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Doctoral Defense

Optical absorption and photoconductivity in barrier heterostructures with short period InAsSb x/InAsSb y strained layer superlattices

Ye Xu

October 1, 2018
10:30 AM
Light Engineering room 250
Advisor: Prof. Dmitri Donetski

The dissertation is dedicated to the characterizations and applications of new Long Wavelength Infrared (LWIR) devices with short period Strained Layer Superlattices (SLS) InAsSb x/InAsSb y heterostructures. The band structure of SLS material InAsSb 0.3/InAsSb 0.6 was studied with barrier Al xGa yIn 1−x−ySb with different Ga compositions: 0%, 5%, 10% and 15%. Valence band offset was estimated and was experimentally proved 15% Ga material with the best band match. The 28% Quantum Efficiency at 3.2 μm was demonstrated with minority hole lifetime 106 ns. An in-plane Negative Photoconductivity (NPC) phenomenon was observed in InAsSb-based SLS under laser excitation at 77K. NPC modelling of InAsSb 0.3/InAsSb 0.6 was done with acoustic and optical phonon scattering in τ-approximation of collision integral. The NPC was explained by reduction of electron mobility due to electron momentum relaxation time decrease and effective mass increase. It was concluded that NPC is a fundamental phenomenon in narrow gap semiconductor materials. The absorption modulation was studied in bulk InAsSb 0.42 alloys and SLS InAsSb 0.32/InAsSb 0.62 heterostructure under electric injection. A significant change of transparency of bulk material was observed in the range of 7 to 10 μm. It was found that the transmission spectrum extends to smaller wavelength at higher injection current, which was explained by population of states. With a 8.6 μm quantum cascade laser in continuous wave mode, the modulation depth was measured at 9.1%. The position of quasi Fermi level was estimated to be at 20 meV above the bottom of the conduction band. For SLS InAsSb 0.32/InAsSb 0.62 heterostructure, the modeling of current spreading has showed a current inhomogeneity spreading within SLS heterostructure. SLS with 2 μm thickness was demonstrated a 7% modulation depth at 10.6 μm. It was concluded that InAsSb-based SLS heterostructures have the potential to provide deep and fast modulation at LWIR in the optical systems with solid-state, gas and semiconductor lasers.


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