Atomic layer deposition of zinc indium sulfide films: Mechanistic studies and evidence of surface exchange reactions and diffusion processes

Abstract

The authors present the elaboration of zinc indium sulfide (ZnInxSy) thin films in the context of a cadmium-free buffer layer development for copper indium gallium diselenide photovoltaic solar cells. The films were deposited by atomic layer deposition (ALD) from ZnEt2 (DEZ), In(acac)3 (acac = acetylacetonate), and H2S at 200 °C. In situ growth kinetics studies were performed with the quartz crystal microbalance technique to determine the respective mass gain per cycle of ZnS and In2S3 layers, allowing determination of the atomic compositions of the ZnInxSy thin films to be expected if the deposition strictly follows the rule of mixtures. As the experimental atomic compositions of the ZnInxSy films differ significantly from this rule, a comprehensive study of the growth mechanism was performed to determine the nature of the side reactions. First, an exchange reaction between In2S3 and the Zn precursor was identified, though this process is not sufficient to account for the experimental data, and therefore, a second process which corresponds to the diffusion of species within the film was also found to take place. Ultimately, the atomic compositions of the ZnInxSy films can be explained by a rate-limited exchange reaction at the surface between DEZ and the In2S3 layer, combined with diffusion of the species in the whole film. More generally, such side reactions should be considered in ALD of multinary compounds, even at low temperature.

In Situ Microgravimetric Study of Ion Exchanges in the Ternary Cu-In-S System Prepared by Atomic Layer Deposition

“…ng.cm −2 ), a progressive mass loss during the purge (−(9 ± 1) ng.cm −2 ), and a mass loss during the H 2 S pulse (−(19 ± 1) ng.cm −2 ). These mechanisms correspond to the ligand exchange reactions, as explained elsewhere [27,28]. This description will serve as a reference for the study of ion exchanges with an ultrathin film of Cu x S. explained elsewhere [27,28].…”

“…These mechanisms correspond to the ligand exchange reactions, as explained elsewhere [27,28]. This description will serve as a reference for the study of ion exchanges with an ultrathin film of Cu x S. explained elsewhere [27,28]. This description will serve as a reference for the study of ion exchanges with an ultrathin film of CuxS.…”

“…Growth mechanism of In 2 S 3 film was first studied and compared with the literature [26][27][28][31][32][33]. Figure 1a depicts the mass variation of In 2 S 3 deposited on an Al 2 O 3 substrate, monitored by the QCM during 900 cycles.…”

“…First, pure films of In 2 S 3 and Cu x S were grown on Al 2 O 3 . While In 2 S 3 had already been investigated with by in-situ QCM [26][27][28], the growth of ALD-Cu x S was investigated, to the best of our knowledge, for the first time with this set of precursors. Then, ion exchanges in the Cu-In-S ternary system were investigated by monitoring the growth of In 2 S 3 on Cu x S and of Cu x S on In 2 S 3 but also the behavior of the Cu precursor on In 2 S 3 .…”

Atomic Layer Deposition of p‐Type Semiconducting Thin Films: a Review

“…Despite the vast ALD literature on metal sulfides, only a handful of the papers confirm the p-type semiconductivity for the films; the possible reason may be related to the inherent difficulties with sulfide ALD processes such as H2S toxicity, air reactivity, and ion-exchange problems. [58,201,202] However, with the recent advances in the field of ALD of metal chalcogenides it should be possible to find appropriate ways to overcome these problems. For example, ALD of sulfides has been successfully implemented over the past decade in each of the three roles common to most of the photovoltaic cells, i.e.…”