Deep Ultraviolet Region Research Articles

Pulsed laser deposition of gallium oxide films for high performance solar-blind photodetectors

Monoclinic gallium oxide thin films were grown on (0001) sapphire at various substrate temperatures ranging from 400 to 1000 °C by pulsed laser deposition using a KrF excimer laser. The structural, optical and compositional properties of the films were analyzed by using x-ray diffraction, transmission electron microscopy, optical transmittance, and Rutherford backscattering spectroscopy. As the substrate temperature was increased to 800 °C, the gallium oxide film possesses single crystalline phase with a preferred growth orientation of (−201) plane and higher crystal quality than those at the other temperatures. Optical transmittance measurements reveal the films grown at 600-1000 °C exhibit a clear absorption edge at the deep ultraviolet region around 250 nm wavelength. Based on the results of Rutherford backscattering spectroscopy, the O/Ga ratio of gallium oxide film increased gradually with increasing substrate temperature. When the substrate temperature was raised to 800-1000 °C, the film composition was close to the formation of Ga2O3, indicating the O vacancies and defects were reduced. Furthermore, the films grown at 600 and 800 °C were chosen to fabricate solar-blind metal-semiconductor-metal photodetectors. At an applied bias of 5 V, the photodetector prepared with 800 °C-grown film has a lower dark current of 1.2 × 10−11 A and a higher responsivity of 0.903 A/W (at a wavelength of 250 nm) than those with 600 °C-grown films. The better device performance is ascribed to the higher crystal quality and fewer O vacancies in the 800 °C-grown film. Moreover, the results indicate the gallium oxide films presented in this study have high potential for deep ultraviolet photodetector applications.

Structural and optical properties of low temperature grown AlN films on sapphire using helicon sputtering system

AlN thin films have been deposited directly on c-plane sapphire substrates at low temperatures by a helicon sputtering system. The structural quality of AlN epitaxial films was characterized by x-ray diffractometry and transmission electron microscopy. The films exhibit smooth surface with root-mean-square roughness as small as 0.7 nm evaluated by atomic force microscope. The optical transmittance spectra show a steep absorption edge at the wavelength of 200 nm and a high transmittance of over 80% in the visible range. The band-edge transition (6.30 eV) of AlN film was observed in the cathodoluminescence spectrum recorded at 11 K. The spectral response of metal–semiconductor–metal photodetectors constructed with AlN/sapphire reveals the peak responsivity at 200 nm and a UV/visible rejection ratio of about two orders of magnitude. The results of this low temperature deposition suggest the feasibility of the epitaxial growth of AlN on sapphire substrates and the incorporation of the AlN films in the surface acoustic wave devices and the optical devices at deep ultraviolet region.

A novel Raman optical activity instrument operating in the deep ultraviolet spectral region

Raman optical activity (ROA) has been exclusively observed in the visible (VIS) and near-infrared (NIR) spectral regions to date. During the last few years, we have designed, constructed and tested the first ROA instrument, operating in the deep-ultraviolet (DUV) spectral region employing 244-nm excitation. This novel DUV ROA instrument is based on a backscattering geometry and incident circular polarization modulation (ICP); it makes use of a fast DUV imaging lens-based spectrograph and specially designed DUV grade polarization optics. The performance of this instrument has been evaluated by analysing measured non-resonant DUV ROA spectra of non-absorbing enantiomeric liquid samples and by comparing these with corresponding ROA spectra recorded in the visible spectral region. Copyright © 2015 John Wiley & Sons, Ltd.

Alkali-metal/alkaline-earth-metal fluorine beryllium borate NaSr3Be3B3O9F4 with large nonlinear optical properties in the deep-ultraviolet region

The linear optical response and second harmonic generation (SHG) in alkali-metal/alkaline-earth-metal fluorine beryllium borate NaSr3Be3B3O9F4 are investigated by means of density functional theory. Calculations are performed using four types of exchange correlations: Ceperley-Alder local density approximation, Perdew Burke and Ernzerhof general gradient approximation, Engel-Vosko generalized gradient approximation, and the recently modified Becke-Johnson potential (mBJ). The mBJ approach brings the calculated band gap (7.20 eV) in excellent agreement with the experimental one (7.28 eV). The calculated values of the uniaxial anisotropy δε=−0.076 and the birefringence Δn(0)=0.052 indicate considerable anisotropy in the linear optical properties, which makes it favorable for the second harmonic generation. The dominant component of the second harmonic generation is χ111(2)(ω). The value of |χ111(2)(ω)| is about 1.2 pm/V at λ = 1064 nm in agreement with previous calculations. To analyze the origin of the high SHG of NaSr3Be3B3O9F4 single crystals, we have correlated the features of |χ111(2)(ω)| spectra with the features of ε2(ω) spectra as a function of ω/2 and ω. From the calculated dominant component |χ111(2)(ω)|, we find that the microscopic first hyperpolarizability, β111, the vector components along the dipole moment direction is 0.5 × 10−30 esu at static limit and 0.6 × 10−30 esu at λ = 1064 nm.

Beryllium-free Rb3Al3B3O10F with reinforced interlayer bonding as a deep-ultraviolet nonlinear optical crystal.

A new beryllium-free borate Rb3Al3B3O10F has been synthesized and characterized by single-crystal X-ray diffraction. It features a framework structure consisting of alveolate [Al3(BO3)3OF]∞ layers tightly bound via Al-O and Al-F bridged bonds, with the in-layer [BO3](3-) groups in nearly coplanar and aligned arrangement. This compound is transparent down to 200 nm and is phase-matchable with a powder second-harmonic generation efficiency of 1.2 times that of KH2PO4. Remarkably, it exhibits a strong interlayer bonding which is about one order larger than that of the benchmark KBe2BO3F2, thus no layering tendency was observed during the crystal growth. In addition, it is nonhygroscopic and thermally stable up to ∼1462 K. These attributes make Rb3Al3B3O10F a promising nonlinear optical crystal in the deep-ultraviolet region. First-principles calculations, combined with the anionic group theory, were adopted to rationalize the optical properties.

AlGaN-based MQWs grown on a thick relaxed AlGaN buffer on AlN templates emitting at 285 nm

We report on the growth of Al0.57Ga0.43N/Al0.38Ga0.63N MQWs grown on a relaxed Al0.58Ga0.42N buffer on AlN template by Metal Organic Vapor Phase Epitaxy. The MQW structure is designed so that the strain in the quantum wells induced by their lattice mismatch with barriers is sufficient to enhance TE polarized emission (E-field ⊥ c). A 630-nm thick relaxed Al0.58Ga0.42N buffer grown on AlN template serves as a pseudo-substrate to release the strain in the barriers and to avoid related defects or composition fluctuation in the active region. Thin (< 2 nm) quantum wells allow preservation of the overlapping of electron and hole wavefunctions considering the strong quantum-confined Stark effect in AlGaN-based MQW structures. Scanning transmission electron microscopy (STEM) coupled to energy-dispersive X-ray spectroscopy (EDX) analysis is used to optimize the growth conditions and to determine the composition of wells and barriers. Optical characterizations of the grown structure reveal a well-defined band-edge emission peak at 285 nm. Based on macro-transmission measurements and simulations, the absorption coefficient of the wells is estimated to be 3 × 105 cm−1 (E-field ⊥ c), attesting that the oscillator strength is preserved for these AlGaN MQWs with high Al content, which is promising for efficient surface-emitting devices in the deep ultra-violet (DUV) region.

Linear and nonlinear properties of ultraviolet fluorine crystal: A first-principles study

The electronic structure and optical properties of Deep Ultraviolet (DUV) nonlinear optical crystal BaMgF4 and KBe2BO3F2 have been studied in the framework of many-body perturbation theory as well as hybrid functionals. For the electronic properties, the hybrid functionals method makes significant improvement on description of electronic band gap for both crystals. However, the results still underestimate the band gap comparing with the GW results. By considering the self-energy of electrons, electronic band gap and the optical band gap are both described well comparing with the experimental results, which is crucial for prediction the performance of DUV crystals in the DUV region. In addition to the remarkable self-energy effect, the macroscopic dielectric function and related optical properties, such as refractive index n(ω), excitation coefficient k(ω), absorption coefficient α(ω), energy-loss function L(ω), and reflectivity R(ω) have been calculated by solving Bethe–Salpeter equation (BSE). By comparing the RPA results and BSE results, we found that the excitonic effects play an important role in description of optical properties, which is absence in the previous work. Furthermore, taking advantage of 2n+1 theorem, the nonlinear optical susceptibility have been calculated as well. As all key factors that determine the performance of DUV nonlinear crystals have been addressed theoretically, our present work pave the way using first-principle theory to assist the crystal engineering for other candidates of DUV nonlinear optical crystal.

Tunable deep ultraviolet femtosecond sum frequency laser based on Ba1-xB2-y-zO4SixAlyGaz crystal

Tunable coherent deep ultraviolet (DUV) light sources, especially ultrashort pulse DUV lasers have great applications in the fields of time-resolved, material processing, spectroscopy, laser spectroscopy and laser fusion. In the UV region, the best choice of generating the laser pulses in the femtosecond or picosecond regime is the frequency up-conversation technique based on second order nonlinearities. Over the past three decades, quite a lot of nonlinear crystals, such as LiB3O5, βup-BaB2O4, KBe2BO3F2 and Ba1-xB2-y- zO4SixAlyGaz have been developed and employed for generating the femtosecond pulses in the blue, ultraviolet, and even the deep-ultraviolet region. A tunable deep ultraviolet femtosecond laser is experimentally studied based on the new nonlinear crystal Ba1-xB2-y-zO4SixAlyGaz It is a kind of low-temperature phase barium metaborate single crystal belonging to a trigonal system, doped with one or more elements selected from Si, Al and Ga. As an optimized β-BaB2O4 crystal, Ba1-xB2-y-zO4SixAlyGaz completely overcomes the shortcomings of deliquescence compared with β-BaB2O4, and its nonlinear efficiency and optical damage threshold have also been greatly improved. Using two crystals as second harmonic generation is to compensate for the spatial walk-off effect and the light path walk-off due to refraction effect The optical axis of the second Ba1-xB2-y-zO4SixAlyGaz is twice the phase matching angle with respect to the first one. In a femtosecond regime, short pulse provides high efficient frequency conversation due to their high peak powers, but the group velocity mismatch is a cognitive factor to limit conversion efficiency. It is obvious that after the frequency doubling, the second harmonic pulse and fundamental pulse separate from each other. The second harmonic pulse lags behind the fundamental pulse as they propagate through the crystal and the second harmonic pulse is broadened into a longer pulse duration than the fundamental pulse The method to compensate for the group velocity mismatch is to adjust the path length between the fundamental and second harmonic pulse by means of time delay line. It consists of beam splitters and mirrors. Tunable deep ultraviolet pulse within a wavelength range from 192.5 to 210 nm is produced, with a maximum average power of 5.8 mW, under a 2.78 W fundamental power. The average power of second harmonic, third harmonic and fourth harmonic are 1.28 W, 194 mW and 5.8 mW at the fundamental wavelength of 800 nm, corresponding to conversion efficiencies of 46.14%, 15.16% and 3% from the previous stage, respectively. The duration of the third harmonic pulse is 640.4 fs at 266.7 nm as measured by the cross-correlation technique.

Enhanced photoelectron emission from aluminum thin film by surface plasmon resonance under deep-ultraviolet excitation

We report photoelectron emission enhancement of aluminum thin films by surface plasmon excitation in the deep-ultraviolet region. Deep-ultraviolet light with a wavelength of 266 nm has enough energy to cause electron emission from aluminum and excite surface plasmons on aluminum. We applied the Kretschmann configuration to excite surface plasmons. The enhancement factor of the emission current is found to depend on the enhanced electric field intensity excited by surface plasmons. The maximum emission efficiency of photoelectrons is 2.9 nA mW−1 for an aluminum thickness of 19 nm with an alumina thickness of 4 nm. The characteristics of the dependence of the photoelectron emission efficiency on the applied bias between the anode and cathode are investigated.

Na3Ba2(B3O6)2F: Next Generation of Deep-Ultraviolet Birefringent Materials

Birefringent materials are of great importance in optical communication and the laser industry, as they can modulate the polarization of light. Limited by their transparency range, few birefringent materials, except α-BaB2O4 (α-BBO), can be practically used in the deep ultraviolet (UV) region. However, α-BBO suffers from a phase transition and does not have enough transparency in the deep UV region. By introducing the relatively small alkali metal Na+ cation and the F– anion to keep the favorable structural features of α-BBO, we report a new birefringent crystal Na3Ba2(B3O6)2F (NBBF), which has the desirable optical properties. NBBF not only maintains the large birefringence (Δn = no – ne = 0.2554–0.0750 from 175 nm to 3.35 μm) and extends its UV cutoff edge to 175 nm (14 nm shorter than α-BBO) but also eliminates the phase transition and has the lowest growth temperature (820 °C) among birefringent materials. These results demonstrate that NBBF is an attractive candidate for the next generation of deep UV birefringent materials.

Quantum state engineering with ultra-short-period (AlN)m/(GaN)n superlattices for narrowband deep-ultraviolet detection.

Ultra-short-period (AlN)m/(GaN)n superlattices with tunable well and barrier atomic layer numbers were grown by metal-organic vapour phase epitaxy, and employed to demonstrate narrowband deep ultraviolet photodetection. High-resolution transmission electron microscopy and X-ray reciprocal space mapping confirm that superlattices containing well-defined, coherently strained GaN and AlN layers as thin as two atomic layers (∼ 0.5 nm) were grown. Theoretical and experimental results demonstrate that an optical absorption band as narrow as 9 nm (210 meV) at deep-ultraviolet wavelengths can be produced, and is attributable to interband transitions between quantum states along the [0001] direction in ultrathin GaN atomic layers isolated by AlN barriers. The absorption wavelength can be precisely engineered by adjusting the thickness of the GaN atomic layers because of the quantum confinement effect. These results represent a major advance towards the realization of wavelength selectable and narrowband photodetectors in the deep-ultraviolet region without any additional optical filters.

Deep-ultraviolet surface plasmon resonance of Al and Alcore/Al2O3shell 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 × 107 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.

Optical properties of UV-transparent aluminum oxide / aluminum fluoride mixture films, prepared by plasma-ion assisted evaporation and ion beam sputtering

Electron beam evaporation (without and with plasma assistance) as well as ion beam sputtering are used to prepare optical mixture coatings for applications in the ultraviolet spectral range. It is demonstrated that intermixing aluminum oxide/ aluminum fluoride materials by these physical vapor deposition techniques results in optical coatings with flexible refractive indices varying between 1.40 and 1.75 in the deep ultraviolet spectral region. At the same time, extinction coefficients vary between less than 1x10−4 and 2x10−3. For evaporated layers, at certain mixture ratios, mechanical stress appears to be close to zero.

A computational study of the double-bands plasmonic light scattering of Al 2 O 3 coated Al nanoshells in the deep-ultraviolet range

Abstract The tunable scattering cross section of oxide layer-coated Al nanoshell has been computationally studied as a function of wavelength. The calculation results show that the two resonance light scattering (RLS) peaks of Al nanoshell have higher frequency and greater intensity than that of Au and Ag nanoshell. Both of the two scattering peaks, which are corresponding to anti-symmetric and symmetric plasmon coupling, take place in the ultraviolet range. And the anti-symmetric scattering peak could be tuned down to the deep-ultraviolet wavelength below 200 nm. By increasing radius and dielectric constant of the inner core, or decreasing the thickness of the oxide layer, the anti-symmetric scattering peak with short wavelength could be enhanced and has greater intensity than that of symmetric scattering peak with long wavelength, which is different from that of Au and Ag nanoshells. Obtaining intense RLS peaks from Al nanoshell at deep-ultraviolet region presents a potential for the application of ultra-sensitive biosensing, ultraviolet material characterization, and ultraviolet nanoscale imaging.

Indium for Deep-Ultraviolet Surface-Enhanced Resonance Raman Scattering

The dielectric constant of indium in the deep-ultraviolet (DUV) region satisfies the conditions for localized surface plasmon resonance with low absorption loss. We report that indium acts as an agent of efficient surface-enhanced resonance Raman scattering (SERRS) in the DUV. Indium-coated SERRS substrates were prepared by depositing indium on fused silica glass substrates with control of the deposition thickness to tailor the plasmon resonance in the DUV. With excitation at 266 nm, SERRS was observed from thin adenine films deposited on the indium-coated substrates, and the signal intensity was up to 11 times higher than that of a bare fused silica glass substrate. FDTD calculations showed that an enhanced electromagnetic field can be locally generated on the indium-coated substrates. Considering the volume of the enhanced field region in the excitation spot, we estimated the average enhancement factor to be 102 or higher. Our results indicate that indium is a promising and easy-to-use metal for efficie...

Structural, morphological, FTIR and photoluminescence properties of gallium oxide thin films

Thin films of β-Ga2O3 are prepared on sapphire substrates via electron beam evaporation and annealed at 1000 °C for 1 h. The effect of the annealing treatment upon the crystal structures, surface morphologies, and optical properties of β-Ga2O3 films are investigated by x-ray diffraction, scanning electron microscopy, atomic force microscopy, Fourier transform infrared spectroscopy, and photoluminescence and optical transmittance spectra. The easily prepared β-Ga2O3 films present a mixed structure of amorphous and crystalline phases. The annealed β-Ga2O3 films exhibit a clear absorption edge in the deep ultraviolet region. Ultraviolet and red emissions are also observed in the photoluminescence spectra of the annealed β-Ga2O3 films.

Effects of substrate temperatures on the characterization of magnesium fluoride thin films in deep-ultraviolet region

As a low refractive index material widely used in coatings for deep-ultraviolet optical systems, magnesium fluoride (MgF2) films were prepared by electron beam evaporation at different substrate temperatures. The effects of the substrate temperatures on the optical properties in vacuum and in air, microstructures, and composition were investigated, as were the microstructures, their composition, and the relation between them. In vacuum, the substrate temperature directly affected the microstructures which dominated the packing density and inhomogeneity along the film thickness. When the films were exposed to air, the refractive index increased and a nonmonotonic change trend of the refractive index with substrate temperature was observed due to adsorbed water and magnesium oxide (MgO) formed in the film. While a moderate amount of MgO reduced absorption loss by decreasing vacancy defects, excessive MgO increased the absorption loss because of the high extinction coefficient of the oxide.

Optical detection of nonradiative recombination centers in AlGaN quantum wells for deep UV region

AbstractBy superposing a chopped below‐gap excitation (BGE) light on a cw above‐gap excitation and observing the intensity change of photoluminescence (PL) due to the BGE, we clarified a distribution of nonradiative recombination (NRR) centers in AlGaN multi‐quantum well (MQW) structures for the wavelength region of deep ultraviolet (UV). The decrease in band‐edge PL peak intensity at 10 K exemplified the presence of a pair of NRR centers in AlGaN well layers whose transition energy corresponds to that of the BGE. The BGE power dependence of the normalized PL intensity showed a quenching saturation due to trap‐filling effect of electrons in one of the centers which were activated by the BGE. The BGE energy dependence of the normalized PL intensity revealed that the optical activation energy of the dominant NRR process via these trap states in the AlGaN MQW is around 1.17 eV. The purely optical scheme of detection enables us to obtain clues for identifying defect levels in DUV region and eliminating them during growth process without any preparation of electrodes. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Structural, electronic and optical properties of novel carbonate fluorides ABCO3F (A=K, Rb, Cs; B=Ca, Sr)

In the present study, we performed first principles calculations on the electronic and optical properties of layered alkali–alkaline earth carbonate fluorides which attract attention in the domain of nonlinear optics. The calculated lattice parameters and volumes with and without inclusion of van der Waals (vdW) correction methods to standard density functional methods were compared with experiments. We observed that vdW interactions are predominant in RbCaCO3F and CsCaCO3F as compared with other computed compounds. The calculated bulk modulus from single crystal elastic constants reveals that these materials are all relatively harder than the KH2PO4 (KDP) crystal. We also found that these materials are wide band gap insulators as obtained from Tran–Blaha modified Becke–Johnson potential. The linear optical properties such as dielectric function, refractive indices, birefringence and absorption spectra are presented. Finally, the calculated birefringence values indicate that these crystals could be promising for producing phase matching in the deep ultra-violet region.

Deposition of Al nanoparticles and their nanocomposites using a gas aggregation cluster source

The study describes the preparation and properties of Al/Al x O y nanoparticles and their nanocomposites with hydrocarbon plasma polymer. The nanoparticles were deposited using a simple gas aggregation cluster source based on a planar magnetron. The influence of deposition parameters on size distribution was studied. It was found that Al nanoparticles may be deposited with mean diameter ranging from 30 to 60 nm by varying magnetron current from 0.4 to 0.2 A. Attention was also paid to the investigation of the charge of the nanoparticles. It is shown that a large portion of Al x O y nanoparticles leave the gas aggregation source as negatively charged. The nanoparticles were also embedded into the plasma polymer matrix by their simultaneous co-deposition with plasma-polymerized n-hexane. Morphology and chemical composition of such fabricated nanocomposites were determined. It was shown that Al x O y nanoparticles embedded into the polymeric matrix exhibit localized surface plasmon resonances in the deep ultraviolet region of the electromagnetic spectrum and the position of the plasmon peak is strongly correlated with nanoparticle size.