GaN-based Light-emitting Diodes Research Articles

Planarization of p-GaN surfaces on MOCVD grown V-defect engineered GaN-based LEDs

The large polarization barriers between the quantum wells and quantum barriers in long-wavelength GaN-based light-emitting diodes (LEDs) inhibit their performance by requiring excess driving voltages to reach standard operating current densities. Lateral injection of carriers directly into quantum wells is required to circumvent this issue. V-defects are naturally occurring inverted hexagonal defects with semipolar 101¯1-plane sidewalls generated on surface depressions from threading dislocations. LEDs engineered to intentionally generate V-defects below the active region of the LED can achieve lateral carrier injection through the V-defect sidewalls and have already been able to demonstrate world record wall-plug efficiencies for LEDs in the green-red wavelengths. V-defects can be enlarged during kinetically limited growth where the growth rate of the c-plane GaN is faster than that of their sidewalls, leaving them unfilled. We report on the metal organic chemical vapor deposition growth conditions required to fill in V-defects with p-GaN during epitaxial growth of the LED post the active region. Circular transmission length measurements of Pd/Au contacts processed on p-GaN surfaces with various amounts of unfilled V-defects showed no significant difference in their sheet resistance and specific contact resistance. J–V measurements of LEDs grown with varying unfilled V-defect densities showed no significant difference in the forward bias regime. However, in the reverse bias regime, catastrophic breakdown occurred at markedly lower voltages for samples with larger unfilled V-defect densities. This suggests that unfilled V-defects may act as hotspots for device failure, and planarizing LED surfaces may help prevent early degradation of LED devices.

Enhancing the Performance of GaN-Based Light-Emitting Diodes by Incorporating a Junction-Type Last Quantum Barrier

In this paper, an n-i-p-type GaN barrier for the final quantum well, which is closest to the p-type GaN cap layer, is proposed for nitride light-emitting diodes (LEDs) to enhance the confinement of electrons and to improve the efficiency of hole injection. The performances of GaN-based LEDs with a traditional GaN barrier and with our proposed n-i-p GaN barrier were simulated and analyzed in detail. It was observed that, with our newly designed n-i-p GaN barrier, the performances of the LEDs were improved, including a higher light output power, a lower threshold voltage, and a stronger electroluminescence emission intensity. The light output power can be remarkably boosted by 105% at an injection current density of 100 A/cm2 in comparison with a traditional LED. These improvements originated from the proposed n-i-p GaN barrier, which induces a strong reverse electrostatic field in the n-i-p GaN barrier. This field not only enhances the confinement of electrons but also improves the efficiency of hole injection.

Analysis of Wet-Etched Structures on R-Plane Substrates of Sapphire in the Etchant of Sulfuric Acid and Phosphoric Acid

The occurrence and variation of wet-etched structures on R-plane substrates of sapphire were analyzed according to the distribution of drilling rates of typical crystal planes. First, the experiment for etching the sapphire hemisphere was conducted to obtain the distribution of etching rates with C-plane as the rotation center in the experimental etchant (236 °C, three parts H2SO4 and one part H3PO4 in terms of volume). Then, the transfer matrix was applied to transform the distribution of etching rates with C-plane as the rotation center in the experimental etchant (236 °C, three parts H2SO4 and one part H3PO4 in terms of volume) into the distribution of etching rates with R-plane as the rotation center. The positive curvature maximum identification method was then applied to obtain the distribution of drilling rates of typical crystal planes on R-plane substrates. Finally, the occurrence and variation of polygonal grooves with different mask configurations on R-plane substrates in the experimental etchant (236 °C, three parts H2SO4 and one part H3PO4 in terms of volume) were analyzed based on the distribution of drilling rates of typical crystal planes. This provides a basis for the application and development of GaN-based light-emitting diode devices.

Investigation into the Effects of Cross-Sectional Shape and Size on the Light-Extraction Efficiency of GaN-Based Blue Nanorod Light-Emitting Diode Structures

We investigated the effect of cross-sectional shape and size on the light-extraction efficiency (LEE) of GaN-based blue nanorod light-emitting diode (LED) structures using numerical simulations based on finite-difference time-domain methods. For accurate determination, the LEE and far-field pattern (FFP) were evaluated by averaging them over emission spectra, polarization, and source positions inside the nanorod. The LEE decreased as rod size increased, owing to the nanorods’ increased ratio of cross-sectional area to sidewall area. We compared circular, square, triangular, and hexagonal cross-sectional shapes in this study. To date, nanorod LEDs with circular cross sections have been mainly demonstrated experimentally. However, circular shapes were found to show the lowest LEE, which is attributed to the coupling with whispering-gallery modes. For the total emission of the nanorod, the triangular cross section exhibited the highest LEE. When the angular dependence of the LEE was calculated using the FFP simulation results, the triangular and hexagonal shapes showed relatively high LEEs for direction emission. The simulation results presented in this study are expected to be useful in designing high-efficiency nanorod LED structures with optimum nanorod shape and dimensions.

Spontaneous Emission Studies for Blue and Green InGaN-Based Light-Emitting Diodes and Laser Diodes

We investigated the efficiency droop phenomenon in blue and green GaN-based light-emitting diodes (LEDs) and laser diodes (LDs), which poses a significant challenge in high-power LEDs and is characterized by a reduction in external quantum efficiency at higher injection currents. Utilizing identical epi-structures for blue and green LEDs and LDs, with variations only in indium composition, our experiments revealed a gradual blue shift in the emission wavelengths as the injection current increased. Notably, the blue LED demonstrated a smaller shift compared to the green LED. In addition, the full width at half maximum of emission spectra increased with increasing injection current density, indicative of efficiency droop. Significantly, LDs consistently exhibited lower junction temperatures despite operating at higher current densities. This is attributed to the enhanced heat dissipation capability of the ridge waveguide LD structure, which results in a narrower emission spectrum and reduced efficiency droop compared to mesa LED structures. These outcomes highlight the efficiency of the ridge waveguide LD structure in heat dissipation from the active layer, offering crucial insights for the advancement of high-power light-emitting devices.

Orthogonally and linearly polarized green emission from a semipolar InGaN based microcavity

Abstract Polarized light has promising applications in biological inspections, displays, and precise measurements. Direct emission of polarized light from a semiconductor device is highly desired in order to reduce the size and energy-consumption of the whole system. In this study, we demonstrate a semipolar GaN-based microcavity light-emitting diode (MCLED) that could simultaneously produce green light with perpendicular and parallel polarizations to the c*-axis. Orthogonally polarized emission with a narrow linewidth (∼0.2 nm) arises from the valence band splitting and birefringent nature of the semipolar GaN material, as well as the mode selection of the resonant cavity. By modulating the cavity length, the device is capable of switching between single- and multi-mode emission spectra. We believe that the approach of employing a cavity structure and semipolar GaN can be extended to produce orthogonally and linearly polarized blue, red, and violet light by adjusting the material compositions.

Optimizing the design of highly linearly polarized white LED with multilayer metal-dielectric grating structure

In order to emit a linearly polarized light and reduce energy loss for a white light emitting diode (LED), a multilayer metal-dielectric grating structure using GaN-based blue LED is proposed. This multilayer structure is composed of three major functional layers: a fluorescent ceramics layer, a dielectric anti-reflection layer and a linear polarization grating layer. Fluorescent ceramics layer is integrated on the top of GaN-based blue LED, to achieve white light emission. In turn, a dielectric anti-reflection layer and a linear polarization grating layer are also integrated to obtain a higher transmission of white light and improve the conversion efficiency of linear polarization. The structural parameters of each functional layer, including period (150 nm), duty cycle (0.5), height of metal-dielectric grating (55 nm and 80 nm) and thickness of dielectric layer (15 nm), are simulated and optimized by finite-difference time-domain method. Then the multilayer structure is fabricated by UV nanoimprint and its transmission and polarization characteristics are tested. The results show that the transmittance and extinction ratio are higher than 50% and 33.8 dB in the wavelength range of 400–800 nm, and the maximum value can reach 91.3% and 41.3 dB, respectively. It is well verified by experiments that the proposed structure has a steady polarization performance of TM mode in the range of ±20°, which provides a fundamental guarantee for developing the potential applications of polarized white LED.

Optical-Emission Properties of GaN-Based Nanopillar Light-Emitting Diodes Prepared on Non-Polished and Non-Single-Crystalline Substrates

GaN-based nanopillar crystals are directly grown on multicrystalline Si and amorphous-carbon-coated graphite substrates whose surfaces are not mirror-polished. Light-emitting diodes (LEDs) of a double-hetero structure are prepared from the nanopillar crystals, and their optical–emission properties are investigated. Despite the substrate type and surface conditions, moderate light emissions are obtained from nanopillar LEDs though the light emissions are not always homogeneous, especially in the LEDs prepared on the graphite-based substrate. Nevertheless, these results will lead to realizations of novel large-area light-emitting devices.

Determining the Turn-On Voltage of GaN-Based Light-Emitting Diodes: From Near-Ultraviolet to Green Spectra

This work investigates the turn-on voltage (V on) estimation methods from experimental data of InGaN-based multiple-quantum-well light-emitting diodes (LEDs) with emission spectra from near-ultraviolet to green wavelengths. Three different methods are examined: fixed current intercept, linear fitting of current-voltage curve, and the minimum ideality factor (n ideal). The voltage at minimum n ideal consistently represents the on-state of LEDs since it describes the carrier recombination processes reflected in light output power (LOP) and the external quantum efficiency (EQE). Beyond the voltage at minimum n ideal, samples’ LOPs start to increase abruptly, with the EQEs reaching ≥ 54% of the peak EQE values. For V on determined by other methods, samples’LOPs are already turned on and the EQEs exceed their peak value. Therefore, using the minimum n ideal is a suitable method for determining Von of an LED.

Enhanced luminescence efficiency in Eu-doped GaN superlattice structures revealed by terahertz emission spectroscopy

Eu-doped Gallium nitride (GaN) is a promising candidate for GaN-based red light-emitting diodes, which are needed for future micro-display technologies. Introducing a superlattice structure comprised of alternating undoped and Eu-doped GaN layers has been observed to lead to an order-of-magnitude increase in output power; however, the underlying mechanism remains unknown. Here, we explore the optical and electrical properties of these superlattice structures utilizing terahertz emission spectroscopy. We find that ~0.1% Eu doping reduces the bandgap of GaN by ~40 meV and increases the index of refraction by ~20%, which would result in potential barriers and carrier confinement within a superlattice structure. To confirm the presence of these potential barriers, we explored the temperature dependence of the terahertz emission, which was used to estimate the barrier potentials. The result revealed that even a dilutely doped superlattice structure induces significant confinement for carriers, enhancing carrier recombination within the Eu-doped regions. Such an enhancement would improve the external quantum efficiency in the Eu-doped devices. We argue that the benefits of the superlattice structure are not limited to Eu-doped GaN, which provides a roadmap for enhanced optoelectronic functionalities in all rare-earth-doped semiconductor systems.

The preparation of CuO film under different annealing atmospheres and investigation of the excellent electrical and undoped tunable electroluminescence characteristics of Au/i-CuO/n-GaN LED

Copper oxide (CuO) film is widely used in optoelectronic devices. The CuO films were fabricated by radio frequency (RF) magnetron sputtering and then annealed at 700 °C under different atmospheres (unannealed, O2, N2, vacuum). The physical characteristics of the films were analyzed systematically. In addition, we prepared Au/i-CuO/n-GaN metal-insulator-semiconductor (MIS) structure devices using optimized conditions. The devices had excellent rectifying characteristics at different ambient temperatures and the turn-on voltage was about 1.7 V. The working mechanism of the diode was analyzed through electroluminescence (EL) spectra and band structure. The EL spectra observed at positive and negative bias were compounded by holes and electrons on the gold electrode passing through the insulating layer into the n-GaN side. The EL spectra exhibited that a dominant ultraviolet (UV) emission peak at ∼376 nm was observed under forward bias (FB), and a strong near-white light emission peak at ∼530 nm was observed under reverse bias (RB). The device exhibited unique electroluminescent spectra tunability varying with bias conditions. The chrominance coordinate of the device under RB was (0.3067,0.3604), and the color temperature was 6585 K. This study proved the application potential of i-CuO as an insulating layer in the preparation of new reference for MIS structured GaN-based tunable light-emitting diodes.

Enhancing current spreading performance of GaN-based ultraviolet light-emitting diode chips through periodic doping composite barrier structures

In response to the issue of current crowding in GaN-based ultraviolet (UV) light-emitting diode (LED) chips, a periodic doping composite barrier structure is proposed. The results of the optical intensity distribution demonstrate that the device with the periodic doping composite barrier structure exhibits a more uniform light distribution compared to the device with traditional structures. Moreover, when the injected current is 20 mA, the light output power (LOP) and wall-plug efficiency (WPE) are improved by 11.99% and 13.5%, respectively. Experimental and simulation results indicate that the periodic doping composite barrier structure enhances the current spreading performance through two mechanisms: (1) the introduction of an electron barrier layer enhances lateral current transport; (2) increase the ratio of longitudinal to transverse resistivity of the material to further enhances current spreading performance.

Research on the Performance Improvement of GaN-Based Ultraviolet Light-Emitting Diode Chips with Magnetron-Sputtered AlN Nucleation Layers

In this paper, the influence of magnetron-sputtered AlN nucleation layers with different thicknesses on ultraviolet (UV) light-emitting diode (LED) chips is studied. First, magnetron-sputtered AlN layers with thicknesses of 8, 25, and 42 nm were prepared on a patterned sapphire substrate (PSS). Subsequently, a 2.8 μm-thick GaN layer was epitaxially grown. We found that when the magnetron-sputtered AlN was 25 nm thick, GaN had the best crystal quality and morphology. Therefore, we grew an 800 nm GaN layer on PSS to analyze the initial growth mechanism of the material. The scanning electron microscopy (SEM) images showed that when the thickness of the sputtered AlN layer was 25 nm, the GaN grains on the sidewall of the PSS cone were the least abundant. In addition, according to the selected area electron diffraction (SAED) test, the (011) crystal plane of GaN appeared on the sidewall of the PSS cone. The (011) crystal plane is the semipolar plane of GaN, which is not conducive to the growth of polar GaN materials. Finally, on this basis, we fabricated UV-LED chips. Among them, the UV-LED chips with the 25 nm magnetron-sputtered AlN layer exhibited the best electrical and optical performance. Overall, this work provides a pathway to improve the performance of UV-LED chips.

Formation and Optical Characteristics of GaN:Eu/GaN Nanowires for Applications in Light-Emitting Diodes

This paper reviews our recent research about the formation and optical characteristics of GaN:Eu/GaN nanowires (NWs) by metalorganic vapor phase epitaxy for application in GaN-based red light-emitting diodes (LEDs). Two types of GaN:Eu/GaN NWs with different configurations are introduced, core–shell and axial geometries. The configuration of GaN:Eu layers on GaN core NWs can be controlled by changing the growth conditions, and affects the properties of Eu luminescence in the GaN NWs. Next, the optimization of the p-GaN growth conditions is performed to allow to form the p-GaN shell layers on the NWs with the pedestal of the NWs free from radial overgrowth, resulting in efficient electrical isolation between top and bottom part of the NWs. Then, the fabrication process of the NW LEDs towards future possible realization of flexible devices is established, including an etch-back process of the PDMS membranes to expose the top p-GaN contact layers. Finally, a proto-type of p-GaN/GaN:Eu/n-GaN NW LEDs on sapphire substrates is fabricated to characterize the device properties. Sharp red luminescence at room temperature from Eu3+ ions is observed under current injection. These results would pave the way towards the realization of flexible light-emitting devices utilizing NW structures based on compound semiconductors.

Composition-dependent trapezoidal quantum barrier effect on efficiency droop in GaN-based light-emitting diodes

Composition-dependent trapezoidal quantum barrier effect on efficiency droop in GaN-based light-emitting diodes

Enhanced performance of GaN-based long-wavelength blue light-emitting diodes with graphene-ITO composite film as transparent conductive electrode

Abstract We present enhanced performance of GaN-based long-wavelength blue light emitting diodes (LEDs) with hybrid transparent conductive electrodes (TCE) made of graphene and indium tin oxide (ITO) composite. The nearly 100% transmittance TCE were gained when graphene grown by chemical-vapor-deposition was annealed with ITO on the top of it. Compared to conventional LEDs, the work voltage was reduced to 3.5 V at 5 mA forward current. The light emission power was improved about 190%. The good peformance can be attributed to the improved lateral current spreading over the composite graphene-ITO TCE before p-GaN layer injected by carriers.

Improved crystal quality and enhanced optical performance of GaN enabled by ion implantation induced high-quality nucleation.

Hetero-epitaxial growth of GaN often leads to high density of threading dislocations, which poses a significant challenge to the promotion of the performance of GaN-based devices. In this study, we address this issue by utilizing an Al-ion implantation pretreatment on sapphire substrates, which induces high-quality regularly arranged nucleation and promotes the crystal quality of GaN. Specifically, we demonstrate that an Al-ion dose of 1013 cm-2 leads to a reduction of full width at half maximum values of (002)/(102) plane X-ray rocking curves from 204.7/340.9 arcsec to 187.0/259.5 arcsec. Furthermore, a systematic investigation of GaN film grown on the sapphire substrate with various Al-ion doses is also performed, and the nucleation layer growth evolution on different sapphire substrates is analyzed. As confirmed by the atomic force microscope results of the nucleation layer, the ion implantation induced high-quality nucleation is demonstrated, which results in the improved crystal quality of the as-grown GaN films. Transmission electron microscope measurement also proves the dislocation suppression through this method. In addition, the GaN-based light-emitting diodes (LEDs) were also fabricated based on the as-grown GaN template and the electrical properties are analyzed. The wall-plug efficiency at 20 mA has risen from 30.7% to 37.4% of LEDs with Al-ion implantation sapphire substrate at a dose of 1013 cm-2. This innovative technique is effective in the promotion of GaN quality, which can be a promising high-quality template for LEDs and electronic devices.

Enhanced output efficiency of GaN-based light-emitting diodes by silver nanowires-induced localized surface plasmon

Enhanced output efficiency of GaN-based light-emitting diodes by silver nanowires-induced localized surface plasmon

Evolution of Frenkel defect pairs in β-Ga2O3 thin film with the introduction of oxygen and its application in GaN-based ultraviolet light-emitting diode

In this work, β-Ga2O3 thin films were deposited by radio frequency (RF) magnetron sputtering technique. The effect of oxygen-argon flow ratio (OAFR) on the formation of Frenkel defect pairs was systematically analyzed. The Frenkel defect pairs would become abundant when the OAFR was 25:25 sccm, resulting in a severe damage in crystalline quality and an increase in the density of disorder. With the further increase of OAFR, Frenkel defect pairs would dramatically decrease or even vanish due to the significant reduction of oxygen vacancies (V0). Immediately after that, on the basis of avoiding the influence of Frenkel defect pairs inside the β-Ga2O3 films to the devices, the p-NiO/i-Ga2O3/n-GaN diode was fabricated from the as-prepared high-quality β-Ga2O3 film after post-deposition annealing as the electron-blocking layer. Subsequently, its electrical characteristics were investigated in detail which exhibited excellent rectification characteristics under all test temperatures. Correspondingly, the device achieved electroluminescence (EL) with a dominant sharp emission peak in the ultraviolet range (∼375 nm). In the end, the energy band diagram was used for in-depth analysis of the EL mechanism.

Recent Advances on GaN-Based Micro-LEDs.

GaN-based micro-size light-emitting diodes (µLEDs) have a variety of attractive and distinctive advantages for display, visible-light communication (VLC), and other novel applications. The smaller size of LEDs affords them the benefits of enhanced current expansion, fewer self-heating effects, and higher current density bearing capacity. Low external quantum efficiency (EQE) resulting from non-radiative recombination and quantum confined stark effect (QCSE) is a serious barrier for application of µLEDs. In this work, the reasons for the poor EQE of µLEDs are reviewed, as are the optimization techniques for improving the EQE of µLEDs.