Heat-Treatment Induced Defects in CZ-Silicon
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HEAT-TREATMENT INDUCED DEFECTS IN CZ-SILICON
S. DANNEFAER-, T. BRETAGNON, K. ABDURAHMAN, D. KERR, AND S. HAHN* *Department of Physics, University of Winnipeg, Winnipeg, Canada, R3B 2E9 "**Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, U.S.A. ABSTRACT Positron lifetime results show that vacancies can be retained after growth of Czochralski silicon at concentrations of -3x10' 6 /cm 3 . Rapid thermal annealing as well as furnace annealing increase the vacancy concentration. The vacancies are predominantly trapped by oxygen interstitial clusters in lightly B-doped materials, and these complexes appear to have temperature dependent configurations which can be quenched-in by rapid cooling. Heavy Sb doping results in trapping of vacancies by the Sb impurities. INTRODUCTION During the cool-down of Czochralsld-grown silicon ingots several types of defects may be formed. One type is the oxygen interstitial which gives rise to an infrared absorption band around 9/Am. Despite the expectation that oxygen clusters should also be formed, these have so far only been found after subsequent heat-treatments at temperatures above -400°C[l]. Traditionally some of these oxygen clusters have been associated with thermal donors. It appears, though, that the techniques usually employed to investigate the structure of defects in semiconductors, such as hyperfine magnetic resonance methods and local mode vibration investigations, do not yield information on other grown-in defects in Cz-Si. Recent positron annihilation experiments[2] have altered this situation by indicating that vacancy-type defects as well as oxygen clusters are present in as-grown Cz-Si. The vacancies detected are monovacancies and/or divacancies, and the interstitial clusters are likely oxygen clusters[3]. In this work we will present some further investigations using the positron annihilation method aimed mainly at investigating the thermal stability of these grown-in vacancies. EXPERIMENTAL Method When positrons are injected into a sample which contains no positron traps, they will annihilate with a characteristic lifetime (the bulk lifetime), determined by the bulk electron density distribution in the sample. When open volume defects are present some of the positrons can be trapped, and the lifetime of these trapped positrons will be larger than the bulk lifetime because the local electron density is smaller than in the bulk. The converse will be the case if the positrons are trapped by interstitial type defects. The rate by which the positrons are trapped by the defects is proportional to the defect concentration, but this trapping rate is not directly obtainable from the experimental data. Based on the simple Mat. Res. Soc. Symp. Proc. Vol. 262. 01992 Materials Research Society
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trapping model[4], which assumes that all positrons initially occupy only the bulk state, even when traps are present, the trapping rate can be calculated from: K(2= 12(1/TB - l/lr)/(1-12).
(1)
Here rB is the bulk lifetime, r2 the lifetime a
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