Interfaces and Composition Profiles in Metal–Sulfide Nanolayers Synthesized by Atomic Layer Deposition

The sharpness of interfaces in multilayer metal–sulfide thin films synthesized by atomic layer deposition (ALD) is virtually unexplored. Presented here are some first results that indicate metal–sulfide multilayer thin films deposited by ALD are in an entirely different regime than metal–oxides in terms of their composition profile. We propose a mixing number to characterize interfacial sharpness. The mixing number is the diffusion distance of mobile atomic species during layer deposition divided by the layer thickness in which the mobile species is diffusing. Ultrathin metal–sulfide multilayers with the structure ZnS/SnS 2 /Cu 2 S/Si substrate were synthesized by ALD because of the relevance of these binaries for formation of the photovoltaic alloy Cu 2 ZnSnS 4 (CZTS). The composition profiles were measured by time-of-flight secondary-ion mass spectrometry (TOF SIMS) with high depth resolution, both as deposited and after annealing at different temperatures in argon. Diffuse interfaces between layers containing the intended elemental species were found in the as-deposited case, indicating a mixing number similar to unity at the synthesis temperature of 135 °C for several pairs of adjacent layers. Annealing the metal–sulfide multilayer structure at 425 °C for 60 min was sufficient to fully mix the layers. Composition profiles were also measured for 20 nm ZnO and 22 nm ZnS capping layers on annealed CZTS films. ZnO suppresses diffusion relative to ZnS but does not prevent it completely. This indicates that the real-time mobility of atomic species in the substrate or underlying layer, as synthesis takes place, plays a critical role in determining the composition profile of ALD multilayer films.