Properties Of InxGa1 Research Articles

Evolution of stratification in high-alloy content InGaN epilayers grown on (0001) AlN

ABSTRACTThe structural properties of InxGa1−xN epilayers, deposited on (0001) AlN templates by plasma-assisted molecular beam epitaxy, were studied by transmission electron microscopy and Raman spectroscopy, as a function of growth temperature. Single phase films with high indium content and well-ordered heteroepitaxial interfaces were attained at lower temperatures. Delayed plastic relaxation resulted in the structural stratification of high-temperature films due to the compositional pulling. Such films relaxed by a stacking fault mechanism, contrary to low temperature ones that exhibited relaxation by misfit dislocations. Despite the higher defect content of the former, their phonon mean free path was also higher, showing that alloying-induced fluctuations of the periodic potential constitute a more critical parameter. Cubic interfacial zones were suppressed at lower growth temperatures.This is part of a thematic issue on Nanoscale Materials Characterisation and Modeling by Advances Microscopy Methods - EUROMAT.

Composition and surface deformation variance in highly strained InxGa1−xSb structures on (100) GaSb

InxGa1−xSb is a ternary semiconductor material that offers excellent electronic properties as well as a widely tunable bandgap range (1.7–7.3 μm). However, because of the potentially large lattice mismatch between InxGa1−xSb and GaSb (up to ∼6%), it is inherently difficult to produce large area, high-quality, defect-free InxGa1−xSb epilayers. Studying crystal deformation processes that ultimately enable gliding dislocations in InxGa1−xSb epilayers, as well as the morphologies that result from these processes, is critical for controlling quantum properties in InxGa1−xSb devices. In this study, InxGa1−xSb nanostructures were produced by a solid-source molecular beam epitaxy on undoped GaSb (100) substrates and were examined using various techniques including scanning electron microscopy, energy dispersive spectroscopy, and (micro) Raman spectroscopy. Characterization data demonstrates that with increasing lattice mismatch (compressive strain), there are two distinct regions across the sample, specifically along the 〈110〉 dislocation direction: those with and without epilayer strain. Both regions can be exploited and exhibit high-quality single crystal material, but the strained regions also consist of a wetting layer, strained alloys, and clusters. Epilayer strain, lateral compositional gradients, and biaxial stress were analyzed as a function of Raman shift in these layers and revealed dependencies on the influence of dislocation slip planes.

Analysis of InxGa1−xN/Si p-n heterojunction solar cells and the effects of spontaneous and piezoelectric polarization charges

The band structures, current-voltage characteristics under solar illumination, and photovoltaic (PV) properties of InxGa1−xN/Si p-n heterojunction solar cells (SCs), as well as the effects of spontaneous and piezoelectric polarization (Psp-PPZ) induced charges are investigated theoretically and numerically. We find that the energy peaks on the conduction and valence bands could exponentially reduce the diffusion currents and photocurrents, thus profoundly affect the PV properties of the SCs. Except for large values, the Psp-PPZ induced interface charges have little influence on the band structures and the PV properties. These results should be useful in analysis and design for multijunction tandem InxGa1−xN/Si SC devices.

Composition-dependent structural, optical and electrical properties of In x Ga1−x N (0.5 ≤ x ≤ 0.93) thin films grown by modified activated reactive evaporation

In this report, we have studied the compositional dependence of structural, optical and electrical properties of polycrystalline InxGa1−xN thin films grown by modified activated reactive evaporation. The growth was monitored by optical emission spectroscopy. The thickness of the films was in the range ~600–800 nm. The phase, crystallinity and composition of the films were determined by X-ray diffraction, Raman spectroscopy and energy dispersive X-ray analysis. The surface morphology was studied by atomic force microscopy. The band gaps of these films obtained from transmittance and photoluminescence measurements were found to vary from 1.88 to 3.22 eV. All the films show n-type conductivity. The carrier concentration was found to be decreasing with increase in gallium incorporation which is in good agreement with the free carrier absorption observed in transmittance spectra.

Properties of InxGa1−xN films in terahertz range

In this letter, we report the characterization of the refractive indices and complex conductivities of a set of GaN films with different carrier concentrations, InN film, and InxGa1−xN films with indium content varying from x = 0.07 to x = 0.14 grown by metalorganic chemical vapor deposition for frequencies ranging from 0.3 to 3 THz using terahertz time-domain spectroscopy (THz-TDS). The refractive indices of InxGa1−xN films at THz range are reported. The carrier density and mobility determined using THz-TDS method show good agreement with four-probe Hall measurements.

Field dependence of barrier heights and luminescence properties in polar and nonpolar InGaN/GaN single quantum wells

The external field dependence of barrier heights and the internal field dependence of luminescence properties in InxGa1−xN/GaN single quantum wells (SQWs) with polar (x=0.13) and nonpolar (x=0.15) orientations were investigated. The conduction band offset of a SQW was characterized by using deep level transient spectroscopy. At a reverse bias of −3 V, the barrier height of the nonpolar SQW was estimated to be 0.42 eV, which is smaller than the 0.60 eV seen in the polar SQW due to the absence of internal fields along the nonpolar direction. Both samples showed a redshift of barrier heights with increasing reverse bias. The carrier recombination affected by carrier localization, quantum-confined Stark effect, and Varshni’s shift was analyzed through temperature-dependent photoluminescence. Numerical simulations of the barrier heights and internal fields showed good agreement with experimental results.

Molecular simulation study of the structural properties in InxGa1−xAs alloys: Comparison between Valence Force Field and Tersoff potential models

The thermodynamic and structural properties of compound semiconductor alloys have been generally modelled using either the Valence Force Field model or the Tersoff potential model. This work compares the properties, such as lattice constant and bond length, of the InxGa1−xAs alloy as predicted by Monte Carlo simulations in the semigrand isothermal isobaric ensemble using both the potential models, with experimental data. The lattice constants are expected to follow the Vegard’s law at any given temperature. Valence Force Field model predicts bond length data which follows the experimentally determined values at 300K; whereas the Tersoff model forecasts that the virtual crystal approximation will be followed. The VFF model, with its experimentally determined parameters, is found to be better for modelling the alloy at room temperature. The Tersoff model, with its fitted parameters, on the other hand predicts the effect of temperature on the microscopic structure of the alloy better. The parameters of the Tersoff potential characterizing the In–Ga interactions can be further improved to predict bond lengths more accurately.

Thermoelectric properties of InxGa1−xN alloys

Thermoelectric (TE) properties of InxGa1−xN alloys grown by metal organic chemical vapor deposition have been investigated. It was found that as indium concentration increases, the thermal conductivity decreases and power factor increases, which leads to an increase in the TE figure of merit (ZT). The value of ZT was found to be 0.08 at 300K and reached 0.23 at 450K for In0.36Ga0.64N alloy, which is comparable to those of SiGe based alloys. The results indicate that InGaN alloys could be potentially important TE materials for many applications, especially for prolonged TE device operation at high temperatures, such as for recovery of waste heat from automobile, aircrafts, and power plants due to their superior physical properties, including the ability of operating at high temperature/high power conditions, high mechanical strength and stability, and radiation hardness.

Electron transport anisotropies in pseudomorphic InGaAs channel materials and their structural origin

AbstractMeasurements of the electron transport properties in Inx Ga1−x As (x = 0.2 and x = 0.3) channel heterostructures have consistently shown the transport mobility to be higher in the [01&1macr;] direction compared to [011] on a (100) wafer. Low temperature (1.6 K) measurements of these materials show that the mobility difference can be as large as 30% whilst the differences in carrier concentrations for the two directions are negligible in comparison. The mobility anisotropy decreases to below 5% at room temperature as the effects of phonon scattering, which is isotropic, begin to dominate. The anisotropy in the quantum mobility is much less pronounced than that in the transport mobility. This suggests that the low mobility in the [011] direction is primarily a result of large angle scattering. Transmission electron microscopy (TEM) analysis was performed on cross‐sections of material in the two directions. Thickness fluctuations in the channel were observed whilst traversing the channel of a (01&1macr;) section in the [011] direction where as none were observed in (011) sections along the [01&1macr;] direction. The length scale of the fluctuations was variable but typically around 50–100 nm. Transport simulations reveal that a 1‐dimensional potential modulation with a wavelength similar to that of the fluctuations produces a mobility anisotropy with the observed sign. Therefore it is likely that the thickness variations, possibly combined with synchronous fluctuations in the indium concentration along the [011] direction, are responsible for the reduced mobility in [011] transport. Although to the best of our knowledge these thickness fluctuations are reported here for the first time, similar growth anisotropies are well known for Inx Ga1−x As layers deposited on InP. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Investigation of graded InxGa1−xP buffer by Raman scattering method

The compositional changes of InxGa1−xP graded buffer inserted between GaP substrate and subsequently grown In0.36Ga0.64P homojunction LED structure were investigated by Raman spectroscopy. The indium content of InxGa1−xP interlayers was increased in eight steps with thickness of 300nm and constant compositional change ΔxIn between the steps. The properties of InxGa1−xP graded buffer along the structure cross-section have been studied by Raman back scattering method and the changes in GaP LO and TO phonons were investigated. Raman shift of 13cm−1 in GaP-like LO1 phonon was measured on beveled [100]surface for compositional change of InxGa1−xP layer in the range of 0<xIn<0.32. The measurements on the cleaved edge of the sample in [011] direction revealed a strong TO phonon at 366cm−1 and weak LO phonon peak at 405cm−1 in GaP substrate. By reaching the graded InxGa1−xP region the intensity of TO phonon decreases and appearance of considerable TO1 phonon shift up to 350cm−1 for In content xIn=0.16was observed. For upper graded layers with xIn from 0.16 to 0.24 the position of GaP-like TO1 was constant and can be ascribed to relaxation of lattice mismatched thin InxGa1−xP graded upper layers in the structure.

Study of structural properties of InxGa1−x As/InyAl1−y As heterosystems on InP substrates

The structural properties of InxGa1−x As/InyAl1−y As samples on InP substrates are studied as functions of growth conditions by the method of high-resolution diffractometry. The results obtained and the photoluminescence spectroscopy data are used to optimize the technology of preparation of high-quality heterostructures with sharp interfaces. The parameters of the two-dimensional electron gas of such heterostructures measured at 77 and 300 K are comparable with the best world standards in this field, so these heterostructures may be used to manufacture transistors and integral amplifiers operating at the frequency 40 GHz and even higher.

In-plane optical anisotropy in InxGa1−xN∕GaN multiple quantum wells induced by Pockels effect

We have investigated the crystal orientation dependence of optical properties in InxGa1−xN∕GaN multiple quantum wells. The spectral peaks and intensity of the microphotoluminescence signal for different crystal orientations were found to have six-fold symmetry. Quite interestingly, the refractive index, obtained from the interference pattern, also varies with the crystal orientation. The 60° periodic anisotropy of electronic transitions as well as optical parameters was interpreted in terms of the Pockels effect induced by the strong built-in field in nitride heterojunctions. The linear dependence of the change of the refractive index on electric field is consistent with the prediction of the Pockels effect. Our result provides an alternative solution to improve the designs of photonic and electronic devices based on nitride semiconductors.

Photoluminescence analysis of Te-doped In0.14Ga0.86As0.13Sb0.87 epitaxial layers as a function of temperature

The influence of tellurium doping on the optical properties of InxGa1–xAsySb1–y epitaxial layers has been studied by photoluminescence (PL) spectroscopy. PL was carried out by exciting the sample with the 488-nm line of an Ar-ion laser, changing the exciting power in the range between 40–200 mW and varying the temperature in the range 15–300 K. For the low-doped sample the PL spectrum showed a narrow exciton-related peak at 648.6 meV with a full width at half maximum (FWHM) of about 7 meV, which is an evidence of the good crystalline quality of the layers. For higher Te doping, the PL spectra showed the presence of band-to-band and donor-to-acceptor transitions which overlap as the Te concentration increases. The peak of the PL band shifts to higher energies as Te doping increases due to a band-filling effect as the Fermi level enters into the conduction band. The temperature dependence of the PL spectra for those samples with higher Te doping show a broad band at intermediate energies which disappear for temperatures above 80 K which is a typical behavior for a transition associated to DX deep defects. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Exact determination of Indium incorporation in (In x Ga1– x N/GaN)-multiple quantum well structures by X-ray diffraction and -reflectivity and its impact on optical properties

Abstract We studied the structural and optical properties of In x Ga1– x N/GaN multiple quantum wells (MQWs) on Si(111) grown by metal organic vapor phase epitaxy by means of photoluminescence, X-ray reflectometry (XRR), and high resolution X-ray diffractometry (HRXRD). The combination of both X-ray analysis methods allows a very precise determination of the structural parameters of the layer systems. In particular, the Indium incorporation and the well thickness of the optically active layers were investigated. Thus, in dependence on crystallographic quality and thicknesses, their error bars of determined values could be reduced to an accuracy of lower than 0.2 nm of well thickness and Δx = 0.02 of Indium proportion. By growing systematic series with various MOVPE deposition temperatures and times the correlations to the optical properties were determined.

In‐Plane Optical Anisotropy in InxGa1—xN/GaN Multiple Quantum Wells

We have investigated the crystal orientation dependence of optical properties in InxGa1—xN/GaN multiple quantum wells. The spectral peak and intensity of the micro-photoluminescence emission for different crystal orientations were found to have sixfold symmetry. Quite interestingly, the refractive index obtained from the interference pattern, also varies with the crystal orientation. The 60 degree periodic anisotropy of electronic transitions as well as optical parameters were interpreted in terms of the formation of hexagonal truncated pyramidal In-rich clusters. Our result provides an alternative solution to improve the designs of photonic and electronic devices based on nitride semiconductors.

Temperature Dependence of Optical Absorption Spectra in In x Ga 1— x N Epitaxial Layers

Optical properties of InxGa1—xN epitaxial layers with indium compositions of x = 0.02, 0.05, and 0.09 have been studied by means of optical absorption (OA) and photoluminescence (PL) spectroscopy. The clear peak due to the absorption of InxGa1—xN ternary alloys was observed up to 300 K for the first time. As a result, temperature dependence of Stokes shift was investigated. The Stokes shift was estimated to be 51 meV for the sample with x = 0.05. With increasing temperature up to about 50 K, the Stokes shift slightly increased. With further increasing temperature from 50 to 100 K, the Stokes shift conversely decreased. Above 100 K, the Stokes shift was independent of temperature and indicated almost a constant value up to 300 K. The Stokes shift at 300 K was estimated to be 44 meV. This temperature dependence of Stokes shift was a characteristic common to all of the samples used in the present work.

Structural and optical investigation of InGaN/GaN multiple quantum well structures with various indium compositions

We have studied the influence of indium (In) composition on the structural and optical properties of Inx Ga1−xN/GaN multiple quantum wells (MQWs) with In compositions of more than 25% by means of high-resolution x-ray diffraction (HRXRD), photoluminescence (PL), and transmission electron microscopy (TEM). With increasing the In composition, structural quality deterioration is observed from the broadening of the full width athalf maximum of the HRXRD superlattice peak, the broad multiple emission peaks oflow temperature PL, and the increase of defect density in GaN capping layers and InGaN/GaN MQWs. V-defects, dislocations, and two types of tetragonal shape defects are observed within the MQW with 33% In composition by high resolution TEM. In addition, we found that V-defects result in different growth rates of the GaN barriers according to the degree of the bending of InGaN well layers, which changes the period thickness of the superlattice and might be the source of the multiple emission peaks observed in the InxGa1−xN/GaN MQWs with high in compositions.

Nonlinear properties in InxGa1 − xAs/GaAs multiple quantum well laser structures

In this paper we investigated nonlinear properties and lasing in InxGa1−xAs/GaAs multiple quantum wells grown by metal-organic chemical vapour deposition. A systematic study, performed by high excitation photoluminescence measurements as a function of excitation intensity, allowed us to identify the minimum well width for observing stimulated emission from well states. We also determined the threshold for stimulated emission for well and barrier lasing. Radiative recombination energies are identified by using theoretical data obtained in the effective mass approximation, including boundary conditions and strain.

Comparison Study of Structural and Optical Properties of InxGa1-xN/GaN Quantum Wells with Different in Compositions

AbstractThe effect of In on the structural and optical properties of InxGa1−xN/GaN multiple quantum wells (MQWs) was investigated. These were five-period MQWs grown on sapphire by metalorganic chemical vapor deposition. Increasing the In composition caused broadening of the high-resolution x-ray diffraction superlattice satellite peak and the photoluminescence-excitation bandedge. This indicates that the higher In content degrades the interface quality because of nonuniform In incorporation into the GaN layer. However, the samples with higher In compositions have lower room temperature (RT) stimulated (SE) threshold densities and lower nonradiative recombination rates. The lower RT SE threshold densities of the higher In samples show that the suppression of nonradiative recombination by In overcomes the drawback of greater interface imperfection.

Microphotoluminescence characterization of cleaved edge overgrowth T-shaped InxGa1−xAs quantum wires

Microphotoluminescence (micro-PL) characterization was performed for T-shaped InxGa1−xAs quantum wires (T-QWRs) fabricated by the cleaved edge overgrowth method with molecular beam epitaxy. The spatial distribution of optical properties in InxGa1−xAs T-QWRs was examined by means of PL intensity imaging and scanning micro-PL spectroscopy with about 1 μm spatial resolution. In the successfully fabricated 3.5-nm-scale In0.17Ga0.83As T-QWRs with Al0.3Ga0.7As barriers, uniform PL image and PL spectra from QWRs along the wire direction were observed, which indicates spatially uniform and high quality QWRs were formed. The effective lateral confinement energy of one-dimensional excitons was found to be 34 meV, showing the usefulness of InxGa1−xAs as a well material in T-QWR structures. On the other hand, in the unsuccessfully fabricated 4-nm-scale In0.09Ga0.91As T-QWRs with Al0.3Ga0.7As barriers, PL image and spectra were inhomogeneous. These results demonstrate the importance of flat cleaved surface and optimized overgrowth condition to fabricate uniform T-QWRs.