Size and dopant-concentration dependence of photoluminescence properties of ion-implanted phosphorus- and boron-codoped Si nanocrystals

We investigate the nanocrystallite-size and dopant-concentration dependence of the photoluminescence (PL) properties of heavily phosphorus- (P) and boron- (B) codoped Si nanocrystals (Si NCs), prepared using a combination of sputtering and ion implantation techniques. We find that the heavily doped Si NC exhibits three exotic luminescence bands, A, B, and C. The peak energy of band A redshifts with increasing dopant concentration. This band is due to the band-to-band transition at the reduced Si-NC band gap caused by the formation of impurity bands together with band-tailing effects. The PL redshift becomes large when the nanocrystallite size decreases, suggesting the occurrence of the quantum-confinement-induced carrier doping effect. The peak energies of bands B and C are independent of both the concentration and size, indicating that these bands are due to transitions between defect- and/or impurity-related localized states. Band A shows stronger thermal quenching than the PL band in pure (undoped) Si NCs, the magnitude of which depends on the dopant concentration. The stronger thermal quenching in band A is probably due to the thermally induced migration of electrons in the impurity band.

Figure 1

PL spectra of Si-NC samples, S1, S2, and S3, at room temperature.

Figure 2

PL spectra of PB-codoped Si NCs for various ion doses of $0.1–4.5 \times {10}^{16} \mathrm{cm}{}^{-2}$: Samples (a) S1; (b) S2; and (c) S3. Gaussian multipeak fit results of PL spectra for various ion doses of $0.1–4.5 \times {10}^{16} \mathrm{cm}{}^{-2}$: Samples (d)–(f) S1 and (g)–(i) S3.

Figure 3

(a) Peak energies of bands A, B, and C in PL spectra of PB-codoped Si NCs as a function of ion dose. (b) Peak energy shift of band A with respect to that of the intrinsic Si NC. The inset shows those of bands B and C. The solid lines in (b) show fit results for the $n^{1/3}$ law.

Figure 4

Schematic illustrations for PB-codoped Si NC energy band diagram for various (a) sizes and (b) impurity concentrations.

Figure 5

Temperature dependence of PL spectra for (a) pure Si NCs and PB-codoped Si NCs for ion doses of (b) 0.1 and (c) $4.5 \times {10}^{16} \mathrm{cm}{}^{-2}$, together with Gaussian multipeak fitting results of PL spectra for PB-codoped Si NC (S3) samples with 0.1 and $4.5 \times {10}^{16} \mathrm{cm}{}^{-2}$ doses measured at 300 [(d) and (e)], 100 [(f) and (g)], and 15 K [(h) and (i)].

Figure 6

Arrhenius plots of $I_{PL}$ for (a) pure Si NCs and PB-codoped Si NCs for ion doses of (b) 0.1 and (c) $4.5 \times {10}^{16} \mathrm{cm}{}^{-2}$. The solid lines are fit results.

Figure 7

PL decay curves of Si NCs for S3 samples doped at $0–4.5 \times {10}^{16} \mathrm{cm}{}^{-2}$ and measured at (a) 300 and (b) 15 K. The emission energy is 1.25 eV. PL lifetimes estimated from the PL decay curves as a function of photon energy at (c) 300 and (d) 15 K.

Figure 8

Emission-energy dependence of lifetime together with deconvoluted PL bands for PB-codoped Si NC sample with a $4.5 \times {10}^{16} \mathrm{cm}{}^{-2}$ dose and measured at 15 K.