Fabrication And Characterization Of Si-based SiGe Relaxed Buffer Layer By Oxidation

Si-based microelectronic and optoelectronic devices with high performance were almost based on the strain-relaxed SiGe buffer layers.To obtain high quality relaxed SiGe buffers,various growth techniques have been developed.The conventional method for obtaining compositionally graded SiGe buffer is to gradually increase the Ge content with slow grading rate(about 10%/um),and thus,thick buffer layers are required for obtaining higher Ge concentration at the surface.The thick layers are not only time consuming but also enhance surface roughness.A suitable method was needed for the fabrication of relaxed SiGe buffer layers with high quality.During oxidation of Si_1-xGe_x,Ge was rejected from the oxide and piles up at the oxidizing interface to form a Ge-enriched layer.In this paper,the Ge-condensation technique was utilized to fabricate a thin SiGe compositionally graded buffer on Si substrate and relaxed SiGe on insulator.The following studies were carried out in the present thesis:1.Based on the D-G model,we proposed a kinetic model for the oxidation of SiGe. This model was capable of predicting the oxide thickness as a function of time.The simulation results were in good agreement with experimental data.2.â‘ An ultra-low dislocation density of 1.2×10⁵cm^-2,98%strain relaxed, compositionally graded SiGe layers were prepared by dry oxidizing the strained Si_0.88Ge_0.12 alloy on Si-sub at 1000℃.â‘¡By the comparison of the samples for various oxidation times,the relaxation mechanisms of the strained SiGe layers during the oxidation processes were analysed.The saturation of oxidation is observed for all the SiGe samples under investigation at different temperature in both wet and dry ambient. We suggest that the oxidation limiting issue should be due to the absence of oxygen diffusing through the oxide to the oxidizing interface.It is found that the oxide at the surface is not pure SiO₂,but mixing of GeO₂ and SiO₂,which may be the real reason for terminating oxidation of the SiGe layers.â‘¢Different from the monotonous increase with oxidation time for the SiGe layer oxidized in dry oxygen,the degree of strain relaxation in the SiGe layer increases up to about 90%in a short time and then decreases slowly during wet oxidation at 1000℃.The results suggest that the expansion in volume due to the transformation of SiGe to oxide has great contribution to the anomalous strain relaxation process for fast oxidation in wet ambient.Thermal annealing effects on a thin compositionally graded SiGe buffer layer on Si-sub fabricated by oxidizing a SiGe layer are investigated.It is suggested that the strain relaxation of such a SiGe buffer layer during thermal annealing is dominated by Si-Ge intermixing,rather than generation and propagation of misfit dislocations or surface undulation.â‘£Finally,a thick highly relaxed Si_0.72Ge_0.28 layer was grown on the SiGe buffer layer.3.High quality SiGeOI buffer layers with small surface roughness,dislocation free and uniform Ge content(0.24,0.33) were fabricated by dry oxidation at 1150℃. Furthermore,by using cyclical thermal oxidation and annealing of 15min.at 900℃, Si_0.56Ge_0.44OI & Si0.4Ge_0.6OI buffer layer had been obtained.The strain relaxation during the oxidation process had been discussed and a relaxed Si_0.72Ge_0.28 layer was grown on the Si_0.67Ge_0.33OI buffer layer.