Cross-sectional Cathodoluminescence Study in Ga-polar and N-polar GaN Epilayers
- PDF / 1,263,142 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 102 Downloads / 185 Views
Cross-sectional Cathodoluminescence Study in Ga-polar and N-polar GaN Epilayers X. L. Du1, D. H. Lim1, K. Xu1, B. L. Liu2, A. W. Jia2, K. Takahashi3, and A. Yoshikawa1,2 1 Center for Frontier Electronics & Photonics, Chiba University-Venture Business Laboratory 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan 2 Department of Electronics and Mechanical Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan 3 Department of Media Science, Teikyo University of Science & Technology, 2525 Yatsuzawa, Uenohara, Kitatsurugun, Yamanashi 409-0193, Japan ABSTRACT Highly spatial resolved cross-sectional cathodoluminescence (CL) has been used to study the difference in the defect-formation and growth-mechanism between Ga-polar and N-polar GaN epitaxial layers. These epilayers were grown on sapphire substrates by low pressure MOCVD. Their polarities were controlled by the sapphire nitridation and the trimethylaluminum (TMAl) pre-flow just before the conventional two-step growth, and were confirmed with the coaxial impact collision ion scattering spectroscopy (CAICISS). For the Ga-polar GaN epilayers, cross-sectional CL images show distinct two layers, the upper layer with dark lines along c-direction and the layer close to the film-substrate interface with pyramidal dark regions. These two layers correspond to quasi 2D and 3D growth mode respectively. Whereas, the crosssectional CL image taken on the N-polar epilayers is predominated with small dark spots which were randomly distributed throughout the whole GaN epilayer, illustrating the predominant island growth mode in N-polar films. INTRODUCTION GaN and related III-nitride materials have already generated tremendous interest due to their applications in high-efficiency light emitting diodes, and blue lasers [1]. Naturally, much attention has been paid on the polarity of GaN recently [2-4]. Wurtzite GaN is a material of noncentrosymmetrical crystal structure, thus, Ga-polar or N-polar layers can be deposited. The surface chemistry, epitaxial growth, doping, and defect structure in the Ga- and N- polar epilayers should be quite distinct. However, the understanding of the GaN polarity still remains uncompleted. This can be attributed to the difficulty of measurement and manipulation of GaN polarity [5]. As for the GaN epilayers grown on the sapphire substrate which is the most extensively used in the deposition of GaN and related III-nitride materials, both Ga-polar and N-polar GaN epilayers can be deposited with different morphologies. Usually, Ga-polar sample has a smooth surface. Conversely, the N-polar sample always exhibits hexagonal faceted surface morphology [6]. Due to the centrosymmetrical crystal structure of sapphire, the pre-treatment of substrate and initial growth procedure have strong influence on the determination of the polarity of the subsequent GaN epilayer because the polarity of GaN epilayer is determined by the stacking sequence of atomic planes at the initial growth stage. For example, sapphire nitridation before the conventional two-
Data Loading...
