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The electronic structure of the epitaxial GaN/Si(111) layers and the Li/GaN/Si(111) interface with a monolayer Li coverage has been studied in situ under ultrahigh vacuum conditions. The experiments were carried out using photo electron spectroscopy with synchrotron radiation in the photon energy range 75–850 eV. The photoemission spectra in the valence band and the core levels of Ga 3d, N 1s, and Li 1s are studied for the monolayer Li coating. It is found that Li adsorption causes a significant decrease in the intensity of the photoemission line of the intrinsic surface state and the appearance of an induced surface state due to charge transfer between the adsorbed Li layer and surface Ga atoms. It has been found that the GaN/Si(111) surface has predominantly Ga polarity. The Li/GaN/Si(111) interface has a semiconductor character. [ABSTRACT FROM AUTHOR]

The electronic structure of the clean AlN surface and the ultrathin K/AIN interface has been studied in situ by synchrotron-based photoelectron spectroscopy using the photon energies in the range of 100–650 eV The effect of K adsorption was studied. Changes in the valence band and in the Al 2p, N 1s, and K 3p core levels spectra have been investigated using K submonolayer deposition. Modification of the surface electronic structure of the AlN caused by K adsorption is found to originate from the local interaction of N surface atoms and K adatoms. As a results the suppression of intrinsic surface state and appearance of a new induced state are observed. It was found the K-induced electron redistribution effect that causes the positive energy shift of N 1s surface peak and increasing N-ionicity. [ABSTRACT FROM AUTHOR]

Strain engineering as one of the most powerful techniques for tuning optical and electronic properties of Ill-nitrides requires reliable methods for strain investigation. In this work, we reveal, that the linear model based on the experimental data limited to within a small range of biaxial strains (< 0.2%), which is widely used for the non-destructive Raman study of strain with nanometer-scale spatial resolution is not valid for the binary wurtzite-structure group-III nitrides GaN and AlN. Importantly, we found that the discrepancy between the experimental values of strain and those calculated via Raman spectroscopy increases as the strain in both GaN and AlN increases. Herein, a new model has been developed to describe the strain-induced Raman frequency shift in GaN and AlN for a wide range of biaxial strains (up to 2.5%). Finally, we proposed a new approach to correlate the Raman frequency shift and strain, which is based on the lattice coherency in the epitaxial layers of superlattice structures and can be used for a wide range of materials. [ABSTRACT FROM AUTHOR]

We have numerically examined the effect of the combination of graded last barrier as well as electron blocking layer in GaN-based multiquantum light-emitting diodes (LEDs). The structure shows improved hole injection into the quantum wells, which is a crucial performance-limiting factor in GaN LEDs. In our proposed structure, valence band offset is reduced, hole injection is enhanced and therefore optoelectronic properties are enhanced significantly. [ABSTRACT FROM AUTHOR]

In this research work, InGaN triple compound was grown under low nitrogen gas flows by using the sputtering technique. The structural, optical and morphological characteristics of the InGaN compound have been studied in detail. X-ray diffraction (XRD) and Raman for structural analysis; absorption measurement technique for optical properties; scanning electron microscopy and atomic force microscopy (AFM) measurement techniques were used for the study of the morphological characteristics. In the XRD analysis, the film deposited at 0 sccm gas flow exhibits a (0002) peak of InN, (0002) and (10–11) peaks of GaN. Other films show dendritic structure. In the Raman analysis, the optical phonon modes of the InGaN compound are A1(LO) and E2(high). Optical band gaps are found to be 2.57 eV, 2.54 eV, 3.03 eV and 2.93 eV for 0–0.4–0.8–1.2 sccm, respectively. These changes are ascribed to the degraded crystallinity of the grown films at high nitrogen flow rates. The surface morphology of the InGaN films grown at 0 sccm displays clusters of near-spherical-shaped nanoparticles over the surface. In the results of the AFM, the surface topography of the InGaN thin films deposited with lower nitrogen content exhibited fewer grains on the surface, especially on 0.4 sccm gas flow rate. The number of grains increased with higher N2 gas flow rates. The surface roughness of the films decreased with increasing N2 gas flows. It is clear that surface morphology of the films depends on the gas flow rate very much. Due to their morphological properties, we can say that they are suitable structures for optoelectronic applications and friction applications in engineering. We can also say that films with a hexagonal crystal structure and different optical band gaps can be used in device applications such as LED, laser diode and power electronics. [ABSTRACT FROM AUTHOR]

We report an original method of GaN/AlN quantum dots (QDs) formation with low density by ammonia MBE on the (0001)AlN surface using a decomposition process of GaN thin layer. The QDs formation has been investigated in situ by reflection high-energy electron diffraction technique. Low density of quantum dots has been obtained in the range 107-109 cm−2. Single quantum dots photoluminescence lines corresponding to exciton and biexiton transitions were observed in micro-photoluminescence spectra. A lattice gas model was developed for correct description of the GaN QDs statistical ensemble on the surface. Effective interaction between QDs results in the discontinuous phase transitions (first-order phase transitions) from low density of QDs (gas branch) to condensed phase of QDs. The GaN QDs formation has been confirmed by high-resolution transmission electron microscopy and micro-photoluminescence of single quantum dots. [ABSTRACT FROM AUTHOR]

Chemical kinetics of a two-dimensional (2D) AlN layer formation on the (0001) sapphire (Al2O3) surface during nitridation as function of ammonia flux and temperature is investigated by reflection high-energy electron diffraction. The process on the surface is described in framework of a chemical reactions kinetic model including interaction between partially reduced aluminum oxide species (AlO) and chemisorbed NH2 particles for the temperature range < 1210 K. The experimentally determined AlN formation rates as functions of both the temperature and the ammonia pressure are successfully described by a simple set of kinetic equations. Calculated maximum rate of the process well agrees with the experimental values. It was found that AlN formation rate is independent from temperature for the temperature range > 1210 K. In this range, the process is described as a phase transition in frame of lattice gas model. Precision measurement of 2D AlN lattice parameters during the nitridation process detects the value of 0.301 nm. This value strongly differs from bulk value of wurtzite AlN structure 0.311 nm, but it coincides exactly with a characteristic structure parameter of the oxygen-deficient (0001) Al2O3 surface with reconstruction (√31×√31)R ± 9°. We assume that this coincidence is the result of minimizing the elastic stresses at the heterojunction of the 2D AlN and the (0001) Al2O3 layer. [ABSTRACT FROM AUTHOR]

Electron relaxation process and transport property of hot electrons in wurtzite GaN, InN and AlN are investigated by an ensemble Monte Carlo (EMC) method using the classic three valleys model. Both momentum and energy relaxation processes are studied here, taking into account the effect of temperature and electric field. It can be found that under different electric fields the relaxation mechanisms are much different for the existence of satellite valley structures in III-nitrides. The momentum and energy relaxation processes of InN are different from that of GaN and AlN for the much smaller effective electron mass in the lowest valley of InN. The polar optical phonon scattering is the main energy relaxation process when the electric field is lower, but inter-valley scattering will dominate the energy relaxation process when the electric field is high. The main momentum relaxation mechanisms are ionized impurity, acoustic phonon, and polar optical phonon scattering at lower electric field, but polar optical phonon and inter-valley scattering are the main momentum relaxation mechanisms when the electric field becomes higher. The momentum relaxation time is much lower than the energy relaxation time, the reason is that nearly all scattering mechanisms relax momentum, but only polar optical phonon and inter-valley scattering relax electron energy. The temperature can affect the momentum and energy relaxation by the role in scattering. Finally, the momentum and energy relaxation times as a function of electron temperature are given, which can be used for relaxation time modeling and device transport research. [ABSTRACT FROM AUTHOR]

A fully self-consistent method for solution of the Schrödinger-Poisson equation is presented for III-nitride quantum cascade structures (QCSs). The impact of internal polarization fields on the band structure is incorporated through the coupled Schrödinger-Poisson equation with proper boundary conditions. The method is validated by comparison of the results with reported values for various QCSs. The absorption coefficient for a QCS is obtained and detailed theoretical analysis of the absorption spectrum performed. The influences of temperature, electric field, and active-layer doping on the absorption coefficient were examined. Devices with III-nitride heterostructures show some superior characteristics due to the presence of strong internal polarization fields. It is observed that system doping and temperature do not change the peak absorption wavelength significantly, which is very helpful for operation of high-performance and high-temperature devices. The operating wavelength of the device can be tuned slightly by variation of the external bias voltage. [ABSTRACT FROM AUTHOR]

The interplay between nonradiative recombination and carrier localization in AlGaN epilayers (0.11 < x < 0.78) was studied using the photoluminescence (PL) and light-induced transient grating (LITG) techniques. The carrier localization conditions were estimated from the temperature dependence of the PL band peak position. The room-temperature carrier lifetimes estimated by LITG were limited by the competition between the carrier localization and nonradiative recombination. Strong carrier localization was shown to be insufficient to achieve high internal quantum efficiency in AlGaN epilayers. [ABSTRACT FROM AUTHOR]

The paper reviews applications of ABC-model to interpret internal quantum efficiency and its droop in III-nitride light-emitting diodes. Advantages of the model, its intrinsic limitations, and tentative mechanisms responsible for deviation of the model predictions from available observations are discussed. New experimental information on recombination processes in the LED active regions coming in terms of the ABC-model is considered along with still open questions and tasks for further experimental and theoretical studies. [ABSTRACT FROM AUTHOR]

A theoretical model to calculate the thermal expansivity and lattice constant are described and used to evaluate the GaN semiconductor. This method is based on the use of integer and noninteger n-dimensional Debye functions for analytical evaluation of the thermal expansivity and lattice constant. The agreement between calculated and experimental values for GaN is generally satisfactory. [ABSTRACT FROM AUTHOR]

The piezoelectric coefficients of all the binary III-N wurtzite semiconductors for both first and second order has been used to evaluate the impact of non linear piezoelectricity on quantum wells and quantum dots of III-N materials and its alloys. The often neglected second order coefficients have instead significant effects on the strain induced piezoelectric polarization. The comparison of our predictions with the experimentally obtained values reveals a much better match with the internal piezoelectric fields in quantum wells and superlattices. The impact of our semi empirical approach is quite pronounced in III-N quantum dots as the excitonic properties remain highly influenced. [ABSTRACT FROM AUTHOR]

III-nitride InGaN-based solar cells have gained importance because their band gap can potentially cover most of the solar spectrum, spanning 0.7 eV to 3.4 eV. However, to use these materials to harvest additional energy, other properties such as their thermoelectric properties should be exploited. In this work, the Seebeck coefficient and the electrical conductivity of three InGaN alloys with various indium concentrations and Gd-doped GaN (GaN:Gd) were measured, and the power factor was calculated. We report a Seebeck value of ∼209 μV/K for Gd-doped GaN. [ABSTRACT FROM AUTHOR]

First ever Ce based GaN diluted magnetic semiconductor is reported. MOCVD grown GaN thin films were implanted with 3×10 cm dose of cerium ions. Photoluminescence (PL), optical transmission, Raman, high-resolution X-ray diffraction (HRXRD) measurements were performed on samples to study the optical and structural properties of the materials. Band gap narrowing is observed in optical transmission measurements, which points to incorporation of cerium ions into GaN host lattice. Superconducting Quantum Interference device (SQUID) was used in order to investigate the magnetic properties of implanted samples as a function of temperature and applied field. Hysteresis loops were recorded at 100 K and 300 K for implanted and as-grown samples. Hysteresis behavior and temperature-dependent magnetization measurements revealed the presence of ferromagnetic ordering in Ce implanted GaN samples, which points to the realization of Ce:GaN diluted magnetic semiconductor. [ABSTRACT FROM AUTHOR]

In this work we report on the magnetron reactive ion etch (MRIE) technology for gallium nitride (GaN) and aluminum gallium nitride (AlxGa1-xN) using dichlorodifluoromethane (CCl2F2), commonly known as halocarbon 12, with etch rates greater than 1,000 and 840 Å/min, respectively. Magnetic confinement of a very low pressure (10-4 Torr range) radio frequency (RF) discharge generates high-density plasmas, with low sheath voltages at the bounding surfaces, and very high dissociation of the source gas. Furthermore, the very low pressure of the etch process is characterized by long mean free paths so that sputtering contamination is reduced. MRIE chemistry has been monitored in situ by means of mass spectroscopy. Finally, we report on the successful fabrication of an indium gallium nitride (InxGa1-xN) blue light emitting diode (LED), whose fabrication sequence included the MRIE etching of GaN in CCl2F2. [ABSTRACT FROM AUTHOR]

First-principles calculations are performed to investigate dynamical and dielectric properties of group-III-nitrides. We focus on vibrational frequencies, dynamical charges, and the phonon density of states of cubic AlN, GaN, and InN. Chemical trends are derived and discussed. The first ab initio calculations of phonon branches and density of states are presented for an ordered InxGa1−xN structure. [ABSTRACT FROM AUTHOR]

GaN layers have been grown by plasma-assisted molecular beam epitaxy on AlN-buffered Si(111) substrates. An initial Al coverage of the Si substrate of aproximately 3 nm lead to the best AlN layers in terms of x-ray diffraction data, with values of full-width at half-maximum down to 10 arcmin. A (2×2) surface reconstruction of the AlN layer can be observed when growing under stoichiometry conditions and for substrate temperatures up to 850°C. Atomic force microscopy reveals that an optimal roughness of 4.6 nm is obtained for AlN layers grown at 850°C. Optimization in the subsequent growth of the GaN determined that a reduced growth rate at the beginning of the growth favors the coalescence of the grains on the surface and improves the optical quality of the film. Following this procedure, an optimum x-ray full-width at half-maximum value of 8.5 arcmin for the GaN layer was obtained. Si-doped GaN layers were grown with doping concentrations up to 1.7×1019 cm−3 and mobilities approximately 100 cm2/V s. Secondary ion mass spectroscopy measurements of Be-doped GaN films indicate that Be is incorporated in the film covering more than two orders of magnitude by increasing the Be-cell temperature. Optical activation energy of Be acceptors between 90 and 100 meV was derived from photoluminescence experiments. [ABSTRACT FROM AUTHOR]

Various Al-Ga-In Nitride alloys have been grown in AIXTRON Planetary Reactors®. GaN is grown with an excellent optical quality and very good thickness uniformity. GaInN with photoluminescence emission wavelengths in the visible blue region have also been produced. AlGaN is demonstrated as the material with the widest bandgap. Furthermore, we will present data on doped material. Characterization of the heterostructures by photoluminescence and sheet resistivity measurements is presented as well. [ABSTRACT FROM AUTHOR]

Transmission electron microscopy has been used to study undoped and Si-doped InGaN/GaN layers. The doped layers show formation of extrinsic dislocation loops. These defects are not formed in the undoped samples. The highly Si-doped layers show failure of selective photoelectrochemical wet-etching used for device fabrication. This loss of etching selectivity is attributed to Si-induced defects evenly distributed in the InGaN layers and their vicinities. [ABSTRACT FROM AUTHOR]

Seeded growth of AlN single crystals was demonstrated in an induction-heated, high-temperature reactor via a physical vapor transport (PVT) process. AIN seeds were prepared from a self-seeded boule containing large single-crystalline grains. Seeded growth was interrupted several times in order to refill the AlN powder source, and a dedicated process scheme was used to ensure epitaxial growth on the seed surface, after prior exposure to air. The growth temperatures were in the range of 2200-2300°C, and the reactor pressure was in the range of 500-900 torr of UHP-grade nitrogen during each growth run. Under these growth conditions, a seed (10 mm diameter) expanded at an angle of 45°, and a larger single crystal up to 18 mm in diameter was obtained. The as-grown surface had three facets, of which facet (11̅20) was smooth and featureless while the other two, (41̅20) and (25̅20), showed serrated morphologies. The double-crystal x-ray rocking curve and glow discharge mass spectroscopy analysis confirmed that the grown crystal was of high crystalline quality with low impurity incorporation. [ABSTRACT FROM AUTHOR]