Solar-blind UV Photodetectors Research Articles

MOCVD growth of MgGa2O4 thin films for high-performance solar-blind UV photodetectors

Epitaxial MgGa2O4 thin films with a bandgap of 5.18 eV were grown on the c-plane sapphire substrate by using metal organic chemical vapor deposition. The structure, optical, electronic, and optoelectronic properties of MgGa2O4 thin films have been investigated before and after high-temperature annealing in an oxygen or nitrogen atmosphere. In particular, the O2-annealed MgGa2O4 thin film reveals a high crystalline quality and a low concentration of oxygen vacancies. Moreover, a quick response speed (tr = 20 ns, td = 400 ns), a low dark current (∼17 pA at 10 V), and a high UV/Visible rejection ratio (>105) can be demonstrated in the O2-annealed MgGa2O4 photodetector, indicating the excellent solar-blind ultraviolet (SBUV) photodetection characteristics. The effect of annealing atmosphere on the photoelectric properties of the film and its physical mechanism were studied. This research provides an effective way to realize high-performance SBUV photodetectors and opens up the application of MgGa2O4 spinel in the field of semiconductor optoelectronic and microelectronic devices.

Fast-speed self-powered PEDOT:PSS/α-Ga2O3 nanorod array/FTO photodetector with solar-blind UV/visible dual-band photodetection

The α-Ga2O3 nanorod array is grown on FTO by hydrothermal and annealing processes. And a self-powered PEDOT:PSS/α-Ga2O3 nanorod array/FTO (PGF) photodetector has been demonstrated by spin coating PEDOT:PSS on the α-Ga2O3 nanorod array. Successfully, the PGF photodetector shows solar-blind UV/visible dual-band photodetection. Our device possesses comparable solar-blind UV responsivity (0.18 mA/W at 235 nm) and much faster response speed (0.102 s) than most of the reported self-powered α-Ga2O3 nanorod array solar-blind UV photodetectors. And it presents the featured and distinguished visible band photoresponse with a response speed of 0.136 s at 540 nm. The response time is also much faster than the other non-self-powered β-Ga2O3 DUV/visible dual-band photodetectors due to the fast-speed separation of photogenerated carries by the built-in electric field in the depletion regions of PEDOT:PSS/α-Ga2O3 heterojunction. The results herein may prove a promising way to realize fast-speed self-powered α-Ga2O3 photodetectors with solar-blind UV/visible dual-band photodetection by simple processes for the applications of multiple-target tracking, imaging, machine vision and communication.

Enhancement of Responsivity in Solar-Blind UV Detector With Back-Gate MOS Structure Fabricated on β-Ga2O3 Films

Monoclinic Ga2O3 (β-Ga2O3) films were grown on Si/SiO2 by using MOCVD. Then, we fabricated the solar-blind photodetector with a back-gate MOS structure. The device exhibited obvious photoresponse under 254-nm UV light illumination, and the photocurrent increased by five orders of magnitude, which could be controlled by VGS. The current generated under dark conditions could also be regulated by VGS and tended to constant when the regulation of VGS was reaching saturation. Meanwhile, VGS was confirmed to have a certain ability to regulate the photocurrent. The present device demonstrated excellent stability and fast response (rise) and recovery (decay) times under the 254-nm light illumination as well as a responsivity of 417.5 A/W, suggesting a valuable application in solar-blind UV 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.

Reinforcement of double built-in electric fields in spiro-MeOTAD/Ga2O3/Si p–i–n structure for a high-sensitivity solar-blind UV photovoltaic detector

The spiro-MeOTAD/Ga2O3/Si p–i–n structure was developed for a high-sensitivity solar-blind UV photodetector.

The growth of HVPE α-Ga2O3 crystals and its solar-blind UV photodetector applications

While Ga2O3 crystals are considered as the next generation optoelectronic materials, the exploration of HVPE α-Ga2O3 crystals and their use as solar-blind UV photodetector are still insufficient. In this paper, we investigated the structural characterization of HVPE α-Ga2O3 materials and their properties as solar-blind photodetector. HVPE α-Ga2O3 exhibited excellent structural properties with relatively good crystallinity, and smooth surface morphology, without any complicated process. Raman investigations confirmed the existence of slight compressive strain in the as-grown HVPE α-Ga2O3. UV–visible spectrophotometry proved that HVPE α-Ga2O3 had an optical bandgap of 5.15 eV, evidencing the suitability of this material for application as solar-blind photodetector. Furthermore, the photodetector synthesized using HVPE α-Ga2O3 showed excellent photo-response characteristics. We believe that this study will support further development of functional oxide semiconductor materials and opto-electronic devices.

Effects of annealing process on the structural and photodetection properties of new thin-film solar-blind UV sensor based on Si-photonics technology

In this paper, a new heterojunction structure based on ultrathin-film ITO sputtered on non-hydrogenated amorphous-silicon (a-Si) is developed for high-detectivity solar-blind UV-photodetector (UV-PD) based on silicon (Si) photonics technology. A strategic combination of Particle Swarm Optimization (PSO) and numerical analysis is used to find out the best design offering superior optoelectronic performance. The optimized design is then elaborated using RF magnetron sputtering technique. A comprehensive investigation of the device structural and optoelectronic properties was carried out, incorporating the influence of heat treatment at temperature values ranging from 300°C to 600°C. X-Ray Diffraction (XRD) measurements indicate that the crystallinity of the sputtered layers was enhanced by increasing the annealing temperature. Significantly, photoelectrical characterization showed that the annealed ITO/a-Si UV-PD exhibits high detectivity exceeding 1013 Jones with a highly improved UV-to-Vis rejection ratio of 5.7 × 103. The ITO/a-Si heterojunction generated a built-in potential, enabling effective separation and transport of photo-induced carriers, thereby reducing recombination losses. Therefore, by well optimizing the proposed heterostructure and the annealing conditions, we were able to elaborate new highly detective, thin-film solar-blind UV-PD based on Si-photonics platform, which can be a promising alternative for future high-performance and cost-effective optoelectronic systems.

Photo-Electrochemical Etching of Single Crystalline β-Ga2O3 Thin Film

β-Ga2O3 is a next-generation semiconductor material having ultra-wide bandgap energy of 4.8 eV and excellent electrical properties. So it is expected to be suitable for various applications such as power electronics, solar-blind UV photodetectors and gas sensors. However, there are some issues to overcome in order for β-Ga2O3 to be commercialized. Among them, crystal defects are important factor that significantly affects device performance. Defects are formed intrinsically during the growth process, or caused by high-energy plasma ion during dry etching. It has been reported that the electrical characteristics are deteriorated in the device such as MOSFET and MISFET fabricated by inductively coupled plasma etching. Unlike dry etching, which can cause damage by plasma ions, wet etching is a damage-free process that uses a chemical reaction between the target material and etchant. In addition, wet etching may be used for removing defects by using the etching characteristic, which the chemically unstable defect areas are preferentially etched. However, β-Ga2O3 hardly reacts with etchant at room temperature, and has been reported to have etch rate of several tens of nm/min when acid etchant are used at higher than 100 °C.In our study, we have tried to increase the etch rate of β-Ga2O3 and remove defects effectively by Photo-electrochemical (PEC) etching. PEC etching is a method that promotes the reaction by irradiated light having energy above bandgap and applied external bias. Single crystalline β-Ga2O3 thin films are cleaved in the [100] direction with mechanical force from single crystalline β-Ga2O3 substrate. Ti/Au was deposited to form Ohmic contact with β-Ga2O3 thin films. With the PEC etching method, we could etch at a lower temperature than conventional wet etching. In addition, it was confirmed that the etch rate can be controlled according to the external bias condition. The details of our experiment conditions and results will be presented at the meeting.

Characterization of Epitaxial β-(Al,Ga,In)2O3-Based Films and Applications as UV Photodetectors

Epitaxial films of β-(AlxGa1−x)2O3, β-Ga2O3, and β-(InxGa1−x)2O3 were grown on (001) sapphire substrates via metalorganic chemical vapor deposition (MOCVD). The compositions of the films as determined from energy dispersive x-ray analysis (EDX) and x-ray photoelectron spectroscopy (XPS) results were XAl = 0.57 ± 0.05 and 0.76 ± 0.05 and XIn = 0.12 ± 0.05 and 0.21 ± 0.05. The optical bandgap was found to correspondingly vary between 6.0 ± 0.2 and 3.9 ± 0.1 eV, as a function of composition via XPS and UV–visible spectroscopy (UV–Vis). X-ray diffraction, scanning electron microscopy, and atomic force microscopy revealed the films to be highly-oriented $$ \left( {\bar{2}01} \right) $$-epitaxial films with nanocrystalline domains. Schottky- and MSM-based solar-blind UV photodetectors were fabricated on the films and showed responsivities at 20 V varying from > 104 A/W for the Ga2O3 devices, > 103 A/W for the (AlxGa1−x)2O3 devices and > 102 A/W for the (InxGa1−x)2O3 devices. Modest shifts in wavelength selectivity corresponding with the changes in composition/bandgap were also measured. Time response measurements on Schottky and MSM detectors reveal rise and dwell times on the order of a minute, indicating the presence of photoconductive gain. Noise-equivalent powers were in the fW–pW regime with specific detectivities ($$ D^{*} $$) between 1010 and 1012 Jones. Scanning photocurrent maps display large photocurrent generation at the Schottky interface in the case of a β-Ga2O3 Schottky detector, whereas for an β-(InxGa1−x)2O3 MSM detector the photocurrent generation occurs in the device channel and at the Schottky interface.

Highly sensitive, ultra-low dark current, self-powered solar-blind ultraviolet photodetector based on ZnO thin-film with an engineered rear metallic layer

In this paper, novel self-powered, solar-blind UV photodetector (PD) designs based on a ZnO thin-film with engineered back metal layer (BML) were fabricated by RF magnetron sputtering and e-beam evaporation techniques. An exhaustive study concerning the impact of dissimilar BML (Au and Ni) on the device structural, optical and electrical properties was carried out. The measured I–V curves illustrated an asymmetrical behavior, enabling a clear and distinctive photovoltaic mode. Superb sensitivity of 107, high ION/IOFF ratio of 149dB, ultralow dark-noise current less than 11pA and responsivity exceeding 0.27A/W were reached for the prepared ZnO-based UV-PDs in self-powered mode. The role of the engineered BML in promoting effective separation and transfer of the photo-induced carriers was discussed using the band-diagram theory. The influence of the annealing process on the UV-sensor performance was also investigated. The annealed device at 500°C demonstrated a lower dark current of a few picoamperes and a high rejection ratio of 2.2×103, emphasizing its exciting visible blindness characteristics. Therefore, the use of an engineered BML with optimized annealing conditions open up new perspectives to realizing high-performance, self-powered solar-blind UV-PDs based on simple thin-film-ZnO structure strongly desirable for various optoelectronic applications.

Nanovoid-driven highly crystalline aluminum nitride and its application in solar-blind UV photodetectors

Surface treatment in AlN layer introduces nanovoids, which suppressed the propagation of threading dislocations to the surface, thus encouraging the growth of high crystalline AlN.

High sensitivity and fast response self-powered solar-blind ultraviolet photodetector with a β-Ga2O3/spiro-MeOTAD p–n heterojunction

A β-Ga2O3/spiro-MeOTAD organic–inorganic p–n heterojunction was fabricated for use in a high sensitivity and fast response self-powered solar-blind UV photodetector.

An ultrahigh responsivity self-powered solar-blind photodetector based on a centimeter-sized β-Ga2O3/polyaniline heterojunction.

Wide band gap semiconductors are promising UV photodetector materials due to their suitable bandgap, high crystal quality, strong absorption and large carrier mobility. Up to now, deep UV photodetectors are mainly based on epitaxial thin films, which have some undesired properties such as p-type doping difficulty. Lattice mismatch hinders the further development of these devices. Here, a high performance self-powered solar-blind UV photodetector was realized by a facile combination of a centimeter-sized single crystal β-Ga2O3 microwire and polyaniline. Owing to the excellent organic/inorganic hybrid p-n junction, the device shows an ultrahigh responsivity of 21 mA W-1 at 246 nm with a sharp cut-off wavelength of 272 nm without an external power supply. Moreover, the dark current is 0.08 pA, which is smaller than those of almost all the previous metallic oxide based solar-blind UV photodetectors. The photodetector also shows a high UV/visible rejection ratio (102) at zero bias voltage. Finally, a physical model of the self-powered photodetector is also proposed. This work provides a simple, low-cost, and effective method for preparing high performance self-powered solar-blind UV photodetectors based on organic/inorganic heterojunctions.

Responsivity improvement of a packaged ZnMgO solar blind ultraviolet photodetector via a sealing treatment of silica gel

A packaged high performance ZnMgO solar-blind UV photodetector is prepared via a silica gel sealing treatment. The responsivity and stability of the device is improved according to this sealing treatment.

High Responsivity and High Rejection Ratio of Self-Powered Solar-Blind Ultraviolet Photodetector Based on PEDOT:PSS/β-Ga2O3 Organic/Inorganic p-n Junction.

A high responsivity self-powered solar-blind deep UV (DUV) photodetector with high rejection ratio was proposed based on inorganic/organic hybrid p-n junction. Owing to the high crystallized β-Ga2O3 and excellent transparent conductive polymer PEDOT:PSS, the device exhibited ultrahigh responsivity of 2.6 A/W at 245 nm with a sharp cutoff wavelength at 255 nm without any power supply. The responsivity is much larger than that of previous solar-blind DUV photodetectors. Moreover, the device exhibited an ultrahigh solar-blind/UV rejection ratio (R245nm/R280nm) of 103, which is two orders of magnitude larger than the average value reported in Ga2O3-based solar-blind photodetectors. In addition, the photodetector shows a narrow bandpass response of only 17 nm in width. This work might be of great value in developing a high wavelength selective DUV photodetector with respect to low cost for future energy-efficient photoelectric devices.

High-Responsivity MSM Solar-Blind UV Photodetector Based on Annealed ITO/Ag/ITO Structure Using RF Sputtering

In this paper, a novel ITO/Ag/ITO (IAI) multilayer UV photodetector was successfully prepared via RF magnetron sputtering technique to achieve the dual-benefit of improved photoresponse and outstanding visible blindness properties. The influence of the annealing process on the performance of the elaborated IAI UV photodetector was carried out. Moreover, the structural and photoelectrical properties of the sputtered tri-layered structures with and without annealing effects were also analyzed. It was found that the elaborated UV -photodetector provides near perfect UV absorbance of 96%. This achievement is attributed to the improved light trapping capability and tunable plasmonic resonance effects induced by the efficient optical coupling between free-space UV photons and Ag plasmons. It was also revealed that the heat treatment paves a new path toward preparing efficient visible-blind UV photodetectors, where the annealed sample demonstrated a superior rejection ratio of 225 as compared to that of the unannealed counterpart. Besides, electrical characterization underlined the ability of the annealed IAI UV photodetector at 500°C for offering exciting opportunities, where it exhibits a high responsivity of 3.8mA/W at self-powered condition. Therefore, by well optimizing both IAI multilayer geometry and annealing conditions, we were able to elaborate an ultrathin photodetector suitable for high-performance and flexible UV sensing applications.

Zero-biased deep ultraviolet photodetectors based on graphene/cleaved (100) Ga2O3 heterojunction.

In this paper, fast response, zero-biased, solar-blind UV photodetectors based on graphene/β-Ga2O3 heterojunctions were fabricated by transferring a monolayer graphene onto fresh cleaved β-Ga2O3 (100) single crystal substrate. At zero bias, the photo responsivity at 254 nm and the UV/visible rejection ratio (R235 nm/R400 nm) and the response time are obtained to be 10.3 mA/W and 2.28 × 102 and 2.24 μs, respectively, for the graphene/β-Ga2O3 (100) detector. The fast response and the high sensitivity can be attributed to the high mobility and UV transparency of graphene top-electrode and the low defect density of the β-Ga2O3 (100) cleaved surface. Such zero-biased detectors are very promising for next-generation solar-blind UV photodetection.

Solution Grown Single-Unit-Cell Quantum Wires Affording Self-Powered Solar-Blind UV Photodetectors with Ultrahigh Selectivity and Sensitivity.

As crystalline semiconductor nanowires are thinned down to a single-unit-cell thickness, many fascinating properties could arise pointing to promising applications in various fields. A grand challenge is to be able to controllably synthesize such ultrathin nanowires. Herein, we report a strategy, which synergizes a soft template with oriented attachment (ST-OA), to prepare high-quality single-unit-cell semiconductor nanowires (SSNWs). Using this protocol, we are able to synthesize for the first time ZnS and ZnSe nanowires (NWs) with only a single-unit-cell thickness (less than 1.0 nm) and a cluster-like absorption feature (i.e., with a sharp, strong, and significantly blue-shifted absorption peak). Particularly, the growth mechanism and the single-unit-cell structure of the as-prepared ZnS SSNWs are firmly established by both experimental observations and theoretical calculations. Thanks to falling into the extreme quantum confinement regime, these NWs are found to only absorb the light with wavelengths shorter than 280 nm (i.e., solar-blind UV absorption). Utilizing such a unique property, self-powered photoelectrochemical-type photodetectors (PEC PDs) based on the ZnS SSNWs are successfully fabricated. The PDs after interface modification with TiO2 exhibit an excellent solar-blind UV photoresponse performance, with a typical on/off ratio of 6008, a detectivity of 1.5 × 1012 Jones, and a responsivity of 33.7 mA/W. This work opens the door to synthesizing and investigating a new dimension of nanomaterials with a wide range of applications.

Gallium oxide solar-blind ultraviolet photodetectors: a review

This review introduces the developments in β-Ga2O3 materials growth and solar blind UV photodetectors in the last decade, summarizes their advantages and potential for improvement, and puts forward some suggestions for actual application requirements.

Synthesis of free-standing Ga2O3 films for flexible devices by water etching of Sr3Al2O6 sacrificial layers**Project supported by the National Natural Science Foundation of China (Grant Nos. 51572033, 51572241, 61774019, 61704153, and 11404029), the Fund of State Key Laboratory of IPOC (BUPT), and the Open Fund of IPOC (BUPT), Beijing Municipal Commission of Science and Technology, China (Grant No. SX2018-04).

Flexible electronic devices have attracted much attention due to their practical and commercial value. Integration of thin films with soft substrate is an effective way to fabricate flexible electronic devices. Ga2O3 thin films deposited directly on soft substrates would be amorphous mostly. However, the thickness of the thin film obtained by mechanical exfoliation method is difficult to control and the edge of the film is fragile and easy to be damaged. In this work, we fabricated free-standing Ga2O3 thin films using the water-soluble perovskite Sr3Al2O6 as a sacrificial buffer layer. The obtained Ga2O3 thin films were polycrystalline. The thickness and dimension of the films were controllable. A flexible Ga2O3 solar-blind UV photodetector was fabricated by transferring the free-standing Ga2O3 film on a flexible polyethylene terephthalate substrate. The results displayed that the photoelectric performances of the flexible Ga2O3 photodetector were not sensitive to bending of the device. The free-standing Ga2O3 thin films synthesized through the method described here can be transferred to any substrates or integrated with other thin films to fabricate electronic devices.