The Concentration of (SiH2)n Sites in Low and High Defect Density a-Si:H
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0910-A09-04
The Concentration of (SiH2)n Sites in Low and High Defect Density a-Si:H D. C. Bobela1, T. Su2, P. C. Taylor3, A. Madan4, and G. Ganguly5 1 University of Utah, Salt Lake City, Utah, 84112-0830 2 Colorado School of Mines, Golden, Colorado, 80401 3 Colorado School of Mines, Golden, 80401 4 MVSystems, Inc., Golden, Colorado, 80401 5 United Solar Ovonics Corp., Auburn Hills, Michigan, 48326 ABSTRACT The concentration of polysilane chains (SiH2)n, where n ≥ 1, is estimated for higher quality hydrogenated amorphous silicon (a-Si:H) by pulsed proton nuclear magnetic resonance 1 techniques ( H NMR). Our measurements indicate the minimum hydrogen content of approximately 10% of the total hydrogen is in the (SiH2)n configuration. Similar 17
measurements in a high defect density sample (10 silicon dangling bond defects cm-3) show that (SiH2)n sites account for ~ 15% of the total hydrogen. While the (SiH2)n infrared absorption (IR) modes are observed in the highly defective sample, no such modes are seen in the higher quality material. The results indicate that a significant amount of the total hydrogen content exists as (SiH2)n regardless of film quality. INTRODUCTION The hydrogen bonding environments and their correlation to film quality has been the subject of an enormous body of research; see for instance [1] and references therein. It is generally believed that a more ordered a-Si:H matrix leads to smaller void structures and therefore improved opto-electronic properties. As such, the concentration of large polysilane hydrogen structures, such as (SiH2)n , is thought to be small compared to the total hydrogen content for device quality films. IR studies seem to support this trend; higher quality films show no evidence of the (SiH2)n scissor or wagging modes near 800-900 cm-1. Most of the hydrogen is bonded to silicon and can be found either in clusters, such as on the internal surfaces of voids, or in isolated regions of the matrix. Since these sites occur in sufficient quantity, >1 atomic percent (at. %), they can be detected via their IR modes near 2000 cm-1. Below the IR sensitivity of approximately1 at %, the hydrogen bonding structures cannot be observed, and hence the (SiH2)n content is unknown. We have recently reported the 1H NMR spectra of (SiH2)n in a high defect density film. At these sites the average hydrogen-hydrogen separation is 1.8 Å [2]. We report here similar (SiH2)n structures in as-grown higher quality material where the IR modes are absent. Of course, n may be unity for these sites; i.e., isolated silicon dihydride sites. Using 1H NMR, which is a more sensitive probe of the bonded hydrogen, we have estimated the (SiH2)n content for both films. We report the measurement details and compare estimates of the (SiH2)n content. Contrary to current beliefs, we find a significant fraction of the total hydrogen content can be bonded in (SiH2)n configurations.
EXPERIMENTAL METHODS and ANALYSIS The higher quality film was grown by plasma enhanced chemical vapor deposition (PECVD) at MVSystems Inc
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