Vision, Challenge, Goal and Impact
Our vision is to develop high quality, stable (Si)GeSn alloys to revolutionize infrared imaging and to answer fundamental questions that are unique to this alloy system. (Si)GeSn is a disruptive technology, which at 20% to 40% Sn, can meet DoD need for lighter, faster, higher signal-to-noise, and more energy efficient 2-5 µm infrared imaging devices at significantly lower cost.
The challenge is that Group IV materials and their synthesis are underdeveloped as evidenced by: (i) most compositions have not even been made and the stability, energy bands, carrier lifetime, defects, are not known for nearly all possible compositions, including those already grown. Also (ii) Group IV materials are very special compared to competitive III-V or II-IV materials in which Si, Ge, Sn, and Pb atoms can occupy any lattice position, randomly placed, clustered, or coherently organized, so that the same composition can have very different properties depending on the atomic ordering, however, these differences, are also unknown. Likewise, (iii) many different compositions can have nearly the same bandgap, however, there is no knowledge about their difference in stability, quality, or properties. Moreover, (iv) different non-thermal equilibrium growth methods are more ideal for different composition regions, but they also bring an unknown trade-off of material quality and stability. Added to this, (v) we are well aware that substrates which create tensile strain readily incorporate Sn while those that create compressive strain reject Sn, but we have no knowledge of how to manage strain to achieve high Sn, and stable (Si)GeSn. Similarly, (vi) we observe Sn droplets to form during growth of high Sn content (Si)GeSn but have no knowledge on how Sn nucleates.
Our goal is targeted to meet these challenges by bringing together a team already at the frontier of (Si)GeSn from material growth, modeling, properties, to high performance devices who have researched together for many years on photonics and IR imaging. This team will leverage and build on its initial breakthroughs on GeSn lasers and detectors, using their (1) unique but complementing non-thermal equilibrium growth techniques, (2) new and exciting ideas to probe and understand group IV growth mechanisms, quality and material stability, and (3) the ability to measure and deliver the fundamental properties needed to design, fabricate and demonstrate (Si)GeSn as the new dominating IR detector technology.
DoD Impact: We will demonstrate infrared detectors with higher performance, broader spectral range, and at significantly lower cost than currently available. As a result, this MURI will have a huge impact on military IR imaging systems, especially for aircrafts which must scan the battlefield in poor visibility situations, ground-based night vision systems from the soldier to vehicles, and on missile tracking systems.
Scientific impact: At the same time, the team will answer fundamental questions that are unique to (Si)GeSn, and inspire new opportunities for the group IV material system. For example, (i) if order-disorder can improve material quality; (ii) can an improved thermodynamic equilibrium diagram point to compositions that improve high Sn stability; (iii) By changing the composition ratio in the growth of group IV alloys, can we reveal metastable points to fabricate new stable high Sn compositions; (iv) will we find that solubility of Sn, as well as the trade-off on quality and stability; (v) by tuning the composition for the same bandgap energy, we can have an indirect or direct bandgap. Direct bandgap is generally better for lasers, but will the indirect/direct border i with high absorption and long carrier lifetimes, be better for detectors, and (vi) can in-situ STM observe and understand nucleation of Sn atoms to form Sn clusters, control stability, and reduce dislocations?
The impact to education is that this MURI will deliver the next generation of researchers to lead progress on IR detector technology. Our plan is to give our students a broad but in-depth training that encompasses: (i) growth and characterization, (ii) investigating and modeling their properties, and (iii) demonstrating these materials as the new dominating IR detector technology.