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Effect of Interface State Density of the AlGaN Electron Blocking Layer/GaN Barrier Layer in InGaN Blue Light-Emitting Diodes
摘要： Herein, two kinds of blue InGaN light-emitting diodes (LEDs) were compared in order to understand the effect of the strain originating from two different sapphire substrates. One of the LED types was grown on a patterned sapphire substrate (PSS), and the other was grown on a conventional sapphire substrate (CSS). The internal strains biased on InGaN quantum wells were compared by studying the time-resolved photoluminescence and the electroreflectance. Through analysis of the dynamic capacitance dispersion measurements, the relation between the trap states and trap density of the AlGaN electron blocking layer/GaN barrier interface above the InGaN quantum wells can be understood in great detail. The trapped charges of the CSSs have faster response times than those of the PSSs; this is attributed to the stronger piezo-field, which induces the AlGaN EBL/GaN barrier interface trap states to be effectively located at shallower levels from the conduction band.
关键词： Strain,Patterned sapphire substrate,InGaN quantum wells
更新于2025-09-23 15:19:57
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Light Extraction Efficiency Optimization of AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes
摘要： Using finite-difference time-domain method, the light extraction efficiency (LEE) of AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) is investigated. Simulation results show that compared to flat sapphire substrate, the nano-patterned sapphire substrate (NPSS) expands the extraction angles of top surface and sidewalls. As a result, the LEE of transverse-magnetic (TM) polarized light is improved significantly. Roughening on the backside of n-AlGaN surface significantly enhances the LEE of top surface of thin-film flip-chip DUV LEDs. However, the LEE of sidewalls of thin-film flip-chip DUV LEDs is greatly weakened. For bare DUV LED, the LEE of flip-chip LED on NPSS is estimated to be about 15%, which is around 50% higher than that of thin-film flip-chip DUV LED with roughening on the backside of n-AlGaN surface.
关键词： AlGaN,nano-patterned sapphire substrate,light-emitting diodes,deep-ultraviolet,light extraction efficiency
更新于2025-09-23 15:19:57
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Low pressure chemical vapor deposition of In <sub/>2</sub> O <sub/>3</sub> films on off-axis c-sapphire substrates
摘要： Heteroepitaxial In2O3 films were grown on transparent c-sapphire substrates with 0°, 3.5°, 6° and 8° off-axis angles via low pressure chemical vapor deposition (LPCVD). Metallic solid indium and oxygen gas were used as the precursors. X-ray diffraction (XRD) spectroscopy and cross-sectional scanning transmission electron microscopy (STEM) were used to study the effects of substrate off-axis on the crystalline quality of the as-grown films. The XRD 2θ-ω spectra confirmed the growth of body centered cubic bixbyite In2O3 films with (111) out-of-plane orientation. STEM images revealed the suppressed dislocation density in the films grown on substrates with off-axis angles. The crystal structure was further confirmed by Raman spectroscopy. Fifteen peaks corresponding to bcc-In2O3 Raman modes were observed at room temperature (RT). XRD ω-rocking curves, XRD Φ-scan profiles and STEM images indicate improved crystalline quality in films grown on substrates with, particularly, 3.5° and 6° off-cut angles. Growth rates in a wide range of ~ 0.5 - 30 μm/hr were achieved. RT photoluminescence peak at ~2.15 eV is attributed to the deep level defect transitions. RT photoluminescence excitation peak at ~3.38 eV corresponds to the optical bandgap of bcc-In2O3. RT electron Hall mobilities of ~ 88 - 116 cm2/V.s were measured with background carrier concentrations of ~ 6 - 9x1017 cm-3.
关键词： low pressure chemical vapor deposition,Indium oxide,off-axis substrate,c-sapphire
更新于2025-09-19 17:15:36
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Evolution of Mechanically Formed Bow Due to Surface Waviness and Residual Stress difference on Sapphire (0001) Substrate
摘要： Single crystalline sapphire is one of the most widely used wafers for modern high-technology electronic and optical devices owing to the well-established mass production, own physical properties with high thermal, mechanical, and chemical stability, and lattice constants suitable for growth of highly crystalline films. When single crystalline wafers are used as substrates, the curvature of the wafer surface usually called bow is inevitable. Since bow can degrade the quality of deposited film by defects generation or undesirable strain at the interface, to reduce the bow and surface roughness, procedures such as lapping, annealing, and polishing are followed after wafer slicing. Although single crystalline sapphire has sufficiently high hardness for the device applications, surface waviness after abrasion and residual stress within wafer can induce bow. In this study, we revealed the mechanism of bow evolution using high-resolution X-ray diffraction and Raman spectroscopy following each wafering steps. We found that bow in single crystalline sapphire wafer is mainly determined by the irregularly abraded surface after wafer slicing at earlier wafering process. This kind of bow is significantly reduced after lapping and annealing procedure. However, additional surface polishing can inflict stress on the surface and increase bow due to residual stress difference between the polishing surface and the opposite surface. While bow can be effectively reduced by further polishing on the opposite surface, the surface lattice remains still strained.
关键词： HR-XRD,Machining process,Residual stress,Sapphire,Bow,Raman spectroscopy
更新于2025-09-19 17:15:36
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Laser multi-focus precision cutting of thick sapphire by spherical aberration rectification
摘要： Laser multi-focus separation technology is an effective way to cut thick, transparent and hard materials. In this paper, factors affecting the quality of multi-focus cutting sapphire were investigated. Optical simulations analysis was performed based on experimental results to analyze multi-focal laser beam propagation from air into sapphire. The results show that the focus interval is linearly dependent on refractive index. Spherical aberration due to refractive-index mismatch disperses the laser foci, leading to a reduction in laser power density which has a significant influence on the cutting quality. The aberration is strongly dependent on the refractivity of the material and the effective numerical aperture (NA). By redesigning the distribution of multiple foci to correct spherical aberration, sapphire with a thickness of 2 mm was cut successfully at a scanning speed of 4 mm/s. The roughness of the cutting section was ～2.5 μm , without obvious observed micro-cracks.
关键词： Optical simulation,Spherical aberration,Multi-focus,Sapphire
更新于2025-09-19 17:13:59
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Improvement of ablation capacity of sapphire by gold film-assisted femtosecond laser processing
摘要： Sapphire is widely used in civilian and military equipment owing to its superior optical and mechanical properties. Femtosecond laser has been demonstrated to be an effective tool to process sapphire material. However, the direct processing of sapphire by femtosecond laser still meets some challenges, such as poor ablation morphology and low laser energy absorption. In this work, femtosecond laser processing of sapphire coated with a 12-nm-thick gold film (Au-coated sapphire) has been investigated. The experimental results have revealed that the ablation morphology of Au-coated sapphire has been improved, featuring fewer molten materials and thermal cracks, as well as regular crater shape and uniform periodic surface structures. It has also been found that, under 100 shots condition, the threshold fluence of Au-coated sapphire is reduced by about 56% compared to that of uncoated one. Meanwhile, the incubation effect of Au-coated sapphire is stronger than that of uncoated one. We also illustrate that the material removal rate of Au-coated sapphire is increased up to about two times higher than that of uncoated one. In order to reveal the effective mechanism of the gold film in the laser processing of sapphire, the energy transfer process among incident photons, free electrons and sapphire lattice phonons was studied. Our study provides a guidance for improving the laser ablation capacity of sapphire.
关键词： Gold film,Sapphire,Material removal rate,Ablation morphology,Femtosecond laser processing
更新于2025-09-19 17:13:59
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[IEEE 2019 SBFoton International Optics and Photonics Conference (SBFoton IOPC) - Sao Paulo, Brazil (2019.10.7-2019.10.9)] 2019 SBFoton International Optics and Photonics Conference (SBFoton IOPC) - Evaluation of the Adhesive Strength in Dentin after Irradiation with Ti:Sapphire Ultrashort Laser Pulses
摘要： This study was done to evaluate whether the irradiation of dentine with ultrashort laser pulses prior to adhesive procedures contributes to increase the adhesive resistance to microtraction essays. Twenty-four human teeth (third molars) were used, divided into 4 groups (n=6): standard adhesive procedures with etch-rinse adhesive were used in the control group, and in the experimental groups, the dentin was irradiated by 25 fs pulses prior to the adhesive procedures, with varying fluences. One tooth from each group was used to evaluate its surface roughness. After 24h, the teeth were sectioned with perpendicular cuts, producing toothpicks that were submitted to the microtraction test. The results showed that the groups irradiated with fluences under 4 J/cm2 presented similar results among themselves, comparable to the control group, while the group irradiated with 8 J/cm2 showed lower adhesive strength. We propose that the adhesive strength and surface roughness reduction resulting from the fluence increase is probably related to the shielding arising from an electron plasma formation during the ablation, decreasing the material removal efficiency. Nevertheless, the lower energy densities did not affect the adhesiveness, maintaining values similar to the control.
关键词： dentin,femtoseconds laser pulses,microtensile bond strength,adhesion,Ti:Sapphire laser irradiation
更新于2025-09-19 17:13:59
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[Laser Institute of America ICALEO? 2014: 33rd International Congress on Laser Materials Processing, Laser Microprocessing and Nanomanufacturing - San Diego, California, USA (October 19–23, 2014)] International Congress on Applications of Lasers & Electro-Optics - Laser microjet? cutting of up to 3 mm thick sapphire
摘要： The Laser Microjet? is a world-wide patented technology which uses ns-laser pulses which is guided to the work piece in a water jet. The sample is therefore cooled by the water and thermal damage during cutting is reduced which is relevant for metals and semiconductor materials. For the case of sapphire cutting this leads to lower thermal load of the samples and therefore no cracking during cutting. Sapphire cutting was until now not possible with the Laser Microjet? although it is used for micro machining of a lot of other materials like semiconductors, metals, diamond and ceramics. Cutting up to 3 mm thick sapphire with parallel walls with a roughness of < 0.5 μm and a kerf width of < 100 μm is achieved. The cutting speed of up to 0.1 mm/s for 1 mm samples has been achieved. The cut samples exhibit parallel walls which is a big advantage of the Laser Microjet? technology compared to other laser cutting technologies, not only for sapphire. The edges at the front side exhibit high quality with a radius of curvature of less than 20 μm and without any chipping. The back side quality still needs to be improved although chips with less than < 20 μm are already achievable. For laser cutting of sapphire high peak intensities are required in order to reach sufficient high absorption of the laser radiation and achieve material ablation. The process is not yet fully understood. It is expected that the water jet interaction with the material plays an important role together with laser light-material interaction. The geometry of the kerf of the work piece has a strong influence on the cutting process. Water jet interruptions due to e.g. non planar surfaces lead to loss of energy on the way to the work piece and therefore interruption of the absorption and ablation process. That is why certain processing strategies are necessary to achieve cutting for samples with the thickness of 3 mm. We expect to be able to cut even thicker samples of > 5 mm as well as further improve the backside edge quality in the future.
关键词： high peak intensities,thermal damage reduction,water jet coupled laser,Laser Microjet?,sapphire cutting
更新于2025-09-19 17:13:59
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A comparative clinical trial to evaluate efficacy and safety of the 308‐nm excimer laser and the gain‐switched 311‐nm titanium:sapphire laser in the treatment of vitiligo
摘要： Background/purpose: Ultraviolet B (UVB) laser irradiation in a targeted manner is a reasonable treatment option for localized vitiligo. Recently, narrow‐band UVB gain‐switched 311‐nm titanium:sapphire lasers (TSL) were developed for the treatment of localized vitiligo. We aimed to compare efficacy, patient satisfaction, and safety between the conventional 308‐nm excimer laser (EL) and gain‐switched 311‐nm TSL in patients with vitiligo. Methods: The 13‐paired lesions from 10 patients who had small vitiligo patches were included in this prospective intra‐patient comparison trial. Each pair was randomly assigned to each laser treatment group and treated twice weekly for 12 weeks. The global photographic assessments by dermatologists, objective numerical assessments by imaging analyzer, and patient's satisfaction were used to evaluate the effectiveness. Adverse effects were also investigated at every visit. Results: All treated lesions showed improvement of about 50% after 12 weeks. There was no significant difference between EL‐ and TSL‐treated groups. Patient satisfaction and preference among the groups were also similar. Regarding safety, there were no serious adverse effects requiring cessation of the treatments; however, the severity score for persistent erythema (lasting >24 hours) was significantly lower in the TSL group than in the EL group. Conclusions: The gain‐switched 311‐nm TSL exhibited similar efficacy to the 308‐nm EL in treating vitiligo as well as improved safety. Therefore, the 311‐nm TSL is considered as a candidate device to replace the EL as a new and promising treatment option for localized vitiligo.
关键词： 308‐nm Excimer laser,gain‐switched 311‐nm titanium:sapphire laser,vitiligo
更新于2025-09-19 17:13:59
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Modification characteristics of filamentary traces induced by loosely focused picosecond laser in sapphire
摘要： The ability of loosely focused picosecond laser to induce filamentation (4.8 times the Rayleigh length) in sapphire was confirmed. The morphology and microstructure of the filamentary traces under typical pulse energy were studied, in which the colorful birefringence phenomenon and damage defects were detected in sapphire. Irreversible ceramic-like polycrystalline microstructures were formed in filamentary traces, and the phase transition from α-Al2O3 to γ-Al2O3 directly reflected the thermal effect of picosecond laser filamentation. According to the filamentary characteristics, a continuous filamentary channel with a uniform diameter of 33 μm and a length of 7443 μm was obtained by gradually raising the focal position. The research provides first-hand information for the high throughput micro-cutting, micro drilling and thin slicing of sapphire based on ultrafast laser non-linear effects.
关键词： Phase transition,Modification,Picosecond laser,Filamentation,Sapphire
更新于2025-09-19 17:13:59