We present a technique of patterned in situ molecular beam epitaxy (MBE) selective area sublimation etching of p-type GaN and direct MBE regrowth of n-type GaN to form lateral pn junction structures without air exposure of the junction region. The Mg-doped p-GaN has 2.1 & times; 10(18 )/ cm(3) mobile holes of mobility 4 cm(2)/V s and the Si-doped n-GaN has 3 & times; 10(18) / cm(3) mobile electrons of mobility >100 cm /V s, both at room temperature. The resulting lateral pn junction exhibits rectification and GaN band-edge electroluminescence at 3.4 eV at forward bias, in addition to blue electroluminescence associated with Mg-doped GaN. We systematically describe the entire MBE etch and regrowth process and expect this vacuum-based technique to enable lateral structures of higher complexity than shown in this initial demonstration.

We report ultrawide-bandgap AlGaN pin diodes with high forward current density and deep-ultraviolet (DUV) electroluminescence signaling efficient carrier injection. The devices incorporate a 100 nm unintentionally doped intrinsic layer between polarization-doped n- and p-type regions, grown by plasma-assisted molecular beam epitaxy on bulk AlN substrates. Electrical characterization reveals current densities up to ∼28 kA/cm2, differential on-resistances below 1 mΩ cm2, ideality factors approaching 1.7, and a cutoff frequency fco = (2πRonCoff)−1 = 30.8 GHz. Capacitance–voltage measurements confirm a reduced junction capacitance relative to a pn diode without the i-layer, while electroluminescence shows ultraviolet emission at 248 nm from the polarization-doped regions. These results establish AlGaN pin diodes as a promising platform for high-speed, high-voltage, and DUV optoelectronic devices, offering a scalable approach to next-generation ultrawide-bandgap electronics and photonics.

ABSTRACT Resonant tunneling is a quantum mechanical effect that enables electrons to traverse classically forbidden regions of space. The engineering of this quantum effect in wide‐bandgap semiconductors promises important technological benefits as it seamlessly combines ultra‐fast electron transport dynamics with superior power‐handling capabilities. Here, we report the first realization of highly coherent electronic quantum interference and resonant tunneling injection in wide‐bandgap triple‐barrier heterostructures. Enabled by the high structural quality of the GaN/AlN triple‐barrier active region, we observe multiple resonant tunneling peaks and negative differential conductance at room temperature. The robustness of the inter‐well resonant tunneling current is experimentally confirmed via temperature‐dependent electronic transport and the generation of electrically tunable microwave oscillations. These results represent a stepping stone in the engineering of intersubband tunneling transport in wide‐bandgap III‐nitride semiconductors, raising hopes for the realization of intersubband optical amplification and frequency‐modulated resonant tunneling oscillators.

Ultrawide bandgap (UWBG) semiconductor pn junction diodes are desired for high voltage power electronics and UV optoelectronic devices. Making low-resistance contacts to the UWBG junction region is difficult due to low solubility of dopants in AlGaN, nonavailability of readily available metals with suitable work functions, processing induced damage and the lack of dislocations in single-crystal bulk AlN substrates. We report dielectric assisted liftoff, a processing scheme to simultaneously achieve n- and p-contacts with low turn-on voltages (von), and low specific differential contact resistances (& rhov;c) in heterostructure diodes on bulk AlN. We report & rhov;cn = 5-6 & times; 10-5 Omega cm2 and vonn approximate to 0 V to etched n-type Al0.75Ga0.25N for current densities between 0 and 5 kA cm-2. We also report & rhov;cp approximate to 10-3-10-5 Omega cm2 and vonp approximate to 0-0.01 V to p-In0.07Ga0.93N contacts over the same current range. The key design principle in fabrication of the diodes is to avoid exposing the nitride semiconductor surfaces to photoresist by using a dielectric layer, which also simultaneously protects the p-InGaN during the n-contact annealing step.

Pseudomorphic growth of a UV-C laser diode heterostructure is demonstrated on a bulk AlN substrate by MBE. While stimulated emission is not observed under current injection, spontaneous emission at 265 nm is observed. Current densities up to 800 A/cm(2), > 5 orders on/off ratio at +/- 5 V, and a low differential on-resistance of 2.6 m Omega cm(2) at the highest current density are achieved. The heterostructure has a high refractive index waveguide core surrounded by n- and p-cladding layers for mode confinement at 270 nm to facilitate edge emission and collection of photons. Edge-collection electroluminescence of the devices is achieved by cleaving the fabricated devices along the m-plane of the wurtzite crystal. Electrical injection results in the emission of high-energy 4.7 eV photons that are collected from the cleaved edge of the heterostructure, corresponding to the optical bandgap of the AlGaN active region. The contribution of power dissipation across the n- and p-regions of the diode is discussed. The n-contact resistance to n-AlGaN is identified as the largest contributor to the series resistance of these devices. Reduction of these resistances will allow these heterostructures to achieve lasing in the future.

Electron blocking layers (EBLs) are instrumental in visible light emitters based on nitride semiconductor heterostructures. AlScN (a) can be lattice-matched to GaN, (b) can be grown at GaN compatible conditions, and (c) offers favorable band offsets required for EBLs, but has not been used in light emitting devices to date. In this work, we test if lattice-matched AlScN can be integrated as an EBL layer in a blue light emitting diode (LED) pn junction heterostructure. We find that blue LED operation with peak electroluminescence (EL) at similar to 460 nm can indeed be realized with an AlScN EBL. In the LED with the AlScN EBL, we observe low reverse leakage current density, and similar to 6 nm lower full-width at half maximum of the EL peak compared to a control sample. Though the turn-on voltage of the AlScN EBL containing blue LED is high, this demonstration proves its feasibility and provides guidance for improved performance in the future.

We report step-flow homoepitaxial growth of Al2O3 on m-plane sapphire at a relatively low substrate temperature of 650 °C using molecular beam epitaxy (MBE). The films exhibit atomically smooth terraces with step heights of 0.58 nm, corresponding to two-thirds of the m-plane lattice spacing. A decrease in growth rate of sapphire with increasing aluminum flux beyond a certain value is observed, a growth regime not previously reported for sapphire homoepitaxy. Oxygen plasma surface pre-treatment is found to produce highly uniform step heights and terrace widths upon MBE, enabling reproducible step-flow growth. The m-plane orientation stabilizes the growth of (110)-oriented TiN, eliminating the twin domains observed in TiN grown on c-plane sapphire. TiN films grown on homoepitaxial m-plane Al2O3 show superconductivity below Tc = 5.4 K, room-temperature resistivity of 31.9 μΩ cm, and residual resistivity ratio (RRR) of 2.7, compared to 18.2 μΩ cm and RRR = 3.3 grown directly on as-received substrates. The findings indicate that for qubit applications, m-plane sapphire homoepitaxial layers/epitaxial TiN superconductor interfaces without air exposure are feasible.

Ultraviolet (UV) light-emitting diodes (LEDs) based on the AlGaN ultrawide bandgap semiconductor material system hold significant promise for a range of applications, including sterilization, water purification, and high-density optical storage. However, the external quantum efficiency (EQE) of these devices remains limited, primarily due to incomplete understanding of the microscopic processes governing carrier dynamics and recombination mechanisms in these ultrawide bandgap semiconductor materials. Lowtemperature electroluminescence (LT-EL) is a powerful technique for probing these mechanisms. Here we deploy LT-EL to elucidate the fundamental device physics of AlGaN-based UV LEDs to identify key avenues for improving EQE for more efficient UV diode technologies.

Development of a high-performance, p-type oxide channel is crucial to realize all-oxide complementary metal–oxide semiconductor technology that is amenable to 3D integration. Among p-type oxides, α-SnO is one of the most promising owing to its relatively high hole mobility {as high as 21 cm2 V−1 s−1 has been reported [M. Minohara et al., J. Phys. Chem. C 124, 1755–1760 (2020)]}, back-end-of-line compatible processing temperature (≤400 °C), and good optical transparency for visible light. Unfortunately, doping control has only been demonstrated over a limited range of hole concentrations in such films. Here, we demonstrate systematic control of the hole concentration of α-SnO thin films via potassium doping. First-principles calculations identify potassium substitution on the tin site (KSn) of α-SnO to be a promising acceptor that is not (self)-compensated by native vacancies or potassium interstitials (Ki). We synthesize epitaxial K-doped α-SnO thin films with controlled doping concentration using suboxide molecular-beam epitaxy. The concentration of potassium is measured by secondary ion mass spectrometry, and its incorporation into the α-SnO structure is corroborated by x-ray diffraction. The effect of potassium doping on the optical response of α-SnO is measured by spectroscopic ellipsometry. Potassium doping provides systematic control of hole doping in α-SnO thin films over the 4.8 × 1017 to 1.5 × 1019 cm−3 range without significant degradation of hole mobility or the introduction of states that absorb visible light. Temperature-dependent Hall measurements reveal that the potassium is a shallow acceptor in α-SnO with an ionization energy in the 10–20 meV range.

Better wall plug efficiency of deep-ultraviolet light emitting diodes (DUV-LEDs) requires simultaneous low resistivity p-type and n-type contacts, which is a challenging problem. In this study, the co-optimization of p-InGaN and n-AlGaN contacts for DUV LEDs is investigated. We find that using a thin In0.07Ga0.93N cap is effective in achieving ohmic p-contacts with specific contact resistivity of 3.10 x 10(-5) Omega cm(2). Upon monolithic integration of p- and n-contacts for DUV LEDs, we find that the high-temperature annealing of 800 degrees C required for the formation of low resistance contacts to n-AlGaN severely degrades the p-InGaN layer, thereby reducing the hole concentration and increasing the specific contact resistivity to 9.72 x 10(-4) Omega cm(2). Depositing a SiO2 cap by plasma-enhanced atomic layer deposition (PE-ALD) prior to high-temperature n-contact annealing restores the low p-contact resistivity, enabling simultaneous low-resistance p- and n-contacts. DUV-LEDs emitting at 268 nm fabricated with the SiO2 capping technique exhibit a 3.5 V reduction in operating voltage at a current level of 400 A/cm(2) and a decrease in differential ON-resistance from 6.4 to 4.5 m Omega cm(2). This study highlights a scalable route to high-performance, high-Al-content bipolar AlGaN devices.

UV-C LEDs pseudomorphically grown by MBE on bulk AlN substrates emitting at 265 nm are demonstrated. High current density up to 800 A/cm^2, 5 orders of on/off ratio, and low differential on-resistance of 2.6 mΩ·cm^2 at the highest current density is achieved. The LED heterostructure has a high refractive index waveguide core surrounded by n- and p-cladding layers similar to a laser diode designed for mode confinement at 270 nm to facilitate edge emission and collection of photons. Edge-emitting devices are made by cleaving the fabricated LEDs along the m-plane of the wurtzite crystal. Electrical injection results in emission of high energy 4.7 eV photons that are collected from the cleaved edge of the LEDs corresponding to the optical bandgap of the AlGaN active region. The contribution of power dissipation across the n- and p-regions of the diode is discussed. The n-contact resistance to n-AlGaN is identified as the largest contributor to the series resistance of the LED in the present generation of devices.

Photocathodes activated to negative electron affinity with a cesium-based activation layer, such as GaAs and GaN, can be used for generating spin-polarized electron beams, but their extreme sensitivity to chemical poisoning limits their operational lifetimes. This work demonstrates that applying and subsequently heating a cesium iodide (CsI) coating can produce a more durable activation layer lacking iodine, but rich in stable cesium suboxides (formal O oxidation state >−2), which significantly extend the dark lifetimes of both GaAs and GaN photocathodes. Through x-ray photoelectron spectroscopy, we examine the stability and formation of these Cs suboxides, which exhibit remarkable resistance to chemical poisoning. Additionally, we investigate the subsequent surface quality using atomic force microscopy. Our findings show that CsI-based surface treatments not only prolong photocathode lifetime but also maintain high spin polarization, positioning this method as a promising approach for enhancing photocathode durability in demanding applications.

Here, we report that a source of Si impurities commonly observed on (010) β-Ga2O3 is from exposure of the surface to air. Moreover, we find that a 15 min hydrofluoric acid (HF) (49%) treatment reduces the Si density by approximately 1 order of magnitude on (010) β-Ga2O3 surfaces. This reduction in Si is critical for the elimination of the often observed parasitic conducting channel, which negatively affects transport properties and lateral transistor performance. After the HF treatment, the sample must be immediately put under vacuum, for the Si fully returns within 10 min of additional air exposure. Finally, we demonstrate that performing a 30 min HF (49%) treatment on the substrate before growth has no deleterious effect on the structure or on the epitaxy surface after subsequent Ga2O3 growth.

Ultrawide bandgap heterojunction p-n diodes with polarization-induced AlGaN p-type layers are demonstrated using plasma-assisted molecular beam epitaxy on bulk AlN substrates. Current-voltage characteristics show a turn on voltage of $V_{\text{bi}}\approx5.5$ V, a minimum room temperature ideality factor of $\eta\approx 1.63$, and more than 12 orders of current modulation at room temperature. Stable current operation of the ultrawide bandgap semiconductor diode is measured up to a temperature of 300$^\circ$C. The one-sided n$^{+}$-p heterojunction diode design enables a direct measurement of the spatial distribution of polarization-induced mobile hole density in the graded AlGaN layer from the capacitance-voltage profile. The measured average mobile hole density is $p \sim 5.7 \times 10^{17}$ cm$^{-3}$, in close agreement with what is theoretically expected from distributed polarization doping. Light emission peaked at 260 nm (4.78 eV) observed in electroluminescence corresponds to interband radiative recombination in the n$^{+}$ AlGaN layer. A much weaker deep-level emission band observed at 3.4 eV is attributed to cation-vacancy and silicon complexes in the heavily Si-doped AlGaN layer. These results demonstrate that distributed polarization doping enables ultrawide bandgap semiconductor heterojunction p-n diodes that have wide applications ranging from power electronics to deep-ultraviolet photonics. These devices can operate at high temperatures and in harsh environments.

Polarization-induced two-dimensional electron gases (2DEGs) in AlN/GaN/AlN quantum well high-electron-mobility transistors on ultrawide bandgap AlN substrates offer a promising route to advance microwave and power electronics with nitride semiconductors. The electron mobility in thin GaN quantum wells embedded in AlN is limited by high internal electric field and the presence of undesired polarization-induced two-dimensional hole gases (2DHGs). To enhance the electron mobility in such heterostructures on AlN, previous efforts have resorted to thick, relaxed GaN channels with dislocations. In this work, we introduce n-type compensation delta-doping in a coherently strained single-crystal (Xtal) AlN/GaN/AlN heterostructure to counter the 2DHG formation at the GaN/AlN interface, and simultaneously lower the internal electric field in the well. This approach yields a delta-doped XHEMT structure with a high 2DEG density of similar to 3.2x10(13) cm(-2) and a room temperature (RT) mobility of similar to 855 cm(2)/Vs, resulting in the lowest RT sheet resistance 226.7 Omega/square reported to date in coherently strained AlN/GaN/AlN HEMT heterostructures on the AlN platform.

To enhance the electron mobility in quantum-well high-electron-mobility transistors (QW HEMTs), we investigate the transport properties in AlN/GaN/AlN heterostructures on Al-polar single-crystal AlN substrates. Theoretical modeling combined with experiment shows that interface roughness scattering due to high electric field in the quantum well limits mobility. Increasing the width of the quantum well to its relaxed form reduces the internal electric field and scattering, resulting in a binary QW HEMT with a high two-dimensional electron gas (2DEG) density of 3.68×1013 cm–2, a mobility of 823 cm2/Vs, and a record-low room temperature (RT) sheet resistance of 206 Ω/□. Further reduction of the quantum well electric field yields a 2DEG density of 2.53×1013 cm–2 and RT mobility > 1000 cm2/V s. These findings will enable future developments in high-voltage and high-power microwave applications on the ultrawide bandgap AlN substrate platform.

Polarization-induced carriers play an important role in achieving high electrical conductivity in ultrawide bandgap semiconductor AlGaN, which is essential for various applications ranging from radio frequency and power electronics to deep UV photonics. Despite significant scientific and technological interest, studies on polarization-induced carriers in N-polar AlGaN are rare. We report the observation and properties of polarization-induced two-dimensional electron gases (2DEGs) in N-polar AlGaN/AlN heterostructures on single-crystal AlN substrates by systematically varying the Al content in the 8 nm top layers from x = 0 to x = 0.6, spanning energy bandgaps from 3.56 to 4.77 eV. The 2DEG density drops monotonically with increasing Al content, from 3.8 × 1013/cm2 in the GaN channel, down to no measurable conductivity for x = 0.6. Alloy scattering limits the 2DEG mobility to below 50 cm2/V s for x = 0.49. These results provide valuable insights for designing N-polar AlGaN channel high electron mobility transistors on AlN for extreme electronics at high voltages and high temperatures, and for UV photonic devices.

Resonant tunneling transport in polar heterostructures is intimately connected to the polarization fields emerging from the geometric Berry-phase. In these structures, quantum confinement results not only in a discrete electronic spectrum, but also in built-in polarization charges exhibiting a broken inversion symmetry along the transport direction. Thus, electrons undergo highly asymmetric quantum interference effects with respect to the direction of current flow. By employing doping to counter the broken symmetry, we deterministically control the resonant transmission through GaN/AlN resonant tunneling diodes and experimentally demonstrate the recovery of symmetric resonant tunneling injection across the noncentrosymmetric double-barrier potential.

We propose a special design enclosure device that promotes isotropic distribution of germicidal UV-C light for the effective disinfection of difficult to reach surfaces. We used experimental and computational approaches to investigate the disinfection efficacy of this device against Escherichia coli and Listeria innocua. Stainless steel, Copper metal, and a Copper polymer were used as solid substrates of varying roughness and hydrophobicity. Bacteria reductions of up to 6.9 log CFU were achieved at various locations relative to the UV-C source after 3 min of treatment (20–990 mJ/cm2 cumulative fluence depending on the location). Inactivation kinetics was nonlinear and followed the Weibull model (0.77 ≤ R2 ≤ 0.97). Optical ray tracing simulation was used to generate maps of spatial light distribution, which were then coupled with microbial inactivation kinetics to create spatial maps of inactivation. The modeling approach used accurately predicted microbial inactivation at various locations, with only small discrepancies (±8%) between predicted and experimental data. These findings demonstrate that the proposed device is suitable for disinfecting various hard to reach surfaces, with numerous possible applications in the food and healthcare industries. Additionally, the modeling approach used here can be used to aid in the design of a highly effective Ultraviolet treatment system.