Structural design and characterization of fluorescent/phosphorescent multilayer top-emitting organic light-emitting diode (OLED) are investigated numerically with the Advanced Physical Model of Semiconductor Devices (APSYS) simulation program in this work. Specifically, the carrier balance and control of the migration of the triplet exciton diffusion avoiding the serious quenching which contributes to the roll-off in quantum efficiency at high current density, and limiting the singlet and triplet excitons at a better emitting zone can be optimized with appropriate cavity design of top-emitting OLED structure. Comparison between the results obtained numerically in this investigation and those obtained experimentally is made. Optimization of the optical and electronic performance of the multilayer OLED devices is attempted. The simulation results show that a better choice for the trade-off between color stability and electroluminescence efficiency can be achieved by properly adjusting the microcavity effect. An optimized performance is achieved if the recombination zone is designed to be located at the maximum of relative power, i.e., the anti-nodal region of the standing wave.
In this study, the effect of exciton-blocking layer (EBL), employed between the electron-transporting layer (ETL) and the undoped host spacer layer, on the characteristics of fluorescent/phosphorescent multilayer white organic lightemitting diode (OLED) is investigated numerically with the APSYS (Advanced Physical Model of Semiconductor Devices) simulation program. The validation of simulation model is confirmed by the good agreement of photoelectric characteristics between the results obtained numerically and those obtained experimentally. Simulation results suggest that singlet excitons and triplet excitons are generated at both hole-transporting layer (HTL)/emitting layer (EML) and EML/ETL interfaces, where electrons and holes accumulate and recombine, with certain thickness of host spacer layers employed on both sides of EML of white OLED structure. Further study shows that a better choice for the trade-off between color stability and electroluminescence (EL) efficiency can be achieved by properly adjusting the number of EBLs. An optimized performance is achieved if two pairs of EBLs are used.
In this paper, we investigated the failure mechanisms of blue InGaN LEDs grown on patterned sapphire substrates and demonstrated the influence of patterned sapphire substrates on the reliability of GaN LED by comparing with conventional LEDs grown on planar sapphire substrates. From experimental results, we found that InGaN LEDs grown on patterned substrates had a higher turn-on voltage but a smaller series resistance compared with conventional LEDs owing to rough inner patterns and small threading dislocation density. Both samples were then acceleratedly aged under a high DC current for two hours. Failure modes were studied with various measurements taken before and after aging. From the power evolution performance, we found that output power of LEDs with patterned substrates increased slightly due to fewer defects while output power of conventional LEDs decayed. This can be inferred from small reverse leakage currents and tunneling currents observed from Log I-V characteristics and EMMI measurement of P-LEDs. A slight redshift in emission wavelength was also found during aging because of possible leakage shunt paths caused by defect generation. Moreover, operation voltage increased slightly after aging which was caused by contact degradation induced by thermal annealing.
We have theoretically investigated the optimized quantum well structure for the ultra-deep ultraviolet (UV) AlGaN light emitting diodes (LEDs) with the consideration of band structure deformation caused by polarization effect. In this paper, we further employ an asymmetric active region to reduce the polarization field in the well-barrier interface and modify the band structure to enhance the power efficiency of the AlGaN LED. By increasing the thickness of p-side barrier from 5 nm to 15 nm, the deformation slope of energy band in the well region is reduced due to the reduction of polarization field, which is caused by the large polarization charges in the interface of p-side barrier and carrier blocking layer. Accordingly, the hole concentration is increased and the carrier distributions are more uniform caused by the less-tilted energy band in the well. Therefore, a higher recombination rate and a higher output power can be obtained. Moreover, the power efficiency of AlGaN LED is barely related to the n-side barrier thickness due to the less polarization field. However, a thinner n-side barrier is preferred to enhance the current spreading. Therefore, an asymmetric QW with a thinner n-side barrier and a thicker p-side barrier is a better choice to enhance the power efficiency for the deep UV AlGaN LED.
An optimized 650-nm AlGaInP multiple-quantum-well (MQW) laser, which has a compressively strained graded-index separate confinement heterostructure (GRIN-SCH), with improved characteristic temperature, is described. We theoretically show that the parabolic GRIN-SCH has a better carrier injection and smaller overflow than the conventional step-SCH for the AlGaInP LD under identical optical confinement. We have also calculated the electron distribution in the quantum wells for both GRIN-SCH-4QW and SCH-4QW at high temperature. The results indicate that the electron leakage to the p-cladding layer is greatly reduced if the GRIN-SCH-4QW structure is used. We have also compared the performance of LDs with different GRIN-SCH profiles and found that the parabolic GRIN-SCH is better than linear GRIN-SCH in terms of carrier confinement. We have further demonstrated the performance of AlGaInP LDs with four different structures (4-QW step-SCH, 5-QW step-SCH, 4-QW parabolic-GRIN-SCH and 5-QW parabolic-GRIN-SCH). Both theoretical and experimental results indicate that the laser diode with GRIN-SCH-4QW shows the best laser performance among the three structures. A characteristic temperature of 110 K has been demonstrated.
The vertical-cavity surface-emitting lasers (VCSEL) operating in the spectral range near 850 nm usually utilize GaAs/AlGaAs as the active layer materials. In this work, in addition to the traditional unstrained GaAs/AlGaAs semiconductor laser, the characteristics of the strained InGaAs/AlGaAs vertical-cavity surface-emitting laser and the distributed Bragg reflectors (DBR) used in this semiconductor laser are investigated with a PICS3D (abbreviation of Photonic Integrated Circuit Simulator in 3D) simulation program. The simulation results show that the strained InGaAs/AlGaAs VCSEL has a better optical performance than that of the traditional unstrained GaAs/AlGaAs VCSEL. That is, when compared with the unstrained GaAs/AlGaAs quantum well structures, the strained InGaAs/AlGaAs VCSEL has a higher stimulated recombination rate, a lower threshold current, a higher main-side mode suppression ratio, and a higher characteristic temperature, which might be owing to its narrower well width and smaller carrier effective masses.
AlGaInP light emitting diode (LED) with a mirror substrate has been successfully fabricated by wafer bonding. The bonding technique using a metallic interlayer has been developed to eliminate handling the fragile, free-standing epilayers. Various structures of the mirror substrate have been studied, and a suitable structure of Au/AuBe/SiO2/Si is proposed. From the observation of the chip fabrication process, it was found that the SiO2 layer could isolate the stress causing from the Si substrate. The device performance of bonded LED is obviously far superior to that of the standard absorb-substrate LED. It exhibits normal p-n diode behavior with a low series resistance. Moreover, the emission wavelength of the bonded LED was independent of the injection current. The low forward series resistance and a good heat sink provided by Si substrate solve the joule heating inhering in conventional LED problem. Furthermore, the bonded LED with high reliability has been demonstrated.
AlGaInP LEDs with emission wavelengths near 570 nm are important in liquid crystal display backlight application. However, high brightness in this spectral region is difficult to achieve due to the reduction of the radiation efficiency in the high-aluminum-containing active region and the smaller band offset between the active and the cladding region. In order to improve the performance of the 570-nm AlGaInP LEDs, we have grown several wafers with different structure designs and studied the optical properties as functions of the device temperature and the excitation power experimentally with a photoluminescence measurement system and numerically with a commercial Latsip simulation program. Specifically, important factors such as the barrier height in quantum wells, the tensile strain barrier cladding next to the MQW region, the compensated strain in MQW, and the disturbed Bragg reflector are investigated. Good agreement between the experimental and numerical results is observed.
A1GaInP visible laser diode is one of the most attractive light sources because it is of great importance in many applications such as optical information storage systems, laser printers, bar code readers and laser pointers. A1GaInP laser diode has a broad emission spectrum of 610 to 690 nm that makes it a versatile and outstanding light source. In addition, AlGaInP laser diodes with low threshold currents have also been realized. The laser diode used for our study is a 660-nm compressively strained A1GaInP with a structure of double-channel ridge waveguide (DCRW). Laser diodes with DCRW structure are widely used for commercialized low-cost and low-power laser diode applications owing to their relatively low threshold currents, easy fabrication and high yield as compared to laser diodes with selectively buried ridge waveguide structure. However, the top surfaces of DCRW laser diodes are non-flat and heat dissipation becomes a main problem for DCRW A1GaInP laser diodes. Especially, if comparing A1GaInP laser diodes with A1GaAs laser diodes, A1GaInP laser diodes have lower thermal conductivity and higher thermal resistance. Therefore, a good die bonding becomes important for improving the heat dissipation of DCRW AlGaInP laser diode chips. Most studies for die bonding have been focused on the choice of submounts or heat sinks with large heat conductivity. Few investigators study how to improve the quality of die bonding and avoid leading voids inside bonded interface. In this study, different p-metal materials, p-metal annealing environments, die-bonding steps and die-bonding equipment were adopted to change die-bonding conditions. Their influences on thermal dissipation capability were also investigated.
General requirements of AlGaInP/InGaP laser diode (LD) for digital versatile disk (DVD) optical pick-up head application, such as wavelength, output power, astigmatism, mode profile, and relative intensity noise will be discussed in this paper. Several efforts which have been made to develop AlGaInP/InGaP laser diodes suitable for DVD application will be reviewed. To record or erase signals in a DVD system, an output power of about 30 mW from AlGaInP/InGaP laser diode is required. Several methods which were proposed to increase catastrophic optical damage level will also be reviewed. Several methods which were proposed to increase catastrophic optical damage (COD) level will also be reviewed. A low-power 650-nm-band AlGaInP/InGaP laser diode utilizing double-channel ridge waveguide structure has been developed at OES/ITRI. Good characteristics of this laser diode such as very low operation current, small astigmatism, and stable fundamental transverse mode operation at a power level of more than 10 mW were obtained. A hybrid optical pick-up head utilizing this laser diode was incorporated in a commercial DVD-video player and functioned with a very good quality.
KEYWORDS: Near field scanning optical microscopy, Near field optics, Near field, Cladding, Semiconductor lasers, Quantum wells, Visible radiation, Optical spectroscopy, Optical fibers, Spatial resolution
Both collection and excitation modes of scanning near-field optical microscopy (SNOM) were used to study a low power visible multiquantum-well laser diode (LD). Collection mode SNOM provides the near-field optical propagating intensity distribution at the facet of LD. Excitation mode SNOM gives local photoconductivity information of the structure of LD facet. Results show highly localized spatial correlation of LD structure and its optical performance at the facet. Different sizes of apertures were used in both modes, and results of near-field interactions can be quite different. Results show obvious difference of photocurrent distribution caused by the different sizes of apertures in excitation mode. Two wavelengths of 543.5 nm and 632.8 nm were used in excitation mode SNOM. It can be deduced from the two pump photon energies that there exists defect level in the energy range of 60 - 380 meV below the conduction band edge in the n-(Al0.7Ga0.3)0.5In0.5P cladding layer. In addition to the highly localized images of topography, optical output, and optical beam induced current at the facet of LD, local near- field optical spectroscopy was performed as well. Spatially resolved near-field optical spectra of both stimulated and spontaneous emissions were obtained at the facet of LD. Longitudinal modes of stimulated emission of LD were observed locally.
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