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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-0.3 eV commonly observed in ZnO crystals grown by all techniques. [ABSTRACT FROM AUTHOR]

Electrical photoelectrical, and microcathodoluminescence properties were measured on doped p-GaN superlattices prepared by molecular beam epitaxy on c-plane sapphire substrates. The photosensitivity of such superlattices is at least two orders of magnitude higher and the microcathodoluminescence intensity is about five times lower than for similarly grown uniformly doped p-GaN films. The difference is explained by the spatial separation of the photoexcited electrons and holes by the superlattice potential. The sheet carrier concentration in the superlattice is shown to be several times (6.6×1012 cm-2 vs 2×1013 cm-2) lower and the effective hole mobility almost two times higher (20 cm2/V s vs 12 cm2/V s) than for bulk p-GaN samples with similar doping. [ABSTRACT FROM AUTHOR]

Electrical properties, admittance, and microcathodoluminescence spectra are compared for p-GaN samples grown by hydride vapor phase epitaxy (HVPE) and by molecular beam epitaxy (MBE). The former are characterized by a high 300 K hole concentration and a weak temperature dependence of conductivity. The latter samples show strongly temperature-activated conductivity due to ionization of Mg acceptors. The main effects of neutron irradiation were similar for the p-HVPE and the p-MBE materials: a compensation of p-type conductivity starting with neutron fluences exceeding 2×1016 cm-2 and conversion to high resistivity n type with the Fermi level pinned near Ec-(0.8–0.9) eV after irradiation with high doses of 1018 cm-2. For the heavily neutron irradiated p-HVPE samples, a strong increase was observed in the c-lattice parameter which indicates an important role for interstitial-type defects. [ABSTRACT FROM AUTHOR]