Efficiency improvement of light-emitting diodes by the magnetic field from the ferromagnetic materials

Abstract

Groupt-III nitride semiconductor such as AlN, GaN, InN, and any compunds composed of the mix: (Al, Ga, In)N have a direct band gap and the possibility of a wide band gap anywhere from 6.3 eV (AlN) to 0.7 eV (InN). Recently, the various field of applications of GaN based light-emitting diodes (LEDs) has broadened greatly, from general lighting to information displays, traffic signals, and automotive lighting, because of their intrinsic advantages such as low power consumption, brighteness, reliability, and environmental benefits. The external quantum efficiency (EQE) of GaN-based LEDs, which is defined as the product of the internal quantum efficiency (IQE) and the light extraction efficiency (LEE). The IQE can be improved by adopting surface plasmon, increase the crystal quality, modify localized stated of InGaN active layer, optimize hole concentration in p-type GaN layer and electron overflow, and reducing piezoelectric effect. Furthermore, according to Snell’s law, LEE is limited by the total internal reflection caused by the large difference in the refractive index (n) of GaN (n = 2.4) and air (n = 1). In addition, the light within the escape cone of GaN-based LEDs undergoes Fresnel reflection. Thus, introducing the various method such as surface texturing, photonic crystals, the patterned sapphire substrate (PSS), and hierarchical structures are very important to the enhancement of light scattering at the interfaces and overcoming the abrupt change of refractive index between GaN and air. In case of the quatum dot (QD) LEDs also have drawn a lot of attention owing to their narrow bandwidth, wide emission spectrum, high color purity, good photo stability, and low cost. These advantages make QDLEDs superior for replacing organic light-emitting diodes (OLEDs) or polymer light-emitting diodes (PLEDs). In particular, their emission wavelength strongly depends on the size and material change resulting in a very wide bandgap from 6.14 eV (SrO) to 0.28 eV (PbSe). Because of these advantages, QDLEDs have been applied to flexible light sources, optoelectronics, and other biomedical fields. However, QD-LEDs also require further improvement of their IQE and LEE by adopting various methods such as high quality core-shell material growth techniques and modify carrier injection structures to have a high efficiency QD based light source. Although InGaN/GaN MQW LEDs and QD-LED are commercially available, their IQE and LEE require further improvement to realize high-efficiency and high-power LEDs. IQE of GaN based LED is strongly influenced by threading dislocations originated from the mismatch of the lattice constants and thermal expansion coefficients of GaN and the underlying sapphire substrate. These threading dislocations act as nonradiative recombination centers, suppressing emission from nearby quantum wells (QWs). Furthermore, holes and electrons that do not participate in radiative recombination are wasted, and carriers inside the radiative recombination center escape due to thermal vibration, etc. to enter non-radiative recombination and combine to prevent maximizing the efficiency of the LED. Therefore, it is very important that prevention of the carrier capturing in non-radiative recombination centers and localizes them in radiative recombination centers until they combined efficiently. In addition, QDLEDs still have low electroluminescence (EL) efficiency because of low carrier injection, high nonradiative energy transfer, and poor out-coupling efficiency. Surface or interface defect-related trapping and carrier release through thermal escape also cause low QDLED efficiency. Overcoming carrier release requires localizing the carriers inside the QDs to attain a high efficiency. However, there have not been many studies on preventing carrier release from a QD before radiative recombination. Although methods such as shell thickness optimization and multi-shell structure have been introduced to enhance carrier confinement in the QD core region, they require a precise and optimized process for each QD core and shell material. Therefore, in this thesis, we improve the efficiency and EQE of GaN-based LEDs and QD-based LEDs by introducing the magnetic field and magnetic field gradient for realization of the high-efficiency and high-power LEDs. In the first part of this dissertation, we optimize the growth conditions of various ferrommagnetic materials and magnetization process. Additionally, we show that the maximized magnetic properties can be obtained by the measurement results of the vibrating sample magnetometer (VSM) and Magnetic Force Microscopy (MFM). The VSM results showed that the ferromagnetic Fe layer of high-quality could be secured if conditions such as DC power, chamber pressure, and distance between Fe target and substrate were precisely controlled. In addition, these ferromagnetic materials generally need to optimize the magnetization process to form a magnetic field in the desired direction, thus compare various conditions such as process pressure, process temperature, and external magnetic field strength of the magnetization equipment to check the magnetic properties. Furthermore, since it is possible to generate a magnetic field in a certain direction without a magnetization process in a combination of 3d transition metal and 4d or 5d noble metals, thus we stacked Co and Pt multilayers alternately by optimized DC sputtering process to produce a magnetic field in perpendicular direction. In the second and third part of this dissertation, we demonstrate the enhanced optical and electrical properties of InGaN/GaN MQW LEDs with various ferromagnetic materials. We also showed the magnetic field profile using simulation results in the MQW of LEDs with various magnetic field directions and patterned magnetic layers to help understand carrier trajectory changes inside the LED. In this regard, the optical and electrical properties of InGaN/GaN based LEDs in which ferromagnetic Co/Pt multi-layer was deposited as film and circular disks are confirmed. At an injection current of 20 mA, the ferromagnetic Co/Pt multilayer film increased the optical output power of the LED by 20% compared to an LED without a ferromagnetic Co/Pt multilayer. Furthermore, the optical output power of the LED with circular disks was 40% higher at 20 mA than the output of the LED with a film. We also observed that in the case of InGaN/GaN LEDs with different thickness of the Fe layer and various line patterned Fe layer, show higher optical output power after the magnetization process. The optical output power of LED with 200, 400, and 600 nm-thick ferromagnetic Fe layers were increased 14, 17, 21%, respectively, at an injection current of 20 mA after the magnetization. Furthermore, the optical output power of LEDs with 200 nm-thick Fe layer deposited as 300 µm film, 70 µm width and 10 µm spaced line patterns, and 10 µm wide and 10 µm spaced line patterns increase 14, 18, and 27% after the magnetization, respectively. Moreover, the efficiency of InGaN/GaN MQWs flip-chip LEDs in various wavelengths with a 200 nm-thick Fe layer on the p-ohmic reflector was examined. The varied wavelength of InGaN/GaN LED is owing to different indium contents in MQWs, and the difference of indium density which is known as potential minima causes various increased optical output power in each LEDs with different wavelength. More specifically, the optical output power of the UV, blue, and green InGaN/GaN LEDs with 200 nm-thick Fe layers at an injection current of 20 mA is increased by 7.7%, 13.0%, and 27.0%, respectively, after magnetization. In the last part of this dissertation, we demonstrate that the luminescence of the CdSe/CdS QD-LEDs with Ferromagnetic Co/Pt multil-layer increases by 33.31% at 7.5 V, 22.34% at 7.5 V, and 16.73% at 7.0 V compared with that of QDLEDs. The red QD is larger than the green and blue QDs, so an increase of luminescence enhances with QD size. This shows that the magnetic field gradient affects larger QDs more. This is because QD size determines the carrier capturing potential. Although the magnetic field intensity produced by the circular FCPM disks is the same for all QDs, the magnetic field gradient localizes more carriers in larger QDs because such QDs have sufficient room to trap more carriers in their cores. The broaden full width at half maximum (FWHM) of electroluminescence result also represent that the carrier localization inside of QD is increased after deposition of ferromagnetic Co/Pt multi-layer. These results indicate that the relationship between the magnetic field, magnetic field gradient, and potential fluctuations can provide guidelines and alternative routes to improve design and efficiency of LEDs when adopting proper magnetic materials.