Relaxed Si1-xGex layers serve as ‘virtual substrates’ for strained Si devices. By means of He + -ion implantation and annealing we have achieved strain-relaxed Si1-xGex (x=0.15 - 0.21) layers less than 500 nm thick with a threading dislocation density typically <2x10 5 cm -2 , comparable to the lowest published values for graded SiGe buffer layers. The root mean square (RMS) surface roughness is less than 0.8 nm, about an order of magnitude lower than for graded SiGe buffer layers. As much as ~85% of the lattice mismatch strain is relieved, depending on the thickness of the SiGe layer. After He + implantation into the Si(001) substrate below a strained SiGe layer grown by ultra-high vacuum chemical vapor deposition (UHVCVD), platelets that emit prismatic dislocation loops are formed below the Si/SiGe interface during subsequent annealing. These dislocations glide towards the interface where they break, and glide further to form a misfit dislocation network that contributes to strain relaxation. A novel method to determine threading dislocation densities in these samples using atomic force microscopy is also discussed.
By: Silke Christiansen, Patricia M. Mooney, Jack O. Chu, Alfred Grill
Published in: RC22691 in 2003
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