Improved light extraction of GaN-based light-emitting diodes with surface-patterned ITO

The surface patterning of the indium tin oxide (ITO) transparent current layer has been investigated to improve the light extraction efficiency of GaN-based light-emitting diodes (LEDs). LEDs with periodic micro-hexagon patterned ITO have been fabricated utilizing standard lithography techniques and inductively coupled plasma (ICP) technology. The luminance intensity of the LED chips with patterned ITO following 160 s ICP etching was enhanced by about 50% compared to the LED chips with unpatterned ITO. Detailed processing parameters are provided. scanning electron microscopy (SEM) and atomic force microscopy (AFM) are used to examine the micro-structures. The results indicate that the surface-patterned ITO technique could have potential applications in high-power GaN-based LEDs.     

GaN-based light-emitting diodes directly grown on sapphire substrate with holographically generated two-dimensional photonic crystal patterns

As an approach to enhance light extraction from GaN-based light-emitting diodes (LEDs), we inserted a submicron period photonic crystal (PC) pattern at the interface between GaN epilayer and sapphire substrate. A two-dimensional square-lattice pillar array of 600-nm period was produced directly onto the sapphire substrate by a combination of laser holography and inductively-coupled-plasma etching. A standard GaN LED heterostructure was grown on top of the nano-patterned substrate, which was then processed to conventional bottom-emitting LED chips. At the drive current of 20 mA, the PC-LED produced surface-normal output power about 40% higher than that of the reference LED (with no PC integrated). Temperature-dependent photoluminescence measurement indicated that the emission enhancement was solely a structural effect by the integrated PC pattern.     

Damage/ablation morphology of laser conditioned sapphire under 1064 nm laser irradiation

The damage/ablation morphologies and laser induced damage threshold (LIDT) of three different sapphire samples: original, 1064 nm laser conditioned and 10.6 μm CO₂ laser polished substrates are investigated with ns pulses laser irradiation. The results indicate that the damage resistance capability cannot be enhanced by 1064 nm laser conditioning or CO₂ laser polishing. The damage/ablation morphology of 1064 nm-laser conditioned samples is same as that of the original sapphire. But it is different from the damage/ablation morphology of the CO₂ laser polished sapphire. The “gentle and strong” ablation phases are observed in this work and several phenomena are observed in the two ablation phases. Ripple is observed in the “gentle” ablation processes, while convex spots and raised rims are observed in the “strong” ablation processes. Meanwhile, stripe damage and pin-points are observed in the CO₂ laser conditioned sapphire after ns laser irradiation. The formation mechanisms of the phenomena are also discussed for the explanation of related damage/ablation morphology. The results may be helpful for the damage/ablation investigation of sapphire in high power laser systems.     

全息技术制作二维光子晶体蓝宝石衬底提高发光二极管外量子效率

为了提高GaN基发光二极管(LED)的外量子效率,在蓝宝石衬底制作了二维光子晶体.衬底上的二维光子晶体结构采用激光全息技术和感应耦合等离子体(ICP)干法刻蚀技术制作,然后采用金属氧化物化学气相沉积(MOCVD)技术在图形蓝宝石衬底(PSS)上生长2μm厚的n型GaN层,4层量子阱和200nm厚的p型GaN层,形成LED结构.衬底上制作的二维光子晶体为六角晶格结构,晶格常数为3.8μm,刻蚀深度为800nm.LED器件光强输出测试结果显示,在PSS上制作的LED(PSS-LED)的发光强度普遍高于蓝宝石平 关键词： 全息 发光二极管 图形蓝宝石衬底 外量子效率     

Mid-wavelength type II InAs/GaSb superlattice infrared focal plane arrays

We have demonstrated 384 × 288 pixels mid-wavelength infrared focal plane arrays (FPA) using type II InAs/GaSb superlattice (T2SL) photodetectors with pitch of 25 μm. Two p-i-n T2SL samples were grown by molecular beam epitaxy with both GaAs-like and InSb-like interface. The diode chips were realized by pixel isolation with both dry etching and wet etching method, and passivation with SiN_x layer. The device one with 50% cutoff wavelength of 4.1 μm shows NETD ∼ 18 mK from 77 K to 100 K. The NETD of the other device with 50% cutoff wavelength at 5.6 μm is 10 mK at 77 K. Finally, the T2SL FPA shows high quality imaging capability at the temperature ranging from 80 K to 100 K which demonstrates the devices’ good temperature performance.     

Micro-solid oxide fuel cell using thick-film ceria

A fabrication method that does not use lithography or etching processes for thick-film based micro-SOFCs (Solid Oxide Fuel Cells) was described and discussed. In this study, a new type of micro-SOFC was fabricated using a free-standing thick-film electrolyte with ~ 20 μm thickness. This structure has the advantages of both electrolyte-support and electrode-support type SOFCs. Generally, the electrolyte should be thicker than e.g., ~ 150 μm since a thinner electrolyte easily cracks in a self-supporting mode during the fabrication procedure. Thus, a new mounting method was developed in order to use a thin-electrolyte film. In this study, a ~ 20 μm-thick GDC (Gd-doped ceria) electrolyte film was successfully mounted on a ~ 400 μm-thick GDC ring by sintering these two pieces together. Ni-GDC and Sm_0.5Sr_0.5CoO₃ were brush painted as an anode and a cathode, respectively. With this new configuration, it was possible to construct an electrolyte-supported SOFC using a thick-film ceria-based electrolyte and measure the power density. The open-circuit voltage (OCV) of the cell in 97%H₂ + 3%H₂O/air was ~ 0.87 V and the maximum power density was ~ 270 mW/cm² at 600 °C. The result shows that the high performance is achievable for the micro-SOFCs using a thick-film ceria-electrolyte operating at 600 °C.     

Nonselective vertical etching of GaAs and AlGaAs/GaAs in high pressure capacitively coupled BCl3/N2 plasmas

We studied nonselective, vertical dry etching of GaAs and AlGaAs/GaAs structure in high pressure capacitively coupled BCl₃/N₂ plasmas. The operating pressure was fixed at 150 m Torr. We found that there was an optimized process condition for nonselective and vertical etching of GaAs and AlGaAs/GaAs at the relatively high pressure. It was noted that there was a range of % N₂ (i.e. 20–40%) where nonselective etching of GaAs over AlGaAs could be achieved in the BCl₃/N₂ mixed plasma. We also found that dry etching of GaAs and AlGaAs/GaAs structure provided quite vertical and smooth surface when % N₂ was in the range of 0–20% in the BCl₃/N₂ plasma. The maximum etch rates for GaAs (0.41 μm/min) and AlGaAs/GaAs structure (0.42 μm/min) were obtained with 20–30% N₂ composition in the plasma.     

AFM characterization of bovine enamel and dentine after acid-etching

Teeth are constituted mainly of hydroxyapatite molecules (Ca₁₀(PO₄)₆(OH)₂), grouped in different microstructural arrangements, depending on the dental layer considered (enamel or dentine). In the present work, these dental microstructural arrangements were characterized by atomic force microscopy. Enamel and dentine samples were cut from freshly extracted bovine incisor teeth. After metallographic polishing, the dental surfaces were etched with lactic acid (113.8 mmol/L, pH 3.3). Three etching times were tested: 1, 3 and 5 min. Atomic force micrographs showed that 1 min of etching time was effective to remove the smear layer, polishing debris and scratches, and display the characteristics of interest for both enamel and dentine. Although the bovine dental enamel rod cross-section presented keyhole-like shape, its measured dimensions (8.8 μm of major axis and 3.7 μm of minor axis) exhibited an insignificant discrepancy from human prisms diameters. Bovine dentinal tubules displayed larger mean diameters (4.0 μm) and a lower density (～17,100 tubules/mm²) than human dentine, suggesting that the use of bovine dentine as a substitute for human dentine in resin adhesion investigations should be reconsidered. Apatite nanoparticles presented a mean radius (22–23 nm) considerably smaller than that of human teeth.     

10.6 μm Infrared light photoinduced insulator-to-metal transition in vanadium dioxide

Vanadium dioxide has excellent phase transition characteristic. Before or after phase transition, its optical, electrical, magnetic characteristic hangs hugely. It has a wide application prospect in many areas. Now, the light which can make vanadium dioxide come to pass photoinduced phase transition range from soft X-ray to medium infrared light (6.9 μm, 180 meV). However, whether 10.6 μm (117 meV) long wave infrared light can make vanadium dioxide generate photoinduced phase transition has been not studied. In this paper, we researched the response characteristic of vanadium dioxide excited by 10.6 μm infrared light. We prepared the vanadium dioxide and test the changes of vanadium dioxide thin film’s transmittance to 632.8 nm infrared light when the thin film is irradiate by CO₂ laser. We also test the resistivity of vanadium dioxide. Excluding the effect of thermal induced phase transition, we find that the transmittance of vanadium dioxide thin film to 632.8 nm light and resistivity both changes when irradiating by 10.6 μm laser. This indicates that 10.6 μm infrared light can make the vanadium dioxide come to pass photoinduced phase transition. The finding makes vanadium has a potential application in recording the long-wave infrared hologram and making infrared detector with high resolution.     

Femtosecond laser processing of indium-tin-oxide thin films

Micro- and nano-scale crystalline indium-tin-oxide (c-ITO) patterns fabricated from amorphous ITO (a-ITO) thin films on a glass substrate using a (low NA 0.26) femtosecond laser pulse that is not tightly focused are demonstrated. Different types of c-ITO patterns are obtained by controlling the laser pulse energies and pulse repetition rate of a femtosecond laser beam at a wavelength of 1064 nm: periodic micro c-ITO dots with diameters of ~1.4 μm, two parallel c-ITO patterns with/without periodic-like glass nanostructures at a laser scanning path and nano-scale c-ITO line patterns with a line width ~900 nm, i.e. ~1/8 of the focused beam׳s diameter (7 μm at 1/e²).     

High-power eye-safe KTA-OPO driven by YVO4/Nd:YVO4 composite laser

We present a high-power 1.53 μm laser based on intracavity KTA-OPO driven by diode-end-pumped acousto-optical Q-switched YVO₄/Nd:YVO₄ composite. The composite crystal was utilized for reducing the thermal effect, and the mode mismatch compensating OPO cavity was designed for efficient OPO conversion. The output power of eye-safe laser at 1535 nm was up to 4.4 W with the pump power of 27 W, corresponding to a diode-to-signal conversion efficiency of 16.3%. To our knowledge, this is the highest output power in diode-end-pumped circumstances. In the experiment, the strong yellow light generated by Raman conversion and frequency doubling in the KTA crystal was observed.     

Exchange bias in patterned FeMn/NiFe bilayers

Electron beam lithography and ion beam etching have been used to pattern a wire-like array in FeMn/NiFe bilayers. The variation of hysteresis loops with the etching depth in FeMn layer has been presented, and it has been found that with increasing etching depth the coercivity increases and M–H loops show an asymmetric kink. Detailed studies of the magnetic behaviors of the asymmetric kink in the patterned sample with 3.5 nm thick FeMn layer have been performed, and a magnetization component perpendicular to the wire direction has been observed.     

Layer optimization in wight organic light emitting diodes (WOLEDs) to reduce the portion of guided waves in ITO/Glass interface

In this paper, our main attempt was to reduce Total Internal Reflection (TIR) happening at Indium Tin Oxide (ITO, n_ito = 1.8 + 0.01i) and Glass (n_glass = 1.51) interface, which is due to ITO's higher index in comparison with Glass's, that makes light guided in ITO layer, 50% of generated light in Wight Organic Light Emitting Diodes (WOLED) are trapped in ITO layer; here we tried to reduce this portion of trapped light by implying 12-fold quasi-photonic crystal to the mentioned interface. With some gentle changes in 12-fold's structure we could reduce TIR in this interface to less than 9%. Also, far field results before and after adding the structure to WOLED were studied, which represents suitability of using this structure for lighting applications.     

Compact WDM demultiplexer for seven channels in photonic crystal

We demonstrate a new device concept for wavelength division demultiplexing based on planar photonic crystal waveguides. The filtering of wavelength channels is realized by shifting the cutoff frequency of the fundamental photonic bandgap mode in consecutive sections of the waveguide. The shift is realized by modifying the size of the border holes.The proposed demultiplexer has an area equal to (16.5 μm × 6.5 μm) and thus it is verified that this structure is very small and can be integrated easily into optical integrated circuits with nanophotonic technologies. The output wavelengths of designed structure can be tuned for communication applications, around 1550 nm. The wavelengths of demultiplexer channels are λ₁ = 1.590 μm, λ₂ = 1.566 μm, λ₃ = 1.525 μm, λ₄ = 1.510 μm, λ₅ = 1.484 μm, λ₆ = 1.450 μm, λ₇ = 1.400 μm respectively. Designs offering improvement of number of the separate wavelengths (seven), miniaturization of the structure (107.25 μm²) is our aim in this work.In our structure, we consider that the 2D triangular lattice photonic crystal is composed of air holes surrounded by dielectric. Its parameters are: radius of holes (r = 0.130 μm), lattice constant (a = 0.380 μm), and index of membrane (n = 3.181:InP). The numerical model used to simulate the structure of the demultiplexer is based on the finite difference time domain (FDTD).     

Measurement of optical and surface properties of PLZT thin films

Lanthanum-modified lead zirconate titanate (Pb_0.93La_0.07(Zr_0.3Ti_0.7)_0.93O₃, PLZT7/30/70) thin films with and without a seeding layer of PbTiO₃ (PT) were successfully deposited on indium-doped tin oxide (ITO) coated glass substrate via spin coating in conjunction with a sol–gel process, and a top transparent conducting thin film of SnO₂ was also prepared in the same way. The thicknesses of PLZT and PT layers are 0.5 μm and 24 nm, respectively. The retardance of PLZT film was measured by a new heterodyne interferometer and enhanced by application of a seeding layer of PT. The Pockels linear electro-optical coefficient of PLZT film with a PT layer was determined to be 3.17 × 10^?9 m/V when the refractive index is considered as 2.505, which is one order larger than 1.4 × 10^?10 m/V for PLZT12/40/60 doped with Dy reported in the literature. The root-mean-square (rms) roughness of PLZT thin film with a PT layer (R_rms = 6.867 nm) was larger than that of PLZT film (R_rms = 0.799 nm). From the comparisons, the average transmittance of PLZT film with a PT seeding layer was 77.01%, which was a little smaller than that of PLZT film (around 80.75%). Experimental results imply that the PT seeding layer plays a key role in the increase of retardance value, leading to a higher Pockels coefficient.     

Development of an efficient coherent optical source at 6.04 μm

Efficiency as high as 26% is obtained for generation of mid-infrared radiation at 6.04 μm by frequency doubling of ammonia laser emission at 12.08 μm in a 15 mm long type-I cut AgGaSe₂ crystal. The NH₃ laser used for this work is optically pumped by a commercial TEA CO₂ laser operating on 9.22 μm and produces pulsed output of ∼210 mJ with a duration of ∼200 ns at 12.08 μm. The generated radiation at 6.04 μm is separated out from the residual radiation at 12.08 μm by exploiting the principle of polarization dependent diffraction of reflection grating.     

Ferrofluidic masking of solid state nuclear track detectors during etching

This work presents a novel method for determining bulk etch rate of CR-39 during prolonged etching by masking the surface with a ferrofluidic film held in position by magnetostatic forces. The CR-39 etching conditions were 6.25 M NaOH solution for 24 h at temperatures ranging from 50 to 80 °C. After etching, the heights of the resulting un-etched plateaus were measured using a Talyscan 150 profilometer. The removed layer thicknesses ranged from 12 to 85 μm, giving corresponding bulk etch rates in the range 0.5–3.54 μm/h.     

The influence of thermal lens effect on pulse repetition rate in diode-pumped passively Q-switched Nd:GdVO4/V:YAG laser

The influence of thermal lens effect on pulse repetition rate in diode-pumped Nd:GdVO₄ 1.34 μm laser has been demonstrated. The repetition rate first increased, then reduced with augment of the pump power, for V:YAG with initial transmission (T₀) of 96%. The maximum repetition rate was 70 kHz, obtained at the pump power of 6.78 W. The best output laser characteristics—the pulse width of 39.2 ns, the peak power of 2.5 kW and the single pulse energy of 102 μJ—were obtained by V:YAG of T₀=89%. With the Gaussian spatial distribution of intracavity photon density being considered, the rate equations were solved numerically and the theoretical calculations agreed with the experimental results.     

Chemical etchant dependence of surface structure and morphology on GaN(0001) substrates

We studied the procedure of cleaning GaN(0001) substrate surfaces by wet etching and subsequent annealing in ultrahigh vacuum for two different types of freestanding GaN wafers: hydride vapor phase epitaxy (HVPE) crystal and Na flux liquid phase epitaxy (LPE) crystal wafers. A flat surface containing GaN(0001)2 × 2 reconstruction was successfully achieved on both HVPE and LPE surfaces by etching in HF and subsequent annealing at ～ 550 °C but was not achieved by etching in HCl, NaOH, and HNO₃.     

Recent progress in LaGaO3 based solid electrolyte for intermediate temperature SOFCs

Partial electronic and oxide ionic conduction in LaGaO₃ doped with Sr and Mg, Co for Ga site was studied with the ion blocking method. It was found that doping small amount of Co into Ga site is effective for elevating the oxide ion conductivity. However, it is seen that the partial electronic conduction monotonically increases with increasing Co amount and P_O2 at p–n transition was shifted to lower value. Even at X = 0.1, the oxide ion conductivity in LSGMC is still dominant. Calculation on the theoretical leakage of electrolyte of solid oxide fuel cells suggests that the highest efficiency of the electrolyte was achieved around 100 μm in thickness for La_0.8Sr_0.2Ga_0.8Mg_0.15Co_0.05O₃ (LSGMC). Preparation of LSGMC film on Ni–Sm_0.2Ce_0.8O₂ porous anode was studied with the colloidal spray method. In order to prevent the reaction between substrate and film, La doped CeO₂ was used for the interlayer film. In accordance with the theoretical calculation, open circuit potential of the cell using LSGMC film electrolyte with 40 μm thickness becomes much smaller than the theoretical value. However, fairly large maximum power density (0.21 W/cm²) can be achieved at 873 K and even at 773 K, the maximum power density of the cell as high as 0.12 W/cm² was exhibited on the SOFC using 40 μm thickness LSGMC electrolyte.