InxGa1-xN Alloys Research Articles

Optical band gap energy values in wurtzite InxGa1-xN

The narrow bandgap in InN has been known as a notorious example of local density approximation or generalized gradient approximation (LDA or GGA) calculations to give a metallic state. Various density functional methods are applied to optimize the atomic structures of the systems. These numerical results are used as the input values for the subsequent GW calculations, which can be applied to estimate the band gap value without phenomenological parameters. It is found that LDA with GW0 or the hybrid functional with self-consistent GW0 approximation provides sufficient theoretical results for both of the investigated compounds of GaN and InN. Although they are still time-consuming, due to less computational cost the former method is selected as a trial to compute the electronic structure in the entire range in ternary InxGa1-xN alloys without any arbitrary parameters. The present theoretical studies in ternary InxGa1-xN alloy were carried out by LDA with GW0. As a result, a good agreement between theoretical and experimental results is obtained, and it is also shown that zone bending could be well-approximated using a quadratic function with a constant, independent of x, parameter equal to 1.85 eV, which is close to the recent experimental results.

Behaviour of Raman B1 (high) mode and evaluation of crystalline quality in the InxGa1–xN alloys grown by RF-MBE

InxGa1–xN ternary alloys are very promising for a variety of applications. However, high-quality growth of InxGa1–xN alloys, particularly in the intermediate In composition range, is very difficult. This study reports on a systematic analysis of the Raman spectra from the InxGa1–xN alloys grown by radio-frequency molecular beam epitaxy (RF-MBE) for the whole In compositional range, particularly in the intermediate range of In composition. The B1 (high) mode, which is inherently Raman inactive is observed for the InxGa1–xN alloys grown by RF-MBE. The behaviour of Raman inactive B1 (high) mode is studied for the evaluation of InxGa1–xN quality, which is found to vary with the In composition and the temperature of growth. The crystallinity of the InxGa1–xN alloys can be assessed using B1 (high) mode’s relative signal intensity and full-width at half-maximum, which are well agreed with the reflection high energy electron diffraction and X-ray diffraction analyses. The optimum growth temperature for the InxGa1–xN alloys grown by RF-MBE in the intermediate range of In composition is also discussed.

High nitrogen flux plasma-assisted molecular beam epitaxy growth of InxGa1-xN films

Growth of efficient III-N based light emitting devices by plasma assisted molecular beam epitaxy has been elusive, even though the technique has attractive advantages in comparison to metal organic chemical vapor deposition. Modern high-flux radio frequency plasma systems could remedy this issue by enabling growth of InxGa1-xN at higher temperatures than previously possible, likely improving the material quality. In this work, active nitrogen fluxes of up to 3.5 μm/h GaN-equivalent growth rate were employed to grow InxGa1-xN alloys. InxGa1-xN growth rates of 1.3 μm/h were demonstrated at growth temperatures of 550 °C and 600 °C with maximum film compositions of In0.25Ga0.75N and In0.21Ga0.79N, respectively. A composition of In0.05Ga0.95N was observed in a film grown at 700 °C with smooth step-terrace morphology.

Decorated Dislocations against Phonon Propagation for Thermal Management

The impact of decorated dislocations on the effective thermal conductivity of GaN is investigated by means of equilibrium molecular dynamics simulations via the Green-Kubo approach. The formation of "nanowires" by a few atoms of In in the core of dislocations in wurtzite GaN is found to affect the thermal properties of the material, as it leads to a significant decrease of the thermal conductivity, along with an enhancement of its anisotropic character. The thermal conductivity of In-decorated dislocations is compared to the ones of pristine GaN, InN, and random and ordered InxGa1-xN alloy, to examine the impact of doping. Results are explained by the stress maps, the bonding properties and the phonon density of states of the aforementioned systems. The decorated dislocations engineering is a novel way to tune, among other transport properties, the effective thermal conductivity of materials at the nanoscale, which can lead to the manufacturing of interesting candidates for thermoelectric or anisotropic thermal dissipation devices.

RF-MBE and MOVPE InxGa1-xN films over the entire composition range: A study on growth method dependence

Abstract This paper reports the influence of different growth methods on the InxGa1-xN ternary alloys over the entire composition range. We have successfully fabricated the single crystalline InxGa1-xN alloys without phase separation and metal indium (In) inclusion over the entire In composition region by the RF-MBE and MOVPE methods. The fluctuation of the c-axis is found smaller in the RF-MBE InxGa1-xN alloys while the fluctuation of a-axis is found smaller in the MOVPE InxGa1-xN alloys. We show that the Raman inactive B1 (High) mode has appeared in addition with the E2 (High) and A1 (LO) modes in this ternary alloy. The behavior of E2 (High) and B1 (High) modes of the InxGa1-xN alloys greatly depends on the composition and growth method. It is also found that the growth method dependence behavior of the FWHM of E2 (High) mode is similar to the a-axis fluctuation behavior in the XRC analysis. Our results spectacle that the local lattice distortion in the MOVPE grown InxGa1-xN alloys in the intermediate In composition region is very large compared with the RF-MBE samples. These findings are very important to better understand the fabrication of high-quality InxGa1-xN alloys aimed at the further development of InGaN-based high-performance optoelectronics and solar cells.

DFT Calculation on the Electronic Structure and Optical Properties of InxGa1-xN Alloy Semiconductors

Using the density functional theory (DFT) of the first principle and Generalized gradient approximation method, the electronic structures and optical properties of the InxGa1-xN crystals with different x (x = 0.25, 0.5, 0.75, 1) have been calculated in this paper. The influence of the electronic structure on the properties has been analyzed. Then the influence of doping quantity on the characteristics has been summarized, which also indicates the trend of complex dielectric function, absorption spectrum and transitivity. With the increase of x, the computational result shows that the optical band gap (i.e.Eg) of the InxGa1-xN crystal tends to be narrow, then the absorption spectrum shifts to the low-energy direction. And the Fermi energy slightly moves to the bottom of conduction band which would cause the growth of conductivity by increasing x. In a word, the InxGa1-xN compound can be achieved theoretically the adjustable Eg and photoelectric performance with x, which will be used in making various optoelectronic devices including solar cell and sensors.

Review—Defect-Tolerant Luminescent Properties of Low InN Mole Fraction InxGa1-xN Quantum Wells under the Presence of Polarization Fields

Different from the case of GaN or AlGaN alloys, the near-band-edge (NBE) emission of quantum wells (QWs) and even epilayers of InxGa1-xN alloys of low InN mole fractions (x) exhibits high quantum-efficiency (QE) against the presence of threading dislocations (TDs) as high as 109 cm−2. Accordingly InxGa1-xN alloys are exclusively used as an active region of green to ultraviolet light-emitting diodes (LEDs) and laser diodes, as well as the heart of white LEDs. Here, current understandings on the emission mechanisms of InxGa1-xN QWs, especially the defect-tolerant emission probability of localized excitons, are reviewed. There exist three disadvantages in obtaining high QE, namely a high density of TDs that cause the nonradiative recombination, kinetic immiscibility of In-containing alloys that introduces high-concentration Shockley-Read-Hall nonradiative recombination centers (NRCs) consisting of vacancy complexes, and high internal electric-fields across the QWs induced by the polarization discontinuities at heterointerfaces along the c-axis, which decrease the radiative recombination rate. The use of InxGa1-xN alloys of low x overcomes such disadvantages with the aid of In-originated localization effects that prevent excitons from trapping by TDs or NRCs. Simultaneous observation of short diffusion length and sufficiently long nonradiative recombination lifetime at room temperature indicates strong localization of holes or excitons.

Review—Recent Advances and Challenges in Indium Gallium Nitride (InxGa1-xN) Materials for Solid State Lighting

In recent times, the demand for electrical energy is increased to such an extent that the scientific research has to be focused on the development of materials that fulfil the growing demands of energy for efficient solid state lighting purposes and provide clean and green energy to mitigate the alarming effects of climate change. The ternary Indium Gallium Nitride (InxGa1-xN) alloys have emerged as the potential candidate for Solid State lighting as they inherent such attributes that make them capable for these applications. In this review, the attributes of InxGa1-xN alloys have been discussed. The dependence of bandgap and bowing parameter on the composition of InxGa1-xN alloys along with various techniques employed for the growth of these alloys in bulk and nanostructure forms have been reviewed. The recent advances in InxGa1-xN based nanostructures for Solid State lighting have also been extensively reviewed. The challenges that are to be overcome for potential use of InxGa1-xN alloys like phase segregation, unavailability of a suitable substrate, polarization and doping have been thoroughly highlighted. In the end, the conclusion and future scope of work on these wonderful classes of materials has been drawn.

Optimized valence force field model for the lattice properties of non-ideal III-nitride wurtzite materials

It has been discussed the optimization of the valence force field model for non-ideal III-nitrides wurtzite materials. To describe the static and dynamical lattice properties of nitrides within the same model, the anisotropy of the wurtzite structure and additional second-nearest-neighbor atomic interaction terms are included in the lattice strain energy expression. The sets of the force constants have been optimized through an iterative scheme in which calculated lattice properties are fitted to experimentally reported data. The model has been applied to the binary wurtzite compounds (GaN, InN) and random InxGa1-xN alloys. The calculated phonon dispersions, elastic constants and internal strain parameters of GaN and InN are in reasonable agreement with the experimental/ab initio values.

Raman spectroscopy of epitaxial InGaN/Si in the central composition range

InGaN alloys are of raising interest for many applications. The possibility of tuning their functional properties with the composition sets the importance of finding methods to characterize these materials in a fast and non-destructive way. Raman spectroscopy is one of these techniques, being able to yield information about the composition, strain, and other relevant parameters. However, the method of measuring the composition with a calibration of the maximum A1(LO) band is today limited to regions which are either rich in In or in Ga. The middle composition range still needs a calibration. By measuring the Raman spectra of different InxGa1-xN alloys grown epitaxially on Si and by comparing them with the results from X-ray diffraction, we investigated this missing region of compositions. Within the range of 30%–65%, we have found that the position of the maximum of A1(LO) scales with the In fraction x as ωx = 736 – 135x – 24x2 cm−1. With this calibration, it is possible to determine, by Raman spectroscopy, the composition of an unknown alloy with an uncertainty of 5%.

Theoretical study of structural, electronic and optical properties of InxGa1-xN alloys

In this work, structural, electronic and optical properties of InxGa1-xN are calculated by using full potential linearized augmented plane wave method. The calculated lattice parameters are found to be increased with the increase in In concentration and slightly deviating from the Vegard’s law. Band gaps are calculated by generalized gradient approximation (GGA) and modified Beck-Johnson (mBJ) GGA. The calculated band gaps are found to be decreased with the increase in In Concentration. The mBJ-GGA band gaps are very close to experimental values. Optical characteristics such as refractive index and dielectric constants are also calculated. The calculated parameters suggested that the materials are suitable for optoelectronics applications.

Tunable Band Gaps of InxGa1–xN Alloys: From Bulk to Two-Dimensional Limit

Using first-principles calculations combined with a semiempirical van der Waals dispersion correction, we have investigated structural parameters, mixing enthalpies, and band gaps of buckled and planar few-layer InxGa1–xN alloys. We predict that the band gaps of buckled InxGa1–xN alloys with hydrogen passivation can be tuned from 5.6 to 0.7 eV with preservation of direct band gap and well-defined Bloch character, making them promising candidate materials for future light-emitting applications. Unlike that in their bulk counterparts, the phase separation could be suppressed in these two-dimensional systems because of reduced geometrical constraints. The disordered planar thin films undergo severe lattice distortion, nearly losing the Bloch character for valence bands, whereas the ordered planar ones maintain the Bloch character yet with the highest mixing enthalpies.

The activation energy for Mg acceptor in the Ga-rich InGaN alloys

The activation energy for Mg acceptor in InxGa1-xN alloys is investigated. It is found that there are three factors to influence the activation energy for Mg acceptor. One is the stronger dependence of the VBM of InxGa1-xN depending on In content than that of the Mg acceptor energy level. The other is the concentration of Mg acceptors. Another is the extending of the valence band-tail states into the band gap. In addition, a model based on modifying the effective mass model is developed. It is found that the model can describe the activation energy for Mg acceptor in the Ga-rich InxGa1-xN alloys well after considering the influence of the valence band-tail states.

Investigation of phase separation in InGaN alloys by plasmon loss spectroscopy in a TEM

ABSTRACTPhase separation of InxGa1-xN alloys into Ga-rich and In-rich regions was observed by a number of research groups for samples grown with high indium content, x. Due to the radiation sensitivity of InGaN to beam damage by fast electrons, high-resolution imaging in transmission electron microscopy (TEM) or core-loss electron energy-loss spectroscopy (EELS) may lead to erroneous results. Low-loss EELS can yield spectra of the plasmon loss regions at much lower electron fluxes. Unfortunately, due to their delayed edge onset, the low energetic core losses of Ga and In partially overlap with the plasmon peaks, all of which shift with indium content.Here we demonstrate a method to quantify phase separation in InGaN thin films from the low-loss region in EELS by simultaneously fitting both plasmon and core losses over the energy range of 13-30eV. Phase separation is shown to lead to a broadening of the plasmon peak and the overlapping core losses, resulting in an unreliable determination of the indium concentration from analyzing the plasmon peak position alone if phase separation is present. For x=0.3 and x=0.59, the relative contributions of the binary compounds are negligibly small and indicate random alloys. For xnom.=0.62 we observed strong broadening, indicating phase separation.

Simulation of the EXAFS and Raman spectra of InxGa1-xN utilizing the equation of motion routine of FEFF8.

A combined analysis of EXAFS and Raman spectra is applied for the study of InxGa1-xN alloys with 0.3<x<0.5. Alloying causes relaxation of the selection rules resulting in Raman spectra that resemble the vibrational density of states. On the other hand, theoretical simulation of the Raman spectra using the Equation of Motion routine of FEFF8 provides the vibrational component of the Debye-Waller factor (DWF). The static disorder component of the DWFs was obtained by fitting the Ga and In K-edge EXAFS spectra. The analysis revealed that the nearest neighbor distances of the 1st and 2nd shell deviate from the values predicted by the law of Vegard and the virtual crystal approximation. The static disorder in the first nearest neighboring shell (In-N and Ga-N) is null whereas in the cation-cation neighboring shells the static component is generally smaller than the vibrational.

Effect of built-in-polarization field on relaxation time and mean free path of phonons in InxGa1-xN/GaN quantum well

The built-in-polarization field at the interface of InxGa1-xN/GaN quantum well enhances the elastic constant, phonon velocity and Debye temperature of InxGa1-xN alloy and their respective bowing constants. As a result, the phonon scattering processes in InxGa1-xN are modified. The combined phonon relaxation time and phonon mean free path has been computed for with and without built-in-polarization field for different indium (In) contents at room temperature. Our result shows that the built-in-polarization field suppresses the scattering mechanisms and enhances the phonon relaxation time and mean free path. The result is used to determine the effect of built-in-polarization field on electrical and thermal properties of InxGa1-xN.

Thermal expansion behaviors of epitaxial film for wurtzite GaN studied by using temperature-dependent Raman scattering

III-nitride materials have attracted considerable attention in the last decade due to their wide applications in solidstate light devices with their direct wide band-gaps and higher quantum efficiencies. InGaN/GaN multiple quantum well is important active region for light-emitting diode, which can be tuned according to indium composition in the InxGa1-xN alloy system. Owing to difficulty in fabricating bulk materials, GaN thin films are heteroepitaxially grown on latticemismatched and thermal-expansion-mismatched substrates, such as sapphire (Al2O3), Si and SiC, which subsequently results in a mass of threading dislocations and higher residual strains. On the one hand, dislocations and defects existing in GaN epifilms trap the carriers as scattering centers in the radiative recombination process between electrons and holes, and play an important role in drooping the internal quantum efficiency. On the other hand, higher built-in electric field induced by residual strains existing in GaN epifilm could make the emission wavelength red-shifted.It is common knowledge that temperature is one of the important factors in the growth process of epitaxial films, as a result, further research on thermal expansion behaviors is needed. Based on the above analysis, an in-depth study of thermal expansion behavior of wurtzite GaN epitaxial film is of vital importance both in theory and in application.In this study, we investigate the thermal expansion behaviors of wurtzite GaN epitaxial films by using temperaturedependent Raman scattering in a temperature range from 83 K to 503 K. According to the physical implication, Gruneisen parameter is almost a constant (Gruneisen parameters of all phonon modes are in a range between 1 to 2 for GaN) that characterizes the relationship between the phonon shift and the volume of a solid-state material. More importantly, Gruneisen parameter is relatively insensitive to temperature and suitable for building the connection between the phonon shift and thermal expansion coefficient. Therefore, the linear relationship between the phonon shift and temperature is built and utilized to calculate the thermal expansion coefficient according to the physical implication of the Gruneisen parameter. Conclusions can be obtained as follows. (1) The thermal expansion coefficient of GaN epifilm can be calculated in a certain temperature range by measuring the phonon modes of E2 (high), A1 (TO) and E1 (TO) through using temperature-dependent Raman scattering when the corresponding Gruneisen parameters are determined. (2) The calculated thermal expansion coefficients of GaN epifilm are consistent with the theoretical values.Conclusions and methods in this paper provide an effective quantitative analysis method to characterize the thermal expansion behaviors of other III-nitride epitaxial thin films, such as AlN, InN, AlGaN, InGaN, InAlN etc., which can be of benefit to reducing the dislocation density and improving the luminescence efficiency of light emitting diode. Therefore, research on thermal expansion behaviors of epifilms using temperature-dependent Raman scattering has a direction for further studying the latter-mismatch and thermal-expansion-mismatch between the epitaxial film and substrate.

First-Principles Investigation on Electronic Structure and Optical Properties of Wurtzite InxGa1-xN Alloys

Abstract First-principles calculations of plane wave ultra-soft pseudo-potential method based on the density functional theory (DFT) were used to study lattice constants, band structure, density of states and optical properties of wurtzite InxGa1-xN alloys. The calculated results indicate that the lattice constant gradually increases, the conduction band shifts to low energy direction and the band gap becomes narrower with the increase in In-doped concentration. Meanwhile, the dielectric function moves towards low energy direction, which is red-shift. The first dielectric peak appears near 2.0 eV, and the maximum dielectric peak close to 4.76 eV is found. There are two intersection points at about 5.6 and 9.2 eV. When the photon energy is less than 5.6 eV or higher than about 9.2 eV, the absorption coefficient of InxGa1-xN in low doping concentration (less than 50%) is smaller than that of high doping concentration of InxGa1-xN. However, when the photon energy is located between 5.6 and 9.2 eV, the absorption spectra of InxGa1-xN in low doping concentration are greater than those of high doping concentration. Therefore, the study result shows that InxGa1-xN alloys can be applied as solar cell and the transparent conductive materials.

Electrical and optical properties of transparent conducting InxGa1−xN alloy films deposited by reactive co-sputtering of GaAs and indium

Thin films of InxGa1–xN alloys were deposited by reactive sputtering using a GaAs target, covered partially with indium and co-sputtered with nitrogen. X-ray and electron diffraction studies indicate the formation of single phase InxGa1–xN films at ~500°C. Hall effect and resistivity measurements show that the alloy films with x≥0.5 have high carrier concentrations in the range of 1020–1021cm−3 and mobility of ~10cm2V−1s−1. Optical measurements of the alloy films show a strong dependence of the band gap on carrier concentration, which is attributed to the Burstein–Moss shift and free carrier effects in the near-infrared region. The values of electron effective mass obtained from plasma resonance data and the Burstein–Moss shift show good agreement. Over a limited composition window of x in the range of 0.5–0.6, the alloy films exhibit low electrical resistivity (≈10−3Ω-cm) and high transparency in part of the visible and near infrared regions, followed by high reflectance in the infrared region, which show their potential for applications as transparent electrodes in photovoltaic and photonic devices and as heat mirrors.

Growth and Characterization of In x Ga1−x N Multiple Quantum Wells Without Phase Separation

Efficient conversion of photon energy into electricity is a crucial step toward a sustainable solar-energy economy. Likewise, solid-state lighting devices are gaining prominence because of benefits such as reduced energy consumption and reduced toxicity. Among the various semiconductors investigated, InxGa1–xN alloys or superlattices are fervently pursued because of their large range of bandgaps between 0.65 eV and 3.4 eV. This paper reports on the fabrication of multiple quantum wells on LiGaO2 (001) substrates by plasma-assisted molecular beam epitaxy. Metal modulated epitaxy was utilized to prevent formation of metal droplets during the growth. Streaky patterns, seen in reflection high-energy electron diffraction, indicate two-dimensional growth throughout the device. Postdeposition characterization using scanning electron microscopy also showed smooth surfaces, while high-resolution x-ray diffraction and high-resolution transmission electron microscopy confirm the epitaxial nature of the overall quantum well structure.