SILICON SPIKES AND IMPURITY ACCUMULATION AT INTERRUPTED GROWTH INTERFACES DURING MOLECULAR-BEAM EPITAXY

Recent proposals by Wood and Wilson [C. E. C. Wood and R. A. Wilson, J . Vac. Sci. Technol. B 11, 1036 (1993)], to explain the formation of i mpurity spikes at substrate/epitaxial layer interfaces in GaAs prepare d by molecular-beam epitaxy (MBE), have been experimentally investigat ed. Their suggestion that the spikes form due to suboxide transport vi a reactions that involve the As2O3 released from the substrate during oxide desorption and hot Knudsen cells, is not supported by the experi mental data. The same authors have also speculated that there may be s ignificant flux leakage from nominally closed cells. For this to occur , reflection and scattering of flux by inadequately cooled cryoshroud baffle surfaces are necessary. Secondary ion mass spectrometry analyse s of interfaces, at which the growth of GaAs and AlAs was interrupted for times up to 30 min, confirm that this takes place. However, flux l eakage is only found to be significant for the high vapor pressure gro up III elements. For these elements, incorporation levels in the range 0.02%-0.1% are found under normal deposition conditions. These result s suggest that careful attention should be given to increasing the int ernal MBE system baffling in order to eliminate cross contamination pr oblems.