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Hexagonal LuFeO3 exhibiting ferroelectricity and weak ferromagnetism is grown on metal-polar GaN by molecular-beam epitaxy. The oxide films exhibit smooth surface morphologies and are found to be single crystalline with an epitaxial relationship related by a 30° in-plane rotation relative to the GaN crystal structure. The LuFeO3 layers grown on GaN exhibit room-temperature ferroelectricity and low-temperature magnetic ordering. This epitaxial integration creates a heterostructure platform to explore and exploit the coupling of the ferroelectricity and magnetism of oxides with the strong spontaneous and piezoelectric polarization and the unique electronic and photonic properties of nitride semiconductors.

This report showcases a vertical tunnel field effect transistor (TFET) fabricated from a GaN/InGaN heterostructure and compares it to a gated vertical GaN p-n diode. By including a thin InGaN layer, the interband tunneling in the TFET is increased compared to the gated homojunction diode. This leads to an increased drain current of 57 μA/μm and a reduced subthreshold swing of 102 mV/dec, from 240 mV/dec. However, trap assisted tunneling prevents devices from realizing subthreshold slopes below the Boltzmann limit of 60 mV/dec. Nevertheless, this work shows the capability of tunnel field effect transistors to be realized in GaN by taking advantage of the spontaneous and piezoelectric polarization in the III-N material system.

We report a fully transparent thin-film transistor utilizing a La-doped BaSnO3 channel layer that provides a drain current of 0.468 mA/μm and an on-off ratio of 1.5 × 108. The La-doped BaSnO3 channel is grown on a 100–150 nm thick unintentionally doped BaSnO3 buffer layer on a (001) MgO substrate by molecular-beam epitaxy. Unpatterned channel layers show mobilities of 127–184 cm2 V−1 s−1 at carrier concentrations in the low to mid 1019 cm−3 range. The BaSnO3 is patterned by reactive ion etching under conditions preserving the high mobility and conductivity. Using this patterning method, a sub-micron-scale thin film transistor exhibiting complete depletion at room temperature is achieved.

Mn4N is a compound magnetic material that can be grown using MBE while exhibiting several desirable magnetic properties such as strong perpendicular magnetic anisotropy, low saturation magnetization, large domain size, and record high domain wall velocities. In addition to its potential for spintronic applications exploiting spin orbit torque with epitaxial topological insulator/ferromagnet bilayers, the possibility of integrating Mn4N seamlessly with the wide bandgap semiconductors GaN and SiC provides a pathway to merge logic, memory and communication components. We report a comparative study of MBE grown Mn4N thin films on four crystalline substrates: cubic MgO, and hexagonal GaN, SiC and sapphire. Under similar growth conditions, the Mn4N film is found to grow single crystalline on MgO and SiC, polycrystalline on GaN, and amorphous on sapphire. The magnetic properties vary on the substrates and correlate to the structural properties. Interestingly, the field dependent anomalous Hall resistance of Mn4N on GaN shows different behavior from other substrates such as a flipped sign of the anomalous Hall resistance.

High-performance p-channel transistors are crucial to implementing efficient complementary circuits in wide-bandgap electronics, but progress on such devices has lagged far behind their powerful electron-based counterparts due to the inherent challenges of manipulating holes in wide-gap semiconductors. Building on recent advances in materials growth, this work sets simultaneous records in both on-current (10 mA/mm) and on-off modulation (four orders) for the GaN/AlN wide-bandgap p-FET structure. A compact analytical pFET model is derived, and the results are benchmarked against the various alternatives in the literature, clarifying the heterostructure trade-offs to enable integrated wide-bandgap CMOS for next-generation compact high-power devices.

In this work we report on synchronized plasma wave resonances in ultrathin-membrane GaN heterostructures. In contrast to commonly employed grating-gate configurations, the analyzed structure contains periodically-patterned ohmic contacts to the two-dimensional electron gas (2DEG), which are laid-out parallel to the gate fingers. Our work demonstrates that the proposed approach allows: more efficient excitation of high order plasmon modes, and superior overall coupling, even in configurations having less number of devices per unit area.

We report a comparative study of the mobility of two-dimensional electron gases (2DEG) formed at AlGaN/GaN heterostructures by simultaneously growing on substrates with very different dislocation densities. The mobility is seen to depend on the 2DEG charge density directly, but surprisingly, dislocations do not cause a discernible impact on the mobility of the samples within the measured region

Tunnel field-effect transistors (TFETs) offer the means to surpass the subthreshold swing (SS) limit of 60 mV/dec that limits MOSFETs. While MOSFETs rely on modulating a potential barrier, which is subject to a Boltzmann tail in the density of states (DOS), interband tunneling in TFETs enables a sharp turn off of the DOS because the transport is no longer governed by an exponential tail of carriers. These devices have been investigated in Si & III-V material systems1, achieving SS's as low as 20 mV/dec2. GaN is advantageous to these other material systems because its large bandgap is ideal for suppressing leakage current. Unfortunately impurity doping in GaN alone is not enough to achieve the internal fields required to promote interband tunneling[Fig l(a)]. However, by taking advantage of the difference in polarization fields between InGaN and GaN, a device structure favoring interband tunneling can be made [Fig l(b)]. Li et. al.3 have theoretically predicted that a GaN heterojunction TFET could obtain an SS of 15 mV/dec and a peak current of $1\times 10^{-4}\ \mathrm{A}/\mu \mathrm{m}$ . For the work being presented, GaN TFETs were fabricated using a surrounding gate (SG) architecture utilizing both nanowires and fins formed from a top-down approach.

Defect engineering of2D semiconducting transition metal dichalcogenides (TMDCs) has been demonstrated to be a promising way to tune both their bandgaps and carrier concentrations. Moreover, controlled introduction of defects in the source/drain access regions of a TMDC FET can boost its performance by decreasing the contact resistance at the metallTMDC interface [1]. While chemical functionalization offers a facile route towards defect engineering in 2D TMDCs, several chemically-treated TMDCs have not been fully understood at the molecular level. In this study, chemical sulfur treatment (ST) utilizing ammonium sulfide [(NH 4 ) 2 S] solution is shown to enhance the p-type behavior in 2D WSe2 via introduction of acceptor defect states near its valence band edge (VBE), with the results verified using detailed scanning tunneling microscopy (STM)/spectroscopy (STS) studies, field-effect transistor (FET) measurements and theoretical density-of-states (DOS) calculations.

$\beta-\mathrm{Ga}_{2}\mathrm{O}_{3}$ electronic devices for high power applications have seen rapid development over the recent years, due to the excellent material properties including an extremely large band-gap, high critical electric field, decent electron mobility and the availability of low-cost bulk substrates. As unipolar devices, Ga 2 O 3 vertical Schottky barrier diodes (SBDs) have fast switching capability, while enjoying all the superior properties of Ga 2 O 3 . With the development of halide vapor phase epitaxy (HVPE) capable of delivering high quality thick n– epitaxial layers [1], $\mathrm{Ga}_{2}\mathrm{O}_{3}$ vertical SBDs have shown promising results with up to 1 kV breakdown voltage (BV) together with decent on-resistance $(\mathrm{R}_{\mathrm{on}})$ of $2-6\ \mathrm{m}\Omega\cdot \mathrm{cm}^{2}\ [1-3]$ . However, the results are still far from the projected performance which surpasses GaN and SiC [4]. One important reason is the high reverse leakage current due to the high surface electric field, which causes thermionic-field emission and barrier height lowering, especially at the device edge where field crowding occurs. The leakage current can be much reduced by edge termination techniques such as field-plating [3]. More effectively, a trench-metal-insulator-semiconductor (MIS) structure can be utilized to reduce the leakage current [5], taking advantage of the reduced surface field (RESURF) effect [6]. In this work, we demonstrate Ga 2 O 3 trench-MIS SBDs with a record-high 1.5 kV breakdown voltage without edge termination, together with a ~104 times reduction in reverse leakage current compared with regular SBDs.

The effect of polarization induced (Pl)-doping in GaN buried p-type layer on reverse blocking is studied for the first time. Forward and reverse I-V characteristics is measured on n-p-n diodes. With PI-doping in the buried p-type layer, the reverse punch-through voltage increases from 30 V to 240 V, even with hydrogen passivating the Mg acceptors, indicating the unique advantage of PI-doping on reverse blocking. The enhanced punch-through voltage is attributed to the polarization fixed charge in the p-layer, which is extracted to be ∼1.3×1017 cm−3 and closely matched with the expected value of 1.4×1017 cm−3. Vertical trench-MOSFETs with a breakdown voltage of 225 V are also demonstrated on the same sample.

GaN-on-GaN vertical devices are expected to find wide application in power electronics, thanks to the high current densities, the low on-resistance and the high breakdown voltage. So far, only few papers on the reliability of GaN-on-GaN vertical devices have been published in the literature. This paper investigates the degradation of GaN-on-GaN pn diodes submitted to stress at high current density. The study was carried out by means of electrical characterization and electroluminescence (EL) measurements. We demonstrate that: (i) when submitted to stress at high current density, the devices show significant changes in the electrical characteristics: an increase in on-resistance/turn-on voltage, an increase in the generation/recombination components, the creation of shunt-paths. (ii) the increase in on-resistance is strongly correlated to the decrease in the EL signal emitted by the diodes. (iii) the degradation kinetics have a square-root dependence on time, indicative of a diffusion process. The results are interpreted by considering that stress induces a diffusion of hydrogen from the highly-p-type doped surface towards the pn junction. This results in a decrease in hole concentration, due to the creation of Mg H bonds, and in a lower hole injection. As a consequence, on-resistance increases while EL signal shows a correlated decrease.

Layered materials have drawn intense attention lately due to the possibility of realizing highly scalable low power electronic and other novel devices. Although several proof-of-concept devices have been demonstrated recently with exfoliated micron-size flakes, controllable growth of large-area electronic grade material, essential for mass production, is in its infancy. Molecular beam epitaxy (MBE) holds the promise to allow large area, layer-controlled uniform growth and in-situ probing. To this end, in this work we have summarized the literature in the field and fortified it with observations made by us on MBE grown materials like MoSe2, MoTe2, WSe2, SnSe2, MoSe2-xTex, their heterostructures and superlattices. We have summarized investigations on several substrates under different growth regimes that are being pursued in the community. This analysis sheds light on the strengths and limitations of various characterization techniques and the challenges to overcome defects in MBE 2D materials.

The frozen internal polarization-induced electric fields due to broken inversion symmetry in all conventional blue and green nitride semiconductor light emitting semiconductor quantum well heterostructures point in a direction opposite to what is desired for efficient flow of electrons and holes. This state of affairs has persisted because of the desire to have p-type hole injectors on top of the quantum well active region. Because of the internal polarization fields in nitride heterostructures, there exist four permutations of doping and polarization for the realization of such light emitters. Which permutation is the most desirable for efficient light emission? In this work, we answer this question by demonstrating a fundamentally new approach towards efficient light emission with bottom-tunnel junctions. The bottom-tunnel junction design aligns the polarization fields in a desired direction in the quantum well, while simultaneously eliminating the need for p-type contacts, and allowing efficient current spreading. By preventing electron overshoot past quantum wells, it disables carrier recombination in undesired regions of the quantized heterostructures, and opens up the possibility for new geometries of integrating and stacking multiple light emitters. Due to the inherent advantages, the bottom-tunnel junction light emitting diode enables a 200-300% increase in the light emission efficiency over alternate heterostructure designs., Comment: 23 pages, 9 figures

We report a novel approach for characterization of terahertz (THz) antennas using the photoinduced coded-aperture imaging (PI-CAI) technique. For a prototype demonstration, a WR-1.5 (500–750 GHz) diagonal horn antenna has been fully characterized. The THz beam radiated from the antenna was imaged using the PI-CAI technique at different distances from the antenna aperture. The measured beam profiles show near-Gaussian shapes, with the expected propagation properties. The far-field antenna radiation patterns were then extracted from the far-field beam image and compared with conventional measurement results, theoretical calculation and full-wave simulation. Finally, the antenna Gaussian coupling efficiency was calculated to be ∼83% based on the two-dimensional (2-D) beam mapping technique. Antenna radiation parameters obtained using the above approach show good agreement with published literature and manufacturer datasheets, demonstrating the potential of the PI-CAI approach for fast and accurate characterization of THz antennas. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:1180–1184, 2015

This work studies the spontaneous self-assembly of Ge QDs on AlAs, GaAs, and AlGaAs by high temperature in-situ annealing in molecular beam epitaxy (MBE). The morphology of Ge dots formed on AlAs are observed by atom probe tomography, which revealed nearly spherical QDs with diameters approaching 10 nm and confirmed the complete absence of a wetting layer. Reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM) of Ge annealed under similar conditions on GaAs and Al0.3Ga0.7As surfaces reveal the gradual suppression of QD formation with decreasing Al-content of the buffer. To investigate the prospects of using encapsulated Ge dots for upconverting photovoltaics, in which photocurrent can still be generated from photons with energy less than the host bandgap, Ge QDs are embedded into the active region of III-V PIN diodes by MBE. It is observed that orders of magnitude higher short-circuit current is obtained at photon energies below the GaAs bandgap compared with a reference PIN diode without Ge QDs. These results demonstrate the promise of Ge QDs for upconverting solar cells and the realization of device-quality integration of group IV and III-V semiconductors.

For the past two decades, repeatable resonant tunneling transport of electrons in III-nitride double barrier heterostructures has remained elusive at room temperature. In this work we theoretically and experimentally study III-nitride double-barrier resonant tunneling diodes (RTDs), the quantum transport characteristics of which exhibit new features that are unexplainable using existing semiconductor theory. The repeatable and robust resonant transport in our devices enables us to track the origin of these features to the broken inversion symmetry in the uniaxial crystal structure, which generates built-in spontaneous and piezoelectric polarization fields. Resonant tunneling transport enabled by the ground state as well as by the first excited state is demonstrated for the first time over a wide temperature window in planar III-nitride RTDs. An analytical transport model for polar resonant tunneling heterostructures is introduced for the first time, showing a good quantitative agreement with experimental data. From this model we realize that tunneling transport is an extremely sensitive measure of the built-in polarization fields. Since such electric fields play a crucial role in the design of electronic and photonic devices, but are difficult to measure, our work provides a completely new method to accurately determine their magnitude for the entire class of polar heterostructures.

Epitaxy is a process by which a thin layer of one crystal is deposited in an ordered fashion onto a substrate crystal. The direct epitaxial growth of semiconductor heterostructures on top of crystalline superconductors has proved challenging. Here, however, we report the successful use of molecular beam epitaxy to grow and integrate niobium nitride (NbN)-based superconductors with the wide-bandgap family of semiconductors-silicon carbide, gallium nitride (GaN) and aluminium gallium nitride (AlGaN). We apply molecular beam epitaxy to grow an AlGaN/GaN quantum-well heterostructure directly on top of an ultrathin crystalline NbN superconductor. The resulting high-mobility, two-dimensional electron gas in the semiconductor exhibits quantum oscillations, and thus enables a semiconductor transistor-an electronic gain element-to be grown and fabricated directly on a crystalline superconductor. Using the epitaxial superconductor as the source load of the transistor, we observe in the transistor output characteristics a negative differential resistance-a feature often used in amplifiers and oscillators. Our demonstration of the direct epitaxial growth of high-quality semiconductor heterostructures and devices on crystalline nitride superconductors opens up the possibility of combining the macroscopic quantum effects of superconductors with the electronic, photonic and piezoelectric properties of the group III/nitride semiconductor family.

THz technology offers multiple applications in areas such as remote sensing, spectroscopy, biomedical imaging, and ultra-wide bandwidth communications [1]. However, obtaining high-frequency performance at THz frequencies has proven challenging in conventional electronic devices. This difficulty motivated the exploration of unconventional transport mechanisms such as electron plasma waves. Two dimensional electron gases (2DEGs) in semiconductor heterostructures can allow for collective motion of electrons, i.e. plasma waves, whose group velocity is >10X larger than typical electron drift velocities (i.e. vg >108 cm/s) [2-3]. Devices based on electron plasma waves have attracted significant attention during recent years for THz generation, detection and amplification [4]. In this context, efficient coupling of external THz radiation into and out of plasmons in semiconductor heterostructures is essential for the operation of these devices. A conventional approach to excite plasmons in a 2DEG is via a grating gate coupler as illustrated in Fig. 1(a). In a grating gate configuration, adjacent unit-cells interact with each other making this a coupled resonant system. In contrast, via addition of source (S) and drain (D) electrodes, in a HEMT array configuration as depicted in Fig. 1(b), every unit cell becomes effectively independent. In this configuration, the THz to plasmon coupling is enhanced due to a cooperative effect by synchronizing the electron plasma waves in each unit-cell of the array as theoretically discussed by Popov et al [5]. Here we present the first experimental demonstration of enhanced THz coupling to electron plasma wave or plasmon in ultra-thin membrane HEMT arrays via plasmon synchronization. A thin-membrane configuration enables us to remove substrate effects and further enhance the coupling. The proposed approach allows: (i) more efficient excitation of high order plasmonic modes, and (ii) superior overall coupling-even in configurations having less number of devices per unit area-. Our results reveal a simple way to enhance the THz to plasmon coupling and thus improve the performance of electron plasma wave based devices; this effect can be exploited, for example, to improve the response of HEMT THz detectors.

GaN vertical power transistors have gained increasing interest in recent years due to the advantages over lateral transistors in high voltage/high current applications. To date, two major topologies have been studied most: gate-on-epi-surface (GoE) and gate-on-sidewall (GoS). The GoE devices include CAVET [1] and VDMOSFET-like transistors [2, 3]. The GoS devices include U-MOS or trench-MOSFETs with inversion channel [4, 5] or regrown AlGaN/GaN semi-polar channel [6], as well as depletion-mode MISFET [7]. The vertical MISFET is the simplest to fabricate, however, it does not have avalanche capabilities inherently besides being difficult to achieve sufficiently large V th . It is easier for trench MOSFETs to achieve normally-off operation, high breakdown voltage (BV) and small footprint. However, it is challenging to achieve high mobility in the inversion channel. In contrast, CAVETs, VDMOS-like transistors and PolarMOS [3] utilize high mobility AlGaN/GaN channel to achieve low Ron, but the channel regrowth posts challenges in achieving low off-state leakage in un-gated regrowth interface. Recently, a novel design based on trench MOSFET is realized by MOCVD regrowth of a thin GaN interlayer [8]. Low Ron and high BV is achieved in the gated regrown channel. Similar to the other MOCVD regrown devices, the buried Mg-doped p-GaN needs to be re-activated by exposing the p-GaN surface during high temperature anneal. This leads to high thermal budget and poses limitations on device geometry. Furthermore, any incomplete activation of buried p-GaN leads to reduced BV. In this work, we design a V-shaped trench MOSFET with MBE regrown UID GaN channel. −600 V breakdown voltage with normally-off operation is demonstrated without the need for re-activation of the buried p-GaN. To our knowledge, this is the highest BV achieved in GaN vertical transistors with MBE regrown channel.

Wide-bandgap materials, particularly Gallium Nitride, have emerged as the platform underlying many of the most promising technologies in the high-power and high-frequency domain. However, GaN p-channel devices lag far behind their popular n-channel counterparts, due to lower mobilities as well as difficulties in doping and forming ohmic contacts. There is a strong need for wide-bandgap p-channel FETs — this missing piece would enable energy-efficient, high-voltage CMOS electronics, a critical technique for on-chip power conditioning and management.

Group III-Nitride semiconductor quantum heterostructures have revolutionized efficient visible light emitters [1]. Even though efficient nitride light emitting diodes (LEDs) and laser diodes (LDs) are now commercially mature, there as several physical effects in them that are poorly understood. Furthermore, light emitters in the UV [2] and green and longer wavelengths [3, 4] remain challenging. The presence of built-in electric fields due to spontaneous and piezoelectric polarization in these quantum heterostructures leads to rather remarkable effects in the N-shaped negative differential resistance (NDR) in resonant tunnel diodes (RTDs) [5]. Much rarer are S-shape NDR, which was reported recently in GaN tunnel switch diodes [6]. To our surprise, we have observed strong, persistent, S-shaped NDR in GaN quantum well laser diodes at room temperature. The S-NDR in the laser diode is strong enough to drive an external circuit into sustained oscillations, and is likely caused by the strong internal polarization fields.

Deep UV (DUV) light-emitting diodes (LEDs) are finding increased application in many areas including water purification and sterilization. Sub-270 nm emission is ideal for these applications since bacterial DNA absorbs strongly in this wavelength regime. To extract high energy photons (∼5 eV), the LED cladding regions must be transparent and therefore consist of high Aluminum content (>60%) n- and p-AlGaN. Ensuring high electrical conductivity in such cladding regions becomes increasingly difficult with increasing Al content due to the large dopant activation energies, especially for the acceptor dopant, Magnesium (Mg) [1]. To maximize Mg incorporation by MBE, a commonly utilized approach is to grow the p-cladding region at a low temperature (∼630 °C [2]). A low growth temperature causes defects and undesired compositional inhomogeneity, leading to inferior vertical hole transport (Fig. 1). To address these issues, a combination of high temperature MBE growth of the p-AlGaN cladding and polarization-induced doping is employed. While high growth temperature ensures good crystal quality and compositional uniformity for the Al-rich p-AlGaN, the polarization-induced doping compensates for the reduced Mg incorporation at high temperature. A sub-250 nm DUV LED is demonstrated for the first time using MBE-grown p-AlGaN layers at a high temperature of 730 °C. SIMS analysis (Fig. 2) showed that Mg incorporation decreases by ∼5 times over the temperature range from 650–730°C for 75% AlGaN, and that Mg incorporation decreases at high Al content, consistent with thermodynamic principles [3]. Based on the observations from Fig. 1 and 2, a substrate temperature of 730 °C was chosen for the p-cladding layer for a sub-250 nm DUV LED structure.

GaN vertical power devices have many advantage over lateral device in device scaling, reliability and thermal management, etc. Traditional power transistors employ p-type pockets to achieve E-mode, RESURF and avalanche capabilities. However, this topology in GaN vertical power transistors has been challenging to implement [1] due to the difficulty to achieve selective area doping without compromising breakdown: p-type pockets in n-type regions or vice versa. The GaN UMOS-FETs or trench MOSFETs can be realized using epitaxial p-layers, however, suffer from low channel mobility in the inversion channel [2, 3]. Using n-type GaN only, depletion mode vertical MISFETs can be achieved with attractive current densities and breakdown voltages [4]. To get normally-off operation, Fin or nanowire (NW) pillars are necessary geometries. Compared with Fins, GaN nanowires have added advantages including superior electrostatic control and possibility for low-cost growth on foreign substrates [5, 6]. In this work, we report the first experimental demonstration of NW-MISFETs on bulk GaN substrates and compare them with Fin-MISFETs with the state-of-the-art performance fabricated on the same sample. The benefit of better electrostatic gate control in nanowire MISFETs are highlighted.

Recently, Ga 2 O 3 has become an attractive material for both power electronic and optoelectronic device applications since large-size electronic-grade Ga 2 O 3 substrates can be readily produced by melt-grown methods. Furthermore, high quality epitaxy and n-type doping schemes have been demonstrated [1, 2]. Due to its ultra-wide band gap (∼4.5–4.9 eV), Ga 2 O 3 is estimated to have a critical breakdown field >6 MV/cm, comparing favorably with ∼3 MV/cm in SiC and ∼4 MV/cm in GaN. This allows devices capable of handling large switching voltages. Devices such as lateral channel MOSFETs, MESFETs [3], MISFETs [4], nano-membrane FETs [5] and lateral FinFETs [6], vertical Schottky Barrier Diodes (SBDs) [7, 8], and deep-UV solar-blind photodetectors [9] have all been demonstrated using Ga 2 O 3 . Here, we report the first Ga 2 O 3 vertical power transistors with a breakdown voltage (BV) of 185 V.

Van der Waals (vdW) bonded heterojunctions composed of diverse two-dimensional (2D) layered materials are emerging as a novel material platform that exhibits intriguing physical phenomena and promises compelling device applications. Here we discuss the development of tunneling devices based on vdW bonded heterojunctions, specially focusing on an Esaki diode with broken gap alignment. Beyond that, an electric oscillator built upon this diode is also demonstrated.

III-nitride based ultraviolet (UV) light emitting diodes (LEDs) are of considerable interest in replacing gas lasers and mercury lamps for numerous applications. Specifically, AlGaN quantum well (QW) based LEDs have been developed extensively but the external quantum efficiencies of which remain less than 10% for wavelengths

N-polar GaN p-n diodes are realized on single-crystal N-polar GaN bulk wafers by plasma-assisted molecular beam epitaxy growth. The current-voltage characteristics show high-quality rectification and electroluminescence characteristics with a high on/off current ratio and interband photon emission. The measured electroluminescence spectrum is dominated by strong near-band edge emission, while deep level luminescence is greatly suppressed. A very low dislocation density leads to a high reverse breakdown electric field. The low leakage current N-polar diodes open up several potential applications in polarization-engineered photonic and electronic devices., Comment: 13 pages, 3 figures, to appear in Applied Physics Letters

The 246 nm AlN-delta-GaN quantum well ultraviolet light-emitting diode was proposed and realized experimentally, with the dominant transverse electric-polarized emission been verified by both the k·p simulation and the room-temperature polarization-dependent electroluminescence measurements.

We demonstrate a 243 nm deep-ultraviolet (deep-UV) GaN/AlN heterostructure light-emitting diode with and without a tunnel-junction p-contact. Optical emission occurs from 2 monolayer thick GaN quantum structure active regions. Use of a tunnel-junction enhances the current density under forward bias.

We report terahertz coded-aperture imaging using photo-induced reconfigurable aperture arrays on a silicon wafer. The coded aperture was implemented using programmable illumination from a commercially available digital light processing projector. At 590 GHz, each of the array element apertures can be optically turned on and off with a modulation depth of 20 dB and a modulation rate of ~ 1.3 kHz. Prototype demonstrations of 4 ×4 coded-aperture imaging using Hadamard coding have been performed. Continuous THz imaging with 8 ×8 pixels has also been demonstrated, using a slowly moving metal strip as the target. In addition, this technique has been successfully applied to mapping THz beams by using a 6 ×6 aperture array at 590 GHz. The imaging results agree closely with theoretical calculations based on Gaussian beam propagation, demonstrating that this technique is promising for realizing real-time and low-cost terahertz cameras for many applications. The reported approach provides a simple but powerful means to visualize THz beams, which is highly desired in quasi-optical system alignment, quantum-cascade laser design and characterization, and THz antenna characterization.

The effects of fringing capacitances on the high-frequency performance of T-gate GaN high-electron mobility transistors (HEMTs) are investigated. Delay time components have been analyzed for gate-recessed InAlN/GaN HEMTs with a total gate length of 40 nm and fT/fmax of 225/250 GHz. It is found that the gate extrinsic capacitance contributes significantly to the parasitic delay-approximately 50% of the total delay in these highly scaled devices. The gate extrinsic capacitance comprises two components: 1) parallel plate capacitances between the T-gate and the surrounding electrodes and 2) the fringing capacitance between the gate stem and the access regions. Detailed study of the gate electrostatics reveals that the later, the fringing capacitance between the T-gate stem and the device access region, ultimately determines the lower limit of the extrinsic capacitance Cext; this minimum Cext can be realized experimentally using a large gate stem height and employing low- k passivation dielectric. Since the corresponding parasitic delay can be expressed as Cext/gm,int, this paper also highlights the importance of maximizing gm,int in ultrascaled HEMTs by adopting strategies to enhance carrier velocity.

Atomically thin molybdenum disulfide (MoS2) offers potential for advanced devices and an alternative to graphene due to its unique electronic and optical properties. The temperature-dependent Raman spectra of exfoliated, monolayer MoS2 in the range of 100-320 K are reported and analyzed. The linear temperature coefficients of the in-plane E-2g(1) and the out-of-plane A(1g) modes for both suspended and substrate-supported monolayer MoS2 are measured. These data, when combined with the first-order coefficients from laser power-dependent studies, enable the thermal conductivity to be extracted. The resulting thermal conductivity kappa = (34.5 +/- 4) W/mK at room temperature agrees well with the first-principles lattice dynamics simulations. However, this value is significantly lower than that of graphene. The results from this work provide important input for the design of MoS2-based devices where thermal management is critical.

Oxide passivation is an active field of investigation for reducing surface state densities on Ge surfaces, namely below high-K gate stacks. However, the quality of Ge oxide passivation has not been as widely investigated in the context of performance in minority carrier photonic devices. p-Ge substrates with a foundry-applied oxide passivation are compared with oxide-stripped substrates passivated by dry thermal oxides grown between 400 ◦ C-650 ◦ Ci n O2. By using p-Ge, only the behavior of minority conduction electrons and their associated traps become the relevant focus of this study. Passivation quality, as evaluated by photoluminescence (PL),isfoundtovaryasafunctionoftemperaturebetween400‐650 ◦ C.ChangesinPLintensityareattributedtodifferencesinsurface terminationofthep-Gesubstrate.X-rayphotoelectronspectroscopy(XPS)studiesfindthatthepresenceofsuboxides(GeOx)increase with oxidation temperature, with GeO2 dominant at 400 ◦ C and mixed suboxides at higher temperatures. Nonuniform desorption of Ge oxide is confirmed above 550 ◦ C. Thermal oxidation is also shown to improve the quality of ion-damaged Ge surfaces. We conclude that suboxides confer better passivation than a GeO2 interface, as determined by PL intensity.

We review the conception and development of two-dimensional heterojunction interlayer field effect transistor (Thin-TFET), where a steep subthreshold swing (SS) and a high on-current are estimated theoretically. The Thin-TFET has been experimentally demonstrated using WSe 2 /SnSe 2 stacked heterostructures, where the SS is mostly likely limited by the interfacial trap density of states and the parasitic MOSFET. Due to its vertical stacking structure, Thin-TFET intrinsically has a smaller gate-drain capacitance compared to the conventional lateral pin-TFET. In turn, this results in mitigated Miller Effect in Thin-TFET thus reducing dynamic energy dissipation in circuits.

RF plasma assisted MBE growth of scandium nitride (ScN) thin films on Ga-polar GaN (0001)/SiC, Al-polar AlN (0001)/Al2O3, and Si-face 6H-SiC (0001) hexagonal substrates is found to lead to a face centered cubic (rock salt) crystal structure with (111) out-of-plane orientation instead of hexagonal orientation. Cubic (111) twinned patterns in ScN are observed by in situ electron diffraction during epitaxy, and the twin domains in ScN are detected by electron backscattered diffraction and further corroborated by X-ray diffraction. The epitaxial ScN films display very smooth, subnanometer surface roughness at a growth temperature of 750 °C. Temperature-dependent Hall-effect measurements indicate a constant high n-type carrier concentration of ∼1 × 1020/cm3 and an electron mobility of ∼20 cm2/V s.

Bismuth-containing compounds inherit the high spin-orbit coupling and bandgap bowing effects of the Bi atom. Here, we report the growth of InBi films using molecular beam epitaxy. By growing in a Bi-rich regime, we obtain coalesced and crystalline films with a sharp interface to the high-resistivity Si(111) substrate. Temperature-dependent transport and resistivity measurements exhibit a nonlinear Hall effect and parabolic magnetoresistance, suggesting two-carrier semimetallic behavior. In In-rich films, metallic temperature-dependent resistivity is observed. In Bi-rich films, we observed semiconductorlike temperature-dependent resistivity as well as superconductivity.

In this talk, I will show our recent achievements for GaN p-n junction diodes with 3.48 kV and 0.95 mΩ·cm2: a record high figure-of-merit (BV2/Ron ) of >12.8 GW/cm2, 600 V GaN vertical V-trench MOSFETs with MBE regrown channel and GaN vertical nanowire and Fin power MISFETs. I. Introduction GaN has been long touted as a promising material for compact and efficient power electronics, owing to its large critical field (>10× of Si), high electron mobility (~2000 cm2/Vs at a carrier concentration of ~1×1016cm-3) and high thermal conductivity (~ 2× of Si). Nowadays GaN vertical power transistors have gained increasing interest in recent years due to the advantagesover lateral transistors in high voltage/high current applications. II. Device fabrication and results A. GaN-on-GaN p-n power diodes The GaN p-n junction epi-structures were grown by MOVPE on bulk-GaN substrates. As shown in Fig.1, three device layer structures have been grown with a varying n-GaN drift layer thickness of 20, 25 and 32 μm. For the 20 μm epi-design, the drift layer doping concentration was kept constant, which is confirmed by the capacitance-voltage (C-V) measurement on these power diodes; the net ND-NA concentration extracted from C-V measurements showed a constant value of 5×1015 cm-3 in the top 10 μm. For the 25 and 32 μm epi-designs, the top 6 μm of the n-GaN drift layer was grown to have the lowest doping concentration: a net ND-NA concentration of ~1×1015 cm-3 was extracted from C-V measurements while a Si concentration of 2.5×101 5 cm-3 was measured by SIMS on a similar epi-wafer. For the 2nd n-GaN drift layer, a net ND-NA concentration of 2~3×1015 cm-3 was extracted in both the 25 and 32 μm power diodes. The benchmark plot is shown in Fig. 2, demonstrating a record Baliga’s figure of merit of >12 GW/cm2 obtained in the 32 μm epi-design.Also shown is that the p-n power diode BV increases with the increasing n-GaN drift-layer thickness while Ron remains nearly the same ~1.0 mΩ·cm2. If the minority carrier lifetime is short, a p-n diode can be assumed to operate like a unipolar device thus Ron is dominated by the n-GaN drift region. Assuming a carrier concentration of 5×1015 cm-3 and a mobility of 2000 cm2/Vs, the specific resistance of a 20 μm n-GaN drift layer is calculated to be ~1.25 mΩ·cm2. The observed record low Ron might arise from other mechanisms including high level injection, photon recycling effects, which warrants further investigation. B. GaN Vertical V-trench MOSFETs The schematic of the device is shown in Fig. 3, which consists of a MBE regrown UID GaN channel covering the sidewall of the V-shaped trench.On-off ratio of 109 and normally-off operation with a threshold voltage of ~16 V is achieved. The output characteristics in Fig. 4 show goodsaturation behavior and an on-current of ~18 A/cm2 at Vgs =25 V. Ron is determined from the linear region to be 0.3 Ω·cm2. The relatively poor Ron and Ion is determined to be limited by the lateral portion of the regrown channel from gated-TLM measurement. We found the sheet resistance of the lateralregrown channel to be ~2 MΩ/sq. at Vgs =25 V. Since the lateral channel length Lg , lateral is 3~4 μm in the measured device, Ron is dominated by the lateral channel. The high resistivity of the lateral regrown channel is likely due tothe carrier compensation by Mg diffusion from the p-GaN underneath. A thicker UID channel or growing a thin n+-GaN counter layer before the channel regrowth could improve the channel conductivity dramatically and is being investigated. The off-state characteristics in shown in Fig. 5. Low drain leakage and a BV of 596 V ismeasured with Vgs =-15 V, indicating good quality of the regrowth p-n junction interface. C . GaN vertical nanowire and fin power MISFETs The NWs and Fins are formedby a top-down approach: dry etch followed by a hot TMAH wet etch to form vertical side walls. Images of fabricated NWs are shown in Fig. 6. The output characteristics of the Fin-MISFET are shown in Fig. 7. An on-current of 14 kA/cm2 and Ron of 0.4 mΩ·cm2 are extracted. The off-state characteristics of the Fin-MISFETs under different gate-bias is shown in Fig. 8. Under more negative gate-bias, the breakdown voltage of the same device increases, reaching a highest value 513 V under Vgs =-15 V where the device undergoes a hard breakdown. Acknowledgement: This work was supported in part by the ARPA-E SWITCHES program (DE-AR0000454) and carried out at the Cornell Nanoscale Science and Technology Facilities(CNF) sponsored by the NSF NNCI program (ECCS-15420819) and New York State. Figure 1

A unique combination of high mobility, high velocity and high sheet density of the 2DEG formed in GaN-based heterostructures has enabled GaN-based HEMTs to be used in a wide range of applications from RF power amplifiers to efficient power converters. Today’s complex communication systems require transceivers to process RF signals efficiently with large bandwidth and high fidelity. While GaN-based HEMT technology has advanced to reach higher power densities, it has not fundamentally changed the power requirements for the linearity performance. To address fundamental limitations of HEMT’s power/linearity/efficiency/frequency tradeoff, we proposed a transistor structure called BRIDGE FET (buried dual gate FET) where gate electrodes are buried into AlGaN/GaN heterostructures and contact laterally with multiple 2DEG channels [1]. A deliberate elimination of a conventional top-contact gate leads to a unique device operation principle and performance advantages for improved linearity and efficiency at large signal operations; (1) The drain-source current is controlled solely by modulating the width of the 2DEG channels by the lateral gate electric field while maintaining the 2DEG density. (2) The MESFET-like device operation enables gradual pinch-off, greatly reducing g m derivatives near pinch-off. (3) Lack of density modulation with V gs leads to a constant electron velocity at high electric field, eliminating a typical g m roll-off at high V gs. This results in a constant gain along a resistive load line. (4) The buried gates forms Schottky contacts to the GaN channels below the 2DEG layers. This enhances electron confinement and improves electrostatic isolation between the source and drain, significantly reducing g d at high V ds. (5) Elimination of the top-contact gate prevents electrons from being trapped on the surface, suppressing current collapse at high voltage operations. (6) An absence of inverse piezoelectric effect due to the reduced vertical electric field at the drain-side of the gate improves device reliability under high voltage stress. In this presentation, current status of our development of multi-2DEG channel BRIDGE FETs will be discussed. This work was sponsored by DARPA-MTO DREaM program under DARPA/CMO Contract No. FA8650-18-C-7807. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressly or implied, of the Defense Advanced Research Projects Agency or the U.S. Government. [1] K. Shinohara et al., IEEE EDL, vol. 39, no. 3, p. 417, March 2018.

Bismuth ferrite layers, ∼200-nm-thick, are deposited on SrRuO 3 -coated DyScO 3 (110) o substrates in a step-flow growth regime via adsorption-controlled molecular-beam epitaxy. Structural characterization shows the films to be phase pure with substrate-limited mosaicity (0.012 ○x-ray diffraction ω-rocking curve widths). The film surfaces are atomically smooth (0.2 nm root-mean-square height fluctuations) and consistof 260-nm-wide [11̄1] o -oriented terraces and unit-cell-tall (0.4 nm) step edges. The combination of electrostatic and symmetry boundaryconditions promotes two monoclinically distorted BiFeO 3 ferroelectric variants, which self-assemble into a pattern with unprecedentedlycoherent periodicity, consisting of 145 ± 2-nm-wide stripe domains separated by [001] o -oriented 71 ○ domain walls. The walls exhibit electricalrectification and enhanced conductivity.

Ga2O3 vertical trench Schottky barrier diodes with four different fin-channel orientations are realized on (001) substrates and compared. Fin-channels along the [010] direction with (100)-like sidewalls result in the highest forward current, while other channel orientations all lead to a shallow turn-on behavior and much lower forward current, indicative of severe sidewall depletion attributed to negative interface charges. The comparison indicates that the interface charge density is the smallest on the (100)-like surfaces. The breakdown voltage of the diodes with 1-μm fin width is around 2.4 kV, with no apparent dependence on the channel orientation.

GaN PolarMOS is a vertical power transistor incorporating the unique polarization-induced bulk doping scheme in III-nitrides for the body p-n junction. We report the realization of this device, wherein the vertical channel, source contact, and body contact regions are successfully formed using three steps of selective-area epitaxial regrowth, all by molecular beam epitaxy (MBE). The fabricated PolarMOS has an excellent on-current of >500 mA mm−1 and a specific on-resistance of 0.66 mΩ · cm2. The reverse breakdown mechanisms of the PolarMOS are investigated. First, a pronounced source-drain vertical leakage is identified and attributed to the passivation of the buried p-type body, which is subsequently resolved by the sidewall activation method. With the body leakage eliminated, the breakdown voltage is found to be limited by a highly conductive path along the regrowth sidewall interface using the conductive scanning probe technique, despite the absence of apparent structural defects.

The nitride semiconductor materials GaN, AlN, and InN, and their alloys and heterostructures have been investigated extensively in the last 3 decades, leading to several technologically successful photonic and electronic devices. Just over the past few years, a number of new nitride materials have emerged with exciting photonic, electronic, and magnetic properties. Some examples are 2D and layered hBN and the III-V diamond analog cBN, the transition metal nitrides ScN, YN, and their alloys (e.g. ferroelectric ScAlN), piezomagnetic GaMnN, ferrimagnetic Mn4N, and epitaxial superconductor/semiconductor NbN/GaN heterojunctions. This article reviews the fascinating and emerging physics and science of these new nitride materials. It also discusses their potential applications in future generations of devices that take advantage of the photonic and electronic devices eco-system based on transistors, light-emitting diodes, and lasers that have already been created by the nitride semiconductors., 16 pages, 3 figures