Low Temperature a-Si:H TFTs with a SiO 2 Gate Insulator Deposited by Liquid Phase Deposition
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Low Temperature a-Si:H TFTs with a SiO2 Gate Insulator Deposited by Liquid Phase Deposition Richard B. M. Cross, David P. Oxley, Meenakshi Manhas, Ekkanath. M. Sankara Narayanan Emerging Technologies Research Centre, De Montfort University, The Gateway, Leicester, United Kingdom, LE1 9BH. ABSTRACT A systematic study has been made of the influence of the deposition conditions on the properties of SiO2 grown by liquid phase deposition (LPD), and a-Si:H manufactured by plasma enhanced chemical vapour deposition (PECVD) with the novel facility of source-gas heating. It is demonstrated that LPD-SiO2 can be grown at 50 °C with good dielectric properties. Material has been produced with a resistivity of 1015 Ωcm and a dielectric strength of 9 MVcm-1.The oxide was found to have a negative fixed oxide charge of 4 x 1011 cm-2, with a dielectric constant of 3.08 and a refractive index of 1.44. In the case of a-Si:H, pre-heating the source gases has enabled material to be grown at 125 °C with a hydrogen content of ~ 10 at%, with a predominance of monohydride bonding and a photosensitivity of ~ 104. Inverted-staggered thin film transistors have been fabricated incorporating these films with an On/Off ratio of five orders of magnitude, a sub-threshold slope of 1.3 Vdecade-1 and a field effect mobility of 0.20 cm2V-1s1 . INTRODUCTION Thin Film Transistors (TFTs) based on hydrogenated amorphous silicon (a-Si:H) are widely used as the pixel switching elements in Active Matrix Liquid Crystal Displays (AMLCDs). Conventionally, a-Si:H and the gate insulator material are deposited by plasma enhanced chemical vapour deposition (PECVD) at deposition temperatures greater than 200 °C [1]. Lowering the growth temperature is usually accompanied by a severe degradation of the film properties and consequently device performance. However, the increasing desire to improve process compatibility with inexpensive, flexible substrates has initiated great efforts by many workers to develop device quality layers at reduced substrate temperatures. In this paper, we describe the physical and electrical properties of hydrogenated amorphous silicon (a-Si:H) grown at 125 °C by PECVD with source-gas heating, and silicon dioxide (SiO2) deposited at 50 °C by an economical and low-temperature method known as liquid phase deposition (LPD). In addition, the device characteristics of inverted-staggered TFTs incorporating these films are also discussed. EXPERIMENTAL DETAILS The SiO2 layer was deposited from a supersaturated solution of hydrofluorosilicic acid (H2SiF6) at 50 °C. Silicic acid powder (99.999%) was added to commercially available 3.09M H2SiF6 and stirred at room temperature for 6 hours. To test if the solution was saturated, a small volume was extracted and hydrogen peroxide (H2O2) was added. A saturated solution appears a
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yellow/gold colour, whilst an unsaturated solution appears clear. The solution was then filtered through 0.1 µm bore size filter paper to remove any undissolved silicic acid powder. The saturated H2SiF6 was then c
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