Structural, microstructural, and electrical properties of gold films and Schottky contacts on remote plasma-cleaned, n-type ZnO{0001} surfaces

Abstract

Current–voltage measurements of Au contacts deposited on ex situ cleaned, n-type ZnO(0001) [(0001¯)] surfaces showed reverse bias leakage current densities of ∼0.01(∼0.1)A∕cm2 at 4.6 (3.75) V reverse bias and ideality factors >2 (both surfaces) before sharp, permanent breakdown (soft breakdown). This behavior was due primarily to the presence of (1.6–2.0)±0.1[(0.7–2.6)±0.1] monolayers (ML) of hydroxide, which forms an electron accumulation layer and increases the surface conductivity. In situ remote plasma cleaning of the (0001) [(0001¯)] surfaces using a 20vol%O2∕80vol%He mixture for the optimized temperatures, times, and pressure of 550±20°C(525±20°C), 60 (30) min, and 0.050 Torr reduced the thickness of the hydroxide layer to ∼0.4±0.1ML and completely eliminated all detectable hydrocarbon contamination. Subsequent cooling of both surfaces in the plasma ambient resulted in the chemisorption of oxygen and a change from 0.2 eV of downward band bending for samples cooled in vacuum to 0.3 eV of upward band bending indicative of the formation of a depletion layer of lower surface conductivity. Cooling in either ambient produced stoichiometric ZnO{0001} surfaces having an ordered crystallography as well as a step-and-terrace microstructure on the (0001¯) surface; the (0001) surface was without distinctive features. Sequentially deposited, unpatterned Au films, and presumably the rectifying gold contacts, initially grew on both surfaces cooled in the plasma ambient via the formation of islands that subsequently coalesced, as indicated by calculations from x-ray photoelectron spectroscopy data and confirmed by transmission electron microscopy. Calculations from the current–voltage data of the best contacts revealed barrier heights on the (0001) [(0001¯)] surfaces of 0.71±0.05(0.60±0.05)eV, a saturation current density of (4±0.5)×10−6A∕cm2(2.0±0.5×10−4A∕cm2), a lower value of n=1.17±0.05(1.03±0.05), a significantly lower leakage current density of ∼1.0×10−4A∕cm2(∼91×10−9A∕cm2) at 8.5 (7.0) V reverse bias prior to sharp, permanent breakdown (soft breakdown). 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 full width at half maximum of the core levels for Zn 2p(1.9±0.1eV) and O 1s(1.5±0.1eV), before and after sequential in situ Au depositions, indicated an abrupt, unreacted Au∕ZnO(0001) interface. Transmission electron microscopy confirmed the abruptness of an epitaxial interface. Annealing the contacts on the (0001) surface to 80±5 and 150±5°C resulted in decreases in the ideality factors to 1.12±0.05 and 1.09±0.05 and increases in saturation current density to 9.05 and 4.34μA∕cm2, the barrier height to 0.82±0.5 and 0.79±0.5eV, and in the leakage current densities to ∼2×10−3A∕cm2 at 6 V and ∼20×10−3A∕cm2 at 7 V, respectively.