A Study of the Origin of Band-A Emission in Homoepitaxial Diamond Thin Films
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OKUSHI, a AND KOJI KAJIMURA b Electrotechnical Laboratory,#0501 1-1-4 Umezono, Tsukuba 305-8568 Ibaragi, Japan
HIDETAKA SAWADA, a HIDEKI ICHINOSE a Department of Material Science, The University of Tokyo 7-3-1 Hongo, Bunkyo-ku 113-8656 Tokyo, Japan TAKASHI SEKIGUCHI a Institute of Material Science in Tohoku University 2-1-1 Katahira, Sendai 980-8577, Miyagi, Japan ABSTRACT The band-A emission (around 2.8 eV) observed in high quality (device-grade) homoepitaxial diamond films grown by microwave-plasma chemical vapor deposition (CVD) was studied by means of scanning cathodoluminescence spectroscopy and high-resolution transmission electron microscopy. Recent progress in our study on homoepitaxial diamond films was obtained through the low CH 4/H2 conditions by CVD. These showed atomically flat surfaces and the excitonic emission at room temperature, while the band-A emission (2.95 eV) decreased. Using these samples, we found that the band-A emission only appeared at unepitaxial crystallites (UC) sites, while other flat surface parts still showed the excitonic emission. High-resolution transmission electron microscopy revealed that there were grain boundaries which contained it-bonds in UC. This indicates that one of the origin of the band-A emission in diamond films is attributed to it bonds of grain boundaries. INTRODUCTION The band-A emission is commonly observed under photo, e-beam or x-ray excitation in all types of diamonds --- natural, synthetic (at high pressure and high temperature: HPHT) or lowpressure chemical vapor deposition (CVD) diamonds. By means of both the cathodoluminescence (CL) and the transmission electron microscopy (TEM) data, it has been considered that the band-A emission seems to be related to a structural defect like lattice dislocations. [1-5] But in a given sample neither all dislocations are luminescent [1,2] nor is there any correlation with the type of the dislocation. [3] Iyer et al. suggested vibronic model for the band-A emission, and simulated with the AMI semi-empirical molecular calculation. [5] They prepared dangling bonds in a dislocation core of diamond, and found that the dangling bonds reconstructed by forming a distorted a bond between the adjacent atoms along the dislocations with large lattice relaxation. Their results showed good agreement with the experimental data, while the structural model of dislocation core has not been revealed yet in any experiments because of insufficient quality of the diamond. Recent progress in our study on homoepitaxial diamond films was obtained through the low CH 4/H2 conditions by CVD. These showed atomically flat surfaces and the excitonic emission at room temperature, while the band-A emission (2.95 eV) decreased. The growth mode of these homoepitaxial diamond films was mainly the step-flow mode, and unepitaxial crystallites (UC) and pyramidal hillocks (PH) were often formed on the surface of the films. The PH included an 87 Mat. Res. Soc. Symp. Proc. Vol. 588 ©2000 Materials Research Society
UC at the center of itself. The intensity
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