Improvements in method and apparatus for determining minority carrier diffusion length
The constant-magnitude steady-state surface photovoltage (SPV) method for determining the minority carrier diffusion length L is in principle an excellent technique. It has, however, recevied relatively limited use because of practical difficulties in carrying out the required measurements. This paper describes an improved measurement system that virtually eliminates these difficulties and allows a rapid straightforward determination of L. These measurements in silicon monitor wafers have enabled a routine quality control check on many factory processing steps, particularly those that are carried out at high temperature and have the potential for significantly degrading L.
Silicon-wafer-surface damage revealed by surface photovoltage measurements
Anomalous results of surface photovoltage (SPV) measurements on Si wafers are shown to be associated with a damaged region beneath the illuminated surface of the wafer being measured. The anomaly is a concave‐upward curvature of the I0(α−1) plot with an r2 value, derived from linear regression analysis, less than the normally observed minimum value (∼0.98). Removal of the damaged region by an appropriate etching procedure allows subsequent SPV measurements whose results are substantially free of the previously observed anomaly. The qualitative character of the anomaly can be reproduced by a simple theoretical model in which only one effect of the damage is considered; this effect is a diminished quantum efficiency for hole‐electron pair generation by photon absorption in the damaged region. The results suggest the use of SPV measurements as a test procedure for revealing the presence of surface damage in Si wafers.
Surface properties of germanium
The contact potential (c.p.) and the change of contact potential with illumination (Δc.p.) L of several germanium surfaces have been measured. The reference electrode used was platinum. It was found that the c.p. could be cycled between two extremes about 0.5 volts apart by changing the gaseous ambient. Ozone or peroxide vapors gave the c.p. extreme corresponding to the largest dipole at the Ge surface. Vapors with OH radicals produced the other extreme. There is a one to one correlation between c.p. and (Δc.p.) L . For 12-ohm cm n-type Ge (Δc.p.) L was large and positive when the surface dipole was largest, decreased to zero and became slightly negative as the surface dipole decreased to its smallest value. The variation for 12-ohm cm p-type Ge was just opposite as regards both sign and dependence on surface dipole. The surface recombination velocity was found to be independent of c.p. For a chemically prepared surface it was 50–70 cm/sec and 180–200 cm/sec for n and p-type surfaces respectively. A theory is given that explains the results in terms of surface traps, N a per cm2 donor-type traps near the conduction band and B b per cm2 acceptor-type traps near the filled band. A quantitative fit with experiment is obtained with only one free parameter. The results are direct evidence for the existence of a space charge layer at the free surface of a semiconductor.