(Invited) Role of Charges in Non-Classical Crystallization of Thin Films and Nanostructures in the Plasma CVD Process

Abstract

Extensive studies have been made on non-classical crystallization, which refers to the crystal growth by the building unit of nanoparticles. In parallel with the non-classical crystallization in the crystal growth in solution, the non-classical crystallization in the gas phase synthesis of thin films and nanostructures by chemical vapor deposition (CVD) and some physical vapor deposition (PVD) has been studied extensively. Here, the charged nanoparticles (CNPs) are spontaneously generated in the gas phase and become the building block of thin films and nanostructures. Charged nanoparticle-based crystallization appears to be very general, including the growth of diamond, Si, ZrO2, GaN, ZnO films as well as nanowires. The generation of CNPs in the gas phase was experimentally confirmed in many systems and their mass distribution was shown to play a decisive role in the microstructure evolution of films, nanowires, and nanotubes. The fact that CNPs can be a building block of crystals without leaving any void behind and of nanowires with smooth surface indicates that CNPs are quasi-solid, having a liquid-like property in diffusion. This means that the charge enhances the atomic diffusion, which is a newly discovered physical phenomenon. This means again that charge weakens the bond strength. Small nanoparticles can be liquid-like even if singly charged but large nanoparticles should be multiply charged to be liquid-like. Based on this understanding, the microcrystalline silicon without an amorphous incubation layer could be deposited on a glass substrate at 320oC and on a polymer substrate at 200oC. Using the liquid-like property of CNPs, the epitaxial growth of silicon was successfully achieved at 550oC by the plasma CVD process. Weakening of bond strength by charges would enhance chemical reactions at low temperature, which can explain the low temperature decomposition in the plasma CVD process. Charge-enhanced diffusion would favor the crystallization and produce charged crystalline nanoparticles at low temperature in the plasma, which explain the low temperature deposition of crystalline films by the plasma CVD process. References N.M. Hwang and D.K. Lee, J. Phys. D: Appl. Phys. 43, 483001 (2010) (Topical Review).A. Badzian and N.M. Hwang, Diamond: Low-Pressure Synthesis, Ref. Module of Mater. Sci. & Mater. Eng. (2016).