Si Nanocrystals Research Articles

3D microstructure analysis of silicon-boron phosphide mixed nanocrystals.

The microstructure of boron (B) and phosphorus (P) codoped silicon (Si) nanocrystals (NCs), cubic boron phosphide (BP) NCs and their mixed NCs (BxSiyPz NCs) has been studied using atom probe tomography (APT), transmission electron microscopy (TEM), and Raman scattering spectroscopy. The BxSiyPz NCs inherit superior properties of B and P codoped Si NCs such as high dispersibility in aqueous media and near infrared (NIR) luminescence and those of cubic BP NCs such as high chemical stability. The microanalyses revealed that BxSiyPz NCs are composed of a crystalline core and an amorphous shell. The core possesses a lattice constant between that of Si (diamond-cubic) and BP (cubic). The amorphous shell is comprised of B, Si and P, though the composition is not uniform and there are local B-rich, Si-rich and P-rich domains connected contiguously. The amorphous shell is proposed to be responsible for their superior chemical properties such as high dispersibility in polar solvents and high resistance to acids, and the crystalline core is responsible for the stable NIR luminescence.

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.

Spectral tuning of colloidal Si nanocrystal luminescence by post-laser irradiation in liquid.

We report a simple technique to tune the luminescence spectra of blue-emitting colloidal silicon nanocrystals (Si-ncs) to the ultraviolet region via post-laser irradiation. The post-laser irradiation of the as-prepared colloidal Si-ncs which exhibited a broad photoluminescence (PL) centered at ∼390 nm with a bandwidth of 105 nm caused a spectral peak shift to ∼330 nm and a decrease in the bandwidth to 80 nm. The degree of the spectral tuning can be controlled by changing the irradiated laser power. The spectroscopic and transmittance electron microscopy analysis revealed that this spectral tuning effect is due to the size reduction of Si-ncs by selective absorption of post-irradiated laser light by particular-sized Si-ncs that have bandgap energies resonant with the irradiated photon energy, as well as the resultant ablation of these Si-ncs.

Photophysical properties of ball milled silicon nanostructures.

Luminescent silicon nanocrystals (SiNCs) have attracted scientific interest for their potential use in LEDs, displays, lasers, photovoltaic spectral-shifting filters and for biomedical applications. A lot of efforts have been made to improve the radiative emission rate in SiNCs, mostly using quantum confinement, strain and ligands. Existing methods, however, are not easily upscalable, as they do not provide the high material yield required for industrial applications. Besides, the photoluminescence (PL) efficiency of SiNCs emitting in the visible spectral range also remains very low. Hence, there is a need to develop a low-cost method for high material yield of brightly emitting SiNCs. Theoretically, strain can be used alongside quantum confinement to modify the radiative emission rates and band-gaps. In view of that, high-energy ball milling is a method that can be used to produce large quantities of highly strained SiNCs. In this technique, balls with high kinetic energy collide with the walls of a chamber and other balls, crushing the particles in between, followed by welding, fracture and re-welding phenomena, reducing the particle size and increasing strains in the samples. In this study, we have used high-energy ball milling in an inert gas atmosphere to synthesize SiNCs and study their photophysical properties. The induced accumulation of high strain, quantum confinement and possibly also impurities in the SiNCs resulted in visible light spectrum PL at room temperature. This method is low cost and easily up-scalable to industrial scale.

Forming Si nanocrystals on insulator by wet anisotropic etching

Implementing the Silicon-On-Insulator platform, we reveal an approach to fabricate Si nanocrystals on insulator by a single step, wet anisotropic etching of thin crystalline layers. The evolution of the Si layer was investigated from the onset of the dissolution process up to the development of isolated nanocrystals. Our results also provide several insights into the understanding of the early stages of the hillocks formation characteristic for the anisotropic etching of Si(001). We discuss several related mechanisms and we give experimental evidences that support the stabilization of the pyramidal hillocks apices by short-lived silicate particles.

Photoluminescence origin of lightly doped silicon nanowires treated with acid vapor etching

In this work, we carry out a comprehensive photoluminescence (PL) study to elucidate the origin of light emission from porous silicon nanowires (pSiNWs). SiNWs were first grooved in lightly doped Si wafer by silver assisted chemical etching, and then treated with an acid vapor emanating from HF/HNO3 aqueous solution heated at 60 °C. Scanning and transmission electron microscopies were used to investigate the effect of the acid vapor etching on morphological properties of SiNWs. The as prepared pSiNWs exhibited a strong room temperature PL emission centered at 1.93 eV. An increase of the PL intensity was observed with the increase of HNO3 in the acid solution. By varying the laser excitation density from 60 to 300 W/cm2, we shed the light on the radiative recombination modes occurring within the Si nanocrystals (SiNCs) generated along the pSiNWs. We study as well the temperature-dependent PL of the pSiNWs in the range 10 to 300 K. Based on both laser excitation density and temperature-dependent PL, we propose a multilevel transition scheme resuming the PL origin taking into account the size distribution, shape and surface states of the SiNCs trimming the wire sidewalls.

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.

Transmission electron microscope imaging of plasma grown electroformed silicon nitride-based light emitting diode for direct examination of nanocrystallization

We report for the first time a direct transmission electron microscope (TEM) imaging of a cross-section of a silicon nitride-based light emitting diode (LED), produced via a method patented by our research group. Grown by plasma enhanced chemical vapor deposition (PECVD) technique the LED structure (glass/Cr/p+-nc-Si:H/i-SiNx:H/n+-nc-Si:H/ITO) was then subjected to a high forward voltage stress for one time only, i.e. electroforming process. After electroforming the LED exhibited a boosted visible light emission and memory effect. To study the structural effect of the electroforming on the as-deposited LED the cross-section was extracted by focused ion beam (FIB) technique directly from the electroformed diode and thus prepared for TEM imaging. Since the electroforming process caused crystallization of ITO and its breakup in some parts of the diode surface, the FIB was conducted for the cross-section containing some regions with ITO layer and some without ITO. TEM examination revealed the nanocrystalline phase formation within the intrinsic layer (i-SiNx:H) caused by the electroforming process. The average size and distribution of Si nanocrystallites formed inside i-SiNx:H was determined. The Si nanocrystallization within i-SiNx:H was compared for the regions with and without ITO layer. The previously proposed model describing the changes taken place in the diode during electroforming process was reconsidered in the light of this TEM analysis.

Electronic Structure Shift of Deeply Nanoscale Silicon by SiO2 versus Si3N4 Embedding as an Alternative to Impurity Doping

Conventional impurity doping of deep nanoscale silicon (dns-Si) used in ultra large scale integration (ULSI) faces serious challenges below the 14 nm technology node. We report on a new fundamental effect in theory and experiment, namely the electronic structure of dns-Si experiencing energy offsets of ca. 1 eV as a function of SiO$_2$- vs. Si$_3$N$_4$-embedding with a few monolayers (MLs). An interface charge transfer (ICT) from dns-Si specific to the anion type of the dielectric is at the core of this effect and arguably nested in quantum-chemical properties of oxygen (O) and nitrogen (N) vs. Si. We investigate the size up to which this energy offset defines the electronic structure of dns-Si by density functional theory (DFT), considering interface orientation, embedding layer thickness, and approximants featuring two Si nanocrystals (NCs); one embedded in SiO$_2$ and the other in Si$_3$N$_4$. Working with synchrotron ultraviolet photoelectron spectroscopy (UPS), we use SiO$_2$- vs. Si$_3$N$_4$-embedded Si nanowells (NWells) to obtain their energy of the top valence band states. These results confirm our theoretical findings and gauge an analytic model for projecting maximum dns-Si sizes for NCs, nanowires (NWires) and NWells where the energy offset reaches full scale, yielding to a clear preference for electrons or holes as majority carriers in dns-Si. Our findings can replace impurity doping for n/p-type dns-Si as used in ultra-low power electronics and ULSI, eliminating dopant-related issues such as inelastic carrier scattering, thermal ionization, clustering, out-diffusion and defect generation. As far as majority carrier preference is concerned, the elimination of those issues effectively shifts the lower size limit of Si-based ULSI devices to the crystalization limit of Si of ca. 1.5 nm and enables them to work also under cryogenic conditions.

Analysis of the conduction mechanisms responsible for multilevel bipolar resistive switching of SiO2/Si multilayer structures

Abstract We report on the bipolar resistive switching behavior of a SiO2/Si multilayer (ML) structure measured without current compliance for non-volatile memory applications. The SiO2/Si ML structure was thermally annealed to induce the formation of Si nanocrystals, which act as conductive nodes. It was found that the SET/RESET voltages and the ON/OFF resistance ratio (107-102) depend on the current-voltage measurement cycles. Moreover, the low resistance state from the initial cycle can be recovered by increasing the stop voltage in a subsequent cycle. The space charge limited conduction (SCLC) is the mechanism responsible for the charge transport in the high resistance state (HRS) during the first measurement cycle, while the Ohmic conduction predominates in the low resistance state (LRS). This suggest the formation of a conductive path within the SiO2/Si ML, which breaks apart and becomes reconstructed upon switching between the LRS to HRS and vice versa. Performing a sequence of switching cycles, one can observe that the SCLC becomes predominant for the both LRS and HRS, suggesting the partial formation and breakdown of several different conductive paths.

Near-IR Emitting Si Nanocrystals Fabricated by Thermal Annealing of SiNx/Si3N4 Multilayers

Silicon nanocrystals in silicon nitride matrix are fabricated by thermal annealing of SiNx/Si3N4 multilayered thin films, and characterized by transmission electron microscopy, X-ray reflectivity and diffraction analysis, photoluminescence and X-ray photoelectron spectroscopy techniques. Si nanocrystals with a mean size of about 4 nm are obtained, and their properties are studied as a function of SiNx layer thickness (1.6–2 nm) and annealing temperature (900–1250 °C). The effect of coalescence of adjacent nanocrystals throughout the Si3N4 barrier layers is observed, which results in formation of distinct ellipsoidal-shaped nanocrystals. Complete intermixing of multilayered film accompanied by an increase of nanocrystal mean size for annealing temperature as high as 1250 °C is shown. Near-IR photoluminescence with the peak at around 1.3–1.4 eV is detected and associated with quantum confined excitons in Si nanocrystals: Photoluminescence maximum is red shifted upon an increase of nanocrystal mean size, while the measured decay time is of order of microsecond. The position of photoluminescence peak as compared to the one for Si nanocrystals in SiO2 matrix is discussed.

Electrically active boron doping in the core of Si nanocrystals by planar inductively coupled plasma CVD

An improvement in the doping efficiency in p-type nc-Si:H, a two-phase structure consisting of Si-nanocrystallites embedded in an amorphous matrix, has been pursued via low-temperature, spontaneous, plasma processing of low-pressure and high-density SiH4 plasma with added B2H6, diluted in H2, in inductively coupled plasma CVD. With increased dopant incorporation, the gradually reduced overall crystallinity systematically dominates via an ultrananocrystalline component in the thermodynamically stable ⟨220⟩ orientation. The microstructure factor decreases continuously, and the bonded-H content of the network increases arbitrarily, with a significant fraction being associated with Si in an Si-H-Si plateletlike configuration. Effective doping by B atoms, mostly in fourfold coordination with Si in its nanocrystalline core, contributes to the rapid increase in conductivity of the doped p-nc-Si:H network, up to a moderate B2H6 flow rate. However, gradually enhanced doping by threefold coordinated electrically inactive B-atoms, mostly at the nanocrystalline grain boundary at a higher B2H6 flow rate, induces gross structural deviation in the degree of nanocrystallinity and reduces dark conductivity substantially. Further, the logarithmic magnitude of the conductivity prefactor (σ0) demonstrates the negative characteristic energy (EMN) in the Meyer–Neldel (MN) relation. The high density of charge carriers in the heavily doped configuration of the nc-Si network forms a deep and continuous band tail near the valence-band edge and induces subsequent narrowing of the bandgap as well as a shift of the Fermi level into the valence band. Degenerate-semiconductor type behavior has been accomplished in a heavily doped p-nc-Si:H network, demonstrating reverse MN characteristics in electrical transport.

Site discrimination and size effect of B-doping in Si nanocrystals by second-neighbor atom consideration

The site discrimination and the size effect on the formation energy Ef of the B atoms in B-doped Si nanocrystals (SiNCs) were studied by introducing a new parameter, the adjusted coordination number (ACN), which includes second-nearest neighbor atoms. A series of Ef values were obtained by using the first-principle calculation. It was found that the B-doping sites in the SiNCs can be well discriminated by ACN, and the change trends of Ef with either size or site become clear if one uses ACN as a describer. When the site is given, Ef has a decreasing trend when the size increases. However, when considering the sites with the same coordination number in a given Si nanocrystal, their Ef values increase with increasing ACN. Moreover, it was observed that sites with ACN = 3.375 are the usually preferred B-doping sites in these SiNCs when they exist.

B-P codopant effects on Raman spectra of Si nanocrystals using real space pseudopotentials

We calculate vibrational and Raman spectra for B-doped, P-doped, and B-P codoped Si nanocrystals using real-space pseudopotentials constructed within density functional theory. An experimental extrinsic peak in the Raman spectra near 650 cm−1 is observed in codoped nanocrystals. We analyze the origin of this peak and find that it can be best explained by the presence of B-P bonds, which are located near the surface of the nanocrystal. We propose that the spectral details of this peak are related to quantum confinement and the breaking of local symmetry associated with the phonon modes involving dopant bonds.

Analytical description of nanowires. I. Regular cross sections for zincblende and diamond structures.

Semiconductor nanowires (NWires) experience stress and charge transfer from their environment and impurity atoms. In response, the environment of a NWire experiences a NWire stress response which may lead to propagated strain and a change in the shape and size of the NWire cross section. Here, geometric number series are deduced for zincblende- (zb-) and diamond-structured NWires of diameter dWire to obtain the numbers of NWire atoms NWire(dWire[i]), bonds between NWire atoms Nbnd(dWire[i]) and interface bonds NIF(dWire[i]) for six high-symmetry zb NWires with the low-index faceting that occurs frequently in both bottom-up and top-down approaches of NWire processing. Along with these primary parameters, the specific lengths of interface facets, the cross-sectional widths and heights and the cross-sectional areas are presented. The fundamental insights into NWire structures revealed here offer a universal gauge and thus could enable major advancements in data interpretation and understanding of all zb- and diamond-structure-based NWires. This statement is underpinned with results from the literature on cross-section images from III-V core-shell NWire growth and on Si NWires undergoing self-limiting oxidation and etching. The massive breakdown of impurity doping due to self-purification is shown to occur for both Si NWires and Si nanocrystals (NCs) for a ratio of Nbnd/NWire = Nbnd/NNC = 1.94 ± 0.01 using published experimental data.

Determination of crystallite size of nanodiamond by Raman spectroscopy

Although the phonon confinement model (PCM) was claimed to be successfully used to accurately calculate the size of larger Si nanocrystals, quantitative size characterization by Raman spectra still remains a challenge in the case of nanodiamonds due to its complexity. Here, we find that a local-mode model of Raman spectra developed recently can be employed to determine the bond number of the ordered diamond core in nanodiamonds, and then furtherly determine the size of nanodiamonds. The Raman lines of nanodiamonds of 3.0 nm, 2.0 nm, 2.2 nm, 3.3 nm, 3.7 nm 4.42 nm and 6.3 nm are calculated. Results are in good agreement with the measured Raman spectra. It not only provides a new approach to predict the size of nanodiamonds accurately by Raman spectra, but also helps to clarify issues in Raman spectra of nanodiamond and other carbon nanomaterials.

Generation of White Light by Hybridization of Red–Green–Blue–Luminescent Materials

Herein, the fabrication of a white luminescent material with photoluminescence (PL) in the region ranging from 350 to 900 nm is reported. Formation of the luminescent material is achieved simply by the high‐temperature (1100 °C) annealing of silicon‐rich oxide nanowires containing a ZnO phase, which are grown by exposing a Zn film to Si and O plasma generated using a mixture of N2O and SiH4 gases at a low temperature (380 °C). The PL spectral response is easily tuned by varying the mixing ratio of the two gases or the Zn film thickness. The generation of white luminescence can be attributed to the hybridization of Si nanocrystals (red), silicon oxide (green), and ZnO (blue)‐based materials.

Forward to Backward Scattering Ratio of Dielectric–Metal Heterodimer Suspended in Almost Free‐Space

AbstractA designed nanoantenna exhibiting unidirectional scattering is a promising building block for novel nanophotonic devices. For device applications using nanoantennas, quantitative information on the directional scattering property is indispensable. However, experimental determination of the forward to backward scattering intensity ratio of a nano‐object is not straightforward. Here, a scheme to determine the ratio quantitatively is proposed. A key technology of the scheme is placing a nano‐object on a thin SiO2 membrane. Because of the low refractive index and the small thickness of the membrane, the nano‐object can be treated as being suspended in free‐space. This allows us to use an ideal Mie scatterer such as a spherical silicon (Si) nanocrystal as a reference to calibrate optical setups for the measurements of forward and backward scattering. Using the developed scheme, the directional scattering property of a heterodimer composed of Si and gold nanospheres synthesized by a solution process is investigated. By combining transmission electron microscopy and scattering measurements, systematic studies on the directional scattering property of a heterodimer by changing the size of a Si nanosphere is performed. It is demonstrated that strong Kerker‐type forward scattering is achieved by tuning the resonance wavelength of the magnetic dipole resonance of a Si nanosphere.

Nearly perfect near-infrared luminescence efficiency of Si nanocrystals: A comprehensive quantum yield study employing the Purcell effect

Thin layers of silicon nanocrystals (SiNC) in oxide matrix with optimized parameters are fabricated by the plasma-enhanced chemical vapor deposition. These materials with SiNC sizes of about 4.5 nm and the SiO2 barrier thickness of 3 nm reveal external quantum yield (QY) close to 50% which is near to the best chemically synthetized colloidal SiNC. Internal QY is determined using the Purcell effect, i.e. modifying radiative decay rate by the proximity of a high index medium in a special wedge-shape sample. For the first time we performed these experiments at variable temperatures. The complete optical characterization and knowledge of both internal and external QY allow to estimate the spectral distribution of the dark and bright NC populations within the SiNC ensemble. We show that SiNCs emitting at around 1.2–1.3 eV are mostly bright with internal QY reaching 80% at room temperature and being reduced by thermally activated non-radiative processes (below 100 K internal QY approaches 100%). The mechanisms of non-radiative decay are discussed based on their temperature dependence.

Modulation of lattice constants by changing the composition and strain in incommensurate Nowotny chimney-ladder phase FeGeγ epitaxially grown on Si

The single phase FeGeγ nanocrystals (NCs) with incommensurate Nowotny chimney-ladder structure were epitaxially grown on Si substrates by reaction between deposited Fe atoms and Ge nanostructured layers on Si. The Ge nanostructured layers worked as blocking layers of the Fe diffusion to Si substrates. We used the two kinds of the Ge nanostructured layers: Ge wetting layers formed by direct Ge deposition on Si and Ge NCs formed by Ge deposition on ultrathin SiO2 films with ultrasmall nanowindows on Si. In the case of Ge wetting layers, the FeGeγ NCs were directly formed on Si, where there was a-axis strain due to the 2.9% lattice mismatch between FeGeγ and Si. On the other hand, in the case of Ge NCs, formed FeGeγ NCs were elastically-strain-relaxed due to the small contact with Si substrates through nanowindows in the ultrathin SiO2 films. Furthermore, the c-axis lattice constant in the epitaxial FeGeγ NCs changed due to the variation of the Fe-Ge composition. These results demonstrated that a-axis lattice constant was modulated by strain control of FeGeγ NCs due to the change of contact area with Si substrate and that c-axis lattice constant of FeGeγ NCs with incommensurate Nowotny chimney-ladder structure was modulated by manipulating Fe-Ge composition. These modulation of lattice constants will open a road for manipulating phonon group velocity related to the lattice constant and managing the waste heat in various devices implemented on Si substrates.