Collaborative Research: Quantum-Cascade-Laser Active Materials Based on Silicon-Germanium Nanomembranes

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

Technical. This project addresses synthesis/processing/fabrication research to explore quantum-cascade (QC) mid- and far-infrared light-emitting materials based on SiGe quantum wells (QWs). Since QC lasers are semiconductor light sources based on intersubband (ISB) transitions (i.e., electronic transitions between quantized states within the same energy band), their operation is essentially unaffected by the nature of the energy band gap of the underlying materials. Hence such considerations provide an approach for the demonstration of laser action in silicon ? a goal complicated by the indirect band gap of (Si)(Ge). Strain, however, associated with Si/SiGe QWs appears as a significant challenge to progress. This project takes a new approach, in which elasti-cally relaxed SiGe nanomembranes are used as the growth substrates and/or the active material. Such nanomembranes will be grown epitaxially on Si(Ge)-on-insulator substrates and then re-leased from the handle wafer by removing the underlying oxide layer via wet etching. The de-sired result would be a free-standing heterostructure where strain relaxation occurs via elastic strain sharing among the constituent epilayers without the formation of defects, and thus be vir-tually free of dislocations. The released nanomembranes could then be transferred onto other substrates. Further challenges complicating the demonstration of silicon-based QC lasers are provided by design issues specific to the SiGe QW materials system, which are also being ad-dressed on this project. An approach based on electronic ISB transitions in the L valleys of Ge/SiGe QWs will be explored. Recent calculations indicate that this approach may have advan-tages over the p-type structures that have been investigated so far, including longer nonradiative lifetimes, larger oscillator strengths, and more efficient tunneling transport. Non-Technical. The project addresses fundamental research issues in a topical area of elec-tronic/photonic materials science having technological relevance. The proposed use of nanomembrane technology for the fabrication of complex semiconductor quantum structures has the potential for broad technological impact beyond the SiGe materials system and the QC-laser device application described above. Moreover, the project activities will promote education through the training of students across disciplines, ranging from semiconductor epitaxial growth to nanomembrane synthesis and processing, bandgap engineering, and THz (terahertz) photonics. To increase the effectiveness and scope of the program, the involvement of undergraduates and high-school interns will be emphasized, by leveraging existing programs with a strong focus on under-represented minorities.