Fundamental aspects of VLS growth

The kinetics and the mechanism of the vapor-liquid-solid (VLS) growth are discussed. Emphasis is placed on the dependence of the growth rate on the whisker diameter. It is found that the rate decreases abruptly for submicron diameters and vanishes at some critical diameter d_c ? 0,1 μm according to the Gibbs-Thomson effect. A new method for simultaneous determination of kinetic coefficients and of supersaturations has been developed. The method can be used to measure the coefficients of some materials as well as the temperature dependence of the coefficient for silicon and the activation energy of the process. From the dependence of supersaturation on the diameter we conclude that whiskers grow by a poly-nuclear mechanism. The periodic instability of the diameter is discussed and the rate-determining step is analysed. We conclude that phenomena on the liquid-solid interface are the decisive ones. In determining the role of liquid phase in vapor growth we measured the “liquid phase effectivity coefficient” as a function of crystallization condition; the coefficient typically was about 10²?10³. It is stressed that the liquid phase reduces the activation energy both on vapor-liquid interfaces (for chemical reactions) and on liquid-solid interfaces (for nucleation). The liquid phase ensures growth rates as high as 1 cm/sec, provided there are no barriers between the interfaces. The growth mechanism of the side faces was studied, and it was observed that the faces grow mainly by a chain mechanism rather than by two-dimensional nucleation. In work on surface diffusion in the VLS whiskers growth by CVD, we found that the whiskers grow mainly by direct deposition rather than by diffusion on the side faces. It is concluded that the VLS mechanism is important also for the vapor growth of platelets, films, and bulk crystals.