Surface nitridation by hydrazine-sulfide solution, which is known to produce surface passivation of GaAs crystals, was applied to GaAs nanowires (NWs). We studied the effect of nitridation on conductivity and microphotoluminescence (μ-PL) of individual GaAs NWs using conductive atomic force microscopy (CAFM) and confocal luminescent microscopy (CLM), respectively. Nitridation is found to produce an essential increase in the NW conductivity and the μ-PL intensity as well evidence of surface passivation. Estimations show that the nitride passivation reduces the surface state density by a factor of 6, which is of the same order as that found for GaAs/AlGaAs nanowires. The effects of the nitride passivation are also stable under atmospheric ambient conditions for six months.

A mild wet nitridation procedure using hydrazine-based solutions has been developed for GaAs (100) surface passivation. Both x-ray photoelectron spectroscopy and spectroscopic ellipsometry show that this nitridation procedure results in a very thin, coherent, and homogeneous GaN layer that is very stable in air. Photoluminescence data show a strong enhancement of the intensity as compared to that of an as-cleaned GaAs sample, indicating that this nitrided layer provides both chemical and electronic passivation of GaAs surfaces. The chemical mechanism of nitridation is discussed.

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Reflectance anisotropy (RA) spectrosocpy has been used to study at 300 K the intrinsic optical transitions on sulfur-passivated surfaces. The spectra allow to follow the modification of surface dimers after annealing at a temperature Ta. For Ta≊440 °C, the S-treated surface is covered by arsenic and sulfur dimers oriented along the [11̄0] direction. Upon subsequent heating, sulfur is desorbed and there appear gallium dimers oriented along [110]. Sulfur passivation has allowed to obtain stable surface structures, with a strongly reduced band bending, which are consistent with the known (2×1) and (4×6) reconstructions.

We have investigated the correlation between the change of the surface electronic properties ͑surface recombination velocity, surface barrier͒ and the change of the surface chemical bonds under annealing in ultrahigh vacuum of sulfide-passivated ͑001͒GaAs. The electronic properties of a ͑NH 4 ͒ 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 ͑i͒ reflectance anisotropy spectroscopy, which essentially monitors the optical transitions due to surface dimers, and ͑ii͒ core-level spectroscopy results on the same sample ͑companion paper in the same issue͒. We find that breaking of arsenic-related chemical bonds, which induces arsenic dimers on the surface, produces an increase of photoluminescence intensity ͑PLI͒. 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: ͑i͒ The improvement of electronic properties of ͑001͒ GaAs arises due to formation of S-Ga bonds on the gallium-terminated part of the surface, ͑ii͒ electronic properties of the astreated surface are deteriorated by excess arsenic, which produces midgap levels, and ͑iii͒ the passivation efficiency can also be reduced by formation of additional surface defects due to etching of the surface in sulfide solutions.