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

技术。本项目致力于基于SiGe量子阱（QWs）的量子级联（QC）中远红外发光材料的合成/加工/制造研究。由于QC激光器是基于子带间（ISB）跃迁（即同一能带内量子化状态之间的电子跃迁）的半导体光源，因此它们的工作基本上不受底层材料能带间隙性质的影响。因此，这些考虑为在硅中演示激光作用提供了一种方法。（Si）（Ge）的间接带隙使目标复杂化。然而，与Si/SiGe量子阱相关的应变似乎是进展的重大挑战。本项目采用了一种新的方法，将弹性松弛的SiGe纳米膜用作生长基质和/或活性材料。这种纳米膜将外延生长在绝缘体上的硅（锗）衬底上，然后通过湿法蚀刻去除底层的氧化层，从柄晶圆上释放出来。期望的结果将是一个独立的异质结构，其中应变松弛通过组成层之间的弹性应变共享而发生，而不会形成缺陷，因此实际上没有位错。释放出来的纳米膜可以转移到其他底物上。硅基QC激光器演示的进一步挑战是SiGe QW材料系统特有的设计问题，该项目也正在解决这些问题。将探索一种基于Ge/SiGe量子阱L谷中的电子ISB跃迁的方法。最近的计算表明，这种方法可能比目前已研究的p型结构具有优势，包括更长的非辐射寿命，更大的振荡器强度和更有效的隧道传输。本项目涉及电子/光子材料科学中具有技术相关性的主题领域的基础研究问题。纳米膜技术用于制造复杂半导体量子结构的提议，具有超越SiGe材料系统和上述qc激光器件应用的广泛技术影响的潜力。此外，该项目活动将通过培训跨学科的学生来促进教育，从半导体外延生长到纳米膜合成和加工，带隙工程和太赫兹光子学。为了提高项目的有效性和范围，将强调本科生和高中实习生的参与，充分利用现有项目，重点关注代表性不足的少数民族。

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.