We report the molecular beam epitaxial growth conditions to realize coherently strained nitrogen-polar gallium nitride quantum wells on single-crystal bulk aluminum nitride substrates. The structural, optical, and electronic properties of these binary N-polar GaN/AlN heterostructures are discussed. The sharpness of the GaN/AlN interface and the preservation of the polarity across the heterojunction is studied by electron microscopy. Photoluminescence measurements reveal two peaks: one at similar to 3.6 eV corresponding to a GaN layer under compressive strain that produces a blue shift and the other at similar to 6.0 eV from the epitaxial AlN buffer layer. A high-density polarization-induced 2D electron gas is formed in the ultrathin N-polar GaN quantum well, whose transport properties are measured to cryogenic temperatures.

Epitaxial aluminum nitride (AlN) thin-film bulk acoustic resonators (FBARs) enable low loss filtering for future 6G systems. They also provide a compact approach for qubit sensing at cryogenic temperatures. However, these devices are rarely characterized systematically from room temperature to cryogenic temperatures, and the mechanisms that limit their cryogenic performance remain unclear. In this work, we study a 15.6 GHz epitaxial AlN FBAR from room temperature to cryogenic temperatures to identify losses from the AlN film and those introduced by the electrodes, anchors, and other device layers. Small signal RF measurements from 294 K down to 6.5 K show an increase in the raw Qmax from 363 to 1589. A temperature dependent model that includes phonon phonon scattering, thermoelastic damping, dielectric loss, electrical loss, and anchor loss helps explain the measured Q(T) trend and identifies a transition from the Landau Rumer to the Akhiezer regime near 270 K. The model indicates that acoustic energy leakage through the anchors limits Q at cryogenic temperatures, while electrical loss dominates at higher temperatures. These results point to two routes toward higher cryogenic Q: better acoustic isolation of the anchors and lower loss electrodes, including superconducting electrodes. Improved anchor design benefits both high frequency 6G filters and cryogenic quantum microwave circuits, while superconducting electrodes are particularly useful for cryogenic operation.

AlN has the largest bandgap in the wurtzite III-nitride semiconductor family, making it an ideal barrier for a thin GaN channel to achieve strong carrier confinement in field-effect transistors, analogous to silicon-on-insulator technology. Unlike /Si/, AlN/GaN/AlN can be grown fully epitaxially, enabling high carrier mobilities suitable for high-frequency applications. However, developing these heterostructures and related devices has been hindered by challenges in strain management, polarization effects, defect control, and charge trapping. Here, the AlN single-crystal high electron mobility transistor (XHEMT) is introduced, a new nitride transistor technology designed to address these issues. The XHEMT structure features a pseudomorphic GaN channel sandwiched between AlN layers, grown on single-crystal AlN substrates. XHEMTs demonstrate RF performance on par with the state-of-the-art GaN HEMTs, achieving 5.92 W/mm output power and 65% peak power-added efficiency at 10 GHz under 17 V drain bias. These devices overcome several limitations present in conventional GaN HEMTs, which are grown on lattice-mismatched foreign substrates that introduce undesirable dislocations and exacerbated thermal resistance. With the recent availability of 100-mm AlN substrates and AlN's high thermal conductivity (340 W/), XHEMTs show strong potential for next-generation RF electronics.

Aluminum scandium nitride (AlScN) is a promising barrier material for gallium nitride (GaN)-based transistors for the next generation of radio-frequency electronic devices. In this work, we examine the transport properties of two-dimensional electron gases (2DEGs) in single- and multi-channel AlScN/GaN heterostructures grown by molecular beam epitaxy and demonstrate the lowest sheet resistance among AlScN-based systems reported to date. Assorted schemes of GaN/AlN interlayers are first introduced in single-channel structures between AlScN and GaN to improve conductivity, increasing electron mobility up to 1370 cm2/Vs at 300 K and 4160 cm2/Vs at 77 K, reducing the sheet resistance down to 170 and 70 Ω/□, respectively. These improvements are then leveraged in multi-channel heterostructures, reaching sheet resistances of 65 Ω/□ for three channels and 45 Ω/□ for five channels at 300 K, further reduced to 21 and 13 Ω/□ at 2 K, respectively, confirming the presence of multiple 2DEGs. Structural characterization indicates pseudomorphic growth with smooth surfaces, while partial barrier relaxation and surface roughening are observed at high scandium content, with no impact on mobility. This first demonstration of ultra-low sheet resistance multi-channel AlScN/GaN heterostructures places AlScN on par with state-of-the-art multi-channel Al(In)N/GaN systems, showcasing its capacity to advance existing and enable new high-speed, high-power electronic devices.

Ultrawide bandgap (UWBG) AlGaN electronics are being developed for next-generation power conversion and wireless communication systems. However, these devices are prone to overheating due to the poor thermal conductivity (k) of AlGaN. In this work, a novel AlN/AlGaN/AlN-on-SiC platform with a thermal performance that exceeds that of today's GaN-on-SiC HEMTs is demonstrated. The device self-heating behavior was characterized via Raman thermometry and thermal design optimization was performed via 3D thermal modeling. The AlN/AlGaN/AlN-on-SiC high electron mobility transistor (HEMT) exhibits a similar to 20% lower channel temperature rise compared to today's GaN-on-SiC HEMTs. This was accomplished by minimizing the thickness of the AlGaN channel and employing a high k AlN buffer and SiC substrate. Thermal design rules presented in this work will facilitate the full exploitation of the electrical benefits offered by the UWBG semiconductor.

Gallium nitride (GaN) is used in solid-state lighting and in high-performance radio frequency and power electronics. However, due to inefficient hole doping and low hole mobility, quantum oscillations in p-type GaN have not been observed, which limits studies of valence bands and hole transport engineering. Here we report high hole mobilities in a polarization-induced two-dimensional hole gas at a gallium nitride/aluminium nitride interface. The holes degenerately occupy two valence bands of GaN-the light-hole and heavy-hole bands-and have mobilities of 2,000 cm2 V-1 s-1 and 400 cm2 V-1 s-1 at 2 K, respectively. We use Shubnikov-de Haas oscillations of holes from both valence bands to extract their respective sheet densities and quantum scattering times and the effective masses of light holes and heavy holes. The hole mobilities of our heterostructure highlight the possibility of developing cryogenic GaN complementary metal-oxide-semiconductor technology with potential applications in quantum computing control electronics.

Neuromorphic computing demands synaptic elements that store and update analogue weights with high precision while minimizing read disturbance. Conventional ferroelectric synapses typically encode weights in remanent polarization states and infer them from polarization-dependent electrical characteristics. Here, we demonstrate a ferroelectric MEMS (FeMEMS) synapse in which analog weights are stored in the effective piezoelectric coefficient d31,eff of a released unimorph beam with a 20 nm HZO layer. Partial domain switching modulates d31,eff, and a low-amplitude AC drive under subcoercive conditions converts the programmed state into beam displacement proportional to d31,effVac, realizing single-device analogue multiplication during readout. The switching-threshold distribution follows a Lorentzian form, and the median threshold obeys a Merz-type field-time law. Using this framework, we demonstrate ∼200 electromechanical weight levels. We further show representative retention and endurance, establishing a compact FeMEMS synaptic weight element for calibration-aware electromechanical multiplication in neuromorphic hardware.

Al1-xScxN (AlScN) has emerged as a promising ferroelectric material for next-generation memory and logic devices; yet its performance remains limited by poor understanding of nanoscale structural instabilities. Here, we uncover intrinsic short-range chemical ordering and strain-defect coupling in molecular-beam-epitaxy grown Al0.66Sc0.34N using a combination of atom probe tomography and transmission electron microscopy. The heterostructure exhibits wurtzite structure with a sharp and clean AlScN/AlN interface with negligible interdiffusion. Frequency distribution and radial distribution analyses reveal statistically significant Sc-Sc enrichment and Sc-N depletion, indicating nanoscale clustering accompanied by local nitrogen deficiency. Corresponding lattice-spacing variations observed by inverse FFT mapping confirm strain variation correlated with Sc-rich domains. These results provide the first atom-by-atom experimental evidence of scandium clustering and its coupling to nitrogen-vacancy formation in the ferroelectric composition regime. These findings identify short-range ordering and strain-defect coupling as intrinsic structural features of ferroelectric AlScN, with important implications for polarization switching behavior and reliability.

We report growth and electron transport of distributed polarization-doped (DPD) AlGaN channels on N-polar single-crystal AlN substrates by plasma-assisted molecular beam epitaxy, along with the initial field-effect transistors (FETs) demonstrated in these AlGaN channels. The AlGaN channels were realized by linearly grading Al-x Ga1-x N from binary AlN to terminal Al compositions of x = 0.07 , 0.17, and 0.50 over a thickness of 100 nm without any intentional doping, enabling the formation of polarization-induced three-dimensional electron gases (3DEGs). Reciprocal space mapping confirmed coherent growth on AlN substrates. Hall-effect measurements revealed systematic control of integrated electron sheet density from 3.7 & times; 10(13) cm(-2) ( x = 0.07 ) to 1.8 & times; 10(13) cm(-2 )( x = 0.50 ), and capacitance-voltage measurements confirmed the 3DEG spatial distribution. FETs fabricated on the structure with x = 0.50 exhibited effective field modulation of the 3DEG, with a maximum drain current density of 32 mA/mm, an on/off ratio exceeding 10 4 , and a breakdown voltage of 445 V at a gate-to-drain spacing of 2.5 mu m . Comparison between modeled and measured 3DEG mobilities suggests that the moderate mobility achieved can be attributed to alloy scattering and point defects in the high-Al composition ( x > 0.5 ) layers in these early demonstrations. Future investigations should focus on improving the high-Al composition epitaxy and transport in order for N-polar DPD-AlGaN FETs on bulk AlN to establish as a viable platform for high-power, ultrawide-bandgap electronics.

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.

Using molecular beam epitaxy, we have realized thin films of titanium nitride (TiN) on c-plane sapphire that exhibit the lowest observed full-width at half maximum X-ray rocking curve width of 18 arcsec. Though the (111) oriented TiN exhibits an abrupt and crystalline interface with sapphire, for the first time we observe sub-surface defects in the sapphire substrate, which nucleate structural defects in the epitaxial TiN layer. Using quarter-wavelength coplanar waveguide (CPW) resonators in a 3 µm/6 µm/3 µm gap/strip/gap lines in a hanger geometry, we find the internal quality factor of the TiN resonators to be >10^6 in the single-photon ⟨ n ⟩∼ 1 limit at 5.8 GHz and 10 mK, rising to >20 × 10^6 at ⟨ n ⟩∼ 10^6. The results are of high interest for applications of superconducting TiN in several areas, and provide a path towards epitaxial Josephson junctions with crystalline barriers in the future for high coherence qubits.

Self-activated growth of ScN was recently reported owing to the ability of scandium to self-catalyze N 2 bond breaking, facilitating nitride film growth from molecular N 2 without plasma. In this report, we expand this novel synthesis method to explore the possibility of self-activated growth of aluminum-scandium-nitride, an emerging ternary nitride system with enhanced functionalities. We find that the rock salt AlScN film growth of low Al composition from 0% to 48% is possible by self-activated growth. The cubic rock salt AlScN films are phase pure up to 21% Al, whereas mixed rock salt AlScN and intermetallic cubic Al x Sc phases appear in films with 28% to 48% Al. Self-activated nitride crystal growth was not observed for films grown with high Al content of 93.7% and 100% Al, which showed cubic Al x Sc and FCC alpha-Al phases, respectively, without any sign of nitrogen incorporation. Lattice parameters of crystalline rock salt AlScN films decrease with increasing Al composition. The self-activated AlScN films, as well as cubic Al x Sc and FCC alpha-Al phase, exhibit twinning and are epitaxial to the 6H-SiC substrate. The findings open up new avenues for the ultralow-energy synthesis of ternary nitrides and beyond.

Accurate characterization of the in-plane thermal conductivity (kappa in) of thin films that comprise a semiconductor device is of utmost importance to ensure sufficient heat spreading and device-level cooling. Time-domain thermoreflectance (TDTR) offers means to measure this property; however, measurement results for the kappa in of thin films using conventional approaches suffer from high uncertainty levels propagated from errors associated with the premeasured out-of-plane thermal conductivity (kappa out), thermal boundary conductance at the film/substrate interface (G2), thermal conductivity of the substrate (kappa s), and controlled parameters such as the thickness of the metal transducer deposited over the film for TDTR measurements (ht). In addition, as films get thinner, the thermal resistance of the film becomes comparable to that of G2, which renders analysis of the transient thermal response associated with the kappa in difficult. In this work, a dual-spot-size TDTR approach based on a ratio estimator was developed to facilitate measurements of the kappa in through existing sequential approaches by improving numerical conditioning and reducing the sources of error. The ratio of the TDTR signal obtained from a measurement using a small laser spot to that of one using a large laser spot is calculated and fitted to a thermal model, from which the kappa in can be determined with low error propagation from other parameters. This workflow was used to measure the in-plane thermal conductivity of aluminum nitride (AlN) thin films grown on silicon carbide (SiC) with thicknesses between 0.4 and 1.3 & micro;m at temperatures between 300 and 450 K. The uncertainty level of the measurement results showed a 2-fold improvement over those acquired using a conventional approach.

Power amplifiers (PAs) using HEMTs with aluminum nitride (AlN) buffer layers promise superior thermal performance over the current generation of GaN-buffer HEMT PAs. However, the continuous-wave (CW) output power densities from AlN-buffer HEMTs have been limited to <6 W/mm so far. This work presents the advancement in the large-signal RF performance of AlN-buffer HEMTs with a 2 & times; improvement in CW output powers. AlN-buffer HEMTs with 180 nm gate length and 2-m source-drain distance fabricated on 100 mm SiC substrates, show on-currents of 1.1 A/mm and breakdown voltage 100 V. On-wafer load-pull power sweep measurements conducted at 10 GHz demonstrated a maximum CW power output (P-out) of 5.5 W/mm at 28 V and 11.7 W/mm at 50 V bias. At 50 V, the peak PAE of 49% had an associated P-out 11.32 W/mm and linear gain of 9.35 dB. This significant power enhancement in AlN-buffer HEMT performance, coupled with its intrinsic thermal advantage over GaN-buffer HEMTs, paves the way for adoption in critical RF applications.

We report the molecular beam epitaxial growth and characterization of wurtzite InScN thin films. We find that wurtzite InScN can be grown on GaN substrates under metal-rich conditions, resulting in smooth surfaces with <1 nm roughness and a two-dimensional growth mode. We further find that InScN can be stabilized in the wurtzite crystalline phase for Sc content up to at least 10.4%. We measure and report the lattice parameters of wurtzite InScN as a function of Sc content and find that between similar to 5% and 8% Sc mole fraction InScN has a similar a lattice constant to InN. The c / a ratio deviates from the ideal tetrahedral value of 1.633 as Sc content increases. The optical absorption edge of wurtzite InScN monotonically increases over a Sc content range of 0%-10.4% from 0.8 to 1.05 eV, accompanied by a shift in the peak index of refraction and extinction coefficient. The E-2(high) Raman peak of InScN shows negligible shift with increasing Sc content compared to a gradual increase in the A 1 (LO) peak position. These fundamental observations can facilitate the future design of InN-based devices.

Ternary aluminum scandium nitride (AlScN) and aluminum yttrium nitride (AlYN) are emerging wide-bandgap piezoelectric and ferroelectric materials with significant potential for next-generation electronic devices. This work investigates the impact of post-growth annealing on the structural, electrical, and piezoelectric properties of AlScN and AlYN thin films grown by plasma-assisted by molecular beam epitaxy on tungsten electrodes. Annealing is performed in vacuum, air, and nitrogen atmospheres at temperatures ranging from 750 to 1100 °C. It is observed that annealing in air and nitrogen lowers the leakage current in AlScN and AlYN films. Piezoelectric coefficient (d33) enhancements as high as 137 pm/V for AlScN and 8.05 pm/V for AlYN are observed for less leaky, nitrogen-annealed samples at 750 °C. Preservation of ferroelectric switching properties post-annealing is confirmed by piezo-response force microscopy and PUND measurements. These findings highlight the role of thermal treatments on the performance of AlScN and AlYN for piezoelectric, ferroelectric, and high-K dielectric device applications.

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.

Gallium nitride (GaN) is a key semiconductor for optoelectronic and power electronic applications. However, achieving low-resistance Ohmic contacts to p-GaN remains challenging, particularly after plasma processing. Annealing of metallic magnesium (Mg) film on p-GaN has emerged as a promising approach, but severe surface roughening limits its applicability to thin (<100 nm) p-GaN layers widely used in practical devices. Here, we demonstrate a capless ultrathin (<10 nm) Mg deposition followed by a soft anneal (600 °C, 300 s) that forms a smooth, ultra-shallow, heavily acceptor-doped surface layer. The process maintains surface smoothness while achieving a low specific contact resistivity of (1–3) × 10−4 Ω cm2 at zero-bias with linear I–V characteristics, even on plasma-etched surfaces. Quasi-vertical p–i–n diodes fabricated using this approach exhibit low leakage currents and high breakdown voltages, indicating preserved junction integrity. This work provides a practical and scalable strategy for forming robust Ohmic contacts on thin p-GaN, with broad implications for GaN-based electronic and optoelectronic devices.