The scaling of traditional silicon transistor is soon reaching its physical limits. To enable a lower power consumption and higher performance, we perform research to integrate new materials on silicon (III-V alloys) and explore novel device architectures like tunnel FETs.

As device scaling reaches its limits, increasing complexity forces the physical distance between computational units to increase. Data communication technologies with high performance and high integration become increasingly important. Integrating photonic components like lasers, detectors, and modulators on a silicon chip will offer new possibilities for scaling and extending Moore’s law.

Strain engineering allows the band structure of semiconductors to be manipulated and control electronic properties as mobility or charge carrier concentration, to improve transistor performance. It also allows the control of optical properties as the wavelength of emission or light polarization, to engineer the performance of photonic devices.

Strain is therefore a key technology to improve the performance of next-generation transistors and optoelectronic devices on chip.

An open-source platform to study uniaxial stress effects on nanoscale devices

G. Signorello, M. Schraff, P. Zellekens , U. Drechsler, M. Buerge, H.R. Steinauer, R. Heller, M. Tschudy, and H. Riel

Review of Scientific Instruments 88, 053906, 2017 - arXiv: 1704.01394

We present an automatic measurement platform that enables the characterization of nanodevices by electrical transport and optical spectroscopy as a function of uniaxial stress. We provide insights into and detailed descriptions of the mechanical device, the substrate design and fabrication, and the instrument control software, which is provided under open-source license. The capability of the platform is demonstrated by characterizing the piezo-resistance of an InAs nanowire device using a combination of electrical transport and Raman spectroscopy. The advantages of this measurement platform are highlighted by comparison with state-of-the-art piezo-resistance measurements in InAs nanowires. We envision that the systematic application of this methodology will provide new insights into the physics of nanoscale devices and novel materials for electronics, and thus contribute to the assessment of the potential of strain as a technology booster for nanoscale electronics.

Manipulating surface states of III–V nanowires with uniaxial stress

G. Signorello, S. Sant, N. Bologna, M. Schraff, U. Drechsler, H. Schmid, S. Wirths, M. D. Rossell, A. Schenk, and H. Riel

Nano Letters 5, 3655 (2017).

III–V surfaces and interfaces play the leading role in determining device performance, and therefore, methods to control their electronic properties have been developed. Typically, surface passivation studies demonstrated how to limit the density of surface states. Strain has been widely used to improve the electronic transport properties of III–Vs, but its potential to modify the surface properties still remains to be explored. We show that uniaxial stress induces a shift in the energy of the surface states of III–V nanowires, modifying their electronic properties. We demonstrate this phenomenon by modulating the conductivity of InAs nanowires over 4 orders of magnitude with axial strain ranging between −2.5% in compression and 2.1% in tension. The band bending at the surface of the nanostructure is modified from accumulation to depletion reversibly and reproducibly. The deformation potentials for the surface states are quantified. These results reveal that strain technology can be used to shift surface states away from energy ranges in which device performance is negatively affected and represent a novel route to engineer the electronic properties of III–V devices.

Inducing a direct-to-pseudodirect bandgap transition in wurtzite GaAs nanowires with uniaxial stress

G. Signorello, E. Lörtscher, P.A. Khomyakov, S. Karg, D.L. Dheeraj, B. Gotsmann, H. Weman, and H. Riel

Nature Communications 5, 3655 (2014).

Many efficient light-emitting devices and photodetectors are based on semiconductors with, respectively, a direct or indirect bandgap configuration. The less known pseudodirect bandgap configuration can be found in wurtzite (WZ) semiconductors: here electron and hole wave-functions overlap strongly but optical transitions between these states are impaired by symmetry. Switching between bandgap configurations would enable novel photonic applications but large anisotropic strain is normally needed to induce such band structure transitions. Here we show that the luminescence of WZ GaAs nanowires can be switched on and off, by inducing a reversible direct-to-pseudodirect band structure transition, under the influence of a small uniaxial stress. We envisage a new generation of devices that can simultaneously serve as efficient light emitters and photodetectors by leveraging the strain degree of freedom.

Tuning the light emission from GaAs nanowires over 290 meV with uniaxial strain

Giorgio Signorello, Siegfried Karg, Mikael T. Björk, Bernd Gotsmann, and Heike Riel

Nano Lett. 13(3), 917–924, 2013.

Semiconducting nanowires can withstand exceptionally high elastic strain, making them ideally suited for novel device applications that require a tuning of the band structure over a broad range. We show that stress can be applied continuously and reversibly to GaAs/Al0.3Ga0.7As core/shell nanowires, both in compression and tension, resulting in a remarkable decrease of the bandgap of up to 296 meV at 3.5% of strain. Raman spectroscopy provides insights on the crystal deformation and the strain tensor.