Research background
Photodetectors are an important part of many optoelectronic applications. In addition to traditional photoelectric detection, polarized light detection has attracted great attention as a promising technology that can be used in various fields such as quantum information, cryptography, and three-dimensional (3D) displays. In particular, line polarization (lp) sensitive photodetectors can be widely used in emerging imaging systems. Image sensors based on lp-sensitive detectors can eliminate unwanted glare and reflections, visualizing artificial objects that would otherwise be difficult to identify. Therefore, the exploration of asymmetric low-dimensional semiconductors with intrinsic polarization sensitivity has aroused considerable research interest. In this case, a 2d van der Waals (VDW) layered semiconductor with intra-plane anisotropy is a promising candidate material for LP-sensitive photoelectric detection. In addition, vdw heterojunction can be easily achieved by simply stacking different 2D materials without being limited by atomic lattice matching, which provides a device platform for high-performance optoelectronic applications.
Presentation of results
In view of this, recently, the Korea Institute of Science and Technology (kist) do kyung Hwang, min-chul park, and Yonsei University seongil im (co-corresponding author) and other collaborations to demonstrate a 2d wse2/rese2 van Der Waals heterojunction of the self-powered line polarization sensitive near-infrared (nir) photodetector. Wse2/rese2 heterojunction photodiodes with semi-vertical geometry exhibit excellent performance: an ideal factor of 1.67, a wide spectral light response of 405-980 nm with significant photovoltaic effects, excellent linearity over a linear dynamic range of more than 100 db, and fast light switching behavior with a cutoff frequency of up to 100 khz. Strong polarization exciton transitions around the edges of the energy band in rese2 result in significant photocurrents associated with polarization of the 980 nm nir line. This linear polarization sensitivity remains stable even after exposure to air for more than five months. In addition, by utilizing the NIR (980 nm) selective linear polarization detection of this photodiode under photovoltaic operation, this paper demonstrates digital incoherent holographic 3D imaging. The article was published in the prestigious journal acs nano under the title "near-infrared self-powered linearly polarized photodetection and digital incoherent holography using wse2/rese2 van der waals heterostructure".
Illustrated reading
Figure 1. Schematic of the 2d wse2/rese2 heterojunction and device. (a) 3D schematic, om image and band diagram of a 2d wse2/rese2 heterojunction photodiode. (b) Afm height map of rese2 and wse2 nanosheets. (c) Microfocus absorption spectra of rese2 and wse2 for various LP light angles.
Band plot of the wse2/rese2 heterojunction, as shown in Figure 1a. The wse2/rese2 heterojunction is a staggered (type ii) heterojunction with the potential to improve photodiode performance. In addition, a large effective band gap of more than 1 ev (the energy difference between the electron affinity of rese2 and the ionization energy of wse2) will result in a considerable open-circuit voltage in photovoltaic behavior. A 3D schematic and om image of the wse2/rese2 heterojunction photodiode is also shown in Figure 1a. In a semi-vertical structure, the bottom electrode of pt overlaps with the wse2/rese2 heterojunction, reducing the quasi-neutral region as a parasitic resistance. As a result, significant photovoltaic effects and high photoelectric detection performance can be achieved. According to the afm height map, as shown in Figure 1b, the thicknesses of rese2 and wse2 are 50 and 6 nm, respectively. To obtain sufficient lp-associated light absorption in heterojunction photodiodes, a significantly thicker rese2 layer is used compared to wse2. Absorption spectroscopy is an important factor that can be directly related to the photodiode's photoelectric properties. Figure 1c shows the microfocused absorption spectra of rese2 and wse2 at different lp light angles (30°). The lp light in the direction parallel to the b-axis of rese2 is set to 0°. In the rese2 layer, strong lp-associated exciton transitions caused by in-plane asymmetry can be clearly observed. In contrast, the wse2 layer with intra-plane symmetry shows exciton transitions unrelated to lp. It is worth noting that the first exciton peaks of rese2 and wse2 are different. The lp-independent absorption behavior in wse2 can bleach respeal-related properties in rese2 in the spectral range below about 800 nm, indicating that it is feasible to use wse2/rese2 heterojunction for NIR (~980 nm) lp selective detection.
Figure 2. Wse2/rese2 photodiode characteristics. (a)Dark and photoinogenic i-v characteristics of wse2/rese2 heterojunction photodiodes under near-infrared (nir; 980, 850 and 808 nm) and visible light (638, 532, 450 and 405 nm). (b) The relationship between the filling factor and the power conversion efficiency and wavelength. (c) Spectral light response rate in photovoltaic (v=0 v) and photoconductivity mode (v=-1 v). (d) Photocurrent distribution in relation to applied voltage and 980 nm nir light power density. (e) The relationship between photocurrent and light power density. (f) Normalize the relationship between the light response and frequency.
Before the lp-sensitive characterization of wse2/rese2 photodiodes, their general photoelectric detection performance was studied. As shown in Figure 2a, the wse2/rese2 photodiode exhibits typical type II heterojunction p-n diode behavior, has relatively good electrical performance (ideal factor of 1.67, rectifier ratio greater than 104), and has a wide spectral light response over the spectral range of 405 nm visible light to 980 nm nir light. The relationship between fill factor and power conversion efficiency (pce) and light wavelength is shown in Figure 2b, achieving a maximum pce of 9.77% at 638 nm illumination, comparable to the most advanced 2D photovoltaic devices. Such excellent PV behavior is attributed to the synergistic effect of the higher effective band gap (greater open-circuit voltage) of the wse2/rese2 heterojunction and the semi-vertical device geometry (reduced parasitic resistance). The spectral light response rate is also extracted directly from the photogenerated current-voltage (I-v) curves of v=0 V (photovoltaic mode) and v=-1 V (photoconductivity mode), as shown in Figure 2c. For each mode, the same spectral behavior was observed under 638 nm illumination, with peak response rate values of 0.57 a/w (photoconductivity mode) and 0.48 a/w (photovoltaic mode), respectively. Several factors, such as the absorption rate of photosensitive materials, the penetration depth of light, and the efficiency of charge collection, affect the spectral response rate. There is a trade-off between light absorption and penetration depth of the wse2/rese2 heterojunction, so the highest value under red light may indicate the best point. The change of performance parameters with light power is also a very important feature for evaluating photodetectors. Two different wavelengths of light, NIR 980 nm and red 638 nm, were chosen because 980 nm nir light is related to the lp characteristics, while 638 nm red light provides the maximum photocurrent (and response rate). As the optical power increases, both the photocurrent and the open circuit voltage increase, as shown in Figures 2d and e. Using a photovoltaic bandwidth system, the cutoff frequency (3 db) of the device under PV mode operation is estimated to be 30 khz (980 nm) and 100 khz (638 nm), as shown in Figure 2f. These fast dynamic responses are superior to res2 or res2-based phototransistors.
Figure 3. Linear polarization (lp) sensitivity characteristics. (a) Dark and photo-induced i-v characteristics with various 980 nm nir lp light angles. (b) Normalized photocurrent plot in polar coordinates. (c&d) Photodiode LP-related photocurrent imaging.
Figure 3a shows the I-V characteristics corresponding to various lp light angles. The maximum photocurrent is achieved at an angle of 0° lp (parallel to the b-axis). As the lp angle increases, the photocurrent gradually decreases, and a minimum value is observed at an lp angle of 90° (perpendicular to the b-axis). Under short-circuit conditions (photovoltaic mode), the ratio of maximum/minimum photocurrent (polarization sensitivity) is approximately 2. The corresponding normalized photocurrent change is shown in the polar plot of Figure 3b. This lp-related photocurrent can be directly correlated with lp-associated exciton transitions in rese2. Notably, the wse2/rese2 photodiode is capable of selectively detecting 980 nm nir lp light, which involves strong polarized exciton transitions around the edge of the rese2 energy band.
To further investigate the effects of semi-vertical device geometry, photocurrent imaging tests were performed at different lp angles. As expected, the photocurrent image observed under 0° lp light is higher than the photocurrent image observed at 90° lp light. Interestingly, photocurrents are strongly generated on the only heterojunction region at the top of the bottom pt electrode, as shown in Figures 3c and d. These results demonstrate the superiority of the semi-vertical device geometry. The bottom pt electrode below the wse2/rese2 junction reduces the parasitic resistance quasi-neutral region and generates a strong electric field. Therefore, exciton separation and charge collection efficiency can be improved. In addition, more excitons can be produced by reabsorbing the reflected light from the pt electrode. Thus, higher and lp-correlated photocurrents can be clearly observed in the wse2/rese2/pt overlap region.
Figure 4. Digital incoherent holography. (a) Schematic of a finch-based holographic recording system consisting of a rotary linear polarizer, a geometric phase (gp) lens, and a NIR LP-sensitive photodetector. (b) Holographic recording results.
In addition to the basic photodetector unit, self-powered digital incoherent holographic 3D imaging is demonstrated using the nir (980 nm) selective LP detection of this photodiode and significant photovoltaic effects. A schematic of a holographic recording system is shown in Figure 4a, which is used as a Fresnel incoherent correlation hologram (finch) system by a combination of a rotary linear polarizer, a geometric phase (gp) lens, and an LP-sensitive photodetector. Figure 4b shows the output of two targets using a holographic recording system. Using the lp-sensitive photoelectric detection capabilities of the wse2/rese2 devices, four phase-shifted holograms were obtained.
Summary and outlook
In this paper, a self-powered lp-sensitive NIR photodetector based on wse2/rese2 VDW heterojunction was fabricated. This device with semi-vertical geometry exhibits excellent photoelectric probing performance with a wide spectral light response, significant photovoltaic effects, excellent linearity, and fast optical switching behavior. Strongly polarized exciton transitions around the edges of the energy band in rese2 result in significant lp-related properties in photodiodes. Even after exposure to air for more than five months, this lp sensitivity can still be maintained. Finally, the NIR digital incoherent holographic recording was demonstrated using the LP detection and significant photovoltaic effect of the WSE2/RESE2 photodiode.
Literature information
near-infrared self-powered linearly polarized photodetection and digital incoherent holography using wse2/rese2 van der waals heterostructure
(acs nano, 2021, doi:10.1021/acsnano.1c06234)
Literature links: https://pubs.acs.org/doi/10.1021/acsnano.1c06234