Growth of crack-free GaN on AlN quantum dots on Si (111) substrates by MOCVD
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Growth of crack-free GaN on AlN quantum dots on Si(111)substrates by MOCVD W.H. Sun,a) J. L. Chen,b) L. S. Wang, a) and S. J. Chua a) a) Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore b) Department of Materials Science, National University of Singapore
AlN self-assembled quantum dots (QDs) with high density of ~ 4.4 X1010/cm2 on Si(111) substrates have been grown by low-pressure chemical vapor deposition under a very low V/III ratio of 350. We found that using AlN-QD/AlN buffer two-inch GaN epilayers without cracks were grown, indicating the underlying quantum dots play a crucial role in relaxing the stain of GaN epilayer. The quality and morphology were investigated by atom force microscopy, transmission electron microscopy, X-ray diffraction and optical microscope.
The excellent properties of GaN together with the technical problems of growth are already well known. GaN is grown in many laboratories worldwide applying different techniques in order to get appropriate layers for devices. LEDs over Si substrates are already available commercially,[1,2] while laser diodes are under development at many R&D laboratories.[2] Si as a substrate material for GaN growth promises two advantages. First, the well known cleaning and other technological procedures developed for Si can be used. Second, the substrate can be chemically removed in order to get self supporting GaN templates for further growth. Therefore GaN grown on Si is of great interest despite the large misfit. Most research work has been done on the growth of GaN on Si substrates, and high-bright GaN LEDs have been successfully fabricated from the subsequent structural materials.[1-3] Furthermore, researchers are intending to get blue laser devices.[2] On the other hand, the topics on the mechanisms of growth of GaN on Si substrates along with the improvement of its quality presently need to investigate further and relevant problems are being solved step by step. Due to the large differences in the thermal expansion coefficient and lattice constants between GaN and Si substrates, the cracks on the surface of GaN epilayers can not be usually eliminated.[4-6] As we know, the buffer layer is important for the quality of GaN on either Sapphires or Si substrates. To improve the material quality even without any cracks, research workers have utilised many kinds of buffer on Si substrates, such as SiC, AlO and GaAs ect.[3-5] Meanwhile, Honda et. al used trench method to get crack –free GaN. [7] In this study, we used AlN self-assembled quantum dots as a buffer and grew very uniform crackfree GaN epilayer. GaN was investigated by atom force microscopy(AFM), transmission electron microscopy (TEM) and high-resolution x-ray diffraction ect. and corresponding growth mechanisms are discussed. The AlN self –assembled quantum dots and sequential GaN epilayers were grown onto Si(111) substrates by metal-organic chemical deposition using Emcore D125. The growth started with the deposition of an AlN buffer layer. Total lay
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