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We report experiments for recovery of the specular beam intensity in reflection high-energy electron diffraction (RHEED) after exposure to As flux over a pre-deposited Al film of thickness equivalent to several monolayers on a GaAs or AlAs substrate in a temperature range between room temperature and 700°C. Four types of recoveries have been found. They form a phase diagram with the substrate temperature and the Al film thickness as parameters. The phase diagram suggests that the thinner Al film on a substrate has lower melting temperature than the bulk melting point of Al if we assume that As atoms do not enhance diffusion of Al atoms in a solid Al film.

Anisotropy in surface migration of Ga atoms on GaAs (001) has been found by a study of the reflection high-energy electron diffraction (RHEED) intensity oscillations. The RHEED intensity oscillations have been investigated on GaAs (001) vicinal surface inclined to both directions of [110] and [1 1 0]. After the method given by Neave et al. [Appl. Phys. Letters 47 (1985) 100], it is found that the diffusion coefficient at [1 1 0] azimuth is 4 times larger than that of [110]. Isotropic surface diffusion of Ge on Ge (001) is also discussed in comparison with GaAs (001).

Tunnelings of X-point electrons are observed in MBE grown AlAs-well, Al 0.5 Ga 0.5 As-barrier triple barrier diodes. In this structure, X-point minima are the lowest conduction band. At 85 K, tunneling peaks are observed both in the thicker and the thinner barrier diode. Although there is a large difference in current density, bias voltages for respective current peaks are very similar for these two diodes. This rules out the possibility of resonant tunnelings through a parasitic Γ -point quantum well in a Al 0.5 Ga 0.5 As barrier. Five current peaks are identified and these are assigned as the tunnelings from the ground subband in one well to the ground, the first excited, and the second excited subband in the other well. Comparison with a Γ -point resonant tunneling diode shows that the electron mass which defines the subband energy levels in the well is the X-point transverse mass.