Chongfan Technology
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24
2025
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01
Quantum dot photodiode array for near-infrared imaging, featuring global shutter and charge merging functions.
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In the near-infrared (NIR) and short-wave infrared (SWIR) bands, traditional silicon-based image sensors exhibit poor sensitivity, while new applications such as depth measurement and multispectral imaging have an urgent demand for image sensors that can efficiently sense in these bands. Colloidal quantum dot (CQD) technology, with its high quantum efficiency at the exciton peak and support for high-resolution imaging, has become a highly promising candidate to meet these new application demands.
According to MEMS Consulting, recently, a research team formed by STMicroelectronics and the University of Toulouse proposed an electrical model for characterizing the electrical behavior of colloidal quantum dot photodiodes designed and manufactured. With this model, researchers explored a new architecture that collects holes instead of electrons. This architecture allows for precise control of charge collection within the colloidal quantum dot film through an electric field. This feature not only enables global shutter functionality but also allows for charge binning operations from multiple photodiodes or alternately driving two physically interleaved photodiode arrays with different types of pixel circuits. These operating modes greatly expand the potential of colloidal quantum dot image sensors in various applications. This research was published under the title "Global Shutter and Charge Binning With Quantum Dots Photodiode Arrays for NIR Imaging" in theIEEE Journal of the Electron Devices Society.
The photosensitive layer in colloidal quantum dot photodiodes is composed of semiconductor nanocrystals. For infrared applications, the photosensitive layer of colloidal quantum dots is mainly made of PbS, InAs, InSb, or HgTe, embedded in an organic or inorganic ligand matrix. By controlling the nanoparticle diameter through engineered chemical synthesis conditions, the bandgap of the colloidal quantum dots and the absorption peak corresponding to the first 1Sh-1Se transition can be precisely adjusted to match specific wavelengths in the infrared band.
Figure 2a shows the colloidal quantum dot photodiode developed in this study for collecting electrons, Figure 2b is its schematic diagram, and Figure 2c presents the manufacturing process flow of the colloidal quantum dot photodiode. The researchers conducted comprehensive characterization of the device to extract physical parameters and validated the device model based on technology computer-aided design (TCAD).
Subsequently, to achieve band structure engineering, the researchers explored an architecture that collects holes. The new architecture of this photodiode is shown in Figure 3a, where the core is the inversion of the electron transport layer (ETL) and hole transport layer (HTL) in the photodiode carrier transport layer. This layer inversion brings two process integration challenges: (1) the presence of the HTL/Cu and ETL/top electrode interface replaces the original HTL/top electrode and ETL/Cu electrode; (2) the thermal budget issue required for the deposition layer above the colloidal quantum dot film. The researchers then discussed in detail the impact of various physical parameters related to the colloidal quantum dot film or associated with ETL and HTL on device performance.
In subsequent research, the researchers focused on demonstrating the possibility of directing all photo-generated carriers to specific bias electrodes. Figure 4a shows this new design of the colloidal quantum dot film photodiode, which interleaves two pixel arrays with different resolutions and sensitivities, providing both simultaneous and alternating operating modes. (1) In the simultaneous operating mode, all photodiodes are in the ON state (conducting state), and the photosensitivity of each pixel is closely related to the relative volume controlled by the electrostatic control of its bottom electrode. If the initial bias of all electrodes during exposure is higher than their reach-through voltage, and then suddenly lowered below their reach-through voltage, all bottom electrodes can be simultaneously blocked, thus achieving global shutter functionality. (2) In the alternating operating mode (as shown in Figure 4b), all bottom electrodes of the first pixel array (Pixel A) collect all charges, while the bottom electrodes associated with the second pixel array (Pixel B) are in the blocking state.
In summary, this research first reviews the physical properties of colloidal quantum dot films and introduces the use of existing physical models in TCAD software to simulate the performance of colloidal quantum dot photodiodes. Next, the study proposes a photodiode architecture that collects holes instead of electrons. This new architecture is optimized by inverting the ETL and HTL in the photodiode carrier transport layer. Subsequently, the study elaborates on methods to enhance these barriers to construct the OFF state for the photodiode in reverse mode. Finally, the study conducted simulation demonstrations confirming that utilizing this OFF state and the electrostatic blocking capability of holes at the bottom electrode obtained through band engineering can achieve global shutter functionality by simultaneously turning off all photodiodes, as well as alternating operation of two pixel arrays with different resolutions or circuits. Additionally, the study can achieve pixel binning by collecting charges only on specific electrodes, thereby improving sensitivity while reducing resolution.
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