Donor band formation in Si quantum dots using hot P+-ion implantation

We demonstrated the formation of a donor band even in Si quantum dots (Si-QDs) with fewer than ten donors. Hot P+-ions were implanted at 800 °C into Si-QDs fabricated by implanting hot Si+ ions into a SiO2 layer. After post-N2 annealing at 1000 °C, the P+-doped Si-QDs with a diameter of 2.5 nm were embedded into the SiO2 layer. The P+-ion dose (DP) varied from 1 × 1015 to 9 × 1015 cm−2. Energy-dispersive x-ray spectroscopy revealed that the implanted P atoms clustered in the Si-QDs, which led to the experimental verification of the co-clustering of hot Si+/P+-ion implantation. Thus, the DP dependence of the P-atom concentration (NP-EDX) in Si-QDs was accurately determined. Additionally, the P 1s spectrum obtained by hard x-ray photoelectron spectroscopy revealed that the P–Si bond of the P-doped Si-QDs, including substantial P atoms, directly verifies donor formation in the Si-QDs. The upper limit of activation rate RACTUP of the implanted P atoms in the Si-QDs was obtained by the P–Si bond ratio. Therefore, the upper limit of donor concentration in the Si-QDs (NDUP) was determined by NDUP =NP-EDX × RACTUP, resulting in 1.4 × 1020 ≤ NDUP ≤ 1.3 × 1021 cm−3. The upper limit of number of donors in the Si-QDs (nDUP) was overestimated to be between 1 and 12. Additionally, the photoluminescence revealed the bandgap EG narrowing (ΔEG), even in the Si-QDs with nDUP 12 caused by donor band tailing. ΔEG was much lower than those of two- and three-dimensional Si.