Gallium nitride materials - Progress, status, and potential roadblocks
Metal-organic vapor phase epitaxy (MOVPE) and molecular beam. epitaxy (MBE) are the principal techniques for the growth and n-type (Si) and p-type (Mg) doping of III-nitride thin films on sapphire and silicon carbide substrates as well as previously grown GaN films. Lateral and pendeoepitaxy via MOVPE reduce significantly the dislocation density and residual strain in GaN and AlGaN films. However tilt and coalescence boundaries are produced in the laterally growing material. Very high electron mobilities in the nitrides have been realized in radio-frequency plasma-assisted MBE GaN films and in two-dimensional electron gases in the AlGaN/GaN system grown on MOVPE-derived GaN substrates at the crossover from the intermediate growth regime to the droplet regime. State-of-the-art Mg doping profiles and transport properties have been achieved in MBE-derived p-type GaN. The Mg-memory effect, and heterogeneous growth, substrate uniformity, and flux control are significant challenges for MOVPE and MBE, respectively. Photoluminescence (PL) of MOVPE-derived unintentionally doped (UID) heteroepitaxial GaN films show sharp lines near 3.478 eV due to recombination processes associated with the annihilation of free-excitons (FEs) and excitons bound to a neutral shallow donor (DdegreesX). All six allowed Raman modes were observed with small full-width at half-maximum values. The introduction of Si or Mg introduces an intense edge emission assigned to a recombination process associated with X-Sidegrees or a deep compensating donor respectively. The PL spectrum of hydride-vapor-phase-epitaxy-derived freestanding GaN templates showed, within the 3.46-3.52-eV region, emission lines assigned to the excited state of the FE A (FXA1) and the dominant DdegreesX. Significant concentrations of Si and O donors were revealed by Fourier transform infrared and secondary ion mass spectrometry studies of UID freestanding samples. The results of magnetic resonance studies of shallow donors and acceptors, the 2.2-eV "yellow" PL band, and defects created by electron irradiation of native defects and dopants in GaN andlor AlGaN are detailed.