Deep-ultraviolet integrated photonic and optoelectronic devices:A prospect of the hybridization of group Ⅲ–nitrides, Ⅲ–oxides,and two-dimensional materials

Progress in the design and fabrication of ultraviolet and deep-ultraviolet groupⅢ–nitride optoelectronic devices,based on aluminum gallium nitride and boron nitride and their alloys,and the heterogeneous integration with two-dimensional and oxide-based materials is reviewed.We emphasize wide-bandgap nitride compound semiconductors(i.e.,(B,Al,Ga)N)as the deep-ultraviolet materials of interest,and two-dimensional materials,namely graphene,two-dimensional boron nitride,and two-dimensional transition metal dichalcogenides,along with gallium oxide,as the hybrid integrated materials.We examine their crystallographic properties and elaborate on the challenges that hinder the realization of efficient and reliable ultraviolet and deep-ultraviolet devices.In this article we provide an overview of aluminum nitride,sapphire,and gallium oxide as platforms for deep-ultraviolet optoelectronic devices,in which we criticize the status of sapphire as a platform for efficient deep-ultraviolet devices and detail advancements in device growth and fabrication on aluminum nitride and gallium oxide substrates.A critical review of the current status of deep-ultraviolet light emission and detection materials and devices is provided.

ZnO-based deep-ultraviolet light-emitting devices

Deep-ultraviolet（DUV） light-emitting devices（LEDs） have a variety of potential applications.Zinc-oxide-based materials,which have wide bandgap and large exciton binding energy,have potential applications in high-performance DUV LEDs.To realize such optoelectronic devices,the modulation of the bandgap is required.This has been demonstrated by the developments of Mg_xZn_（1-x）O and Be_xZn_（1-x）O alloys for the larger bandgap materials.Many efforts have been made to obtain DUV LEDs,and promising successes have been achieved continuously.In this article,we review the recent progress of and problems encountered in the research of ZnO-based DUV LEDs.

A 2DEG back-gated graphene/AlGaN deep-ultraviolet photodetector with ultrahigh responsivity

A graphene/AlGaN deep-ultraviolet(UV)photodetector is presented with ultrahigh responsivity of 3.4×105 A/W at 261 nm incident wavelength and 149 pW light power.A gain mechanism based on electron trapping at the potential well is proposed to be responsible for the high responsivity.To optimize the trade-off between responsivity and response speed,a back-gate electrode is designed at the AlGaN/GaN two-dimensional electron gas(2DEG)area which eliminates the persistent photocurrent effect and shortens the recovery time from several hours to milliseconds.The 2DEG gate is proposed as an alternative way to apply the back gate electrode on AlGaN based devices on insulating substrates.This work sheds light on a possible way for weak deep-UV light detection.

A picosecond widely tunable deep-ultraviolet laser for angle-resolved photoemission spectroscopy

We develop a picosecond widely tunable laser in a deep-ultraviolet region from 175 nm to 210 nm, generated by two stages of frequency doubling of a 80-MHz mode-locked picosecond Ti：sapphire laser. A β-BaB2O4 walk-off compensation configuration and a KBe2BO3F2 prism-coupled device are adopted for the generation of second harmonic and fourth harmonics, respectively. The highest power is 3.72 mW at 193 nm, and the fluctuation at 2.85 mW in 130 rain is less than ±2%.

Deep-ultraviolet surface plasmon resonance of Al and Al_(core)/Al_2O_(3shell) nanosphere dimers for surface-enhanced spectroscopy

The localized surface plasmon resonance properties of Al and Alcore/Al2O3shell nanosphere dimers with Al and Al core nanosphere radii of 20 nm and Al2O3 shell of 2 nm in the deep-ultraviolet region have been studied using the finite difference time domain method. The extinction spectra and the electric field distribution profiles of the two dimers for various gap distances between two individual nanospheres are compared with those of the corresponding monomers to reveal the extent of plasmon coupling. It is found that with the interparticle distance decreasing, a strong plasmon coupling between two Al or Alcore/Al2O3shell nanospheres is observed accompanied by a significant red shift in the extinction spectra at the parallel polarization direction of the incident light related to the dimer axis, while for the case of the perpendicular polarization direction, a weak plasmon coupling arises characterized by a slight blue shift in the extinction spectra. The electric field distribution profiles show that benefiting from the dielectric Al2O3 shell, the gap distance of Alcore/Al2O3shell nanosphere dimers can be tailored to 〈 1 nm scale and results in a very high electric field enhancement. The estimated surface-enhanced Raman scattering enhancement factors suggests that the Alcore/Al2O3shell nanosphere dimers with the gap of 〈 1 nm gave rise to an enhancement as high as 8.1 × 10^7 for interparticle gap = 0.5 nm. Our studies reveal that the Alcore/Al2O3shell nanosphere dimers may be promising substrates for surface-enhanced spectroscopy in the deep-ultraviolet region.

基于锥形超晶格p-AlInGaN层的AlGaN基深紫外发光二极管性能优化

为了解决AlGaN基深紫外(DUV)发光二极管(LED)中的严重电子溢出和低空穴注入的问题,本文提出了一种新型锥形超晶格p-AlInGaN层,它大幅改善了基于AlGaN的DUV LED的光电特性.与传统结构相比,所提出结构的输出功率提高了337.8%;同时它的内部量子效率(IQE)也高达96%,并且没有效率下降现象.仿真计算结果表明,锥形超晶格p-AlInGaN层的引入明显增加了多量子阱(MQWs)内载流子的浓度并降低了量子阱(QWs)内的电场,导致了更高的辐射复合率,为改善DUV LED的性能提供了一个有吸引力的解决方案.

Enhanced nonlinear optical functionality in birefringence and refractive index dispersion of the deep-ultraviolet fluorooxoborates

As a promising candidate,the fluorooxoborate has enkindled new explorations of nonlinear optical materials to meet the deep-ultraviolet criteria.However,big challenges and open questions still remain facing this exciting new field,especially the birefringence and dispersion of refractive index which are fundamental parameters for determining the phasematching second harmonic generation wavelength.Here we designed possible anionic groups in fluorooxoborates,and analyzed the optical anisotropy to check its influence on birefringence,which was proved further by the response electronic distribution anisotropy approximation.The functional modules modulating birefringence in fluorooxoborates were explored systematically.We developed an approach for evaluating the behavior of the refractive index dispersions and found that the fluorooxoborates had small refractive index dispersions owing to the introduction of fluorooxoborate modules.Our results demonstrate that fluorooxoborates can be utilized to realize short phase-matching wavelength markedly and offer a path toward novel performance-driven materials design.

Review of encapsulation materials for AlGaN-based deep-ultraviolet light-emitting diodes

This paper reviews and introduces the techniques for boosting the light-extraction efficiency(LEE) of AlGaNbased deep-ultraviolet(DUV: λ < 300 nm) light-emitting diodes(LEDs) on the basis of the discussion of their molecular structures and optical characteristics, focusing on organoencapsulation materials. Comparisons of various fluororesins, silicone resin, and nonorgano materials are described. The only usable organomaterial for encapsulating DUV-LEDs is currently considered to be polymerized perfluoro(4-vinyloxy-1-butene)(p-BVE)terminated with a —CF3 end group. By forming hemispherical lenses on DUV-LED dies using p-BVE having a —CF3 end group with a refractive index of about 1.35, the LEE was improved by 1.5-fold, demonstrating a cost-feasible packaging technique.

Deep-ultraviolet nonlinear optical crystals by design:A computer-aided modeling blueprint from first principles

Deep-ultraviolet(DUV)nonlinear optical(NLO)crystal is indispensable in current DUV all-solid-state laser technology,which is the key device to generate DUV light by frequency conversion.Due to stringent criteria,DUV NLO crystals are scarce and their discovery faces a big challenge.Although KBe2BO3F2(KBBF)is already excellent for current uses,the development of DUV science requires the materials with shorter output wavelengths and larger NLO effects,so as to meet the needs of DUV lasers with higher precision and higher power.Therefore,how to efficiently design DUV NLO materials has always been the core issue in NLO materials science.Looking back on the development of NLO materials,it turns out that theoretical modeling and simulation is an effective and efficient method,not only for mechanism understandings,but also for experimental exploration.In this article,in order to accelerate the process of DUV materials discovery,we summarize and propose a powerful computeraided modeling system and design blueprint that can be used to evaluate the DUV NLO performance in a faster way than pure experiments.With this in hand,we enrich the understanding of NLO structure-property correlation,and systematically prospect the DUV NLO properties on the basis of many existing and designed structures according to different structural types and chemical compositions.Seven novel structures are predicted for the first time to exhibit potential DUV NLO capabilities.All the results enable us to believe that the computer-aided modeling blueprint will play an important role in the exploration of new DUV NLO crystals.

Surface plasmon coupled fluorescence in deep-ultraviolet excitation by Kretschmann configuration

We report the experimental demonstration of fluorescence of CdSe quantum dots with surface plasmon excitation in deep-ultraviolet （deep-UV） region. Surface plasmon resonance in deep-UV is excited by aluminum thin film in the Kretschmann-Raether geometry. Considering the oxidation thickness of aluminum, the experimental results of incident angle dependence of reflectance show good agreement with Fresnel theory. Surface plasmon resonance with 19 nm-thick aluminum and 5 nm-thick almnina was excited at the incident angle of 48 degrees for 266 nm excitation. Fluorescence of CdSe quantum dots coated on this aluminum film was observed by the surface plasmon excitation.

BaYOBO_(3):A deep-ultraviolet rare-earth oxy-borate with a large second harmonic generation response

Deep-ultraviolet(UV)nonlinear optical(NLO)crystals with excellent performances are of great importance in laser science and technology.However,to design and synthesize an ideal deep-UV NLO material with a balance between second harmonic generation(SHG)response and UV cut-off edge is still a huge challenge.Herein,a new oxy-borate BaYOBO_(3)(BaYBO)was designed and synthesized by substituting Be^(2+)with Y^(3+)from Sr_(2)Be_(2)B_(2(O_(7).The BaYBO features a stable three-dimensional framework and satisfies the property balance between a large SHG response(~2.6×KH_(2)PO_(4)KDP))and a short UV cut-off edge(<190 nm)as a promising deep-UV NLO candidate.The study of structure-property relationship indicates that the large SHG response of BaYBO is mainly attributed to the coplanar arrangement and large number density of BO_(3 )triangles.These results demonstrate that substituting Be^(2+)with Y^(3+)cations is also a feasible strategy for developing new deep-UV NLO crystals.

Syntheses, characterization, and theoretical calculation of Rb2Mg3（P2O7）2 polymorphs with deep-ultraviolet cutoff edges

By the combination of the isolated P2O7 dimers and Mg O4 tetrahedra,α-andβ-Rb2Mg3(P2O7)2 polymorphs were synthesized by a high-temperature solution method.α-Rb2Mg3(P2O7)2 crystallizes in non-centrosymmetric space group P212121,whileβ-Rb2Mg3(P2O7)2 crystallizes in centrosymmetric P21/c.Both structures contain a three dimensional[Mg3P4O14]^2- anionic framework,while Rb^+ cations are in the space.Structure analyses show that the isolated P2O7 dimers can easily adjust their variable configurations and orientations to fit the different coordination environments of the cations,which is conducive to the formation of polymorphs.The phase transformation process fromα-toβ-Rb2Mg3(P2O7)2 was further investigated by powder X-ray diffraction and thermal gravimetric/differential scanning calorimetry measurements.In addition,UV-vis-NIR diffusion spectra indicate both materials have deep-ultraviolet cut-off edges(below 190 nm).α-Rb2Mg3(P2O7)2 is second-harmonic generation(SHG)-active and the origin of SHG response was investigated by the SHG density calculations.The first-principle calculations were also carried out to illuminate their structure-property relationships.

LiB_(5)O_(5)F_(2)(OH)_(4):A new deep-ultraviolet birefringent crystal with[B_(5)O_(5)F_(2)(OH)_(4)]anionic group

A new deep-ultraviolet(DUV)birefringent crystal,LiB_(5)O_(5)F_(2)(OH)_(4)(LBOFH),was designed and synthesized by a high-temperature solution method.Its structure features a[B_(5)O_(5)F_(2)(OH)_(4)]anionic group that is constructed by four π-conjugated units,i.e.,two[BO_(3)]units and two[BO(OH)_(2)]units connected by the[BO_(2)F_(2)]unit,which forms a near-coplanar configuration.The group exhibits large anisotropic polarizability and highest unoccupied molecular orbital-lowest unoccupied molecular orbital(HUMO-LUMO)gap,thus providing a good balance between the large birefringence(0.124 at 546 nm)and band gap(7.99 eV)in LBOFH.This work suggests new possibilities for the design and fabrication of advanced DUV birefringent crystals using the novel anionic group and the expansion of their alternative system with new hydroxyfluorooxoborates.

Rb_(5)Ba_(2)(B_(10)O_(17))_(2)(BO_(2)):The formation of unusual functional [BO_(2)]^(-) in borates with deep-ultraviolet transmission window

The design and synthesis of new deep-ultraviolet(DUV)optical materials are important but challenging.It is well known that the polarizability anisotropy of linear[BO_(2)]^(−)is larger than those of[BO_(3)]^(3−)and[BO_(4)]^(5−),which is the better choice for exploring excellent optical materials.Here,we search for such crystals in borate systems based on the unusual fundamental structural unit[BO_(2)]^(−).By combining the linear[BO_(2)]^(−)with the traditional triangular[BO_(3)]^(3−)and tetrahedral[BO_(4)]^(5−)units,a new mixed metal borate,Rb_(5)Ba_(2)(B_(10)O_(17))_(2)(BO_(2))was obtained via high-temperature solution method.It exhibits excellent optical properties including a DUV transmission window(188 nm)and suitable birefringence(0.06).It also has good thermal stability and its crystal plates can be prepared easily.Furthermore,the strategies of designing borates with[BO_(2)]^(−)units have been proposed from the transformation of crystal framework and the preparation method.

Ba_(2)B_(9)O_(13)F_(4)·BF_(4):first fluorooxoborate with unprecedented infinite [B_(18)O_(26)F_(8)] tubes and deep-ultraviolet cutoff edge

Fluorooxoborate has emerged as a promising system for exploring optical materials due to their rich structural chemistry and physicochemical properties,yet so far not fully studied.Herein,we report a new fluorooxoborate,Ba_(2)B_(9)O_(13)F_(4)·BF_(4),which contains the greatest variety of structural basic units,[BO_(3)],[BO_(2)F_(2)],[BO_(3)F],and[BF_(4)],and exhibits unprecedented 1D infinite[B_(18)O_(26)F_(8)]tubes.The unique structure avoids the dangling bonds caused by terminal oxygens,which is beneficial to the large bandgap and deep-ultraviolet cutoff edge.This article expands the crystal structure of fluorooxoborates and contributes to the exploration of deep-ultraviolet optical materials in fluorooxoborate systems.

Recent progress in ultraviolet and deep-ultraviolet nonlinear optical aluminoborates

Aluminoborates(ABOs)show wide ultraviolet(UV)transmittance,and are one of the important types of UV and deep-UV nonlinear optical(NLO)materials.In recent years,the combination of AlO_(n)(n=4,5,6)polyhedra or their fluorinated analogues with different polyborate anions leads to a rich variety of crystalline ABOs.In this review,we summarize the recent developments of UV and deep-UV NLO inorganic ABOs.These ABOs can be grouped into three types in terms of their compositions:(i)aluminum oxyborates,(ii)alkali/alkaline earth metal aluminoborates,and(iii)alkali/alkaline earth metal aluminoborate fluorides and fluoroaluminoborates.We discuss the synthesis,structural features,NLO properties,as well as related structure-performance relationships of these non-centrosymmetric ABOs.Finally,the future perspectives in this field are discussed.

Fluorooxoborates:A precious treasure of deep-ultraviolet nonlinear optical materials

As the key components of all-solid-state deep-ultraviolet(DUV,λ<200 nm)laser sources,exploring DUV nonlinear optical(NLO)materials has been a research hotspot for a long time.However,as the only material that has been practically used to generate DUV coherent light through the direct second harmonic generation(SHG)process,KBe_(2)BO_(3)F_(2)(KBBF)still meets bottlenecks,namely,severe layered crystal growth tendency,for extensive application.In the search for the next generation of DUV NLO materials,fluorooxoborates are highly expected due to their excellent NLO performances,flexible structures,and rich structural chemistry.So far,a large number of materials with prominent properties have emerged in the fluorooxoborate system.By investigating the differences in structures and properties among fluorooxoborates and borates and borate fluorides,the excellent NLO per-formances of fluorooxoborates are attributed to functionalized B-O/F chromophores.In this review,by analyzing the published fluorooxoborates,the fluorooxoborate system is a precious treasure for exploring DUV NLO ma-terials.The synthesis,structures,and properties of some representative fluorooxoborates in the research history are introduced and summarized.Finally,we put forward the opportunities and challenges of future research in fluorooxoborate DUV NLO crystals.

Ba(SO_(3)CH_(3))-(2):A Deep-Ultraviolet Transparent Crystal with Excellent Optical Nonlinearity Based on a New Polar Non-π-Conjugated NLO Building Unit SO_(3)CH_(3)^(−)

In this study,the first methyl sulfonate deepultraviolet(DUV)nonlinear optical(NLO)crystal,Ba(SO_(3)CH_(3))_(2),was successfully synthesized with the polar non-π-conjugated tetrahedron,SO_(3)CH_(3)^(−),as a novel DUV NLO building unit.Results showed that Ba(SO_(3)CH_(3))_(2)not only had a very short absorption edge of 159 nm,which is the shortest among reported phase-matchable sulfates or sulfonates,but also exhibited excellent optical properties.(Powder X-ray diffractionwas ca.1.5×KH_(2)PO_(4)and birefringence was 0.04 at 589.3 nm.)It also exhibited high thermal stability and remarkable stability against air and moisture.Additionally,bulk Ba(SO_(3)CH_(3))_(2)crystal could be obtained conveniently from a simple solution evaporation process.Therefore,Ba(SO_(3)CH_(3))_(2)should have great potential as a DUV NLO crystal in the near future.

The effect of the multi-period on the properties of deep-ultraviolet transparent conductive Ga_2O_3/ITO alternating multilayer films

Ga2O3/ITO alternating multilayer films were deposited on quartz glass substrates by rnagnetron sputtering. The effect of the multi-period on the structural, optical and electrical properties of Ga2O3/ITO alternating multilayer films was investigated by an X-ray diffractometer, a double beam spectrophotometer and the Hall system, respectively. A low sheet resistance of 225.5 Ω/□ and a high transmittance of more than 62.9% at a 300 nm wavelength were obtained for the two-period alternating multilayer film with a thickness of 72 nm.

基于大厚度六方氮化硼中子探测器的制备

六方氮化硼是一种中子敏感材料。介绍了使用低压气相化学沉积在1673K下以大约20μm/h的生长速率制备了高质量203μm厚的六方氮化硼。在氮化硼的两侧沉积厚度为100nm的金电极,制备了垂直结构的六方氮化硼中子探测器。该器件的电学性质表明制备的六方氮化硼材料的迁移率寿命的乘积(μτ)为2.8×10^(-6)cm^(2)/V,电阻率为1.5×10^(14)Ω·cm。在850V电压下,该探测器对热中子的探测效率为34.5%,电荷收集效率为60%。