Publications

ULTRA Publications

Ultrawide-bandgap (UWBG) semiconductors, with bandgaps significantly wider than the 3.4 eV of GaN, represent an exciting and challenging new area of research in semiconductor materials, physics, devices, and applications. Because many figures-of-merit for device performance scale nonlinearly with bandgap, these semiconductors have long been known to have compelling potential advantages over their narrower-bandgap cousins in high-power and RF electronics, as well as in deep-UV optoelectronics, quantum information, and extreme-environment applications. Only recently, however, have the UWBG semiconductor materials, such as high Al-content AlGaN, diamond and Ga2O3, advanced in maturity to the point where realizing some of their tantalizing advantages is a relatively near-term possibility. In this article, the materials, physics, device and application research opportunities and challenges for advancing their state of the art are surveyed.

2021 || Applied Physics Letters || 1

MBE growth and donor doping of coherent ultrawide bandgap AlGaN alloy layers on single-crystal AlN substrates

Single-crystal  Aluminum Nitride (AlN) crystals enable the epitaxial growth of ultrawide bandgap Al(Ga)N alloys with drastically lowe extended defect densities. Here, we report the plasma-MBE growth conditions for high Al-composition AlGaN alloys on single-crystal AlN substrates. An AlGaN growth guideline map is developed, leading to pseudomorphic AlₓGa₁₋ₓN epitaxial layers with x  ~0.6–1.0 Al contents at a growth rate of  ~0.3 lm/h. These epitaxial  layers exhibit atomic steps, indicating step flow epitaxial growth, and room-temperature band edge emission from ~4.5 to 5.9 eV. Growth conditions are identified in which the background impurity concentrations of O, C, Si, and H in the MBE layers are found to be very near or below detection limits. An interesting Si segregation and gettering behavior is observed at the epitaxial AlGaN/AlN  heterojunction  with significant  implications for the formation and transport of 2D electron or hole  gases.  Well-controlled intentional Si doping ranging from ~2 x 10¹⁷ to 3 x 10¹⁹ atoms/cm³ is obtained, with sharp dopant density transition profiles. In Si-doped Al₀.₆Ga₀.₄N epilayers, a room-temperature free electron concentration of  ~3 x 10¹⁹/cm³, an electron mobility of ~27 cm²/V s, and an n-type resistivity of ~7.5 m cm are obtained. The implications of these findings on electronic and photonic devices on single-crystal AlN substrates are discussed.
 

2021 || Applied Physics Letters || 2

The impact of interfacial Si contamination on GaN-on-GaN regrowth for high power vertical devices

The development of gallium nitride (GaN) power devices requires a reliable selective-area doping process, which is difficult to achieve because of ongoing challenges associated with the required etch-then-regrow process. The presence of silicon (Si) impurities of unclear physical origin at the GaN regrowth interface has proven to be a major bottleneck. This paper investigates the origin of Si contamination at the epitaxial GaN-on-GaN interface and demonstrates an approach that markedly reduces its impact on device performance. An optimized dry-etching approach combined with UV-ozone and chemical etching is shown to greatly reduce the Si concentration levels at the regrowth interface, and a significant improvement in a reverse leakage current in vertical GaN-based p–n diodes is achieved.

2021 || Applied Physics Letters || 3

Noncured Graphene Thermal Interface Materials for High-Power Electronics: Minimizing the Thermal Contact Resistance

We report on experimental investigation of thermal contact resistance, RC, of the noncuring graphene thermal interface materials with the surfaces characterized by different degree of roughness, Sq. It is found that the thermal contact resistance depends on the graphene loading, ξ, non-monotonically, achieving its minimum at the loading fraction of ξ ~15 wt %. Decreasing the surface roughness by Sq~1 μm results in approximately the factor of ×2 decrease in the thermal contact resistance for this graphene loading. The obtained dependences of the thermal conductivity, KTIM, thermal contact resistance, RC, and the total thermal resistance of the thermal interface material layer on ξ and Sq can be utilized for optimization of the loading fraction of graphene for specific materials and roughness of the connecting surfaces. Our results are important for the thermal management of high-power-density electronics implemented with diamond and other wide-band-gap semiconductors

2021 || Nanomaterials || 4

Carbon complexes in highly C-doped GaN

We investigate the properties of heavily C-doped GaN grown by hydride vapor phase epitaxy using both optical experiments and hybrid density functional theory calculations. Previous work has established that carbon acceptors (CN) give rise to a yellow luminescence band near 2.2 eV along with a blue luminescence band near 2.9 eV. Photoluminescence measurements show the yellow band shifting as a function of carbon concentration, suggesting a change in the behavior of carbon species as carbon content increases. With hybrid density functional theory we calculate the electrical and optical behavior of carbon centers containing multiple carbon impurities, which may arise in heavily doped material. We compare the behavior of these complexes to the isolated centers, and find that the dicarbon donor-acceptor (CGa−CN) complex is a candidate to explain the shift in the yellow luminescence peak. Tricarbon complexes have high formation energies and modest binding energies, and also give rise to optical transitions that are inconsistent with the observed spectra. We also identify the split dicarbon interstitial on the gallium site as a low-energy species with a large binding energy that may act to compensate carbon acceptors. Local vibrational modes are calculated for carbon impurity centers, and we compare these results to recent experiments. Dicarbon and tricarbon complexes involving CGa and CN exhibit modes that are only slightly higher than the isolated species, while carbon interstitials and related complexes give rise to vibrational modes much higher than CGa and CN.

2021 || Physical Review B || 5

Electrical and Thermal Performance of Ga₂O₃–Al₂O₃–Diamond Super-Junction Schottky Barrier Diodes

The design space of Ga 2 O 3 -based devices is severely constrained due to its low thermal conductivity and absence of viable p-type dopants. In this work, we discuss the limits of operation of a novel Ga 2 O 3 –Al 2 O 3 –diamond-based super-junction device concept, which can alleviate the constraints associated with Ga 2 O 3 -based devices. The improvements achieved using the proposed device concept are demonstrated through electrical and thermal simulations of Ga 2 O 3 –Al 2 O 3 –diamond-based super-junction Schottky barrier diodes (SJ-SBDs) and non-punch-through or conventional Schottky barrier diodes (NP-SBDs). The SJ-SBD enables operation below the RON -breakdown voltage limit of Ga 2 O 3 NP-SBD, enabling >4 kV blocking voltage at RON of 1–3 mΩ cm 2 . The maximum switching frequency of SJ-SBD may be only a few kHz, as it is limited by the activation energy of acceptors (0.39 eV) in the diamond. Crucially, compared with NP-SBD, the use of diamond also results in ~60% reduction in temperature rise during static power dissipation. Polycrystalline diamond (PCD) properties depend on detailed microstructure and benefits compared to ideal Ga 2 O 3 NP-SBD arise for diamond critical fields ≥6 MV/cm and thermal conductivities as low as 50–150 W/(m ⋅ K).

2021 || IEEE Transactions on Electron Devices || 6

Reviews, Reports, and General Reference Publications

Ultrawide-bandgap (UWBG) semiconductors, with bandgaps significantly wider than the 3.4 eV of GaN, represent an exciting and challenging new area of research in semiconductor materials, physics, devices, and applications. Because many figures-of-merit for device performance scale nonlinearly with bandgap, these semiconductors have long been known to have compelling potential advantages over their narrower-bandgap cousins in high-power and RF electronics, as well as in deep-UV optoelectronics, quantum information, and extreme-environment applications. Only recently, however, have the UWBG semiconductor materials, such as high Al-content AlGaN, diamond and Ga2O3, advanced in maturity to the point where realizing some of their tantalizing advantages is a relatively near-term possibility. In this article, the materials, physics, device and application research opportunities and challenges for advancing their state of the art are surveyed.

2017 || WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim || 1

The cost of computing has declined approximately eight orders of magnitude over the past fifty years, enabling the digital revolution that has influenced almost every facet of human life, including energy, public health, national security, and business. All of science and engineering now depends upon advanced computing, from the design of molecules to the furthering of our understanding of the universe. This remarkable progress has been driven by the miniaturization of integrated circuit technology. The transistors on computer chips, based on the element silicon, have roughly halved in size every two years (this is known as Moore’s Law), becoming faster, smaller, more energy efficient, and cheaper over the past five decades. Throughout these technological advances, the computing approach, the von Neumann model, has largely remained unchanged since its introduction 75 years ago. This computing model consists of an instruction and processing unit connected to a memory that contains both instructions and data. The delay in moving instructions and data to and from the memory for processing is called the “von Neumann bottleneck” and is increasingly a limit on the performance of computer systems for both scientific simulation and data intensive computing. This combination of silicon microelectronics technology and the von Neumann computing model forms the basis for almost all computing appliances in the world today.

2018 || U.S. Department of Energy || 2