In this report,an effective wet cleaning process is proposed for surface optimization of GOI and germanium-rich SGOI,which can be combined with low temperature thermal cleaning.In addition,the epitaxial growth of(strain)germanium layers on(S)GOI substrates using this cleaning method has also been demonstrated.
(S)GOI substrates with thickness of 25-30nm and active component XEFF of 0.5,0.6 and 1.0 were prepared by germanium condensation process.Here,the active component implies the assumed in-plane germanium component of the SiGe layer of a completely relaxed SiGe crystal.These(S)GOI substrates and bulk germanium substrates are used to check the cleaning process.After removing the silica layer formed during Ge condensation with a diluted HF solution,the(S)GOI substrate was immersed in a diluted ammonium OH solution for 2 min to remove the remaining metal and organic materials and rinsed in ultra-pure water for 5 min.They are then immersed in various acid solutions(hydrofluoric acid,hydrochloric acid,hydrogen bromide,and their mixtures)to clean their surfaces.Soak for 2 minutes unless otherwise stated.After the wet cleaning,the GOI substrate was thermally annealed in a chemical vapor deposition chamber in a hydrogen atmosphere of 700 pa at various temperatures from 400℃to 600℃.Germanium layers were then grown on these(sulfur)GOI substrates using germanium IV as the source gas in the same chemical vapor deposition chamber.Growth temperature from 300℃to 400℃,growth pressure from 1 pa to 100 pa.The substrate surface after chemical cleaning was analyzed by X-ray photoelectron spectroscopy(XPS)and thermal desorption spectroscopy(TDS).The structure and surface morphology of the samples were observed by transmission electron microscope and atomic force microscope,respectively.The composition and relaxation ratio of germanium were measured by out-of-plane and in-plane X-ray diffraction.
Results and discussion
GOI Substrate hot cleaning-Hydrogen baking:
First,evaluate the upper temperature limit for hot cleaning.Root mean square values( 10 m*10 m)of and germanium substrates measured by atomic force microscopy are shown in FIG.1 as a function of calcination temperature.Even if the cleaning temperature rises to 600 degrees Celsius,the surface roughness of germanium blocks will not appear.However,at temperatures above 450 degrees Celsius,the surface of the GOI layer becomes rougher.It has been confirmed that the upper limit of heat cleaning for GOI substrates is lower than that for germanium substrates.Therefore,it is necessary to remove residual surface oxides and impurities more effectively during wet cleaning.
FIG.1 Root mean square values of thin GOI and germanium substrates as a function of hydrogen baking temperature
Wet cleaning of germanium surface:
Next,optimize the wet cleaning process.Germanium-3D XPS spectra of germanium substrates after various wet cleaning are shown in Figure 2.The photoelectron emission Angle of these samples was set to 30°to enhance the signal from the surface.As mentioned earlier,GeO2 and GeOx(x<2)can be etched most efficiently by HBr solution.However,in our experiment,as shown in Figure 3(a),granular products were produced on the germanium surface that remained in the atmosphere for about 30 minutes after HBr etching,although they were not observed on the surface after HBr etching.It was confirmed that these granular products were not attributed to HBr solution because they did not appear on the surface of silica treated by HBr.In addition,atomic force microscopy confirmed that their size and density increased over time.As a result,the granular product may be any product produced after the etching process due to the termination of the unstable surface.On the other hand,HF+HCl solutions can etch GeO2 and GeOx as efficiently as HBr solutions(x<2)without granular products and surface roughness.Therefore,HF+HCl etching is considered to be the most suitable method for removing residual oxides before epitaxy.
Germanium homoepitaxy growth on GOI Substrate:
After HF+substrate was cleaned by HCl and baked in H2 at 450℃,the Ge layer was epitaxial grown on GOI substrate at 400℃.Here,substrates cleaned by different solutions are loaded together into the deposition chamber,and germanium layers grow on them simultaneously.Many pits were observed on the surface of the germanium layer on the GOI substrate cleaned by HF,and these pits are thought to be formed by the remaining surface oxides on the GOI substrate,while the germanium layer with flat surface grows on the GOI substrate cleaned by HF+HCl.It is found that HF+HCl cleaning is the most suitable wet cleaning method,which can be combined with low temperature thermal cleaning.
Strain germanium epitaxial growth on SGOI substrates:
Finally,it is proved that strain germanium layer is epitaxial grown on thin SGOI substrate by the above method.It is found that in order to avoid surface roughness,the growth temperature needs to be reduced in the epitaxy on the lattice mismatch substrate compared with that on the lattice mismatch substrate.It is found that strain germanium layer with flat surface can be obtained at growth temperature below 325℃.As shown in Figure 7:
FIG.7 Root mean square value of 10 nm thick strain germanium layer grown on SGOI(Xeff=0.6)substrate with growth temperature
XTEM images and in-plane XRD measurements also showed that the strained germanium layer successfully grew on the SGOI substrate at 300℃without lattice relaxation,while the germanium layer grown at 325℃with the same flat surface partially relaxed due to the higher growth temperature,as shown in Table 1.Therefore,the strain GOI substrate was successfully fabricated by HF+HCl cleaning,low temperature H2 baking and low temperature germanium epitaxy.
In conclusion,we propose an efficient surface cleaning process that optimizes low temperature epitaxial growth on GOI and germanium rich composite SGOI substrates.It is found that HF+HCl cleaning is the most suitable wet cleaning method for growing epitaxial films on thin GOI layer,requiring a thermal cleaning temperature of less than 450℃.Finally,the successful formation of strain-GOI structures has been demonstrated by applying these wet and hot cleaning processes to a thin SGOI initial substrate.