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

Abstract

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 °C (L1) and 400 °C (S4). The test samples were depth profiled by secondary ion mass spectrometry using 500 eV O2+ at normal beam incidence. The surface of S4 was quite smooth, with a root mean square roughness σ<0.1 nm. By contrast, L1 was rather rough, σ≅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 concentrations below about 40% of the peak level, are attributed to defect-promoted boron diffusion during growth. Sample L1 showed boron spikes of larger width above the 10%–20% peak level, but a much more rapid, roughly exponential falloff on both sides. This sharpness of the dopant spikes implies the absence of boron diffusion during low-temperature growth. The “best” deltas (those with small width and sharp falloff) were obtained with boron contaminants of ambient origin that resided at the (oxidized) interface between the substrate and the silicon buffer layer. This observation suggests that boron 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°, also with oxygen flooding). Silicon matrix signals measured in parallel did not 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 artifact calls for recalibration of previously reported profiles of shallow boron implantations in silicon.