Modulation of Carrier Surface Lifetime and the Time Constants of Surface States in Si

The effects of ac fields on the surface potential (φs) and recombination velocity (s) (or surface lifetime) of carriers in Si surfaces have been studied by means of a method used earlier in Ge. The field is applied normally to the ``back surface'' of large area p-n junctions and its effects on the surface are detected by means of changes in the reverse saturation current of the diode. The study yielded the following results: The range of the equlibrium values of φs at 300°K is about ±0.5 V. Mainly one recombination energy level is found at ±16 kT from mid-gap. The effectiveness of the modulation of s is a measure of surface response. The data are compared with frequency response curves derived for some specific distributions of time constants for the surface states. The response to applied fields is much larger in the higher frequency range (5 to 50 kc/sec) rather than at 10–1000 cps. The time constants for the Si surface states involved in this behavior are ≈10−4 sec, one to two orders to magnitude shorter than in Ge, and this may account for the difficulties encountered earlier in modulating φs in Si at the low frequencies used in Ge. These surface states, that inhibit modulation at low frequencies, may be the usual states on the outside of the oxide (``slow'' states), part of the ``fast'' states at the semiconductor-oxide interface, or may be spread in the space-charge region.