Solar-blind Ultraviolet Photodetectors Research Articles

Solar-blind ultraviolet photodetectors with thermally reduced graphene oxide formed on high-Al-content AlGaN layers

Solar-blind deep-ultraviolet (UV) photodetectors (PDs) with high responsivity and fast response have attracted significant attention in environmental, industrial, biological, and military applications. AlGaN is a representative semiconductor material in the field of solar-blind detection; semiconductor performance can be accelerated by combining it with high-transparency, high-stability contact electrode materials. In this study, solar-blind deep-UV metal–semiconductor–metal (MSM) PDs were fabricated based on two-dimensional reduced graphene oxide (rGO) contacts formed on various high-Al-content AlGaN semiconductors. A low dark current in the order of a few picoamperes and a fast photoresponse time of a few tens of milliseconds were confirmed. The investigation of the effects of front- and back-side illumination showed that the photocurrents and corresponding responsivities of the PDs drastically improved under back-side illumination. In detail, the peak locations of the responsivity–wavelength curves were downshifted from 290 nm with a responsivity of 0.0518 A/W for the rGO/Al0.5Ga0.5N MSM PD to 250 nm with a responsivity of 0.0113 A/W for the rGO/Al0.7Ga0.3N MSM PD under back-side illumination. These results indicate that rGO contacts on AlGaN provide a viable approach for developing solar-blind deep-UV PDs.

Preparation of all-oxide β-Ga2O3/α-MoO3 heterojunction towards self-driven deep ultraviolet photosensor

A self-driven all-oxide β-Ga2O3/α-MoO3 heterojunction solar-blind ultraviolet (UV) photodetector is introduced in this work. The photodetector shows photo responsivity (R) of 0.59 mA W−1, specific detectivity (D*) of 1011 Jones and linear dynamic region (LDR) of 162.9 dB at 10 V, and R of 0.26 mA W−1, D* of 4 1010 Jones and LDR of 89.44 dB at −10 V. In addition, it could also operate repeatably and stably at zero bias, illustrating that it is a self-driven photodetector. In one word, the fabricated β-Ga2O3/α-MoO3 heterojunction has a potential to work as a self-driven solar-blind UV sensing device.

Zn/Mg co-alloyed for higher photoelectric performance and unchanged spectral response in β-Ga2O3 solar-blind photodetector

Alloying with the divalent elements (such as Zn and Mg) in β-Ga2O3 thin film can reduce the dark current and improve the photorespond speed for the solar-blind ultraviolet (UV) photodetector. However, alloying only with one element will change the spectral sensitivity out of solar-blind area due to an increase of band gap for the Mg alloyed and a decrease for Zn alloyed. Herein, a unchange band gap in β-Ga2O3 thin films were obtained by Zn/Mg co-alloyed through radio frequency magnetron sputtering. The photodetector based on Zn/Mg co-alloyed β-Ga2O3 thin films shows a low dark current (6.2 × 10−12 A at 1 V), a high ratio of photo-to-dark current (1.8 × 104), a high responsivity of 25.7 mA W−1 (to 254 nm), a high rejection ratio (R 254 nm/R 280 nm) of 1.2 × 103 under the bias of 10 V, and a fast photoresponse time (less than 0.5 μs for the rise time), which can be practically used to detect weak solar-blind deep UV signals.

Post-annealing effects on RF sputtered all-amorphous ZnO/SiC heterostructure for solar-blind highly-detective and ultralow dark-noise UV photodetector

The rapid progress of wide band gap SiC semiconductor material opens up new opportunities to develop efficient monolithically integrated ultraviolet (UV) photonic and power systems for a wide range of advanced applications. In this paper, low-noise solar-blind UV photodetector (PD) based on all-amorphous ZnO/SiC heterostructure was fabricated via RF magnetron sputtering technique. The device structural and optical properties were investigated before and after thermal treatment at different annealing temperature values varying from 300 °C to 600 °C. UV-Visible spectroscopy revealed that the annealing process has a beneficial effect in terms of high UV absorbance and solar-blindness properties. Photoelectrical characterization demonstrated the high UV photoresponse and low dark noise of the prepared UV PD based on all-amorphous ZnO/SiC structure. Improvement of the device performances were achieved by an appropriate annealing process. After post-annealing, the thermally treated ZnO/SiC UV PD at 500 °C exhibits a high detectivity of 2.4 × 1012 Jones, high signal to noise ratio of 2.64×105 and a giant UV–Vis rejection ratio of 5.9 × 103. Therefore, the present study may provide new perspectives for fabricating ultralow dark noise solar-blind UV PD based on all-amorphous ZnO/SiC heterostructure, which promotes the development of integrated UV photonic systems based on SiC platform.

Fully transparent InZnSnO/β-Ga2O3/InSnO solar-blind photodetectors with high schottky barrier height and low-defect interfaces

Ultra-wide bandgap β-Ga2O3 photodiodes have received great attention due to transparent solar-blind deep ultraviolet photodetectors. We demonstrated an all oxide-based single-crystal β-Ga2O3 photodiode incorporated with transparent conductive InZnSnO and InSnO as Schottky and Ohmic contacts, respectively. High work function and low resistivity of InZnSnO allow for clear rectifying characteristics with a low dark current (<0.1 nA) of up to −100 V of reverse bias. The Schottky barrier height and junction defects at the Schottky interface were modified using post-annealing treatment, thereby influencing the deep ultraviolet photoresponse. The responsivity of the annealed device was 9.6 mA/W, decreasing the Schottky barrier height engineering, while the photo responding speed was degraded and caused a persistent photocurrent effect due to the generation of defect states.

Two-dimensional Al2O3 with ultrawide bandgap and large exciton binding energy for solar-blind ultraviolet photodetectors

New Al2O3 monolayers were predicted using density functional theory, and their stability, geometric structures, and mechanical, electronic, and optical properties were investigated. Among them, Te-Al2O3 with Pmmb space group was the most stable structure with markedly mechanical anisotropy. At the HSE06 level, Te-Al2O3 exhibited an ultra-wide indirect bandgap of 6.338 eV, which was verified to have a relatively reliable value by contrastive analysis on experiments. An exciton with a large binding energy of 1.356 eV was revealed in Te-Al2O3 by solving Bethe–Salpeter equation, and such large binding energy intensely affected the optical properties. As a result, the anisotropic absorption spectra were located at ~4.0–16 eV, nearly covering the spectrum ranging from the near- ultraviolet (UV) to far-UV regions. In particular, the first optical absorption peak appearing at ~5.0 eV indicated that Te-Al2O3 is suitable for solar-blind photodetectors. This work revealed that the ultra-wide bandgap semiconductor Te-Al2O3 has a great application in UV photodetection.

An enhanced solar-blind ultraviolet photodetector based on polyvinyl alcohol/carbon nanodots film

An enhanced solar-blind ultraviolet photodetector based on polyvinyl alcohol/carbon nanodots film

A study for the influences of temperatures on ZnGa2O4 films and solar-blind sensing performances

In this work, ZnGa2O4 metal–semiconductor–metal solar-blind ultraviolet (UV) photodetectors were fabricated and discussed by using the radio-frequency (r.f.) magnetron sputtering method with considerations of the influences of growing and measuring temperatures on photoelectric performances in detail. With the increase of growing temperature, the grain size and optical bandgap of the films increased. Ohmic contacts were formed between the films and Ti/Au electrodes for all the samples, and the photoconductive photodetectors shown decent solar-blind photoelectric performances. At 10 V biasing voltage, the devices had fast response speed and large photo-to-dark current ratio of ∼107 at 650 °C. Moreover, we measured the dark current (I dark) and photocurrent (I photo) of these devices in the temperature range from 298 K to 598 K. With the increase of temperature, the I dark increased monotonously, while the I photo first decreased and then increased. Furthermore, this study about the effects of temperatures on the ZnGa2O4 films and photodetectors may well provide a meaningful guide for its relevant developments in the future.

Fabrication and properties of a solar-blind ultraviolet photodetector based on Si-doped β-Ga2O3 film grown on p-Si (111) substrate by MOCVD

A solar-blind ultraviolet photodetector based on p-Si/n-β-Ga2O3:Si heterojunction, using the Si-doped β-Ga2O3 as the n-type layer, was fabricated successfully by metal-organic chemical vapor deposition (MOCVD). The current-voltage and photoresponse characteristics of photodetector were investigated. The fabricated p-Si/n-β-Ga2O3:Si heterojunction photodetector exhibited typical rectification behavior and good solar-blind ultraviolet photoresponse. Sensitive photodetection of current at the reverse bias voltage and stable on-off switching performance were achieved. The photodetector showed a responsivity of 3.76 A/W and a photo-to-dark current ratio of 37.9 under 254 nm illumination. The rise time constants (τr1, τr2) and the decay time constants (τd1, τd2) of the photodetector under 254 nm illumination at the reverse voltage of − 20 V were estimated to be (0.30 s, 0.76 s) and (0.15 s, 6.15 s), respectively. The fabricated p-Si/n-β-Ga2O3:Si heterojunction photodetector displayed relatively faster response speed compared to the reported photodetectors based on p-Si/undoped β-Ga2O3 heterojunction. The results indicate that photodetectors based on p-Si/n-β-Ga2O3:Si heterojunction have potential development for the high-performance solar-blind ultraviolet photodetectors.

Performance enhancement of a self-powered solar-blind UV photodetector based on ZnGa2O4/Si heterojunction via interface pyroelectric effect

The photodetectors based on the wide bandgap semiconductor (WBS)/Si heterojunction have attracted more and more attention in recent years due to their excellent photoelectric characteristics and easy integration capabilities. In this work, we have demonstrated a self-powered solar-blind ultraviolet (UV) photodetector based on the ZnGa2O4/Si heterojunction. A typical rectification characteristic with a rectification ratio exceeding 103 within ±5 V can be obtained. At 0 V bias, the −3 dB cutoff wavelength of ∼255 nm and the UV-visible rejection ratio of ∼3 × 102 show that the device has excellent self-powered solar-blind UV detection performance. In addition, the responsivity and the response speed of ZnGa2O4/Si heterojunction can be efficiently enhanced by a transient spike current at 0 V bias when turning on and off the 254 nm UV light. The interface pyroelectric effect of the ZnGa2O4 film should be responsible for this transient spike photocurrent phenomenon. Our findings in this work pave a feasible way to realize high-performance WBS/Si heterojunction self-powered solar-blind photodetectors.

Self-powered solar-blind ultraviolet photodetector based on α-Ga2O3 nanorod arrays fabricated by the water bath method

In this paper, α-Ga2O3 nanorod arrays (NRAs) with preferential growth along the (110) direction were successfully prepared on the FTO substrate by the water bath method. With the help of a scanning electron microscope (SEM), X-ray diffractometer (XRD), and Raman spectrometer (Raman), the crystal structure and morphology characteristics of α-Ga2O3 NRAs were studied. On this basis, a photoelectrochemical (PEC) solar-blind ultraviolet photodetector based on the α-Ga2O3 NRAs was fabricated, and the photoelectric performance of the device was analyzed in detail through the PEC test system, and the working mechanism of the device was further discussed. The results show that the prepared α-Ga2O3 NRAs have good crystal quality which is closely arranged on the substrate and a quadrangular prism shape from the top view. The constructed α-Ga2O3 NRAs PEC photodetector shows typical solar-blind ultraviolet response characteristics and stable self-powered ability. Meanwhile, the device exhibited a high photo-dark current ratio (PDCR), responsivity (R) and detectivity (D*) of 1.01×103, 11.34 mA/W and 2.68×1011 Jones, respectively, as well as superior wavelength selectivity and fast response. This work confirms that α-Ga2O3 NRAs prepared by the water bath method have potential application prospects in highly sensitive and fast response PEC self-powered solar-blind ultraviolet photodetectors.

Heteroepitaxial growth of high Mg-content single-phased W-MgZnO on ZnO matrixes in various nucleation states for solar-blind and visible-blind dual-band UV photodetectors

High Mg-content single-phase wurtzite Mg0.51Zn0.49O (W-Mg0.51Zn0.49O) films were realized by radio frequency (RF) magnetron sputtering technique on quartz (SiO2) substrates. The metal-semiconductor-metal (MSM) solar-blind (SB) and visible-blind (VB) dual-band ultraviolet (DB-UV) photodetectors (PDs) were also fabricated based on heteroepitaxial growth of W-Mg0.51Zn0.49O films on ZnO buffer layer (BL) at different nucleation states. Under 270 nm of irradiation, the Au/Mg0.51Zn0.49O/ZnO (30 min) PD exhibited an optimal external quantum efficiency (EQE, 304.4%) and specific detectivity (1012), which indicated that the ZnO BL obtained with a 30 min treatment time was most efficient to improve the detection of SB-UV wavelength. The origin of these results was attributed to the contact area and adhesion strength between MgZnO and ZnO matrices. The results showed a feasible way to obtain high Mg-content wurtzite MgZnO alloys and performance improvement through ZnO BL, which is favorable to the applicability in SB/VB DB-UV PDs.

Photodetectors: Cross‐Substitution Promoted Ultrawide Bandgap up to 4.5 eV in a 2D Semiconductor: Gallium Thiophosphate (Adv. Mater. 22/2021)

In article number 2008761, Congxin Xia, Zhongming Wei, and co-workers describe substitution with lighter elements significantly increasing the bandgap of GaPS4, and simultaneously inheriting its in-plain anisotropy. GaPS4 thus becomes a promising candidate material for solar-blind ultraviolet and polarization-sensitive photodetectors. The balance demonstrates the main idea of the cross-substitution, with edge-sharing [GeS4]–[GeS4] atomic packing in the left pan and the tetrahedra centered by lighter elements (Ga and P) forming the [GaS4]–[PS4] motif in the right pan.

Energy Down-Conversion Cs3Cu2Cl5 Nanocrystals for Boosting the Efficiency of UV Photodetector

Zero-dimension (0-D) lead halide perovskite nanocrystals (NCs) have attracted a sight of interest in the field of optoelectronic devices due to their outstanding properties, such as high photoluminescence quantum yield (PLQY) and size- and composition-controlled tunable emission wavelengths. However, the toxicity of lead (Pb) element in the lead perovskite NCs is the bottleneck for the commercial application of perovskite NCs. Herein, we report a facile ligand-assisted synthesis to achieve lead-free Cs3Cu2Cl5 NCs with a high PLQY of ∼70% and good stability against environmental oxygen/moisture as a promising down-conversion material. It has good merits of high PLQY and large Stokes shift (∼300 nm) originated from the effect of Jahn–Teller distortion and self-trapped excitons (STEs). Furthermore, the Cs3Cu2Cl5 NCs embedded composite films (NCCFs) were utilized to enhance the ultraviolet (UV) response of silicon (Si) photodetectors. External quantum efficiency (EQE) measurements show that the UV response can be greatly improved from 3.3 to 19.9% @ 295 nm based on NCCFs combined with Si photodiodes. Our work offers an effective approach to develop highly efficient and stable lead-free Cs3Cu2Cl5 NCs for the application in the solar-blind UV photodetector.

Influence of N2O/TEGa Ratio on Deposition of β-Ga2O3 Films and Performance of Au-β-Ga2O3-Au Solar-Blind Photodetectors

In this study, a series of β-Ga2O3 films are prepared by using triethylgallium (TEGa) and nitrous oxide (N2O) as precursors to explore the effect of N2O/TEGa ratio on the characteristics of β-Ga2O3 films. A metal/semiconductor/metal (MSM)-type solar blind ultraviolet (UV) photodetector (PD) is fabricated using as-prepared β-Ga2O3 film. It is found that an increment of N2O/TEGa ratio tends to suppress the oxygen vacancies in β-Ga2O3 film so the device performance can be significantly improved. This work gives a deep insight into the impact of TEGa/N2O ratio for depositing β-Ga2O3 on the film quality, the surface morphology, the chemical composition and the device performance for UV PDs.

Effect of off-axis substrate angles on β-Ga2O3 thin films and solar-blind ultraviolet photodetectors grown on sapphire by MOCVD

β-Ga2O3 thin films were heteroepitaxially grown on c-plane sapphire substrates with different off-axis angles toward <11‾00> direction by metalorganic chemical vapor deposition (MOCVD), where the crystal quality of the films and the deep-ultraviolet photodetectors (PDs) fabricated on these films were characterized. The effect of sapphire substrates with different off-axis angles on the β-Ga2O3 thin films and the optical performance of β-Ga2O3 PDs are investigated for the first time. (2‾01)-oriented β-Ga2O3 thin films were grown on sapphire substrates with off-axis angles of 0°, 1.5°, 4°, and 6°. The full width at half maxima (FWHM) of rocking curves decreased significantly with the increase of off-axis angles. The β-Ga2O3 thin films grown on the 6° off-axis angle substrate showed a FWHM of 0.55°, indicating excellent crystal quality. Substrate off-axis angles play a critical role in the nucleation process in the growth of β-Ga2O3 thin films. The step edge on the off-axis substrates serves as the preferential incorporation sites for Ga adatoms. The step terrace width is controlled by the off-axis angle and influences the growth modes depending on the relative magnitude of the step terrace width compared with the Ga adatom diffusion length. In addition, β-Ga2O3 photodetectors with a metal-semiconductor-metal (MSM) structure have been fabricated using these β-Ga2O3 thin films. The β-Ga2O3 PDs fabricated on 6° off-axis sapphire substrate showed excellent optical performance with a large photocurrent of 61 μA, an outstanding photo/dark current ratio of 2.3 × 103, and peak responsivity of 74.6 mA/W. These results can provide important guidance on heterogeneous epitaxy of β-Ga2O3 thin films and photodetectors on cost-effective mass-producible sapphire substrates by MOCVD.

Highly solar-blind ultraviolet selective metal-semiconductor-metal photodetector based on back-illuminated AlGaN heterostructure with integrated photonic crystal filter

In this paper, we proposed a back-illuminated metal-semiconductor-metal AlGaN heterostructure solar-blind ultraviolet (UV) photodetector integrated with a SiO2/Si3N4 one-dimensional photonic crystal (1D PC). The light absorption outside solar-blind ultraviolet caused by impurity and defect energy levels is significantly suppressed by the 1D PC. The fabricated device exhibits extremely low dark current of 2 pA at 20 V applied voltage, where light/dark current ratio exceeds 4000. Meanwhile, the photodetector demonstrates a manifest narrow-band solar-blind detection property. The optical modulations of heterostructure energy-band engineering and photonic crystal filter both contribute to the solar-blind absorbing selectivity. In addition, the finger-scaling effects are also investigated based on carrier transport mechanism. These results are anticipated to promote the evolution on design and fabrication of solar-blind UV photodetector.

Stable and Self-Powered Solar-Blind Ultraviolet Photodetectors Based on a Cs3Cu2I5/β-Ga2O3 Heterojunction Prepared by Dual-Source Vapor Codeposition

Self-powered solar-blind ultraviolet (UV) photodetectors have drawn worldwide attention in recent years because of their important applications in military and civilian areas. In this study, a dual-source vapor codeposition technique was employed, for the first time, to prepare a nontoxic copper halide Cs3Cu2I5, which was integrated with the β-Ga2O3 wafer to construct a type-II heterojunction for photodetection applications. By optimizing the annealing conditions, high-quality Cs3Cu2I5 films with dense morphology, high crystallinity, and a long carrier lifetime of 1.02 μs were acquired. Because of the high material integrity of Cs3Cu2I5 films and effective interfacial carrier transfer from Cs3Cu2I5 to β-Ga2O3, a heterojunction device demonstrates a good solar-blind UV response property and operates at zero bias. Typically, the photodetector presents a low dark current (∼1.2 pA), a high solar-blind/UVA rejection ratio (∼1.0 × 103), a relatively fast photoresponse speed (37/45 ms), and a high photo-to-dark current ratio (∼5.1 × 104) at zero bias. Moreover, even after 12-h continuous working and 2-month storage without encapsulation in ambient air, the photodetection ability of the device can almost be maintained, demonstrating outstanding air stability. Our results suggest that nontoxic Cs3Cu2I5 is able to serve as a prospective candidate for stable solar-blind UV photodetection.

Photoresponsive characteristics of EFG-grown iron-doped (100) Ga2O3 substrate with low dark current

Low dark current is more desired in photodetector, that contributes to high photo-to-dark current ratio and high sensitivity. Therefore, in this paper, we use the edge-defined-film-fed (EFG)-grown iron (Fe)-doped Ga2O3 substrate to fabricate a solar-blind ultraviolet (UV) photodetector. Fortunately, the photodetector shows a low dark current of ∼10–13 A, which are more promised to advance the solar-blind photodetections. The responsivity (R) of 0.145 mA W−1, specific detectivity (D*) of 6 1010 cm Hz1/2 W−1 (Jones), linear dynamic region (LDR) of 75.56 dB and rise/decay time of 0.28 s/0.43 s are achieved in this Fe-doped β-Ga2O3 solar-blind UV photodetector.

Amorphous gallium oxide (a-Ga2O3)-based high-temperature bendable solar-blind ultraviolet photodetector

A bendable and thermally stable solar-blind ultraviolet (UV) photodetector has been demonstrated based on Ni/amorphous Ga2O3 (a-Ga2O3)/a-AlN/Cu foil structure. Here, Cu foil can simultaneously act as a bendable substrate and withstand a high-temperature environment. The ultra-wide bandgap a-AlN insulating layer can withstand mechanical tensile stress and effectively act as an insulating layer between a-Ga2O3 and Cu. Thus, the a-Ga2O3-based photodetector shows stable UV response characteristics with different bending radii and temperatures. The photodetector has high responsivity of 0.518 A W−1 and a fast response time of 0.17 s under 200 °C temperature with a 1.46 cm bending radius. With exceptional bendability and thermal stability, this a-Ga2O3-based photodetector has potential applications in harsh environments such as high-power bendable electron devices, flame detection, etc.