Study on Characterization of Semiconducting Metal-Oxide Thin-Films by Atomic Layer Deposition and Their Application in High-Performance Field-Effect Transistors

Abstract

Silicon-based semiconductor materials, which are currently used as the main channel layer in the memory industry, are facing fundamental problems such as increased leakage current and deterioration of reliability due to continuous device scaling. Amorphous In-Ga-Zn-O substance which is currently adopted as a channel layer for thin-film transistors in the display industry, are has a wide band gap of > 3.0 eV and low sub-gap states due to the intercalation of orientation insensitive In 5s orbital below the conduction band edge. These intriguing features renders the resulting field-effect transistors to have an ultra-low-leakage current characteristic and the strong immunity to short channel effects such as gate-induced-drain leakage (GIDL) and drain-induced barrier lowering (DIBL). It makes them one of the promising semiconducting channels that can replace Si semiconductors. Up to now, amorphous InGaZnO film have been mainly deposited using a sputtering method. Advanced 3D memory devices such as DRAM and NAND are enabled by developing novel ALD process because it has an excellent step-coverage and thickness controllability of the deposited film on the complicate nanoscale trench structure. This thesis proposes ALD-derived metal-oxide (metal = In, Ga, Zn) thin film deposition and fabrication of high mobility metal-oxide field-effect transistors with an excellent bias stability applicable to next-generation memory devices. This thesis consisted of three parts including the crystalline IGO film, ZnO/InGaO heterojunction and InGaZnO cation composition-gradient heterojunction. First, the ALD-derived InGaO thin films with different Ga cation concentration were prepared under the annealing temperature of 400 and 700 °C. It was found that the increasing incorporation of Ga impedes the formation of crystalline structure due to disparity of crystal structure in Ga2O3 and In2O3 motif. Depending on the Ga-fraction and annealing temperatures, the facile control including amorphous phase, randomly oriented polycrystalline and highly aligned polycrystalline state was achieved. In particular, the highly aligned polycrystalline IGO FET exhibited the high mobility of 60.7 cm2/Vs, gate swing of 0.40 V/decade and ION/OFF ratio of 5.1  109 as well as the good device uniformity, which was attributed to the efficient densification and high degree of lattice ordering. In the second part, the heterojunction stack consisting of 3-nm-thick ZnO film and 10-nm-thick InGaO film, which is well-known in high-efficiency III-V semiconductor LED, was designed for the high mobility electron transistor. The necessary condition to realize the high –mobility heterojunction-based transistor was examined in detail where the bandgap engineering and work-function criterion for InGaO and ZnO were carefully considered. The fabricated heterojunction ZnO/InGaO FET exhibited the high mobility of 63.2 cm2/Vs, gate swing of 0.26 V/decade and ION/OFF ratio of 9.0  108 as well as the good electrical stability, which was explained by the formation of the charge confinement layer. In the last part, the heterojunction channel was realized on basis of InGaZnO bilayer structure with different cation composition ratios, which consisted of the 5-nm-thick charge confinement layer and 2-nm-thick carrier barrier layer. The field-effect mobility of a heterojunction IGZO channel device was higher than that of a single IGZO channel FET. The boosting in terms of field-effect mobility depends on the difference in the Fermi level (EF) between the charge confinement layer and the capping layer where the EF values can be modulated in its cation ratio.