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Mg doped lithium niobate (Mg:LN) exhibits several advantages over undoped LN such as resistance to photorefraction, lower coercive fields, and p-type conductivity that is particularly pronounced at domain walls and opens up a range of applications, e.g., in domain wall electronics. Engineering of precise domain patterns necessitates well founded knowledge of switching kinetics, which can differ significantly from that of undoped LN. In this work, the role of humidity and sample composition in polarization reversal has been investigated under application of the same voltage waveform. Control over domain sizes has been achieved by varying the sample thickness and initial polarization as well as atmospheric conditions. In addition, local introduction of proton exchanged phases allows for inhibition of domain nucleation or destabilization, which can be utilized to modify domain patterns. Polarization dependent current flow, attributed to charged domain walls and band bending, demonstrates the rectifying ability of Mg:LN in combination with suitable metal electrodes that allow for further tailoring of conductivity. [ABSTRACT FROM AUTHOR]

An experimental study was conducted on controlling the growth mode of La0.7Sr0.3MnO3 thin films on SrTiO3 substrates using pulsed laser deposition (PLD) by tuning growth temperature, pressure, and laser fluence. Different thin film morphology, crystallinity, and stoichiometry have been observed depending on growth parameters. To understand the microscopic origin, the adatom nucleation, step advance processes, and their relationship to film growth were theoretically analyzed and a growth diagram was constructed. Three boundaries between highly and poorly crystallized growth, 2D and 3D growth, stoichiometric and non-stoichiometric growth were identified in the growth diagram. A good fit of our experimental observation with the growth diagram was found. This case study demonstrates that a more comprehensive understanding of the growth mode in PLD is possible. [ABSTRACT FROM AUTHOR]

This work presents a tri-gate GaN junction high-electron-mobility transistor (JHEMT) concept in which the p–n junction wraps around the AlGaN/GaN fins in the gate region. This tri-gate JHEMT differs from all existing GaN FinFETs and tri-gate HEMTs, as they employ a Schottky or a metal-insulator-semiconductor (MIS) gate stack. A tri-gate GaN JHEMT is fabricated using p-type NiO with the gate metal forming an Ohmic contact to NiO. The device shows minimal hysteresis and a subthreshold slope of 63 ± 2 mV/decade with an on-off current ratio of 108. Compared to the tri-gate MISHEMTs fabricated on the same wafer, the tri-gate JHEMTs exhibit higher threshold voltage (VTH) and achieve positive VTH without the need for additional AlGaN recess. In addition, this tri-gate JHEMT with a fin width of 60 nm achieves a breakdown voltage (BV) > 1500 V (defined at the drain current of 1 μA/mm at zero gate bias) and maintains the high BV with the fin length scaled down to 200 nm. In comparison, the tri-gate MISHEMTs with narrower and longer fins show punch-through at high voltages. Moreover, when compared to planar enhancement mode HEMTs, tri-gate JHEMTs show significantly lower channel sheet resistance in the gate region. These results illustrate a stronger channel depletion and electrostatic control in the junction tri-gate compared to the MIS tri-gate and suggest great promise of the tri-gate GaN JHEMTs for both high-voltage power and low-voltage power/digital applications. [ABSTRACT FROM AUTHOR]

Zika virus detection was demonstrated using antibody-functionalized cover glasses externally connected to the gate electrode of an AlGaN/GaN high electron mobility transistor (HEMT). A pulsed bias voltage of 0.5 V was applied to an electrode on the region of the cover glass region functionalized with antibody, and the resulting changes of drain current of the HEMT were employed to determine the presence of Zika virus antigen concentration ranging from 0.1 to 100 ng/ml. The dynamic and static drain current changes as a function of Zika virus concentration were modeled with a spring-like elastic relaxation model and the Langmuir extension model, respectively. Excellent fits to the data were found with relaxation time constants of antibody and antigen molecules in the range of 11 μs and 0.66–24.4 μs, respectively, for the concentration range investigated. The ratio of antibody bound with antigen to the total available antibody on the functionalized contact window was in the range of 0.013–0.84 for the Zika antigen concentration range of 0.1–100 ng/ml. Since the HEMT is not exposed to the bio-solution, it can be used repeatedly. The functionalized glass is the only disposable part in the detection system, showing the potential of this approach for hand-held, low cost sensor packages for point-of-care applications. [ABSTRACT FROM AUTHOR]

The use of ITO interlayers between Ga2O3 and Ti/Au metallization is shown to produce Ohmic contacts after annealing in the range of 500-600 °C. Without the ITO, similar anneals do not lead to linear current-voltage characteristics. Transmission line measurements were used to extract the specific contact resistance of the Au/Ti/ITO/Ga2O3 stacks as a function of annealing temperature. Sheet, specific contact, and transfer resistances all decreased sharply from as-deposited values with annealing. The minimum transfer resistance and specific contact resistance of 0.60 Ω mm and 6.3 x 10-5 Ω cm² were achieved after 600 °C annealing, respectively. The conduction band offset between ITO and Ga2O3 is 0.32 eV and is consistent with the improved electron transport across the heterointerface. [ABSTRACT FROM AUTHOR]

AZO interlayers between n-Ga2O3 and Ti/Au metallization significantly enhance Ohmic contact formation after annealing at 300°C.Without the presence of the AZO, similar anneals produce only rectifying current-voltage characteristics. Transmission Line Measurements of the Au/Ti/AZO/Ga2O3 stacks showed the specific contact resistance and transfer resistance decreased sharply from as-deposited values with annealing. The minimum contact resistance and specific contact resistance of 0.42 -mm and 2.82×10-5 Ω-cm2 were achieved after a relatively low temperature 400°C annealing. The conduction band offset between AZO and Ga2O3 is 0.79 eV and provides a favorable pathway for improved electron transport across this interface. [ABSTRACT FROM AUTHOR]

The effects of proton irradiation on the dc performance of InAlN/GaN metal-oxide-semiconductor high electron mobility transistors (MOSHEMTs) with Al2O3 as the gate oxide were investigated. The InAlN/GaN MOSHEMTs were irradiated with doses ranging from 1 x 1013 to 1 x 1015cm-2 at a fixed energy of 5MeV. There was minimal damage induced in the two dimensional electron gas at the lowest irradiation dose with no measurable increase in sheet resistance, whereas a 9.7% increase of the sheet resistance was observed at the highest irradiation dose. By sharp contrast, all irradiation doses created more severe degradation in the Ohmic metal contacts, with increases of specific contact resistance from 54% to 114% over the range of doses investigated. These resulted in source-drain current- voltage decreases ranging from 96 to 242mA/mm over this dose range. The trap density determined from temperature dependent drain current subthreshold swing measurements increased from 1.6 x 1013cm-2V-1 for the reference MOSHEMTs to 6.7 x 1013cm-2V-1 for devices irradiated with the highest dose. The carrier removal rate was 1287 ± 64 cm-1, higher than the authors previously observed in AlGaN/GaN MOSHEMTs for the same proton energy and consistent with the lower average bond energy of the InAlN. [ABSTRACT FROM AUTHOR]

AlGaN/GaN high electron mobility transistors (HEMTs) have been grown on sapphire substrates, using ZrTi buffer layers to provide in-plane lattice-matching to hexagonal GaN. X-ray diffraction (XRD) as well as cross-section transmission electron microscopy (TEM) were used to assess the quality of the HEMT structure. The XRD 2h scans showed full-width-at-half-maximum values of 0.16, 0.07°, and 0.08° for ZrTi alloy, GaN buffer layer, and the entire HEMT structure, respectively. TEM studies of the GaN buffer layer and the AlN/ZrTi/AlN stack showed the importance of growing thin AlN buffer layers on the ZrTi layer prior to growth of the GaN buffer layer. The density of threading dislocations in the GaN channel layer of the HEMT structure was estimated to be in the 108 cm2 range. The HEMT device exhibited a saturation drain current density of 820mA/mm, and the channel of the fabricated HEMTs could be well modulated. A cutoff frequency (fT) of 8.9GHz and a maximum frequency of oscillation (fmax) of 17.3GHz were achieved for HEMTs with gate dimensions of 1 x 200 μm. [ABSTRACT FROM AUTHOR]

The effects of proton irradiation energy on dc characteristics of AlGaN/GaN metal-oxide semiconductor high electron mobility transistors (MOSHEMTs) using Al2O3 as the gate dielectric were studied. Al2O3/AlGaN/GaN MOSHEMTs were irradiated with a fixed proton dose of 5?×?1015?cm-2 at different energies of 5, 10, or 15MeV. More degradation of the device dc characteristics was observed for lower irradiation energy due to the larger amount of nonionizing energy loss in the active region of the MOSHEMTs under these conditions. The reductions in saturation current were 95.3%, 68.3%, and 59.8% and reductions in maximum transconductance were 88%, 54.4%, and 40.7% after 5, 10, and 15MeV proton irradiation, respectively. Both forward and reverse gate leakage current were reduced more than one order of magnitude after irradiation. The carrier removal rates for the irradiation energies employed in this study were in the range of 127-289cm-1. These are similar to the values reported for conventional metal-gate high-electron mobility transistors under the same conditions and show that the gate dielectric does not affect the response to proton irradiation for these energies. [ABSTRACT FROM AUTHOR]

The effect of a back gate on the dc performance of AlGaN/GaN high electron mobility transistor was investigated. The back gate was fabricated directly under the device active area by etching off the Si substrate, AlN nucleation layer, and graded AlGaN transition layer and depositing Ni/Au-based gate metal on the exposed GaN buffer layer. The reverse bias gate leakage current decreased from 3.9 x 10-5 to 1.2 x 10-5 mA/mm by applying -10V at the back gate. Because of the suppression of gate leakage current by the back gate, the drain on/off ratio improved from 1.8 x 105 to 1.2 x 106 and the subthreshold swing from 204 to 137 mV/dec. Moreover, the drain breakdown voltage could be improved by 40% when the back gate was biased at -25 V. [ABSTRACT FROM AUTHOR]

The use of two-dimensional (2D) materials in optoelectronics has attracted much attention due to their fascinating optical and electrical properties. However, the low optical absorption of 2D materials arising from their atomic thickness limits the maximum attainable external quantum efficiency. For example, in the visible and near-infrared regimes monolayer MoS2 and graphene absorb only ~10% and 2.3% of incoming light, respectively. Here, we experimentally demonstrate the use of Fano-resonant photonic crystals to significantly boost absorption in atomically thin materials. Using graphene as a test bed, we demonstrate that absorption in the monolayer thick material can be enhanced to 77% within the telecommunications band, the highest value reported to date. We also show that the absorption in the Fano-resonant structure is non-local, with light propagating up to 16 lm within the structure. This property is particularly beneficial in harvesting light from large areas in field-effect-transistor based graphene photodetectors in which separation of photo-generated carriers only occurs ~0.2 *#956;m adjacent to the graphene/electrode interface. [ABSTRACT FROM AUTHOR]

The degradation mechanism of Ti/Al/Ni/Au-based Ohmic metallization on AlGaN/GaN high electron mobility transistors upon exposure to buffer oxide etchant (BOE) was investigated. The major effect of BOE on the Ohmic metal was an increase of sheet resistance from 2.89 to 3.69 Ω/  after 3min BOE treatment. The alloyed Ohmic metallization consisted 3–5 µm Ni-Al alloy islands surrounded by Au-Al alloy-rings. The morphology of both the islands and ring areas became flatter after BOE etching. Energy dispersive x-ray analysis and Auger electron microscopy were used to analyze the compositions and metal distributions in the metal alloys prior to and after BOE exposure. [ABSTRACT FROM AUTHOR]

The effects of postprocess annealing on the gate leakage current and breakdown voltage characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) was investigated. The fabricated AlGaN/GaN HEMTs were postannealed at 250, 300, 350, 400, or 450°C under a nitrogen (N2) atmosphere by using rapid thermal annealing, and both direct current (dc) and pulsed measurements were performed to characterize the changes in device performance. The reverse gate leakage current (IG) at VG = -10 V was reduced by one order of magnitude and the off-state drain breakdown voltage (Voff) increased by over three-fold after postprocess annealing at 450 °C. The reverse gate leakage current was found to be independent of gate-to-drain potential after annealing. The gate pulse measurements revealed the activation of deep traps during the postannealing at elevated temperatures. [ABSTRACT FROM AUTHOR]

AlGaN/GaN based metal-insulator-semiconductor high-electron-mobility transistors (HEMTs) using a plasma enhanced atomic layer deposited 10 nm AlN as the gate insulator and passivation layer were demonstrated. A refractive index of 1.92 for the deposited AlN was measured using an ellipsometer, which was slightly lower than that of bulk AlN. The deviation of the refractive index from the ideal value was caused by AlN surface oxidation, and this was confirmed by X-ray photoelectron spectroscopy and Auger depth profiling analyses. The HEMT drain current was modulated with gate voltages ranging from -3 to +4 V. The HEMT exhibited an on-off ratio of 3.3 × 108 due to the low gate leakage current and a maximum saturation drain current of 600 mA/mm. Beside reducing the gate leakage current, the effectiveness of the HEMT passivation was confirmed by gate pulse measurements, which showed only a 7% decrease of the drain current. [ABSTRACT FROM AUTHOR]

AlGaN/GaN high electron mobility transistors (HEMTs) with three different types of buffer layers, including a GaN/AlGaN composite layer, or 1 or 2 µm GaN thick layers, were fabricated and their reliability compared. The HEMTs with the thick GaN buffer layer showed the lowest critical voltage (Vcri) during off-state drain step-stress, but this was increased by around 50% and 100% for devices with the composite AlGaN/GaN buffer layers or thinner GaN buffers, respectively. The Voff-state for HEMTs with thin GaN and composite buffers were ~100 V, however, this degraded to 50-60 V for devices with thick GaN buffers due to the difference in peak electric field near the gate edge. A similar trend was observed in the isolation breakdown voltage measurements, with the highest Viso achieved based on thin GaN or composite buffer designs (600-700 V), while a much smaller Viso of ~200 V was measured on HEMTs with the thick GaN buffer layers. These results demonstrate the strong influence of buffer structure and defect density on AlGaN/GaN HEMT performance and reliability. [ABSTRACT FROM AUTHOR]

Hydrothermally grown SnO2 was integrated with AlGaN/GaN high electron mobility transistor (HEMT) sensor as the gate electrode for oxygen detection. The crystalline of the SnO2 was improved after annealing at 400 °C. The grain growth kinetics of the SnO2 nanomaterials, together with the O2 gas sensing properties and sensing mechanism of the SnO2 gated HEMT sensors were investigated. Detection of 1% oxygen in nitrogen at 100 °C was possible. A low operation temperature and low power consumption oxygen sensor can be achieved by combining the SnO2 films with the AlGaN/GaN HEMT structure. [ABSTRACT FROM AUTHOR]

The effects of proton irradiation energy on dc and rf characteristics of InAlN/GaN high electron mobility transistors (HEMTs) were investigated. A fixed proton dose of 5 x 1015 cm-2 with 5, 10, and 15 MeV irradiation energies was used in this study. For the dc characteristics, degradation was observed for sheet resistance, transfer resistance, contact resistivity, saturation drain current, maximum transconductance, reverse-bias gate leakage current, and sub-threshold drain leakage current for all the irradiated HEMTs; however, the degree of the degradation was decreased as the irradiation energy increased. Similar trends were obtained for the rf performance of the devices, with ~10% degradation of the unity gain cut-off frequency (fT) and maximum oscillation frequency (fmax) for the HEMTs irradiated with 15 MeV protons but 30% for 5 MeV proton1 irradiation. The carrier removal rate was in the range 0.66-1.24 cm-1 over the range of proton energies investigated. [ABSTRACT FROM AUTHOR]

Graphene layers on SiO2/Si substrates were exposed to chemicals or gases commonly used in semiconductor fabrication processes, including solvents (isopropanol, acetone), acids, bases (ammonium hydroxide), UV ozone, H2O, and O2 plasmas. The recovery of the initial graphene properties after these exposures was monitored by measuring both the layer resistance and Raman 2D peak position as a function of time in air or vacuum. Solvents and UV ozone were found to have the least affect, while oxygen plasma exposure caused an increase of resistance of more than 3 orders of magnitude. Recovery is accelerated under vacuum but changes can persist for more than 5 h. Careful design of fabrication schemes involving graphene is necessary to minimize these interactions with common processing chemicals. [ABSTRACT FROM AUTHOR]

The authors report successful patterning of transparent, conductive single-walled carbon nanotube films down to 100 nm lateral dimensions by photolithography or e-beam lithography and subsequent O2 plasma etching using an inductively coupled plasma reactive ion etching (ICP-RIE) system. They systematically study the effect of ICP-RIE etch parameters, such as substrate bias power, chamber pressure, and substrate cooling, on the nanotube film etch rate and etch selectivity. They also characterize the effect of the linewidth etched on the nanotube film etch rate for widths ranging from 50 μm down to 100 nm. Furthermore, by fabricating standard four point probe structures using the patterning capability developed, the authors investigate the effect of different resist processes on the resistivity of patterned single-walled carbon nanotube films and the effect of ICP reactive ion etching on the resistivity of partially etched nanotube films. In addition, they demonstrate that using an ICP-RIE system provides significant advantages, such as faster etch rates and better etch selectivity, over conventional parallel plate RIE plasma systems, making it possible to pattern lateral features as small as 100 nm in nanotube films. The simple and efficient “top-down” patterning capability developed in this article could open up many opportunities for integrating single-walled nanotube films into a wide range of electronic and optoelectronic devices. [ABSTRACT FROM AUTHOR]

Optimized UV ozone cleaning of graphene layers on SiO2/Si substrates is shown to improve contact resistance of e-beam evaporated Ti/Au contacts by three orders of magnitude (3 x 10-6 Ω-cm²) compared to untreated surfaces (4 x 10-3 Ω-cm²). Subsequent annealing at 300 °C lowers the minimum value achieved to 7 x 10-7 Ω-cm². Ozone exposure beyond an optimum time (6min in these experiments) led to a sharp increase in sheet resistance of the graphene, producing degraded contact resistance. The UV ozone treatment is a simple and effective method for producing high quality contacts to graphene. [ABSTRACT FROM AUTHOR]

The effects of high energy proton irradiation dose on dc performance as well as critical voltage of the drain-voltage step-stress of AlGaN/GaN high electron mobility transistors (HEMTs) were investigated to evaluate the feasibility of AlGaN/GaN HEMTs for space applications, which need to stand a variety of irradiations. The HEMTs were irradiated with protons at a fixed energy of 5 MeV and doses ranging from 109 to 2 × 1014 cm-2. For the dc characteristics, there was only minimal degradation of saturation drain current (IDSS), transconductance (gm), electron mobility, and sheet carrier concentration at doses below 2 × 1013 cm-2, while the reduction of these parameters were 15%, 9%, 41% and 16.6%, respectively, at a dose of 2 × 1014 cm-2. At this same dose condition, increases of 37% in drain breakdown voltage (VBR) and of 45% in critical voltage (Vcri) were observed. The improvements of drain breakdown voltage and critical voltage were attributed to the modification of the depletion region due to the introduction of a higher density of defects after irradiation at a higher dose. [ABSTRACT FROM AUTHOR]

The effects of proton irradiation energy on dc, small signal, and large signal rf characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) were investigated. AlGaN/GaN HEMTs were irradiated with protons at fixed fluence of 5 × 1015/cm2 and energies of 5, 10, and 15 MeV. Both dc and rf characteristics revealed more degradation at lower irradiation energy, with reductions of maximum transconductance of 11%, 22%, and 38%, and decreases in drain saturation current of 10%, 24%, and 46% for HEMTs exposed to 15, 10, and 5 MeV protons, respectively. The increase in device degradation with decreasing proton energy is due to the increase in linear energy transfer and corresponding increase in nonionizing energy loss with decreasing proton energy in the active region of the HEMTs. After irradiation, both subthreshold drain leakage current and reverse gate current decreased more than 1 order of magnitude for all samples. The carrier removal rate was in the range 121-336 cm-1 over the range of proton energies employed in this study. [ABSTRACT FROM AUTHOR]