Enhancing light extraction efficiency of InGaN/GaN multi quantum well light emitting diodes with embedded two dimensional photonic crystal structures
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Advance in the growth methods of III-Nitrides and researches in order to eliminate doping problems of gallium nitride (GaN) resulted in high band gap materials with increased crystal quality which have led to tremendous improvement in opto-electronic devices. Their durability under harsh environmental and operational conditions such as high pressure and high temperature, and large spectral coverage range including 200 nm deep ultra-violet (deepUV) through 1500 nm infra-red (IR) make them excellent candidates for opto-electronic applications such as full color LCD panels, biomedical sensor devices, high resolution printers, high density storage devices, defense systems. Among III-Nitrides, GaN has attracted the most interest with its high electronic band gap of 3.4 eV at room temperature and ability to form compounds with other group III elements aluminum and indium which have band gaps of 6.2 eV and 0.7 eV respectively which results in a large optical spectrum. However, potential performance of III-Nitride based devices is hindered mainly because of an optical phenomenon called total internal reflection (TIR). High dielectric constant of these materials prevents light to escape from the structure. Light is totally reflected back from the air-nitride interface for incident angles larger than critical angle. With a refractive index of 2.7, GaN material has 22°-24° critical angle which means less than 12% of light can just escape the structure. While 66% of the light generated in quantum well region of a GaN based light emitting diode (LED) is trapped in the GaN layer, 22% of the light is guided in the sapphire substrate. Because of TIR, although internal quantum efficiency of GaN based LED with emission wavelength of around 400 nm is almost unity (higher than 90%), external quantum efficiency is very low. To enhance extraction, a lot of geometrical methods including surface roughening and facet shaping have been tried to reduce the effects of dielectric contrast between the device and medium. In order to increase the extraction efficiency of GaN based LEDs, two dimensional photonic crystals were used in this thesis. LED wafers used were fabricated in collaboration with University of Santa Barbara, California (UCSB) which are InGaN/GaN multi quantum well (MQW) structures that emit light at 390 nm and 410 nm respectively. These LED wafers were processed in the scope of the thesis and photonic crystal (PC) structures were patterned on the p layer of the device. This thesis work is concentrated on two parts; first part is characterization and fabrication, and second part is simulation. In characterization and fabrication part, firstly GaN material etching characterizations were completed using dry etching method by reactive ion etcher (RIE) since there had been no optimized recipe for GaN processing in the clean room know-how. In this characterization, main parameter was the etching anisotropy since vertical side-wall is crucial for LED processing and PC formation. After characterization step was completed, p doping activation was done in Middle East Technical University, Ankara (METU) and then LED wafers were processed in clean room class-100 environment at Bilkent University, Ankara by using the know-how and recipe obtained from characterization stage. As a final step, two dimensional square and triangular photonic crystal lattice structures were patterned by using electron beam writer and these structures were transferred on to p layer of GaN LED by using RIE. Measurements and imaging regarding material and optical properties, fabrication quality and extraction enhancement were done by using scanning electron microscope (SEM), atomic force microscopy (AFM), spectrophotometer (UV-Vis), ellipsometer and probe station. I-V characteristics, optical power measurements and intensity plots on black&white CCD camera images were taken. In simulations part, two and three dimensional simulations using plane wave expansion method, integral method and finite difference time domain method were completed. More than 40000 simulations were run in total during this thesis work. As a result, PCs with 520 nm lattice period and 260 nm hole diameters in square and triangular geometries were modeled and fabricated. Final depth of lattices was around 100-120 nm. Results of 2D integral simulations suggested around 15-20% of error between modeling and experiments because of imperfectness regarding fabrication of PC structures. Furthermore, damaging effects of RIE and focused electron beams were not considered. In measurements, extraction efficiency enhancement factors of about 2.2 and about 2.6 were found using square and triangular PC lattices with respect to LED devices without PC structure patterning. In simulations, while square PC lattice models showed 2.8 times enhancement in extraction efficiency, triangular lattice resulted in 3.1 times with respect to no PC models. In comparison of measurements and simulations, difference in the range of 15- 25% was found which were expected as stated above with also considering the effect of processing damages on top p and p++ layer and quantum well region.
KeywordsGallium nitride (GaN)
Light emitting diode (LED)
Finite difference time domain (FDTD) simulation