Electrical properties and deep traps in ZnO films grown by molecular beam epitaxy

Capacitance-voltage (C-V), capacitance-frequency (C-f), admittance spectroscopy, deep trap spectra, and far infrared reflectance measurements were performed on undoped and N-doped ZnO films deposited on sapphire by molecular beam epitaxy. The results show existence of a heavily doped n+ layer near the interface with the substrate. The presence of these layers explains the large difference between the electron concentrations measured in the films by Hall effect and C-V profiling or calculated from the plasma minimum frequency in reflectance. C-V data obtained at low temperatures show a prominent persistent photocapacitance in the films. Admittance spectra were dominated by electron traps with ionization level EC−0.3eV commonly observed in ZnO crystals grown by all techniques.