Reflection high-energy electron diffraction intensity oscillations and surface reconstructions measured during epitaxial growth of Si(001) from Si2H6 molecular beams

Abstract

The growth of Si(001) from a gas source molecular-beam epitaxy system using disilane (Si2H6) was investigated using reflection high-energy electron diffraction (RHEED). Surface reconstructions which occurred during growth were studied as a function of both substrate temperature and surface coverage. Growth below 600 °C was found to be initiated by the formation of three-dimensional (3D) islands. Near 645 °C, (2×2) and c(4×4) reconstructions occurred on the growing Si(001) surface. At higher temperatures, above 680 °C, growth was found to proceed two dimensionally. The Si(001) surface was found to have undergone a series of reconstructions which have been attributed to the number of hydrogen adatoms and Si dimers covering the surface. RHEED intensity oscillations have been used to monitor Si(001) during growth. Oscillations are easily obtained on well prepared surfaces on which a buffer layer has been grown. Oscillations of the specular beam in both the [010] and [110] azimuths have been measured as a function of temperature and disilane flow rate. Strong and damped oscillations are observed between 610 and 680 °C in the two-dimensional growth regime. At higher temperatures growth by step propagation dominated, while at lower temperatures growth became three dimensional (3D) and consequently oscillations were weak or absent. Oscillations of the fractional order and specular beams in the [110] azimuths indicate that growth occurs in a monolayer rather than a bilayer fashion, and the alternating intensity of sequential oscillations in the specularly reflected beam in the [110] azimuth may be indicative of the relative step density in the two orthogonal directions. Growth rates, as determined from the oscillations, are found to be independent of incident beam flux at substrate temperatures below 600 °C. Above this temperature the growth rate increases proportionally to the incident flux. An Ahrrenius plot indicates an activation energy (EA) of 40.7 kcal/mol in the low temperature regime (T<600 °C) and an apparent EA dependence on disilane flux in the high temperature regime.