Observation of steplike sheet resistance increase of shallow doped bare silicon during initial contact to air

Shallow doping of silicon was carried out by vapour phase surface adsorption of phosphine and subsequent diffusion and activation of phosphorus by conventional rapid thermal lamp annealing. Time-dependent measurements by the four-point probe resistivity method revealed a remarkable steplike increase of the sheet resistance of the shallow doped samples during the initial contact with air after the doping process in vacuum and after native oxide removal. A kink in the increase was clearly visible after about 30 min to one hour of air contact. The sheet resistance did not reach a constant value until three to four hours of keeping the samples in air. Interestingly, such an effect does not seem to have been described before. It could be associated with the growth of native oxide on the silicon in conjunction with a silicon consumption and deactivation of dopants in the shallow doped subsurface layer. Comparative time-dependent measurements of the oxide thickness by spectroscopic ellipsometry supported direct-logarithmic oxide growth behaviour and did not show a steplike growth characteristic. The observation suggests studying appropriate time-dependent surface related processes, such as the observed native oxidation of shallow doped samples, by means of the simple four-point probe setup.