Jimy Encomendero

We observe a 2D hole gas at room temperature in an as-grown, undoped 10 nm coherently strained GaN within an AlN/GaN/AlN heterostructure grown on bulk AlN by MBE. Analogous to the inversion channel in silicon MOSFETs, we grow n++ GaN source/drain regions and observe the p-GaN channel turns n-type under a positive gate voltage on a MOS gate overlapping the channel, achieving normally-off enhancement-mode n-channel transistor operation. We present the design principles to achieve E-mode AlN/GaN/AlN HEMTs. The structure can also be realized on silicon, SiC, or sapphire to exploit the ultrawide bandgap of AlN while significantly reducing gallium consumption.

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 ~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.

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 2.1×1018/cm3 mobile holes of mobility 4 cm2/V s and the Si-doped n-GaN has 3×1018/cm3 mobile electrons of mobility >100 cm2/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.

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.

AlYN and AlScN have recently emerged as promising nitride materials that can be integrated with GaN to form two-dimensional electron gases (2DEGs) at heterojunctions. Electron transport properties in these heterostructures have been enhanced through careful design and optimization of epitaxial growth conditions. In this work, we report for the first time Shubnikov-de Haas (SdH) oscillations of 2DEGs in AlYN/GaN and AlScN/GaN heterostructures, grown by metalorganic chemical vapor deposition. SdH oscillations provide direct access to key 2DEG parameters at the Fermi level: (1) carrier density, (2) electron effective mass (m*≈0.24 me for AlYN/GaN and m*≈0.25 me for AlScN/GaN), and (3) quantum scattering time (τq≈68 fs for AlYN/GaN and τq≈70 fs for AlScN/GaN). These measurements of fundamental transport properties provide critical insights for advancing emerging nitride semiconductors for future high-frequency and power electronics.

AlN/GaN/AlN high electron mobility transistors (HEMTs) have demonstrated exceptional potential for surpassing the electrical limitations of conventional AlGaN/GaN HEMTs. This study investigates the thermal performance of two types of AlN/GaN/AlN HEMTs with homoepitaxial AlN buffer layers grown on AlN substrates: an AlN/GaN/AlN single-crystal HEMT (AlN XHEMT) featuring a pseudomorphic/thin GaN channel and a conventional structure with a relaxed/thick GaN channel. Frequency- and time-domain thermoreflectance measurements reveal bulk-like thermal conductivity in the homoepitaxial AlN buffer layer, with negligible thermal boundary resistance at the AlN buffer/substrate interface. Consequently, Raman thermometry demonstrates that the AlN XHEMT with a thin (∼20 nm) pseudomorphically strained GaN channel exhibits better thermal performance than identical HEMT layer structures grown on a 4H-SiC substrate, despite 4H-SiC possessing a higher thermal conductivity. In addition, the AlN XHEMT exhibits a 22% lower channel temperature under 14 W/mm power density than the AlN/GaN/AlN-on-AlN HEMT that employs a thick (275 nm) relaxed GaN channel. These findings highlight that AlN XHEMTs offer not only electrical but also thermal advantages for high-power and high-frequency applications.

The development of large-wafer single-crystal AlN substrates has expanded their role beyond UV photonics to enable next-generation integrated electronics. In this study, we investigated δ-doped AlN/GaN/AlN heterostructures, where an n-type δ-doping layer is introduced to suppress the undesired two-dimensional hole gas at the bottom GaN/AlN interface, thereby enhancing the conductivity of the two-dimensional electron gas at the top AlN/GaN interface. We began by systematically identifying epitaxial growth conditions to achieve high crystalline quality, as confirmed by cross-sectional transmission electron microscopy images. To understand the impact of δ-doping density on transport properties, we combined theoretical modeling with experimental measurements, revealing that an optimal δ-doping density of ∼5×1013cm−2 minimizes interface roughness scattering and enhances mobility. Finally, we demonstrated scalability by extending the growth to large-area wafers, supported by structural and transport characterization. A sheet resistance of 246.8 ± 38.1 Ω/□ measured across a 3-in. (75 mm) wafer highlights the uniformity and performance potential of δ-doped AlN/GaN/AlN heterostructures for high-power, high-frequency electronic applications.

We report measurements and an improved analysis methodology to characterize the velocity-field characteristics of a polarization-induced two-dimensional hole gas in a GaN/AlN heterostructure at both room and cryogenic temperatures, using pulsed voltage and current through a micrometer-scale constriction. These high-bias measurements are made possible by Ohmic contacts that remain sufficiently transparent (<50 Ω mm above 10 mA/mm current) at cryogenic temperatures. We observe a room temperature saturation velocity of (2.1 ± 0.2)×106 cm/s and associated mobility of ∼7–12 cm2/V s for a hole density of 4×1013 cm−2, which increases to (4.4 ± 0.4)×106 cm/s at 4.2 K, with an associated mobility of ∼25–50 cm2/V s. The measured ensemble hole saturation velocity in this geometry, which is suitable for field-effect transistor channels, is lower than that of holes measured in lightly-doped n-type 3D bulk GaN (6.63 ×106 cm/s), owing to the 2D geometry and high carrier density of the two-dimensional hole gas, and degraded hole mobility from recess etching. Measured velocity-field contours are corroborated against bulk density functional theory and two-dimensional full-band real-space cellular Monte Carlo simulations under different surface boundary conditions.