Center for Functional Nanomaterials Seminar

In-situ TEM allows us to monitor metal-catalyzed Si nanowire growth and measure the growth kinetics. We are also interested in fabricating heterojunction nanowires and relating their electronic properties and performance to the structural properties of the interfaces, such as the abruptness of the composition change and the strain distribution. Recent advances in nanowire growth using liquid catalysts (the vapor-liquid-solid, VLS, method) have shown that the interfaces between materials such as Si and Ge show a diffuse compositional change due to a reservoir effect in the liquid catalyst. We therefore studied nanowire growth using solid catalysts (the vapor-solid-solid, VSS, method). Through in-situ TEM observations, we find that VSS-grown Si nanowires exhibit a richer morphology compared with VLS-grown Si nanowires. Growth occurs by step flow at the catalyst/silicon interface, presenting significantly different kinetics from that in the VLS case. We will describe a simple model for step-flow that shows that step flow kinetics is controlled by a single parameter, determined mostly by the free energy function of the catalyst. The model also shows that reducing the reservoir effect requires a solid catalyst with low Si and Ge concentrations. We have developed one particular alloy, AlAu2, which fulfills these requirements, and have used it to grow axial Si-Ge heterojunction nanowires with compositionally abrupt interfaces. This growth method is also useful for making precisely controlled structures such as Si/Ge(1nm)/Si. We will discuss the large lattice-mismatch-induced strain in such structures.