Semiconductor Surface Potential and Surface States from Field-Induced Changes in Surface Recombination

Experiments are reported on the effects of ac electric fields and ambients on the surface recombination velocity in germanium and silicon. The variations in surface recombination are detected by changes in the reverse current of large area "back surface" diodes. The experimental method is an improved version of that of Thomas and Rediker and is a convenient means of exploring surface type, stability, time effects at the surface, and effects induced by high ac fields and ambients on the slow surface states. Observation of a maximum of surface recombination in terms of applied field provides a reference point from which the zero-field value of the surface potential can be evaluated, and the dependence of the surface recombination velocity ($s$) on the surface potential (${\phi }_s$) can be established. Values of ${\phi }_s$ are in the range of ±0.25 v in Ge and ±0.5 v in Si. The energies of the main recombination states are at $6\mathrm{kT}$ to $9\mathrm{kT}$ from mid-gap in Ge and at $+16\mathrm{}\mathrm{kT}$ or $-16\mathrm{}\mathrm{kT}$ in Si. Information on the surface potential and the surface states obtained by the present technique is in reasonable agreement with that derived from other types of measurements. The present results are independent of uncertainties due to surface mobility. Experimental patterns show directly that the "charge" surface states are also the ones that give rise to surface recombination. The width of the observed curves of $s$ vs induced charge (and ${\phi }_s$) depends on bulk resistivity and is smaller for the higher resistivity samples. Such behavior could arise from contributions to $s$ from recombination states distributed in the surface space-charge region.