Solar-blind Ultraviolet Detection Research Articles

Performance improvement of amorphous Ga2O3 ultraviolet photodetector by annealing under oxygen atmosphere

We have demonstrated the fast-speed and high-rejection-ratio solar-blind ultraviolet (UV) photodetectors based on amorphous Ga2O3 (a-Ga2O3) films grown by atomic layer deposition. The effect of the annealing under oxygen atmosphere on the performance of a-Ga2O3 photodetectors is investigated. By the oxygen annealing at 500 °C, the 90-10% decay time of a-Ga2O3 photodetector can be decreased to ∼150 ns, and the UV/Visible rejection ratio of the photodetector can reach as high as 2.74×105, due to the significant suppression of the visible light response. Moreover, the dark current of the 500 °C-annealed photodetector is only 9.43 pA at 10 V bias. These phenomena can be explained by the decrease in oxygen vacancy concentration of the a-Ga2O3 films after the oxygen annealing. The combination of high rejection ratios and fast operating speeds offers a viable way for facile and scalable fabrication of the oxide semiconductor solar-blind UV detectors.

Recent progress on the electronic structure, defect, and doping properties of Ga2O3

Gallium oxide (Ga2O3) is an emerging wide bandgap semiconductor that has attracted a large amount of interest due to its ultra-large bandgap of 4.8 eV, a high breakdown field of 8 MV/cm, and high thermal stability. These properties enable Ga2O3 a promising material for a large range of applications, such as high power electronic devices and solar-blind ultraviolet (UV) photodetectors. In the past few years, a significant process has been made for the growth of high-quality bulk crystals and thin films and device optimizations for power electronics and solar blind UV detection. However, many challenges remain, including the difficulty in p-type doping, a large density of unintentional electron carriers and defects/impurities, and issues with the device process (contact, dielectrics, and surface passivation), and so on. The purpose of this article is to provide a timely review on the fundamental understanding of the semiconductor physics and chemistry of Ga2O3 in terms of electronic band structures, optical properties, and chemistry of defects and impurity doping. Recent progress and perspectives on epitaxial thin film growth, chemical and physical properties of defects and impurities, p-type doping, and ternary alloys with In2O3 and Al2O3 will be discussed.

Size Regulation and Photoluminescence Properties of <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> Nanomaterials

Gallium oxide (Ga<sub>2</sub>O<sub>3</sub>) nanomaterials have great potential in the fields of ultraviolet transparent electrodes, high-temperature gas sensors, solar blind ultraviolet detectors and power devices, while achieving Ga<sub>2</sub>O<sub>3</sub> nanomaterials with high crystalline quality and controllable size and morphology still remains challenge. Herein, size-controllable Gallium oxide hydroxide (GaOOH) nanorods, nanorod bundles, and spindles were prepared by hydrothermal method. After high temperature calcination, GaOOH nanomaterials were successfully transformed into higher-quality single-crystal β-Ga<sub>2</sub>O<sub>3</sub> nanomaterials which well retained the morphological characteristics of the pristine GaOOH.With the help of X-ray diffraction (XRD), Raman scattering spectroscopy (Raman) and field emission scanning electron microscope (FE-SEM), we systematically studied the influence of the pH value and the concentration of anionic surfactants in the precursor solution on the crystal structure and surface morphology of GaOOH and β-Ga<sub>2</sub>O<sub>3</sub> nanomaterials, and explored the different growth mechanism of GaOOH nanomaterials under different conditions. Simultaneously, room temperature photoluminescence (PL) tests revealed that β-Ga<sub>2</sub>O<sub>3</sub> nanomaterials with different morphologies exhibit typical broad blue-green emission and sharp red emission, which are closely related to the existence of intrinsic defects in nanomaterials.The above research results provide valuable information for the controllable preparation of high-quality β-Ga<sub>2</sub>O<sub>3</sub> nanomaterials.

Analysis of High-Temperature Carrier Transport Mechanisms for High Al-Content Al0.6Ga0.4N MSM Photodetectors

An AlGaN-based solar blind ultraviolet (UV) metal–semiconductor–metal (MSM) photodetector (PD) with a high Al-content of 0.6 has been successfully fabricated. The device exhibits a cutoff wavelength of 255 nm corresponding to the sharp cutoff transmission spectrum of Al0.6Ga0.4N. In addition, dislocations in the Al0.6Ga0.4N epi-layer has been analyzed by high-resolution transmission electron microscope (TEM). The Al0.6Ga0.4N-based solar blind PD exhibits a responsivity of 0.51 A/W at 10 V with a high breakdown voltage of 470 V. Suggesting its potential applications for high-temperature solar blind UV detection, the ${I}$ – ${V}$ – ${T}$ characteristics have been comprehensively investigated to explore its high-temperature carrier transport mechanisms. It is convincingly demonstrated that the bias leakage current across the device is dominated by thermionic-field emission transport at low bias voltages and Poole–Frenkel emission at high bias voltages from room temperature up to 425 K.

Self-powered solar-blind ultraviolet photodetector based on Au/ZnMgO/ZnO:Al with comb-shaped Schottky electrode

We have demonstrated a self-powered solar-blind ultraviolet (UV) photodetector based on the Au/ZnMgO/ZnO:Al vertical structure. The comb-shaped Au was selected as the Schottky electrode due to its high transmittance (more than 60%) in the wavelength region from 200 nm to 300 nm. At 0 V bias, the detector showed a responsivity of 55 mA/W under 265 nm light illumination with the intensity of 80 nW/cm2, which is larger than that of any other previously reported ZnO-based self-powered UV photodetectors. And a fast response speed can be also clearly observed with 10%–90% rise and 90%-10% decay times of 20 μs and 300 μs, respectively. The excellent self-powered solar-blind UV detection of our Au/ZnMgO/ZnO:Al device should be associated with the high external quantum efficiency, especially for the weak signal detection. Our findings offered new chances in high-performance self-powered photodetectors fabrication.

Solar-blind ultraviolet detection based on TiO2 nanoparticles decorated graphene field-effect transistors

Abstract Sensitive solar-blind ultraviolet (UV) photodetectors are important to various military and civilian applications, such as flame sensors, missile interception, biological analysis, and UV radiation monitoring below the ozone hole. In this paper, a solar-blind UV photodetector based on a buried-gate graphene field-effect transistor (GFET) decorated with titanium dioxide (TiO2) nanoparticles (NPs) was demonstrated. Under the illumination of a 325-nm laser (spot size ~2 μm) with a total power of 0.35 μW, a photoresponsivity as high as 118.3 A/W was obtained, at the conditions of zero gate bias and a source-drain bias voltage of 0.2 V. This photoresponsivity is over 600 times higher than that of a recently reported solar-blind UV photodetector based on graphene/vertical Ga2O3 nanowire array heterojunction (0.185 A/W). Experiments showed that the photoresponsivity of the TiO2 NPs decorated GFET photodetectors can be further enhanced by increasing the source-drain bias voltage or properly tuning the gate bias voltage. Furthermore, the photoresponse time of the TiO2 NPs decorated GFET photodetectors can also be tuned by the source-drain bias and gate bias. This study paves a simple and feasible way to fabricate highly sensitive, cost-efficient, and integrable solar-blind UV photodetectors.

N-ZnO/p-GaN heterojunction ultraviolet (UV) photo detectors with high responsivity and fast response time grown by chemical vapor deposition technique

High quality c-axis oriented n-type ZnO epitaxial films are grown on p-type c-GaN/sapphire templates using a chemical vapor deposition technique where metallic zinc is used as the Zn source. n-ZnO/p-GaN heterojunctions were thus formed and show rectifying behaviour with a turn-on voltage of ∼2.4 V and a very low leakage current of 1 × 10−11 A. Study of the spectral distribution of the photo response properties of these heterostructures shows a maximum at a wavelength of 366 nm with a peak responsivity of ∼0.4 mA/W at zero bias condition. The peak responsivity increases further with the applied forward bias and reaches ∼191 mA/W at 1 V. The spectral profile shows a sharp reduction in responsivity for wavelengths larger than ≈400 nm, making these devices suitable for application in solar blind UV detection. These devices are also found to show a fast response with a rise/decay time of only a few milliseconds. The study also reveals that the photo-responsivity of these devices depends crucially on the microcrystalline quality of the ZnO layers.

Design of a solar-blind ultraviolet band-pass filter based on frequency domain superposition

In this paper, a band-pass filter based on photonic crystals is designed by using the principle of frequency domain superposition. The influence of the heterostructure position, period numbers and incidence angles on transmission spectrums are analyzed. The filter can achieve efficient filtering in 240–280 nm solar-blind ultraviolet band, with an average transmittance of 72.2%. Simultaneously, a good band-stop appears in near ultraviolet and visible region (300–700 nm) and its average transmittance is below 3.4%. It provides a choice in the application of solar-blind ultraviolet detection technology.

Selective area deposited n-Al0.5Ga0.5N channel field effect transistors with high solar-blind ultraviolet photo-responsivity

We report on AlGaN field effect transistors over AlN/sapphire templates with selective area grown n-Al0.5Ga0.5N channel layers for which a field-effect mobility of 55 cm2/V-sec was measured. Using a pulsed plasma enhanced chemical vapor deposition deposited 100 A thick SiO2 layer as the gate-insulator, the gate-leakage currents were reduced by three orders of magnitude. These devices with or without gate insulators are excellent solar-blind ultraviolet detectors, and they can be operated either in the photoconductive or the photo-voltaic modes. In the photo-conductive mode, gain arising from hole-trapping in the depletion region leads to steady-state photoresponsivity as high as 1.2 × 106A/W at 254 nm, which drops sharply below 290 nm. A hole-trapping limited detector response time of 34 ms, fast enough for real-time flame-detection and imaging applications, was estimated.

Ultrafast Solar-Blind Ultraviolet Detection by Inorganic Perovskite CsPbX3 Quantum Dots Radial Junction Architecture.

Inorganic CsPbX3 (X = Cl, Br, I, or hybrid among them) perovskite quantum dots (IPQDs) are promising building blocks for exploring high performance optoelectronic applications. In this work, the authors report a new hybrid structure that marries CsPbX3 IPQDs to silicon nanowires (SiNWs) radial junction structures to achieve ultrafast and highly sensitive ultraviolet (UV) detection in solar-blind spectrum. A compact and uniform deployment of CsPbX3 IPQDs upon the sidewall of low-reflective 3D radial junctions enables a strong light field excitation and efficient down-conversion of the ultraviolet incidences, which are directly tailored into emission bands optimized for a rapid photodetection in surrounding ultrathin radial p-i-n junctions. A fast solar-blind UV detection has been demonstrated in this hybrid IPQD-NW detectors, with rise/fall response time scales of 0.48/1.03 ms and a high responsivity of 54 mA W-1 @200 nm (or 32 mA W-1 @270 nm), without the need of any external power supply. These results pave the way toward large area manufacturing of high performance Si-based perovskite UV detectors in a scalable and low-cost procedure.

Solar-blind-ultraviolet extraordinary transmission for ultrasensitive photoconductive detector based on plasmonic subwavelength interdigital electrodes

The solar-blind-ultraviolet (SBUV) detection industry demands high sensitivity as well as easy processability for its semiconductor devices. Photoconductive detectors have the simplest structure. However, the electrodes covering the illuminated side cause optical shielding losses, resulting in a relatively low sensitivity of such devices. Through finite-difference time-domain (FDTD) simulation, we demonstrated that surface-plasmon-based enhanced SBUV transmission is achievable for Al interdigital electrodes (IDEs) with a period ⩽200 nm and an interval ⩾140 nm. Under this parameter setting, a larger interval and smaller period leads to further enhancement of SBUV transmission. Particularly, we have found that different possible dielectric environments, such as Ni insertion, Al oxidization, and MgF2 anti-oxidation, would not exert fatal effects on this enhancement. Besides, such an enhancement is maintained under the angle of incidence within 10°, which is large enough for practical SBUV detection. Our research reveals the feasibility of high sensitivity by a simple photoconductive device, showing profound significance for an applicable SBUV detector.

Enhanced solar-blind ultraviolet single-photon detection with a Geiger-mode silicon avalanche photodiode

Solar-blind ultraviolet detection is of great importance in astronomy and industrial and military applications. Here, we report enhanced solar-blind ultraviolet single-photon detection by a normal silicon avalanche photodiode (Si APD) single-photon detector with a specially designed photon-collecting device. By re-focusing the reflected photon from the Si chip surface on the detection area by the aluminum-coated hemisphere, the detection efficiency of the Si APD at 280 nm can be improved to 4.62%. This system has the potential for high-efficiency photon detection in the solar-blind ultraviolet regime with low noise.

Design of nanophotonic, hot-electron solar-blind ultraviolet detectors with a metal-oxide-semiconductor structure

Solar-blind ultraviolet (UV) detection refers to photon detection specifically in the wavelength range of 200 nm–320 nm. Without background noises from solar radiation, it has broad applications from homeland security to environmental monitoring. The most commonly used solid state devices for this application are wide band gap (WBG) semiconductor photodetectors (Eg > 3.5 eV). However, WBG semiconductors are difficult to grow and integrate with Si readout integrated circuits (ROICs). In this paper, we design a nanophotonic metal-oxide-semiconductor structure on Si for solar-blind UV detectors. Instead of using semiconductors as the active absorber, we use Sn nano-grating structures to absorb UV photons and generate hot electrons for internal photoemission across the Sn/SiO2 interfacial barrier, thereby generating photocurrent between the metal and the n-type Si region upon UV excitation. Moreover, the transported hot electron has an excess kinetic energy >3 eV, large enough to induce impact ionization and generate another free electron in the conduction band of n-Si. This process doubles the quantum efficiency. On the other hand, the large metal/oxide interfacial energy barrier (>3.5 eV) also enables solar-blind UV detection by blocking the less energetic electrons excited by visible photons. With optimized design, ∼75% UV absorption and hot electron excitation can be achieved within the mean free path of ∼20 nm from the metal/oxide interface. This feature greatly enhances hot electron transport across the interfacial barrier to generate photocurrent. The simple geometry of the Sn nano-gratings and the MOS structure make it easy to fabricate and integrate with Si ROICs compared to existing solar-blind UV detection schemes. The presented device structure also breaks through the conventional notion that photon absorption by metal is always a loss in solid-state photodetectors, and it can potentially be extended to other active metal photonic devices.

Enhanced spectral response of an AlGaN-based solar-blind ultraviolet photodetector with Al nanoparticles.

An enhanced spectral response was realized in an AlGaN-based solar-blind ultraviolet (SB-UV) detector using aluminum (Al) nanoparticles (NPs) of 20-60 nm. The peak responsivity of the detector (about 288 nm) with 60 nm Al NPs is more than two times greater than that of a detector without Al NPs under a 5-V bias, reaching 0.288 A/W. To confirm the enhancement mechanism of the Al NPs, extinction spectra were simulated using time-domain and frequency-domain finite-element methods. The calculation results show that the dipole surface plasmon resonance wavelength of the Al NPs is localized near the peak responsivity position of AlGaN-based SB-UV detectors. Thus, the improvement in the detectors can be ascribed to the localized surface plasmon resonance effect of the Al NPs. The localized electric field enhancement and related scattering effect result in the generation of more electron-hole pairs and thus a higher responsivity. In addition, the dark current of AlGaN-based SB-UV detectors does not increase after the deposition of Al nanoparticles. The results presented here is promising for applications of AlGaN-based SB-UV detectors.

Wide range bandgap modulation based on ZnO-based alloys and fabrication of solar blind UV detectors with high rejection ratio.

Theoretical calculations on formation energies of MgZnO, BeZnO and BeMgZnO alloys are presented. The ternary alloy MgZnO (BeZnO) is found to be unstable with high Mg (Be) contents. However, the quaternary system BeMgZnO is predicted to be stable with small Be/Mg atom ratio. Subsequently, a wurtzite Be0.17Mg0.54Zn0.29O alloy with a bandgap of 5.15 eV has been acquired experimentally. Its bandgap is in the middle of solar blind region and thus it is an ideal material for realizing a high rejection ratio solar blind ultraviolet (UV) detector, which has long been a problem. A metal-semiconductor-metal (MSM) structured solar blind UV detector based on this material is then fabricated, realizing a much higher rejection ratio than reported MgZnO-based detectors. One more interesting thing is, as a complicated quaternary system, BeMgZnO can maintain its crystal quality in a wide compositional range, which is not happening in MgZnO and BeZnO. To get some microscopic insight into the Be-Mg mutual stabilizing mechanism, more calculations on the lattice constants of BeZnO and MgZnO alloys, and the coordination preference of Be ions in alloy were conducted. The a-axis lattice compensation and 4-fold coordination preference of Be atom are confirmed the major origins for Be-Mg mutual stabilizing in ZnO lattice.

M面蓝宝石上外延(110)取向立方MgZnO薄膜及其日盲紫外探测器件研究

A series of MgxZn1-xO alloy films were grown on m-plane sapphire by low-pressure metalorganic chemical vapor deposition (LP-MOCVD). Optical and structural characterizations indicated that the films with Zn mole fraction up to 55% are single-cubic-phased, and the optical bandgap of Mg0.45Zn0.55O film is 4.7 eV. The epitaxial relationships between substrate and rocksalt MgZnO layers were determined as (110)MgznO (1010)sapphire, [001]MgZnO [1210]sapphire, and [110]MgZnO [0001]sapphire. The fixed uniform orientation of the films was accompanied by an improvement in crystal quality. A metal-semiconductor-metal structured solar-blind ultraviolet detector fabricated on Mg0.45Zn0.55O film showed a response peak at 260 nm and a sharp cutoff at 278 nm.

Background limited ultraviolet photodetectors of solar-blind ultraviolet detection

The background noise in solar-blind ultraviolet (UV) detectors from the solar-irradiance leakage photons has been compared with the detector noise from the dark current. It has been found that the background noise is the deterministic limiting factor in solar-blind UV photodetection. The detector performance of background limited ultraviolet photodetector limit and the ultimate signal fluctuation limit are therefore proposed. It proves to be an effective method for device optimization by suppressing the declining tails above 285 nm in the response curves. The absorptive spectral filter requirement has been discussed for common solar-blind UV detectors to achieve the detectivity improvement.

Molecular beam epitaxy-grown wurtzite MgS thin films for solar-blind ultra-violet detection

Molecular beam epitaxy grown MgS on GaAs(111)B substrate was resulted in wurtzite phase, as demonstrated by detailed structural characterizations. Phenomenological arguments were used to account for why wurtzite phase is preferred over zincblende phase or its most stable rocksalt phase. Results of photoresponse and reflectance measurements performed on wurtzite MgS photodiodes suggest a direct bandgap at around 5.1 eV. Their response peaks at 245 nm with quantum efficiency of 9.9% and enjoys rejection of more than three orders at 320 nm and close to five orders at longer wavelengths, proving the photodiodes highly competitive in solar-blind ultraviolet detection.

Crack-free AlGaN for solar-blind focal plane arrays through reduced area epitaxy

We report on crack reduction for solar-blind ultraviolet detectors via the use of a reduced area epitaxy (RAE) method to regrow on patterned AlN templates. With the RAE method, a pre-deposited AlN template is patterned into isolated mesas in order to reduce the formation of cracks in the subsequently grown high Al-content AlxGa1−xN structure. By restricting the lateral dimensions of the epitaxial growth area, the biaxial strain is relaxed by the edges of the patterned squares, which resulted in ∼97% of the pixels being crack-free. After successful implementation of RAE method, we studied the optical characteristics, the external quantum efficiency, and responsivity of average pixel-sized detectors of the patterned sample increased from 38% and 86.2 mA/W to 57% and 129.4 mA/W, respectively, as the reverse bias is increased from 0 V to 5 V. Finally, we discussed the possibility of extending this approach for focal plane array, where crack-free large area material is necessary for high quality imaging.

$\hbox{Zr}_{x}\hbox{Ti}_{1 - x}\hbox{O}_{2}$-Based Ultraviolet Detectors Series

In this letter, Zr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Ti <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -based ultraviolet (UV) detector series with Pt electrodes were fabricated. The Zr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Ti <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thin films were prepared by a sol-gel method and characterized by means of X-ray diffraction and UV-visible absorption spectra. At 5 V bias, the dark currents of the detectors were less than 7 nA; under radiation of UV light, a high responsivity rate of 470 A/W was achieved at 270 nm for the Zr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.1</sub> Ti <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.9</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> detector. The results showed that, at low Zr doping, the photoresponse was greatly improved, whereas at high Zr doping, the response peaks shift to the short-wavelength direction, which provides a new platform to fabricate solar-blind UV detectors.