Improving Thermal Stability Of Electrical Properties Of Al-Doped ZnO

Transparent conductive oxide(TCO)thin film is the key to manufacture and optimize various optoelectronic devices.Although ITO(indium tin oxide)is currently the most widely used TCO film in commercial applications,it has many shortcomings,such as the rare source of In,which leads to its high price,easy to produce toxic smoke in the production process,low toughness and so on.Therefore,it is of great significance to develop new TCO materials.Zinc oxide(ZnO)has been considered as one of the most potential alternative materials for ITO due to its excellent properties such as wide band gap,high abundance and non-toxicity.Aluminium-doped zinc oxide(AZO)is one of the most promising candidates for ZnO-based TCO with both high transmittance(85%,in the visible range)and low resistivity(～10-4 Ω cm).However,the electrical properties of AZO deteriorate significantly after exposing to oxygen-rich high temperature environment,which greatly limits its application in related devices.Therefore,improving the thermal stability of AZO is of great interest.This work aims to explore the method to improve the thermal stability of AZO and shed light on the underlying mechanism:1、The experimental results show that,with the increase of annealing temperature,the electrical properties of AZO decrease significantly,which is mainly due to the increase of zinc vacancies.This is attributed to decrease of carrier concentrations due to desorption of zinc atoms in AZO,and the decrease of carrier mobility due to the increase of grain boundary defects.2、The ZnO protective layer was prepared by chemical vapor deposition(CVD)to improve the thermal stability of AZO.The experimental results show that the morphology of ZnO protective layer has a significant effect on the thermal stability of AZO.The optical and electrical properties of continuous ZnO films prepared at the substrate temperature of 600℃ achieve the best performance in protecting AZO.The possible reasons are as follows:1)During the process of CVD preparation,the crystal structure and defect density of AZO were improved by high temperature recrystallization in zinc-rich atmosphere,and the carrier mobility was increased;2)Meanwhile,ZnO prepared by CVD tends to sacrifice its own structural order to form a surface layer with large defect density,so as to effectively capture O atoms from the external environment to avoid their penetration into AZO,and thus maintain the carrier concentration of AZO.3、In order to identify the impact of high temperature annealing in zinc rich environment and coating of protective layer on the improvement of AZO thermal stability,and to analyze the influence of preparation technique,the ZnO protective layer prepared by low substrate temperature magnetron sputtering(MS)and atomic layer deposition(ALD)was further studied.The results show that the low temperature process can obtain ZnO films with similar morphology and crystal structure as the high temperature CVD process,but it can’t improve the thermal stability of ZnO/AZO as effectively,which indicates that the high temperature CVD process has unique advantages.In the absence of recrystallization in zinc-rich atmosphere,AZO could not improve carrier mobility while ensuring high carrier concentration.In addition,even if the three methods of preparation of ZnO layer can sacrifice their own structure in order to protect the AZO,thus inhibiting the Zn desorption from the AZO layer and the diffusion of external O atoms/ions into AZO.But due to in the process of MS and ALD the substrate of AZO without the effect of high temperature recrystallization in zinc rich environment,fails to improve the crystal quality and defect density.Therefore,the instability of inherent defects in AZO has an unavoidable influence on the thermal stability of AZO.These results provide a new avenue for the optimization of related TCO materials and the improvement of their thermal stability,which will beneficial for device optimization.