Compound semiconductors of chalcopyrite structure with high optical absorption coefficient (α = 10^-5 cm^-1) such as CuInX₂ (X = S, Se) warrant interest as absorber materials for thin film solar cells. Solar cells up to 17 % and 11 % have been realized with polycristalline p- CuInSe₂/n-CdS and p-CuInS₂/n-CdS thin film junctions. Such efficiencies seem to represent the upper limit of polycrystalline systems.
Single crystals of adequate doping and geometry, however, may overcome this limit. Single crystalline (112)-oriented CuInS₂ surfaces in the cm₂-range can be obtained using a lamellar eutectic growth mechanism crystallizing a CuInS₂ melt by horizontal gradient freeze in a temperature gradient ranging from 5 - 15 øC/cm under elevated argon pressure of ~20 bar.
In order to identify single crystalline lamellae, an analytical technique of ultimate lateral resolution was required. Auger microprobe analysis (AES) together with scanning electron microscopy SEM (Perkin-Elmer, PHI 595) could be applied on single lamellae edge faces with high resolution at U_ac = 0.5 kV. Layer sequences were cut and polished perpendicular to the cleavage plane of the chalcopyrite matrix. Fig. 1 shows the lamellar nature of the sample with a matrix consisting of chalcopyrite and irregularly intergrown lamellae of an additional phase with at most 1.5 um thickness. Auger line scans with lateral resolution of less than 2 um show the semiquantitative distribution across the strata. Position 5 is due to a surface void which has been filled by undefined abraded materials during the polishing process. An Auger point analysis of the locations 1, 2, 3, 4 yielded the composition of an In-rich CuInS₂ matrix and the stoichiometry of CuIn₅S₈ lamellae.
The appearance of 1.5 um thick lamellae of the spinel phase CuIn₅S₈ which are epitaxially intergrown with the CuInS₂ matrix in the cubic [111] direction can only be explained assuming an incongruent melting point for CuInS₂. Recent investigations of the phase relations in the CuS - InS and Cu₂S - In₂S₃ join, respectivelly, confirm this assumption. The defect chemistry and the growth mechanism will be presented as well as the use of the growth pattern to improve the material for thin film solar cells.

Fig. 1 Secondary electron micrograph of a polished cross section of a CuInS2 - CuIn5S8 crystal fragment showing eutectic coupled growth features (above); Auger microprobe scanning analysis along the cross section (below).

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