parameters necessary for optimizing the InSb epifilm growth [31].
In this study, we present the results of our comprehensive investigations using SE, XRD, and SR-XAS, respectively, to assess the InSb film thickness and report the effects of V/III source ratios on the films crystalline quality for optimizing the MOCVD growth parameters, which were not exactly achieved from previous reports [31].e SR-XAS was used to acquire the structural properties of the material at the atomic scale.Furthermore, temperature-dependent optical constants of InSb thin films between 25 °C and 300 °C (n, k, and ε) were probed by exploiting SE.
is work provides a helpful guide to thin film characterization procedures required to monitor the growth processes, understanding the chemical and physical properties of materials and guiding the designs of high-performance InSb thin film devices.

Material Growth.
e growth of ultrathin uniform InSb thin epilayers was carried out on 4-inch semi-insulating (SI) GaAs (100) substrates using a low-pressure MOCVD method in vertical configuration by exploiting a high-speed rotating disk (180 mm diameter) reactor [31,47].e substrate orientation of 4-inch SI GaAs (100) was 2-4 °off towards <110>.e trimethyl indium and trimethyl antimony sources were used as vapor-phase constituents of In and Sb into a reaction chamber at approximately ∼20 °C and with the bubbler pressures set at 400 Torr for trimethyl indium and 323 Torr for trimethyl antimony, respectively.Hydrogen acted as carrier gas, and the pyrometer growth temperature was 395 °C.In this paper, we have studied a series of five InSb/GaAs samples grown with different V/III source flow ratios and thickness.Table 1 summarizes the growth relevant parameters of the samples used from the MOCVD growth [31] and their thicknesses determined in this study.For InSb growth using trimethyl indium and trimethyl antimony, the surface morphology was found to be very sensitive to the V/III ratio and closely reflected the crystallinity of as-grown films.All InSb films with V/III ratio varied between 3.91 and 5.38 exhibit mirror-like surface morphology having thickness ranging from 34 to 64 nm.To examine the effects of V/III ratio on the InSb film characteristics, we named the five samples S1 to S5 in the order of V/III ratio corresponding to their growth run numbers (IA376, 380, 373, 370, and 371, resp.[31]).thickness, optical constants of materials; the method has also been employed for a complete depth pro ling in semiconducting epitaxially grown ultrathin or thin lms.While the conventional SE approach su ers from the drawback of slow data acquisition process and covers a limited spectral range, the phase-modulated method that we have employed here o ers the fast and precise data acquisition over a large wavelength range.A dual rotating-compensator Mueller matrix ellipsometer (ME-L ellipsometer, Wuhan Eoptics Technology Co. Ltd., China), equipped with Linkam Scienti c heating and cooling stage device (THMSG600E), was used for the acquisition of ellipsometric spectra on ultrathin InSb epi lms.At room temperature, we acquired (Figure 1) two parameters Ψ and ∆, represented the ratio of amplitude decay between and p and s polarization of the re ected light and their phase di erence, respectively, as a function of wavelength (λ) from 200 nm to 1670 nm at 1 nm step (or energy from 0.74 eV to 6.2 eV) at three angles of incidence 55 °, 60 °, and 65 °.Advances in Materials Science and Engineering 2.2.2.Raman Scattering Spectroscopy.RSS is a powerful and nondestructive technique for providing valuable information on materials' characteristics-especially for assessing epilayer thickness, strain, disorder, and site selectivity of defects.e RSS method is particularly suited for probing the local atomic and/or nanoscale structural changes in the InSb ultrathin lms grown on SI GaAs while making the careful analysis of its subtle spectral variations.At the earlier time, we had quickly measured RSS on our InSb samples using a Raman spectrometer with the excitation from a HeNe laser 633 nm line.Later on, after more years, we performed further Raman measurements using 514-nm laser excitation on these samples, with similar results obtained.To avoid duplication, these data were not presented here.

X-Ray Di raction Spectroscopy.
As a common technique for material characterization, the XRD is widely used to evaluate the quality of crystal structure.It is sensitive for estimating stress/strains in epitaxially grown thin lms.Furthermore, the peak position and the full width at half maximum (FWHM) of X-ray di raction spectra give the information of crystal orientation and crystal quality.e di ractometer we used in experiments is Rigaku MiniFlex 600, Japan.e ve InSb samples were measured by this X-ray di ractometer using a Cu Kα radiation (λ 1.5406 Å).

Synchrotron Radiation X-Ray Absorption Spectroscopy.
XAFS spectra were collected for the InSb thin lm samples in X-ray uorescence yield mode at the beam line 01C1 of the National Synchrotron Radiation Research Center in Hsinchu, Taiwan.e photon energy for the XAFS covered the range from 27,640 to 28,840 eV. e intensity of X-ray was monitored by a liquid N 2 -lled ionization chamber, and uorescence emitted from the sample was measured by an argon-lled Stern-Held-Lytle detector.A Si (111) doublecrystal monochromater with a 0.5-mm entrance slit was used.A lter was inserted between the sample and the detector window to reduce the noise from scattering and to improve the spectrum quality.e incident photon direction was 45 degree to surface of the sample, and the ux I 0 of incident photon was monitored simultaneously by an ion chamber located just before the sample chamber.All measurements were made at RT, and all X-ray absorption spectra were normalized to I 0 .

Spectroscopic Ellipsometry at Room Temperature.
e measured SE spectra are tted empirically by minimizing the squared di erences between the observed and calculated Ψ and Δ values, which generated from the tting model at the corresponding wavelengths.e quality of the t can be judged by the mean square error (MSE) de ned as follows: where n stands for the total number of experimental measurements of Ψ and Δ for each chosen wavelength, m represents the number of tting parameters used, the superscripts "mod" and "exp" represent the appropriate values of model and experimental data, and σ signi es the MSE between the calculated and experiment data at each wavelength.
In this study, the SE data were tted using Tauc-Lorentz multiple oscillator modes [48]: where E is photon transition energy, A and C are tting constants, and estimates are obtained by tting Ψ and ∆.Equation ( 2) is useful for evaluating the dielectric function ε In (3), the term P is the main part of the Cauchy integral, where ε 1 (∞) is added as a tting parameter.
Figure 1 shows experimental SE Ψ and ∆ spectra of ve InSb/GaAs samples, measured with three incident angels at room temperature (RT).e spectral oscillations of the ve samples are similar, indicating that they have similar optical properties.All of the Ψ and ∆ data with variable angles were considered in the calculation of the optical parameters like n and k.
e thicknesses of InSb lms were extracted through analyzing the SE data.
e three-layer model, substrate/ lm/oxidation, or surface roughness was rst tested for modeling.
e simulation results showed the surface oxidation layer with the thickness of 0.01 nm or less for all sample.is is physically meaningless, that is, the thickness of the surface oxidation layer should be zero.erefore, the two-layer model, substrate/ lm, was used for modeling on ve InSb/GaAs samples here.Fitting results are listed in Table 1.

e X-Ray Di raction Data and Fits.
Figure 2 shows the wide-scan XRD spectrum of a typical InSb/GaAs sample (S2) in the angle range (2θ) from 55 °to 69 °at 0.1 °step.e intensity of the GaAs (400) peak was observed to be weaker than that of the InSb (400) peak. is phenomenon was due to that the semi-insulating GaAs wafer cut 2-4 °o (100) towards <110> was used as the substrate.e surface of GaAs wafer was deviated from the real (100) orientation.e GaAs wafer was not the real GaAs (100) substrate or not with the real (100) orientation surface. is type of semi-insulating GaAs (100) 2 °o towards <110> substrate was successfully used for the growth of InSb thin lms [47,49].During the growth process, it could lead the heteromismatch-induced dislocations to spread along the InSb/GaAs interface plane but not along the lm growth normal direction, that is, not to the InSb lm surface, so as to greatly decrease the dislocation density at the surface area of InSb lm.
From this rough scan, the InSb (400) peak appeared as a single peak with the FWHM of 0.18 °.Inset of Figure 2 shows ne-scan XRD spectrum in the angle (2θ) of 56 °-58 °at 0.005 °step.e observation of the peak doublet splitting is caused due to the X-ray source Cu Kα 1 and Cu Kα 2 radiation.e well-separation of the InSb (400) peak doublet and the narrow FWHM of 0.07 °from the InSb (400) Kα 1 peak indicated good crystallinity of the lm.A sharp diffraction peak was observed at 2θ 56.88 °, which corresponds to the (400) crystalline plane of InSb (S2).e other diffraction peak located at 66.20 °can indexed to the (400) crystal plane of GaAs substrate.Table 2 lists the values of   Advances in Materials Science and Engineering XRD peak and FWHM obtained by Lorentz tting, for ve InSb samples with di erent V/III ratios.
Farag et al. [51] calculated the crystallite size (D) of the InSb lm grown on GaAs from the broadening of few XRD peaks using the Debye-Scherrer equation (4): where λ is the X-ray wavelength of Cu Kα (0.15418 nm), β is the width of the peak at half maximum intensity for the thin lm, K S is the Scherrer constant of the order of unity (0.95 for powder and 0.89 for lm), and θ is the corresponding Bragg's angle.
Usually, nite size gives broader peaks.e contribution of our InSb lm thickness, D, to the XRD broad peak width, β, can be estimated by modi ed Debye-Scherrer formula (5), e so-calculated β values using the InSb lm thickness obtained from SE are also listed in Table 2.It is seen that these β values are much larger than the XRD FWHMs measured in our experiments.is predicts the high crystalline quality of our MOCVD grown InSb lms on GaAs.

Simulations of SR-XAS Data.
Indium K-edge extended X-ray absorption ne structure (EXAFS) was employed to study the local structure of InSb thin lms on GaAs. Figure 3 plots the four In K-edges EXAFS of InSb, with all of In absorption edges near 27,940 eV.Replicate EXAFS scans were coadded to improve the signal-to-noise ratio.Due to numerous di raction peaks in the In-EXAFS spectra, the samples were spun to average out the di raction peaks in the EXAFS spectra.All EXAFS spectra were collected to 1,200 eV beyond the In K-edges.All EXAFS data were analyzed for wave vectors (k) from 3.0 Å−1 to 11.5 Å−1 .e chi data were k 2 χ (k) weighted as shown in Figure 4 and Fourier transformed with a window width of k 0.5 Å−1 to yield the R-space data.As shown in Figure 5, the simulated EXAFS spectra were generated, based on the documented crystallographic properties for In and Sb using ab initio-based theory.
We used ATHENA program to remove the background and extract the EXAFS oscillations from the k-space signals.
To extract the bond length of R In-Sb , a structural model was built using the package IFEFFIT.Fitting results of the Fourier transforms show a good agreement with the measurement shown in Figure 5.With the rst neighbor peak being well de ned in the In absorption, we extracted the R In−Sb from the In data listed in Table 3. e EXAFS tting for ve InSb/GaAs gave bond length values around 2.80 Å, and the sample S5 with the highest V/III ratio of 5.38 has its R In−Sb slightly less than 2.80 Å. e coordination number (CN) for all samples is close to but slightly below 4 (the regular value) with samples S4 and S5 nearest to 3.96-3.98,samples S2 and S3 at 3.92, and only sample S1 with the lowest V/III ratio of 3.91 signi cantly lower at 3.80.ese variations could be caused by residual strain and too low/too high might cause higher lattice mismatch in the InSb/GaAs heterointerface, leading to higher residual strain within the epitaxial InSb lm. is nding suggests that V/III ratios of 4.20 (S3) and 4.78 (S4) are the better growth parameter values, which is in consistent with the results from Raman scattering.

Further Analyses and Temperature-Dependent SE.
e real part ε 1 and imaginary part ε 2 of the dielectric function of the bulk InSb and bulk GaAs as function of photon energy is shown in Figure 6.
e peaks in these spectra re ect the high-energy-state transitions of bulk InSb and can be used to obtain several related critical energy points (E 1 , E 1 + Δ 1 , E ′ 0 , E 2 , and E ′ 1 ) [52]. e high-energystate transitions of bulk GaAs are E 0 + W 0 , E 1 , E ′ 0 , and E 2 [52].erefore, Figure 6 shows the peaks those are the highenergy-state transitions of the InSb lm and none of GaAs substrate.D'Costa et al. [15] reported the SE study at room temperature for an InSb/GaAs grown by molecular beam epitaxy, showing similar critical energy points from InSb.
From the simulation of the SE Ψ and Δ data in Figure 1, the re ective index (n) and extinction coe cient (k) versus wavelength or energy can be extracted.Figures 7(a) and 7(b) exhibit the comparative n and k variation with energy (eV), for ve InSb/GaAs, respectively.e critical energy points (E 1 , E 1 + Δ 1 , E ′ 0 , E 2 , and E ′ 1 ) are marked as discussed in last paragraph.
Temperature-dependent (TD) SE measurements were performed in the temperature range from 20 °C to 300 °C. Figure 8 presents a set of TD-SE experimental data for sample S2, which were measured at three di erent angles, 55 °, 60 °, and 65 °.Simulation ts were performed for all SE Ψ and Δ data.As mentioned in Section 3.1 for RT-SE, the three-layer model-substrate/ lm/surface oxide-was rst used for modeling.e best tting simulation results showed the surface oxide layer with the thickness of 0.01 nm or less, that is, zero, for all SE data measured from 20 °C to 250 °C.However, for the SE spectra (Ψ and Δ) data measured at 300 °C, the three-layer model-substrate/ lm/surface oxide-has to be used, resulting in a surface oxide thickness of 5.4 nm for sample S2.Other samples had similar results, that is, SE data between RT and 250 °C showed no surface oxide layer but SE at 300 °C revealed a near 5-nm surface oxide layer, corresponding to about two atomic layer of indium oxides.
Figure 9 shows the variation of refractive index n and extinction coe cient k of InSb lms in a temperature range from 20 °C to 300 °C.As seen, the InSb lm contains several absorption peaks in the wavelength range from 200 to Energy (eV)   Advances in Materials Science and Engineering 1500 nm.Some absorption peaks shift to the longer wavelength (redshift) with increasing temperature.Kim et al. [53] performed TD-SE on bulk InSb over a temperature of 31 K to 675 K.A similar redshift was observed in our experimental InSb lms on GaAs over the temperature range of 20 °C to 300 °C, that is, 293 K to 573 K, comparable with that observed in other study [53].Figure 10 exhibits the complex dielectric function, real and imaging parts, ε 1 and ε 2 , calculated by ε 1 n 2 − k 2 and ε 2 2nk, of InSb lm S2 with di erent temperatures.e high-energy critical points of E 1 , E 1 + Δ 1 , E ′ 0 , E 2 , and E ′ 1 are indicated in the gure.e gradual redshifts of these critical energy peaks with temperature increasing from 20 °C to 250 °C are seen.However, these redshifts of critical energy peaks in Figure 10 and n and k spectra in Figure 9 become much more pronounced as temperature increases to °C.
ese large variations in n and k and ε 1 and ε 2 are due to the formation of an InO surface layer from the severe oxidation of the surface of InSb lm at 300 °C. is oxidation could be ignored for InSb lm measured in 20 °C-250 °C because the SE ts with three-layer model leaded to an InO surface thickness of <0.01 nm, much thinner than one monoatomic layer.But the oxide layer cannot be ignored in sample heated at 300 °C, that is, higher than 250 °C, the severe oxidation

Conclusions
In summary, a series of InSb thin lms grown on 4-inch GaAs substrates by MOCVD technique, with di erent V/III ratios, were investigated by SE, XRD, and SR-XAS, respectively.InSb thin lm thickness was extracted by tting the experimental SE data.e crystallinity of the lms was high quality extracted from the well separation of the InSb (400) peak doublet and the narrow FWHM of the InSb (400) Kα 1 peak in the XRD spectrum.rough advanced synchrotron radiation technique of XAS and data simulation, the atomic scale bonding length and coordination number were obtained.
rough combined multiple technological analyses, the results showed that InSb lms on GaAs grown under too high or too low V/III ratios are with poor quality, while those grown with proper V/III ratios of 4.20 and 4.78 possess the high crystalline quality.
ese results are useful to the material growers for improving the growth processing.
e temperature-dependent (TD)-SE measurements (20-300 °C) and simulation revealed a signi cant phenomenon: the SE spectra, optical constants (n, k, ε 1 , ε 2 ), and critical energy points (E 1 , E 1 + Δ 1 , E ′ 0 , E 2 , and E ′ 1 ) of the InSb thin lms varied with temperature (T) gradually and smoothly as T increased from 20 °C to 250 °C, and all SE spectra can be tted well by the two-layer model (substrate/ lm).However, up to 300 °C, the SE spectra appeared to show large changes, and simulation revealed the existence of an indium-oxide (InO) layer of ∼5.4 nm, that is, about two atomic layers.e optical constants (n, k, ε 1 , ε 2 ) and critical energy points (E 1 , E 1 + Δ 1 , E ′ 0 , E 2 , and E ′ 1 ) had showed transilient changes from 250 °C to 300 °C, which are due to the top about two atomic layers oxidized.is indicates the high temperature limitation for the use of InSb/GaAs materials, up to 250 °C, which provides a hinder to the device designers using InSb materials.

Data Availability
All types of data used to support the ndings of this study are included within the article.Advances in Materials Science and Engineering

Figure 1 :
Figure 1: Experimental SE data and model tting for ve MOCVD grown InSb/GaAs samples.

Figure 2 :
Figure 2: XRD spectrum and Lorentz t of S2 in the angle range of 55 °-69 °.

Figure 4 :
Figure 4: e In K-edge oscillation k 2 χ (k) for all the measured InSb thin lm samples.

Figure 5 :
Figure 5: e magnitude of the Fourier transforms of measured samples (black lines) and the results of tting (red lines).

Figure 6 :
Figure 6: e dielectric functions of the bulk InSb and bulk GaAs.

Figure 7 :
Figure 7: (a) Comparative re ective index (n) and (b) comparative extinction coe cient (k) for ve InSb/GaAs samples, deduced from SE Ψ and Δ data in Figure 1.

Figure 8 :
Figure 8: SE experimental data and model tting for S2 at di erent temperatures.

Figure 9 :
Figure 9: e refraction index n (a) and the extinction coe cient k (b) of InSb thin lm (S2) with varied temperatures of 20 °C-300 °C.

Figure 10 :
Figure 10: e real and imaginary part of the dielectric function ε of the InSb thin lm (S2) at varied temperatures of 20 °C-300 °C.

Table 1 :
icknesses and MSE of InSb thin film samples from SE.

Table 2 :
e tting and calculation results from XRD of ve InSb samples with di erent V/III ratios.
(or n, k). e real part of the dielectric function ε 1 is obtained by exploiting the Kramers-Kronig integrations, that is,

Table 3 :
e tting results of the EXAFS data for rst coordination shell atom around In atom.