Ion Channeling Studies of CdTe Films on GaAs
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Ion Channeling Studies of CdTe Films on GaAs Barry Wilkens, Bellcore, Red Bank, NJ Ahmet Erbil, Georgia Institute of Technology, Atlanta, Georgia Abstract Thin films of 11111 oriented CdTe have been MOCVD grown onto 11111 GaAs substrates. When thicknesses exceed 1000A the epitaxy is quite good (backscattering minimum yield of approximately 15%) in spite of a 14% lattice mismatch. A narrowing of the Cd angular scan suggests a displacement of some of the Cd atoms in the lattice. A model based on a Te vacancy is presented to describe the data. Experiment: High quality thin films of II-VI compound semi-conductors (e.g. CdTe) can be epitaxially grown on Group III-V materials, such as GaAs, through various UHV techniques. In the present experiment [1111 CdTe was MOCVD grown onto 11111 GaAs substrates. Ion channeling with MeV He ions was used to study the effects of the large (14%) lattice mismatch between the two materials, on the crystalinity of the film and its interface. Fig. 1 shown the minimum backscatter yield vs film thickness for the series of films measured. It can be seen
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CdTe Film Thickness (Angs.) Figure 1) Minimum yield vs film thickness for [1111 alignment.
Mat. Res. Soc. Syrup. Proc. VoL 130. 0198• Materials Research Society
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from this figure that after approximately 1000 A the minimum yield reaches a value under 20%. In the [100] CdTe/ [100] GaAs system it has been shown that the mismatch is taken up at the interface by edge dislocations with a periodicity 1 This is most likely what is corresponding to the 14% lattice mismatch. occuring in the present system, however confirmation awaits TEM results. Angular scans of the films about the [111] axis (fig. 2) show a narrowing in the 1.2
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Cd angular minimum yield with respect to the Te and GaAs substrate, indicating a small displacement of some of the Cd atoms from their lattice sites. Channeling along the off-normal [2111 axis reveals, along with poorer overall channeling, an offset (.150 + .020) of the Cd minimum yield (fig. 3). The Te minimum yield remains aligned with that of the GaAs substrate indicating an effect in the Cd sublattice itself.
Discussion: A model based on To vacancies gives a geometric description for the angular scan observations. A To vacancy would allow its four tetragonally bonded Cd atoms to relax or flatten out to a trigonal configuration with their three Te nearest neighbors. This results in Cd atoms moving off lattice sites and protruding into channels by as much as .3A depending on the "flatness* of the trigonal bond. The lattice displacements are symmetrical about the [111] axis resulting only in a narrowing of the Cd scan as shown in fig. 2.. However, the displacements are
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