TRhe band-A emission (around 2.8 eV) observed in high quality (deveice-grade) homoepitaxial diamond films grown by microwave-plasma chemical vapor deposition (CVD) was studied by means of scanning cathodoluminescence spectroscopy and igh-resolution transmission electron microscopy. Recent progress in our study on homoepitaxial damond films was obtained through the low CH_4/H_2 conditions by CVD. These showed atomically flat surfaces and the excitionic 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 lfat surface parts still showed the excitonic emission. High-resolution transmission elecron microscopy revealed that there were grain boundaries which contanined pi -bonds in UC. This indicates that one of the origin of the band-A emission in diamond films is attributed to pi bonds of grain boundaries.
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