Mechanical properties of GaAs crystals
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INTRODUCTION Extensive studies have been conducted on the mechanical behavior of elemental semiconductors such as Si and Ge crystals in terms of dislocation dynamics.1"7 Using an empirical expression for the dislocation velocity as a function of the stress and the temperature, the present authors6'7 have shown that the experimentally observed stress-strain characteristics of Si crystals are described quantitatively well with the model including the assumption that most dislocations are generated by means of multiplication of themselves and that they interact with each other through their elastic stress fields. Dislocations of a high density move together during plastic deformation of a crystal to give rise to a macroscopically observable strain rate. The mean velocity and density of such dislocations in Si and Ge crystals have been determined by means of the so-called strain-rate cycling tests as a function of the strain, the temperature, the strain rate, and the initial state of the crystal. The experimental results have been found to be described quantitatively well with a theoretical model proposed by Sumino.5 In comparison with elemental semiconductors, only a small number of works have been conducted on mechanical properties of compound semiconductors such as GaAs crystals.8"1' It seems to be reasonable to suppose the mechanical behavior of a GaAs crystal to be similar to that of Si and Ge crystals to some extent because of the similarities in atomic bonding and crystal structure. Indeed, a GaAs crystal is brittle at room temperature, while it becomes ductile at elevated tempera252
J. Mater. Res. 2 (2), Mar/Apr 1987
tures; plastic deformation at high temperature takes place by means of a slip of the {l 11}/( 110) system, as in Si and Ge crystals. However, a compound semiconductor has peculiar problems associated with polarity and composition. Glide dislocations with an edge component are distinguished into two types, namely, a dislocations and /? dislocations, depending on the species of atoms located at the dislocation core. These two types of dislocations have different dynamic characteristics. The mobility of a dislocations has been reported to be higher than that of/3 dislocations by orders of magnitude in an undoped GaAs crystal. 12~16 In this paper the characteristics in plastic deformation of GaAs crystals are investigated. Stress-strain behavior and yield strength are measured as a function of the temperature and strain rate. The effect of the initial state of a crystal that is controlled by heat treatment prior to the deformation test and the effect of the surface condition are also examined. Moreover, characteristics in collective motion of dislocations during deformation are determined. II. EXPERIMENTAL Specimens for compression tests were prepared from two kinds of GaAs crystals grown by either the boat technique or the liquid encapsulated Czochralski (LEC) technique. Main impurities are shown in Table I together with the concentration. The specimen had the compression axis along the [ 123 ] direc
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