Study On GaN LEDs On Carbon Nanotube Patterned Sapphire Substrates

In recent years,as a new type of solid-state lighting,GaN-based light-emitting diode(LED)has advantages of low power consumption,long lifetime,high luminous efficiency,etc.It also plays an important role in automotive lighting,display backlight units,etc.Nowaday,patterned sapphire has been widely utilized in the fabrication of GaN based LED.However,GaN-based LEDs and related patterned substrates still suffered following problems.1)Poor heat dissipation of conventional LEDs and inferior crystalline quality caused by mismatch between sapphire and GaN;2)The preparation of conventional patterned substrates is difficult and expensive;3)As an essential processes in fabrication of vertical GaN-based LEDs,traditional laser lift-off technology needed high laser energy threshold,would cause degradation to LED.Focusing on those problems,we investigated the effect of the layer number of carbon nanotube onto the crystalline quality and stress of the GaN epitaxial layer,the optoelectrical and thermal performances of the GaN-based lateral LED device,and also the improvement of laser lift-off process based on carbon nanotube and optoelectrical performance of vertical structure LED devices.The results include:1.Studied the effect of the layer number of carbon nanotube on the crystalline and stress of GaN materials:Two-layer carbon nanotube was found to be the optimal choice to achieve best crystalline quality,lowest dislocation density,and uniform surface flatness.In addition,the insertion of more than two layers carbon nanotubes can also effectively reduce the stress of the epitaxial layer.With the increasing of layer numbers,the stress release is more profound,which plays an important role in the improvement of device performance.2.Fabrication of lateral LED devices on different layers of carbon nanotube:The optical and heat dissipation performance of the lateral LED with 2-layer carbon nanotubes was the best.Compared with the LED without carbon nanotube,the saturation optical power improved 555%,and the peak value of the external quantum efficiency improved 874%.3.The laser lift-off process is improved by carbon nanotube,which the energy threshold of laser lift-off is reduced from 26.0mJ to 20.7mJ,and the energy threshold gradually decreases with the increase of the layer number of carbon nanotube and saturated at 3-layer.It was proved that the reduction of laser threshold was efficient to alleviate the degradation of GaN material and LED devices in LLO process.Fabrication of GaN-based vertical structure LED devices based on carbon nanotube with different layers:The vertical structure LED with 2-layer carbon nanotube has the strongest optical power and external quantum efficiency,which were increased by 452%and 522%compared with 0-CNT LED,respectively.4.Compared and analyzed the optoelectric performance of the vertical structure LED and the lateral LED:It was found that the optoelectric perforamnce of the vertical structure LED was improved compared with lateral one.The vertical structure LED is found to have a lower QCSE due to the release of stress after laser stripping,significantly improving the thermal effect and current crowding effect.