High detectivity solar-blind high-temperature deep-ultraviolet photodetector based on multi-layered (l00) facet-oriented β-Ga₂O₃ nanobelts

An Aerosol-Assisted Chemical Vapor Deposition Route to Tin-Doped Gallium Oxide Thin Films with Optoelectronic Properties

Gallium oxide is a wide-bandgap compound semiconductor material renowned for its diverse applications spanning gas sensors, liquid crystal displays, transparent electrodes, and ultraviolet detectors. This paper details the aerosol assisted chemical vapor deposition synthesis of tin doped gallium oxide thin films using gallium acetylacetonate and monobutyltin trichloride dissolved in methanol. It was observed that Sn doping resulted in a reduction in the transmittance of Ga2O3 films within the visible spectrum, while preserving the wide bandgap characteristics of 4.8 eV. Furthermore, Hall effect testing revealed a substantial decrease in the resistivity of Sn-doped Ga2O3 films, reducing it from 4.2 × 106 Ω cm to 2 × 105 Ω cm for the 2.5 at. % Sn:Ga2O3 compared to the nominally undoped Ga2O3.

High Electron Mobility in Si-Doped Two-Dimensional β-Ga2O3 Tuned Using Biaxial Strain

Two-dimensional (2D) semiconductors have attracted much attention regarding their use in flexible electronic and optoelectronic devices, but the inherent poor electron mobility in conventional 2D materials severely restricts their applications. Using first-principles calculations in conjunction with Boltzmann transport theory, we systematically investigated the Si-doped 2D β-Ga2O3 structure mediated by biaxial strain, where the structural stabilities were determined by formation energy, phonon spectrum, and ab initio molecular dynamic simulation. Initially, the band gap values of Si-doped 2D β-Ga2O3 increased slightly, followed by a rapid decrease from 2.46 eV to 1.38 eV accompanied by strain modulations from -8% compressive to +8% tensile, which can be ascribed to the bigger energy elevation of the σ* anti-bonding in the conduction band minimum than that of the π bonding in the valence band maximum. Additionally, band structure calculations resolved a direct-to-indirect transition under the tensile strains. Furthermore, a significantly high electron mobility up to 4911.18 cm2 V-1 s-1 was discovered in Si-doped 2D β-Ga2O3 as the biaxial tensile strain approached 8%, which originated mainly from the decreased quantum confinement effect on the surface. The electrical conductivity was elevated with the increase in tensile strain and the enhancement of temperature from 300 K to 800 K. Our studies demonstrate the tunable electron mobilities and band structures of Si-doped 2D β-Ga2O3 using biaxial strain and shed light on its great potential in nanoscale electronics.

Amorphous Gallium Oxide-Based Crosstalk-Suppressing Solar-Blind Imaging Array Prepared by One-Step Method

The solar-blind ultraviolet band presents a unique opportunity for low-background-noise detection due to limited atmospheric light transmission. Especially, with the ability to obtain size, shape and position information on the objects, the solar-blind image sensors are receiving increasing attention. However, because the inhibition of the crosstalk in crossbar arrays induces the complexity of the preparation process, rarely are applicable solar-blind UV imaging arrays reported. This work prepared an amorphous gallium oxide (a-Ga2O3)-based diode with a remarkable rectifier ratio of up to 106. Then an imaging array was prepared by one-step method in that the diodes working as the detection elements and the switching elements inhibiting crosstalk were simultaneously deposited. Moreover, a 2 × 2 crossbar array clearly demonstrates the inhibition of the crosstalk. This work presents a simple and cost-effective method for preparing applicable solar-blind imaging arrays that inhibits crosstalk, promoting the practical application of the Ga2O3-based solar-blind UV detectors.

Recent Progress in Solar-Blind Photodetectors Based on Ultrawide Bandgap Semiconductors

Ultrawide bandgap (UWBG) semiconductors, including Ga2O3, diamond, Al x Ga1-x N/AlN, featuring bandgaps greater than 4.4 eV, hold significant promise for solar-blind ultraviolet photodetection, with applications spanning in environmental monitoring, chemical/biological analysis, industrial processes, and military technologies. Over recent decades, substantial strides in synthesizing high-quality UWBG semiconductors have facilitated the development of diverse high-performance solar-blind photodetectors (SBPDs). This review comprehensively examines recent advancements in UWBG semiconductor-based SBPDs across various device architectures, encompassing photoconductors, metal-semiconductor-metal photodetectors, Schottky photodiodes, p-n (p-i-n) photodiodes, phototransistors, etc., with a systematic introduction and discussion of their operational principles. The current state of device performance for SBPDs employing these UWBG semiconductors is evaluated across different device configurations. Finally, this review outlines key challenges to be addressed, aiming to steer future research endeavors in this critical domain.

Organic High-Temperature Synaptic Phototransistors for Energy-Efficient Neuromorphic Computing

Organic optoelectronic synaptic devices that can reliably operate in high-temperature environments (i.e., beyond 121°C) or remain stable after high-temperature treatments have significant potential in biomedical electronics and bionic robotic engineering. However, it is challenging to acquire this type of organic devices considering the thermal instability of conventional organic materials and the degradation of photoresponse mechanisms at high temperatures. Here, high-temperature synaptic phototransistors (HTSPs) based on thermally stable semiconductor polymer blends as the photosensitive layer are developed, successfully simulating fundamental optical-modulated synaptic characteristics at a wide operating temperature range from room temperature to 220°C. Robust optoelectronic performance can be observed in HTSPs even after experiencing 750 h of the double 85 testing due to the enhanced operational reliability. Using HTSPs, Morse-code optical decoding scheme and the visual object recognition capability are also verified at elevated temperatures. Furthermore, flexible HTSPs are fabricated, demonstrating an ultralow power consumption of 12.3 aJ per synaptic event at a low operating voltage of -0.05 mV. Overall, the conundrum of achieving reliable optical-modulated neuromorphic applications while balancing low power consumption can be effectively addressed. This research opens up a simple but effective avenue for the development of high-temperature and energy-efficient wearable optoelectronic devices in neuromorphic computing applications.

Fabrication of β-Ga2O3 Nanotubes via Sacrificial GaSb-Nanowire Templates

β-Ga2O3 nanostructures are attractive wide-band-gap semiconductor materials as they exhibit promising photoelectric properties and potential applications. Despite the extensive efforts on β-Ga2O3 nanowires, investigations into β-Ga2O3 nanotubes are rare since the tubular structures are hard to synthesize. In this paper, we report a facile method for fabricating β-Ga2O3 nanotubes using pre-synthesized GaSb nanowires as sacrificial templates. Through a two-step heating-treatment strategy, the GaSb nanowires are partially oxidized to form β-Ga2O3 shells, and then, the residual inner parts are removed subsequently in vacuum conditions, yielding delicate hollow β-Ga2O3 nanotubes. The length, diameter, and thickness of the nanotubes can be customized by using different GaSb nanowires and heating parameters. In situ transmission electron microscopic heating experiments are performed to reveal the transformation dynamics of the β-Ga2O3 nanotubes, while the Kirkendall effect and the sublimation process are found to be critical. Moreover, photoelectric tests are carried out on the obtained β-Ga2O3 nanotubes. A photoresponsivity of ~25.9 A/W and a detectivity of ~5.6 × 1011 Jones have been achieved with a single-β-Ga2O3-nanotube device under an excitation wavelength of 254 nm.

Miniaturized solar-blind ultraviolet imaging system enabled by a diffractive/refractive hybrid

In this paper, we demonstrated a miniaturized diffractive/refractive hybrid system based on a diffractive optical element and three refractive lenses to achieve solar-blind ultraviolet imaging within a range of 240-280 nm. We experimentally demonstrate the optical system has both outstanding resolution and excellent imaging capability. The experiments demonstrate that the system could distinguish the smallest line pair with a width of 16.7 µm. The modulation transfer function (MTF) at the target maximum frequency (77 lines pair/mm) is great than 0.76. The strategy provides significant guidance for the mass production of solar-blind ultraviolet imaging systems towards miniaturization and lightweight.

Ultrahigh-Sensitivity and Fast-Speed Solar-Blind Ultraviolet Photodetector Based on a Broken-Gap van der Waals Heterodiode

Broad-bandgap semiconductor-based solar-blind ultraviolet (SBUV) photodetectors have attracted considerable research interest because of their broad applications in missile plume tracking, flame detectors, environmental monitoring, and optical communications due to their solar-blind nature and high sensitivity with low background radiation. Owing to its high light absorption coefficient, abundance, and wide tunable bandgap of 2-2.6 eV, tin disulfide (SnS₂) has emerged as one of the most promising compounds for application in UV-visible optoelectronic devices. However, SnS₂ UV detectors have some undesirable properties such as slow response speed, high current noise level, and low specific detectivity. This study reports a metal mirror-enhanced Ta_0.01W_0.99Se₂/SnS₂ (TWS) van der Waals heterodiode-based SBUV photodetector with an ultrahigh photoresponsivity (R) of ∼1.85 × 10⁴ AW^-1 and a fast speed with rising time (τ_r) of 3.3 μs and decay time (τ_d) of 3.4 μs. Notably, the TWS heterodiode device exhibits a significantly low noise equivalent power of ∼1.02 × 10^-18 W Hz^-1/2 and a high specific detectivity of ∼3.65 × 10¹⁴ cm Hz^1/2 W^-1. This study provides an alternative method for designing fast-speed SBUV photodetectors with enormous potential in applications.

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.

A High-Performance ε-Ga2O3-Based Deep-Ultraviolet Photodetector Array for Solar-Blind Imaging

One of the most important applications of photodetectors is as sensing units in imaging systems. In practical applications, a photodetector array with high uniformity and high performance is an indispensable part of the imaging system. Herein, a photodetector array (5 × 4) consisting of 20 photodetector units, in which the photosensitive layer involves preprocessing commercial ε-Ga₂O₃ films with high temperature annealing, have been constructed by low-cost magnetron sputtering and mask processes. The ε-Ga₂O₃ ultraviolet photodetector unit shows excellent responsivity and detectivity of 6.18 A/W and 5 × 10¹³ Jones, respectively, an ultra-high light-to-dark ratio of 1.45 × 10⁵, and a fast photoresponse speed (0.14/0.09 s). At the same time, the device also shows good solar-blind characteristics and stability. Based on this, we demonstrate an ε-Ga₂O₃-thin-film-based solar-blind ultraviolet detector array with high uniformity and high performance for solar-blind imaging in optoelectronic integration applications.

High Performance of Zero-Power-Consumption MOCVD-Grown β-Ga2O3-Based Solar-Blind Photodetectors with Ultralow Dark Current and High-Temperature Functionalities

In this article, we report on high-performance deep ultraviolet photodetectors (DUV PDs) fabricated on metal-organic chemical vapor deposition (MOCVD)-grown β-Ga₂O₃ heteroepitaxy that exhibit stable operation up to 125 °C. The fabricated DUV PDs exhibit self-powered behavior with an ultralow dark current of 1.75 fA and a very high photo-to-dark-current ratio (PDCR) of the order of 10⁵ at zero bias and >10⁵ at higher biases of 5 and 10 V, which remains almost constant up to 125 °C. The high responsivity of 6.62 A/W is obtained at 10 V at room temperature (RT) under the weak illumination of 42.86 μW/cm² of 260 nm wavelength. The detector shows very low noise equivalent power (NEP) of 5.74 × 10^-14 and 1.03 × 10^-16 W/Hz^1/2 and ultrahigh detectivity of 5.51 × 10¹¹ and 3.10 × 10¹⁴ Jones at 0 and 5 V, respectively, which shows its high detection sensitivity. The RT UV-visible (260:500 nm) rejection ratios of the order of 10³ at zero bias and 10⁵ at 5 V are obtained. These results demonstrate the potential of Ga₂O₃-based DUV PDs for solar-blind detection applications that require high-temperature robustness.

Extended linear detection range of a Bi0.5Na0.5TiO3 thin film-based self-powered UV photodetector via current and voltage dual indicators

Ferroelectric materials are widely recognized for their ability to generate photovoltaic voltages larger than their bandgap, making them ideal candidates for photodetector applications. Here, we report a self-powered UV photodetector based on a Bi_0.5Na_0.5TiO₃ (BNT) thin film prepared by the sol-gel method. Compared with conventional photodetectors based on a single detection indicator, the demonstrated photodetector realizes UV light intensity detection over a wide linear range using a current and voltage dual indicator detection method. When the UV light intensity is lower than 1.8 mW cm^-2, the voltage can be used to detect the light signal. Conversely, the current can be utilized to detect the signal. This method not only broadens the linear detection range of UV light intensity, making it possible to detect weak UV light of 45.2 nW cm^-2, but also allows the detector to maintain relatively high sensitivity within the detectable range. To investigate the distribution of spatial UV light intensity, a self-powered photodetector array system has been utilized to record the output voltage signals as a map.

High-temperature flexible WSe2 photodetectors with ultrahigh photoresponsivity

The development of high-temperature photodetectors can be beneficial for numerous applications, such as aerospace engineering, military defence and harsh-environments robotics. However, current high-temperature photodetectors are characterized by low photoresponsivity (<10 A/W) due to the poor optical sensitivity of commonly used heat-resistant materials. Here, we report the realization of h-BN-encapsulated graphite/WSe2 photodetectors which can endure temperatures up to 700 °C in air (1000 °C in vacuum) and exhibit unconventional negative photoconductivity (NPC) at high temperatures. Operated in NPC mode, the devices show a photoresponsivity up to 2.2 × 10⁶ A/W, which is ~5 orders of magnitude higher than that of state-of-the-art high-temperature photodetectors. Furthermore, our devices demonstrate good flexibility, making it highly adaptive to various shaped surfaces. Our approach can be extended to other 2D materials and may stimulate further developments of 2D optoelectronic devices operating in harsh environments.

High-temperature photodetectors are desired for aerospace applications and harsh-environment robotics, but their responsivity is usually limited. Here, the authors report flexible hBN-encapsulated graphite/WSe2 photodetectors which can endure temperatures up to 700 °C in air with enhanced photoresponsivity.

Non-Ultrawide Bandgap Semiconductor GaSe Nanobelts for Sensitive Deep Ultraviolet Light Photodetector Application

In this paper, the authors report the fabrication of a sensitive deep ultraviolet (DUV) photodetector by using an individual GaSe nanobelt with a thickness of 52.1 nm, which presents the highest photoresponse at 265 nm illumination with a responsivity and photoconductive gain of about 663 A W^-1 and 3103 at a 3 V bias, respectively, comparable to or even better than other reported devices based on conventional wide bandgap semiconductors. According to the simulation, this photoelectric property is associated with the wavelength-dependent absorption coefficient of the GaSe crystal, for which incident light with shorter wavelengths will be absorbed near the surface, while light with longer wavelengths will have a larger penetration depth, leading to a blueshift of the absorption edge with decreasing thickness. Further finite element method (FEM) simulation reveals that the relatively thin GaSe nanobelt exhibits an enhanced transversal standing wave pattern compared to its thicker counterpart at a wavelength of 265 nm, leading to an enhanced light-matter interaction and thereby more efficient photocurrent generation. The device can also function as an effective image sensor with acceptable spatial resolution. This work will shed light on the facile fabrication of a high-performance DUV photodetector from non-ultrawide bandgap semiconductors.

Epitaxial Growth of 2D Ultrathin Metastable γ-Bi2 O3 Flakes for High Performance Ultraviolet Photodetection

Ultraviolet detection is of great significance due to its wide applications in the missile tracking, flame detecting, pollution monitoring, and so on. The nonlayered semiconductor γ-Bi₂ O₃ is a promising candidate toward high-performance UV detection due to the wide bandgap, excellent light sensitivity, environmental stability, nontoxic and elemental abundance properties. However, controllable preparation of ultrathin 2D γ-Bi₂ O₃ flakes remains a challenge, owing to its nonlayered structure, metastable nature, and other competing phases. Moreover, the UV photodetectors based on 2D γ-Bi₂ O₃ flake have not been implemented yet. Here, ultrathin (down to 4.8 nm) 2D γ-Bi₂ O₃ flakes with high crystal quality are obtained via a van der Waals epitaxy method. The as-synthesized single-crystalline γ-Bi₂ O₃ flakes show a body-centered cubic structure and grown along (111) lattice plane as revealed by experimental observations. More importantly, photodetectors based on the as-synthesized 2D γ-Bi₂ O₃ flakes exhibit promising UV detection ability, including a responsivity of 64.5 A W^-1 , a detectivity of 1.3 × 10¹³ Jones, and an ultrafast response speed (τ_rise  ≈ 290 µs and τ_decay  ≈ 870 µs) at 365 nm, suggesting its great potential for various optoelectronic applications.

Wide Dynamic Range Thermometer Based on Luminescent Optical Cavities in Ga2 O3 :Cr Nanowires

Remote temperature sensing at the micro- and nanoscale is key in fields such as photonics, electronics, energy, or biomedicine, with optical properties being one of the most used transducing mechanisms for such sensors. Ga₂ O₃ presents very high chemical and thermal stability, as well as high radiation resistance, becoming of great interest to be used under extreme conditions, for example, electrical and/or optical high-power devices and harsh environments. In this work, a luminescent and interferometric thermometer is proposed based on Fabry-Perot (FP) optical microcavities built on Cr-doped Ga₂ O₃ nanowires. It combines the optical features of the Cr³⁺ -related luminescence, greatly sensitive to temperature, and spatial confinement of light, which results in strong FP resonances within the Cr³⁺ broad band. While the chromium-related R lines energy shifts are adequate for low-temperature sensing, FP resonances extend the sensing range to high temperatures with excellent sensitivity. This thermometry system achieves micron-range spatial resolution, temperature precision of around 1 K, and a wide operational range, demonstrating to work at least in the 150-550 K temperature range. Besides, the temperature-dependent anisotropic refractive index and thermo-optic coefficient of this oxide have been further characterized by comparison to experimental, analytical, and finite-difference time-domain simulation results.

Microstructures and electronic characters of β-Ga2O3 on different substrates: exploring the role of surface chemistry and structures

Unveiling the microstructural and electronic properties of β-Ga₂O₃ on different substrates is vital to realize the high quality and performance of β-Ga₂O₃. Here, the microstructure disorder, defect characters and orbital structures of β-Ga₂O₃ on the Al₂O₃, MgO, and SiC substrates with different terminations are studied. Although several growth mechanisms for β-Ga₂O₃ are observed on the same substrate, β-Ga₂O₃ prefers to deposit the octahedral Ga atom firstly on the Al₂O₃ and MgO substrates, and the latter can restrain the oxygen-vacancy formation and migration. The structural disorder, band offsets and gap states can be improved upon depositing β-Ga₂O₃ on a substrate with metal terminations under the oxygen-poor conditions. Compared to the Al₂O₃ substrate, β-Ga₂O₃ on the SiC substrate shows a smaller structure disorder and a higher defect formation energy, in particular under the oxygen-rich conditions, since β-Ga₂O₃ prefers to deposit the tetrahedral Ga atom firstly on the SiC substrate to form a SiC-Ga₂O₃ interface with less dangling bonds. The type-II band alignment of the SiC-Ga₂O₃ interface can be changed into the type-I character with larger band offsets when β-Ga₂O₃ is deposited under the oxygen-rich conditions, irrespective of the termination of the SiC substrate. These results provide a useful understanding of the effect of substrates on the quality and performance of β-Ga₂O₃ and a scientific basis for the application of substrate-Ga₂O₃ interfaces.

Self-catalyst β-Ga2O3 semiconductor lateral nanowire networks synthesis on the insulating substrate for deep ultraviolet photodetectors

Monoclinic gallium oxide (β-Ga2O3) is a super-wide bandgap semiconductor with excellent chemical and thermal stability, which is an ideal candidate for detecting deep ultraviolet (DUV) radiation (100-280 nm). The growth of β-Ga2O3 is challenging and most methods require Au as the catalyst and a long reacting time (more than 1 hour). In this work, the self-catalyst β-Ga2O3 lateral nanowire networks were synthesized on an insulating substrate rapidly by a simple low-cost Chemical Vapor Deposition (CVD) method. A thin film of β-Ga2O3 nanowire networks was synthesized within a reacting time of 15 minutes, which possesses a huge possibility for the rapid growth of β-Ga2O3 metal oxide nanowires networks and application in the future solar-blind photodetector. MSM (metal-semiconductor-metal) photodetectors based on the β-Ga2O3 nanowire networks revealed fast response (on-off ratios is about 103), which is attributed to the unique cross-junction barrier-dominated conductance of the nanowire networks. In addition, the self-catalyst β-Ga2O3 nanowires grown on insulating SiO2 are achieved and could be expected to find important applications in a bottom-up way of fabricating the next generation semiconductor nanoelectronics.

Ultrasensitive Flexible Solar-Blind Photodetectors Based on Graphene/Amorphous Ga2O3 van der Waals Heterojunctions

Flexible photodetectors (PDs) have become the latest research interest owing to their potential applications in future implantable sensors and foldable/wearable optoelectronics. Ga₂O₃, an emerging ultrawide band gap semiconductor, is considered as the native photosensitive material for solar-blind PDs. The reported fabrication temperature of Ga₂O₃ films is usually above 600 °C, which hinders its practical application for flexible devices. In this work, flexible PDs based on graphene/amorphous Ga₂O₃ van der Waals heterojunctions are fabricated, which demonstrate promising photoresponse to solar-blind ultraviolet light. The device yields a high photo-to-dark current ratio (∼10⁵) and large responsivity (22.75 A/W) under 254 nm light illumination, which could be ascribed to the efficient photogenerated electron-hole pair separation by the strong built-in field. Moreover, flexible PDs also show long-term environmental stability and outstanding mechanical flexibility without any encapsulation. Our work provides a new potential candidate for realizing cost-effective high-performance flexible optoelectronic applications.

Synthesis of High-Quality Multilayer Hexagonal Boron Nitride Films on Au Foils for Ultrahigh Rejection Ratio Solar-Blind Photodetection

Solar-blind photodetectors have widespread applications due to the unique merit of a "black background" on the earth. However, most solar-blind photodetectors reported previously exhibited quite low rejection ratios (R_200nm/R_280nm < 10³) and were interfered with by light longer than 280 nm. Herein, by an ambient pressure chemical vapor deposition (CVD) method, large-area, clean, and uniform two-dimensional (2D) multilayer h-BN films with different thicknesses have been successfully synthesized on Au foils. The synthesized multilayer h-BN film is transparent to light longer than 280 nm, showing excellent optical and optoelectronic properties to weak solar-blind light (μW/cm²). This sensitive solar-blind h-BN photodetector exhibits ultrahigh rejection ratios (R_220nm/R_280nm > 10³ and R_220nm/R_290nm > 10⁴), a low dark current (10² fA), and a large detectivity (3.9 × 10¹⁰ Jones). It is noteworthy that the rejection ratio (R_220nm/R_290nm) here is superior to most of those previously reported based on traditional semiconductors. This large-scale, clean, and uniform multilayer h-BN film will contribute to the progress of next-generation optoelectronic devices.

Atomically Thin Oxyhalide Solar-Blind Photodetectors

2D wide-bandgap semiconductors demonstrate great potential in fabricating solar-blind ultraviolet (SBUV) photodetectors. However, the low responsivity of 2D solar-blind photodetectors still limits their practical applications. Here, high-responsivity solar-blind photodetectors are achieved based on 2D bismuth oxychloride (BiOCl) flakes. The 2D BiOCl photodetectors exhibit a responsivity up to 35.7 A W^-1 and a specific detectivity of 2.2 × 10¹⁰ Jones under 250 nm illumination with 17.8 µW cm^-2 power density. In particular, the enhanced photodetective performances are demonstrated in BiOCl photodetectors with increasing ambient temperature. Surprisingly, their responsivity can reach 2060 A W^-1 at 450 K under solar-blind light illumination, maybe owing to the formation of defective BiOCl grains evidenced by in situ transmission electron microscopy. The high responsivity throughout the solar-blind range indicates that 2D BiOCl is a promising candidate for SBUV detection.

High-Performance Deep Ultraviolet Photodetector Based on NiO/β-Ga2O3 Heterojunction

Ultraviolet (UV) photodetector has attracted extensive interests due to its wide-ranging applications from defense technology to optical communications. The use of wide bandgap metal oxide semiconductor materials is of great interest in the development of UV photodetector due to their unique electronic and optical properties. In this work, deep UV photodetector based on NiO/β-Ga₂O₃ heterojunction was developed and investigated. The β-Ga₂O₃ layer was prepared by magnetron sputtering and exhibited selective orientation along the family of ([Formula: see text] 01) crystal plane after annealing. The photodetector demonstrated good performance with a high responsivity (R) of 27.43 AW^-1 under a 245-nm illumination (27 μWcm^-2) and the maximum detectivity (D*) of 3.14 × 10¹² cmHz^1/2 W^-1, which was attributed to the p-NiO/n-β-Ga₂O₃ heterojunction.

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.

Structural Engineering of Si/TiO2/P3HT Heterojunction Photodetectors for a Tunable Response Range

To meet the demands of next-generation optoelectronic circuits, the design and construction of photodetectors with a tunable photoresponse range and self-powered feature are urgently required. To achieve selective wavelength detection, a band-pass filter is usually required to dislodge the interference of a certain wavelength light, which inevitably enhances the weight and increases the cost. Here, we demonstrate a self-powered photodetector with a tunable response range by constructing a heterojunction structure consisting of poly(3-hexylthiophene) (P3HT), a TiO₂ interlayer, and silicon nanowires. By controlling the P3HT concentration, both core-shell and embedded configurations can be obtained, which exhibit different response ranges. This work provides a convenient route to construct self-powered wavelength-selective photodetectors, which may find applications in light communication and biomedical engineering.

Synthesis of ternary oxide Zn2GeO4 nanowire networks and their deep ultraviolet detection properties

Ternary oxide Zn₂GeO₄ with a wide bandgap of 4.84 eV, as a candidate for fourth generation semiconductors, has attracted a great deal of attention for deep ultraviolet (DUV) photodetector applications, because it is expected to be blind to the UV-A/B band (290–400 nm) and only responsive to the UV-C band (200–290 nm). Here, we report on the synthesis of Zn₂GeO₄ nanowire (NW) networks by lower pressure chemical vapor deposition and investigate their corresponding DUV detection properties. We find that pure Zn₂GeO₄ NWs could be obtained at a growth pressure of 1 kPa. The DUV detection tests reveal that growth pressure exerts a significant effect on DUV detection performance. The Zn₂GeO₄ NW networks produced under 1 kPa show an excellent solar-blind photoresponsivity with fast rise and decay times (t_rise ≈ 0.17 s and t_decay ≈ 0.14 s).

Ternary oxide Zn₂GeO₄ with a wide bandgap of 4.84 eV, as a candidate for fourth generation semiconductors, has attracted lots of attention for deep UV photodetector applications, as it is blind to the UV-A/B band and only responds to the UV-C band.

Vacuum-Ultraviolet Photodetection in Few-Layered h-BN

Over the past 20 years, astro and solar physicists have been working hard to develop a new-generation semiconductor-based vacuum-ultraviolet (VUV, 100-200 nm) photodetector with small size and low power consumption, to replace the traditional microchannel detection system, which is ponderous and has high energy consumption, and finally to reduce the power load and launch costs of explorer satellites. However, this expectation has hardly been achieved due to the relatively low photoresponsivity and external quantum efficiency (EQE) of the reported VUV photoconductive detectors based on traditional wide-band-gap materials and structures. Here, on the basis of few-layer h-BN, we fabricated a high-performance two-dimensional photodetector with selective response to VUV light. Typically, it has high sensitivity (EQE = 2133%, at 20 V) to the extremely weak 160 nm light (3.25 pW). This excellent photoresponsivity can be attributed to the high carrier collection efficiency and existing surface trap states of few-layer h-BN. In addition, this device can maintain a stable performance in a wide temperature range (80-580 K), which is quite favorable for application in deep space with huge temperature fluctuation.

Photoelectric Detectors Based on Inorganic p-Type Semiconductor Materials

Photoelectric detectors are the central part of modern photodetection systems with numerous commercial and scientific applications. p-Type semiconductor materials play important roles in optoelectronic devices. Photodetectors based on p-type semiconductor materials have attracted a great deal of attention in recent years because of their unique properties. Here, a comprehensive summary of the recent progress mainly on photodetectors based on inorganic p-type semiconductor materials is presented. Various structures, including photoconductors, phototransistors, homojunctions, heterojunctions, p-i-n junctions, and metal-semiconductor junctions of photodetectors based on inorganic p-type semiconductor materials, are discussed and summarized. Perspectives and an outlook, highlighting the promising future directions of this research field, are also given.

High-Performance and Self-Powered Deep UV Photodetectors Based on High Quality 2D Boron Nitride Nanosheets

High-quality two-dimensional (2D) crystalline boron nitride nanosheets (BNNSs) were grown on silicon wafers by using pulsed plasma beam deposition techniques. Self-powered deep ultraviolet (DUV) photodetectors (PDs) based on BNNSs with Schottky contact structures are designed and fabricated. By connecting the fabricated DUV photodetector to an ammeter, the response strength, response time and recovery time to different DUV wavelengths at different intensities have been characterized using the output short circuit photocurrent without a power supply. Furthermore, effects of temperature and plasma treatment on the induced photocurrent response of detectors have also been investigated. The experimental data clearly indicate that plasma treatment would significantly improve both induced photocurrent and response time. The BNNS-based DUV photodetector is demonstrated to possess excellent performance at a temperature up to 400 °C, including high sensitivity, high signal-to-noise ratio, high spectral selectivity, high speed, and high stability, which is better than almost all reported semiconducting nanomaterial-based self-powered photodetectors.

Ultrawide Band Gap β-Ga2O3 Nanomechanical Resonators with Spatially Visualized Multimode Motion

Beta gallium oxide (β-Ga₂O₃) is an emerging ultrawide band gap (4.5 eV-4.9 eV) semiconductor with attractive properties for future power electronics, optoelectronics, and sensors for detecting gases and ultraviolet radiation. β-Ga₂O₃ thin films made by various methods are being actively studied toward such devices. Here, we report on the experimental demonstration of single-crystal β-Ga₂O₃ nanomechanical resonators using β-Ga₂O₃ nanoflakes grown via low-pressure chemical vapor deposition (LPCVD). By investigating β-Ga₂O₃ circular drumhead structures, we demonstrate multimode nanoresonators up to the sixth mode in high and very high frequency (HF/VHF) bands, and also realize spatial mapping and visualization of the multimode motion. These measurements reveal a Young's modulus of E_Y = 261 GPa and anisotropic biaxial built-in tension of 37.5 MPa and 107.5 MPa. We find that thermal annealing can considerably improve the resonance characteristics, including ∼40% upshift in frequency and ∼90% enhancement in quality (Q) factor. This study lays a foundation for future exploration and development of mechanically coupled and tunable β-Ga₂O₃ electronic, optoelectronic, and physical sensing devices.

Solar-Blind Photodetector with High Avalanche Gains and Bias-Tunable Detecting Functionality Based on Metastable Phase α-Ga2O3/ZnO Isotype Heterostructures

The metastable α-phase Ga₂O₃ is an emerging material for developing solar-blind photodetectors and power electronic devices toward civil and military applications. Despite its superior physical properties, the high quality epitaxy of metastable phase α-Ga₂O₃ remains challenging. To this end, single crystalline α-Ga₂O₃ epilayers are achieved on nonpolar ZnO (112̅0) substrates for the first time and a high performance Au/α-Ga₂O₃/ZnO isotype heterostructure-based Schottky barrier avalanche diode is demonstrated. The device exhibits self-powered functions with a dark current lower than 1 pA, a UV/visible rejection ratio of 10³ and a detectivity of 9.66 × 10¹² cm Hz^1/2 W^-1. Dual responsivity bands with cutoff wavelengths at 255 and 375 nm are observed with their peak responsivities of 0.50 and 0.071 A W^-1 at -5 V, respectively. High photoconductive gain at low bias is governed by a barrier lowing effect at the Au/Ga₂O₃ and Ga₂O₃/ZnO heterointerfaces. The device also allows avalanche multiplication processes initiated by pure electron and hole injections under different illumination conditions. High avalanche gains over 10³ and a low ionization coefficient ratio of electrons and holes are yielded, leading to a total gain over 10⁵ and a high responsivity of 1.10 × 10⁴ A W^-1. Such avalanche heterostructures with ultrahigh gains and bias-tunable UV detecting functionality hold promise for developing high performance solar-blind photodetectors.

A Solar-Blind UV Detector Based on Graphene-Microcrystalline Diamond Heterojunctions

An ultraviolet detector is demonstrated through a whole-wafer, thin diamond film transfer process to realize the heterojunction between graphene and microcrystalline diamond (MCD). Conventional direct transfer processes fail to deposit graphene onto the top surface of the MCD film. However, it is found that the 2 µm thick MCD diamond film can be easily peeled off from the growth silicon substrate to expose its smooth backside for the graphene transfer process for high-quality graphene/MCD heterojunctions. A vertical graphene/MCD/metal structure is constructed as the photodiode device using graphene as the transparent top electrode for solar-blind ultraviolet sensing with high responsivity and gain factor. As such, this material system and device architecture could serve as the platform for next-generation optoelectronic systems.

Zero-Power-Consumption Solar-Blind Photodetector Based on β-Ga2O3/NSTO Heterojunction

A solar-blind photodetector based on β-Ga₂O₃/NSTO (NSTO = Nb:SrTiO₃) heterojunctions were fabricated for the first time, and its photoelectric properties were investigated. The device presents a typical positive rectification in the dark, while under 254 nm UV light illumination, it shows a negative rectification, which might be caused by the generation of photoinduced electron-hole pairs in the β-Ga₂O₃ film layer. With zero bias, that is, zero power consumption, the photodetector shows a fast photoresponse time (decay time τ_d = 0.07 s) and the ratio I_photo/I_dark ≈ 20 under 254 nm light illumination with a light intensity of 45 μW/cm². Such behaviors are attributed to the separation of photogenerated electron-hole pairs driven by the built-in electric field in the depletion region of β-Ga₂O₃ and the NSTO interface, and the subsequent transport toward corresponding electrodes. The photocurrent increases linearly with increasing the light intensity and applied bias, while the response time decreases with the increase of the light intensity. Under -10 V bias and 45 μW/cm² of 254 nm light illumination, the photodetector exhibits a responsivity R_λ of 43.31 A/W and an external quantum efficiency of 2.1 × 10⁴ %. The photo-to-electric conversion mechanism in the β-Ga₂O₃/NSTO heterojunction photodetector is explained in detail by energy band diagrams. The results strongly suggest that a photodetector based on β-Ga₂O₃ thin-film heterojunction structure can be practically used to detect weak solar-blind signals because of its high photoconductive gain.

A simple synthesis of Ga2O3 and GaN nanocrystals

A simple top-down strategy to α-Ga₂O₃ and GaN nanocrystals is developed and the morphology-dependent optical properties of α-Ga₂O₃ crystals are established.

Graphene-β-Ga2 O3 Heterojunction for Highly Sensitive Deep UV Photodetector Application

A deep UV light photodetector is assembled by coating multilayer graphene on beta-gallium oxide (β-Ga₂ O₃ ) wafer. Optoelectronic analysis reveals that the heterojunction device is virtually blind to light illumination with wavelength longer than 280 nm, but is highly sensitive to 254 nm light with very good stability and reproducibility.

Self-Powered Solar-Blind Photodetector with Fast Response Based on Au/β-Ga2O3 Nanowires Array Film Schottky Junction

Because of the direct band gap of 4.9 eV, β-Ga2O3 has been considered as an ideal material for solar-blind photodetection without any bandgap tuning. Practical applications of the photodetectors require fast response speed, high signal-to-noise ratio, low energy consumption and low fabrication cost. Unfortunately, most reported β-Ga2O3-based photodetectors usually possess a relatively long response time. In addition, the β-Ga2O3 photodetectors based on bulk, the individual 1D nanostructure, and the film often suffer from the high cost, the low repeatability, and the relatively large dark current, respectively. In this paper, a Au/β-Ga2O3 nanowires array film vertical Schottky photodiode is successfully fabricated by a simple thermal partial oxidation process. The device exhibits a very low dark current of 10 pA at -30 V with a sharp cutoff at 270 nm. More interestingly, the 90-10% decay time of our device is only around 64 μs, which is much quicker than any other previously reported β-Ga2O3-based photodetectors. Besides, the self-powering, the excellent stability and the good reproducibility of Au/β-Ga2O3 nanowires array film photodetector are helpful to its commercialization and practical applications.

Electrochemical Energy Storage Application and Degradation Analysis of Carbon-Coated Hierarchical NiCo2S4 Core-Shell Nanowire Arrays Grown Directly on Graphene/Nickel Foam

We developed a new electrode comprising thin carbon layer coated hierarchical NiCo₂S₄ core-shell nanowire arrays (NiCo₂S₄@C CSNAs) on graphene/Ni foam (Ni@G) substrates. The electrode showed outstanding electrochemical characteristics including a high specific capacitance of 253 mAh g⁻¹ at 3 A g⁻¹, high rate capability of 163 mAh g⁻¹ at 50 A g⁻¹ (~64.4% of that at 3 A g⁻¹), and long-term cycling stability with a capacity retention of 93.9% after 5000 cycles. Comparative studies on the degradation of hierarchical NiCo₂S₄ CSNA electrodes with and without carbon coatings revealed that the morphology pulverization, structural separation at core/shell interface, and irretrievably chemical composition change of NiCo₂S₄ CSNAs electrode are major factors that deteriorate the electrochemical performance of the electrodes without carbon coating. The favorable roles of carbon coatings on hierarchical NiCo₂S₄ CSNAs were further clarified: (1) serving as a physical buffering layer that suppresses the structural breakdown; (2) retarding the chemical composition conversion of the NiCo₂S₄ CSNAs; and (3) providing extra path for charge transition in addition to the NiCo₂S₄ core nanowires. Understanding of the degradation mechanisms and the significance of the surface carbon coatings would provide useful guidelines for the design of new electrode materials for high-performance electrochemical devices.

Nanostructured Photodetectors: From Ultraviolet to Terahertz

Inspired by nanoscience and nanoengineering, numerous nanostructured materials developed by multidisciplinary approaches exhibit excellent photoelectronic properties ranging from ultraviolet to terahertz frequencies. As a new class of building block, nanoscale elements in terms of quantum dots, nanowires, and nanolayers can be used for fabricating photodetectors with high performance. Moreover, in conjunction with traditional photodetectors, they exhibit appealing performance for practical applications including high density of integration, high sensitivity, fast response, and multifunction. Therefore, with the perspective of photodetectors constructed by diverse low-dimensional nanostructured materials, recent advances in nanoscale photodetectors are discussed here; meanwhile, challenges and promising future directions in this research field are proposed.

Solar-Blind Avalanche Photodetector Based On Single ZnO-Ga₂O₃ Core-Shell Microwire

High-performance solar-blind (200-280 nm) avalanche photodetectors (APDs) were fabricated based on highly crystallized ZnO-Ga2O3 core-shell microwires. The responsivity can reach up to 1.3 × 10(3) A/W under -6 V bias. Moreover, the corresponding detectivity was as high as 9.91 × 10(14) cm·Hz(1/2)/W. The device also showed a fast response, with a rise time shorter than 20 μs and a decay time of 42 μs. The quality of the detectors in solar-blind waveband is comparable to or even higher than that of commercial Si APD (APD120A2 from Thorlabs Inc.), with a responsivity ∼8 A/W, detectivity ∼10(12) cm·Hz(1/2)/W, and response time ∼20 ns. The high performance of this APD make it highly suitable for practical applications as solar-blind photodetectors, and this core-shell microstructure heterojunction design method would provide a new approach for realizing an APD device.

High gain Ga₂O₃ solar-blind photodetectors realized via a carrier multiplication process

Ga2O3 photodetectors with interdigitated electrodes have been designed and fabricated, and the Ga2O3 area exposed to illumination acts as the active layer of the photodetector, while the area covered by Au interdigital electrode provide an arena for carrier multiplication. The photodetectors show a maximum responsivity at around 255 nm and a cutoff wavelength of 260 nm, which lies in the solar-blind region. The responsivity of the photodetector reaches 17 A/W when the bias voltage is 20 V, which corresponds to a quantum efficiency of 8228%, amongst the best value ever reported in Ga2O3 film based solar-blind photodetectors.

Synthesis and characterization of monodispersed β-Ga₂O₃ nanospheres via morphology controlled Ga₄(OH)₁₀SO₄ precursors

To our best knowledge, monodispersed β-Ga2O3 nanospheres were successfully synthesized for first time via morphology-controlled gallium precursors using the forced hydrolysis method, followed by thermal calcination processes. The morphology and particle sizes of the gallium precursors were strongly dependent on the varying (R = SO4(2-)/NO3(-)) concentration ratios. As R decreased, the size of the prepared gallium precursors decreased and morphology was altered from sphere to rod. The synthesized S2 (R = 0.33) consists of uniform and monodispersed amorphous nanospheres with diameters of about 200 nm. The monodispersed β-Ga2O3 nanospheres were synthesized using thermal calcination processes at various temperatures ranging from 500 to 1000 °C. Monodispersed β-Ga2O3 nanospheres (200 nm) consist of small particles of approximately 10-20 nm with rough surface at 1000 °C for 1 h. The UV (375 nm) and broad blue (400-450 nm) emission indicate recombination via a self-trapped exciton and the defect band emission. Our approach described here is to show the exploration of β-Ga2O3 nanospheres as an automatic dispersion, three-dimensional support for fabrication of hierarchical materials, which is potentially important for a broad range of optoelectronic applications.

Enhanced performance of a TiO2 ultraviolet detector modified with graphene oxide

The performance of a Schottky metal–semiconductor–metal (MSM) ultraviolet (UV) photodetector is limited by the insufficient gain and the uncontrollable noise current.