GaN-based Vertical Light-emitting Diodes Research Articles

Numerical Investigation of Light Extraction Efficiency of GaN-based Vertical Blue Micron-Scale Light-Emitting Diode Structures.

The light extraction efficiency (LEE) of GaN-based vertical blue micron-scale light-emitting diode (μ-LED) structures was investigated numerically using three-dimensional finite-difference timedomain (FDTD) methods. The entire μ-LED chip was included in the FDTD computational domain to determine the LEE accurately. As the lateral dimensions of μ-LEDs increased from 5 to 30 μm, the LEE decreased gradually because of the increased portion of light trapped inside the LED chip and the increased light absorption in the GaN layers with increasing chip size. The LEE varied strongly with the p-GaN thickness for the μ-LED with a flattop surface, which could be explained by the strong dependence of the spatial distribution of the emission patterns on the p-GaN thickness. This dependence on the p-GaN thickness decreased when the surface of the μ-LED chip was patterned. A high LEE of >80% could be achieved in LEDs with properly chosen parameters. The FDTD simulation results presented in this study are expected to be employed advantageously in designing μ-LED structures with a high LEE.

Stripping GaN/InGaN epitaxial films and fabricating vertical GaN-based light-emitting diodes

The growth technology of GaN-based light-emitting-diodes (LEDs) greatly restricts their preparation of vertical structure. Now we can get GaN-based LEDs structure on silicon (Si) substrate, which makes us a big step forward in the preparation of vertical GaN-based LEDs. Herein, we report the fabrication of vertical GaN-based LEDs through a chemical etching process to removing Si substrate combing with inductively coupled plasma reactive ion etching (ICP-RIE) to remove the un-doped buffer followed by copper (Cu) electroplating technology. By collecting photoluminescence (PL) and Raman scattering spectra of the GaN-based epitaxial structure, it is confirmed that the residual strain between the epitaxial structure and growth substrate is effectively released. The corresponding electroluminescence (EL) measurement of the vertical GaN-based LEDs was conducted by using a probing test with an optical power meter, showing a good electrical property and optical property. The vertical GaN-based LEDs have better output power and better heat dissipation capacity. This research proposes a low-cost method to fabricate freestanding and high performance vertical GaN-based LEDs.

Optical Characterization of GaN-Based Vertical Blue Light-Emitting Diodes on P-Type Silicon Substrate

Fabricating GaN-based light-emitting diodes (LEDs) on a silicon (Si) substrate, which is compatible with the widely employed complementary metal–oxide–semiconductor (CMOS) circuits, is extremely important for next-generation high-performance electroluminescence devices. We conducted a systematic investigation of the optical properties of vertical LEDs, to reveal the impacts of the manufacturing process on their optical characteristics. Here, we fabricated and characterized high-efficiency GaN-based LEDs with integrated surface textures including micro-scale periodic hemispherical dimples and nano-scale random hexagonal pyramids on a 4 inch p-type Si substrate. The highly reflective Ag/TiW metallization scheme was performed to decrease downward-absorbing light. We demonstrated the influence of transferring LED epilayers from a sapphire substrate onto the Si substrate on the emission characteristics of the vertical LEDs. The removal of the sapphire substrate reduced the adverse impacts of the quantum-confined Stark effect (QCSE). The influence of integrated surface textures on the light extraction efficiency (LEE) of the vertical LEDs was studied. With the injection current of 350 mA, vertical LEDs with integrated surface textures demonstrated an excellent light output power of 468.9 mW with an emission peak wavelength of 456 nm. This work contributes to the integration of GaN-based vertical LEDs into Si-based integrated circuits.

Light Extraction Efficiency of GaN-Based Micro-Scale Light-Emitting Diodes Investigated Using Finite-Difference Time-Domain Simulation

We conducted a systematic investigation of the light extraction efficiency (LEE) of GaN-based vertical micro-scale light-emitting diode (μ-LED) structures using three-dimensional finite-difference time-domain (FDTD) simulations. The LEE of μ-LED structures was found to have a strong dependence on structural parameters such as the shape of the chip cross section, chip dimension, and p-GaN thickness. The LEE of a μ-LED with a circular cross section was lower than that of a μ-LED with a square cross section by 5∼10% owing to the coupling of light with high-quality-factor whispering gallery modes. The LEE of a μ-LED structure decreased as the chip dimension increased, which could be attributed to the increased portion of trapped light inside the LED chip and increased light absorption in the GaN with an increasing chip dimension. In addition, the LEE varied significantly with the thickness of the p-GaN layer, which could be explained by the strong dependence of the angular distribution of the emission pattern on the p-GaN thickness. The FDTD simulation method presented in this study is expected to be advantageously employed in designing μ-LED structures with high LEE.

Rectification behavior of polarization effect induced type-II n-GaN/n-type β-Ga2O3 isotype heterojunction grown by metal organic vapor phase epitaxy

We demonstrated the growth of high-quality and compressive-stressed single-crystal c-GaN on (2¯01) β-Ga2O3 utilizing metal organic vapor phase epitaxy. Rectification behavior of the n-GaN/n-Ga2O3 isotype heterojunction was observed. Valence and conduction band offsets were determined to be 1.625 eV ± 0.07 eV and –0.165 eV ± 0.07 eV, respectively, confirming a type-II heterojunction due to the polarization effect. The extracted forward (0.702 V) and reverse (0.178 V) turn-on voltages could be ascribed to the band bending (0.695 eV) effect and the conduction band offset, respectively. The present work may lead to a deeper understanding of the GaN/β-Ga2O3 heterojunction and play a guiding role in the development of the GaN-based vertical structure light-emitting diodes on β-Ga2O3.

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.

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.

Efficient performance enhancement of GaN-based vertical light-emitting diodes coated with N-doped graphene quantum dots

Novel vertical light-emitting diodes (VLEDs) decorated with N-doped graphene quantum dots (N-GQDs) have been fabricated, based on the metal organic chemical vapor deposition method (MOCVD), wafer bonding technique, laser lift-off (LLO) technique, inductively coupled plasma (ICP) etching and impregnation processing. There is an obvious luminescence enhancement and resistance reduction for VLEDs after coating N-GQDs. The results show an improvement of light output power (LOP), dependent on the improvement of optical extraction efficiency and current spreading. The improvement of external quantum efficiency (EQE) is mainly owing to the roughened surfaces. Meanwhile, an enhancement of electroluminescence (EL) intensity and photoluminescence (PL) intensity has been achieved when the VLEDs were immersed in 0.9 mg/mL of N-GQDs solution, 4 times that of bare VLEDs. After coating N-GQDs, the performance of VLEDs is greatly improved, which could be attributed to the photons recovered by extracting light from waveguide modes.

Effect of interfacial composition on Ag-based Ohmic contact of GaN-based vertical light emitting diodes

By investigating the effect of a defective interface structure on Ag-based Ohmic contact of GaN-based vertical light-emitting diodes, we found a direct relationship between the interfacial composition and the Schottky barrier height of the Ag(111)/GaN(0001) interface. It was demonstrated that the Schottky barrier height of a defect-free Ag(111)/GaN(0001) interface was 2.221 eV, and it would be dramatically decreased to 0.375 eV with the introduction of one Ni atom and one Ga vacancy at the interface structure. It was found that the tunability of the Schottky barrier height can be attributed to charge accumulations around the interfacial defective regions and an unpinning of the Fermi level, which explains the experimental phenomenon of Ni-assisted annealing improving the p-type Ohmic contact characteristic. Lastly, we propose a new method of using Cu as an assisted metal to realize a novel Ag-based Ohmic contact. These results provide a guideline for the fabrication of high-quality Ag-based Ohmic contact of GaN-based vertical light-emitting diodes.

High-performance vertical GaN-based near-ultraviolet light-emitting diodes on Si substrates

High-performance vertical GaN-based near-ultraviolet (UV) light-emitting diodes (LEDs) on Si substrates with an electroluminescence emission wavelength of 395 nm have been fabricated by designing epitaxial structures to reduce the dislocation density and enhance the electron confinement and hole injection.

Performance-improved vertical GaN-based light-emitting diodes on Si substrates through designing the epitaxial structure

Performance-improved vertical GaN-based light-emitting diodes (LEDs) have been fabricated on Si substrates through designing the epitaxial structures with a combination of an AlN interlayer and a SiNxinterlayer.

High-Efficiency InGaN/GaN Quantum Well-Based Vertical Light-Emitting Diodes Fabricated on β-Ga2O3 Substrate.

We demonstrate a state-of-the-art high-efficiency GaN-based vertical light-emitting diode (VLED) grown on a transparent and conductive (-201)-oriented (β-Ga2O3) substrate, obtained using a straightforward growth process that does not require a high-cost lift-off technique or complex fabrication process. The high-resolution scanning transmission electron microscopy (STEM) images confirm that we produced high quality upper layers, including a multiquantum well (MQW) grown on the masked β-Ga2O3 substrate. STEM imaging also shows a well-defined MQW without InN diffusion into the barrier. Electroluminescence (EL) measurements at room temperature indicate that we achieved a very high internal quantum efficiency (IQE) of 78%; at lower temperatures, IQE reaches ∼86%. The photoluminescence (PL) and time-resolved PL analysis indicate that, at a high carrier injection density, the emission is dominated by radiative recombination with a negligible Auger effect; no quantum-confined Stark effect is observed. At low temperatures, no efficiency droop is observed at a high carrier injection density, indicating the superior VLED structure obtained without lift-off processing, which is cost-effective for large-scale devices.

On the metallic bonding of GaN-based vertical light-emitting diode

In this work, metallic bonding in GaN-based vertical light-emitting diode (VLED) is systematically characterized by using combined methodology of transmission Kikuchi diffraction (TKD) and energy dispersive X-ray spectroscopy (EDS) in a scanning electron microscope (SEM). SEM-based TKD with EDS identifies chemical composition, grain morphology, orientation, and phases at metallic bonding, while transmission electron microscopy (TEM) provides nanoscale characteristics of metallic diffusion bonding, and its interface-related defects and nano-twinned boundaries. Our results from SEM-TKD and TEM techniques provide unparalleled insight into the metallic bonding, and its future optimization.

Effect of plasma surface treatment on embedded n-contact for GaN-based blue light-emitting diodes grown on Si substrate

Unlike the finger-like n-contact that is prepared after the wafer bonding and the N-polar GaN surface roughening for GaN-based vertical structure light-emitting diodes (LEDs) grown on Si substrates, the embedded via-like n-contact is formed prior to the wafer bonding. The high temperature process of the wafer bonding often causes the electrical characteristics of the via-like embedded n-contact to degrade. In this paper, we study in detail the effect of plasma treatment of the n-GaN surface on the forward voltage of GaN-based LED grown on Si substrate. It is shown that with no plasma treatment on the n-GaN surface, the forward voltage (at 350 mA) of the 1.1 mm1.1 mm chip with a highly reflective electrode of Cr (1.1 nm)/Al is 3.43 V, which is 0.28 V higher than that of the chip with a pure Cr-based electrode. The LED forward voltages for both kinds of n-contacts can be reduced by an O2 plasma treatment on the n-GaN surface. But the LED forward voltage with a Cr/Al-based electrode is still 0.14 V higher than that of the chips with a pure Cr-based electrode. However, after an Ar plasma treatment on the n-GaN surface, the LED forward voltage with a Cr/Al-based electrode is reduced to 2.92 V, which is equal to that of the chip with a pure Cr-based electrode. The process window of the n-GaN surface after the Ar plasma treatment is broader. X-ray photoelectron spectroscopy is used to help elucidate the mechanism. It is found that Ar plasma treatment can increase the concentration of N-vacancies (VN) at the n-GaN surface. VN acts as donors, and higher VN helps improve the thermal stability of n-contact because it alleviates the degradation of the n-contact characteristics caused by the high temperature wafer bonding process. It is also found that the O content increases slightly after the Ar plasma treatment and HCl cleaning. The O atoms are mainly present in the dielectric GaOx film before the Ar plasma treatment and the HCl cleaning, and they exist almost equivalently in the conductive GaOxN1-x film and the dielectric GaOx film after Ar treatment and HCl cleaning. The conductive GaOxN1-x film and the VN donors formed during the plasma treatment can reduce the contact resistance and the LED forward voltage.

Investigation of n-ohmic contact of vertical GaN-based light-emitting diodes on graphite substrate with Ag-In bonding

Vertical light-emitting diodes (VLEDs) were successfully transferred from a GaN-based sapphire substrate to a graphite substrate by using low-temperature and cost-effective Ag-In bonding, followed by the removal of the sapphire substrate using a laser lift-off (LLO) technique. One reason for the high thermal stability of the AgIn bonding compounds is that both the bonding metals and Cr/Au n-ohmic contact metal are capable of surviving annealing temperatures in excess of 600°C. Therefore, the annealing of n-ohmic contact was performed at temperatures of 400°C and 500°C for 1min in ambient air by using the rapid thermal annealing (RTA) process. The performance of the n-ohmic contact metal in VLEDs on a graphite substrate was investigated in this study. As a result, the final fabricated VLEDs (chip size: 1000µm×1000µm) demonstrated excellent performance with an average output power of 538.64mW and a low operating voltage of 3.21V at 350mA, which corresponds to an enhancement of 9.3% in the light output power and a reduction of 1.8% in the forward voltage compared to that without any n-ohmic contact treatment. This points to a high level of thermal stability and cost-effective Ag-In bonding, which is promising for application to VLED fabrication.

Device characteristics and thermal analysis of GaN-based vertical light-emitting diodes with different types of packages

We investigated the device characteristics of GaN-based blue vertical light-emitting diodes (VLEDs) with two different package structures (i.e., lead frame with metal/plastic body (MPLF package) and lead frame with metal body (MLF package)) under various measurement conditions. In comparison with the MPLF packaged VLEDs, the MLF packaged VLEDs exhibited relatively lower junction temperature and thermal resistance values due to the better heat dissipation capability, leading to further improved optical, spectral, and thermal device characteristics. Thermal simulations of the VLEDs with two different packages were performed using three-dimensional steady-state device models to theoretically calculate their thermal and mechanical behaviours. The maximum temperatures, internal temperature distributions, and thermomechanical stresses were analysed by a finite element method.

Enhanced light extraction of GaN-based vertical LEDs with patterned trenches and nanostructures

An efficient surface texturing technique that uses patterned trench etching and the selective formation of GaN nanostructures on the trench bottoms to improve the light extraction of vertical GaN-based light-emitting diodes (VLEDs) is proposed and demonstrated. Theoretical and experimental results that show the effectiveness of the proposed surface roughening scheme in improving light output power (LOP) and wall-plug efficiency (WPE) are presented and discussed. Compared with conventional VLEDs, significant improvements in LOP and WPE at 350mA of about 37.6% and 5.1%, respectively, are obtained for the proposed VLEDs. The effective lateral light emission harvested by patterned trenches and the strongly enhanced angular randomization of photons that minimizes the total internal reflection at the GaN/air interface are responsible for the LOP and WPE improvements.

Enhanced wall-plug efficiency in monolithically integrated vertical light-emitting-diode cells based on III-nitride heterostructures

This work presents the enhancement of the wall-plug efficiency in GaN-based vertical light-emitting diodes (LEDs) by splitting into nine LED cells. The cells are monolithically integrated and connected in series to develop high-voltage vertical LEDs (HV-VLEDs). Wall-plug efficiency at an input power of 1 W for the optimum-design HV-VLEDs is improved to 43.5% from the value of 40.6%, as compared to that of conventional VLED with the same active device area. The result indicates that the HV-VLEDs with uniform current spreading improve the conversion efficiencies of VLEDs and are expected to be more beneficial in larger-chip-size and higher-power LEDs.

Titanium oxide nanotube arrays for high light extraction efficiency of GaN-based vertical light-emitting diodes.

TiO2 nanotube (NT) arrays were fabricated on the surface of n-GaN through a liquid-phase conversion process using ZnO nanorods (NRs) as a template for high-efficiency InGaN/GaN multiple quantum well (MQW) vertical light-emitting diodes (VLEDs). The optical output power of the VLEDs with TiO2 NTs was remarkably enhanced by 23% and 189% at an injection current of 350 mA compared to those of VLEDs with ZnO NRs and planar VLEDs, respectively. The large enhancement in optical output is attributed to a synergistic effect of efficient light injection from the n-GaN layer of the VLED to TiO2 NTs because of the well-matched refractive indices and superior light extraction into air at the end of the TiO2 NTs. Light propagation along various configurations of TiO2 NTs on the VLEDs was investigated using finite-difference time domain simulations and the results indicated that the wall thickness of the TiO2 NTs should be maintained close to 20 nm for superior light extraction from the VLEDs.

Investigation of the Purcell effect in GaN-based vertical LED structures using FDTD simulation

The spontaneous emission (SE) rate in GaN-based vertical light-emitting diode (LED) structures can be modified significantly depending on the local density of states near the high-reflectance mirror. In this study, this Purcell effect in vertical LED structures is investigated numerically using finite-difference time-domain simulation. The Purcell factor of vertical LED structures is found to vary periodically with the p-GaN thickness, while it is almost independent of the n-GaN thickness, indicating strong influence of the p-type electrode reflector on the SE modification. The peak of the Purcell factor is obtained to be as high as 1.5 at a properly chosen p-GaN thickness, which leads to substantial increase in the internal quantum efficiency (IQE) of LEDs. Since the influence of the Purcell factor on IQE becomes more conspicuous as IQE decreases, the Purcell effect of vertical LED structures is expected to mitigate the green-gap and the efficiency droop problems of current LED technologies.