Hydrogenated nanocrystalline silicon (nc-Si:H) thin films grown by plasma-enhanced chemical vapor deposition (PECVD) from feed gases containing silane (SiH4) and hydrogen (H2) have tremendous potential for electronic, optoelectronic, and photovoltaic device fabrication technologies. Plasma-surface interactions during PECVD and subsequent H2 plasma treatment of these films determine their structure and properties. The films may be either polycrystalline, where nanometer-size grains are separated by grain boundaries, or polymorphous where the nanocrystals are embedded in a hydrogenated amorphous Si (a-Si:H) matrix. Fundamental understanding of the plasma-surface interactions that govern the nucleation and growth of the nanocrystalline phase during deposition or post-deposition processing, as well as control of the nanocrystalline grain size distribution are essential for tailoring the electronic and optical properties of the deposited films.
This research aims at developing strategies for controlling the grain size and crystalline fraction in nc-Si:H films formed through low-temperature PECVD from SiH4 heavily diluted in H2 or through post treatment of a-Si:H films with H atoms created by plasma dissociation of H2. Toward these goals, the PIs propose a research plan that integrates in situ plasma and surface diagnostics with atomic-scale simulations. They seek a fundamental and quantitative understanding of the role of hydrogen in the nucleation and growth of nanocrystalline silicon films that will aid in manipulating synthesis methods and choosing plasma-processing parameters to gain a better control over the film properties than is currently possible. As a result, the proposed study will set the stage for establishing quantitative relationships between the film's structure (e.g., grain size and crystalline fraction) and plasma processing parameters, such as the H flux and the substrate temperature.
The proposed experimental work will focus on synthesizing silicon films containing nanocrystals through H-atom post treatment of a-Si:H films deposited by PECVD. Fluxes of H atoms will be measured using line-of-sight threshold-ionization mass spectrometry. In situ multiple total internal reflection Fourier transform infrared (MTIR-FTIR) specroscopy will be used to detect silicon hydrides in the growing film and on its grain boundaries. High-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and Raman spectroscopy will provide information on the grain sizes and grain-size distribution, as well as crystalline fraction; spectroscopic ellipsometry will be used in situ during deposition and post treatment to monitor the evolution of these same parameters. In conjunction with the experimental work, molecular-dynamics (MD) simulations of a-Si:H film growth and H2 plasma post treatment will be carried out aiming at both fundamental understanding of the growth and crystallization mechanisms and comprehensive identification of chemical reaction and diffusion processes for subsequent quantitative energetic and rate analysis. The resulting reaction/diffusion database will be used as input for implementing hybrid off-lattice kinetic Monte Carlo (KMC) simulations that are capable of capturing the long-time- scale dynamics of silicon film growth and H2 plasma post treatment. The computational results will be compared directly with the experimental data; the insights gained from the simulations will be used to guide new experimental studies and design new deposition strategies.
Intellectual Merit - The proposed research is pioneering in linking experimental diagnostic measurements and structural characterization analyses with computational atomic-scale studies of chemical reactions and crystallization mechanisms. The research is particularly timely given our recent developments of in situ experimental techniques for monitoring plasma-surface interactions and atomic-scale simulation tools. The PIs anticipate that their research findings will enable systematic engineering strategies for controlling thin-film crystallinity and the grain size distribution of nanocrystalline silicon films, which in turn determine the films' electronic and optical properties. In addition, they expect that their research strategy and methodology will be applicable to studying the growth and processing of various other technologically important materials.
Broader Impact - The scientific underpinnings of nanostructured materials synthesis and thin-film deposition & processing are multidisciplinary and cut across traditional boundaries between physics, chemistry, chemical engineering, materials science, as well as applied and numerical mathematics. Thus, the proposed systematic study of silicon thin-film deposition and post-deposition processing provides ideal means for training students to address technologically important problems using an integrated, state-of-the-art experimental and computational approach. The results of the research will be disseminated broadly in the physics, chemistry, electronic materials, and plasma engineering communities through publications and conference presentations. The proposed research has the potential to enable technological advancements in low-temperature plasma deposition of nc-Si:H films which will have tremendous impact on fabrication of solar cells for renewable energy production and flexible display manufacturing for consumer electronics.
