Optimization of charge carrier extraction in colloidal quantum dots short-wave infrared photodiodes through optical engineering

Colloidal quantum dots (QDs) have attracted scientific interest for infrared (IR) optoelectronic devices due to their bandgap tunability and the ease of fabrication on arbitrary substrates. In this work, short-wave IR photodetectors based on lead sulfide (PbS) QDs with high detectivity and low dark current is demonstrated. Using a combination of time-resolved photoluminescence, carrier transport, and capacitance-voltage measurements, it is proved that the charge carrier diffusion length in the QD layer is negligible such that only photogenerated charges in the space charge region can be collected. To maximize the carrier extraction, an optical model for PbS QD-based photodiodes is developed, and through optical engineering, the cavity at the wavelength of choice is optimized. This universal optimization recipe is applied to detectors sensitive to wavelengths above 1.4 mu m, leading to external quantum efficiency of 30% and specific detectivity (D*) in the range of 10(12) Jones

Thin-film quantum dot photodiode for monolithic infrared image sensors

Imaging in the infrared wavelength range has been fundamental in scientific, military and surveillance applications. Currently, it is a crucial enabler of new industries such as autonomous mobility (for obstacle detection), augmented reality (for eye tracking) and biometrics. Ubiquitous deployment of infrared cameras (on a scale similar to visible cameras) is however prevented by high manufacturing cost and low resolution related to the need of using image sensors based on flip-chip hybridization. One way to enable monolithic integration is by replacing expensive, small-scale III-V-based detector chips with narrow bandgap thin-films compatible with 8- and 12-inch full-wafer processing. This work describes a CMOS-compatible pixel stack based on lead sulfide quantum dots (PbS QD) with tunable absorption peak. Photodiode with a 150-nm thick absorber in an inverted architecture shows dark current of 10(-6) A/cm(2) at 2 V reverse bias and EQE above 20% at 1440 nm wavelength. Optical modeling for top illumination architecture can improve the contact transparency to 70%. Additional cooling (193 K) can improve the sensitivity to 60 dB. This stack can be integrated on a CMOS ROIC, enabling order-of-magnitude cost reduction for infrared sensors

Monolithic near infrared image sensors enabled by quantum dot photodetector

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Monolithic Infrared Image Sensors based on Thin-Film Quantum Dot Photodiodes

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Carrier Mobility, Lifetime, and Diffusion Length in Optically Thin Quantum Dot Semiconductor Films

We propose a method to measure the fundamental parameters that govern diffusion transport in optically thin quantum dot semiconductor films and apply it to quantum dot materials with different ligands. Thin films are excited optically, and the profile of photogenerated carriers is modeled using diffusion-based transport equations and taking into account the optical cavity effects. Correlation with steady-state photoluminescence experiments on different stacks comprising a quenching layer allows the extraction of the carrier diffusion length accurately from the experimental data. In the time domain, the mapping of the transient PL data with the solutions of the time-dependent diffusion equation leads to accurate calculations of the photogenerated carrier mobility. These findings allow the estimation of the speed limitations for diffusion-based transport in QD absorbers.status: publishe

Optoelectronic design of thin-film infrared sensitive photodiodes based on lead-sulfide quantum dots

status: Published onlin

Optimization of Charge Carrier Extraction in Colloidal Quantum Dots Short-Wave Infrared Photodiodes through Optical Engineering

© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Colloidal quantum dots (QDs) have attracted scientific interest for infrared (IR) optoelectronic devices due to their bandgap tunability and the ease of fabrication on arbitrary substrates. In this work, short-wave IR photodetectors based on lead sulfide (PbS) QDs with high detectivity and low dark current is demonstrated. Using a combination of time-resolved photoluminescence, carrier transport, and capacitance–voltage measurements, it is proved that the charge carrier diffusion length in the QD layer is negligible such that only photogenerated charges in the space charge region can be collected. To maximize the carrier extraction, an optical model for PbS QD-based photodiodes is developed, and through optical engineering, the cavity at the wavelength of choice is optimized. This universal optimization recipe is applied to detectors sensitive to wavelengths above 1.4 µm, leading to external quantum efficiency of 30% and specific detectivity (D*) in the range of 10 12 Jones.status: publishe

Determining charge carrier extraction in lead sulfide quantum dot near infrared photodetectors

Colloidal quantum dots (QDs) based on lead sulfide (PbS) have acquired scientific interest for infrared optoelectronic devices with potential bandgap tunability and ease of fabrication on arbitrary substrates. In this work, we show how device analysis data feed back into process optimization, towards the realization of high performance QD NIR photodetectors. Using the combination of transient PL, carrier transport and CV measurements we obtain the carrier density, lifetime and diffusion length in the layers. From the measured short diffusion length of the minority carriers, we deduce the need to achieve a wide depletion region to minimize recombination and thus enhance the carrier harvesting. Process optimization lead to a depletion region of more than 150 nm, resulting in high photon to carrier conversion. Furthermore, the complex index of refraction of all layers is characterized using ellipsometry and reflection/transmission, and these values are used as input for a transfer matrix method. Using the first interference peak, we show that a maximum EQE of 25% can be expected from optical modeling, a value that we almost reach experimentally (20%). Combining all of the above, we demonstrate 1450-nm photodetectors with dark current in the range of μA and specific detectivity (D*) of 10^11 Jones

Integration of PbS quantum dot photodiodes on silicon for NIR imaging

Colloidal quantum dots based on lead sulfide (PbS) are very attractive materials for the realization of novel image sensors. They offer low cost synthesis, compatibility with a variety of substrates and processing on large area. The tunable band gap enables selective light detection from the visible wavelengths up to the short-wave-infrared (SWIR). This work describes the roadmap towards the integration of quantum dot photodiodes (QDPD) on top of a Si based CMOS read-out circuit. Photodiodes using an n-p junction architecture are fabricated on Si substrates, showing a dark current of 30 nA/cm(2) at -1 V reverse bias, EQE above 20% and specific detectivity above 10(12) cm Hz(1/2) W-1 at the wavelength of 940 nm. Efficiency is improved by reducing absorption in the top contact through optical design. Furthermore, photolithographic patterning of the thin-film stack is introduced for the first time, showing the feasibility of pixel pitches down to 40 mu m, opening the way towards high resolution monolithic infrared imagers and the incorporation of infrared and visible sensitive pixels side by side