| Factors affecting inter-diffusion in AlGaAs heterostructures have been studied using photoluminescence measurements of single and multiple quantum well structures grown by both Molecular Beam Epitaxy and Metal-Organic Chemical Vapor Deposition techniques. Results of anneals done on these samples indicate a depth dependence for inter-diffusion, arising from incomplete native-defect diffusion due to interactions between the crystal and the annealing ambient. These data confirm previous observations of incomplete defect diffusion and show that defect kinetics must be included in models describing inter-diffusion. Additionally, inter-diffusion coefficients calculated from post-anneal energy shifts of identical quantum well structures, grown on different substrates, differ by an order of magnitude, suggesting that defect incorporation in the epitaxial layers can also be influenced by growth conditions. Such influences are not presently treated in models of inter-diffusion.Experiments were also conducted to examine the effect of the crystal Fermi level on inter-diffusion. In contrast to previous work, variation of the Fermi level has been achieved by placing quantum wells in different sections of the intrinsic regions of p-i-n and n-i-p samples, thereby separating the Fermi level effect from other influences of the dopants. Inter-diffusion coefficients calculated for annealed quantum wells do not agree with those predicted by current theory. Possible reasons for this discrepancy are discussed.These observations were used to design a self-consistent experiment in which the time evolution of transition energies from five quantum wells is monitored to obtain information on the relative defect concentration profile in the epitaxial layers. Experimental data obtained from a sequence of anneals using this structure clearly illustrate the diffusion of defects through the structure, thereby providing a more accurate picture of the inter-diffusion process.A numerical model in which defect diffusion is allowed to proceed concurrently with inter-diffusion has also been developed. Parametric analysis using this model indicates that defect diffusion is rarely complete for typical heterostructures and annealing conditions of interest. The calculations also show that defect diffusion can be completely masked if inter-diffusion rates are much larger than defect diffusion rates. |
