Understanding Nucleation and Growth of Crystalline Germanium Through in-situ Studies of the ec-LLS Technique

Abstract

The electrochemical liquid liquid solid (ec-LLS) technique is a novel method for the synthesis of crystalline inorganic covalent semiconductor materials. The crux of the technique involves using a liquid metal working electrode to serve both as a source of electrons for electrochemical reduction of semiconductor precursors and as a growth solvent to facilitate the growth of crystalline semiconductor material. Ec-LLS combines the precise control of electrochemical reactions with the ability to furnish crystalline semiconductor material of melt crystal growth techniques. Previous research on ec-LLS focused on ex-situ, macroscale methods to identify methods to control crystal nucleation and growth, probed how the identity of the liquid metal influenced crystal morphology, and explored the possibility of alloying reactions between a group III liquid metal and group V precursor. These studies were significant and impactful, but several key questions remain.

This thesis describes multiple strategies to study directly ec-LLS growth processes using in-situ techniques. The central goal of this dissertation is to provide atomic-level insight into the nucleation and growth crystalline germanium (Ge) via ec-LLS. This thesis details the electrochemical synthesis and simultaneous characterization of crystalline Ge in both real-time and with high spatial resolution.

The first portion of this thesis details a general methodology for the study of electron beam stimulated ec-LLS nanowire growth via liquid cell transmission electron microscopy (LC-TEM). Specifically, Chapter 2 describes the use of liquid metal Ga and In nanodroplets for the growth of Ge nanowires at room temperature. A variety of conditions were explored including liquid metal nanodroplet surface condition, liquid metal nanodroplet size and density, formal concentration of dissolved GeO2, and electron beam intensity. This work revealed that nanowire growth rate was limited by the heterogeneous reduction of dissolved GeO2, the high activation barrier for nucleation in ec-LLS, and the influence of growth rate on defect formation in the growing nanowires.

The second portion of this thesis focuses on the use of X-Ray Reflectivity (XRR) to investigate the liquid metal-liquid electrolyte interface during potentials commonly employed during ec-LLS. Chapter 3 probes the surface of a liquid mercury working electrode at potentials positive of the 4e- reduction of GeO2 to Ge0. Three primary findings were revealed. 1) When the electrolyte only contained Na¬2B4O7, a pristine mercury-electrolyte interface was observed. 2) When GeO2 was introduced to solution, a solid adlayer formed on the surface of Hg when E ≥ 0.2 V vs. SCE. 3) When the applied potential was between -0.5 V and -0.9 V an anion adsorbate layer consistent was HGeO3- was instead observed on the mercury surface. Chapter 4 shifts the focus to more negative potentials in the same system. Under these conditions, XRR was used to gauge the atomic level structure of the liquid Hg/liquid water interface during the growth of crystalline Ge by ec-LLS. A principal finding was that nucleation and growth occur in the near surface region of the liquid metal rather than deep in the bulk. Nevertheless, the surface ordering of the liquid Hg was maintained throughout, indicating poor wetting of crystalline Ge by the liquid metal.

The final chapter of this thesis details unresolved work that could serve as future research pathways. Six different projects encompassing LC-TEM, XRR, and general growth strategies for group III-V semiconductor materials are outlined. These works serve to further elucidate the knowledge gaps present in ec-LLS.