We present a study of the growth of cubic GaN films on (001) GaAs by molecular beam epitaxy. Our investigations focus on the nucleation stage as well as on the subsequent growth of GaN. The phenomenon of epitaxial growth at this extreme mismatch (20%) is demonstrated to arise from a coincidence lattice between GaAs and GaN. The presence of a high-density of stacking faults in the GaN layer is explained within this understanding as being a natural consequence of the coalescence of perfectly relaxed nuclei. We furthermore analyze the growth kinetics of GaN via the surface reconstruction transitions observed upon an impinging Ga flux, from which we obtain both the desorption rate of Ga as well as the diffusion coefficient of Ga adatoms on the Ga-stabilized GaN surface. The diffusivity of Ga is found to be very low at the growth temperatures commonly used during molecular beam epitaxy, which provides an explanation for the microscopic surface roughness observed on our samples.

At hydrostatic pressures up to 12 GPa, photoluminescence experiments were performed simultaneously on zincblende (cubic) and wurtzite (hexagonal) gallium nitride. The linear band gap pressure coefficient of hexagonal GaN was determined to be (0.94 +/- 0.04) meV/GPa larger than the one of cubic GaN.

We review our investigations of the optical properties of β-GaN and β-(In,Ga)N layers grown on GaAs(001) by plasma-assisted molecular beam epitaxy. For In contents up to 17%, the band gap of β-(In,Ga)N layers, estimated by combining photoluminescence with spectroscopic ellipsometry as well as transmittance spectroscopy, shows good agreement with theoretical predictions from the literature. Using the theoretically predicted band gaps, we calculate 2.75eV isoenergy contours for both β-(In,Ga)N/GaN and α-(In,Ga)N/GaN quantum wells. Comparing these contours to calculations of the critical thickness, it becomes evident that blue-emitting α-(In,Ga)N/GaN quantum wells are above or at the border to plastic relaxation except for very thin quantum wells of high In content. In contrast, blue-emitting β-(In,Ga)N/GaN quantum wells lie below this border for a wide range of thickness and In content.

We discuss the conditions to be satisfied for the growth of cubic GaN films on GaAs(001) and Si(001) by plasma-assisted MBE. A strictly strichiometric group V/III Our ratio is required for single-phase growth, which is controlled in real time by means of RHEED. While phase purity of cubic GaN-on-GaAs is induced by a well-defined orientation relationship, growth on Si(001) often results in a phase transition due to the formation of amorphous SixNy, inclusions at the GaN-on-Si interface during nucleation. A GaAs-on-Si(001) starting surface overcomes this problem as evidenced by the phase purity of the resulting epitaxial cubic GaN films.

We discuss the conditions to be satisfied for the growth of cubic GaN films on GaAs(001) and Si(001) by plasma-assisted MBE. A stoichiometric group V/III flux ratio is required for single-phase growth, which is controlled via the surface reconstruction in real time by means of RHEED. While phase purity of cubic GaN-on-GaAs is induced by a well-defined orientation relationship, the growth on Si(001) often results in a phase transition due to the formation of amorphous SixNy, inclusions at the GaN-on-Si interface, which act as nucleation centers for the growth of the hexagonal GaN phase. A suitable template, such as a thin GaAs or SiC insertion layer, prevents the formation of the SixNy inclusions. Cubic GaN grown under optimized conditions exhibit a narrow excitonic luminescence line at 3.272 eV at 5 K and high luminescence efficiency at room temperature. In addition, we discuss the origin of the n-type background doping in the as-grown layers and we present the new concept of codoping to achieve high p-type conductivity in GaN at room temperature.

We discuss the growth of cubic GaN and (In,Ga)N films on GaAs by plasma-assisted molecular beam epitaxy. Conditions to be satisfied for the synthesis of single-phase films are pointed out. In the case of the binary compound GaN, strictly stoichiometric growth is required, while the ternary compound (In,Ga)N has to be grown N rich for reducing the amount of In segregating on the growth front. Finally, we discuss our finding of high p-type conductivities in (Be,O)-codoped GaN films in the light of recent theoretical studies of this subject.

We present a theoretical investigation of the carrier recombination in GaN. Analytical expressions of the radiative recombination coefficients of the coupled exciton–free-carrier system are derived by employing the van Roosbroeck–Shockley relation between absorption and spontaneous emission. Screening to the first order for the discrete exciton state is taken into account. Our results demonstrate the importance of exciton effects, which persist up to high temperatures and excitation densities, for the spontaneous emission in GaN. Finally, "best-case" simulations of the particle densities after pulsed excitation as a function of time or depth show the importance of both bulk and interface nonradiative recombination in GaN.

The reconstruction and topography of the β-GaN(001) surface are investigated with scanning tunneling microscopy on GaN/GaAs(001) layers grown by molecular beam epitaxy. Two Ga-terminated reconstructions are observed: a (2 × 2)- and a novel (√10×√10)R18.4°. The experimental results of filled and empty state STM images are in good agreement with a simulation based on structure models that include a dimerization of the surface Ga atoms. The formation of the unique (√10×√10)R18.4° is related to the high ionicity of GaN and to steric reasons, not allowing a full Ga coverage of the surface.

Cubic GaN films were grown on GaAs substrates by plasma-assisted molecular beam epitaxy (MBE). Using real-time control of the group V/ III flux ratio by means of RHEED, single-phase cubic GaN films with superior structural and optical properties were obtained. A narrow excitonic luminescence line at 3.272 eV is observed at 5 K, yielding a low-temperature energy gap of 3.30 eV. The nucleation of the metastable cubic GaN phase with a well-defined orientation relationship to the GaAs substrate is discussed. In addition, we discuss the origin of the n-type background doping in the as-grown layers, and we present a new concept of codoping to achieve high p-type conductivity in GaN at room temperature.

Single-phase cubic GaN on (001)GaAs with superior structural and electronic properties was grown by plasma-assisted MBE using the in-situ control of the surface stoichiometry via PHEED intensity transients which in turn allows one to exactly determine the VIII flux ratio. The phase purity degrades with deviations from the flux ratio of unity. Detailed cathodoluminescence measurements allow us to identify the exciton (free at 3.272 and bound at 3.263 eV, both at 5 K) and the donor-acceptor pair (3.150 eV) transitions, revealing an energy gap of 3.30 eV for cubic GaN at 5 K.

We report on the epitaxial growth and the microstructure of cubic GaN. The layers investigated are deposited by plasma-assisted molecular beam epitaxy on GaAs (001) and (311)A substrates. Transmission electron microscopy reveals that, despite of the extreme lattice mismatch between these two materials, GaN grows in the metastable cubic phase with a well-defined orientation-relationship to the substrate and a sharp heteroboundary. This preference of the metastable phase and its epitaxial orientation originate in the initial stage of growth which is discussed in connection with a coincidence lattice for the investigated interface structures.

Ultrathin (5–7 monolayers) nucleation layers of GaN are deposited on (2×4)-GaAs(001) by plasma-assisted molecular beam epitaxy. Reflection high-energy electron diffraction applied in situ reveals these layers to be epitaxial β-GaN. Transmission electron microscopy confirms this result and reveals in addition that the layers are highly connected and have an atomically abrupt interface to the GaAs substrate. The rms roughness of these layers, as measured by atomic force microscopy, is as low as 1.4 Å.

Using scanning tunneling microscopy and reflection high energy electron diffraction, we study the surface reconstructions of zincblende GaN(001) prepared by molecular beam epitaxy on GaAs(001). We observe a (2 x 2) and a (root 10 x root 10)R18.4 degrees reconstruction at a Ga coverage of 0.5 and 0.8 monolayers, respectively. Local density map calculations of the highest occupied and lowest unoccupied molecular orbitals for the experimentally deduced structure models based on Ga dimer formation show excellent agreement with the filled and empty state STM images, respectively. (C) 1997 Elsevier Science B.V.

The valence band discontinuity of the n-type cubic GaN/GaAs heterojunction is measured by means of angle-resolved photoemission spectroscopy using synchrotron radiation. High quality cubic GaN films are grown on GaAs(100) substrates by nitrogen plasma-assisted molecular beam epitaxy, and the valence band discontinuity is determined by a combination of core and valence level spectra. A value of ΔEV=(1.84±0.1) eV across the GaN/GaAs heterojunction is obtained, which means that the discontinuity in the conduction bands at this interface is very small, such that a vertical contact scheme may be realized for GaN/GaAs heterojunctions.

We investigate the transient surface emission and the optical gain of cubic GaN on GaAs(001) upon pulsed optical pumping at room temperature. The initial decay time of the transient surface emission drastically decreases with increasing excitation density, reaching a value as short as 20 ps at a fluence of 50 μJ cm−2. This rapid decay suggests the presence of laterally amplified spontaneous emission. In fact, gain-stripe measurements of the edge emission reveal an optical gain exceeding 100 cm−1 at a fluence of 20 μJ cm−2.

We compare the recombination dynamics of GaN single crystals to that of epitaxial cubic GaN layers on GaAs(001) using picosecond photoluminescence (PL) spectroscopy. At low temperatures, the band-edge PL decay time of the layer is much shorter than that of the crystals, evidencing the importance of nonradiative processes in the case of the layer. However, at room temperature both the emission spectra and their decay times of layer and crystals are almost identical. At 300 K the decay times are short (10 ps) for low excitation density and increase to values of 100 ps at moderate excitation density reflecting the saturation of nonradiative recombination channels.

The hexagonal and cubic phases of GaN are characterized by spatially resolved cathodoluminescence (CL) spectra from micrometer-size single crystals with either hexagonal or cubic habits grown by plasma-assisted molecular-beam epitaxy. At 5 K, distinct narrow excitonic lines are found at 3.472 and 3.272 eV for the hexagonal and cubic phase, yielding energy gaps of 3.500 and 3.300 eV, respectively. Detailed temperature- and intensity-dependent CL measurements on cubic GaN crystals enable us to clearly identify the exciton (free: 3.272 eV, bound: 3.263 eV) and the donor-acceptor pair (3.150 eV) transition. Moreover, we determine the donor-band and acceptor-band transition energy for this phase. In addition, phonon replicas of the exciton line and of the donor-acceptor pair transition are observed at 3.185 and 3.064 eV, respectively.

Spatially resolved cathodoluminescence (CL) spectroscopy in connection with scanning electron microscopy performed on cubic (c) GaN between 5 and 300 K reveals that at low temperatures the CL spectra of c-GaN single crystals consist of four well-separated lines. The two lines highest in energy were previously identified as excitonic and donor-acceptor transitions, respectively. Here, we show that the lines lowest in energy are due to an additional free-to-bound transition, involving an impurity different from those related to the donor-acceptor transition, and its phonon replica. The CL spectra of c-GaN layers, while being rather broad, are composed of these four lines. Moreover, at 300 K the spectra of the layers and of the crystals are both dominated by the excitonic transition and closely resemble each other.

P-type room-temperature conductivities as high as 50/Ω cm are achieved in cubic GaN layers by the concept of reactive codoping. We use Be as the acceptor species and O as the reactive donor to render isolated Coulomb scatterers into dipole scatterers. This concept allows us to achieve high hole mobilities and thus p-type conductivities.