Bin Cao

Conformal phosphor coating using capillary microchannel for controlling color deviation of phosphor-converted white light-emitting diodes

We demonstrated a conformal phosphor coating method for phosphor-converted white light-emitting diodes (LEDs) using capillary microchannel formed by a fixture. The mixture of the phosphor particles and the silicone spontaneously flows in capillary microchannel and was pinned at the edge of the fixture due to the surface tension effect. The present coating method was applied to different packaging types with both the conventional chip and the vertical injection chip. Experimental results show this method can efficiently improve the angular color uniformity (ACU). Compared with those samples packaged by conventional dispensing coating, angular color correlated temperature (CCT) deviation of the test samples by the present method can reduce from 1500K to 200K for the average CCT 5200K from -90° to +90 view angles, and reduce from 5000K to 1000K for the average CCT 9150K. Additionally, this method can prevent the packaging non-consistency that the average CCT varies from package to package due to the deviation of phosphor-silicone mixture volume. In the experiments, even when the phosphor-silicone mixture volume varies from 0.3 µl to 0.5 µl, the deviation of the average CCT is less than 80K.

Modeling the Light Extraction Efficiency of Bi-Layer Phosphors in White LEDs

In this letter, we modeled the bi-layer phosphors in light-emitting diode packages and optimized the concentration gradient to enhance the light extraction efficiency (LEE). The modeling processes were presented in detail. Based on our bi-layer model, we changed the concentration from 0.05 to 0.4 g/cm3 in both the upper and lower phosphor layers. It was found that when the concentration gradient between the upper layer and lower layer increases from 0.05:0.4 to 0.4:0.05, the LEE would be enhanced as much as 17.63%. The trends were validated by experiments. Some potential ways were proposed to realize this kind of phosphor configuration where the concentration of the upper layer was larger than that of the lower layer.

Effects of current crowding on light extraction efficiency of conventional GaN-based light-emitting diodes.

Current crowding effects (CCEs) on light extraction efficiency (LEE) of conventional GaN-based light-emitting diodes (LEDs) are analyzed through Monte Carlo ray-tracing simulation. The non-uniform radiative power distribution of the active layer of the Monte Carlo model is obtained based on the current spreading theory and rate equation. The simulation results illustrate that CCE around n-pad (n-CCE) has little effect on LEE, while CCE around p-pad (p-CCE) results in a notable LEE droop due to the significant absorption of photons emitted under p-pad. LEE droop is alleviated by a SiO₂ current blocking layer (CBL) and reflective p-pad. Compared to the conventional LEDs without CBL, the simulated LEE of LEDs with CBL at 20 A/cm² and 70 A/cm² is enhanced by 7.7% and 19.0%, respectively. It is further enhanced by 7.6% and 11.4% after employing a reflective p-pad due to decreased absorption. These enhancements are in accordance with the experimental results. Output power of LEDs with CBL is enhanced by 8.7% and 18.2% at 20 A/cm² and 70 A/cm², respectively. And the reflective p-pad results in a further enhancement of 8.9% and 12.7%.

Silicon-based system in packaging for light emitting diodes

Silicon-based packaging for light emitting diodes (LEDs) is the tendency of packaging for improving high-power LED performance. In this paper, a simple and high efficient wafer level packaging (WLP) based on silicon with TSV is proposed. Monte-Carlo ray-trace method is implemented to analyze the effect of the size of silicone lens on light extraction efficiency (LEE). The results show that LEE of hemisphere lens is higher than that of spherical segment lens. LEE increases with radius increasing and becomes stable when radius is above 1.2 mm due to the absorption of silicon for small part of light reflected back. Temperature distribution is uniform because of high thermal conductivity of copper and silicon; however, the mismatch of CTE between them causes high thermal stress which is alleviated in TSV when it partly filled with copper from analysis by three-dimensional finite element method. Six-axis micro-tester experiment shows the nominal shear strength of the silicone lens formed on a bare silicon with four TSVs improved 10% compared with that without TSVs. Wafer level packaging for LEDs is proposed. LEDs are encapsulated with silicon substrate with four TSVs filled with copper and other four TSVs filled with silicone. The processes of silicon substrate based on MEMS technology are used. TSV is filled with copper partly by two-step electroplating in order to avoid thinning of the substrate for blind vias. The phosphor conformal coating is implemented using capillary micro-channel, which can improve the angular color uniformity and consistency of WLP. The array of silicone lens using the mould method shows high coherence and few defects.

Analysis of light extraction efficiency of GaN-based light-emitting diodes

GaN-based light emitting diodes (LEDs) as one of the most important light source in next-generation solid-state lighting have been extensively studied and remarkable progress has been obtained. However, the light extraction efficiency (LEE) is not sufficient to satisfy application requirements. Most of the photons generated in multiple quantum well (MQW) always are trapped inside the semiconductor because both the reflection index of GaN and InGaN are higher than that of air, which results in total internal reflection (TIR). Great efforts were made in enhancing light extraction of LEDs experimentally in previous investigation. However, detailed theoretical studies in predicting the LEE of different types of LEDs are not available. In this paper the light extraction efficiency(LEE) of conventional chip (CC), flip chip (FC) and vertical chip (VC) is investigated using Monte Carlo ray tracing method is presented in conventional chip (CC). Monte Carlo ray tracing simulation based on statistics is known to be one of the most suitable methods to analyze the dependence of the light extraction efficiency of LEDs on the variety of the device parameters. Diffused bottom surface was found to be better for improvement of LEE than the perfect mirror surface. When there is a material with higher refraction index on the top surface the VC structure has the highest LEE no matter the bottom surface is perfect mirror or diffuse surface, which is above 80%. It is indicated that VC is potential in high-power GaN-based LEDs from the results.

3D silicon-based packaging for light emitting diodes

The three-dimensional (3D) light-emitting diodes (LEDs) integration package based on silicon with through silicon vias (TSV) attracts a lot of attention because of a better performance, lighter package, and higher integration density. Two main kinds of LED chip including conventional chip (CC) and vertical chip (VC) encapsulated in silicon platform with and without a reflector cup are analyzed through simulation to understand the optical and thermal characteristics of the module. The results show that the light extraction efficiency (LEE) is more dependent on the type of the chip instead of the structural parameters of the reflector cup. However, it has a significant effect on the far-field pattern of the package module. The thermal analyses show that the temperature distribution is uniform due to the high thermal conductivity and convection coefficient. Mismatch of thermal expansion coefficients (CTE) between TSV-full-filled copper of lOOum-thickness and silicon causes large thermal stress, the maximum von Mises stress reaching 272MPa, which would be decreased by 11.0% through half filling TSV with the copper of 20um-thickness.

A study of GaN-based LED structure etching using inductively coupled plasma

GaN as a wide band gap semiconductor has been employed to fabricate optoelectronic devices such as light-emitting diodes (LEDs) and laser diodes (LDs). Recently several different dry etching techniques for GaN-based materials have been developed. ICP etching is attractive because of its superior plasma uniformity and strong controllability. Most previous reports emphasized on the ICP etching characteristics of single GaN film. In this study dry etching of GaN-based LED structure was performed by inductively coupled plasmas (ICP) etching with Cl2 as the base gas and BCl3 as the additive gas. The effects of the key process parameters such as etching gases flow rate, ICP power, RF power and chamber pressure on the etching properties of GaN-based LED structure including etching rate, selectivity, etched surface morphology and sidewall was investigated. Etch depths were measured using a depth profilometer and used to calculate the etch rates. The etch profiles were observed with a scanning electron microscope (SEM).

Evaluation of GaN-based blue light emitting diodes based on temperature/humidity accelerated tests

The degradation of high power GaN-based blue light-emitting diodes (LEDs) was investigated by considering the electrical, optical and electroluminescence spectrum aging characteristics. The LED samples are stressed at the condition of 85°C and 85% RH using an injection current of 1000mA. Optical output power decreases to 80% of initial value after 1000 hours of temperature/humidity accelerated tests. Changes in the series resistance can be observed from the current-voltage characteristics during the stress tests. The increase of nonradiative paths and discoloration of encapsulation materials are possibly responsible for the degradation of optical output power. The mechanisms of increasing nonradiative recombination paths may be related to the generation of defects in the active region due to the high injection current and the increase of LED chip temperature. The influence of degradation on the electroluminescence spectrum has also been analyzed. The peak position of electroluminescence spectrum under the same current showed a redshift after the temperature/humidity accelerated tests, indicating a decreased band gap. The particular humidity effects on the degradation of blue LED are investigated, and the luminous flux of the packaged blue LEDs decreased with the increasing stress time due to the prolonged humidity test.

Realization of high-quality light output based on a novel LED packaging

A white LED modeling method is proposed to implement the colorimetric simulation and a novel LED packaging integrated with a specific freeform lens is designed to realize the high-quality light output. High angular color uniformity (ACU) is achieved by mixing the central bluish white light and the marginal yellowish white light via the specific freeform lens. Based on the white LED modeling method, the angular colorimetric characteristics of the LED packaging can be obtained and demonstrate that the AC U in terms of angular correlated color temperature (CCT) deviation is improved from 580K to 204K compared with the traditional LED packaging. Furthermore, experiment is conducted and results show a good accordance with the simulation. The novel LED package presents not only an improved ACU, but also a noninferior luminous flux compared with the hemispherical LED packaging.

Enhancing Light Output of GaN-Based LEDs With Graded-Thickness Quantum Wells and Barriers

GaN-based light-emitting diodes (LEDs) with graded-thickness quantum wells and barriers (GMQW-LEDs) are fabricated and researched in this letter. The light power and carrier distribution of GMQW-LEDs are compared with those of LEDs with original uniform MQW (OR-LEDs), graded-thickness quantum wells (GQW-LEDs), and graded-thickness quantum barriers (GQB-LEDs) through numerical simulation, respectively. The experimental results show that light power of GMQW-LEDs is enhanced significantly compared with that of OR-LEDs. The simulation results reveal that GMQW-LEDs show light output power enhancements of 25.7%, 14.3%, and 9.2% compared with OR-LEDs, GQW-LEDs, and GQB-LEDs at current density of 100 A/cm2, respectively. This is due to the superior hole distribution in quantum wells, which inhibits the electron leakage and enhances the radiative recombination.