Doping Dependence of Local Hydrogen Motion in Hydrogenated Amorphous Silicon

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P. HARI, P.C. TAYLOR AND R.A. STREET* Department of Physics, University of Utah, Salt Lake City UT 84112 *Xerox Palo Alto Research Center, 3333 Coyote Hill Rd, Palo Alto CA 94304.

ABSTRACT 1 H NMR dipolar echo measurements have been performed on a series of samples of phosphorus- and boron-doped a-Si:H. The dipolar echo sequence consists of three rf pulses 2 followed by an echo in the form: (ir/ )x - TI - (7r/4)y - 72 - (7r/4)y - echo. The echo height is plotted against T2 and the slope yields the dipolar spin-lattice relaxation time (TID). TID is a measure of fluctuations in the local dipolar field surrounding each hydrogen atom in a-Si:H, and measurement of this quantity can be employed as a probe of hydrogen motion on a microscopic 3 scale. The TID measurements of 10-5 B-doped, 10- P-doped and undoped a-Si:H are compared to the previously measured TID of 10-4 B-doped a-Si:H. The TID values for 10-4 B-doped, 10-5 B-doped and undoped a-Si:H are, respectively, 1.7 ms, 11 ms and 22 ms at 300 K. The T1D for 10-3 P-doped is found to be the same as for 10-5 B-doped within experimental error. These trends are similar to the variation of the macroscopic diffusion of hydrogen with respect to various doping levels, but the details of the local motion are very different from those of the macroscopic diffusion.

INTRODUCTION Previous dipolar echo measurements [1] of hydrogen in amorphous silicon indicate that the dipolar spin lattice relaxation time, can be used as a measure of the local motion of hydrogen in amorphous silicon. TID measurements as a function of various doping levels can give information about the influence of dopants on microscopic hydrogen motion. Also macroscopic diffusion [2,31 measurements using secondary ion mass spectroscopy (SIMS) yield a definite trend as a function of doping. By comparing the microscopic and macroscopic dependences of hydrogen motion on doping, we can obtain valuable information regarding local hydrogen motion in hydrogenated amorphous silicon. In this study we use the Jeener-Broekaert [4] pulse sequence to measure the dipolar spin lattice relaxation time (TID). We have measured T1D for the slow component (narrow line) of the free induction decay for four different samples. The temperature dependence of TID has been measured for one sample from which we estimate a thermal activation energy for microscopic motion of hydrogen. TID,

EXPERIMENTAL DETAILS The NMR measurements were made using an Oxford superconducting magnet and a Matec 5100 main frame with 525 gated amplifier plug-in. A Wavetec model 3006 frequency generator was used as a cw source. The pulse sequence was generated using a Tektronix PG5110 programmable pulse generator and a homemade flip-flop switch. Phase sensitive detection was achieved by a Matec 625 broadband receiver and a tuned pre-amplifier. Signal averaging was done using a LeCroy 9400 A dual 175 MHz oscilloscope. A typical 90° pulse width was about 2 /is. Mat. Res. Soc. Symp. Proc. V