Lateral Light-emitting Diodes Research Articles

Efficiency and Forward Voltage of Blue and Green Lateral LEDs with V-shaped Defects and Random Alloy Fluctuation in Quantum Wells

For nitride-based blue and green light-emitting diodes (LEDs), the forward voltage $V_\text{for}$ is larger than expected, especially for green LEDs. This is mainly due to the large barriers to vertical carrier transport caused by the total polarization discontinuity at multiple quantum well and quantum barrier interfaces. The natural random alloy fluctuation in QWs has proven to be an important factor reducing $V_\text{for}$. However, this does not suffice in the case of green LEDs because of their larger polarization-induced barrier. V-defects have been proposed as another key factor in reducing $V_\text{for}$ to allow laterally injection into multiple quantum wells (MQWs), thus bypassing the multiple energy barriers incurred by vertical transport. In this paper, to model carrier transport in the whole LED, we consider both random-alloy and V-defect effects. A fully two-dimensional drift-diffusion charge-control solver is used to model both effects. The results indicate that the turn-on voltages for blue and green LEDs are both affected by random alloy fluctuations and V-defect density. For green LEDs, $V_\text{for}$ decreases more due to V-defects, where the smaller polarization barrier at the V-defect sidewall is the major path for lateral carrier injection. Finally, we discuss how V-defect density and size affects the results.

Effect of carbon nanotube pattern on the laser lift off and quantum efficiencies of near UV vertical LEDs

Nowadays, ultraviolet light emitting diodes (UVLEDs) have aroused great interest in past few years for their promising application in adhesive curing, security identification and solid-state lighting etc. In gallium nitride (GaN) based LEDs, the patterned substrates and vertical devices' structure both played pivotal roles in elevating devices' performance. In this work, vertical near-UV-LEDs (405 nm) on different layers of carbon nanotube (CNT) patterns were fabricated by laser lift off (LLO), compared with the conventional lateral counterparts. The LLO threshold energy was reduced by CNTs layers. Although conventional lateral LEDs (LLEDs) exhibited highest external quantum efficiency (EQE) on 2CNTs pattern, the EQE of 3CNTs vertical LEDs (VLEDs) surpassed after LLO. The intrinsic physics mechanism, including: internal quantum efficiency (IQE), light extraction efficiency (LEE), Shockley-Read-Hall (SRH) recombination, radiative recombination and Auger effects, were analyzed by ‘ABC model’, Raman spectrum, X-ray Diffraction (XRD) and temperature dependence photoluminescence (TDPL).

Single-photon emission from single-electron transport in a SAW-driven lateral light-emitting diode

The long-distance quantum transfer between electron-spin qubits in semiconductors is important for realising large-scale quantum computing circuits. Electron-spin to photon-polarisation conversion is a promising technology for achieving free-space or fibre-coupled quantum transfer. In this work, using only regular lithography techniques on a conventional 15 nm GaAs quantum well, we demonstrate acoustically-driven generation of single photons from single electrons, without the need for a self-assembled quantum dot. In this device, a single electron is carried in a potential minimum of a surface acoustic wave (SAW) and is transported to a region of holes to form an exciton. The exciton then decays and creates a single optical photon within 100 ps. This SAW-driven electroluminescence, without optimisation, yields photon antibunching with g(2)(0) = 0.39 ± 0.05 in the single-electron limit (g(2)(0) = 0.63 ± 0.03 in the raw histogram). Our work marks the first step towards electron-to-photon (spin-to-polarisation) qubit conversion for scaleable quantum computing architectures.

High-power and reliable GaN-based vertical light-emitting diodes on 4-inch silicon substrate.

High-power and reliable GaN-based vertical light-emitting diodes (V-LEDs) on 4-inch silicon substrate were fabricated and characterized in this article. The metallization scheme reliability was improved by depositing the Pt/Ti films that surround the compressed Ag/TiW films to protect it from environmental humidity. We demonstrated that although current crowding in V-LEDs was not as severe as that in lateral light-emitting diodes (L-LEDs), high current density around the opaque metal n-electrode in V-LEDs remained a problem. A SiO2 current blocking layer (CBL) was incorporated in V-LEDs to modify the current distribution. Roughening the emitting surface of V-LEDs with KOH and H3PO4 etchant was compared and the influence of surface roughening on the emission property of V-LEDs was studied. The high-power V-LEDs showed low forward voltage with small series resistance and high light output power (LOP) without saturation up to 1300 mA. Under 350 mA injection current, V-LEDs achieved an excellent light output power (LOP) of 501 mW with the peak emission wavelength at 453 nm. The prominent output performance of V-LEDs demonstrated in this work confirmed that integrating the optimized metallization scheme, SiO2 CBL and surface texturing by KOH wet etching is an effective approach to higher performance V-LEDs.

Current diffusion and efficiency droop in vertical light emitting diodes* *Project supported by the National Key Research and Development Program of China (Grant No. 2018YFB0406702), the Professorship Start-up Funding (Grant No. 217056), the Innovation-Driven Project of Central South University, China (Grant No. 2018CX001), the Project of State Key Laboratory of High-Performance Complex Manufacturing, Central South University, China (Grant No. ZZYJKT2018-01),

Current diffusion is an old issue, nevertheless, the relationship between the current diffusion and the efficiency of light emitting diodes (LEDs) needs to be further quantitatively clarified. By incorporating current crowding effect (CCE) into the conventional ABC model, we have theoretically and directly correlated the current diffusion and the internal quantum efficiency (IQE), light extraction efficiency (LEE), and external quantum efficiency (EQE) droop of the lateral LEDs. However, questions still exist for the vertical LEDs (V-LEDs). Here firstly the current diffusion length L s(I) and L s(II) have been clarified. Based on this, the influence of CCE on the EQE, IQE, and LEE of V-LEDs were investigated. Specifically to our V-LEDs with moderate series resistivity, L s(III) was developed by combining L s(I) and L s(II), and the CCE effect on the performance of V-LEDs was investigated. The wall-plug efficiency (WPE) of V-LEDs ware investigated finally. Our works provide a deep understanding of the current diffusion status and the correlated efficiency droop in V-LEDs, thus would benefit the V-LEDs' chip design and further efficiency improvement.

The engagement of cortical areas preceding exogenous vergence eye movements.

Source analyses on event related potentials (ERPs) derived from the electroencephalogram (EEG) were performed to examine the respective roles of cortical areas preceding exogenously triggered saccades, combined convergences, and combined divergences. All eye movements were triggered by the offset of a central fixation light emitting diode (LED) and the onset of a lateral LED at various depths in an otherwise fully darkened room. Our analyses revealed that three source pairs, two located in the frontal lobe–the frontal eye fields (FEF) and an anterior frontal area–, and one located within the occipital cortex, can account for 99.2% of the observed ERPs. Overall, the comparison between source activities revealed the largest activity in the occipital cortex, while no difference in activity between FEF and the anterior frontal area was obtained. For all sources, increased activity was observed for combined vergences, especially combined convergences, relative to saccades. Behavioral results revealed that onset latencies were longest for combined convergences, intermediate for combined divergences, and the shortest for saccades. Together, these findings fit within a perspective in which both occipital and frontal areas play an important role in retinal disparity detection. In the case of saccades and combined divergences stimulus-locked activity was larger than response-locked activity, while no difference between stimulus- and response-locked activity was observed for combined convergences. These findings seem to imply that the electrophysiological activity preceding exogenous eye movements consists of a sensory-related part that is under cortical control, while subcortical structures may be held responsible for final execution.

(Electronics and Photonics Division Award Address) Technological Issues and Design Rules of Electrodes for High-Efficiency GaN-Based Light-Emitting Diodes

Group III-nitride-based LEDs are of great technological importance because of their applications including display, solid-state lighting, automotive headlight, and water/air purification. For these applications, the attainment of high external quantum efficiency (EQE) of LEDs is essential. On the one hand, their applications necessitate the fabrication of different geometry LEDs, such as lateral-type, vertical-type, and flip-chip LEDs, which require different design rules of electrodes and current spreaders. To maximize the EQE, it is important to increase the light extraction efficiency (LEE). In addition, the improvement of current injection (by forming low contact resistivity) and current spreading is vital for the fabrication of high-efficiency LEDs. It is well known that lateral LEDs suffer from the light extraction limit due to the absorption of the metal contacts, such as n and p contacts, and bond pads. In addition, p-GaN has characteristic problems, which make it difficult to achieve device-quality ohmic contacts with the specific contact resistance lower than ~10–4 Ωcm2 because of the difficulty in growing a highly doped p-GaN and the absence of appropriate metals or conducting oxides having work function larger than that of p-GaN. Thus, for the formation of ohmic and reflective contacts to p-GaN, also serving as light extractor and/or current spreader, transparent conducting oxides (TCOs)-based and Ag-based schemes have been extensively investigated. In this talk, we present methods of enhancing current injection efficiency through the formation of transparent and reflective ohmic electrodes. Design rules are suggested to enhance current injection efficiency, such as the control of surface Fermi-level through surface treatment, barrier tunneling by forming interfaces with inhomogeneous Schottky barriers, the modification of Schottky barrier heights (SBHs), control of native defects in GaN surface regions. Furthermore, nanostructures, such as Ag nano-dots (NDs) and Ag nano-wires (NWs), are used to enhance the light output powers of GaN-based LEDs. Finally, on the basis of scanning transmission electron microscopy, X-ray photoemission spectroscopy, secondary ion mass spectroscopy and Auger electron spectroscopy results, possible ohmic formation and electrical degradation mechanisms are described and discussed.

Degradation effect of Auger recombination and built-in polarization field on GaN-based light-emitting diodes

In this work, theoretical investigation of the influence of Auger recombination coefficient and built-in polarization field on the internal quantum efficiency (IQE) in terms of lateral-vertical single quantum well (SQW) as well as multiquantum well (MQW) GaN-based blue light-emitting diodes are presented. The degradation effect of the built-in polarization field on the IQE of vertical light-emitting diodes is used to strengthening the Auger recombination coefficient in comparison to lateral light-emitting diodes. This result has been found consistent in both single-as well as multi-quantum well structures. In addition, when Auger recombination coefficient has been included in the analysis, vertical multiquantum well structure shows more degradation in the IQE in comparison to the lateral structures. The effect has been dominant in vertical MQW case.

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.

Effect of device package on optical, spectral, and thermal properties of InGaN/GaN near-ultraviolet lateral light-emitting diodes

We investigated the package effect on the temperature-dependent optical and spectral characteristics of InGaN/GaN near-ultraviolet (NUV) lateral light-emitting diodes (LLEDs) on the metal heatsink (MH) and package (PKG) in the injection current range of 0 - 500 mA at 298 and 358 K. For the NUV LLEDs on the MH, the device characteristics reflected directly its chip performance. For the NUV LLEDs on the PKG, the rapidly varied spectral shift as well as the reduced device efficiency was observed due to the increased number of layers with relatively low thermal conductivities. The junction temperature (T j ) and thermal resistance of the NUV LLEDs on the PKG were also significantly increased compared to the NUV LLEDs on the MH. The three-dimensional heat transfer simulations for both the devices were carried out to obtain the temperature distributions by finite element method. The theoretically calculated T j values showed a good agreement with the experimentally measured T j values.

A Low-Temperature External Electron Retarding Electrode for Improving Vertical Green LED Performance

To alleviate the mismatch between electron/hole velocities and improve the quantum efficiencies, the cobalt-doped ZnO (CZO) dilute magnetic films grown by pulsed-laser deposition at a low temperature of 100 °C were served as the external electron retarding n-electrodes for vertical InGaN light-emitting diodes (LEDs). The retardation of the electron mobility is owing to the scatter of electrons via the spin-orbit interaction of Co2+ ions and their corresponding ferromagnetic properties. A 150-nm-thick CZO film was chosen as the n-electrode for the vertical green LED (530 nm). In comparison to conventional lateral LED, the vertical LEDs without and with the CZO n-electrode had 21.3% and 39.6% improvements in the output power (at 350 mA), respectively. The vertical LED with the CZO n-electrode showed an increment in the light output power (at 350 mA) by 15.1% as compared with the vertical LED without the CZO n-electrode. Obviously, after inserting the CZO n-electrode, the excessively large mobility difference between the electron and hole carriers in the conventional vertical LED is reduced significantly, which can decrease the nonradiative recombination rate and improve the emission characteristic. The results also reveal the CZO film served as an external electron retarding electrode is highly potential for vertical LED applications.

Improvement of luster consistency between the p-Pad and the n-Pad of GaN-based light-emitting diodes via the under-etching process

For improvement of the light extraction efficiency of GaN-based lateral light-emitting diodes (LEDs), a p-GaN surface was textured through a low-temperature (850 °C) p-GaN growth process. However, the p-GaN texturing process caused luster inconsistency between the n-pad and the p-pad due to the roughness difference between the indium-tin oxide (ITO) and the n-GaN beneath the pads, which decreased the image recognition rate and accuracy during the wire bonding process for LED packaging. Therefore, an under-etching process was proposed to improve the luster consistency between the p-pad and the n-pad of GaN-based LEDs with a naturally textured p-GaN surface. The under-etching process decreased the roughness of the exposed n-GaN surface from 109 nm to 73.1 nm, which was similar to the roughness (74.8 nm) of the ITO surface. Optical microscopy showed that LEDs with a naturally textured p-GaN surface exhibited excellent luster consistency between the n-pad and the p-pad after the proposed under-etching process had been applied. Further analysis indicated that the LEDs with a naturally textured p-GaN surface showed no degradation of optical or the electrical performance after the proposed under-etching process had been applied. At a 20-mA injection current, the light output power of a LED with naturally a textured p-GaN surface was 8.7% higher than that of a LED with a smooth p-GaN surface.

Reduced efficiency droop of nonpolar a-plane (11-20) GaN-based light-emitting diodes by vertical injection geometry

Vertical nonpolar a-plane (11-20) InGaN/GaN light-emitting diodes (LEDs) have been demonstrated by using laser lift-off technique. The forward voltage of the a-plane vertical LEDs was 4.3 V at 350 mA, which was reduced by 0.8 V compared to that of the a-plane lateral LEDs. The vertical geometry of the a-plane LEDs produced the higher quantum efficiency with a low efficiency droop and also enhanced the output power by more than 40%, when compared to those of a-plane lateral LEDs. These results can be attributed to the high thermal dissipation as well as uniform current spreading of the vertical geometry of the a-plane LEDs. Furthermore, elimination of the highly defected GaN nucleation layer after removing the sapphire substrates during the fabrication process can also enhance current injection efficiency, followed by the increase in the output power.

Influence of the quantum well models on the numerical simulation of planar InGaN/GaN LED results

Within this paper, we present electric model of a light emitting diode (LED) made of gallium nitride (GaN) followed by examples of simulation results obtained by means of Sentaurus software, which is the part of the TCAD package. The aim of this work is to answer the question of whether physical models of quantum wells used in commercial software are suitable for a correct analysis of the lateral LEDs made of GaN.

Hybrid Tunnel Junction-Graphene Transparent Conductive Electrodes for Nitride Lateral Light Emitting Diodes.

Graphene transparent conductive electrode (TCE) applications in nitride light emitting diodes (LEDs) are still limited by the large contact resistance and interface barrier between graphene and p-GaN. We propose a hybrid tunnel junction (TJ)-graphene TCE approach for nitride lateral LEDs theoretically and experimentally. Through simulation using commercial advanced physical models of semiconductor devices (APSYS), we found that low tunnel resistance can be achieved in the n(+)-GaN/u-InGaN/p(+)-GaN TJ, which has a lower tunneling barrier and an enhanced electric field due to the polarization effect. Graphene TCEs and hybrid graphene-TJ TCEs are then modeled. The designed hybrid TJ-graphene TCEs show sufficient current diffusion length (Ls), low introduced series resistance, and high transmittance. The assembled TJ LED with the triple-layer graphene (TLG) TCEs show comparable optoelectrical performance (3.99 V@20 mA, LOP = 10.8 mW) with the reference LED with ITO TCEs (3.36 V@20 mA, LOP = 12.6 mW). The experimental results further prove that the TJ-graphene structure can be successfully incorporated as TCEs for lateral nitride LEDs.

High-Performance Nitride Vertical Light-Emitting Diodes Based on Cu Electroplating Technical Route

GaN-based vertical-geometry light-emitting diodes (V-LEDs) are considered as ideal candidates for future high-power and high-efficiency lighting devices. However, the optoelectrical performance, yield, and stability of V-LEDs are still inferior than the conventional lateral LEDs (L-LEDs) due to some fabrication process hurdles, such as the p-type reflective electrode, nitrogen-polar (N-polar) n-type contact, surface texturization, insufficient current diffusion, and the foreign conductive substrate transfer. Here, we present a minireview of our long-term efforts on V-LEDs fabrication based on Cu electroplating foreign substrate transfer technical route. The electroplating approach shows its inherent advantages of moderation, flexibility, and reliability, yet still hampered by some technical hurdles specifically for V-LEDs fabrication: nonuniformity, easily thermal deformation, difficult to dice out. Since the foreign substrate transfer process is the most critical step, here, first, we introduced our efforts on Cu electroplating manipulation in detail: optimized the polishing and grinding process, Cu:W pseudoalloy and Cu/Ni bilayer electroplating, hybrid wet-etching plus dry laser scribing, and fully wet-etching approaches. Then, we summarized our investigation results for the p-type reflective electrode, nitrogen-polar (N-polar) n-type contact deposition, surface texturization, and current diffusion boost design. Based on the rationally manipulated process, the assembled V-LEDs show excellent optoelectrical performance: record-low forward voltage ( $V_{F}$ , 2.75 V at 350 mA, 3.04 V at 1000 mA, 1 mm2), low reverse leakage current ( $IR$ , 0.1– $0.25~\mu \text{A}$ at −10 V), high lumen efficiency (115 lm/W at 350 mA), low thermal resistance (RT, 1.58 °C/W for V-LEDs chip and 12.06 °C/W for V-LEDs lamp), high yield (>90%), and long-term stability.

Thermal Annealing Effects on the Performance of a Ga-Doped ZnO Transparent-Conductor Layer in a Light-Emitting Diode

To identify the individually optimized thermal annealing conditions for reducing the contact resistivity between highly Ga-doped ZnO (GaZnO) and doped-GaN and for improving the electrical and optical properties of GaZnO, the results of the one-step and two-step growth/annealing processes of GaZnO at various growth and thermal annealing temperatures are compared. The one-step (two-step) process corresponds to the condition of thermal annealing for the whole GaZnO layer (only for a 10-nm GaZnO layer of the first-step growth). The two-step process always results in lower contact resistivity on either p-GaN or n-GaN at any annealing temperature. Lateral and vertical light-emitting diodes (LEDs) with GaZnO layers on the top are fabricated to show that the LED samples with the two-step process have the lower device resistance levels, higher emission efficiencies, and weaker efficiency droop effects, when compared with those with the one-step process. In the lateral LED sample with the two-step process, the combination of the effective atomic interdiffusion at the GaZnO/p-GaN junction under the optimized annealing condition and the preservation of the superior electrical property in the major GaZnO layer without annealing leads to its better performance, when compared with that of the lateral LED sample with the one-step process.

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.

Thermally induced variations of strain condition and emission behavior in flat and bendable light-emitting diodes on different substrates.

The emission behaviors of four light-emitting diodes (LEDs) of different substrate structures, including a lateral LED grown on sapphire, a vertical LED wafer-bonded onto Si (111), a bendable LED Ag-epoxied onto a flat metal, and another bendable LED Ag-epoxied onto a metal of a curved surface, under different duty cycles of current injection are compared. Their different variation trends of emission behavior with injection duty cycle are attributed to the different thermally-induced strain conditions in the epitaxial layers, which are controlled by their substrate structures, in increasing injection duty cycle or current level. The results of Raman scattering measurements during LED operation show that a stronger tensile strain is generated under heating for reducing the quantum-confined Stark effect and hence increasing emission efficiency when the epitaxial layer is not tightly bonded onto a hard substrate. Such a behavior is particularly stronger when the epitaxial layer is bent.

(Invited) Vertical-Geometry GaN-Based Light-Emitting Diodes: Improving Current Injection and Light Extraction Efficiencies

For GaN-based light-emitting diodes (LEDs) that are expected to replace incandescent and fluorescent lighting lamps, chip-area and driving current are best optimized. To use such LEDs as a lighting source, effective heat dissipation is one of the major problems to be solved. The heat generated during operation can accelerate the aging degradation of LEDs. LEDs for solid-state lighting applications could be realized by vertical-type configuration having a robust conductive support where emitted light is extracted through the n-type semiconductor rather than the p-type semiconductor. Compared to conventional lateral LEDs, vertical LEDs (VLEDs) were known to show better performance in particular at high current injection due to the excellent heat dissipation capability. To fabricate wafer-level GaN-based vertical LEDs, the formation of a robust conductive supporter is vital. A great deal of efforts has been so far made to fabricate a robust conductive supporter using Si, Cu, and Ni mainly by wafer-bonding and electroplating techniques. In this study, a multi-functional bonding material system, which consists of thick Cu layer and a Sn-based bonding layer, was employed to fabricate wafer level GaN-based VLEDs. It is shown that fully packaged VLEDs fabricated using the multi-functional bonding material system give an operating voltage of <3.35 V at 350 mA and after 1500 hrs, their reverse currents remain stable.Furthermore, VLEDs require the formation of low resistance and thermally stable ohmic contacts to N-polar n-type GaN and to Ga-polar p-type GaN. However, unlike n-ohmic contacts to Ga-polar n-GaN, n-ohmic contacts to N-polar GaN are usually known to produce poor electrical characteristics. Thus, in this study, we investigated the electrical characteristics of Ohmic contacts to N-polar n-GaN. To form reliable n-type electrodes to N-polar n-GaN, different fabrication processes, such as laser-annealing, use of a blocking layer were employed. The results of the designed n-type electrodes are much more stable than conventional untreated samples. Based on the X-ray photoemission spectroscopy, Auger electron spectroscopy, transmission electron microscopy, and secondary ion mass spectroscopy results, Ohmic formation mechanisms for the N-polar contact are described and discussed.In addition, to enhance the light extraction, the formation of Ga-polar p-type reflectors with high reflectance and low contact resistance is essential. Because of the reasonable electrical property, Ag is the most frequently used reflector. However, Ag only reflector suffers from thermal degradation when annealed above 300ºC in air. Thus, in this study, we presented ways of improving both the thermal and electrical properties of Ag-based reflectors. It is shown that the use of interlayers, middle layers, and capping layers significantly improves the optical reflectance as well as the electrical properties. The combined Ag-based reflectors give better electrical property than the Ag only sample. Blue LEDs fabricated with the annealed combined Ag-based reflectors show a lower forward voltage at 20 mA than LEDs with the annealed Ag only contacts. On the basis of scanning electron microscopy and X-ray pole figure and phi scan results, possible mechanisms for the electrical and thermal improvement are described and discussed.