D. J. As

The near band edge photoluminescence of cubic GaN epilayers

The near band edge photoluminescence (PL) of cubic GaN epilayers grown by radio frequency (rf) plasma-assisted molecular beam epitaxy on (100) GaAs is measured. Since the PL is excited with an unfocused laser beam it resembles the layer properties rather than the properties of micron-size inclusions or micro crystals. The low temperature PL spectra show well separated lines at 3.26 and 3.15 eV which are due to excitonic and donor-acceptor pair transitions (donor binding energy 25 meV, acceptor binding energy 130 meV). No emission above the band gap of the cubic phase is detected. PL results are confirmed by x-ray diffraction and atomic force microscopy which reveal only negligible contributions from hexagonal inclusions and micron size single crystals. The room temperature PL consists of an emission band at about 3.21 eV with a full width at half maximum of 117 meV.

Phase separation suppression in InGaN epitaxial layers due to biaxial strain

In this letter, we show that external biaxial strain suppress spinodal phase separation in thin InGaN epitaxial layers pseudomorphically grown on thick unstrained cubic ~c! GaN~001! buffer layers. The InGaN films are terminated by a top GaN layer forming GaN/InGaN/GaN double heterostructures. By monitoring the alloy composition and thickness for a fixed growth temperature, we control the presence of biaxial strain induced by the rigid GaN buffer in the InGaN layers. We start by first showing from ab initio calculations of the alloy free energy taking strain into account that the biaxial strain is expected to induce a suppression of the miscibility gap leading to a single homogeneous phase for the InGaN alloys. We use high resolution x-ray diffraction ~HRXRD! reciprocal space maps to select the strained layers. We have shown recently that micro-Raman is an accurate tool to observe separate phases in InGaN epitaxial layers. 4,8 Micro-Raman spectroscopy measurements are also used in this work to demonstrate conclusively the suppression of the spinodal phase separation process in strained quantum wells. The c-GaN/In x Ga 12x N/GaN double heterostructures were grown on GaAs~001! substrates by molecular-beam epitaxy using a rf plasma nitrogen source. The GaN buffer layers were grown at T5720 °C with thicknesses of about 400 nm. The c-InGaN films were deposited at lower growth temperatures of 600 °C. The films were deposited at growth rates of 40 nm/h. The GaN cap layers, of about 30 nm thick, were grown at low temperatures of about 600 °C in order to reduce In desorption and interdiffusion. The growth front was continuously monitored by reflection high-energy electron diffraction and the diffraction patterns exhibited a cubic symmetry along all major azimuths.

Temperature dependence of exciton peak energies in ZnS, ZnSe, and ZnTe epitaxial films

High-quality ZnS, ZnSe, and ZnTe epitaxial films were grown on (001)-GaAs-substrates by molecular beam epitaxy. The 1s-exciton peak energy positions have been determined by absorption measurements from 2 K up to about room temperature. For ZnS and ZnSe additional high-temperature 1s-exciton energy data were obtained by reflectance measurements performed from 300 up to about 550 K. These complete E1s(T) data sets are fitted using a recently developed analytical model. The high-temperature slopes of the individual E1s(T) curves and the effective phonon temperatures of ZnS, ZnSe, and ZnTe are found to scale almost linearly with the corresponding zero-temperature energy gaps and the Debye temperatures, respectively. Various ad hoc formulas of Varshni type, which have been invoked in recent articles for numerical simulations of restricted E1s(T) data sets for cubic ZnS, are discussed.

Lattice dynamics of hexagonal and cubic InN: Raman-scattering experiments and calculations

We present results of first- and second-order Raman-scattering experiments on hexagonal and cubic InN covering the acoustic and optical phonon and overtone region. Using a modified valence-force model, we calculated the phonon dispersion curves and the density of states in both InN modifications. The observed Raman shifts agree well the calculated Γ-point frequencies and the corresponding overtone density of states. A tentative assignment to particular phonon branches is given.

Universality of electron accumulation at wurtzite c- and a-plane and zinc-blende InN surfaces

Electron accumulation is found to occur at the surface of wurtzite (112¯0), (0001), and (0001¯) and zinc-blende (001) InN using x-ray photoemission spectroscopy. The accumulation is shown to be a universal feature of InN surfaces. This is due to the low Г-point conduction band minimum lying significantly below the charge neutrality level.

Molecular beam epitaxy of phase pure cubic InN

Cubic InN layers were grown by plasma assisted molecular beam epitaxy on 3C-SiC (001) substrates at growth temperatures from 419to490°C. X-ray diffraction investigations show that the layers have zinc blende structure with only a small fraction of wurtzite phase inclusions on the (111) facets of the cubic layer. The full width at half maximum of the c-InN (002) x-ray rocking curve is less than 50arcmin. The lattice constant is 5.01±0.01A. Low temperature photoluminescence measurements yield a c-InN band gap of 0.61eV. At room temperature the band gap is about 0.56eV and the free electron concentration is about n∼1.7×1019cm−3.

Electroluminescence of a cubic GaN/GaAs (001) p–n junction

A cubic GaN p–n diode has been grown on n-type GaAs (001) substrates by plasma assisted molecular epitaxy. For p- and n-type doping, elemental Mg and Si beams have been used, respectively. The optical properties are characterized by photoluminescence at room temperature and 2 K. Current–voltage and capacitance–voltage measurements of the cubic GaN n+–p junction are performed at room temperature. The electroluminescence at 300 K is measured through a semitransparent Au contact. A peak emission at 3.2 eV with a full width at half maximum as narrow as 150 meV is observed, indicating that near-band edge transitions are the dominating recombination processes in our device. A linear increase of the electroluminescence intensity with increasing current density is measured.

Refractive index and gap energy of cubic InxGa1−xN

Spectroscopic ellipsometry studies have been carried out in the energy range from 1.5 to 4.0 eV in order to determine the complex refractive indices for cubic InGaN layers with various In contents. The films were grown by molecular-beam epitaxy on GaAs(001) substrates. By studying GaN films, we prove that for the analysis of optical data, a parametric dielectric function model can be used. Its application to the InGaN layers yields, in addition, the composition dependence of the average fundamental absorption edge at room temperature. From the latter, a bowing parameter of 1.4 eV is deduced.

Room temperature green light emission from nonpolar cubic InGaN∕GaN multi-quantum-wells

Cubic InGaN∕GaN multi-quantum-wells (MQWs) with high structural and optical quality are achieved by utilizing freestanding 3C-SiC (001) substrates and optimizing InGaN quantum well growth. Superlattice peaks up to fifth order are clearly resolved in x-ray diffraction. Bright green room temperature photoluminescence (PL) from c-InxGa1−xN∕GaN MQWs (x=0.16) is observed. The full width at half maximum of the PL emission is about 240meV at 300K. The PL intensity increases with well thickness, prooving that polarization fields which can limit the performance of the wurtzite III-nitride based devices are absent. The diffusion length of excess carriers is about 17nm.

In situ growth regime characterization of cubic GaN using reflection high energy electron diffraction

Cubic GaN layers were grown by plasma-assisted molecular beam epitaxy on 3C-SiC (001) substrates. In situ reflection high energy electron diffraction was used to quantitatively determine the Ga coverage of the GaN surface during growth. Using the intensity of the electron beam as a probe, optimum growth conditions of c-GaN were found when a 1 ML Ga coverage is formed at the surface. 1μm thick c-GaN layers had a minimum surface roughness of 2.5nm when a Ga coverage of 1 ML was established during growth. These samples revealed also a minimum full width at half maximum of the (002) rocking curve.