Relationship between Microscopic Structure and Optical Property of Polycrystalline GaN on Silica Glass
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Relationship between Microscopic Structure and Optical Property of Polycrystalline GaN on Silica Glass
Hidetaka Kagatsume, Hiroaki Aono, Tsutomu Araki and Yasushi Nanishi Department of Photonics, Ritsumeikan University, 1-1-1 Noji-Higashi, Kusatsu 525-8577, Japan. ABSTRACT 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.
INTRODUCTION GaN and related compounds have attracted much attention in view of its application to optoelectronic devices such as light emitting diodes (LEDs) and laser diodes (LDs) that operate in the visible-ultraviolet range, because of their wide bandgaps. However, production of the GaN bulk crystal is difficult, due to the high equilibrium pressure of nitrogen at the melting point of GaN. At present, these optoelectronic devices have been fabricated using sapphire or SiC substrates [1-3], although various substrates have been attempted for hetroepitaxial growth. On the other hand, Iwata et al. reported that strong photoluminescence (PL) intensity was obtained from polycrystalline GaN films grown on silica glass substrates [4]. Moreover, it has been reported about impurity doping [5] and improvements of the properties of polycrystalline GaN [6]. We have also demonstrated that satisfactory optical properties were obtained from polycrystalline GaN and suggested the optical properties were closely related to the structure of polycrystalline GaN [7, 8]. More recently, D.P.Bour et al. reported polycrystalline LEDs that used silica glass substrates [9]. Thus, with polycrystalline GaN on silica glass substrates, there is the possibility of being able to apply it to optoelectronic devices. It has the advantage of low-cost and large-area optoelectronic devices. However, concerning strong luminescence as seen from the polycrystalline, the crystal structure and luminescence mechanism have not become clear yet. Thus in this study, we demonstrate the recent results obtained from careful examination of relationships between the microstructure and luminescence from polycrystalline GaN grown on silica glass by electron-cyclotron-resonance plasma-excited molecular beam epitaxy (ECR-MBE). The analyses of the GaN films were performed using scanning electron microscope (SEM), cathodoluminescence (CL) and transmission electron microscope (TEM).
EXPERIMENTAL GaN films were grown on silica glass substrates by ECR-MBE. In thi
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