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

技术上的。本项目致力于探索基于SiGe量子井(QW)的量子级联(QC)中红外和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(即同一能带内量子态之间的电子跃迁)的半导体光源，它们的运行基本上不受底层材料带隙性质的影响。因此，这些考虑为演示激光在硅中的作用提供了一种方法。这个目标因(Si)(Ge)的间接带隙而变得复杂。然而，与Si/SiGe量子点相关的应变似乎是进步的重大挑战。本项目采用了一种新的方法，即使用弹性松弛的SiGe纳米薄膜作为生长衬底和/或活性材料。这种纳米膜将在绝缘体上的硅(Ge)衬底上外延生长，然后通过湿法腐蚀去除底层的氧化层，从手柄晶片上重新出租。设计的结果将是一个独立的异质结构，其中应变松弛发生在组成外延层之间的弹性应变共享而不形成缺陷，从而基本上没有位错。然后，释放的纳米膜可以转移到其他基质上。SiGe量子阱材料系统的特定设计问题给基于硅的QC激光器的演示带来了更多的挑战，该系统也在该项目中得到了宣传。将探索一种基于Ge/SiGe量子阱L峡谷中电子ISB跃迁的方法。最近的计算表明，这种方法可能比目前所研究的p型结构具有更长的非辐射寿命、更大的振子强度和更有效的隧道传输。该项目涉及具有技术相关性的电子/光子材料科学专题领域的基础研究问题。将纳米薄膜技术用于制造复杂的半导体量子结构的提议具有潜在的广泛技术影响，超出了上述SiGe材料系统和QC-激光器件的应用。此外，项目活动将通过对学生进行跨学科培训来促进教育，范围从半导体外延生长到纳米薄膜合成和加工、带隙工程和太赫兹(太赫兹)光子学。为了提高方案的有效性和范围，将强调本科生和高中实习生的参与，利用现有方案，重点关注代表性不足的少数族裔。

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.