Deformation-Induced Dislocations in 4H-SiC and GaN
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hand, the dislocations induced by deformation in the temperature range 550 Burgers vector [7]. As in other tetrahedrally-coordinated structures, these dislocations are dissociated into two partials with Burgers vectors b,=1/3 and b,=1/3, where the subscripts 1 and t denote the leading and trailing partials, respectively. The dislocations dissociate as follows: 1/3 = 1/3 + 1/3 In tetrahedrally coordinated compounds, because of the polarity along the [0001] axis, the core of the perfect or partial basal dislocations consists of only one species, i.e. silicon (gallium) or carbon (nitrogen), and, because the partial dislocations belong to the glide plane, they are denoted as Si(g) [Ga(g)] or C(g) [N(g)] [8]. 411-SiC Fig. 2 shows typical bright-field (BF) micrographs of 4H-SiC deformed in compression. Both micrographs in this figure were obtained using the g= 1011 reflection near the [1012] zone axis. Samant [9] has recently shown that plastic deformation of 4H-SiC above -1100°C takes place by the activation of the (0001) slip system with the uncorrelated motion of partial dislocations. This is clearly shown in Fig. 2(a) where dissociated dislocations, consisting of a pair of leading/trailing partials bounding a ribbon of intrinsic stacking fault, are observed. Standard strain contrast experiments indicate that, as expected, the Burgers vectors of the partials are, respectively, parallel to < 1100> and < 1010> directions.
ab
Fig. 2. TEM micrograph of 4H-SiC deformed in compression (a) at 1300°C and (b) at 7009C From the width of partial separation, the stacking fault energy of 4H-SiC has been estimated to be 14.7+2.5 mj/m2; this value is nearly five times larger than that (2.9±0.5mJ/m 2) of 6H-SiC obtained by the same techniques [10]. Fig, 2(b) shows a BF micorgraph of dislocations induced by deformation at 7009C. In this case, the microstructure is dominated by single leading partials without the associated trailing partials; each partial drags a stacking fault. The Burgers vectors of the single leading partials are all parallel to the < 1100> directions, and LACBED experiments on three different segments have shown that they have a silicon core (i.e. they are Si(g) partials) [11 ]. Thus, it appears that in the 4H-SiC single crystals, deformation proceeds by nucleation and glide of single leading Si(g) partials at low-temperatures ( 1100C). It has already been argued 371
[12] that only the leading partials, with a silicon core, nucleate at low temperatures (- 1100°C) is required to nucleate the associated carbon-core trailing partials whereby the glide of leading/trailing pairs (i.e., dissociated dislocations) will carry out the plastic deformation: at a certain applied shear stress, the activation barrier for nucleation of trailing partials is higher than that of the leading partials. Since the formation of the same partial dislocation from the same source cannot occur more than once on the same glide plane, plastic deformation of the crystal can take place to a very limited extent at low temperatures. On the other
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