Optical Properties of Doped Silicon Quantum Dots with Crystalline and Amorphous Structures

Abstract

The interaction of silicon quantum dots with light is remarkable, as electronic transitions are influenced by the interplay of their atomic structure and by electronic quantum confinement in three dimensions. In this study, the optical properties of 4 undoped and 16 doped silicon quantum dots were calculated using time-dependent density functional theory. The HOMO–LUMO gap, maximum absorption wavelength, and oscillator strength at that wavelength were calculated for two crystalline structures, c-Si29H36 and c-Si35H36, and two amorphous structures, a-Si29H36 and a-Si35H36; in addition, optical properties were calculated for each of the structures doped with either phosphorus or aluminum in one of two different positions: in the center of the cluster or at the surface of the cluster. The calculated optical properties reveal that the absorbance spectrum of the amorphous structures is red shifted compared to that of the crystalline structures, and doping causes the spectrum to shift even further toward the red. Additionally, absorption of light at the maximum wavelength in doped structures caused charge density to transfer from the center of the quantum dot to the surface. The combination of strong absorptions in the visible region of the electromagnetic spectrum and the observed charge transfer make doped silicon quantum dots promising candidates as materials for solar energy applications.

Energetics and carrier transport in doped Si/SiO2quantum dots

“…In the case of free-standing hydrogenated Si-QDs, impurities located close to the QD surface are energetically more favorable than others, thanks to a larger atomic mobility that allows a reduction of the stress around the dopant atom. 16,[18][19][20][21] In this case, doping the nanostructure core region could result very difficult, even for materials commonly doped in their bulk phase. 19 Beside strain effects, other chemistry-governed factors, occurring at shorter scales, can determine the energetically favored site of the impurity.…”

“…16,[18][19][20][21] In this case, doping the nanostructure core region could result very difficult, even for materials commonly doped in their bulk phase. 19 Beside strain effects, other chemistry-governed factors, occurring at shorter scales, can determine the energetically favored site of the impurity. For example, in the case of OH-terminated or SiO 2 -embedded QDs, the strong electronegativity of O makes P strongly repelled from the interfacial sites, while conversely attracting B.…”

“…19,26 In our systems, doping with single group-III or group-V impurities results in an odd number of electrons, for which spin-polarized calculations must be employed. For small Si-QDs, a correlation between the energy difference of spin-down and spin-up impurity levels and the magnitude of the Stokes shift of undoped Si-QDs has been reported, signaling structural deformation.…”

“…[6][7][8][9][10] Theoretically, several works have studied the formation and ionization energies, and the opto-electronic properties of freestanding doped SiQDs. [16][17][18][19][20][21][22][23][24][25][26] Instead, only recently theoretical works dealing with structural properties of doped embedded Si-QDs have appeared in literature. 27 In any case, all the above works show that the final properties of these systems are strongly sensitive to the concentration and position of the impurities.…”

Silicon quantum dots embedded in a SiO2matrix: From structural study to carrier transport properties

“…The generation of the QD inside the a-SiO 2 matrix allows the formation of Si-O-Si bridge bonds at the Si/SiO 2 interface, and the reduction of the embedded system energy gap in all the systems, as shown by previous theoretical studies, 16,64 and in accordance with photoluminescence measurements where the energy gap for QDs around 2.5 nm in diameter was determined to be 1.9 eV for amorphous QDs 65 and 2.7 eV for crystalline ones. 66 Similarly to the crystalline case, HOMO and LUMO states of amorphous systems are localized at the surface of the QD and inside it, respectively.…”