Author: Wong, Man Hon
Title: Photoconductivity and photo-assisted gas sensing effect of reactive pulsed radio-frequency sputtered large-area 2-dimensional tin disulfide films
Advisors: Ong, C. W. (AP)
Man, H. C. (ISE)
Degree: Ph.D.
Year: 2018
Subject: Hong Kong Polytechnic University -- Dissertations
Layer structure (Solids)
Department: Department of Applied Physics
Pages: xx, 147 pages : color illustrations
Language: English
Abstract: SnS₂ is one of the 2D layered materials, possibly having similar distinctive properties and expected application potential in ultrathin regime like other members in the family. Ultrathin SnS₂ could have an electron band structure allowing direct transitions under above-bandgap photo excitation, which assists room-temperature photoconductivity and gas sensing effect to redox gas species. This is analogous to the room-temperature photo-assisted gas sensing effect of direct-bandgap metal oxide semiconductors, while the corresponding behavior of ultrathin SnS₂ has not been explored in detail. To date 2D materials prepared by using CVD and exfoliation are mostly micron-scaled crystallites or discontinuous thin films. It is challenging to fabricate large-area uniform SnS₂ thin film in a one-step process, which would lead to extended scope of study on the material and prospect in realizing mass production for practical applications. In this thesis, fabrication of large-area uniform SnS₂ of a few layer thick was attempted by using reactive pulsed radio-frequency magnetron sputtering with a H₂S-containig reactant gas. It was found that a higher substrate surface temperature Ts enhanced crystallization of the deposit, meanwhile sulfur content was reduced. The use of 9%H₂S in sputtering at Ts = 200°C optimized crystallization of the film containing rich enough sulfur content to form SnS2. The film exhibited a layered structure with c-planes preferentially oriented along the substrate surface. Tauc plot indicated that the films have a direct bandgap of around 3 eV. Under 365-nm LED photo excitation, they demonstrated persistent photoconductivity and photo-assisted gas sensing effects to oxygen. The 21.6-nm thick film had a photo sensor response of 300 and a gas sensor response of 0.2. The corresponding values of the 1.8-nm thick film increased significantly to 40000 and 75 respectively, namely 130 and 100 times larger than those of the thicker film. Cyclic tests showed that the film has good stability in 25 cycles, especially after several prior warm-up cycles.
A model has been proposed to explain the photoconductivity decay of the films in different thickness against oxygen concentration. A decay curve was assumed to be composed of three exponential decay components, which were attributed to the effects due to the trap states in the film arising from structural defects and the chemisorbed oxygen species of O₂- and O₂²-. For the ultrathin film, the surface localized states associated with the sulfur vacancies and chemisorbed oxygen species dominated the transport of charge carriers. This explained the prominent persistent photoconductivity decay, and the large gas sensor response due to large surface-area-to-volume ratio. The contributions to electron capturing by O₂- and O₂²- species at different O₂ concentration could be differentiated. The model also provided a practical means for fast quantitative determination of O₂ concentration, which based on fitting to a limited period of dynamic photoconductivity decay, instead of waiting a long time for reaching steady state. It is expected SnS₂ can be a potential alternative to direct bandgap metal oxide semiconductors in gas sensor applications.
Rights: All rights reserved
Access: open access

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