SCANNING-TUNNELING-MICROSCOPY OF FLAT AND VICINAL MOLECULAR-BEAM EPITAXY-GROWN GAAS(001)-(2X4) SURFACES - THE EFFECT OF GROWTH-RATE

Scanning tunneling microscopy (STM) has been used to investigate the e ffect of the deposition rate on the resulting morphology of nominally flat and vicinal GaAs(001)-(2 X 4) surfaces grown by molecular-beam ep itaxy. On the nominally flat surfaces, low deposition rates are found to create smooth surfaces and lead to anisotropic islanding with an A- type step (Ga terminated, parallel to the [110BAR] direction) to B-typ e step (As terminated, parallel to the [110] direction) average aspect ratio which is larger than that produced by standard deposition rates . The results suggest that the increase in the island anisotropy at lo w growth rates reflects the energetics of the step edges. In contrast, the growth rates investigated are found not to have any obvious effec t on the resultant morphology of the 2-degrees A-type vicinal surfaces . In particular, no islanding on the terraces is observed at either de position rate. However, detailed statistical analyses of the STM image s indicate that there is a larger spread in the terrace width and a gr eater probability for forming kinks with increasing growth rate. This observation suggests that the probability of a Ga adatom incorporating at an A-type step (rather than at a B-type kink) is greater for stand ard growth rates than for low growth rates.