Author ORCID Identifier


Date of Award

Fall 12-10-2018

Degree Type


Degree Name

Doctor of Philosophy (PhD)


Physics and Astronomy


p-GaAs/AlxGa1-xAs heterostructure-based extended-wavelength infrared photodetectors have been experimentally studied in great detail. Unlike the conventional IR photodetectors with the wavelength threshold determined by an activation energy (Δ) corresponding to a minimum energy gap, a novel class of IR photodetectors show an extended-wavelength spectral photoresponse far beyond the limit set by Δ (given as λt = hc). p-GaAs/AlxGa1-xAs heterostructure-based IR photodetectors, with non-zero barrier energy offset between the AlxGa1-xAs barriers, were used. These IR photodetectors were initially designed to have a conventional wavelength threshold of ~3 µm, corresponding to Δ ~0.4 eV of p-GaAs/AlxGa1-xAs heterostructure. However, the extended-wavelength photoresponse was observed up to ~ 60 µm. The study included a set of devices with varying values of the barrier energy offset. The offset was found to be necessary for the extended-wavelength mechanism. However, with further increase in the offset by ~0.13 eV, the extended-wavelength threshold showed a very small variation (~0.002 eV). Instead, increasing the barrier energy offset led to an increased strength of spectral photoresponse. Similarly, increasing the barrier gradient in another set of devices caused no significant change in the wavelength threshold. These observations are believed to originate from build-up of a quasi-equilibrium Fermi level, at a fixed level, irrespective of the variation of the device parameter. Split-off (S-O) band was found to be the most probable energy level to build-up a quasi-equilibrium Fermi level as consequence of hot-phonon bottleneck effect. The study of the dark current characteristics of these IR photodetectors confirmed no compromise in the dark current due to the presence of the extended-wavelength mechanism of photoresponse. In addition, from the study of a 2 – 6 µm IR photodetector, based on p-GaAs/AlxGa1-xAs heterostructure, operating at 0 V, the specific detectivity was found to be enhanced by two orders of magnitude, to ~1.9×1011 Jones, due to a current blocking barrier, although a small compromise in the spectral responsivity (by a factor of ~1.5) was observed. Furthermore, a 3 – 5 µm band IR photodetector based on p-InP/InAlAs heterostructure (currently under development), is discussed as an alternative for the 3 – 5 µm band detection.