Diode Chips Research Articles

Fanless, porous graphene-copper composite heat sink for micro devices

Thermal management in devices directly affects their performance, but it is difficult to apply conventional cooling methods such as the use of cooling liquids or fans to micro devices owing to the small size of micro devices. In this study, we attempted to solve this problem by employing a heat sink fabricated using copper with porous structures consisting of single-layer graphene on the surface and graphene oxide inside the pores. The porous copper/single-layer graphene/graphene oxide composite (p-Cu/G/rGO) had a porosity of approximately 35%, and the measured pore size was approximately 10 to 100 µm. The internal GO was reduced at a temperature of 1000 °C. On observing the heat distribution in the structure using a thermal imaging camera, we could observe that the p-Cu/G/rGO was conducting heat faster than the p-Cu, which was consistent with the simulation. Furthermore, the thermal resistance of p-Cu/G/rGO was lower than those of the p-Cu and pure Cu. When the p-Cu/G/rGO was fabricated into a heat sink to mount the light emitting diode (LED) chip, the measured temperature of the LED was 31.04 °C, which was less than the temperature of the pure Cu of 40.8 °C. After a week of being subjected to high power (1000 mA), the light intensity of p-Cu/G/rGO decreased to 95.24%. However, the pure Cu decreased significantly to 66.04%. The results of this study are expected to be applied to micro devices for their effective thermal management.

MEMS-VCSEL as a tunable light source for OCT imaging of long working distance

We present a wavelength tunable vertical-cavity surface-emitting laser (VCSEL) constructed by die-bonding a half-cavity InGaAs laser diode (LD) chip onto a silicon-on-insulator chip with a microelectromechanical system electrostatic diaphragm mirror that functions as a Fabry–Perot interferometer. As a result of the short cavity length, the integrated tunable LD has single-mode lasing characteristics with an extremely large coherence length of 150 m or more. The developed wavelength tunable LD is used to perform swept-source optical coherence tomography with a large scan depth, which is applicable to ophthalmic observation for the diagnosis of pathologic nearsightedness based on the measurement of the axial length of an eye.

Development of an adaptive template for fast detection of lithographic patterns of light-emitting diode chips

With the expansion in light-emitting diode (LED) lighting market and technology, control of product quality has become the focus of LED development. To achieve high online production capacity, automated quality detection with object image has been employed for comparison using mostly the standard templates. However, the resulting poor fitting causes misjudgment of the detection system. This study proposes an adaptive template method to improve the system fitting, reduce the system misjudgment, and enhance the detection efficiency. The severely damaged LED chips were screened out based on their grayscale entropy indices and related coefficient indices, which enhanced the reliability of the adaptive template system and accelerated the overall system detection process. To overcome the displacement and scale changes of the lithographic patterns, the scale-invariant feature transform (SIFT) and Harris–Laplace methods were used for comparison. The scale-invariant feature transform (SIFT) and Harris–Laplace methods were used to search and compare the feature points of the chip patterns, with the aim to overcome the displacement and scale changes of the lithographic patterns and establish the adaptive template in real conditions. The fast correlation coefficient comparison method was compared with the adaptive template comparison method proposed in this study. The results showed that the detection accuracy of the new method was 98.36%, which is 15.79% more accurate than the fast correlation coefficient comparison method. In terms of time performance, the method proposed in this study took 0.08 s less to complete the partition defect template. Moreover, the average detection time per chip was reduced by another 1.4 s, which improved the efficiency by 30.43%. The adaptive template proposed in this study exclusively establish itself based on different lithographic patterns, thus improving the detection efficiency and accuracy of the LED industry, and raising its market competitiveness in the industry.

Flashlight-Induced Strong Self-Adhesive Surface on a Nanowire-Impregnated Transparent Conductive Film.

The flashlight annealing process has been widely used in the field of flexible and printed electronics because it can instantly induce chemical and structural modifications over a large area on an electronic functional layer in a subsecond time range. In this study, for the first time, we explored a straightforward method to develop strong self-adhesion on a metal nanowire-based flexible and transparent conductive film via flashlight irradiation. Nanowire interlocking, for strong mechanical bonding at the interface between the nanowires and polyamide film, was achieved by simple hot pressing. Then, by irradiating the nanowire-impregnated film with a flashlight, several events such as interdiffusion and melting of surface polymers could be induced along with morphological changes leading to an increase in the film surface area. As a result, the surface of the fabricated film exhibited strong interfacial interactions while forming intimate contact with the heterogeneous surfaces of other objects, thereby becoming strongly self-adhesive. This readily achievable, self-attachable, flexible, and transparent electrode allowed the self-interconnection of a light-emitting diode chip, and it was also compatible for various applications, such as defogging windows and transparent organic light-emitting diodes.

Micropatterns Fabricated by Photodimerization-Induced Diffusion.

Pattern technology plays an important role in the generation of microstructures with different functionalities and morphologies. In this report, a straightforward and versatile strategy is presented for spatially regulating the growth of a microstructure on a surface by the photodimerization of maleimide (MI). Upon exposure of ultraviolet (UV) light, photodimerization of MI in a film comprising furan-grafted polymer and bismaleimide (BMI) produces a chemical gradient, which can drive the diffusion of BMI from the unexposed to the exposed region and from the bottom to the surface, resulting in the growth of micropatterns. Sequential crosslinking induced by the Diels-Alder reaction between MI and furan maintains the stability of pattern shape. Theoretical modeling with reaction-diffusion equations reveal that as photodimerization moves the system far from thermodynamic equilibrium, the formation of a chemical potential gradient requires the redistribution of matter, resulting in the formation of topographies. Directional molecular motion induced by UV light can generate complex morphology, and produce materials with unique optical functions, such as charming-ordered gratings. This straightforward method of fabricating micropatterns by photodimerization-induced diffusion is successfully applied to patterned curved surfaces, microfluidic channels and encapsulation of integrated light emitting diode chips.

MMCT-induced high-bright yellow light-emitting phosphor Bi3+-activated Ba2YGaO5 used for WLED

Based on the wide application of Bi3+-activated phosphors in Near-ultraviolet light-emitting diode (NUV-LED) chips for inducing healthy lighting, it is necessary to reveal the characteristics of high-quality Bi3+-activated phosphors. In this study, a high bright yellow light is realized with Bi3+-activated Ba2YGaO5, showing an Internal quantum efficiency (IQE) of up to 92%. Under 350 nm excitation, Ba2YGaO5: Bi3+ emits a broad yellow light peak at 587 nm with full width at half maximum (FWHM) of 135 nm, while simultaneously exhibiting good thermal stability; these results are superior to those of the commercial yellow phosphor Y3Al5O12: Ce3+. Moreover, significant cathodoluminescence properties are observed with Ba2YGaO5: Bi3+. These excellent luminescence properties are demonstrated to result from the D-level state described as the Metal-to-metal charge transfer (MMCT) state. Combined with the crystal and electronic structure of Ba2YGaO5, the origin of the D-level state along with its luminous mechanism is investigated in detail in this work, which may pave the way for developing more high-quality Bi3+-activated phosphors for use in NUV-LED chip-induced healthy lighting.

Investigation on the leakage current characteristics of large size GaN diodes

The leakage mechanism of large size GaN p–i–n diodes is studied, and the possible reasons are analyzed. It is found that (i) the leakage current of most diode chips is not proportional to their area, and the leakage problem deteriorates seriously when the size of the device increases; (ii) the open circuit voltage of the photovoltaic device decreases with the increasing leakage current of GaN diodes; and (iii) low density nanotubes are detected in larger size GaN diodes, which is one of the important reasons for the large leakage of large size GaN diodes.

Highly Thermal Conductive Poly(vinyl alcohol) Composites with Oriented Hybrid Networks: Silver Nanowire Bridged Boron Nitride Nanoplatelets.

With the increasing demand for thermal management materials in the highly integrated electronics area, building efficient heat-transfer networks to obtain advanced thermally conductive composites is of great significance. In the present work, highly thermally conductive poly(vinyl alcohol) (PVA)/boron nitride nanoplatelets@silver nanowires (BNNS@AgNW) composites were fabricated via the combination of the electrospinning and the spraying technique, followed by a hot-pressing method. BNNS are oriented along the in-plane direction, while AgNWs with a high aspect ratio can help to construct a thermal conductive network effectively by bridging BNNS in the composites. The PVA/BNNS@AgNW composites showed high in-plane thermal conductivity (TC) of 10.9 W/(m·K) at 33 wt % total fillers addition. Meanwhile, the composite shows excellent thermal dispassion capability when it is taken as a thermal interface material of a working light-emitting diode (LED) chip, which is certified by capturing the surface temperature of the LED chip. In addition, the out-of-plane electrical conductivity of the composites is below 10-12 S/cm. The composites with outstanding thermal conductive and electrical insulating properties hold promise for application in electrical packaging and thermal management.

Soft and Self‐Adhesive Thermal Interface Materials Based on Vertically Aligned, Covalently Bonded Graphene Nanowalls for Efficient Microelectronic Cooling

AbstractUrged by the increasing power and packing densities of integrated circuits and electronic devices, efficient dissipation of excess heat from hot spot to heat sink through thermal interface materials (TIMs) is a growing demand to maintain system reliability and performance. In recent years, graphene‐based TIMs received considerable interest due to the ultrahigh intrinsic thermal conductivity of graphene. However, the cooling efficiency of such TIMs is still limited by some technical difficulties, such as production‐induced defects of graphene, poor alignment of graphene in the matrix, and strong phonon scattering at graphene/graphene or graphene/matrix interfaces. In this study, a 120 µm‐thick freestanding film composed of vertically aligned, covalently bonded graphene nanowalls (GNWs) is grown by mesoplasma chemical vapor deposition. After filling GNWs with silicone, the fabricated adhesive TIMs exhibit a high through‐plane thermal conductivity of 20.4 W m−1 K−1 at a low graphene loading of 5.6 wt%. In the TIM performance test, the cooling efficiency of GNW‐based TIMs is ≈1.5 times higher than that of state‐of‐the‐art commercial TIMs. The TIMs achieve the desired balance between high through‐plane thermal conductivity and small bond line thickness, providing superior cooling performance for suppressing the degradation of luminous properties of high‐power light‐emitting diode chips.

Cyan Broad-Band Emission Phosphor with Scandium Silicon Multiple-Ring Structure for White Light-Emitting Diodes and Field Emission Displays.

Silicate phosphor KBaScSi2O7:Ce3+ (KBSS:Ce3+), as a novel material, has been prepared by the solid-state method in this study. The crystal structure, luminescent properties, thermal stability, and cathodoluminescence (CL) properties of this material were analyzed systematically. It concludes that the phosphor can emit cyan light with emission peak at 509 nm under n-UV light excitation (300-400 nm). By coating KBSS:Ce3+ with a red-emitting CaAlSiN3:Eu2+ on a n-UV (365 nm) light-emitting diode (LED) chip, the intense warm white light with high RA (83.4) and low CCT (3652) can be produced under a 350 mA forward bias current.In addition, the CL performance shows that KBSS:0.10Ce3+ has high saturation current and voltage and good color stability under low voltage conditions. All these results indicate that KBSS:Ce3+ phosphor will be very promising in LED and field emission display applications.

Waste heat recycling of high-power lighting through chips on thermoelectric generator

A waste heat is inevitably generated in high-power lighting due to the limited efficiency of light-emitting diode (LED) chips and the non-radiative transition of phosphors. In this work, we proposed an integrated module of LED chips on thermoelectric generator (TEG) for the waste heat recycling of high-power lighting. The multi-chips were directly bonded on the hot side of TEG for heat conduction. The chip powers and temperature distribution of chips-on-TEG were investigated under different chip currents, and their output voltage, current, and power were measured. When the chip current increases from 0.4 to 1.4 A, the heat power increases from 1.7 to 9.2 W, resulting in a high temperature gradient of 219 °C for the TE elements at the chip current of 1.4 A. This enough heat energy converts to an electric power with an output voltage of 1.23 V. In addition, a low-power indicating lamp was driven by the electricity from three chips-on-TEGs without additional power consumption. Our work opens a good idea to recover the normally wasted heat from high-power lighting, which could be extended for the heat energy harvesting in many power devices.

Incorporation of zinc ions towards low toxicity and high stability of organic-inorganic methyl ammonium lead bromide perovskite QDs via ultrasonication route for white-LEDs

B-site substitution perovskite quantum dots (PQDs) with amended stability and less toxicity have been acquainted in recent years. However, the PQDs comprising various B-site cations display low photoluminescence quantum yields (PLQYs), instability, and require skimpy synthesis procedure. Herein, we have endeavored to introduce ultrasonication synthesis of MAPb1-xZnxBr3 PQDs as an auspicious route to stable PQDs with enhanced PLQY. The successful synthesis of PQDs resulted in a high PLQY (>93%), which is supposed to be owing to the decrease in non-radiative recombination centers. Furthermore, we have found that the structure, morphology, and emission properties are secured with the substitution of Zn2+. The PQDs have been integrated as color converting layers coated on blue light emitting diode (LED) chips, which exhibit white light with the wide color gamut of National Television Standards Committee (NTSC) 124%. The present research reveals a new synthesis route to obtain stable and efficient PQDs without negotiating color quality for display application.

Multifunctional Pr3+ single doped CaLaMgTaO6: Crystal structure, thermal behavior and applications

This paper, for the first time, presents a comprehensive study of the structure, thermal stability and photoluminescence properties of CaLaMgTaO6:Pr3+ phosphors for LED applications. Pr3+ ions exhibits two clusters of dominant emissions peaked at 495 nm (3P0→3H4) and 657 nm (3P0→3F2) from double perovskite-structure CaLaMgTaO6 compounds. Meanwhile, the samples cover the emission wavelength of commercial blue light-emitting diode (LED) chips with high internal quantum efficiency (IQE) of 30.25% upon 450 nm excitation. The mechanism of thermal quenching for this phosphor was investigated and the corresponding activation energy ΔE was determined to be 0.23 eV. The fabricated Pr3+ single doped CaLaMgTaO6 together with a 450 nm blue LED chip is well-matched with the absorption band of photosynthesis, phototropism, and photomorphogenesis. Furthermore, this phosphor along with commercially available phosphors was applied to fabrication of 450 nm UV chip and YAG:Ce3+ emitted a well-distributed warm white light with high color rendering index (CRI) of 84.1 and a correlated color temperature (CCT) of 3914 K. All of the results indicate that this novel phosphor can compensate for the red light and has potential applications in plant cultivation and warm white LEDs field.

Magnetic field enhanced ionic wind for environment-friendly improvement and thermal management application

High heat flux electronics have encountered difficulties in thermal management. There are some shortcomings in traditional cooling methods. Ionic wind cooling technology based on electrohydrodynamic (EHD) has unique advantages. There are by-product hazards and speed raising limits in the reported ionic wind generators. An electromagnetic field and nanomaterials improved ionic wind cooling system is designed for environment-friendly improvement and thermal management application. The proposed cooling system is optimized experimentally to increase the ionic wind intensity and reduce ozone generation. The results indicate that adding a magnetic field changes the movement trajectory of charged particles and thus regulates the distribution of the flow field. In the macroscopic sense, the ionic wind intensity is also improved, and the greater the magnetic flux density is, the more obvious the effect is. A maximal improvement of 58.6% was obtained. The ionic wind velocity increases by 0.71 m/s due to the synergistic effect of a magnetic field. Meanwhile, the O3 concentration decreased by more than 95% using both the magnetic field and the carbon nanotubes. When the cooling system is applied to a high-power light-emitting diode (LED) chip for thermal management, the cooling effect is remarkable. The results provide an important theoretical basis for the application of EHD technology in the thermal management of electronic devices.

Evaluating the moisture resistance of Y3Al5O12: Ce3+ phosphor used in high power white LED packaging

Owing to its low cost and high efficiency, the blue light-emitting diode (LED) chip covered with the Y3Al5O12:Ce3+ (YAG) phosphor has become a mainstream technology of high power white LED packaging. However, phosphors are often susceptible to degradation under high-temperature and high-humidity conditions, which directly affects the optical and color stability of the white LED package. In this study, the moisture resistance of YAG phosphor was evaluated through both a short-term water-immersion test and long-term aging tests, i.e. 85 °C&85% Relative Humidity (RH) test and high-temperature and saturated humidity test. The phosphor hydrolysis mechanism was then revealed by characterizing the phosphor emission spectrum, photothermal property, crystal structure, micromorphology, and chemical element composition.

LED chip cooling system using ionic wind induced by multi-wire corona discharge

• A T-shaped multi-wire ionic wind pump is developed for cooling a high-power LED chip. • Relationship between air flow in the pump and its electrode configuration is revealed. • Increasing the wire-plate gap in the outlet channel strengthens the reverse discharge effect. • Arranging electrodes in the outlet channel further improves the performance of the T-shaped EHD pump. The high temperature of a light-emitting diode (LED) chip reduces its luminous output and lifetime and can even cause catastrophic damage to the chip. In recent years, ionic wind generators have shown great potential for LED chip cooling owing to their noiselessness, low energy consumption, and compact structure. However, the heat transfer and flow characteristics of ionic wind generated by multi-wire electrodes for LED cooling have not been sufficiently investigated. In this study, a T-shaped multi-wire ionic wind pump, in which each wire electrode corresponds to a pair of parallel plates, was designed for heat dissipation of a 24 W high-power LED chip. The relationship between the air flow characteristics inside the ionic wind pump and the electrode configuration was revealed through numerical simulations. Then, an optimized ionic wind pump was fabricated to test the cooling performance of the LED chip. The results show that each pair of plate electrodes has a critical length, and the average velocity at the pump inlet becomes independent of the plate length when it exceeds this critical value. For fixed wire electrodes in the outlet channel, when their operating voltage remains constant, increasing the wire-plate distance reduces the charge density around these wires, thus strengthening the reverse discharge effect and reducing the average velocity at the pump inlet. When only the wire in the inlet channel discharges electricity at a fixed voltage of 18 kV, the ionic wind pump can stabilize the LED case temperature below its safe temperature (LED operating voltage: 36 V). If all wires discharge electricity at a fixed voltage of 18 kV, the LED case temperature can be further reduced to 74 °C, which is lower than the maximum rated case temperature of 85 °C. In conclusion, when designing multi-wire ionic wind pumps, the electrodes inside them should be elaborately configured to reduce mutual interference and maximize the ionic wind flow rate. The designed ionic wind pump can serve as a noiseless thermal management equipment for high-power LED illumination devices.

Doping effect numerical comparison of band gap energy and active region range for GaN and GaAs based semiconductor

This work reports the effect of doping concentration on the energy-band structure of semiconductor materials. The research focuses on the resultant values of bandgap energy and its depletion region (length/area), based on the initial concentrations of doping which are the donors and acceptors. The energy-band diagram is simulated by initializing the various materials’ properties of Gallium Nitride (GaN) and Gallium Arsenide (GaAs), and solving the Poisson’s equation derived from Maxwell’s equation. The equation is solved by applying the finite difference method (FDM) and using the Newton-Raphson method. Both of these materials are compared with different doping concentrations (1×1013cm−3-1×1017cm−3). Taking the GaAs properties as the controlled variable, the band structure is validated with literature findings. The measured band gap energy of GaN changes from 1.5215eV to 7.6689eV, and GaAs, from 1.1330eV to 5.6431eV. It increases with the proportion to the doping concentration increments. However, when obtaining both of the spatial active regions for GaN and GaAs, it reduce from (1.9990μm-0.0790μm)x1μm2 and from (1.9990μm-0.0890μm)x1μm2 respectively. The findings show the effect of doping concentration on the semiconductor energy-band structure. Thus, the numerical system is expected to be used as the determination of the internal quantum efficiency, and the output spectrum of light-emitting diode (LED) chip.

Nanoimprinting metal-containing nanoparticle-doped gratings to enhance the polarization of light-emitting chips by induced scattering

Polarized radiative luminous semiconductor chips have huge application potential in many highly value-added fields. The integration of a subwavelength grating is recognized to be the most promising method for the development of polarized chips, but still faces the challenge of low polarized radiative performance. This paper describes a proposal for, and the development of, a scattering-induced enhanced-polarization light-emitting diode chip by directly nanoimprinting a metal-containing nanoparticle-doped grating onto the top surface of a common flip chip. The rate at which quantum-well light emission is used by the developed polarized chip is improved by more than 30%. More attractively, the doped scattering nanoparticles function as a scattering-induced polarization state converter that is sandwiched in between the top aluminum grating and the bottom silver reflector of the chips. The originally non-radiated light, with an electric-field vector parallel to the grating lines, is reflected back and forth inside the sandwich until it changes to the perpendicular vibration mode and is radiated outside the chip. Therefore, the polarization extinction ratio is greatly improved, compared to undoped samples.

Highly Efficient Liquid-Quantum Dot/Melamine- Modified Urea-Formaldehyde Microcapsules for White Light-Emitting Diodes

A liquid matrix is beneficial for improving the luminescence performance of quantum dots (QDs), although it is difficult to be packaged with light-emitting diode (LED) chips. In this study, liquid-QDs with chlorobenzene solution and melamine-modified urea-formaldehyde (MUF) were used as the core and shell structures, respectively, successfully yielding liquid-QD/MUF microcapsules for white LED packaging. The concentration of the liquid-QDs in the microcapsules was optimized, demonstrating an increase of 15.9% in the luminous efficiency, as compared to the traditional solid-QDs at a specific QD energy proportion. The efficiency roll-off can be attributed to the increased reabsorption and back-scattering loss at higher liquid-QDs and microcapsules concentrations. It was also confirmed that the liquid-QD/MUF microcapsules have an excellent thermal stability below 200 °C, owing to the protection of the MUF shells. Finally, white LEDs compounding the red liquid-QD/MUF microcapsules and YAG phosphor particles were fabricated, achieving a high luminous efficiency of 149.2 lm/W at 20 mA, correlated color temperature of 3301 K, and color rendering index of 88.8. This study will open new research avenues for liquid-QD packaging in white LEDs.

Hypergravity-Induced Accumulation: A New, Efficient, and Simple Strategy to Improve the Thermal Conductivity of Boron Nitride Filled Polymer Composites.

Thermal conductive polymer composites (filled type) consisting of thermal conductive fillers and a polymer matrix have been widely used in a range of areas. More than 10 strategies have been developed to improve the thermal conductivity of polymer composites. Here we report a new “hypergravity accumulation” strategy. Raw material mixtures of boron nitride/silicone rubber composites were treated in hypergravity fields (800–20,000 g, relative gravity acceleration) before heat-curing. A series of comparison studies were made. It was found that hypergravity treatments could efficiently improve the microstructures and thermal conductivity of the composites. When the hypergravity was about 20,000 g (relative gravity acceleration), the obtained spherical boron nitride/silicone rubber composites had highly compacted microstructures and high and isotropic thermal conductivity. The highest thermal conductivity reached 4.0 W/mK. Thermal interface application study showed that the composites could help to decrease the temperature on a light-emitting diode (LED) chip by 5 °C. The mechanism of the improved microstructure increased thermal conductivity, and the high viscosity problem in the preparation of boron nitride/silicone rubber composites, and the advantages and disadvantages of the hypergravity accumulation strategy, were discussed. Overall, this work has provided a new, efficient, and simple strategy to improve the thermal conductivity of boron nitride/silicone rubber and other polymer composites (filled type).