| Self-assembled GeSi islands grown via the Stranski-Krastanov mode on Si(OOl) substrates have been widely investigated during the last decades.The interest is mainly driven by their promising applications in a new generation of devices compatible with the existing Si technology and by the understanding of strained layer epitaxy.It is well known that the composition of the islands,together with their shape and size, is a critical parameter in determining their optoelectronic properties.As a consequence,to determine the distribution of the composition and misfit strain in the islands is important to understand their structure-property relationship as well as their growth mechanism.In the first part,the composition and the strain distributions in the GeSi/Si(001) coherent islands have been determined by digital analysis of HRTEM images.The results show high Ge content at the central region of island,whereas a lower Ge content near the outmost shell. The island is partially relaxed by the substrate deformation(tensile strain) and strain concentrated around the edge of islands.When the GeSi islands are put into actual applications in optoelectronic and microelectronic devices,it is necessary to bury islands in Si,i.e.to grow an additional Si layer on the GeSi islands.Previous investigations revealed that the GeSi island shape changes after Si capping at a high temperature,whereas by means of low temperature deposition,GeSi islands are embedded into a Si matrix without appreciably altering their shape and composition.The strain energy and/or surface energy may play a key role in the shape transformation.The knowledge about evolution of composition and stain energy distribution is very important to fully understand the island growth mechanism during Si capping.In the second part,the changes of the Ge content and the strain in the self-assembled islands on Si(001) at initial Si capping are investigated by Raman spectroscopy.Both peaks of Ge-Ge and Ge-Si vibration modes show blueshifts after Si capping at 640℃with a layer thickness of 0.32 and 0.64 nm.According to the peak positions of Ge-Ge and Ge-Si,the strain and the Ge composition in the islands are analyzed. It is found that the strain energy in an island increases remarkably after Si capping.After simple analysis on the surface energy,it is concluded that the strain energy in the substrate in association with an island formation and evolution should be included in accounting for the resulting island shape transition during Si capping.For the application of the microelectronic device,if one wants to use the islands as stressors to introduce a strain,thus to increase the electron mobility in the Si capping layer, the preservation of island morphology is useful.However,little attention has been paid on the quality of the low-temperature grown Si capping layer,although such information is also critical for the final devices.In the third part,planar defects were observed in the Si capping layers by TEM,which were overgrown on the GeSi islands at low temperature of 300℃.The generation of the planar defects benefits the strain energy relaxation in the Si capping layer.In the fourth part,after the quantum dots with a low-temperature-grown Si capping layer annealing at a high temperature of 640℃,pits were observed on the surface.The density of the pits was nearly equal to that of the quantum dots,and it's found that the pits were exactly over the quantum dots by TEM.Similar to the case of the planar defects,the generation of the pits benefits the strain energy relaxation in the Si capping layer.
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