Deposition And Nanostructure Evolution Of SIGe Films

SiGe material has a higher carrier mobility relative to Si,and a tunable band structure,being compatible with CMOS technology.With the delelopment of band engineering,SiGe films have been widely used in the field of both microelectronics and optoelectronics.The lattice and band structure can be modulated by introducing nanostructures into SiGe films.Affected by composition,interface and temperature,the nanostructures would transform and appear new characteristics.This thesis is devoted to controlling the compositions and structures in the epitaxial growth of SiGe films,and the formation and evolution of nanostructures during annealing treatments.In this thesis,we developed a new deposition system named Dual-sources Jet-ICPCVD for depositing SiGe films.In the Dual-sources Jet-ICPCVD system,SiH4 and GeH4 source gases are introduced from different positions into active zone of plasma,consequently,the probability of reactions between Si precursors and Ge precursors is highly reduced.Phase separation appears in the SiGe films,and generates a large number of localized Si and Ge clusters.The composition of the doposited SiGe films can be regulated by adjusting the RF power and the flow rate of SiH4,GeH4 and H2,respectively.SiGe nanocrystalline(NCs)formed in the SiGe films during the annealing process.The formation and structure of NCs,in particular,the structural evolution of NCs in Ge rich films were studied.At the beginning of crystallization process,localized Si and Ge clusters crystallize to form small-scale Si and Ge NCs.With the increase of the crystallization rate,the disorder of amorphous films decreases,and this would lead to the percolation of clusters to form larger Si and Ge NCs.As the annealing time is prolonged,atoms in NCs obtain enough energy and start migrating to reduce the Gibbs free energy of the system.In Ge rich films,affected by strain,Ge atoms would concentrate in the center of the NCs,and Si atoms would diffuse into the outer shell of NCs to form a Ge core/SiGe shell nanostructure.The atom migration and the resulting morphology,structure evolution during high temperature annealing was investigated.Besides the free energy,surface energy plays a more important roll in atoms transportation.Driven by the lower surface energy of Ge atoms compared to that of Si,Ge atoms segregate to the surface of the columnar grains and the surface of the film.When there are enough Ge atoms piled up on the surface,Ge atom would desorp from the film,consequently affected the morphology of the film.At the same time,with the large amount of Ge atoms transferring from the inside of the film,holes were left.The migration of atoms is effected by the composition of the film.In samples with high Si content,the probability that atoms migrate is lower than that in samples with high Ge content,which which indicates that silicon atoms can prohibit the migration of atoms in SiGe films.By using molecular mechanics method,furthermore,formation energy was simulated for understanding the stability of SiGe NCs.Due to the large proportion of surface atoms in the NCs,Ge atoms with small surface energy occupy the surface of NCs.In the SiGe NCs,the distributions of Si and Ge atoms are related to the overall composition of the nanocrystals.As Ge content is less than 0.5,phase separation does not occur in SiGe NCs.By analyzing the formation energy of SiGe NCs with different compositions and nanostructures,it revealed that SiGe NCs tend to form a Ge/SiGe/Ge structure when Ge content increases over 0.5.