Aggregate structure in CuBSe₂/Mo films (B=In,Ga): Its relation to their electrical activity

CuBSe₂/Mo films of about 3 μm in thickness prepared on different substrates (Si single crystal, glass, alumina) by three-source evaporation have been investigated by electron and optical microscopic techniques: Scanning electron microscopy [including (EBIC) electron-beam-induced current mode], conventional transmission electron microscopy (TEM), and optical Nomarski microscopy. They show, on top of their well-known dense polycrystalline structure (≊1 μm average grain size), a more coarse "aggregate" structure with aggregate dimensions of 20-100 μm, depending on the substrate used. The aggregate boundaries are characterized by very poor EBIC collection efficiency. For samples on glass substrates, this structure, as detected in EBIC, correlates with deformation patterns of the Mo layer seen by Nomarski contrast when viewed through the glass side of the samples. Local electrical measurements made on small Schottky contacts reveal a correlation between the aggregate structure and the I-V characteristics. TEM studies show the presence of both cubic (sphalerite) and tetragonal (chalcopyrite) phases with very clear intergranular phase separation. In many of the grains, twinning was observed. Separate studies of the aggregate boundaries show them to be made up of very tiny crystallites (≊50 Å), apparently mixed with amorphous material. Some of these crystallites in the CuInSe ₂ films are identified as CuSe and In₂Se₃ particles.