Effect of Cryogenic Temperature Deposition of Various Metal Contacts to Bulk, Single-Crystal n-type ZnO
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0957-K09-02
Effect of Cryogenic Temperature Deposition of Various Metal Contacts to Bulk, SingleCrystal n-type ZnO Jon Wright1, L. Stafford1, B. P. Gila1, D. P. Norton1, S. J. Pearton1, Hung-Ta Wang2, and F. Ren2 1 Materials Science & Engineering, University of Florida, Gainesville, FL, 32611 2 Chemical Engineering, University of Florida, Gainesville, FL, 32611
ABSTRACT The effect of cryogenic temperatures during metal deposition on the contact properties of Pd, Pt, Ti, and Ni on bulk single-crystal n-type ZnO has been investigated. Deposition at both room and low temperature produced contacts with Ohmic characteristics for Ti and Ni metallizations. By sharp contrast, both Pd and Pt contacts showed rectifying characteristics after deposition with barrier heights between 0.37-0.69 eV. Changes in contact behavior were measured on Pd to anneal temperatures of ~300˚C, showing an increase in barrier height along with a decrease in ideality factor with increasing annealing temperature. This difference with annealing temperature is in sharp contrast to previous results for Au contacts to ZnO. There were no differences in near-surface stoichiometry for the different deposition temperatures; however low temperature contacts demonstrated some peeling/cracking for Pt and Pd. INTRODUCTION The development of reliable and thermally stable Ohmic and Schottky contacts to ZnO is one of the critical issues related to the fabrication of ZnO-based UV light emitters/detectors and field effect transistors. To date, a number of different metallization schemes have been examined for Ohmic and rectifying contacts on n-ZnO [1-20]. Metals such as Au, Ag and Pd form rectifying contacts with low Schottky barrier heights in the 0.6-0.8 eV range [6-13] and in addition, the thermal stability of these contacts is usually extremely poor, with degradation occurring even at 60°C for Au/n-ZnO [14-19]. The trend in barrier heights often does not correlate with the metal work functions, indicating that surface states or surface contamination is playing an important role in determining the electrical transport properties of the contacts [7-10, 16-19].
One approach to achieving increased Schottky barrier heights that has proven successful for other semiconductors such as GaAs and InP is the use of cryogenic deposition temperatures [20-24]. The mechanism for the barrier height enhancement is still not firmly established. In the case of Au contacts on InP [24], room temperature deposition produced an ideality factor of 1.02 which was nearly independent of temperature and the current transport was controlled by thermionic emission (TE). In the case of 77K deposition of the Au, the ideality factor was increased and the current transport was controlled by thermionic field emission (TFE) [24]. The barrier height enhancement and the difference in transport mechanism were both attributed to the formation of an amorphous layer at the metal/semiconductor interface. This amorphous layer was suggested to act as an insulator to create a metal-insulator-semiconducto
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