Solution- Processed Alkali Metals-Doped Amorphous Zinc Tin Oxide Thin-Film Transistors and Analysis of Alkali Metal Doping Mechanism through the UV-Visible Spectroscopic
Authors: Lim Keon-Hee , Lee Eungkyu , Kim Kyongjun , Kim Youn Sang 
Published: February 2020

Recently, alkali metal doped ZnO has received much positive attention owing to its high performance and good electrical stability. In particular, since alkali metals such as Li and Na are general elements and show good potential as doping elements in ZnO semiconductors, several researchers have been interested in alkali metal doping process of polycrystalline zinc oxide semiconductors. However, only a few studies have been reported the doping mechanism to enhance the mobility. In addition, there have been few studies on alkali metal doping mechanism in amorphous metal oxide semiconductors. Most studies about alkali metal doped oxide semiconductor have been inquired in term of crystallinity, oxygen vacancy, and surface morphology properties. For these reasons, we also studied the mechanism of alkali metal doping in view of oxygen vacancy by spectroscopic study using XPS and surface morphology using AFM. However, although we observed that oxygen vacancy contents are reduced slightly with a correlation by interstitial or substitutional mechanism, it seems to be difficult to identify the correlation between the enhancement of electrical properties induced by interstitial doping and the changes of oxygen vacancy contents, because the difference of XPS spectra in doped ZTO films is too small. Also, as the decrease of oxygen vacancy contents means the decrease of electron mobility in metal oxide semiconductors, the tendency of oxygen vacancy spectra in XPS is inconsistence with general phenomena. It means that other mechanisms are required to precisely explain the electrical properties of the doped amorphous ZTO TFTs related to alkali metal doping mechanism. Otherwise, the change in the optical band gap supported by the Burstein-moss theory showed successfully that the enhancement of mobility was related to the interstitial doping of alkali metals. This result shows that the change of the optical band gap clearly applied to the study of the relationship between the change in mobility and interstitial doping concentrations.

Here in, we introduce alkali metals doped and solution processed amorphous zinc tin oxide (ZTO) semiconductor TFTs, which show better electrical properties, such as field effect mobility and conductivity, than that of intrinsic amorphous ZTO based TFTs. We also analyze that the doping mechanism of alkali metals in the amorphous ZTO based TFTs using various techniques such as, atomic force microscopy (AFM), X-ray photoemission spectroscopy (XPS), Hall mobility. Furthermore, through the UV-visible spectroscopy, we suggest a comprehensive technique for investigating the enhanced electrical performance induced by alkali metal doping in terms of the change in optical band gap. Specially, we showed that UV-visible spectroscopic analysis for investigating the electrical performance of alkali metal doped metal oxide semiconductor TFTs has good potential as a fast and non-destructive analytical technique.