Transparent Conducting Oxides by Atomic Layer Deposition

Abstract

Energy demand and emissions of greenhouse gases are increasing. The use of renewable energy such as solar energy, hydro power and wind power can help mitigate further emissions. Transparent conducting oxides (TCO’s) are an important class of materials of relevance for renewable energy. ZnO and SnO2 are well-known n-type TCO materials that are good candidates to replace the commonly used Sn-doped In2O3 (ITO). Delafossite type oxides, CuMO2, are potential candidates as p-type TCO materials. By realization and combination of n- and p- type transparent oxides into p-n junctions one will enable the fabrication of transparent solar cells and transparent transistors that can lead to functionalization of already available surfaces, such as windows etc. This study has investigated the deposition of a selection of different materials suitable as TCO’s. The selection comprises the materials ZnO, SnO2, CuO, Cu-Al-O and Cu-Y-O, which has been deposited using Atomic Layer Deposition (ALD). CuO was deposited as a critical step to enable growth of films in the Cu-M-O system (M=Al/Y). The work related to deposition of ZnO focused on the purity of the precursors used and whether they were a source for impurities in the resulting film, especially with respect to Al content. It was seen that the precursors might lead to Al doping in the film and that this level might not be easily controlled if the purity grade of the precursor is not sufficiently high. Related to this work, the heterojunction between the film and Si substrate was studied. This is important whenever ZnO is to be used as a transparent electrode for solar cells. It was seen that the junction was improved by annealing the film at 400 ºC. A process using SnI4 and O3 to deposit SnO2 has been studied at temperatures below 300 ºC in order to investigate the crystal structure, electrical and optical properties of the as-deposited films. The process enables growth from a temperature of 110 ºC. Films deposited at temperatures of 300 ºC are crystalline while at lower temperatures the films are amorphous. UV-vis-NIR spectroscopy shows absorption due the fundamental absorption in the films. For films deposited at 300 ºC the specific resistivity is 7.1.10-3 ?? cm, carrier concentration of 5.1019 cm-3 and the mobility is 17 cm2/V.s. The SnO2 process has also been used as a buffer layer during high temperature annealing of Cu-Al-O films due to its chemical robustness. A process to deposit CuO has been studied using copper acetylacetonate, [Cu(acac)2] and O3. The process was found to give uniform films following the self-limiting growth pattern of ALD in a temperature range of 140 – 230 ºC with a growth rate of ~0.038 nm/cycle. The films were crystalline as deposited with the tenorite structure. Good control of stoichiometry in the Cu-Al-O and Cu-Y-O systems by combining the processes for deposition of CuO, Al2O3 and Y2O3 has been achieved. Films with the delafossite type structure were obtained by high temperature annealing of films containing Cu and Al. The current compilation of work shows that the ALD technique is well suited for deposition of conducting and transparent materials.