Fabrication of less strained and more efficient GaN LED

Abstract

The wide-bandgap GaN and related materials have extensively been studied and utilized in important optoelectronic device applications such as light emitting diodes (LEDs) and laser diodes. However, in order to realize high-performance and reliable optoelectronic devices, high quality GaN epitaxial layers are definitely required. GaN-based epitaxial layers are often grown on foreign substrates such as ZnO, Si, LiGaO2 and Al2O3. Among these substrates, sapphire (Al2O3) substrates are extensively used because of high quality, transparency, high temperature stability, and availability in large-area wafers. However, large mismatches in lattice constant and thermal expansion coefficient between GaN and sapphire substrates cause several severe problems in the fabrication of high efficiency optoelectronic devices. The three major problems in the GaN-based LED structures grown on sapphire substrates are the high dislocation density in GaN due to lattice mismatch, poor light extraction and significant wafer bowing. High density dislocations, regarded as major non-radiative recombination centers in GaN-based LEDs, typically lower the LED external efficiency and shorten the device lifetime. Moreover, the large difference in refractive index between GaN (2.4) and sapphire (1.7) results in poor light extraction due to total internal reflection. Severe wafer bowing also hinders the mass production of LEDs in large-area wafers. The thermal expansion coefficient of sapphire is much larger than that of GaN so that severe biaxial compressive stress is generated within GaN during cooling process after high temperature deposition. As a result, severe wafer bowing is occurred. Wafer bowing often causes cracks in the GaN epitaxial layers during laser radiation for lift-off process used for the fabrication of vertical LEDs. Moreover, it has been reported that the convex wafer bowing increases the X-ray rocking curve full width at half maximum (XRC FWHM) value of GaN (002) plane up to 7%, indicating the reduction of GaN crystal quality. To overcome these problems, two methods were proposed in this study. Firstly, GaN thin film was grown on silica hollow nanosphere(S-HNS) coated sapphire substrate. Secondly, GaN thin film was grown using sapphire substrate with SiO2 thin film on its backside. To grow GaN thin film using S-HNS coated substrate, S-HNS coated sapphire substrate was made by using nanoscale polystyrene(PS)/SiO2 coreshell sphere monolayer fabricated by modified dip coating method. After dip coating of PS/SiO2 coreshell structure, thermal annealing was followed to remove PS and fixation of S-HNS. And un-doped GaN thin film and LED structure was grown by metalorganic chemical vapor deposition(MOCVD). LED device was fabricated by conventional photolithogra phy, dry etching, and metal electrode deposition using LED structure with and without containing S-HNS monolayers. By insertion of S-HNS into LED structure, XRC FWHM value of (102) plane was reduced from 480 to 345 arcsec, dislocation density was reduced from 4 x 108 cm-2 to 1 x 108 cm-2, compressive stress of GaN thin film was reduced almost 20% and output power of LED device was increased almost 2 times. S-HNS induces nanoscale lateral epitaxial overgrowth so internal quantum efficiency is enhanced. And low refractive index of S-HNS causes photon scattering as a result light extraction efficiency is increased. To reduce the wafer bowing, LED structure was grown on sapphire substrate with SiO2 thin film on its backside. By using this method, wafer bowing is reduced almost 60% and compressive stress is also greatly reduced.