A Photodetector Based on p-Si/n-ZnO Nanotube Heterojunctions with High Ultraviolet Responsivity

Synthesis of InAl-alloyed Ga2O3 nanowires for self-powered ultraviolet detectors by a CVD method

Ga2O3 is a kind of wide-band gap semiconductor, which has great potential in deep ultraviolet detection because of its high efficiency and fast response. Doping can improve the photoelectric properties of Ga2O3 materials. In this paper, In and Al elements alloyed Ga2O3 nanowires (InAl-Ga2O3 NWs) were successfully grown on p-GaN using a cost-effective chemical vapor deposition method and a vertical structure. The GaN/InAl-Ga2O3 NWs p-n self-powered wide-gap UV photodetector (PD) was constructed based on sputtered gold film as the bottom and top electrodes, and spin coated with polymethyl methacrylate as the insulating layer in the vertical direction. The GaN/InAl-Ga2O3 UV PD exhibits excellent performances, including an extremely low dark current of 0.015 nA, a maximum photocurrent of about 16 nA at zero-bias voltage under 265 nm illumination, and a light-to-dark current ratio greater than 103. The responsivity is 0.94 mA W-1, the specific detectivity is 9.63 × 109 jones, and the good fast response/attenuation time is 31.2/69.6 ms. The self-powered characteristics are derived from the internal electric field formed between p-type GaN and n-type InAl-Ga2O3 NWs, which is conducive to the rapid separation and transfer of photogenerated carriers. This work provides an innovative mechanism of high-performance metal oxide nanowires for the application of p-n junction photodetectors, which can operate without any external bias.

Wide Bandgap Semiconductors for Ultraviolet Photodetectors: Approaches, Applications, and Prospects

Ultraviolet (UV) light, invisible to the human eye, possesses both benefits and risks. To harness its potential, UV photodetectors (PDs) have been engineered. These devices can convert UV photons into detectable signals, such as electrical impulses or visible light, enabling their application in diverse fields like environmental monitoring, healthcare, and aerospace. Wide bandgap semiconductors, with their high-efficiency UV light absorption and stable opto-electronic properties, stand out as ideal materials for UV PDs. This review comprehensively summarizes recent advancements in both traditional and emerging wide bandgap-based UV PDs, highlighting their roles in UV imaging, communication, and alarming. Moreover, it examines methods employed to enhance UV PD performance, delving into the advantages, challenges, and future research prospects in this area. By doing so, this review aims to spark innovation and guide the future development and application of UV PDs.

Investigations on Optical Absorption and the Pyro-phototronic Effect with Selectively Patterned Black Silicon for Advanced Photodetection

A novel property existing in the stain-etching technique that eliminates the need for expensive etchant masks in the texturization process of silicon wafers was identified. Through the combination of grayscale lithography and stain-etching methodologies, selective patterning of silicon with AR-P 3510 T, a positive-photoresist mask, was carried out. The etch area ratio was varied in nine different patterns of various feature sizes ranging from 400 to 1500 μm. The optical characteristics of the patterned substrates were determined from diffuse reflectance spectroscopy analysis, and the results were supported with finite-difference time-domain simulations. Complimenting the improvement in optical properties, the electrical losses in microwell-patterned photodetector devices have been reduced with an electro-optic optimum value of the surface enhancement factor, γ. The photodetecting efficiency of a selectively patterned microwell photodetector device exceeded the planar and black silicon photodetector devices with a considerable improvement in the pyro-phototronic effect. This work suggests an alternative for black silicon optoelectronic devices providing a new route to fabricate selectively patterned substrates with an achieved detectivity 16- and 20-fold higher than black and planar silicon photodetector devices, respectively.

Enhanced Spectral Response of ZnO-Nanorod-Array-Based Ultraviolet Photodetectors by Alloying Non-Isovalent Cu-O with CuAlO2 P-Type Layer

CuAlO₂ was synthesized by a hydrothermal method, in which the Cu-O dimers were incorporated by simply altering the ratio of the reactants and the temperature. The incorporation process increases the grain size in CuAlO₂, and modulates the work function and binding energies for CuAlO₂ due to the partial substitution of Cu⁺ 3d¹⁰ with Cu²⁺ 3d⁹ orbitals in the valence band maximum by alloying non-isovalent Cu-O with a CuAlO₂ host. Based on the ZnO nanorod arrays (NRs) ultraviolet photodetector, CuAlO₂/Cu-O fabricated by the low-cost drop-coating method was used as the p-type hole transport layer. The incorporation of the Cu-O clusters into CuAlO₂ lattice to enhance the conductivity of CuAlO₂ is an effective way for improving ZnO NRs/CuAlO₂ device performance. The photodetectors exhibit significant diode behavior, with a rectification ratio approaching 30 at ±1 V, and a dark saturation current density 0.81 mA cm^-2. The responsivity of the ZnO-NRs-based UV photodetector increases from 13.2 to 91.3 mA/W at 0 V bias, with an increase in the detectivity from 2.35 × 10¹⁰ to 1.71 × 10¹¹ Jones. Furthermore, the ZnO NRs/[CuAlO₂/Cu-O] photodetector exhibits a maximum responsivity of 5002 mA/W at 1.5 V bias under 375 nm UV illumination.

Gold-Promoted Electrodeposition of Metal Sulfides on Silicon Nanowire Photocathodes To Enhance Solar-Driven Hydrogen Evolution

Constructing composite structures is the key to breaking the dilemma of slow reaction kinetics and easy oxidation on the surface of lightly doped p-type silicon nanowire (SiNW) array photocathodes. Electrodeposition is a convenient and fast technique to prepare composite photocathodes. However, the low conductivity of SiNWs limits the application of the electrodeposition technique in constructing composite structures. Herein, SiNWs were loaded with Au nanoparticles by chemical deposition to decrease the interfacial charge transfer resistance and increase active sites for the electrodeposition. Subsequently, co-catalysts CoS, MoS₂, and Ni₃S₂ with excellent hydrogen evolution activity were successfully composited by electrodeposition on the surface of SiNWs/Au. The obtained core-shell structures exhibited excellent photoelectrochemical hydrogen evolution activity, which was contributed by the plasma property of Au and the abundant hydrogen evolution active sites of the co-catalysts. This work provided a simple and efficient solution for the preparation of lightly doped SiNW-based composite structures by electrodeposition.

Enhanced Photoresponsivity UV-C Photodetectors Using a p-n Junction Based on Ultra-Wide-Band Gap Sn-Doped β-Ga2O3 Microflake/MnO Quantum Dots

Solar-blind self-powered UV-C photodetectors suffer from low performance, while heterostructure-based devices require complex fabrication and lack p-type wide band gap semiconductors (WBGSs) operating in the UV-C region (<290 nm). In this work, we mitigate the aforementioned issues by demonstrating a facile fabrication process for a high-responsivity solar-blind self-powered UV-C photodetector based on a p-n WBGS heterojunction structure, operating under ambient conditions. Here, heterojunction structures based on p-type and n-type ultra-wide band gap WBGSs (i.e. both are characterized by energy gap ≥4.5 eV) are demonstrated for the first time; mainly p-type solution-processed manganese oxide quantum dots (MnO QDs) and n-type Sn-doped β-Ga₂O₃ microflakes. Highly crystalline p-type MnO QDs are synthesized using cost-effective and facile pulsed femtosecond laser ablation in ethanol (FLAL), while the n-type Ga₂O₃ microflakes are prepared by exfoliation. The solution-processed QDs are uniformly dropcasted on the exfoliated Sn-doped β-Ga₂O₃ microflakes to fabricate a p-n heterojunction photodetector, resulting in excellent solar-blind UV-C photoresponse characteristics (with a cutoff at ∼265 nm) being demonstrated. Further analyses using XPS demonstrate the good band alignment between p-type MnO QDs and n-type β-Ga₂O₃ microflakes with a type-II heterojunction. Superior photoresponsivity (922 A/W) is obtained under bias, while the self-powered responsivity is ∼86.9 mA/W. The fabrication strategy adopted in this study will provide a cost-effective means for the development of flexible and highly efficient UV-C devices suitable for energy-saving large-scale fixable applications.

Enhanced Deep Ultraviolet Photoresponse in Ga doped ZnMgO Thin Film

High Mg content (60%) ZnMgO samples with and without Ga dope were grown by an RF magnetron sputtering system. The effect of Ga dope on the ZnMgO sample and the respective ultraviolet photodetectors (UVPD) device’s performance were carefully studied by various experimental methods. The investigations of the structure and optical properties of the ZnMgO sample established that the Ga doped sample has a better crystal quality and larger band gap (5.54 eV). The current-voltage characteristics indicate that both the photocurrent and dark current were enhanced after Ga dope. Under 12 V bias, the undoped UVPD show two spectral response peaks at 244 nm and 271 nm with a responsivity of 1.9 A/W and 0.38 A/W, respectively. While the Ga doped UVPD showed only one response peak at 241 nm and the deep UV responsibility up to 8.9 A/W;, as the bias increased from 12 V to 60 V, the responsiveness raised to 52 A/W, with a signal to noise ratio (241 nm/700 nm) as high as 10⁵. Combining the results of XRD, PL spectrum and XPS, the enhanced ultraviolet photoresponse of the Ga dope device contributed to improving the crystal quality and “dopant-defect pairing effect” caused by Ga doping, which led to a considerable reduction in the number of ionized impurities in the scatting centers, and enhanced the carrier’s mobility. Our work demonstrates that even a high Mg content ZnMgO can exhibit enhanced UV performance after a Ga dope due to the dopant-defect pairing effect, which confirmed the advantage of the use of ZnMgO in the deep-UV region.

Pt/ZnGa2O4/p-Si Back-to-Back Heterojunction for Deep UV Sensitive Photovoltaic Photodetection with Ultralow Dark Current and High Spectral Selectivity

In this work, a strategy of constructing a back-to-back heterojunction is proposed to fabricate Si-based photovoltaic photodetectors with high deep ultraviolet (DUV) spectral selectivity. By combining Pt with a thickness of 4 nm with a ZnGa₂O₄/Si heterojunction, a back-to-back heterojunction is successfully constructed. Based on that, a Pt/ZnGa₂O₄/p-Si DUV photovoltaic detector with a low dark current density (∼9.6 × 10^-5 μA/cm²), a large photo-to-dark current ratio (PDCR, >10⁵), and a fast response speed (decay time <50 ms) is fabricated. At 0 V bias, this device displays a photoresponsivity of about 1.36 mA/W and a high deep ultraviolet-visible (DUV-vis) rejection ratio (R_258 nm/R_420 nm) of ∼1.1 × 10⁵, which are 1-2 orders of magnitude higher than those of most photovoltaic DUV detectors reported currently. Even at a working temperature of 470 K, the detectivity of this device can still reach ∼1.23 × 10¹⁰ Jones. In addition, compared with Au/ZnGa₂O₄/Si devices, the dark current and PDCR of this Pt/ZnGa₂O₄/Si device decrease by 2 orders of magnitude and increase by 1 order of magnitude, respectively. The enhanced performance of this ZnGa₂O₄/Si device can be attributed to the higher Schottky barrier established between Pt with a higher work function and ZnGa₂O₄. This strategy of adopting a back-to-back heterojunction device structure to hinder the visible light photoresponse of Si-based photodetectors and thus to reduce the dark current of a device can provide a reference for preparing photovoltaic DUV detectors with excellent performance.

Enhanced luminescence/photodetecting bifunctional devices based on ZnO:Ga microwire/p-Si heterojunction by incorporating Ag nanowires

With the disadvantages of indirect band gap, low carrier mobility, and large lattice mismatch with other semiconductor materials, one of the current challenges in Si-based materials and structures is to prepare low-dimensional high-performance optoelectronic devices. In this work, an individual ZnO microwire via Ga-incorproration (ZnO:Ga MW) was employed to prepare a light-emitting/detecting bifunctional heterojunction structure, combined with p-type Si crystal wafer as a hole transporting layer. In a forward-bias regime, red luminescence peaking at around 680 nm was captured. While, the fabricated heterojunction device also exhibited an obvious photoresponse in the ultraviolet wavelengths. Interestingly, the introduction of Ag nanowires (AgNWs) are utilized to increase light output with amplitude 4 times higher than with that of naked wire-based LEDs. Similarly, the performance parameters of the fabricated n-AgNWs@ZnO:Ga MW/p-Si heterojunction photodetector are significantly enhanced, containing a responsivity of 5.52 A W^-1, detectivity of 2.34 × 10¹² Jones, external quantum efficiency of 1.9 × 10³% illuminated under 370 nm at -1 V. We compare this work with previous reported photodetectors based on various ZnO/Si-based materials and structures, some performance parameters are not superior, but our constructed n-AgNWs@ZnO:Ga MW/p-Si heterojunction photodetector has comparable overall characteristics, and our findings stand out especially for providing an inexpensive and suitable pathway for developing low-cost, miniaturized and integrated ultraviolet photodetectors. The demonstration of AgNWs enhanced low-dimensional light-emitting/detecting bifunctional photodiodes can offer a promising scheme to construct high-performance Si-based optoelectronic devices.

Recent advances in development of nanostructured photodetectors from ultraviolet to infrared region: A review

Herein, we aim to evaluate the photodetector performance of various nanostructured materials (thin films, 2-D nanolayers, 1-D nanowires, and 0-D quantum dots) in ultraviolet (UV), visible, and infrared (IR) regions. Specifically, semiconductor-based metal oxides such as ZnO, Ga₂O₃, SnO₂, TiO₂, and WO₃ are the majority preferred materials for UV photodetection due to their broad band gap, stability, and relatively simple fabrication processes. Whereas, the graphene-based hetero- and nano-structured composites are considered as prominent visible light active photodetectors. Interestingly, graphene exhibits broad band spectral absorption and ultra-high mobility, which derives graphene as a suitable candidate for visible detector. Further, due to the very low absorption rate of graphene (2%), various materials have been integrated with graphene (rGO-CZS, PQD-rGO, N-SLG, and GO doped PbI₂). In the case of IR photodetectors, quantum dot IR detectors prevails significant advantage over the quantum well IR detectors due to the 0-D quantum confinement and ability to absorb the light with any polarization. In such a way, we discussed the most recent developments on IR detectors using InAs and PbS quantum dot nanostructures. Overall, this review gives clear view on the development of suitable device architecture under prominent nanostructures to tune the photodetector performance from UV to IR spectral regions for wide-band photodetectors.

Low-Temperature Induced Enhancement of Photoelectric Performance in Semiconducting Nanomaterials

The development of light-electricity conversion in nanomaterials has drawn intensive attention to the topic of achieving high efficiency and environmentally adaptive photoelectric technologies. Besides traditional improving methods, we noted that low-temperature cooling possesses advantages in applicability, stability and nondamaging characteristics. Because of the temperature-related physical properties of nanoscale materials, the working mechanism of cooling originates from intrinsic characteristics, such as crystal structure, carrier motion and carrier or trap density. Here, emerging advances in cooling-enhanced photoelectric performance are reviewed, including aspects of materials, performance and mechanisms. Finally, potential applications and existing issues are also summarized. These investigations on low-temperature cooling unveil it as an innovative strategy to further realize improvement to photoelectric conversion without damaging intrinsic components and foresee high-performance applications in extreme conditions.

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.

A highly efficient bilayer graphene/ZnO/silicon nanowire based heterojunction photodetector with broadband spectral response

This paper presents three self-powered photodetectors namely, p⁺-bilayer graphene (BLG)/n⁺-ZnO nanowires (NWs), p⁺-BLG/n⁺-Si NWs/p^--Si and p⁺-BLG/n⁺-ZnO NWs/p^--Si. The Silvaco Atlas TCAD software is utilized to characterize the optoelectronic properties of all the devices and is validated by analytical modeling. The proposed dual-junction photodetectors cover broadband spectral response varying from ultraviolet to near-infrared wavelengths. The dual-heterojunction broadband photodetector exhibits photocurrent switching with the rise and fall time of 1.48 and 1.27 ns, respectively. At -0.5 V bias, the highest external quantum efficiency, photocurrent responsivity, specific detectivity, and the lowest noise equivalent power of 71%, 0.28 A W^-1, 4.2 × 10¹² cmHz^1/2 W^-1, and 2.59 × 10^-17 W, respectively, are found for the dual-heterojunction device with a wavelength of 480 nm at 300 K. The proposed nanowires based photodetectors offer great potential to be utilized as next-generation optoelectronic devices.

Self-Powered Filterless Narrow-Band p-n Heterojunction Photodetector for Low Background Limited Near-Infrared Image Sensor Application

Photonic detection with narrow spectrum selectivity is very important to eliminate the signal from obtrusive light, which can improve the anti-interference ability of the infrared imaging system. While the self-driving effect inherent to the p-n junction is very attractive in optic-electronic integration, the application of the p-n junction in narrow-band photodetectors is limited by the usual broad absorption range. In this work, a self-powered filterless narrowband near-infrared photodetector based on CuGaTe₂/silicon p-n junction was reported. The as-fabricated photodetector exhibited typical narrow-band response which shall be ascribed to the slightly smaller band gap of Si than CuGaTe₂ and the restricted photocurrent generation region in the p-n heterojunction by optimizing CuGaTe₂ thickness. It is observed that when the thickness of CuGaTe₂ film is 143 nm, the device exhibits a response peak centered around 1050 nm with a full-width at half-maximum of ∼118 nm. Further device analysis reveals a specific detectivity of ∼10¹² Jones and a responsivity of 114 mA/W under 1064 nm illumination at zero bias. It was also found that an image system based on the narrowband CuGaTe₂/Si photodetector showed high noise immunity for its spectral selective characteristics.

Optimized performance III-nitride-perovskite-based heterojunction photodetector via asymmetric electrode configuration

Organometal halide perovskite photodetectors have recently drawn significant attention due to their excellent potential to perform as broadband photodetectors. However, the photoresponse in the ultraviolet (UV) spectrum can be improved by introducing wide bandgap semiconductors. In this work, we report on a methylammonium lead iodide/p-type gallium nitride (MAPI/p-GaN) heterojunction photodetector. We demonstrate that the device is capable of detecting in the UV region by p-GaN being hybridized with MAPI. We further investigate different symmetric and asymmetric metal-electrode contacts to enhance the device performance including the response time. The asymmetric electrode configuration is found to be the most optimal configuration which results in high photoresponse (photo-responsivity is 55 mA W⁻¹ and fall time < 80 ms). As the light illumination occurs through the GaN side, its presence ultimately reduces MAPI degradation due to efficient absorption of the UV photons by GaN film.

Enhanced perovskite/GaN-based broad-band photodetector is demonstrated by optimizing electrode configurations. The detection capability of the optimized perovskite/GaN structure was extended to UV range with fast response and high responsivity.

Solar-Blind Self-Powered Photodetector Using Solution-Processed Amorphous Core-Shell Gallium Oxide Nanoparticles

Solution-processed deep ultraviolet (DUV) photodetectors based on wide band gap oxide semiconductors (WBGS) working in the <280 nm wavelength range are drawing increasing attention of the research community because of their cost-effective production and potential use in diverse applications. Here, we report on the synthesis of novel core-shell amorphous gallium oxide nanoparticles (NPs) (a-Ga₂O_x/GaO_x NPs) that have not been previously obtained. The amorphous gallium oxide NPs were synthesized from gallium nitride using the femtosecond laser ablation in liquid technique. Transmission electron microscopy and electron energy-loss spectroscopy measurements revealed the amorphous NP nature with a Ga-rich core and oxide-rich shell. Optical properties of these core-shell amorphous gallium oxide NPs were investigated by time-resolved spectroscopy and photoluminescence. As a proof of concept, the amorphous gallium oxide NPs were used as an active layer in a solar-blind DUV photodetector with high responsivity (778 mA/W) at 244 nm, which is the highest responsivity recorded to date for any solution-processed DUV photodetector. This work on a high-performance solution-processed device paves the way for large-scale industrial application of the WBGS.

Ultrasensitive, Superhigh Signal-to-Noise Ratio, Self-Powered Solar-Blind Photodetector Based on n-Ga2O3/p-CuSCN Core-Shell Microwire Heterojunction

Solar-blind photodetectors have captured intense attention due to their high significance in ultraviolet astronomy and biological detection. However, most of the solar-blind photodetectors have not shown extraordinary advantages in weak light signal detection because the forewarning of low-dose deep-ultraviolet radiation is so important for the human immune system. In this study, a high-performance solar-blind photodetector is constructed based on the n-Ga₂O₃/p-CuSCN core-shell microwire heterojunction by a simple immersion method. In comparison with the single device of the Ga₂O₃ and CuSCN, the heterojunction photodetector demonstrates an enhanced photoelectric performance with an ultralow dark current of 1.03 pA, high photo-to-dark current ratio of 4.14 × 10⁴, and high rejection ratio (R₂₅₄/R₃₆₅) of 1.15 × 10⁴ under a bias of 5 V. Excitingly, the heterostructure photodetector shows high sensitivity to the weak signal (1.5 μW/cm²) of deep ultraviolet and high-resolution detection to the subtle change of signal intensity (1.0 μW/cm²). Under the illumination with 254 nm light at 5 V, the photodetector shows a large responsivity of 13.3 mA/W, superb detectivity of 9.43 × 10¹¹ Jones, and fast response speed with a rise time of 62 ms and decay time of 35 ms. Additionally, the photodetector can work without an external power supply and has specific solar-blind spectrum selectivity as well as excellent stability even through 1 month of storage. Such prominent photodetection, profited by the novel geometric construction and the built-in electric field originating from the p-n heterojunction, meets greatly well the "5S" requirements of the photodetector for practical application.

Catalyst-Free Vertical ZnO-Nanotube Array Grown on p-GaN for UV-Light-Emitting Devices

One-dimensional (1D) structures-based UV-light-emitting diode (LED) has immense potential for next-generation applications. However, several issues related to such devices must be resolved first, such as expensive material and growth methods, complicated fabrication process, efficiency droop, and unavoidable metal contamination due to metal catalyst that reduces device efficiency. To overcome these obstacles, we have developed a novel growth method for obtaining a high-quality hexagonal, well-defined, and vertical 1D Gd-doped n-ZnO nanotube (NT) array deposited on p-GaN films and other substrates by pulsed laser deposition. By adopting this approach, the desired high optical and structural quality is achieved without utilizing metal catalyst. Transmission electron microscopy measurements confirm that gadolinium dopants in the target form a transparent in situ interface layer to assist in vertical NT formation. Microphotoluminescence (PL) measurements of the NTs reveal an intense ZnO band edge emission without a defect band, indicating high quality. Carrier dynamic analysis via time-resolved PL confirms that the emission of n-ZnO NTs/p-GaN LED structure is dominated significantly by the radiative recombination process without efficiency droop when high carrier density is injected optically. We developed an electrically pumped UV Gd-doped ZnO NTs/GaN LED as a proof of concept, demonstrating its high internal quantum efficiency (>65%). The demonstrated performance of this cost-effective UV LED suggests its potential application in large-scale device production.

Crystal-Structure-Dependent Piezotronic and Piezo-Phototronic Effects of ZnO/ZnS Core/Shell Nanowires for Enhanced Electrical Transport and Photosensing Performance

We report the crystal-structure-dependent piezotronic and piezo-phototronic effects of ZnO/ZnS core/shell nanowires (CS NWs) having different shell layer crystalline structures. The wurtzite (WZ) ZnO/WZ ZnS CS NWs showed higher electrical transport and photosensing properties under external strain than the WZ ZnO/zinc blende (ZB) ZnS CS NWs. The WZ ZnO/WZ ZnS CS NWs under a compressive strain of -0.24% showed 4.4 and 8.67 times larger increase in the output current (1.93 × 10^-4 A) and photoresponsivity (8.76 × 10^-1 A/W) than those under no strain. However, the WZ ZnO/ZB ZnS CS NWs under the same strain condition showed 3.2 and 2.16 times larger increase in the output current (1.13 × 10^-4 A) and photoresponsivity (2.16 × 10^-1 A/W) than those under no strain. This improvement is ascribed to strain-induced piezopolarization charges at both the WZ ZnO NWs and the grains of the WZ ZnS shell layer in WZ ZnO/WZ ZnS CS NWs, whereas piezopolarization charges are induced only in the ZnO core region of the WZ ZnO/ZB ZnS CS NWs. These charges can change the type-II band alignment in the ZnO and ZnS interfacial region as well as the Schottky barrier height at the junction between the semiconductor and the metal, thus facilitating electrical transport and reducing the recombination probability of charge carriers under UV irradiation.

High-Performance Graphene/β-Ga2O3 Heterojunction Deep-Ultraviolet Photodetector with Hot-Electron Excited Carrier Multiplication

Solar-blind ultraviolet (SBUV) detection has important applications in wireless secure communication, early warning, and so forth. However, the desired key device for SBUV detection and high-sensitivity and low-noise "sandwich" photodetector with large detective area is difficult to be fabricated because it is usually hard for traditional wide band gap semiconductors to boast both high conductivity and high SBUV transparency. Here, we proposed to use graphene as the transparent conductive layer to form graphene/β-Ga₂O₃ heterojunction. With the help of large-area graphene and hot carrier multiplication, a SBUV photodetector with large detective area, low dark current, and high sensitivity was successfully assembled. Its photoresponsivity is 1-3 orders of magnitude higher than that of the conventional SBUV photodetectors, and its response speed can rival the best device ever reported.

Induced Photonic Response of ZnO Nanorods Grown on Oxygen Plasma-Treated Seed Crystallites

We examined the influence of O₂ plasma treatment for the ZnO seed layer (SL) crystallites on the material characteristics of ZnO nanorods (NRs) synthesized by the hydrothermal method. Diode photocurrent and photo-response transient characteristics of the p-Si/n-ZnO-NR heterojunction-based ultraviolet (UV) photodetectors were also examined according to the plasma treatment for the SLs. The superior optical properties of NRs were measured from the photoluminescence by exhibiting 4.6 times greater near-band edge emission when grown on the O₂-plasma-treated SL. The degree of (002) orientation of the NR crystals was improved from 0.67 to 0.95, as revealed by X-ray diffraction analysis, and a higher NR surface density of ~80 rods/μm² with a smaller mean diameter of 65 nm were also observed by the SL modification using plasma-treatment. It was shown by X-ray photo-electron spectroscopy that this improvement of NR crystalline quality was due to the recovery of stoichiometric oxygen with significant reduction of oxygenated impurities in the SL crystals and the subsequent low-energy growth mode for the NRs. UV PDs fabricated by the proposed SL plasma treatment technique showed significantly enhanced UV-to-dark current ratio from 2.0 to 83.7 at a forward bias of +5 V and faster photo-response characteristics showing the reduction in recovery time from 16 s to 9 s.

Surface-Engineered Nanostructure-Based Efficient Nonpolar GaN Ultraviolet Photodetectors

Surface-engineered nanostructured nonpolar (112̅0) gallium nitride (GaN)-based high-performance ultraviolet (UV) photodetectors (PDs) have been fabricated. The surface morphology of a nonpolar GaN film was modified from pyramidal shape to flat and trigonal nanorods displaying facets along different crystallographic planes. We report the ease of enhancing the photocurrent (5.5-fold) and responsivity (6-fold) of the PDs using a simple and convenient wet chemical-etching-induced surface engineering. The fabricated metal-semiconductor-metal structure-based surface-engineered UV PD exhibited a significant increment in detectivity, that is, from 0.43 to 2.83 (×10⁸) Jones, and showed a very low noise-equivalent power (∼10^-10 W Hz^-1/2). The reliability of the nanostructured PD was ensured via fast switching with a response and decay time of 332 and 995 ms, which were more than five times faster with respect to the unetched pyramidal structure-based UV PD. The improvement in device performance was attributed to increased light absorption, efficient transport of photogenerated carriers, and enhancement in conduction cross section via elimination of recombination/trap centers related to defect states. Thus, the proposed method could be a promising approach to enhance the performance of GaN-based PD technology.