X-ray diffraction study of interdiffusion and growth in (GaAs)<i>n</i>(AlAs)<i>m</i> multilayers

Abstract

A detailed study has been made of interdiffusion in (GaAs)n(AlAs)m multilayer structures grown by molecular beam epitaxy. The subscripts n and m indicate that the structure is a repeating sequence of n monolayers of GaAs followed by m monolayers of AlAs. The time dependence of the Fourier components of the composition modulation at a constant annealing temperature was obtained by measuring the intensities of the superlattice satellites by x-ray diffraction. For a sample with (n,m) = (12,9) at a temperature of 860 °C, the data can be fit by a composition-dependent diffusion coefficient of the form D (C) =8.8×10−20 exp(αC) cm2/sec, where α=2.06 and C is the gallium concentration. The parameter α is proportional to the difference in the diffusion activation energy in GaAs and AlAs. An α of 2.06 at 860 °C corresponds to an energy difference of 0.201 eV. Calculations which assume vacancy diffusion via the arsenic sublattice give an energy difference of 0.25 eV in reasonable agreement with the experimental value. The average diffusion constant for a sample with (n,m) = (1.1,1.1) is 5.3×10−21 cm2/sec at T=800 °C. These results are in good agreement with those reported in the literature. In addition, a large growth anisotropy is observed in the (100) plane with a larger correlation length in the direction parallel to the dangling bonds on the As-rich surface of GaAs.