Formation of a reliable intermediate band in Si heavily coimplanted with chalcogens (S, Se, Te) and group III elements (B, Al)

This first-principles study describes the properties of Si implanted with several chalcogen species (S, Se, Te) at doses considerably above the equilibrium solubility limit, especially when coimplanted with the group III atoms B and Al. The measurements of chalcogen-implanted Si show strong optical absorption in the infrared range. The calculations carried out show that substitution of Si by chalcogen atoms requires lower formation energy than the interstitial implantation. In the resulting electronic structure, at concentrations close to 0.5%, an impurity band determined by the properties of the chalcogens introduced is observed in the forbidden energy gap of Si. Although this band is a few tenths of an electron volt wide, it remains energetically isolated from both the valence and the conduction bands. Appropriate coimplantation with group III elements allows control over the occupation of the intermediate band while modifying its energies only slightly. A moderate energy gain (especially small for B) seems to be obtained when $p$-doping atoms occupy the sites next to those of the chalcogens. Therefore, the apparent electrostatic attraction between species that in isolation would act as acceptors and double donors is smaller than expected. The intermediate-band properties have been preserved for all of the coimplanted compounds analyzed here, regardless of the species involved or the distance between them, which constitutes an appreciable advantage for the design of new experimental materials.

Figure 1

(Color online) Density of states diagrams obtained for the relaxed (a) ${\text{S}}_{\text{Si}}{\text{Si}}_{215}$, (b) ${\text{Se}}_{\text{Si}}{\text{Si}}_{215}$, and (c) ${\text{Te}}_{\text{Si}}{\text{Si}}_{215}$.

Figure 2

(Color online) Density of states diagrams obtained for the relaxed ${\text{S}}_i{\text{Si}}_{216}$.

Figure 3

(Color online) Optical absorption coefficient calculated for ${\text{S}}_{\text{Si}}{\text{Si}}_{215}$, ${\text{Se}}_{\text{Si}}{\text{Si}}_{215}$, and ${\text{Te}}_{\text{Si}}{\text{Si}}_{215}$ compounds compared to that of pure Si. The AM1.5G solar spectrum (Ref. 38) in the background is shown as a reference.

Figure 4

(Color online) Imaginary part of the dielectric function calculated for the ${\text{S}}_{\text{Si}}{\text{Si}}_{215}$ compound separated into the contributions of the two possible absorption processes.

Figure 5

(Color online) Density of states diagrams obtained for the remotely coimplanted ${\text{B}}_{\text{Si}}{\text{S}}_{\text{Si}}{\text{Si}}_{214}$ (red solid) and ${\text{Al}}_{\text{Si}}{\text{S}}_{\text{Si}}{\text{Si}}_{214}$ (green dotted). Vertical lines indicate the Fermi energies of both compounds.

Figure 6

(Color online) The electronic band structure for (a) remotely and (b) closely coimplanted ${\text{B}}_{\text{Si}}{\text{S}}_{\text{Si}}{\text{Si}}_{214}$.

Figure 7

(Color online) Density of states diagrams obtained for the remotely coimplanted ${\text{B}}_{\text{Si}}{\text{Se}}_{\text{Si}}{\text{Si}}_{214}$ (red solid) and ${\text{Al}}_{\text{Si}}{\text{Se}}_{\text{Si}}{\text{Si}}_{214}$ (green dotted).

Figure 8

(Color online) Density of states diagrams obtained for the remotely coimplanted ${\text{B}}_{\text{Si}}{\text{Te}}_{\text{Si}}{\text{Si}}_{214}$ (red solid) and ${\text{Al}}_{\text{Si}}{\text{Te}}_{\text{Si}}{\text{Si}}_{214}$ (green dotted).

Figure 9

(Color online) Optical absorption coefficients of (a) $X_{\text{Si}}{\text{S}}_{\text{Si}}{\text{Si}}_{214}$, (b) $X_{\text{Si}}{\text{Se}}_{\text{Si}}{\text{Si}}_{214}$, and (c) $X_{\text{Si}}{\text{Te}}_{\text{Si}}{\text{Si}}_{214}$ (with $X=\text{B}$ and Al) compared to those of the compounds implanted only with chalcogens.

Figure 10

(Color online) The imaginary part and its components of the dielectric function calculated for ${\text{B}}_{\text{Si}}{\text{S}}_{\text{Si}}{\text{Si}}_{214}$.