Increasing the Electron Mobility of ZnO-Based Transparent Conductive Films Deposited by Open-Air Methods for Enhanced Sensing Performance

Abstract

The development of open-air, high-throughput, low-cost thin film fabrication techniques has immense potential and interest in optoelectronics. However, the oxygen-rich atmosphere associated with such processes can have detrimental effects on the electrical properties of the deposited films. An example of this is found in materials based on ZnO, for which atmospheric processing results in low mobility values. This stems mainly from adsorbed oxygen species at the grain boundaries, which limit carrier transport. This paper describes the effect of a low-temperature UV treatment on the electrical properties of ZnO and aluminum doped zinc oxide (ZnO:Al) films deposited by atmospheric pressure spatial atomic layer deposition (AP-SALD). Because of the mild UV treatment, a significant decrease in the amount of oxygen traps at the grain boundaries has been observed. This results in a large improvement of the carrier mobility, up to 47 times for undoped ZnO and 16 times for ZnO:Al. The effect of temperature (RT to 220 °C) during the UV treatment on the conductivity of undoped ZnO and ZnO:Al films is discussed. The study of the time-dependent conductivity of ZnO and ZnO:Al films using tunneling emission based models provides a simple means for extracting the grain boundary trap density, a critical parameter in semiconductors that is usually not easy to estimate. We show that the high conductivity of the UV-treated films can be preserved when exposed to oxygen at high temperature because of a very thin alumina (Al2O3) barrier layer. Finally, we demonstrate that the effect of UV illumination of thin ZnO films deposited in oxidizing atmospheres can be used to design improved UV or oxygen sensors.

In-situ spatial and temporal electrical characterization of ZnO thin films deposited by atmospheric pressure chemical vapour deposition on flexible polymer substrates

“…It has previously been shown that annealing ZnO films in air results in a reduction in the concentration of oxygen vacancies, or an increase in oxygen interstitial trap states, and hence a decrease in the available charge carriers 51 . As noted earlier, it has also been reported that oxygen tends to accumulate at the grain boundaries of ZnO-based films, slowing transport of charges from one grain to another as the electrons must tunnel through this barrier 39,40 . www.nature.com/scientificreports/ This effect leads to an increase in film resistance of a couple orders of magnitude, and is particularly noted in AP-SALD films.…”

“…In addition, the outer substrate positions are likely to be exposed to more oxygen from the surrounding atmosphere, whereas the central portion of the substrate remains better isolated from the external environment. It has recently been reported that oxygen exposure limits the mobility and hence conductivity of ZnO-based films deposited by open-air methods via barrier formation at the grain boundaries, such that this may also contribute to the higher resistances observed at the outer substrate positions 39 , 40 . The ZnO photoluminescence spectra at different positions on the substrate can be seen in Fig.…”

Planar and Transparent Memristive Devices Based on Titanium Oxide Coated Silver Nanowire Networks with Tunable Switching Voltage

“…The TiO 2 films were deposited using a home‐made AP‐SALD [ 44 ] using TiCl 4 as titanium precursor and water as oxygen precursor. The deposition was performed at a substrate temperature of 150 °C and in the open air.…”

Transparent Heaters: A Review

“…It is also worth mentioning that ZnO‐based TCOs may not be the best candidates for high temperature applications, since ZnO tends to absorb oxygen species in the grain boundaries, which has a detrimental effect on mobility, as explained and modelled recently. [ 19 ] It has been shown that the absorption of oxygen species increases rapidly with temperature, [ 134 ] thus most likely limiting the applications of THs based on AZO.…”