Abstract
We deposited Ge layers on (001) Si substrates by molecular beam epitaxy and used them to fabricate suspended membranes with high uniaxial tensile strain. We demonstrate a CMOS-compatible fabrication strategy to increase strain concentration and to eliminate the Ge buffer layer near the Ge/Si hetero-interface deposited at low temperature. This is achieved by a two-steps patterning and selective etching process. First, a bridge and neck shape is patterned in the Ge membrane, then the neck is thinned from both top and bottom sides. Uniaxial tensile strain values higher than 3% were measured by Raman scattering in a Ge membrane of 76 nm thickness. For the challenging thickness measurement on micrometer-size membranes suspended far away from the substrate a characterization method based on pump-and-probe reflectivity measurements was applied, using an asynchronous optical sampling technique.
Highlights
Electrical and optical properties of semiconductor materials can be tailored by applying strain.[1]
Our goal is to study the effects of uniaxial strain in Ge membranes with thicknesses in the range of tens of nanometers
Suspended Ge membranes were fabricated with high uniaxial tensile strain from layers deposited by MBE on Si substrates
Summary
Electrical and optical properties of semiconductor materials can be tailored by applying strain.[1] Strained materials have found application in devices such as semiconductor lasers,[2] transistors,[3] optical modulators,[4] and photodetectors,[5] among others. Tensile stress applied to Ge has the potential to improve light emission by modifying the electronic band structure, transforming this indirect band gap material into a direct gap semiconductor.[6,7] This transition is predicted to occur at around 2% biaxial strain parallel to the (100) crystallographic plane[8,9,10] and above 4% uniaxial tensile strain along the [100] crystallographic direction.[11] Uniaxial strain exceeding the threshold for obtaining the direct band gap has been demonstrated[12,13] but its implementation in optoelectronic devices is technologically challenging. Ge has the important advantage of being suitable for monolithic integration on Si substrates and, eventually, allowing integration of laser diodes in CMOS technology
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