ANALYSIS OF CAPACITANCE-VOLTAGE MEASUREMENTS ON HEAT-TREATED CU2-XS-CDS HETEROJUNCTIONS

The capacitance-voltage characteristics of a p-type (metalllike)/n-type semiconductor junction are described in terms of a simple three-region space charge in the semiconductor. The assumed space charge consists of a narrow (∼100 Å) high-density space charge at the interface, followed by an extended low-density space charge (insulating layer), and finally the bulk space charge. The calculations assume that the equilibrium space-charge density does not change with applied reverse voltage. The electric field at the junction is calculated analytically, and the electrostatic potential is calculated for the case of abrupt junctions between the various space-charge regions. The results indicate effective barrier-height lowering as a consequence of interface charges, even in the presence of the insulating layer which dominates capacitance-voltage measurements. An interpretation is given to the slope and voltage-axis intercept of (1/C)2-vs-V plots for a variety of special cases of the general space-charge distribution. Dark-capacitance-voltage data gathered on Cu2-xS/CdS photovoltaic cells are interpreted in terms of the proposed space-charge distribution. The change in junction capacitance with the air-heat treatments of this heterojunction is qualitatively explained by this model.