Current Spreading Performance Research Articles

Enhancing current spreading performance of GaN-based ultraviolet light-emitting diode chips through periodic doping composite barrier structures

In response to the issue of current crowding in GaN-based ultraviolet (UV) light-emitting diode (LED) chips, a periodic doping composite barrier structure is proposed. The results of the optical intensity distribution demonstrate that the device with the periodic doping composite barrier structure exhibits a more uniform light distribution compared to the device with traditional structures. Moreover, when the injected current is 20 mA, the light output power (LOP) and wall-plug efficiency (WPE) are improved by 11.99% and 13.5%, respectively. Experimental and simulation results indicate that the periodic doping composite barrier structure enhances the current spreading performance through two mechanisms: (1) the introduction of an electron barrier layer enhances lateral current transport; (2) increase the ratio of longitudinal to transverse resistivity of the material to further enhances current spreading performance.

29‐4: Higher External Quantum Efficiency with Lower Current Density Injection of <10 μm Pixel Size Arrays for Display Application

In this paper, the gallium nitride (GaN) based Micro‐LED was fabricated based on a self‐aligned process with hydroxide treatment and ALD passivation from 100 μm down to 3 μm. The different current spreading performance was characterized based on the series resistance analysis. Then the size‐dependent carrier concentration profile was demonstrated via the capacitancevoltage measurement, identifying the various carrier injection behavior by different size. Finally, the external quantum efficiency and luminance versus current density dependence for array device and the single device was compared, revealing a higher efficiency at lower current density for array device, which is favorable for display application.

3.3: Higher External Quantum Efficiency with Lower Current Density Injection of <10 μm Pixel Size Arrays for Display Application

In this paper, the gallium nitride (GaN) based Micro‐LED was fabricated based on a self‐aligned process with hydroxide treatment and ALD passivation from 100 μm down to 3 μm. The different current spreading performance was characterized based on the series resistance analysis. Then the size‐dependent carrier concentration profile was demonstrated via the capacitancevoltage measurement, identifying the various carrier injection behavior by different size. Finally, the external quantum efficiency and luminance versus current density dependence for array device and the single device was compared, revealing a higher efficiency at lower current density for array device, which is favorable for display application.

Using a Multi-Layer Stacked AlGaN/GaN Structure to Improve the Current Spreading Performance of Ultraviolet Light-Emitting Diodes.

To obtain excellent current spreading performance of ultraviolet light-emitting diodes (UVLEDs), a 60-period stacked Si modulation-doped n-AlGaN/u-GaN structure is proposed to replace the traditional n-AlGaN structure. The high-resolution X-ray diffraction ω-scan rocking curves show that the periodic growth of AlGaN and GaN layers plays a positive role in reducing dislocation density. Compared with the conventional UV light-emitting diodes (LEDs), light emission micrographs of devices with a multi-layer stacked n-AlGaN/u-GaN structure reveal higher brightness and a more uniform distribution. In addition, the output power and external quantum efficiency under a 20-mA injection current are increased by 22% and 26.5%, respectively. Experimental and simulation results indicate that a multi-layer stacking structure can alleviate the current crowding effect in four ways: (1) a reduction in dislocation density; (2) replacement of quasi-two-dimensional electron transport with electronic bulk transport to enhance electron mobility; (3) an increase in electron concentration without improving the impurity concentration; and (4) a weakening of the electron scattering effect by reducing the impurity concentration.

High-Power GaN-Based Vertical Light-Emitting Diodes on 4-Inch Silicon Substrate.

We demonstrate high-power GaN-based vertical light-emitting diodes (LEDs) (VLEDs) on a 4-inch silicon substrate and flip-chip LEDs on a sapphire substrate. The GaN-based VLEDs were transferred onto the silicon substrate by using the Au–In eutectic bonding technique in combination with the laser lift-off (LLO) process. The silicon substrate with high thermal conductivity can provide a satisfactory path for heat dissipation of VLEDs. The nitrogen polar n-GaN surface was textured by KOH solution, which not only improved light extract efficiency (LEE) but also broke down Fabry–Pérot interference in VLEDs. As a result, a near Lambertian emission pattern was obtained in a VLED. To improve current spreading, the ring-shaped n-electrode was uniformly distributed over the entire VLED. Our combined numerical and experimental results revealed that the VLED exhibited superior heat dissipation and current spreading performance over a flip-chip LED (FCLED). As a result, under 350 mA injection current, the forward voltage of the VLED was 0.36 V lower than that of the FCLED, while the light output power (LOP) of the VLED was 3.7% higher than that of the FCLED. The LOP of the FCLED saturated at 1280 mA, but the light output saturation did not appear in the VLED.

Study of a GaN-Based LED With an Al/AZO Composite Transparent Conductive Layer

A new and simple Al/aluminum-doped zinc oxide (AZO) composite structure is proposed to act as a transparent conductive layer (TCL) for GaN-based LEDs. The Al/AZO composite layers effectively improve the current spreading performance, compared with an AZO TCL. Experimentally, the specific contact resistance of the studied Al/AZO LED is reduced by about 27.6%, compared with an AZO LED. In addition, under an operating current of 20 mA, the forward voltage and light output power (LOP) of the studied Al/AZO LED are decreased by 0.15 V and increased by 19.5%, respectively. Uniform and enhanced intensities are also found in light emission mapping images of the studied Al/AZO LED. Consequently, based on benefits including simple structure and improved electrical and optical properties, the studied Al/AZO TCL is suitable for GaN-based LED applications.

Effect of p-GaN layer grown with H2 carrier gas on wall-plug efficiency of high-power LEDs

The effect of employing different carrier gases (H2 only and 1:1 vol% N2:H2) in the p-type GaN (p-GaN) layer on the wall-plug efficiency (WPE) of high-power light-emitting diodes (LEDs) is studied. Since GaN crystal could be a two-dimension (2-D) growth mode in H2 ambient, better quality and smoother surface of the p-GaN were obtained. The current spreading performance of the p-GaN layer using H2 alone as the carrier gas was enhanced, resulting in advanced light output power (LOP). In addition, turn-on voltage and dynamic resistance at 500mA, which can strongly contribute to the WPE, were also reduced by 0.12V and 0.13Ω, respectively. The studied device with H2 as the carrier gas in the p-GaN layer (p-H2 layer) exhibits 9.5% and 12.4% improvements in LOP and WPE at 500mA over the device (N2/H2=1:1), as well as significantly better electrostatic discharge robustness. Therefore, the use of a p-H2 layer can effectively improve the performance of GaN-based LEDs for high power applications.

Improved current spreading performance of a GaN-based light-emitting diode with a stair-like ITO layer

A GaN-based light-emitting diode (LED) with a stair-like ITO layer is studied. The stair-like ITO structure is achieved by using a simple wet etching process. The current distribution could be adjusted by the different thickness of each step in this stair-like ITO layer. The current injected from p-pad is forced to be spread outside by the thicker ITO layer below the p-pad, which exhibits larger parasitic series resistance, instead of flowing downward directly. The current spreading effect could be enhanced. As compared with a conventional LED, at 20mA, the studied device with a stair-like ITO layer shows 13.8% improvement in light output power. A lower turn-on voltage is also achieved.

Effects of the Use of an Aluminum Reflecting and an ${\rm SiO}_{2}$ Insulating Layers (RIL) on the Performance of a GaN-Based Light-Emitting Diode With the Naturally Textured p-GaN Surface

A GaN-based light-emitting diode (LED) with an aluminum (Al) reflecting and an SiO2 insulating layers (RILs) deposited on the naturally textured p-GaN surface is fabricated and studied. The use of RIL could enhance the current spreading performance and reduce the photon absorption by the p-pad metal. The textured surface is used to limit the total internal reflection and increase photon scattering. In this paper, effects of the use of an Al RL and/or an SiO2 insulating layer on the performance of GaN-based LEDs are systematically studied and compared in detail. At 20 mA, as compared with a conventional LED with naturally textured (planar) p-GaN surface, the studied device exhibits 12.2% (55.5%) enhancement in light output power. Additionally, a 28.5% (95%) increment of luminous flux is achieved. The studied device also shows 15.6% light intensity improvement of far-filed pattern. Experimentally, although power consumption and junction temperature are slightly increased because of the insertion of RIL structure, these drawbacks could be surpassed by the mentioned optical improvements. Therefore, for conventional GaN-based LEDs, light extraction efficiency could be further improved by the employment of RIL structure.

Enhancement in light output power of LEDs with reflective current blocking layer and backside hybrid reflector

A GaN-based light-emitting diode (LED) with reflective current blocking layer (CBL) underneath p-electrode pad and backside hybrid reflector was fabricated and investigated. With a 20 mA injection current, the LED with SiO2/ITO/Al reflective CBL deposited on naturally textured p-InGaN/p-GaN surface exhibited a light output power that was 7.6% and 18.5% higher than those of the textured LEDs with SiO2 CBL and without SiO2 CBL, respectively. The LED with backside hybrid reflector exhibited a light output power that was 30% higher than that of LED without the hybrid reflector. The enhancement in light output power is attributed to the improved current spreading performance via the SiO2 CBL, the Al omnidirectional metal reflector to prevent the light absorption by the opaque p-electrode pad, and the backside hybrid reflector to extract bottom-emitting light.

Enhancement in light extraction of LEDs with SiO2 current blocking layer deposited on naturally textured p-GaN surface

GaN-based light-emitting diodes (LEDs) with a SiO2 current blocking layer (CBL) deposited on naturally textured V-shaped pits (V-pits), grown on the p-GaN surface, was fabricated and investigated. The V-pits grown on p-GaN surface were used to increase photon scattering probability, and the SiO2 CBL was used to improve current spreading performance of the textured LED. Compared to the planar LED, the LED with naturally textured p-GaN surface exhibited 9.3% enhancement in light output power under 20mA injection current. At 20mA injection current, the LED with SiO2 CBL deposited on naturally textured p-GaN surface exhibited a 11.8% higher light output power than naturally textured LED without SiO2 CBL. Therefore, for a LED with V-pits grown on p-GaN surface, the light extraction efficiency can be further improved by employing a SiO2 CBL. The 20mA-driven forward voltage of textured LED with SiO2 CBL was only 0.04V higher than that of the textured LED.

Improved Performance of an InGaN-Based Light-Emitting Diode With a p-GaN/n-GaN Barrier Junction

An InGaN-based light-emitting diode with a p-GaN/n-GaN barrier junction is fabricated and investigated. Due to the built-in potential induced by this junction, superior current spreading performance is achieved. In addition, the suppression of the current-crowding phenomenon yields a reduced parasitic effect. Therefore, under an injection current of 20 mA, improved behaviors, including lower turn-on voltage, lower parasitic series resistance, and reduced p-n junction temperature, are achieved. In addition, due to the improved current-spreading ability, longer life-time, driving at medium current injection (60 mA), as well as significantly enhanced electrostatic discharge performance, are obtained.

Improved current-spreading performance of an InGaN-based light-emitting diode with a clear p-GaN/n-GaN barrier junction

The InGaN-based light-emitting diode (LED) with a clear p-GaN/n-GaN barrier junction is fabricated and investigated. Due to the built-in potential induced by this junction, superior current spreading performance is achieved. In addition, the suppression of current crowding phenomenon yields the reduced parasitic effect. Therefore, under an injection current of 20 mA, improved behaviors including lower turn-on voltage, lower parasitic series resistance, and significantly enhanced electrostatic discharge (ESD) performance are presented.

Improved performance of GaN-based light-emitting diodes by using short-period superlattice structures

AbstractAn InGaN/GaN multiple-quantum-well (MQW) light-emitting diode (LED) with a ten-period i (undoped) -InGaN/p (Mg doped) -GaN (2.5 nm/5.0 nm) superlattice (SL) structure, was fabricated. This SL structure that can be regarded as a confinement layer of holes to enhance the hole injection efficiency is inserted between MQW and p-GaN layers. The studied LED device exhibits better current spreading performance and an improved quality, compared with a conventional one without SL structure. Due to the reduced contact resistance as well as more uniformity of carriers injection, the operation voltage at 20 mA is decreased from 3.32 to 3.14 V. A remarkably reduced reverse-biased leakage current (10−7–10−9 A) and higher endurance of the reverse current pulse are found. The measured output power and external quantum efficiency (EQE) of the studied LED are 13.6 mW and 24.8%, respectively. In addition, significant enhancement of 25.4% in output power as well as increment of 5% in EQE for the studied devices is observed, as the studied devices show superior current spreading ability and reduction in dislocations offered by the SL structure.

Characteristics of a GaN-Based Light-Emitting Diode With an Inserted p-GaN/i-InGaN Superlattice Structure

An interesting GaN-based light-emitting diode (LED) with a ten-period i (undoped)-InGaN/p (Mg doped)-GaN (2.5 nm/5 nm) superlattice (SL) structure, inserted between a multiple-quantum well (MQW) structure and a p-GaN layer, is fabricated and studied. This inserted SL can be regarded as a confinement layer of holes to enhance the hole injection efficiency. As compared with a conventional LED device without the SL structure, the studied LED exhibits better current spreading performance and an improved quality. The turn-on voltage, at 20 mA, is decreased from 3.32 to 3.14 V due to the reduced contact resistance as well as the more uniformity of carriers injection. A substantially reduced leakage current (10- 7 to 10- 9 A) and higher endurance of the reverse current pulse are found. The measured output power and external quantum efficiency (EQE) of the studied LED are 13.6 mW and 24.8%. In addition, as compared with the conventional LED without the SL structure, the significant enhancement of 25.4% in output power as well as the increment of 5% in EQE are observed due to the superior current spreading ability and reduction of dislocations offered by the SL structure.

On a GaN-Based Light-Emitting Diode With a p-GaN/i-InGaN Superlattice Structure

An interesting GaN-based light-emitting diode (LED) with a ten-period i-InGaN/p-GaN (5-nm/5-nm) superlattice (SL) structure, inserted between a multiple-quantum-well structure and a p-GaN layer, is fabricated and studied. This inserted SL can be regarded as a confinement layer of holes to enhance the hole injection efficiency. As compared with a conventional LED device without the SL structure, the studied LED exhibits better current-spreading performance and an improved quality. The turn-on voltage, at 20 mA, is decreased from 3.32 to 3.14 V due to the reduced contact resistance as well as the more uniformity of carrier injection. A substantially reduced leakage current (10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> -10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> A) and higher endurance of the reverse current pulse are found. As compared with the conventional LED without the SL structure, the significant enhancement of 25.4% in output power and the increment of 5% in external quantum efficiency are observed.

Impact of an indium oxide/indium-tin oxide mixed structure for GaN-based light-emitting diodes

An interesting GaN-based light emitting diode (LED) using a 50 nm indium oxide (In2O3)/250 nm indium-tin oxide (ITO) mixed structure to replace the commonly used ITO (250 nm) current spreading layer is fabricated and studied. Use of the In2O3 layer could reduce the contact resistance of p-GaN in LEDs. In addition, this highly-resistive In2O3 layer, below the ITO layer could improve the current spreading performance. Experimentally, at room temperature, using this mixed structure, the luminous and EL intensities are enhanced by 17.7 and 17.1%, respectively.

Characterization of Large-Area AlGaInP/Mirror/Si Light-Emitting Diodes Fabricated by Wafer Bonding

High-brightness AlGaInP/mirror/barrier/Si light-emitting diodes (LEDs) with vertical electrodes were fabricated using a wafer bonding technique. The high-thermal-conductivity Si substrate provides a better heat sink (compared with GaP or GaAs), which is a particularly important characteristic for high-power, large-area emitter applications. It allows for saturation of the small-area LED (300×300 µm2) after 150 mA injection and for the large-area LED (1200×1200 µm2) to increase output power even at 300 mA injection. The light output of the large-area vertical-conducting LED is mainly affected by the top-side (i.e., n-AlGaInP cladding) electrode design, where an interdigitated-finger electrode provides the better current spreading performance and prevents the current crowding problem. The extent of junction heating on LED samples (300×300–1200×1200 µm2) can also be evaluated from the corresponding electroluminescence spectra and emission-peak-wavelength driven by different injection current values. Finally, using a natural lithography technique, the mirror-substrate (MS) LEDs with textured surfaces present a brightness of 10 cd, which is about 4 cd brighter than that of the MS LED without a textured surface. With a combination of the MS wafer bonding and surface texturing techniques, thermal management in packaging becomes the key factor in further enhancing the external quantum efficiency of large-area LEDs under high flux operation.