Colloidal quantum dot (CQD) based mid-wavelength infrared optoelectronics

Colloidal quantum dot (CQD) photodetectors are a rapidly emerging technology with a potential to significantly impact today’s infrared sensing and imaging technologies. To date, CQD photodetector research is primarily focused on lead-chalcogenide semiconductor CQDs which have spectral response fundamentally limited by the bulk bandgap of the constituent material, confining their applications to near-infrared (NIR, 0.7-1.0 um) and short-wavelength infrared (SWIR, 1-2.5 um) spectral regions. The overall goal of this dissertation is to investigate a new generation of CQD materials and devices that advances the current CQD photodetector research toward the technologically important thermal infrared region of 3-5 ?m, known as mid-wavelength infrared (MWIR). In this dissertation, electronic and optoelectronic characteristics of Ag2Se CQD based devices are analyzed by different device architectures with detailed analysis of detector performance parameters. The first part of the dissertation includes the report on the fabrication of solution-processed lateral photoconductive photodetectors. Significant photoresponse is demonstrated in MWIR with the lateral photoconductor at room temperature. The detailed analysis on the effect of ligand exchange as well as temperature and spectral dependent photoresponses is presented. In the second device structure, vertically stacked quantum dot devices are demonstrated. In this device architecture, a barrier QD layer is placed in between mid-wavelength absorber intraband Ag2Se QD layer. The insertion of barrier layer reduces dark current significantly since 1Se Ag2Se QD-1Se PbS QD conduction offset serves as a potential barrier, blocking the transport of thermally generated electrons and holes. In addition, vertical device design improves detector performance parameters significantly at room temperature. At the last part of the dissertation, development of p-n heterojunction diode devices is presented as third device structure. High performance detectors can be realized using a traditional p-n junction device design, however, the heavily-doped nature of intraband quantum dots present a new challenge in realizing diode devices. To address this challenge, an unique trait of blending two different QDs is employed to control electrical property. The fabricated p-n junction devices demonstrate reduced noise current density due to reverse bias operation, which shows improvement in the specific detectivity of the detector at room temperature. Consequently, this dissertation presents the feasibility of uncooled, room-temperature photodetection in the MWIR with intraband silver selenide quantum dots that has the potential to impact numerous applications ranging from all-weather night vision, machine vision, biomedical imaging, to free-space optical communication