Current Spreading Layer Research Articles

Enhancing the Static and Dynamic Performance of High-Speed VCSELs by Zn-Diffused Shallow Surface Relief Apertures

In this paper, we demonstrate a novel structure for 850- and 940-nm wavelength high-speed vertical-cavity surface-emitting lasers (VCSELs). Extra shallow apertures (~20 nm) are etched on the topmost current spreading (CS) layer of 850- or 940-nm VCSELs, which have Zn-diffusion and oxide-relief apertures inside. Such a structure simultaneously allows a significant enhancement of the output power and a reduction in the number of optical modes in the optical spectrum, which migrates toward the quasi-single-mode (QSM). Comparison is made to multi-mode (MM) reference VCSELs produced without etching of the CS layer. The etched devices exhibit a larger signal-to-noise ratio for error-free 32 Gbit/s transmission over 100-m MM fibers (MMFs) at both wavelengths (850 and 940 nm). In addition, the dynamic/static performance of the etched samples is also superior to that of a QSM reference sample, produced by utilizing only Zn-diffusion apertures without etching of the CS layer. The demonstrated device structure opens the door to greatly improve the performance of SM and high-power VCSELs for high-speed data transmission.

Lateral Current Spreading in III-N Ultraviolet Vertical-Cavity Surface-Emitting Lasers Using Modulation-Doped Short Period Superlattices

Lateral hole injection into AlGaN-based ultraviolet (UV) vertical-cavity light-emitting lasers (VCSELs) has been studied via numerical simulation. For blue and violet vertical cavity light emitters, indium tin oxide (ITO) is most commonly used as a transparent current spreading layer to increase the overlap between the optical mode and the radial current profile. However, ITO has very high optical losses in the UV spectrum, so alternative schemes for lateral current spreading have been investigated for use in UV-VCSELs. A modulation doped short-period superlattice (MD-SPSL) has been proposed as a transparent lateral current spreading layer in UV-VCSELs. The narrow bandgap unintentionally doped (uid) material maintains a high mobility due to reduced impurity scattering and has a high free hole concentration due to modulation doping, thus forming highly conductive channels which aid lateral hole transport. This has been shown to partially mitigate current crowding around the current aperture. To account for imperfect modulation doping due to the magnesium memory effects and other factors, the effect of varying the hole mobility in the uid-narrow bandgap layer of the MD-SPSL from 13–300 cm2/( $\text {V}\cdot \text {s}$ ) on the threshold current and slope efficiency has also been studied. Employing an MD-SPSL results in a significant reduction in the threshold current and slope efficiency compared to ITO, and the extent of the improvement depends on the hole mobility in the uid-AlGaN layer.

Direct formation of transfer-free graphene as current spreading layers on n-ZnO nanorods/p-GaN light-emitting diodes

Transfer-free graphene has been directly grown on ZnO nanorods (NRs)/p-GaN LEDs by a feasible approach involving rapid thermal annealing of amorphous carbon and nickel layers. Compared with conventional ZnO NRs/p-GaN LEDs without a current spreading layer, the current–voltage characteristic and electroluminescence performance are enhanced, mainly attributed to more efficient current spreading by the transfer-free graphene that conformally covers the entire surface of ZnO NRs, leading to better carrier injection. It is hence expected that our scheme will be of great importance for nanostructured LED arrays with large-scale fabrication.

InGaN/GaN ultraviolet LED with a graphene/AZO transparent current spreading layer

We report an approach of using an interlayer of single layer graphene (SLG) for electroluminescence (EL) enhancement of an InGaN/GaN-based near-ultraviolet (NUV) light-emitting diode (LED) with an aluminum-doped zinc oxide (AZO)-based current spreading layer (CSL). AZO-based CSLs with and without a SLG interlayer were fabricated on the NUV LED epi-wafers. The current-voltage (I-V) characteristic and the EL intensity were measured and compared. We find that the LED without the SLG interlayer can possess a 40% larger series resistance. Furthermore, a 95% EL enhancement was achieved by the employment of the SLG interlayer.

Development of a High-Performance 3,300V Silicon Carbide MOSFET

To address stringent performance and reliability requirements of industrial and traction power conversion systems we have developed planar 3,300V MOSFETs at a 6-inch SiC-compatible silicon CMOS foundry. By optimizing the unit cell structure and using a deep current-spreading layer we demonstrated a low MOSFET specific on-resistance RDSA=11.2 mΩ·cm2 (ID=5A, VGS=15V) and fast switching for the baseline design. Robust short-circuit handling (7.5μs at Vds=1500V and 5.0μs at Vds=2200V) was demonstrated with an alternative unit cell design with RDSA=14.8 mΩ·cm2 (ID=5A, VGS=15V).

Graphene as current spreading layer on AlGaInP light emitting diodes

Due to high transmittance and high mobility, graphene is one of the promising candidates for a current spreading layer, which is crucial to light emitting diode (LED) performance. In this paper, improved AlGaInP LED performance was reported after graphene was applied on the GaP surface. Due to its lowered work function difference than with the GaN material, the electrical properties remain the same without additional voltage bias. The light output power is enhanced by about 40% under the current injection of 5 mA at room temperature, which was confirmed by the light emission profile analysis in this study. Such results indicate that raphene is a promising candidate as a current spreading layer under low current injection.

Performance optimization of AlGaN-based LEDs by use of ultraviolet-transparent indium tin oxide: Effect of in situ contact treatment

Ultraviolet (UV)-transparent indium tin oxide (ITO) grown by metal–organic chemical vapor deposition (MOCVD) is used as the current-spreading layer for 368 nm AlGaN-based light-emitting diodes (LEDs). By performing in situ contact treatment on the LED/ITO interface, the morphology, resistivity, and contact resistance of electrodes become controllable. Resistivity of 2.64 × 10−4 Ω cm and transmittance at 368 nm of 95.9% are realized for an ITO thin film grown with Sn-purge in situ treatment. Therefore, the high-power operating voltage decreases from 3.94 V (without treatment) to 3.83 V (with treatment). The improved performance is attributed to the lowering of the tunneling barrier at the LED/ITO interface.

Influence of photo-generated carriers on current spreading in double diode structures for electroluminescent cooling

Current crowding close to electrical contacts is a common challenge in all optoelectronic devices containing thin current spreading layers (CSLs). We analyze the effects of current spreading on the operation of the so-called double diode structure (DDS), consisting of a light emitting diode (LED) and a photodiode (PD) fabricated within the same epitaxial growth process, and providing an attractive platform for studying electroluminescent (EL) cooling under high bias conditions. We show that current spreading in the common n-type layer between the LED and the PD can be dramatically improved by the strong optical coupling between the diodes, as the coupling enables a photo-generated current through the PD. This reduces the current in the DDS CSL and enables the study of EL cooling using structures that are not limited by the conventional light extraction challenges encountered in normal LEDs. The current spreading in the structures is studied using optical imaging techniques, electrical measurements, simulations, as well as simple equivalent circuit models developed for this purpose. The improved current spreading leads further to a mutual dependence with the coupling efficiency, which is expected to facilitate the process of optimizing the DDS. We also report a new improved value of 63% for the DDS coupling quantum efficiency.

Nano-imprinting of refractive-index-matched indium tin oxide sol-gel in light-emitting diodes for eliminating total internal reflection.

Refractive-index (RI)-matched nanostructures are implemented in GaN-based light-emitting diodes (LEDs) for enhancing light output efficiency. The RI-matched indium tin oxide (ITO) nanostructures are successfully implemented in GaN-based lateral LEDs by using ITO sol–gel and nanoimprint lithography. The ITO sol–gel nanostructures annealed at 300 °C have RI of 1.95, showing high transparency of 90% and high diffused transmittance of 34%. Consequently, the light output power in LEDs with the RI-matched nanostructures increases by 8% in comparison with that in LEDs containing flat ITO. Ray tracing and finite-difference time-domain (FDTD) simulations show that the RI-matched nanostructures on the transparent current spreading layer dramatically reduce Fresnel reflection loss at the interface of the current spreading layer with the nanostructure and extract confined waveguide lights in LEDs.

Optimization design of transparent conductive Al-doped ZnO and mechanism for performance-enhancing GaN light emitting diodes

The current crowding effect is one of the key factors that reduce the performance of light emitting diodes (LEDs). The transparent Al-doped ZnO (AZO) film, which was employed as a current spreading layer for a GaN-based blue LED, had been prepared and optimized by adjusting deposition parameters. Experimental results showed an obvious enhancement of the electroluminescence performance of the LEDs by using the highly transparent and low-resistance AZO layer. The mechanisms were discussed on the basis of the optical and electrical characteristics. The ANSYS simulation analysis proved that the optimized AZO layer weakened the current crowding effect in the GaN-based blue LED.

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.

ITO/AlN rod-based hybrid electrodes: effect of buffer layers in AlN rods on performance of 365-nm light-emitting diodes

In this study, hybrid electrodes consisting of AlN rod arrays and surrounding indium tin oxide (ITO) films are proposed, and the effect of buffer layers in AlN rods on the performance of 365-nm light-emitting diodes (LEDs) is investigated. The AlN rod arrays were introduced in the form of ITO/AlN/buffer layers (Cr/Ni or thin ITO) to reduce the area and voltage for electrical breakdown (EBD), which is used to form conductive channels. The surrounding ITO film was used as a current spreading layer. Using this electrode design, we observed improved ohmic behavior and a higher transmittance at 365 nm compared to those of the reference ITO. In addition, both conduction and ohmic conduction mechanisms in the structure comprising a metal, AlN rod film, and p-AlGaN surface were investigated using various analysis tools. As a result, the 365-nm LEDs with thin Cr/Ni buffer layers exhibited a significantly higher light output power, lower forward voltages, and lower leakage currents than the LEDs with thin ITO buffer layers and reference ITOs.

Reduction of on-resistance and current crowding in quasi-vertical GaN power diodes

This paper studies the key parameters affecting on-resistance and current crowding in quasi-vertical GaN power devices by experiment and simulation. The current distribution in the drift region, n−-GaN, was found to be mainly determined by the sheet resistance of the current spreading layer, n+-GaN. The actual on-resistance of the drift region significantly depends on this current distribution rather than the intrinsic resistivity of the drift layer. As a result, the total specific on-resistance of quasi-vertical diodes shows a strong correlation with the device area and sheet resistance of the current spreading layer. By reducing the sheet resistance of the current spreading layer, the specific on-resistance of quasi-vertical GaN-on-Si power diodes has been reduced from ∼10 mΩ·cm2 to below 1 mΩ·cm2. Design space of the specific on-resistance at different breakdown voltage levels has also been revealed in optimized quasi-vertical GaN power diodes.

The role of ITO resistivity on current spreading and leakage in InGaN/GaN light emitting diodes

The effect of a transparent ITO current spreading layer on electrical and light output properties of blue InGaN/GaN light emitting diodes (LEDs) is discussed. When finite conductivity of ITO is taken into account, unlike in previous models, the topology of LED die and contacts are shown to significantly affect current spreading and light output characteristics in top emitting devices. We propose an approach for calculating the current transfer length describing current spreading. We show that an inter-digitated electrode configuration with distance between the contact pad and the edge of p-n junction equal to transfer length in the current spreading ITO layer allows one to increase the optical area of LED chip, as compared to the physical area of the die, light output power, and therefore, the LED efficiency for a given current density. A detailed study of unpassivated LEDs also shows that current transfer lengths longer than the distance between the contact pad and the edge of p-n junction leads to increasing surface leakage that can only be remedied with proper passivation.

Characteristics of GaN-Based LEDs With Hybrid Microhole Arrays and SiO2Microspheres/Nanoparticles Structures

The Characteristics of GaN-based LEDs with hybrid microhole arrays and SiO2 microspheres (MSs)/ nanoparticles (NPs) are comprehensively studied. The SiO2 MSs/NPs antireflection coating, deposited by a rapid convection deposition, acts as a passivation layer of GaN-based LEDs. Since the critical angle could be enlarged by antireflection coating, Fresnel reflection could be reduced. In addition, due to the roughened surface of SiO2 MSs/NPs antireflection coating, the scattering effect could also be increased. Thus, the light extraction efficiency could be further enhanced. As compared with a conventional LED with a planar aluminum-doped zinc oxide current spreading layer (Device A), the studied device with the proposed hybrid structure and a sputtered SiO2 passivation layer (Device E) causes a suppressed leakage current and % enhancements on light output power, external quantum efficiency, and wall-plug efficiency performance.

Zinc oxide-based current spreading layer behavior on the performance of P-side-up thin-film red light emitting diodes

Transparent conductive layers deposited on a GaP window layer were used to fabricate high-brightness p-side-up thin-film AlGaInP light emitting diodes (LEDs). Zinc oxide (ZnO) and aluminum-doped zinc oxide (ZAO) with an Zn:Al cycle ratio of 20:1 were grown by atomic layer deposition for comparison. The ZAO thin films reduced the droop in the external quantum efficiencies of the LEDs as well as the junction temperature, which result in increases in the light output power and thermal stability of the LEDs. The efficiency droops of the LEDs with the ZnO and ZAO thin films were 41% and 15%, respectively. The junction temperature of the LED with the ZAO thin film can be reduced to 39.6°C at an injection current of 700mA, which is lower than that of the LED with the ZnO thin film (58.1°C). The above results are promising for the development of ZAO thin films for applications in AlGaInP LEDs.

High-Performance GaN-Based LEDs With AZO/ITO Thin Films as Transparent Contact Layers

The Al-doped ZnO (AZO)/indium tin oxide (ITO) bilayer films were proposed as transparent contact layers (TCLs) for the fabrication of GaN-based light-emitting diodes (LEDs). In TCLs on the p-type GaN layer, the ITO film serves as the ohmic contact layer with the thickness of only 20 nm, whereas the 300–750nm AZO films act as the current spreading layer. At an optimal AZO thickness of 500 nm, GaN-based LEDs with AZO/ITO TCLs have a forward voltage of 3.36 V, an electroluminescence emission at 526 nm with highest brightness and a large light intensity of 386 mcd at 20 mA, which are almost the same as the commercial LEDs with a 300-nm ITO TCL. The 500-nm AZO/20-nm ITO bilayer films exhibit a high transparency above 90% in the visible region, but they display a low conductivity with resistivity $\sim 7 \times 10^{-3}~\Omega \cdot $ cm. The success of GaN-based LEDs with such a relatively high-resistivity electrode strongly demonstrates that the AZO/ITO film is a very effective and practical TCL on the p-type GaN layer. The indium-saving AZO/ITO TCLs may be actually a universal approach as p-type electrodes in high-performance GaN-based LEDs for commercially mass production with low cost.

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.

AlGaInP-Based LEDs With Al-doped ZnO Transparent Conductive Layer Grown by MOCVD

In this paper, Al-doped ZnO (AZO) grown by metal-organic chemical vapor deposition (MOCVD) using H2O as an oxidizer was employed as a transparent conductive layer (TCL) in AlGaInP-based light-emitting diodes (LEDs). A direct ohmic contact structure was observed between the AZO and the carbon-doped p+-GaP (p+-GaP:C) contact layer with a specific contact resistance of $\mathsf {2.2} \times \mathsf {10}^{{\mathsf {-4}}} $ cm $^{{\mathsf {2}}}$ . It is attributed to the unintentional hydrogen-doped n++-AZO interfacial layer at the clear interface of the AZO/p+-GaP:C. A forward voltage ( ${V}_{f}$ ) of 1.99 V at 20 mA was achieved, which is lower than that of the ITO-TCL LED with a ${V}_{f}$ of 2.07 V. In addition, compared with the conventional LED without TCL and the ITO-TCL LED, the studied LED demonstrates high output power and high wall-plug efficiencies. These results indicate that the AZO-TCL grown by MOCVD using H2O as an oxidizer is an effective current spreading layer for high-performance AlGaInP-based LED application.

Impact of AlN-aperture on optical and electrical properties of nitride VCSEL

This paper reports a numerical analysis of nitride VCSELs with indium-tin-oxide current spreading layers and AlN apertures. The results show that the thickness, location and diameter of the AlN aperture significantly influence the optical and electrical properties of the VCSEL.