Low Temperature Molecular Beam Epitaxial Growth And Characterization Of Ge_xSi_1-xFilms On Si

Silicon and germanium are a type of well-known semiconductor materials.They are the most important materials which build the foundation of today’s microelectronics industry.High quality Ge_xSi_1-x alloys are highly desired in the field of semiconductor devices,such as optoelectronic devices,detectors,thermoelectric devices,and high speed devices.Besides,Ge_xSi_1-x can be used to integrate Ⅲ-Ⅴmaterials and GeSn alloys to silicon based materials.However,because of the large lattice mismatch between silicon and germanium（4.2%）,growing Ge_xSi_1-x thin films especially pure germanium on silicon substrate with good crystalline quality and low surface roughness has remained a challenge.The contents of this work include:the growth optimization of Ge_xSi_1-x（0<x≤1）thin films on Si（100）substrates by molecular beam epitaxy;the effects of growth temperature,Ge composition and thickness on the film quality;the growth of Ge_xSi_1-x/Si heterojunction structures and the effects of in-situ annealing and ex-situ annealing on the samples.Major results and conclusions are summarized as follows:（1）We have discovered that the direct growth of Ge_xSi_1-x（0<x≤1）thin films,especially those with higher germanium contents,on Si（100）substrates can be realized using a low growth temperature（100-400℃）.（2）We have successfully grown a series of Ge_xSi_1-x（0<x≤1）thin films with the Ge composition x covering the whole range on Si（100）substrates at low temperature.X-ray diffraction（XRD）and atomic force microscopy（AFM）measurements show that the films have good crystalline quality and low surface roughness.As the germanium composition gets higher,the relaxation of the film is easier to occur.（3）Reciprocal space mapping（RSM）study has shown that as the Ge_xSi_1-x film thickness increases,the relaxation level increases and the surface becomes rougher.For samples,with a specific composition,such as x=0.67,the relaxation at different depth within the sample seems to be different,however,the overall relaxation can be improved by in-situ annealing.（4）The critical thickness for the Ge_xSi_1-x film relaxation on Si（100）substrates can be increased by lowering the growth temperature.（5）Ge_xSi_1-x/Si heterojunction structures with good crystalline quality and abrupt interfaces have been grown at low temperature by molecular beam epitaxy.（6）Annealing can improve the relaxation of the sample,but it is necessary to generate defects and dislocations to release the stress,and the sample surface fluctuation normally becomes slightly bigger.From our study,500-600℃ is the annealing temperature limit for pure germanium samples in a nitrogen environment.To summarize,we have systematically studied the epitaxial growth of Ge_xSi_1-x films directly on Si（100）substrates and successfully grown a series of Ge_xSi_1-x films with x covering from 0 to 1.We have studied the relaxation and the way to tune the relaxation in these films.This work has provided an important material platform for the integration of Si and GeSn alloys,III-V semiconductors,etc.