scispace - formally typeset
Search or ask a question
Author

Kevin D. Leedy

Bio: Kevin D. Leedy is an academic researcher from Air Force Research Laboratory. The author has contributed to research in topics: Thin film & Pulsed laser deposition. The author has an hindex of 31, co-authored 180 publications receiving 3443 citations. Previous affiliations of Kevin D. Leedy include University of Illinois at Urbana–Champaign & Air Force Institute of Technology.


Papers
More filters
Journal ArticleDOI
TL;DR: In this article, a Sn-doped (100) $\beta $ -Ga2O3 epitaxial layer was grown via metal-organic vapor phase epitaxy onto a single-crystal, Mg-Doped semi-insulating (100, β)-Ga 2O3 substrate.
Abstract: A Sn-doped (100) $\beta $ -Ga2O3 epitaxial layer was grown via metal–organic vapor phase epitaxy onto a single-crystal, Mg-doped semi-insulating (100) $\beta $ -Ga2O3 substrate. Ga2O3-based metal–oxide–semiconductor field-effect transistors with a 2- $\mu \text{m}$ gate length ( $L_{G})$ , 3.4- $\mu \text{m}$ source–drain spacing ( $L_{\textrm {SD}})$ , and 0.6- $\mu \text{m}$ gate–drain spacing ( $L_{\textrm {GD}})$ were fabricated and characterized. Devices were observed to hold a gate-to-drain voltage of 230 V in the OFF-state. The gate-to-drain electric field corresponds to 3.8 MV/cm, which is the highest reported for any transistor and surpassing bulk GaN and SiC theoretical limits. Further performance projections are made based on layout, process, and material optimizations to be considered in future iterations.

455 citations

Journal ArticleDOI
TL;DR: In this paper, a top-down BCl3 plasma etching on a native semi-insulating Mg-doped (100) β-Ga2O3 substrate was used to construct fin-array field effect transistors (finFETs).
Abstract: Sn-doped gallium oxide (Ga2O3) wrap-gate fin-array field-effect transistors (finFETs) were formed by top-down BCl3 plasma etching on a native semi-insulating Mg-doped (100) β-Ga2O3 substrate. The fin channels have a triangular cross-section and are approximately 300 nm wide and 200 nm tall. FinFETs, with 20 nm Al2O3 gate dielectric and ∼2 μm wrap-gate, demonstrate normally-off operation with a threshold voltage between 0 and +1 V during high-voltage operation. The ION/IOFF ratio is greater than 105 and is mainly limited by high on-resistance that can be significantly improved. At VG = 0, a finFET with 21 μm gate-drain spacing achieved a three-terminal breakdown voltage exceeding 600 V without a field-plate.

284 citations

Journal ArticleDOI
TL;DR: Preliminary results indicate potential for monolithic or heterogeneous integration of power switch and RF devices using inline-formula LaTeX, as well as power gain, efficiency, and power-added efficiency of 0.23 W/mm, 5.1 dB, and 6.3%.
Abstract: We demonstrate a $\beta $ -Ga2O3 MOSFET with record-high transconductance ( ${g}_{m}$ ) of 21 mS/mm and extrinsic cutoff frequency ( ${f}_{T}$ ) and maximum oscillating frequency ( ${f}_{\max }$ ) of 3.3 and 12.9 GHz, respectively, enabled by implementing a new highly doped ohmic cap layer with a sub-micron gate recess process. RF performance was further verified by CW Class-A power measurements with passive source and load tuning at 800 MHz, resulting in ${P}_{{OUT}}$ , power gain, and power-added efficiency of 0.23 W/mm, 5.1 dB, and 6.3%, respectively. These preliminary results indicate potential for monolithic or heterogeneous integration of power switch and RF devices using $\beta $ -Ga2O3.

236 citations

Journal ArticleDOI
TL;DR: In this article, a new theoretical formalism and several different experimental techniques were used to demonstrate that the addition of 1.4 \ifmmode\times\else\texttimes\fi{} 10${}^{21}$-cm${}^{\ensuremath{-}3}$ Ga donors in ZnO causes the lattice to form 1.7 \ifmodes\times \else\ texttimes\fa{}
Abstract: Self-compensation, the tendency of a crystal to lower its energy by forming point defects to counter the effects of a dopant, is here quantitatively proven. Based on a new theoretical formalism and several different experimental techniques, we demonstrate that the addition of 1.4 \ifmmode\times\else\texttimes\fi{} 10${}^{21}$-cm${}^{\ensuremath{-}3}$ Ga donors in ZnO causes the lattice to form 1.7 \ifmmode\times\else\texttimes\fi{} 10${}^{20}$-cm${}^{\ensuremath{-}3}$ Zn-vacancy acceptors. The calculated ${V}_{\mathrm{Zn}}$ formation energy of 0.2 eV is consistent with predictions from density functional theory. Our formalism is of general validity and can be used to investigate self-compensation in any degenerate semiconductor material.

163 citations

Journal ArticleDOI
TL;DR: In this paper, a Ge-doped Ga2O3 homoepitaxial material grown by molecular beam epitaxy on (010) Fe-Doped semi-insulating substrates was used for MOSFETs.
Abstract: We report on MOSFETs fabricated on Ge-doped $\beta $ -Ga2O3 homoepitaxial material grown by molecular beam epitaxy on (010) Fe-doped semi-insulating substrates. The Ge-doped channel devices performed similar to previously reported devices with Sn- and Si-doped channels with the drain current ON/OFF ratios of $> 10^{8}$ and the saturated drain current of >75 mA/mm at $V_{G}=0$ V. Hall effect measurements showed a high carrier mobility of 111 cm2/( $\text{V}\cdot \text{s}$ ) with $4\times 10^{17}$ cm $^{-3}$ active carriers. A MOSFET with a gate-drain spacing of $5.5~\mu \text{m}$ had a three-terminal breakdown voltage of 479 V.

141 citations


Cited by
More filters
Journal ArticleDOI

4,756 citations

Journal ArticleDOI
TL;DR: The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed in this article.
Abstract: Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (e) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

1,535 citations

Journal ArticleDOI
TL;DR: Puurunen et al. as discussed by the authors summarized the two-reactant ALD processes to grow inorganic materials developed to-date, updating the information of an earlier review on ALD.
Abstract: Atomic layer deposition (ALD) is gaining attention as a thin film deposition method, uniquely suitable for depositing uniform and conformal films on complex three-dimensional topographies. The deposition of a film of a given material by ALD relies on the successive, separated, and self-terminating gas–solid reactions of typically two gaseous reactants. Hundreds of ALD chemistries have been found for depositing a variety of materials during the past decades, mostly for inorganic materials but lately also for organic and inorganic–organic hybrid compounds. One factor that often dictates the properties of ALD films in actual applications is the crystallinity of the grown film: Is the material amorphous or, if it is crystalline, which phase(s) is (are) present. In this thematic review, we first describe the basics of ALD, summarize the two-reactant ALD processes to grow inorganic materials developed to-date, updating the information of an earlier review on ALD [R. L. Puurunen, J. Appl. Phys. 97, 121301 (2005)], and give an overview of the status of processing ternary compounds by ALD. We then proceed to analyze the published experimental data for information on the crystallinity and phase of inorganic materials deposited by ALD from different reactants at different temperatures. The data are collected for films in their as-deposited state and tabulated for easy reference. Case studies are presented to illustrate the effect of different process parameters on crystallinity for representative materials: aluminium oxide, zirconium oxide, zinc oxide, titanium nitride, zinc zulfide, and ruthenium. Finally, we discuss the general trends in the development of film crystallinity as function of ALD process parameters. The authors hope that this review will help newcomers to ALD to familiarize themselves with the complex world of crystalline ALD films and, at the same time, serve for the expert as a handbook-type reference source on ALD processes and film crystallinity.

1,160 citations

Journal ArticleDOI
TL;DR: 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.
Abstract: J. Y. Tsao,* S. Chowdhury, M. A. Hollis,* D. Jena, N. M. Johnson, K. A. Jones, R. J. Kaplar,* S. Rajan, C. G. Van de Walle, E. Bellotti, C. L. Chua, R. Collazo, M. E. Coltrin, J. A. Cooper, K. R. Evans, S. Graham, T. A. Grotjohn, E. R. Heller, M. Higashiwaki, M. S. Islam, P. W. Juodawlkis, M. A. Khan, A. D. Koehler, J. H. Leach, U. K. Mishra, R. J. Nemanich, R. C. N. Pilawa-Podgurski, J. B. Shealy, Z. Sitar, M. J. Tadjer, A. F. Witulski, M. Wraback, and J. A. Simmons

785 citations

Journal ArticleDOI
TL;DR: ZnO nanostructures have shown the binding of biomolecules in desired orientations with improved conformation and high biological activity, resulting in enhanced sensing characteristics, and compatibility with complementary metal oxide semiconductor technology for constructing integrated circuits makes ZnO Nanostructure suitable candidate for future small integrated biosensor devices.

512 citations