Correlation Between Phototransport and Network Order in a-Si:H
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ABSTRACT Hydrogenated amorphous silicon (a-Si:H) films prepared by the glow discharge (GD) technique show superior optoelectronic properties over those prepared by rf sputtering (RFS). To find out whether this is associated to structural differences in the amorphous network, we have carried out a comprehensive comparison of the Raman spectra of the two types of films grown at different substrate temperatures. The use of two properly chosen excitation radiations allowed the observation of the Raman spectra from the near surface versus that from the bulk of the films. The results show that the short-range order in the bulk of GD films is close to that of the ideal tetrahedral network, having an rms bond angle deviation (AO), of •9°. In contrast, the smallest value of AO found in the RFS films was &150. There is also a short-range order inhomogeneity in both sets of materials that can be reduced significantly by selecting the appropriate substrate temperature. The intermediate-range disorder is relatively small and uniform in GD films, while large differences exist in this parameter between the surface and bulk of RFS films. In general, the results indicate that the short-range order and the inhomogeneity in intermediate-range order present in the RFS films cannot be improved to equal those of GD materials by annealing at temperatures low enough that no substantial hydrogen effusion occurs. These structural differences are argued to be the: reasons for the superior phototransport properties of GD over RFS materials and are interpreted in terms of the differences between the two deposition processes.
INTRODUCTION Amorphous silicon is described as a distorted tetrahedrally coordinated network, with a limited amount of order at the short-range (bond angle distribution) and medium-range (dihedral angle distribution) levels."12 Since the main interest is in its electronic properties, there were numerous attempts to correlate the structural disorder with these properties. It is now quite well established that the electronic structure 3' 4 as well as the transport 5'6' 7 and phototransport 8 properties are related to well defined features of the structural disorder. In particular, there exist theoretical studies able to predict the dependence of the electronic density of states on characteristic features of the Si network. 9'10 Since a-Si:H is thermodynamically metastable3,' both its structural and electronic properties depend critically on the deposition and post-deposition conditions. In particular, the deposition 6. technique itself is of significant importance in the determination of the above 612 properties. ' Of the many methods proposed for the deposition of a-Si:H, the one utilized thus 479 Mat. Res. Soc. Symp. Proc. Vol. 377 0 1995 Materials Research Society
far for device production is the glow discharge (GD) decomposition of silane, which is considered to yield the best photoelectronic properties. Another method popular among many researchers is the radio frequency sputtering (RFS) in which the sputtered silico
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