Growth of CuInSe2 crystals in Cu-rich Cu-In-Se thin films

A Cu-rich CuInSe2 (CIS) thin film with an atomic ratio of Cu/In = 3.6 was characterized using high-resolution and analytical transmission electron microscopy (TEM). The film was deposited on a Mo coated soda-lime glass substrate by physical vapor deposition. Rutherford backscattering spectroscopy (RES) and Auger electron spectroscopy (AES) showed that a secondary impurity phase such as Cu2Se segregated on the CIS surface. The three-dimensional crystallographic relationship between the Cu2Se and CIS was found to be (11 (1) over bar)(Cu2Se) parallel to (11 (1) over bar)(CIS) and [011](Cu2Se) parallel to [011](CIS) where the Cu2Se and CIS had pseudocubic structures with a = 5.8 Angstrom and a = 11.6 Angstrom, respectively. CuPt type CIS could be observed near the interface between the Cu2Se and CIS. A growth model of CIS crystals under Cu and Se excess condition is proposed based on the results of TEM. The characteristics of the CIS growth model in Cu-rich CIS film are summarized as follows: (i) CIS crystals are produced from Cu2Se crystals by a ''topotactic reaction,'' (ii) sphalerite and/or CuPt type CIS are produced first after the reaction, and (iii) the metastable sphalerite and/or CuPt type CIS is then transformed to the stable chalcopyrite CIS phase.