A statistical method to optimize the chemical etching process of zinc oxide thin films.

Zinc oxide (ZnO) is an attractive material for microscale and nanoscale devices. Its desirable semiconductor, piezoelectric and optical properties make it useful in applications ranging from microphones to missile warning systems to biometric sensors. This work introduces a demonstration of blending statistics and chemical etching of thin films to identify the dominant factors and interaction between factors, and develop statistically enhanced models on etch rate and selectivity of c-axis-oriented nanocrystalline ZnO thin films. Over other mineral acids, ammonium chloride (NH4Cl) solutions have commonly been used to wet etch microscale ZnO devices because of their controllable etch rate and near-linear behaviour. Etchant concentration and temperature were found to have a significant effect on etch rate. Moreover, this is the first demonstration that has identified multi-factor interactions between temperature and concentration, and between temperature and agitation. A linear model was developed relating etch rate and its variance against these significant factors and multi-factor interactions. An average selectivity of 73 : 1 was measured with none of the experimental factors having a significant effect on the selectivity. This statistical study captures the significant variance observed by other researchers. Furthermore, it enables statistically enhanced microfabrication processes for other materials.

VO2-based micro-electro-mechanical tunable optical shutter and modulator.

VO2-based MEMS tunable optical shutters are demonstrated. The design consists of a VO2-based cantilever attached to a VO2-based optical window with integrated resistive heaters for individual mechanical actuation of the cantilever structure, tuning of the optical properties of the window, or both. Optical transmittance measurements as a function of current for both heaters demonstrates that the developed devices can be used as analog optical shutters, where the intensity of a light beam can be tuned to any value within the range of VO2 phase transition. A transmittance drop off 30% is shown for the optical window, with tuning capabilities greater than 30% upon actuation of the cantilever. Unlike typical mechanical shutters, these devices are not restricted to binary optical states. Optical modulation of the optical window is demonstrated with an oscillating electrical input. This produces a transmittance signal that oscillates around an average value within the range off VO2's phase transition. For an input current signal with fixed amplitude (fel= 0.28 Hz), tuned to be at the onset of the phase transition, a transmittance modulation of 14% is shown. Similarly, by modulating the DC-offset, a transmittance modulation of VO2 along the hysteresis is obtained.

Spalling-Induced Liftoff and Transfer of Electronic Films Using a van der Waals Release Layer.

Heterogeneous integration strategies are increasingly being employed to achieve more compact and capable electronics systems for multiple applications including space, electric vehicles, and wearable and medical devices. To enable new integration strategies, the growth and transfer of thin electronic films and devices, including III-nitrides, metal oxides, and 2D materials, using 2D boron nitride (BN)-on-sapphire templates are demonstrated. The van der Waals (vdW) BN layer, in this case, acts as a preferred mechanical release layer for precise separation at the substrate-film interface and leaves a smooth surface suitable for vdW bonding. A tensilely stressed Ni layer sputtered on top of the film induces controlled spalling fracture that propagates at the BN/sapphire interface. By incorporating controlled spalling, the process yield and sensitivity are greatly improved, owed to the greater fracture energy provided by the stressed metal layer relative to a soft tape or rubber stamp. With stress playing a critical role in this process, the influence of residual stress on detrimental cracking and bowing is investigated. Additionally, a back-end selected area lift-off technique is developed which allows for isolation and transfer of individual devices or arbitrary shapes.

Two-dimensional BN buffer for plasma enhanced atomic layer deposition of Al2O3 gate dielectrics on graphene field effect transistors.

Here, we investigate the use of few-layer metal organic chemical vapor deposition (MOCVD) grown BN as a two-dimensional buffer layer for plasma enhanced atomic layer deposition (PE-ALD) of Al2O3 on graphene for top gated field effect transistors (FETs). The reactive nature of PE-ALD enables deposition of thin (2 nm) dielectrics directly on graphene and other two-dimensional materials without the need for a seed or functionalization layer; however, this also leads to significant oxidation of the graphene layer as observed by Raman. In FETs, we find this oxidation destroys conductivity in the graphene channel. By transferring thin (1.6 nm) MOCVD BN layers on top of graphene channels prior to PE-ALD, the graphene is protected from oxidation enabling BN/Al2O3 layers as thin as 4 nm. Raman and X-ray photoelectron spectroscopy on BN films show no significant oxidation caused by PE-ALD of Al2O3. Inserting the BN layer creates an atomically abrupt interface significantly reducing interface charges between the graphene and Al2O3 as compared to use of a 2 nm Al buffer layer. This results in a much smaller Dirac voltage (- 1 V) and hysteresis (0.9 V) when compared to FETs with the Al layer (VDirac = - 6.1 V and hysteresis = 2.9 V).

Classical and quantum conductivity in ß-Ga2O3.

The conductivity σ, quantum-based magnetoconductivity Δσ = σ(B) - σ(0), and Hall coefficient RH (= µH/σ) of degenerate, homoepitaxial, (010) Si-doped ß-Ga2O3, have been measured over a temperature range T = 9-320 K and magnetic field range B = 0-10 kG. With ten atoms in the unit cell, the normal-mode phonon structure of ß-Ga2O3 is very complex, with optical-phonon energies ranging from kTpo ~ 20-100 meV. For heavily doped samples, the phonon spectrum is further modified by doping disorder. We explore the possibility of developing a single function Tpo(T) that can be incorporated into both quantum and classical scattering theory such that Δσ vs B, Δσ vs T, and µH vs T are all well fitted. Surprisingly, a relatively simple function, Tpo(T) = 1.6 × 103{1 - exp[-(T + 1)/170]} K, works well for ß-Ga2O3 without any additional fitting parameters. In contrast, Δσ vs T in degenerate ScN, which has only one optical phonon branch, is well fitted with a constant Tpo = 550 K. These results indicate that quantum conductivity enables an understanding of classical conductivity in disordered, multi-phonon semiconductors.

Efficient broadband energy detection from the visible to near-infrared using a plasmon FET.

Plasmon based field effect transistors (FETs) can be used to convert energy induced by incident optical radiation to electrical energy. Plasmonic FETs can efficiently detect incident light and amplify it by coupling to resonant plasmonic modes thus improving selectivity and signal to noise ratio. The spectral responses can be tailored both through optimization of nanostructure geometry as well as constitutive materials. In this paper, we studied various plasmonic nanostructures using gold for a wideband spectral response from visible to near-infrared. We show, using empirical data and simulation results, that detection loss exponentially increases as the volume of metal nanostructure increases and also a limited spectral response is possible using gold nanostructures in a plasmon to electric conversion device. Finally, we demonstrate a plasmon FET that offers a broadband spectral response from visible to telecommunication wavelengths.

Flyweight, Superelastic, Electrically Conductive, and Flame-Retardant 3D Multi-Nanolayer Graphene/Ceramic Metamaterial.

A ceramic/graphene metamaterial (GCM) with microstructure-derived superelasticity and structural robustness is achieved by designing hierarchical honeycomb microstructures, which are composited with two brittle constituents (graphene and ceramic) assembled in multi-nanolayer cellular walls. Attributed to the designed microstructure, well-interconnected scaffolds, chemically bonded interface, and coupled strengthening effect between the graphene framework and the nanolayers of the Al2 O3 ceramic (NAC), the GCM demonstrates a sequence of multifunctional properties simultaneously that have not been reported for ceramics and ceramics-matrix-composite structures, such as flyweight density, 80% reversible compressibility, high fatigue resistance, high electrical conductivity, and excellent thermal-insulation/flame-retardant performance simultaneously. The 3D well-ordered graphene aerogel templates are strongly coupled with the NAC by the chemically bonded interface, exhibiting mutual strengthening, compatible deformability, and a linearly dependent relationship between the density and Young's modulus. Considerable size effects of the ceramic nanolayers on the mechanical properties are revealed in these ceramic-based metamaterials. The designed hierarchical honeycomb graphene with a fourth dimensional control of the ceramic nanolayers on new ways to scalable fabrication of advanced multifunctional ceramic composites with controllable design suggest a great potential in applications of flexible conductors, shock/vibration absorbers, thermal shock barriers, thermal insulation/flame-retardant skins, and porous microwave-absorbing coatings.

Biofunctionalized zinc oxide field effect transistors for selective sensing of riboflavin with current modulation.

Zinc oxide field effect transistors (ZnO-FET), covalently functionalized with single stranded DNA aptamers, provide a highly selective platform for label-free small molecule sensing. The nanostructured surface morphology of ZnO provides high sensitivity and room temperature deposition allows for a wide array of substrate types. Herein we demonstrate the selective detection of riboflavin down to the pM level in aqueous solution using the negative electrical current response of the ZnO-FET by covalently attaching a riboflavin binding aptamer to the surface. The response of the biofunctionalized ZnO-FET was tuned by attaching a redox tag (ferrocene) to the 3' terminus of the aptamer, resulting in positive current modulation upon exposure to riboflavin down to pM levels.

Linearity and temperature dependence of large-area processed high-q barium strontium titanate thin-film varactors.

Ba(0.6)Sr(0.4)TiO(3) (BST) thin-films with large dielectric tunability as high as 4:1 were obtained using a large-area pulsed laser deposition process, with low loss-tangents below 0.01 at zero-bias and 10 GHz. This paper summarizes experimental results obtained on large-area processed BST thin films on 100-mm-diameter sapphire substrates characterized using a varactor shunt switch test structure. Varactors with 0.25-mumthick BST films exhibited large dielectric tunability, the relative dielectric permittivity at zero bias of 990 tuned to 250 at an electric field of 320 kV/cm. The leakage current through the BST film was below 2 nA up to 6 V dc bias. The quality factor (Q) exceeded 300 at relatively low 6 V dc bias for the BST varactors at 1 GHz. These results confirm that large-area processed BST thin films are ready to compete with semiconductor varactors for commercial applications at RF, microwave, and millimeterwave frequencies.