Si NCs Research Articles

Absence of free carriers in silicon nanocrystals grown from phosphorus- and boron-doped silicon-rich oxide and oxynitride.

Phosphorus- and boron-doped silicon nanocrystals (Si NCs) embedded in silicon oxide matrix can be fabricated by plasma-enhanced chemical vapour deposition (PECVD). Conventionally, SiH4 and N2O are used as precursor gasses, which inevitably leads to the incorporation of ≈10 atom % nitrogen, rendering the matrix a silicon oxynitride. Alternatively, SiH4 and O2 can be used, which allows for completely N-free silicon oxide. In this work, we investigate the properties of B- and P-incorporating Si NCs embedded in pure silicon oxide compared to silicon oxynitride by atom probe tomography (APT), low-temperature photoluminescence (PL), transient transmission (TT), and current–voltage (I–V) measurements. The results clearly show that no free carriers, neither from P- nor from B-doping, exist in the Si NCs, although in some configurations charge carriers can be generated by electric field ionization. The absence of free carriers in Si NCs ≤5 nm in diameter despite the presence of P- or B-atoms has severe implications for future applications of conventional impurity doping of Si in sub-10 nm technology nodes.

Tuning Confinement in Colloidal Silicon Nanocrystals with Saturated Surface Ligands.

The optical properties of silicon nanocrystals (Si NCs) are a subject of intense study and continued debate. In particular, Si NC photoluminescence (PL) properties are known to depend strongly on the surface chemistry, resulting in electron-hole recombination pathways derived from the Si NC band-edge, surface-state defects, or combined NC-conjugated ligand hybrid states. In this Letter, we perform a comparison of three different saturated surface functional groups-alkyls, amides, and alkoxides-on nonthermal plasma-synthesized Si NCs. We find a systematic and size-dependent high-energy (blue) shift in the PL spectrum of Si NCs with amide and alkoxy functionalization relative to alkyl. Time-resolved photoluminescence and transient absorption spectroscopies reveal no change in the excited-state dynamics between Si NCs functionalized with alkyl, amide, or alkoxide ligands, showing for the first time that saturated ligands-not only surface-derived charge-transfer states or hybridization between NC and low-lying ligand orbitals-are responsible for tuning the Si NC optical properties. To explain these PL shifts we propose that the atom bound to the Si NC surface strongly interacts with the Si NC electronic wave function and modulates the Si NC quantum confinement. These results reveal a potentially broadly applicable correlation between the optoelectronic properties of Si NCs and related quantum-confined structures based on the interaction between NC surfaces and the ligand binding group.

Effect of Si3N4‐Mediated Inversion Layer on the Electroluminescence Properties of Silicon Nanocrystal Superlattices

AbstractThe achievement of an efficient all‐Si electrically‐pumped light emitter is a major milestone in present optoelectronics still to be fulfilled. Silicon nanocrystals (Si NCs) are an attractive material which, by means of the quantum confinement effect, allow attaining engineered bandgap visible emission from Si by controlling the NC size. In this work, SiO2‐embedded Si NCs are employed as an active layer within a light‐emitting device structure. It is demonstrated that the use of an additional thin Si3N4 interlayer within the metal–insulator–semiconductor device design induces an enhanced minority carrier injection from the substrate, which in turn increases the efficiency of sequential carrier injection under pulsed electrical excitation. This results in a substantial increase in the electroluminescence efficiency of the device. Here, the effect of this Si3N4 interlayer on the structural, optical, electrical, and electro‐optical properties of a Si NC‐based light emitter is reported, and the physics underlying these results is discussed.

Size-dependent phosphorus doping effect in nanocrystalline-Si-based multilayers

Doping in Si nanocrystalline (Si NCs) is currently an interesting issue since it can modulate electronic structures of Si NCs and consequently affect the electric and optical properties. However, it has been reported that doping in Si NCs is quite difficult due to the self-purification effect and the locations of dopants such as phosphorus (P) and boron (B), as well as the influences of dopants on physical properties is still unclear. In the present work, the P-doped Si NCs/SiO2 multilayers with various dot sizes (2–20 nm) were fabricated. It is found that the electronic properties are enhanced significantly after doping. However, for P-doped samples, the conductivity is size-dependent and reduces with decreasing the dot size, which can be attributed to the reduction of activated carrier concentration in the film with small dot size. Moreover, we find P dopants can induce deep-level for sample with small size of 2 nm, and a sub-band near infrared light emission near 1250 nm is detected. The possible mechanism are discussed briefly.

SPUTTER-GROWN Sb-DOPED SILICON NANOCRYSTALS EMBEDDED IN SILICON-RICH CARBIDE FOR Si HETEROJUNCTION SOLAR CELLS

Sb-doped silicon nanocrystals (Si–NCs) films were fabricated by magnetron co-sputtering combined with rapid-thermal annealing. The effects of Sb content on the structural and electrical properties of the films were studied. The dot size increased with the increasing Sb content, and could be correlated to the effect of Sb-induced crystallization. The variation in the concentration of Sb shows a significant impact on the film properties, where as doped with 0.8[Formula: see text]at.% of Sb exhibited major property improvements when compared with other films. By employing Sb-doped Si–NCs films as emitter layers, Si–NCs/monocrystalline silicon heterojunction solar cells were fabricated and the effect of the Sb doping concentration on the photovoltaic properties was studied. It is found that the doping level in the Si–NCs layer is a key factor in determining the short-circuit current density and power conversion efficiency (PCE). With an optimized doping concentration of 0.8[Formula: see text]at.% of Sb, a maximal PCE of 7.10% was obtained. This study indicates that the Sb-doped Si–NCs can be good candidates for all-silicon tandem solar cells.

MoS2 monolayers on Si and SiO2 nanocone arrays: influences of 3D dielectric material refractive index on 2D MoS2 optical absorption.

Heterostructures enable the control of transport and recombination of charge carriers, which are either injected through electrodes, or created by light illumination. Instead of full 2D-material-heterostructures in device applications, using hybrid heterostructures consisting of 2D and 3D materials is an alternative approach to take advantage of the unique physical properties of 2D materials. In addition, 3D dielectric nanostructures exhibit useful optical properties such as broadband omnidirectional antireflection effects and strongly concentrated light near the surface. In this work, the optical properties of 2D MoS2 monolayers conformally coated on 3D Si-based nanocone (NC) arrays are investigated. Numerical calculations show that the absorption in MoS2 monolayers on SiO2 NC is significantly enhanced, compared with that for MoS2 monolayers on Si NC. The weak light confinement in low refractive index SiO2 NC leads to greater absorption in the MoS2 monolayers. The measured photoluminescence and Raman intensities of the MoS2 monolayers on SiO2 NC are much greater than those on Si NC, which supports the calculation results. This work demonstrates that 2D MoS2-3D Si nano-heterostructures are promising candidates for use in high-performance integrated optoelectronic device applications.

Ion-beam formation of light-emitting structures based on silicon nitride layers on silicon

The paper shows the advantage of silicon nitride for use as a light-emitting element in integrated circuits. Alongwith this, the chosen methods for obtaining the studied samples enable us to determine all possible causesresponsible for radiative recombination. The effect of irradiation with Xe+ ions of 200 MeV in the dose range109 – 1014 ions/cm2 on the optical properties and the structure of silicon enriched silicon nitride (SRN) filmsdeposited on the Si substrate by chemical vapor deposition under low pressure was studied. Based on the Ramanscattering (RS) data, it was concluded that the irradiation by 200 MeV Xe ions with a dose of 1×1014 ions/cm2leads to the dissolution of the amorphous Si phase in the nitride matrix. According to the results of transmissionelectron microscopy, preliminary irradiation with swift heavy ions (SHI) enhances the phase separation processin the nitride layer with a 22% excess of Si during the subsequent annealing at 1100°C for 60 minutes. The SHIirradiation, followed by the heat treatment, leads to a further increase in the intensity of the photoluminescence(PL) in the spectral range 600–750 nm compared with that in case of annealed films without preliminaryirradiation. It is known that radiation in this spectral range is due to Si nanocrystals (Si NCs).

Characterization of Silicon Nanocrystal Surfaces by Multidimensional Solid-State NMR Spectroscopy

The chemical and photophysical properties of silicon nanocrystals (Si NCs) are strongly dependent on the chemical composition and structure of their surfaces. Here we use fast magic angle spinning (MAS) and proton detection to enable the rapid acquisition of dipolar and scalar 2D 1H–29Si heteronuclear correlation (HETCOR) solid-state NMR spectra and reveal a molecular picture of hydride-terminated and alkyl-functionalized surfaces of Si NCs produced in a nonthermal plasma. 2D 1H–29Si HETCOR and dipolar 2D 1H–1H multiple-quantum correlation spectra illustrate that resonances from surface mono-, di-, and trihydride groups cannot be resolved, contrary to previous literature assignments. Instead the 2D NMR spectra illustrate that there is large distribution of 1H and 29Si chemical shifts for the surface hydride species in both the as-synthesized and functionalized Si NCs. However, proton-detected 1H–29Si refocused INEPT experiments can be used to unambiguously differentiate NMR signals from the different surf...

Changes of the absorption cross section of Si nanocrystals with temperature and distance.

The absorption cross section (ACS) of silicon nanocrystals (Si NCs) in single-layer and multilayer structures with variable thickness of oxide barriers is determined via a photoluminescence (PL) modulation technique that is based on the analysis of excitation intensity-dependent PL kinetics under modulated pumping. We clearly demonstrate that roughly doubling the barrier thickness (from ca. 1 to 2.2 nm) induces a decrease of the ACS by a factor of 1.5. An optimum separation barrier thickness of ca. 1.6 nm is calculated to maximize the PL intensity yield. This large variation of ACS values with barrier thickness is attributed to a modulation of either defect population states or of the efficiency of energy transfer between confined NC layers. An exponential decrease of the ACS with decreasing temperature down to 120 K can be explained by smaller occupation number of phonons and expansion of the band gap of Si NCs at low temperatures. This study clearly shows that the ACS of Si NCs cannot be considered as independent on experimental conditions and sample parameters.

Determination of Shape and Sphericity of Silicon Quantum Dots Imaged by EFTEM‐Tomography

The shape of size‐controlled silicon nanocrystals (Si NCs) embedded in SiO2 is investigated by tomographic energy‐filtered transmission electron microscopy (EFTEM). The sphericity of the quantum dots is determined by computational analyses. In contrast to other fabrication methods, we demonstrate that the NCs in superlattices are non‐agglomerated, individual clusters with slightly oblate spheroidal shape. This allows for low surface‐to‐volume ratios and thereby low non‐radiative defect densities as required by optoelectronic or sensing applications. A near‐spherical shape is also a prerequisite for the direct comparison of Si quantum dots (QDs) with theoretical simulations.

Luminescent Capillary‐Based Whispering Gallery Mode Sensors: Crossing the Lasing Threshold

Silicon nanocrystals (Si NCs) present many advantages for sensor applications, including relatively bright and stable luminescence, low‐to‐negligible toxicity, and high physical and chemical robustness. In this work, the efforts in creating capillary‐based sensors from smooth films of oxide‐embedded SiNCs are summarized, and their responses to dye‐doped polymer films are compared. The main method is to form sub‐micrometer luminescent coatings on microcapillary channel walls. The coating must have a high index of refraction so that it can support the luminescence whispering gallery modes (WGMs) which propagate through the film and extend into the channel medium. Using Si NCs, a general refractometric sensing and the detection of layer‐by‐layer polyelectrolyte deposition on the capillary channel are demonstrated. The Si NC sensors are exceptionally robust and can be cleaned and re‐used multiple times. The main limitation of the method currently involves the relatively slow detection (requiring typically more than 20 s per collected luminescence spectrum) due to the low light levels associated with the Si NC luminescence. Finally, our most recent work is discussed, which aims to extend the luminescent capillary sensor into the lasing regime.

Light-Converting Polymer/Si Nanocrystal Composites with Stable 60-70% Quantum Efficiency and Their Glass Laminates.

Thiol-ene polymer/Si nanocrystal bulk hybrids were synthesized from alkyl-passivated Si nanocrystal (Si NC) toluene solutions. Radicals in the polymer provided a copassivation of "dark" Si NCs, making them optically active and leading to a substantial ensemble quantum yield increase. Optical stability over several months was confirmed. The presented materials exhibit the highest photoluminescence quantum yield (∼65%) of any solid-state Si NC hybrid reported to date. The broad tunability of thiol-ene polymer reactivity provides facile glass integration, as demonstrated by a laminated structure. This, together with extremely fast polymerization, makes the demonstrated hybrid material a promising candidate for light converting applications.

Fully radiative relaxation of silicon nanocrystals in colloidal ensemble revealed by advanced treatment of decay kinetics

A comprehensive study of the spectrally resolved photoluminescence (PL) decay kinetics of dodecyl-passivated colloidal silicon nanocrystals (Si NCs) is presented. The correct treatment of average decay lifetime is demonstrated. We report on importance to distinguish the external quantum efficiency (QE) from the internal QE. The external QE of the ensemble of Si NCs is measured to be ∼60%, while the internal QE of Si NCs emitting around ∼1.5 eV is evaluated to be near unity. This difference between internal and external QE is attributed to a fraction of “dark” (absorbing but non-emitting) Si NCs in the ensemble. This conclusion is based on the analysis of deconvoluted size-selected decay curves retrieved by the presented mathematical procedure. The homogeneous line-broadening is estimated to be around 180 meV by experimentally challenging single-NC PL measurements. In addition, radiative lifetimes are calculated by the envelope function approximation and confirm the observed exponential increase of lifetime with decreasing emission photon energy.

Microstructure analysis of silicon nanocrystals formed from silicon rich oxide with high excess silicon: Annealing and doping effects

Thin films consisting of silicon nanocrystals fabricated by high silicon content in silicon rich oxide show unique properties of decreasing resistivity and increasing light absorption while maintaining quantum confinement effects. With that said, the effect of the annealing temperature and doping element on the microscopic structure of silicon nanocrystals (Si NCs) and the film are still under research. In this study, individual intrinsic, boron-, and phosphorus-doped films are annealed at various temperatures, and their structural properties are analyzed via atom probe tomography together with glancing incidence x-ray diffraction, Raman spectroscopy (Raman), transmission electron microscopy (TEM), and energy filtered TEM. In addition, photoluminescence (PL) is performed and linked with their microstructural properties. The Si NC growth is confirmed at annealing temperatures of 1000 °C and 1100 °C. The microstructure of the Si NCs in the whole film is dramatically changed by increasing the annealing temperature from 1000 °C to 1100 °C. In addition, doping changes the arrangement of the Si NCs by assisting their penetration across the SiO2 barrier layers. This study helps to understand the relationship between the microscopic and macroscopic properties of the Si NC film, showing that the size and distribution of the Si NCs are correlated with the obtained PL profiles.

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.

Doping effect in Si Nanocrystals/SiO2 multilayers

Doping in Si nano-crystals (Si NCs) is currently a great challenge to develop the high-performance nano-devices. Here, we fabricated P-doped Si NCs by PECVD in Si/SiO2 multilayered structures after annealing at high temperature. It is demonstrated experimentally that P dopants can be incorporated into Si NCs substitutionally. Furthermore, we found that the photoluminescence properties of Si NCs can be drastically modified by P doping in Si NCs/SiO2 multilayers with the ultra-small size. A subband light emission centred at 1200nm is observed, which can be ascribed to the radiative recombination via the P-induced deep level in the gap of the Si NCs. Interestingly, it is also found that the subband emission can be enhanced obviously by B and P co-doping. Our results suggest that doping plays an important role in the electronic structures and optoelectronic characteristics of Si NCs, which provide a new route to realize the Si NCs-based optical and electronical devices.

Density functional theory study on the B doping and B/P codoping of Si nanocrystals embedded in SiO2

Doping silicon nanocrystals (Si NCs) embedded in silicon dioxide ($\mathrm{Si}{\mathrm{O}}_{2}$) with boron (B) and phosphorus (P) is a promising way of tuning the properties of Si NCs. Here we take advantage of density functional theory to investigate the dependence of the structural and electronic properties of Si NCs embedded in $\mathrm{Si}{\mathrm{O}}_{2}$ on the doping of B and P. The locations and energy-level schemes are examined for singularly B-doped or B/P-codoped Si NCs embedded in $\mathrm{Si}{\mathrm{O}}_{2}$ with a perfect or defective Si/$\mathrm{Si}{\mathrm{O}}_{2}$ interface at which a Si dangling bond exists. A dangling bond plays an important role in the doping of Si NCs with B or B/P. The doping behavior of B in Si NCs embedded in $\mathrm{Si}{\mathrm{O}}_{2}$ vastly differs from that of P. The electronic structure of a B/P-codoped Si NC largely depends on the distribution of the dopants in the NC.

Water-dispersible near-infrared luminescent silicon nanocrystals -immobilization on substrate

Abstract

Erratum to: Atom probe tomography of phosphorus- and boron-doped silicon nanocrystals with various compositions of silicon rich oxide — ERRATUM

“The P and B concentration in Si NCs and the oxide region was extracted from the distance around 0.5 nm and -2.5 nm, respectively.”

Integration of colloidal silicon nanocrystals on metal electrodes in single-electron transistor

We develop a facile process to integrate colloidal silicon nanocrystals (Si NCs) with metal electrodes in a single-electron transistor by self-assembly. Gold (Au) surface is modified by an amine-terminated self-assembled monolayer to have a positive potential. All-inorganic boron (B) and phosphorus (P) codoped Si NCs, with a negative surface potential and size-controllability, are selectively adsorbed on an amine-terminated Au surface by electrostatic attraction. We demonstrate the fabrication of SETs consisting of electroless-plated Au nanogap electrodes and codoped Si NCs using this process and observation of clear Coulomb diamonds at 9 K.