Wide spectral photoresponse of template assisted out of plane grown ZnO/NiO composite nanowire photodetector

Self-powered, wide spectral UV response out-of-plane photodetector based on ZnO/porous silicon heterostructure

The photoresponse of the ZnO/porous silicon (p-Si) heterojunction is studied in an out-of-plane contact configuration. p-Si substrate is fabricated by anodic etching followed by the electrochemical deposition of ZnO NR film, forming ZnO/p-Si heterojunction. XRD study is done to understand the effect of the substrate on ZnO film growth in terms of strain and crystal size. UV-vis absorbance spectrum shows a broad absorption for wavelengths from 230 to 380 nm. The PL emission shows two narrow and prominent electron transition peaks at 263 and 383 nm and a peak of ∼550 nm corresponding to defects. The 263 nm wavelength responsivity of the photodetector from UV-vis and PL data suggests the presence of a defective SiOxas an intermediate layer between ZnO and p-Si. The photodetector is measured for its spectral selectivity and responsivity for both 266 and 370 nm. Under self-powered conditions, the device shows a low dark current of a few nA and enhancement of ∼100 nA and ∼1.37μA for both wavelengths. A responsivity of 527 mA W-1and 10.5μA W-1and detectivity of 2.5 × 1010and 2.9 × 107Jones at 1 V bias under 266 and 370 nm UV illumination are observed. The fast rise/decay time of 67/65 ms and 29/18 ms is observed for the self-powered condition of the device under both wavelengths respectively. The photoresponse of the modified ZnO/SiOx/p-Si heterojunction for both wavelengths is analyzed for the electron transfer mechanism using the heterojunction band bending model. The short circuit current and open circuit voltage of the photodetector is estimated to be 293 nA, 56.33 mV, and 13.63μA, 124.8 mV for 266 and 370 nm, respectively. It is concluded that the 266 nm responsivity comes from the defects in SiOxintermediate layer, and the photocurrent generated in the device is due to tunneling across the junction.

3D Printed Lattice Template by Material Extrusion Technique for Fabrication of Pixelated Photodetector

Rigid and flexible, pixelated ultraviolet photodetectors (PD) based on ZnO have been fabricated by material extrusion 3D printing technique. The photoresponse is studied in an out-of-plane configuration. An open lattice structure is printed using PLA over ITO/Glass substrate for rigid, and TPU over ITO/PET substrate for flexible PDs. ZnO slurry is filled selectively into the columnar matrix by the microdispensing technique. The optical detector printed on ITO/Glass substrate shows a sensitivity of 25 and responsivity of 1.55 nA/mW with a rise and decay time of 1.6 and 0.6 s, respectively. Similarly, the flexible PD printed using TPU lattice shows a sensitivity of 9.5 and responsivity of 0.38 nA/mW with a rise and decay time of 1.8 and 0.6 s, respectively. The charge transport mechanism is studied using band diagram analysis. 3D printed open lattice structure is found to be a potential template for sensor fabrication. This work demonstrates the capability of material extrusion 3D printing with an open lattice structure for the fabrication of high-resolution pixelated PDs.

Flexible assembly of the PEDOT: PSS/ exfoliated β-Ga2O3 microwire hybrid heterojunction for high-performance self-powered solar-blind photodetector

Motivated by the goals of fabricating highly reliable, high performance, and cost-efficient self-powered photodetector (PD) for numerous scientific research and civil fields, an organic-inorganic hybrid solar-blind ultraviolet (UV) PD based on PEDOT: PSS/exfoliated β-Ga₂O₃ microwire heterojunction was fabricated by a flexible and cost-effective assembly method. Benefiting from the heterojunction constructed by the highly crystalline β-Ga₂O₃ and the excellent hole transport layer PEDOT: PSS, the device presents a high responsivity of 39.8 mA/W at 250 nm and a sharp cut-off edge at 280 nm without any power supply. Additionally, the ultra-high normalized photo-to-dark current ratio (> 10⁴ mW^-1cm²) under reverse bias and the superior detectivity of 2.4×10¹² Jones at zero bias demonstrate the excellent detection capabilities. Furthermore, the hybrid PD exhibits a rapid rise time (several milliseconds) and high rejection ratio (R₂₅₀/R₃₆₅: 5.8 × 10³), which further highlights its good spectral selectivity for solar-blind UV. The prominent performance is mainly ascribed to the efficient separation of the photogenerated carriers by the large built-in electric field of the advanced heterojunction. This flexible assembly strategy for solar-blind UV PD combines the advantages of high efficiency, low cost and high performance, providing more potential for PD investigation and application in the future.

Base-width modulation effects on the optoelectronic characteristics of n-ITO/p-NiO/n-ZnO heterojunction bipolar phototransistors

ITO/NiO/ZnO npn heterojunction bipolar phototransistors (HBPTs) with various base widths are fabricated using a radio-frequency sputtering system. The effects of base-width modulation on the optoelectronic characteristics of the prepared HBPTs are studied. The dark current of HBPTs decreases with increasing base width because the injected electrons from the emitter are recombined in the wide base region. The photocurrent increases with decreasing base width, which is attributed to higher emitter-base injection efficiency. The responsivity increases with the collector-emitter voltage (V_CE) in the HBPTs with a 100 nm base width, whereas the responsivity sharply decreases atV_CE> 4 V for the HBPTs with a thinner base width (80 nm) due to the punch-through effect. In contrast, the responsivity approaches saturation at largeV_CEfor HBPTs with a thicker base width (120 nm). The responsivity and detectivity decrease with increasing incident light intensity, which is caused by an increase in the base recombination loss. The HBPTs with a base width of 100 nm exhibits the largest responsivity and detectivity; their detectivity is higher than that of HBPTs with base widths of 80 and 120 nm by approximately two and three orders, respectively.

Separate absorption and multiplication solar-blind photodiodes based on p-NiO/MgO/n-ZnO heterostructure

High-performance solar-blind separate absorption and multiplication avalanche photodiodes (SAM-APDs) were fabricated based on a p-NiO/MgO/n-ZnO dual heterojunction structure. The prepared SAM-APDs exhibited a separated absorption and multiplication structure that used NiO and ZnO as absorption layers, and ultrawide-bandgap MgO as a multiplication layer. When the reverse-bias voltage exceeded 6 V, carrier avalanche multiplication occurred, and the avalanche gain reached a high value of 2.7 × 10³, corresponding to a 1120% quantum efficiency, at a reverse-bias voltage of 10 V. These solar-blind SAM-APDs had an ultraviolet (UV) (310 nm)/visible (500 nm) rejection ratio as high as 563.6 at a 2 V reverse-bias voltage. These features render the SAM-APDs highly suitable for practical applications as UV solar-blind photodetectors.

Tuning optical and optoelectronic properties of gold nanoparticle and ZnO thin film hetero-structures

Thin film hetero-structures (TFHSs) involving metal oxide thin films and noble metal nanoparticles are very important for many optoelectronics based device applications. This work reports the growth, characterization, and tuning of photoluminescence and I–V properties of TFHSs involving zinc oxide (ZnO) and gold nanoparticles (GNPs). ZnO thin films and GNPs were respectively deposited by the Pulsed Laser Deposition (PLD) and DC sputtering with subsequent annealing. Three different TFHSs were prepared by varying the relative positions of ZnO and GNPs, namely Si-GNPs-ZnO, Si-ZnO-GNPs, and Si-ZnO-GNPs-ZnO. X-ray diffraction results confirmed the high crystallinity of the films, with single phase nature of the ZnO and GNPs. Scanning electron microscopy micrograph analysis confirmed that the morphology of structures containing both GNPs and ZnO is influenced by the bottom layer. Diffuse reflectance spectroscopy results also indicated that the position of GNPs relative to ZnO affects the plasmon resonance of GNPs as well as the overall optical properties of the TFHSs. Photoluminescence studies revealed that the presence of GNPs affects the defect concentration in the TFHSs. The I–V characteristics showed that the TFHSs where ZnO contains GNPs in embedded form are better suited for photodiode application. This study adds a new dimension to the research on optoelectronics devices.

Anisotropic interfacial properties of monolayer C2N field effect transistors

Monolayer C2N is promising for next-generation electronic and optoelectronic applications due to its appropriate band gap and high carrier efficiency. However, relative studies have been held back due to the lack of high-quality electrode contacts. Here, we comprehensively study the electronic and transport properties of monolayer C2N with a series of electrode materials (Al, Ti, Ni, Cu, Ag, Pt, V2C, Cr2C and graphene) by using the nonequilibrium Green's function (NEGF) method combined with density functional theory (DFT). The monolayer C2N forms Ohmic contacts with the Ti/Cu/Ag electrode material in both armchair and zigzag directions, whereas Ohmic contact is only formed in the zigzag direction of the C2N-Al field effect transistor. However, the C2N-Ni, -Pt, -V2C, -Mo2C, -graphene contact systems form n-type Schottky contacts in either the armchair or zigzag direction owing to the relatively strong Fermi level pinning (the pinning factor S = 0.32 in the armchair direction and S = 0.26 in the zigzag direction). By insertion of BN or graphene between the C2N and Pt electrode in the armchair direction of contact systems, the Fermi level pinning can be effectively weakened due to the suppression of metal-induced gap states. Conspicuously, an Ohmic contact is realized in the C2N field effect transistors with the BN-Pt electrode, suggesting a possible approach to fabricating high-performance devices. Our study is conducive to selecting appropriate electrode materials for C2N-based field effect transistors.

Ag Nanowire-Plasmonic-Assisted Charge Separation in Hybrid Heterojunctions of Ppy-PEDOT:PSS/GaN Nanorods for Enhanced UV Photodetection

The surface states, poor carrier life, and other native defects in GaN nanorods (NRs) limit their utilization in high-speed and large-gain ultraviolet (UV) photodetection applications. Making a hybrid structure is one of the finest strategies to overcome such impediments. In this work, a polypyrrole (Ppy)-poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/GaN NRs hybrid structure is introduced for self-powered UV photodetection applications. This hybrid structure yields high photodetection performance, while pristine GaN NRs showed negligible photodetection properties. The ability of the photodetector is further boosted by functionalizing the hybrid structure with Ag nanowires (NWs). The Ag NWs-functionalized hybrid structure exhibited a responsivity of 3.1 × 10³ (A/W), detectivity of 3.19 × 10¹⁴ Jones, and external quantum efficiency of 1.0⁶ × 10⁶ (%) under a UV illumination of λ = 382 nm. This high photoresponse is due to the huge photon absorption rising from the localized surface plasmonic effect of a Ag NWs network. Also, the Ag NWs significantly improved the rising and falling times, which were noted to be 0.20 and 0.21 s, respectively. The model band diagram was proposed with the assistance of X-ray photoelectron spectroscopy to explore the origin of the superior performance of the Ag NWs-decorated Ppy-PEDOT:PSS/GaN NRs photodetector. The proposed hybrid structure seems to be a promising candidate for the development of high-performance UV photodetectors.