Ga-rich GaAs[001] surfaces observed by STM during high-temperature annealing in MBE chamber

Si-doped GaAs[001] 1/spl deg/ off <111>A (n = 2 /spl times/ 10/sup 18/ cm/sup -3/) substrates were prepared by standard procedures in MBE and then annealed at 550/spl deg/C without As overpressure in order to produce a (4 /spl times/ 2) phase surface as determined by RHEED and RAS(or RDS). Under the same conditions as these measurements, STM images were obtained. The STM data strongly points to a co-existence of reconstructions. One set of candidates is predicted as the /spl zeta/(4 /spl times/ 6), /spl zeta/(4 /spl times/ 4), and /spl zeta/(4 /spl times/ 4) reconstructions. All models are based on /spl zeta/(4 /spl times/ 2) by Lee et al. and satisfy electron-counting heuristics. The models differ in the presence and location of Ga atoms. At elevated temperatures Ga adatoms can detach and diffuse to make Ga clusters. Mobile Ga would result in different surface reconstructions on different parts of the surface. Each of these reconstructions does not form large domains and is distributed randomly on the [001] surface. This reasonably explains why we do not observe the 1/4- and 1/6-order reflections in RHEED patterns obtained along the [1 - 10] direction. However, since all reconstructions are derived from the /spl zeta/(4 /spl times/ 2), the surface dynamics associated with Ga motion will produce transient regions with this symmetry. Therefore, it is natural to observe the 1/2-order reflection along the [1 - 10] direction.