| The high-efficiency and high-power LEDs for white light illumination attractmore and more attentions for world-wide GaN-based semiconductor LED researcherand manufacturer. It will be future development trend of LED industry. It is wellknown that patterned sapphire substrate (PSS) cannot only decrease the threadingdislocation density of GaN epitaxial films, but also increase the light extractionefficiency of GaN-based LEDs as well.In this article, nanopatterned sapphire substrates (NPSS) were prepared bydual-stage annealing of patterned Al thin films prepared by electron beamlithography (EBL), which can overcome the puzzles of common direct etching forsapphire crystal.Based on it, low-cost, high-through and large-scale NPSS were then obtainedby solid phase reaction of patterned Al thin films prepared by laser interferencelithography (LIL). Finally, the evaluation of NPSS fabricated by LIL forhigh-efficiency and high-power LED epitaxial wafers were achieved by crystalquality comparasion of GaN-based blue LED epitaxial wafers grown on NPSS andun-patterned sapphire substrate (UPSS).The experiments of sputtering Al thin films on sapphire substrates for PSSshow that the surface roughness of Al thin films increases with increasing DCmagnetron power, Ar gas pressure, and substrate temperature, respectively.Eventually, the smallest mean surface roughness (Ra=1.34nm) for200nm Al thinfilms on sapphire substrates can be obtained with DC magnetron power of60W, Argas pressure equal to0.2Pa, substrate temperature of70°C.The experiments of patterning Al thin films by EBL show that the proximityeffect of EBL could be corrected by dose modification and shape modification.Moreover, the size of Al patterns prepared by EBL increases with increasingexposure dosage. It can be successfully obtained on sapphire substrates for a seriesof Al patterns with different shapes (circular and square), fill factors (0.5,0.67, and0.75) and aspect ratios (2.31,3.85, and5.39).The theoretical calculation results for patterning Al thin films by LIL show theperiod of patterns prepared by LIL increases with decreasing the angle (θ) of twointerfered beams. It can be successfully obtained with the angle of two beamsinterfered θ and exposure time were set to9.35°and8min on sapphire substrates forsquare patterned Al thin films with the diagonal length of615nm, period of990nmand height of200nm.NPSS can be fabricated by annealing of patterned Al thin films prepared by EBL and LIL, respectively. Firstly, it happens low-temperature oxidation annealing.The hillocks formation on Al thin films is minimized with an oxidation annealing at450°C for24h. The little change in the morphology of patterned Al thin films isobserved at450°C for24h. It is observed for following high-temperature annealingat900-1200°C results in the growth of the underlying sapphire single crystal byconsuming the oxide layer. SEM results show the patterns are retained on sapphiresubstrates after high-temperature annealing at less than1200°C. The XRD andRaman results reveal that the composition and orientation of island patterns by dualstage annealing of patterned Al thin films for24h at450°C, and1h1000°C are thesame as that of sapphire (0001) substrates.The experiments of the preparation of GaN-based blue LED epitaxial wafers onNPSS by LIL show that the1.67×1010cm-2dislocation reduction of GaN epitaxialfilms on NPSS prepared by LIL is associated with the bending of threadingdislocations during the overgrowth. The crystal quality of InGaN/GaN multiplequantum wells (MQWs) of GaN-based blue LED epitaxial wafers on NPSS preparedby LIL is better than that of GaN-based blue LED epitaxial wafers on UPSS.Moreover, the interface of InGaN/GaN MQWs of GaN-based blue LED epitaxialwafers on NPSS prepared by LIL is more sharp than that of GaN-based blue LEDepitaxial wafers on UPSS. The8times increase of PL intensity confirms thereduction of quantum confined stack effect caused by the stress of InGaN/GaNMQWs for GaN-based blue LED epitaxial wafers on NPSS prepared by LIL. |
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