Atomic Layer Deposition of Ternary Compounds on Cu(In,Ga)Se2: An in Situ Quartz Crystal Microbalance Study

Abstract

For ternary compounds grown by atomic layer deposition (ALD), some compositional variations during the first nanometers of deposition can occur if the initial cycles of the process differ from film-on-film growth. This is problematic for applications that require a precise control of the composition near the interface to the underlying material. One such application is thin-film solar cells, where most emerging buffer layers are ternary compounds. However, such compositional variations can be very difficult to quantify with conventional material characterization methods. A method based on in situ quartz crystal microbalance (QCM) was therefore employed, which allows for detailed studies of the initial growth of ternary compound buffer layers on Cu(In,Ga)Se2 (CIGS) solar cell absorbers, namely, Sn1-xGaxOy and Zn1-xSnxOy. Here, conventional Cr/Au-coated quartz crystals were precoated with a Mo/CIGS thin-film stack before being mounted in an ALD reactor, after which the initial growth on the as-deposited CIGS surfaces was recorded using a QCM monitor. The mass gain for each individual subcycle was extracted and used to estimate compositional depth profiles. It was found that the cation ratio differed for up to the first 3 nm of the deposited film compared to bulk composition for both processes. More specifically, Sn1-xGaxOy was slightly enriched in Ga at the interface (Δx ≈ 10%) and Zn1-xSnxOy was enriched in Sn (Δx ≈ 29%). These compositional variations were suppressed when using RbF-treated CIGS surfaces, which is attributed to a decreased nucleation delay. Similar variations are expected to be present in solar cells fabricated with ternary compound buffer layers, which can potentially influence charge transport and recombination near the absorber/buffer interface. The methodology demonstrated in this study can both improve the understanding of interface formation and allow for an enhanced control of interface properties, in thin-film solar cells and other structures that employ ternary compounds grown by ALD.