Sb doping and electrical characteristics of ultrathin SinGem superlattices

Abstract

Ultrathin strained layer Si_nGe_m superlattices (SLS) with periods of a few atomic monolayers (n‐ML Si, m‐ML Ge; n+m≤20) have been grown by molecular‐beam epitaxy. On a 〈100〉‐oriented, n⁺ ‐doped silicon substrate a buffer with a Si_1−yBGe_yB alloy layer has been grown applying the concept of strain symmetrisation for the subsequent SLS layer. The thickness of the SLS layer was chosen to be around 200 nm. The buffer and SLS layer have been n doped by incorporation of an antimony adlayer deposited prior to growth. A thin (20 nm), p⁺ ‐doped (B, 6×10¹⁹ cm⁻³ ) Si_1‐ycGe_yc contact layer was grown on top of the SLS layer. We have fabricated mesa photodiodes (A_M=1.3×10⁻⁵ cm²) by standard semiconductor processing techniques and characterized them by current‐voltage (I‐V) and capacitance‐voltage (C‐V) measurements. We compare the Sb‐dopant profiles of the buffer and the SLS layer measured by secondary‐ion mass spectroscopy and by C‐V analysis. Due to depletion of the Sb adlayer the Si_nGe_m SLS samples exhibit a declining Sb‐dopant profile in the growth direction where the integral concentration of the SLS layer is found to depend on the n/m ratio and is up to 60% lower than a reference Si homodiode grown under the same conditions. The Si_0.6Ge_0.4 alloy sample shows a 2–3 times lower Sb concentration than the corresponding superlattice samples with the same integral Ge content (Si₆Ge₄ and Si₁₂Ge₈ SLS). We conclude that the Sb incorporation in a SLS can be used as a probe for the quality of the SLS growth.