Glow discharge decomposition

Hamano et al. (1982) have fabricated an a-Si H photodiode array linear image sensor. The sensor structure is shown in Fig. 5. The sensor is constructed by first forming individual electrodes on a glass or a ceramic substrate. Then l-/mi-thick undoped a-Si H is produced at 230°C by glow-discharge decomposition of silane and finally 1500-A-thick ITO common electrode, which also acts as an antireflection coating, is deposited by dc sputtering. 

It should be noted that both Feldman et al. and Dey and Fong used a-Si films deposited by vacuum evaporation. This probably accounts for the relatively low OFF-state resistances which they both found (100-100 k 2). It is now well established that vacuum-evaporated a-Si is a very different material from the hydrogenated form of a-Si obtained, for example, by the carefully controlled glow-discharge decomposition of silane (e.g., Spear, 1977). 

The most common deposition technique is the glow-discharge decomposition (gdd) of volatile inorganic compounds - e.g., SiH4 to make a-Si H. Several variants are available where the ionized plasma is driven by a dc electric field (with the substrate on either the anode or the cathode), an ac field (60 Hz), an rf inductive, or rf capacitive (sometimes with a superposed dc bias). These techniques can be used in the presence of a magnetic field. The pressure of SiH4 is in the range 1 to 700 Pascal with the substrate at 200 300°C. Above 350°C, H2 evolves from the a-Si H layer ... 

Inchcations that Mott s rule on the doping insensitivity of amorphous semiconductors may not be strictly obeyed in amorphous silicon could be taken already from the work of Chittick et al. (1969). For a-Si films prepared by glow-discharge decomposition of SiH4 (monosilane) these authors reported a rise in the conductivity by factors of 10 and—10 when 200 ppm and 4% PH3 (phosphine), respectively, were added to the silane. The observed instability of these films, however, sheds doubt about whether this was a true doping effect or rather an eflfect of defects, which were known to cause even higher conductivity changes (Beyer and Stuke, 1972). 

Despite these limitations a wealth of useful information has been obtained from the charge drift experiments, particularly for the high resistivity semiconductors Se, As2Sc3, AS2S3, S and a-Si obtained by glow discharge decomposition of SiH4. 

Fig. 5.30. (a) T-dependence of the electron drift mobility juj) of an a-Si film, 1.3 jum thick, produced by glow discharge decomposition of silane, (b) Model of the electronic state distribution (after Le Comber and Spear (1970)). 

Fig. 5.36. The density of localized states g(E) plotted against - E for a number of a-Si specimens produced by glow discharge decomposition of silane. The samples were prepared on substrates held at different temperatures as noted in the figure. The point E Si indicates the lower limit for g(E) set by these experiments for an evaporated Si film. L and S denote linear and square low interpolations of g(E) to a value at E estimated from drift mobility (after Spear and LeComber (1972)).

Samples were prepared by rf glow discharge decomposition of gas mixtures in a system designed to reduce contamination by water vapor or oxygen. 

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