Amorphous structure of semiconducting CdAs2 doped with Ge
In crystalline CdAs2, Cd and As atoms are arranged spirally, to form comparatively large (A- site) and small (B-site) vacant sites, bordered by As atoms. This crystalline structure is shown in Fig. 1. The structure of amorphous CdAs2 has been taken to closely resemble that of the crystalline form; doping the compound with Ge atoms would initially cause Ge occupation of the large A-sites with minimal local lattice distortion. Larger amounts of the dopant, in excess of the solubility limit, would cause the rejection of Ge atoms from the lattice, since occupation of the comparatively smaller B-site occupation would otherwise cause severe lattice distortion. With germanium additions in excess of CdGe0.6As2, the amorphous CdAs2 structure transforms to the CdGeAs2 structure, wherin the cation-sites are equally occupied by Cd and Ge at alternate sites.Samples with appropriate compositions were fabricated by water-quenching the molten compound,vacuum sealed in carbon coated (by pyrolysis) fused silica tubes. The samples were mechanically thinned and ion-milled using a liquid nitrogen cold stage, for examination in the TEM and for EDS.