GaN-based Flip-chip Light-emitting Diodes Research Articles

Effect of distributed Bragg reflectors on optoelectronic characteristics of GaN-based flip-chip light-emitting diodes

Distributed Bragg reflectors have been widely utilized in GaN-based flip-chip light-emitting diodes (FCLEDs) owing to their excellent reflection performance. Recently, wide reflected angle DBR (WRA-DBR) has been suggested to enhance the optical characteristics of GaN-based FCLEDs by incorporating multiple sub-DBRs with varying central wavelengths. However, the reflectivity of WRA-DBR decreases at large incident angle from 425 nm to 550 nm, which restricts further optical performance improvement of FCLEDs. Here, we demonstrate a quintuple-stack DBR comprised of five sub-DBRs. The quintuple-stack DBR possesses a high reflectivity (>97.5%) for incident angles below 50° within the blue and green light wavelength ranges. Compared to WRA-DBR, quintuple-stack DBR exhibits a higher reflectivity in wavelength range of 425 nm to 550 nm and thinner multilayer thicknesses. Furthermore, stronger electric field intensities exist in the top facet and sidewalls of FCLED with quintuple-stack DBR, revealing that quintuple-stack DBR is beneficial for enhancing the light extraction efficiency. As a result, the light output power of FCLED with quintuple-stack DBR is ∼3% higher than that of FCLED with WRA-DBR at 750 mA.

Strategically constructed high-reflectivity multiple-stack distributed Bragg reflectors for efficient GaN-based flip-chip mini-LEDs

Full-angle distributed Bragg reflectors (DBRs) consisting of numerous sub-DBRs with discrete central wavelengths have been developed to enhance performance of GaN-based flip-chip mini light-emitting diodes (FC mini-LEDs). However, relatively low reflectivity of full-angle DBRs at large angle incidence restricts further enhancement in performance of FC mini-LEDs. Here, we introduce a reflectivity optimization strategy for constructing high-reflectivity multiple-stack DBRs by rationally engineering the number of sub-DBRs and adjusting central wavelength distribution of sub-DBRs. Based on the reflectivity optimization strategy, we devise a Ti3O5/SiO2 quintuple-stack DBR which is composed of five sub-DBRs. Our quintuple-stack DBR maintains a high reflectivity (>97.5%) over a wide range of incident angles of light. Notably, compared with the full-angle DBR, our quintuple-stack DBR exhibits higher reflectivity at large angle incidence and thinner multilayer thickness. Furthermore, we demonstrate two types of GaN-based blue FC mini-LEDs with indium-tin oxide (ITO)/quintuple-stack DBR and ITO/full-angle DBR p-type ohmic contacts. Benefiting from superior reflection performance, blue FC mini-LED with ITO/quintuple-stack DBR achieves an enhancement of ∼5.8% in light output power at 10 mA, in comparison with blue FC mini-LED with ITO/full-angle DBR. Our work signifies an advancement towards high-reflectivity DBRs, which enables higher-performance FC mini-LEDs.

Ni-Cu Nanoparticles-Embedded Ag-Based Reflector for High-Efficiency Light-Emitting Diodes.

We investigated the use of a silver reflector embedded with Ni-Cu nanoparticles to achieve low resistance and high reflectivity in GaN-based flip-chip light-emitting diodes. Compared to a single layer of Ag, the NC-NPs/Ag reflector exhibits a higher light reflectance of ~90% at a wavelength of 450 nm, a lower contact resistance of 4.75 × 10-5 II cm², and improved thermal stability after annealing at 400°C. The NC-NPs formed after the annealing process prevents agglomeration of the Ag layer, while also reducing the Schottky barrier height between the p-GaN layer and metal reflector. The LED fabricated with a NC-NPs/Ag reflector exhibited a forward-bias voltage of 3.13 V and an improvement in light output power of 36.6% (at 20 mA), when compared with the LED composed of a Ag SL. This result indicates that the NC-NPs/Ag reflector is a promising p-type reflector for high-intensity light-emitting diodes.

Origins of Inhomogeneous Light Emission From GaN-Based Flip-Chip Green Micro-LEDs

Spatially resolved electroluminescence emitted through the sapphire substrate of GaN-based flip-chip green microscale light-emitting diodes with $20\times 20\,\,\mu \text{m}^{\text {2}}$ mesas has been collected via microscopic hyperspectral imaging. The current crowding is identified in the central region of the mesa, corresponding to the position of the p-pad. The bright-spot emissions at the edges of the mesa originate from localized potential energy valleys. This letter suggests that the comprehensive information about current crowding and the distribution of the light emission across the mesa of microscale light-emitting diode can be obtained by capturing spatially resolved light emission from the side of the transparent substrate.

High-stability reflective bonding pads for GaN-based flip-chip light-emitting diodes packaged by reflow soldering

We provide a detailed insight into the effects of metal bonding pads on the performance of GaN-based flip-chip blue light-emitting diodes. The flip-chip blue light-emitting diodes with four kinds of reflective bonding pads were fabricated and compared. During the chip package process by reflow soldering, different bonding pads exhibited distinct voidage and bonding force with the solder layers on Cu-coated AlN ceramic substrates. Compared with other samples in this work, the packaged chips with Ti/Al/Ti/Pt/Au bonding pads showed the lowest voidage of 15.48% and thermal resistance, resulting in the highest bonding shear force and light output power. Meanwhile, the optical power of these samples with Ti/Al/Ti/Pt/Au bonding pads only exhibited a degradation rate of 1.8% after accelerated ageing at 100 °C for more than 1000 h. It is concluded that GaN-based flip-chip light-emitting diodes with such Ti/Al/Ti/Pt/Au bonding pads bear good mechanical stability, excellent thermal performance and long-term reliability.

Performance enhancement of GaN-based near-ultraviolet flip-chip light-emitting diodes with two-step insulating layer scheme on patterned sapphire substrate

Nitride-based near-ultraviolet (NUV) flip-chip (FC) light-emitting diodes (LEDs) on patterned sapphire substrate (PSS) with n-contact full-via-holes (FVH) structure have been fabricated by a two-step insulating layer (IL) method. Comparing with the conventional one-step IL method, the NUV FCLEDs manufactured by two-step IL method exhibit a reduction of 0.19 V in forward voltage and an enhancement of 36.2% in the wall-plug efficiency (WPE). In addition, the WPE of two-step IL NUV FCLEDs was enhanced by 108% as compared to that of the one-step IL FCLEDs on flat sapphire substrate. The current density distribution simulation shows a low current density and uniform distribution for the FCLEDs with FVH structure. The enhanced performance could be attributed to the PSS extraction structure and the more uniform current distribution at the active region due to the uniformly distributed via-holes. The results indicate that the proposed two-step IL method is an effective way to achieve high-power and high-efficiency FCLEDs.

Effect of Dielectric Distributed Bragg Reflector on Electrical and Optical Properties of GaN-Based Flip-Chip Light-Emitting Diodes.

We demonstrated two types of GaN-based flip-chip light-emitting diodes (FCLEDs) with distributed Bragg reflector (DBR) and without DBR to investigate the effect of dielectric TiO2/SiO2 DBR on optical and electrical characteristics of FCLEDs. The reflector consisting of two single TiO2/SiO2 DBR stacks optimized for different central wavelengths demonstrates a broader reflectance bandwidth and a less dependence of reflectance on the incident angle of light. As a result, the light output power (LOP) of FCLED with DBR shows 25.3% higher than that of FCLED without DBR at 150 mA. However, due to the better heat dissipation of FCLED without DBR, it was found that the light output saturation current shifted from 268 A/cm2 for FCLED with DBR to 296 A/cm2 for FCLED without DBR. We found that the use of via-hole-based n-type contacts can spread injection current uniformly over the entire active emitting region. Our study paves the way for application of DBR and via-hole-based n-type contact in high-efficiency FCLEDs.

Comparative Study of Highly Reflective ITO/DBR and Ni/Ag ohmic Contacts for GaN-Based Flip-Chip Light-Emitting Diodes

Heat removal and current spreading uniformity are important for the realization of highly efficient light-emitting diodes (LEDs). Here we have compared two types of GaN-based flip-chip LEDs (FCLEDs) with highly reflective indium-tin oxide (ITO)/distributed Bragg reflector (DBR) and Ni/Ag p-type ohmic contacts. The via-based n-type contact structure was implemented to increase the utilization ratio of active region area and improve current spreading over the entire LEDs. Compared with FCLED with Ni/Ag, the FCLED with ITO/DBR shows a 6.3% higher light output power (LOP) at low current density of 31 A/cm2 (90 mA) due to higher reflectance of ITO/DBR ohmic contact. However, at high injection currents, the Ni/Ag ohmic contact in FCLED alleviates self-heating issue by means of its superior heat dissipation performance. We also found that the less area coverage between ITO and metallization layer have a negative influence on heat dissipation performance of FCLED with ITO/DBR. As a result, the maximum light output power (LOP) of the FCLED with Ni/Ag obtained at 292.8 A/cm2 is 1.3 times larger than that of FCLED with ITO/DBR obtained at 196.3 A/cm2.

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.

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.

Output power enhancement of GaN-based flip-chip light-emitting diodes via conical structures generated by a monolayer of nanospheres

This letter describes the output power enhancement of the GaN-based flip-chip light-emitting diodes (FC LED) featuring conical structures fabricated by etching a self-assembled monolayer SiO2 spheres as the hard mask. By roughening the surface of FC LED components, it increases structural size of the components and elevates the light extraction efficiency of FC LED. At a constant current of 400 mA, the output power of the FC LED with 1200 nm conical structures is 638.1 mW and enhanced by 6.1% compared with the FC LED without surface roughening.

Improved light extraction efficiency of GaN-based flip-chip light-emitting diodes with an antireflective interface layer

GaN-based flip-chip light-emitting diodes (FC-LEDs) grown on nanopatterned sapphire substrates (NPSS) are fabricated using self-assembled SiO2 nanospheres as masks during inductively coupled plasma etching. By controlling the pattern spacing, epitaxial GaN can be grown from the top or bottom of patterns to obtain two different GaN/substrate interfaces. The optoelectronic characteristics of FC-LED chips with different GaN/sapphire interfaces are studied. The FC-LED with an antireflective interface layer consisting of a NPSS with GaN in the pattern spacings demonstrates better optical properties than the FC-LED with an interface embedded with air voids. Our study indicates that the two types of FC-LEDs grown on NPSS show higher crystal quality and improved electrical and optical characteristics compared with those of FC-LEDs grown on conventional planar sapphire substrates.

Effects of pre-annealed ITO film on the electrical characteristics of high-reflectance Ni/Ag/Ni/Au contacts to p-type GaN

In this study, a Ni/Ag/Ni/Au multilayer with first Ni layer of 0.5nm was first optimized for high reflectivity (92.3%), low specific contact resistance (2.1×10−3Ωcm2) and good attachment strength to p-type GaN. To further decrease the contact resistance, the p-type GaN surface was previously treated with pre-annealed indium-tin-oxide (ITO) film before deposition of the Ni/Ag/Ni/Au multilayer, and resulted in a lower specific contact resistance of 1.9×10−4Ωcm2. The X-ray photoelectron spectroscopy results indicated that Ga 2p core level of the p-type GaN surface with the pre-annealed ITO film had a lower binding energy, leading to a reduction in the contact resistance. Furthermore, GaN-based flip-chip light-emitting diodes (LEDs) with and without the pre-annealed ITO film were fabricated. The average forward voltage of the flip-chip LEDs fabricated with the pre-annealed ITO film is 3.22V at an injection current density of 35A/cm2, which is much lower than that (3.49V) of flip-chip LEDs without the pre-annealed ITO film. These results reveal that the proposed approach is effectively to fabricate high quality p-type contacts toward high power GaN-based LEDs.

Modification of internal quantum efficiency and efficiency droop in GaN-based flip-chip light-emitting diodes via the Purcell effect.

The Purcell effect in GaN-based flip-chip (FC) light-emitting diode (LED) structures is investigated numerically using finite-difference time-domain simulations. Depending on the thickness of the p-GaN layer, the variation of the Purcell factor of FC LEDs is obtained to be as high as 20%, which results in the relative modification of the internal quantum efficiency (IQE) as large as 8% and 2.5% for the unmodified IQE of 0.4 and 0.8, respectively. Since the influence of the Purcell effect becomes more conspicuous as the IQE decreases, the Purcell enhancement can be advantageously used to mitigate the efficiency droop problem to some extent. When the Purcell effect is positively applied to the blue LED with the peak IQE of 0.8 and the droop ratio of 29.1%, the peak IQE and the droop ratio are found to be improved to 0.82 and 26.3%. This small but non-negligible effect on IQE is expected to be importantly adopted for industry development of high efficiency LEDs.

GaN-Based Power Flip-Chip LEDs With SILAR and Hydrothermal ZnO Nanorods

In this paper, we present a simple and low-cost method combined successive ionic layer adsorption and reaction with Hydrothermal to form ZnO nanorods for GaN-based power flip-chip light-emitting diodes (LEDs). With 350-mA current injection, it was found that forward voltages were all 2.97 V when the 350-mA output powers were 361.7, 420.7, 426.8, 448.2, and 430.4 mW for LED I, LED II, LED III, LED IV, and LED V, respectively. It was also found that the light output power of LED IV was 24% larger than that of LED I. Furthermore, it was also found that the formation of ZnO nanorods on the top of sapphire will not degrade the electrical properties.

Dual enhancement of light extraction efficiency of flip-chip light-emitting diodes with multiple beveled SiC surface and porous ZnO nanoparticle layer coating

A porous ZnO nanoparticle layer coating composed of columnar ZnO nanoparticle piles and a multiple-beveled substrate was used to enhance the light extraction efficiency of GaN-based flip-chip light-emitting diodes (FC-LEDs), which were grown on high-purity SiC substrates. The SiC substrate was multiple-beveled by fabricating an ‘X’ pattern on the face of it, followed by a deposition of a porous ZnO nanoparticle layer on the ‘X’-patterned surface. A porous ZnO nanoparticle layer was fabricated with gas phase cluster beam deposition in a glancing incidence. The incident angular-resolved light transmission of the ZnO nanostructure beyond the critical angle of total internal reflection (TIR) was greatly enhanced. The light output power of the LED was improved by nearly 60% compared to the original planar GaN-based LED on an SiC substrate (FC-SLED), which contained a significant enhancement supplemental to the 18% electroluminescence (EL) enhancement realized with the ‘X’-pattern beveling. We demonstrated that a dual enhancement of light extraction efficiency was achieved by using the hierarchical surface consisting of microscale textures (the multiple-beveled surfaces) and nanoscale structures (the ZnO nanoparticle layers).

Improved Thermal Stability of GaN-Based Flip-Chip Light-Emitting Diodes with TiW-Based Diffusion Barrier

Improved Thermal Stability of GaN-Based Flip-Chip Light-Emitting Diodes with TiW-Based Diffusion Barrier

Performance enhancement of GaN-based flip-chip ultraviolet light-emitting diodes with a RPD AlN nucleation layer on patterned sapphire substrate

In this work, flip-chip ultraviolet light-emitting diodes (FCUV-LEDs) on patterned sapphire substrate (PSS) at 375 nm were grown by an atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD). A specialized reactive plasma deposited (RPD) AlN nucleation layer was utilized on the PSS to enhance the quality of the epitaxial layer. By using high-resolution X-ray diffraction, the full-width at half-maximum of the rocking curve shows that the FCUV-LEDs with RPD AlN nucleation layer had better crystalline quality when compared to conventional GaN nucleation samples. From the transmission electron microscopy (TEM) image, it can be observed that the tip and incline portion of the pattern was smooth using the RPD AlN nucleation layer. The threading dislocation densities (TDDs) are reduced from 7 × 107 cm−2 to 2.5 × 107 cm−2 at the interface between the u-GaN layers for conventional and AlN PSS devices, respectively. As a result, a much higher light output power was achieved. The improvement of light output power at an injection current of 20 mA was enhanced by 30%. Further photoluminescence measurement and numerical simulation confirm such increase of output power can be attributed to the improvement of material quality and light extraction.

Effect of Beveled SiC Substrate on Light Extraction of Flip-Chip Light-Emitting Diodes

We fabricated GaN-based flip-chip light-emitting diodes (FC-LEDs), which were grown on both conventional and high purity SiC substrates. The influence of beveling, thickness, and absorption of the SiC substrate on the light extraction efficiency (LEE) of the FC-LED was investigated by simulation and experiment. From the simulation results, the LEE of the FC-SLED with a 60° beveling angle is higher than that on a rectangular substrate. The experimental results demonstrate that about 15% enhancement of the LEE was achieved by increasing the thickness of the high purity SiC substrate from 120 to 470 μm. A sample with an X pattern on the top face exhibits the highest light output power. The LEE of the FC-SLED can be efficiently enhanced by suitably beveling the substrate with no degradation of the electrical properties.

Thermal simulation and analysis of flat surface flip-chip high power light-emitting diodes

Conventional GaN-based flip-chip light-emitting diodes (CFC-LEDs) use Au bumps to contact the LED chip and Si submount, however the contact area is constrained by the number of Au bumps, limiting the heat dissipation performance. This paper presents a flat surface high power GaN-based flip-chip light emitting diode (SFC-LED), which can greatly improve the heat dissipation performance of the device. In order to understand the thermal performance of the SFC-LED thoroughly, a 3-D finite element model (FEM) is developed, and ANSYS is used to simulate the thermal performance. The temperature distributions of the SFC-LED and the CFC-LED are shown in this article, and the junction temperature simulation values of the SFC-LED and the CFC-LED are 112.80 °C and 122.97 °C, respectively. Simulation results prove that the junction temperature of the new structure is 10.17 °C lower than that of the conventional structure. Even if the CFC-LED has 24 Au bumps, the thermal resistance of the new structure is still far less than that of the conventional structure. The SFC-LED has a better thermal property.