Strain-driven Band Structure Engineering in Ge/Si-Nanowires

The objective of the proposed project is to explore combined effects of quantum confinement, field effect modulation and most notably strain related effects, to tune the electrical and optical properties of Ge/Si-nanowire structures. Though Si and Ge are established materials in CMOS technology, the inefficient light emission due to the indirect band gap inhibits their use as on-chip light sources or detectors. Several works have been published recently, proposing the transition of Si and Ge from indirect to direct band gap semiconductors by means of strain. Particularly Ge, due to the small difference between direct and indirect band gap energy has been suggested even as a gain medium for lasing. Recent attempts to minimize the 136 meV gap between the direct - and the indirect L-valley, include band structure modifications by quantum confinement, alloying with Sn, or strain engineering. The direct band gap transition of Ge was predicted at biaxial strains of ~2% or uniaxial strains of ~4%. In situ tuning of such high strain levels appeared to be challenging as the strain will be relieved by the formation of dislocations, plastic deformation or even result in fracture. However, the superior mechanical robustness of single-crystal nanowires enables to apply such high strain without fracturing to various materials. Therefore in any application where crystallinity and strain are important, the idea of making nanowires should be of a high value. Thus we will monolithically integrate vapour-liquid-solid grown nanowires - which appeared to be the most mechanically sound - into micromechanical straining devices to guarantee electrical reliable as well as rigid mechanical contacts. Based on our previous investigations we are convinced that we can control sufficiently high strain levels in a reliable manner and adapt the device in a way to enable further field modulation by adding a gate all around architecture. The highly beneficial mechanical robustness of the nanowire geometry together with dynamically tuneable application of ultra-high strain levels in the percent range will allow unprecedented investigations of strain effects on the electronic band structure of Ge-nanowires, Ge/Si core/shell-nanowires as well as doped nanowire heterostructures. Further we will explore an electrostatically actuated straining device enabling to characterization the piezoresistivity as well as the modifications of the optical properties by -photoluminescence, -Raman and scanning photocurrent microscopy investigations at various temperatures down to T=4K. From these electrical and optical investigations of ultra-strained nanowires in combination with ab- initio simulations we will obtain a general understanding of strain related electrical and optical effects in Ge and Si based systems. The ultimate goal is a new generation of tuneable light emitting devices as well as detectors by leveraging the strain degree of freedom.

The various effects of strain on Si and Ge have been studied since the 1950s. It was recognized early, that the band structure and thus the electrical and optical properties can be tuned by strain. Particularly for Ge there have been several attempts to minimize the gap between the direct - and the indirect L-valley, including band structure modifications by quantum confinement, alloying with Sn, or strain engineering. For bulk materials, such strain tuning of material properties is restricted to strain levels below 1%, as higher strain will result in the formation of dislocations, plastic deformation and finally fracture. The ability to fabricate single-crystal nanowires (NWs), which are widely free of structural defects and their monolithic integration into straining devices, enabled us to apply ultra-high strain levels up to 10% proven NWs as an ideal platform for the exploration of strain-related effects. The highly beneficial mechanical robustness of the NW geometry and reliable contacts together with dynamically tunable application of ultra-high strain levels in the percent range allowed unprecedented investigations of strain effects on the electrical and optical properties of Ge/Si based NW devices. Therefore strained Si and Ge NWs were subjected to various analysis techniques like field emission scanning electron microscopy, -Raman spectroscopy, spatially resolved photoluminescence as well as temperature dependent electrical transport investigations. The potential of this approach was demonstrated by e.g. the demonstration of a giant piezoresistive effect, observed for Si as well as Ge NWs strained to 10% and 5% respectively. A model based on stress induced carrier mobility change and surface charge modulation was proposed to interpret the actual piezoresistive behavior of the NWs. Using strained p-n junction NWs we explored an approach for a direct measure of the band gap narrowing for tensile strained NWs. Within the collaboration with Ricardo Rurallis group we simulated the implications of strain on the band structure, providing fundamental physical understanding, and materials development guidance. The flexibility of our approach was further demonstrated by integrating CdS NWs into our straining module. Such direct semiconductor NWs are of particular interest as these have already demonstrated e.g. continuous wave laser emission and ultrafast modulation capabilities. However individual NW laser devices currently suffer from fixed emission spectra determined by the material band gap. In cooperation with the group of Carsten Ronning from the Univ. Jena, exemplarily tunable nanolaser devices were fabricated by integrating individual CdS NWs into the straining device which enabled the realization of dynamically tunable nanoscale coherent light sources. Within the project the electrical contacts to the strained NWs appeared to be one of the most critical components. Within the optimization process of these contacts we explored a novel contact formation mechanism resulting in Al-Ge-Al NW heterostructures which appeared to be a promising material combination for other applications ranging from quantum ballistic transport, single electron transistors, to photo detectors and even plasmonics.