Epitaxy Of Strain-relaxed SiGe, Ge Films And Fabrication Of Ge Photodetectors On Si Substrates

Si-based SiGe materials have been extensively applied in optoelectronic devices such as photodetectors and high mobility metal-oxide-semiconductor field effect transistors due to their advantageous properties,especially in compatibility with Si microelectronic processing.Si-based SiGe materials and their related devices have attracted great attention.However,it is a great challenge to directly deposit high-quality SiGe,especially Ge films on Si substrates due to the large lattice mismatch between Ge and Si.This thesis focuses on the growth of relaxed-SiGe and Ge virtual substrates(VSs)in ultra-high vacuum chemical vapor deposition (UHV/CVD)system using low temperature buffer technique and the fabrication of Si-based Ge photodetectors.The following are the details:An approach for the growth of relaxed-SiGe VSs with low temperature Ge (LT-Ge)islands buffer was proposed and intensively studied.The role of LT-Ge islands buffer in the mechanism of strain adjustment and dislocation annihilation was analyzed.High-quality relaxed-Si_0.72Ge_0.28films were grown on Si(100)substrates in UHV/CVD system using LT-Ge islands buffers.Si_0.72Ge_0.28film with a thickness of only 380 nm has a strain relaxation degree of 99%and a threading dislocation density less than 10⁵ cm^-2.No cross-hatch pattern is observed on the SiGe surface and the surface root-mean-square(rms)roughness is less than 2 nm.Annealing experiment indicated that SiGe films had great thermal stability.The influence of growth temperature and thickness of LT-Ge buffers on the surface morphology and strain relaxation of epitaxial Ge(epi-Ge)films was systemically investigated,and the role of LT-SiGe and Ge double buffers to improve the quality of epi-Ge films was studied.High quality relaxed-Ge films were grown on Si(100)substrates in UHV/CVD system with optimized buffer layers.X-ray diffraction peak of 400 nm-Ge epitaxially deposited on Si substrate is symmetric with a full width at half maximum(FWHM)of 460 arc sec,the threading dislocation density is measured to be 5×10⁵ cm^-2using etching pits counting method.There is no cross-hatch pattern on the Ge surface and the surface rms roughness is only 0.7 nm.The origin of tensile strain in epi-Ge and its effect on the band structure was investigated.Tensile strain was developed when cooling down due to the difference in thermal expansion coefficients between Ge and Si.The experimental data for tensile strain agrees quite well with the theoretical calculation.Tensile strain leads to the direct band-gap shrinkage of epi-Ge at a rate of 140 meV per percent calculated based on deformation potential theory.Normal-incident Si-based Ge PIN photodetectors were designed and fabricated. The dark current density is 20 mA/cm² at-1 V reverse bias.At wavelength 1.55μm, the responsivity is 0.23 A/W at-2 V reverse bias.The absorption coefficient of tensile-strained epi-Ge at 1.55μm is calculated to be 3000 cm^-1,which is 3 times that of bulk Ge.Response spectrum of epi-Ge photodetector fabricated on Silicon-On-Insulator substrate shows strong resonant enhancement effect.