Spin-valve, weak localization/antilocalization, and scanned probe microscopy measurements are used to investigate the influence of sulfur-based surface treatments and electrically insulating barrier layers on spin injection into, and spin transport within, the two-dimensional electron layer at the surface of p-type InAs at 4.2 K. An electrically insulating barrier layer is found to be required to achieve nonzero spin injection efficiency, with a 3 nm Al(2)O(3) electrically insulating barrier providing a spin injection efficiency of 5 +/- 2%. Conductive atomic force microscopy suggests that localized leakage through the InAs native oxide is sufficient to suppress spin-polarized current injection in the absence of a more highly insulating barrier layer. Spin scattering lengths are determined experimentally from both weak localization/antilocalization and spin-valve measurements. Spin and elastic scattering lengths of 230 +/- 20 and 85 +/- 5 nm, respectively, are measured, with a sulfur-based surface treatment increasing the spin scattering length to 250 +/- 20 nm and decreasing the elastic scattering length to 65 +/- 5 nm. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3502674]