Materials Research Areas of the Ringel Group

III-V Compound Semiconductors. Research involves the growth and characterization (structural, chemical, electronic, optical) properties of III-V compounds, heterostructures and nanostructures based on (Ga,In,Al)/)(As,P,Sb). Focus is on MBE growth, defects, interface and surface properties and growth/relaxation dynamics in these materials and how these correlate with electronic and optoelectronic properties.

III-V/Si Heterogeneous Integration and Substrate Engineering. Research on the use of engineered SiGe substrates via the > 15 year Ringel-Fitzgerald (MIT) collaboration to enable lattice mismatched integration of a wide range of III-V compounds and heterostructures with Si is ongoing. Focus is on heterovalent interfaces, all forms of materials mismatch, optimized bandstructure engineering and defect control.

Metamorphic III-V Compounds and Lattice-Mismatched Heteroepitaxy. Research on the optimization of lattice constant grading based on both the anion (group V) and cation (group III) sublattice is being conducted to span lattice constants between those of Si and InAs. This is augmented through the newly formed Heterogeneous Materials Integration Initiative (HMI2) partnership with the Air Force Research Laboratory at Wright Patterson Air Force Base. Studies include comparative studies of the interrelationships between growth, relaxation dynamics, electron transport and defects as a function of material system.

Defects and Trap Characterization in Semiconductors. Detailed exploration of the physics and impact of deep level defects in semiconductors that include GaN and AlGaN, III-As, III-P, and SiGe materials are ongoing using a wide range of state of the art deep level spectroscopy methods. The roles played by materials growth method and parameters, processing conditions, high-energy radiation, alloy composition, and other issues are all part of detailed explorations that are critical for photovoltaics, electronics, optoelectronics and other device technologies.

Metal-Semiconductor Nanostructures. Crystalline metal-semiconductor structures are being grown and characterized for research on plasmonic and spintronic properties.

Device Research Areas of the Ringel Group

Solar Energy Conversion Devices. Intense research is ongoing to continue our group's pioneering activities to develop hybrid III-V/Si solar cell technologies for both space and terrestrial solar energy conversion applications. Work on multi and single junction III-V/SiGe solar cells, 3-D engineered photovoltaic (PV) concentrators is ongoing, and methods to integrate super efficient thin crystalline films with inexpensive and thin substrates are being explored.

Optoelectronics. Primary emphasis is on the monolithic integration of visible and infrared band laser diodes and light emitting diodes with Si, analogous to our photovoltaics programs. Quantum and Nanostructured III-V's incorporating multiple quantum well, superlattice and Bragg reflectors, with process science to achieve optimized optical confinement using methods that are compatible with Si processing are being explored.

Thermal Energy Conversion Devices. Research includes efforts to advance both thermophotovoltaic (TPV) and thermoelectric (TE) devices based on unique tandem structures, bandgap engineering and selective use of nanostructures. Devices are developed using the metamorphic narrow bandgap materials discussed above. The TE effort is a collaboration with Prof. Heremans and his group in Mechanical Engineering.

Ultra-High-Speed Electronics and Metamorphic Nanoelectronics. Research is intensely focused to incorporate the ultra-high mobility large lattice constant III-V semiconductors explored in the metamorphic materials group, described above, into various FET and HBT transistor structures and nanoelectronic circuits. This activity is in collaboration with Prof. Wu Lu and his research group.

Wide Bandgap RF Electronics. As part of an ONR MURI team, we are exploring the properties and impact of electronic traps within AlGaN/GaN HEMT devices. This effort is a device-driven extension of the efforts described above in the Defect Characterization component of the Ringel group.