Deep Ultraviolet Photodetectors Research Articles

Highly sensitive solar-blind deep ultraviolet photodetector based on graphene/PtSe2/β-Ga2O3 2D/3D Schottky junction with ultrafast speed

There is an emerging need for high-sensitivity solar-blind deep ultraviolet (DUV) photodetectors with an ultra-fast response speed. Although nanoscale devices based on Ga2O3 nanostructures have been developed, their practical applications are greatly limited by their slow response speed as well as low specific detectivity. Here, the successful fabrication of two-/three-dimensional (2D/3D) graphene (Gr)/PtSe2/β-Ga2O3 Schottky junction devices for high-sensitivity solar-blind DUV photodetectors is demonstrated. Benefitting from the high-quality 2D/3D Schottky junction, the vertically stacked structure, and the superior-quality transparent graphene electrode for effective carrier collection, the photodetector is highly sensitive to DUV light illumination and achieves a high responsivity of 76.2 mA/W, a large on/off current ratio of ~ 105, along with an ultra-high ultraviolet (UV)/visible rejection ratio of 1.8 × 104. More importantly, it has an ultra-fast response time of 12 µs and a remarkable specific detectivity of ~ 1013 Jones. Finally, an excellent DUV imaging capability has been identified based on the Gr/PtSe2/β-Ga2O3 Schottky junction photodetector, demonstrating its great potential application in DUV imaging systems.

Enhanced deep-ultraviolet sensing by an all-inorganic p-PZT/n-Ga2O3 thin-film heterojunction

In this work, an enhanced-performance deep-ultraviolet (DUV) photodetector based on a Ga2O3/lead zirconate titanate (Pb(Zr0.52Ti0.48)O3, PZT) p-n heterojunction is fabricated and characterized for the first time. Compared to a Ga2O3-based device, the heterojunction device achieved a lower dark current of 2.7 10−11 A, a higher photo- to dark-current ratio of 2 103, a higher responsivity of 5.5 mA W−1, a larger specific detectivity of 2.6 1011 Jones and a higher external quantum efficiency of 2.7% at −2 V under 254 nm UV light illumination with an intensity of 500 μW cm−2. The p-PZT/n-Ga2O3 was confirmed to be a potential candidate for the construction of DUV photodetectors with enhanced sensing performance.

Zn2GeO4 nanowires synthesized by dual laser-hydrothermal method for deep-ultraviolet photodetectors

Ternary Oxide Nanocrystals (TONs) and its ultraviolet detection capability have achieved widespread attention in recent years due to its wide commercial and military applications. However, how to make stable and environmentally-friendly TONs have always bothered us. In this paper, we introduced an advanced dual parallel 1064 nm laser ablation in liquid (LAL) plus hydrothermal process method to synthesize Zn2GeO4 nanowires. In addition, we innovatively added H2O2 during LAL process. The results prove that the samples synthesized in water with H2O2 not only have a longer size, but also perform better in subsequent deep-ultraviolet (DUV) photoelectric tests. DUV detectors based on Zn2GeO4 nanowires exhibited excellent photoconductive performance in terms of high current switching ratio, excellent stability and reproducibility, and fast response and recovery time due to this universal and green synthesis method. This also provides a new idea and application method for the synthesis of TONs.

Bandgap engineering of Gallium oxides by crystalline disorder

Gallium oxide (Ga2O3) has recently emerged as a promising candidate for applications in high-power and radio frequency electronics, deep-ultraviolet optoelectronics, etc. The engineering of bandgap and constructing of heterostrucutres are fundamental steps towards such applications. However, efficient bandgap engineering of Ga2O3 is still a huge challenge. Herein, by the combination of experiments and first-principles calculations, we report that the oxygen vacancy (VO) density and crystalline disorder of the Ga2O3 can be tuned continuously by modulating the O/Ga ratio during the growth process. The VO can introduce localized defect states right above the valence band, thus improving the conductivity of the films. While the crystalline disorder can lead to the shift of the valence band towards the conduction band, thus narrowing the bandgap of the Ga2O3 significantly. As a demonstration of the practical applications of the bandgap engineering by the crystalline disorder, Ga2O3-based deep-ultraviolet homojunction photodetectors have been developed. The device shows a peak responsivity of 22.1 mA/W and a detectivity of 8.7 × 1012 Jones at 0 V bias, which are among the best values for zero-biased Ga2O3 photodetectors. The present findings on tuning the bandgap of Ga2O3via structural disorder are expected to pave a new avenue to achieving high performance Ga2O3 optoelectronic and electronic devices.

Integrated Flexible Ga2O3 Deep UV Photodetectors Powered by Environmental Electromagnetic Radiation Energy

AbstractThis work reports an integrated flexible deep ultraviolet (UV) photodetection system hosting an amorphous Ga2O3 (a‐Ga2O3) photodetector (PD) and an energy harvesting component including a receiving electrode and a full‐wave rectifier. An alternating signal is induced by the coupling of human body with environmental electromagnetic radiation through human hand's contact with the receiving electrode. The signal is subsequently converted into direct current (DC) by the rectifier which is composed of four thin‐film diodes with high rectification ratio (≈106) and fast response time (rising time ≈240 µs and falling time ≈680 µs). Driven by the DC output, the a‐Ga2O3 PD achieves a photoresponsivity of 0.10 A W−1 and a detectivity of 2.24 × 1011 Jones under 254 nm UV illumination. The integrated flexible a‐Ga2O3 UV PD manifests a repeatable and pleasurable response behavior with a bending radius of 15 mm, providing a new paradigm for utilizing the environmental energy as the power source in wearable electronics.

Investigating the role of oxygen and related defects in the self-biased and moderate-biased performance of β-Ga2O3 solar-blind photodetectors

High-performance, low-cost, self-powered deep-ultraviolet photodetectors (DUV-PDs) are essential for military and civil applications. β-Ga2O3 stands alone among all the solar-blind materials in its suitability for use in next-generation DUV-PDs. However, deep traps by oxygen vacancies critically affect the photogenerated carriers, and hence the photodetector’s final efficiency. Notwithstanding, both a lack of and an excess of oxygen in β-Ga2O3 ultimately lead to leakage channels, carrier scattering and sub-bandgap absorption. However, no studies on the impact of extremes of oxygen (oxygen-poor and oxygen-rich) on β-Ga2O3 photodetector efficiency are available in the literature. Therefore, in the present work, we aim to understand the impact of varied oxygen flow rates from 0% to 4% on material properties and photodetector performance. Photoluminescence, time-resolved photoluminescence (TRPL), x-ray photoelectron spectroscopy and the electrical properties of fabricated photodetectors confirmed the critical role of oxygen in β-Ga2O3. TRPL measurements revealed that β-Ga2O3 with 1% oxygen flow had a reported shortest decay time of nearly 50 ps. A very low dark current of 0.9 pA and a maximum photo-to-dark current of >103 were achieved at zero bias for β-Ga2O3 under optimum oxygen flow. The responsivity, external quantum efficiency, detectivity and dark current for a sample at moderate bias fabricated under optimum oxygen flow were found to be 190.08 A W−1, 9.42 × 104%, 1.22 × 1015 Jones and 21 nA, respectively. Hence, the measurements showed that for better device performance and self-powered response, oxygen concentrations that are neither too low nor too high are needed, and the detailed mechanism behind this is discussed. Comparison of the figures of merit with those of other reported devices in both self-powered and high bias mode reveals the far superior performance of the present device.

Ultrahigh Deep-Ultraviolet Responsivity of a β-Ga2O3/MgO Heterostructure-Based Phototransistor

Deep-ultraviolet (DUV) photodetectors based on wide-band-gap semiconductors have attracted significant interest across a wide range of applications in the industrial, biological, environmental, and military fields due to their solar-blind nature. As one of the most promising wide-band-gap materials, β-Ga2O3 provides great application potential over detection wavelengths ranging from 230 to 280 nm owing to its superior optoelectronic performance, stability, and compatibility with conventional fabrication techniques. Although various innovative approaches and device configurations have been applied to achieve highly performing β-Ga2O3 DUV photodetectors, the highest demonstrated responsivity of the β-Ga2O3 photodetectors has only been around 105 A/W. Here, we demonstrate a β-Ga2O3 phototransistor with an ultrahigh responsivity of 2.4 × 107 A/W and a specific detectivity of 1.7 × 1015 Jones, achieved by engineering a photogating effect. A β-Ga2O3/MgO heterostructure with an Al2O3 encapsulation layer is employed not only to reduce photogenerated electron/hole recombination but also to suppress the photoconducting effects at the back-channel surface of the β-Ga2O3 phototransistor via a defect-assisted charge transfer mechanism. The measured photoresponsivity is almost 2 orders of magnitude higher than the highest previously reported value in a β-Ga2O3-based photodetector, to the best of our knowledge. We believe that the demonstrated β-Ga2O3/MgO heterostructure configuration, combined with its facile fabrication method, will pave the way for the development of ultrasensitive DUV photodetectors utilizing oxide-based wide-band-gap materials.

Position-sensitive solar-blind deep-ultraviolet detectors

β-Ga2O3 is a direct ultra-wide bandgap semiconductor with an energy gap of 4.9 eV, which can serve as the ideal candidate for solar-blind deep-ultraviolet (DUV) detection. Despite the extensive research in Ga2O3 detectors, the positive sensitive ones have not been reported yet. Recently, researchers demonstrated the position sensitive solar-blind DUV photodetector by using the β-Ga2O3 thin film as an active layer. The photodetectors have high uniformity, fast response, and high DUV/visible rejection ratio. The DUV detector was successfully utilized to align the light beam. This work favors the applications of β-Ga2O3 in DUV or X-ray beam alignment or imaging.

Growth of hexagonal boron nitride films on silicon substrates by low-pressure chemical vapor deposition

By respectively using Boron trichloride and ammonia as the boron source and the nitrogen source, and nitrogen as the carrier gas, hexagonal boron nitride films were grown on (100) silicon substrates via a low-pressure chemical vapor deposition method in a deposition temperature range of 900–1300 °C with a total pressure of 100 Pa. The effect of temperature on crystalline quality of the hexagonal boron nitride films was investigated using X-ray diffraction and Raman spectra. It was demonstrated that stoichiometric polycrystalline hexagonal boron nitride films were synthesized at 1200 °C. A deep-ultraviolet photodetector based on a 2.3 µm hBN film exhibited a photo-to-dark-current ratio of ~ 312 at 20 V bias under the irradiation of a UV-enhanced xenon lamp. It indicates that hBN films are promising functional materials for high-performance solar-blind photodetectors.

Flexible transparent solar blind ultraviolet photodetector based on amorphous Ga<sub>2</sub>O<sub>3</sub> grown on mica substrate

Solar-blind deep-ultraviolet (UV) photodetectors (PDs) based on the super-wide bandgap semiconductor material Ga<sub>2</sub>O<sub>3</sub> is one of the hot topics of current research, but how to prepare high-performance Ga<sub>2</sub>O<sub>3</sub>-based solar-blind PDs in the field of flexible and transparent optoelectronics still faces challenges. In this work, an amorphous Ga<sub>2</sub>O<sub>3</sub> film with high transmittance is grown on a flexible mica substrate by using the radio frequency magnetron sputtering technology. On this basis, using AZO as an electrode material, a transparent metal-semiconductor-metal (MSM) structured solar-blind deep ultraviolet photodetector based amorphous Ga<sub>2</sub>O<sub>3</sub> film is fabricated, and the performance of PD in the planar state and after multiple bending are systematically compared and analyzed. The results show that the amorphous Ga<sub>2</sub>O<sub>3</sub> based transparent PD has ultra-high visible light transparency and shows good solar-blind ultraviolet photoelectric characteristics. The responsivity of the PD under 254 nm light is 2.69 A/W, and the response time and the recovery time are 0.14 s and 0.31 s, respectively. After bending 300 times, the PD has a photoresponse behavior similar to its planar state, and the performance of the PD has no obvious attenuation phenomenon, showing good flexibility and stability. This work proves that AZO can be used as the electrode material of the next generation of flexible and visible light transparent Ga<sub>2</sub>O<sub>3</sub> based photodetectors, and provides a reference for developing the high-performance flexible and transparent solar-blind deep ultraviolet photodetectors.

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.

Effect of N-doping on performance of <inline-formula><tex-math id="Z-20210908134859">\begin{document}${\boldsymbol\beta}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20210434_Z-20210908134859.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20210434_Z-20210908134859.png"/></alternatives></inline-formula>-Ga<sub>2</sub>O<sub>3</sub> thin film solar-blind ultraviolet detector

<i>β</i>-Ga<sub>2</sub>O<sub>3</sub>-based deep-ultraviolet photodetector (PD) has versatile civil and military applications especially due to its inherent solar-blindness. In this work, pristine and N-doped <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> thin films are prepared on <i>c</i>-plane sapphire substrates by radio frequency magnetron sputtering. The influences of N impurity on the micromorphology, structural and optical properties of <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> film are investigated in detail by scanning electron microscopy, X-ray diffraction, and Raman spectra. The introduction of N impurities not only degrades the crystal quality of <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> films, but also affects the surface roughness. The <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> films doped with N undergoes a transition from a direct optical band gap to an indirect optical band gap. Then, the resulting metal-semiconductor-metal (MSM) PD is constructed. Comparing with the pure <i>β</i>-Ga<sub>2</sub>O<sub>3</sub>-based photodetector, the introduction of N impurities can effectively depress dark current and improve response speed of the <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> device. The N-doped <i>β</i>-Ga<sub>2</sub>O<sub>3</sub>-based photodetector achieves a dark current of 1.08 × 10<sup>–11 </sup>A and a fast response speed (rise time of 40 ms and decay time of 8 ms), which can be attributed to the decrease of oxygen vacancy related defects. This study demonstrates that the acceptor doping provides a new opportunity for producing ultraviolet photodetectors with fast response for further practical applications.

Highly selective ozone-treated β -Ga2O3 solar-blind deep-UV photodetectors

The ultra-wide energy bandgap (4.6–4.9 eV) of the β-Ga2O3 semiconductor offers intrinsic solar blindness, which is a great advantage as the absorber material of a deep ultraviolet (UV) photodetector. Although the band-to-band excitation transition in β-Ga2O3 is allowed solely by the UV-C wavelength, the defective sites including oxygen vacancies can induce sub-bandgap absorption, resulting in high background noise. The UV-ozone treatment was performed at elevated temperatures to investigate its effect on removing these oxygen vacancies; it creates reactive oxygen radicals that can reach the β-Ga2O3 lattice and passivate the defective sites. The chemical analysis through x-ray photoelectron and micro-Raman spectroscopies revealed an increase in Ga–O bonding after UV-ozone treatment. The optoelectrical measurements on the β-Ga2O3 UV-C photodetectors showed that the UV-ozone treatment significantly decreased the response to UV-A light. Thus, the photodetector performance (photo-to-dark current ratio, responsivity, detectivity, and rejection ratio) was greatly enhanced; especially, the rejection ratio was increased to 4.56 × 108 by eight orders of magnitude after UV-ozone treatment. The remarkably improved UV-C selectivity in the β-Ga2O3 solar-blind photodetector highlights its potential of realizing truly solar-blind photodetectors using a simple UV-ozone treatment technique.

In situ and tunable structuring of semiconductor-in-glass transparent composite.

SummarySemiconductor-in-glass composites are an exciting class of photonic materials for various fundamental applications. The significant challenge is the scalable elaboration of composite with the desirable combination of tunable structure, high semiconductor loading ratio, and excellent transparency. Here we report that the topological engineering strategy via hybridization of the glass network former enables to surmount the aforementioned challenge. It not only facilitates the in situ precipitation of (Ga2-xAlx)O3 domains with continuously tunable composition but also allows to simultaneously refine the grain size and enhance the crystallinity. In addition, the composites exhibit excellent transparency and can host various active dopants. We demonstrate the attractive broadband optical response of the composite and achieve the pulse laser operation in mid-infrared waveband. The findings are expected to provide a fundamental principle of in situ modification in hybrid system for generation of high-performance semiconductor-in-glass composites.

Study of temperature dependent behavior of h-BN nanoflakes based deep UV photodetector

Here we report on a robust deep ultraviolet (UV) photodetector based on hexagonal-boron nitride (h-BN) flakes. Metal-semiconductor-metal (MSM) photodetector was fabricated on mechanically exfoliated multilayered h-BN flake using high work function metal platinum (Pt). In this way, Schottky barrier height of 0.85 eV and ideality factor of 1.01 was obtained. The photocurrent at 205 nm was found to be 10 times higher than the dark current. The photodetector exhibited a responsivity of 5.2 mA/W at 10 V with incident power density of only 44.6 μW/cm2. A stable temporal response was observed by repeatedly switching on and off the incident light. The robustness of the photodetector was tested by varying the temperature from room temperature (RT) to 200 °C. Responsivity of 32 mA/W and photo to dark current ratio (PDCR) of 0.35 was obtained at 10 V, indicating good thermal stability of the photodetector.

Exceptional Responsivity (>6 kA/W) and Dark Current (<70 fA) Tradeoff of n-Ga2O3/p-CuO Quasi-Heterojunction-Based Deep UV Photodetector

Reduction of high dark current has been a challenge for high responsivity photodetector (PD). In this context, the present article demonstrates an ultralow dark current of Ga2O3-based deep ultraviolet (UV)-photodetectors (UV-PDs) with enhanced photoresponses by tailoring a p/n heterojunction. An n- Ga2O3/p-CuO quasi-heterostructure-based deep UV-PDs has been fabricated on a sapphire (0001) substrate using an inexpensive electrospraying technique. After Ga2O3 deposition, platinum (Pt) electrodes ( $\sim 50$ nm) are fabricated as a metal–semiconductor–metal (MSM) device using sputtering. The device exhibits a very low dark current in the order of few fA $(6.94\times 10^{-14}$ A) at 5 V because of the enhanced depletion width at p/n heterojunction and Pt/ Ga2O3 metal–semiconductor contacts, which provides a narrow path for free carriers to travel from one metal contact to other under dark condition. The depletion layers get thinner due to the absorption of UV-photons in the UV-illumination condition. Hence, the photogenerated carriers get a wider channel to get collected at Pt-electrodes. Thus, in addition to ultralow dark current, the device exhibits an extraordinary photodetection characteristics, such as a high responsivity ( $\sim 6.33 \times 10^{3} \mathrm{AW}^{-1}$ ), remarkable phototo-dark current ratio ( $\sim 2.99 \times 10^{6}$ ), very high detectivity ( $\sim 4.44 \times 10^{14} \mathrm{mHz}^{0.5} \mathrm{~W}^{-1}$ ), and exceptional external quantum efficiency of $\sim 3.1 \times 10^{6}$ % at 5 V.

Solar-blind imaging based on 2-inch polycrystalline diamond photodetector linear array

Diamond is a promising ultrawide-bandgap semiconductor, which has drawn much attention in deep-ultraviolet optoelectronics and high-power electronics, due to its unique properties, including the highest thermal conductivity, strong radiation resistance, high breakdown electric field, chemical and thermal stability, and high carrier mobility. However, the optoelectronics applications have been restricted greatly by the small growth size of the single-crystal diamond. In this work, we demonstrate a solar-blind photodetector linear array based on high-quality 2-inch polycrystalline diamond, which has the advantage of large-area growth. The photodetector cells exhibit good uniformity, a responsivity of 45 mA/W at 228 nm with a cut-off wavelength of 240 nm, and a short response time of <20 μs. Clear solar-blind images are obtained by using the photodetector linear array as sensing units. The results reported in this work promise the potential applications of polycrystalline diamond in deep-ultraviolet photodetectors.

Zinc Gallium Oxide-A Review from Synthesis to Applications.

Spinel ZnGa2O4 has received significant attention from researchers due to its wide bandgap and high chemical and thermal stability; hence, paving the way for it to have potential in various applications. This review focuses on its physical, optical, mechanical and electrical properties, contributing to the better understanding of this material. The recent trends for growth techniques and processing in the research and development of ZnGa2O4 from bulk crystal growth to thin films are discussed in detail for device performance. This material has excellent properties and is investigated widely in deep-ultraviolet photodetectors, gas sensors and phosphors. In this article, effects of substrate temperature, annealing temperature, oxygen partial pressure and zinc/gallium ratio are discussed for device processing and fabrication. In addition, research progress and future outlooks are also identified.

On the response of gamma irradiation on atomic layer deposition-grown β-Ga2O3 films and Au-β-Ga2O3-Au deep ultraviolet solar-blind photodetectors

β-Ga2O3 films are deposited on (0001) sapphire substrates using triethylgallium (TEGa) and nitrous oxide (N2O) under high N2O/TEGa ratios by atomic layer deposition (ALD). Au-β-Ga2O3-Au metal/semiconductor/metal (MSM) solar-blind deep ultraviolet (DUV) photodetectors (PDs) are prepared using Au interdigitated electrodes deposited by thermal evaporation. The ALD-grown β-Ga2O3 films and Au-β-Ga2O3-Au DUV MSM PDs are irradiated with gamma ray to explore the response of gamma irradiation on the β-Ga2O3 films and β-Ga2O3 DUV MSM PDs. It is found that gamma irradiation tends to deteriorate the structural properties of the β-Ga2O3 films and dark current of the β-Ga2O3 DUV MSM PDs. Nevertheless, it also results in an increase in the 254 nm illuminated photocurrent of the Au-β-Ga2O3-Au DUV MSM PD. Energy band diagram schematics of the biased Au-β-Ga2O3-Au DUV MSM PDs are presented to interpret the influence of gamma irradiation-induced defects on the performances of the Au-β-Ga2O3-Au DUV MSM PDs.

Recent Progress of Deep Ultraviolet Photodetectors using Amorphous Gallium Oxide Thin Films

Deep ultraviolet (UV) photodetectors have wide applications both in civil and military fields. Many materials have been explored to realize deep UV photodetection. Amorphous gallium oxide (a‐GaOx), as a member of transparent amorphous oxide semiconductors (TAOSs), has attracted a great deal of attention due to its ultrawide bandgap and scalable synthesis at room temperature. Plenty of researches have been focused on this topic in recent years. Herein, the latest progresses in the preparation methods of a‐GaOx using radio‐frequency sputtering, pulsed laser deposition, atomic layer deposition, and other deposition techniques are summarized. Dependence of the stoichiometry, crystallinity, optical, electrical, and morphological properties on different preparation parameters and doping/alloying elements is tentatively discussed, as well as those deep UV photodetectors based on a‐GaOx and related thin films. Finally, a short summary with further possible investigations is provided for a better understanding and development of a‐GaOx materials and photodetectors.