Influence of Al precursor and oxidant on the performance of ALD-Al2O3 ultrathin films embedded in SiO2 for modulation acceptor doping of silicon

Modulation acceptor doping (MAD) induces high hole densities in silicon nanostructures via trivalent acceptor states in ultrathin SiO2 coatings. Thereby, conventional dopant impurity incorporation into the Si lattice, with all its detrimental side effects on the nanoscale and concerning mobility deterioration, is circumvented. In this work, we study the influence of the atomic layer deposition (ALD) chemistry of Al2O3 in SiO2/Al2O3 stacks on MAD by systematically varying the metal precursor, i.e., trimethylaluminum and dimethylaluminum isopropoxide, as well as the oxidant, i.e., H2O, O3, and O2-plasma. MAD metal–oxide–semiconductor capacitors and van der Pauw Hall structures are investigated to correlate the net fixed charge density in the dielectric with the actual hole sheet density in Si, thereby testing whether the electrically active charge is dominated by Al-induced acceptor states. Remarkably, all ALD combinations yield large negative fixed charge densities of ∣Qfix∣ ≈ (5.5–6.9)×1012 cm−2, which directly correlate with equivalently high hole sheet densities of psheet ≈ (5.0–5.8) × 1012 cm−2. This yields a high hole density to oxide fixed charge conversion ratio (η) ranging from 0.84 to 1.0. Overall, the results demonstrate that the electrical MAD response is intrinsically robust and largely insensitive to the specific ALD-Al2O3 chemistry, although the number of ALD cycles required to reach saturation is chemistry dependent. In addition, the close agreement between |Qfix| and psheet shows that the C–V-derived Qfix, although a net dielectric charge, can serve as a direct proxy for the hole density induced by MAD in Si.