Photodetectors based on Weyl semimetal promise extreme performance in terms of highly sensitive, broadband and self-powered operation owing to its extraordinary material properties. Layered Type-II Weyl semimetal that break Lorentz invariance can be further integrated with other two-dimensional materials to form van der Waals heterostructures and realize multiple functionalities inheriting the advantages of other two-dimensional materials. Herein, we report the realization of a broadband self-powered photodetector based on Type-II Weyl semimetal Td -MoTe2 . The prototype metal-MoTe2 -metal photodetector exhibits a responsivity of 0.40 mA W-1 and specific directivity of 1.07 × 108 Jones with 43 μs response time at 532 nm. Broadband responses from 532 nm to 10.6 μm are experimentally tested with a potential detection range extendable to far-infrared and terahertz. Furthermore, we identify the response of the detector is polarization angle sensitive due to the anisotropic response of MoTe2 . The anisotropy is found to be wavelength dependent, and the degree of anisotropy increases as the excitation wavelength gets closer to the Weyl nodes. In addition, with power and temperature dependent photoresponse measurements, the photocurrent generation mechanisms are investigated. Our results suggest this emerging class of materials can be harnessed for broadband angle sensitive, self-powered photodetection with decent responsivities.

Anisotropic Broadband Photoresponse of Layered Type-II Weyl Semimetal MoTe2.

For nearly a century, oxygen has been widely accepted as the key element that triggers photo-response in polycrystalline PbSe photoconductive detectors. Our photoluminescence and responsivity studies on PbSe samples, however, suggest that oxygen only serves as an effective sensitization improver and it is iodine rather than oxygen that plays the key role in triggering the photo-response. These studies shed light on the sensitization process for detector applications and ways to passivate defects in IV–VI semiconductors. As a result, high peak detectivity of 2.8 × 1010 cm·Hz1/2·W−1 was achieved at room temperature.

Study of sensitization process on mid-infrared uncooled PbSe photoconductive detectors leads to high detectivity

Lead selenide layers were obtained by thermal evaporation in vacuum on thermally oxidized silicon. As-deposited layers present no response to infrared radiation and an activation procedure is mandatory. The sensitization process described in this work consists of a thermal treatment developed in two different stages: The first step in an iodine and oxygen rich atmosphere at 220 °C, and the second one at a higher temperature (450 °C) on air. After this treatment, the developed photodetectors show high sensitivity to medium wavelength infrared radiation at room temperature. We have analyzed the structural, compositional, electrical, and morphological changes of the films occurred during the activation process. After processing many films, we conclude that iodine plays a key role in the PbSe sensitization. This halogen behaves as a transport agent during the PbSe recrystallization process, and promotes a fast growth of PbSe microcrystals. Oxygen is trapped into the PbSe lattice during the recrystallization p...

Role of halogens in the mechanism of sensitization of uncooled PbSe infrared photodetectors

We introduce a charge separation model in this work to explain the mechanism of enhanced photoconductivity of polycrystalline lead salt photoconductors. Our results show that this model could clarify the heuristic fabrication processes of such lead salt detectors that were not well understood and often considered mysterious for nearly a century. The improved lifetime and performance of the device, e.g., responsivity, are attributed to the spatial separation of holes and electrons, hence less possibility of carrier recombination. This model shows that in addition to crystal quality the size of crystallites, the depth of outer conversion layer, and doping concentration could all affect detector performance. The simulation results agree well with experimental results and thus offer a very useful tool for further improvement of lead salt detectors. The model was developed with lead salt family of photoconductors in mind, but may well be applicable to a wider class of semiconducting films.

M.F. Ugarte

Papers

PbSe photodetector arrays for IR sensors

Optimized geometry in infrared arrays for remote sensing of forest fires

An approach for the compensation of the background variability in fire detection systems