Improved efficiency in GaN-based light-emitting diodes by strain engineering

Abstract

III-nitride based semiconductors such as AlN, GaN, InN, and any alloy 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). In recent years, the range 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 low power consumption anovement to realize high-efficiency and high-power LEDs. IQE 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). IQE is also lowered by the separation of the electron and hole wave functions driven by the polarization-induced internal electrostatic field. 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. And the efficiency droop, which is a gradual reduction in the EQE with increasing injection current density, has still to be investigated in greater depth for high-power GaN-based LEDs. Several mechanisms, including electron overflow from the active region, the piezoelectric field that occurs in MQWs, Auger recombination, carrier delocalization, and inefficient hole injection into MQWs, have been proposed to explain the origin of the efficiency droop phenomenon. Among these mechanisms, the piezoelectric field that occurs due to lattice mismatch between the quantum well and quantum barrier materials is currently under active study because the internal electrostatic field that is generated by piezoelectric polarization reduces the overlap between the electron and hole wave functions in the active region. Therefore, in this thesis, we improve the efficiency droop and EQE of GaN-based LEDs by strain engineering for realization of the high-efficiency and high-power LEDs. In the first part of this dissertation, we investigated the effec ts of trapezoidal quantum barriers (QBs) on efficiency droop in InGaN/GaN MQW LEDs. Simulations showed that the electrostatic field in QWs of LEDs with trapezoidal barriers is reduced because of the reduced sheet charge density at the QW-QB interface caused by the thin GaN layer in trapezoidal QBs. Additionally, the InGaN grading region in trapezoidal QBs suppresses hot carrier transport and this enhances efficient carrier injection into the QWs. The electroluminescence intensity of an LED with trapezoidal QBs is increased by 10.2% and 6.7% at 245 A/cm2. when compared with the intensities of LEDs with square-type GaN barriers and multilayer barriers, respectively. The IQE droop of an LED with trapezoidal QBs is16% at 300 A/cm2, while LEDs with square-type GaN barriers and multilayer barriers have IQE droop of 31% and 24%, respectively. This IQE droop alleviation in LEDs with trapezoidal QBs is attributed to the reduced energy band bending, efficient hole injection, and more uniform hole distribution in the MQWs that results from reduction of the piezoelectric field by the trapezoidal QBs. In the second part of this dissertation, we demonstrate the enhanced optical and electrical properties of InGaN/GaN MQW LEDs with strain-relaxing Ga-doped ZnO transparent conducting layers (TCLs). Ga-doped ZnO was epitaxially grown on p-GaN by metal-organic chemical vapor deposition. The optical output power of LED with a 500-nm-thick-Ga-doped ZnO TCL increased by 30.9% at 100 mA, compared with that of LED with an indium tin oxide (ITO) TCL. Raman spectroscopy measurement and the simulation of wavefunction overlap of electron and hole in MQWs revealed that the enhanced optical output power was attributed to the increased IQE due to the decreased compressive strain in the active region. The increase of optical output was also attributed to the increased optical transmittance of the Ga-doped ZnO TCL owing to its higher refractive index compared to that of ITO TCL. In the third part of this dissertation, we demonstrate that the continuos wavelength tuning in GaN based LED on flexible tungsten (W) substrate by using bending process. The wavelength of LED is shifted due to the external strain. When the LED on W substrate was bended by external force (convex bending process), the applied strain in the active region of LED was determined by thickness and Young's modulus of W substrate the blue LED with a 450 nm wavelength show a blue-shift of 12 nm at 3.12 x 10-3 of tensile strain and a green LED with a 530 nm wavelength show a blue-shift of 15 nm at the same tensile strain due to the relaxation of QCSE resulted from the decrease of band bending of MQWs by the convex bending of LED on the flexible W substrate. These results indicate that the continuous tuning of emission wavelength of LEDs can be achieved by applying the external strain on LEDs for many applications in medical, healthcare, and visible light communication systems.