It was recalled that high n+-doping, of the cap layers of heterojunction structures, produced anomalous Zn diffusion in the base region during metalorganic vapor phase epitaxial growth. This was attributed to non-equilibrium group-III interstitials that were generated in the cap layer, and created highly diffusive Zn interstitials via the kick-out mechanism. It was shown here that low-temperature (550C) growth was effective in reducing the effect of the n+ cap layer. Due to a large time constant for the recovery of thermal point defect equilibrium, the last-to-grow n+ cap layer could not inject excessive numbers of group-III interstitials into the base region during growth. However, during low-temperature growth the first-to-grow n+ sub-collector produced group-III interstitials and thus caused anomalous Zn diffusion. In order to prevent this effect, it was suggested that growth should be interrupted for 0.5h before growing the base layer, and that growth of the n+ sub-collector should be carried out at 600C. These changes were effective in removing undesirable group-III interstitials.

K.Kurishima, T.Kobayashi, H.Ito, U.Gösele: Journal of Applied Physics, 1996, 79[8], 4017-23