Study of atomic layer deposition of indium oxy-sulfide films for Cu(In,Ga)Se2 solar cells

This paper explores the growth mechanism of plasma enhanced atomic layer deposition of In2(S,O)3 films. The films were deposited using indium acetylacetonate (In(acac)3), hydrogen sulfide (H2S) and Ar/O2 plasma as oxygen precursor. The films were characterized using X-ray reflectometry, spectrophotometry, energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. To understand the growth mechanism and especially the interactions between Ar/O2 plasma and In2(S,O)3 growing film, in-situ analyses were performed using quadrupole mass spectrometry. In-situ qualitative analysis revealed good correlation between the species detected in vapor phase and thin film properties. High concentrations of atomic and molecular oxygen were measured in the vapor phase during O2 plasma pulses. Significant decrease of these species could be observed by varying the plasma power from 2600 to 300 W, while the optical band gap remained at high values (> 2.6 eV). The analysis of the O2-free/Ar plasma process showed that some of these oxygen species originate either from the indium precursor or from the substrate surface. This study explains the high oxygen content of the films, and allows us to reduce and control it. Generally, this report provides keys to understand the effect of plasma reactivity for the elaboration of oxide based materials.