In situ transmission electron microscopy studies of initial stages of vapor-liquid-solid growth of silicon nanowire

Bong Joong Kim, Purdue University

Abstract

A critical component in the successful application of Si nanowire in electronic and optical devices is a fundamental understanding of the initial stages of vapor-liquid-solid (VLS) process via which most of semiconducting nanowires are grown. In this dissertation, in-situ transmission electron microscopy is used to investigate each of the stages of the VLS process in detail, in order to understand the kinetics of each stage and to assess how they impact overall nanowire growth process. Utilizing the unique capabilities of a specially constructed ultra-high vacuume transmission electron microscopye (UHV-TEM) equipped with in-situ ultra-high vacuum chemical vapor deposition (UHV-CVD) growth facilities, detailed measurements and analysis of the nucleation and growth process in nanoscale systems are presented. Observations of individual events, as opposed to data from ensembles, allow direct probing of classical nucleation theories in detail. The system described here is directly related to nanowire formation: nucleation of Si from a Au-Si eutectic liquid is examined at increasing supersaturation, as in the VLS process, in contrast to nucleation on cooling as in most other nucleation studies. A simple theoretical model is presented which provides an accurate description of the measurements of nucleation at the nanoscale. No size effects on nucleation are observed, even in systems below 12 nm. The result that the phase diagram is insensitive to size down to this value is in stark contrast to recent reports for AuGe, which lacked the quantitative rigor of this study, and suggests that it may be relatively simple to design processes to create reproducible structures at these size scales in technologically relevant applications. Moreover, this model allows determination of the supersaturation required to initiate nucleation. This supersaturation exhibits a strong temperature dependence, and - surprisingly - a higher supersaturation is required at higher nucleation temperatures. The examination of the three stages of the vapor-liquid-solid process, prior to the axial growth of Si nanowires - the dissolution of the solid Au catalyst in the AuSi eutectic liquid, Si saturation in the liquid AuSi, and Si nucleation was also carried out using the in situ transmission electron microscope (TEM). Our quantitative analyses show that the incubation time for Si nucleation linearly increases with liquid droplet radius. This linear dependence is explained by taking into account the invariant characteristics of the following two factors in time; the shape of liquid AuSi droplet, and the sticking coefficient of disilane on the AuSi. Cross sectional TEM imaging shows the shape of the liquid drop remains constant with time. From dark field imaging and measurement of the dissolution rate of solid Au, we find that the solid to liquid transformation occurs from the surface inwards and the solid volume linearly decreases with time. Based upon these results, we suggest that the rate of Si addition to the droplet is constant over time – a simple conclusion to estimate the supersaturation at the moment of Si nucleation. Finally, quantitative in situ measurements of coarsening/decay of individual AuSi droplets with simple models accounting for kinetic behaviors of the droplets were implemented. From the measurements of droplet volumes as a function of time in combination with both surface diffusion - and attachment - models of droplet decay, it is found that attachment/detachment of Au adatoms on the Si (100) surface is the rate-limiting step. Both coarsening and decay kinetics agree well with a simple kinetic equation, and the “reservoir effect” reflected in it would result in the apparent size dependence of decay rate. These results have important implications for acquiring the capability of tailoring the structural properties of Si nanowires (i.e. diameter, distribution and density). Throughout this dissertation, emphases are placed on the advantages of quantitative measurements of all the early stages of VLS growth of Si nanowires. These kinetic analyses yield insight into such fundamental process as crystal growth, phase transformation and nucleation, as well as into technologically important aspects of designing nano-scale devices.

Degree

Ph.D.

Advisors

Stach, Purdue University.

Subject Area

Materials science

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