Indium Tin Oxide Current Spreading Layer Research Articles

Current spreading structure of GaN-based vertical-cavity surface-emitting lasers.

Indium tin oxide (ITO) is often used as a current spreading layer in the GaN-based vertical-cavity surface-emitting lasers (VCSELs). However, the absorption coefficient of ITO is significant, which reduces the laser output power, raises the threshold, and makes VCSELs hardly lase in the ultraviolet range. To find a transparent conductive structure that can replace ITO, we propose a periodic p-AlGaN/u-GaN/p-GaN structure. In the simulation of light-emitting diodes, the optimized parameter is obtained with multi-period 10 nm p-Al0.1Ga0.9N/2 nm u-GaN/8 nm p-GaN combined with n-GaN/n-Al0.2Ga0.8N in the n region. Applying the structure to 435 nm VCSELs and comparing it to a common VCSEL with the ITO current spreading layer, it can be found that the new structure reduces the threshold from 9.17 to 3.06 kA/cm2. The laser power increases from 1.33 to 15.4 mW. The optimized structure has a high laser power and a lower threshold, which can be used in future investigations.

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.

Strategy and mechanics for bendable micro-light emitting diode array integrated by polymer

The strategy for fabricating bendable micro-light emitting diode arrays is presented and the mechanical properties of bent arrays are analysed in this paper. The key procedures of the fabrication process consist of grooving the wafer, welding the upside electrode with an indium tin oxide current-spreading layer, integrating the micro-pixels, removing the substrate, electroplating the backside electrode, and encapsulating the device. In order to prevent potential physical destruction and improve the reliability and fatigue of the arrays, the position of the neutral mechanical plane was calculated. Strain and stress distributions in bent LED arrays showed that strain concentrated in the polymers and the generated stress preferentially concentrated in the electrodes and semiconductor. These results indicate that physical destruction, rupture, or delamination may first occur in the electrodes or semiconductor interface.

Effect of an ITO current spreading layer on the performance of InGaN MQW solar cells

AbstractInGaN‐based solar cells have been investigated through fabrication with and without using an indium‐tin‐oxide (ITO) current spreading layer (CSL). For the devices with a planar top surface, utilization of the ITO CSL leads to enhanced performance under 1 sun air‐mass 1.5 global spectrum illumination. In contrast, when surface‐texturing is applied to significantly improve the light absorption, the efficiency of the device without using the ITO CSL is higher compared to the one with the ITO. Furthermore, measurements on reflectance of the corresponding surfaces and transmission of the ITO CSL are carried out. The influence of the ITO CSL has been discussed in terms of surface reflection, the light loss due to the ITO CSL shading and the power loss associated with the absence of the ITO CSL. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Enhancement of light extraction efficiency of vertical LED with patterned graphene as current spreading layer

In this paper we report an optimised pattern of surface textured graphene current spreading layer (CSL) for the enhancement of light extraction efficiency (LEE) in InGaN/GaN vertical light emitting diodes (V-LEDs). It is found that by texturing graphene surface LEE improves drastically. This improvement is attributed to better current spreading of graphene and increased random and multiple scattering of light through textured surfaces. Simulation results illustrate that V-LEDs with surface textured (hexagonal pattern) ITO as CSL shows threefold improvement in light extraction efficiency compared to V-LEDs with no surface texturing on ITO CSL. Further, LEE of V-LEDs having patterned graphene CSL is compared with that for indium tin oxide (ITO) CSL. V-LEDs with optimised hexagonal patterning on graphene CSL shows 13.42% enhancement of LEE compared to that of LED with hexagonal patterning on ITO surface.

Influence of the ITO current spreading layer on efficiencies of InGaN-based solar cells

A systematic investigation of the utilization of indium-tin-oxide (ITO) as a current spreading layer (CSL) with and without a textured surface has been performed on InGaN-based solar cells, demonstrating a difference in the influence on the performance of the devices. It is found that employing an ITO CSL improves the conversion efficiency for devices with planar surfaces; whereas it reduces the efficiency for the surface textured devices. Consequently, best performance is achieved for the surface-textured solar cell without employing an ITO CSL, with an enhancement of 75% in the conversion efficiency compared to the planar cell without ITO. Our reflectance measurements show that the ITO CSL can effectively suppress surface reflection for the planar devices, while it becomes less effective for the surface-textured devices. Furthermore, a transmission measurement is carried out to estimate the absorption of the ITO CSL. The influence of the ITO CSL is discussed in terms of surface reflection and light loss due to the ITO shading effect as well as power loss associated with the absence of the ITO CSL.

Design and comparative study of lateral and vertical LEDs with graphene as current spreading layer

This study analyzes the current spreading effect of graphene on lateral and vertical light emitting diodes (LEDs). We observe an improvement in uniformity of current distribution, light output power and wall-plug efficiency in lateral LEDs (L-LEDs) with graphene current spreading layer (CSL) as compared to those with indium tin oxide (ITO) CSL. From the results we conclude that graphene CSL may be better alternative to ITO CSL. We further carried out a comparative study of lateral and vertical LEDs with graphene CSL. We observe 17% higher light output power, 16% higher wall-plug efficiency and 62% lower series resistance in the case of V-LEDs with graphene CSL when compared to that of L-LEDs with a graphene CSL. Reasons behind these results have been discussed.

Implant-Defined 3 $\times$ 3 In-Phase Coherently Coupled Vertical Cavity Surface Emitting Lasers Array

Two-dimensional in-phase coherently coupled vertical cavity surface emitting laser (VCSEL) array with high beam quality was fabricated by proton implantation. Indium-tin-oxide (ITO) current spreading layer was employed to improve the uniformity of the injection current. This makes a coherent array to operate in in-phase mode even in a relatively larger scale. The appropriate thickness of ITO current spreading layer was obtained by experiment (80 nm). The procedure of proton implantation defined array is considerably simple and low cost. A 3 × 3 in-phase coherently coupled array with 2.4° (1.28 × D.L.) of lobe beamwidth was achieved. It sufficiently demonstrates the advantages of the high beam quality of the in-phase coherently coupled arrays. The output power of the array was up to 4.5 mw under pulse-wave condition.

Enhanced Optical Property of InGaN Light-Emitting Diodes with SiO2 Nano-Bowl Photonic Crystal by Nanosphere Lithography

Recently, nitride-based compound semiconductors have been attracting extensive attention in high performance electroluminescent devices due to their broad range of emission wavelength. InGaNbased Light-emitting diodes (LEDs) characterized by small size, high efficiency and long lifetime have been indispensable components in various applications including general lighting, traffic lights, automobile headlights, and backlights in liquid crystal displays. However, it is still necessary to further improve the light output power (LOP) of these modern LEDs to approach the next generation applications such as high-end solid-state lighting. The LOP of the LEDs depends on the external quantum efficiency (EQE) and is severely restricted by poor light extraction efficiency (LEE) due to the total internal reflection (TIR) effect caused by the large contrast in refractive index at the interface between the GaN film and the surrounding air. 1 Various approaches have been reported to improve the LEE of the LEDs by increasing the light scattering at the GaN/air interface including patterned sapphire substrate (PSS), 2,3 surface texturing, 4,5 and graded index structures. 6,7 Lately, many efforts have been taken to demonstrate the fabrication and application of the two-dimensional photonic crystal (2D-PC) structures to improve the LEE of the LEDs by suppressing the unwanted guiding modes. 8‐10 However, previous researches either require complicated process or expensive equipment 11 in the formation of wavelength periodic structures which greatly limited the application of 2D-PC structures to the LEDs. On the other hand, the 2D-PC structures can deteriorate the electrical properties 12 of the LED devices and affect the device lifetime. 13 To avoid these negative effects, 2D-PC structures can be formed onto the indium-tin oxide (ITO) current spreading layer Instead. During the fabrication of these structures in the characteristic length scales on the order of the wavelength, nano-patterning techniques are involved. Compared to nano-patterning using E-beam, 14 nano-imprint 15 lithographic techniques, nano-sphere lithography (NSL) 16‐18 has been demonstrated as a high-efficiency low-cost, and high throughput nano-fabrication method. In this paper, the fabrication and characterization of the InGaN LEDs with SiO2 nano-bowl photonic crystal (SNPC) atop the ITO current spreading layer patterned by self-assembled NSL were reported. The optical and electrical performances of the LEDs with SNPCs were evaluated in comparison with the conventional LEDs, together with an investigation of the mechanisms involved. The optical performances were also confirmed by the numerical simulations using the finite difference time domain (FDTD) method.

Reliability of AlGaInP light emitting diodes with an ITO current spreading layer

Three aging experiments were performed for AlGaInP light emitting diodes (LED) with or without indium tin oxide (ITO), which is used as a current spreading layer. It was found that the voltage of the LED with an ITO film increased at a high current stressing, while there was little change for that of the LED without the ITO. The results of the LEDs with different thicknesses of the ITO film show that the LED with a thicker ITO has a higher reliability. The main reason for the voltage increase of the LED with the ITO film might be the current crowding in the ITO film around the P-type electrode.

Electromagnetic absorption in transparent conducting films

In this article we calculate the skin depth of indium tin oxide (ITO) films (and related materials) as a result of free electron absorption within the visible spectrum using the simple Drude model. We also discuss the consequences of finite skin depth for the transparency of current spreading layers for light emitting diode (LED) applications. Low sheet resistances are highly desirable for these layers, but the free electron density n in ITO cannot be increased much beyond 2×1021 cm−3 without pulling the plasma frequency into the red end of the visible spectrum (thus making it highly reflective); furthermore, any increases in the film thickness cause reduced transparency due to the finite skin depth δ. However, above the plasma frequency for n≈1021 cm−3 we find that increases in the electron mobility μe cause increases in δ, since then approximately δ∝μe. Therefore, if μe in ITO can be increased above present state-of-the-art values around 50 cm2 V−1 s−1 to intrinsic limiting values around 100 cm2 V−1 s−1 by improved film processing, then substantial increases in transparency are possible whilst not sacrificing the high conductivity. The output optical power of a LED using an ITO current spreading layer with high n is also approximately proportional to μe, so mobility increases also have a direct impact on the external power efficiency of these devices.