Optics, Photonics & Lasers

Biography

Biography: Cheng Yen Chien

Abstract

Defect reduction is always an important topic for the researches of epitaxy improvement. Commercial dome-shaped patterned-sapphire substrates (CDPSS) had been designed to tackle this problem during the epitaxy of gallium nitride (GaN), and they did reduce the density of defect considerably. In order to reveal the veiled mechanism of defect reduction, we had executed Raman scattering and x-ray diffraction (XRD) measurements on various samples with different growth time to verify the behavior of defects during epitaxy 1. The results of etch pits density (EPD) had been included in figure 2, too. All the measurements show a trend of rapidly decreasing rate initially, but become smooth after 20 minutes. The reason could be figured out from the TEM cross section images. The empty spaces surrounding the sidewall of slope indicate that the growing rate here is so slow that the lateral growth takes place. When the accumulated strain reaches to a critical level, it forces dislocations to turn toward the interface to release the strain, as the red lines and yellow arrows indicate in the left part of figure 3. These lateral dislocations can block other up growing dislocations under them; therefore the defects reduce rapidly. When the growth of GaN reaches the summit of domes (about 20 minutes), only few thread dislocations (TDs) are left. With the continuous growing of GaN, these TDs could join other TDs as the yellow arrow indicated in the right part of figure 3, and the total TDs reduce gradually further. With knowing of the mechanism of defect reduction, further investigations can be designed. The performance of devices with fairly low defect density can be improved greatly. Even defect free region also be expected. It will improve the performance of electronic device and optoelectronic device. And we believe that not only feasible for GaN, but also for other III-V materials.