Growth of SnO2 and TiO2 thin films by PE-ALD : their structural characteristics and H2 gas sensing properties

Abstract

One of the potential alternatives for the fossil fuels is hydrogen (H2) gas because it is renewable and clean. However, a high flammability and explosiveness of H2 in the gas storage are great problems. Therefore, the detection of the H2 gas from leakage is indispensable for safety. Gas sensors for H2 gas have been studied and commercially available for many years. In order to meet the demands of future H2 gas applications, however, many researches are conducted to improve low cost, low working temperature (below 100 oC), selectivity (NOx, EtOH, H2O, etc) and reliability in addition to reducing sensor size. In this research, semiconductor type metal oxide gas sensor is attracted due to low cost and high sensitivity toward H2 gas and plasma enhanced atomic layer deposition (PEALD) is used to control variables such as thickness and morphology. As sensing materials, SnO2 and TiO2 which are most widely studied materials for H2 gas sensor are ii chosen. The three main topics will be discussed to investigate gas sensing mechanism and enhance gas sensing performance: 1) Oriented SnO2 thin films grown on TiO2 single crystals, 2) Brookite TiO2 thin film epitaxially grown on YSZ substrates, 3) SnO2-TiO2 dual layer gas sensors. The first and second topics are focused on investigating gas sensing mechanism. Theoretical studies of gas adsorption and desorption on specific crystallographic planes or phases have been conducted. Although gas sensing performance is strongly depended on gas adsorption on a specific crystallographic plane or phases, the study of relationships between gas sensing performance and crystallographic planes or phases is very limited due to morphology change caused by crystallographic change. Therefore, in this study, I tried to separate of variables using epitaxial deposition methods to control the crystallographic planes or phases without morphology change. First, epitaxial SnO2 films were deposited on TiO2 single crystals with various orientations by PE-ALD, and their structural characteristics and gas sensing properties were investigated, particularly focusing on the crystallographic orientation dependence of H2 gas response. Dibutyltindiacetate (DBTDA) was used as Sn source, and (100), (001), (110), and (101) TiO2 were employed as substrates for SnO2 deposition. All the SnO2 films were ∼90 nm thick after 1000 ALD cycles and epitaxially grown on TiO2 substrates, which were confirmed by X-ray pole figure and high resolution transmission electron microscopy (HRTEM). Differently oriented epitaxial SnO2 films iii showed the different H2 gas response and different temperature dependence of gas response. The (101) SnO2 films grown on (101) TiO2 exhibited the highest H2 gas response of ∼380 toward 1000 ppm H2/air at 400 °C, which was associated with the different temperature dependence of resistance in (101) film rather than the microstructural characteristics and chemical composition compared to the other films. Next, epitaxial brookite TiO2 (B-TiO2) film was deposited on (110) YSZ substrate using PE-ALD and its structural, optical, and gas sensing properties were investigated. Chemical states and morphology of the TiO2 film were investigated by XPS and AFM. Xray diffraction, X-ray pole figure, and high resolution TEM analyses revealed that deposited TiO2 film was pure B-TiO2 and highly oriented to (120) plane. The determined in-plane orientation relationships were B-TiO2 YSZ [210] [110] and B-TiO2 YSZ [001] [001] and lattice mismatches were -1.91 and 0.06 %. Phase of BTiO2 film was unchanged at 700 °C heat treatment and the sensor showed stable and high sensing properties for H2 gas. The highest magnitude of the gas response (Rair/Rgas) was determined to be ~150 toward 1000 ppm H2/air at 150 °C. In addition, B-TiO2 sensor showed a high selectivity for H2 against CO, EtOH, and NH3. The last topic is concentrated to enhance high sensing ability and selectivity in addition to low working temperature and low cost using silicon substrate. Although SnO2 thin film is extensively studied in H2 gas sensor owing to high gas sensing performance, most SnO2 iv thin film sensors have great problems to be commercialized because of high working temperature and low selectivity. First, to reduce working temperature, the thickness effect of SnO2 gas senor is studied. When the thickness of SnO2 thin film is ~ 4 nm which is near debye length, the maximum gas response was exhibited at low temperature (below 100 oC) but the sensors have still poor selectivity. To enhance the gas selectivity, SnO2- TiO2 dual layer gas sensor was suggested. When TiO2 was deposited on SnO2, the gas response was increased and the optimum thickness was ~ 4 nm of SnO2 and ~ 4 nm of TiO2. Dual layer sensor showed the excellent gas selectivity against NOx gas compared with SnO2 single layer sensor. Although the dual layer sensors exhibited poor selectivity against humidity, the problem is solved with adjusting of operating temperature up to 100 oC.