本論文主要研究具有反向極化電場的InGaN量子井發光二極體特性，因為此反向極化電場預期會提高量子井導電帶的等效位障，降低電子溢流的機率；同時，AlGaN/GaN電流阻擋層介面之價電帶的凸起會消失，故而增加電洞注入量子井的機率，使發光二極體發光效率提高並改善大電流下效率衰退的現象。 此研究以兩種結構來實現反向極化發光二極體，第一種結構是利用磊晶再成長的方式製作p-side down發光二極體，此結構有p-GaN 阻值太高與電流擁塞嚴重的問題，導致元件操作電壓過大，影響元件光電轉換效率。第二種結構是加入一層穿隧結構的p-side down發光二極體，以解決上述問題。實際製作之元件顯示，在當電流密度提升至60 A/cm2 時，傳統發光二極體與上層n型氮化鎵厚度為150 nm的穿隧發光二極體效率衰退率分別為33.8%與26.3%，確實觀察到效率衰退的改善。將上層n型氮化鎵厚度從150 nm增加到600 nm，會使穿隧發光二極體的操作電壓從10.2 V降到6.7 V，但是增加n型氮化鎵厚度會使材料吸光率增加，導致發光效率較差。未來尚須解決高電壓與材料吸光的問題，才能使光電轉換效率超越傳統發光二極體。 ;This research is aimed at InGaN quantum well light-emitting diodes (LEDs) with reversed polarization field to reduce efficiency droop issue. Simulation shows that compared with conventional LEDs, reversing the polarization field results in high effective barrier in the conduction band, which reduces electron overflow, and diminished energy spike at the AlGaN/GaN interface in the valence band, which enhances hole injection. These properties would reduce the efficiency droop that often observed on conventional LEDs . Two approaches to realize LEDs with reversed polarization field have been investigated in this work. One is to grow the LED structure on sapphire starting with the p-side layer. However, high sheet resistance of the p-GaN layer and severe current crowding effect lead to a high on-voltage, i.e. forward voltage at 20 mA, of the LEDs prepared by this approach. The lowest on-voltage observed on the p-side down LEDs is 10.4 V. The other approach is to insert a tunneling structure into the p-side down LEDs. Compared with conventional LEDs, the tunneling LEDs with a 150 nm top n-GaN layer exhibit improved efficiency droop at 60 A/cm2 form 33.8% and 26.3%. However, the on-voltage of tunneling LEDs is still as high as 10.2 V. It can be reduced to 6.7 V by using a 600 nm top n-GaN layer at the expense of light output power. In conclusion, the light absorption and high on-voltage issues must be resolved to achieve wall-plug efficiency better than the conventional LEDs.
