Development of Thin Film and Nanorod ZnO-Based LEDs and Sensors
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Development of Thin Film and Nanorod ZnO-Based LEDs and Sensors S. J. Pearton1, L. C. Tien1, H S Kim1, D P Norton1, J. J. Chen2, H. T. Wang2, B. S. Kang2, F. Ren2, W. T. Lim1, J. Wright1, R. Khanna1, L. F. Voss1, L Stafford1, J. Jun3, and Jenshan Lin3 1 MSE, University of Florida, Gainesville, FL, 32611 2 Chemical Engineering, University of Florida, Gainesville, FL, 32611 3 ECE, University of Florida, Gainesville, FL, 32611
ABSTRACT The development of new etching and contact metallurgies for the ZnO/ZnMgO/ZnCdO materials system and various approaches for realizing ZnO LEDs are reviewed. ZnO nanorod MOSFETs and pH sensors have been demonstrated. In addition, selective detection of hydrogen with Pt-coated single ZnO nanorods is discussed discussed. The Pt-coated single nanorods show a current response approximately a factor of three larger at room temperature upon exposure to 500ppm H2 in N2 than thin films of ZnO. The power consumption of these sensors can be very small (in the nW range) when using discontinuous coatings of Pt. Once the Pt coating becomes continuous, the current required to operate the sensors increases to the µW range. The ZnO nanorods are insensitive to oxygen in the measurement ambient. INTRODUCTION ZnO is attracting attention for transparent electronics, sensors and UV light emitters. ZnO has the advantage of a relative low growth temperature which is suitable for deposition on cheap glass substrates and much higher excition binding energy (~ 60meV) than GaN (25meV). The excitons in the ZnO semiconductor will not dissociate into free electrons or holes due to the heat at room temperature or the scattering between the excitons. Additionally, commercial ZnO substrates are available. The ZnO system also has a simpler processing relative to GaN which cannot be wet-etched in conventional acid mixtures at safe temperatures. The curvature of conduction band and valence band for ZnO is smaller than GaN, meaning that the electron effective mass in ZnO will be larger. Some groups have published N-doped or P-doped ZnObased p-n junction or MIS LEDs[1-14]. Tsukazaki et al, studied ZnO p-i-n homojunction structure on (0001) ScAlMgO4 grown by laser MBE [2,3]. Lim et al. have made p-n homojunction ZnO LED on sapphire by r-f sputtering[5]. Jiao et al. have demonstrated ZnO p-n junction LED on a-plane Al2O3 substrate by plasma-assisted MBE [8]. However, much more work is needed on all aspects of ZnO device and materials technology. In this paper we describe recent progress in developing processes for improved ZnO LEDs and also on development of ZnO nanowire devices. RESULTS AND DISCUSSION (i) Contacts To improve ZnO LED device performance, the development of low resistance Ohmic contacts to ZnO is essential. The conventional contact formation involves the deposition of metal contact on ZnO followed by annealing at elevated temperatures. Some groups have found
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Specific contact resistance (Ohm-cm )
low contact resistivity for N-type and P-type ZnO on the order of 10-7 and 10-5 Ω-cm2, respectively
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