A INTRODUCTION

Molecular beam epitaxy (MBE) consists of growth by evaporation and condensation of elemental species. These species generally evaporate as either single atoms or as molecules of 2 4 atoms. For Si, Mella and Brodsky [1] have determined that evaporation is primarily as single atoms, making the name MBE, strictly, a misnomer. The ideal species is one with a vapour pressure high enough that it can be evaporated from thermal sources, but low enough that it will condense on all other surfaces within the MBE system. If that atom is evaporated in a vacuum free of scattering, deposition will then be a line-of-sight process from the source to the first cool surface. Fluxes can then be tuned by simple geometrical considerations and stopped, instantly, by interposing deposition 'shutters'. These attributes make MBE the most conceptually simple of the epitaxial growth techniques and provide control of composition, doping and layer thickness down to a scale of single atomic layers [2 4].

These considerations can be used to define generalized MBE operating conditions. To be free of scattering, deposition must occur over dimensions small compared to the vacuum mean free path [5]

(1)

where P is the total pressure of the residual gas, which is assumed to be air-like in composition and at room temperature. The mean free path varies inversely as pressure and reaches 5 m at 10 ^-5 torr. Given typical MBE system dimensions...