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The ability to tailor the potential landscape of two-dimensional (2D) transition-metal dichalcogenides could have significant impact in many functional device applications. Here we demonstrate that...

The defect ∼0.8 eV below the conduction band edge of β-Ga2O3 wide bandgap semiconductor is investigated using the matched Arrhenius-equation projection technique that offers substantial improvement over the conventional deep level transient spectroscopy technique. An experimental technique is developed to extract activation energy Ea and attempt-to-escape frequency ν0 of defects bypassing both the rate-window treatment and the Arrhenius plot. Only raw capacitance transients in the time domain are needed with this technique. The capacitance transients are projected between the temperature and time domains as well as to Ea and ν0 domains. Extraction of Ea and ν0 is accomplished by matching the projected and experimental capacitance transients to each other.

AlN/GaN digital alloys (i.e., short period superlattices) have the advantage of being an ultra-wide bandgap due to the quantum confinement effect, preservation of high mobility (no strong alloy scattering), and higher thermal conductivity than AlGaN random alloys. Herein, we investigate the influence of AlN/GaN thickness combinations on digital alloy (DA) thermal conductivity. Using time-domain thermoreflectance and steady-state thermoreflectance, we measure the anisotropic thermal conductivity of AlN/GaN DA’s, with thickness combinations: (2 nm/1 nm), (4 nm/1 nm), (6 nm/1 nm), (11 nm/1 nm). DA's with thick AlN layers exhibit highly anisotropic thermal conductivity (i.e., 4x difference between cross-plane and in-plane). Using finite-element modeling, we thermally simulate AlGaN and AlN/GaN DA devices. Improved lateral heat spreading in DA devices results in lower hot-spot temperatures compared to AlGaN devices. As the first thermal measurements of AlN/GaN DA’s, this study provides guidance for future device design.

Using molecular beam epitaxy, we prepared seven p-type AlGaN samples of ~25% in Al content, including six samples with Mg-doped/un-doped AlGaN alternating-layer structures of different layer-thickness combinations, for comparing their p-type performances. Lower sheet resistance and higher effective hole mobility are obtained in a layer-structured sample, when compared with the reference sample of uniform Mg doping. The improved p-type performance in a layer-structured sample is attributed to the diffusion of holes generated in an Mg-doped layer into the neighboring un-doped layers, in which hole mobility is significantly higher because of weak ionized impurity scattering. Among the layer-structured samples, that of 6/4 nm in Mg-doped/un-doped thickness results in the lowest sheet resistance (the highest effective hole mobility), which is 4.83 times lower (4.57 times higher) when compared with the sample of uniform doping. The effects of the Mg-doped/un-doped layer structure on p-type performance in AlGaN and GaN are compared.

In this work, we investigated the edge doping effect to graphene plasmon resonances in graphene nanoribbons (GNRs), which is known to affect the electronic doping of GNRs but has not been systematically studied. We found the Fermi levels, which reflect that the sheet charge carrier densities, extracted from the graphene plasmon resonance frequencies, vary across GNRs of different widths on the same sample. Using Raman spectroscopy, we confirmed that the variation of the sheet charge carrier density is caused by edge doping, which has a stronger effect on narrower GNRs. To further understand the edge doping effect, electron beam irradiation (EBI) is applied to modify the charge state of the edge. Using EBI, we successfully demonstrated the tuning of the graphene plasmon resonances due to the change of the edge doping states. These findings demonstrated the importance of the edge doping effect in determination of the surface plasmon frequency in GNRs.

We report the measurement of majority carrier concentration, depletion width, mobility, and resistivity in a thin-film based Cu2ZnSnSe4 (CZTSe) photovoltaic device. The carrier transport properties were measured using coordinated admittance spectroscopy and capacitance-voltage technique. The bias dependence of the modified dielectric relaxation in the absorber of the CZTSe solar cell was investigated to extract the mobility and resistivity. Hall measurement was also performed at room temperature for the verification of carrier concentration, resistivity, and mobility. The temperature dependent resistivity and mobility exhibit thermally activated behaviors characterized by a thermal activation energy ≈ 60 meV. The positive temperature dependence of the mobility indicates a carrier-transport impeding effect caused by the band-edge fluctuation in poly-crystalline CZTSe, whose magnitude is measurable by the aforementioned activation energy.

In this study, gel-based electrolytes were produced by mixing a liquid-based electrolyte with poly(methyl methacrylate) (PMMA), which served as a gelator to enhance the stability of the electrolytes and dye-sensitized solar cells (DSSCs). Furthermore, bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) and camphorsulfonic acid (C10H16O4S) were separately added to the gel-based electrolytes. The results indicate that adding LiTFSI and C10H16O4S to the gel-based electrolytes effectively enhanced the device efficiency. In particular, the device with the 1:1 + 0.1 M LiTFSI gel-based electrolyte exhibited the optimal efficiency (7.33%), which was higher than that observed in the device with the liquid-based electrolyte. The results also proved that the DSSC devices with the gel-based electrolytes exhibited a longer stability. Therefore, using optimized gel-based electrolytes can enhance the power conversion efficiency and stability of DSSC devices.

We have studied the surface structure of single-crystal, wide-gap semiconductor $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}(010)$ using x-ray photoelectron diffraction (XPD), low-energy electron diffraction (LEED), and x-ray photoelectron spectroscopy (XPS). The XPS measurements show typical spectra for stoichiometric ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}(010)$. Annealing in vacuum produced a sharp ($1\ifmmode\times\else\texttimes\fi{}1$) LEED pattern, characteristic of the monoclinic structure. The XPD angular anisotropies were collected for the Ga $2{p}_{3/2}$ and O $1s$ core levels. Surface interlayer relaxation up to 8% of the bulk interplanar distance and 0.11--0.14 $\AA{}$ rumpling are observed at the $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}(010)$ surface. At the surface, the oxygen atoms shift toward the vacuum with respect to the gallium atoms. The rumpling decreases to zero and and the interplanar distance reaches the bulk value of 1.52 $\AA{}$ by the sixth atomic layer. The surface structure agrees with that predicted by first-principles density functional theory calculations which, in addition, suggest a significant band gap narrowing of $\ensuremath{\approx}1$ eV in the surface layer, due to surface states spatially localized on surface oxygen atoms of ${\mathrm{O}}_{\mathrm{II}}$ type.

In this work, the structural and electrical properties of metalorganic chemical vapor deposited Si-doped β-(AlxGa1−x)2O3 thin films grown on (010) β-Ga2O3 substrates are investigated as a function of Al composition. The room temperature Hall mobility of 101 cm2/V s and low temperature peak mobility (T = 65 K) of 1157 cm2/V s at carrier concentrations of 6.56 × 1017 and 2.30 × 1017 cm−3 are measured from 6% Al composition samples, respectively. The quantitative secondary ion mass spectroscopy (SIMS) characterization reveals a strong dependence of Si and other unintentional impurities, such as C, H, and Cl concentrations in β-(AlxGa1−x)2O3 thin films, with different Al compositions. Higher Al compositions in β-(AlxGa1−x)2O3 result in lower net carrier concentrations due to the reduction of Si incorporation efficiency and the increase of C and H impurity levels that act as compensating acceptors in β-(AlxGa1−x)2O3 films. Lowering the growth chamber pressure reduces Si concentrations in β-(AlxGa1−x)2O3 films due to the increase of Al compositions as evidenced by comprehensive SIMS and Hall characterizations. Due to the increase of lattice mismatch between the epifilm and substrate, higher Al compositions lead to cracking in β-(AlxGa1−x)2O3 films grown on β-Ga2O3 substrates. The (100) cleavage plane is identified as a major cracking plane limiting the growth of high-quality Si-doped (010) β-(AlxGa1−x)2O3 films beyond the critical thicknesses, which leads to highly anisotropic and inhomogeneous behaviors in terms of conductivity.

Nanostructure morphologies of transition metal dichalcogenides (TMDs) are gaining much interest owing to their catalytic, sensing, and energy storage capabilities. Here, we report the synthesis of highly dense MoO

Through magneto-transport measurements and analysis of the observed Shubnikov de Haas oscillations in (010) (AlxGa1-x)2O3/Ga2O3 heterostructures, spin-splitting of the Landau levels in the (010) Ga2O3 two-dimensional electron gas (2DEG) has been studied. Analysis indicates that the spin-splitting results from the Zeeman effect. By fitting the both the first and second harmonic of the oscillations as a function of magnetic field, we determine the magnitude of the Zeeman splitting to be 0.4$\hbar\omega_c$, with a corresponding effective g-factor of 2.7, for magnetic field perpendicular to the 2DEG., Comment: 7 pages, 5 figures

Studies of Si, Ge donors and Fe, Mg as acceptors in β-Ga2O3 through temperature-dependent van der Pauw and Hall effect measurements of samples grown by a variety of methods are presented in this chapter. Si and Ge are identified as shallow donors with donor energy of 30 meV instead of DX states. Fe is a deep acceptor with its energy level 860 meV below the conduction band edge. Mg-doped samples present an activation energy of 1.1 eV, but the type could not be resolved. Unintentional donors and acceptors are also discussed including intrinsic defects (i.e., vacancies) and extrinsic impurities. Last, an unintentional donor with energy of 110 meV is presented and the impacts of its incomplete ionization to power devices are discussed.

This paper introduces a growth method---suboxide molecular-beam epitaxy (S-MBE)---which enables the growth of Ga2O3 and related materials at growth rates exceeding 1 micrometer per hours with excellent crystallinity in an adsorptioncontrolled regime. Using a Ga + Ga2O3 mixture with an oxygen mole fraction of x(O) = 0.4 as an MBE source, we overcome kinetic limits that had previously hampered the adsorption-controlled growth of Ga2O3 by MBE. We present growth rates up to 1.6 micrometer per hour for Ga2O3--Al2O3 heterostructures with unprecedented crystalline quality and also at unparalleled low growth temperature for this level of perfection. We combine thermodynamic knowledge of how to create molecular-beams of targeted suboxides with a kinetic model developed for the S-MBE of III-VI compounds to identify appropriate growth conditions. Using S-MBE we demonstrate the growth of phase-pure, smooth, and high-purity homoepitaxial Ga2O3 films that are thicker than 4 micrometer. With the high growth rate of S-MBE we anticipate a significant improvement to vertical Ga2O3-based devices. We describe and demonstrate how this growth method can be applied to a wide-range of oxides. S-MBE rivals leading synthesis methods currently used for the production of Ga2O3-based devices., Comment: 15 pages, 12 figures

In aiming to achieve red-shifted single-photon emitters that result from localized excitons in monolayer WSe2, we report in this work a theoretical investigation on the optical properties of nanostructures with vacancies and rotational defects. We find that for pristine monolayer WSe2, the complex excitonic manifold, namely, the energies of the bright and dark excitons and the exciton binding energy, agree well with the experimental data when using the GW (Green's function approximation with a screened Coulomb interaction W)-Bethe Salpeter Equation (GW-BSE) method, including spin-orbit coupling. The predicted second and third lowest dark excitons are close in energy and appear below the second bright exciton. Upon introduction of single or double Se vacancies, or a single W vacancy within monolayer WSe2, accurate computational results demonstrate emergence of deeper defect excitons in comparison to shallower values observed for edges, which are consistent with measured emissions upon Ar+ plasma treatment of WSe2 for longer periods of time. Furthermore, using corrected RPA (random phase approximation) calculations, we find that defect excitons red-shift significantly for large rotational defects that pattern the monolayer. Finally, interestingly, first-order Raman intensity calculations demonstrate that a comparison between pristine and defective monolayer WSe2 with a single vacancy can provide a fingerprint for defect characterization. Overall, our results will encourage experimental defect engineering to enable the development of red-shifted single-photon emitters, such as by inducing extended patterning of monolayer WSe2.

An electrically inactive layer (EIL) near the edge of micro-patterned graphene lines was investigated using both scanning microwave microscopy (SMM) and Raman spectroscopy. Monolayer graphene sheets grown by chemical vapor deposition (CVD) were transferred from the copper foil growth substrate to different substrates, and structured into periodic lines using lithography and oxygen plasma etching. SMM images were recorded at half- and quarter- wavelength resonances showing distinguished imaging contrast of the EIL from the center of the graphene lines. The width of the EIL is determined around 0.17 μm. Correlation of the SMM imaging contrast indicates that the local electrical conductivity of the EIL is significantly reduced, which agrees with Raman spectroscopic images.

β-Ga2O3 is a promising ultra-wide bandgap semiconductor whose properties can be further enhanced by alloying with Al. Here, using atomic-resolution scanning transmission electron microscopy, we find the thermodynamically unstable γ-phase is a ubiquitous structural defect in both β-(AlxGa1−x)2O3 films and doped β-Ga2O3 films grown by molecular beam epitaxy. For undoped β-(AlxGa1−x)2O3 films, we observe γ-phase inclusions between nucleating islands of the β-phase at lower growth temperatures (∼500–600 °C). In doped β-Ga2O3, a thin layer of the γ-phase is observed on the surfaces of films grown with a wide range of n-type dopants and dopant concentrations. The thickness of the γ-phase layer was most strongly correlated with the growth temperature, peaking at about 600 °C. Ga interstitials are observed in the β-phase, especially near the interface with the γ-phase. By imaging the same region of the surface of a Sn-doped β-(AlxGa1−x)2O3 after ex situ heating up to 400 °C, a γ-phase region is observed to grow above the initial surface, accompanied by a decrease in Ga interstitials in the β-phase. This suggests that the diffusion of Ga interstitials toward the surface is likely the mechanism for growth of the surface γ-phase and more generally that the more-open γ-phase may offer diffusion pathways to be a kinetically favored and early forming phase in the growth of Ga2O3. However, more modeling and simulation of the γ-phase and the interstitials are needed to understand the energetics and kinetics, the impact on electronic properties, and how to control them.

Understanding the origin of unintentional doping in Ga2O3 is key to increasing breakdown voltages of Ga2O3 based power devices. Therefore, transport and capacitance spectroscopy studies have been performed to better understand the origin of unintentional doping in Ga2O3. Previously unobserved unintentional donors in commercially available $$(\bar{2}01)$$ ( 2 ¯ 01 ) Ga2O3 substrates have been electrically characterized via temperature dependent Hall effect measurements up to 1000 K and found to have a donor energy of 110 meV. The existence of the unintentional donor is confirmed by temperature dependent admittance spectroscopy, with an activation energy of 131 meV determined via that technique, in agreement with Hall effect measurements. With the concentration of this donor determined to be in the mid to high 1016 cm−3 range, elimination of this donor from the drift layer of Ga2O3 power electronics devices will be key to pushing the limits of device performance. Indeed, analytical assessment of the specific on-resistance (Ronsp) and breakdown voltage of Schottky diodes containing the 110 meV donor indicates that incomplete ionization increases Ronsp and decreases breakdown voltage as compared to Ga2O3 Schottky diodes containing only the shallow donor. The reduced performance due to incomplete ionization occurs in addition to the usual tradeoff between Ronsp and breakdown voltage.

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.

We report on the growth of β-Ga2O3 thin films using trimethylgallium (TMGa) as a source for gallium and pure O2 for oxidation. The growth rate of the films was found to linearly increase with the increase in the molar flow rate of TMGa and reach as high as ∼6 μm/h at a flow rate of 580 μmol/min. High purity, lightly Si-doped homoepitaxial β-Ga2O3 films with a good surface morphology, a record low temperature electron mobility exceeding 23 000 cm2/V s at 32 K, and an acceptor concentration of 2 × 1013 cm−3 were realized, showing an excellent purity film. Films with room temperature (RT) electron mobilities ranging from 71 cm2/V s to 138 cm2/V s with the corresponding free carrier densities between ∼1.1 × 1019 cm−3 and ∼1.5 × 1016 were demonstrated. For layers with the doping concentration in the range of high-1017 and low-1018 cm−3, the RT electron mobility values were consistently more than 100 cm2/V s, suggesting that TMGa is suitable to grow channel layers for lateral devices, such as field effect transistors. The results demonstrate excellent purity of the films produced and confirm the suitability of the TMGa precursor for the growth of device quality β-Ga2O3 films at a fast growth rate, meeting the demands for commercializing Ga2O3-based high voltage power devices by metalorganic chemical vapor deposition.

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.

Nanostructure morphologies of transition metal dichalcogenides (TMDs) are gaining much interest owing to their catalytic, sensing, and energy storage capabilities. Here, we report the synthesis of highly dense MoO2/MoS2 core–shell nanoparticles, a new form of TMD nanostructure, via chemical vapor deposition using new growth geometry where a thin film of MoO3 was used as a source substrate for Mo as opposed to using MoO3 powder used in conventional studies. To grow the MoO2/MoS2 core–shell nanoparticles, we precisely control the carrier gas flow rate and sulfur vapor introduction time with respect to the melting of a MoO3 thin film used for Mo precursor. Scanning electron microscope image shows dense coverage of nanoparticles of 50–120 nm in size. The transmission electron microscopy image shows that the nanoparticles consist of crystalline MoO2 core covered with a few layer MoS2 shell. Raman and energy dispersive spectroscopy characterizations further confirm the chemical composition of the nanoparticle containing MoO2 and MoS2. We discuss the growth conditions under which the nanoparticles grow and elucidate its growth mechanism. We also discuss how a small but controllable changes in growth condition could lead to other highly dense growth of vertical/lateral MoO2/MoS2 plates in both source and growth substrates due to the unique growth geometry used in this study.

We investigate the minority carrier lifetime of Be-doped InAs/InAsSb type-II superlattices as a function of doping density and temperature using time-resolved photoluminescence (TRPL) to determine if switching the superlattice type from the typical n-type residual carrier concentration to p-type may improve device performance by improving the lifetime–mobility product. The introduction of the Be dopant to the superlattice reduces the carrier lifetime, first by a factor of ∼3 for doping densities near or below the n-type residual carrier concentration, then by an order of magnitude for samples doped well above the residual carrier concentration. Further, the higher-doped p-type samples demonstrate two distinct TRPL decay regimes and two peaks in the PL spectra, suggesting the formation of an additional acceptor-related recombination pathway leading to the observed shorter carrier lifetime.

The authors investigate unintentional Si doping in β-Ga2O3 thin films grown via plasma-assisted molecular beam epitaxy. By identifying the Si dopant cell and the radio frequency oxygen plasma source as the two major sources of unintentional Si present in the growth chamber, adjustments to the parameters controlling these sources allow for the reduction of unintentional Si doping in β-Ga2O3 films by over two orders of magnitude. Decreasing the temperature of the Si dopant cell when idle reduces the unintentional doping from 2 × 1018 to 3 × 1017 cm−3. Furthermore, decreasing the oxygen plasma power from 300 to 250 and to 200 W results in the reduction of the unintentional Si doping from 3 × 1017 to 1 × 1016 cm−3. The reduction in unintentional Si doping concentration allows for low doping density in the drift region of β-Ga2O3 devices to design for high breakdown voltage via plasma-assisted molecular beam epitaxy.

We have studied the properties of Si, Ge shallow donors and Fe, Mg deep acceptors in $\beta$-Ga2O3 through temperature dependent van der Pauw and Hall effect measurements of samples grown by a variety of methods, including edge-defined film-fed (EFG), Czochralski (CZ), molecular beam epitaxy (MBE), and low pressure chemical vapor deposition (LPCVD). Through simultaneous, self-consistent fitting of the temperature dependent carrier density and mobility, we are able to accurately estimate the donor energy of Si and Ge to be 30 meV in $\beta$-Ga2O3. Additionally, we show that our measured Hall effect data are consistent with Si and Ge acting as typical shallow donors, rather than shallow DX centers. High temperature Hall effect measurement of Fe doped $\beta$-Ga2O3 indicates that the material remains weakly n-type even with the Fe doping, with an acceptor energy of 860 meV relative to the conduction band for the Fe deep acceptor. Van der Pauw measurements of Mg doped Ga2O3 indicate an activation energy of 1.1 eV, as determined from the temperature dependent conductivity., Comment: 19 pages, 6 figures

In aiming to achieve red-shifted single-photon emitters that result from localized excitons in monolayer WSe2, we report in this work a theoretical investigation on the optical properties of nanostructures with vacancies and rotational defects. We find that for pristine monolayer WSe2, the complex excitonic manifold, namely, the energies of the bright and dark excitons and the exciton binding energy, agree well with the experimental data when using the GW (Green's function approximation with a screened Coulomb interaction W)-Bethe Salpeter Equation (GW-BSE) method, including spin-orbit coupling. The predicted second and third lowest dark excitons are close in energy and appear below the second bright exciton. Upon introduction of single or double Se vacancies, or a single W vacancy within monolayer WSe2, accurate computational results demonstrate emergence of deeper defect excitons in comparison to shallower values observed for edges, which are consistent with measured emissions upon Ar+ plasma treatment of WSe2 for longer periods of time. Furthermore, using corrected RPA (random phase approximation) calculations, we find that defect excitons red-shift significantly for large rotational defects that pattern the monolayer. Finally, interestingly, first-order Raman intensity calculations demonstrate that a comparison between pristine and defective monolayer WSe2 with a single vacancy can provide a fingerprint for defect characterization. Overall, our results will encourage experimental defect engineering to enable the development of red-shifted single-photon emitters, such as by inducing extended patterning of monolayer WSe2.

Guanine and poly(methyl methacrylate) (PMMA) layers 60 nm thick were fabricated by physical vapor deposition and spin-coating, respectively, for potential application as bio-based field-effect transistors. A few layers of graphene on a variety of non-flexible and flexible supports were used as substrates for the guanine and PMMA layers. The interactions between these layers and graphene were studied and their effects on the electrical properties of the graphene were monitored over an extended period of time. Guanine had no noticeable effects on the transport properties of graphene. The presence of PMMA reduced the charge-carrier mobility of graphene by up to 42%. This combination has potential for use as a device in biosensors, environmental science, computer-processing, and graphene-based electronics.

Firing temperature and gas pressure effect of synthesizing Sr2Si5N8:Eu2+ were investigated. The emission intensity is positively correlated with the firing temperature under 0.1 and 0.5 MPa gas pressure. The Sr2Si5N8:Eu2+ with the highest emission intensity was found at 1700°C and 1980°C under 0.1 and 0.5 MPa gas pressure, respectively. Although the maximum emission intensity of Sr2Si5N8:Eu2+ obtained under 0.5 MPa gas pressure condition is higher than that under 0.1 MPa. The Sr2Si5N8:Eu2+ synthesized under 0.5 MPa gas pressure in the temperature range from 1600°C to 1800°C have lower emission intensities than that synthesized under 0.1 MPa indicating that the melting of Sr3N2 is an important step for the formation of Sr2Si5N8:Eu2+. Moreover, the Sr2Si5N8:Eu2+ undergoes phase transition into SrSi6N8:Eu2+ completely after elongating the heating duration to 6 h at 1980°C under 0.5 MPa gas pressure. The same feature was observed under 0.1 MPa gas pressure after firing 8 h at 1750°C. Different heating durations led to different degrees of phase transition.

As a transparent conducting oxide with a large bandgap of ∼4.9 eV and associated large estimated critical electric field (E c ) strength of 8 MV/cm, β-Ga 2 O 3 (BGO) has been touted for its tremendous potential as a power switch. Power switch metrics such as Baliga's figure of merit (BFOM) estimating dc conduction losses and Huang's material figure of merit (HMFOM) incorporating dynamic switching losses are functions of E C 3 and E C respectively [1, 2]. BFOM for BGO is expected to exceed that of GaN by 400% and HMFOM for BGO is expected to be comparable to GaN. It can also be shown that for a given power loss during switching, the switch frequency (f) varies as E C 2 suggesting the potential of BGO power conversion in the GHz regime [1, 2]. The manufacturability and cost value proposition of BGO based on large area native substrate availability as shown by Huang's chip area manufacturing FOM (HCAFOM) indicate a disruptive cost advantage over GaN (330%). Additionally, the Johnson figure of merit (JFOM) representing the power-frequency product for RF amplification for BGO is similar to that of GaN indicating potential for integration of power conversion and RF applications in the same platform. Finally, Huang's high temperature figure of merit (HTFOM) shows that BGO has the lowest metric for all semiconductors compared here due to low thermal conductivity and high field strength. However, all wide bandgap power semiconductors, including diamond, face significant thermal engineering challenges relative to Si because of the inherent high energy densities of materials with large E c values. A summary of unipolar FET figure-of-merit comparisons for power semiconductors is shown in Table 1.

We measure the ultrafast nonlinear dynamics of two-layer graphene. One instantaneous electronic and three non-instantaneous free carrier responses are observed. Measurements indicate a large saturable absorption effect due to Pauli blocking.

The authors use isothermal capacitance transient spectroscopy to detect three deep defects located at approximately 0.6, 0.8, and 1.0 eV below the conduction band of β-Ga2O3 materials. The electrical properties of these defects such as their capture cross sections and concentrations are characterized, and their potentially adverse effects on power electronic devices are discussed. Because the isothermal capacitance transient spectroscopy method is implementable by instruments intended for steady-state capacitance-voltage measurement, it is promising for use as an effective characterization and monitoring tool for deep defects in β-Ga2O3 and other wide bandgap semiconductors.

Electron paramagnetic resonance (EPR) and infrared absorption are used to detect Ir4+ ions in β-Ga2O3 crystals. Mg and Fe doped crystals are investigated, and concentrations of Ir4+ ions greater than 1 × 1018 cm−3 are observed. The source of the unintentional deep iridium donors is the crucible used to grow the crystal. In the Mg-doped crystals, the Ir4+ ions provide compensation for the singly ionized Mg acceptors and thus contribute to the difficulties in producing p-type behavior. The Ir4+ ions replace Ga3+ ions at the Ga(2) sites, with the six oxygen neighbors forming a distorted octahedron. A large spin-orbit coupling causes these Ir4+ ions to have a low-spin (5d5, S = 1/2) ground state. The EPR spectrum consists of one broad line with a significant angular dependence. Principal values of the g matrix are 2.662, 1.815, and 0.541 (with principal axes near the crystal a, b, and c directions, respectively). Ionizing radiation at 77 K decreases the Ir4+ EPR signal in Mg-doped crystals and increases the signal in Fe-doped crystals. In addition to the EPR spectrum, the Ir4+ ions have an infrared absorption band representing a d-d transition within the t2g orbitals. At room temperature, this band peaks near 5153 cm−1 (1.94 μm) and has a width of 17 cm−1. The band is highly polarized: its intensity is maximum when the electric field E is parallel to the b direction in the crystal and is nearly zero when E is along the c direction.Electron paramagnetic resonance (EPR) and infrared absorption are used to detect Ir4+ ions in β-Ga2O3 crystals. Mg and Fe doped crystals are investigated, and concentrations of Ir4+ ions greater than 1 × 1018 cm−3 are observed. The source of the unintentional deep iridium donors is the crucible used to grow the crystal. In the Mg-doped crystals, the Ir4+ ions provide compensation for the singly ionized Mg acceptors and thus contribute to the difficulties in producing p-type behavior. The Ir4+ ions replace Ga3+ ions at the Ga(2) sites, with the six oxygen neighbors forming a distorted octahedron. A large spin-orbit coupling causes these Ir4+ ions to have a low-spin (5d5, S = 1/2) ground state. The EPR spectrum consists of one broad line with a significant angular dependence. Principal values of the g matrix are 2.662, 1.815, and 0.541 (with principal axes near the crystal a, b, and c directions, respectively). Ionizing radiation at 77 K decreases the Ir4+ EPR signal in Mg-doped crystals and increases the s...

Niobium nitride thin films were deposited using reactive magnetron sputtering simultaneously on sapphire substrates with TiN, VN, and AlN buffer layers. Deposition temperature was varied from 400 to 840 °C. It was found that the crystal structure, surface roughness, and transition temperatures of the resulting NbN films depend strongly on both the growth temperature and the type of the buffer layer. The use of VN and TiN buffer layers for growing NbN at 400 °C improved transition temperatures compared to NbN grown at 840 °C on sapphire. While increasing the temperature improved the superconducting performance of films grown directly on sapphire, it caused hexagonal δ′-NbN and e-NbN phases to emerge on the buffered films. A highly oriented hexagonal e-NbN film was achieved by using a TiN buffer and an 840 °C deposition temperature. The ability to deposit high performance NbN at a lower temperature will improve and simplify the fabrication of advanced superconducting devices such as superconducting single photon detectors.Niobium nitride thin films were deposited using reactive magnetron sputtering simultaneously on sapphire substrates with TiN, VN, and AlN buffer layers. Deposition temperature was varied from 400 to 840 °C. It was found that the crystal structure, surface roughness, and transition temperatures of the resulting NbN films depend strongly on both the growth temperature and the type of the buffer layer. The use of VN and TiN buffer layers for growing NbN at 400 °C improved transition temperatures compared to NbN grown at 840 °C on sapphire. While increasing the temperature improved the superconducting performance of films grown directly on sapphire, it caused hexagonal δ′-NbN and e-NbN phases to emerge on the buffered films. A highly oriented hexagonal e-NbN film was achieved by using a TiN buffer and an 840 °C deposition temperature. The ability to deposit high performance NbN at a lower temperature will improve and simplify the fabrication of advanced superconducting devices such as superconducting single p...

Various nitrogen-based bridging ligands at the sites of a Cu(111) surface, including μ3-imido and μ2-azavinylidene species, have been identified by surface-sensitive spectroscopies and density functional theory (DFT) calculations. The isolated intermediates demonstrate how several metal atoms acting in concert can usher the conversion from a primary azide into a nitrile end-product. The collection of mechanistic details, the N–N2 bond scission, the sequential β-hydrogen eliminations, and the alterations of the NC unsaturation, illustrates rich information about structures and transformations of N-donor ligands.

We report the electrical transport characteristics of a series of molecular wires, fc-C≡C-C6H4-SAc (fc = ferrocenyl; Ac = acetyl) and AcS-C6H4-C≡C-(fc)n-C≡C-C6H4-SAc (n = 2, 3), consisting of multiple redox-active ferrocenyl centers. The self-assembled monolayers of these molecular wires on Au surfaces were comprehensively characterized by electrochemistry and conductive atomic force microscopy techniques. Characterization of the wires revealed that electron transport is made extremely efficient by the organometallic redox states. There is a strong electronic coupling between ferrocenyl moieties, and superior electron-transport ability exists through these semirigid molecular wires. Standard rate constants for the electron transfer between the electrode and the ferrocenyl moieties were measured for the monolayers by a potential-step chronoamperometry technique. The electron conduction through the molecular wires was estimated using the monolayers as a bridge from the Au(111) metal surface to the gold tip of a conductive atomic force microscope (CAFM). Using the CAFM, Coulomb blockade behavior arising from the capacitive charging of the multinuclear redox-active molecules was observed at room temperature. The conductance switching was mediated by the presence of various ferrocenyl redox states and each current step corresponded to a specific redox state.

Single defects in monolayer WSe2 have been shown to be a new class of single photon emitters and have potential applications in quantum technologies. Whereas previous work relied on optical excitation of single defects in isolated WSe2 monolayers, in this work we demonstrate electrically driven single defect light emission by using both vertical and lateral van der Waals heterostructure devices. In both device geometries, we use few layer graphene as the source and drain and hexagonal boron nitride as the dielectric spacer layers for engineered tunneling contacts. In addition, the lateral devices utilize a split back gate design to realize an electrostatically defined p-i-n junction. At low current densities and low temperatures (∼5 K), we observe narrow spectral lines in the electroluminescence (EL) whose properties are consistent with optically excited defect bound excitons. We show that the emission originates from spatially localized regions of the sample, and the EL spectrum from single defects has a doublet with the characteristic exchange splitting and linearly polarized selection rules. All are consistent with previously reported single photon-emitters in optical measurements. Our results pave the way for on-chip and electrically driven single photon sources in two-dimensional semiconductors for quantum technology applications.