.  Therefore, it is of great interest to explore a reliable and quick method for investigating dislocations in AlGaN/GaN heterostructures. To characterize the dislocation density of AlGaN/GaN heterostructures, various evaluation techniques such as high resolution X-ray diffraction (HRXRD), cathodoluminescence (CL), photoluminescence (PL), atomic force microscopy (AFM), and transmission electron microscopy (TEM) have been adopted [3].

In the present study, the Al_xGa_1-xN epilayers with 0 ≤ x ≤ 0.51 on GaN layers were grown by Metal Organic Chemical Vapour Deposition (MOCVD) method. Fig (1) shows typical cross-sectional view of Al_xGa_1-xN /GaN heterostructures on sapphire substrate. The dislocation density of the samples has been estimated through wet-chemical etching and HRXRD. The FWHM of (002) plane of GaN layers, estimated from ω-scan was in the range of 342-404 arc-sec, whereas FWHM of (102) plane was in the range 632-999 arc-sec. Ortho phosphoric acid (H₃PO₄) has been used as chemical etchant for AlGaN/GaN samples. The wet etching process has been optimized by varying the etching time and temperature. H₃PO₄ at 180 ºC for 90 seconds has been found as the optimal etching condition for AlGaN/GaN samples. After etching, the AlGaN/GaN samples have been analysed by SEM and AFM. Figure 2(a) shows the SEM image of etch pits and figure 2( b) shows the AFM image of etch pits formed on the surface of AlGaN layers. The dislocation density of the samples was found to be between 2.1x10⁸cm^-2 and 1.9x10⁹cm^-2. Photoluminescence studies reveal that the near band edge (NBE) emission intensity of AlGaN and GaN layers increases after etching due to increases the surface roughness. The dislocation density estimated from etch pits has been correlated with HRXRD results.

References

[1] Yitao Liao, Christos Thomidis, Chen-kai Kao, and Theodore D. Moustakas, “AlGaN based deep ultraviolet light emitting diodes with high internal quantum efficiency grown by molecular beam epitaxy” Appl. Phys. Lett., Vol. 98, pp. 081110, 2011.

[2] D. Kapolnek, X. H. Wu, B. Heying, S. Keller, B. P. Keller, U. K. Mishra, S. P. DenBaars, and J. S. Specka, “Structural evolution in epitaxial metalorganic chemical vapor deposition grown GaN films on sapphire” Appl. Phys. Lett., Vol. 67, pp. 1541-1543, 1995.

[3] J.L. Weyher, S. Lazar, L. Macht, Z. Liliental-Weber, R.J. Molnar, S. Mu¨ ller,V.G.M. Sivel, G. Nowak and  I. Grzegory “Orthodox etching of HVPE-grown GaN” Journal of Crystal Growth, Vol. 305,pp. 384–392, 2007.

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1. Structural and optical characterization of AlGaN/GaN layers
2. Growth and characterization of Al_xGa_1-xN/GaN/ Al₂O₃ heterostuctures  
3. Effect of nucleation islands coalescence on determining the quality of AlN layers grown by MOCVD