氮化蓝宝石衬底上GaN薄膜的微结构与光学性质

用透射电子显微镜(TEM),X射线衍射(XRD)和光荧光谱(PL)等测量手段研究了GaN薄膜的微结构和光学性质。样品是用光辐射加热MOCVD在蓝宝石衬底上制备的。随着衬底氮化时间的增加,扩展缺陷的密度显著增加。在位错密度增加一个数量级时,XRD摇摆曲线半宽度(FWHM)由11″增加到15″,PL谱的黄光发射从几乎可忽略增加到带边发射强度的100倍。结合生长条件,我们对黄光与微结构的关系作了讨论。

Microstructural and Optical Properties of GaN Films on Nitrided Sapphire Substrates

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.