Engineering silicon nanocrystals: Theoretical study of the effect of codoping with boron and phosphorus

Abstract

We show that the optical and electronic properties of nanocrystalline silicon can be efficiently tuned using impurity doping. In particular, we give evidence, by means of ab initio calculations, that by properly controlling the doping with either one or two atomic species, a significant modification of both the absorption and the emission of light can be achieved. We have considered impurities, either boron or phosphorous (doping) or both (codoping), located at different substitutional sites of silicon nanocrystals with size ranging from 1.1 to 1.8 nm in diameter. We have found that the codoped nanocrystals have the lowest impurity formation energies when the two impurities occupy nearest neighbor sites near the surface. In addition, such systems present band-edge states localized on the impurities, giving rise to a redshift of the absorption thresholds with respect to that of undoped nanocrystals. Our detailed theoretical analysis shows that the creation of an electron-hole pair due to light absorption determines a geometry distortion that, in turn, results in a Stokes shift between adsorption and emission spectra. In order to give a deeper insight into this effect, in one case we have calculated the absorption and emission spectra beyond the single-particle approach, showing the important role played by many-body effects. The entire set of results we have collected in this work give a strong indication that with the doping it is possible to tune the optical properties of silicon nanocrystals

Room-temperature below bulk-Si band gap photoluminescence from P and B co-doped and compensated Si nanocrystals with narrow size distributions

“…The optical and electrical transport properties of Si-ncs can also be controlled by impurity doping [8,9]. Because of the very small number of constituent atoms of Si-ncs, doping of a few impurities significantly modifies their properties [10,11].…”

Electronic properties and dielectric response of surfaces and nanowires of silicon from ab-initio approaches

“…1 DFT total energy calculations are used here, 2 to evaluate what amount of energy (i.e the formation energy (FE)) is required to sustain the doping process (with acceptor and donor impurities, such as Boron (B) and Phosphorus (P)) in Si-NWs. Starting from the undoped Si n H m -NW, the FE of the neutral B or/and P impurities can be defined as the energy needed to insert one B and/or one P atom within the Si-NW after removing one (or two) Si atoms (transferred to the chemical reservoir, assumed to be bulk Si) [4][5][6][7][8][9][10] …”

Optical Properties of Intrinsic and Shallow Impurity‐Doped Silicon Nanocrystals

“…Although a precise control of the number of p-and n-type impurities in a Si nanocrystal seems to be impossible, recent calculations demonstrated preferential formation of nanocrystals with equal number of p-and n-type impurities. The group of Ossicini [20,21,23] demonstrated by first-principles calculations that the formation energy of Si nanocrystals drastically decreases when pairs of B and P are doped into Si nanocrystals; the formation energy of a pair of B-and P-doped Si nanocrystals (Si 147 H 100 clusters) is about 1 eV smaller than that of B-doped ones and about 0.7 eV smaller than that of P-doped ones. This indicates that if Si nanocrystals are grown by the procedure described in Section 3.3.1, that is, the phase separation of P-and B-doped Si-rich SiO 2 (Si-rich BPSG) by annealing, nanocrystals with equal number of B and P are preferentially grown because they are energetically favorable.…”

“…Therefore, impurity-doped semiconductor nanostructures have been a subject of intensive research [9][10][11][12][13]. Unfortunately, contrary to many theoretical studies [14][15][16][17][18][19][20][21][22][23][24][25], experimental work on shallow impurity-doped Si nanocrystals makes a poor progress because of difficulties in their preparation and characterization. The main difficulty arises from the fluctuation of impurity number per nanocrystal in a nanocrystal assembly.…”