Sunday, May 3, 2015

Review on ALD of Metal Sulfides

Tuomo Suntola demonstrated the growth of ZnS thin films by ALD 40 years ago growing ZnS. This was the starting point of ALD development in Finland and later ALD research and industrialization of the method worldwide. Today novel applications in energy storage, catalysis, and nanophotonics have lead to an increased interest in metal sulfide materials. The recent focus on 2D layered materials like single-layer MoS2 researched as transistor channel material, is probably the driver in this renewed interest in chalcogenide ALD. Here is a rather fresh review paper on ALD of metal sulfides from University of Michigan and Argonne National Laboratory.


Suntola investigating ALD of ZnS 40 yaeras ago (Picture from 40 years of Atomic Layer Deposition, Riikka Puurunen)

Atomic Layer Deposition of Metal Sulfide Materials 
Neil P. Dasgupta, Xiangbo Meng, Jeffrey W. Elam, and Alex B. F. Martinson
Acc. Chem. Res., 2015, 48 (2), pp 341–348, DOI: 10.1021/ar500360d

The field of nanoscience is delivering increasingly intricate yet elegant geometric structures incorporating an ever-expanding palette of materials. Atomic layer deposition (ALD) is a powerful driver of this field, providing exceptionally conformal coatings spanning the periodic table and atomic-scale precision independent of substrate geometry. This versatility is intrinsic to ALD and results from sequential and self-limiting surface reactions. This characteristic facilitates digital synthesis, in which the film grows linearly with the number of reaction cycles. While the majority of ALD processes identified to date produce metal oxides, novel applications in areas such as energy storage, catalysis, and nanophotonics are motivating interest in sulfide materials. Recent progress in ALD of sulfides has expanded the diversity of accessible materials as well as a more complete understanding of the unique chalcogenide surface chemistry.

Abstract Image

ALD of sulfide materials typically uses metalorganic precursors and hydrogen sulfide (H2S). As in oxide ALD, the precursor chemistry is critical to controlling both the film growth and properties including roughness, crystallinity, and impurity levels. By modification of the precursor sequence, multicomponent sulfides have been deposited, although challenges remain because of the higher propensity for cation exchange reactions, greater diffusion rates, and unintentional annealing of this more labile class of materials. A deeper understanding of these surface chemical reactions has been achieved through a combination of in situ studies and quantum-chemical calculations. As this understanding matures, so does our ability to deterministically tailor film properties to new applications and more sophisticated devices.

This Account highlights the attributes of ALD chemistry that are unique to metal sulfides and surveys recent applications of these materials in photovoltaics, energy storage, and photonics. Within each application space, the benefits and challenges of novel ALD processes are emphasized and common trends are summarized. We conclude with a perspective on potential future directions for metal chalcogenide ALD as well as untapped opportunities. Finally, we consider challenges that must be addressed prior to implementing ALD metal sulfides into future device architectures