Final Report Summary - III-V NWS ON SI (Self-assembled growth of III–V Semiconductor Nanowires on Si for Future Photonic and High Electron Mobility Applications)
Executive Summary:
This project entitled “Self-assembled growth of III-V semiconductor nanowires on silicon for future photonic and high electron mobility applications” aims to combine nanoscale compound semiconductor materials with Si technology to develop high-performance, ultrahigh-density solid-state devices for integrated nanophotonics, efficient light emitters and communication technology. As high-performance nanoscale materials we explored direct band gap semiconductor nanowires (NW) of the gallium arsenide (GaAs) family, which are materials of high optical activity, high electron mobility and which further enable precise tuning of many intrinsic properties (emission wavelengths, absorption coefficients, electron mobilities, etc.) through heterostructure and band gap engineering with related alloys (InAs, InGaAs, AlGaAs). In addition, in their one-dimensional (1D) NW geometry these structures of very high aspect ratio offer many unique characteristics not present in the bulk form, most importantly the possibility for exploiting quantum physical effects and significantly reduced lattice mismatch restrictions that enable their direct integration on very dissimilar substrates such as Si.
In this work significant progress and several important milestones were achieved, ranging from the development of growth and integration strategies of (In,Ga,Al)As-based nanowires on Si platform, to the characterization of fundamental NW properties, and finally to the realization of high-performance nanophotonic emitters and high-electron mobility device structures.
Utilizing molecular beam epitaxy (MBE), a well-known method for the fabrication of crystalline semiconductor materials with the highest precision and highest purity, we were able to realize completely catalyst-free growth of free-standing InAs, GaAs and composition-tuned InGaAs NWs on Si (111) with superior morphological and compositional homogeneity as compared to other methods. Based on these we characterized many of the intrinsic properties of the NWs (structural, optical and electronic properties) by employing various different nanometrology techniques, mandatory to gain both fundamental understanding as well as to tune the NW properties for best possible functionality in related devices. Remarkably, we found that in the NW geometry different crystal phases such as a wurtzite phase can be stabilized in contrast to what is observed as bulk crystal phase for these materials. We elaborated many physical features of these different crystal phases, such as different electronic band gap energies and therefore luminescence wavelengths, as well as changes in the electron transport behaviour.
Ultimately, by precisely designing complex NW heterostructures (radial core-shell NWs and axial NW heterointerfaces) we could realize various novel photonic and electronic device concepts. As a major milestone we achieved, for instance, the first optically pumped room-temperature infrared NW laser using surface passivated GaAs-AlGaAs core-shell NWs with very low lasing threshold. Furthermore, we demonstrated a concept for high-quality NW-based tunnel diodes on Si as major ingredient in so-called tunnelling field effect transistors (TFET) for superior power-efficient switches and future post-CMOS applications. Finally, we have also successfully realized a new nanoelectronic device structure, i.e. a core-shell NW heterostructure that incorporates modulation doping, leading to much higher electron mobilities than in current state-of-the-art NW structures which have been suffering from surface defects and further hampered high-speed electronic performance.
In summary, with this project we could achieve substantial research output and establish high competitiveness within the international scientific community. These achievements enabled a gradual increase in the number of research personnel, attracted further research grants and increased the international visibility of our activities to gain new very high-quality collaborations. Furthermore, this program has benefitted strongly from the excellent support by the host institution, which has led to a successful reintegration and career development of the fellow.
Overall, the results obtained in this project present an important drive forward toward ultra-small, highly efficient solid-state devices reaching out to communities working in integrated nanophotonics, light emitters and absorbers (photovoltaics), sensing, and communication and information technology.
Web address: http://www.wsi.tum.de
Contact: Gregor.Koblmueller@wsi.tum.de