Micro-area Molecular Beam Epitaxial Growth Of Sige / Si Heterostructure

In this thesis the influence of the edge effect of the epitaxial film and the mask material on the strain of the SiGe/Si heterostructure grown in limited area is firstly studied.Further studies indicate that this kind of the edge induced strain relaxation will affect some properties of the epitaxial film in the window.In the experiments different morphologies of the SiGe film in the window with different mask materials are observed,we suggest this result originates from the different strain distribution caused by the edge effect of the mask material.The thermal stability of the SiGe/Si heterostructure is greatly improved by growing the SiGe film in the window.We attribute this result to the edge induced strain relaxation and the blocking effect for the propagation of the misfit dislocation by the edge of the window.Besides we have developed a new method for growing the strained Si film of high quality, which bases on the understanding of the experimental results.The thesis consists of six chapters.In chapter one,the basic properties of the SiGe heterostructure and the aim of the project are introduced.In chapter two,we will introduce the MBE technology for growing the SiGe/Si heterostructure,and the characterization for the SiGe/Si heterostructure.In chapter three,we will report the experimental results about the influence of the edge effect of the epitaxial film and the mask material on the strain and the dislocation density of the SiGe/Si structure grown in micron size windows.Using the Raman scattering technology,we observed the different strain distribution at the boundary area of the SiGe film grown in the window with different mask material(the SiO₂ film or the Si₃N₄ film).We suggest this result originates from the edge effect of the mask material.Chapter four contains two parts.In the first part we will report the experimental results that the different morphology of the SiGe film was observed for the SiGe/Si heterostructure grown in the windows with different mask materials,and we attribute this result to the different strain distribution in the SiGe film,which is discussed in chapter three.In the second part we discuss the thermal stability of the SiGe/Si heterostructure in the micron-size window.Experiments showed that the thermal stability of the SiGe/Si heterostructure in the window is greatly improved.This result may be caused by the edge induced strain relaxation and the blocking effect for the misfit dislocation propagation by the edge of mask. Chapter 5 concems the SiGe film edge induced strain relaxation effect on density and lateral dimension of the nanostructures(quantum dot molecule) grown in micron-size windows.The experimental data showed that the density of the nanostructures decreased along with the reducing of the window size and that the lateral dimension of nanostructures increased along with the decreasing of the window size.It indicated that density and size of nanostructures could be modulated by choosing the window size,when the Ge fraction was fixed in SiGe films.Chapter six includes the following three sections.In the first section,MBE growth of highly relaxed Si_0.45Ge_0.55 virtual substrate with very low dislocation density was studied.By using the Si₃N₄ film as the mask material,Si_0.45Ge_0.55 film was grown on a compositionally stepwise graded SiGe buffer layer in 3μm×3μm windows on Si(001) substrate.The results showed that more than 90%strain of the Si_0.45Ge_0.55 film relaxed,and threading dislocation density is lower than 2×10⁵cm^-2, which is less than that of the film grown on the unpattemed area by a factor of 20.In the second part,we will study the strained Si film,which was grown on the virtual substrate described above.The result showed that for the strained Si layer grown in the window of 3×3μm²,the tense strain reached 1.5%,and no misfit dislocations could be observed,as the sample was etched by the modified Schimmel etchant.We suggest that the strain relaxation and the suppression for the misfit dislocation in the virtual substrate are caused by the combined contribution of the edge effect of the epitaxial film and the compositionally stepwise graded SiGe buffer layer.