Nitride-based Light-emitting Diodes Research Articles

Passivation to Improve the Efficiency and Reliability of III-Nitride Micro-Light-Emitting Diode

Because group III-V and nitride-based micro light-emitting diodes (micro-LEDs) exhibit excellent performance, such as fast response, low power consumption, and thermal stability, they have been widely applied for display and biomedical applications, including optogenetics and multi-site neuron. However, unlike conventional LEDs (> 300 × 300 µm2), micro-LEDs (> 100 × 100 µm2) suffer from severe Shockley Read-Hall (SRH) non-radiative recombination, which is related to sidewall damage introduced by ICP-RIE dry-etching process for LED isolation. As the size of LEDs decreases, such non-radiative recombination phenomenon increases due to the increase in the sidewall surface ratio. Many attempts have been made to improve such defect-related recombination in micro-LEDs. In other words, sidewall passivation using dielectric materials was reported to be effective in alleviation sidewall-related leakage characteristics.In this study, we investigated the electrical and optical properties of GaN-based micro-LEDs (blue) as a function of the number of passivation layers. The passivation layers were deposited using plasma enhanced chemical vapor deposition (PECVD) or/and atomic layer deposition (ALD). It is shown that by employing combined deposition techniques of ALD and PECVD, the optical and electric performance of 10 µm-size micro-LEDs are significantly improved. Compared with the conventional PECVD passivation method, the combined ALD/PECVD passivation showed the smaller ideality factor. For the 10 µm-size micro-LEDs, maximum external quantum efficiency (EQE) was obtained at 35 A/cm2 when the stacked ALD/PECVD passivation layer was used, whereas it was attained at 79 A/cm2 when the PECVD passivation was used. Based on the emission microscopy results, leakage paths were observed. The results show that the use of stacked ALD/PECVD passivation layer could serve as a potentially important process for the fabrication of high-performance micro-LEDs (< 10 × 10 µm2).

Improved Light Extraction of Nitride-Based Light-Emitting Diodes by Al2O3-Doped UV-Curing Encapsulant

Silicone-based UV curing has recently been developed as a novel kind of encapsulation technology, which is time-saving and cost-effective for the package of light-emitting diodes (LEDs). In this work, to improve the light output power (LOP) and device reliability of LEDs packaged by UV curing, Al2O3 nanoparticles (NPs) are doped into silicone. Different compositions of Al2O3 NPs are used and the optimal composition is found to be 0.3%. Compared with LEDs packaged by UV curing without Al2O3 doping (256.3 mW) and by traditional thermal-curing (264.1 mW) encapsulants, the LOP is increased to 272.9 mW for LED with Al2O3-NP-doped encapsulant. The enhancement of LOP is attributed to both the increased refractive index and light scattering effect of the Al2O3 NPs, which improves the light extraction efficiency (LEE) of the encapsulant. Furthermore, the improvement of LEE also suppresses the heat generation and thus improving the long-term reliability of the devices. These results indicate that using high refractive index materials as doping, UV curing could be an excellent choice as a novel package technology for the LED industry.

III-nitride blue light-emitting diodes utilizing hybrid tunnel junction with low excess voltage

Tunnel junctions (TJs) offer alternative designs and promise in some cases improved performances for nitride-based light-emitting diode (LEDs) and laser diodes (LDs) and are widely used in academic studies. However, the voltage penalty of the LEDs and LDs, in comparison with standard contact technologies, has been a major concern especially for commercial applications. In this study, we investigated methods to achieve low excess voltage. Using ammonia molecular beam epitaxy (NH3 MBE), GaN TJs were grown on commercial metalorganic chemical vapor deposition (MOCVD) grown blue LED wafers. Atom probe tomography (APT) and secondary ion mass spectrometry (SIMS) indicate 1 min buffered HF (BHF) clean of the regrowth interface reduced Mg and impurity incorporation into the n++ regrown TJ layers. The wafers were processed and measured in parallel to reference wafers using both university processes and industry processes. At 20 A cm−2, TJ LEDs grown with Si δ-doping at the junction interface processed in the university cleanroom had a forward voltage of 3.17 V in comparison to 2.86 V for LEDs processed with a standard indium tin oxide (ITO) contact. Unencapsulated TJ LEDs processed by industrial process without ITO or current blocking layer had about 0.3 V excess voltage compared to reference LEDs. The TJ LEDs also had more uniform light emission profile. The low excess voltage and consistent results acquired in both settings suggest that TJ can be scaled for industrial processes.

Increasing the Luminescence Efficiency of Long-Wavelength ( In , Ga ) N Quantum Well Structures by Electric Field Engineering Using an ( Al , Ga ) N Capping Layer

Despite the very high efficiency at blue wavelengths, nitride-based light-emitting diodes suffer from low efficiencies in the green and red spectral ranges. Different solutions have been proposed to mitigate the ``green gap,'' including the addition of an ($\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{N}$ capping layer to the quantum well structure. In this work, we show how the increased polarization field due to such an ($\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{N}$ capping layer has profound effects on carrier location and recombination probabilities. The proper choice of ($\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{N}$ composition leads to an increased electron-hole overlap as well as an enhanced confinement in the quantum well region. Therefore, the combination of band structure and electric field engineering can be a promising approach to mitigate the green gap problem.

Gallium nitride micro-light-emitting diode structured light sources for multi-modal optical wireless communications systems.

Gallium nitride-based light-emitting diodes (LEDs) have revolutionized the lighting industry with their efficient generation of blue and green light. While broad-area (square millimetre) devices have become the dominant LED lighting technology, fabricating LEDs into micro-scale pixels (micro-LEDs) yields further advantages for optical wireless communications (OWC), and for the development of smart-lighting applications such as tracking and imaging. The smaller active areas of micro-LEDs result in high current density operation, providing high modulation bandwidths and increased optical power density. Fabricating micro-LEDs in array formats allows device layouts to be tailored for target applications and provides additional degrees of freedom for OWC systems. Temporal and spatial control is crucial to use the full potential of these micro-scale sources, and is achieved by bonding arrays to pitch-matched complementary metal-oxide-semiconductor control electronics. These compact, integrated chips operate as digital-to-light converters, providing optical signals from digital inputs. Applying the devices as projection systems allows structured light patterns to be used for tracking and self-location, while simultaneously providing space-division multiple access communication links. The high-speed nature of micro-LED array devices, combined with spatial and temporal control, allows many modes of operation for OWC providing complex functionality with chip-scale devices.This article is part of the theme issue ‘Optical wireless communication’.

Near-ultraviolet chip-based phosphor-converted solar-spectrum white light-emitting diode

As efficient artificial light sources, nitride-based light-emitting diodes (LEDs) have been widely used. However, III nitride white light-emitting diodes (WLEDs) are mostly fabricated by combining a blue LED chip with yellow phosphors, which results in inevitable problems, such as low color-rendering index (CRI) and detrimental effect to human eyes. As a solution for healthy lighting, we report a strategy to produce a WLED with an emission spectrum perfectly matched with natural daylight. By utilizing near-ultraviolet LED chips and a mixture of blue/cyan/amber/red phosphors, a CRI (Ra) of 97.9 is achieved for the WLEDs at a correlated color temperature of around 5000 K. The resemblance ratio of these solar-spectrum WLEDs with the standard normalized daylight spectrum (5000 K) is found to as high as 93.4%. Residual UV light in the normalized spectrum is &lt;5 % . The method will benefit the development of high-efficiency healthy artificial light sources.

Investigating the Efficiency Droop of Nitride-Based Blue LEDs with Different Quantum Barrier Growth Rates

In this study, GaN-based blue InGaN/GaN light-emitting diodes (LEDs) with different growth rates of the quantum barriers were fabricated and investigated. The LEDs with quantum barriers grown with a higher growth rate exhibit a lower leakage current and less non-radiative recombination centers in the multiple quantum wells (MQWs). Therefore, the LED with a higher barrier growth rate achieves a better light output power by 18.4% at 120 mA, which is attributed to weaker indium fluctuation effect in the QWs. On the other hand, the localized states created by indium fluctuation lead to a higher local carrier density, and Auger recombination in the QWs. Thus, the efficiency droop ratio of the LEDs with a higher barrier growth rate was only 28.6%, which was superior to that with a lower barrier growth rate (39.3%).

Carrier accumulation enhanced Auger recombination and inner self-heating-induced spectrum fluctuation in CsPbBr3 perovskite nanocrystal light-emitting devices

Light-emitting devices (LEDs) based on colloidal semiconductor nanocrystals (NCs) are promising next-generation thin-film display and lighting devices owing to their high luminescence efficiency and size-tunable color. However, the studies of the carrier transport and recombination mechanisms of the NC-LEDs lag far behind. We study the operation mechanisms of the CsPbBr3 NC-LEDs with a working voltage as high as 40 V. They exhibit an anomalous hump-shaped bias dependence of luminescence intensity. A theoretical model is proposed to explain this, and it indicates that there is enhanced Auger recombination in the CsPbBr3 NC-LEDs caused by imbalanced carrier accumulation, which causes efficiency droop similar to that in the nitride-based light-emitting diodes. We also report the intriguing phenomenon of electroluminescence spectrum fluctuation of the CsPbBr3 NC-LEDs, which is ascribed to the inner self-heating effect in the emission layer.

Regulating the circular polarization in nitride-based light-emitting diodes through the spin injection

Circular polarization in nitride-based light-emitting diodes is investigated and regulated through the spin injection. Ferromagnet/dielectric are grown on n-type GaN as electron injector. Ultrathin quantum wells with quantum confinement effect are designed for enhancement of the circular polarizations in electroluminescence. By modulating the tunneling layer thickness and injection polarizer material, the preferred spin injection structure is determined. Room temperature circular polarization for surface-emission is optimized to 9.0% at most under a vertical magnetic field for the spin injecting contact. Spin relaxation time and diffusion length of about 34.9 ps and 134.5 nm, respectively, are revealed through the non-local three-terminal spin valve measurements.

Improving the luminous efficiency of gallium nitride-based light-emitting diodes using Ag nanograting structure

In order to improve the traditional gallium nitride (GaN)-light-emitting diode (LED) luminous efficiency, a structure of the GaN-LED is designed based on the local field enhancement of surface plasmon (SP). The principle of improving LED luminous properties using the SP features is described in detail. The factors that affect the light extraction efficiency of this LED structure are simulated with the COMSOL software. The thickness of p-GaN layer, the thickness of indium tin oxide (ITO) layer, the grating period T , the duty ratio r , as well as the shape of the interface between p-GaN and ITO are studied. The normalized radiated powers, the normalized absorbed powers, and the electric field distribution of this structure under different structure parameters have been achieved. The results show that when the thickness of p-GaN layer is 110 nm, the thickness of ITO buffer layer is 120 nm, the period T is 370 nm, the duty ratio is 0.5, and the value of D 1 / T is 0.6. The luminous intensity of LED is higher than that of the traditional LED nearly 43 times and is nearly 36 times higher than that of the Ag nanosphere structure, and SPs can be coupled with the active layer effectively to improve the luminous efficiency of LED, providing a theoretical guide for the preparation of high-performance GaN-LED chips.

Stability and electronic properties of GaN phases with inversion symmetry to inherently inhibit polarization

Two centrosymmetric gallium nitride phases, one with a body-centered-tetragonal structure (bct-GaN) and the other base-centered orthorhombic ($Z$-GaN), are predicted. The inversion symmetry inherently inhibits spontaneous polarization, and more importantly, piezoelectric polarization regardless of heteroepitaxial growth direction and even in the presence of shear strain, offering alternative solutions to the polarization-induced electrostatic field problem encountered by group-III nitride-based light-emitting diodes (LEDs), beyond those promised by nonpolar and semipolar wurtzite (w-GaN) and zinc-blende GaN. Density functional theory calculations, validated by agreement with experiment for w-GaN, reveal equilibrium structures, phonon dispersions, and band structures of bulk bct-GaN and $Z$-GaN. To ensure their stability at room temperature, we first carry out an analysis of dynamical and thermal stability using phonon calculations and molecular dynamics simulations, which indicate that these centrosymmetric structures are dynamically stable and remain stable at high temperature. Moreover, the relative stability of GaN polymorphs is highly dependent on sample thickness. We predict that bct-GaN and $Z$-GaN are energetically more favorable than w-GaN for freestanding ultrathin films up to 47 and 70 layers, respectively, thus suggesting the possibility of the kinetic growth of these two phases. The $\mathit{GW}$ self-energy corrected, direct band gaps of bct-GaN and $Z$-GaN are close to the w-GaN value, promising LED applications in the same spectral ranges.

Degradation mechanisms of bias stress on nitride-based near-ultraviolet light-emitting diodes in salt water vapor ambient

Degradation mechanisms of nitride-based near-ultraviolet (near-UV) light-emitting diodes (LEDs) were systematically analyzed by applying forward- and reverse-bias stresses to them in a salt water vapor ambient. The surface temperature of the forward-bias stress sample was higher than that of the reverse-bias stress sample. The high temperature of the forward-bias stress sample accelerated the chemical reaction of the device structure with salt water vapors and led to faster degradation. Composition analyses of the sample surface and cross-section were conducted to investigate the failure mechanism. The analyses results indicated that the erosion of the indium–tin–oxide layer enhanced the diffusion of the conducting metal into the LED crystal. The proposed method can effectively characterize the quality of near-UV LEDs in a short duration.

P-type silicon as hole supplier for nitride-based UVC LEDs

The ineffective p-type doping of nitrides using magnesium (Mg), the best available dopant, has limited the development and performance of all III-nitride-based devices, including bipolar junction transistors and light emitting diodes (LEDs). For nitride-based ultraviolet (UV) LEDs, as the Al composition increases for achieving shorter wavelengths (e.g. <280 nm) into the UVC spectral range, the p-type doping issue, which causes very inefficient hole injection, becomes more severe than ever. In this work, we report the detailed study of using p-type Si as a hole supplier for high-Al composition UVC LEDs. We first describe the method of Si/GaN junction formation, where the lattice-mismatch challenge between Si and GaN is overcome by using a 0.5 nm thick Al2O3 layer at the interface. This serves as a physical separation layer between the two materials as well as a passivation, tunneling, and thermal buffer layer. High-resolution transmission electron microscope image illustrates the high-quality interface between Si and GaN. We further detail the hole transport mechanism of the p–p Si/GaN isotype junction through both simulations and experiments. The enhanced hole concentration in the AlGaN/AlN multiple quantum wells (MQWs) due to the use of p-type Si as the hole supplier is verified through comparison with conventional UVC LEDs. Finally, high-performance UVC LEDs made with AlN/AlGaN (Al: 72%) MQWs employing p-type Si as their hole suppliers are demonstrated experimentally to serve as an example of the novel hole injector strategy.

On the polarization self-screening effect in multiple quantum wells for nitride-based near ultraviolet light-emitting diodes

The tilted energy band in the multiple quantum wells (MQWs) arising from the polarization effect causes the quantum confined Stark effect (QCSE) for [0001] oriented III-nitride-based near ultraviolet light-emitting diodes (NUV LEDs). Here, we prove that the polarization effect in the MQWs for NUV LEDs can be self-screened once the polarization-induced bulk charges are employed by using the alloy-gradient InxGa1-xN quantum barriers. The numerical calculations demonstrate that the electric field in the quantum wells becomes weak and thereby flattens the energy band in the quantum wells, which accordingly increases the spatial overlap for the electron-hole wave functions. The polarization self-screening effect is further proven by observing the blueshift for the peak emission wavelength in the calculated and the measured emission spectra. Our results also indicate that for NUV LEDs with a small conduction band offset between the quantum well and the quantum barrier, the electron injection efficiency for the proposed structure becomes low. Therefore, we suggest doping the proposed quantum barrier structures with Mg dopants.

Investigation on Modulation Speed of Photon-Recycling White Light-Emitting Diodes With Vertical-Conduction Structure

The speed performance of a high lumen and high color rendering index (CRI) gallium nitride-based white-light light-emitting diode (LED) has been investigated in detail by measuring its electrical-to-optical frequency responses with different color filters at the receiver end. The ultra-violet/blue/green multiple quantum wells (MQWs) are stacked in the studied LED chips, which have a vertical conducting structure and red phosphor coating for generating white light with high CRI. The typical studied LED exhibits a smaller 3 dB bandwidth (0.6 MHz versus 2 MHz) compared with a reference LED with ultra-violet/blue MQWs and a yellow phosphor coating. This finding suggests that the fluorescent decay time of red phosphor is much longer than that of yellow ones (approximately 270 ns versus approximately 80 ns). In addition, the extracted carrier lifetime inside blue/green InxGa1-xN/GaN MQWs under UV light pumping/modulation is as long as approximately 50 ns, which is much slower than that of forward-biased current modulation (approximately 50 ns versus on–off keying modulation scheme are demonstrated under a moderate operation bias current (approximately 0.75 A) of studied LEDs.

Impurity resonant state p-doping layer for high-efficiency nitride-based light-emitting diodes

Impurity resonant state p-doping was recently proposed as an alternative to the commonly used p-GaN for high-efficiency nitride-based light-emitting diodes (LEDs) in order to address the issue of electron leakage and efficiency droop. We demonstrated that due to the effective acceptor ionization and reduction of the potential barrier for hole injection, a more symmetrical electron and hole injection and distribution could be achieved. Our experimental results revealed that the LEDs with this alternative p-type layer exhibited a higher quantum efficiency and reduced efficiency droop. We expect this approach to be equally applicable for deep-UV laser diodes and LEDs to replace the commonly used absorptive p-GaN layer.

Optical and thermal performance of nitride-based thin-film flip-chip light-emitting diodes

Nitride-based light-emitting diodes (LEDs) were fabricated based on initial patterned sapphire substrate by thin film (TF) and flip-chip (FC) techniques. An enhancement of 7.7% in light output power was first obtained by optimizing the p-type contact layer. Then, periodic patterned surface was optimized by wet etching after removal of the sapphire substrate, and the light output power of the TFFC-LEDs was enhanced by 19.8% as compared to that of the TFFC-LEDs with original patterned surface. The device performance characteristic including illuminant pattern, emission wavelength movement and junction temperature variation of the optimized TFFC-LEDs were shown as compared with commercialized LEDs. The results indicate that the TFFC-LEDs possess uniform current distribution and lower junction temperature, thus show promising applications in various areas such as automotive lighting, illegal capture and solid-state lighting.

Luminescence Spectra of High-Power Violet and Ultraviolet Gallium Nitride-Based LEDs

The electroluminescence spectra of high-power light-emitting diodes (LEDs) based on p–n heterostructures of the InGaN/AlGaN/GaN type emitting in the visible short-wavelength and ultraviolet spectral regions (the range from 370 to 460 nm) are investigated. The shape of spectra is described by a model taking into account a two-dimensional combined density of states and potential fluctuations. Additional long-wavelength peaks are found in emission spectra. The emission power of ultraviolet and violet LEDs reaches 233 mW at a current of 350 mA, and the external quantum yield reaches 23% at the maximum (near a current of 100 mA).

Localization landscape theory of disorder in semiconductors. III. Application to carrier transport and recombination in light emitting diodes

This paper introduces a novel method to account for quantum disorder effects into the classical drift-diffusion model of semiconductor transport through the localization landscape theory. Quantum confinement and quantum tunneling in the disordered system change dramatically the energy barriers acting on the perpendicular transport of heterostructures. In addition they lead to percolative transport through paths of minimal energy in the 2D landscape of disordered energies of multiple 2D quantum wells. This model solves the carrier dynamics with quantum effects self-consistently and provides a computationally much faster solver when compared with the Schr\"odinger equation resolution. The theory also provides a good approximation to the density of states for the disordered system over the full range of energies required to account for transport at room-temperature. The current-voltage characteristics modeled by 3-D simulation of a full nitride-based light-emitting diode (LED) structure with compositional material fluctuations closely match the experimental behavior of high quality blue LEDs. The model allows also a fine analysis of the quantum effects involved in carrier transport through such complex heterostructures. Finally, details of carrier population and recombination in the different quantum wells are given.

Selective-area growth of periodic nanopyramid light-emitting diode arrays on GaN/sapphire templates patterned by multiple-exposure colloidal lithography

Gallium nitride-based nanopyramid light-emitting diodes are a promising technology to achieve highly efficient solid-state lighting and beyond. Here, periodic nanopyramid light-emitting diode arrays on gallium nitride/sapphire templates were fabricated by selective-area metalorganic chemical vapor deposition and multiple-exposure colloidal lithography. The electric field intensity distribution of incident light going through polystyrene microspheres and photoresist are simulated using finite-different time-domain method. Nitrogen as the carrier gas and a low V/III ratio (ratio of molar flow rate of group-V to group-III sources) are found to be important in order to form gallium nitride nanopyramid. In addition, a broad yellow emission in photoluminescence and cathodoluminescence spectra were observed. This phenomena showed the potential of nanopyramid light-emitting diodes to realize long wavelength visible emissions.