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 (<1%), respectively, were determined. Detailed characterisations of the layer structure and the composition revealed phase-separated micrometer sized features.
High-quality wurtzite GaN layers were grown by MBE on sapphire(0001) substrates with ~15% lattice-mismatch, showing 18 meV photoluminescence (PL) linewidth, a free electron concentration of to 1x1017cm-3, typical bulk mobility values of ~100 cm2/Vs, and a surface roughness of ~10 nm. The effects of the V/III-ratio, AlN buffer layer, impurities and defects were characterised, revealing correlations between morphology, optical and electrical properties. The effects of the oxygen donors, carbon acceptors, and defect related donors and acceptors were determined by a systematic series of epilayers and characterisations.
Low aluminium concentrations (0.10-3.30%) were investigated for wurtzite AlxGa1-xN on sapphire (0001). The electron mobility and the surface roughness improved significantly around 0.17% Al, compared to pure GaN. The incorporation of 0.10% Al resulted in an improved PL linewidth of 15 meV. The PL spectra indicated a linear increase of the band gap above 0.23% Al, while at lower concentrations it remained constant.
The epitaxial growth of the wurtzite InxGa1-xN is difficult, since In is known to segregate. Optical characterisation on a series of different InxGa1-xN layers indicated the formation of phases with discrete In-contents, even increasing above the supplied In/Ga-flux ratio. A clear vertical compositional change was also revealed, formed due to In-droplet formation during the layer growth. Perfect hexagonally shaped crystallites were revealed under the droplets, formed under the liquid In phases. The surfactant effects of In were also observed, providing improved Ga-faced surfaces.
The optimisation of epitaxial growth for GaN and AlxGa1-xN layers on sapphire(0001) substrates resulted in high-quality materials. Using this know-how, piezo-induced AlGaN/GaN heterostructure field-effect transistor (HFET) structures were grown. The formation of a two-dimensional electron gas (2DEG) at the interface effectively improved the bulk mobility by a factor of three.