Paper EM-ThM3
Correlation of Native Point Defects to Thermal Stability of Schottky Barrier Formation at Metal-ZnO Interfaces

Thursday, October 18, 2007, 8:40 am, Room 612

ZnO is an important semiconductor system for spintronic, nanoelectronic and optoelectronic devices. Important to realization of these devices is control and thermal stability of the metal-ZnO interface. We investigated this interface on bulk single crystal substrates grown by different methods from 5 different vendors. Using a remote oxygen plasma to remove ZnO surface adsorbates, subsurface defects and hydrogen, we studied Al, Au, Ir, Ni, Mo, Pd, Pt, and Ta contacts. Depth-resolved cathodoluminescence spectroscopy (DRCLS) reveals the presence of 3 defects at energies of 2.1, 2.5 and 3.0 eV. These deep level states vary in concentration with vendor, with depth from the interface, and with metal contact. Current-voltage measurements show that material containing high concentrations of defects in the subsurface strongly affects reverse currents, idealities and barrier heights acquired from current-voltage measurements. After annealing these contacts at temperatures of 350ºC, 450 ºC, 550 ºC, and 650 ºC in an argon ambient, DRCLS spectra identify defect formation that correlates to the nature of the metal-ZnO interface. Metals that form oxides show increased deep-level emissions that have been attributed to oxygen vacancies, while metals that form eutectics with Zn reveal increased luminescence from defects associated with Zn vacancies. Ta contacts annealed at 550 ºC create blocking contacts to ZnO, and DRCLS in the interface region reveal the formation of a Ta oxide. Al contacts also form blocking contacts at temperatures that depend on the native point defect densities. DRCLS of the subsurface oxide interface reveal increases in a 2.5eV transition often associated with oxygen vacancies. Au contacts that are annealed above the eutectic temperature for Au-Zn exhibit an increase of the 2.1eV defect level that correlates to Zn vacancies. Elevated temperature results demonstrate that the thermal stability of Schottky barriers also correlates to the density of native point defects. These differences in native point defect densities have a significant impact on defect formation at both elevated and room temperatures. Samples with high native defect concentrations initially can increase reactions in the subsurface, thus creating more defects associated with the metal-ZnO surface chemistry. Overall we find that metal-ZnO chemical reactions introduce interface native defects.These and native bulk defects dominate Schottky barrier properties and thermal stability.