T-Site-Trapped Molecular Hydrogen in a-Si:H
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T-Site-Trapped Molecular Hydrogen in a-Si:H R. E. Norberg, D. J. Leopold, P. A. Fedders, R. Borzi, P. H. Chan, J. Herberg, N. Tomic Department of Physics, Washington University, St. Louis, Mo 63130 ABSTRACT Proton-29Si double resonance NMR measurements on high quality plasma-enhanced chemical vapor deposition a-Si:H deposited from SiH4 show that more than one third of the contained hydrogen is present as H2 molecules residing in the amorphous equivalent of T sites. The NMR signal from these trapped H2 appears in the narrow 4 kHz proton line, which arises from the less clustered hydrogen population. Very little of the molecular component is in the broad ~24 kHz line, which arises mostly from clustered hydrogen tightly bonded to silicon. INTRODUCTION For many years proton NMR measurements on hydrogenated amorphous silicon have shown two principal components, including a narrow Lorentzian line and a broad Gaussian line corresponding respectively to less- and more-clustered hydrogens. Now 29Si-1H double resonance NMR experiments show that the narrow proton line arises primarily from H2 molecules trapped in the amorphous equivalent of tetragonal T sites. EXPERIMENT Proton NMR at 200 MHz and 29Si NMR at 39.6 MHz have examined high quality amorphous silicon films prepared [1] by plasma enhanced chemical vapor deposition (PECVD) from SiH4. Results of proton-29Si double resonance measurements permit separate identification of Si-bonded H and of an abundant component of molecular H2 singly resident in interstitial T sites. The basic idea is to perturb the 29Si nuclei while observing proton NMR signals. The resultant change in the proton spectrum then allows investigation of interactions between 29Si and protons. RESULTS Figure 1 shows three proton NMR components commonly found in high quality PECVD a-Si:H. The stimulated echo spectra shown here at 20 and 95 K are not fully relaxed, but have been chosen to show three components which include (at 20 K): a 175 kHz rather rectangular doublet from ortho-hydrogen (ortho-H2) molecules on internal surfaces; a ~24 kHz FWHM more or less Gaussian line from “more-clustered hydrogen”; and a 3 to 4 kHz FWHM Lorentzian line from “less-clustered hydrogen”. Much of the work reported here will indicate that the narrow Lorentzian proton NMR line arises from molecular H2 interstitially singly-trapped in the amorphous equivalents of tetragonal T sites and that the ~24 kHz broad proton line arises from clusters of Si-bonded H. Figures 1 and 2 show that, by 95 K, the 24 and 3 kHz lines are essentially unchanged, but the 175 kHz ortho-H2 line has narrowed substantially – not because of translational motion, but rather from phonon-driven molecular ∆mJ transitions within the J=1 rotational manifold for ortho-H2 in the presence of large crystal fields arising from the internal surfaces. Fully relaxed proton spectra at 20 K and above show fractional populations of 42% for the narrow Lorentzian,
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57% for the broad Gaussian, and about 1% for the 175 kHz ortho-H2. In Fig. 2 the 23 and 3 kHz linew
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