Solar-blind Detection Research Articles

Nano tree-like branched structure with α-Ga2O3 covered by γ-Al2O3 for highly efficient detection of solar-blind ultraviolet light using self-powered photoelectrochemical method

For photoelectrochemical ultraviolet detector, carrier recombination, low light trapping efficiency and long-distance charge carries transport are the main factors restricting its detection performance. Herein, a novel tree-like branched structure with α-Ga2O3 covered by γ-Al2O3 has been successfully fabricated by simple two-step hydrothermal treatment and applied into the self-powered photoelectrochemical solar-blind detector. Owning to the introduction of γ-Al2O3 layer with ultra-wide bandgap to coat outside of α-Ga2O3, an energy barrier has been established at the semiconductor/electrolyte interface accelerating the photogenerated carriers separation and thus suppressing interfacial charge recombination. Besides, the obtained tree-like branched structure significantly expands the contact surface area between carries and electrolyte to shorten the distance of charge transport improving response speed and brings a larger specific surface area to capture and scatter incident photons enhancing light absorption. Campared with the pure α-Ga2O3, the α-Ga2O3-γ-Al2O3 based on the photoelectrochemical device shows higher responsivity, larger photocurrent density and faster response time to solar-blind ultraviolet signal, some performance improvements exceed 100%, indicating the highly efficient detection.

Controlled thin-film deposition of α or β Ga2O3 by ion-beam sputtering

The wide bandgap of oxide semiconductors enables solid-state devices with functionalities beyond the reach of technologically more well-established materials such as silicon, germanium, or gallium nitride. These functionalities, among others, include optical transparency paving the way for transparent (opto-)electronics like ultraviolet light emitters or solar-blind detectors as well as large breakdown fields enabling high-power electronic applications. In particular, Ga2O3 is considered as oxide compatible with the nitride III-V materials. As a particular challenge, Ga2O3 may crystallize in different polymorphs. All possess large optical bandgaps beyond 4.8 eV and, thus, are of great interest for such devices. Nowadays, Ga2O3 thin films are prepared by a plethora of different synthesis methods including physical as well as chemical variants, however, reports on a controlled manufacturing process of individual Ga2O3 phases with sputtering techniques are currently scarce. Here, we grow Ga2O3 thin films by ion-beam sputtering on sapphire substrates of different orientation and demonstrate that selective controlled synthesis of different Ga2O3 polymorphs is feasible. The monoclinic β-phase and the corundum-type α-phase are stabilized when deposited on (0001) and (10–10) sapphire substrates, respectively. A stacking rather than a mixture of both phases is observed when thin films of α-phase exceed a certain thickness. Thus, thickness control enables the transition between the two phases. The proposed technique holds promise for ion-beam sputter methods to be employed in Ga2O3-related device build-up.

High performance solar-blind UV detector based on β-Ga2O3/GaN nanowires heterojunction

Gallium nitride nanowires and β-Ga2O3 nanowires were prepared by chemical vapor deposition. A solar-blind ultraviolet detector was fabricated based on β-Ga2O3/GaN nanowires heterojunction. The UV detector exhibits excellent response properties to deep ultraviolet with wavelength of 254 nm. The device exhibits a photo-to-dark current ratio of 1.375 × 103 and a high detectivity of 1.2 × 1011 Jones at 10 V bias Therefore, the responsivity of the UV detector is 27.5 mA/W under 254 nm ultraviolet light (8 mW/cm2) at 10 V bias voltage. The detector also has the advantages of easy preparation, low dark current, high rejection ratio, fast response, good stability and repeatability. The design method of β-Ga2O3/GaN nanowires heterojunction has the advantages of novelty and simplicity, which provides a new approach to fabricate a heterojunction detector.

Research on the Theoretical Model Between Solar-blind UV and Atmospheric Temperature during Atmospheric Transmission

In recent years,solar blind ultraviolet detection has been widely used in power fault detection.Based on atmospheric temperature profile and MODTRAN,we conduct simulation analysis on the influence of atmospheric temperature on solar blind ultraviolet atmospheric transmission.And finally the relationship between atmospheric temperature and ultraviolet atmospheric transmission was obtained through wavelength analysis and curve fitting.Based on the simulation relational expression,the quantitative analysis of atmospheric temperature and atmospheric transmission can be carried out,which can promote the study of the characteristics of solar blind ultraviolet transmission in the atmosphere and improve the accuracy of corona detection.

Y2O3: Eu3+/PMMA hybrid film as a converter for enhanced harvesting of broadband solar-blind UV light.

At present, the most common materials for solar-blind UV light detectors are wide band-gap semiconductors, which generally have high requirements and complex methods for preparation. Ordinary semiconductor materials such as silicon, TiO2, and Cu2O were industrialized, but they were excluded for direct harvest of solar-blind UV light due to their inability to absorb solar-blind light photons. Here, inorganic-organic hybrid film of Y2O3:Eu3+/PMMA was used as a spectral converter to realize the detection of broadband solar-blind UV light by ordinary semiconductor, converting broadband solar-blind UV luminescence to visible luminescence based on down-conversion process, after which the visible luminescence was detected by the Si photo-resister. The results show that hybrid film based on rare earth luminescence materials is particularly valuable for broadband solar-blind UV detection.

High-performance high-temperature solar-blind photodetector based on polycrystalline Ga2O3 film

In this paper, high-temperature solar-blind photodetectors were fabricated based on polycrystalline Ga2O3 (poly-Ga2O3) film which was prepared by annealing amorphous Ga2O3 (a-Ga2O3) film in oxygen atmosphere. The effects of annealing temperature on the properties of the material and the performances of the corresponding Metal–semiconductor–metal photodetector were investigated. The main factors affecting device performance are attributed to the oxygen vacancy defect concentration in Ga2O3 and the effective Schottky barrier height and width between gold and Ga2O3. We found that the detector based on 650 °C annealed poly-Ga2O3 showed the best solar-blind detection photoelectric performance and good thermal stability. More interestingly, the device has fast response recovery speed (0.16 s), low dark current (≤0.1 nA) and high photo-to-dark current ratio (≥97) at 200 °C high temperature operation. These performances are even better than the reported high-temperature photodetectors based on high-quality crystal Ga2O3 thin film or Ga2O3 bulk. Besides, our method that avoid direct high-temperature growth of poly-Ga2O3 and may be beneficial to its commercial large-area production and further cost reduction.

Polarization-enhanced AlGaN solar-blind ultraviolet detectors

AlGaN solar-blind ultraviolet detectors have great potential in many fields, although their performance has not fully meet the requirements until now. Here, we proposed an approach to utilize the inherent polarization effect of AlGaN to improve the detector performance. AlGaN heterostructures were designed to enhance the polarization field in the absorption layer, and a high built-in field and a high electron mobility conduction channel were formed. As a result, a high-performance solar-blind ultraviolet detector with a peak responsivity of 1.42 A/W at 10 V was achieved, being 50 times higher than that of the nonpolarization-enhanced one. Moreover, an electron reservoir structure was proposed to further improve the performance. A higher peak responsivity of 3.1 A/W at 30 V was achieved because the electron reservoir structure could modulate the electron concentration in the conduction channel. The investigation presented here provided feasible approaches to improve the performance of the AlGaN detector by taking advantage of its inherent property.

Spectrally Selective and Highly Sensitive UV Photodetection with UV‐A,C Band Specific Polarity Switching in Silver Plasmonic Nanoparticle Enhanced Gallium Oxide Thin‐Film

AbstractTraditional photodetectors generally show a unipolar photocurrent response when illuminated with light of wavelength equal or shorter than the optical bandgap. Here, it is reported that a thin film of gallium oxide (GO) decorated with plasmonic nanoparticles, surprisingly, exhibits a change in the polarity of the photocurrent for different UV bands. Silver nanoparticles (Ag NPs) are vacuum‐deposited onto β‐Ga2O3 and the AgNP@GO thin films show a record responsivity of 250 A W−1, which significantly outperforms bare GO planar photodetectors. The photoresponsivity reverses sign from +157 µA W−1 in the UV‐C band under unbiased operation to −353 µA W−1 in the UV‐A band. The current reversal is rationalized by considering the charge dynamics stemming from hot electrons generated when the incident light excites a local surface plasmon resonance in the Ag nanoparticles. The Ag nanoparticles improve the external quantum efficiency and detectivity by nearly one order of magnitude with high values of 1.2 × 105 and 3.4 × 1014 Jones, respectively. This plasmon‐enhanced solar blind GO detector allows UV regions to be spectrally distinguished, which is useful for the development of sensitive dynamic imaging photodetectors.

Fabrication of ϵ-Ga2O3 solar-blind photodetector with symmetric interdigital Schottky contacts responding to low intensity light signal

Owing to the constraint of carrier transport, the dark current is relatively lower in Schottky contacted devices than that in Ohmic contacted devices, leading to a high specific detectivity in photodetectors. In this work, we prepared ϵ-Ga2O3 thin film by using metal-organic chemical vapor deposition, then constructed a three-pair interdigital ultraviolet solar-blind photodetector with Au electrodes as Schottky contacts. Seen from the results, this photodetector displays an outstanding wavelength selectivity with responsivity of 0.52 A W−1 responding to 250 nm wavelength light. In addition, it shows a photo-to-dark current ratio of 1.82 × 104/6.03 × 102 at 5 V under 40/5 μW cm−2 254 nm light illuminations, respectively, and a low dark current of 1.87 × 10−11 A. Correspondingly, the specific detectivity is 1.67 × 1013/4.45 × 1012 Jones, and the photoresponsivity is 0.198 A W−1/52.54 mA W−1. Overall, ϵ-Ga2O3 prepared here is certified to be an excellent candidate material to perform high-performance solar-blind detection.

Metal–Semiconductor–Metal ε-Ga2O3 Solar-Blind Photodetectors with a Record-High Responsivity Rejection Ratio and Their Gain Mechanism

In recent years, Ga2O3 solar-blind photodetectors (SBPDs) have received great attention for their potential applications in solar-blind imaging, deep space exploration, confidential space communication, etc. In this work, we demonstrated an ultra-high-performance ε-Ga2O3 metal–semiconductor–metal (MSM) SBPD. The fabricated photodetectors exhibited a record-high responsivity and fast decay time of 230 A/W and 24 ms, respectively, compared with MSM-structured Ga2O3 photodetectors reported to date. Additionally, the ε-Ga2O3 MSM SBPD presents an ultrahigh detectivity of 1.2 × 1015 Jones with a low dark current of 23.5 pA under an operation voltage of 6 V, suggesting its strong capability of detecting an ultraweak signal. The high sensitivity and wavelength selectivity of the photodetector were further confirmed by the record-high responsivity rejection ratio (R250 nm/R400 nm) of 1.2 × 105. From the temperature-dependent electrical characteristics in the dark, the thermionic field emission and Poole–Frenkel emission were found to be responsible for the current transport in the low and high electric field regimes, respectively. In addition, the gain mechanism was revealed by the Schottky barrier lowering effect due to the defect states at the interface of the metal contact and Ga2O3 or in the bulk of Ga2O3 based on current transport mechanism and density functional theory calculations. These results facilitate a better understanding of ε-Ga2O3 photoelectronic devices and provide possible guidance for promoting their performance in future solar-blind detection applications.

Material-to-device performance correlation for AlGaN-based solar-blind p–i–n photodiodes

We report on crystalline quality-to-device performance correlation for self-powered Al0.40Ga0.60N, p–i–n ultraviolet (UV) photodetectors on c-plane sapphire. The active p–i–n detector stack was grown over an AlN buffer. Careful optimization of the nucleation density on growth surface helped achieve a two-orders and one-order of magnitude reduction in the screw and edge dislocation density in the buffer layer, respectively. This resulted in a nine-orders of magnitude reduction in the reverse leakage current from 4.3 mA to 4.2 pA (at 10 V). Correspondingly, a thirteen-fold enhancement in the zero-bias external quantum efficiency (EQE) from 3.4% to 45.5%, when measured under 289 nm front-illumination was also observed. The detector epi-stack grown over the optimal AlN buffer layers led to the realization of high-performance p–i–n detectors with a dark current density below 4 nA cm−2 at 10 V and a zero-bias EQE of 74.7% under back-illumination. This is one of the highest zero-bias EQE reported for solar-blind detectors realized on template-free and mask-free III-nitrides grown using metal organic chemical vapor deposition on any substrate. The deep-UV-to-visible rejection ratio exceeded 106 while the deep-UV-to-near UV rejection exceeded 103. The thermal-noise limited detectivity was estimated to be 4 × 1014 cm Hz1/2 W−1. Hopping conduction along screw dislocation-mediated localized trap states was found to be the dominant carrier transport mechanism in the samples exhibiting high reverse leakage. For these samples, the responsivity (photocurrent) exhibited an exponential variation with reverse bias and a nonlinear variation with input optical power. This is explained using a hole-trapping associated gain mechanism and its impact on the transient characteristics of the detectors is investigated. A 6 × 1 linear array of the highest EQE detectors was realized and detector performance parameters were found to be comparable before and after wire bonding. This study is expected to enhance the understanding of III-nitrides-based vertical, self-powered detectors and benefit the development of high-performance, focal plane arrays using less complicated growth techniques.

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.

Construction of a β-Ga2O3-based metal–oxide–semiconductor-structured photodiode for high-performance dual-mode solar-blind detector applications

A dual-mode, sensitive β-Ga2O3 MOS-structured photodiode is constructed to perform solar-blind detection, showing high-performances and operations at zero bias with a high external quantum efficiency of 16.37% and specific detectivity of 1011 Jones.

(Invited) Recent Advances in III-Nitride Devices Using Ultrawide Bandgap AlxGa1-Xn Active Layers

III-Nitride devices with high-Al AlxGa1-xN (x>0.4) active layers have the potential of addressing several key application areas. For these high Al-compositions the direct bandgap leads to a cut-off wavelength<290 nm which makes the material ideal for solar blind detectors. Similarly, AlxGa1-xN (x>0.4) multiple quantum wells (MQWs) are used in the active region of the deep ultraviolet (DUV) light emitting diodes. More recently, to take advantage of the higher breakdown field for high voltage devices, several researchers including us have explored Heterojunction Field-Effect Transistors (HFETs) and metal-oxide-semiconductor (MOS) HFETs with high-Al AlxGa1-xN (x>0.4) channel layers. Currently nearly all the (UWBG) AlGaN devices are fabricated on 3-5 µm thick MOCVD grown high quality AlN/sapphire templates. This choice is dictated by the deep UV optical transparency, the easy availability and the low-cost of sapphire. Increasing the AlN template thickness to larger than 5 µm leads to stress related cracking. The low thermal conductivity of sapphire and this AlN growth thickness limitation leads to device performance limitations especially for high power operation. To avoid this, recently using a pulsed MOCVD approach, we have for the first time succeeded in growing 16 µm thick, crack-free, low-defect AlN template layers on basal plane sapphire substrates. These were then used to deposit epitaxial structures for the AlGaN channel HFET/MOSHFETs. In this presentation we will present studies to establish the role that the high conductivity thick AlN buffers play in improving the devices’ electrical and thermal performance. Recently we have also for the first time conducted optical waveguiding studies at UVC wavelengths. For these studies we fabricated monolithically integrated UVC LEDs, detectors and waveguides on an AlN/sapphire template platform. Optical attenuation coefficient as low as 4.8 cm-1 was measured for Al0.5Ga0.5N waveguide with an AlN clad layer. Details of our integrated device fabrication and optical losses measurements scheme will also be discussed. Acknowledgement: This work was partially supported by Defense Advanced Research Projects Agency, Army Research Office and Office of Naval Research (Monitors: Dr. M. Gerhold. Dr. Paul Maki and Dr. Lynn Petersen).

Influence of growth temperature on the characteristics of β-Ga2O3 epitaxial films and related solar-blind photodetectors

Abstract High-quality (-201)-oriented β-Ga2O3 thin films were epitaxially grown on c-sapphire substrates by pulsed laser deposition (PLD) at various substrate temperatures using a β-Ga2O3 ceramic target. These films were then used to fabricate metal–semiconductor–metal (MSM) solar-blind photodetectors (PDs) based on an Au/β-Ga2O3/Au structure. The crystal quality, atomic ratio of O/Ga, and bandgap of the β-Ga2O3 films all increased with increasing growth temperature, causing the dark current and response time of the PDs to decrease dramatically. The PD based on the β-Ga2O3 film grown at 700 °C exhibited the best performance, with a low dark current of 10.6 pA at 10 V and a high peak responsivity of 18.23 A/W (at 255 nm). Furthermore, the response time of the fabricated PD was fast (τrise: 0.062/0.379 s, τdeacy: 0.058/0.663 s). These results represent the state-of-art performance in dark current for PLD-grown β-Ga2O3-based MSM solar-blind detectors.

Visible- and solar-blind photodetectors using AlGaN high electron mobility transistors with a nanodot-based floating gate

AlGaN-channel high electron mobility transistors (HEMTs) were operated as visible- and solar-blind photodetectors by using GaN nanodots as an optically active floating gate. The effect of the floating gate was large enough to switch an HEMT from the off-state in the dark to an on-state under illumination. This opto-electronic response achieved responsivity >108 A/W at room temperature while allowing HEMTs to be electrically biased in the off-state for low dark current and low DC power dissipation. The influence of GaN nanodot distance from the HEMT channel on the dynamic range of the photodetector was investigated, along with the responsivity and temporal response of the floating gate HEMT as a function of optical intensity. The absorption threshold was shown to be controlled by the AlN mole fraction of the HEMT channel layer, thus enabling the same device design to be tuned for either visible- or solar-blind detection.

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.

Review of gallium-oxide-based solar-blind ultraviolet photodetectors

Solar-blind photodetectors are of great interest to a wide range of industrial, civil, environmental, and biological applications. As one of the emerging ultrawide-bandgap semiconductors, gallium oxide (Ga2O3) exhibits unique advantages over other wide-bandgap semiconductors, especially in developing high-performance solar-blind photodetectors. This paper comprehensively reviews the latest progresses of solar-blind photodetectors based on Ga2O3 materials in various forms of bulk single crystal, epitaxial films, nanostructures, and their ternary alloys. The basic working principles of photodetectors and the fundamental properties and synthesis of Ga2O3, as well as device processing developments, have been briefly summarized. A special focus is to address the physical mechanism for commonly observed huge photoconductive gains. Benefitting from the rapid development in material epitaxy and device processes, Ga2O3-based solar-blind detectors represent to date one of the most prospective solutions for UV detection technology towards versatile applications.

Solar blind Schottky photodiode based on an MOCVD-grown homoepitaxial β-Ga2O3 thin film

We report on a high performance Pt/n−Ga2O3/n+Ga2O3 solar blind Schottky photodiode that has been grown by metalorganic chemical vapor deposition. The active area of the photodiode was fabricated using ∼30 Å thick semi-transparent Pt that has up to 90% transparency to UV radiation with wavelengths &amp;lt; 260 nm. The fabricated photodiode exhibited Schottky characteristics with a turn-on voltage of ∼1 V and a rectification ratio of ∼108 at ±2 V and showed deep UV solar blind detection at 0 V. The Schottky photodiode exhibited good device characteristics such as an ideality factor of 1.23 and a breakdown voltage of ∼110 V. The spectral response showed a maximum absolute responsivity of 0.16 A/W at 222 nm at zero bias corresponding to an external quantum efficiency of ∼87.5%. The cutoff wavelength and the out of band rejection ratio of the devices were ∼260 nm and ∼104, respectively, showing a true solar blind operation with an excellent selectivity. The time response is in the millisecond range and has no long-time decay component which is common in photoconductive wide bandgap devices.

Transition of photoconductive and photovoltaic operation modes in amorphous Ga2O3-based solar-blind detectors tuned by oxygen vacancies**Project supported by the National Key Research and Development Project, China (Grant No. 2017YFB0403003), the National Natural Science Foundation of China (Grant Nos. 61774081, 61322403, and 91850112), the State Key Research and Development Project of Jiangsu Province, China (Grant No. BE2018115), Shenzhen Fundamental Research Project,

We report on the transition of photovoltaic and photoconductive operation modes of the amorphous Ga2O3-based solar-blind photodetectors in metal–semiconductor–metal (MSM) configurations. The conversion from Ohmic to Schottky contacts at Ti/Ga2O3 interface is realized by tuning the conductivity of amorphous Ga2O3 films with delicate control of oxygen flux in the sputtering process. The abundant donor-like oxygen vacancies distributed near the Ti/Ga2O3 interface fascinate the tunneling process across the barrier and result in the formation of Ohmic contacts. As a consequence, the serious sub-gap absorption and persistent photoconductivity (PPC) effect degrades the performance of the photoconductive detectors. In contrast, the photovoltaic device with a Schottky contact exhibits an ultra-low dark current less than 1 pA, a high detectivity of , a fast response time of , and a high ultraviolet C (UVC)-to-ultraviolet A (UVA) rejection ratio of 103. The promoting performance is attributed primarily to the reduction of the sub-gap states and the resultant suppression of PPC effect. With simple architecture, low fabrication cost, and easy fusion with modern high-speed integrated circuitry, these results provide a cost-effective way to realize high performance solar-blind photodetectors towards versatile practical applications.