Chemical and phase compositions of silicon oxide films with nanocrystals prepared by carbon ion implantation

The chemical and phase compositions of silicon oxide films with self-assembled nanoclusters prepared by ion implantation of carbon into SiO _x (x < 2) suboxide films with subsequent annealing in a nitrogen atmosphere have been investigated using X-ray photoelectron spectroscopy in combination with depth profiling by ion sputtering. It has been found that the relative concentration of oxygen in the maximum of the distribution of implanted carbon atoms is decreased, whereas the relative concentration of silicon remains almost identical over the depth in the layer containing the implanted carbon. The in-depth distributions of carbon and silicon in different chemical states have been determined. In the regions adjacent to the layer with a maximum carbon content, the annealing results in the formation of silicon oxide layers, which are close in composition to SiO₂ and contain silicon nanocrystals, whereas the implanted layer, in addition to the SiO₂ phase, contains silicon oxide species Si²⁺ and Si³⁺ with stoichiometric formulas SiO and Si₂O₃, respectively. The film contains carbon in the form of SiC and elemental carbon phases. The lower limit of the average size of silicon nanoclusters has been estimated as ∼2 nm. The photoluminescence spectra of the films have been interpreted using the obtained results.     

Temperature behaviour of optical properties of Si+-implanted SiO2

Silicon nanocrystals were prepared by Si⁺-ion implantation and subsequent annealing of SiO₂ films thermally grown on a c-Si wafer. Different implantation energies (20-150 keV) and doses - cm ^-2 ) were used in order to achieve flat implantation profiles (through the thickness of about 100 nm) with a peak concentration of Si atoms of 5, 7, 10 and 15 atomic%. The presence of Si nanocrystals was verified by transmission electron microscopy. The samples exhibit strong visible/IR photoluminescence (PL) with decay time of the order of tens of μs at room temperature. The changes of PL in the range 70-300 K can be well explained by the exciton singlet-triplet splitting model. We show that all PL characteristics (efficiency, dynamics, temperature dependence, excitation spectra) of our Si⁺-implanted SiO₂ films bear close resemblance to those of a light-emitting porous Si and therefore we suppose similar PL origin in both materials. Received 1st September 1998 and Received in final form 7 September 1999     

Quenching of Si nanocrystal photoluminescence by doping with gold or phosphorous

Si nanocrystals embedded in SiO₂ doped with P and Au at concentrations in the range of 1×10¹⁸-3×10²⁰ cm⁻³ exhibit photoluminescence quenching. Upon increasing the Au concentration, a gradual decrease in nanocrystal photoluminescence intensity is observed. Using a statistical model for luminescence quenching, we derive a typical radius of ∼3 nm for nanocrystals luminescing around 800 nm. Au doping also leads to a luminescence lifetime reduction, which is attributed to energy transfer between adjacent Si nanocrystals, possibly mediated by the presence of Au in the form of ions or nanocrystals. Doping with P at concentrations up to 3×10¹⁹ cm⁻³ leads to a luminescence enhancement, most likely due to passivation of the nanocrystal-SiO₂ interfaces. Upon further P doping the nanocrystal luminescence gradually decreases, with little change in luminescence lifetime.     

Erbium excitation in a SiO<Subscript>2</Subscript>: Si-nc matrix under pulsed pumping

The photoluminescence of Er³⁺ ions in a SiO₂ matrix containing silicon nanocrystals 3.5 nm in diameter is studied under resonant and nonresonant pulsed pumping with pulses 5 ns in duration. The effective erbium excitation cross section under pulsed pumping, σ_eff = 8.7 × 10^?17 cm₂, is close to that for nanocrystals. Comparison of the erbium photoluminescence intensity obtained for a SiO₂ matrix with and without nanocrystals made it possible to determine the absolute concentration of optically active nanocrystals capable of exciting erbium ions, the concentration of optically active erbium, and the average number of erbium ions excited by one nanocrystal. The study revealed that excitation transfer from one erbium ion to another is a relatively slow process, which accounts for the low efficiency of erbium ion excitation under pulsed pumping in a SiO₂ matrix containing silicon nanocrystals.     

掺纳米锗氧化硅的发光性能研究

由溶胶/凝胶法制备得到的GeO₂/SiO₂玻璃在700 ℃的条件下经H₂还原,得到具有奇特光致发光性质的Ge/SiO₂玻璃,该玻璃在室温条件下用246 nm（5.01 eV）的光激发时,能发射出很强的 392 nm（3.12 eV）、较强的 600 nm（2.05 eV）和次强的770 nm（1.60 eV） 的荧光.利用X射线衍射（XRD）、X射线光电子能谱（XPS）及透射电镜（TEM）实验证明,该玻璃能够发射3种不 关键词： 2')" href="#">Ge/SiO₂ Ge纳米晶粒 溶胶/凝胶法 光致发光     

Temperature dependence of compositional changes of SiO2 surface during ion and electron bombardment

The influence of ion (Ar⁺ 0.5 keV, 2 microA/cm²) and electron (2 keV, 2 mA/cm²) bombardment on the elemental composition of SiO₂ was investigated in the temperature range of 270–790 K. Elemental composition was controlled by AES. It was found that both ion and electron bombardment resulted in an increasing amount of Si₉₂ (elemental silicon) and in decreasing amounts of both O₅₁₀ and Si₇₈ (silicon bound to oxygen). The temperature influence on the composition of SiO₂ is negligible under ion bombardment while the amount of Si₉₂ strongly increases under electron bombardment at temperatures exceeding 600 K. The mechanism of temperature dependence is discussed.     

Fabrication and Characterization of Si Nanocrystals Synthesized by Electron Beam Evaporation of Si and SiO2 Mixture

Silicon nanocrystals synthesized by electron beam （e-beam） evaporation of Si and SiO2 mixture are studied. Rutherford backscattering spectrometry of the as-deposited Si-rich silicon dioxide or oxide （SRO） thin film shows that after evaporation, the Si and SiO2 concentration is well kept, indicating that the e-beam evaporation is suitable for evaporating mixtures of Si and SiO2. The SRO thin films are annealed at different temperatures for two hours to synthesize silicon nanoerystals. For the sample annealed at 1050℃, silicon nanoerystals with different sizes and the mean diameter of 4.5 nm 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 results.     

Optical transitions in Ge nanocrystals formed by high-pressure annealing of Ge ion implanted SiO2 films

Optical transitions in Ge nanocrystals formed by high-pressure annealing of the Ge⁺ ion implanted SiO₂ films have been studied by Raman and photoluminescence spectroscopy. It has been found that the E₁,E₁+Δ₁ Raman resonance shift observed from the unstrained and hydrostatically compressed nanocrystals corresponds to the quantization of the electron-hole state spectrum of the Ge band. It has also been established that the appearance of a green photoluminescence band centered at 420-520 nm correlates with the formation of strained nanocrystals. Comparisons of the PL data with HRTEM results have been made, which suggest that the green PL arises from strained Ge nanocrystals of a radius of less than 5 nm. The direct electron-hole recombination at Γ is discussed as a possible origin of the observed photoluminescence band.     

Influence of the ion synthesis and ion doping regimes on the effect of sensitization of erbium emission by silicon nanoclusters in silicon dioxide films

The photoluminescence spectra of erbium centers in SiO₂ films with ion-synthesized silicon nanoclusters under nonresonant excitation were investigated. Erbium was introduced into thermal SiO₂ films by ion implantation. The dependences of photoluminescence intensity on the dose, the order of ion implantation of Si and Er, the annealing temperature, and additional Ar⁺ and P⁺ ion irradiation regimes, i.e., factors determining the influence of radiation damage and doping on sensitization of erbium luminescence by silicon nanoclusters, were determined. It was found that the sensitization effect and its amplification due to doping with phosphorus are most pronounced under the conditions where nanoclusters are amorphous. The quenching of photoluminescence due to radiation damage in this case manifests itself to a lesser extent than for crystalline nanoclusters. The role of various factors in the observed regularities was discussed in the framework of the existing concepts of the mechanisms of light emission and energy exchange in the system of silicon nanoclusters and erbium centers.     

Photoluminescence detection of impurities introduced in silicon by dry etching processes

We report on a photoluminescence study of silicon samples subjected to different dry etching processes. Several luminescence lines, known from defects produced by high-energy irradiation, manifest damage of the crystalline material. Noble gas ion beam etching (using Ne⁺, Ar⁺, Kr⁺, and Xe⁺) with ion energies as low as 400 eV produces characteristic luminescence lines which correspond to defects within a 200–300 Å thick surface layer. Incorporation of carbon during CF₄ reactive ion etching produces the familiar G-line defect. The G-line photoluminescence intensity in our samples is directly correlated with the substitutional carbon concentration, as determined by infrared absorption measurements before the etch process; we therefore suggest that a simple method to determine the substitutional carbon concentration in a crystalline silicon sample is a standard dry etching process and a comparison of the resulting G-line photoluminescence intensity to a calibrated sample. The sensitivity of this method seems to be better than 10¹⁴ carbon atoms/cm³.     

镶嵌在氧化硅薄膜中纳米晶硅的可见光荧光谱与发光机制的研究

采用对非晶氧化硅薄膜退火处理方法，获得纳米晶硅与氧化硅的镶嵌结构．室温下观察到峰位为2.40eV光致发光．系统地研究了不同退火温度对薄膜的Raman谱、光荧光谱及光电子谱的影响．结果表明，荧光谱可分成两个不随温度变化的峰位为1.86和2.30eV的发光带．Si2p能级光电子谱表明与发光强度一样Si⁴⁺强度随退火温度增加而增加．Si平均晶粒大小为4.1—8.0nm，不能用量子限制模型解释蓝绿光的发射．纳米晶硅与SiO₂界面或SiO₂中与氧有关的缺陷可能是蓝绿光发射的主要原因 关键词：     

Scanning near-field cathodoluminescence microscopy of an Si+ implanted and thermally annealed SiO2 layer

A scanning near-field cathodoluminescence microscope (SNCLM) is successfully used to image the cathodoluminescence of an Si⁺ implanted and thermally annealed submicronic SiO₂ layer. Owing to the subwavelength resolution of the system a “cross-sectional” cathodoluminescence image was obtained. The intensity image profile shows that sample luminescence results from the whole SiO₂ layer confirming a preceding electroluminescence study. Sample luminescence is attributed to point defects generated into the whole SiO₂ layer during Si⁺ ion implantation and thermal annealing.     

Characterization of ion species of silicon oxide films using positive and negative secondary ion mass spectra

Secondary ion species of silicon oxide films have been investigated using time-of-flight secondary ion mass spectrometry (TOF-SIMS). Characterization of thermally grown SiO₂ films on silicon has been performed. A diagram showing secondary ion spectra of SiO₂ films in both positive and negative polarities indicates the pattern of change in polarities and intensities of ion species from SiO⁺ to Si₅O₁₁⁻. The ions mostly change from positive to negative polarity between Si_nO_2n−1 and Si_nO_2n. Ion peaks with the strongest intensities in the respective cluster ions correspond to the Si_nO_2n+1 negative ion. Intensities of ion species of Si_nO_2n+2 appear negligibly small. Ion species of Si₃O⁺, Si₃O₂⁺ and Si₃O₃⁺ have been found at the interface between silicon and SiO₂ films. The intensity patterns of these ion species compared to those of SiO₂ films indicate that most of these species are not emitted from the SiO₂ films, but likely from the SiO structures.     

Structure and photoluminescence properties of Er3+-doped TiO2-SiO2 powders prepared by sol-gel method

Er 3+-doped TiO 2-SiO 2 powders are prepared by the sol-gel method,and they are characterized by high resolution transmission electron microscopy (HR-TEM),X-ray diffraction (XRD) spectra,and Raman spectra of the samples.It is shown that the TiO 2 nanocrystals are surrounded by an SiO 2 glass matrix.The photoluminescence (PL) spectra are recorded at room temperature.A strong green luminescence and less intense red emission are observed in the samples when they are excited at 325 nm.The intensity of the emission,which is related to the defect states,is strongest at the annealing temperature of 800 C.The PL intensity of Er 3+ ions increases with increasing Ti/Si ratio due to energy transfer between nano-TiO 2 particles and Er 3+ ions.     

Effect of surface Si-Si dimers on photoluminescence of silicon nanocrystals in the silicon dioxide matrix

The effect of surface states of silicon nanocrystals embedded in silicon dioxide on the photoluminescent properties of the nanocrystals is reported. We have investigated the time-resolved and stationary photoluminescence of silicon nanocrystals in the matrix of silicon dioxide in the visible and infrared spectral ranges at 77 and 300 K. The structures containing silicon nanocrystals were prepared by the high-temperature annealing of multilayer SiO _x /SiO₂ films. The understanding of the experimental results on photoluminescence is underlain by a model of autolocalized states arising on surface Si-Si dimers. The emission of autocatalized excitons is found for the first time, and the energy level of the autolocalized states is determined. The effect of these states on the mechanism of the excitation and the photoluminescence properties of nanocrystals is discussed for a wide range of their dimensions. It is reliably shown that the cause of the known blue boundary of photoluminescence of silicon nanocrystals in the silicon dioxide matrix is the capture of free excitons on autolocalized surface states.     

Infrared photoluminescence from GeSi nanocrystals embedded in a germanium–silicate matrix

We investigate the structural and optical properties of GeO/SiO₂ multilayers obtained by evaporation of GeO₂ and SiO₂ powders under ultrahigh vacuum conditions on Si(001) substrates. Both Raman and infrared absorption spectroscopy measurements indicate the formation of GeSi nanocrystals after postgrowth annealing at 800°C. High-resolution transmission electron microscopy characterizations show that the average size of the nanocrystals is about 5 nm. For samples containing GeSi nanocrystals, photoluminescence is observed at 14 K in the spectral range 1500–1600 nm. The temperature dependence of the photoluminescence is studied.     

Luminescent properties of R doped Sr2SiO4:Eu(R=Li, Na and K) red-emitting phosphors for white LEDs

Sr₂SiO₄:Eu³⁺ and Sr₂SiO₄:Eu³⁺ doped with R⁺(R⁺=Li⁺, Na⁺ and K⁺) phosphors were prepared by conventional solid-state reaction and investigated by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and photoluminescence spectroscopy. XRD patterns and SEM reveal that the optimal firing condition for Sr₂SiO₄:Eu³⁺ was 1300 °C for 4 h. The excitation and emission spectra indicate that the phosphor can be effectively excited by ultraviolet (395 nm) and blue (466 nm) light and emits intense red light peaked at around 614 nm corresponding to the ⁵D₀→⁷F₂ transitions of Eu³⁺. In the research work, the effect of R⁺ contents on luminescence property and the Eu³⁺ concentration quenching process have also been investigated. The Eu³⁺ concentration quenching mechanism was verified to be a multipole-multipole interaction and the critical energy-transfer distance was calculated to be around 14.6 Å. The dopant R⁺(R⁺=Li⁺, Na⁺ and K⁺) as charge compensator in Sr₂SiO₄:Eu³⁺ can further enhance luminescence intensity, and the emission intensity of Sr₂SiO₄:Eu³⁺ doping Li⁺ is higher than that of Na⁺ or K⁺.     

Silicon nanoparticles: Their photoluminescence, complex refractive index, and relationship with the band structure

The photoluminescence, IR transmission, and Raman scattering spectra of nc-Si nanocrystals (5 nm in size) have been investigated. The shape and spectral position of maxima in the photoluminescence and IR transmission spectra are theoretically described. It is shown that nc-Si particles consist of a Si core and a SiO₂ shell. The existence of surface Si-O and Si-H states in Si nanocrystals enhances photoluminescence. It is established that the spectral position of the main Raman scattering peak in the frequency range 500–520 cm^?1 undergoes a significant red shift for small-size nanostructures. Based on direct determination of the complex refractive index of n-Si nanoparticles by measuring spectral ellipsometric parameters, nanostructured silicon is found to be an indirect-gap semiconductor.     

Formation of Ge nanocrystals and evolution of the oxide matrix in as-deposited and annealed LPCVD SiGeO films

SiGeO films were deposited by LPCVD using Si₂H₆, GeH₄ and O₂ as reactive gases and furnace annealed to segregate the possible excess of Si and Ge in the form of nanocrystals embedded in an oxide matrix. For low GeH₄:Si₂H₆ flow ratios and deposition temperatures of 450 ^°C or lower, the deposited film consists of a SiO₂ matrix incorporating Ge. No Ge oxides and no nanocrystals are detected. After annealing of the samples with SiO₂ matrices at temperatures of 600 ^°C or higher, quasi-spherical isolated Ge nanocrystals with diameters ranging from 4.5 to 9 nm and homogeneously distributed throughout the whole film thickness are formed. In the samples deposited with low GeH₄:Si₂H₆ flow ratios, the original SiO₂ matrix holds its composition.     

Growth mechanism and photoluminescence of the SnO2 nanotwists on thin film and the SnO2 short nanowires on nanorods

SnO2 nanotwists on thin film and SnO2 short nanowires on nanorods have been grown on single silicon substrates by using Au-Ag alloying catalyst assisted carbothermal evaporation of SnO2 and active carbon powders.The morphology and the structure of the prepared nanostructures are determined on the basis of field-emission scanning electron microscopy(FESEM),transmission electron microscopy(TEM),selected area electronic diffraction(SAED),high-resolution transmission electron microscopy(HRTEM),x-ray diffraction(XRD),Raman and photoluminescence(PL) spectra analysis.The new peaks at 356,450,and 489 nm in the measured PL spectra of two kinds of SnO2 nanostructures are observed,implying that more luminescence centres exist in these SnO2 nanostructures due to nanocrystals and defects.The growth mechanism of these nanostructures belongs to the vapour-liquid-solid(VLS) mechanism.