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Thermal Imaging with Plasmon Resonance Enhanced HgTe Colloidal Quantum Dots Photovoltaic Devices Xin Tang, Matthew M Ackerman, and Philippe Guyot-Sionnest ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b03871 • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 11, 2018
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Thermal Imaging with Plasmon Resonance Enhanced HgTe Colloidal Quantum Dots Photovoltaic Devices Xin Tang, Matthew M. Ackerman, and Philippe Guyot-Sionnest*, James Franck Institute, The University of Chicago, 929 East 57th Street, Chicago, IL 60637
ABSTRACT: Thermal imaging in the mid-wave infrared plays an important role for numerous applications. The key functionality is imaging devices in the atmospheric window between 3 and 5 µm, where disturbance from fog, dust, and other atmospheric influence could be avoided. Here, we demonstrate sensitive thermal imaging with HgTe colloidal quantum dots photovoltaic detectors by integrating the HgTe CQDs with plasmonic structures. The responsivity at 5 µm is enhanced two to three-fold over a wide range of operating temperatures from 295 K to 85 K. A detectivity of 4 × 1011 Jones is achieved at cryogenic temperature. The noise equivalent temperature difference is 14 mK at an acquisition rate of 1kHz for a 200 microns pixel. Thermal images are captured with a single-pixel scanning imaging system.
KEYWORDS: HgTe CQDs; thermal imaging; plasmon resonance; photovoltaic devices; midwave infrared
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Nowadays, thermal imaging in the mid-wave infrared mainly relies on bulk semiconductors with narrow bandgap.1 Mercury cadmium telluride (MCT), indium antimonide (InSb), and superlattice structures are all mature materials for focal plane array imaging chips with good quantum efficiency; however, the high fabrication and processing costs hinder potential applications like autonomous vehicles,2 face recognition,3 and industrial inspection.4 Cheaper materials with fast response, high sensitivity, and compatible with large-scale production, are the key to facilitate civilian applications. In recent years, mercury telluride (HgTe) colloidal quantum dots (CQDs) have emerged as a promising nanomaterial for broadband infrared detection.5 The absorption edge of HgTe CQDs has been steadily extended from the nearinfrared (NIR),6 to the mid-wave infrared (MWIR) 7 and long-wave infrared (LWIR).8 By simply drop-casting HgTe CQDs on a commercial read-out integrated circuit, MWIR imaging at 15Hz frame rate with noise equivalent temperature difference (NETD) of 102 mK was achieved,9 showing a promising route to low-cost thermal cameras. Such 5 microns HgTe CQD cameras, operated in photoconducting mode, suffered from 1/f noise,10 which limited the detectivity to ~1010 Jones at 90K. Prior HgTe CQDs Schottky photovoltaic detectors had reduced 1/f noise and achieved background limited performance (BLIP) with detectivity up to 3.8 × 1010 Jones and a 2% external quantum efficiency (EQE) at cryogenic temperature.11 A vast improvement was demonstrated recently with a pn junction of HgTe CQDs,12 with an EQE of 17% with detectivity of 1.2 × 1011 Jones at 85 K. The corresponding internal quantum efficiency (IQE) was estimated to be as high as 90%. To further improve the performance, the absorption of the CQD films must be increased. However, depositing uniform thick CQD films that absorb all the light is a practical difficulty. Alternatively, the light absorption of thin HgTe CQDs layer can be enhanced using interference or plasmonic structures. A previous report integrated the HgTe CQDs detector with
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an interference structure leading to a detectivity of 3.3 × 1011 Jones and EQE of 32% at 85 K.12 However, the interference structure used a 5 nm thin metal contact for transparency requirements, which may not be optimal for device performance. In this work, we show that this contact significantly increased the series resistance and limited the output photocurrent at higher operating temperature. We demonstrate an interference electrode structure with improved electrical conductivity and better performance at room temperature. Other strategies to enhance light absorption in CQDs include nano-structuring quantum dot film surface,13 gratings,14 and antennas.15 Previous studies integrated plasmonic nanostructures with CQD films to enhance the near-field electromagnetic field and to increase the light absorption by scattering.16–18 Over 240% enhancement of the photocurrent was achieved with plasmonic nanorods in a HgTe CQDs photovoltaic NIR detector.17 Plasmon resonance enhancement was also extended to the MWIR and LWIR with sub-wavelength metal-disk arrays under films of HgSe CQDs photoconductors.18 We therefore explore plasmonic enhancement with HgTe CQDs MWIR photovoltaic detectors with plasmonic disk arrays. Experiments were guided by COMSOL simulations, and we obtained significant improved performances over a wide range of operating temperature from 295 K to 85 K. RESULTS AND DISCUSSION Improved performance by lowering the electrical resistance. Ref.12 introduced a simple interference structure to significantly enhance the photocurrent in a thin CQD film. As shown in Figure 1, such an interference structure requires a semitransparent top contact, which is then coated with a MWIR transparent film, defining the optical resonance, and a final reflector film.
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In these previous devices, the semi-transparent electrode was 5 nm of evaporated Au, the dielectric was evaporated SiO2, and the top reflector was a 30 nm gold layer. The temperaturedependent photocurrent of a detector with and without an interference structure is shown in Figure 1a. For the reference detector with a 30 nm top contact, no interference structure was added. As previously reported, at low temperatures, the photocurrent of the detector with the interference structure is significantly better than the reference detector. However, at higher operating temperatures, the photocurrent drops dramatically, such that at 295 K the photocurrent of the detector with an interference structure is one order of magnitude smaller than the reference detector. We show here that the reason for the loss of photocurrent at room temperature is the large series resistance, Rs, introduced by the thin contact layer (5 nm Au) in the interference structure. For a photovoltaic device, the output photocurrent Isc can be expressed by
q (V + I sc Rs ) V + I sc Rs I sc = I L − I 0 ⋅ exp − nkT Rsh
(1)
where IL is the light generated photocurrent, I0 is the dark saturation current, V is the bias voltage, n is the ideal factor, k is the Boltzmann constant, Rsh is the shunt resistance, and T is the operation temperature. Based on typical experimental results of detectors with an area of 100 × 200 µm2, IL, I0, and n are estimated to be 50 nA, 5 nA, and 1.77, respectively (Figure S1). With zero bias and when qIscRs
