AlN, GaN, AlxGa1 − xN nanotubes and GaN/AlxGa1 − xN nanotube heterojunctions

Semiconductor optoelectronic devices based on GaN and on InGaN or AlGaN alloys and superlattices can operate in a wide range of wavelengths, from far infrared to near ultraviolet region. The efficiency of these devices could be enhanced by shrinking the size and increasing the density of the semiconductor components. Nanostructured materials are natural candidates to fulfill these requirements. Here we use the density functional theory to study the electronic and structural properties of (10,0) GaN, AlN, Al_xGa_1 − xN nanotubes and GaN/Al_xGa_1 − xN heterojunctions, 0<x<1. The Al_xGa_1 − xN nanotubes exhibit direct band gaps for the whole range of Al compositions, with band gaps varying from 3.45 to 4.85 eV, and a negative band gap bowing coefficient of −0.14 eV. The GaN/Al_xGa_1 − xN nanotube heterojunctions show a type-I band alignment, with the valence band offsets showing a non-linear dependence with the Al content in the nanotube alloy. The results show the possibility of engineering the band gaps and band offsets of these III-nitrides nanotubes by alloying on the cation sites.     

Effect of pressure on the energy band gaps of wurtzite GaN and AlN and electronic properties of their ternary alloys AlxGa1−xN

Using the Empirical Pseudopotential Method (EPM) combined with an improved Virtual Crystal Approximation (VCA), where the effect of compositional disorder is included as an effective periodic potential, we have calculated the electronic band structure of GaN and AlN under hydrostatic pressure up to 200 kbar and 160 kbar, respectively. The behavior of electronic properties of their alloys Al_xGa_1−xN in the wurtzite structure have been predicted at zero pressure, for the entire range of alloy concentrations.     

Magnetotransport, optical, and electronic subband properties in AlxGa1−xN/AlN/GaN heterostructures

The Shubnikov-de Haas (S-dH) results at 1.5 K for Al_xGa_1−xN/AlN/GaN heterostructures and the fast Fourier transformation data for the S-dH data indicated the occupation by a two-dimensional electron gas (2DEG) of one subband in the GaN active layer. Photoluminescence (PL) spectra showed a broad PL emission about 30 meV below the GaN exciton emission peak at 3.474 eV that could be attributed to recombination between the 2DEG occupying in the AlN/GaN heterointerface and photoexcited holes. A possible subband structure was calculated by a self-consistent method taking into account the spontaneous and piezoelectric polarizations, and one subband was occupied by 2DEG below the Fermi level, which was in reasonable agreement with the S-dH results. These results can help improve understanding of magnetotransport, optical, and electronic subband properties in Al_xGa_1−xAs/AlN/GaN heterostructures.     

Numerical optimization of carrier confinement characteristics in (AlxGa1−xN/AlN)SLs/GaN heterostructures

We present numerical optimization of carrier confinement characteristics in (Al_xGa_1−xN/AlN)SLs/GaN heterostructures in the presence of spontaneous and piezoelectrically induced polarization effects. The calculations were made using a self-consistent solution of the Schrödinger, Poisson, potential and charge balance equations. It is found that the sheet carrier density in GaN channel increases nearly linearly with the thickness of AlN although the whole thickness and equivalent Al composition of Al_xGa_1−xN/AlN superlattices (SLs) barrier are kept constant. This result leads to the carrier confinement capability approaches saturation with thicknesses of AlN greater than 0.6 nm. Furthermore, the influence of carrier concentration distribution on carrier mobility was discussed. Theoretical calculations indicate that the achievement of high sheet carrier density is a trade-off with mobility.     

Compositional contrast in AlxGa1−xN/GaN heterostructures using scanning spreading resistance microscopy

Scanning spreading resistance microscopy has found extensive use as a dopant-profiling technique for silicon-based devices, and to a lesser extent for some III-V materials. Here we demonstrate its efficacy for wide bandgap nitrides and, in particular, show that it may be used to differentiate between layers of different Al-content in an Al_xGa_1−xN/GaN heterostructure. A monotonic increase in resistance signal with increasing Al-content is demonstrated, under optimal imaging conditions. The variation in measured resistance with applied bias is shown to be dependent on the aluminium content, and this is discussed, along with other issues, in the context of potential quantification of unknown samples. The procedure for forming an optimal image is different from that for silicon, in terms of contact forces and applied biases.     

Electronic and structural properties of zincblende AlxIn1−xN

Numerical calculations based on first-principles are applied to study the electronic and structural properties of ternary zincblende AlInN alloy. The results indicate the lattice constant has a small deviation from the Vegard’s law. The direct and indirect bowing parameters of 4.731 ± 0.794 eV and 0.462 ± 0.285 eV are obtained, respectively, and there is a direct-indirect crossover near the aluminum composition of 0.817. The bulk modulus is monotonically increased with an increase of the aluminum composition, and the deviation parameter of bulk modulus of 10.34 ± 9.37 GPa is obtained. On the contrary, the pressure derivative of bulk modulus is monotonically decreased with an increase of the aluminum composition.     

Room temperature growth of InxGa1−xN thin films by mixed source modified activated reactive evaporation

Polycrystalline In_xGa_1−xN thin films were prepared by mixed source modified activated reactive evaporation (MARE) technique. The films were deposited at room temperature on glass substrates without any buffer layer. All the films crystallize in the hexagonal wurtzite structure. The indium concentration calculated from XRD peak shift using Vegard's law was found to be varying from 2% to 92%. The band gap varies from 1.72 eV to 3.2 eV for different indium compositions. The indium rich films have higher refractive indices as compared to the gallium rich films. The near infra-red absorption decreases with gallium incorporation into InN lattice which is mainly due to decrease in the free carrier concentration in the alloy system. This fact is further supported from Hall effect measurements. MARE turns out to be a promising technique to grow In_xGa_1−xN films over the entire composition range at room temperature.     

First-principles study of the electronic and magnetic properties of a Nickel-Zinc ferrite: ZnxNi1−xFe2O4

Using first-principles density functional theory within the generalized gradient approximation method, the effect of Zn doping on electronic and magnetic properties of NiFe₂O₄ ferrite spinel has been studied. The crystal structure of the compounds is assigned to a pseudocubic structure and the lattice constant increases as the Zn concentration increases. Our spin-polarized calculations give a half-metallic state for NiFe₂O₄ and a normal metal state for Zn_xNi_1−xFe₂O₄ (0<x≤0.5). Based on the magnetic properties calculations, it is found that the saturation magnetic moment enhances linearly with increase in the Zn content in NiFe₂O₄. The Zn doping in NiFe₂O₄ also induces strong ferrimagnetism since it decreases the magnetic moment of A-sites.     

Theoretical study on InxGa1−xN/GaN quantum dots solar cell

In this work, the structure of In_xGa_1−xN/GaN quantum dots solar cell is investigated by solving the Schrödinger equation in light of the Kronig-Penney model. Compared to p-n homojunction and heterojunction solar cells, the In_xGa_1−xN/GaN quantum dots intermediate band solar cell manifests much larger power conversion efficiency. Furthermore, the power conversion efficiency of quantum dot intermediate band solar cell strongly depends on the size, interdot distance and gallium content of the quantum dot arrays. Particularly, power conversion efficiency is preferable with the location of intermediate band in the middle of the potential well.     

Polar oscillation and dispersion properties of quasi-confined optical phonon modes in a wurtzite GaN/AlxGa1−xN nanowire

Under the dielectric continuum model and Loudon’s uniaxial crystal model, the properties of the quasi-confined (QC) optical phonon dispersions and the electron-QC phonons coupling functions in a cylindrical wurtzite nanowire are deduced via the method of electrostatic potential expanding. Numerical computations on a GaN/Al_0.15Ga_0.85N wurtzite nanowire are performed. Results reveal that, for a definite axial wave number k_z and a certain azimuthal quantum number m, there are infinite branches of QC modes. The frequencies of these QC modes fall into two regions, i.e. a high frequency region and a low frequency region. The dispersion of the QC modes are quite apparant only when k_z and m are small. The lower-order QC modes in the higher frequency region play more important role in the electron-QC phonon interactions. Moreover, for the higher-order QC modes in the high frequency region, the electrostatic potentials “escaping” out of the well-layer material nearly could be ignored.     

Numerical optimization of In-mole fractions and layer thicknesses in AlxGa1−xN/AlN/GaN high electron mobility transistors with InGaN back barriers

The effects of the In-mole fraction (x) of an In_xGa_1−xN back barrier layer and the thicknesses of different layers in pseudomorphic Al_yGa_1−yN/AlN/GaN/In_xGa_1−xN/GaN heterostructures on band structures and carrier densities were investigated with the help of one-dimensional self-consistent solutions of non-linear Schrödinger-Poisson equations. Strain relaxation limits were also calculated for the investigated Al_yGa_1−yN barrier layer and In_xGa_1−xN back barriers. From an experimental point of view, two different optimized structures are suggested, and the possible effects on carrier density and mobility are discussed.     

The influence of chemical treatment and thermal annealing on AlxGa1−xN surfaces: An XPS study

The influences of chemical treatment and thermal annealing of Al_xGa_1−xN (x = 0.20) have been investigated by X-ray photoelectron spectroscopy (XPS). XPS analysis showed that successive chemical treatments and annealing produced changes in the stoichiometry of the Al_xGa_1−xN surface, with the surface concentration of N increasing and Al and Ga decreasing with increasing temperature. Band bending occurred at the Al_xGa_1−xN surface, in parallel with the observed changes in stoichiometry. These results are discussed in the context of the creation of surface states via the activation of vacancies and induced by defects. These findings point towards the possibility of selecting and/or engineering the band structure at Al_xGa_1−xN surfaces through a combination of surface preparation and annealing.     

Structural and optical properties of an InxGa1−xN/GaN nanostructure

The structural and optical properties of an In_xGa_1−xN/GaN multi-quantum well (MQW) were investigated by using X-ray diffraction (XRD), atomic force microscopy (AFM), spectroscopic ellipsometry (SE) and photoluminescence (PL). The MQW structure was grown on c-plane (0 0 0 1)-faced sapphire substrates in a low pressure metalorganic chemical vapor deposition (MOCVD) reactor. The room temperature photoluminescence spectrum exhibited a blue emission at 2.84 eV and a much weaker and broader yellow emission band with a maximum at about 2.30 eV. In addition, the optical gaps and the In concentration of the structure were estimated by direct interpretation of the pseudo-dielectric function spectrum. It was found that the crystal quality of the InGaN epilayer is strongly related with the Si doped GaN layer grown at a high temperature of 1090 °C. The experimental results show that the growth MQW on the high-temperature (HT) GaN buffer layer on the GaN nucleation layer (NL) can be designated as a method that provides a high performance InGaN blue light-emitting diode (LED) structure.     

Theoretical studies of structural, elastic, electronic and lattice dynamic properties of AlxYyB1−x−yN quaternary alloys

A theoretical study on the structural, elastic, electronic and lattice dynamic properties of Al_xY_yB_1−x−yN quaternary alloys in zinc-blend phase has been carried out with first-principles methods. Information on the lattice parameter, the lattice matching to available substrates and energy band-gaps is a prerequisite for many practical applications. The dependence of the lattice parameter a, bulk modulus B, elastic constants C₁₁, C₁₂ and C₄₄, band-gaps, optical phonon frequencies (ω_TO and ω_LO), the static and high-frequency dielectric coefficients ε (0) and ε (∞) and the dynamic effective charge Z^? were analyzed for y=0, 0.121, 0.241, 0.362 and 0.483. A significant deviation of the bulk modulus from linear concentration dependence was observed. A set of isotropic elastic parameters and related properties, namely bulk and shear moduli, Young's modulus, Poisson's ratio are numerically estimated in the frame work of the Voigt-Reuss-Hill approximation. The resistance to changes in bond length and lateral expansion in Al_xY_yB_1−x−yN increase with increasing y concentration. We observe that at y concentration about 0.035 and 0.063, Al_xY_yB_1−x−yN changes from brittle to ductile and Γ-X indirect fundamental gap becomes Γ-Γ direct fundamental gap. There is good agreement between our results and the available experimental data for the binary compound AlN, which is a support for those of the quaternary alloys that we report for the first time.     

Electronic structural properties and formation energy of Sn1−xPbxO2 solid solutions electrode

First-principles calculations based on density functional theory within the generalized gradient approximation have been performed for the Sn_1−xPb_xO₂ solid solution. The doped formation energies and electronic structures are also analyzed. Results show that the Sn_0.9375Pb_0.0625O₂ solid solution has the highest stability because of its minimum formation energy value of 0.04589 eV at a doping ratio of 0.0625. The SnO₂ lattice constants expand in a distorted rutile structure after Pb doping. The band structure and density of states calculations indicate that the band gap of SnO₂ narrowed due to the presence of the Pb impurity energy levels in the forbidden band, namely, Pb 6s energy band overlaps with the conductivity band in the F–Q direction. In addition, the number of electrons filled at the bottom of the conduction band increases from 0.13 to 3.96 after doping, resulting in the strengthening of the conductivity of the solid solution after doping of plumbum. The results provide a theoretical basis for the development and application of the Sn_1−xPb_xO₂ solid solution electrode.     

Sn1−xBixO2 and Sn1−xTaxO2 (0≤x≤0.75): A first-principles study

The structural, elastic, electronic and optical (x=0) properties of doped Sn_1−xBi_xO₂ and Sn_1−xTa_xO₂ (0≤x≤0.75) are studied using the first-principles pseudopotential plane-wave method within the local density approximation. The independent elastic constants C_ij and other elastic parameters of these compounds have been calculated for the first time. The mechanical stability of the compounds with different doping concentrations has also been studied. The electronic band structure and density of states are calculated and the effect of doping on these properties is also analyzed. It is seen that the band gap of the undoped compound narrowed with dopant concentration, which disappeared for x=0.26 for Bi doping and 0.36 for Ta doping. The materials thus become conductive oxides through the change in the electronic properties of the compound for x≤0.75, which may be useful for potential application. The calculated optical properties, e.g. dielectric function, refractive index, absorption spectrum, loss-function, reflectivity and conductivity of the undoped SnO₂ in two polarization directions are compared with both previous calculations and measurements.     

Electronic band structure of InPxSb1−x alloys

The investigation of optoelectronic properties of zinc-blende InP_xSb_1−x, semiconducting alloys by pseudopotential calculations is studied. The scheme uses the local empirical pseudopotential method, which involves the disorder effect into the virtual crystal approximation by introducing an effective potential disorder. Various quantities for the alloy of interest are calculated. The obtained results show a reasonable agreement with the available experimental data. Special attention has also been given to the compositional dependence of these studied quantities.     

Optical characterization of InxGa1−xN alloys

InGaN layers were grown by molecular beam epitaxy (MBE) either directly on (0 0 0 1) sapphire substrates or on GaN-template layers deposited by metal-organic vapor-phase epitaxy (MOVPE). We combined spectroscopic ellipsometry (SE), Raman spectroscopy (RS), photoluminescence (PL) and atomic force microscopy (AFM) measurements to investigate optical properties, microstructure, vibrational and mechanical properties of the InGaN/GaN/sapphire layers.The analysis of SE data was done using a parametric dielectric function model, established by in situ and ex situ measurements. A dielectric function database, optical band gap, the microstructure and the alloy composition of the layers were derived. The variation of the InGaN band gap with the In content (x) in the 0 < x ≤ 0.14 range was found to follow the linear law E_g = 3.44-4.5x.The purity and the stability of the GaN and InGaN crystalline phase were investigated by RS.     

Intersubband transition in symmetric AlxGa1 − xN/GaN double quantum wells with applied electric field

Influence of the applied electric field (AEF) on the intersubband transitions (ISBTs) in symmetric Al_xGa_1 − xN/GaN double quantum wells (DQWs) is investigated by self-consistent calculation. It is found that three- and four-energy-level DQWs can be realized when suitable electric field is applied. When the AEF is 0.93 MV/cm, the 1_odd-2_even ISBT has a comparable absorption coefficient with the 1_even-2_odd ISBT, and the four-energy-level DQWs are realized. The wavelengths of the 1_odd-2_even and 1_even-2_odd ISBTs are located at 1.31 and 1.62 μm, respectively. When the AEF is 1.10 MV/cm, the 1_odd-2_even and 1_odd-2_odd ISBTs have comparable absorption coefficients, and the three-energy-level DQWs are realized. The wavelengths of the 1_odd-2_even and 1_odd-2_odd ISBTs are located at 1.30 and 1.55 μm, respectively. The results suggest promising application to two-color optoelectronic devices operating within optical communication wavelength band, and this study also provides a method for realizing the two-color optoelectronic devices by using the AEF.     

A model for the band gap energy of the N-rich GaN1−xAsx (0<x≤0.07) and the As-rich GaN1−xAsx (0.95≤x≤1)

In this paper, a model for the band gap energy of the N-rich GaNAs (0<x≤0.07) and the As-rich GaNAs (0.95<x≤1) is developed. For the N-rich GaNAs, The parameters describing the variation of the CBM and the VBM in the N-rich GaNAs are obtained by fitting the experimental data. For the As-rich GaNAs, the parameters in the model are obtained by fitting the experimental data of the band gap energy. It is found that the band gap evolution of the N-rich energy is different from that of the As-rich band gap energy. The model may be used to calculate the band gap energy of other dilute group III-N–V nitrides.