Photo-effects in isotype heterojunctions

Abstract

Isotype heterojunctions were made by alloying n-type Ge pellets on monocrystalline n-type Si platelets. The investigations concern a number of junctions which showed current saturation for both polarities of voltage. In a previous publication I–V characteristics and capacitance measurements were reported. An interpretation of the measurements was given using the model of two Schottky diodes back-to-back. In the present paper results are discussed of similar measurements under illumination with white light of varying intensity. Further the spectral and geometrical dependences of the photovoltage were measured. The results confirm the validity of the assumed model. Strong evidence was found for photo-emission of electrons from the Ge valence band through the interface to the Si conduction band. This emission is analogous to valence band photo-emission from a free semiconductor surface into vacuum. It is shown that interface valence band emission can also explain the spectral photoresponse data reported by other investigators for GeSi and GeGaP n-n heterojunctions. A method is developed with the aid of which the discontinuities in the band edges at the interface can be evaluated quite easily, even for isotype heterojunctions where double current saturation is obscured by leakage phenomena. In the Appendix the same method is applied to two rectifying contacts of different metals on a single semiconductor substrate: in spite of high leakage currents it renders possible a direct and simple evaluation of the difference in barrier heights.