High Performance Green LEDs for Solid State Lighting

Despite enormous efforts and investments, the efficiency of InGaN-based green and yellow-green light emitters remains relatively low, and that limits progress in developing full color display, laser diodes, and bright light sources for general lighting. The low efficiency of light emitting devices in the green-to-yellow spectral range, also known as the “Green Gap”, is considered a global concern in the LED industry. The polar c-plane orientation of GaN, which is the mainstay in the LED industry, suffers from polarization-induced separation of electrons and hole wavefunctions (also known as the “quantum confined Stark effect”) and low indium incorporation efficiency that are the two main factors that contribute to the Green Gap phenomenon. One possible approach that holds promise for a new generation of green and yellow light emitting devices with higher efficiency is the deployment of nonpolar and semi-polar crystallographic orientations of GaN to eliminate or mitigate polarization fields. In theory, the use of other GaN planes for light emitters could also enhance the efficiency of indium incorporation compared to c-plane. In this thesis, I present a systematic exploration of the suitable GaN orientation for future lighting technologies. First, in order to lay the groundwork for further studies, it is important to discuss the analysis of processes limiting LED efficiency and some novel designs of active regions to overcome these limitations. Afterwards, the choice of nonpolar orientations as an alternative is discussed. For nonpolar orientation, the (1-100)-oriented (m-plane) structures on patterned Si (112) and freestanding m-GaN are studied. The semi-polar orientations having substantially reduced polarization field are found to be more promising for light-emitting diodes (LEDs) owing to high indium incorporation efficiency predicted by theoretical studies. Thus, the semi-polar orientations are given close attention as alternatives for future LED technology. One of the obstacles impeding the development of this technology is the lack of suitable substrates for high quality materials having semi-polar and nonpolar orientations. Even though the growth of free-standing GaN substrates (homoepitaxy) could produce material of reasonable quality, the native nonpolar and semi-polar substrates are very expensive and small in size. On the other hand, GaN growth of semi-polar and nonpolar orientations on inexpensive, large-size foreign substrates (heteroepitaxy), including silicon (Si) and sapphire (Al2O3), usually leads to high density of extended defects (dislocations and stacking faults). Therefore, it is imperative to explore approaches that allow the reduction of defect density in the semi-polar GaN layers grown on foreign substrates. In the presented work, I develop a cost-effective preparation technique of high performance light emitting structures (GaN-on-Si, and GaN-on-Sapphire technologies). Based on theoretical calculations predicting the maximum indium incorporation efficiency at θ ~ 62º (θ being the tilt angle of the orientation with respect to c-plane), I investigate (11-22) and (1-101) semi-polar orientations featured by θ = 58º and θ = 62º, respectively, as promising candidates for green emitters. The (11-22)-oriented GaN layers are grown on planar m-plane sapphire, while the semi-polar (1-101) GaN are grown on patterned Si (001). The in-situ epitaxial lateral overgrowth techniques using SiNx nanoporous interlayers are utilized to improve the crystal quality of the layers. The data indicates the improvement of photoluminescence intensity by a factor of 5, as well as the improvement carrier lifetime by up to 85% by employing the in-situ ELO technique. The electronic and optoelectronic properties of these nonpolar and semi-polar planes include excitonic recombination dynamics, optical anisotropy, exciton localization, indium incorporation efficiency, defect-related optical activities, and some challenges associated with these new technologies are discussed. A polarized emission from GaN quantum wells (with a degree of polarization close to 58%) with low non-radiative components is demonstrated for semi-polar (1-101) structure grown on patterned Si (001). We also demonstrated that indium incorporation efficiency is around 20% higher for the semi-polar (11-22) InGaN quantum wells compared to its c-plane counterpart. The spatially resolved cathodoluminescence spectroscopy demonstrates the uniform distribution of indium in the growth plane. The uniformity of indium is also supported by the relatively low exciton localization energy of Eloc = 7meV at 15 K for these semi-polar (11-22) InGaN quantum wells compared to several other literature reports on c-plane. The excitons are observed to undergo radiative recombination in the quantum wells in basal-plane stacking faults at room temperature. The wurtzite/zincblende electronic band-alignment of BSFs is proven to be of type II using the time-resolved differential transmission (TRDT) method. The knowledge of band alignment and degree of carrier localization in BSFs are extremely important for evaluating their effects on device properties. Future research for better understanding and potential developments of the semi-polar LEDs is pointed out at the end.

IntechOpen eBooks, 2023

Gallium nitride (GaN)-based solid state lighting technology has revolutionized the semiconductor industry. The GaN technology has played a crucial role in reducing world energy demand as well as reducing the carbon footprint. As per the reports, the global demand for lighting has reduced around 13% of total energy consumption in 2018. The Department of Energy (USA) has estimated that bright white LED source could reduce their energy consumption for lighting by 29% by 2025. Most of the GaN LEDs are grown in c-direction, and this direction gives high growth rate and good crystal integrity. On the other hand, the c-plane growth induces piezoelectric polarization, which reduces the overall efficiency of LEDs since the last decade researchers round the globe working on III-N material to improve the existing technology and to push the limit of III-V domain. Now, the non-polar and semi-polar grown LEDs are under investigation for improved efficiency. With the recent development, the GaN is not only limited to lighting, but latest innovations also led the development of micro-LEDs, lasers projection and point source. These developments have pushed GaN into the realm of display technology. The miniaturization of the GaN-based micro-LED and integration of GaN on silicon driving the application into fast response photonic integrated circuits (ICs). Most of the recent advancements in GaN LED field would be discussed in detail.

Optical and Quantum Electronics, 2012

Nitride-based quantum well (QW) LEDs for lighting applications suffer from efficiency issues related to the strong built-in fields due to the difference in electric polarization of the constituent materials. In this paper we present a study based on device simulation showing the beneficial impact of band gap engineering approaches on device performance in particular for green LEDs.

SPIE Proceedings, 2009

The green spectral region provides a formidable challenge for energy efficient light emitting diodes. In metal organic vapor phase epitaxy we developed GaInN/GaN quantum well material suitable for 500-580 nm LEDs by rigorous defect reduction and thrive for alloy uniformity. We achieve best results in homoepitaxy on polar c-plane, and non-polar a-plane and m-plane bulk GaN. By the choice of crystal orientation, the dipole of piezoelectric polarization in the quantum wells can be optimized for highest diode efficiency. We report progress towards the goal of reduced efficiency droop at longer wavelengths.

physica status solidi (RRL) – Rapid Research Letters, 2007

2015

The green gap problem refers to the lack of high-efficacy LEDs in the green–yellow range of the visible spectrum. From the side of the III–nitride LEDs, it is caused mainly by the increase of the efficiency droop, but also in part by the increase of the electrical losses, with respect to shorter-wavelength LEDs. This study clarifies some points about the causes of the green gap. Empirical measurements of the recombination rate coefficients show that the droop aggravates with increasing wavelength mainly due to the increase of the radiative carrier lifetime with increasing wavelength. Being clear that the problem of the droop could be overcome by operating the quantum wells at lower carrier injection rates, the possibility of using an InGaN phosphor to convert the blue light of a more efficient blue LED into green light was considered. A device based on this concept was realized and its efficacy surpassed that of a traditional green LED.

physica status solidi c, 2007

Energy efficient lighting can be achieved by help of high efficiency light emitting diodes (LEDs). Substantial performance gains are feasible in the green-500-555 nm-spectral region by implementing proper design of the active region. Here we analyze external quantum efficiency of high performing LED dies as a function of current and temperature in order to formulate relevant optimization rules. Among a large sample set we identify traits that clearly correlate with respective device performance.

IEEE Photonics Technology Letters, 2000

We report the fabrication of GaN-based blue light-emitting diodes (LEDs), which separately incorporate the three different electron blocking layers (EBLs), namely, a conventional AlGaN, a uniform multiquantum barrier (UMQB), and a chirped multiquantum barrier (CMQB). On the administration of 20 mA injection current, the corresponding LED output powers measured were 27.5, 27.2, and 25.4 mW for CMQB LED, UMQB LED, and LED, respectively, with a conventional AlGaN EBL. It was also found that the LED with CMQB EBL exhibited a significantly lower drooping effect and a smaller forward bias as compared with LEDs with a conventional AlGaN EBL and UMQB EBL.

Far-ultraviolet-C (Far-UVC) light-emitting-diodes (LEDs) offer a promising technology for the disinfection of surface, air, water, food and airborne disease transmission in occupied spaces, including COVID-19 (SARS-CoV-2) and other viral diseases, when it is meticulously designed, engineered, and applied. Research should continue on both the safety and efficacy of AlGaN-based Far-UVC LEDs, as well as the material choices and device designs to develop highly efficient solid-state UV germicidal irradiation (UVGI) at 222 nm emission to replace toxic low-pressure mercury lamps emitting at 253.7 nm. However, the key issue of hole concentration inside the multi-quantum-wells (MQWs) of AlGaN-based Far-UVC LEDs with high Al-contents is quite critical. Therefore, theoretical studies of AlGaN-based Far-UVC LEDs may suggest sufficient evidence for immediate consideration and implementation for the epitaxial growth of 222 nm-band Far-UVC LED technology during this worldwide health crisis. In this paper, the initial design of the Al-graded p-AlGaN hole source layer (HSL) on the performances of Far-UVC LED was compared with conventional bulk p-AlGaN HSL (non-graded)based LED devices. For the evaluation of the device's performances, the energy band diagram, internal quantum efficiency (IQE), electrons and holes concentration, radiative recombination rate, and current density vs voltage characteristic were compared. It was found that LEDs at 222 nm emission without using the undoped (ud)-AlGaN final-quantum-barrier (FQB) and only keeping the Al-graded Mg-doped p-AlGaN HSL showed high carrier injection into the MQWs. The variation in the energy band diagram around the p-AlGaN electron-blocking layer (EBL)/p-AlGaN HSL region and p-AlGaN HSL/p-GaN contact-layer (CL) indicates that the introduction of the Al-graded p-AlGaN HSL, as well as the special choice of Al composition at the interfaces, are quite promising for the enhancement of hole injection toward MQWs. The simulation results suggest that the proposed structure of the Al-graded p-AlGaN HSL after omitting the ud-AlGaN FQB structure in the Far-UVC LED is quite useful for achieving high peak efficiency, as well as for suppressing the efficiency droop when compared to the conventional bulk Far-UVC LED. After introducing a new design of 40 nm-thick p-AlGaN HSL in the Far-UVC LED, the radiative recombination rate in the first two quantum-wells of MQWs has been improved up to B50%. The enhanced radiative recombination rate is attributed to the enhanced level of electron and hole concentrations by B26% and 53%, respectively, in the MQWs. Ultimately, after removing the ud-AlGaN FQB and using 40 nm-thick Al-graded (Al: 100% to 20%) p-AlGaN HSL, the efficiency droop has been remarkably reduced from B39% (Bulk-LED) to B19% in the new design of Far-UVC LED structure.

| Light-emitting diodes (LEDs) have become quite a high-performance device of late and are revolutionizing the display and illumination sectors of our economy. Due to demands for better performance and reduced energy consumption there is a constant race towards converting every single electron hole pair in the device to photons and extracting them as well while using only the minimum required voltage.