B. Schöttker

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.

STRUCTURAL PROPERTIES AND RAMAN MODES OF ZINC BLENDE INN EPITAXIAL LAYERS

We report on x-ray diffraction and micro-Raman scattering studies on zinc blende InN epitaxial films. The samples were grown by molecular beam epitaxy on GaAs(001) substrates using a InAs layer as a buffer. The transverse-optical (TO) and longitudinal-optical phonon frequencies at Γ of c-InN are determined and compared to the corresponding values for c-GaN. Ab initio self-consistent calculations are carried out for the c-InN and c-GaN lattice parameters and TO phonon frequencies. A good agreement between theory and experiment is found.

Mechanisms of optical gain in cubic gallium nitrite

We report on the mechanisms of optical gain in cubic GaN. Intensity-dependent gain spectra allow a distinction of the processes involved in providing optical amplification. For moderate excitation levels, the biexciton decay is responsible for a gain structure at 3.265 eV. With increasing excitation densities, gain is observed on the high energy side of the cubic band gap due to band filling processes. For the highest pump intensities, the electron-hole plasma is the dominant gain process. Gain values up to 210 cm−1 were obtained, indicating the high potential of cubic GaN for device applications. The observed gain mechanisms are similar to those of hexagonal GaN.

Optical constants of cubic GaN in the energy range of 1.5-3.7 eV

The refractive index and extinction coefficient of cubic GaN in the energy range of 1.5–3.7 eV were determined with high accuracy using combined reflectivity and spectroscopic ellipsometry studies of layers grown by molecular beam epitaxy on GaAs(001). A comparison of the experimental reflectivity data with theoretical calculations demonstrates that the data analysis has to be performed by taking into account both surface roughness and a nonabrupt substrate–film interface. In the transparent region the refractive index of cubic GaN was found to be slightly higher than that of the hexagonal modification.

INCORPORATION AND OPTICAL PROPERTIES OF MAGNESIUM IN CUBIC GAN EPILAYERS GROWN BY MOLECULAR BEAM EPITAXY

The incorporation and optical properties of Mg in cubic GaN (c-GaN) epilayers grown by rf plasma-assisted molecular beam epitaxy on (100) GaAs are investigated by secondary ion mass spectroscopy and low-temperature photoluminescence (PL). By varying the Mg flux by more than four orders of magnitude, the incorporation of Mg saturates at high Mg flux and is limited to a value of about 5×1018 cm−3 due to the high volatility of Mg at growth temperature. In addition, we observe an accumulation of Mg at the GaN/GaAs interface due to a diffusion of Mg to the GaAs substrate. Low-temperature PL spectra reveal several well-separated lines. Besides a shallow acceptor level at EA≅0.230 eV, additional Mg-related deep defect levels indicate an incorporation of Mg at off-gallium sites or as complexes.

Pressure and temperature effects on optical transitions in cubic GaN

Pressure and temperature effects on optical transitions in cubic GaN grown on a GaAs substrate have been studied by photoluminescence (PL) spectroscopy at hydrostatic pressures up to 9 GPa (10 K) and as a function of temperature (10–300 K) at ambient pressure. The dominant emissions at 10 K and ambient pressure are assigned to the bound-exciton transition (zero-phonon line), the donor-acceptor-pair (DAP) emission, and, tentatively, to the first three LO-phonon replicas of the bound exciton. These PL features shift to higher energy with increasing pressure. The pressure coefficients indicate that the observed recombination processes involve states which are closely related to the band edges. Temperature-induced evolutions from bound to free-exciton (FE) transition and DAP emission to free-to-bound transition are resolved. The binding energies of the FE and donor and acceptor levels in cubic GaN have been determined from the temperature and power-density dependence of the PL emission energies.

Cathodoluminescence of homogeneous cubic GaN/GaAs(001) layers

The cathodoluminescence (CL) of cubic (c-) GaN epitaxial layers is investigated at temperatures between 50 K and 300 K. The low temperature CL spectra show three well resolved emission lines (3.26 eV, 3.17 eV and 3.08 eV) which are due to excitonic, donor-acceptor and free to acceptor transitions. Spatially resolved measurements of the intensity of the excitonic emission demonstrate the homogeneity of the layers which are free of microcrystalline inclusions. The room temperature CL of the layers has a full width at half maximum of 56 meV and is due to excitonic recombination as is concluded from the zero-shift of the line position when the excitation intensity is varied over some orders of magnitude. The intensity of a broad emission band at 2.4 eV shows a strong nonlinear variation of the intensity at high excitation levels. Using a rate equation model for the near band edge and the deep 2.4 eV emission we are able to describe the intensity variation of these radiative transitions as a function of the excitation intensity. Depth resolved CL measurements reveal a homogeneous depth distribution of deep recombination centres responsible for the deep 2.4 eV luminescence band.

Optical gain and stimulated emission of cleaved cubic gallium nitrite

In this letter, we report on the observation of optically excited stimulated emission of c-GaN layers grown by molecular-beam epitaxy (MBE). Stimulated emission was observed at 1.8 K and room temperature. The threshold intensity for excitation of stimulated emission from our MBE-grown c-GaN layers is significantly lower than that reported for c-GaN grown by metalorganic chemical vapor deposition (MOCVD). The experimental data of optical gain and stimulated emission presented in this letter demonstrate that this material has a good potential for the future realization of cleaved cavity blue light-emitting laser diodes.

Micro-Raman analysis of cubic GaN layers grown by MBE on (001) GaAs substrate

Cubic GaN layers are grown by molecular beam epitaxy on (001) GaAs substrates. Optical micrographs of the GaN epilayers intentionally grown at Ga excess reveal the existence of surface irregularities such as bright rectangular structures, dark dots surrounded by rectangles and dark dots without rectangles. Micro-Raman spectroscopy is used to study the structural properties of these inclusions and of the epilayers in greater detail. We conclude that the observed irregularities are the result of a melting process due to the existence of a liquid Ga phase on the growing surface.

Magneto-optical investigations on cubic GaN in high magnetic fields

Abstract We report on direct IR–cyclotron resonance (CR) spectra of p-type cubic GaN (c-GaN) using semidestructive single-turn coil megagauss field generation. Two strong temperature dependent CR transitions were detected using measurement techniques with optimized signal to noise ratio resolving transmission changes smaller then 2%. On the basis of group theoretical relations between the valence-band energy-parameters of the effective Rashba–Sheka–Pikus Hamiltonian for the wurtzite and the effective Kohn–Luttinger Hamiltonian for the zincblende structure the Landau level scheme was calculated for c-GaN and compared with the observed transitions.