Characterization of Defects in Heavily Si-Doped GaAs by a Monoenergetic Positron Beam
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CHARACTERIZATION OF DEFECTS IN HEAVILY Si-DOPED GaAs BY A MONOENERGETIC POSITRON BEAM A. UEDONO*, Y. UJIHIRA*, L. WEI** AND S. TANIGAWA** *Department of Industrial Chemistry, Faculty of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan **Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305, Japan ABSTRACT Native defects in Si-doped, Zn-doped and undoped GaAs grown by horizontal Bridgman (HB) method and molecular beam epitaxiy (MBE) were studied by a monoenergetic positron beam. Positron lifetime spectra and Doppler broadening profiles were also measured by using energetic positrons. It was found that monovacancies were usually found in Si-doped HB-GaAs, however, divacancies were created in a specimen with low impurity concentration. For Zn-doped HB-GaAs, interstitial clusters were found to be major type of defects. The high concentration of monovacancies and of divacancies coexist in heavily Si-doped MBE-GaAs. These vacancy-complexes were found to act to reduce the concentration of free carriers. INTRODUCTION The positron annihilation technique is now a powerful method for the study of point-defects in semiconductors [1-3). By using this technique, the concentration of defects for a detectable response is about 0.1 ppm, and there are no restrictions regarding specimen conductivity nor specimen temperature. A positron, being a positive particle, has a tendency to become trapped by neutral and negatively charged trapping centers, but not by positively charged ones. The implanted positrons into condensed matter eventually annihilate with electrons, producing two annihilation y-rays. Doppler broadening of the y-ray energy spectrum is determined by the momentum distribution of a positron-electron system. The shape of Doppler broadening profiles is characterized by the lineshape parameter S which corresponds to the ratio of the counts in the central part to the total counts (1] . The value of the S parameter differs depending on the type of defects. The lifetije of positrons trapped by defects also strongly depend on the size of defects. Thus the positron annihilation technique provides one with the ability to determine a species of defects, and one can also estimate the concentration of defects by using the specific trapping rate of these defects [3]. For III-V compound semiconductor such as GaAs, native point defects can exist in both sublattices. In principle, six types of point defects can be formed during crystal growth. Further, depending on the position of the Fermi level, defects can be neutral, negatively or positively charged. The positron annihilation in GaAs has been extensively studied [4-15) . Because of the complex nature and the large variety of defects in GaAs, however, there are notable differences among interpretations of the annihilation characteristics in GaAs (10). These studies were mainly carried out with measurements of lifetime spectra and of Dopper broadening profiles by using energetic (E=0-0.5 MeV) positrons. Therefore, in the present Mat. Res.
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