Growth and characterization of Ge-substituted Cu2ZnSnSe4 thin films

The incorporation of germanium into Cu2ZnSn(S,Se)4 thin films is gaining a massive attention because of its potential to tailor the properties of kesterite absorbers resulting in high efficiency solar photovoltaic devices. The present work reports the growth of Cu2Zn(Sn,Ge)Se4 (CZTGSe) thin films by incorporation of germanium during the deposition of precursor film consisting of multiple stacks of (Sn/Se/Ge/Se/ZnSe/Se/Cu/Se). The precursor stacks were sequentially evaporated onto soda lime glass substrates held at 100 °C in high vacuum. Subsequent selenization of precursor films in a horizontal tubular furnace at 425 °C led to the formation of single phase Cu2Zn(Sn0.7Ge0.3)Se4 films. X-ray diffraction pattern of stacked layers selenized at 425 °C revealed the formation of Cu2Zn(Sn,Ge)Se4 films with a preferred orientation along (112) plane. Rietveld refinement corroborated the growth of kesterite-type CZTGSe having tetragonal structure with lattice constants of a = 5.678 Å and c = 11.304 Å. Raman measurements performed using multiple excitation wavelengths confirmed the growth of single phase Cu2Zn(Sn,Ge)Se4 films. Secondary ion mass spectroscopy (SIMS) depth profiles of stacked layers selenized at 425 °C illustrated a uniform distribution of constituent elements in Cu2Zn(Sn0.7Ge0.3)Se4 films. The FESEM images showed relatively uniform and spherical grains of 100-150 nm in size. Optical absorption studies of the films showed an optical band gap of 1.19 eV with high absorption coefficient (> 104 cm−1). The films exhibited p-type conductivity with a resistivity of 69.8 Ω cm, mobility of 38.35 cm2(Vs)−1 and carrier concentration of 2.33 × 1015 cm−3.