Defect levels in electrodeposited n-type CdTe thin films

Abstract

Admittance spectroscopy is used to measure electron traps near the conduction band of the n-type electrodeposited CdTe thin films. Several samples of Ni-CdTe Schottky-barrier and Ni-TeO2-CdTe devices are used in the measurement. Two donor levels with activation energies around 0.12 eV and between 0.19 and 0.26 eV are detected. In addition, there may be one or more shallow donor levels within 0.04 eV from the conduction band. Capacitance loops are observed in the capacitance-voltage measurements performed at different temperatures and bias sweep rates. The activation energies, densities, and electron-capture cross sections of the two slow electron-trap levels responding to the cyclical ramp bias are estimated. The rise and decay photocapacitance transient measurements at 295 K reveal two electron-trap levels with photoionization threshold energies of 0.8 and 1.3 eV. These two levels are suspected to correspond to the other two slow electron-trap levels estimated in the capacitance-voltage measurement and located around or above the midgap. The differences between thermal and optical ionization energies and the slow electron-capture rates of these two levels may be explained by the nonradiative, thermally activated capture processes with large lattice relaxation. Numerous defects around and below the midgap are detected by the steady-state photocapacitance measurement at 100 K. A band of seven defect levels spreading within 0.6 to 1.0 eV is observed. Large capacitance changes are recorded at 1.37, 1.41, and 1.45 eV. These three levels are speculated to be the major compensating acceptor levels. Based on both the photocurrent and photocapacitance measurements, electrons emitted from the 1.02-, 1.06-, 1.21-, and 1.28-eV levels are believed to arise from the defects at the TeO2-CdTe interface.