Development of Terahertz Quantum Well Photodetector at 3 THz

Abstract

This thesis reports a new terahertz (𝑓=3.22 THz) quantum well photodetector (THz QWP) as well as comprehensive numerical models simulating device active region and different grating couplers including traditional diffraction metal grating and novel patch antenna structure. Among all the terahertz detectors, terahertz quantum well photodetector (THz QWP), has proved its fast optical response speed and remarkable sensitivity and shown great potential for applications in security, bio-medical technology and space communication. However, due to high requirements in experiment condition and growth quality, THz QWPs absorbed at 3 THz or lower have not been much developed yet. Furthermore, THz QWP working around 3 THz can be combined with one of the strongest terahertz emitters, terahertz quantum cascade laser (THz QCL), for ultrafast spectroscopy and imaging applications. Recently, THz QWPs integrated with different grating couplers have been explored and are now intensively investigated to improve device temperature performance. In that regard, we aim to improve the simulation model and develop a new THz QWP absorbing at 3 THz. Moreover, numerical COMSOL models are built to analyze optical properties of traditional diffraction metal grating coupler and novel patch antenna coupler in this work. According to measured current density-voltage (j-V) profiles and absorption spectrum, the proposed THz QWP with a new active region design has its background-limited infrared performance (BLIP) temperature at 10 K and manages to achieve terahertz range absorption at lower frequency around 3 THz. Results show a peak responsivity of 1.9 A/W, a peak detectivity of 4.63×1010 cmHz1/2/W and an absorption range from 94.5 cm-1 (2.83 THz) to 142.7 cm-1 (4.25 THz). To our knowledge, this THz QWP has the lowest peak absorption frequency (3.22 THz) and it is the second one that works near 3 THz. Measured j-V profiles show a size-dependent shifting, indicating the existence of sidewall leakage currents, and the comparison between simulation and measurement results of j-V profiles reveals the consistency, especially at higher temperature. In addition, simulation results on 1D metal grating coupler and patch antenna coupler are demonstrated and they strongly agree with the experiment data from literatures as well as general rules of thumb.