NUCLEATION OF GAAS ON CAF2 SI(111) SUBSTRATES

Two different nucleation modes in epitaxial growth of GaAs on CaF2/Si( lll) substrates ave been identified. At low temperature (below 500-deg rees-C). GaAs nucleates on CaF2(111) surface with a three-dimensional mode. When T(s) is high (higher than 590-degrees-C), two-dimensional n ucleation of GaAs can be achieved on the CaF2/Si(111) surface under a proper As flux. A thorough investigation of this phenomenon has been c arried out using Rutherford backscattering spectrometry. A stable As l ayer has been observed on the CaF2(111) surface when the CaF2/Si(lll) substrates are treated at high temperatures and under a high enough As flux (greater-than-or-equal-to 2 X 10(-5) mbar). The formation of thi s As layer is understood as a consequence of a chemical reaction betwe en As adatoms and the CaF2 surface. The existence of the As layer modi fies the surface free energy of CaF2/Si(lll) substrates and leads to a transition in the GaAs nucleation mode. By controlling the nucleation mode, high quality GaAs has been grown on the top of CaF2/Si(111) sub strates without using electron-beam modification of the CaF2 surface, and, thus, avoids irradiation damage of the epitaxial initiation surfa ce.