Electron paramagnetic resonance (EPR), photoluminescence, and infrared optical absorption have been used to investigate a ZnO crystal before and after a thermal anneal for 1 h in air at 900 °C. The sample was an undoped high quality crystal grown by the chemical vapor transport method. In addition to shallow donor impurities, the crystal contained trace amounts of copper ions. Prior to the thermal anneal, these ions were all in the Cu+ (3d10) state and the observed luminescence at 5 K, produced by 364 nm light, consisted of a broad structureless band peaking at 500 nm. After the high-temperature anneal, the Cu2+ (3d9) EPR spectrum was observed and the luminescence had changed significantly. The emission then peaked near 510 nm and showed structure identical to that reported by Dingle [Phys. Rev. Lett. 23, 579 (1969)]. Our data reaffirm that the structured green emission in ZnO is associated with Cu2+ ions. We suggest that the unstructured green emission (observed before the high-temperature anneal) is donor–acceptor pair recombination involving the Cu+ acceptors.

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Micro-Electromechanical Systems (MEMS) have a variety of potential civil and military applications. As MEMS become widely implemented, the application of closed-loop control methods to MEMS will lead to higher degrees of certainty and reliability of microelectromechanical operation in physically demanding environments. This paper presents an overview of recent research and development progress in the area of MEMS control at West Virginia University. The implementation and simulation of a closed-loop stroke-length control of a lateral comb actuator will be presented. The closed-loop scheme implemented is based on a common proportional, integral, derivative (PID) linear feedback control. The position signal of the lateral comb actuator device was obtained using a through-wafer free-space optical probe. The paper will also include further progress in both linear and nonlinear feedback position control schemes.

We present modeling and experimental results from the use of a 1310-nm-wavelength through-wafer optical microprobe in conjunction with a microstructure grating to monitor the motion of a lateral comb resonator stage. The optical signal that results from shuttle interaction with the microprobe beam exhibits a peak-to-valley dynamic range that corresponds to 2-microm microstructure displacement, facilitating submicrometer positional resolution on digitization. This signal was used to achieve microstructure positional feedback and effective microsystem model parameter extraction, which are essential for structure control and model-based fault detection.