Monday, March 31, 2014

Single-crystal aluminum nitride nanomechanical resonators

Aluminum nitride is a light, stiff, piezoelectrically active material that can be epitaxially grown on single-crystal Si. AlN is beginning to play a role in the integration of semiconducting electronic and surface acoustic wave devices, and may prove useful for the integration of other types of mechanical devices as well. We describe the growth and subsequent electron-beam patterning and etching of epitaxial AlN-on-silicon films into nanomechanical flexural resonators. We have measured resonators with fundamental mechanical resonance frequencies above 80 MHz, and quality factors in excess of 20000.

Source:IEEE

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Electrostatic actuation of surface/bulk micromachined single-crystal silicon microresonators

In fabricating microelectromechanical systems (MEMS), bulk micromachining using (100) and (110)single crystal silicon and surface micromachining using polycrystalline silicon are used. However, both micromachining methods have drawbacks, and micromachining actuating or sensing MEMS using single crystal silicon has been an active research topic in resent years. This paper presents electrostatic actuation of a resonator fabricated by the SBM (surface/bulk micromachining) process. The SBM process allows fabricating released structures in single crystal silicon. To fabricate electrodes and to electrically isolate them, a junction isolation method using reverse-biased diodes is developed. The breakdown voltage of this isolation method is measured to be larger than 150 volts. A SBM processed microresonator is actuated at 36 kHz. A displacement of several μm is achieved in atmosphere with a 20 volts peak-to-peak supply

Source:IEEE

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Friday, March 28, 2014

Photoconductive Response of Single Crystal Germanium Layers Prepared by the Pyrolytic Decomposition of GeI2

Layers of single crystal germanium have been prepared by pyrolytic decomposition of GeI2 at about 350°C upon single crystal seeds of germanium by J. C. Marinace. The substrate germanium could be removed and the resultant thin layers (5×10-3 cm) studied. The Hall coefficient and resistivity measured on these layers by Dunlap et al. showed that the layers as grown were usually n type and contained about 1016 donors/cm3. These donors are situated at 0.22±0.04 ev below the conduction band. In the work reported here, the photoresponse of such layers at 77°K was found to extend to about 6 μ in substantial agreement with this thermal activation energy. The 0.2ev donors apparently can be annealed out by prolonged heating at 550°C. The layers could also be intentionally doped with standard acceptors such as Al during formation. It was hoped that it would be possible to obtain the incorporation of Au into the layers. For this purpose, layers for which Au doping was attempted were annealed to remove the 0.2ev donors. Although some of the layers became p type, a comparison of the photoresponse at 77°K with the photoresponse obtained for a 5×10-3 cm thick ptype plate of regularly Audoped germanium showed no evidence for the 0.15 ev Au level. Likewise, annealed layers for which Ag doping was attempted were examined but yielded no positive identification of the 0.14ev level of Ag.

Source:IEEE

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Harsh environments minimally invasive optical sensor using free-space targeted single-crystal silicon carbide

To the best of our knowledge, for the first time, a single-crystal silicon carbide (SiC)-based minimally invasive smart optical sensor suited for harsh environments has been designed and demonstrated. The novel sensor design is based on an agile wavelength source, instantaneous single-wavelength strong two-beam interferometry, full optical power cycle data acquisition, free-space targeted laser beams, multiple single-crystal-thick SiC optical front-end chips, and multiwavelength signal processing for unambiguous temperature measurements to form a fast and distributed smart optical sensor system. Experiments conducted using a 1550-nm eye-safe band-tunable laser and a 300-mum coating-free thick SiC chip demonstrate temperature sensing from room temperature to 1000degC with an estimated average 1.3degC resolution. Applications for the proposed sensor include use in fossil fuel-based power systems, aerospace/aircraft systems, satellite systems, deep-space exploration systems, and drilling and oil mining industries

Source:IEEE

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Thursday, March 13, 2014

Ultraprecision Micro-Machining of Single Crystal Germanium by Applying Elliptical Vibratlon Cutting

'Ultrasonic elliptical vibration cutting' technology, which has been developed by the authors, is applied to ultraprecision machining of single crystal germanium in the present research. The germanium is an excellent material for infrared optical devices, whereas ultraprecision micro-machining of thegermanium is difficult due to its brittleness. Micro grooving experiments are carried out on the (100), (110) and (111) surfaces of single crystal germanium substrates, and basic effects of the elliptical vibration on the ductile micro-machining are clarified as follows. Critical depth of cut to generate a completely fracture-free surface on the single crystal germanium is increased significantly in all crystalorientations on the (100) and (111) surfaces, and ultraprecision micro grooving is realized successfully at a large depth of cut of 1 mum by applying the elliptical vibration cutting. This improvement by the elliptical vibration cutting is considered to be caused by reduction of instantaneous uncut chip thickness and cutting force in each cycle of the elliptical vibration. When the rake angle becomes negative, influence of the crystal orientation on the critical depth of cut becomes negligible, while it is considerable at 0 degree. On the other hand, the rake angle does not have considerable influence on average values of the critical depth of cut between -20 to 0 degree. The critical depth of cut is increased in all crystalorientations as the cutting speed is reduced in the elliptical vibration cutting

Source:IEEE

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Synthesis of nanoscale structures in single crystal silicon carbide by electron beam lithography

Nanostructures were formed on diced specimens of single crystal 4H-SiC silicon carbide using electron beam lithography. A scanning electron microscope was retrofitted with a commercially available electron beam lithography package and an electrostatic beam blanker to permit nanoscale lithography to be performed. A process was first developed and optimized on silicon substrates to expose, poly-methyl-methacrylate (PMMA) resist with an electron beam to make nanoscale nickel masks for reactive ion etching. The masks consist of an array of nickel dots that range in size from 20 to 100 nm in diameter. Several nanoscale structures were then fabricated in silicon carbide using electron beam lithography. The structures produced are characterized by field emission scanning electron microscopy.

Source:IEEE

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Thursday, March 6, 2014

Thermally evaporated single-crystal Germanium on Silicon

Using conventional and polarization-dependent Raman spectroscopy we investigate the structural properties of Germanium films thermally evaporated on Silicon under various conditions. The analysis suggests that the Ge films can be crystalline, amorphous and poly-oriented, depending on the substrate temperature. We use both comparison with Raman spectra of Ge films grown on amorphous substrates and polarization-dependent Raman measurements to demonstrate that in the 250–450 °C interval, crystalline Ge films are epitaxial. This result is validated by means of large angle X-ray diffraction measurements. We employ these films to fabricate and characterize near infrared heterojunction photodiodes that exhibit high responsivities and low dark current densities.

Source: Thin Solid Films

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Brittle–ductile transition during diamond turning of single crystal silicon carbide

In this experimental study, diamond turning of single crystal 6H-SiC was performed at a cutting speed of 1 m/s on an ultra-precision diamond turning machine (Moore Nanotech 350 UPL) to elucidate the microscopic origin of ductile-regime machining. Distilled water (pH value 7) was used as a preferred coolant during the course of machining in order to improve the tribological performance. A high magnification scanning electron microscope (SEM FIB- FEI Quanta 3D FEG) was used to examine the cutting tool before and after the machining. A surface finish of Ra=9.2 nm, better than any previously reported value on SiC was obtained. Also, tremendously high cutting resistance was offered by SiC resulting in the observation of significant wear marks on the cutting tool just after 1 km of cutting length. It was found out through a DXR Raman microscope that similar to other classical brittle materials (silicon, germanium, etc.) an occurrence of brittle-ductile transition is responsible for the ductile-regime machining of 6H-SiC. It has also been demonstrated that the structural phase transformations associated with the diamond turning of brittle materials which are normally considered as a prerequisite to ductile-regime machining, may not be observed during ductile-regime machining of polycrystalline materials.
Highlights
► SPDT trial of single crystal 6H-SiC using distilled water as coolant was performed.
► A direct evidence of brittle–ductile transition was found.
► An average machined surface roughness of 9.2 nm was obtained.
► Phase transformations may not be observed during ductile-regime machining of RB-SiC.

Source:International Journal of Machine Tools and Manufacture

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