Doped Si Nanocrystals Research Articles

Heavily Doped Si Nanocrystals Formed in P-(SiO/SiO2) Multilayers: A Promising Route for Si-Based Infrared Plasmonics

As building blocks of multifunctional materials involving coupling at the nanoscale, highly doped semiconductor nanocrystals are of great interest for potential applications in nanophotonics. In this work, we investigate the plasmonic properties of highly doped Si nanocrystals embedded in a silica matrix. These materials are obtained by evaporation of heavily phosphorus-doped SiO/SiO2 multilayers in an ultrahigh vacuum chamber followed by rapid thermal annealing. For P contents between 0.7 and 1.9 atom %, structural investigations at the nanoscale give clear evidence that P atoms are mainly located in the core of Si nanocrystals with concentrations reaching up to 10 atom %, i.e., well beyond the solid solubility limit of P in bulk Si. Alloying and formation of SiP nanoparticles are observed for P contents exceeding 4 atom % in the multilayer. Infrared absorption measurements give evidence of a localized surface plasmon resonance located in the 3–6 μm range. A core–shell structure was used to model Si nanocrystals embedded in a silica matrix. Based on the Mie theory and the Drude model, both the mobility and the free charge carrier density were extracted from the simulation, with values reaching 27 cm2 V–1 s–1 and 2.3 × 1020 cm–3, respectively. This results in a dopant activation rate of about 8%.

Hyperdoped Si nanocrystals embedded in silica for infrared plasmonics.

We present the experimental realization of plasmonic hyperdoped Si nanocrystals embedded in silica via a combination of sequential low energy ion implantation and rapid thermal annealing. We show that phosphorus dopants are incorporated into the nanocrystal cores at concentrations up to six times higher than P solid solubility in bulk Si by combining 3D mapping with atom probe tomography and analytical transmission electron microscopy. We shed light on the origin of nanocrystal growth at high P doses, which we attribute to Si recoiling atoms generated in the matrix by P implantation, which likely increase Si diffusivity and feed the Si nanocrystals. We show that dopant activation enables partial nanocrystal surface passivation that can be completed by forming gas annealing. Such surface passivation is a critical step in the formation of plasmon resonance, especially for small nanocrystals. We find that the activation rate in these small doped Si nanocrystals is the same as in bulk Si under the same doping conditions.

Ab initio studies of the optoelectronic structure of undoped and doped silicon nanocrystals and nanowires: the role of size, passivation, symmetry and phase.

Silicon nanocrystals and nanowires have been extensively studied because of their novel properties and their applications in electronic, optoelectronic, photovoltaic, thermoelectric and biological devices. Here we discuss results from ab initio calculations for undoped and doped Si nanocrystals and nanowires, showing how theory can aid and improve comprehension of the structural, electronic and optical properties of these systems.

Synthesis of PEG-grafted boron doped Si nanocrystals.

Silicon nanocrystals are intriguing materials for biomedical imaging applications because of their unique optical properties and biological compatibility. We report a new surface functionalization route to synthesize biological buffer soluble and colloidally stable silicon nanocrystals, which is enabled by surface boron doping. Harnessing the distinctive Lewis acidic boron surface sites, postsynthetic modifications of plasma synthesized boron doped nanocrystals were carried out with polyethylene glycol (PEG-OH) ligands in dimethyl sulfoxide under photochemical conditions. The influence of PEG concentration, PEG molecular weight, and boron doping percentage on the nanocrystal solubility in a biological buffer has been investigated. The boron doping facilitates the surface functionalization via two probable pathways, by providing excellent initial dispersiblity in polar solvents and providing available acidic boron surface sites for bonding. These boron doped silicon nanocrystals have nearly identical absorption features as intrinsic silicon nanocrystals, indicating that they are promising candidates for biological imaging applications.

Improved device performance of Si-based heterojunction solar cells by using phosphorus doped Si nanocrystals embedded in SiC host matrix

Phosphorus-doped Si nanocrystals embedded in amorphous SiC (Si NCs:SiC) films were fabricated by annealing phosphorus-doped Si-rich amorphous SiC materials at 900°C to get n-type Si NCs/p-Si heterojunction for photovoltaic device applications. The film compositions and the microstructure were characterized by X-ray photoelectron spectra and Raman scattering technique. After phosphorus doping, the dark conductivity can reach to be as high as 48 S/cm which is increased by six orders of magnitude compared with the un-doped one, while the bandgap keeps almost unchanged around 2.14 eV. The improved device performance was confirmed with the fill factor of 58% and the power conversion efficiency of 6.11%, which can be attributed to the good conductivity of phosphorus-doped Si NCs and the improved rectification characteristics of heterojunction structures.

Free electron-driven photophysics in n-type doped silicon nanocrystals.

Although there have been extensive speculation regarding the applicability of doped silicon nanocrystals (Si NCs) in optoelectronic technologies, a quantitative analysis on the photophysical workings of introduced free carriers remains elusive. Here, we present a comprehensive study on the photophysics of ∼7.5 nm heavily phosphorous-doped Si NCs, using a combination of spectroscopic techniques. We correlate the carrier dynamics with the location of the free carriers - which we tune from NC core to surface depending on the state of oxidation. The strength of the Coulomb interactions between the photoexcited electron-hole pairs and the doping-induced free carriers depends on (1) the concentration of free carriers, (2) the location of these carriers, and (3) the diameter of the NCs. In contrast to prior studies, the photoexcited carrier dynamics in these n-type doped Si NCs are dominated by strong Coulomb interactions with doping-induced free electrons, characterized by a negative trion lifetime of around 9 ns. While radiative recombination in doped direct bandgap NCs can often still compete with trion recombination (allowing emission to be present), emission in our doped Si NCs is completely quenched due to the relatively slow radiative recombination in these indirect bandgap NCs. Furthermore, multi-exciton interaction times are slightly shortened compared to those of intrinsic Si NCs, which we attribute to an increased number of free electrons, enhancing the oscillator strength of Auger recombination. These results constitute a framework for the optimization of doped Si NC synthesis techniques and device engineering directions for future doped-Si NC-based optoelectronic and photovoltaic applications.

First Principle Studies of B and P Doped Si Nanocrystals

The properties of n‐ and p‐doped silicon nanocrystals obtained through ab initio calculations are reviewed here. The aim is the understanding of the effects induced by substitutional doping on the structural, electronic and optical properties of free‐standing and matrix‐embedded Si nanocrystals. The preferential positioning of the dopants and their effects on the structural properties with respect to the undoped case, as a function of the nanocrystals diameter and termination, are identified through total‐energy considerations. The localization of the acceptor and donor related levels in the band gap of the Si nanocrystals, together with the impurity activation energy, are discussed as a function of the nanocrystals size. The dopant induced differences in the optical properties with respect to the undoped case are presented. Finally, the case of B and P co‐doped nanocrystals is discussed showing that if carriers are perfectly compensated, the Si nanocrystals undergo a minor structural distortion around the impurities inducing a significant decrease of the impurities formation energies with respect to the single doped case. Due to co‐doping, additional peaks are introduced in the absorption spectra, giving rise to a size‐dependent red shift of the absorption spectra.

The phosphorus and boron co-doping behaviors at nanoscale in Si nanocrystals/SiO2 multilayers

Phosphorus (P) and Boron (B) co-doping effects at the nanoscale in Si nanocrystals/SiO2 multilayers have been studied in the present work. Several interesting experimental results are achieved which are in contrast to the case in bulk-Si and the previous observations on the doped Si nanocrystals. It is found that all the co-doping samples are n-type regardless of B doping ratios. The P doping efficiency in Si NCs is higher than B dopants, and it can be improved via B co-doping with suitable levels. Raman and ESR spectra indicate that the different occupation preferences of P and B in Si NCs are responsible for the interesting co-doping behaviors. It looks like that the electronic structures and the physical properties of Si NCs can be modulated via the impurities co-doping approach.

Accurate analysis of the size distribution and crystallinity of boron doped Si nanocrystals via Raman and PL spectra

Through fitting of Raman and PL spectra, Si nanocrystal size distributions and Si crystalline fractions are obtained.

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.

First-principles calculations of Raman spectra in Li-doped Si nanocrystals

We examine the vibrational and Raman spectra for Li doped Si nanocrystals using real-space pseudopotentials constructed within density functional theory. We calculate differences in the Raman spectra using the Placzek approximation. The insertion of Li atoms into Si nanocrystals disrupts the Si crystal structure forming a region of Li–Si alloy in which the regular crystal structure is significantly disrupted. The Raman spectrum for this alloy exhibits a Li induced peak at 440–480cm−1. We find an accompanying reduction in the size of the dominant bulk-like Si peak at 520cm−1. Both of these results are consistent with experiment. Our analysis of the calculated spectrum confirms the utility of using Raman spectroscopy, coupled with first principle computations, to predict the structural and electronic properties of Li doped Si nanocrystals.

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.

Influence of doping on the optical properties of silicon nanocrystals embedded in SiO2

AbstractCo‐implantation, with overlapping implantation projected ranges, of Si and doping species (P, As, B) followed by a thermal annealing step is a viable route to form doped Si nanocrystals (NCs) embedded in SiO2. This presentation deals with optical characterizations of both doped and undoped Si‐NC prepared by this method. The NC effective presence in the oxide layer and their crystallinity is verified by Raman spectrometry. Photoluminescence (PL) and PL excitation measurements reveal quantum confinement effects and a gradual PL quenching with increasing dopant concentrations. The measured Stokes shift remains constant and its value ∼ 0.2 eV is almost twice the Si–O vibration energy. This suggests that a possible radiative recombination path is a fundamental transition assisted by a local phonon. Lifetime investigations show that PL time‐decays follow a stretched exponential. Atomic probe tomography analyses demonstrate that n‐type dopants (P, As) are efficiently introduced in the NC core, whereas p‐type dopant (B) are located at the NC/SiO2 interface. All together these experimental observations question on possible different carrier recombination paths in P or As doped NC compared to B one's. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical characterizations of doped silicon nanocrystals grown by co-implantation of Si and dopants in SiO2

Co-implantation, with overlapping implantation projected ranges, of Si and doping species (P, As, and B) followed by a thermal annealing step is a viable route to form doped Si nanocrystals (NCs) embedded in silica (SiO2). In this paper, we investigate optical characterizations of both doped and un-doped Si-NCs prepared by this method. The effective NC presence in the oxide layer and their crystallinity is verified by Raman spectrometry. Photoluminescence (PL) and PL excitation measurements reveal quantum confinement effects and a gradual PL quenching with increasing dopant concentrations. In un-doped NC, the measured Stokes shift remains constant and its value ∼0.2 eV is almost twice the Si–O vibration energy. This suggests that a possible radiative recombination path is a fundamental transition assisted by a local phonon. PL lifetime investigations show that PL time-decays follow a stretched exponential. Using a statistical model for luminescence quenching, a typical NC diameter close to 2 nm is obtained for As- and P-doped samples, consistent with our previous atomic probe tomography (APT) analyses. APT also demonstrated that n-type dopant (P and As) are efficiently introduced in the NC core, whereas p-type dopant (B) are located at the NC/SiO2 interface. This last observation could explain the failure of the luminescence-quenching model to determine NC size in B-doped samples. All together, these experimental observations question on possible different carrier recombination paths in P or As doped NC compared to B one's.

Charge transfer of single laser crystallized intrinsic and phosphorus-doped Si-nanocrystals visualized by Kelvin probe force microscopy

Isolated intrinsic and phosphorus doped (P-doped) Si-nanocrystals (Si-NCs) on n- and p-Si substrates are fabricated by excimer laser crystallization techniques. The formation of Si-NCs is confirmed by atomic force microscopy (AFM) and conductive AFM measurements. Kelvin probe force microscopy (KPFM) is then carried out to visualize the trapped charges in a single Si-NC dot which derives from the charge transfer between Si-NCs and Si substrates due to their different Fermi levels. The laser crystallized P-doped Si-NCs have a similar Fermi level around the mid-gap to the intrinsic counterparts, which might be caused by the inactivated impurity atoms or the surface states-related Fermi level pinning. A clear rise of the Fermi level in P-doped Si-NCs is observed after a short time thermal annealing treatment, indicating the activation of dopants in Si-NCs. Moreover, the surface charge quantity can be estimated using a simple parallel plate capacitor model for a quantitative understanding of the KPFM results at the nanoscale.

Electrical observation of non-radiative recombination in Er doped Si nano-crystals during thermal quenching of intra-4f luminescence

Thermal quenching of luminescence of Er dopants in Si nano-crystals (Si-nc’s) was investigated employing an impedance model for the analysis of photo-injected charges. Relaxation response indicated that Er doping forms not only optical centers but also trapping centers near the Si-nc’s. The response time constant of trapped charges was dependent on temperature, with the dependence correlating to thermal quenching. These findings indicate that quenching occurs by trapping followed by consumption of charges. The complex analyses revealed that the response represents non-radiative recombination at the centers rather than release of confined charges from the Si-nc through the centers. We propose a possible energy diagram for the non-radiative recombination.

Efficient n-type doping of Si nanocrystals embedded in SiO2 by ion beam synthesis

It is shown that co-implantation, with overlapping projected ranges of Si and P or As, followed by a single thermal annealing step is an efficient way to form doped Si nanocrystals (Si-nc's) embedded in SiO2 with diameters of a few nanometers. Atom probe tomography is used to image directly the spatial distribution of the various species at the atomic scale, evidencing that the P and As atoms are efficiently introduced inside the Si nanocrystals. In addition, we report on the influence of the dopant doses on the Si-nc's related photoluminescence as well as on the I(V) characteristics of MOS structures including these Si-nc's.

Exciton localization in doped Si nanocrystals from single dot spectroscopy studies

The results of low-temperature photoluminescence characterization of single silicon nanocrystals prepared from highly doped silicon-on-insulator wafers are presented. The effect of B, P, As, and Sb impurities on ensemble as well as individual emission spectra are determined by comparison with the line shapes of undoped nanocrystals. From the statistical analysis of the luminescence spectra, the donor ionization energies for nanocrystals emitting in the range of 1.5--2.0 eV are estimated to be 140--200 meV, while the exciton-impurity binding energy for As- and Sb-doped nanocrystals is found to be about 40--45 meV.

Acceptor-related low-energy photoluminescence from boron-doped Si nanocrystals

Boron (B) doped Si nanocrystals (Si-ncs) dispersed in hydrofluoric (HF) acid solution are prepared by dissolving borosilicate films containing B-doped Si-ncs in HF solution. We find that the etching rate of B-doped Si-ncs is much smaller than that of undoped Si-ncs. The difference of the etching rate allows us to extract only doped Si-ncs in the mixture of doped and undoped Si-ncs and observe the photoluminescence (PL) due to the transition from the conduction band to the acceptor state. The PL was very broad with the maximum around 1.15 eV. From the analysis of the PL data obtained for the samples prepared under different conditions and different etching time, preferential doping sites of B atoms are estimated. The data suggests that B-doped Si-ncs consists of intrinsic cores and heavily B-doped shells.

Size- and doping-dependent time-resolved photoluminescence of doped Si nanocrystals

Time-resolved photoluminescence (PL) has been studied for B- and Sb-dopedSi nanocrystals (NCs) fabricated by ion beam sputtering and annealing.For B-doped Si NCs, the PL intensity as well as the PL lifetime (τPL) increases asNC size (d) varies from 1.5 to 2.6 nm, similar to the case for undoped Si NCs, but with further increase ofd, they decrease, possibly resulting from the increase of optically less active NCswith the increase of NCs containing more dopants. The PL intensity andτPL monotonically decrease with increasing doping concentration (nD), irrespective of doping element. Si NCs show smallerτPL in B doping than in Sb doping over the full range ofnD. The sharp decrease in PL intensity, accompanied by the gradual decrease inτPL for thehigher nD of Sb, may be attributed to Auger recombination due to the presence of Sb inside Si NCs.The higher PL quench rate by Sb compared to B could be attributed to better ionization ofSb dopants in Si NCs.