Proton NMR Spin-Lattice Relaxation Time Characterization of a-Si(H) Structure
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PROTON NMR SPIN-LATTICE RELAXATION TIME CHARACTERIZATION OF a-Si(H) STRUCTURE
M. E. LOWRY, R. G. BARNES, D. R. TORGESON, AND F. Ames Laboratory-USDOE* and Department of Physics, University, Ames, Iowa 50011
R. JEFFREY Iowa State
ABSTRACT NMR data are presented for reactively sputtered amorphous silicon-hydrogen alloys (a-Si(H)). Measured differences in two of the samples are attributed to two distinct morphologies: a mixed phase (monohydride and dihydride) and a purely monohydride composition. Features of the mixed phase morphology have been modeled. Room temperature, 35 MHz spin-lattice relaxation times are presented for a series of monohydride samples prepared with systematically varied sputtering parameters. A correlation of proton T1 with the density of ESR states tentatively is suggested. *Operated for the U.S. Department of Energy by Iowa State University under contract No. W-7405-Eng-82. This research was supported by the Director for Energy Research, Office of Basic Energy Sciences, WPAS-KC-02-02-02.
INTRODUCTION Electron Spin Resonance (ESR) spectroscopy has been a fundamental tool in probing the structure of amorphous silicon-hydrogen alloys [a-Si(H)] for several years [1]. However, Nuclear Magnetic Resonance (NMR) is relatively new to the field [2]. One of the more important triumphs of ESR spectroscopy in defining the structure of a-Si(H) has been its role in unmasking the function of the incorporated hydrogen, which act to satisfy "dangling bonds" or defects in the ideal continuously connected, tetrahedrally coordinated network of pure a-Si [3]. This discovery led to the now commonly held belief that glow discharge (GD) a-Si(H) and the reactively sputtered (in the presence of H) material were essentially the same. However, there seems to be at least one difference between the two materials. Infrared transmission spectroscopy has identified two main classes of bonding type for the incorporated hydrogen: monohydride and dihydride [4,5]. Using the sputtering process one is able to reliably control the type of bonding [5]. Verification of the existence of dihydride and monohydride configurations by NMR techniques, addressed by at least one group of workers [2], forms a major portion of this communication. THEORY AND METHOD Our NMR measurements have concentrated on the spin-lattice relaxation time T1 of the protons in various a-Si(H) samples. (TI)-I represents the rate of transfer of absorbed rf energy from the proton spin system to the "lattice". This decay of energy content of the spin system can be represented by the Bloch equation [6]: dMz (T)/dT = [Mo0 -Mz (T) /T 1 (1) here, Mz(T) represents the component of the nuclear magnetization parallel to the static field Ho and MO represents the value of Mz at equilibrium. The
342 particular method used to measure T 1 will, of course, determine the boundary conditions to be satisfied by Eq. (1). We have utilized the inversion recovery (IR) method which uses a 180, T, 90 NMR pulse seauence [7] and dictates the initial condition that Mz(T=0)=-M0. This
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