Room Temperature Atomic Layer Deposition of ZnO Observed By in Situ IR Absorption Spectroscopy

Abstract

Zinc oxide (ZnO) is an attractive material for flexible electronics since it has various merits of nontoxicity, wide bandgap, low resistivity, and a high optical transmittance. The ZnO deposition has been intensively studied as transparent conductive films for the liquid crystal displays and thin film solar cells. Furthermore, ZnO has been used as buffer layers for gas barrier films on organic electronics devices. The ZnO deposition has been already reported by DC magnetron sputtering, chemical vapor deposition (CVD), and metal organic CVD (MOCVD). However, it was difficult to control the film thickness with a nanometer precision. In addition, these require high-temperature treatments exceeding 300 ºC, which might be problems for organic electronics and micromachines. To achieve the room temperature growth with a nanometer scale precision, we developed RT atomic layer deposition (ALD). In this study, we report a new RT ALD with dimethyl zinc (DMZ) and an oxidation gas of plasma excited humidified Ar. We used an in-situ multiple-internal-reflection IR absorption spectroscopy (MIR-IRAS) equipped ALD system to investigate the saturation behavior in precursor adsorption and oxidization. We determined injection conditions of the precursor and oxidization gas for the RT ALD based on the IRAS experiment. The RT ALD of ZnO was performed to confirm the ZnO formation on a silicon substrate. In figure 1, we show ZnO thicknesses as a function of ALD cycle which were measured by spectroscopic ellipsometry. The growth per cycle was estimated as 0.046 nm/cycle. We evaluate its composition by using X-ray photoelectron spectroscopy and confirmed that it was fully oxidized. We assume that the deposited film was amorphous from the X-ray diffraction pattern. In addition, we discuss the reaction mechanism in this ALD with Langmuir equation fitting applied to the saturation curve plotted from the result of IR absorption spectroscopy. We depict the surface reaction scheme of the ZnO RT ALD in this paper. Acknowledgment; This work was partly supported by JST-CREST Grant Number JPMJCR14F3, Japan. It was also partly supported by JSPS KAKENHI Grant Numbers 15H03536 and 16K17494. Figure 1 Growth thicknesses of ZnO as a function of the ALD cycle number. Figure 1