Mechanical properties and dislocation dynamics of GaP
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namical properties of individual dislocations are deduced. The results are compared with those in Si and GaAs so far investigated. II. EXPERIMENTAL Specimens for compression tests were prepared from a GaP crystal doped with sulfur impurity at a concentration of 3.3 X 1017 cm" 3 grown by the liquid encapsulated Czochralski technique. They were finished by chemical polishing with hot aqua regia following the mechanical lapping into a rectangular shape, approximately 2.7 x 2.7 x 10.6 mm3 in size. The compression axis was parallel to the [T23] direction and the side surfaces parallel to the (111) and (541) planes. The densities of grown-in dislocations ranged between 2 X 104 and 3 X 105 cm"2. Compression tests were carried out under constant strain rates with the use of an Instron-type machine at various temperatures in a high purity argon gas atmosphere. The effective stress was determined by means of the strain-rate cycling technique, the principle of which was described elsewhere in detail.8 This is the stress needed to move dislocations at a certain velocity against the intrinsic resistance of the crystal lattice. Such a velocity is determined by the externally given deformation condition. The nature of collective motion of dislocations in deformation was deduced from the behavior of the effective stress. III. RESULTS AND DISCUSSION A. Stress-strain characteristics Figure 1 shows stress-strain curves of specimens with a density of grown-in dislocations of 2 x 104 cm"2 under a shear strain rate of 2 x 10~4 s"1 at various temperatures between 600 and 900 °C. The curves are characterized by a noticeable drop in the stress after yielding, followed by a gradual increase in the stress with the strain due to workhardening. The magnitude of the upper yield stress and the amount of stress drop after yielding both depend very 355
1989 Materials Research Society
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I. Yonenaga and K. Sumino: Properties and dynamics of GaP
10 20 Shear strain, %
10 20 Shear strain, % FIG. 1. Stress-strain curves of GaP with a density of grown-in dislocations of 2 x 104 cm~2 under a shear strain rate of 2 x 10~4 s' 1 at various temperatures between 600 and 900 °C. The effective stress Teff determined by the strain-rate cycling technique is also shown as a function of the strain.
sensitively on the temperature, diminishing rapidly as the temperature is raised. The stress drop is absent at 900 °C. Such characteristics in the temperature dependence of stress-strain behavior are commonly observed in Si,6 Ge,2 InSb,16 GaAs8 so far investigated. The noticeable drop in the stress after yielding is a direct consequence of the fact that the velocity of a dislocation in the crystal is insensitive to the stress.17 Thus, we know that the magnitude of the so-called stress exponent is small in GaP as in Si, Ge, InSb, and GaAs. Figure 2 shows stress-strain curves of specimens with a density of grown-in dislocations of 2 x 104 cm" 2 at 700 °C under various shear strain rates. T
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