MEMS resonators bear great potential for applications as RF sensors, filters and oscillators, e.g., in life sciences or information technology. A semiconductor fabrication process has been applied to prepare resonant AlN and SiC beams operating at frequencies between 0.1 and 2.1 MHz. The metallized beams were actuated in a permanent magnetic field of about 0.5 T by the Lorentz force. For systematic studies of the resonant frequencies and quality factors, the induced voltage was measured using time domain and frequency domain techniques. Resonator geometry, material and ambient pressure were varied to attain a generalized understanding of the RF performance. The dependence of the resonant frequency on tensile axial strain has been derived analytically and extended to include highly strained beams. Based on these formulas, accurate detection of the residual layer strain after fabrication is presented. To describe the quality factor a chain of beads model has been applied successfully. The influences of the beam width and the pressure-dependent viscosity on the model parameters are analyzed.
A detailed analysis of the piezoelectric response of (GaN/)AlGaN/GaN heterostructures is reported. The electromechanical properties of two types of heterostructures with an Al content of 31% are compared. Only a single two-dimensional electron gas (2DEG) is formed for samples with thin GaN cap layers, while both a 2DEG and a two-dimensional hole gas coexist in the case of thick GaN caps. The lower GaN layer represents the mechanically supporting layer, while the AlGaN film, and in some cases an additional GaN cap layer, serves as the piezoelectrically active layers for actuation. The 2DEG (at the lower AlGaN/GaN interface) provides the conducting channel which was used as back electrode for the applied external voltage. Electroreflectance spectroscopy is applied in order to determine the electric field distribution across the whole structure as a function of the applied voltage. It is found that only a part of the modulation voltage drops across the active region. Piezoelectric force microscopy yields the field (voltage)-dependent actuation of the layers. By correlating the results of the two experimental techniques we are able to determine the piezoelectric modulus d33 with considerably improved reliability. A value for Al0.31Ga0.69N of 5 pm/V is found which is higher than an estimation based on previously reported data for GaN and AlN
One-step device fabrication through the integration of nanowires (NWs) into silicon microchips is still under intensive scientific study as it has proved difficult to obtain a reliable and controllable fabrication technique. So far, the techniques are either costly or suffer from small throughput. Recently, a cost-effective method based on thin-film fracture that can be used as a template for NW fabrication was suggested. Here, a way to integrate NWs between microcontacts is demonstrated. Different geometries of microstructured photoresist formed by using standard photolithography are analyzed. Surprisingly, a very simple "stripe" geometry is found to yield highly reproducible fracture patterns, which are convenient templates for fault-tolerant NW fabrication. Microchips containing integrated Au, Pd, Ni, and Ti NWs and their suitability for studies of conductivity and oxidation behavior are reported, and their suitability as a hydrogen sensor is investigated. Details of the fabrication process are also discussed.
In this work, coalescence aspects of wurtzite-III-nitride epitaxy are addressed. The coalescence phenomena have been studied in thin epilayers by means of electron and atomic force microscopies, and electron and x-ray diffractions. This study generalizes the growth parameters responsible for the rapid coalescence of III-nitride films, and describes the coalescence qualitatively and, partly, analytically for the case of heteroepitaxy in nonequilibrium conditions. Coalescence time and the corresponding diffusion coefficients at elevated temperatures were estimated for GaN and InN depositions. The rate of coalescence has been found to impact on the structure and morphology of III-nitride epilayers. A simple growth model was suggested to explain the formation of domain boundaries and ͑0001͒ stacking faults formed during the coalescence. In particular, it is shown that two adjacent and tilted, hexagonal-shaped 2H domains may form a noncoherent boundary explicitly along a ͕11 ¯00͖ plane. We also suggest that the interaction between tilted domains induces the localized lateral growth of the most epitaxially oriented domain forming a basal ͑0001͒ stacking fault followed by the formation of surface macrosteps, and consequently the termination of a threading dislocation by its dissociation and propagation under the formed ͑0001͒ stacking fault.
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