The group-Ⅲ nitrides and their related ternary alloys have been becoming the most attractive material for light emitting diodes (LEDs) and laser diodes (LDs) in the UV and blue spectral range. However

the origins of the yellow luminescence (YL)

which is commonly observed in almost undoped and n-type GaN

remain unclear. The nature and the role of initial nitridation of sapphire surface and the generation of threading dislocation (TD) are ambiguous too. It is a possible way to resolve the above problems by studying the microstructures and optical properties combining the growth conditions. In this work

two kinds of GaN films were grown by MOCVD under different initial nitridation time (180 seconds for Sample A

and 90s for Sample B). The growth temperature is about 950℃. The cross-sectional transmission electron microscope (TEM) is performed near the GaN/sapphire interface. Corresponding the growth processes

there are three zones in the GaN layer: buffer layer

"faulted" zone and "sound" zone. There are "haystack-like" domains in the faulted zone

in which there are high density of extended defects. The thickness of the faulted zone is about 0.4μm. Just above this region

the defects density is reduced sharply

and the quality of the layer is improved

which is identified by electron diffraction (ED) patterns. In comparison

Sample A shows about an order lower density of extended defects than Sample B

their columnar diameter is larger and its ED pattern is sharper

too. The buffer layer of Sample B is more smooth than Sample A. The appropriate rougher morphology of the buffer layer may cause that the structural disorder between the high-temperature (HT) grown island and the buffer layer is accommodated by Frank and Shockley partial dislocations. However

the TD is likely to propagate into the HT GaN from the smooth surface of the buffer layer grown after extensively nitridation of substrate. According to the position and width of X-ray diffraction peaks of (0002) and (0004)

the size of crystalline grain

D and the residual strain could be calculated. The values of(D

ε

in

)of Sample A and B are (175nm

0.167%) and (90nm

0.141%)

respectively. Small size of crystalline grain may be advantageous to relieve the lattice and thermal mismatch stress

but it lead to higher density of extended defects. The width of GaN (0002) reflections of Sample A is 4min lower than that of Sample B. This result may be due to the lower screw TD. The higher density of screw TD may widen the GaN (0002) peak in X-ray rocking curve. In room temperature PL spectra

the ratio of band-edge emission intensity to YL one of Sample A is 3 orders larger than that of Sample B. Furthermore the YL in Sample B is stronger and has fine structure

while that in Sample A is nearly invisible in the PL spectrum. The GaN epilayer with high quality exhibits almost no YL emission. It is well known that the YL corresponds to the deformed crystal structure. We assigned that the screw TD and mixed TD are attributed to the YL referring to our XRD results.