Colloidal Quantum Dots and Integration with Silicon for Near-Infrared Light Detection
Creators: Xu, Qiwei
Publisher: name: University of Alberta Library
Year: 2023

<p>Near-infrared (NIR) light detection underpins various applications such as night vision, healthcare imaging and optical communications. While commercial silicon-based photodetector is transparent to wavelengths that are longer than about 1100 nm due to the absorption cut-off of silicon (Si), lead sulphide (PbS) colloidal quantum dots (CQDs) have received attention as the materials for NIR photodetection given their strong and tunable absorption in NIR region and room-temperature solution processability. This thesis presents our progress in realizing fast and efficient NIR photodetection based on PbS CQD. The particular focus considers the integration of PbS CQDs with Si for on-Si light detection. <br>
The CQDs exhibit low carrier mobility (~10-5-10-2 cm2V-1s-1) compared to their crystalline counterparts, which has severely hindered the realization of high-speed PbS CQD photodetection. We demonstrate a fast PbS CQD photodiode fabricated from low-mobility CQDs (~10-3 cm2V-1s-1) with rise/fall times as short as 0.33 μs at zero voltage bias. The device achieves a fast response by engineering RC time delay and charge transport. <br>
Integrating PbS CQDs onto Si opens a plethora of applications in NIR light sensing for the CQDs. An integration strategy is proposed based upon an on-chip CQD photodetector structure, photodiode-oxide-semiconductor field-effect transistor (POSFET). This device architecture utilizes Si as a channel for carrier transport and PbS CQD as the light-absorbing material controlling the channel conductivity. Simulations showed that the photodiode could provide photovoltage to the semiconductor, forming an inversion layer at the oxide-semiconductor interface, and the electron density at the interface is significantly enhanced. For a proof-of-concept demonstration, we experimentally connected a CQD photodiode with a commercial Si transistor and proved that the current from the transistor could be increased by photovoltage provided by the photodiode under NIR light illumination. The device shows a responsivity of 5.9A/W at the wavelength of 1250 nm.<br>
Another on-Si integration strategy based upon a structure of PbS CQD/Si heterojunction has recently received attention. We conducted a simulation work, in which we leverage and incorporate the heterojunction structure in a Si photonics platform. The study comprehensively compares the CQDs with the other two candidates for NIR light detection, germanium (Ge) and indium gallium arsenide (InGaAs), for a photodetector integrated with a Si waveguide. Results find that the CQD photodetector is slow but has the lowest dark current, which is suitable for medium-speed but ultralow noise applications.<br>
CQD/Si heterojunction photodetectors that have been previously reported suffer from low EQE at the NIR region. We demonstrate that Si surface passivation is crucial for building a high-performance CQD/Si heterojunction photodetector. We have studied one-step methyl iodine (CH3I) and two-step chlorination/methylation processes for Si surface passivation. Transient photocurrent (TPC) and transient photovoltage (TPV) decay measurements reveal the two-step passivated CQD/Si interface exhibits fewer trap states and decreased recombination rates. These passivated substrates were incorporated into prototype CQD/Si infrared photodiodes, and the best performance photodiode based upon the two-step passivation shows an EQE of 31% at 1280 nm which represents a nearly two-fold increase over the standard device based upon the one-step CH3I passivated Si. <br>
Further, we demonstrate the successful integration of PbS CQD Inks with Si. Thanks to the well-passivated Si surface by the two-step method and high-quality CQD Inks, the heterojunction photodiode yields a low density of trap states. With an insertion layer of p-type CQDs capped with1,2-ethanedithiol (EDT) ligands, the built-in electric field is much enhanced, leading to improved charge extractions. As a result, we obtained an EQE of 44% at the excitonic wavelength of 1280 nm. The EQE values are maintained without detectable degradation throughout more than 600 h, achieving superior device stability. <br>
The aforementioned studies reveal the potential of large-area and room-temperature integration of PbS CQDs on Si read-out integrated circuits (Si-ROCIs) for building a NIR CMOS image sensor (CIS). Finally, we conduct a simulation study, in which we incorporate CQD/Si heterojunction into a NIR CMOS CIS. Results clearly show that each pixel on the CIS sensitively responds to NIR light, and signals are read out through the bottom circuits as validated by the readout timing sequence simulation.<br>
The findings in this thesis are invaluable to the research of developing low-cost and large areas, especially on-Si NIR light detection.</p>