Radio frequency sputter deposition of Cu2ZnSnS4 thin films with a temperature-controlled reflector wall: Effects of H2 addition to the sputtering gas

Cu2ZnSnS4 (CZTS) thin films were deposited from a quaternary ceramic compound target using radio-frequency magnetron sputtering in Ar and Ar-10% H-2 to investigate the effects of H-2 addition to Ar on the film structure and optical and electrical properties. The sputter-deposition system was equipped with a cylindrical reflector wall between the target and substrate to enhance the probability of reimpingement of Zn and S atoms re-evaporated from the growing thin-film surface to the film surface. The thin films deposited in both Ar and Ar-10% H-2 were identified as kesterite Cu2ZnSnS4 with < 112 > preferred orientation by X-ray diffraction measurements and Raman spectroscopy. The crystallite size evaluated from the half width at full-maximum of the < 112 > diffraction peak increased from 10 to 12 to 24 nm with the H-2 addition to Ar at substrate temperatures < 200 degrees C. The lattice spacing of (112) also increased by 2-3 nm with the H-2 addition to Ar. Cross-sectional scanning electron microscopy images of the thin films revealed that the thin-film structure changed from a fine to coarse columnar structure with the H-2 addition to Ar. X-ray photoelectron spectroscopy analysis indicated that the O content decreased, whereas the Cu content increased with the H-2 addition. The band gap range of the CZTS thin films deposited in Ar-10% H-2 was 1.4-1.5 eV, which was narrower than that of the films deposited using Ar. The Hall mobility and carrier density were not greatly affected by the H-2 addition to Ar. Although other factors than O content, such as cation ratio, cation/anion ratio, and film crystallinity affect band gap and other film properties, from the fact that the H-2 addition affected the lattice spacing and the band gap clearly, it was concluded that suppression of the incorporation of oxygen into CZTS thin films by the H-2 addition to Ar was the foremost factor for the decrease in the lattice spacing and the narrowing of the band gap.