Growth characteristics of Cu(In,Ga)Se2 thin films using 3-stage deposition process with a NaF precursor

Cu(In,Ga)Se2 or CIGS thin films with NaF precursor are grown on Mo coated soda-lime glass (SLG) substrates using a molecular beam deposition (MBD) technique. The growth characteristics of the CIGS films deposited using the 3-stage process are examined by interrupting the deposition process at the end of each stage and the transition temperature at the beginning of the second stage. The evolution of the CIGS films derived from the compounds, e.g. γ-(In,Ga)2Se3, Cu(In,Ga)Se2+CuxSe, is investigated by comparing the properties of the films to those without an NaF precursor. The XRD spectra show the dominant peak of (1 0 5) preferred orientation of the γ-(In,Ga)2Se3 enhanced by the NaF precursor which is seen at the end of the first stage. The decrease of (2 2 0)(2 0 4) intensity of the CIGS film is found at the end of the second stage followed by the increase of (1 1 2) intensity at the end point. The AFM images at the end of the first stage show a smooth surface with similar grain shape in the films with the NaF precursor. During the second stage, the grain size of the Cu-rich CIGS film increases with slightly sharper grain boundaries. However, at the end point, the CIGS film is fully obtained and shows small sharp grains corresponding to the increase of (1 1 2) orientation. The cross-section SEM images show small columnar grains with deep grain boundaries at the end point. The AES depth profiles show that most Na atoms are located near the bottom layer in the first stage and diffuse to the surface of the film after increasing the temperature. Then the Na atoms uniformly distribute into the Cu-rich CIGS film during the second stage. Significantly high Na content is found at the surface of the CIGS film at the end point. In addition, a gradient of Ga composition in the CIGS film is also observed in the AES measurement. A simple model of Na-enhanced CIGS thin film growth based on the experimental results is proposed to describe the growth and doping mechanisms.