T-Site Trapped Molecular Hydrogen in Hot Wire a-Si:H
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T-Site Trapped Molecular Hydrogen in Hot Wire a-Si:H J. Herberg, P. A. Fedders, D. J. Leopold, R. E. Norberg, and R. E. Schropp1 Department of Physics, Washington University, St. Louis, Mo 63130 1 Debye Institute, Utrecht University, P. O. Box 80000, NL-3508 TA Utrecht, The Netherlands ABSTRACT Proton NMR and proton-29Si double resonance NMR have been performed on hot wire a-Si:H films deposited from SiH4. Results are compared with those from conventional plasma enhanced chemical vapor deposition a-Si:H. Proton nutational angle studies and proton-29Si spin-echo double resonance (SEDOR) signals at 80 and 294 K indicate that a significant proton resonance population arises from T-site-trapped molecular H2. The hot wire films also display a N+] ):+0 Vuperbroad proton line and a sharp feature diamagnetically shifted by 25 ppm.
INTRODUCTION Previously reported proton-29Si spin echo double resonance (SEDOR) NMR measurements1-3 on PECVD hydrogenated amorphous silicon have demonstrated that most of the contained hydrogen which comprises the less-clustered narrow proton NMR line in fact arises from hydrogen molecules (H2) individually trapped in the amorphous equivalents of interstitial tetragonal T sites. Most of the remaining contained hydrogen appears in a ~24 kHz broad proton NMR line, which arises from clustered H bonded to Si. In the present, work we have used 200 MHz proton NMR and proton-29Si SEDOR NMR to examine the hydrogenic structrure of two hot wire HW a-Si:H films prepared at Utrecht and at NREL.
EXPERIMENT Using NMR sample designations, film S45 is a 127 mg portion of a HWCVD film prepared at the Debye Institute in Utrecht. The substrate temperature was kept at 240oC and the deposition rate 5.4 PKRXUIRUQHDUO\IRXUKRXUVWRSHUPLWXVHRIDQ$OVXEVWUDWHVXEVHTXHQWO\ removed. HW film B42 was prepared at NREL by deposition onto Al foil wrapped on stainless steel. The deposition temperature increased from 330 to 540oC for an estimated deposition rate RI
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B42 powder sample loaded for NMR was 143 mg. X-ray diffraction did not show any crystalline silicon peaks. Figure 1 shows peak-normalized room temperature proton NMR spectra from free induction decays (FIDs) in the two HW a-Si:H films S45 and B42. The spectra were taken following a 90o proton pulse at a pulse repetition interval of 5 sec. Both the B42 and the S45 signals reflect 400 acquisitions. The relative proton spin counts in the two films are in the ratio 18 reflecting the very different sample preparation conditions. Normalized to a standard PECVD a-Si:H sample, the proton spin populations are S45 23 at.% and B42 1.3 at.%. These
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two sample films were examined at 80 and 294 K using 200 MHz proton NMR and proton-29Si spin echo double resonance (SEDOR).
Figure 1. Room temperature proton resonance line shapes from free-induction decays following SURWRQSXOVHV
Figure 2. Comparison of S45 line shape from a FIDDQGD VHF
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