Design, fabrication and characterization of quantum dot infrared photodetectors

Mid- and long-wavelength infrared (3--20 mum) detection is a key technology for numerous commercial, military and space applications, e.g., night vision, thermal imaging, chemical analysis, non-destructive detecting, remote sensing, and missile guidance and defense. Due to the long carrier capture and relaxation times, quantum dot infrared photodetectors (QDIPs) have the potential for higher photoconductive gain and photoresponse than quantum well infrared photodetectors (QWIPs). Most importantly, the three-dimensional confinement of electrons in the quantum dots permits QDIPs to operate in the normal-incidence mode, unlike QWIPs which are not sensitive to radiation that is incident perpendicular to the quantum wells.; In the work presented in this dissertation, we established two ways to successfully lower the dark current and noise: use un-intentionally doped active regions and introduce AlxGa1-xAs current blocking layers. The combination of these two significantly enhanced the performance of the QDIPs and yielded a 5-layer InAs QDIPs with a high detectivity of 1 x 1010 cmHz1/2/W for 6.2 mum photoresponse at 77 K.; The tunability of the absorption wavelength is directly correlated to the manipulation of electronic states in the QDs, which can be controlled by altering the QD confining potentials through the change of the QD cap layers. Studies on a series of InAs QDs with InxGa1-xAs cap layers showed a red shift of the photoluminescence peaks with increase in the In content, which suggests a decrease in the intraband transition energy. Strong photoluminescence and narrow spectral widths indicated no decrease in performance. 2.0-ML InAs QDIPs with In0.15Ga0.85As cap layers demonstrated peak absorption at 8.4 mum with a detectivity of 5 x 109 cmHz1/2/W at 78 K.; Taking the advantage of the narrow photoresponse spectral widths that enable very low cross channel interference, we demonstrated the first mutli-wavelength QDIP. This voltage-controllable multi-wavelength detection is a result of a bimodal size distribution in the QDIP structure. QDIPs with variable deposition amount QDs took this concept a step further to provide control of the two absorption wavelengths and optimization of the device performance for each wavelength peak.; New structures that focus on the increasing the responsivity have been exploited to further improve the QDIPs performance. QDIPs with increased absorption length were realized by incorporating ten QD layers, which gave rise to a significant enhancement of detectivity. At 78 K, the detectivity reached 3 x 1011 cmHz1/2/W at 9.3 mum. This is the highest detectivity reported for any QDIP and this record high detectivity exceeded the detectivity of QWIPs in the long-wavelength infrared regime.; A microcavity that consists of Au as a high reflectivity back mirror and 3.5 pairs Ge(140 nm)/SiO2(331 nm) as a low-loss front mirror, respectively, showed the potential to increase the responsivity by a factor of 5∼10. By incorporating the microcavity structure with this 10-layer QDIP, we anticipate that performance of QDIPs can be made comparable to that of Mercury Cadmium Telluride (Hg1-xCdxTe) photodetectors.