Electrical and optoelectrical properties of CdS-Ge heterojunctions

Abstract

The electrical and optoelectrical characteristics are presented for n-CdS/ p-Ge heterojunctions fabricated by depositing Ge onto (0001) faces of CdS monocrystals. The experimental results are interpreted in terms of an energy band model proposed for this heterojunction. It is found that a Schottky barrier is present on the CdS side of the interface. With an In (injecting) contact to the CdS and an ohmic contact to the Ge, the device acts as a photoconductor in which the current is space-charge-limited for forward bias (Ge positive, CdS negative). For the opposite bias polarity the photoconductivity is reduced substantially and the current is limited by tunneling through the barrier spike in the conduction band. The photoconductivity at room temperature results from electrons being excited in the CdS bulk by photons of 2·34 eV energy. For forward bias the photocurrent is found to be proportional to the light intensity at moderate voltages, as for a normal photoconductor. However, at low voltages the photocurrent is found to be proportional to the two-thirds power of the light intensity.