Study On The Epitaxial Growth And Defect Modulation Of Heterostructures For Si Photonics

Due to the potential failing of Moore's Law,silicon photonics,which has many advantages such as high performance,small sizes,low power consumption,and low cost,has become one of the means to overcome the bottleneck of current Si-based integrated circuits.As for the material choices,Ge on Si and Ga As on Si are very promising for silicon photonics due to their advantages including adjustable direct band gaps and high mobilities.However,due to the large mismatch in both lattices and thermal expansion,high-quality epitaxial growths of Ge and Ga As on Si substrates are still full of challenges.The high defect density has been limiting the performance of the hetero-structures and devices grown on Si substrates.This thesis aims to solve the problems in highly mismatched heteroepitaxial growths of Ge on Si and Ga As on Si.We have used molecular beam epitaxy(MBE)technique to achieve high-quality epitaxial growth,and in-situ annealing technique in ultra-high vacuum(UHV)to modulate the interfaces of Ge/Si and Ga As/Ge,and finally achieved the rearrangement of misfit dislocations at the Ge/Si interface and high-quality single-crystalline Ge films.Moreover,the anti-phase domain(APD)density in Ga As/Ge/Si is reduced by 70%.Both high-quality Ge/Si virtual substrates and the integration with Ga As are realized on Si substrates.The key results of this work can be summarized as follows:(i)the surface structures of the grown Ge and Ga As together with the kinetics of the initial nucleation during the MBE growth were elaborated in detail;(ii)the effects of in-situ annealing on the strain status and defect density of the Ge film were studied,and results showed that after annealing the strain status changed from compressive to tensile,and the dislocation density decreased from 109 cm-2 to 107cm-2;(iii)through the optimization of the annealing conditions,the rearrangement of mismatch dislocations at the Ge/Si interface was achieved,and periodic 90° interfacial misfit(IMF)dislocations were formed which were attributed to the high mobility and low defect density in the Ge films;(iv)the Hall effect measurements revealed the scattering mechanisms by the defects in the Ge films,and the hole mobility of the Ge film was close to 1300 cm2/V·s,which is the highest reported value for Ge on Si;(v)the effects of growth temperature,V/III ratio and annealing conditions on the quality of the Ga As epitaxial films were studied,and it was found that the optimization of in-situ annealing conditions could significantly reduce the surface roughness from 1.695 nm to 0.350 nm and the APD density from 3.23 ?m-1 to 0.96 ?m-1.In this study,high-quality epitaxial growth and effective defect control are achieved in the Ge films grown on Si by appropriate epitaxial techniques and annealing treatments.Ge/Si virtual substrates with high hole mobilities are obtained,making it possible to fabricate various photonic devices(such as photodetectors,lasers,waveguides,etc.)with superior performance on Ge/Si.The high-quality Ga As films on the Ge/Si virtual substrates provide a good platform for the integration of III-V photonic devices with Si.The study on the defect modulation and the mechanisms behind,especially the successful regulation of the misfit dislocations at the interface,will provide a deeper understanding and helpful guidance to realize more heterostructures with a large mismatch.