GaN is a wide bandgap Ⅲ-Ⅴ compound semiconductor material applicable to optical devices (LED, LD, and full color display) and high density storage devices. For several years, GaN have been attracted tremendous attention as a promising for optoelectronic devices applications in the operating visible to ultraviolet (UV) wavelength region. Due to the lack of GaN substrate, sapphire (α-$Al_2O_3$) is generally used as a substrate for the growth of GaN films. However large lattice mismatch between GaN and $Al_2O_3$ leads to the high defect density in the GaN films causing degradation of optical property.
In this paper, growth of high quality GaN epilayer using metalorganic chemical vapor deposition (MOCVD) method was demonstrated to solve one of the main problems in GaN epitaxial growth. Using low temperature (LT) GaN buffer layer (BL), high quality GaN epilayer can be easily grown with lower defect density. In this study, we tried to optimize the growth condition of high quality undoped GaN by varying LT buffer layer growth temperature. It is found that the LT buffer layer plays a key role in improving the quality of GaN film on a sapphire substrate. For analysis, 2 ㎛-thick GaN was characterized by atomic force microscopy (AFM), X-ray diffraction (XRD), transmission electron microscopy (TEM), photoluminescence (PL) for structural and optical properties of GaN epilayer.
As a results of AFM, good surface morphology was observed GaN epilayers grown on 540℃ BL growth temperature. GaN epilayers grown on 560℃ BL growth temperature observed rough surface morphology. For XRD characterization, different biaxial strain operated interface between GaN and sapphire substrate by changing buffer layer temperature. The biaxial compressive strain of interface was highly operated at GaN epilayers grown on 540℃ BL growth temperature. Also FWHM and CHL was decreased at 540℃. This high compressive strain of interface effects on microstructure of GaN epilayers and it was observed by TEM. Defect density was decreased at 540℃ due to high compressive strain and low thermal stress. Also threading dislocation was bending at 540℃. According to the decreasing of defect density, optical properties were increased and that phenomenon was observed by PL characterization. Therefore it was observed good structural and optical properties at 540℃ BL growth temperature.
