Investigations of Ultra High Vacuum Chemical Vapor Deposition of Silicon: Germanium Alloys on Silicon-on-Sapphire Substrates for Applications to Device Fabrication Technology

The goal of this project was to investigate the growth of SiGe device structures on silicon-on-sapphire substrates to determine the advantages of combining these two technologies. We have found that device quality epitaxial layer structures can be grown using UHV/CVD epitaxy, provided that the silicon-on-sapphire substrates are of the highest quality, i.e. that the stacking fault density is extremely low. Functioning devices were fabricated at SPAWARSYSCEN, San Diego on UHV/CVD-grown Si/SiGe/Si/sapphire wafers provided by IBM. Enhancements in the performance of these SiGe devices compared to UHV/CVD-grown Si/sapphire devices, similar to those observed for similar SiGe structures on bulk Si substrates, were found. However, an analysis of the wafers after device fabrication showed that SiGe heterostructures were degraded due to interdiffusion of Si and Ge during device processing at T>850°C. This work emphasizes the requirement for low temperature device fabrication processes for SiGe devices.
Recent publications showed that greater enhancements in p-channel FETs are achieved in modulation-doped structures having a strained SiGe (x>0.7) carrier channel grown on a strain-relaxed SiGe (x=0.35) buffer layer on Si(001). These layer structures were grown on silicon-on-sapphire substrates and hole mobilities of 804 cm²/Vsec were measured by the Hall effect. This result demonstrates that extraordinarily high mobilities can be achieved on sapphire substrates.