Micro-structural and optical investigation of semi-polar (11-22) III-nitrides overgrown on regularly arrayed micro-rods

This project aims to systematically study the micro-structure and optical properties of semi-polar (11-22) GaN with a step-change in crystal quality achieved by means of overgrowth on regularly arrayed micro-rods. The optical properties of (11-22) InGaN-based quantum wells grown on such high quality GaN templates have been investigated in comparison with their c-plane counterparts.
The overgrowth of semi-polar (11-22) GaN grown is performed using a metal organic chemical vapour deposition (MOCVD) technique on specially designed micro-rod arrays on standard m-plane sapphire. Owing to the specially designed patterns of the micro-rod template, our overgrowth technique effectively mitigates the intrinsic issue of the anisotropic lateral growth rate. Thanks to the massive overgrowth work of semi-polar (11-22) GaN on sapphire which were mainly carried out by Dr. Yipin Gong, I have the chance to investigate the defect reduction mechanism in details by using transmission electron microscopy (TEM) measurements. It has been found that most of the dislocations and the basal stacking faults (BSFs) have been effectively blocked by the SiO2 masks on each micro-rod and then the coalescence processes during the overgrowth. By favouring the growth along the c-direction (the c-direction growth leads to free-defect, while the defects can be penetrated through the growth along the a-direction), the defects as a result of the growth along the a-direction could be blocked. As a type of two-dimensional defect, BSFs can expand within the basal plane, propagating with a component parallel the m-direction. This leads to a distribution of BSF-free regions periodically separated by the BSF regions along the [-1-123] direction. Furthermore, each BSF region typically consists of low density BSF clusters and high density BSF clusters with a periodic distribution along the m-direction. Finally, a defect reduction model has been established in order to study the influence of the micro-patterning on defect reduction.
Further investigation was also carried out on the influence of micro-rod diameter on the crystal quality of overgrown semi-polar (11-22) GaN in order to optimize our micro-rod design. It has been found that the BSF density decreases monotonically with increasing micro-rod diameter from 2 to 5 μm, and then starts to be saturated when the micro-rod diameter further increases. However, the dislocation density reduces significantly when the micro-rod diameter increases from 2 to 4 μm, and then increases slightly with further increasing the diameter to 5 μm. In addition, it has been found that it is effective to employ shorter micro-rods to further reduce BSFs, allowing for further improvement in crystal quality. The best crystal quality of the overgrown (11-22) GaN has been achieved by overgrowth on micro-rods with a 4 μm diameter and a 0.4 μm height, corresponding to a dislocation density of 2.0 × 108 cm-2 which is better or at least equivalent to standard c-plane GaN with a similar thickness grown on c-plane sapphire. The best sample achieved so far exhibits a BSF density of 2.8 × 104 cm-1.
In this project, a number of advance optical characterization methods have been performed on a large number of semi-polar (11-22) InGaN/GaN MQWs with a wide spectral range of up to yellow spectra region grown on high quality semi-polar GaN, aiming to systemically investigate the mechanisms which form the large Stokes shift that generally occurs to InGaN/GaN MQWs. This is a long-standing issue. The semi-polar (11-22) samples exhibit a lower Stokes shift than their c-plane counterparts, although they show larger exciton localization than their c-plane counterparts. The Stokes shift in the semi-polar samples shows a linear relationship with the emission energy in long wavelength region, but with a smaller gradient compared with that in the c-plane counterparts. The time-resolved photoluminescence (PL) measurements reveal a significant reduction in the piezoelectric fields of the semi-polar sample. It is suggested that the piezoelectric field induced polarization is the major mechanism for causing the large Stokes shift in conventional c-plane InGaN/GaN MQWs.