InxGa1 xN Research Articles

Structural and optical properties of composition-graded InGaN nanowires

At the moment, InGaN ternary compounds are of a great interest for the development of devices for sunlight driven water splitting. However, the synthesis of such materials is hindered by the fact that InxGa1–xN layers are susceptible to phase decomposition at x from 20 to 80%. Nanowires can be a promising solution to this problem. The purpose of our study was to analyze the structural and optical properties of InxGa1–xN nanowires with a gradient x content being inside the miscibility gap. InxGa1–xN nanowires were grown on silicon substrates using plasma-assisted molecular beam epitaxy. The structural properties of nanowires were studied using scanning and transmission electron microscopy. The chemical composition and optical properties of nanowires were analyzed using energy-dispersive microanalysis and photoluminescence spectroscopy. It was shown for the first time that the composition-graded InxGa1–xN nanowires with x from 40 to 60% can be grown using plasma-assisted molecular beam epitaxy. The grown samples exhibit photoluminescence at room temperature with a maximum at about 890 nm, which corresponds to an In content of about 62% according to the modified Vegard’s rule and the transmission electron microscopy data. The obtained results can be of practical interest for the development of devices for water splitting induced by sunlight or sources of near IR radiation

Highly Stable White Light Emission from III-Nitride Nanowire LEDs Utilizing Nanostructured Alumina-Doped Mn4+ and Mg2.

In this report, red-emitting alumina nanophosphors doped with Mn4+ and Mg2+ (Al2O3:Mn4+, Mg2+) are synthesized by a hydrothermal method using a Pluronic surfactant. The prepared samples are ceramic-sintered at various temperatures. X-ray diffraction shows that Al2O3:Mn4+, Mg2+ annealed at 500 °C exhibits a cubic γ-Al2O3 phase with the space group Fd3m-227. The tetragonal δ-Al2O3 and rhombohedral α-Al2O3 phase is obtained at 1000 and 1300 °C, respectively. Cube-like nanoparticles in a size of ∼40 nm are observed for the alumina heated at 500-1000 °C. The size and red-emitting intensity of the phosphors remarkably increased with annealed temperature ∼1300 °C. Emission spectra of the phosphors show strong peaks at 678 and 692 nm due to 2 E g → 4 A 2 transitions of the Mn4+ ion, under a light excitation of 460 nm. A strong zero-phonon line (ZPL) emission is observed in the luminescence spectra of δ-Al2O3:Mn4+, Mg2+ at 298 K, whereas a weak one is observed in those of α- and γ-Al2O3:Mn4+, Mg2+. The alumina phosphors exhibited an excellent waterproof ability during 60 days in water and good thermal stability in the range of 77-573 K. A warm-white light-emitting diode (WLED) fabricated using In x Ga1-x N nanowire chips with Al2O3:Mn4+, Mg2+ red-emitting nanophosphors presents a high color rendering index of ∼95.1 and a low correlated color temperature of ∼4998 K. Moreover, the current-voltage characteristic of the nanowire LEDs could be improved using Al2O3:Mn4+, Mg2+ nanophosphors which is attributed to the increased heat dissipation in the nanowire LEDs.

Synthesis of CaSnN2 via a High-Pressure Metathesis Reaction and the Properties of II-Sn-N2 (II = Ca, Mg, Zn) Semiconductors.

A novel ternary nitride semiconductor, CaSnN2, with a layered rock-salt-type structure (R3̅m) was synthesized via a high-pressure metathesis reaction. The properties and structures of II-Sn-N2 (II = Ca, Mg, Zn) semiconductors were also systematically studied, and the differences among them were revealed by comparison. These semiconductor materials showed a rock-salt- or wurtzite-type structure depending on the combined effect of the synthetic conditions and the characteristics of the group II elements. Additionally, the rock-salt-type structures of CaSnN2 and MgSnN2 (i.e., the ambient-pressure phase) were different from those predicted using first-principles calculations. Further, on the basis of first-principles calculations and consideration of the pressure effect, the recovered CaSnN2 sample showed an R3̅m structure. CaSnN2 and MgSnN2 showed a band gap of 2.3-2.4 eV, which is suitable for overcoming the green-light-gap problem. These semiconductors also showed a strong cathode luminescence peak at room temperature, and generalized gradient approximation (GGA) calculations revealed that CaSnN2 has a direct band gap. These inexpensive and nontoxic semiconductors (II-Sn-N2 semiconductors (II = Ca, Mg, Zn)), with mid band gaps are required as pigments to replace cadmium-based materials. They can also be used in emitting devices and as photovoltaic absorbers, replacing InxGa1-xN semiconductors.

A 3D simulation comparison of carrier transport in green and blue c-plane multi-quantum well nitride light emitting diodes

Until recently, the electrical efficiency of green nitride light-emitting diodes (LEDs) was considerably lower than that of blue LEDs. This is particularly surprising as one would expect a reduced forward voltage with increasing emission wavelength. In this paper, we theoretically investigated the impact of the number of quantum wells on the forward voltage of III-nitride LEDs with x = 0.15 (blue) and x = 0.24 (green) InxGa1–xN QWs. The simulated dependence of current density (J) on applied diode bias (V) shows a significant increase of 1.9 V in the forward voltage between one and five quantum well (QW) c-plane green LED structures. Artificially turning off the polarization fields in the simulation does not entirely suppress this effect. Due to the large band offsets in the green LED multiple QW stack, simulations indicate a sequential band filling of the QW sequence. This mechanism should not be limited to c-plane LEDs and could also be present in nonpolar or semipolar devices.

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.

Investigation of the Structural and Optical Properties of Compositionally V‐Graded Strained InxGa1–xN Layers

Compositionally graded InxGa1–xN‐based materials have been receiving more attention recently due to both their novel structure and intrinsic properties. However, high‐quality material with a well‐controlled and optimized grade to high In composition remains challenging to grow. Herein, the growth and characterization of continuous 2D films of compositionally graded InxGa1–xN are investigated using molecular beam epitaxy (MBE) on (0001) GaN templates at 575 °C. Each film is formed by compositionally grading the growth from GaN to InxGa1–xN and back to GaN, symmetrically, such that the bandgap and the composition of Ga follow a V‐shaped pattern. Here x has been chosen to be ≈0.20 and 0.22. These compositions are confirmed through high‐resolution X‐ray diffraction simulations. Furthermore, asymmetric reciprocal space mapping reveals complete coherence with the GaN substrate. Finally, photoluminescence is measured at low temperature to demonstrate the luminescent properties of the films.

Hot Electrons in InxGa1–xN and InxAl1–xN Binary Solid Solutions

The nonlinear field dependences of the hot electron drift velocity have been calculated by means of numerical solution of the Boltzman kinetic equation in two families of nitride semiconductor solid solutions: InxGa1–xN and InxAl1–xN. The calculations have been carried out in the electric field up to 30 kV/cm at lattice temperatures of 77 and 300 K, and for electron concentrations of 1 × 1018 and 1 × 1019 cm–3. In the calculations, the composition x has been taken as 0, 0.25, 0.5, 0.75, and 1 for both alloys.

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.

P–n junction diodes with polarization induced p-type graded InxGa1–xN layer

In this study, p–n junction diodes with polarization induced p-type layer are demonstrated on Ga polar (0001) bulk GaN substrates. A quasi-p-type region is obtained by linearly grading the indium composition in un-doped InxGa1–xN layers from 0% to 5%, taking advantage of the piezoelectric and spontaneous polarization fields which exist in group III-nitride heterostructures grown in the typical (0001) or c-direction. The un-doped graded InxGa1–xN layers needed to be capped with a thin Mg-doped InxGa1–xN layer to make good ohmic contacts and to reduce the on-resistance of the p–n diodes. The Pol-p–n junction diodes exhibited similar characteristics compared to reference samples with traditional p-GaN:Mg layers. A rise in breakdown voltage from 30 to 110 V was observed when the thickness of the graded InGaN layer was increased from 100 to 600 nm at the same grade composition.

Interface dislocations in In x Ga1-x N/GaN heterostructures

Interface dislocations have been investigated by transmission electron microscopy for InxGa1–xN (50 nm)/GaN heterostructures grown by metal-organic vapor phase epitaxy for 0.13 < x < 0.20. Structural properties of the dislocations were analysed by conventional transmission electron microscopy by diffraction contrast. We observed two kinds of dislocations lying in the interface: screw type dislocations with Burgers vector b = a = 1/3 <11−20> and pure edge misfit dislocations. The screw type dislocations were observed for x ≤ 0.17 and misfit dislocations for x ≥ 0.18. While the formation of MDs may be explained in the framework of conventional interface strain relaxation, the presence of screw-type dislocations may bring new insight on processes in hexagonal materials heteroepitaxy.

Microtube Light-Emitting Diode Arrays with Metal Cores.

We report the fabrication and characteristics of vertical microtube light-emitting diode (LED) arrays with a metal core inside the devices. To make the LEDs, gallium nitride (GaN)/indium gallium nitride (In(x)Ga(1-x)N)/zinc oxide (ZnO) coaxial microtube LED arrays were grown on an n-GaN/c-aluminum oxide (Al2O3) substrate. The microtube LED arrays were then lifted-off the substrate by wet chemical etching of the sacrificial ZnO microtubes and the silicon dioxide (SiO2) layer. The chemically lifted-off LED layer was then transferred upside-down on other supporting substrates. To create the metal cores, titanium/gold and indium tin oxide were deposited on the inner shells of the microtubes, forming n-type electrodes inside the metal-cored LEDs. The characteristics of the resulting devices were determined by measuring electroluminescence and current-voltage characteristic curves. To gain insights into the current-spreading characteristics of the devices and understand how to make them more efficient, we modeled them computationally.

Emission color-tuned light-emitting diode microarrays of nonpolar In(x)Ga(1-x)N/GaN multishell nanotube heterostructures.

Integration of nanostructure lighting source arrays with well-defined emission wavelengths is of great importance for optoelectronic integrated monolithic circuitry. We report on the fabrication and optical properties of GaN-based p–n junction multishell nanotube microarrays with composition-modulated nonpolar m-plane InxGa1–xN/GaN multiple quantum wells (MQWs) integrated on c-sapphire or Si substrates. The emission wavelengths were controlled in the visible spectral range of green to violet by varying the indium mole fraction of the InxGa1–xN MQWs in the range 0.13 ≤ x ≤ 0.36. Homogeneous emission from the entire area of the nanotube LED arrays was achieved via the formation of MQWs with uniform QW widths and composition by heteroepitaxy on the well-ordered nanotube arrays. Importantly, the wavelength-invariant electroluminescence emission was observed above a turn-on of 3.0 V because both the quantum-confinement Stark effect and band filling were suppressed due to the lack of spontaneous inherent electric field in the m-plane nanotube nonpolar MQWs. The method of fabricating the multishell nanotube LED microarrays with controlled emission colors has potential applications in monolithic nonpolar photonic and optoelectronic devices on commonly used c-sapphire and Si substrates.

Thick (~1 μm) p‐type InxGa1–xN (x ~ 0.36) grown by MOVPE at a low temperature (~570 °C)

This paper reports the post‐growth annealing effects of low‐temperature grown Mg‐doped InGaN. By using MOVPE, 1 μm‐thick Mg‐doped InxGa1–xN (x ~ 0.36) films are grown at 570 °C. In order to activate the Mg acceptors, grown samples are treated by the conventional furnace annealing (FA) or the rapid thermal annealing (RTA). In the case of the FA at 650 °C for 20 min, the InGaN film is phase‐separated. On the other hand, the RTA at a temperature higher than 700 °C enables us to get p‐type samples. By using the RTA at 850 for 20 s, p‐type samples with a hole concentration 1018–1019 cm−3 are successfully obtained without phase separation.

Effect of thermal annealing in a-InxGa1–xN films prepared by reactive RF-sputtering

We deposited amorphous indium gallium nitride (a-InxGa1–xN) films at room temperature by reactive radio frequency sputtering with GaN and InN targets and investigated the change of their properties from thermal annealing at annealing temperatures, Ta, below 200 °C. A large change in the indium and nitrogen composition ratios was not observed by thermal annealing at a Ta below 200 °C. In the X-ray diffraction patterns of the films annealed at a Ta below 200 °C, no perceivable peaks assigned to crystalline InxGa1–xN were found. The photoconductivty, σp, increased with an increase in Ta. On the other hand, the increase of the dark conductivity, σd, was very small with an increase in Ta below 200 °C. As a result, the photosensitivity, σp/σd, increased from 252 to 2500 by thermal annealing at a Ta of 200 °C.

The effect of growth parameters on the Mg acceptor in InxGa1‐xN:Mg and AlxGa1‐xN:Mg

AbstractInxGa1–xN and AlxGa1–xN alloys are used in many optoelectronic applications due to their tunable band gap, but p‐type doping remains a challenge. To better understand the Mg acceptor in nitride alloys, we investigate the effects of In or Al mole fraction, growth temperature and sample thickness on the amount of un‐ionized (neutral) Mg using electron paramagnetic resonance (EPR) spectroscopy. The results show that neither temperature nor thickness effects the concentration of the neutral Mg‐related acceptor defects; however, the mole fraction of metal, In or Al, alters the behavior of the dopant. For InxGa1–xN, a broadening of the EPR linewidth is shown to be directly related to the presence of a nearby In and is consistent with a lowering of the acceptor level. Incorporation of Al into GaN, on the other hand, produces a systematic decrease in the concentration of neutral Mg‐related acceptors as the amount of Al increases. Earlier studies indicate that the reduction is caused by incomplete hydrogen removal from the acceptor impurity. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

The effects of varying threading dislocation density on the optical properties of InGaN/GaN quantum wells

AbstractThe effects of the threading dislocation (TD) density on the optical properties of a series of comparable InxGa1–xN/ GaN multiple QW structures were studied. The TD density ranged from 2 × 107 cm–2, for a structure grown on a free‐standing GaN substrate, to 5 × 109 cm–2 grown on a sapphire substrate. Room temperature internal quantum efficiencies (IQEs) were determined by temperature dependent photoluminescence (PL); no systematic dependence of the IQE on the TD density was found. The excitation power density dependence of the efficiency was investigated, which also showed no systematic dependence on TD density. PL excitation spectroscopy was used to verify that equivalent carrier densities were generated within the QWs of each structure. The lack of systematic dependence of the optical properties on TD density is attributed to the strong carrier localisation in InGaN/GaN QWs. At the highest density of TDs studied, it is estimated that the average defect separation greatly exceeds the in‐plane diffusion lengths of electrons and holes; consequently the majority of carriers in the system are isolated from the TDs. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

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.

Strain Engineering of Nanowire Multi-Quantum Well Demonstrated by Raman Spectroscopy

An analysis of the strain in an axial nanowire superlattice shows that the dominating strain state can be defined arbitrarily between unstrained and maximum mismatch strain by choosing the segment height ratios. We give experimental evidence for a successful strain design in series of GaN nanowire ensembles with axial InxGa1-xN quantum wells. We vary the barrier thickness and determine the strain state of the quantum wells by Raman spectroscopy. A detailed calculation of the strain distribution and LO phonon frequency shift shows that a uniform in-plane lattice constant in the nanowire segments satisfactorily describes the resonant Raman spectra, although in reality the three-dimensional strain profile at the periphery of the quantum wells is complex. Our strain analysis is applicable beyond the InxGa1-xN/GaN system under study, and we derive universal rules for strain engineering in nanowire heterostructures.

Strain induced variations in band offsets and built-in electric fields in InGaN/GaN multiple quantum wells

The band structure, quantum confinement of charge carriers, and their localization affect the optoelectronic properties of compound semiconductor heterostructures and multiple quantum wells (MQWs). We present here the results of a systematic first-principles based density functional theory (DFT) investigation of the dependence of the valence band offsets and band bending in polar and non-polar strain-free and in-plane strained heteroepitaxial InxGa1-xN(InGaN)/GaN multilayers on the In composition and misfit strain. The results indicate that for non-polar m-plane configurations with [12¯10]InGaN//[12¯10]GaN and [0001]InGaN//[0001]GaN epitaxial alignments, the valence band offset changes linearly from 0 to 0.57 eV as the In composition is varied from 0 (GaN) to 1 (InN). These offsets are relatively insensitive to the misfit strain between InGaN and GaN. On the other hand, for polar c-plane strain-free heterostructures with [101¯0]InGaN//[101¯0]GaN and [12¯10]InGaN//[12¯10]GaN epitaxial alignments, the valence band offset increases nonlinearly from 0 eV (GaN) to 0.90 eV (InN). This is significantly reduced beyond x ≥ 0.5 by the effect of the equi-biaxial misfit strain. Thus, our results affirm that a combination of mechanical boundary conditions, epitaxial orientation, and variation in In concentration can be used as design parameters to rapidly tailor the band offsets in InGaN/GaN MQWs. Typically, calculations of the built-in electric field in complex semiconductor structures often must rely upon sequential optimization via repeated ab initio simulations. Here, we develop a formalism that augments such first-principles computations by including an electrostatic analysis (ESA) using Maxwell and Poisson's relations, thereby converting laborious DFT calculations into finite difference equations that can be rapidly solved. We use these tools to determine the bound sheet charges and built-in electric fields in polar epitaxial InGaN/GaN MQWs on c-plane GaN substrates for In compositions x = 0.125, 0.25,…, and 0.875. The results of the continuum level ESA are in excellent agreement with those from the atomistic level DFT computations, and are, therefore, extendable to such InGaN/GaN MQWs with an arbitrary In composition.

Luminous Efficiency of Axial InxGa1–xN/GaN Nanowire Heterostructures: Interplay of Polarization and Surface Potentials

Using continuum elasticity theory and an eight-band k·p formalism, we study the electronic properties of GaN nanowires with axial InxGa1-xN insertions. The three-dimensional strain distribution in these insertions and the resulting distribution of the polarization fields are fully taken into account. In addition, we consider the presence of a surface potential originating from Fermi level pinning at the sidewall surfaces of the nanowires. Our simulations reveal an in-plane spatial separation of electrons and holes in the case of weak piezoelectric potentials, which correspond to an In content and layer thickness required for emission in the blue and violet spectral range. These results explain the quenching of the photoluminescence intensity experimentally observed for short emission wavelengths. We devise and discuss strategies to overcome this problem.