High-Vacuum Studies of Surface Recombination Velocity for Germanium

Abstract

The surface recombination velocity, $s$, of minority carriers for (100) faces of germanium crystals has been studied under various vacuum conditions. Values of mean lifetime were obtained by the decay-of-photoconductivity method and the corresponding values of surface recombination velocity were determined from geometrical considerations. Increases in $s$ were observed due to initial evacuation of the experimental tube. Following outgassing of the samples in vacuum at temperatures above 500°C and for a time in excess of 30 hours, $s$ was found to be insensitive to exposure to oxygen at pressures as high as ${10}^{-4\mathrm{}}$ mm Hg. As a result of cleaning the surfaces of samples by the ion-bombardment technique, $s$ increased to a value of (5 to 7)×${10}^3$ cm/sec. Heating the ion-bombarded crystals in vacuum at temperatures above 500°C for prolonged periods of time, followed by slow cooling, resulted in a decrease in $s$ to a value below 500 cm/sec but greater than the value obtained following a CP-4 etch and the value obtained after pumping. The recombination velocity of clean and annealed (100) germanium surfaces was found to be insensitive to room-temperature adsorption of oxygen at pressures as high as 5×${10}^{-5\mathrm{}}$ mm Hg. Exposure of these clean surfaces to oxygen at temperatures of about 100°C resulted in small decreases in mean lifetime, corresponding to a possible increase in $s$, assuming that this was entirely a surface phenomenon. The clean-surface value of $s$ could be restored by heating at 500°C in vacuum for short periods of time. The characteristics of a well-outgassed oxide-covered surface and a clean surface are found to be similar. The observed similarities are discussed and the conclusion drawn that the oxide layer, of itself, has little or no effect on the surface recombination process.