Electronic drift mobilities and space-charge-limited currents in lithium-doped zinc oxide

Abstract

Electron drift mobilities and lifetimes were measured from 250 to 400°K in oriented single crystals of high-resistivity, Li-doped ZnO by determining the transit time across a plane-parallel specimen for electrons which had been photoinjected at one surface by light pulses of about 10 sec duration. Drift mobilities at 300°K ranged from 2–8 cm 2 V −1 sec −1 for the specimens investigated with no orientational dependence being apparent. The observed exponential temperature dependence of the mobility indicated that the electron drift mobility is primarily controlled by thermal release from trapping states, of estimated density 10 16cm −3, lying in a 0·29 eV region below the conduction band. Electron lifetimes, estimated from low-field data, showed some orientation dependence and ranged from 30−100μ sec at room temperature, while electron ranges were (1−10) × 10 −4 cm 2 V −1 and were temperature independent. Ohmic electrodes of indium were used to inject electrons into plane-parallel monocrystals of Li-doped ZnO, producing space-charge-limited currents at high fields. The observed current-voltage and current-temperature relations indicated that discrete or narrow bands of electron traps existed in the 0·4 eV and in the 0·72−0·80 eV regions below the conduction band edge with densities of 4 × 10 13 and 10 17 cm −3, respectively.