| Green lighting is an unavoidable necessity in modern society; however the most used lighting technology, the incandescent lamp, is by far outdated and extremely inefficient yet. In contrast, light emitting diodes(LEDs), the so-called ultimate lamp for the future, enables direct conversion of electrical energy into light with little waste. Highly efficient LEDs with long lifetime are available nowadays; however, they are still of big challenges to cope with the market for high-power LED lighting. In general, sapphire is used as substrate to the epitaxial growth of GaN. A good number of defects, particularly dislocations, can be formed due to the large lattice mismatch and thermal expansion coefficient mismatch between sapphire and GaN, which will critically decrease the internal quantum efficiency. Besides, the total reflection between Ga N and the air can also diminish LED light extraction efficiency. Nowadays, patterned sapphire substrate(PSS) is used as a fundamental technology to overcome such above problems, but is also of technical problems.In this thesis, the main technical problems of patterned sapphire substrates are summaried. The nucleation behaviors of GaN epitaxially grown on PSS at different growth stages are investigated in detail. It is demonstrated that Ostwald ripening process during the recovery procedure of GaN islands leads to the acceleration for GaN coalescence. On PSS, GaN islands grow mainly in the intervals via the Ostwald ripening process. The ordered distribution of patterns provides the same surroundings for islands nucleation. All GaN islands grow up between patterns, and eventually cover the patterns around simultaneously, which thereby shortens the growth period and improves the uniformity of surface morphology, Compared with GaN grown on non-PSS, the PSS case is of 12% improvement in thickness.The LED structures are hence deposited to reveal the reason for quality improvement of LED epitaxial layers on PSS. After studying the defects in the PSS case, the dominant mechanism of defect multiplication and the effects of patterns in terms of materials science and device technology are elucidated. It is demonstrated that the proposed patterns can on the one hand induce the formation of stacking faults, and on the other hand, change the climbing path of dislocation with the lateral growth of GaN. Consequently, the patterns work successfully for the reduction in the number of dislocations spreading to multiple quantum wells, thereby the improvement in crystal quality.To solve the problem of pattern design, a cost- and time effective approach are proposed based on a ray-tracing software, TracePro. The so-called widebottomed cone-shaped patterns whose width is exceeded 3 μm and height is exceeded 1.6 μm are studied in the first time. Simulation reveals that, compared with commercial PSS currently available, PSS with widebottomed cone-shaped patterns is of higher yield in LED light extraction efficiency. The subsequent pattern fabrication and epitaxial growth of LED structure on proposed wide-bottomed PSS verify the simulation results experimentally. The effects of widebottomed patterns on light extraction, crystal quality of wafer as well as stress relaxation are subsequently discussed in detail. Compared with the LED on commercial PSS, the designed PSS-LED with widebottomed patterns is of 12.7% improvement in light output power.To sum up, this thesis focuses on the nucleation behaviors of GaN epitaxially grown on PSS, and address the design of novel PSS based on an unconventional research method. Such results provides supports for the fabrication of highly efficient LEDs. |
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