Photoconductors based on wide band gap semiconductors are potential devices for UV light detection due to internal
photoelectrical gain and fabrication simplicity. Photoresponses of photoconductors based on GaN and ZnO show high
values in UV range under large biases and relatively low values in visible range. Although photoresponse of ZnO
photoconductors is similar to that of GaN-based photoconductors, mechanisms of photoconductance between two
materials are very different. This difference can be found in optical power dependence of photocurrent and I-V
characteristics, and has an impact on device design. In this paper we report experimental studies of photoresponse for
newly developed ZnO photoconductors. The ZnO film was grown on a 6H-SiC substrate by hybrid beam deposition. The
photoconductor device is formed with interdigitated finger-shaped Ti/Au ohmic contacts on the ZnO film. Electrical
characteristics, spectral photoresponse, and persistence properties were studied for the device under variable biases. We
find that there are at least three mechanisms involved in the device. At low biases and low incident light power, the
photoresponse is mainly due to photocreation. At higher light power and lower biases, the space charge regions are
responsible for the photocurrent. At higher biases, the contribution from surface states is dominant.
Among wide bandgap materials that are sensitive to photons in the ultraviolet (UV) region, ZnO is a promising photonic
material because of its unique optoelectronic properties. Based on the lateral interdigitated back-to-back Schottky
contact structure on ZnO film, metal-semiconductor-metal (MSM) photodetectors have substantially lower parasitic
capacitance compared with vertical p-i-n photodetectors, which leads to a very high speed photodetection. In this paper,
we report optical characteristics of MSM ZnO UV photodetectors for which ZnO films were fabricated by hybrid beam
deposition. An annealing process was used in oxygen ambient. The MSM ZnO photodetector consists of two
interdigitated electrodes both with Ti/Au metals on an n-type ZnO thin film. The electrodes on the photodetector are
finger-shaped. We found that the annealing process decreases contact resistance and photoresponse time. The possible
mechanism of annealing process is the removal of surface defects created in the fabrication process. A sublinear power
dependence of photocurrent reveals the existence of a light induced space charge region inside the ZnO film. The device
displays fast pulse response with a very short rise time and a relatively long relaxation time with applied bias. The
exponential decay tail indicates an RC type time response.
Aluminum nitride (AlN) films were grown by chemical vapor deposition (CVD) on boron-doped diamond films deposited by the hot-filament CVD (HFCVD) method. The films were characterized by scanning electron microscopy, x-ray diffraction, and Raman spectroscopy. The electrical characterization of the AlN/diamond interface was performed by current-voltage (I-V) and capacitance- voltage measurements. The resulting films showed one x-ray diffraction peak of (100) oriented AlN and three diamond diffraction peaks of (111), (220) and (331) orientation. The Raman spectra showed two peaks, one at 660 cm-1 due to scattering by the AlN lattice and the other at 1335 cm-1 by the diamond lattice. The I-V measurements on the metal(W)/diamond/Si/Al structure showed ohmic behavior from which the diamond film resistivity of 5 X 105 (Omega) -cm was estimated. The I-V measurements on the W/AlN/diamond/Si/Al structure showed rectifying behavior. The capacitance of the film was independent of the applied voltage and was dominated by the diamond bulk capacitance.
Tunable diode laser spectroscopy offers unique advantages for high resolution infrared reflection absorption spectroscopy (IRRAS) studies of monolayers on single crystal surfaces. Its greatest potentials lie in analyses of lineshapes less than 1 cm (FWHM), for resolving narrow features on absorption peaks, and for in situ solid-liquid interfacial studies, where high intensity is often required. Other IRRAS techniques introduce instrumental broadening sufficient to create considerable uncertainty in the intrinsic linewidth of very narrow peaks, and can mask sharp features. Using isolated or single mode output from a diode laser source provides resolution of iO3 cm, and thereby eliminates the need for instrumental deconvolution. However, the signal-to-noise (SIN) ratio is generally lower for spectra obtained with diode sources than with Fr-JR techniques, so that the most important applications using diode lasers have been studies of strong scatterers, such as CO on Pt(1 1 1). For this study, linewidths were measured for the C-O stretch mode for the ontop site of CO adsorbed at saturation coverage on a Pt(1 1 1) single crystal using diode laser sources. The measured linewidth at 100 K, using isolated modes, was 2.3 0.3 cm. The linewidth at 0 K was estimated to be 2.0 0.4 cm from variation of linewidths with temperature. Using the uncertainty principle, the 0 K linewidth corresponds to a lifetime of 2.7 0.6 ps, in reasonable agreement with recent lifetime measurements using time-resolved techniques.