Deep Ultraviolet Region Research Articles

From β-KB3O4F2 to α-KB3O4F2: Phase Transition-Directed Great Changes in Structure and Optical Anisotropy.

Herein, a new fluorooxoborate α-KB3O4F2 is synthesized successfully in a closed system. It crystallizes in the P21/n space group and features the rare one-dimensional (1D) zigzag [B6O8F4]∞ chains built by fundamental building blocks (FBBs) [B6O9F4]. To the best of our knowledge, KB3O4F2 is a unique example of inorganic anhydrous borate whose two polymorphs show 1D B-O/F chains constructed by different FBBs but the same symmetry operation. Different from β-KB3O4F2, the inconsistent arrangement of π-conjugated [B2O5] units in α-KB3O4F2 makes it exhibit a small birefringence (0.011@546 nm). Further, α-KB3O4F2 owns a deep-ultraviolet (DUV) cutoff edge (<200 nm), suggesting that it has a potential application as the zero-order waveplate material in the DUV region.

Elucidating the Photo-Induced Electronic Structure and Mechanisms of ZnAl-Layered Double Hydroxides: A DFT Study.

Layered double hydroxides (LDHs) have been regarded as excellent catalysts for a variety of photocatalytic applications including the hydrogen production, carbon dioxide reduction, and nitrogen fixation, etal. The elucidation of the photocatalytic mechanism of LDH-based photocatalysts under light irradiation, especially at the ultraviolet (UV) and deep ultraviolet (DUV) region, at the molecular level has remained elusive. In this study, the photo-induced electronic structure of ZnAl-LDH materials were investigated, and a comprehensive understanding of its underlying mechanism, both in the UV and DUV region, was gained using density functional theory (DFT) calculations. The UV and DUV regions exhibit distinct excitation characteristics, revealing the complex interactions between electrons and holes within the system. The DUV region significantly promotes electron transfer, indicating the potential application of LDH materials as a DUV catalysis material. This study elucidates the electron transfer kinetics in LDHs upon UV and DUV irradiation, thereby offering new perspective for the development of photocatalytic materials under different light region.

First-principle study of the effect of Hf doping and VO-Hi co-existence on absorption spectrum, conductivity and carrier activity of β-Ga2O3

At present, some progress has been made in the study of Hf doping on the conductivity. However, few studies on the effects of Hf doping, and O vacancy and interstitial H co-existence on photoelectric properties of β-Ga2O3. O vacancy and interstitial H will inevitably exist during the preparation of β-Ga2O3. Given these problems, the effects of Hf doping, O vacancy and interstitial H co-existence on photoelectric properties of β-Ga2O3 were studied using first-principles calculations. Results show that doping and defects affect the photoelectric properties of β-Ga2O3 to some extent. With increased Hf doping concentration, the bandgap of the system decreases gradually, the absorption spectrum in the deep ultraviolet region red shifts, the carrier activity and carrier lifetime are enhanced. The mobility and conductivity of the system also increase with increased doping concentration. The Hf doping and interstitial H can effectively improve the conductivity of the system. In summary, Hf can be used as a powerful candidate material for designing and preparing β-Ga2O3 optoelectronic devices.

Localized surface plasmon resonance enhanced deep ultraviolet photodetectors based on Pd@Nb2CTx/AlGaN van der Waals heterojunctions

Localized surface plasmon resonance (LSPR) has been proven as an effective means to improve the performance of optoelectronic devices from infrared to ultraviolet region. However, due to the lack of suitable plasmon materials in the deep ultraviolet (DUV) region, studies in this field were relatively rare. Herein, a simple solution reduction method was proposed to decorate palladium nanoparticles (Pd NPs) onto two-dimensional (2D) niobium carbide Nb2CTx (MXene) nanosheets to fabricate Pd@Nb2CTx/aluminum-gallium nitride (AlGaN) van der Waals heterojunction (vdWH) DUV photodetectors (PDs). Thanks to the plasmon coupling between Pd@Nb2CTx and AlGaN, the obvious enhanced optical absorption and carrier excitation of the as-fabricated DUV PDs have been observed with a peak responsivity of 0.86 A/W, as well as a fast response (rise/decay time of 37.8/14.5 ms) under −3 V bias and 254 nm DUV illumination. This study provides direct evidence for LSPR of Pd NPs in the DUV region, which will develop an optional pathway for the structure design of DUV PDs.

Deep-trap ultraviolet persistent phosphor for advanced optical storage application in bright environments

Extensive research has been conducted on visible-light and longer-wavelength infrared-light storage phosphors, which are utilized as promising rewritable memory media for optical information storage applications in dark environments. However, storage phosphors emitting in the deep ultraviolet spectral region (200–300 nm) are relatively lacking. Here, we report an appealing deep-trap ultraviolet storage phosphor, ScBO3:Bi3+, which exhibits an ultra-narrowband light emission centered at 299 nm with a full width at half maximum (FWHM) of 0.21 eV and excellent X-ray energy storage capabilities. When persistently stimulated by longer-wavelength white/NIR light or heated at elevated temperatures, ScBO3:Bi3+ phosphor exhibits intense and long-lasting ultraviolet luminescence due to the interplay between defect levels and external stimulus, while the natural decay in the dark at room temperature is extremely weak after X-ray irradiation. The impact of the spectral distribution and illuminance of ambient light and ambient temperature on ultraviolet light emission has been studied by comprehensive experimental and theoretical investigations, which elucidate that both O vacancy and Sc interstitial serve as deep electron traps for enhanced and prolonged ultraviolet luminescence upon continuous optical or thermal stimulation. Based on the unique spectral features and trap distribution in ScBO3:Bi3+ phosphor, controllable optical information read-out is demonstrated via external light or heat manipulation, highlighting the great potential of ScBO3:Bi3+ phosphor for advanced optical storage application in bright environments.

Breaking through the ‘‘200 nm deep-ultraviolet wall’’ of phase matching region by cation structural modulation

Adjusting birefringence to push the phase-matching (PM) of nonlinear optical (NLO) crystals blue-shifting to the deep-ultraviolet (DUV) region is the inorganic chemists and material scientists' pursuit, while refractive dispersion as an important but easily overlooked optical properties generally brings the unexpected result. Herein, two new Y-based borates, K2YB3O6F2 and Rb2YB3O6F2 have been synthesized by the modulation of A-site cation. In both structures, owing to the structure-directing properties of Y-based polyhedra, the natural advantage of π-conjugated [B3O6] groups was pushed to the greatest extent, theoretical calculations and structure analysis revealed that the coplanar arrangement of [B3O6] groups results in the large birefringence of K2YB3O6F2 and Rb2YB3O6F2. Importantly, the aligned orientation of [B3O6] rings makes Rb2YB3O6F2 possess a strong SHG response of 5.0 × KDP, meanwhile the small dispersion of the refractive indices makes Rb2YB3O6F2 the first borate crystal containing the strong π-conjugation [B3O6] rings with the PM limit breaking through the ‘‘200 nm DUV wall’’. This work indicates that refractive dispersion can be effectively reduced through the cation effect, thereby broadening the PM range.

A theoretical study on deep ultraviolet supercontinuum generation in subwavelength optical fiber

In order to obtain supercontinuum (SC) spectra in the deep ultraviolet (UV) region, a generation method of UV SC is proposed based on subwavelength fiber (SWF) composed of background material MgF2. The nonlinear Schrödinger equation is solved by the split-step Fourier method, the optical fiber transmission characteristics are analyzed by the full-vector finite element, and the evolution of the pumping light into SC is numerically simulated in detail. The results of the numerical analysis show that the SC spectra have a spectral width extending from 229.11 nm to 2366.68 nm when 350 fs input pulses of 6 kW peak power, centered about λ0 = 400 nm, are launched into a 4 cm long MgF2 SWF. Such a deep UV SC spectrum has many applications, especially in telecommunication and military fields.

Sputter deposition of ZnO–AlN pseudo-binary amorphous alloys with tunable band gaps in the deep ultraviolet region

ZnO–AlN pseudo-binary amorphous alloys (a-ZAON hereinafter) with tunable band gaps in the deep ultraviolet (DUV) region have been synthesized using magnetron sputtering. The miscibility gap between ZnO and AlN has been overcome using room-temperature sputtering deposition, leveraging the rapid quenching abilities of sputtered particles to fabricate metastable but single-phase alloys. X-ray diffraction patterns and optical transmittance spectra revealed that the synthesized films with chemical composition ratios of [Zn]/([Zn] + [Al]) = 0.24–0.79 likely manifested as single-phase of a-ZAON films. Despite their amorphous structures, these films presented direct band gaps of 3.4–5.8 eV and thus high optical absorption coefficients (105 cm−1). Notably, the observed values adhered to Vegard’s law for crystalline ZnO–AlN systems, implying that the a-ZAON films were solid solution alloys with atomic-level mixing. Furthermore, atomic force microscopy analyses revealed smooth film surfaces with root-mean-square roughness of 0.8–0.9 nm. Overall, the wide-ranging band gap tunability, high absorption coefficients, amorphous structures, surface smoothness, and low synthesis temperatures of a-ZAON films position them as promising materials for use in DUV optoelectronic devices and power devices fabricated using large-scale glass and flexible substrates.

High gain NiO/ZnO heterojunction photodiodes operated in Deep-ultraviolet region

Epitaxially matched (0001) Al2O3 single substrates are utilized to design p-n heterojunctions (p-NiO/n-ZnO) as they offer low defect density for device fabrication due to small lattice mismatches. Using the radio frequency sputtering technique, the NiO/ZnO thin film-based heterojunction has been fabricated by varying the n-type film's (ZnO) thickness for ultra-violet (UV) photodiode application. It has been investigated how different ZnO film thicknesses affect junction parameters and photoresponse properties. The high photosensitivity (∼ 104), enhanced current gain (∼ 105) and fast response obtained at very low UV intensity (0.62μW/cm2) at 300 nm wavelength are attributed to the internal gain mechanism (recombination tunnelling and space-charge limited current conduction with the remarkable effect of the presence of excess free oxygen) in these devices. The prepared heterojunction photodiode with high speed and deep-UV sensitivity can be a potential solution for visible blind UV detection.

Boosting UV responsivity of silicon photodetectors through efficient quantum-cutting with La3+, Yb3+ co-doped perovskite quantum dots

Quantum cutting involves the absorption of a single high-energy photon, followed by the release of its energy through the emission of two or more lower-energy photons. This approach holds promise as a method to improve the ultraviolet (UV) performance of silicon photodetectors (Si PDs). In this study, we successfully fabricated La3+, Yb3+ co-doped CsPbClBr2 perovskite quantum dots (PQDs) using a modified hot-injection method, achieving a remarkable photoluminescent quantum yields (PLQYs) of 181 %. Subsequently, we spin-coated a film of La3+, Yb3+ co-doped CsPbClBr2 PeQDs, serving as a light conversion material, onto the front surface of Si PDs. The resulting configuration exhibited enhanced responsivity (0.131 A/W) and external quantum efficiency (55.6 %), long-term stability at 360 nm, and expanded responsiveness into the deep UV region (200 nm). Simultaneously, the responsivity and external quantum efficiency in the visible and near-infrared regions exhibited minimal reduction. This significant performance can be attributed to two key factors: (1) The higher efficient quantum cutting PLQYs (181 %) of La3+, Yb3+ co-doped CsPbClBr2 PeQDs and (2) the presence of fewer traps and a larger tolerance factor resulting from La3+ and Yb3+ co-doping.

β-Ga2O3 van der Waals p-n homojunction

The van der Waals (vdW) p-n junctions are crucial to develop multifunctional and high-performance electronic and optoelectronic devices. The asymmetric doping effect in wide-bandgap (WBG) semiconductors poses a fundamental obstacle for fabricating the vdW p-n homojunction and impedes the development of full WBG semiconductors-based bipolar devices. In this study, we demonstrate the β-Ga2O3 vdW p-n homojunctions with 2.0 nm-thick vdW gap, 2.6 eV built-in potential and 1.5 μm-wide depletion region, via combining quasi-two-dimensional n-type β-Ga2O3 nanosheets with p-type β-Ga2O3 films. Various tunneling transports including direct tunneling, Fowler-Nordheim tunneling, and exciton-assisted tunnelling are observed and explored in detail. The β-Ga2O3 vdW p-n homojunction diodes possess high forward current density (3.0 × 10−3 A/cm2), extremely-low reverse leakage current density (3.0 × 10−9 A/cm2), high rectification ratio (106 under dark and 107 under illumination), high photoresponsivity (13.4 A/W) and detectivity (9.38 × 1013 Jones) at 10 V bias under 250 nm illumination, and narrowband detection for the deep-ultraviolet solar-blind spectral region. This work lays the foundation for β-Ga2O3 homogeneous bipolar vdW devices and paves the way to advance the next-generation electronic and optoelectronic multifunctional devices based on the vdW integration.

First-principles study on the electronic and optical properties of pn-type SnO/MoS[formula omitted] heterojunction tuned by various intrinsic vacancy defects and layer thicknesses

The impact of various intrinsic point defects and layer thicknesses on the electronic structure and optical properties of pn-type SnO/MoS2 heterojunctions has been investigated using first-principles calculations in this study. SnO/MoS2 heterojunctions are indirect bandgap semiconductors, with a narrower bandgap compared to monolayer SnO and MoS2, which decreases with increasing thickness of SnO or MoS2 layers. Intrinsic vacancy defects with different valence states were introduced using the transfer to the real state model method, which ensures that ionized electrons or holes are placed in the real band edge states. The primary intrinsic vacancy defects in the SnO/MoS2 heterojunction are Sn vacancies, and the valence states of the defects significantly influence the defect formation energy. Under different growth conditions, the dominant defects transition between VSn2−, VSn−, VO, and VS as the Fermi level varies. The intrinsic monolayer (1/1) SnO/MoS2 heterojunction exhibits weak n-type doping characteristics, while the introduction of VSn− defects leads to a shift towards p-type doping. As the number of SnO layers increases, VSn2− and VS defects introduce defect levels of different degrees in the bandgap, acting as electron–hole recombination centers and facilitating the transition between p-type doping and metallic conductivity characteristics. Increasing the number of MoS2 layers introduces localized defect energy levels in the bandgap, leading to p-doping or n-doping characteristics. The intrinsic SnO/MoS2 heterojunction primarily absorbs in the near-ultraviolet region, while also exhibiting a certain absorption capacity in the visible and deep ultraviolet regions. The presence of Sn and S defects can enhance the response to visible light absorption.

Deep ultraviolet applications of mono-bilayer boron nitride nanoribbons: a first-principles study

The optical properties of mono-bilayer boron nitride nanoribbons (BNNRs) on deep ultraviolet (DUV) region were calculated by adjusting the width, interlayer spacing and layer stacking. The band structure of monolayer BNNRs is sensitive to the width. DUV response also changes with the width, interlayer spacing and layer stacking in the mono-bilayer BNNRs. The N-N layer stacking can improve the reflectivity and absorption coefficient of DUV response. Therefore, the electronic properties and DUV response of mono-bilayer BNNRs can be tuned through changing the width, interlayer spacing and layer stacking. Our results show that BNNRs can be applied for DUV communication and DUV detectors, etc.

Cascaded Second Harmonic Generation for Deep‐UV Radiations with a 2D Nonlinear Photonic Quartz Crystal

AbstractHarmonic generation can broaden the laser wavelength in a great scope and is rich in physics, but the efficient cascaded second harmonic generation especially those directly deep into deep‐ultraviolet (DUV) with one single crystal seems to be impossible, due to the extremely strict phase‐matching conditions and relative narrow bandgap of present nonlinear optical crystals. Here, this work designs a nonlinear photonic quartz crystal where the phase difference of interacted light waves is manipulated in two dimensions and realizes an unprecedented cascaded phase‐matched 2nd–5th harmonic generation simultaneously with the photon energy up to 6 eV, representing the shortest wavelength by cascaded second harmonic generation in the solid‐state nonlinear medium. The generated light is located in the visible‐DUV region with a total conversion efficiency of 2.7%. The present nonlinear photonic quartz scheme not only provides a multi‐wavelength solid‐state laser source applied in modern devices but also develops a new route for studying the light‐matter nonlinear interaction in the DUV region.

Integrated Deep Ultraviolet Doublet Metalens for Projection Imaging

Metalenses, with their unique modulation of light, are in great demand for many potential applications. As a proof-of-principle demonstration, we focus on designing SiO2 metalenses that operate in the deep ultraviolet region, specifically around 193 nm. Based on the deep ultraviolet metalens proposed in this paper, an integrated deep ultraviolet doublet metalens is further offered. When the incident light is a plane wave with a wavelength of 193 nm, the integrated doublet metalens can reduce the beam size by a factor of 4:1, and the emitted light is flat. The integrated doublet metalens can project the reticle image proportionally, making the projection image clear. The integrated doublet metalens has the best imaging effect at the propagation distance of 2 μm and can tolerate ±3 degrees of incident angle deviation. Our findings establish general and systematic strategies to guide the design of traditional optical lens arrays with excellent integrated doublet metalenses and pave the way for enhanced optical performance in the application of large-relative-aperture deep ultraviolet detection, deep ultraviolet microscope systems, laser beam combining systems, deep ultraviolet lithography systems, etc.

Effects of different atomic passivation on conductive and dielectric properties of silicon carbide nanowires

Based on the transport and polarization relaxation theories, the effects of hydrogen, fluorine, and chlorine atom passivation on the conductivity and dielectric properties of silicon carbide nanowires (SiCNWs) were numerically simulated. The results show that passivation can decrease the dark conductivity of SiCNWs and increase its ultraviolet photoconductivity. Among them, the photoconductivity of univalent (H) passivated SiCNWs is better than that of seven-valent (Cl, F) passivated SiCNWs. In terms of dielectric properties, the passivated SiCNWs exhibit a strong dielectric response in both deep ultraviolet and microwave regions. Hydrogen passivation SiCNWs produce the strongest dielectric response in deep ultraviolet, while fluorine passivation SiCNWs produce the strongest dielectric relaxation in the microwave band, which indicates that atomic passivation SiCNWs have a wide range of applications in ultraviolet optoelectronic devices and microwave absorption and shielding.

Angular Engineering Strategy for Enhanced Surface Nonlinear Frequency Conversion in Centrosymmetric Topological Semimetal HfGe0.92Te.

Surface nonlinear optics are essential for developments in integrated photonics and micro/nano optoelectronics. However, the nonlinear optical conversion efficiency on a surface is restricted by the finite nonlinear susceptibility of matter and the intrinsic atomic-layered interaction length between light and matter. In this work, based on an angular engineering strategy, it is demonstrated that the centrosymmetric topological semimetal HfGe0.92Te crystal has a giant and anisotropic surface second-order nonlinear susceptibility up to 5535 ± 308pm V-1 and exhibits efficient and unprecedented second-harmonic generation (SHG). The maximum optical conversion efficiency is found to be up to 3.75‰, which is 104 times higher than that obtained from a silicon surface. Because of the linear dispersion over a wide range of energies around the Dirac points, this high conversion efficiency can be maintained with SHG wavelengths ranging from the visible region (779nm) to the deep-UV region (257.5nm). This study can facilitate the development of topological photonics and integrated nonlinear photonics based on topological semimetals.

Thermodynamic, electronic, and optical properties of ultra-wide bandgap zirconium-doped tin dioxide from a DFT perspective.

The effects of zirconium doping on the thermodynamic, electronic, and optical properties of tin dioxide are investigated by using density functional theory calculations combined with the cluster expansion method. In the whole composition range, the formation enthalpies of all structures are positive, indicating that SnO2-ZrO2 is an immiscible system and the ZrSnO2 alloy has a tendency of phase separation at low temperature. The x-T phase diagram of ZrSnO2 ternary alloy shows that the critical temperature is 979 K, which means that when the growth temperature of ZrSnO2 crystal is higher than the critical temperature, it is possible to realize the full-component solid solution. The bandgaps of ZrxSn1-xO2 alloys (0 ≤ x ≤ 1) are direct and increase as the Zr composition increases. Zr doping can tune the bandgap of SnO2 from the ultraviolet-B region to the deep ultraviolet region, and has a strong optical response to deep ultraviolet light. The projected density of states and band offsets clearly reveal the reason for the increase of bandgap, which provides useful information to design relevant optoelectronic devices such as quantum wells and solar-blind deep ultraviolet photodetectors.

Al-rich AlGaN Alloys: Unique Materials for Deep UV LEDs

Energy bandgap of AlxGa1−xN alloys can be tuned systematically from ∼ 3.4 to 6.1 eV by changing the alloy composition (x) from 0 to 1 and the direct bandgap nature is maintained in the entire range of alloy compositions which make the AlGaN alloys suitable materials for the development of light emitting diodes (LEDs) covering the ultraviolet (UV) spectral region from 210 to 400 nm. For LEDs in the deep UV regions (λ &lt; 300 nm), Al-rich AlGaN alloys of Al content higher than 50% are required. Deep UV LEDs have applications in a wide range of fields including display, disinfection, medical, sensing, and communication. With recent progress in the material growth and electrical conductivity, Al-rich AlGaN alloys have emerged as unique wideband gap materials for the development of deep UV LEDs. In this review article, how the progress of Al-rich AlGaN alloys has made in terms of the material growth and electrical conductivity leading its emergence as deep UV materials have been reviewed. Challenges and prospects of the deep UV LEDs to improve the performance of the devices will also be discussed.

Design of high-entropy films as ultra-violet light reflector

This study proposes the design and implementation of high-entropy alloy (HEA) thin films as materials for the reflector of deep ultraviolet light-emitting devices and marks the first application of HEA in optical field. In the material design phase, an Al-Co-Zn-Ni alloy is designed by considering the reflectivity, phase stability and binary formation energy. Reverse Monte Carlo calculation is employed to establish stable atomic structure. Frequency-dependent dielectric equations are derived from first principles calculations, with dielectric constants incorporating contributions from ions and electrons and combining classical Drude and Lorentz models. The dielectric equations yield optical constants that are used to calculate the reflectance. In the experimental phase, we systematically vary the aluminum content and prepare Al-Co-Zn-Ni thin films ranging from pure aluminum to equimolar compositions. In the deep ultraviolet region, the reflectance of thermally treated Al65Zn12Co12Ni11 and Al50Zn16Co17Ni17 thin films is found superior to that of gold by three times. Structural and crystalline changes with varying aluminum content are found, as well as intriguing phase separation and superlattice formation that persist until equimolar compositions. This unique nanoscale structural variation causes the Young's modulus and hardness of Al-Co-Zn-Ni thin films to initially increase and then decrease with decreasing aluminum content. There is a significant increase in the corrosion potential difference when reducing Al contents in Al-Co-Zn-Ni, indicating improved passivation effects as the equimolar composition is approached. By modulating the aluminum content, the nanoscale structure of Al-Co-Zn-Ni can be adjusted, thereby improving its optical, mechanical, corrosion resistance, and thermal stability properties. This demonstrates the high versatility of this material for practical optical applications.