Study of the rate-limiting processes in liquid-phase epitaxy of thick YBaCuO films

Abstract

Liquid-phase epitaxy was used to grow c-axis YBa 2Cu 3O 7−δ thick films on NdGaO 3 (1 1 0) substrates in an Y supersaturated BaO–CuO solvent. In the case of stationary substrates, the familiar √ t growth kinetics is found. Substrate rotation was employed in order to induce forced convection in the liquid, thereby reducing the thickness of the diffusion boundary layer. The resulting growth kinetics showed transient and steady-state growth regimes. The transient regime extends to about 200 s with √ t growth kinetics. In the steady-state regime, diffusion across an established diffusion boundary layer led to a linear increase of film thickness with time. This diffusion boundary layer was estimated to be ∼35 μm for a substrate rotating at 200 rpm. Detailed study of the film growth suggested the presence of an interface kinetics that limited the growth on a competitive basis with volume diffusion in the liquid. The phenomenological kinetic coefficient is found to be ∼2.5×10 −6 m s −1, which is significantly higher than theoretically predicted. The diffusion coefficient of Y in the solution was estimated from viscosity (Stokes–Einstein and Sutherland relations) to be around 2.9–4.3×10 −10 m 2 s −1 at 990°C, while the growth data gave a value of ∼4×10 −10 m 2 s −1.