Objective  Mid-wave infrared imaging plays an important role in various fields including military reconnaissance, remote sensing, and aerospace. The existing mid-wave infrared focal planes mainly use bulk semiconductor materials such as mercury cadmium telluride, type-II superlattices, and indium antimonide, which have excellent performance and high stability. However, the complex material preparation and flip-chip bonding processes limit the production volume and their usage in cost-sensitive application. As an emerging infrared semiconductor material, colloidal quantum dots (CQDs) have the advantages of wide spectral tunability, large-scale synthesis, and low-cost preparation, providing a new route towards high-performance and low-cost infrared focal plane arrays. For this purpose, HgTe CQDs have been investigated and a mid-wave infrared focal plane array imager has been proposed in this paper.

  Methods  Oleylamine was used as the reaction solvent for the synthesis of HgTe CQDs. Inorganic mercury salts and tellurium were dissolved in oleylamine and trioctylphosphine, respectively, at 100 ℃. After mixing them in an anhydrous and oxygen-free environment, the size of the HgTe CQDs can be precisely controlled by the reaction time, thus the response wavelength can be accurately adjusted. The transmission electron microscopy (TEM) image of the HgTe quantum dots used in this experiment is shown (Fig.1), with a diameter of about 8 nm. The response spectra of quantum dots at room temperature and 80 K are shown (Fig.2). The response cut-off wavelength of the quantum dot detector reaches 4.6 μm at 80 K. The HgTe CQDs mid-wave infrared detector uses a trapping-mode photodetector configuration. The device structure and energy band diagram are shown (Fig.3).

  Results and Discussions   The diagram of signal extraction and dewar test package is shown (Fig.4). The performance of the trapping-mode infrared focal plane detector is quantitatively analyzed by testing parameters including photoresponse non-uniformity, noise voltage, specific detectivity, and operable pixel rate. A calibrated blackbody is used as the excitation light source, and the temperature of the blackbody is stabilized with a feedback control circuit. The blackbody emitting cavity is about 4 cm in diameter and the distance between the imager and the emitting cavity is about 25 cm. The experimental results show that the non-uniformity of the photoresponse of the focal plane array device is as low as 3.42% (Fig.5(a)). The noise of the detector is an important indicator of performance, which is determined by the noise of the readout circuit itself and the uniformity of the film thickness of the detector pixel points. The overall noise of the detector is low, and the average noise voltage is as low as 0.66 mV at an integration time of 2 ms and a device bias of 2.3 V (Fig.5(b)). The distribution of the specific detectivity, and the average peak specific detectivity is about 2 × 10¹⁰ Jones (Fig.5(c)). The operable pixel rate can reach 99.99% (Fig.6).

  Conclusions  In this paper, we report a CMOS-compatible trapping-mode HgTe CQDs mid-wave infrared focal plane and demonstrate the infrared thermal imaging capability. With a noise equivalent temperature difference of 51.26 mK (F#=2), a low photoresponse nonuniformity of 3.42%, an operable pixel rate of 99.99%, a response cutoff wavelength of 4.6 μm, and a peak specific detectivity of 2×10¹⁰ Jones at 80 K, the HgTe CQDs-based focal plane array is expected to potentially solve the bottlenecks faced by traditional bulk semiconductors. In the future, HgTe CQDs will be combined with 3D nanostructure embossing and other processing technologies to develop multi-functional and multi-mode infrared detectors.