Solar-blind Ultraviolet Detection Research Articles

Heterojunction polarization enhancement and shielding for AlGaN-based solar-blind ultraviolet avalanche detectors.

AlGaN-based solar-blind ultraviolet avalanche detectors have huge potentials in the fields of corona discharge monitoring, biological imaging, etc. Here, we study the impact of the heterojunction polarization-related effects on the AlGaN-based solar-blind ultraviolet avalanche detectors. Our work confirms that the polarization heterojunction is beneficial to reducing avalanche bias and lifting avalanche gain by improving the electric field in the depletion region, while the polarization-induced fixed charges will lead to a redistribution of the electrons, in turn shielding the charges and weakening the electric field enhancement effect. This shielding effect will need external bias to eliminate, and that is why the polarization heterojunction cannot work at relatively low bias but has an enhancement effect at high bias. Controlling the doping level between the hetero-interface can affect the shielding effect. An unintentionally doped polarization heterojunction can effectively reduce the shielding effect, thus reducing the avalanche bias. The conclusions also hold true for the negative polarization regime. We believe our findings can provide some useful insights for the design of the AlGaN-based solar-blind ultraviolet detectors.

Solar-blind UV detector with fast response speed and an ultrahigh detectivity based on SrTiO3 (111) substrate grown β-Ga2O3 thin film

Wide band gap semiconductor β-Ga2O3 thin film is widely used in photodetectors. Along the β-Ga2O3 [102] direction, the lattice mismatch between (2‾ 01) β-Ga2O3 and (111) SrTiO3 (STO) is approximately 2.4 %. STO can be used as a substrate for heterostructure fabrication. Here, taking advantage of the symmetry and interatomic super-cellular matching between β-Ga2O3 and the STO substrate, β-Ga2O3 epitaxial films were cultivated on STO (111) substrates via an LPCVD technique. Photoconductive solar blind ultraviolet (UV) detectors have been fabricated based on cultivated Ga2O3 thin films, and the intrinsic linkage of the detector performance with temperature variation has been analyzed in detail. The composition, surface morphology and electrical properties of the films were characterized. The optimized device showed a response rate of 66.64 A/W. The fast rise time (τr1/τr2) and decay time (τd1/τd2) at 20 V bias were 0.15 s/0.18 s and 0.20 s/0.25 s, respectively, with the detectivity of 4.1x1012 Jones.

Transmission enhancement in coupled nanohole and nanodisk arrays for solar blind UV filter

Extraordinary optical transmission (EOT) based on metallic nanohole array has great potential for optical filtering, owing to its spectral selectivity and structure-dependent tunability. However the transmittance of EOT is relatively low owing to the large loss of the metal film, particularly in the UV waveband. Herein, we propose a high transmission narrowband ultraviolet filter based on aluminum compound nanostructures on a UV-grade fused silica substrate. These compound nanostructures are consisted of periodic nanodisk and nanohole arrays with the same period in a staggered rectangular arrangement. Numerical simulations using the finite-difference time-domain (FDTD) method have shown that the compound structures exhibit high transmittance of over 70% and a narrower bandwidth of less than 50 nm in the 200–300 nm spectral region compared with the conventionally EOT of pure metallic nanohole arrays. Moreover, a broad suppression in the wavelength ranges of 300 to 1100 nm was achieved. The enhanced performance is attributed to the coupling between the surface plasmon polariton (SPP) of nanohole arrays and the localized surface plasmon resonance (LSPR) of nanodisk arrays. The compound coupled nanostructures can be used in solar-blind ultraviolet detectors and the enhancement mechanism has potential for use in other spectral regions.

Enabling Broadband Solar-Blind UV Photodetection by a Rare-Earth Doped Oxyfluoride Transparent Glass-Ceramic.

Oxyfluoride transparent glass-ceramics (GC) are widely used as the matrix for rare-earth (RE) ions due to their unique properties such as low phonon energy, high transmittance, and high solubility for RE ions. Tb3+ doped oxyfluoride glasses exhibit a large absorption cross section for ultraviolet (UV) excitation, high stability, high photoluminescence quantum efficiency, and sensitive spectral conversion characteristics, making them promising candidate materials for use as the spectral converter in UV photodetectors. Herein, a Tb3+ doped oxyfluoride GC isdeveloped by using the melt-quenching method, and the microstructure and optical properties of the GC sample are carefully investigated. By combining with a Si-based photo-resistor,a solar-blind UV detector is fabricated, which exhibits a significant photoelectric response with a broad detection range from 188 to 400nm. The results indicate that the designed UV photodetector is of great significance for the development of solar-blind UV detectors.

Surface chemical composition and HRTEM analysis of heteroepitaxial β-Ga2O3 films grown by MOCVD

β gallium oxide (β-Ga2O3) is an ultrawide bandgap semiconductor with great potential for solar-blind ultraviolet detection, power devices, and gas sensing. However, the understanding of the surface properties of β-Ga2O3 remains insufficient. Here, epitaxial growth of β-Ga2O3 films was carried by metal–organic chemical vapor deposition (MOCVD) on sapphire substrates. The surface chemical composition, surface layer crystalline properties, and surface morphology of the β-Ga2O3 films were discussed and analyzed in detail. By comparing the atomic ratios of O/Ga, the surface morphology, and the crystalline properties of the films, we attribute the component of O 1s with the binding energy around 531.8 eV to surface lattice O atoms. Moreover, the one-dimensional defects inside the surface layers and the near-interface regions of the films were observed at atomic scale by high resolution transmission electron microscopy (HRTEM). By comparing the lattice fringes of different facets, the geometric phase uniformity, and the distribution of strain, we found that the crystalline quality of the surface layers is much higher than the near-interface regions.

Fabrication of h-BN solar-blind ultraviolet detectors by RF magnetron sputtering

Hexagonal boron nitride (h-BN) has become a popular material in solar-blind ultraviolet (SBUV) detection due to its ultra-wide bandgap, high optical absorption coefficient, high breakdown field strength and high thermal conductivity. However, the high deposition temperature restricts the choice of h-BN film substrates and hinders the fabrication of h-BN flexible devices. In this work, several h-BN films have been prepared by RF magnetron sputtering with B as the target at room temperature, and based on the films, SBUV detectors were also fabricated with the metal-semiconductor-metal (MSM) structure, which exhibit an extremely low dark current (0.07 pA) and a high response (1.37 μA/W) at 3 V. These results establish the feasibility of fabricating h-BN films and SBUV detectors at room temperature.

AlGaN-Based Self-Powered Solar-Blind UV Focal Plane Array Imaging Photosensors: Material Growth, Device Preparation, and Functional Verification

This paper reports the development of hybrid AlGaN-based self-powered solar-blind ultraviolet (UV) focal plane array imaging photosensors from material growth and array preparation to functional verification. The detailed process starts with the epitaxial growth by metal-organic chemical vapor deposition (MOCVD). A high-quality AlN template is obtained on the 2-inch double-polished c-plane sapphire substrate by introducing a mesothermal AlN (MT-AlN) interlayer and adjusting the growth rate of the high-temperature AlN epilayer (HT-AlN). Then, a polarization-enhanced p-i-n structure photodetector material is grown on the AlN template. Subsequently, based on the p-i-n structural material, a 320×256 back-illuminated solar-blind photodiode array is fabricated and hybridized to a matching Si-based CMOS readout integrated circuit (ROIC) chip to form a focal plane detector. Solar-blind UV detection is observed throughout the array in the 262-281 nm spectral region with a peak external quantum efficiency (EQE) of 65.3% at zero-bias voltage (no antireflection coating) and a nanosecond transient response time at a reverse bias of 5 V. The visible response of the ROIC is eliminated by developing a masking technology that is opaque to visible light, which enables the focal plane detector to achieve a high UV/visible rejection ratio more than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> .

Solar-Blind Ultraviolet Detectors Based on High-Quality HVPE α-Ga2O3 Films With Giant Responsivity

Solar-Blind Ultraviolet Detectors Based on High-Quality HVPE α-Ga₂O₃ Films With Giant Responsivity

Preparation and photodetection performance of high crystalline quality and large size β-Ga2O3 microwires

Ultrawide band gap semiconductors are promising solar-blind ultraviolet (UV) photodetector materials due to their suitable bandgap, strong absorption and high sensitivity. Here, β-Ga2O3 microwires with high crystal quality and large size were grown by the chemical vapor deposition (CVD) method. The microwires reach up to 1 cm in length and were single crystalline with low defect density. Owing to its high crystal quality, a metal–semiconductor–metal photodetector fabricated from a Ga2O3 microwire showed a responsivity of 1.2 A/W at 240 nm with an ultrahigh UV/visible rejection ratio (R peak/R 400 nm) of 5.8 × 105, indicating that the device has excellent spectral selectivity. In addition, no obvious persistent photoconductivity was observed in the test. The rise and decay time constants of the device were 0.13 and 0.14 s, respectively. This work not only provides a growth method for high-quality Ga2O3 microwires, but also demonstrates the excellent performance of Ga2O3 microwires in solar-blind ultraviolet detection.

High-Performance Self-Driven Solar-Blind Ultraviolet Photodetectors Based on HfZrO2/β-Ga2O3 Heterojunctions.

Ga2O3 is a wide-bandgap semiconductor that has shown great potential for application in solar-blind ultraviolet (UV) photodetectors. However, the responsivity and detectivity of Ga2O3-based self-driven solar-blind UV photodetectors are insufficient for practical applications at present because of the limited separation of photogenerated carriers in the devices. In this work, Hf0.5Zr0.5O2/β-Ga2O3 heterojunction-based self-driven solar-blind UV photodetectors are constructed by combining ferroelectric Hf0.5Zr0.5O2 (HfZrO2) material with Ga2O3, taking advantage of the ultrawide bandgap of HfZrO2 and the favorable II-type energy band configuration between both. Upon optimization, a HfZrO2/β-Ga2O3 heterojunction-based UV photodetector with a HfZrO2 layer thickness of 10 nm is shown to provide remarkable responsivity (R = (14.64 ± 0.3) mA/W) and detectivity (D* = (1.58 ± 0.03) × 1012 Jones), which are much superior to those of a single Ga2O3-based device toward 240 nm light illumination. Further, the device performance is adjustable with varying poling states of HfZrO2 and shows substantial enhancement in the upward poling state, benefiting from the constructive coupling of the ferroelectric depolarization electric field in HfZrO2 and the built-in electric field at the HfZrO2/β-Ga2O3 interface. Under illumination of weak light of 0.19 μW/cm2, the upward poled device shows significantly enhanced R (52.6 mA/W) and D* (5.7 × 1012 Jones) values. The performance of our device surpasses those of most previously reported Ga2O3-based self-driven photodetectors, indicating its great potential in practical applications for sensitive solar-blind UV detection.

A strategy of high-sensitivity solar-blind photodetector for fabricating graphene surface modification ZnGa2O4/Ga2O3 core-shell structure nanowire networks

One-dimensional core-shell nanowires are considered the optimal choice for high-performance photodetectors with the ideal interfacial area, well-controlled structure, and better light absorption ability. There are lots of research about Ga2O3 wide band gap semiconductors applied in solar-blind photodetectors. However, the defects such as relatively slow response/recovery time and poor response rate have limited its application in photodetectors. In this work, a GaZn2O4/Ga2O3 core-shell nanostructure is successfully prepared by the CVD (Chemical Vapor Deposition) technique. Then, a strategy of MSM solar blind ultraviolet detector based on graphene surface modification core-shell nanowires is proposed. The device has a characteristic of nanowire network structure, interface heterojunction, and Schottky contact. According to the results, a significant performance of the device with 0.1% graphene surface modified ZnGa2O4/Ga2O3 nanowires is achieved, like, the faster response time (rise/down time, 130 m s/13 m s), and the higher detection capability of the solar-blind ultraviolet detector (responsivity, 64.5 mA/W). Our work provides a facile fabrication procedure to construct a high-performance photodetector.

Narrow-Band Solar-Blind Ultraviolet Detectors Based on AlSnO Films with Tunable Band Gap.

Semiconductor materials with sufficiently wide band gaps are urgently desired for use in solar-blind ultraviolet detectors. In this work, the growth of AlSnO films was achieved through the magnetron sputtering technique. AlSnO films with band gaps in the range of 4.40-5.43 eV were obtained by varying the growth process, which demonstrates that the band gap of AlSnO is continuously tunable. What is more, based on the films prepared, narrow-band solar-blind ultraviolet detectors were fabricated with good solar-blind ultraviolet spectral selectivity, excellent detectivity, and narrow full widths at half-maximum in the response spectra, showing a great potential to be applied to solar-blind ultraviolet narrow-band detection. Therefore, based on the results above, this study focusing on the fabrication of detectors via band gap engineering can be a significant reference for researchers interested in solar-blind ultraviolet detection.

Band Gap Engineering of AlYN Films for Solar-Blind Ultraviolet Photodetection

Aluminum nitride (AlN) has been proven as a potential photoelectric material with high breakdown field strength, excellent thermal conductivity, and stable physical properties. However, its ultrawide intrinsic band gap (6.2 eV) prohibits the direct application of AlN as an optical absorption layer for solar-blind ultraviolet (SBUV) detectors. To solve this problem, AlYN thin films with a SBUV absorption cut-off edge of 264 nm were prepared first by energy band engineering using yttrium (Y) as the dopant, based on which a photovoltaic detector with a p-Gr/i-AlYN/n-Si structure was fabricated and exhibited excellent performance of SBUV detection at 0 V bias, including an extremely short rise time (tr) of 20 ms, a decay time (td) of 60 ms, an ultrahigh detectivity (D*) of 5.09 × 1012 Jones, a responsivity (R) of 13 mA/W, a maximum external quantum efficiency (EQE) of 6.69%, and a rejection ratio (R255nm/R310nm) of 245. This work has great value as a reference for the preparation of high-performance AlN-based SBUV detectors.

CVD Approach to a Single Gallium Oxide Nanowire for Solar-blind UV Detector

Aims: Finding a simple, energy-saving and low-cost synthesis method to process gallium oxide nanowires by CVD for solar-blind detector. Background: Due to a bandgap of 4.5 ~ 4.9 eV and a high breakdown field strength of 8 MV/cm, gallium oxide (Ga2O3) has great application prospects in solar blind ultraviolet detection and highpower devices. Notably, Ga2O3 NWs are currently the key research objects of solar-blind UV detection materials because of the characteristics of efficient photocarrier separation and collection, and the bandgap width perfectly matches the energy of deep ultraviolet photons. Objective: To find a simple method to synthesize Ga2O3 nanowire with smooth surface and uniform diameter, and the prepared single gallium oxide nanowire UV detector has high photoelectric conversion efficiency. Methods: Ga2O3 NWs are prepared on the SiO2/Si substrate by chemical vapor deposition (CVD) approach at low reaction temperature with gold particles serving as the catalyst and gallium arsenide (GaAs) as a gallium source. Results: X-ray diffraction and Raman spectroscopy characterization indicate the crystal structure of NWs is β-Ga2O3, and scanning electron microscope (SEM) characterization proves that the NWs have a uniform diameter and smooth surface. Moreover, the high-resolution transmission electron microscopy (HRTEM) characterization shows that the material had high crystal quality. Moreover, the photoconductive solar-blind UV detector with a single Ga2O3 NW is prepared, showing the excellent performance of the high responsivity and external quantum efficiency. Conclusion: The effects of growth temperature and the size of gold catalyst on the morphology of β-Ga2O3 NWs have been investigated. The results show that with the reaction temperature is 625 °C and the diameter of Au catalyst is about 30~50 nm, it is more conducive to the formation of NMs with crystal structure, smooth surface and uniform diameter. The performance of the solar-blind UV photodetector shows that the device has higher sensitivity (R = 149.82 A/W), external quantum efficiency (EQE = 73206%), and response rate τrise = 0.66 s, and τdown = 0.45 s.

A High-Performance ε-Ga2O3-Based Deep-Ultraviolet Photodetector Array for Solar-Blind Imaging.

One of the most important applications of photodetectors is as sensing units in imaging systems. In practical applications, a photodetector array with high uniformity and high performance is an indispensable part of the imaging system. Herein, a photodetector array (5 × 4) consisting of 20 photodetector units, in which the photosensitive layer involves preprocessing commercial ε-Ga2O3 films with high temperature annealing, have been constructed by low-cost magnetron sputtering and mask processes. The ε-Ga2O3 ultraviolet photodetector unit shows excellent responsivity and detectivity of 6.18 A/W and 5 × 1013 Jones, respectively, an ultra-high light-to-dark ratio of 1.45 × 105, and a fast photoresponse speed (0.14/0.09 s). At the same time, the device also shows good solar-blind characteristics and stability. Based on this, we demonstrate an ε-Ga2O3-thin-film-based solar-blind ultraviolet detector array with high uniformity and high performance for solar-blind imaging in optoelectronic integration applications.

Thickness dependent optical properties of amorphous/polycrystalline Ga2O3 thin films grown by plasma-enhanced atomic layer deposition

Amorphous/polycrystalline Ga2O3 is attracting considerable attention because its low cost and convenience of use make it a promising material for applications of solar blind ultraviolet detectors and power electronics, except for the widely investigated α/β phase Ga2O3. Spectroscopic ellipsometry (SE) is the preferred technique for determining optical constants that are vital for device design. The bandgap energy of Ga2O3 is close to the upper energy limit of commercial ellipsometers, making it challenging to determine the optical constants of Ga2O3 films. Considering the results obtained using the point-by-point method, we developed a strategy using 2 Tauc-Lorentz (2TL) oscillators that reasonably describes the optical behavior of the amorphous/polycrystalline Ga2O3 films deposited by plasma-enhanced atomic layer deposition on Si, sapphire, and Si/SiO2 substrates. The refractive index of the Ga2O3 films increases after annealing due to the reduced thickness and polycrystalline structure of the films. The polycrystalline samples have larger bandgaps than amorphous samples. The optical constants of Ga2O3 films with thicknesses ranging from ∼ 3 to ∼ 28 nm were extracted through SE fitting by the developed 2TL method. For both the as-deposited and annealed films, the bandgap increases as the film thickness decreases. The obtained results are useful for the design of photodetectors and power devices.

Performance optimization of CZT thick films by CdTe buffer layers for solar-blind ultraviolet photoelectric detectors

CdZnTe (CZT) thick films with CdTe buffer layers (CZT/CdTe) were studied for high-performance ultraviolet photoelectric detector. CZT thick films were prepared on AlN/Si substrates via close-spaced sublimation (CSS) and CdTe buffer layers were introduced to optimize the interface quality of CZT thick films. Results of x-ray diffraction showed that the full width at half maximum (FWHM) and dislocation density (δ) of interface of optimized samples were 0.156 ° and 2.543 × 1014 lin/m2, respectively. Interface detect analysis of free-standing CZT and CZT/CdTe thick films was studied by x-ray photoelectron spectroscopy (XPS) and low-temperature photoluminescence (PL) spectroscopy, and mechanism of in-situ annealing in CdTe atmosphere to reduce interfacial defect states was discussed. Moreover, the photo-dark current ratio of the CZT/CdTe thick film metal-semiconductor-metal (MSM) structural photodetector was as high as 5530, which was nearly double that of the CZT thick film detector, and the carrier mobility lifetime product could reach 5.59 × 10−4 cm2/V at 100 V bias, confirming the improvement of carrier transport capacity and detection efficiency. CZT-based detectors have inestimable prospect in the field of solar-blind ultraviolet detection with the participation of CdTe buffer layers.

Amorphous-AlZnN/graphene heterostructure for solar-blind ultraviolet photovoltaic detectors

Based on the characteristics of ultra-wide bandgap, radiation resistance, high thermal and chemical stability, AlN has been regarded as an ideal material with great potential for ultraviolet detectors. However, its application in solar-blind ultraviolet (SBUV) detection (between 200 nm and 280 nm) is limited by the absorption cutoff edge that is generally less than 200 nm. Here, a photovoltaic SBUV detector with p-Gr/i-AlZnN/n-Si heterojunction structure is first fabricated based on amorphous AlZnN film with an absorption cutoff edge of 258 nm, which is obtained by doping Zn atoms into AlN and controlling the proportion of Zn atoms. The device exhibits outstanding SBUV detection performance, such as the open-circuit voltage (Voc) reaching 0.87 V at 255 nm, a responsivity of 20.81 mA/W, an EQE of 10.13%, a detection rate of 3.98 × 1012 Jones, a rise time of 33.2 ms and a fall time of 155 ms under 0 V bias. The results of this study indicate that amorphous AlZnN films applicable to SBUV detectors can be well prepared by energy band engineering, which also work as a reference for the preparation of AlZnN SBUV detectors with excellent performance.

Photoemission of AlGaN nanocone array cathodes for vacuum ultraviolet detectors

The mushroom growth of ultraviolet detection, the high performance demands of solar-blind ultraviolet detectors have also grown. In our work, the AlGaN heterojunction nanocone array photocathode was used as the core device of the vacuum ultraviolet detector. The method of light absorption is obtained by FDTD Lumerical software, and the photoelectron transmission process is written in Matlab software by connecting the continuity equation and finite difference. The optical absorption model and photoemission model of AlGaN heterojunction nanocone array photocathode are established. The built-in electric field generated by the heterojunction structure promotes the drift diffusion of carriers, and the external electric field accelerates and deflects the flight of the emitted electrons. When the external electric field is rised from 0 V/μm to 2 V/μm, the collection efficiency of the nanocone array with an incident angle of 40° increases from 7.33% to 31.25%, and the electron collection ratio increases from 16.26% to 71.04%. These conclusions demonstrate the feasibility and effectiveness of this scheme, and provide theoretical guidance for the design and fabrication of high-performance solar-blind ultraviolet detectors.

Ternary compound MgTiO3 combined with graphene for solar-blind deep ultraviolet photodetection

Ternary materials have gained great popularity in the field of solar-blind ultraviolet (UV) photodetectors due to their diverse band gaps. Whereas, quality problems brought by lattice damage and phase separation appear frequently during preparation, which greatly limit the application of ternary solar-blind photoresponsive materials. MgTiO3 (MTO), a ceramic material, has extensive application in microwave capacitors and dielectric resonators but takes no place in the field of photodetectors. Faced with this situation, this study investigates the effect of annealing on the structure of MTO thin film, and fabricates a solar-blind UV detector through combining MTO with graphene, which expands the application of ternary compound MTO to the field of photodetectors. Under solar-blind UV illumination, this MTO-based detector exhibits excellent performance with an open circuit voltage as high as 1.7 V. At 0 V bias, the detector exhibits an ultrafast response speed with the rise and decay time as short as 30 ms and 25 ms, respectively. Besides, a responsivity up to 46.5 mA/W, an external quantum efficiency (EQE) up to 20.4%, and a specific detectivity (D*) up to 6.14 × 1011 Jones also prove the great performance this detector owns. All the results achieved in this work not only suggest the great potential of MTO in being applied to fabricating solar-blind UV detectors, but also provide some guidance for the development of photovoltaic detectors based on other ternary semiconductor materials.