Comparison of the top-down and bottom-up approach to fabricate axial nanowire-based silicon/germanium heterostructures.
Axial nanowire-based Silicon/Germanium (Si/Ge) heterostructures are interesting research objects because of the possibility to fabricate and investigate quantum well and quantum dot (QD) structures. Furthermore, they are expected to be building blocks for future devices, e. g. field-effect transistors (FETs), sensors and optoelectronic devices. They have also become interesting as the basic components of thermoelectric elements which are intended to operate at lower temperatures than conventional bulk Si/Ge materials. However, there are still a lot of tasks that have to be accomplished. This includes the exact control of the nanowire (NW) positions for future contacting as well as the positioning of the Ge layers, and also the challenge to produce sharp Si/Ge interfaces and high element concentrations within the Si and Ge part of the NWs.
A variety of different approaches has been developed to fabricate Si NWs and Si/Ge NW heterostructures. They can be divided into two basic categories: 'bottom-up' approaches and 'top-down' approaches. The most common example of the former one is the vapour-liquid-solid (VLS) mechanism, which allows the growth of NWs via a liquid catalyst droplet attached to the surface of the Si substrate. Prominent examples of top-down approaches are combinations of conventional layer growth with etching techniques like, for instance, metal-assisted wet chemical etching or reactive ion etching (RIE).
This work compares bottom-up and top-down approaches concerning the structural properties of the fabricated NWs, focussing firstly on the control of the NW position, dimensions, orientation and morphology, secondly on the positioning of the Ge layers, the achievable Ge concentrations and the abruptness of the Si/Ge interfaces, and thirdly on crystal defects, i. e. inclusions of catalyst atoms and misfit dislocations, which might be generated during the fabrication process. Because it offers a precise growth control under highly reproducible conditions, molecular beam epitaxy (MBE) has been chosen as a representative deposition technique for both the bottom-up NW growth and the fabrication of the Si/Ge heterostructures as part of the top-down approach. In the latter case, the NWs were produced by electron beam lithography (EBL) and RIE.
Based on the results of this investigation, the selected approaches will in the end also be compared concerning their scientific potential, e. g. for the investigation of quantum confinement effects, and their application potential as building blocks for devices.
Keywords: nanowires, silicon, germanium, growth