Hybrid Growth Chamber                       Glow from copper target

The typical layer structure of a current high-efficiency CuInGaSe2 (CIGS) device is as follows: a soda-lime glass substrate, Mo back-contact, CIGS absorber layer (p type), CdS buffer layer (n−), and ZnO (n++) window layer. In a typical device, the CIGS absorber layer is a polycrystalline film of 1.5–2 micrometer thickness. The film is deposited on a Mo-coated soda-lime glass in one of the many multi-stage processes, either by coevaporation or by selenization of metallic precursor films. On top of the CIGS absorber, a thin (»60 nm) CdS buffer layer is grown by chemical bath deposition (CBD), followed by Al-doped ZnO window layer deposited by sputtering or chemical vapor deposition. With evaporated fingershaped Al electrode on ZnO, the device is completed. The photovoltaic cell generates electromotive force through the separation of photo-generated positive and negative charges by a potential barrier. In the CIGS device, the ZnO/CdS/CIGS heterojunction provides the potential barrier. Most of the light is absorbed in the p-type CIGS layer (there is some absorption of radiation above 2.4 eV in the thin CdS layer), and photo-generated electrons are swept into the n type region once they migrate into the built-in field of the heterojunction.

CIGS films in our lab are deposited using a hybrid sputtering and evaporation technique. Commercially supplied “epi-ready” (110), (001) and (111)B and (111)A GaAs wafers are used as substrates. The In, Ga, and Cu fluxes are generated by magnetron sputtering of In and Cu or Cu-Ga alloy targets in 99.9999% pure Ar at 0.3Pa. Se is supplied in excess from an effusion cell. Layers with a range of compositions can be obtained by varying the flux ratio of Cu-Ga relative to In and by changing the Cu-to-Ga ratio in the Cu-Ga target. Growth temperatures range from 480 ±C to 720 ±C. Typical growth rates are 1.0 micron/h and the thickness of grown layers ranges from 0.1 microns to 2 microns.

Copper target being sputtered                           GaAs substrate at 700C  

The surface morphologies of the films are examined by optical microscopy and scanning electron microscopy (SEM). The topology of selected films is characterized with atomic force microscopes (AFM). SEM is also used to study the cross-section of cleaved samples and measure the thicknesses of the layers. The average compositions of the layers are measured by energy dispersive X-ray spectroscopy (EDS) in the SEM calibrated using a polycrystalline CIGS standard. Elemental depth profiles are measured by secondary ion mass spectrometry (SIMS) and Auger electron spectrometry (AES). The structure, orientation, and quality of the deposited layers are characterized by electron backscatter diffraction (EBSD) and X-ray diffraction (XRD).

Polycrystalline CuInSe2                            X-SEM of CuInSe2 Bicrystal