Enhancement of uniformity and mobility of two-dimensional semiconducting thin film

Abstract

Although transition metal dichalcogenides (TMDs) have a lot of advantages such as great on/off current ratio over 107, high mobility over several hundreds of cm$^2$V$^{-1}$s$^{-1}$, flexibility and transparency, most studies related to TMDs are based on the mechanically exfoliated film with limited size. In order for TMDs to reach the actual commercialization level, it is necessary to raise the electrical properties of TMDs which are far below the theoretical value to the limit, and the synthetic technology should be further improved. In this thesis, MoS$_2$ was selected as a representative for TMDs. The overall content is organized in the following bottom-up approach: 1. MoS$_2$ thin film growth, 2. Defect control of MoS$_2$ thin film, and 3. MoS$_2$ FETs structure engineering. 1. MoS$_2$ thin film growth A large-area MoS$_2$ thin film was synthesized through MOCVD method. The growth aspects were summarized in the form of grow map expressed as a function of various growth parameters. Furthermore, by developing a layer-controlled growth method, it was possible to synthesize the MoS$_2$ thin film with one to five layers. The layer-by-layer growth mechanism was also investigated. 2. Defect control of MoS$_2$ thin film To improve the electrical performance of as-grown MoS$_2$ thin film, defects generated during the growth process were controlled using the oxygen gas. Oxygen gas strengthened the n-type properties of MoS$_2$ thin film by creating sulfur vacancies, and the potential barrier originating from grain boundaries was reduced by passivating the dangling bonds in the grain boundaries. In addition, the Schottky barrier height at metal-MoS$_2$ junction was lowered due to the re-arrangement of charge neutrality level. 3. MoS$_2$ FETs structure engineering AlN thin film was inserted as an interfacial layer between the Al$_2$O$_3$ gate insulator and MoS$_2$ channel to enhance the electrical performance. AlN with high surface optical phonon energy served to reduce the surface optical phonon scattering caused by the Al$_2$O$_3$ gate insulator for the MoS$_2$ channel. The effect of AlN insertion was confirmed by the tendency of mobility change according to the temperature. Furthermore, the nitrogen substitutional doping for MoS$_2$ during the AlN deposition was analyzed in the optical and electrical view point. And through this analysis, a research field for strain engineering was suggested.