Role of near-surface states in ohmic-Schottky conversion of Au contacts to ZnO

Abstract

A conversion from ohmic to rectifying behavior is observed for Au contacts on atomically ordered polar ZnO surfaces following remote, room-temperature oxygen plasma treatment. This transition is accompanied by reduction of the “green” deep level cathodoluminescence emission, suppression of the hydrogen donor-bound exciton photoluminescence and a ∼0.75eV increase in n-type band bending observed via x-ray photoemission. These results demonstrate that the contact type conversion involves more than one mechanism, specifically, removal of the adsorbate-induced accumulation layer plus lowered tunneling due to reduction of near-surface donor density and defect-assisted hopping transport.

Interface trap characterization and electrical properties of Au-ZnO nanorod Schottky diodes by conductance and capacitance methods

“…14,15 Schottky barrier heights for bulk ZnO have been reported in the range of 0.6-0.8 eV. 2,4 In low-dimensional systems, the Schottky barrier height depends not only on the work function of the metal but also on the pinning of the Fermi level by the surface states, image force lowering of the barrier, field penetration and the existence of an interfacial insulating layer; these effects change the absolute current value at low bias values by lowering the Schottky barrier. 16 Schottky devices can be used to evaluate the different semiconductor parameters, including the carrier density, the Schottky barrier height, the carrier density profile, and the band gap discontinuity.…”

“…[4][5][6][7] The realization of high quality Schottky contacts on ZnO nanostructures seems to be difficult because of the interface states, the surface morphology, hydroxide surface contamination, and the subsurface defects, which all play important roles in the electrical properties of these contacts. 4 In recent years, a number of process methodologies have been developed for the fabrication of reproducible high quality Schottky contacts on ZnO nanostructures, but controversies remain with regard to the Schottky barrier height and the ideality factor of the ZnO Schottky contacts. 2,[8][9][10][11] The deviations in the barrier heights and the ideality factor have been proposed as having been caused by the effects of asymmetric contacts, and the influence of the interfacial layers and/or surface states.…”

“…3 Although considerable progress has been made in the fabrication of ZnO nanostructure-based Schottky diodes, many questions remain about the nature of the electrical transport and the interface states at the metal-ZnO interface. 4 A stable and good quality rectifying metal contact on the n-ZnO surface is crucial for many optoelectronic applications and remains a challenge despite numerous recent investigations. [4][5][6][7] The realization of high quality Schottky contacts on ZnO nanostructures seems to be difficult because of the interface states, the surface morphology, hydroxide surface contamination, and the subsurface defects, which all play important roles in the electrical properties of these contacts.…”

“…4 A stable and good quality rectifying metal contact on the n-ZnO surface is crucial for many optoelectronic applications and remains a challenge despite numerous recent investigations. [4][5][6][7] The realization of high quality Schottky contacts on ZnO nanostructures seems to be difficult because of the interface states, the surface morphology, hydroxide surface contamination, and the subsurface defects, which all play important roles in the electrical properties of these contacts. 4 In recent years, a number of process methodologies have been developed for the fabrication of reproducible high quality Schottky contacts on ZnO nanostructures, but controversies remain with regard to the Schottky barrier height and the ideality factor of the ZnO Schottky contacts.…”

Electrical characteristics of palladium Schottky contacts to hydrogen peroxide treated hydrothermally grown ZnO

“…However, despite the first SC to ZnO was reported by Mead [51] already in 1965, it is still challenging to realize stable and high quality SCs. For instance, the presence of a conductive surface layer due to group III ions [53], native defects both resident in the bulk and created by the metallization [54], processing [55], adsorbates [56,57], sample orientation [58,59] and presence of surface dipoles [60], can all affect the ZnO surface and the metal-semiconductor junction performance. Similar effects can account for the wide scatter, up to ∼ 2 eV in the ZnO work function (i.e.…”