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A STUDY OF DEFECTS IN CZOCHRALSKI-GROWN SILICON BY POSITRON ANNIHILATION SPECTROSCOPY S. C. Sharma*, N. Hozhabri, and R. C. Hyer Department of Physics, University of Texas at Arlington, Arlington, Texas 76019 T. Hossain, S. Kim, F. 0. Meyer, III, M. F. Pas, and A. E. Stephens Texas Instruments, Dallas, Texas 75265 *To whom correspondence should be addressed ABSTRACT We have studied defects in Cz-grown single crystal silicon by utilizing a variable energy positron beam and positron lifetime spectroscopy in conjunction with surface photovoltage measurements. We present results for the depth profile of defects obtained from the Doppler broadening spectra measured by implanting variable energy positrons at different depths ranging from the surface down to -1 um deep. We have also measured positron lifetime spectra at different locations on a wafer and have obtained a radial variation in the density of the vacancy-type defects.

We present results from a study of the depth and spatial profiles of vacancy-type defects in Cz-grown silicon by utilizing Positron Annihilation Spectroscopy (PAS). PAS is a nondestructive technique which is sensitive to vacancies at 1015 cm-3 level. The positronelectron annihilation process is characterized by the positron lifetime and the electron momentum distribution. A lattice vacancy in the silicon lattice traps positrons [1]. The trapped positrons live for a much longer time and the trapping is reflected by a narrower electron-positron momentum distribution. Thus by measuring the changes in the positron lifetime and/or the electron momentum distributions, one can study the nature and density of the positron-trapping defects in a lattice [2-4]. We have recently applied the variable energy positron beam technique to study the depth profile of defects in MBE grown low temperature buffer layers of GaAs [5,6]. We implant variable energy positrons at different depths and measure the Doppler broadening spectra. The depth-profile of the defects measured for the as-grown Cz-Si wafer is compared with that obtained for a Si epilayer grown on Si < 100> by the decomposition of SiHCl 3 under 1 atmosphere and at 1120'C. The bulk positron lifetime measurements, made by axially displacing a 22Na positron source on the wafer, provide radial distribution of the vacancies and they indicate the presence of divacancies in the wafer. Our variable energy positron beam spectrometer consists of a magnetically guided positron beam, a high-purity Ge solid state detector and computer controlled multichannel analyzer. Positrons emitted from a 22 Na source are moderated by a venetian type vane made of well annealed tungsten foils. The moderated positrons are transported through a curved beam-line by a focusing magnetic field and accelerated to desired energies up to 20 keV just prior to being implanted into the sample. Positrons thermalize within picoseconds and diffuse through the sample. While diffusing, positron experience trapping in defects with a probability that depends on the concentration and nature of the def