In search of optimum conditions for the growth of sharp and shallow B-delta markers in Si by molecular beam epitaxy

Multiple boron delta spikes in silicon, with spacings between 4.3 and 20 nm , have been grown by molecular beam epitaxy at temperatures of about 100 de grees C (LI) and 400 degrees C (S4). The test samples were depth profiled b y secondary ion mass spectrometry using 500 eV O-2(+) at normal beam incide nce. The surface of S4 was guile smooth, with a root mean square roughness sigma<0.1 nm. By contrast, L1 was rather rough, sigma congruent to 0.5 nm. The boron depth profiles of S4 revealed sharp peaks but pronounced tails on either side. The tails, which dominate the dopant distributions at concent rations below about 40% of the peak level, are attributed to defect-promote d boron diffusion during growth. Sample L1 showed boron spikes of larger wi dth above the 10%-20% peak level, but a much more rapid, roughly exponentia l falloff on both sides. This sharpness of the dopant spikes implies the ab sence of boron diffusion during low-temperature growth. The "best" deltas ( those with small width and sharp falloff) were obtained with boron contamin ants of ambient origin that resided at the (oxidized) interface between the substrate and the silicon buffer layer. This observation suggests that bor on atoms in silicon dioxide are rather immobile. Depth profile measurements on crystalline samples, either containing boron deltas or being uniformly doped with boron, revealed severe variations of the B+ signal over a depth of up to 25 nm at normal and oblique beam incidences (up to 50 degrees, als o with oxygen flooding). Silicon matrix signals measured in parallel diet n ot show any variation beyond the transient depth. The initial overshoot of the B+ signal, observed just below the transient depth, sometimes exceeded the stationary signal by more than a factor of 2, and the signal undershoot in extended regions at larger depths was low by up to several 10%. This ar tifact calls for recalibration of previously reported profiles of shallow b oron implantations in silicon. (C) 2000 American Vacuum Society. [S0734-211 X(00)06501-X].