Shining light above 2 µm using nanocrystal based LED
There is currently a technological gap for light source in the 1.5 µm (beyond telecom) to 4 µm (when intraband sources become efficient). To address this issue a consortium of researcher design light emitting diode with emission in the 2 to 2.3 µm range using HgTe nanocrystals as emitting materials.
Infrared light emitting diodes (LEDs) offer a variety of applications including active imaging, telecommunication, organic molecule sensing and airfield lighting. However, there lies a technology gap for powerful sources between 1.5 µm and 4 µm. In the 1.3-1.5 µm range, Indium-based III-V epitaxially-grown quantum well structures have achieved high maturity for telecommunications, whereas in the mid-infrared above 4 µm, quantum cascade lasers have been the most successful solid-state sources. The range of 1.5-4 µm, especially the extended short-wave infrared wavelengths, is nevertheless attractive owing to strong absorption bands of CO2, NH3 and alkanes within 2-2.5 µm. In quest of powerful low-cost LEDs for filling this gap, infrared nanocrystals (NCs) presenting high photoluminescence (PL) efficiency, flexible spectral tunability and suppressed substrate constraint, appear as an ideal candidate.
The first step toward electroluminescence (EL) beyond 2 µm is to select the appropriate NC emitters. Several infrared NCs, such as PbS1 and Ag2S,2 have produced bright EL yet limited below 1.7 µm. HgTe NCs offering a spectral tunability from visible to THz3 are thus promising to address longer wavelengths. The next challenge is their device integration
In traditional III-V LEDs, a quantum well is located at the interface of a p-n junction and works as a recombination center. Its colloidal analogy was proposed recently for a solar-sell-derived PbS-based LED1, where the narrow-bandgap PbS emitters are sandwiched by a n-type electron transport layer (ZnO NCs) and a p-type hole transport layer (short thiol capped PbS NCs). We maintain this configuration and build our LEDs following a multilayer stack of ITO/ZnO/HgTe:ZnO/PbS/Au, see the schematic in Figure 1a. Specially, a HgTe:ZnO mixture is used as emitting layer and the device performance was optimized by tuning the volume ratio of the two materials.
Figure 1a. Schematic of the ITO/ZnO/HgTe:ZnO/PbS/Au stack for LED. b. Electroluminescence spectrum of the LED device compared with the photoluminescence of HgTe NCs. Inset: Infrared image of a working pixel.
Using the proposed LED structure, we demonstrate the first LED operating from 2 to 2.3 µm peak4. Figure 1b shows an infrared image of a working pixel and the corresponding EL and PL spectra. The ZnO content in the HgTe:ZnO layer is found strongly correlated to the device performance. The device, with a ZnO content of 50%, provides the highest EQE of 0.3%, comparable with state-of-the-art epitaxially-grown LEDs in the similar wavelength. A low turn-on voltage of 0.6 eV (≈EG/e) and a bright radiance up to 3 W.Sr-1m-2 are also obtained.
The operation mechanisms of the device, particularly the role of ZnO within the HgTe:ZnO layer, are elucidated with three strategies: (1) electrical transport, (2) transient reflectivity spectroscopy, and (3) time-resolved x-ray photoemission. The results indicate that, in the HgTe:ZnO layer, the HgTe NCs govern the transport while the ZnO NCs content tunes the majority carrier hence the charge balance in the device, which is essential for high performances.
Paper can be accessed at https://www.nature.com/articles/s41566-021-00902-y and read at https://rdcu.be/cC8AT
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