Illumination- and Annealing-Induced Changes in the Infrared and Raman Spectra of a-Si:H
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Illumination- and Annealing-Induced Changes in the Infrared and Raman Spectra of a-Si:H L.-F. Arsenault, S. Lebiba, E. Sacher and A. Yelon Department of Engineering Physics and Materials Engineering Ecole Polytechnique, Montreal, Quebec, H3C 3A7, Canada
ABSTRACT We have investigated the changes, produced by light-soaking, in both the IR and Raman responses of the Si-Hn stretching peaks in the 2000-2100 cm-1 range. Our observations of the IR response are in qualitative agreement with those of Kong and co-workers [1]: that is, short-term light soaking produces an increase in the intensity of the signal and a simultaneous shift to lower frequency. In contrast, short-term light soaking decreases the total intensity of the Raman signal in the 2000-2100 cm-1 range, when normalized to the TO phonon peak at about 480 cm-1. In both cases, these modifications are reversed on annealing at 200Û&:HVXJJHVWWKDWWKHVHFKDQJHVDUH attributable to alterations in the environments of the Si-Hn bonds, with the resultant transfer of intensity between IR and Raman matrix elements. Details of the evolution of the components of the Raman spectrum in the 2000-2100 cm-1 range are presented, and compared with IR changes in the same range. INTRODUCTION Intensive work over the past several years has shown that in addition to creating electronic defects, light soaking produces structural changes in a-Si :H. The nature of these changes and their relation to the Staebler-Wronski effect, or lack thereof, are still far from clear [2]. Here, we present the results of a study of the changes in infrared (IR) absorption and in Raman scattering due to both light soaking and annealing. These results, in combination with those from the literature [1], suggest that changes, different from those previously suggested, take place in the environment of the Si-Hn bonds. EXPERIMENTAL Undoped, device-quality a-Si:H films were deposited, at NREL. The depositions were made from pure silane onto crystalline Si substrates, at 230Û & XVLQJ WKH UI SODVPD-enhanced chemical vapor deposition technique. Pieces were cut from these samples and used in either IR or Raman experiments. All anneals were at 200Û & EHORZ WKH IDEULFDWLRQ WHPSHUDWXUH EXW KLJK enough to assure that times of the order of 1 h resulted in saturated behavior. Infrared measurements were carried out on a Bio-Rad FTS 3000 spectrometer, using an MTEC 300 photoacoustic cell. At a modulation frequency of 800 Hz, the entire sample was probed. Samples for IR experiments were exposed to white light for up to 10 hours before measurements, either in transmission or using photoacoustic spectroscopy (PAS). In principle, by extracting the in-phase and quadrature signals in PAS, it is possible to determine the surface and a
Present address: École Polytechnique, Paris.
A14.3.1
volume contributions to IR absorption [3]. Unfortunately, the signal-to-noise ratio was too low to permit this separation, and only total intensity spectra, essentially identical to absorption spectra, were analyzed.
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