Silicon δ-doping and Isoelectronic Doping in GaAs ans GaN Layers Grown by MBE
This work concerns MBE-grown material, particularly physical effects due to controlled impurities of Si and N in arsenides, and the growth of nitrides and studies of Al- and As-impurities in them. Apart from interesting physical phenomena there are important device applications. The first part is devoted to studies of Si .delta.- to mono-layers in GaAs. The .delta.-doping represents the narrowest possible doping profile, which is important in order to realise small devices (e.g. with the purpose to decrease power consumption and increase device operation frequency). The second part concerns GaN. The direct large band gap of III-nitrides make them suitable for short wavelength, high intensity light emission applications in a vast range of wavelengths.
Thin silicon layers, embedded in GaAs, extending from a low .delta.-doping concentration up to 6 monolayers, were studied using the growth temperatures of 500 - 630°C. The electrical characteristics are discussed and a lattice relaxation of the Si-layer at 4 MLs was determined. The out-diffusion of the .delta.-layers as a function of growth temperature and doping concentration were measured by secondary ion mass spectrometry. For structures grown at temperatures near, or above 600°C, the out-diffusion was significantly larger at high doping concentrations (above 1012 cm-2). We proposed an expression to predict the migration of Si from the layer to the surrounding GaAs-matrix, with doping concentration and growth temperature as parameters. Photoluminescence of thin Si-layers in the monolayer range provided two fairly broad emission bands, which were explained by inter-band transitions in GaAs and Si. Studies of DX-like centres in InxGa1-xAs, were undertaken using InxGa1-xAs-quantum wells, which were silicon .delta.-doped in the centre. Confinement effects from the combination of quantum wells and .delta.-doping, gave a shift of the Fermi-level to far above the conduction band edge into the energy region where trapping, and hence Fermi level pinning, from DX-centres occurred. The Fermi-level energy was determined by measuring the free carrier concentration and self consistently solving the Schrödinger and Poisson equations. The behaviour of the free carrier concentration, and thus the Fermi-level, indicated the presence of DX-centres.
The optimum MBE growth conditions of GaN are very critical and narrow, particularly the N/Ga-flux ratio. We report optimisation of N/Ga-ratio on GaAs substrates using RHEED-reconstruction changes and surface morphology obtained by SEM. Aspects of isoelectronic doping in GaAs and GaN, and alloy concentrations of GaNxAs1-x and AlxGa1-xN are discussed. A series of AlGaN-samples using small amounts of Al were grown and characterised with photoluminescence, Hall effect and SEM. Effects on PL line-width and Hall effect data were observed due to aluminium concentration. Nitrogen was incorporated in GaAs from isoelectronic concentrations up to GaN. X-ray diffraction revealed two peaks from the layer, one close to the GaAs and the other at cubic GaN, respectively. Assuming the peak beside the GaAs was from GaNxAs1-x, about one order of magnitude lower nitrogen concentration from XRD was present as from the SIMS results, which was explained by phase separation. In the intermediate region of the alloy the surface roughness increased as a result of phase separation between the GaAs and GaN.
One work presents new findings of diffusion of N in GaAs. SIMS measurements were made on a 30 nm thick GaAs:N layer which was annealed at various temperatures. Assuming the kick-out mechanism, where interstitial nitrogen atoms replace the substitutional arsenic atoms, a set of coupled diffusion-reaction equations were solved using the SIMS-measured distributions as starting profiles. From these, the diffusion constants from nitrogen and arsenic atoms were obtained.