Search

Thin (40–150 nm), highly doped n+ (1019–1020 cm−3) Ga2O3 layers deposited using pulsed laser deposition (PLD) were incorporated into Ti/Au ohmic contacts on (001) and (010) β-Ga2O3 substrates with carrier concentrations between 2.5 and 5.1 × 1018 cm−3. Specific contact resistivity values were calculated for contact structures both without and with a PLD layer having different thicknesses up to 150 nm. With the exception of a 40 nm PLD layer on the (001) substrate, the specific contact resistivity values decreased with increasing PLD layer thickness: up to 8× on (001) Ga2O3 and up to 16× on (010) Ga2O3 compared with samples without a PLD layer. The lowest average specific contact resistivities were achieved with 150 nm PLD layers: 3.48 × 10−5 Ω cm2 on (001) Ga2O3 and 4.79 × 10−5 Ω cm2 on (010) Ga2O3. Cross-sectional transmission electron microscopy images revealed differences in the microstructure and morphology of the PLD layers on the different substrate orientations. This study describes a low-temperature process that could be used to reduce the contact resistance in Ga2O3 devices.

This report implements for the first time an alkyne-azide “click” chemistry to position dopamine-specific aptamer receptors on InGaZnO (IGZO) thin film transistor (TFT) surfaces for sensitive and selective detection of dopamine. It incorporates catalytic attachment of an alkyne-terminated silane to an azide-anchored aptamer that precludes the need of an intermediate molecular cross-linking bond. The TFTs demonstrate high-gain (~ 1000) Schottky action in deep subthreshold regime achieved through high-quality metal–semiconductor contact engineering which enables almost unity voltage operation. The aptamer is specifically synthesized for selectivity towards dopamine and volt-order-calibrated responses with empirical and theoretical detection limits of 1 nM and 100 pM are achieved.

Disorder arising from random locations of charged donors and acceptors introduces localization and diffusive motion that can lead to constructive electron interference and positive magnetoconductivity. At very low temperatures, 3D theory predicts that the magnetoconductivity is independent of temperature or material properties, as verified for many combinations of thin-films and substrates. Here, we find that this prediction is apparently violated if the film thickness d is less than about 300 nm. To investigate the origin of this apparent violation, the magnetoconductivity was measured at temperatures T = 15 – 150 K in ten, Sn-doped In2O3 films with d = 13 – 292 nm, grown by pulsed laser deposition on fused silica. We observe a very strong thickness dependence which we explain by introducing a theory that postulates a second source of disorder, namely, non-uniform interface-induced defects whose number decreases exponentially with the interface distance. This theory obeys the 3D limit for the thickest samples and yields a natural figure of merit for interface disorder. It can be applied to any degenerate semiconductor film on any semi-insulating substrate. Disorder in semiconductors may lead to quantum interference and positive magnetoconductivity, whose maximum value in 3D is independent of material properties. Here, an apparent violation of this upper bound, in Sn-doped In2O3 films on fused silica, is explained by a model that accounts for additional disorder close to the interface.

Indium tin oxide (ITO) has become a very useful plasmonic and nonlinear optical material because of its highly tunable electrical and optical properties and strong optical nonlinearity. In this work, the authors conducted detailed fabrication process studies by using high-temperature reactive sputtering to finely tune the optical properties of ITO thin films, particularly the epsilon-near-zero (ENZ) wavelength in the near and mid-IR spectrum. Sputtered ITO thin films are characterized by using spectroscopic ellipsometry, surface profilometry, Hall measurements, and 4-point probe testing. Additionally, the effect of post-deposition annealing of ITO films is also investigated.

This study reports the deep subthreshold characteristics (≤1 V) of low thermal budget Indium Gallium Zinc Oxide (IGZO) thin film transistors (TFTs) with Schottky barrier source/drain contacts. The Schottky barrier was analyzed and a consistent ideality factor was observed across the devices. A deep subthreshold region was extracted from the nominal characteristics and barrier influence was observed in the low voltage region. This operation led to high output impedance (~1012Ω) and excellent trans-conductance leading to a high voltage gain (>100) due to hard saturation of the output characteristics. Positive bias stress and stability tests were conducted within this region that showed minimal drift in the transfer characteristics. Such characteristics make these devices an excellent choice for low-power deep subthreshold and weak signal applications.

We demonstrated 500 °C operation of field-effect transistors made using ultra-wide bandgap semiconductor β-Ga2O3. Metal–semiconductor field-effect transistors were fabricated using epitaxial conductive films grown on an insulating β-Ga2O3 substrate, TiW refractory metal gates, and Si-implanted source/drain contacts. Devices were characterized in DC mode at different temperatures up to 500 °C in vacuum. These variable-temperature measurements showed a reduction in gate modulation of the drain current due to an increase in gate leakage across the gate/semiconductor Schottky barrier. Devices exhibited a reduction in transconductance with increasing temperature; despite this, drain current increased with temperature due to a reduction in threshold voltage caused by the de-trapping of electrons from deep-level traps. Devices also showed negligible change in semiconductor epitaxy and source/drain contacts, hence demonstrated recovery to their room-temperature electrical properties after the devices were tested intermittently at different high temperatures in vacuum. The mechanism of gate leakage was also explored, which implicated the presence of different conduction mechanisms at different temperatures and gate electric fields.

Development of electronic-grade gate dielectrics with negligible hysteresis, low interface defect density (N IT ), reduced gate leakage and high breakdown strength (E BD ) have significant importance for the application of β-Ga 2 O 3 in next-generation power electronic devices. In this article, we present a method for fabricating a metal-oxide-semiconductor capacitor (MOSCAP) exhibiting low hysteresis by depositing Al 2 O 3 on a piranha-cleaned, BOE (buffered oxide etchant)-treated, (010) β-Ga 2 O 3 and after performing forming gas post-deposition annealing (FG-PDA). Resultant devices were characterized using different electrical methods and using Atomic Force Microscopy (AFM). An interface defect density of ~ 1011 cm-2 was obtained for the MOSCAP that had the most optimized process showing a V FB ~ 1.0V and a gate leakage of J G ≤ 2x10-8 A/cm2 when the device was accumulated up to ~ 2.7 MV/cm. This is the first demonstration of hysteresis-free MOSCAP using a (010) β-Ga 2 O 3 substrate.

The conductivity σ, quantum-based magnetoconductivity Δσ = σ(B) − σ(0), and Hall coefficient RH (= µH/σ) of degenerate, homoepitaxial, (010) Si-doped β-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 β-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 β-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.

Quantum magnetoconductance, delta-sigma(B)=sigma(B)-sigma(0), offers a new, unique way to study phonons in heavily-doped, complex semiconductors, including beta-Ga2O3 and Sn-doped In2O3 (ITO). At low temperature, theory predicts delta-sigma(B)=2.908B^(1/2) S/cm, shown to be true for thick, but not thin, samples. We grew ten ITO films by PLD on fused silica, d=13–292 nm. The thickness dependence was explained by a new delta-sigma(d)-vs-d theory based on a second source of disorder, interface-generated defects that decrease exponentially with distance from the ITO/FS interface. A fit of delta-sigma(d)-vs-d gives three parameters, including d*, the thickness above which the surface is not affected by interface damage.

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).

This chapter examines homoepitaxial β-Ga2O3 thin films fabricated by pulsed laser deposition. The recent availability of high-quality native substrates affords investigations using a wide range of growth conditions to achieve single crystalline films by pulsed laser deposition. Deposition parameter optimization and structural, electrical and chemical film characterization are presented for undoped and impurity-doped films. Films grown on commercially available (010) Fe compensation-doped substrates fabricated by the edge-defined film-fed growth technique and a developing Czochralski method are examined. Hall effect results from PLD Si-doped β-Ga2O3 films are compared with data produced from other vapor phase epitaxial growth techniques. The influence of substrate orientation on film properties is also studied with (010) and (001) crystals. Implementation of a PLD n+ regrowth layer in a transistor device is demonstrated to achieve low ohmic contact resistance. Results from β-(AlxGa1−x)2O3 film studies show the potential utility of heterostructures for field-effect transistor 2DEG formation.

Since vanadium dioxide (VO2) is a phase change material that shows a metal to insulator transition behavior at a temperature near 68℃, it has many potential applications in the areas of science and engineering. In this study, tungsten (W) -doped VO2 thin films were synthesized by pulsed laser deposition (PLD) technique with a metallic vanadium target and W-wires which were simply attached on the surface of the vanadium target. Also, bowtie RF antenna structures were fabricated by using the films to check the frequency tunability. The experimental results show a decrease of the transition temperature and a broadening of the phase transition as the atomic percentage of W increases in the film structure. The results also reveal that the W-doped VO2 thin film based RF antenna was fully functional with the successful frequency tuning capability.

Epsilon-near-zero (ENZ) materials, when probed at or near wavelengths corresponding to their zero permittivity crossing points, have unique and interesting properties that can be exploited for enhancing nanoscale light–matter interactions. Here, we experimentally show that over an order of magnitude increase in the absorption band of a periodically patterned metal–dielectric–metal structure can be obtained by integrating an indium tin oxide (ITO) subwavelength nanolayer into the insulating dielectric gap region. Through incorporation of a 12 nm thick ITO layer between the patterned gold nanodisks and the SiO2 dielectric layer, a 240 nm wide, flat-top perfect (>98%) absorption centered at 1550 nm wavelength is enabled. The demonstrated wideband, perfect absorption resonance is shown to be due to coupling between the gap plasmon mode of the metasurface and the ENZ mode in the nanoscale ITO film.

Titanium oxide (TiO2) is a semiconducting oxide of increasing interest due to its chemical and thermal stability and broad applicability. In this study, thin films of TiO2 were deposited by pulsed laser deposition on sapphire and silicon substrates under various growth conditions, and characterized by x-ray diffraction (XRD), atomic force microscopy (AFM), optical absorption spectroscopy and Hall-effect measurements. XRD patterns revealed that a sapphire substrate is more suitable for the formation of the rutile phase in TiO2, while a silicon substrate yields a pure anatase phase, even at high-temperature growth. AFM images showed that the rutile TiO2 films grown at 805°C on a sapphire substrate have a smoother surface than anatase films grown at 620°C. Optical absorption spectra confirmed the band gap energy of 3.08 eV for the rutile phase and 3.29 eV for the anatase phase. All the deposited films exhibited the usual high resistivity of TiO2; however, when employed as a buffer layer, anatase TiO2 deposited on sapphire significantly improves the conductivity of indium gallium zinc oxide thin films. The study illustrates how to control the formation of TiO2 phases and reveals another interesting application for TiO2 as a buffer layer for transparent conducting oxides.

We report enhancement-mode $\beta $ -Ga2O3 (BGO) MOSFETs on a Si-doped homoepitaxial channel grown by molecular beam epitaxy. A gate recess process is used to partially remove the epitaxial channel under the 1- $\mu \text{m}$ gated region to fully deplete at ${V}_{\textsf {GS}}= 0$ V. BGO MOSFETs achieve drain current density near 40 mA/mm and ${I}_{ \mathrm{\scriptscriptstyle ON}}/{I}_{ \mathrm{\scriptscriptstyle OFF}}$ ratio ~109 which is the highest reported for homoepitaxial normally-off BGO transistors. At ${V}_{\textsf {GS}}= \textsf {0}$ V, a breakdown voltage of 198 and 505 V is achieved with the source–drain spacing of 3 and $8~\mu \text{m}$ , respectively. The power switching figure of merits for dc conduction and dynamic switch losses meet or exceed the theoretical silicon limit and previously reported depletion-mode BGO transistors.

Conductive homoepitaxial Si-doped β-Ga2O3 films were fabricated by pulsed laser deposition with an as-deposited 2323 S cm−1 conductivity (resistivity = 4.3 × 10−4 Ω-cm, carrier concentration = 2.24 × 1020 cm−3, mobility = 64.5 cm2 V−1 s−1, and electrical activation efficiency = 77%). High quality homoepitaxial films deposited on commercial (010) Fe-compensated β-Ga2O substrates were determined by high-resolution transmission electron microscopy and x-ray diffraction. The β-Ga2O3 films have ∼70% transparency from 3.7 eV (335 nm) to 0.56 eV (2214 nm). The combination of high conductivity and transparency offers promise for numerous ultrawide bandgap electronics and optoelectronic applications.

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 (f el = 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.

Many challenges exist in understanding transport properties in metal-oxide thin film transistors (MO-TFTs). Microstructural disorder, dielectric/active layer interface trap states, and grain boundaries contribute to reductions in device properties such as transistor output current, mobility, and sheet carrier concentration. In this work, we use scheduled interruptions during atomic layer deposition combined with a series of thermal anneals to control properties of ZnO/ A l 2 O 3 TFTs, and to experimentally decouple physical effects within the TFT affecting device performance. Using concurrent current–voltage and gated Hall effect measurements, we observe two sets of trends for these devices. In the first, increasing post-temperature anneal is shown to improve the device characteristics by reducing grain boundary effects. In the second, exposure to ambient conditions is shown to increase the number of interface trap states. Comparisons of measured values show that these trap states enhance some properties of the lower temperature annealed series, while degrading these values in the higher temperature annealed series. We show that this is due to devices acting in two separate transport regimes: localized, and non-localized. We investigate the position of the Fermi level, adjusted by gate bias, to these two regimes under each test condition, and suggest a simple model to describe the results.

Vanadium dioxide (VO2) and vanadium pentoxide (V2O5) thin films change their properties in response to external stimuli such as photons, temperature, electric field and magnetic field and have applications in electronics, optical devices, and sensors. Due to the multiple valence states of V and non-stoichiometry in thin films, it is challenging to grow epitaxial, single-phase V-oxide on a substrate, or a heterostructure of two epitaxial V-oxides. We report the formation of a heterostructure consisting of a few nm thick ultrathin V2O5 epitaxial layer on pulsed laser deposited tens of nm thick epitaxial VO2 thin films grown on single crystal Al2O3(001) substrates without post annealing of the VO2 film. The simultaneous observation of the ultrathin epitaxial V2O5 layer and VO2 epitaxial film is only possible by our unique reflection high energy electron diffraction pole figure analysis. The out-of-plane and in-plane epitaxial relationships are V2O5[100]||VO2[010]||Al2O3[001] and V2O5[03 2 ¯ ]||VO2[100]||Al2O3[1 1 ¯ 0], respectively. The existence of the V2O5 layer on the surface of the VO2 film is also supported by X-ray photoelectron spectroscopy and Raman spectroscopy.

We demonstrate a $\beta $ -Ga2O3 MOSFET with record-high transconductance ( ${g}_{m}$ ) of 21 mS/mm and extrinsic cutoff frequency ( ${f}_{T}$ ) and maximum oscillating frequency ( ${f}_{\max }$ ) of 3.3 and 12.9 GHz, respectively, enabled by implementing a new highly doped ohmic cap layer with a sub-micron gate recess process. RF performance was further verified by CW Class-A power measurements with passive source and load tuning at 800 MHz, resulting in ${P}_{{OUT}}$ , power gain, and power-added efficiency of 0.23 W/mm, 5.1 dB, and 6.3%, respectively. These preliminary results indicate potential for monolithic or heterogeneous integration of power switch and RF devices using $\beta $ -Ga2O3.

We report on MOSFETs fabricated on Ge-doped $\beta $ -Ga2O3 homoepitaxial material grown by molecular beam epitaxy on (010) Fe-doped semi-insulating substrates. The Ge-doped channel devices performed similar to previously reported devices with Sn- and Si-doped channels with the drain current ON/OFF ratios of $> 10^{8}$ and the saturated drain current of >75 mA/mm at $V_{G}=0$ V. Hall effect measurements showed a high carrier mobility of 111 cm2/( $\text{V}\cdot \text{s}$ ) with $4\times 10^{17}$ cm $^{-3}$ active carriers. A MOSFET with a gate-drain spacing of $5.5~\mu \text{m}$ had a three-terminal breakdown voltage of 479 V.

Addition of wavelength selective absorbers on microbolometers tends to increase their thermal mass and slow their infrared response times. Making the bolometric material an integral part of the absorber and minimizing layer thicknesses is one possible way to maintain high detector speeds. Here, we study experimentally the effect on permittivity of adding a layer of semiconducting VOx between two layers of SiO2. Additionally, we investigate theoretically the effect on resonance wavelength of thinning the metal in metal-insulator-metal plasmonic resonant absorbers.

Beta-phase gallium oxide $(\beta-\mathrm{Ga}_{2}\mathrm{O}_{3})$ has shown promise as a next-generation wide-bandgap semiconductor for use in power electronics. It possesses a bandgap and expected critical field strength of ~4.8 eV and ~8 MV/cm, respectively, surpassing the same measured characteristics of GaN and SiC [1]. Early work has been successful in demonstrating lateral metal-oxide-semiconductor field-effect transistors (MOSFETs), predominately for depletion mode operation, with high critical field strength [2], high current density [3], and high breakdown voltage [4], [5]. One limitation of $\beta-\mathrm{Ga}_{2}\mathrm{O}_{3}$ MOSFETs is source access resistance $(\mathrm{R}_{\mathrm{S}})$ , the ungated region between source and gate, with sheet resistance $(\mathrm{R}_{\mathrm{SH}})$ typically in the $\mathrm{k}\Omega/\mathrm{sq}$ range. The $\mathrm{R}_{\mathrm{S}}$ affects key device performance parameters such as transconductance $(\mathrm{G}_{\mathrm{M}})$ and drain-current density $(\mathrm{I}_{\mathrm{DS}})$ . Higher DC and RF performance can be expected from eliminating $\mathrm{R}_{\mathrm{S}}$ by self-aligning the gate and source contacts. We present, for the first time, a self-aligned gate (SAG) $\beta-\mathrm{Ga}_{2}\mathrm{O}_{3}$ MOSFET using a refractory metal gate-first design. MOSFET devices with $2\mathrm{x}50\mu \mathrm{m}$ gate periphery, $7.5\ \mu \mathrm{m}$ source-drain distance $(\mathrm{L}_{\mathrm{SD}})$ and $2\ \mu \mathrm{m}$ gate length $(\mathrm{L}_{\mathrm{G}})$ were directly compared with and without the SAG features. MOSFETs with SAG show a substantial increase in $\mathrm{G}_{\mathrm{M}}$ and $\mathrm{I}_{\mathrm{DS}}$ , from ~2 mS/mm to ~14 mS/mm and ~10 mA/mm to ~45 mA/mm, respectively, up to $\mathrm{V}_{\mathrm{GS}}=4$ V. Lastly, we report a laterally scaled device $(2\mathrm{x}50\mu \mathrm{m})$ with $\mathrm{L}_{\mathrm{SD}}=2.5\mu \mathrm{m}$ and $\mathrm{L}_{\mathrm{G}}=2\mu \mathrm{m}$ , achieving high current density $(\sim 140\mathrm{mA}/\mathrm{mm})$ , and high G M $(\sim 35\mathrm{mS}/\mathrm{mm})$ .

The power switching potential of Ga 2 O 3 has been well established in the last several years due to the material's high critical electric field $(\mathrm{E}_{\mathrm{C}})$ strength. This allows for dramatic device scaling and operation at high voltage with reduced series resistance in power switching applications. The same argument applies for RF power amplification, where the high $\mathrm{E}_{\mathrm{C}}$ of Ga 2 O 3 allows FETs to be scaled to smaller dimensions while operating at higher operating voltages than existing RF FETs. At the same time, ab initio calculations published in 2017 for velocity-field characteristics of Ga 2 O 3 indicate a peak electron velocity of $2\mathrm{x}10^{7}$ cm/s which suggests the power-frequency product for Ga 2 O 3 is significantly greater than that of GaN. In 2017, the first demonstrated CW RF operation for Ga 2 O 3 FETs was in part limited by the thermal resistance and immaturity of heat extraction techniques to ~300 mW/mm. Since then, pulsed RF power measurement techniques have been implemented to accelerate electrical characterization of RF Ga 2 O 3 FETs. Pulsed RF output power $> 500$ mW/mm has been measured at 1 GHz. Advances in laterally scaled devices and topology design to ensure electrons achieve saturated velocity in the channel will be discussed.

The path to achieving integrated RF and power conversion circuitry using the β-Ga2O3 material system is described with regard to the materials high Johnson's RF figure of merit. Recent results, including large signal data at VD = 50 V, are provided, showing progress in achieving high-voltage RF operation. Additionally, progress in achieving high-gain devices through gate length scaling is also benchmarked by a record RF power device with a gate length of 0.5 μm achieving a 2.1 GHz μm f T − L G product. These results are compared with state-of-the-art RF devices, and the expectations for β-Ga2O3 at this point in its maturity throughout this Letter with future milestones laid out to measure progress. The conclusion includes near- and long-term projections for β-Ga2O3 devices for RF based on the results and projected milestones presented.

Fourier transform infrared (FTIR) measurements were carried out on thin films and bulk single crystals of ZnO over a wide temperature range to study the free-carrier and multi-phonon infrared absorptions and the effects of hydrogen incorporation on these properties. Aluminum-doped ZnO thin films were deposited on quartz substrates using atomic-layer deposition (ALD) and sol–gel methods. Hall-effect measurements showed that the ALD films have a resistivity of ρ = 1.11 × 10−3 Ω cm, three orders of magnitude lower than sol–gel films (ρ = 1.25 Ω cm). This result is consistent with the significant difference in their free-carrier absorption as revealed by FTIR spectra obtained at room temperature. By reducing the temperature to 80 K, the free carriers were frozen out, and their absorption spectrum was suppressed. From the FTIR measurements on ZnO single crystals that were grown by the chemical vapor transport method, we identified a shoulder around 3350 cm−1 and associated it with the presence of two or more hydrogen ions in a Zn vacancy. After reducing the hydrogen level in the crystal, the measurements revealed the multi-phonon absorption of ZnO in the range of 700–1200 cm−1. This study shows that the multi-phonon absorption bands can be completely masked by the presence of a large concentration of hydrogen in the crystals.

Aluminum-doped zinc oxide (ZnO:Al) thin films were synthesized by atomic layer deposition on silicon, quartz and sapphire substrates and characterized by x-ray diffraction (XRD), high-resolution scanning electron microscopy, optical spectroscopy, conductivity mapping, Hall effect measurements and positron annihilation spectroscopy. XRD showed that the as-grown films are of single-phase ZnO wurtzite structure and do not contain any secondary or impurity phases. The type of substrate was found to affect the orientation and degree of crystallinity of the films but had no effect on the defect structure or the transport properties of the films. High conductivity of 10−3 Ω cm, electron mobility of 20 cm2/Vs and carrier density of 1020 cm−3 were measured in most films. Thermal treatments in various atmospheres induced a large effect on the thickness, structure and electrical properties of the films. Annealing in a Zn and nitrogen environment at 400°C for 1 h led to a 16% increase in the thickness of the film; this indicates that Zn extracts oxygen atoms from the matrix and forms new layers of ZnO. On the other hand, annealing in a hydrogen atmosphere led to the emergence of an Al2O3 peak in the XRD pattern, which implies that hydrogen and Al atoms compete to occupy Zn sites in the ZnO lattice. Only ambient air annealing had an effect on film defect density and electrical properties, generating reductions in conductivity and electron mobility. Depth-resolved measurements of positron annihilation spectroscopy revealed short positron diffusion lengths and high concentrations of defects in all as-grown films. However, these defects did not diminish the electrical conductivity in the films.

AlN thin film structures have many useful and practical piezoelectric and pyroelectric properties. The potential enhancement of the AlN piezo- and pyroelectric constants allows it to compete with more commonly used materials. For example, combination of AlN with ScN leads to new structural, electronic, and mechanical characteristics, which have been reported to substantially enhance the piezoelectric coefficients in solid-solution AlN-ScN compounds, compared to a pure AlN-phase material. In our work, we demonstrate that an analogous alloying approach results in considerable enhancement of the pyroelectric properties of AlN - ScN composites. Thin films of ScN, AlN and Al1-x ScxN (x = 0 – 1.0) were deposited on silicon (004) substrates using dual reactive sputtering in Ar/N2 atmosphere from Sc and Al targets. The deposited films were studied and compared using x-ray diffraction, XPS, SEM, and pyroelectric characterization. An up to 25% enhancement was observed in the pyroelectric coefficient (Pc = 0.9 μC /m2K) for Sc1-xAlxN thin films structures in comparison to pure AlN thin films (Pc = 0.71 μC/m2K). The obtained results suggest that Al1-x ScxN films could be a promising novel pyroelectric material and might be suitable for use in uncooled IR detectors.

A Sn-doped (100) $\beta $ -Ga2O3 epitaxial layer was grown via metal–organic vapor phase epitaxy onto a single-crystal, Mg-doped semi-insulating (100) $\beta $ -Ga2O3 substrate. Ga2O3-based metal–oxide–semiconductor field-effect transistors with a 2- $\mu \text{m}$ gate length ( $L_{G})$ , 3.4- $\mu \text{m}$ source–drain spacing ( $L_{\textrm {SD}})$ , and 0.6- $\mu \text{m}$ gate–drain spacing ( $L_{\textrm {GD}})$ were fabricated and characterized. Devices were observed to hold a gate-to-drain voltage of 230 V in the OFF-state. The gate-to-drain electric field corresponds to 3.8 MV/cm, which is the highest reported for any transistor and surpassing bulk GaN and SiC theoretical limits. Further performance projections are made based on layout, process, and material optimizations to be considered in future iterations.

Disordered ionic-bonded transition metal oxide thin-film transistors (TFTs) show promise for a variety of dc and RF switching applications, especially those that can leverage their low-temperature, substrate-agnostic process integration potential. In this paper, enhancement-mode zinc-oxide TFTs were fabricated and their switching performance evaluated. These TFTs exhibit the drain-current density of 0.6 A/mm and minimal frequency dispersion, as evidenced by dynamic current–voltage tests. A high-frequency power switch figure of merit $R_{{\mathrm{\scriptscriptstyle ON}}}Q_{G}$ of 359 $\text{m}\Omega \,\cdot \, $ nC was experimentally determined for 0.75- $\mu \text{m}$ long-channel devices, and through scaling 45.9 $\text{m}\Omega \,\cdot \, $ nC is achievable for 11 V-rated devices (where $R_{\mathrm{\scriptscriptstyle ON}}$ is ON-state drain–source resistance, and $Q_{G}$ is gate charge). An RF switch cutoff frequency $f_{c}$ of 25 GHz was measured for the same 0.75- $\mu \text{m}$ TFT, whereas $f_{c}$ exceeding 500 GHz and power handling in the tens of watts are projected with optimization.

InGaZnO (IGZO) is an excellent semiconductor material for thin-film transistors (TFTs) used in direct-current and radio-frequency (RF) switching applications, especially since it can be grown at low temperatures on a wide variety of substrates. IGZO thin films with a composition of InGaZnO4 have been deposited and used as channel layers in TFTs for many applications to date; however, IGZO compositions can also be easily changed to vary their properties. These different compositions of IGZO may have different defect properties. In this study, we report the growth of IGZO with composition of In2O3:Ga2O3:5ZnO (In2Ga2Zn5O11) by pulsed laser deposition (PLD) and its electronic defects studied by thermally stimulated current (TSC) spectroscopy. It was found that the as-grown sample has a DC activation energy of 0.62 eV, and four major traps with activation energies between ∼0.16–0.50 eV and one at ∼0.90 eV. Electrical properties and the deep traps of PLD-grown IGZO thin film were found to be varied by post-processing conditions such as thermal history and measurement conditions such as the bias and light exposure applied to the samples. The instabilities induced by these conditions can be explained by structural and stoichiometric features—the ZnO4 tetrahedra and GaO6 octahedra in the structure may become distorted, caused by oxidation or reduction, so that the different defect states could be changed and/or lattice energy variations from the distortion can be observed. This work demonstrates that current-based trap emission, such as that associated with TSC, can effectively reveal electronic defects in highly-resistive semiconductor materials, especially those that are not amenable to capacitance-based techniques, such as deep-level transient spectroscopy, and provide an effective manner to study the trap instabilities in IGZO.

We study the electrical and optical properties of degenerate ZnGa2O4 films grown by metalorganic chemical vapor deposition (MOCVD) on sapphire and semi-insulating films grown by pulsed laser deposition (PLD) on fused silica. After a forming-gas anneal at 700 °C, the MOCVD film is highly conducting, with a room-temperature carrier concentration of 2 × 1020 cm−3, a mobility of 20 cm2/V s, and direct bandgap absorptions at 3.65 eV and 4.60 eV. Under the same annealing conditions, the PLD film is semi-insulating, with a direct bandgap absorption at 5.25 eV. The phonon structure, important for electrical and thermal conduction as well as superconductivity and other quantum phenomena, is very complicated due to the large number of atoms (and, thus, phonon branches) in the unit cell. However, we show that the phonon contributions to electron mobility (μph) can be directly measured by quantum-based magnetoconductivity over the temperature span T = 10–200 K. From an approximate analytical formula, μph = function (Tph, T), we calculate an effective phonon energy kTph(T) that takes account of all phonon contributions at temperature T. For T = 10–200 K, the value of kTph ranges from about 10 to 90 meV, consistent with the energy range of the ZnGa2O4 phonon density of states (at 0 K) calculated by density functional theory. The total measured mobility can then be modeled by μtot−1 = μii−1 + μph−1, where μii is the mobility due to ionized-impurity scattering. With a high bandgap, controllable conductivity, high breakdown voltage, and bulk-growth capability, ZnGa2O4 offers opportunities for high-power electronics and UV detectors.

Two methods of measuring the electronic transport properties of a material are transistor DC-voltage and the Hall effect. Hall mobility measurements of normally off semiconductors can be done by electrostatic doping to lower resistance in the channel. We show that by measuring both, we can compare any value (raw measured as well as calculated data) directly to any other value along an index of FET gate and drain voltage across the entire safe operating area of the device. Our gated Hall technique intrinsic calculations of Hall mobility, typically possible only for bulk or doped materials, for thin-film transistor materials stack up with thickness scaled to practical values.

This study investigates the process–microstructure–property relationship during a UV laser crystallization of a transparent conductive layer—gallium doped zinc oxide (GZO) films after pulsed laser deposition (PLD). UV laser induced crystallization technique is able to apply ultra-fast post-treatment to modify GZO films with better structural and optoelectronics properties, suggesting a potential for large-scale manufacturing. A physical simulation model coupled laser–matter interaction and heat-transfer was utilized to study pulse laser heating and heat dissipation process. The laser crystallized GZO film exhibits low resistivity of ~ 3.2 × 10−4 Ω cm, high-Hall mobility of 22 cm2/V s, and low sheet resistance of 22 Ω/sq. High-transmittance (T) over 90% at 550 nm is obtained (with glass substrate). The optoelectronic performance improved mainly attributes to grain boundary modification in the polycrystalline film, e.g., decrease of grain boundary density and passivation of electron trap at grain boundaries.

The optical properties of lanthanum nickelate (LNO) films grown by chemical solution decomposition on sapphire substrates were measured and extracted. The relatively low imaginary part of the dielectric function indicates that LNO may perform well in plasmonic applications at traditional telecommunication frequencies.

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.