US-Ireland Joint R&D Partnership: Strained Engineered Germanium Quantum-Well Laser on GaAs and Si for Optical Coherence Tomography

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Optical-coherence-tomography (OCT) is a powerful technique with a wide range of applications, from imaging live tissue to non-destructive industrial testing. Coherent light sources for OCT in the short wave infra-red (SWIR) wavelength can achieve much higher resolution with wider bandwidth and penetration in opaque living tissue, such as, brain and lung tissues. However, there is a lack of SWIR sources with the combined intensity and bandwidth to further enhance the OCT performance. The continued development of affordable and compact coherent light sources in this spectral range is important in many areas of modern technology. The combination of different semiconducting materials and light source architectures offers new paths for highly efficient SWIR sources at reduced cost. The central thrust of the proposed collaborative research is to investigate the design of germanium (Ge) based coherent light sources, with heterogeneous integration of InGaAs/Ge/InGaAs quantum-well structures on GaAs and large area, cost-effective Si substrates. Our objective is to develop tunable SWIR light sources capable of significantly higher penetration depth, image contrast and resolution than available from existing light sources, which will benefit a wide range of important medical, industrial and consumer applications.To demonstrate the viability of the proposed approach, several key technical and scientific challenges must be addressed, including: (i) design and numerical simulation of the proposed strained ε-Ge-based quantum-well (QW) device architectures; (ii) materials synthesis and analysis of InGaAs/ε-Ge/InGaAs QW heterostructures on GaAs and Si using III-V strain template for modified bandgap of Ge; (iii) fabrication and demonstration of ε-Ge QW coherent light sources in wavelength ranges from 1.7 μm to 2.5 μm on GaAs; and (iv) implementation of an integration scheme on large area, cost-effective Si substrates. An international partnership of scientific groups from USA, Ireland and Norther Ireland brings together a synergistic mix of expertise and specialized facilities in materials science, semiconductor fabrication, test and simulation. To address (ii), (iii), and (iv), the proposed research will utilize the state-of-the-art in-house epitaxial growth (interconnected group-IV and III-V molecular beam epitaxy chambers), collaborative materials characterization and simulation (e.g., high-resolution x-ray diffraction, transmission electron microscopy, photoluminescence spectroscopy, deep level transient spectroscopy, and electronic band structure simulation), and in-house/partner fabrication facilities. To address (i), a combination of numerical simulations and electronic structure theory will be leveraged to develop experimentally-calibrated InGaAs/ε-Ge/InGaAs QW device models necessary for broadband light emission in SWIR range. By investigating these topics, this research will elucidate numerous as-of-yet unexplored avenues of fundamental research, including: (a) the amount of strain and doping density in Ge to optical gain and emission wavelength; (b) the role of Ge layer thickness as a function of strain to optical gain; (c) the reduction of current density arising from non-radiative recombination; (d) the threshold current density with amount of strain in Ge; and (e) the realization of device-quality epitaxial Ge QW heterostructures on Si through minimization of dislocations and anti-phase domains in in-situ III-V buffer architectures on Si. Through a comprehensive examination and understanding of the above challenges, this research will establish a pathway to achieving new high performance coherent light sources in the little explored SWIR spectral range that will benefit society as well as industry via medical imaging and non-destructive testing. Furthermore, this international scientific partnership allows for a comprehensive project that trains and mentors students in the field of photonics and nanotechnology through exchange programs. The outcomes of the proposed research results will be disseminated to public through National Science Foundation and lay a foundation for continued and growing US-Ireland collaboration.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.