ICMCTF2016 Session C5-1: Thin Films for Active Devices

Monday, April 25, 2016 10:00 AM in Room Sunset

Monday Morning

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10:00 AM C5-1-1 Microfabricated Thermal Actuators based on SputteredThin-Film Nitinol
Cory Knick, Christopher Morris (US Army Research Laboratory, USA)

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.

10:20 AM C5-1-2 Semiconductor Devices from Energetically Deposited Films: Modelling and Characterisation
James Partridge, Nicholas McDougall, Billy Murdoch, Edwin Mayes, Masturina Kracica, Hiep Tran, Anthony Holland, Dougal McCulloch (RMIT University, Australia)

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.

10:40 AM C5-1-3 Approaching Defect-free Amorphous Silicon Nitride by Plasma-assisted Atomic Beam Deposition for High Performance Gate Dielectric
Chung-Lin Wu, Shu-Ju Tsai, Chiang-Lun Wang, Hung-Chun Lee, Chun-Yeh Lin, Jhih-Wei Chen (National Cheng Kung University, Taiwan, Republic of China)
In the past few decades, gate insulators with a high dielectric constant (high-k dielectric) enabling a physically thick but dielectrically thin insulating layer, have been used to replace traditional SiOx insulator and to ensure continuous downscaling of Si-based transistor technology. However, due to the non-silicon derivative natures of the high-k metal oxides, transport properties in these dielectrics are still limited by various structural defects on the hetero-interfaces and inside the dielectrics. Here, we show that another insulating silicone compound, amorphous silicon nitride (a-Si3N4), is a promising candidate effective electrical insulator for use as a high-k dielectric. We have examined a-Si3N4 deposited using the plasma-assisted atomic beam deposition (PA-ABD) technique in an ultra-high vacuum (UHV) environment and demonstrated the absence of defect-related luminescence; it was also found that the electronic structure across the a-Si3N4/Si heterojunction approaches the intrinsic limit, which exhibits large band gap energy and valence band offset. We demonstrate that charge transport properties in the metal/a-Si3N4/Si (MNS) structures approach defect-free limits with a large breakdown field and a low leakage current. Our results obtained using PA-ABD suggest a general strategy to markedly improve the performance of Si devices with a nearly defect-free dielectric.
11:00 AM C5-1-4 Growth and Morphology Control of β-Ga2O3 Nanostructures by Atmospheric-pressure CVD
Tomoaki Terasako (Graduate School of Science and Engineering, Ehime University, Japan); Yuki Kawasaki (Faculty of Engineering, Ehime University, Japan); Masakazu Yagi (National Instiutute of Technology, Kagawa College, Japan)

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 (Tg) 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 Tg 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 Tg, 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.

11:20 AM C5-1-5 Influence of Repeated Uniaxial Mechanical Strain on Amorphous In-Ga-Zn-O Thin Film Transistors Fabricated on Flexible Polyimide Substrates
Bo-Wei Chen, Ting-Chang Chang, ShinPing Huang, Yu-Ju Hung, Ann-Kuo Chu (National Sun Yat-Sen University, Taiwan, Republic of China); Tai-Jui Wang, Tsu-Chiang Chang (Industrial Technology Research Institute, Taiwan, Republic of China)
This letter investigates the impacts of mechanical strain on amorphous In-Ga-Zn-O (a-IGZO) thin film transistors fabricated on a polyimide substrate. The flexible TFTs were exposed to mechanical tension and compression stress with bending radius of 30mm. Device parameters such as threshold voltage, carrier mobility and sub-threshold swing were extracted from drain current-gate voltage characteristics. Also, the strain-induced trap states generation and distribution of density-of-state within the energy gap were investigated utilizing capacitance-voltage measurements. By investigating the transfer as well as output characteristics of devices, variations in threshold voltage were discovered, while none were found for carrier mobility or trap states.
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