Flip-chip Light-emitting Diodes Research Articles

Electrical and optical characterization of GaN-based light-emitting diodes fabricated with top-emission and flip-chip structures

We investigated the electrical and optical characteristics of GaN-based light-emitting diodes (LEDs) fabricated with top-emission and flip-chip structures. Compared with top-emission LEDs, flip-chip LEDs exhibited a 0.25 V smaller forward voltage and an 8.7 Ω lower diode resistance. The light output power of the flip-chip LED was also larger than that of the top-emission LED by factors of 1.72 and 2.0 when measured before and after packaging, respectively. The improved electrical and optical output performances of flip-chip LEDs were quantitatively analyzed in terms of device resistance and ray optics, respectively.

InGaN-Based High-Power Flip-Chip LEDs With Deep-Hole-Patterned Sapphire Substrate by Laser Direct Beam Drilling

A high-performance flip-chip light-emitting diode (LED) with deep-hole-patterned sapphire substrate was fabricated by laser direct beam drilling. The output power of the LED was measured to be as high as 145 mW at a forward current of 350 mA, which is improved by 19% compared to that of the reference LED. This significant enhancement of the LED with deep-hole-patterned sapphire is attributed to the increase of extraction efficiency, resulting from the increase in photon escape probability due to enhanced light scattering at the deep-hole pattern.

Comment on “Enhancement of flip-chip white light-emitting diodes with a one-dimensional photonic crystal”

We show that research presented in Opt. Lett.34, 301 (2009)OPLEDP0146-959210.1364/OL.34.000301 applied questionable phosphor definitions and a questionable simulation procedure for light-emitting diodes. Our simulation indicates that a one-dimensional photonic crystal is beneficial for color control but cannot improve the light extraction as asserted in that Letter.

GaN-Based Power Flip-Chip LEDs With an Internal ESD Protection Diode on Cu Sub-Mount

The authors demonstrate the fabrication of 1 mm t 1 mm GaN-based power flip-chip light-emitting diodes (LEDs) with an internal electrostatic discharge (ESD) protection diode on Cu sub-mount. With the internal diode, it was found that forward voltage of the LED increased from 3.22 to 3.38 V while output power decreased from 366.5 to 273.9 mW when under 350 mA current injection due to the reduced light emitting area. It was also found that we can achieve a significantly better ESD robustness by building the internal diode inside the LED chip. Furthermore, It was found that 90% of the LEDs with internal diode survived with an applied reverse ESD surge of 12000 V and 25% of the LEDs can even endure 20000 V reverse ESD stressing.

Raising the quantum efficiency of AlGaInN flip-chip LEDs by Reactive ion etching of the outer side of SiC substrates

A method for creating a light-scattering microprofile on the outer side of SiC substrates in order to diminish the light extraction loss caused by the effect of total internal reflection in light-emitting AlGaIn/GaN structures has been further developed. It is suggested to use thin photoresist layers as random masks for reactive ion etching of silicon carbide substrates. A microprofile with the required parameters has been obtained on the substrate surface by optimizing the etching modes, with the result that the external quantum efficiency of light-emitting crystals increased by more than 25%.

Performance of Flip-Chip Thin-Film GaN Light-Emitting Diodes With and Without Patterned Sapphires

We report on improved device performance of flip-chip (FC) GaN-based light-emitting diodes (LEDs) by combining patterned sapphire substrate (PSS) and thin-film techniques. It was found that an FC LED grown on a conventional planar sapphire exhibits a power enhancement factor of only 36.3% after the thin-film processes of substrate removal and surface roughening. In contrast, the as-fabricated FC LED grown on a PSS showed a power enhancement factor of up to 62.3% without any postprocess as compared with the light output power of an original conventional FC LED. Further intensity improvement to 74.4% could be achieved for the FC LED/PSS sample with the thin-film processes.

A study of thermal processes in high-power InGaN/GaN flip-chip LEDs by IR thermal imaging microscopy

Results of an experimental study of temperature fields generated in high-power AlGaInN heterostructure flip-chip light-emitting diodes (LEDs) via their self-heating at high working currents are presented. The method of IR thermal imaging microscopy employed in the study enables a direct measurement of the temperature distribution over the p-n junction area with a high resolution of ∼3 μm at an absolute measurement error of ∼2 K. It is shown that large temperature gradients may arise in high-power LEDs at high excitation levels as a result of current crowding. This effect should be taken into account when designing lightemitting chips and estimating the admissible operation modes. The method of IR thermal imaging microscopy can also reveal microscopic defects giving rise to current leakage channels and impairing device reliability.

A chip-level electrothermal-coupled design model for high-power light-emitting diodes

An advanced three-dimensional electrothermal-coupled simulation model basing on finite-element method numerical simulation is developed to study the electrical and thermal properties of chip-level high-power GaN-based light-emitting diodes (LEDs). The current spreading, heat generation, and transfer in the device are comprehensively considered in this model. The current-spreading effect of the transparent current-spreading layer and the thermal performance of LEDs with interdigitated-electrodes are investigated. The simulation results prove that the temperature distribution in the active layer is strongly affected by the electrode pattern. The obvious heat accumulation in LEDs with conventional interdigitated-electrode patterns can be seen both in the simulated results and the infrared measured results. The heat transfer efficiency can be improved by using a symmetry electrode pattern design. The thermal management of the bump configurations in flip-chip LEDs is also studied. A more reasonable and thermal effective bump configuration is presented, and the simulated results show that a lower average temperature and more uniform heat distribution in the chips can be obtained.

Fabrication and thermal analysis of flip-chip light-emitting diodes with different numbers of Au stub bumps

The thermal performance of flip-chip (FC) light-emitting diodes (LEDs) with different numbers of Au stub bumps has been investigated by using thermosonic bonder. The LEDs were mounted on the aluminium nitride (AlN) sub-mounts which have superior thermal conductivity (230 W/mK), and the high power Chip-on-Plate (COP) package was proposed to be used for our measurement. In order to understand the thermal performance of the high power FC-LEDs, the experimental measurement and finite-element model (FRM) numerical simulation have been used. It is found that the thermal performance of our 1 × 1 mm 2 FC-LEDs can only be improved when using at least 6 Au stub bumps as interconnected metals. Moreover, the surface temperature of FC-LEDs is significantly reduced while using 20 Au stub bumps.

提高LED光提取效率的研究

The impact of the refractive index and thickness of substrate on the light extraction efficiency of flip-chip LED is analyzed through simulation analysis.And a new diamond-shaped structure is proposed.The results show that the light extraction efficiency is improved to a good purpose in the diamond-shaped LED,resulting in an increase in the light extraction efficiency by 1.51,2.03 or 3.65 times when the substrate is Si,Sapphire or SiC espectively.

Reflecting p-contact based on thin ITO films for AlGaInN flip-chip LEDs

A reflecting contact to a p-GaN layer, used in fabrication of blue flip-chip light-emitting diodes, has been produced by deposition of thin indium tin oxide (ITO) films by electron-beam evaporation. The high reflectance of the contact, which exceeds that of a Ni/Ag contact, provides a 15–20% increase in the external quantum efficiency of light-emitting crystals. The forward voltage drops for crystals with an ITO(5 nm)/Ag(220 nm) contact are comparable with the corresponding values for crystals with a Ni(1.5 nm)/Ag(220 nm) contact. The specific resistance of the contact with an ITO layer is 3.7 × 10−3 Ω cm2. It is shown that, for ITO films produced by the given method, the optimal thicknesses providing the best electrical and optical characteristics of the crystals are in the range 2.5–5.0 nm.

A Modified Flip-Chip LED Packaging Design With Enhanced Light Coupling Efficiency for Plastic Optical Fiber Networks

The explosive growth in the use of the Internet and multimedia applications in both the home and the office has fueled the requirement for high-bandwidth communication systems capable of processing huge volumes of data in a prompt and reliable manner. Plastic optical fiber (POF) has emerged as an ideal solution for meeting this requirement and is now specified by many architects as a simple, one-cable solution for home and office data communication networks. This study presents a modified flip-chip light emitting diode (LED) package for use in short-distance, POF-based communication systems. In contrast to the planar surface of the traditional design, the proposed LED package has a curved boundary surface between the underfill and the air. This boundary surface functions as a virtual lens, which focuses the light emitted by the LED into the acceptance cone of the optical fiber such that the amount of light coupled into the fiber core is maximized. The proposed design yields an average coupling efficiency of around 49.6% over an LED-optical core misalignment range of 0.4–3 mm. Furthermore, the curved boundary surface reduces the distance between the emitting source and the ambient environment; therefore, it improves the thermal dispersion efficiency of the LED package.

Improved electrical and thermal properties of Ag contacts for GaN-based flip-chip light-emitting diodes by using a NiZn alloy capping layer

We investigate the effect of a 100 nm-thick NiZn alloy (10 wt% Zn) capping layer on the thermal and electrical properties of Ag reflectors (200 nm) for flip-chip light-emitting diodes (LEDs). It is shown that the introduction of the NiZn capping layer minimizes the formation of interfacial voids and surface agglomeration. Furthermore, LEDs fabricated with the NiZn-capping-layer-combined contacts produce better output power as compared to those with the Ag only reflectors. For example, the LEDs with the 400 ∘C-annealed Ag/NiZn contacts give higher output power by ∼36% than those with the 400 ∘C-annealed Ag only contacts. X-ray photoemission spectroscopy and Auger electron spectroscopy measurements are performed to understand the improved electrical properties of the LEDs fabricated with the NiZn-capping-layer-combined Ag contacts.

Investigation of high extraction efficiency flip-chip GaN-based light-emitting diodes

In order to obtain higher light output power, the flip-chip structure is used. We studied the ratio of the light of GaN sides before and after fabricating metal reflector on p-GaN. The SiO2/SiNx dielectric film reflectors were deposited through plasma enhance chemical vapor deposition following the fabrication of metal reflector, and then the dielectric film reflectors on the electrodes were etched in order to expose the electrodes to the air. It is found that comparing with the flip-chip GaN-LED without dielectric film reflectors, light output power can be increased by as high as 10.2% after the deposition of 2 pairs of SiO2/SiNx dielectric film reflectors on GaN-LEDs, which cover the sidewalls and the areas without the metal reflector. This result indicates that the high reflector formed by multi-layer dielectric films is useful to enhance the light output power of GaN-based LED, which reflects light from step sidewalls and p-GaN without metal reflector to internal, and then light emits from the surface.

GaN-Based Power Flip-Chip LEDs With Cu Submount

Nitride-based power flip-chip (FC) LEDs with Cu submount were proposed and prepared. With a much higher thermal conductivity, it was found that we can achieve a lower operation voltage under high-current injections and lower junction temperature from the FC LEDs with Cu submount. Compared with the power FC LEDs with Si submount, the reliability of the proposed device was also better.

Surface Texturing of High-Power Flip-Chip LEDs by Femtosecond Laser Direct Structuring

We report on the femtosecond laser direct structuring of the sapphire flipside surfaces of high-power flip-chip LEDs. It is found that diameter and depth of the created submicrometer-sized holes can be manipulated by varying the laser power at a constant number of laser pulses. This method enables the control of the structure sizes with high precision. Our study shows that the light extraction from such LEDs increases strongly with increasing hole sizes.

High-Brightness InGaN–GaN Power Flip-Chip LEDs

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We report the fabrication of InGaN–GaN power flip-chip (FC) light-emitting diodes (LEDs) with a roughened sapphire backside surface prepared by grinding. It was found that we can increase output power of the FC LED by about 35% by roughening the backside surface of the sapphire substrate. The reliability of the proposed device was also better, as compared to power FC LEDs with a conventional flat sapphire backside surface. </para>

Effect of the Flat and Pattern Surface Texturing on Light Extraction of GaN Flip-Chip Light-Emitting Diodes

Device performances of GaN-based flip-chip light-emitting diodes (FC LEDs) with planar and patterned sapphire substrates (PSS) were compared in this study. It was found that for the FC LED with planar sapphire, enhancement factor of luminous intensity can be raised to 107.5% after the processes of substrate removal and surface roughening. By contrast, for the FC LED with PSS, the intensity enhancement factor is already up to 169.5% without any post-processes as compared with the intensity of an as-fabricated conventional FC LED. Further intensity improvement to 205.1% can be achieved for the FC LED with PSS by employing subsequent processes such as substrate removal and surface roughening. These results indicate that the PSS approach is useful in improving light extraction of a nitride-based FC LED.

Effect of the Flat and Pattern Surface Texturing on Light Extraction of GaN Flip-Chip Light-Emitting Diodes

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Emission distribution in GaInAsSb/GaSb flip-chip diodes

Results obtained in studies of the near-field pattern of GaInAsSb IR flip-chip light-emitting diodes (LEDs) operating in the 2 μm range are presented. The electrical and reflective properties of ohmic contacts are discussed and the near-field emission distribution is analyzed in relation to the current and electrical and geometrical parameters of the LEDs.