Changes in Short- and Medium-Range Order in a-Si:H Induced by Light-Soaking, Pseudodoping and Doping With Boron
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O.A.GOLIKOVA*,M.M.KAZANIN*,V.Kh.KUDOYAROVA*,G.J.ADRIAENSSENS**, A.ELIAT** * A.F.Ioffe Physico-Technical Institute, 194021, St.Petersburg, Russia. **K.U.Leuven, Laboratorium voor Halfgeleiderfysica, B- 3001, Heverlee, Belgium. ABSTRACT A study of a-Si:H structure on the scales of short- and medium- range order has been carried out by the Raman spectroscopy method. Undoped a-Si:H films were deposited by the conventional rf- PECVD with some special variations in the process conditions ( pseudodoping technique); doping with boron was performed from a gas phase as well as by ion implantation. Changes in short- and medium- range order induced by light-soaking, pseudodoping and doping with boron were determined and compared. INTRODUCTION Light-induced changes in a-Si:H structure have recently attracted much attention. Besides well-known defect creation, changes in medium- range order were expected [1] and were confirmed by NMR measurements [2,3]. Light-soaking was shown to affect also a-Si:H entropy [4], Si bond angles [5], compressive stress [6] and to result in dilatation of a film [7]. Light-soaking is known to lead to the Fermi level shifts toward the midgap. Pseudodoping, i.e., the Fermi level shifts toward the conduction band which are solely controlled by variations in film deposion conditions, was shown to be accompanied by changes in a number of the structural parameters of a-Si:H such as defect density, hydrogen content, Urbach energy, Si bond angle dispersion [8]. Similarly, doping with the electrically active impurities is also known to be accompanied by defect creation. Information on other structural transformations in doped a-Si:H is less readily available. The purpose of this work is to determine and to compare changes in short- and mediumrange order of a-Si:H induced by the different ways: light-soaking, pseudodoping, and doping with boron. EXPERIMENTAL Deposition of the pseudodoped a-Si:H was described in [8]. The films were deposited by the triode PECVD ( f = 13.56 MHz ) at constant temperature Tý = 300'C but with varying the cathode- grid bias ( V= 0 - 200V) and the pressure of SiH 4 ( P = 50 - 120 mT). The material was characterized by IR spectroscopy and by OMS spectroscopy giving information, respectively, on the hydrogen content and on the defect density in the films[91.The temperature dependence of the dark conductivity (Od) was measured at T= 300619
Mat. Res. Soc. Symp. Proc. Vol. 507 © 1998 Materials Research Society
470K to determine its activation energy giving information on the Fermi level position in the gap. Doping with boron during deposition from a gas phase as well as after deposition by ion implantation was described in [10]. The deposition temperature of all films under study was equal to 300'C. Light-induced changes in a-Si:H structure were investigated using the pseudodoped a-Si:H with an accelerated kinetics of the photoconductivity under light-soaking ( W =100 mW/cm2 , X < 0.9 Ptm) :a7 ph_ t - 0,66 attributed to clustered hydrogen ( CH = 16 at.%) in amorphous network [9]. That
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