Room Temperature ZnO/Zn 0.8 Mg 0.2 O Resonant Tunneling Devices For Microwave Applications
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Room Temperature ZnO/Zn0.8Mg0.2O Resonant Tunneling Devices For Microwave Applications Aravind Inumpudi 1, Agis A. Iliadis 1, Supab Choopun 2, R. D. Vispute 2, and T. Venkatesan 2 1 Department of Electrical and Computer Engineering, University of Maryland, College Park, MD 20742 2 Center for Superconductivity Research, Department of Physics, University of Maryland, College Park, MD 20742 ABSTRACT A ZnO/Zn0.8Mg0.2O double barrier resonant tunneling device (DBRTD) is reported here for the first time. The structure consists of 6 nm ZnO quantum wells and 7 nm Zn0.8Mg0.2O barriers grown by pulsed laser deposition (PLD) on c-cut sapphire substrates. Negative differential resistance (NDR) peaks were obtained at room temperature. The structure is developed by using an indium-tin oxide (ITO) layer both as the back contact electrode and as an etch-stopping layer. The PLD growth quality, wet etching processing for developing the mesa structure, and the I-V characteristics of the device are reported. INTRODUCTION Double barrier resonant tunneling devices (DBRTD) show high potential applications in ultra-high frequency mixing and microwave-millimeter wave oscillation circuits, AD converters, multi-valued logic and others. Most of the RTD work has dealt with the AlGaAs/GaAs system, and recently Su et al [1] reported DBRTD’s using the AlInAsSb/InGaAs heterojunction system. Earlier work by Cong, Albrecht, Nathan and Ruden [2][3][4], demonstrated that external uniaxial stress applied along suitable orientations of AlAs/GaAs DBRTD’s, produced interfacial polarization charges due to the piezoelectric properties of AlAs, that affected resonant tunneling conditions and shifted the negative differential resistance (NDR) peaks. ZnO is a wide band gap (Eg=3.3 eV) metal-oxide semiconductor with well known piezoelectric [5] and optical properties in the UV range [6]. When Mg is introduced into the lattice, the ternary MgxZn1-xO combined with ZnO can provide a barrier/well heterostructure suitable for a resonant tunneling device. Furthermore, the important piezoelectric properties of this material system combined with the quantum confinement effects can provide the opportunity to enhance performance and allow tunability in RTD’s made in this system. Despite the promising character of this system for tunability and high frequency operation, a resonant tunneling device has not been demonstrated yet. One of the main requirements for the RTD to provide NDR peaks at room temperature, is the state of the heterointerfaces, which must be “smooth” enough for resonant tunneling to occur. This task can be difficult to achieve in this system, and in general in wide bandgap semiconductors like GaN, due to the lack of widely available suitable lattice matched substrates. For ZnO, c-cut (0001) sapphire is the most common substrate used, but the lattice mismatch is significant at approximately 16.8%, and ZnO buffer layers are necessary to achieve high quality growth [7]. Recently the epitaxial growth of this system on (0001) ScAlMgO4 (SCAM) latticematched su
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