Electrochemical Thin Film Growth Assisted by Pb Monolayer
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The properties of heteroepitaxial thin films are very much dependent on the method used for their deposition. The electrochemical/electroless deposition represents the ambient temperature growth process for many heteroepitaxial metal overlayers while preserving the integrity of their interface. However, in many substrate-overlayer systems the interfacial free energy of the overlayer/electrolyte is smaller than the sum of the interfacial free energies of the substrate/electrolyte and overlayer/substrate and nucleation and growth of rough 3D films is favored. In epitaxial systems where the thermodynamics predicts Frank-van der Merve or 2D layer-by-layer growth, the difficulties in producing 2D single crystal overlayers by electrodeposition are similar to those encountered during the vacuum deposition at ambient temperature. Owing to strain and kinetic issues, 3D growth occurs often. In recent years, a significant progress has been made in understanding the kinetic aspects controlling the growth modes in vacuum deposition. This development has led to various discoveries in which the growth kinetics during electrochemical growth was manipulated to enhance the evolution of atomically flat epitaxial overlayers. In the first part of this PhD work, electrochemical and electroless Pb UPD phenomena were investigated as a critical step in development of more sophisticated approaches to manipulate growth kinetics during electrochemical/electroless growth of thin films. An electroless atomic layer deposition (eALD) method has been developed using the electroless Pb monolayer as the reducing agent and sacrificial layer in surface limited redox replacement reaction with more noble metal ions (Ptn+). This eALD process is highly selective to metal substrates at which Pb forms the electroless monolayer. Repetition of the eALD cycle leads to formation of a highly compact, smooth and conformal noble metal film. In the second part, electrochemical deposition of 2D Cu thin film on Ru(0001) surface was investigated. In the first case, Cu was co-deposited with periodically deposited and stripped Pb mediator, the Pb mediator serves to increase the density of 2D Cu islands during deposition and promote a layer by layer thin film growth mode. In the second case, a full UPD monolayer of Pb was used as a surfactant during Cu deposition and 2D growth of Cu was obtained on the Ru(0001) surface. In the third part of this work, an electroless approach was investigated for Co deposition on the Cu(h, k, l) polycrystalline substrate. The effect of substrate pre-treatment on the final morphology and structure of deposited Co film was studied; the results suggest that proper annealing of the Cu substrate in the forming gas (75%H2, 24%N2 and 1%CO) is beneficial for the ultimate quality of the deposited Co thin film. Our study shows that with the presence of the electroless Pb monolayer surfactant, the morphology and quality of the deposited Co film is improved in terms of uniformity, continuity and smoothness of the film. The experimental result indicates that the electroless Pb monolayer has efficiently reduced the Schwoebel barrier and enhanced the surface diffusion of atoms during deposition.