Embedding SiGe nanostructures, i.e. very narrow films, islands and clusters as well as semiconductor nanotubes, into heterostructures deposited on Si wafers open new routes for a variety of Si devices. This includes high speed electronic devices as well as the monolithic integration of opto-electronic on Si. The development of SiGe hetero-bipolar transistors with transit frequencies exceeding 200 GHz1 and the recent demonstration of the fabrication of ultra-small C-MOS devices2 using strained Si channels deposited on SiGe virtual substrates are prominent examples of the development of Si/SiGe nanotechnology. However, besides these well established routes in SiGe technology, nanostructures allow for new concepts of Si devices, like the Dot-FET3, a C-MOS compatible device based on Ge islands. Furthermore it might be feasible to use Ge islands and quantum wells with high Ge concentrations for the fabrication of Si based optical devices.
Quantum Cascade (QC) structures offer a viable route for the fabrication of a Si based laser, since this device relies on intersubband transitions within one band4 and thus circumventing the problem of the indirect nature of interband transitions in Si. In a previous study we demonstrated the electroluminescence of the heavy hole 2 (HH2) to HH1 transition in a Si/SiGe QC structure grown pseudomorphically on Si (100) substrates5. Here we present results on strain compensated Si/Si0.2Ge0.8 QC structures deposited on relaxed Si0.5Ge0.5 buffer layers. Quantum cascade (QC) structures containing up to 30 cascades, each containing 28 induvidual layers ranging in width from 4 to 28 Å have been deposited. TEM micrographs indicate that there is no structural degradation through the whole stack of layers. The interfaces in the top cascades are as perfect as at the bottom of the structure. Extended studies using x-ray diffractometry and x-ray reflectivity give evidence that the QC structures are strain compensated towards the Si0.5Ge0.5 relaxed buffer layer, having an excellent reproducibility within the structure and from sample to sample and have abrupt interfaces.
First test devices have been fabricated and low temperature (20K) electroluminescence spectrum of a quantum cascade structure containing 3 cascades. Clearly a peak at 185 meV is has been detected. The emission peak is p-polarized and the energy fits to the expected transition energy of the HH2 to HH1 transition in the cascade structure, giving evidence that it stems from the desired intersubband transition between the two heavy hole subbands.
In a second approach Ge quantum dots have been used to improve the opto-electronic properties. The idea is to overcome the limitation of the indirect band gap of Si by a strong localization of the carriers in quantum dots. However, the Ge quantum dots provide a strong carrier confinement only for the holes, the electrons are only weakly confined in the Si. In this study the Ge quantum dots were embedded in strained Si quantum wells grown on relaxed SiGe buffer layers. The strained Si quantum wells provide confinement of the electrons in the vicinity of the Ge dots. The structures were deposited on planar as well as on patterned substrates by MBE. The size of the mesa structures have been used as experimental parameter. Relaxed buffer layers grown on line shaped mesa structures show a strongly reduced dislocation density. Consequently the deep luminescence attributed to dislocations in the buffer layers is strongly reduced and pronounced photoluminescence of the quantum structures grown on top of the buffer layers can be observed.