Enhancements in light output power by MOCVD patterned growth and in situ roughening

Technologies in III-Nitride based Light Emitting Diode (LED) have been developing rapidly in the past decade. Having longer life time and higher light output efficiency, LED is forecasted to be the next generation general light source, replacing the existing incandescent and fluorescent lamps. To realize this, the brightness of III-Nitride LEDs has to be further increased. Currently, the main challenges to enhancing light output power are to reduce the threading dislocation densities in the epilayer, as well as to increase the extraction efficiency of light from the device. This thesis focuses on two areas – the growth of LED on Patterned Sapphire Substrate (PSS) and in situ texturing of LED surface during MOCVD growth. The first part involves the growth and characterizations of LEDs on three chemically wet etched PSSs, which are different in etch patterns and etch depths. It was found that all the LED samples grown on PSSs showed more than 20% enhancement in light output power. Cross sectional TEM study showed that bending of threading dislocations occurred in the bulk GaN grown on stripe-patterned substrate. Such observation suggests that the use of PSS can help reduce threading dislocation density and thus enhance the internal quantum efficiency of the LED. In the second part, four in situ surface texturing techniques are developed and studied. Owing to the large difference in refractive indices between GaN and air (2.4 and 1 respectively), only around 4% of the generated light can be extracted from the surface of an LED. Surface texturing can effectively increase the light extraction efficiency. Electroluminescence (EL) results showed that three of the in situ roughening techniques could effectively increase the light output powers through enhanced extraction efficiencies. It was found that the growth of p-GaN at low temperature was the most efficient technique for improving light extraction efficiency