Room temperature mid-infrared quantum dot detector

The mid-infrared band is an important atmospheric window, which provides additional thermal information compared with the visible light band, and has important value in medical detection, meteorological remote sensing, military reconnaissance, and space exploration. However, this band cannot be directly perceived by the human eye.

Infrared photodetectors use photoelectric technology to break through human visual barriers and detect infrared radiation emitted by objects in a passive way. At present, mid-infrared detectors are mainly based on mercury tellurium, quantum well and type II superlattice, which are not only difficult epitaxial growth methods, complex flip bonding processes coupled with readout circuits, but also must achieve high detection performance at low temperatures, which is very costly.

As an emerging infrared material, colloidal quantum dots are easy to synthesize on a large scale by chemical thermal injection, “ink-type” liquid phase processing can be directly coupled with the readout circuit, and its “quantum confinement” effect limits the generation of thermally excited carriers at the three-dimensional scale, which is expected to realize uncooled, low-cost, high-performance mid-wave infrared detectors. However, the interfacial transport and band mismatch caused by the current colloidal quantum dot and heterojunction design still make the detector still have to reach the background limit at liquid nitrogen (80 K) temperature, and the theoretical predicted room temperature operation is still far away.

The research group of Professor Menglu Chen, School of Optoelectronics, Beijing Institute of Technology, proposed a method of mixed-phase ligand exchange, successfully prepared quantum dot inks with multiple doped states from N type to P type, designed and prepared a homojunction device with “strong P-weak P-intrinsic-weak N-strong N” gradient stack, and greatly optimized the built-in electric field, which increased the “background limit” operating temperature of the wave-infrared detector in quantum dots and successfully realized room temperature operation.

The results were published in Light: Science & Applications under the title “High-operating-temperature mid-infrared photodetectors via quantum dot gradient homojunction.”

Mixed-phase ligand exchange includes three steps: liquid-phase ligand exchange, surface dipole doping regulation, and solid-phase ligand exchange.

Its core is to replace long-chain ligands with short-chain ligands, shorten the gap between quantum dots to achieve dense packing and obtain high carrier mobility of the film, and at the same time replace non-polar ligands with polar ligands to achieve accurate and controllable doping types and degrees of quantum dot films. (as shown in Figure 1).

Figure 1. Schematic diagram of the quantum dot ligand exchange process

On the basis of material optimization, this work draws on the design of the gradient layer of traditional materials, uses the “ink” processing technology of colloidal quantum dots, and realizes gradient homojunction devices through the gradient stacking of thin film layers of “strong P-weak P-intrinsic-weak N-strong N” type quantum dots. (Figure 2)

Figure 2. Schematic diagram of a quantum dot gradient homojunction device and energy band

The structure is cleverly designed. On the one hand, the gradient junction strengthens the built-in electric field, increases the thickness of the depletion layer, and optimizes the generation and separation process of photogenerated carriers. On the other hand, the homojunction avoids the loss of photogenerated carriers caused by the mismatch of interfacial transport, and optimizes the transport and collection process of photogenerated carriers.

This work greatly improves the working temperature of the detector, and the medium-wave 4-5 micron detector is higher than that of 1011Jones at 200 K, and the performance reaches the background limit; At 280 K, it can still maintain a specific detection rate of 1010. Compared with conventional quantum dot detectors, the external quantum efficiency of gradient homojunction quantum dot detectors is increased by nearly 1 order of magnitude, reaching 77%. This work also verifies the practical application functions of the detector such as thermal imaging and gas detection. (Figure 3)

Figure 3. Application verification of spectrometer and infrared camera of quantum dot medium wave infrared detector at room temperature

In summary, this work developed a mixed-phase ligand exchange method, realized the preparation technology of quantum dot materials with medium and high carrier mobility and doping accurately and controllably, and prepared gradient homojunction photodetectors on this basis, which broke through the working temperature limitation of mid-infrared detectors and greatly promoted the development of uncooled, low-cost and high-performance infrared detectors.

Related paper information:

(Source: LightScience Applications WeChat public account)

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