| Semiconductor nanowires have attracted dramatic attention due to their potential applications in electronics and optics. However, rational control of nanowire properties requires the understanding of nanowire growth mechanisms, the knowledge of which remains limited and largely phenomenological. In this work, thermodynamics and kinetics methods are employed to study the growth features of semiconductor nanowires prepared by the vapor-liquid-solid (VLS) method.;A thermodynamic model is first established to examine the semiconductor nanowire size limit. From the derived formulas, it is found that ever smaller wire can be grown via the VLS method without a limit imposed by thermodynamics until reaching some kinetic growth restrictions. A kinetic model is then proposed based on two-dimensional island nucleation-growth process to obtain the nanowire steady state growth rate, with which formulation is performed from basic physical principles. This model seems to be the only one that can fit a set of extensive growth rate data on Si whiskers/nanowires. Next, a general model is developed to describe the instant nanowire morphology from the beginning of growth to either reaching the steady state with a constant diameter or ending up with a hillock with the growth process terminating. The equilibrium of the interface (three-phase contact) configuration is arrived at via the balance of the static physical tensions and the dynamic chemical tension. Finally with the understanding of fundamental characteristics of the VLS method, a model is proposed to describe the concentration profiles of nanowire heterojunctions and pn-junctions. It also yields the reason why these junctions have a graded transition region instead of one with atomic sharpness. |
