Cd3As2 is a representative three-dimensional (3D) Dirac semi-metal material, which is expected to develop high-performance wide spectrum photodetectors due to the unique optical and electrical properties. For instance, the band structure exhibits a unique three-dimensional Dirac structure which express the characteristics of zero band-gap and full spectral absorption, high photocurrent response and the ultra-high charge mobility, these characteristics make it a more potential candidate for photodetection. However, arising from its excellent conductivity, the extremely high dark currents in 3D semi-metal-based photodetector is an imperfection that limit the development of individual Cd3As2 film photoconductive detector. Here in, we developed an Cd3As2/Bi2O2Se heterojunction by thermal depositing Bi2O2Se film on the as-prepared Cd3As2 film. The Cd3As2/Bi2O2Se heterojunction film photodetector demonstrates a relative broad spectrum photodetection from visible (405nm) to near infrared (1310nm) at room temperature with high responsivity (Ri) and fast response time (τ). These results show that the dark current is reduced about a half compared to the individual Cd3As2 film at the same bias voltage. Subsequently, at the bias voltage of three volts, we tested the detector performance under 808nm laser irradiation, in which the maximum photocurrent responsivity (Ri) can be reached to 17.8 mA/W. We analyze that the greatly enhanced-performance improvement of the device maybe originated from its vertical structure advantages, due to the formed internal potential barrier would be accelerates the speed of collecting carrier. This work provides a suitable method and reference for fabricating broad-spectrum and high-speed Cd3As2 based inorganic photodetectors.
Three-dimensional (3D) Dirac semimetal Cd3As2 has an important application prospect in future for broad-spectrum photodetector due to its unique topological energy-band layout and excellent optical absorption capacity. However, extremely high dark current of individual Cd3As2 is the key bottleneck limiting its photoelectric detection performance. Herein, the strategies of Zn element doping and construction hetero-structure with Sb2Se3 are co-adopted for maximize the performance of Cd3As2 based device. The constructed 3D Dirac semimetal (ZnxCd1-x)3As2/Sb2Se3 film heterojunction photodetector prototype has efficiently achieve broad-spectra photoelectric detection from visible (450 nm) to nearinfrared region (1550 nm) wavebands at room temperature with superior performance, including high responsivity (Ri), detectivity (D*) as well as outstanding fast response time (τ). The excellent performance of the device may be attributed to high-quality of hetero-structure interface, strong light absorption capacity and suppressed dark current in (ZnxCd1-x)3As2/Sb2Se3 heterojunction system. This work has laid a solid foundation for the construction of a high-performance new topological semi-metal photodetector.
To extended the effective focusing length (f) and narrowing lateral full width at half maxima (FWHM) of photonic nanojets (PNJs) formed by microlenses with wavelength-scale size, in this paper, we study focusing characteristics of dielectric hemisphere. Refractive index (RI) of hemisphere, radius of covering Au disks on flat-surface of hemisphere and immersed materials effect on focusing characteristics are studied. Simulation study show that hemisphere with radius 4.5 μm, shows narrower focusing beam waist, and shorter f when RI of hemisphere gets bigger. When RI reaches 2.5, lateral FWHM of PNJ and f are 177nm, 324nm, respectively, under illumination of a plane wave with a 365nm wavelength. Comparing with normal hemisphere, hemisphere (RI 1.52, radius 4.5μm and illumination wavelength 365nm) with Au disk covering its flat-surface center, shows obvious smaller FWHM of PNJ and shorter f (still longer than 2 μm). it is because that the engineered hemisphere is like a lens with high numerical aperture, and it only allows incident beam far away from the axis participate in the formation of PNJ. With increasing of Au disk radius, the equivalent numerical aperture gets bigger and thus FWHM of PNJ gets smaller. FWHM of PNJ small than half of illumination wavelength when radius of Au disk gets bigger than 1800nm. The length of PNJ in above study is short, less than 2μm. When the engineered hemisphere (RI 1.52, radius of hemisphere and Au disk 4.5μm and 1800nm, respectively) is immersed in water, MgF2, et al, the f and L get longer than 11μm, 5μm, respectively. Although lateral FWHM of PNJ at this time is bigger than 400nm, it can be narrowed by replacing it with bigger Au disk, optimizing immersed material, et al. Due to low manufacture cost of these hemisphere lenses and lenses array, we believe they have potential in near-field and far-field application with resolution small than 0.5λ.
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