Sulfide-passivated GaAs (001) .2. Electronic properties

We have investigated the correlation between the change of the surface electronic properties (surface re; combination velocity, surface barrier) and the change of the surface chemical bonds under annealing in ultrahigh vacuum of sulfide-passivated (001)GaAs. The electronic properties of a (NH4)(2)S-passivated surface were monitored using room-temperature photoreflectance, which gave the value of the surface barrier, and photoluminescence. The surface chemical bonds; were probed by (i) reflectance anisotropy spectroscopy, which essentially monitors the optical transitions due to surface dimers, and (ii) core-level spectroscopy results on the same sample (companion paper in the same issue). We find that breaking of arsenic-related chemical bonds, which induces arsemic dimers on the surface, produces an increase of photoluminescence intensity (PLI). Conversely, a clear correlation is found between the desorption of sulfur due to breaking of Ga-S chemical bonds, the appearance of the gallium dimer line, and the decrease of PLI. Based on these results, we outline the dominant features of sulfide passivations: (i) The improvement of electronic properties of (001) GaAs arises due to formation of S-Ga bonds on the gallium-terminated part of the surface, (ii) electronic properties of the as-treated surface are deteriorated by excess arsenic,which produces midgap levels, and (iii) the passivation efficiency can also be reduced by formation of additional surface defects due to etching of the surface in sulfide solutions.