Dislocation loops and precipitates in an arsenichyphen;rich GaAs crystal have been studied by transmission electron microscopy to investigate their formation mechanism and establish their relationship to point defects. The precipitates are identified to be hexagonal arsenic phases having a simple orientation relationship with the GaAs matrix. Detailed analyses of the loops indicate that they are composed of two separate defects lying on (111) planes: a faulted loop and a perfect loop. It is proposed that the loops are formed by condensation of excess arsenic interstitials followed by clustering of excess Ga vacancies and subsequent generation and movement of Shockley partial dislocation(s). The faulted loop is interpreted as an hcp arrangement of arsenic atoms. This model supports the hypothesis that arsenic interstitials and Ga vacancies coexist in GaAs at high temperatures although arsenic interstitials initiate the formation of arsenichyphen;related dislocation loops. Implications concerning the formation process of the EL2 deephyphen;level defect are also discussed.
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