MOCVD生长InxGa1-xN合金微观结构和光学特性

利用发光光谱、X射线衍射(XRD)、原子力显微(AFM)等实验方法对MOCVD生长的InxGa1-xN合金进行了研究。原子力显微图样表明样品表面出现纳米尺度为微岛状结构。样品PL和PLE谱表明,其主要吸收峰位于波长为365,474nm,发光峰的位置位于波长为545,493nm处,其中545nm发光峰半高宽较493nm发光峰宽,这两个峰分别起源于In(Ga)N浸润层和InGaN层发光,浸润层局域化激子和岛状微观结构弛豫特性是产生发光峰Stokes移动的重要原因。

Investigation on Optical and Micro-structural Properties of Inx Ga1-x N Alloys Grown by MOCVD

The photoluminescence (PL) and photoluminescence excitation spectra (PLE) of selected InxGa1-xN samples grown by metal organic chemical vapor deposition (MOCVD) have been investigated, and the analysis of structural and optical behaviors has been taken to study the existence of the microstructure InGaN alloys on the basis of X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements.XRD reveals that the sample is a single crystalline InxGa1-xN film formed predominantly in (0002) direction, the In mole composition x in InxGa1-xN film is calculated to be about 0.2 according to the Bragg's angle of InxGa1-xN (0002). The AFM plane-view and 3D images of the selected In0.2Ga0.8N sample show almost uniform island-like microstructure appearing to be composed of granular-crystalline in nanometer scale. The islands composed of the grapes-like have an average height of 10 nm at most and a mean size of 300 nm. The self-organized microstructure appeared to form cone and/or pyramid shape and tended to agglomerate island-like surface with RMS roughness about 10 nm, which attributed to Stranski-Krastanov growth mechanism. The comparison between PLE and PL under different exciting or monitoring energy allows determining the existence of wetting layer (WL)in the sample. The PLE and PL measurements show that the resonant absorption is related to the band gap and wetting layer of the alloy at different exciting and monitoring energy. It has been found that PLE peaks of the In0.2Ga0.8N alloy are dominated by 365 nm and 474 nm with monitoring wavelength 545 nm and 493 nm; the broad 545 nm and narrow 493 nm emission peaks are originated from In(Ga)N wetting layer and InGaN region respectively. The structural and optical analyses suggest that the origin of the exciton localization center is the self-formed islands. Hence, the irrelevancy of the PL spectra to the gap energy was explained in terms of fluctuation resulting from variations in grain size or shape. This type of localized center provides another physical model for the proposed localized states in InxGa1-xN epilayers. Further characterization of the InGaN systems is underway to help fully understand the correlation between the structural and the optical pro-perties of self-organized micro-structure of InGaN system.