A combination of surface-sensitive techniques and electron transport measurements have been used to characterize the effect of argon ion bombardment and annealing on a series of InSb(100) samples. Ex situ electrical conductivity and magnetoresistance measurements at 4.2 K, and in situ high-resolution electron energy loss spectroscopy (HREELS) carried out at 300 K, indicate that all the samples studied exhibit enhanced n-type behaviour after the surface cleaning procedure. This effect is most pronounced after annealing to between 450 and 500 K and arises from the formation of a high-density electron gas with a sheet carrier concentration of approximately (6.5-9.0)*10¹²cm^-2. The carrier concentration is significantly reduced on annealing to higher temperatures up to a maximum of 700 K. Electron-energy-dependent HREELS measurements of the plasmon energy and intensity, in conjunction with model calculations based on dielectric theory, indicate that the n-type layer is approximately 500 AA thick and located approximately 175 AA below a surface depletion layer. The occupancy of the electronic subbands has been obtained by Shubnikov-de Haas measurements and self-consistent calculations. These show that the positive charge which confines the electrons is spread over approximately 300 AA with a best fit being provided by a Gaussian-like potential profile. The calculations demonstrate that the corresponding wavefunction spread for the i=0 subband, which contains approximately 40% of the total carriers induced, has a spatial dimension of approximately 500 AA in good agreement with the HREELS results.