ICMCTF2016 Session C5-1: Thin Films for Active Devices

We report on the microfabrication and thermal characterization of functional shape memory alloy (SMA) thin-film nickel-titanium ( NiTi) bimorph cantilever actuators, which actuate between 1.2 mm radius of curvature and nearly flat states upon heating from room temperature to above 60°C. Our devices are suitable for MEMS applications such as electro-thermal switches and other applications requiring mechanical actuation in response to changes in temperature. Here, we report on the characterization of 270-2300 nm NiTi films on 200 nm Pt layers, with NiTi deposited at 600°C for an in-situ crystallization process. We focused on equiatomic NiTi devices, which thermally transformed between martensite and austenite phases near room temperature, corresponding to significant changes in residual stress, Young’s modulus, and other properties. Reversible phase changes were observed using a differential scanning calorimetry method, with both Austenitic and Martensitic transformations starting at ~55°C. We further characterized reversible SMA behavior for NiTi films on Si through wafer bow measurements ramped at 3 °C/min. Finally, we developed a microfabrication process to create freestanding NiTi/Pt cantilever bimorphs with thicker gold segments electroplated at increments along the devices to ensure that bending only occurred along each cantilever’s length.

Significant levels of residual stress were measured for all sputtering conditions, with possibly higher values for in-situ annealed films—up to 500 MPa—and significant shape memory effects were observed for films down to 270 nm. The released cantilever bimorphs exhibited a 1.2 mm radius of curvature at room temperature, and were actuated into a “down” state that curled slightly into the substrate by heating above 60°C. Predicted folding upon release based on cooling from the deposition temperature and thermal expansion coefficients for NiTi and Pt was 1.22 mm, which agreed well with the experiment. This model also predicted radii of curvature approaching 100 µm with ~200nm NiTi film and correspondingly thin Pt, which will be the subject of future studies. Finally, to explore dynamic heating and actuation effects, we recorded high speed video up to 2,000 fps while irradiating devices with a 532 nm green laser with power density of ~7.2 W/cm². Actuation response time was ~30 ms and reversible, irradiated device-specific actuation was demonstrated across an array of devices. Future work will also involve a dynamic laser-actuation study at various power densities.

Interest is growing in carbonaceous contacts to electronic devices due to the stability, flexibility and wide variety of electronic properties exhibited by this family of materials. We report on modelling, characterization and device applications of graphitic thin films energetically deposited onto semiconductors including Si and 6H-SiC. The control over micro-structural properties afforded by energetic deposition has been exploited to form lithographically defined, highly rectifying contacts with Schottky ideality factors close to unity. The interface regions of these devices are critical to their characteristics and have been investigated using cross-sectional electron microscopy, electron energy loss spectroscopy and modelling. Results will be presented and discussed. In addition, wide bandgap hexagonal boron nitride device layers have been energetically deposited. Defects in hBN have been studied using electron microscopy, electrical/optical measurements, synchrotron-based x-ray absorption spectroscopy and calculations. Their character and influence on device properties will also be discussed.

Gallium oxide with a monoclinic struture (β-Ga₂O₃) is a possible candidate for the gas sensing devices. It is expected that the use of the size- and morphology-controlled quasi-one dimensional nanostrutures is effective for realizing the high sensitive gas sensing devices [1]. Among various growth techniques, we have paid attention to atmospheric-pressure chemical vapor deposition (AP-CVD) utilizing vapor-liquid-solid (VLS) growth mechanism. In this paper, we will discuss the possibility of the shape-controlled growth of β-Ga₂O₃ nanostructures.

Growth experiments of β-Ga₂O₃ nanostuructures were performed by the AP-CVD using gallium (Ga) beads and water (H₂O) as source materials and a gold (Au) film as catalyst in terms of substrate materials, Au film thickness, growth temperature (T_g) and growth time (t). The nanostructures were characterized by θ-2θ scan X-ray diffraction measurements, scanning electron microscope (SEM) observations, transmission electron microscope (TEM) observations and photoluminescence (PL) and PL excitation (PLE) measurements.

Local structure analysis based on TEM observations and selective area electron diffration (SAED) measurements revealed the successful growth of single crystalline β-Ga₂O₃ nanowires (NWs). The increase in T_g or the increase in t resulted in the change in shape from NWs to nanobelts (NBs) together with the increase in width. The diversity of shapes of nanostructures was found to be caused by the coalescences among the neighboring Au catalyst particles due to the rise in T_g, the dgree of the competition between VLS and vapor-solid (VS) growth mechanisms and the epitaxial relation between the substrate and the NW. PL spectra from the β-Ga₂O₃ nanostructures were dominated by the emissions with the peaks at ~370 nm, ~470 nm and ~530 nm, indicating that the nanostructures have the native defects introduced by the deviation from the stoichiometric compositions [2,3].

<<References>> [1] E. Comini, Analytica Chemica Acta 568 (2006) 28. [2] D. Calestani et al., Mater. Sci. Eng. C 25 (2005) 625. [3] L. Binet and D. Gourier, J. Phys. Chem. Solids 59 (2005) 1241.