Effects of ultra-high vacuum treatments on n-type Si contact resistivity

Most electronic and photonic devices include ohmic metal-semiconductor junction(s), of which contact resistivity needs to be minimized for best efficiency of the devices. Interface defects in the junction usually degrade the junction's performance, thus cleaning and passivation of semiconductor surface is crucial during contact fabrication. For silicon devices the RCA (Radio Corporation of America) cleaning has been the most known method. Here we have addressed the question whether it is still possible to develop Si surface treatments to decrease the contact resistivity. We have combined wet chemistry and ultra-high vacuum (UHV) heating for two cases: low and highly phosphorus-doped n-type Si. As compared to silicon surfaces treated only with wet chemistry, the contact resistivity is lowered when (i) lowly doped n-Si is rapidly heated at temperature around 1200 degrees C in UHV followed by hydrofluoric (HF) acid dip before Ni sputtering; (ii) p-Si substrate with highly n-type surface is first immersed in HF, then UHV heated at 400 degrees C followed by immersion to HF. Our results show that the final HF dip decreases surface oxide formation in air during sample transfer to the metal deposition, and that surface phosphorus concentration decreases at highly doped n-Si surfaces during elevated temperature UHV heating.