ON THE ATOMIC-STRUCTURE AND ELECTRONIC-PROPERTIES OF DECAPPED GAAS(001)(2 X 4) SURFACES

We have performed scanning tunneling microscopy and spectroscopy on the GaAs(001)(2 x 4) surface. The measurements were performed on arsenic capped samples grown ex-situ by MBE. The n-doped epilayers were grown on flat GaAs(001) n-doped wafers and substrates cut 2-degrees towards (010). The epilayers were terminated with 100 angstrom of intrinsic material. Using STM we establish that capping does not compromise the surface quality of the (2 x 4) reconstruction. We find that provided the ex-situ MBE grown surfaces are of sufficient quality, the arsenic cap can be stripped off to yield surfaces equal to in-situ samples. Results from vicinal samples indicate that As from the cap adheres more strongly to step edges, which can be removed by subsequent annealing. The surfaces of all the samples studied were a mixture of (2 x 4) and c(2 x 8) domains. Tunneling I-V curves are interpreted as primarily due to tunneling out of the doubly occupied lone-pair states located on each surface As atom. Compared with GaAs(110), the spectra are much less sensitive to the distribution of empty states above E(F). This behavior was independent of spatial location on the surface. The insensitivity to Ga derived empty states persists even when spectra are obtained above exposed Ga sites in the missing dimer rows where they may be expected to contribute. The measurements serve to locate the Fermi level on these samples pinned 0.6 +/- 0.2 eV above the valence band maximum.