A New PDS Study on PECVD a-Si:H
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A NEW PDS STUDY ON PECVD A-SI:H C. MANFREDOTrrI*, F. FIZZOTrI*, M. BOERO*, E. VITTONE*, G. AMATO** AND L. BOARINO** *
Experimental Physics Dept., University of Torino, Via P. Giuria 1, Torino, Italy
**
National Electrotechnical Institute G. Ferraris, Strada delle Cacce 91, Torino, Italy and Consorzio INFM, Genova, Italy
ABSTRACT
The effect of incorporation of SiH 2 on a-Si:H matrix on the optical properties has been investigated on some details. It is shown that SiH2 incorporation is responsible for a further increase of the optical gap, and also for an increase in disorder. For concentrations larger than 15% approximately, microvoids are probably created and SiH 2, which is covering their inner surfaces, seems not to affect the optical properties further on. The results indicate also clearly that the effect of autoannealing of the film during its growth cannot be neglected and that a new parameter should be retained, i.e. the total deposition time. INTRODUCTION
It is well known that H content of a-Si:H is strictly correlated with its electronic and optical properties, such as the optical energy gap EG and the logarithmic slope of Urbach edge E01,2,3,4. The results are generally reported as a function of either the substrate temperature 5 or the deposition rate6 , which are assumed to be in a direct relationship with CH, in cases when it is not directly known or measured. Moreover, there are no direct evidences, to our knowledge, of the separate effect of SiH and SiH2 configurations on electronic properties of a-Si:H. NMR results 7 indicate that randomly distributed SiH groups should be present in a-Si:H together with clustered configurations, which could be either SiH or SiH 2 on internal surfaces of voids. However, no correlation with optical properties is reported. It appears to be generally the case that the presence of H reduces the overall strain in the structure of a-Si 8 but the origin of this H-induced bond angle strain reduction could be ascribed either to the replacement of highly strained Si-Si bonds by the formation of two neighbouring Si-H bonds 8 , or also to the presence of SiH 2 units, the lower effective coordination (two-fold) for such Si atoms allowing for an efficient release of strain 9 . However, in some cases at least 8 , it can be seen that COTO decreases linearly with CH, one can therefore, since the linewidth of the TO-phonon peak increases with decreasing woT1, also conclude that the bond angle disorder increases with increasing CH. In the present work, we present the results on optical properties of a-Si:H, as derived from PDS and optical transmission measurements, strictly correlated with both SiH and SiH 2 concentrations in the sample. Since, at least in the selected range of deposition temperatures, SiH concentration is constant and only SiH 2 is varying, we shall show that a correlation exists between SiH 2 concentration and both EG and Eo and that, in our case, E0 increases with EG. Moreover, self-annealing effects during the deposition cannot be neglected and the total depos
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