Photoconductivity is observed in ZnO epilayers due to photoexcitation in the visible spectral region of 400-700 nm, below the ZnO bandgap energy of 3.4 eV. Photoconductive transients due to visible photoexcitation have time constants in the order of minutes. Treatment of the ZnO surface with SiO 2 passivation layers results in a significant reduction in the photoconductive signal and photoconductive time constant. The photoconductive response is attributed to hole traps in ZnO, where a rate equation model is presented to describe the photoconductive characteristics. A method of extracting the hole trap density spectrum is presented on the basis of the rate equation model and assumptions for hole capture lifetime and carrier recombination lifetime that are validated by experimental time-resolved photoluminescence measurements of the material under study. Traps are found to be distributed near 0.75 eV and 0.9 eV from the valence band edge for SiO 2 passivated and unpassivated ZnO epilayers, respectively.

A strong photoconductive response is observed for ZnO epilayers in the presence of both above bandgap and below bandgap photoexcitation. Photoexcitation for energies larger than the bandgap results in a photoconductive response with fast and slow time constants on the order of nanoseconds and larger than milliseconds, respectively. The fast and slow time constants are attributed to minority carrier recombination and slow escape of holes from traps, respectively. Photoexcitation in the visible spectral region, below the bandgap energy, results in slow rise and fall time constants on the order of minutes and hours. A model for the photoconductive response based on rate equations is presented providing an accurate fit to measured photoconductivity data. The rate equation model suggests the presence of hole trap levels in the energy range of 0.6 eV to 1.0 eV relative to the valence bandedge. The passivation of the ZnO surface with SiO 2 shows significantly reduced photoconductive transient decay time constants, suggesting a significant reduction of deep surface defects on the ZnO material.

HgCdTe remains the material of choice for high-performance infrared (IR) detectors due to its tunable direct bandgap energy corresponding to the IR spectral region, and the advancement of HgCdTe materials growth and processing technologies. Accurate knowledge of the HgCdTe optical absorption coefficient is important for IR detector design, layer screening, and device analysis. The spectral response for IR detectors is dependent on optical absorption above the bandgap energy, where much of the study of absorption coefficient in HgCdTe has focused on the bandtail region. In this work, the optical absorption coefficient was studied by theoretical bandstructure calculations and experimental measurements on HgCdTe layers using techniques of IR spectroscopic ellipsometry and IR transmission. The theoretical and experimental results suggest that the absorption coefficient between 600 cm Ϫ1 and 5,000 cm Ϫ1 is related to energy relative to bandgap with a fractional exponent between 0.6 and 1, rather than the previously used expressions relating to a parabolic or hyperbolic bandstructure. The fitting parameters for Hg 1-x Cd x Te with x ϭ 0.22-0.60 are presented to develop a model for the optical absorption coefficient spectra. The calculated detector spectral response using the new and previously reported absorption coefficient models suggests that next generation IR detectors employing multilayer structures with graded compositional profiles will likely benefit from this new model.

The electrical properties of several metal contacts to n-type ZnO (0001) were studied. The ZnO samples consisted of bulk single-crystal material, epitaxial layers on sapphire grown by molecular beam epitaxy (MBE), and polycrystalline thin films on sapphire obtained by pulsed laser deposition (PLD). Ohmic and rectifying contacts were observed dependent upon both the metal material and the ZnO surface. Ohmic contacts were characterized using the circular transmission line method (c-TLM), where contact resistivity was found to be in the range of 10 Ϫ4 Ϫ10 Ϫ5 Ω-cm 2 . Schottky behavior was observed using Ag contacts exhibiting varying leakage current and breakdown voltage dependent on the polarity of the ZnO surface.

Accurate knowledge of the optical-absorption coefficient in HgCdTe is important for infrared (IR) detector design, production process (layer screening), and interpretation of detector performance. Measurements of the optical-absorption coefficient of HgCdTe layers with uniform composition are presented with the goal of developing a revised model in the interest of IR detector technology. Existing methods of determining HgCdTe alloy composition from IR transmission measurements are compared, where one self-consistent method is suggested and shown to agree with experimental detector data. An exponential Urbach and hyperbolic model are presented to represent band tail and above-bandgap absorption regions, respectively. Parameters associated with these models are extracted for Hg 1Ϫx Cd x Te compositions of x ϭ 0.22-0.60 and temperatures of T ϭ 40-300 K using samples of varying thickness to obtain accurate data for varying spectral regions of the absorption coefficient. An initial analytical expression for the absorption coefficient is presented and compared to experimental detector-response data. Detector-response simulations indicate that accurate optical-absorption models are needed, where detector structures with thin layers and arbitrary compositional profiles in current and future IR detectors will be the most demanding.