用于Ge外延生长的GOI和SGOI衬底的表面清洁研究

An effective wet cleaning process, optimized for low  temperature Ge epitaxy on thin Ge or SiGe structures with reduced  surface roughening, is proposed. It is found that the HF+HCl  cleaning is the most effective wet cleaning method that is  applicable to the low temperature thermal cleaning. It is also found  that temperature of the thermal cleaning appropriate for thin  Germanium on Insulator (GOI) substrates is approximately 450 o C.  Moreover, the successful formation of the strained-GOI structures  is demonstrated by applying these wet and thermal cleaning processes to thin Silicon-Germanium on Insulator (SGOI)  substrates.

In this report, an effective wet cleaning process optimized for GOI and Ge-rich SGOI  surfaces, which can be combined with the low temperature thermal cleaning, is proposed.  Moreover, the epitaxial growth of (strained-) Ge layers on (S)GOI substrates by using  this cleaning procedure is demonstrated.

First, the upper limit of the temperature of the thermal cleaning is evaluated. The  variations of the RMS values (@ 10µm*10µm) of GOI and Ge bulk substrates measured  by AFM are plotted in Fig.1 as a function of the H2 -baking temperature.

Fig1

No surface roughening is observed on Ge bulk surfaces even if the cleaning temperature  is raised up to 600 o C. The surfaces of GOI layers, however, become rougher at  temperatures higher than 450 o C. It has been confirmed that the upper limit of the thermal  cleaning for GOI substrates is lower than that for Ge bulk substrates. Thus, more  effective removal of remaining surface oxides and impurities is strongly needed in the  wet cleaning processes.

Here, both surface conditions of the HF-cleaned Ge surfaces and the HF+HCl-cleaned  substrates have been characterized by TDS as shown in Fig.4 and Fig.5, respectively. No  distinct difference in the hydrogen desorption spectra is observed between HF-cleaning  and HF+HCl-cleaning. On the other hand, the Cl and HCl desorption peaks are observed  only in HF+HCl-cleaning spectra. These results suggest that Ge surfaces are partially Clterminated and Ge-H bonds are not stable, as previously reported .

Here, the substrates cleaned by the different solutions were loaded into the deposition  chamber together and Ge layers were grown on them at the same time. Many pits, which  are considered to be formed by remaining surface oxides on the GOI substrates, are  observed on the surfaces of the Ge layers on the HF-cleaned GOI substrates, while the Ge  layers having flat surfaces are grown on the HF+HCl-cleaned GOI substrates. Thus, it is  found that the HF+HCl cleaning is the most suitable wet cleaning method which can be  combined with the low temperature thermal cleaning.