An investigation of surface states at a silicon/silicon oxide interface employing metal-oxide-silicon diodes

Abstract

A new solid-state device, the M-O-S diode, of which an oxidized silicon surface is an integral part, is introduced, and a theory for its operation in the absence of surface states is obtained. The capacitance of this device may be considerably more voltage sensitive than that of a p-n junction. The existence of surface states with non-zero relaxation times is introduced into the theoretical model. It is shown that the states may increase the capacitance of the device, as well as affect the proportion of applied voltage which appears across the silicon. A small-signal equivalent circuit is derived which includes the effect of the surface states. It is also shown that a comparison of the theoretical capacitance vs. voltage curve without states and a measured high-frequency capacitance vs. voltage curve may be used to obtain the distribution of all states, regardless of their time constants. Results are given of measurements and calculations on two M-O-S diodes having different surface treatments before oxidation. Both surfaces have a total density of about 3 × 10 12 states/cm 2. In both cases, the distribution of states is continuous and has its highest peak about 100 mV above E F (0), the position of the Fermi level at the silicon surface if there is no voltage drop across the silicon The time constants of the states extend from 10 −8 sec to longer than 10 −2 sec. There is a tendency for states located at deeper energy levels to have longer time constants, but some of the states in the high density of states above E F (0) have long time constants. The distribution of time constants with energy level is somewhat different for the two surfaces. A comparison is made between the distribution of states obtained here with the distribution reported by others working in the field. The results are similar in density and location of the peaks of the distribution reported here, but differ in that some other sources report a discrete distribution.