Hydrogen Passivation Studies in Dislocated CZ and FZ Silicon
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HYDROGEN PASSIVATION STUDIES IN DISLOCATED CZ AND FZ SILICON C. Dubi, J. P. Kalejs and S. Rajendran Mobil Solar Energy Corporation, 4 Suburban Park Drive, Billerica, Mass 01821 ABSTRACT 0 Hydrogen passivation using a Kaufmann ion source at 400 C has been carried out on FZ and CZ silicon dislocated by four-point bending at high temperatures. The results differ from those reported for dislocations passivated in silicon sheet grown by the EFG technique. A model for hydrogen diffusion and trapping is presented to argue that the differences observed are not produced by hydrogen transport effects in the bulk.
INTRODUCTION Passivation of dislocations by hydrogen has been shown to lead to increases in the bulk lifetime of silicon sheet grown by the Edge-defined Film-fed Growth (EFG) technique [1]. Passivation mechanisms are not yet well understood, however, because there remain major areas of confusion regarding hydrogen diffusion and trapping behavior in silicon [2]. In this report we examine Kaufmann ion source passivation of dislocations introduced in initially dislocation-free single crystal FZ and CZ silicon. Four point bending at temperatures above 800°C is used to provide dislocated samples with reduced minority carrier diffusion lengths for the passivation studies at 400 0 C. A model for hydrogen diffusion and trapping [3] is used to obtain a consistent interpretation for the passivation response observed in the single crystal materials and in the EFG sheet. EXPERIMENTAL DETAILS The samples used in the present study were (111) and (100) orientation single crystal FZ and CZ silicon wafers, 1-2 ohm-cm p-type. Interstitial oxygen in the FZ material was below about lxlO1 6 /cm 3 , and in the range of 7 3 Some CZ material with comparable carbon2 5-10x10l /cm for the CZ silicon. 3 17 oxygen levels (both about 5x10 /cm ) was also examined. Samples 5x10 cm 0 were stressed at temperatures between 600 and 1400 C using a four-point bending apparatus constructed from graphite according to the specifications of ASTM E 328-78 [41. The bending axis was for the (111) and for the (100) samples. Here we discuss only the results obtained above 800°C where the dislocation arrays were clearly defined in the uniform stress central region of the sample. The experimental procedure consisted of a temperature ramp of about a half to one hour up to the operating temperature without a load on the sample, a load application time of a few seconds to one hour (depending on the operating temperature and stress 0 used), and a cooling time to 400 C of about one half hour with the load removed. The experiments were carried out in a resistance heated furnace used for growth of EFG silicon ribbon. All graphite parts were purified with a high temperature halogen treatment used to prepare graphite for EFG furnaces. The hydrogen passivation was performed with a Kaufmann ion source 0 operated at 2 keV. The substrate temperature was kept at 400 C and the passivation time was 2 min, identical to conditions used for passivation of EFG material reported prev
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