P-doped Si Nanocrystals Research Articles

Electron spin resonance in P-doped Si nanocrystals/SiC stacked structures with various dot sizes

Doping in semiconductor nanocrystals have a significant impact on electronic structures and the relative studies are needed in order to deeply understand the doping effect in nanoscale. Here, we fabricate un-doped and P-doped Si nanocrystals (Si NCs)/Silicon carbide (SiC) multilayers with various dot sizes (8, 4 and 2 nm). Low temperature (77 K) electron spin resonance spectra are measured to study the electronic structures before and after P doping. It is found that the g value is kept at 2.005 for un-doped Si NCs samples, which is independent of the dots size, indicating the existence of dangling bonds at the surface of Si NCs. After P doping, for samples with dot size of 8 nm, the g value is 1.998, which is assumed to the conduction electrons. It suggests that P impurities are introduced into the inner of Si NCs substitutionally to provide the conduction electrons. However, with the reduction of dot size, the signals with g = 2.004, which is associated with the Si vacancies, are identified in P-doped samples, while the peak-to-peak linewidth ΔBpp is narrowed with the dots size decreasing. It shows the possibility to apply P-doped Si NCs/SiC multilayers with ultra-small dot sizes into Si-based spintronics.

Electronic band structures of undoped and P-doped Si nanocrystals embedded in SiO2

Conduction and valence band maxima in Si nanocrystals (d = 4.2 ± 0.9 nm) embedded in SiO2 as a function of P concentration.

Effects of nitrogen impurities on the microstructure and electronic properties of P-doped Si nanocrystals emebedded in silicon-rich SiNx films

Phosphorus doped Si nanocrystals (SNCs) emebedded in silicon-rich SiNx:H films were prepared using plasma enhanced chemical vapor deposition technique, and the effects of nitrogen incorporation on the microstructure and electronic properties of the thin films have been systematically studied. Transmission electron microscope and Raman observation revealed that nitrogen incorporation prevents the growth of Si nanocrystals, and that their sizes can be adjusted by varying the flow rate of NH3. The reduction of photoluminescence (PL) intensity in the range of 2.1–2.6 eV of photon energy was observed with increasing nitrogen impurity, and a maximal PL intensity in the range 1.6–2.0 eV was obtained when the incorporation flow ratio NH3/(SiH4+H2+PH3) was 0.02. The conductivity of the films is improved by means of proper nitrogen impurity doping, and proper doping causes the interface charge density of the heterojunction (H-J) device to be lower than the nc-Si:H/c-Si H-J device. As a result, the proper incorporation of nitrogen could not only reduce the silicon banding bond density, but also fill some carrier capture centers, and suppress the nonradiative recombination of electrons.

Comparative study on the localized surface plasmon resonance of boron- and phosphorus-doped silicon nanocrystals.

Localized surface plasmon resonance (LSPR) of doped Si nanocrystals (NCs) is critical to the development of Si-based plasmonics. We now experimentally show that LSPR can be obtained from both B- and P-doped Si NCs in the mid-infrared region. Both experiments and calculations demonstrate that the Drude model can be used to describe the LSPR of Si NCs if the dielectric screening and carrier effective mass of Si NCs are considered. When the doping levels of B and P are similar, the LSPR energy of B-doped Si NCs is higher than that of P-doped Si NCs because B is more efficiently activated to produce free carriers than P in Si NCs. We find that the plasmonic coupling between Si NCs is effectively blocked by oxide at the NC surface. The LSPR quality factors of B- and P-doped Si NCs approach those of traditional noble metal NCs. We demonstrate that LSPR is an effective means to gain physical insights on the electronic properties of doped Si NCs. The current work on the model semiconductor NCs, i.e., Si NCs has important implication for the physical understanding and practical use of semiconductor NC plasmonics.

Ab initio study on the effect of structural relaxation on the electronic and optical properties of P-doped Si nanocrystals

In contrast to the conventional doping of bulk silicon (Si), the doping of Si nanocrystals (NCs) that are often smaller than 5 nm in diameter may lead to serious structural changes. Since the electronic and optical properties of Si NCs are intimately associated with their structures, it is critical to understand how doping impacts the structures of Si NCs. By means of ab initio calculation we now compare 1.4 nm phosphorus (P)-doped Si NCs without structural relaxation and those with structural relaxation. Structural changes induced by structural relaxation are manifested by the stretching and compressing of bonds and apparent variations in bond angles. With the increase of the concentration of P structural changes induced by structural relaxation become more serious. It is found that structural relaxation makes differences in the energy-level schemes of P-doped Si NCs. Structural relaxation also causes the binding energy of an electron in a P-doped Si NC to more significantly increase as the concentration of P increases. With the increase of the concentration of P structural relaxation leads to more pronounced changes in the optical absorption of P-doped Si NCs.

First-principles study of vibrational modes and Raman spectra in P-doped Si nanocrystals

We have studied the vibrational modes and Raman spectra of P-doped Si nanocrystals using pseudopotential density functional theory and the Placzek approximation. We find that Si nanocrystal vibrations are largely unaffected by the introduction of P dopants. However, the Raman spectra of doped nanocrystals are enhanced relative to those of pristine nanocrystals, and demonstrate a strong dependence on dopant position. Thus, Raman has the potential of being developed as a tool for probing the location of the dopant within the nanocrystal. Our analysis shows that vibrational modes involving atoms in the vicinity of the dopant give the largest contributions to the Raman spectra.

Fabrication of Si heterojunction solar cells using P-doped Si nanocrystals embedded in SiN x films as emitters

Si heterojunction solar cells were fabricated on p-type single-crystal Si (sc-Si) substrates using phosphorus-doped Si nanocrystals (Si-NCs) embedded in SiNx (Si-NCs/SiNx) films as emitters. The Si-NCs were formed by post-annealing of silicon-rich silicon nitride films deposited by electron cyclotron resonance chemical vapor deposition. We investigate the influence of the N/Si ratio in the Si-NCs/SiNx films on their electrical and optical properties, as well as the photovoltaic properties of the fabricated heterojunction devices. Increasing the nitrogen content enhances the optical gap E04 while deteriorating the electrical conductivity of the Si-NCs/SiNx film, leading to an increased short-circuit current density and a decreased fill factor of the heterojunction device. These trends could be interpreted by a bi-phase model which describes the Si-NCs/SiNx film as a mixture of a high-transparency SiNx phase and a low-resistivity Si-NC phase. A preliminary efficiency of 8.6% is achieved for the Si-NCs/sc-Si heterojunction solar cell.

Tight-Binding Calculations of the Optical Response of Optimally P-Doped Si Nanocrystals: A Model for Localized Surface Plasmon Resonance

We present tight-binding calculations in the random-phase approximation of the optical response of Silicon nanocrystals (Si NCs) ideally doped with large concentrations of phosphorus (P) atoms. A collective response of P-induced electrons is demonstrated, leading to localized surface plasmon resonance (LSPR) when a Si NC contains more than ≈10 P atoms. The LSPR energy varies not only with doping concentration but also with NC size due to size-dependent screening by valence electrons. The simple Drude-like behavior is recovered for NC size above 4 nm. Si NCs containing a large number of deep defects in place of hydrogenic impurities do not give rise to LSPR.

Optical gap of P-doped Si nanocrystals

The optical gap of Si nanocrystals strongly doped with phosphorus has been calculated. The energy levels of the electron and hole ground states in the Si nanocrystal were found as functions of the nanocrystal size and the P concentration in the framework of the envelope function approximation and under the assumption of uniform impurity distribution over the nanocrystal volume. It is shown that introducing phosphorus into the nanocrystal leads to a decrease in its optical gap. This decrease is related to a strong shift of the ground electron level produced by the short-range part of the Coulomb electron-P-ion interaction.

Critical Role of Dopant Location for P-Doped Si Nanocrystals

The doping of phosphorus (P) provides an additional means to control the optical properties of silicon nanocrystals (Si NCs). The P-doping-induced changes in the optical properties of Si NCs, however, have not been consistently understood. On the basis of first-principles calculations, we explain the P-doping-induced infrared absorption of Si NCs and the effect of P-doping on the light emission from Si NCs. The explanations are enabled by the investigation of the locations of P in Si NCs, including a variety of locations at the surface of Si NCs. We show that the light emission from Si NCs critically depends on the location of P. Transitions involving P-doping-induced defect energy levels lead to the infrared absorption of Si NCs.

Nonlinear optical properties of phosphorus-doped silicon nanocrystals/nanoclusters

Comprehensive studies have been performed on the nonlinear optical responses of silica films containing phosphorus (P)-doped Si nanoclusters and/or nanocrystals. In P-doped Si nanocrystals, enhancements of the nonlinear refractive indices (n2) and two photon absorption coefficients (β) relative to those of intrinsic Si nanocrystals were observed. The analysis of electron spin resonance and absorption spectra revealed that P donors are responsible for the large enhancements of n2 and β.

Tunnel migration in ensembles of silicon nanocrystals doped with phosphorus

A comparative study of radiative recombination and tunnel migration in ensembles of P-doped Si nanocrystals embedded in wide-band dielectric matrix is performed. We have found the migration process to be strongly suppressed at donor concentration exceeding some critical value. At the same time, the radiative recombination rate gradually rises as the concentration increases. This essentially enhances the radiative-channel efficiency in the photoluminescence process.

Stark effect of confined shallow levels in phosphorus-doped silicon nanocrystals

By envelope-function approximation, we computed the effect of confinement in spherical P-doped Si nanocrystals in a uniform electric field without adjustable parameters. Based on nanocrystal size, we can distinguish several regimes. For a radius $R$ that is larger than ${R}_{t}$ $({R}_{t}\ensuremath{\sim}21\text{ }\text{nm})$ the ground state is ionized at a critical electric field, ${\mathcal{E}}_{cr}$, by tunneling from a $1s$-like state, localized at the impurity, to a $2p$-like state, localized to the well that is formed by the electric field and the potential barrier that is generated by the embedding matrix at the nanocrystal surface. For smaller nanocrystals, for which ${R}_{sp}<R<{R}_{t}$ $({R}_{sp}\ensuremath{\sim}7\text{ }\text{nm})$, there is a range of electric fields in which the ground state is formed by the hybridization of the impurity states and surface-well states. Further, within this hybridization range, there is a value of the electric field at which the ground $1s$-like state and the excited $2{p}_{0}$ state have the same hyperfine coupling. Based on these findings, we envisage a quantum computing scheme in which qubits shuttling relies on excited states when an electric field is applied.

Stark effect, polarizability, and electroabsorption in silicon nanocrystals

Demonstrating the quantum-confined Stark effect (QCSE) in silicon nanocrystals (NCs) embedded in oxide has been rather elusive, unlike the other materials. Here, the recent experimental data from ion-implanted Si NCs is unambiguously explained within the context of QCSE using an atomistic pseudopotential theory. This further reveals that the majority of the Stark shift comes from the valence states which undergo a level crossing that leads to a nonmonotonic radiative recombination behavior with respect to the applied field. The polarizability of embedded Si NCs including the excitonic effects is extracted over a diameter range of 2.5--6.5 nm, which displays a cubic scaling, $\alpha=c D^3$, with $c=2.436\times 10^{-11}$ C/(Vm), where $D$ is the NC diameter. Finally, based on intraband electroabsorption analysis, it is predicted that p-doped Si NCs will show substantial voltage tunability, whereas n-doped samples should be almost insensitive. Given the fact that bulk silicon lacks the linear electro-optic effect as being a centrosymmetric crystal, this may offer a viable alternative for electrical modulation using p-doped Si NCs.

Phosphorus doping of ultra-small silicon nanocrystals

P-doped Si nanocrystals (radius≤2 nm) were synthesized by depositing an ultrathin (0.3 nm) P-SiO2 film close to eachSiO layer of SiO/SiO2 multilayers. During annealing P atoms migrate into the Si-rich region. Due to the low diffusivity of Pin SiO2, P atoms segregate in the Si nanocrystal region and are incorporated in the silicon nanostructures.The P level in the Si nanoclusters can be controlled by changing the P content in the P-SiO2 layer.

Electron spin-resonance studies of conduction electrons in phosphorus-doped silicon nanocrystals

The properties of conduction electrons in P-doped Si nanocrystals embedded in insulating glass matrices have been studied by electron spin-resonance spectroscopy. For heavily P-doped samples, a broad conduction electron signal is observed at low temperatures. The width of the signal is found to be much broader than that of P-doped bulk Si crystals. The temperature dependence of the signal intensity obeys the Curie law even when the P concentration is very high. This suggests that in P-doped nanocrystals donor levels do not merge into the conduction band even at very high P concentration, and also provides evidence that Si nanocrystals smaller than a certain threshold size do not become metallic, at least when they are prepared under an equilibrium condition.