Si NCs Research Articles

Colloidal Si nanocrystals: a controlled organic–inorganic interface and its implications of color-tuning and chemical design toward sophisticated architectures

The optical use of colloidal silicon nanocrystals (Si NCs) has gained increasing attention for its possible contributions to building a sustainable society that ideally uses resources and energy with high efficiency without causing damage to the environment or human health. Si wafers (E(g) ≈ 1.1 eV) dominate modern microelectronics as an impressive electronic material, but they exhibit relatively poor optical performance owing to an indirect bandgap structure. Interestingly, however, full control of the size distribution and surface chemistry of the NCs yields size-dependent light emission in a very wide range from near-ultraviolet through visible to near-infrared wavelengths. In addition to such unique luminescence properties, Si exhibits a high chemical affinity to covalent linkages with carbon, oxygen, and nitrogen, thereby producing almost unlimited variations in organic-Si NCs architectures hybridized at the molecular level. To achieve this goal, I note some parameters, including interfacial chemistry, that are emerging as important elements for increasing our understanding of the effect of quantum confinement in nanostructured Si and for realizing efficient fluorescence emission. This article covers new aspects of derivatives of Si NCs in applications that utilize their optical absorption and emission features.

Dynamical effects of defect photoluminescence from single SiO2 and Si nanoparticles

We present results of photoluminescence (PL) studies of single SiO2 nanoparticles (SiO2 NPs) and single Si nanocrystals (Si NCs). Single particle spectroscopy reveals almost identical PL spectra for both kinds of nanosized systems. The emission curves exhibit a zero-phonon line and one or two phonon bands, which can be assigned to longitudinal optical phonons in SiO2. Using cylindrical vector beams for imaging the fluorescence of single particles we show that they possess a linear excitation transition dipole moment (TDM). Furthermore, the single particle fluorescence patterns demonstrate upon continuous excitation dynamical effects such as blinking, bleaching, and flipping of the TDM. The latter is related to a redistribution of defect states caused by charge fluctuations in the surrounding of the embedded NP. Excitation fluorescence images visualize the intermediate state resulting from the TDM flipping.

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.

Optimization of silicon nanocrystals growth process by low pressure chemical vapor deposition for non-volatile memory application

The processes of silicon nanocrystals (Si–NCs) growth on both SiO 2 and Si 3N 4 substrates by low pressure chemical vapor deposition have been systematically investigated. A two-step process was adopted for Si–NCs growth: nucleation at a high temperature (580–600 °C) and growth at a low temperature (550 °C). By adjusting the pre-deposition waiting time and deposition time, the density, size and uniformity can be effectively controlled. Compared to the growth of Si–NCs on SiO 2, the coalescence speed of Si–NCs on Si 3N 4 is faster. Uniform Si–NCs with a high density of 1.02 × 10 12 cm − 2 and 1.14 × 10 12 cm − 2 have been obtained on SiO 2 and Si 3N 4, respectively. Finally, a Si–NCs-based memory structure with a 2.1 V memory window was demonstrated.

Photoluminescence of as-grown and thermal annealed SiOx/Si-nanocrystals heterolayers grown by reactive rf sputtering

SiO x / Si -nanocrystals (Si NCs) heterolayers were fabricated employing a rf magnetron sputtering system. The synthesis process, through modification of the oxygen partial pressure of the plasma, promotes the synthesis of stoichiometric SiO2 layers and affect the Si NCs layer giving place to SiOx/Si NCs (1.64<x<2) interfaces. All as-grown samples showed strong photoluminescence (PL) bands in the visible and near-infrared regions; transmission electron microscopy measurements confirmed the presence of Si NCs. Thermal annealing at 1100 °C promoted the SiO2 stoichiometry in the interface and the crystallization of more Si NCs. The results allow us to clearly identify the origin of the PL bands; indicating that the near-infrared emission is related to the nonstoichiometric oxide while the red and green bands are originated in Si NCs.

Amorphous Si xC 1− x:H single layers before and after thermal annealing: Correlating optical and structural properties

Amorphous Si x C 1− x :H single layers with varying stoichiometry were prepared using plasma enhanced chemical vapour deposition (PECVD). The as-deposited layers were annealed at different temperatures ( T a ) from 500 to 1100 °C. The influence of annealing on layers with varying composition x was investigated by a variety of analytical techniques including photoluminescence (PL) spectroscopy, reflectance and transmittance measurements, grazing incidence X-ray diffraction (GIXRD) and Fourier transform infrared spectrometry (FTIR). Before annealing, PL measurements show a shift of the spectra to higher energies with increase in carbon content. Calculations of the optical bandgap by other groups confirm this trend. The FTIR investigations show in addition to the expected Si–C bonds also the formation of oxygen and hydrogen related bonds within the layers. After annealing, solely a broad PL signal appears at energies around 2.0 eV. This might be due to luminescence of defects, created by the incorporated amount of carbon and the evolution of crystalline phases in the layers. GIXRD measurements confirmed the formation of both Si NCs after 700 °C and SiC NCs after annealing at 1000 °C. The FTIR spectra exhibit a shift of the SiC absorption band and an increase in the Si–C bond density due to the crystallization of SiC. After annealing a hydrogen passivation was performed, which leads to a strong decrease in the PL intensity. However, annealed Si-rich films show a peak around 1.2 eV after H-passivation, which might originate from quantum confinement effects in Si NCs.

Optimization of Si NC/P3HT Hybrid Solar Cells

AbstractSilicon nanocrystals (Si NCs) are shown to be an electron acceptor in hybrid solar cells combining Si NCs with poly(3‐hexylthiophene) (P3HT). The effects of annealing and different metal electrodes on Si NC/P3HT hybrid solar cells are studied in this paper. After annealing at 150 °C, Si NC/P3HT solar cells exhibit power conversion efficiencies as high as 1.47%. The hole mobility in the P3HT phase extracted from space‐charge‐limited current measurements of hole‐only devices increases from 2.48 × 10−10 to 1.11 × 10−9 m2 V−1 s−1 after annealing, resulting in better transport in the solar cells. A quenching of the open‐circuit voltage and short‐circuit current is observed when high work function metals are deposited as the cathode on Si NC/P3HT hybrid devices.

A Silicon Nanocrystal Schottky Junction Solar Cell produced from Colloidal Silicon Nanocrystals

Solution-processed semiconductors are seen as a promising route to reducing the cost of the photovoltaic device manufacture. We are reporting a single-layer Schottky photovoltaic device that was fabricated by spin-coating intrinsic silicon nanocrystals (Si NCs) from colloidal suspension. The thin-film formation process was based on Si NCs without any ligand attachment, exchange, or removal reactions. The Schottky junction device showed a photovoltaic response with a power conversion efficiency of 0.02%, a fill factor of 0.26, short circuit-current density of 0.148 mA/cm2, and open-circuit voltage of 0.51 V.

Bonding of Oxygen at the Oxide/Nanocrystal Interface of Oxidized Silicon Nanocrystals: AnAb InitioStudy

Ab initio methods based on density functional theory are employed to investigate the bonding of oxygen (O) at the oxide/nanocrystal (NC) interface after hydrogen (H)-passivated silicon (Si) NCs are oxidized. Besides the well-known quantum confinement effect, the type of O bonding and the oxidation state of Si at the oxide/NC interface are found to significantly affect the optical properties of oxidized Si NCs. After oxidation, the excitation energies of Si35 and Si66 increase, while those of Si87 and Si123 decrease. We show that oxidation-induced redshifts in the light emission from Si NCs do not always result from defective O such as doubly bonded O at the oxide/NC interface. When Si atoms at the oxide/NC interface are mainly in low oxidation states, backbond O at the interface per se results in the redshifts in the light emission from Si NCs. When Si atoms at the oxide/NC interface are mainly in high oxidation states, Si3+═O at the interface leads to the redshifts in the light emission from Si NCs. It i...

Structural properties of annealed SiOx

On the basis of recent results, concerning the quantitative determination of the clustered silicon concentration in substoichiometric silicon oxide layer and based on electron energy loss spectroscopy (EELS) and energy-filtered transmission electron microscopy, in this paper we demonstrate that the structural properties of Si nanoclusters (Si ncs) strongly depend on the technique used for specimen preparation. In particular here we report on annealed SiOx films prepared by plasma enhanced chemical vapour deposition (PECVD) and magnetron sputtering, showing that their structural properties are very different. It is shown that in PECVD films only around 30% of the Si excess agglomerates in clusters while an almost complete agglomeration occurs in sputtered films. These data are explained on the basis of the different initial structural properties of the as-deposited films which become crucial for the subsequent evolution.

Enhanced charge storage characteristics of silicon nanocrystals fabricated by electron-beam coevaporation of Si and SiOx(x=1 or 2)

In this article, a simple and flexible electron-beam coevaporation (EBCE) technique has been reported of fabrication of the silicon nanocrystals (Si NCs) and their application to the nonvolatile memory. For EBCE, the Si and SiOx(x=1 or 2) were used as source materials. Transmission electron microscopy images and Raman spectra measurement verified the formation of the Si NCs. The average size and area density of the Si NCs can be adjusted by increasing the Si:O weight ratio in source material, which has a great impact on the crystalline volume fraction of the deposited film and on the charge storage characteristics of the Si NCs. A memory window as large as 6.6 V under ±8 V sweep voltage was observed for the metal-oxide-semiconductor capacitor structure with the embedded Si NCs.

Electrical and optical characteristics of silicon nanocrystal solar cells

We investigated the relationship between the absorption in silicon nanocrystals (Si NCs) and the photocurrent of Si NC solar cells. Here, the absorption of Si NCs in the blue and green light regions was enhanced by up to 14 times compared to bulk Si. In addition, the photocurrent in Si NC solar cells was found to originate from absorption in the Si NCs. The photocurrent of the Si NC solar cells, however, was smaller than the predicted value due to the slow carrier transport in the silicon nitride film containing the Si NCs.

Room-temperature solution route to free-standing SiO 2-capped Si nanocrystals with green luminescence

We report a new solution route for the preparation of SiO 2-capped silicon nanocrystals (Si NCs). The Si NCs terminated with SiO 2 are fully characterized by transmission electron microscopy, X-ray diffraction, UV–vis absorption, photoluminescence decay and Fourier transform infrared spectra. The photoluminescence spectra reveal that the Si NCs solution emits green luminescence at 535 and 578 nm excited at 490 nm. The origin of the green luminescence of Si NCs is studied. Our theoretical calculations reveal that the green emission is due to the surface related localized states of self-trapped excitons rather than the purely quantum-confined states, which agrees well with the experimental results. A self-trapped exciton model is proposed to take into account the stepwise localization of electron and hole at the Si–SiO 2 interface. From the localization energies the effective Bohr radii of the localized electrons and holes are estimated to be about 1.71 and 1.57 nm, respectively.

Effect of exciton migration on the light emission properties in silicon nanocrystal ensembles

Different silicon nanocrystal (Si NC) systems in which Si NCs were either entirely isolated or loosely interconnected were studied by photoluminescence (PL) and time-resolved PL decay measurements in the range between 70 and 290 K, in order to investigate the role of exciton migration in the PL properties. We examined three kinds of samples: (a) two light emitting mesoporous Si (PSi) films with different porosities, grown on p-type Si, (b) a heavily oxidized light emitting anisotropic macroporous Si film, and (c) a film consisted of a Si NC superlattice with six Si NC/SiO2 bilayers, grown by low pressure chemical vapor deposition of amorphous Si (α-Si), followed by high temperature thermal oxidation. In the two mesoporous Si films of the first case, the Si NCs show a degree of interconnection that depends on the porosity, whereas in the two other cases the NCs were isolated by SiO2, the degree of electrical isolation depending on the thickness of the SiO2 interlayer between them. Temperature dependent PL spectra and PL decay times of the different systems correlate well with the ability of excitons to migrate from one NC to another (case of loosely correlated NCs) or remain strongly localized within the Si NCs (case of effectively isolated NCs).

Silicon nanocrystals synthesized by electron-beam co-evaporation method and their application for nonvolatile memory

In this paper, the silicon nanocrystals (Si NCs)/SiO 2 hybrid films designed for nonvolatile memory applications are prepared by electron-beam co-evaporation of Si and SiO 2. Transmission electron microscopy images and Raman spectra verify the formation of Si NCs. Metal-oxide-semiconductor capacitor structure with Si NCs embedded in the gate oxide is fabricated to characterize the memory behaviors. High-frequency capacitance–voltage and capacitance–time measurements further demonstrate the memory effect of the structure resulting from the charging or discharging behaviors of Si NCs. It is found that the memory window can be changed by adjusting the Si/SiO 2 wt. ratio in source material. The memory devices with Si NCs/SiO 2 hybrid film as floating gate yield good retention characteristics with small charge loss.

Size-dependent photocurrent of photodetectors with silicon nanocrystals

We report on the effect of nanocrystal size on the photocurrent of silicon nanocrystal (Si NC) photodetectors. The photocurrent onset energy was increased with decreasing the size of Si NCs, which agreed with the blueshift in the absorption onset. The increase in the quantum efficiency with increasing the size of Si NCs was much larger than the increase in the absorbance with increasing the size of Si NCs. This was attributed to the differences in charge separation for Si NC of different sizes, which was influenced by the recombination rate and binding energy of photogenerated excitons in Si NCs.

Silicon Nanocrystals Synthesized by Electron-beam Co-evaporation Method and Their Application for Nonvolatile Memory

The Silicon nanocrystals designed for nonvolatile memory applications are firstly prepared by electron beam co-evaporation of Si and SiO2 hybrid. Transmission electron microscopy observation and Raman scattering certify the formation of Si NCs. Metal-oxide-semiconductor capacitor structure with Si NCs embedded in the gate oxide is fabricated to characterize the memory behaviors. High-frequency capacitance-voltage and capacitance-time measurements demonstrate the memory effect of the structure resulting from the charging/discharging behaviors of Si NCs. It is found that flat-band voltage can be changed by adjusting the concentration of Si and SiO2 in hybrids. The Si nanocrystals memory device has yielded good retention characteristics with small charge loss.

The Synthesization of Si Nanocrystals Embedded in HfO2 Matrix by Electron Beam co-Evaporation Technique

Silicon nanocrystals synthesized by electron beam co-evaporation (EBCE) of Si and HfO2 mixture are studied. HfO2 and Si powder are uniformly mixed together in certain proportion. The mixed Si & HfO2 powder is evaporated by electron beam evaporation technique. Then the SRO thin films are annealed at different temperatures at 2 hours in N2 ambient to synthesize Silicon nanocrystals. For the sample annealed at 1050 ° at 2 hours, silicon nanocrystals with different size between 3 nm to 5 nm, the mean diameter of 4.0 nm, and the density of 3×1012 cm-2 are evidently observed by high resolution transmission electron microscopy (HRTEM). Then the Raman scattering and photoluminescence spectra arising from Silicon nanocrystals are further confirmed the above result. In addition, the samples annealed at 900 ° for 2 hours to 8 hours are carefully studied by Raman spectra. This way of preparing SRO thin films and Si NCs has unique advantage and is controllable and flexible

Hybrid Solar Cells from P3HT and Silicon Nanocrystals

We are reporting new hybrid solar cells based on blends of silicon nanocrystals (Si NCs) and poly-3(hexylthiophene) (P3HT) polymer in which a percolating network of the nanocrystals acts as the electron-conducting phase. The properties of composite Si NCs/P3HT devices made by spin-coating Si NCs and P3HT from a common solvent were studied as a function of Si NC size and Si NC/P3HT ratio. The open-circuit voltage and short-circuit current are observed to depend on the Si NC size due to changes in the bandgap and surface-area-to-volume ratio. Under simulated one-sun A.M. 1.5 direct illumination (100 mW/cm2), devices made with 35 wt % Si NCs 3-5 nm in size showed 1.15% power conversion efficiency.

Ultra-Low energy Ion Implantation of Si into HfO2-based layers for Non Volatile Memory Applications

AbstractThe fabrication of NCs is carried out using an innovative method, ultra-low energy (≤5 keV) ion implantation (ULE-II) into thin (6-9 nm) HfO2–based layers in order to form after subsequent annealing a controlled 2D array of Si NCs. The implantation of Si into HfO2 leads to the formation of SiO2–rich regions at the projected range due to the oxidation of the implanted Si atoms. This anomalous oxidation that takes place at room temperature is mainly due to humidity penetration in damaged layers. Different solutions are investigated here in order to avoid this oxidation process and stabilize the Si-phase. Finally, unexpected structures as HfO2 NCs embedded with SiO2 matrix are obtained and show interesting memory characteristics. Interestingly, a large memory window of 1.18 V has been achieved at relatively low sweeping voltage of ± 6 V for these samples, indicating their utility for low operating voltage memory device.