Flip-chip Light-emitting Diodes Research Articles

Current crowding and self-heating effects in AlGaN-based flip-chip deep-ultraviolet light-emitting diodes

We thoroughly explored the physical origin of the efficiency decrease with increasing injection current and current crowding effect in 280 nm AlGaN-based flip-chip deep-ultraviolet (DUV) light-emitting diodes (LEDs). The current spreading length was experimentally determined to be much smaller in DUV LEDs than that in conventional InGaN-based visible LEDs. The severe self-heating caused by the low power conversion efficiency of DUV LEDs should be mainly responsible for the considerable decrease of efficiency when current crowding is present. The wall-plug efficiency of the DUV LEDs was markedly enhanced by using a well-designed p-electrode pattern to improve the current distribution.

Numerical Investigation of Purcell Enhancement of the Internal Quantum Efficiency of GaN-based Green LED Structures

GaN-based green light-emitting diode (LED) structures suffer from low internal quantum efficiency (IQE), known as the “green gap” problem. The IQE of LED structures is expected to be improved to some extent by exploiting the Purcell effect. In this study, the Purcell effect on the IQE of green LED structures is investigated numerically using a finite-difference time-domain simulation. The Purcell factor of flip-chip LED structures is found to be more than three times as high as that of epi-up LED structures, which is attributed to the high-reflectance mirror near the active region in the flip-chip LED structures. When the unmodified IQE is 20%, the relative enhancement of IQE can be greater than 50%, without utilizing the surface-plasmon coupling effect. Based on the simulation results, the “green gap” problem of GaN-based green LEDs is expected to be mitigated significantly by optimizing flip-chip LED structures to maximize the Purcell effect.

Numerical simulation and experimental investigation of GaN-based flip-chip LEDs and top-emitting LEDs.

We demonstrate a GaN-based flip-chip LED (FC-LED) with a highly reflective indium-tin oxide (ITO)/distributed Bragg reflector (DBR) ohmic contact. A transparent ITO current spreading layer combined with Ta2O5/SiO2 double DBR stacks is used as a reflective p-type ohmic contact in the FC-LED. We develop a strip-shaped SiO2 current blocking layer, which is well aligned with a p-electrode, to prevent the current from crowding around the p-electrode. Our combined numerical simulation and experimental results revealed that the FC-LED with ITO/DBR has advantages of better current spreading and superior heat dissipation performance compared to top-emitting LEDs (TE-LEDs). As a result, the light output power (LOP) of the FC-LED with ITO/DBR was 7.6% higher than that of the TE-LED at 150mA, and the light output saturation current was shifted from 130.9 A/cm2 for the TE-LED to 273.8 A/cm2 for the FC-LED with ITO/DBR. Owing to the high reflectance of the ITO/DBR ohmic contact, the LOP of the FC-LED with ITO/DBR was 13.0% higher than that of a conventional FC-LED with Ni/Ag at 150mA. However, because of the better heat dissipation of the Ni/Ag ohmic contact, the conventional FC-LED with Ni/Ag exhibited higher light output saturation current compared to the FC-LED with ITO/DBR.

Thermal transient measurement and modelling of a power cycled flip-chip LED module

Reliability testing has become a major focus for light emitting diode (LED) systems to predict its lifetime. One common type of testing methods, which was investigated in this study, is the supply switching test (SST), also called power cycle test. In this paper, reliability testing was used to analyze failure modes appearing within a flip chip LED module. The characterization of the LED module was done by thermal transient measurement and radioscopic imaging. It was shown that the thermal transient measurement was sensitive enough to observe structural changes induced by the SST. To supply the experiments, a finite volume simulation of the LED module was carried out to analyze which physical structures in the LED module caused the changes in the measurements. Furthermore, the simulation enabled the thermal heat path to be visualized within the module via isotherms. To understand the changes seen in the measurement, two different failure type scenarios were implemented in the simulation. The effects of solder joint degradation and delamination in the printed circuit board (PCB) were analyzed and depicted by their isotherms. Conclusions about the failure roots could be drawn by comparing thermal transient measurements of a stressed LED module with its corresponding simulation of an integrated failure.

Design of ITO/SiO2/TiO2 distributed Bragg reflectors as a p-type electrode in GaN-based flip-chip light emitting diodes

Two different SiO2/TiO2 distributed Bragg reflectors (DBR) with ITO ohmic contacts were investigated as p-type reflective electrodes in InGaN/GaN flip-chip light-emitting diodes (FC-LEDs). The DBR structures were designed to have reflectance over 95% at wavelengths of 400–520nm (DBR 1) and 400–720nm (DBR 2). These ranges are wider than those of the electroluminescence spectrum of blue FC-LEDs. The FC-LEDs with DBR 1 showed an output power of 114mW at 200mA, while those with DBR 2 presented higher output power of 135mW. A ray tracing simulation showed that the anomalously low output power of the FC-LEDs with DBR 1 can be attributed to the low incidence angle to the DBR due to the diffraction of the light at the patterned sapphire substrate. A finite-difference time-domain simulation and angle-dependent reflectance measurement of the DBRs were also carried out. The results showed that DBR 2 yielded high reflectance even at low incidence angles, while DBR 1 presented low reflectance at incidence angles lower than 30°, followed by a reduction of the output power.

Numerical and experimental investigation of GaN-based flip-chip light-emitting diodes with highly reflective Ag/TiW and ITO/DBR Ohmic contacts.

We demonstrate two types of GaN-based flip-chip light-emitting diodes (FCLEDs) with highly reflective Ag/TiW and indium-tin oxide (ITO)/distributed Bragg reflector (DBR) p-type Ohmic contacts. We show that a direct Ohmic contact to p-GaN layer using pure Ag is obtained when annealed at 600°C in N2 ambient. A TiW diffusion barrier layer covered onto Ag is used to suppress the agglomeration of Ag and thus maintain high reflectance of Ag during high temperature annealing process. We develop a strip-shaped SiO2 current blocking layer beneath the ITO/DBR to alleviate current crowding occurring in FCLED with ITO/DBR. Owing to negligibly small spreading resistance of Ag, however, our combined numerical and experimental results show that the FCLED with Ag/TiW has a more favorable current spreading uniformity in comparison to the FCLED with ITO/DBR. As a result, the light output power of FCLED with Ag/TiW is 7.5% higher than that of FCLED with ITO/DBR at 350 mA. The maximum output power of the FCLED with Ag/TiW obtained at 305.6 A/cm2 is 29.3% larger than that of the FCLED with ITO/DBR obtained at 278.9 A/cm2. The improvement appears to be due to the enhanced current spreading and higher optical reflectance provided by the Ag/TiW.

Low-Flicker Lighting From High-Voltage LEDs Driven by a Single Converter-Free Driver

This letter reports the design and characterization of high-power flip-chip high-voltage light-emitting diodes (HVLEDs), driven by a single converter-free LED driver for a compact and low-flicker lighting system. The HVLED chips show small forward voltage variation, uniform light emission, good optical output linearity, and stable thermal performance. All these characteristics well satisfy the demanding requirements of the novel quasi-constant power control scheme adopted in the converter-free LED driver. After bonding 12 HVLED chips (60 LED cells in total) and a single driver onto a silicon carrier through metal bonding, a low lighting flicker of 15.2%~17% was demonstrated, showing great potential for high-quality illumination applications that require low flicker, high light output, and good thermal stability.

Chip-scale white flip-chip light-emitting diode containing indium phosphide/zinc selenide quantum dots

A method for preparing a quantum dot (QD)-white light-emitting diode (WLED) is reported. Holes were etched in the SiO2 layer deposited on the sapphire substrate of the flip-chip LED by inductively coupled plasma, and these holes were then filled with QDs. An ultraviolet-curable resin was then spin-coated on top of the QD-containing SiO2 layer, and the resin was cured to act as a protecting layer. The reflective sidewall structure minimized sidelight leakage. The fabrication of the QD-WLED is simple in preparation and compatible with traditional LED processes, which was the minimum size of the WLED chip-scale integrated package. InP/ZnS core-shell QDs were used as the converter in the WLED. A blue light-emitting diode with a flip-chip structure was used as the excitation source. The QD-WLED exhibited color temperatures from 5900 to 6400 K and Commission Internationale De L'Elcairage color coordinates from (0.315, 0.325) to (0.325, 0.317), under drive currents from 100 to 400 mA. The QD-WLED exhibited stable optoelectronic properties.

Reliability and failure analysis of solder joints in flip chip LEDs via thermal impedance characterisation

Flip chip technology has made significant improvements to LED chip on board (COB) packages and modules by reducing thermal resistance compared with traditional wire bonded LEDs. However, one of the critical issues of flip chip packaged LEDs is the reliability of their solder interconnects, which are responsible for the heat transfer to the substrate. Based on a supply switching test, also called power cycle test, a model for monitoring and reliability was done to describe the remaining lifetime and additional the solder fatigue. Therefore, changes in thermal resistance of the LED-module were measured and correlated with solder fatigue of the LED flip chip package. Thermal impedance measurements were performed to monitor the status of the LED-module. Detailed analyses, comparing with the initial state, allow the evaluation of the occurring temperature swing of each LED chip. Single chip analysis combined with a thermal resistance driven monitoring model allows the calculation of remaining lifetime at each time during testing and beyond the first chip failure in a multi chip LED-module. Accordingly, it could be shown that the thermal impedance is a powerful tool for reliability monitoring and failure analysis.

Enhanced Luminescence of InGaN-Based 395 nm Flip-Chip Near-Ultraviolet Light-Emitting Diodes with Al as N-Electrode**Supported by the National Key Research and Development Program of China under Grant Nos 2016YFB0400901 and 2016YFB0400804, the Key Laboratory of Infrared Imaging Materials and Detectors of Shanghai Institute of Technical Physics of Chinese Academy of Sciences under Grant No IIMDKFJJ-15-07, the National Natural Science Foundation of

High-reflectivity Al-based n-electrode is used to enhance the luminescence properties of InGaN-based 395 nm flip-chip near-ultraviolet (UV) light-emitting diodes. The Al-only metal layer could form the Ohmic contact on the plasma etched n-GaN by means of chemical pre-treatment, with the lowest specific contact resistance of 2.211×10−5Ω⋅cm2. The Al n-electrodes enhance light output power of the 395 nm flip-chip near-UV light-emitting diodes by more than 33% compared with the Ti/Al n-electrodes. Meanwhile, the electrical characteristics of these chips with two types of n-electrodes do not show any significant discrepancy. The near-field light distribution measurement of packaged chips confirms that the enhanced luminescence is ascribed to the high reflectivity of the Al electrodes in the UV region. After the accelerated aging test for over 1000 h, the luminous degradation of the packaged chips with Al n-electrodes is less than 3%, which proves the reliability of these chips with the Al-based electrodes. Our approach shows a simplified design and fabrication of high-reflectivity n-electrode for flip-chip near-UV light emitting diodes.

Neighbour-die effect on the measurement of wafer-level flip-chip LED dies in production lines

The light from the side surfaces of the test flip-chip light-emitting diode (FCLED) dies is reflected, refracted or absorbed by neighbour dies during the measurement of wafer-level FCLED dies in production lines. A notable measurement deviation is caused by the neighbour-die effect, which is not considered in current industry practice. In this paper, Monte Carlo ray-tracing simulations are used to study the measurement deviations caused by the neighbour-die effect and extension ratios of the film. The simulation results show that the maximal deviation of radiant flux impinging the photodiode can reach 5.5%, if the die is tested without any neighbour dies, or is surrounded by a set of neighbour dies at an extension ratio of 1.1. Moreover, the dependence between the measurement results and neighbour cases for different extension ratios is also investigated. Then, a modified calibration method is proposed and studied. The proposed technique can be used to improve the calibration and measurement accuracy of the test equipment used for measurement of wafer-level FCLED dies in production lines.

Performance enhancement of InGaN LEDs with Al-graded GaN/AlGaN multiple electron-blocking layers

The characteristics of InGaN-based flip-chip light-emitting diodes (LEDs) with Al-graded-composition GaN/AlGaN multiple electron-blocking layers (MEBLs) are investigated. Results showed that MEBL LED samples with differently graded Al mole fractions possess a higher light output power and a lower efficiency droop than a conventional EBL LED while avoiding the operating voltage penalty. These improvements are mainly attributed to the improvements in electron confinement and hole injection efficiency caused by mitigating the polarization-induced band-bending effect at the interface between the last barrier of multiple quantum wells and the EBL.

Validation methodology to analyze the temperature-dependent heat path of a 4-chip LED module using a finite volume simulation

Thermal management in the solid-state lighting sector has become a main issue, due to reliability and efficiency issues. Herein, thermal structure function analysis provides a powerful tool to understand the heat transfer inside operated light emitting diode (LED) modules. In this paper a combined approach of simulation and experiment, as a heat path analysis of a LED module based on four flip chip LEDs, is presented. A validated simulation was used to visualize on the one hand the heat path as isothermals and on the other hand to show an alternative approach of the electrical transient correction. In addition to that, the structure function analysis also included the consideration of influence parameters in terms of different operating conditions (e.g. heat sink temperature, heating current, the use of different thermal interface materials between the device and the heatsink). This was investigated by the statistical Design of Experiments (DOE) approach. The DOE dissected the effect of each input variation to different features of the structure functions. An experimental setup showed, that the temperature of the heat sink caused the dominating effect on the thermal properties of the device. Finally numerical simulation confirmed that these effects came from the temperature dependencies of the thermal conductivities.

Improved Performance and Heat Dissipation of Flip-Chip White High-Voltage Light Emitting Diodes

Three types of white light emitting diodes (LEDs) were constructed with $4 \times 1$ micro-cells. The first one is the convectional LED series connected with four cells using the wire-bonding process. The other two devices are $4 \times 1$ lateral-type and $4 \times 1$ flip-chip high-voltage LEDs (HV-LEDs) using the interconnection technique. The convectional LED, lateral-type HV-LED, and flip-chip HV-LED are denoted as C-LED, L-HV-LED, and FC-HV-LED, respectively. Moreover, the white LEDs were formed by combining the blue LED chips and the phosphor-dispensing method. The thermal resistances (at 20 mA) of C-LED, L-HV-LED, and FC-HV-LED were 14.5, 59.2, and 12.2 K/W, respectively. In addition, the surface temperatures (at 20 mA) of these three devices were 30.74–31.82, 65.93–68.95, and 27.01–27.96 °C, respectively. Obviously, the heat dissipation of L-HV-LED was much worse than that of C-LED. However, via the fabrication of the flip-chip structure, the heat dissipation of HV-LED can be enhanced significantly. At an injection current of 100 mA, the luminous efficiencies of C-LED, L-HV-LED, and FC-HV-LED were 80.8, 81.0, and 91.8 lm/W, respectively. Furthermore, at the same forward voltage, a higher current can be driven in the FC-HV-LED, leading to an apparent improvement in the luminous efficiency. According to our calculation, the emission size of HV-LED was only 84% compared with that of C-LED. On the other hand, the fabrication of HV-LED can be performed without the wire-bonding process. This indicates that the FC-HV-LED possesses not only lower production costs but also higher optoelectronic performance than that of the C-LED.

Improvement in homogeneity and binning yield of color coordinate by a conformal phosphor sheet technique in white flip-chip LEDs

This paper introduces a conformal phosphor sheet technique for white light-emitting diodes based on Si wafer molding. The use of the Si wafer for molding allowed the fabrication of a flip-chip-level conformal phosphor sheet with a sidewall structure. This phosphor sheet was processed by placing the phosphor layer onto a chip with uniform thickness, which enabled uniform color conversion and an improved angular color temperature.

Improvement of thermal stability of Ag-based reflector in GaN-based flip chip LEDs by electron-beam irradiation

We proposed the effects of electron-beam irradiation (EBI) on the Ag reflector for GaN-based light-emitting diodes (LEDs). To evaluate the thermal stability of Ag, we carried out a thermal aging test. The Ag with EBI showed a high reflectance of 97.9% at a 450 nm wavelength and higher thermal stability than the Ag reflector without EBI. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and X-ray diffraction (XRD) were used to examine the surface and texture evolution of the Ag reflector before and after thermal aging at 300 °C for 120 min. SEM and EBSD showed that unlike the agglomeration occurred in the Ag reflector without EBI, the Ag reflector with EBI only showed an increase in grain size after thermal aging at 300 °C for 120 min. XRD data demonstrated the Ag reflector to have a higher -peak intensity due to EBI. On the other hand, after thermal aging at 300 °C for 120 min, the Ag reflector with EBI becomes less -peak than the Ag reflector without EBI. The difference in the surface and texture between the Ag reflectors with/without EBI is closely related to the changes in grain size and surface energy by thermal aging.

GaN-based flip-chip LEDs with highly reflective ITO/DBR p-type and via hole-based n-type contacts for enhanced current spreading and light extraction

We demonstrate GaN-based double-layer electrode flip-chip light-emitting diodes (DLE-FCLED) with highly reflective indium-tin oxide (ITO)/distributed bragg reflector (DBR) p-type contact and via hole-based n-type contacts. Transparent thin ITO in combination with TiO2/SiO2 DBR is used for reflective p-type ohmic contact, resulting in a significant reduction in absorption of light by opaque metal electrodes. The finely distributed via hole-based n-type contacts are formed on the n-GaN layer by etching via holes through p-GaN and multiple quantum well (MQW) active layer, leading to reduced lateral current spreading length, and hence alleviated current crowding effect. The forward voltage of the DLE-FCLED is 0.31V lower than that of the top-emitting LED at 90mA. The light output power of DLE-FCLED is 15.7% and 80.8% higher than that of top-emitting LED at 90mA and 300mA, respectively. Compared to top- emitting LED, the external quantum efficiency (EQE) of DLE-FCLED is enhanced by 15.4% and 132% at 90mA and 300mA, respectively. The maximum light output power of the DLE-FCLED obtained at 195.6A/cm2 is 1.33 times larger than that of the top-emitting LED obtained at 93A/cm2.

GaN-based flip-chip LEDs with highly reflective ITO/DBR p-type and via hole-based n-type contacts for enhanced current spreading and light extraction

GaN-based flip-chip LEDs with highly reflective ITO/DBR p-type and via hole-based n-type contacts for enhanced current spreading and light extraction

Optical, spectral, and thermal analyses of InGaN/GaN near‐ultraviolet flip‐chip light‐emitting diodes with different package structures

We analyzed and compared the optical, spectral, and thermal characteristics of InGaN/GaN near‐ultraviolet (NUV) flip‐chip (FC) light‐emitting diodes (LEDs) packaged on the board with dielectric/metal layers (DMB package), board with ceramic/metal layers (CMB package), and board with ceramic/metal layers and thermal tape (CMBT package) in the injection current range of 0–800 mA at 298 and 358 K. For the DMB packaged LED, the relatively high junction temperature was observed due to the inefficient heat dissipation, leading to the more optical output power drop and spectral shift. For the CMBT packaged device, it further dissipated heat at high currents where the performance of CMB packaged device was degraded. In comparison with the experimental results, the theoretically calculated temperature and stress distributions of the NUV FC LEDs with the DMB and CMB packages were also investigated by finite element method‐based simulations.

Through-Silicon via Submount for Flip-Chip LEDs

A blue light emitting diode (LED) was prepared by a flip-chip (FC) LED and three-dimensional through-silicon via (3D-TSV) technique. The experimental results indicated that the diameter and length of the Si via were about 180 μm and 400 μm, respectively. The Cu was uniformly and high density filled in each TSV, and the average resistance was about 0.14 mΩ. It was also found that the 96.43Sn-3.57at%Ag bumps were electroplated on the Cu plugged TSVs of a silicon substrate, and these were smoother at 250°C. After reflow, a 3D blue light emitting diode was prepared by peak bonding at 250°C and 1000 N pressure for 30 min. Compared with the output of the LED 417.17 mw/w, that of the 3D LED was 424.67 mw/w.