Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session B4: Advances in ZnO Materials Physics and Applications |
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Sponsoring Units: DMP FIAP Chair: Tom Myers, West Virginia University Room: Baltimore Convention Center 308 |
Monday, March 13, 2006 11:15AM - 11:51AM |
B4.00001: Theory of defects and doping in ZnO Invited Speaker: In spite of rapid progress in materials quality, zinc oxide still suffers from serious problems in controlling its conductivity. We are addressing these issues by performing first-principles calculations based on density-functional theory (DFT) in the local density approximation (LDA). In addition, we have developed an approach (based on “LDA+U”) for overcoming the DFT band-gap problem, allowing us to more accurately compare and predict defect levels. Native point defects are still frequently invoked as sources of n-type conductivity, but our results do not support this hypothesis. Oxygen vacancies are deep donors; our configuration coordinate diagrams for this defect provide a detailed interpretation of the recent ODEPR results of Vlasenko and Watkins [1]. Zinc interstitials are shallow donors, but their formation energy is high and they diffuse very rapidly, with a migration barrier as low as 0.6 eV, making it very unlikely that they would be stable. For the zinc antisite, we find an unexpected low-symmetry configuration. While still high in energy, this defect may play a role under non-equilibrium conditions such as irradiation. We suggest that unintentional incorporation of impurities (such as hydrogen) is a more likely explanation for n-type background doping. However, native defects play a crucial role as compensating centers in p-type material, and we will discuss ways of overcoming this problem. Among the native defects that act as acceptors, zinc vacancies have the lowest formation energy; they introduce deep levels likely responsible for green luminescence. Our calculated migration barriers for the point defects are in good agreement with experimental data where available, and provide insight in the processes that take place during growth, irradiation, or annealing. \par [1] L. S. Vlasenko and G. D. Watkins, Phys. Rev. B 71, 125210 (2005). [Preview Abstract] |
Monday, March 13, 2006 11:51AM - 12:27PM |
B4.00002: Key Materials Aspects for Valence Control of ZnO. Invited Speaker: ZnO has significant advantages for light emitting diodes (LEDs) and lasers from the following reasons; 1) exciton binding energy in ZnO is 60 meV and can be enhanced over 100 meV in superlattices, 2) it is possible to tune the bandgap from 3 eV to 4.5 eV in Zn$_{1-x}$Cd$_{x}$O and Mg$_{x}$Zn$_{1-x}$O alloy films having quite small lattice mismatch, and 3) large and high-quality single-crystal wafers are commercially available. In order to harvest these advantages in real devices, reliable technique for fabricating p-type ZnO has to be properly established. Recently we have reported on the improvements of undoped ZnO film quality with inserting a ZnO self-buffer layer onto lattice matched ScAlMgO$_{4}$ substrate [1]. In view of point defect formation during the epitaxy, we have carefully optimized the growth conditions. We selected nitrogen as an acceptor, because the ionic radius is close to that of oxygen. Here we propose a repeated temperature modulation (RTM) technique for efficient nitrogen doping into ZnO with keeping high crystallinity [2]. By carefully optimizing the conditions, p-type ZnO with a hole concentration of 10$^{16}$ - 10$^{17}$ cm$^{-3}$ can be reproducibly fabricated. We also demonstrated blue electroluminescence from p-i-n homojunction LED [3]. The details of thin film growth, characteristics of p-type ZnO and device performance will be presented. \newline \newline [1] A. Tsukazaki et al. \textit{Nature Mater}. \textbf{4}, 42 (2005). \newline [2] A. Tsukazaki et al. \textit{Appl. Phys. Lett.}\textbf{83}, 2784 (2003). \newline [3]A. Tsukazaki et al. \textit{Jpn. J. Appl. Phys.Lett.}44, L643 (2005). [Preview Abstract] |
Monday, March 13, 2006 12:27PM - 1:03PM |
B4.00003: Localized States and Charge Transfer at ZnO Surfaces and Interfaces Invited Speaker: With the advent of techniques to probe semiconductor electronic properties in the near-interface region on a nanometer scale, it is now possible to understand and control the mechanisms playing a role in localized state formation and charge transfer at ZnO interfaces. While world-wide research activity into this major new semiconductor has increased dramatically, the ability to control ZnO interfaces has been a major challenge to their opto- and microelectronic applications. Nanoscale depth-resolved cathodoluminescence and x-ray photoemission spectroscopies reveal the segregation of point defects and the donor character of hydrogen in the near-surface region. A conversion from ohmic to rectifying behavior is observed for gold contacts on atomically ordered polar ZnO surfaces following remote oxygen plasma treatment. This transition is accompanied by reduction of the well-known ``green'' deep level emission, suppression of the hydrogen donor-bound exciton photoluminescence and a large increase in n-type band bending. These results demonstrate that the contact type conversion involves more than one mechanism, specifically, removal of the adsorbate-induced accumulation layer plus lowered tunneling due to reduction of near-surface donor density and defect-assisted hopping transport. Schottky barriers for a wide array of metals on ZnO reveal that the strength of interface reaction plays a dominant role in forming near-interface defects and the resultant Schottky barriers. Similar correlations for other compound semiconductors indicate that the impact of near-interface native defects on Schottky barriers is more general in nature. \newline \newline [1] H.L. Mosbacker, Y.M. Strzhemechny, B.D. White, P.E. Smith, D.C. Look, D.C. Reynolds, C.W. Litton, and L.J. Brillson, ``Role of Near-Surface States in Ohmic-Schottky Conversion of Au Contacts to ZnO,'' Appl. Phys. Lett. 87, 012102 (2005). \newline [2] Y.M. Strzhemechny, H.L. Mosbacker, D.C. Look, D.C. Reynolds, C.W. Litton, N.Y.Garces, NC. Giles, L.E. Halliburton, S. Niki, and L.J. Brillson, ``Remote Hydrogen Plasma Doping of Single Crystal ZnO,'' Appl. Phys. Lett. 84, 2545 (2004). [Preview Abstract] |
Monday, March 13, 2006 1:03PM - 1:39PM |
B4.00004: Growth, Assembly, and Characterization of ZnO Nanostructures on Ag Films Invited Speaker: In the past decade, significant advances have been made in the synthesis of ZnO nanostructures. The next step in making these nanomaterials useful is to assemble them on surfaces in a controlled and desired fashion. In this talk, I will discuss the growth of complex ZnO nanostructures via a solution method in which organic templates are used to control assembly of these nanostructures on substrate surfaces. The low temperature aqueous growth method used in this work is an environmentally benign process, which is compatible with organic templates and modifiers, can be used to grow large areas uniformly, and has potential for inexpensive manufacturing. To control the assembly of these solution grown nanostructures, we modify the substrate surfaces with patterned self-assembled monolayers, which in turn determines the final spatial organization of the ZnO nanorods. This is a bottom-up approach in which materials are deposited only where they are needed. Using this approach, we have achieved excellent control in spatial placement, selectivity, crystal orientation, and nucleation density. In addition, complex, hierarchical structures have been synthesized by controlling solution chemistry and growth conditions. Due to lack of inversion symmetry in hexagonal crystal, ZnO is a piezoelectric material with Zn (0001) polar and O (000$\bar {1})$ polar surfaces exhibiting drastically different physical and chemical properties. Hence, it is important to determine the orientation of the ZnO nanorods on surfaces. Using piezoelectric force microscopy (PFM) and a well-characterized ZnO single crystal reference, we have measured the amplitude and phase of piezoelectric responses of over 100 individual ZnO rods. The phase of the PFM signal is 180\r{ } from the applied electric field, indicating that the nanorods are [0001] oriented. This result contradicts what would have been expected based on an examination of rod morphology. Also, the PFM amplitude of the nanorods was found to be significantly larger than that of the ZnO single crystal. The origin behind this observation and the variation among different nanorods will be discussed. [Preview Abstract] |
Monday, March 13, 2006 1:39PM - 2:15PM |
B4.00005: Charge- and Spin-Based Devices in ZnO Thin Films and Nanostructures Invited Speaker: ZnO is a wide bandgap semiconductor of potential for device concepts based on charge and/or electron spin. As a direct bandgap material with emission in the ultraviolet, ZnO is being actively pursued in the areas of ultraviolet light emitting diodes and laser diodes. The critical issue in developing such optoelectronic devices is p-type doping. As a dilute magnetic semiconductor, numerous experimental reports indicate ferromagnetism in transition metal doped ZnO. However, the mechanism for this magnetic behavior continues to be a topic of debate. In addition, numerous techniques have been utilized to synthesize ZnO nanoscale structures, many of which appear to be useful for sensors and nanoelectronics. In this talk, pertinent issues for spin and charge-based ZnO devices will be discussed. The focus will be on p-type doping, pn junction formation, magnetic doping, and nanowire-based sensor development. This work is supported by the National Science Foundation (DMR-029086), the Department of Energy (DE-FC26-04NT42271), and the Air Force Office of Scientific Research (030967). [Preview Abstract] |
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