Fabrication and overgrowth of semi-polar and non-polar GaN on sapphire for advanced III-nitride optoelectronics

• Main Author: Xing, Kun
• Other Authors: Wang, Tao
• Published: University of Sheffield 2015
• Subjects: 621.3
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.657007

Fabrication of nano-rod and micro-rod array templates and then overgrowth of semi-polar (11-22) and non-polar (11-20) GaN on the templates on sapphire have been performed. Two simple but cost-effective fabrication techniques based on self-organized nano-masks and ultraviolet (UV) photolithography approaches have been developed, respectively. Optimization in the fabrication of non-/semi-polar nano-rod structures and semi-polar micro-rod structures has been systematically performed. The GaN nano-rod and micro-rod arrays with high reproducibility and good uniformity across two inch wafers have been achieved. Metal organic chemical vapour deposition (MOCVD) has been employed to conduct GaN overgrowth on the semi-polar or non-polar GaN nano-rod array templates which are fabricated based on standard semi-polar or non-polar GaN on r-plane (for non-polar) or m-plane (for semi-polar) sapphire prepared using MOCVD. Great improvements in crystal quality for both semi-polar and non-polar GaN have been achieved, which have been confirmed by both x-ray and transmission electron microscopy (TEM) measurements. It has been found that the diameter of nano-rods plays a significant role in reducing dislocation density. With increasing diameter of nano-rods, the dislocation density of the overgrown GaN is reduced. By using the self-organized masks, a quick coalescence with a less than 1 µm thick layer has been achieved, which is much less than those using any conventional overgrowth techniques (typically 10-20 µm). An approach to the fabrication of semi-polar micro-rod GaN array structures has been developed, which is based on the combination of mask design and UV photolithography techniques. A systematic study on the overgrowth of semi-polar GaN on the semi-polar micro-rod GaN array structures have been performed as a function of the diameter of the micro-rods. A further reduction in the dislocation densities and especially basal stacking faults has been achieved compared with that using self-organized nano-mask approach. The mechanism for defect reduction in the GaN overgrown on nano-rod/micro-rod templates has been investigated by detailed TEM studies. The dislocations are either terminated by the SiO2 masks or the residue voids formed due to the large GaN lateral growth rate. The propagation of basal stacking fault can be blocked during the coalescence initiated from the sidewall of the adjacent nano-rods or micro-rods. InGaN/GaN multiple quantum wells (MQWs) have been grown on the semi-polar and non-polar overgrown GaN, respectively. The temperature dependent photoluminescence (PL) measurements demonstrate a significant enhancement in the internal quantum efficiency of InGaN/GaN MQWs grown on the overgrown GaN compared with that grown on as-grown c-plane, non-polar, and semi-polar GaN, respectively, as a result of the huge improvement in crystal quality and elimination or mitigation of the quantum confined Stark effect, which has been confirmed by excitation power dependent PL measurements. InGaN/GaN MQWs with a significant enhancement in indium incorporation have been achieved, demonstrating a major advantage of utilisation of semi-polar GaN for the growth of long wavelength emitters. So far, high optical performance with an emission wavelength of up to 590 nm has been demonstrated.