Polycrystalline GaN with improved c-axis orientation is successfully grown on silica glass substrates by electron cyclotron resonance molecular beam epitaxy after nitridation of the substrate surface. This substrate nitridation gives rise to the formation of a silicon nitride layer, and results in a marked improvement in the uniformity of c-axis orientation and the surface morphology of subsequently grown polycrystalline GaN.

We have investigated relationships between microscopic structure and cathodoluminescence (CL) property of polycrystalline (poly-) GaN grown by electron-cyclotron-resonance plasma-excited molecular beam epitaxy (ECR-MBE) on ZnO/Si substrates. Very strong CL with a peak of 3.45 eV was observed from the poly-GaN, which mainly showed a columnar structure with a size of 50-100 nm. On the other hand, the intensity of CL from the poly-GaN with few columnar domains was weaker than that of the poly-GaN with the columnar structure. The CL image from the poly-GaN, in which the columnar domains were locally observed, showed a strong contrast between bright domains and a dark background. It is confirmed that these bright regions in the CL images are corresponding to the columnar domains of the poly-GaN, by comparing with the SEM images. These results suggest that the columnar domains are responsible for the strong CL from the poly-GaN grown on the ZnO/Si substrates. Cross-sectional transmission electron microscope (TEM) observation revealed that the columnar domains had high quality crystallinity with few defects.

We have investigated relationships between the microscopic structure and optical property of polycrystalline GaN grown by (electron-cyclotron-resonance plasma-excited molecular beam epitaxy) ECR-MBE on silica glass substrates, using scanning electron microscope (SEM) and cathodoluminescence (CL). It was found that CL intensity was stronger for the samples with a large columnar domain size. These individual columnar domains showed clear luminescence. It was found that the origin of strong luminescence from polycrystalline GaN is due to such a columnar domain. That luminescence was closely related to the morphology of the columnar domains. It was revealed that the columnar domain with a homogeneous and hexagonal shape showed clear luminescence.