Multiple Quantum NMR Study of Hydrogen Clustering in Amorphous Silicon
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MULTIPLE
QUANTUM NMR STUDY OF HYDROGEN CLUSTERING IN AMORPHOUS SILICON
KAREN K. GLEASON-, AND J. A. REIMERý
J.
BAUM--,
A.
N.
GARROWAY...,
A.
PINES-
Department of Chemical Engineering, University of California, Berkeley CA 94720 Department of Chemistry, University of California, Berkeley CA 94720 Code 6120, Naval Research Laboratory, Washington DC 20375-5000
ABSTRACT
Because Multiple Quantum NMR coherences occur only between spins which are coupled together by the dipole interaction, this technique has been used to study the clustering of hydrogen in amorphous silicon. The clustered hydrogen was found to be associated with the broad line of the single quantum NMR spectra. For device quality films, the average cluster size is approximately six protons. The concentration of these five to seven atom defects increases with increasing hydrogen content until, at very high hydrogen content, the clusters are replaced by a continuous network of silicon-hydrogen bonds.
INTRODUCTION
Many researchers have examined the relationship between the structure of amorphous hydrogenated silicon (a-Si:H) and its optical and electronic properties Ell. "Device quality" films J0oi between 8 and 20 atom % hydrogen and approximately contain dangling bond defects/cmO . Films with hydrogen contents outside this range contain higher concentration of defects, rendering them "nondevice" materials. Only monohydride (Si-H) silicon-hydrogen bonds are observed in the infrared spectra of device quality a-Si:H, whereas dihydride (Si-He) , trihydride (Si-H,) and polymeric ((Si-Hv.),) bonds are seen in films of higher hydrogen content. NMR reveals an inhomogeneous distribution of hydrogen in device quality materials [2,3). About 4 atom % hydrogen exists as spatially isolated monohydrides and trapped molecular He, giving rise to a narrow (4 kHz) NMR line. The remaining hydrogen gives rise to a broad (25 kHz) NMR line, indicative of clustering. A conceptual drawing of how these clusters exist on the silicon atom network is shown. in Figure 1. We do not suggest these are actual structures since they would give a single quantum NMR line width a factor of two larger than
Mat. Res. Soc. Symp. Proc. Vol. 70. k1986 Materials Research Society
84
observed. As the hydrogen content varies only the magnitude of the broad component changes; the magnitude of the narrow component and both linewidths remain constant. Anneali ng at 600',C causes the hydrogen associated with the broad line to evolve, leaving only the narrow line in the NMR spectra. We will apply quantitative Multiple Quantum (MQ) of solids to study the size and extent of clustering in a-Si:H thin films.
FIGURE I 3-D Schematic of clustered monohydrides
EXPERIMENTAL
Detailed descriptions of the MQ NMR experiment in solids can be found elsewhere [4]. In summary the experiment can be divided into four parts. First is the preparation period, composed of an integral multiple of eight 90o radio-frequency pulses separated by appropriate delays. During the preparation period multiple quantum coh
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