Electrical, Chemical, and Structural Characterization of Au Schottky Contacts on Remote Plasma-Treated n-Type ZnO[0001]

Abstract

In situ cleaning procedures for ZnO( ) and ZnO(0001) surfaces have been developed and their efficacy evaluated in terms of the removal of residual hydrocarbons and hydroxide and the crystallography, microstructure, and electronic structure of these surfaces. Annealing ZnO( ) in pure oxygen at (600-650°C)±20°C and 0.100±0.001 Torr for 30 min. reduced but did not eliminate all detectable hydrocarbon contamination. However, annealing under similar conditions at 700°C±20°C for 15 min. caused desorption of both the hydrocarbons and the hydroxide constituents to concentrations below the detection limits of X-ray photoelectron spectroscopy. However, thermal decomposition degraded the surface microstructure. Exposure of the ZnO( ) surface to a remote plasma containing an optimized 20% O2/80% He mixture for the optimized time, temperature, and pressure of 30 min, 525 °C, and 0.050 Torr resulted in smooth, highly-ordered, atomically-stepped and stoichiometric surfaces with a nearly flat electronic band structure. A 0.5 eV change in band bending was attributed to the significant reduction of the accumulation layer associated with surface contamination. The hydrocarbons were undetectable and only ~0.4±0.1 ML of hydroxide remained. The latter appeared to be more tightly bound to the ZnO(0001) surface due to unfilled dangling bonds associated with the polarity of this surface. This effect increased the optimal time and temperature of the plasma cleaning process to 60 min. and 550 °C, respectively, at 0.050 Torr. Approximately 0.4±0.1 ML of hydroxide also remained on this surface.

Current-voltage results of Au contacts deposited on as-received, n-type ZnO(0001) surfaces showed an ~0.01 A/cm2 leakage current density to 4.6 V reverse bias and ideality factors (n) >2 before sharp, permanent breakdown. This behavior was due primarily to the presence of hydroxide on this surface, which typically increases the surface conductivity and forms an accumulation layer. Cooling in the plasma ambient caused the chemisorption of oxygen and the formation of a depletion layer of lower surface conductivity. These process steps produced smooth, highly-ordered, stoichiometric ZnO(0001) surfaces that possessed 0.3 eV of upward band bending. Sequentially deposited, unpatterned Au and the most rectifying gold contacts initially grew on these surfaces via the formation of islands that subsequently coalesced. The latter displayed a barrier height of 0.71±0.05 eV, a saturation current density of 4.04 µA/cm2, a value of n = 1.17±0.05, a significantly lower leakage current density of ~0.1 mA/cm2 to 8.5 V reverse bias, and a sharp, permanent breakdown at ~ –8.75 V. All measured barrier heights were lower than the predicted Schottky-Mott value of 1.0 eV, indicating that the interface structure and the associated interface states affect the Schottky barrier. However, the constancy in the FWHM of the core levels for Zn 2p (1.9±0.1 eV) and O 1s (1.5±0.1 eV), before and after sequential in situ Au depositions indicated an abrupt, unreacted Au/ZnO(0001) interface.

Similar results were obtained for Schottky contacts deposited on remote plasma treated ZnO( ) surfaces, which possessed a step and terrace microstructure. Analysis of transmission electron microscopy results revealed the growth of epitaxial, single crystal Au forming an abrupt, unreacted interface.