This work studies growth of two major kinds of semi-polar GaN including (11-22) and (20-21) GaN on novel designed silicon (Si) substrates and growth of GaN nanowires (NWs) on planar Si substrates both by means of metal organic chemical vapour deposition (MOCVD).
In principle, semi-polar GaN cannot be achieved by MOCVD growth on any planar Si substrates. In order to obtain semi-polar GaN on Si, patterned Si substrates are required. Two different kinds of simple but cost-effective patterning approaches have been developed to fabricate (113) Si substrates with either inverted-pyramid or stripe patterns, where a combination of a standard photolithography technique, dry etching and anisotropic wet etching has been employed. As a result of these cost-effective approaches, patterned Si substrates with both good uniformity and high reproducibility have been achieved.
By the growth on the (113) Si substrates with inverted-pyramid patterns, semi-polar (11-22) GaN with high quality has been achieved. Taking the major advantages of such a specially designed patterning, both melt-back etching and surface cracks, which are the two major issues for the growth of GaN-on-silicon, have been successfully eliminated. The (11-22) orientated GaN has been confirmed and characterised by detailed X-ray diffraction (XRD) measurements, and detailed microstructural investigation has been performed by transmission electron microscopy (TEM) measurements. The TEM measurements show a significant reduction in crystal defects like dislocations and basal plane stacking fault (BSFs). As a result, good optical properties have been obtained for the (11-22) GaN, confirmed by photoluminescence (PL) measurements.
Stripe-patterned Si substrates have been fabricated in order to allow to achieve semi-polar (20-21) GaN films through the growth on either both sidewalls or single sidewalls of the Si stripes. In the case of the growth on the single sidewall of the Si stripes, a single crystal (20-21) GaN has been obtained. In the case of the growth on both sidewalls of the Si stripes, the growth of (20-21) GaN coalescences and then leads to a surface with an ‘M’ shape. The melt-back etching has been resolved by introducing a number of extra gaps to truncate the Si stripe patterns, allowing the NH3 as a precursor for group V to cover all the exposed silicon surface so that melt-back etching can be effectively suppressed. The ‘M’ shaped GaN self-forms a good cavity which could be further used for the growth of a laser structure, one of the greatest challenges for the fabrication of semi-polar III-nitride based laser diodes as a result of extreme difficulties in cleaving semi-polar GaN. The microstructural investigation and optical properties of the (20-21) GaN have been carried out by detailed XRD, TEM and PL measurements. Finally, growth of InGaN multiple quantum well (MQWs) structures has been attempted on the (20-21) semi-polar GaN in order to validate the excellent crystal quality of the (20-21) semi-polar GaN. Temperature-dependent and excitation-power dependent PL measurements on the InGaN MQWs have been performed, demonstrating high internal quantum efficiency and effectively suppressed quantum confine stark effect, compared to their c-plane counterparts.
It is well-known that the MOCVD growth of GaN NWs on planar Si substrates without any patterning feature or without any pre-deposited metal catalyst is a great challenge, which is completely different from MBE growth, where Ga metal instead of Ga precursor (metal organic source) is used and the growth is performed under high vacuum. In order to address this challenge, a new approach for the growth of GaN NWs on planar (111) Si substrates has been developed, where the key point is to employ Trimethylaluminum (TMA) pre-flowing. It has been found that the formation of Al-Si alloyed nanodots by the TMA pre-flowing is critical to the initialisation of GaN NW growth. The influence of the growth conditions used on the NW morphology has been systematically investigated. By optimizing the growth conditions, straight and cylindrical GaN NWs have been obtained. Based on XRD and TEM measurements, the NWs demonstrate high crystal quality with a low density of defects, leading to superior optical properties confirmed by PL measurements.
