Quantum dots (QDs) are semiconductor nanocrystals with sizes on the order of several nanometers, whose bandgaps can be tuned by controlling the particle size. Owing to this bandgap tunability, QDs can absorb near-infrared (NIR) and short-wave infrared (SWIR) light, spectral regions that are difficult to access with conventional silicon-based devices. However, colloidal QDbased infrared photodetectors still suffer from intrinsically high dark current, trap-induced noise, and limited response speed. As a result, they exhibit fundamental performance gaps in terms of detectivity and speed–bandwidth product compared to epitaxial infrared detectors, highlighting the need for structural and architectural design strategies to overcome these limitations. In this review, we discuss recent advances in enhancing the spectral selectivity and sensitivity of infrared photodetectors through three-dimensional optical architectures, including metasurfaces and metamaterials. We focus in particular on design strategies and the underlying mechanisms responsible for performance enhancement, and we outline how structural approaches can be leveraged to effectively control the sensitivity and wavelength selectivity of QD-based infrared detectors.

Silicon carbide (SiC), with its wide bandgap and strong resistance to radiation and thermal conditions, is a promising material for ultraviolet (UV) photodetector applications under harsh environments. In this study, porous SiC thin films with thicknesses of 20, 50, and 80 nm were fabricated on 4H-SiC substrates using aerosol deposition (AD), which enables roomtemperature film formation. The device with a 50 nm-thick film exhibited the highest photoresponse under UV-C illumination (260 nm), achieving a maximum photo-to-dark current ratio (PDCR) of 205.2, a responsivity of 0.058 A/W, an external quantum efficiency (EQE) of 27.71%, and a specific detectivity (D*) of 7.9×10¹¹ Jones. These results are attributed to an optimized balance between photon absorption and carrier transport in the porous structure. The findings confirm the potential of ADfabricated porous SiC films for highly sensitive and scalable UV photodetector applications.

ZnO-based transparent conductive films have been widely studied to achieve high performance optoelectronic devices such as next generation flexible and transparent display systems. In order to achieve a transparent flexible ZnO-based device, a low temperature growth technique using a flexible polymer substrate is required. In this work, high quality flexible ZnO films were grown on colorless polyimide substrate using atomic layer deposition (ALD). Transparent ZnO films grown from 80 to 200℃ were fabricated with a metal-semiconductor-metal structure photodetectors (PDs). As the growth temperature of ZnO film increases, the photocurrent of UV PDs increases, while the sensitivity of that decreases. In addition, it is found that the response times of the PDs become shorter as the growth temperature increases. Based on these results, we suggest that high-quality ZnO film can be grown below 200℃ in an atomic layer deposition system, and can be applied to transparent and flexible UV PDs with very fast response time and high photocurrent.

Grain boundaries play a major role in determining device performance, particularly of polysilicon-based photodetectors. Through the post-annealing of as-deposited polysilicon and then, the analysis of electric behavior for a metal-polysilicon-metal (MSM) photodetector, we were able to identify the influence of grain boundaries. A modified model of polysilicon grain boundaries in the MSM structure is presented, which uses a crystalline-interfacial layer-SiO_x layer- interfacial layer-crystalline system that is similar to the Si-SiO₂ system in MOS device. Hydrogen passivation was achieved through a hydrogen ion implantation process and was used to passivate the defects at both interfacial layers. The thin SiO_x layer at the grain boundary can enhance the photosensitivity of an MSM photodetector by decreasing the dark current and increasing the light absorption.

In this study, double-layered TCO (transparent conductive oxide) films were produced by depositing two distinct TCO materials: SnO₂ works as an n-type layer and ITO (indium-doped tin oxide) serves as a transparent conductor. Both transparent conductive oxide-films were sequentially deposited by sputtering. The electrical and optical properties of single-layered TCO films (SnO₂) and double-layered TCO (ITO/SnO₂) films were investigated. A TCO-embedding photodetector was realized through the formation of an ITO/SnO₂/p-Si/Al layered structure. The remarkably high rectifying ratio of 400.64 was achieved with the double-layered TCO device, compared to 1.72 with the single-layered TCO device. This result was attributed to the enhanced electrical properties of the double-layered TCO device. With respect to the photoresponses, the photocurrent of the double-layered TCO photodetector was significantly improved: 1,500% of that of the single-layered TCO device. This study suggests that, due to the electrical and optical benefits, double-layered TCO films are effective for enhancing the photoresponses of TCO photodetectors. This provides a useful approach for the design of photoelectric devices, including solar cells and photosensors.

An all-transparent ultraviolet (UV) photodetector was fabricated by structuring p-NiO/n-SnO₂/ITO on a glass substrate. SnO₂ is an important semiconductor material because of its large bandgap, high electron mobility, high transmittance (as high as 80% in the visible range), and high stability under UV light. For these reasons, SnO₂ is suitable for a range of applications that involve UV light. In order to form a highly transparent p-n junction for UV detection, SnO₂ was deposited onto a device containing NiO as a high-transparent metal conductive oxide for UV detection. We demonstrated that all-transparent UV photodetectors based on SnO₂ could provide a definitive photocurrent density of 4 nA cm^-2 at 0 V under UV light (365 nm) and a low saturation current density of 2.02 nA × cm^-2. The device under UV light displayed fast photoresponse with times of 31.69 ms (rise-time) and 35.12 ms (fall-time) and a remarkable photoresponse ratio of 69.37. We analyzed the optical and electrical properties of the NiO/SnO₂ device. We demonstrated that the excellent properties of SnO₂ are valuable in transparent photoelectric device applications, which can suggest various routes for improving the performance of such devices.

Ultraviolet (UV) photodetectors are used in various industries and fields of research, including optical communication, flame sensing, missile plume detection, astronomical studies, biological sensors, and environmental research. However, general UV detectors that employ Schottky junction diodes and p^-n junctions have high fabrication cost and low quantum efficiency. In this study, we investigated the characteristics of materials used to manufacture UV photodetectors in a low-cost solution process that requires easy fabrication of flexible substrates. We fabricated p-type NiO and n-type ZnO substrates with wide band gap by the sol^-gel method and compared the characteristics of substrates prepared under different spin-coating and heat-treatment conditions.

Molybdenum oxide (MoO₃) offers pivotal advantages for high optical transparency and low light reflection. Considering device fabrication, n-type MoO₃ semiconductor can spontaneously establish a junction with p-type Si. Since the energy bandgap of Si is 1.12 eV, a maximum photon wavelength of around 1,100 nm is required to initiate effective photoelectric reaction. However, the utilization of infrared photons is very limited for Si photonics. Hence, to enhance the Si photoelectric devices, we applied the wide energy bandgap MoO₃ (3.7 eV) top-layer onto Si. Using a large-scale production method, a wafer-scale MoO₃ device was fabricated with a highly crystalline structure. The MoO₃/p-Si heterojunction device provides distinct photoresponses for long wavelength photons at 900 nm and 1,100 nm with extremely fast response times: rise time of 65.69 ms and fall time of 71.82 ms. We demonstrate the high-performing MoO₃/p-Si infrared photodetector and provide a design scheme for the extension of Si for the utilization of long-wavelength light.

An all-transparent photodetector was fabricated by structuring Cu₂O/ZnO/AZO/ITO on a glass substrate. The visible-range transmittance was as high as 80%, which ensures clear vision forhuman eyes. High-transparency metal conductive oxides (p-type Cu₂O and n-type ZnO) were appliedto form the transparent p/n junction. The functional AZO layer was adopted to improve the transparent photodetector performance between the ZnO and ITO, improving the photoresponses because of its electrical conductivity. To clarify the AZO functionality, a comparator device was prepared without the AZO layer in the formation of Cu₂O/ZnO/ITO/Glass. The Cu₂O/ZnO/AZO/ITO device provided a rectifying ratio of 113.46, significantly better than the 9.44 of the Cu₂O/ZnO/ITO device. In addition, the Cu₂O/ZnO/AZO/ITO device`s photoresponses at short wavelengths were better than those of the comparator. The functioning AZO layer provides ahigh-performing transparent Cu oxide photodetector and may suggest a route for the design of efficient photoelectric devices.

A high-performing all-transparent photodetector was created by configuring a MoO_x/NiO/ZnO/ITO structure on a glass substrate. The ITO bottom layer was applied as a back contact. To achieve the transparent p/n junction, p-type NiO was coated on the n-type ZnO layer. Reactive sputtering was used to spontaneously form the ZnO or NiO layer. In order to improve the transparent photodetector performance, the functional MoO_x window layer was used. Optically, the MoO_x window provided a refractive index layer (n=1.39) lower than that of NiO (n=2), increasing the absorption of the incident light wavelengths (λ_s). Moreover, the MoO_x window can provide a lower sheet resistance to improve the carrier collection for the photoresponses. The MoO_x/NiO/ZnO/ITO device showed significantly better photoresponses of 877.05 (at λ=460 nm), 87.30 (λ=520 nm), and 30.38 (λ=620 nm), compared to 197.28 (λ=460 nm), 51.74 (λ=520 nm) and 25.30 (λ=620 nm) of the NiO/ZnO/ITO device. We demonstrated the high-performing transparent photodetector by using the multifunctional MoO_x window layer.

A polysilicon-based metal-semiconductor-metal (MSM) photodetector was fabricated by means of our new methods. Its photoresponse characteristics were analyzed to see if it could be applied to a sensor system. The processes on which this study focused were an alloy-annealing process to form metal-polysilicon contacts, a post-annealing process for better light absorption of as-deposited polysilicon, and a passivation process for lowering defect density in polysilicon. When the alloy annealing was achieved at about 400℃, metal-polysilicon Schottky contacts sustained a stable potential barrier, decreasing the dark current. For better surface morphology of polysilicon, rapid thermal annealing (RTA) or furnace annealing at around 900℃ was suitable as a post-annealing process, because it supplied polysilicon layers with a smoother surface and a proper grain size for photon absorption. For the passivation of defects in polysilicon, hydrogen-ion implantation was chosen, because it is easy to implant hydrogen into the polysilicon. MSM photodetectors based on the suggested processes showed a higher sensitivity for photocurrent detection and a stable Schottky contact barrier to lower the dark current and are therefore applicable to sensor systems.

Transparent n-type metal-oxide semiconductor of MoO_x was applied on a p-type Si substrate for high-performing heterojunction photodetector. The formation of MoO_x on Si spontaneously established a rectifying current flow with a high rectification ratio of 1,252.3%. Under light illumination condition, n-type MoO_x/p-type Si heterojunction device provided significantly fast responses (rise time : 61.28 ms, fall time : 66.26 ms). This transparent metal-oxide layer (MoO_x) would provide a functional route for various photoelectric devices, including photodetectors and solar cells.

NiO serves as a window layer for Si photoelectric devices. Due to the wide energy bandgap of NiO, high optical transparency (over 80%) was achieved and applied for Si photoelectric devices. Due to the high the high mobility, the heterojunction device (Al/n-Si/SiO2/p-NiO/ITO) provide ultimately fast photoresponses of rising time of 38.33 μs and falling time of 39.25 μs, respectively. This functional NiO layer would provide benefits for high-performing photoelectric devices, including photodetectors and solar cells.