Light detection in the deep-ultraviolet (DUV) solar-blind waveband has attracted interest due to its critical applications, especially in safety and space detection. A DUV photodetector based on wide-bandgap semiconductors provides a subversive scheme to simplify the currently mature DUV detection system. As an ultra-wide-bandgap (4.4–5.3 eV) semiconductor directly corresponding to the DUV solar-blind waveband, Ga2O3 has an important strategic position in the prospective layout of semiconductor technology owing to its intrinsic characteristics of high breakdown electric field, excellent tolerance of high/low temperature, high resistance to radiation, and rich material systems. As the only native substrate that can be fabricated from melt-grown bulk single crystals, β-Ga2O3 has attracted a lot of attention both in power-electronic and photo-electronic devices. In addition, other metastable phases (e.g. α, ϵ, γ) of Ga2O3 have attracted great interest due to their unique properties. In this work, we discuss the advances in achieving bulk and film Ga2O3 materials with different crystal phases. In addition, the latest achievements with polymorphous Ga2O3-based solar-blind photodetectors (SBPDs) and the methods to enhance their performance, including doping, annealing, and transparent electrodes, are also discussed. Furthermore, as the most desirable application, DUV imaging technologies based on Ga2O3 SBPDs are systematically summarized. Finally, conclusions regarding recent advances in Ga2O3 SBPDs, remaining challenges, and prospects are presented and discussed.
photodetector (SBPD), as an indispensable part of spectral detectors, plays important roles distinctively in various crucial applications, such as missile tracking, flame prewarning, secure communication, and environment monitoring. [1][2][3][4] In terms of the possible harsh application environment, high-performance SBPDs with excellent tolerance towards high temperature, high voltage, and high radiation, are required inevitably. Based on low cost and mature technology, the currently available Si-based SBPDs are facing the challenges of filter dependence, low penetration depth of high-energy ultraviolet (UV) photons, and finite responsivity (R) towards solar-blind region. [5][6][7] Especially, Si-based SBPDs suffer serious thermal instability owing to the narrow bandgap, which suppresses their high-temperature applications. Advanced SBPDs based on wide bandgap (WBG) materials, such as MgZnO, [8] AlGaN, [9,10] diamond, [11] and Ga 2 O 3 , [12] are believed as subversive substitutes of Si-based SBPDs. Among the various WBG materials, Ga 2 O 3 is the most desirable candidate for SBPDs applications based on the facts that, i) its ultra-wide bandgap (4.5-4.9 eV) corresponds to the solar-blind region directly without the necessity of bandgap modulation by doping or alloying process; [13] ii) its high absorption coefficient for high-energy UV photons benefits outstanding sensitivity in solar-blind region; [14] iii) it balances high solar-bind response and material workability; [15] iv) its large-size bulk single crystals can be put into mass production by low-cost melt-grown methods; [16] and most importantly, v) it has high structural stability toward temperature, radiation, and electric field for harsh-environment application. [17] High-quality Ga 2 O 3 material, including single-crystal substrates, nanostructures, and epitaxial films, [14,18,19] facilitate sharp junction interface, such as P-N heterojunction, [20] N-N heterojunction, [21] phase junction, [22] and Schottky junction, [23] to improve the solar-blind response performance. Nevertheless, so far, high-performance SBPDs based on high-quality Ga 2 O 3 Gallium oxide (Ga 2 O 3 ), with an ultrawide bandgap, is currently regarded as one of the most promising materials for solar-blind photodetectors (SBPDs), which are greatly demanded in harsh environment, such as space exploration and flame prewarning. However, realization of high-performance SBPDs with high tolerance toward harsh environments based on low-cost Ga 2 O 3 material faces great challenges. Here, defect and doping (DD) engineering towards amorphous GaO X (a-GaO X ) has been proposed to obtain ultrasensitive SBPDs for harsh condition application. Serious oxygen deficiency and doping compensation of the engineered a-GaO X film ensure the high response currents and low dark currents, respectively. Annealing item in nitrogen of DD engineering also incurs the recrystallization of material, formation of nanopores by oxygen escape, and suppression of sub-bandgap defect states. As a result, the tailored GaO...
Metal oxide thin-films transistors (TFTs) produced from solution-based printing techniques can lead to large-area electronics with low cost. However, the performance of current printed devices is inferior to those from vacuum-based methods due to poor film uniformity induced by the “coffee-ring” effect. Here, we report a novel approach to print high-performance indium tin oxide (ITO)-based TFTs and logic inverters by taking advantage of such notorious effect. ITO has high electrical conductivity and is generally used as an electrode material. However, by reducing the film thickness down to nanometers scale, the carrier concentration of ITO can be effectively reduced to enable new applications as active channels in transistors. The ultrathin (~10-nm-thick) ITO film in the center of the coffee-ring worked as semiconducting channels, while the thick ITO ridges (>18-nm-thick) served as the contact electrodes. The fully inkjet-printed ITO TFTs exhibited a high saturation mobility of 34.9 cm2 V−1 s−1 and a low subthreshold swing of 105 mV dec−1. In addition, the devices exhibited excellent electrical stability under positive bias illumination stress (PBIS, ΔVth = 0.31 V) and negative bias illuminaiton stress (NBIS, ΔVth = −0.29 V) after 10,000 s voltage bias tests. More remarkably, fully printed n-type metal–oxide–semiconductor (NMOS) inverter based on ITO TFTs exhibited an extremely high gain of 181 at a low-supply voltage of 3 V, promising for advanced electronics applications.
Self‐powered solar‐blind photodiodes with convenient operation, easy fabrication, and weak‐light sensitivity, are highly desired in environmental monitoring and deep space exploration. Ga2O3 with its bandgap directly corresponding to solar‐blind waveband is a promising candidate material for solar‐blind photodetection. However, ever‐reported self‐powered Ga2O3 photodiodes suffer unsatisfactory photoresponse performance, owing to unideal interface and electrode transmittance. Here, Ag nanowire (AgNW) networks with excellent solar‐blind ultraviolet transmittance are introduced to form self‐powered AgNW–Ga2O3 photodiodes with sharp Schottky interfaces. The tradeoff between solar‐blind ultraviolet transmittance and carrier‐collection ability of the sparse AgNW network is systematically studied and the AgNW density is optimized for the best photoresponse. Expansion of depletion region outwards the AgNW–Ga2O3 contact and the field crowding effect facilitate the high photoresponse. As a result, the champion AgNW–Ga2O3 Schottky photodiode exhibits excellent sensitivity for weak‐light detection, including considerable responsivity of 14.8 mA W–1, ultrahigh photo‐to‐dark‐current ratio above 1.2 × 105, high rejection ratio (R254 nm/R365 nm) of 2.6 × 103, and fast response speed (rise/decay time of 20/24 ms) under self‐powered mode. Balancing the transmittance and carrier‐collection ability of elaborate electrode provides an alternative strategy to achieve high‐performance self‐powered Ga2O3 photodetectors for future weak‐light‐sensitive optoelectronic systems.
backgrounds and high signal-to-noise ratio for fire monitoring, corona detection, medical imaging, biological monitoring, and space exploration. [6,7] Rigid commercial silicon-based photomultiplier tubes with inevitable filters are dominated in the market, while facing the challenges of thermal intolerance, low responsivity, high cost, and architectural complexity of the detection system. [8,9] Flexible DUV photodetectors, with the characteristics of harshenvironment resistance, high sensitivity to DUV light, and light weight, conform to the ever-increasing requirements of DUV detection, especially for space exploration and wearable electronics. [10,11] Great efforts have been paid to develop flexible DUV photodetectors by the combination of organic/inorganic materials, elastic substrates, and assistant technologies (e.g., spray coating and transferring methods). [12][13][14][15][16][17][18] As reported by Qiu et al., photodetectors and arrays based on nanofibrils of P3HT-b-PHA are demonstrated with excellent flexibility for highly selective DUV image sensing application. [12] Assisted by a spray coating or transferring method to a flexible PET substrate, the ZnO quantum dots or AlGaN/GaN heterostructures not only maintain the excellent quality of the original materials, but also exhibit high ultraviolet photodetection performance and robust stability under bent condition. [14,16] The high photoresponse characteristics under stress conditions of the ZnO nanocrystal network, h-BN nanosheets, and Ga 2 O 3 films demonstrate the High deep-ultraviolet (DUV) sensitivity and excellent flexibility of ultrathin gallium oxide (Ga 2 O 3 ) film with an ultrawide bandgap endow its extreme propensity in flexible DUV photodetector especially for space exploration and wearable electronics. However, an efficient strategy with high throughput and low cost is highly deficient to realize flexible and robust Ga 2 O 3 DUV photodetectors to face potential harsh environments. In this work, flexible and heat-resistant Ga 2 O 3 DUV photodetectors based on optimized inkjet printing with environmental-friendly aqueous solvent are demonstrated. The dynamic evolution from Ga(NO 3 ) 3 precursor to crystalline Ga 2 O 3 film has been explicitly uncovered. Photodetectors based on printed ultrathin Ga 2 O 3 films on rigid substrate exhibit outstanding performance, including high photo-to-dark current ratio about 10 6 , considerable responsivity of 1.3 A W −1 , superior detectivity of 1.46 × 10 14 Jones, and fast decay time of 0.026 s under 254 nm illumination. In addition, flexible devices on mica substrate not only retain outstanding photo electrical performance, but also demonstrate excellent mechanical flexibility and thermal stability. Moreover, benefiting from the uniformity of the pixels by high-throughput inkjet printing, the Ga 2 O 3 DUV photodetector array presents excellent sharp-imaging capability. This work provides a feasible strategy for printable, flexible, and harsh-environment-resistant Ga 2 O 3 DUV photodetectors toward space exp...
High tunability of photoresponse characteristics under work conditions is desired for a single solar-blind photodetector to be applied in multifarious fields. Three-terminal metal–oxide–semiconductor field-effect phototransistors have shown excellent controllability of performance, but the hysteresis issue impedes their stable operation. In this work, the metal–semiconductor field-effect phototransistor based on the exfoliated Ga2O3 microflake and graphene thin film is demonstrated. The high-quality quasi-van der Waals interface between Ga2O3 and graphene eliminates the hysteresis issue and generates a subthreshold swing as low as 69.4 mV/dec. By regulating gate voltage (Vg), the dominated mechanism of photocurrent generation in the device can be tuned continuously from the fast photoconduction effect to photogating effect with high photogain. Accordingly, the responsivity, dark current, detectivity, rejection ratio, and decay time of the device can be well adjusted by the Vg. At Vg = −1 V and a source to drain voltage of 2 V, the device shows excellent performance with a responsivity of 2.82 × 103 A/W, a rejection ratio of 5.88 × 105, and a detectivity of 2.67 × 1015 Jones under 254 nm illumination. This work shows the possibility of realizing highly tunable solar-blind photodetectors to meet the requirements for different application fields by introducing gate voltage modulation.
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