Defect Structure of Cu-Rich and In-Rich Chalcopyrite CuInSe 2 Films Grown on GaAS
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467 Mat. Res. Soc. Symp. Proc. Vol. 399 0 1996 Materials Research Society
Figure 1. Bright field image of In-rich CulnSe 2 film near the I lTO] zone axis. The film surface appears at the top.
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Figure 2. Diffraction pattern from In-rich film showing weak spots corresponding to twinning.
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Figure 3 shows an image of a nominally Cu-rich film imaged on a [120] zone axis. The [120] axis is chosen because of the appearance of chalcopyrite structure diffraction spots: i.e. diffraction due to the Cu and In sublattice ordering. Thus, images taken on the [120] axis can show sensitivity to defects in the cation sublattice. 7 The horizontal bands in Figure 3 are stacking faults on the cation sublattice: i.e. a translation of the lattice such that the Se sublattice is undisturbed, but which move each cation site onto a non-equivalent cation site. A high resolution image of such a defect is shown in Figure 4. In the chalcopyrite structure, there are four distinct cation sites: two each for Cu and In. Note that they are not all symmetrically distinct: for example, the two Cu sites are indistinguishable from each other in a perfect crystal. The four cation sites correspond to four different stacking possibilities which are allowed with disrupting the anion sublattice. Simulated images of each of these four stacking configurations are shown in figure 5.
Figure 3. Bright field image of Cu-rich film taken on [120] zone axis
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Figure 4. High resolution image of cation sublattice stacking fault taken on the [120] zone axis.
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07711 77 "7ýit Figure 5. Simulated high-resolution image of cation sublattice stacking faults for each of the possi stackings: a) correct stacking, b) [0, 1/2, 1/4] translation, c) [1/2, 1/2, 0] translation, d) [1/2, 0, 1/4 translation. Simulations for defocus of -250A and sample thickness 200A.
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Figure 6 shows a diffraction pattern from near the substrate interface taken from the [120] zone axis. In this pattern, both c-axis and a-axis oriented spots are seen, suggesting that both configurations grow on the GaAs substrate initially. However, most diffraction patterns from the bulk of the films only show the c-axis orientation. The mosaic of defects near the substrate would then be due to mismatch between regions of a-axis and regions of c-axis oriented material. The defects become much more scarce as the film converts to primarily c-axis orientation. It seems possible, however, that some regions of a-oriented material do persist through the entire thickness of films which are very Cu-rich. DISCUSSION There are two especially important conclusions from the results shown here. The first is that the nature of defects in Cu-rich and In-rich films are qualitatively different, both in the type of defect and in spacial distribution: The In-rich films show (112) stacking-fault and twin-type defects throughout the film. Cu-rich films show a high density of dislocations and other ( 112) planar defects near the substrat
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