Sinc Research Articles

Inherent paramagnetic defects in layered nanocrystalline Si/SiO 2 superstructures

With the view to fully disclose the nature of occurring paramagnetic defects, a detailed electron spin resonance (ESR) study has been carried out on entities comprised of regular arrays of Si nanoparticles (np's) (size ∼2 nm) embedded in an SiO 2 matrix, obtained by the SiO/SiO 2 superlattice method. The approach encompasses high-sensitivity first- and second-harmonic low-temperature X, K and Q-band ESR in combination with computer simulations. This enabled disentanglement of the common Si dangling-bond (DB) signal, observed in the as grown state as being composed solely of Si/SiO 2 interface-specific powder patterns of P b(0) and P b1 defects, indicating that the majority, if not all, of the np's are crystalline. The inferred densities are in the range of standard values obtained for thermal SiO 2 grown on Si and remain unchanged over different (V)UV irradiation treatments. Yet, upon (V)UV irradiation, SiO 2-specific defects ( E γ ′ and EX) were activated, in numbers demonstrating standard SiO 2 quality. Only ∼71% of the Si nanocrystals (nc's) house a P b-type center, indicating the structure to be comprised of two subsystems, which may hence reflect in different defect-sensitive properties, such as, e.g., photoluminescence. Relying on the known properties of P b(0) and P b1 defects in standard microscopic Si/SiO 2, the data would comply with Si nc's, in average, predominantly bordered by (1 1 1) and (1 0 0) facets.

Role of ballistic transport in photoluminescence excitation of Si nanocrystals

Photoluminescence of Si NCs with the size (10–300nm) bigger than the exciton Bohr radius in the bulk Si crystals (4.8nm) has been considered. Photoluminescence in such NC systems is analyzed from the point of view of new concept based on the effect of hot carrier ballistic transport in excitation of suboxide defect-related photoluminescence at the Si/SiOx interface. The dependence of the 1.70eV PL band integrated intensity on Si NC sizes was numerically calculated on the base of the hot carrier ballistic PL model. The well correlation between calculated and experimental results has been obtained for Si NCs with the size from the 30–150nm range.

Ballistic effect and optical properties of Si nanocrystallite structures

AbstractThe purpose of this work is to clarify the type of corresponding radiative transitions and the PL mechanism in the big silicon nano‐crystallites with the size from the range of 10‐150 nm, which is higher than an exciton Bohr radius (≤5 nm) in corresponding bulk crystal. Photoluminescence in this type of Si nanocrystals is analyzed from the point of view of new concept based on the hot carrier ballistic transport role in the excitation of oxide‐related defects at the Si/SiOx interface. The role of hot carrier ballistic transport in the modification of photocurrent spectra of Si NC device structures is discussed as well. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

An analysis of erbium excited state absorption in silicon-rich silica

Erbium-doped silicon-rich silica is a material that has generated great interest, as it holds considerable promise for practical silicon-based optical sources. There are numerous reports of sensitization of erbium 1.5 μm luminescence by silicon nanoclusters (Si nc), and it is now accepted that this process increases the effective absorption cross-section of Er ions by three orders of magnitude. There have been reports of optical gain in a planar waveguide pumped at wavelengths away from Er absorption bands. However, no lasing action has yet been achieved. In this paper, we discuss a key interaction—excited state absorption (ESA)—which has been neglected in this material. Typically, Si nc responsible for sensitization have a broad photoluminescence peak centred around 800 nm. Moreover, spectral hole-burning studies suggest that resonant energy transfer occurs from the Si to the Er in this wavelength region. However, this energy transfer route may be problematic, due to an ESA transition known from early work on erbium-doped fibre amplifiers (EDFAs). Excitation around 800 nm causes the Er metastable state to be excited to higher-level states. Such transitions serve no useful purpose, but constitute an energy drain: EDFAs pumped at 800 nm require an order of magnitude more pump power to achieve the same gain as those pumped at 980 nm. We have conducted an analysis of the erbium-doped silicon-rich silica system, incorporating ESA in the model. We compare the results of our analysis with reported experimental data and extract the Si nc–Er excited state energy transfer coefficient, yielding a value of 1×10 −15 cm 3/s. This is comparable to the Si nc–Er ground state transfer coefficient, confirming that ESA is potentially significant.

Charge injection and tunneling mechanism of solid state reaction silicon nanocrystal film

Solid state reaction silicon nanocrystals (Si nc’s) of an average size of 10nm have been synthesized. Charge transport characteristics have been investigated as a function of temperature and voltage. From 305to400K, it is found that space-charge-limited current (SCLC), with an exponential distribution of trapping states, dominates the conduction mechanism. High resolution transmission electron microscope images indicate that microscopic structural defects, such as dislocations, are present in this solid state reaction Si nc. These defects are a possible source of trapping states as described in the SCLC model. Using this model, a trap density of Nt=1.46×1018cm−3 and a characteristic trap temperature Tt=2057K can be extracted. The trap density is two orders of magnitude greater than the Si nc density, showing that the structural defects in Si nc, such as dislocations and grain boundaries, are capable of trapping more carriers in a single solid state reaction Si nc.

The formation mechanism of Si nanocrystals in SiO 2

Si nanocrystals (Si nc) were formed by the implantation of Si + into a SiO 2 film followed by high-temperature annealing. The microstructural evolution of Si nc with annealing time has been investigated by high-resolution transmission electron microscopy (HRTEM) in detail. Three evident growth phases have been observed: formation of amorphous Si-rich SiO x nodules; transformation of the amorphous nodules into Si nc and coalescence of small particles into larger ones. Based on the HRTEM results, a formation mechanism has been proposed for the Si nc embedded in SiO 2.

Charge trapping phenomena of tetraethylorthosilicate thin film containing Si nanocrystalssynthesized by solid-state reaction

In this work, we report on the fabrication of tetraethylorthosilicate (TEOS) thin dielectricfilm containing silicon nanocrystals (Si nc), synthesized by solid-state reaction, in acapacitor structure. A metal–insulator–semi-conductor (MIS) capacitor, with 28 nm thick Sinc in a TEOS thin film, has been fabricated. For this MIS, both electron and holetrapping in the Si nc are possible, depending on the polarity of the bias voltage. AVFB shift greater than 1 V can be experienced by a bias voltage of 16 V applied tothe metal electrode for 1 s. Though there is no top control oxide, the dischargetime for 10% of charges can be up to 4480 s when it is biased at 16 V for 1 s. Itis further demonstrated that charging and discharging mechanisms are due tothe Si nc rather than the TEOS oxide defects. This form of Si nc in a TEOSthin film capacitor provides the possibility of memory applications at low cost.

Correlation between optical properties and Si nanocrystal formation of Si-rich Si oxide films prepared by plasma-enhanced chemical vapor deposition

We have investigated the phase separation and silicon nanocrystal (Si NC) formation in correlation with the optical properties of Si suboxide (SiO x , 0 < x < 2) films by thermal annealing in high vacuum. The SiO x films were deposited by plasma-enhanced chemical vapor deposition at different nitrous oxide/silane (N 2O/SiH 4) flow ratios. The as-deposited films show increased Si concentration with decreasing N 2O/SiH 4 flow ratio, while the deposition rate and surface roughness have strong correlations with the flow ratio in the N 2O/SiH 4 reaction. After thermal annealing at temperatures above 1000 °C, Fourier transform infrared spectroscopy, Raman spectroscopy, and transmission electron microscopy manifest the progressive phase separation and continuous growth of crystalline-Si (c-Si) NCs in the SiO x films with increasing annealing temperature. We observe a transition from multiple-peak to single peak of the strong red-range photoluminescence (PL) with increasing Si concentration and annealing temperature. The appearance of the single peak in the PL is closely related to the c-Si NC formation. The PL also redshifts from ∼1.9 to 1.4 eV with increasing Si concentration and annealing temperature (i.e., increasing NC size). The good agreements of the PL evolution with NC formation and the PL peak energy with NC size distribution support the quantum confinement model.

Characterization of Electrical Properties of Si Nanocrystals Embedded in an Insulating Layer by Scanning Probe Microscopy

Scanning probe microscope (SPM) with a conducting tip was used to electrically probe silicon nanocrystals (Si NCs) embedded in a SiO2 layer. The Si NCs were generated by the laser ablation method with compressed Si powder followed by a sharpening oxidation. The size of Si NCs is in the range of 10-50 nm, and the density is around 1011 /cm2. Using a conducting tip, the charge was injected directly into each Si NC, and the image contrast change and dC/dV curve shift caused by the trapped charges were monitored. The results were compared with those of the conventional MOS capacitor.

Influence of oxidizing ambient to tetraethylorthosilicate thin films containing solid-state reaction silicon nanocrystals

In this work, we present a systematic study on the oxidation of solid-state reaction silicon nanocrystals (Si nc) embedded in tetraethylorthosilicate (TEOS) thin films. The 12.8nm Si nc are spun coated in TEOS thin film and hard baked in O2 ambient at 900°C with varying times. The resulting grain size is investigated using x-ray diffraction and transmission electron microscopy. Si nanoclusters are observed. Upon 900°C oxidation with varying times, three main regions of grain size transition are identified. With short oxidation time, sintering with oxidation at the rim of the agglomerate dominates; at medium oxidation time, full sintering in the agglomerate with pure oxidation retarded by interfacial stress and strain dominates; at long oxidation time, self-limiting effect prevails. Study has been carried out using a combination of classical sintering model and Kao’s two dimensional oxidation model considering the decrease of reaction rate with increasing stress perpendicular to the Si surface. In our developed model, the critical stress for self-limiting oxidation is found to be 2.9×109Pa.

Characteristics of mechanically milled silicon nanocrystals embedded in TEOS thin films

In this work, the unique synthesis of mechanically milled silicon nanocrystals (Si nc) embedded in tetraethylorthosilicate (TEOS) thin films is reported. The Si nc are required to be synthesized externally via inexpensive mechanical milling before it is mixed with the TEOS solution for spin coating. Si nc with a range of sizes, from 10 to 25 nm, have been synthesized using mechanical milling. These milled Si nc and milled Si nc embedded in TEOS thin film are characterized by differential thermal analysis (DTA), Raman spectroscopy and photoluminescence excitation (PLE). The defects, such as non-bridging oxide hole centers (NBOHCs), in amorphous SiO 2 are found to be the most probable dominant mechanism for the PL of milled Si nc embedded in films. The milled Si nc embedded in TEOS thin film is further developed into MOS capacitor. Charge trapping in Si nc with a V FB shift of 0.4 V is observed.

X-ray Photoelectron Spectroscopic Analysis of Si Nanoclusters in SiO2 Matrix

We investigated silicon nanoclusters Si(nc) in a SiO2 matrix prepared by the plasma-enhanced chemical vapor deposition technique, using X-ray photoelectron spectroscopy (XPS) with external voltage stimuli in both static and pulsed modes. This method enables us to induce an additional charging shift of 0.8 eV between the Si2p peaks of the oxide and the underlying silicon, both in static and time-resolved modes, for a silicon sample containing a 6 nm oxide layer. In the case of the sample containing silicon nanoclusters, both Si2p peaks of Si(nc) and host SiO2 undergo a charging shift that is 1 order of magnitude larger (>15 eV), with no measurable difference between them (i.e., no differential charging between the silicon nanoclusters and the oxide matrix could be detected). By use of a measured Auger parameter, we estimate the relaxation energy of the Si(nc) in the SiO2 matrix as -0.4 eV, which yields a -0.6 eV shift in the binding energy of the Si(nc) with respect to that of bulk Si in the opposite direction of the expected quantum size effect. This must be related to the residual differential charging between the silicon nanoclusters and the oxide host. Therefore, differential charging is still the biggest obstacle for extracting size-dependent binding energy shifts with XPS when one uses the oxide peak as the reference.

Si nanocrystal-containing SiO x ( x < 2) produced by thermal annealing of PECVD realized thin films

This paper is focused on the description of relationship between deposition parameters (flow rates of SiH 4 and N 2O precursor gases) and properties of SiO x films. The silicon-rich silicon oxide thin films deposited using plasma enhanced chemical vapour deposition (PECVD) in silane-nitrous oxide-helium discharges were thermally annealed at 1273 K for 1 h. Fourier transform infrared (FTIR) spectroscopy indicated that the chemical composition was dominated by silicon suboxide containing silicon-nitride and silicon-hydrogen bonds. For the as-deposited films and for the annealed films, Raman spectra show a band approximately at 480 cm −1, related to amorphous silicon and a band at 520 cm −1, related to nanocrystallite silicon, respectively. Transmission electron microscopy analysis demonstrated that silicon nanocrystals (Si nc), having a mean radius ranging between 3 and 6 nm were present in the annealed films. Using spectroscopic ellipsometry (SE) in the 0.2–0.88 μm spectral range, the values of layer thickness, optical indices and component volume fractions were determined using the Fourouhi–Bloomer (FB) model for the as-deposited films and the Bruggeman effective medium approximation (BEMA) for the annealed ones.

Stacking faults in Si nanocrystals

Si nanocrystals (Si nc) were formed by the implantation of Si+ into a SiO2 film on (100) Si, followed by high-temperature annealing. High-resolution transmission electron microscopy has been used to examine the microstructure of the Si nc produced by a high-dose (3×1017cm−2) implantation. It is shown that there are only stacking-fault (SF) defects in some nanocrystals; while in others the stacking faults (SFs) coexist with twins. Two kinds of SFs, one being an intrinsic SF, the other being an extrinsic SF, have been observed inside the Si nc. More intrinsic SFs have been found in the Si nc, and the possible reasons are discussed. These microstructural defects are expected to play an important role in the light emission from the Si nc.

Faceting of Si nanocrystals embedded in SiO 2

Faceting has been observed in some Si nanocrystals (Si nc) embedded in SiO 2 using high-resolution transmission electron microscopy (HRTEM). Statistical analyses of the interface-energy (Si nc/SiO 2) ratios for different facets of the single-crystalline Si nc have been carried out. For these single-crystalline Si nc, the interfacial energy ratios of {1 0 0} and {1 1 3} relative to {1 1 1} facets, are 1.1 and 0.89, respectively. The influences of planar defects such as twins and stacking faults on the faceting and interfacial energy of Si nc/SiO 2 are discussed in terms of their possible contribution to the interfacial energy.

Defect-induced photoluminescence from tetraethylorthosilicate thin films containing mechanically milled silicon nanocrystals

In this work, the unique synthesis of mechanically milled silicon nanocrystals (Si nc) embedded in tetraethylorthosilicate (TEOS) thin films is reported. A series of Si nc, with sizes ranging from 10to25nm, have been synthesized using mechanical milling. For both the milled Si nc and milled Si nc embedded in TEOS thin film, infrared absorption and photoluminescence results show that the photoluminescence (PL) is not a consequence of quantum confinement, amorphous Si component, or Si–OH or Si–H bonds. The defects, such as nonbridging oxide hole centers (NBOHCs), in amorphous SiO2 are probably the dominant mechanism for the PL of milled Si nc embedded in TEOS thin films. In addition, PL excitation results reveal oxidation-induced strain between the interfaces of milled Si nc∕SiO2 has also generated a new luminescence center. This luminescence center is similar to the NBOHCs attributed to interfacial strain.

Ordered coalescence of Si nanocrystals inSiO2

The microstructure of the Si nanocrystals (Si nc) has been investigated using conventional and high-resolution transmission electron microscopy (HRTEM). For most of the nanocrystals $(g90%)$ larger than 10 nm, HRTEM observations show that they are formed by the coalescence of smaller ones. Two kinds of coalescence, one being the preferential attachments of small particles to the {111} facets of a seed nanoparticle, and the other being an ordered combination of two or more small nanocrystals with appropriate orientations, have been observed.

Annealing and oxidation of silicon oxide films prepared by plasma-enhanced chemical vapor deposition

We have investigated phase separation, silicon nanocrystal (Si NC) formation and optical properties of Si oxide (SiOx, 0&amp;lt;x&amp;lt;2) films by high-vacuum annealing and dry oxidation. The SiOx films were deposited by plasma-enhanced chemical vapor deposition at different nitrous–oxide/silane flow ratios. The physical and optical properties of the SiOx films were studied as a result of high-vacuum annealing and thermal oxidation. X-ray photoelectron spectroscopy (XPS) reveals that the as-deposited films have a random-bonding or continuous-random-network structure with different oxidation states. After annealing at temperatures above 1000 °C, the intermediate Si continuum in XPS spectra (referring to the suboxide) split to Si peaks corresponding to SiO2 and elemental Si. This change indicates the phase separation of the SiOx into more stable SiO2 and Si clusters. Raman, high-resolution transmission electron microscopy and optical absorption confirmed the phase separation and the formation of Si NCs in the films. The size of Si NCs increases with increasing Si concentration in the films and increasing annealing temperature. Two photoluminescence (PL) bands were observed in the films after annealing. The ultraviolet (UV)-range PL with a peak fixed at 370–380 nm is independent of Si concentration and annealing temperature, which is a characteristic of defect states. Strong PL in red range shows redshifts from ∼600 to 900 nm with increasing Si concentration and annealing temperature, which supports the quantum confinement model. After oxidation of the high-temperature annealed films, the UV PL was almost quenched while the red PL shows continuous blueshifts with increasing oxidation time. The different oxidation behaviors further relate the UV PL to the defect states and the red PL to the recombination of quantum-confined excitions.

Correlation between electroluminescence and structural properties of Si nanoclusters

Abstract We have studied the electrical and optical properties of light emitting MOS devices based on Si nanostructures. In these devices the dielectric layer consists of a SiOx thin film prepared by plasma enhanced chemical vapor deposition. As-deposited SiOx films were annealed at high temperature to induce the separation of the Si and SiO2 phases with the formation of Si nanocrystals (nc) embedded in the insulating matrix. Devices based on Si nc present a strong light emission at room temperature at a wavelength of about 900 nm. Devices emitting at different wavelengths have been fabricated by implanting SiOx films with rare earth ions. Devices based on Er, Tm and Yb-doped Si nanoclusters exhibit an intense room temperature electroluminescence (EL) respectively at 1.54 μm, 0.78 μm and 0.98 μm. The EL properties of the devices have been optimized through the study of the structural properties of their active layers by means of the energy filtered transmission electron microscopy (EFTEM) technique. EFTEM has evidenced the presence of a relevant contribution of amorphous nanostructures, and has given a very reliable quantitative estimation of the size and density of the Si nanostructures, as well as of the concentration of Si atoms remaining dissolved in the matrix. In the case of RE-doped SiOx films, the role of amorphous nanostructures in sensitizing the RE luminescence has been clearly demonstrated. These results open the way to new interesting applications in optoelectronics, such as electrically driven optical amplifiers or light sources.

The effect of implantation dose on the microstructure of silicon nanocrystals inSiO2

Si nanocrystals were formed by the implantation ofSi+ intoa SiO2 film, deposited on (100) Si, followed by high-temperature annealing.Transmission electron microscopy (TEM) was used to examine the effect ofimplantation dose on the microstructure of the Si nanocrystals (Si nc) in theSiO2 film. The size and spatial distribution and concentration of the Si nc in four passivatedsamples with different implantation doses were investigated using the dark-field imagingtechnique. The thickness of all the samples was determined by electron energy-lossspectroscopy (EELS). The structure of the Si nc in all the samples was determined usingselected area electron diffraction. It was found that the average diameter of the Si ncchanges from 2.7 to 3 and to 3.3 nm for the samples with implantation doses of6 × 1016,8 × 1016 and 1 × 1017 cm−2; however, it ranges from 2 to 22 nm in the sample with an implantation dose of3 × 1017 cm−2. The size of the Si nc is comparatively homogeneous throughout thewhole implanted layer in the samples with implantation doses of6 × 1016 and 8 × 1016 cm−2, while in the samples with implantation doses of1 × 1017 and 3 × 1017 cm−2, the Si nc in the middle region of the implanted layer are bigger than those near the surface or thebottom of the layer. From TEM experimental results, the concentration of the Si nc is estimated tobe 6 × 1018,4 × 1018 and4 × 1018 cm−3 for the samples withimplantation doses of 6 × 1016, 8 × 1016 and 1 × 1017 cm−2, respectively. For the sample with an implantation dose of3 × 1017 cm−2, the concentration for the Si nc of 3 nm is around5 × 1018 cm−3; the concentration for Si nc of 6 nm is around2 × 1018 cm−3; and the concentration for the 12 nm group is about1 × 1018 cm−3. In addition, the concentration determined from TEM experiments is compared with thecalculated one. Combining the TEM results with a Monte Carlo simulation, we alsodiscuss the sputtering effect and the depth distribution of the Si ions implanted inSiO2.