Date of Award
Doctor of Philosophy (PhD)
Physics and Astronomy
A. G. Unil Perera
Several types of p-doped Infrared detectors were studied. These include InAs/GaAs quantum dot (QDIP), and dots-in-well (DWELL) and split off band-based heterojunction detectors. In these structures, IR absorption leading to detection is based on valence-band inter-sublevel hole transitions. For a QDIP and DWELL, at 80 K, two response bands observed at 1.5 – 3 and 3 – 10 µm were identified as due to optical transitions from the heavy hole to spin–orbit split-off QD level and from the heavy-hole to heavy/light-hole level, respectively. Unlike the n-type with bias dependent spectral response, the p-type hole response displays a well-preserved spectral profile (independent of the applied bias) observed in both QDIP and DWELL detectors. At a response peak of ~ 5.2 µm, QDIP and DWELL exhibit an external quantum efficiency of 17 % and 9 % respectively. At elevated temperatures between 100 and ~120 K (for QDIP), 130 K (for DWELL), both QDIP and DWELL detectors exhibit a strong far-infrared or terahertz (THz) response up to 70 µm which show promising potential of p-type QDs for developing THz infrared photodetectors.
Based on the dark current and noise power spectral density analysis, structural parameters such as the numbers of active layers, the surface density of QDs, and the carrier capture or relaxation rate, type of material and electric field are some of the optimization parameters identified to improve the photoconductive and dark current gain of detectors. The capture probability of DWELL is found to be more than two times higher than the corresponding QDIP. Based on the noise analysis, QDs based structures suppressed phonon scattering and enhanced carrier life time or photoconductive gain. Furthermore, in a GaAs/AlxGa1-xAs heterostructure, for a given width of AxlGa1-xAs barrier, the barrier thickness can be varied by varying the Al mole fraction x, which is referred to as a graded barrier. Grading the barrier and optimizing the emitter thickness of GaAs/AlGaAs heterostructures enhance the absorption efficiency, the escape probability and lower the dark current; hence, enhances the responsivity and specific detectivity of detectors.
The two important methods (Arrhenius plot and Temperature Dependent Internal photoemission (TDIPS)) of determining detectors threshold wavelengths or band offsets were compared. For detectors with long threshold wavelength (>> 9.3 μm), the Arrhenius plot used to extract activation energy leads to energy values with deviation higher than ~ 10 % from the corresponding TDIPS values and results from the temperature dependent Fermi distribution tailing effect and Fowler–Nordheim tunneling current. Therefore, TDIPS or other methods, that take the temperature effects on the band offset and Fowler–Nordheim tunneling current into account, are needed for a precise band offset characterization of a long threshold wavelength detectors.
Wolde, Seyoum G., "p-TypeE InAs/GaAs Quantum Dot, Dot-In-Well, and Low-Frequency Noise Properties of Infrared Photodetectors." Dissertation, Georgia State University, 2018.