Hagyoung Choi

Tuning the electronic structure of tin sulfides grown by atomic layer deposition.

In this study, tin sulfide thin films were obtained by atomic layer deposition (ALD) using Tetrakis(dimethylamino)tin (TDMASn, [(CH3)2N]4Sn) and hydrogen sulfide (H2S). The growth rate of the tin sulfides (SnSx) was shown to be highly dependent on the deposition temperature, and reaction times of 1 second for the TDMASn and H2S were required to reach the saturation regime. Surface morphologies were smooth or rectangular with rounded corners as observed by a field emission scanning electron microscope (FE-SEM) and were dependent on temperature. X-ray diffraction results confirmed that the crystal structure of SnSx can be tuned by changing the ALD temperature. Below 120 °C, SnSx films appeared to be amorphous. In addition, SnSx films were SnS2 hexagonal at 140 and 150 °C and SnS orthorhombic above 160 °C. Similarly, the values of the optical band gap and binding energy showed significant differences between 150 and 160 °C. The electronic structures of SnSx were extracted by UPS and absorption spectroscopy, and the unsaturated Sn 3d molecular orbital (MO) states in the band edge were found to be responsible for the great improvement in electrical conductivity. This study shows that TDMASn-H2S ALD is an effective deposition method for SnSx films, offering a simple approach to tune the physical properties.

Al2O3 multi-density layer structure as a moisture permeation barrier deposited by radio frequency remote plasma atomic layer deposition

Al2O3 films deposited by remote plasma atomic layer deposition have been used for thin film encapsulation of organic light emitting diode. In this study, a multi-density layer structure consisting of two Al2O3 layers with different densities are deposited with different deposition conditions of O2 plasma reactant time. This structure improves moisture permeation barrier characteristics, as confirmed by a water vapor transmission rate (WVTR) test. The lowest WVTR of the multi-density layer structure was 4.7 × 10−5 gm−2 day−1, which is one order of magnitude less than WVTR for the reference single-density Al2O3 layer. This improvement is attributed to the location mismatch of paths for atmospheric gases, such as O2 and H2O, in the film due to different densities in the layers. This mechanism is analyzed by high resolution transmission electron microscopy, elastic recoil detection, and angle resolved X-ray photoelectron spectroscopy. These results confirmed that the multi-density layer structure exhibits ver...

Moisture Barrier Properties of Al2O3 Films deposited by Remote Plasma Atomic Layer Deposition at Low Temperatures

We report the effect of process temperature on moisture permeation barrier properties of Al2O3 films deposited by remote plasma atomic layer deposition (RPALD) at various low temperatures from 50 to 200 °C. XPS analysis of O 1s peak reveals that the O–H ratio decreases with process temperature from 38.1% at 50 °C to 25.8% at 200 °C. The water transmission rates using electrical Ca degradation test indicates that the 100 nm Al2O3 film enhances the moisture barrier performance from 2.0×10-2 to 5.0×10-4 g m-2 day-1 with increasing the process temperature. This result indicates that increasing the process temperature improves the moisture permeation barrier properties significantly even in RPALD process. It is attributed to the increase in the Al2O3 mass density due to the decrease of relatively O–H ratio with increase in temperature as revealed by XPS O 1s peak deconvolution and FTIR analysis in the Al2O3 films.

Radio frequency plasma power dependence of the moisture permeation barrier characteristics of Al2O3 films deposited by remote plasma atomic layer deposition

In the present study, we investigated the gas and moisture permeation barrier properties of Al2O3 films deposited on polyethersulfone films (PES) by capacitively coupled plasma (CCP) type Remote Plasma Atomic Layer Deposition (RPALD) at Radio Frequency (RF) plasma powers ranging from 100 W to 400 W in 100 W increments using Trimethylaluminum [TMA, Al(CH3)3] as the Al source and O2 plasma as the reactant. To study the gas and moisture permeation barrier properties of 100-nm-thick Al2O3 at various plasma powers, the Water Vapor Transmission Rate (WVTR) was measured using an electrical Ca degradation test. WVTR decreased as plasma power increased with WVTR values for 400 W and 100 W of 2.6 × 10−4 gm−2day−1 and 1.2 × 10−3 gm−2day−1, respectively. The trends for life time, Al-O and O-H bond, density, and stoichiometry were similar to that of WVTR with improvement associated with increasing plasma power. Further, among plasma power ranging from 100 W to 400 W, the highest power of 400 W resulted in the best moi...

Fast spatial atomic layer deposition of Al2O3 at low temperature (<100 °C) as a gas permeation barrier for flexible organic light-emitting diode displays

The authors developed a high throughput (70 A/min) and scalable space-divided atomic layer deposition (ALD) system for thin film encapsulation (TFE) of flexible organic light-emitting diode (OLED) displays at low temperatures (<100 °C). In this paper, the authors report the excellent moisture barrier properties of Al2O3 films deposited on 2G glass substrates of an industrially relevant size (370 × 470 mm2) using the newly developed ALD system. This new ALD system reduced the ALD cycle time to less than 1 s. A growth rate of 0.9 A/cycle was achieved using trimethylaluminum as an Al source and O3 as an O reactant. The morphological features and step coverage of the Al2O3 films were investigated using field emission scanning electron microscopy. The chemical composition was analyzed using Auger electron spectroscopy. These deposited Al2O3 films demonstrated a good optical transmittance higher than 95% in the visible region based on the ultraviolet visible spectrometer measurements. Water vapor transmission r...

Effect of scan speed on moisture barrier properties of aluminum oxide using spatial atomic layer deposition

Atomic layer deposition (ALD) has been shown to produce high-quality thin films with superior moisture barrier performance on polymer substrates. However, the conventional time-sequenced mode is incompatible with industrial needs due to its low deposition rate. One solution to overcome this throughput issue is to use spatial ALD. Recently, various approaches have been reported. The authors also developed a fast spatial ALD system using an industrial 2G (370 × 470 mm2) glass substrate. Using this system, the authors investigated the effect of a scan speed on the moisture barrier properties of aluminum oxide (Al2O3) thin films. While the scan speeds were varied over a wide range of 100–800 mm/s, the water vapor transmission rate increased only slightly, from 1.4 × 10−3 to 3.0 × 10−3 g/m2/day. At a scan speed of 800 mm/s, the deposition rate was 70 A/min, which was about seven times higher than that of conventional ALD. Moreover, the physical and chemical properties of the thin films slightly worsened with t...