Molecular Beam Epitaxy and Characterisation of GaN-compounds on GaAs(001) and Sapphire(0001)

The hexagonal (wurtzite) and the cubic (zinc blende) group-III nitrides and their heterostructures have attracted much attention due to their potential for applications in high-power, high-frequency electronic and optoelectronic devices. The optical emission range of the GaN-based alloys cover the whole visible range from near infrared (IR) to ultraviolet (UV). Considerable effort is being devoted to improve the quality of epitaxial layers, as well as material characterisation methods and techniques for device processing. Heteroepitaxial layers, in the form of the thermodynamically stable wurtzite crystal structure, are used for high electron mobility transistors (HEMTs), light emitting diodes (LEDs) and lasers. Recently, new applications appeared in traffic lights and efficient low-voltage, flat panel white light sources and UV laser diodes, while there is tremendous interest in high-density optical storage systems, UV lithography and projection displays. This work is devoted to the plasma-assisted molecular beam epitaxy (MBE) growth of high-quality GaN-compounds. The lack of a suitable lattice-matched substrate and the large number of growth parameters with a narrow region of optimum condition make the MBE growth of these materials especially difficult. The possibility of device integration motivates the study of GaN-growth on substrates such as GaAs. To obtain GaN epilayers in the less stable zinc blende structure, the MBE technique provides the required non-equilibrium conditions at low growth temperatures. Due to the large difference in lattice constant between GaAs and GaN (~20%), and the reactivity of the active nitrogen, the plasma-assisted nucleation of GaN on GaAs(001) is very sensitive to the concentration of native defects at the surface. In most cases the GaAs surface is degraded through ion damage. Based on the reflection high-energy electron diffraction (RHEED) surface reconstruction transition (2x4)=>(3x3)=>(1x1), a transition diagram was constructed to find near-stoichiometric conditions, resulting in a layer-by-layer growth with a device quality surface roughness of ~20 nm. Real-time RHEED revealed the effects of the first GaN monolayers on the nitridation damage. These in situ results were combined with a unique ex situ morphology characterisation of the damaged interface region, resulting in a detailed model to describe the damage formation. As a mixed group-V ternary, zinc blende GaNxAs1-x layers were grown by MBE on GaAs(001) substrates, with a nitrogen concentration ranging from low N-doping concentration in GaAs up to GaN. The solubility limits for N in GaAs (~9-10%) and As in GaN