Piezoelectric aluminum nitride films were deposited onto 3 in. [0001] sapphire substrates by reactive magnetron sputtering to explore the possibility of making highly (002)-textured AlN films to be used in surface acoustic wave (SAW) devices for high temperature applications. The synthesized films, typically 1 μm thick, exhibited a columnar microstructure and a high c-axis texture. The relationship between the microstructures and process conditions was examined by x-ray diffraction (XRD), transmission electron microscopy, and atomic force microscopy analyses. The authors found that highly (002)-textured AlN films with a full width at half maximum of the rocking curve of less than 0.3° can be achieved under high nitrogen concentration and moderate growth temperature, i.e., 250 °C. The phi-scan XRD reveals the high in-plane texture of deposited AlN films. The SAW devices, based on the optimized AlN films on sapphire substrate, were characterized before and after an air annealing process at 800 °C for 90 min...

Highly textured growth of AlN films on sapphire by magnetron sputtering for high temperature surface acoustic wave applications

InAs has been studied as the active material of metamorphic InAlAs∕InAs modulation-doped heterostructures grown on InP. We show that the main limitation of these structures is the plastic relaxation of the 10–15nm thick InAs channel compressively strained to the underlying InAlAs buffer. The best results are obtained with a composite channel made of 7nm InAs inserted in an InGaAs layer. In this case, the electron mobility reaches 21 500 and 179000cm2∕Vs at 300 and 77K, respectively, for a sheet carrier density of 9×1011cm−2. These values are among the highest ones ever reported for such metamorphic structures.

Performances and limitations of InAs∕InAlAs metamorphic heterostructures on InP for high mobility devices

Piezoelectric semiconductor strained layers can be treated as piezoelectric transducers to generate nanometer-wavelength and THz-frequency acoustic waves. The mechanism of nano-acoustic wave (NAW) generation in strained piezoelectric layers, induced by femtosecond optical pulses, can be modeled by a macroscopic elastic continuum theory. The optical absorption change of the strained layers modulated by NAW through quantum-confined Franz-Keldysh (QCFK) effects allows optical detection of the propagating NAW. Based on these piezoelectric-based optical principles, we have designed an optical piezoelectric transducer (OPT) to generate NAW. The optically generated NAW is then applied to one-dimensional (1-D) ultrasonic scan for thickness measurement, which is the first step toward multidimensional nano-ultrasonic imaging. By launching a NAW pulse and resolving the returned acoustic echo signal with femtosecond optical pulses, the thickness of the studied layer can be measured with <1 nm resolution. This nano-structured OPT technique will provide the key toward the realization of nano-ultrasonics, which is analogous to the typical ultrasonic techniques but in a nanometer scale.

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Optical piezoelectric transducer for nano-ultrasonics

In this paper, the technology and properties of surface acoustic wave (SAW) devices on AlN and GaN films are reviewed. The excellent characteristics of these materials for high frequency applications are demonstrated by the fabrication of SAW filters with central frequencies higher than 2.2 GHz. The thermal behavior of these filters has been analyzed. Finally, the integration of a SAW generator with a metal-semiconductor-metal photodetector is described, showing the important synergy resulting from such integration.

High frequency SAW devices on AlGaN: fabrication, characterization and integration with optoelectronics

In spite of various reports that the density of misfit dislocations threading a growing heteroepitaxial thin film can be considerably reduced by means of using a “compliant substrate,” the underlying physical mechanisms are not well understood. The common theoretical models suppose that the growing film can preferably relax in an elastic way by slipping on this kind of substrate. This idea, however, requires us to suppose macroscopic slip displacements. Such displacements are disregarded in the common theories and not reported to occur experimentally. This very doubtful free-slipping hypothesis has been used to establish a force balance (“strain partitioning”) between the growing film and the template layer below it and, consequently, to theoretically derive an enhanced critical thickness for the onset of slip of misfit dislocations. In the present article, more realistic mechanisms are discussed, including early plastic relaxation at a low film thickness where multiplicative interaction of dislocations h...

Principles of strain relaxation in heteroepitaxial films growing on compliant substrates

Sound propagation velocity of surface acoustic waves (SAWs) and bulk acoustic waves (BAWs) has been investigated by means of high resolution Brillouin spectroscopy. The results show a linear dependence of the BAW propagation velocity with the Al concentration. There is no relevant influence of the substrate chosen in the propagation velocity of BAWs in AlxGa1−xN thin films. SAW propagation velocity of epitaxially grown AlN is clearly lower than the observed one in AlN prepared by reactive dc magnetron sputtering. Numerical simulation results of SAW propagation velocity are compared with the experimental results.

Hypersonic characterization of sound propagation velocity in AlxGa1−xN thin films

Influence of interface dislocations on surface kinetics during epitaxial growth of InGaAs

A way to obtain graphene-based materials on a large-scale level is by means of chemical methods for the oxidation of graphite to obtain graphene oxide (GO), in combination with thermal, laser, chemical and electrochemical reduction methods to produce reduced graphene oxide (rGO). Among these methods, thermal and laser-based reduction processes are attractive, due to their fast and low-cost characteristics. In this study, first a modified Hummer’s method was applied to obtain graphite oxide (GrO)/graphene oxide. Subsequently, an electrical furnace, a fusion instrument, a tubular reactor, a heating plate, and a microwave oven were used for the thermal reduction, and UV and CO2 lasers were used for the photothermal and/or photochemical reduction. The chemical and structural characterizations of the fabricated rGO samples were performed by Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD), scanning electron microscope (SEM) and Raman spectroscopy measurements. The analysis and comparison of the results revealed that the strongest feature of the thermal reduction methods is the production of high specific surface area, fundamental for volumetric energy applications such as hydrogen storage, whereas in the case of the laser reduction methods, a highly localized reduction is achieved, ideal for microsupercapacitors in flexible electronics.

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M. A. Sánchez

Papers

Hypersonic characterization of sound propagation velocity in AlxGa1−xN thin films

Influence of interface dislocations on surface kinetics during epitaxial growth of InGaAs

Comparison of Thermal and Laser-Reduced Graphene Oxide Production for Energy Storage Applications