Boosting Hole Mobility in Coherently Strained [110]-Oriented Ge–Si Core–Shell Nanowires

Abstract

The ability of core–shell nanowires to overcome existing limitations of heterostructures is one of the key ingredients for the design of next generation devices. This requires a detailed understanding of the mechanism for strain relaxation in these systems in order to eliminate strain-induced defect formation and thus to boost important electronic properties such as carrier mobility. Here we demonstrate how the hole mobility of [110]-oriented Ge–Si core–shell nanowires can be substantially enhanced thanks to the realization of large band offset and coherent strain in the system, reaching values as high as 4200 cm2/(Vs) at 4 K and 1600 cm2/(Vs) at room temperature for high hole densities of 1019 cm–3. We present a direct correlation of (i) mobility, (ii) crystal direction, (iii) diameter, and (iv) coherent strain, all of which are extracted in our work for individual nanowires. Our results imply [110]-oriented Ge–Si core–shell nanowires as a promising candidate for future electronic and quantum transport devices.

Multiple Andreev reflections and Shapiro steps in a Ge-Si nanowire Josephson junction

“…We start with estimating the elastic scattering length using l e = µm v F /e [63] with µ the hole mobility, m the effective hole mass and v F the Fermi velocity. We use µ ≈ 3500 cm 2 /Vs (determined at 4 K, see [64]) and m ≈ 0.5m e for the mixed heavy and light holes [54,65] with m e the free electron mass. To obtain the Fermi velocity we use the solutions of the Schrödinger equation for a cylindrical potential well and find the expression for the Fermi energy of the n th subband with quantum number l as E n,l =h 2 α 2 n−1,l /2m R 2 [66] with α n,l the l th root of the n th order Bessel function and R the wire radius.…”

Widely tunable GaAs bandgap via strain engineering in core/shell nanowires with large lattice mismatch

“…The lattice mismatch can be accommodated via elastic deformation of not only the shell, but also the core, depending on the relative thicknesses and chemical compositions 18 . This increases the capabilities for engineering the strain and, thus, the electronic structure and properties of the heterostructure 19–23 . Unlike quantum-dot heterostructures, where elastic accommodation of large misfit stresses is also possible 24 , the hetero-interface in nanowires can be several micrometres long, allowing for practical use in a wide variety of device concepts, e.g., in photovoltaics, lasers, thermoelectrics and electronics 25 .…”

Coulomb blockade in monolithic and monocrystalline Al-Ge-Al nanowire heterostructures

“…Recently, band-structure engineering by controlled epitaxial growth of core-shell NWs provoked the investigation of one-dimensional hole-gas systems [8], attractive for both fundamental studies and future nanoelectronics [9]. Despite the vast body of pioneering experimental work on GeSi [10] and Ge-Si core-shell structures [11,12] the investigation of Coulomb blockade effects in pure Ge based nanostructures is still evasive. This is mainly associated to limitations in fabricating reliable contacts to Ge nanostructures [13].…”