Oxygen and nitrogen uptake during the MBE growth of AlGaAs epitaxial layers

Submission note: A thesis submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy to the School of Engineering and Mathematical Sciences, Faculty of Science, Technology and Engineering, La Trobe University, Bundoora.

The main aim of the work reported in this thesis was to investigate the uptake of impurities during the MBE growth of the compound semiconductor AlGaAs on (100) GaAs substrates. A preliminary investigation of the relationship between perpendicular lattice parameter and Al mole fraction in a set of standard samples revealed that Vegard’s law as it relates to this material system was violated and the deviation from linearity was measured. Subsequently, the uptake of the impurities oxygen, nitrogen, carbon and indium incorporated from the ambient background during the MBE growth of a set of samples containing AlAs epitaxial layers was measured by SIMS and the perpendicular lattice parameters of the layers were measured by HRXRD. The relationship between levels of impurities and resulting AlAs perpendicular lattice parameter was modelled by assuming that the natural AlAs lattice constant was modified by the presence of impurities and quantified by lattice expansion and contraction parameters which exerted effects in proportion to the impurity mole fractions. This allowed predicted lattice constants to be calculated to assess the impact of the various impurities on perpendicular lattice parameters. This modelling approach was then extended to AlGaAs, which was more complex because of the range of stoichiometries possible compared with the particular stoichiometry of AlAs. This work was further extended to an investigation of impurity incorporation when deliberately admitting pure oxygen, pure nitrogen or air into the growth chamber as AlGaAs layers were being grown, although the low levels admitted meant that the impurity levels remained dominated by the ambient background present before any leak was established. The conclusions that could be drawn from this work were that the impurities oxygen and carbon substitute on group V sites only when there was at least one aluminium atom in the group III layer beneath, while indium atoms and nitrogen molecules sit on group III sites. Values of the lattice expansion and contraction variables were determined. The process of incorporation of oxygen into AlGaAs epitaxial layers has also been modelled using a kinetic rate equation approach. The model was applied to SIMS measurements of oxygen levels in AlGaAs layers grown under ambient background conditions to ascertain the mechanisms at play when oxygen is incorporated. Many possible processes were assessed against the experimental data and the conclusions that could be drawn were that: (i) oxygen chemisorbs into an AlGaAs layer only if at least one Al atom sits in one of the two group III sites to which the As site occupied by the oxygen is bonded, (ii) that oxygen can segregate back to the physisorbed state if no more than one Al atom sits in the two group III sites below, (iv) that As4 molecules in the physisorbed state can block potential oxygen bonding sites and that As4 diffusion rates depend on the local surface Al mole fraction.