SiGe based hetero-and nanostructures

Note: Pizza served at 11:45
Speaker: Günther Bauer, Institute for Semiconductor and Solid State Physics, Johannes Kepler University Linz, Austria

When: April 30, 2010 (Fri), 12:00PM to 01:00PM (add to my calendar)
Location: SCI 352
Hosted by: Bennett Goldberg

This event is part of the Biophysics/Condensed Matter Seminar Series.

Abstract:
Due to their compatibility with main-stream Si technology SiGe hetero-and nanostructures have attracted wide-spread interest for possible electronic and optoelectronic applications. The lattice mismatch between Ge and Si is slightly higher than 4% and thus the SiGe system has turned out as a model system for studies of strain-driven evolution from two-dimensional layer to three-dimensional Stranski-Krastanow island growth. I will first present detailed studies on this morphological evolution based on structural as well as on photoluminescence studies with so far unprecedented resolution, as low as 0.025 monolayers of Ge deposition which shed light on the energetics as well as kinetics of this growth mode. The growth of SiGe islands by deposition of SiGe alloys instead of pure Ge allows us to control both the Ge concentration and gradient in the islands. In contrast to the commonly found increasing Ge content with island height, growth conditions for islands with nearly constant and even decreasing Ge profile along the growth direction were found. Atomic force microscopy, transmission electron microscopy and high-resolution x-ray diffraction were employed to determine the islands’ size, shape, lateral distance and Ge composition. Efficient photoluminescence (PL) is emitted from these islands. We show that for islands with higher Ge contents at the bottom than at the apex, transitions between heavy holes and electron xy states in the compressive Si regions around the island’s circumference dominate the photoluminescence spectra instead of the usually observed recombination between heavy holes and electrons in the z valleys in the tensile Si above the island’s apex. A quantitative modelling of the band structure within the islands and in the surrounding Si matrix based on full 3D simulations with the experimentally obtained island shape and composition as input parameters is in excellent agreement with the experimental PL data..
The tensile strain in the Si cap layers on top of buried SiGe islands lends itself to applications of such islands for electronic devices: it lifts the degeneracy of the six-fold Si conduction band states and thus leads in the z valleys to an enhanced electron mobility as compared to unstrained Si. For two-dimensional arrays of SiGe islands grown on pit-patterned Si substrates, field effect tranistor devices using strained Si channels on top of such islands were realized which show indeed an enhanced current driveability.
Finally, I will present how Si/SiGe multiquantum well structures can be used for achieving mid-infrared detection based on intersubband transitions. Voltage-tunable, two-band mid-infrared detection based on Si/SiGe quantum-cascade injector structures has been realized. Such investigations were complemented by a direct measurement of the intersubband hole relaxation time by femtosecond resolved photocurrent experiments. These allow for a quantitative determination of hole relaxation in SiGe quantum wells under electrically active conditions.
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