SWIR detection with thin-film photodetectors based on colloidal quantum dots

Image sensors operating in the short-wavelength infrared (SWIR) wavelength range typically use epitaxially grown III-V semiconductor as the photoactive material. To realize a two-dimensional focal plane array, the detector chip is connected with a CMOS readout chip using solder bump hybridization, imposing a limit on the pixel pitch. One way to realize higher resolution and finer pixel scaling is to use a monolithic approach with the photoactive layer deposited directly on top of the readout chip. In this paper, we describe a pixel stack based on colloidal quantum dots that can be monolithically integrated. Colloidal quantum dots are an interesting material group as their optical properties can be tuned with their size and their electrical properties can be adjusted by the organic ligand selection. For SWIR detection, we are using PbS QDs with diameter larger than 5 nm, with the cut-off wavelength reaching 1600 nm. The QD film is deposited by spin-coating directly on top of the substrate and the process temperature is kept below 150°C. QD active layers for infrared detection are not widely explored, thus we are focusing on optimization of the QD film and development of the multilayer pixel stack. Electron and hole transport layers are selected to improve the photodiode performance while top and bottom contacts are optimized to allow top illumination. Optical interference simulations provide optimum thickness of each layer to enhance the cavity effect (and thus the efficiency). Furthermore, optical design is used for the semi-transparent top contact to improve the light in-coupling effect, supporting top illumination. Devices realized until now have dark current density in the range of 10-6 A/cm2 at a reverse bias voltage of -2 V. The external quantum efficiency at the wavelength of 1450 nm is above 10%, even though the active layer film thickness in only 150 nm. The stack is compatible with integration on top of silicon substrate. In summary, colloidal quantum dots provide a way to realize monolithic infrared imagers in a cost-effective way. Thin-film active layer enables scaling down pixel pitch beyond the limitations of traditional flip-chip hybridization, as the resolution is defined by the readout chip. In this work, we present a QD-based pixel stack design for a next generation, monolithic infrared image sensor. It enables uncooled detection of SWIR radiation up to the wavelength of 1600 nm.