@article{80752eb5e6124275b07429d264d5dcd7,
title = "The structure and morphology of (112)-oriented Cu (In,Ga) Se2 epitaxial films",
abstract = "The properties of the {112} surfaces of Cu (In,Ga) Se2 (CIGS) are important to the performance of photovoltaic devices based on these materials. Epitaxial CIGS films were grown on GaAs (111) A (cation-terminated) and B (anion-terminated) substrates and the structure and morphology of the films were studied. There are a large number of rotational twins in films grown on (111) B substrates, but not for (111) A substrates. The lm surfaces consist of shallow triangular pyramids bounded by one type of 〈 110 〉 step. The steps show both sharp inside and outside corners. New layers nucleated as islands at step edges. The step heights are a mixture of single and multiple atomic layers. The change in surface chemistry from cation to anion terminated affects the measured electronic states comprising the valence band and is probably accompanied by a shift in the valence band edge. There is a distinct difference in step edge shape that shows a difference in step kink density but the expected high kink energy on both surfaces indicates that this may be due to kinetic rather than thermodynamic effects.",
author = "Dongxiang Liao and Angus Rockett",
note = "Funding Information: We gratefully acknowledge the support of the National Science Foundation under Award No. 0602938-0017756000 Materials World Network. At the beginning of the project the program was supported by the Office of Basic Energy Sciences under Contract No. DEFG02-91ER45439, which also supports the Center for Microanalysis of Materials (CMM) at the University of Illinois. All microanalysis was carried out in the Center for Microanalysis of Materials at the University of Illinois, which is supported by the U.S. Department of Energy. The authors thank Mauro Sardela, Jim Mabon, Scott McLaren, and Rick Haasch for their assistance in XRD, EBSD, and AFM experiments, respectively. FIG. 1. EBSD patterns for a GaAs (111) A substrate (top left), the corresponding CIGS (112) A epitaxial layer grown on this substrate (top right) and two patterns for CIGS (112) B epitaxial layers (bottom left and right) showing the presence of twinned domains. The twins result in 180 ° rotation of the EBSD patterns. No twinned regions or rotations of the EBSD patterns were observed on the (112) A surfaces. FIG. 2. AFM images of the surface of CIGS epitaxial films grown on GaAs(111) A (left) and B (right) substrates. The top pair of images shows that the surface consists of pyramidal islands. On the A surface all islands point in the same direction while the B surface includes islands rotated 180 with respect to each other, indicating the presence of growth twin defects. Higher resolution images of the centers of pyramidal islands on each surface are given in the middle. The lower row provides a sketched outline of some of the step edges in the center images to highlight the differences in step morphologies for the two surfaces. FIG. 3. A schematic diagram of the surface atomic structure of an island on the Se-terminated CIGS {112} B surface. Metal atoms are designated with gray and white circles while Se atoms are designated with black circles. FIG. 4. AFM images of two CIGS (112)B surfaces grown at 705 (top) and 715 (bottom) ° C . Both images show 20 × 20 mm 2 areas. FIG. 5. AFM images of the top of triangular islands on (112) A and (112) B surfaces. The left image includes ∼ 2.5 μ m arcs along steps with this curvature. Other similarly curved steps are visible in the image. The (112) B steps are straight to within the ability of our AFM to measure although the image suggests that the steps meander along this straight line by 5 nm. FIG. 6. Valence band spectra of {112} A and B surfaces either air-exposed or prepared clean by sputtering or Se capping in the deposition chamber and subsequent cap evaporation in the XPS preparation chamber. Both linear (a) and logarithmic (b) plots are given to emphasize different portions of the spectra. The normalization of both the linear and logarithmic plots is the same. ",
year = "2008",
doi = "10.1063/1.3009961",
language = "English (US)",
volume = "104",
journal = "Journal of Applied Physics",
issn = "0021-8979",
publisher = "American Institute of Physics",
number = "9",
}