X-ray absorption study of light emitting silicon nanocrystals

X-ray absorption spectra obtained by total electron yield (TEY) at the Si absorption K-edge have been measured to have chemical and structural information about Si nanocrystals (Si-nc) produced by plasma-enhanced chemical vapour deposition (PECVD). The TEY technique has been employed to investigate the formation of Si-nc and the modification of the silica matrix as a function of annealing temperature (500–1250°C) and of silicon content in the film (35–46 at%). The amount of silicon present in the Si-nc has been evaluated by TEY. Thanks to Rutherford backscattering spectrometry measurements, the amount of Si atoms bonded to oxygen and to nitrogen, incorporated by PECVD, has been assessed. A compositional model that interprets the experimental findings is presented.     

Investigation of the formation of silicon nanocrystals by annealing of amorphous SiCx/c-Si structures

In this work, we study the changes in the optical properties of 300-nm-thick hydrogenated amorphous silicon carbide layers after an annealing process. Both intrinsic and phosphorus-doped amorphous silicon carbide layers (a-SiC_x:H) were deposited on silicon wafers by plasma enhanced chemical vapour deposition (PECVD) at 400 °C and annealed in a quartz furnace at 800 °C. The presence of randomly oriented silicon nanocrystals was confirmed by X-ray diffraction (XRD) measurements after the partial recrystallization process only in the doped layers. The presence or the absence of the nanocrystals clearly changes the Fourier transform infrared (FTIR) spectra. From the fitting of the experimental curves with the model of Lorentz oscillators, the refractive index and the extinction coefficient of the different layers were obtained.     

镶嵌有纳米硅的氮化硅薄膜键合特性分析

采用螺旋波等离子体化学气相沉积(HWPCVD)技术制备了非化学计量比的氢化氮化硅薄膜,对所沉积样品及氮气环境中920 ℃退火样品的微观结构及键合特性进行了分析。Raman散射结果表明,薄膜中过量硅以非晶纳米粒子形式存在,退火样品呈现纳米晶硅和氮化硅的镶嵌结构。红外吸收和可见光吸收特性比较结果显示,薄膜样品的微观结构依赖于化学计量比以及退火过程,硅含量较低样品因高的键合氢含量而表现出低的纳米硅表面缺陷态密度;退火过程将引起Si—H和N—H键合密度的减少,因晶态纳米颗粒的形成,退火样品表现出更高的结构无序度。     

Synchrotron investigations of the electron structure of silicon nanocrystals in a SiO2 matrix

Silicon ions are implanted into silicon oxide thin films obtained by the thermal oxidation of silicon wafers in wet oxygen. The implantation dose is accumulated either once or cyclically, and the samples are annealed in dry nitrogen every time after implantation. The second series of samples is prepared in a similar way, but the technology for obtaining the oxide films includes additional annealing at 1100°C in air for three hours. X-ray absorption near-edge structure (XANES) spectra are obtained using synchrotron radiation. In all the Si L _2,3 spectra, two absorption edges are observed, the first corresponding to elemental silicon, and the second corresponding to the SiO₂ matrix. The fine structure of the first edge indicates that nanocrystalline silicon (nc-Si) can form in the SiO₂ matrix, whose atomic and electronic structure depends on the technology of its formation. In both series, the cyclic accumulation of the total dose (Φ = 10¹⁷ cm^?2) and the annealing time (2 h) gives rise to the most pronounced fine structure in the region of the absorption edge of elemental silicon. The probability of forming silicon nanocrystals decreases for the denser silicon oxide in the second series of samples.     

Study on phase separation in a-SiOx for Si nanocrystal formation through the correlation of photoluminescence with structural and optical properties

The phase separation in amorphous silicon suboxide (a-SiO_x) films upon thermal annealing for the formation of light emitting silicon nanocrystals (Si-NCs) was studied through the correlation of photoluminescence (PL) and photoluminescence excitation (PLE) with structural and optical properties. The PL and PLE features and the structural and optical properties show a strong dependence on the annealing process and reveal that the precipitation of the excess Si in a-SiO_x and the formation of Si-NCs from the precipitated Si are two separate processes which should be distinguished in the phase separation in a-SiO_x. They proceed at different temperatures and the formation of Si-NCs is a slow process compared with the precipitation of the excess Si. The nanocrystal size and size distribution evolve with annealing time at the initial stages and are mainly dependent on annealing temperature for a certain O content in the initial a-SiO_x with the density of the formed Si-NCs increasing with longer annealing duration.     

Electroluminescence of silicon nanocrystals in MOS structures

We have studied the structural, electrical and optical properties of MOS devices, where the dielectric layer consists of a substoichiometric SiO_x (x<2) thin film deposited by plasma-enhanced chemical vapor deposition. After deposition the samples were annealed at high temperature (>1000 °C) to induce the separation of the Si and the SiO₂ phases with the formation of Si nanocrystals embedded in the insulating matrix. We observed at room temperature a quite intense electroluminescence (EL) signal with a peak at ∼850 nm. The EL peak position is very similar to that observed in photoluminescence in the very same device, demonstrating that the observed EL is due to electron–hole recombination in the Si nanocrystals and not to defects. The effects of the Si concentration in the SiO_x layer and of the annealing temperature on the electrical and optical properties of these devices are also reported and discussed. In particular, it is shown that by increasing the Si content in the SiO_x layer the operating voltage of the device decreases and the total efficiency of emission increases. These data are reported and their implications discussed. Received: 31 August 2001 / Accepted: 3 September 2001 / Published online: 17 October 2001     

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.     

Hydrogen evolution in amorphous silicon carbide

The influence of the hydrogen content and its evolution in the annealing treatments of the structural properties of hydrogenated amorphous silicon carbide (a-SiC:H) prepared by plasma enhanced chemical vapour deposition under different deposition conditions have been studied. The results indicate how strongly hydrogen affects the presence of defects in the amorphous network of the samples.     

Photoluminescence of Er<Superscript>3+</Superscript> ions in layers of quasi-ordered silicon nanocrystals in a silicon dioxide matrix

The spectra and kinetics of photoluminescence from multilayered structures of quasi-ordered silicon nanocrystals in a silica matrix were studied for undoped samples and samples doped with erbium. It was shown that the optical excitation energy of silicon nanocrystals could be effectively transferred to Er³⁺ ions, which was followed by luminescence at a wavelength of 1.5 µm. The effectiveness of energy transfer increased as the size of silicon nanocrystals decreased and the energy of exciting light quanta increased. The excitation of erbium luminescence in the structures was explained based on dipole-dipole interaction (the Förster mechanism) between excitons in silicon nanocrystals and Er³⁺ ions in silica surrounding them.     

Electrical behavior of size-controlled Si nanocrystals arranged as single layers

A metal–oxide–semiconductor structure containing a single layer of size-controlled silicon nanocrystals embedded into gate oxide was fabricated. Size control for the silicon nanocrystals was realized by using a SiO₂/SiO/SiO₂ layer structure with the embedded SiO layer having the thickness of the desired Si nanocrystals and using a high-temperature annealing for forming the silicon nanocrystals. Current–voltage, capacitance–voltage, and conductance–voltage characteristics were measured for a sample containing 4-nm-sized crystals. From the Fowler–Nordheim plot an effective barrier height of 1.6 eV is estimated for our silicon nanocrystals. Electron trapping, storing, and de-trapping in silicon nanocrystals were observed by capacitance–voltage and conductance–voltage measurements. The charge density was measured to be 1.6×10¹² /cm², which is nearly identical to the silicon-nanocrystal density measured approximately via a transmission electron microscopy image. Conductance measurements reveal a very low interface charge of our structure. PACS 72.80.Sk; 73.63.Bd; 73.40.Qv     

Structural characterisation of the silica and alumina Zener pinning phases in nanocrystalline CeO2 by 29Si and 27Al nuclear magnetic resonance

Nanocrystalline CeO₂ samples have been manufactured using sol-gel techniques, containing either 15 % silica or 10 % alumina by weight to restrict growth of the ceria nanocrystals during annealing by Zener pinning. ²⁹Si and ²⁷Al MAS NMR have been used to investigate the structure of these pinning phases over a range of annealing temperatures up to 1000 °C, and their effect on the CeO₂ morphology has been studied using electron microscopy. The silica pinning phase resulted in CeO₂ nanocrystals of average diameter 19 nm after annealing at 1000 °C, whereas the alumina pinned nanocrystals grew to 88 nm at the same temperature. The silica pinning phase was found to contain a significant amount of inherent disorder indicated by the presence of lower n Qⁿ species even after annealing at 1000 °C. The alumina phase was less successful at restricting the growth of the ceria nanocrystals, and tended to separate into larger agglomerations of amorphous alumina, which crystallised to a transition alumina phase at higher temperatures.     

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.     

Rate equation modelling of erbium luminescence dynamics in erbium-doped silicon-rich-silicon-oxide

Erbium doped silicon-rich silica offers broad band and very efficient excitation of erbium photoluminescence (PL) due to a sensitization effect attributed to silicon nanocrystals (Si-nc), which grow during thermal treatment. PL decay lifetime measurements of sensitised Er³⁺ ions are usually reported to be stretched or multi exponential, very different to those that are directly excited, which usually show a single exponential decay component.In this paper, we report on SiO₂ thin films doped with Si-nc's and erbium. Time resolved PL measurements reveal two distinct 1.54 μm Er decay components; a fast microsecond component, and a relatively long lifetime component (10 ms). We also study the structural properties of these samples through TEM measurements, and reveal the formation of Er clusters. We propose that these Er clusters are responsible for the fast μs decay component, and we develop rate equation models that reproduce the experimental transient observations, and can explain some of the reported transient behaviour in previously published literature.     

Photoluminescence properties of SiNx/Si amorphous multilayer structures grown by plasma-enhanced chemical vapor deposition

Very thin (nanometric) silicon layers were grown in between silicon nitride barriers by SiH₂Cl₂/H₂/NH₃ plasma-enhanced chemical vapor deposition (PECVD). The multilayer structures were deposited onto fused silica and silicon substrates. Deposition conditions were selected to favor Si cluster formation of different sizes in between the barriers of silicon nitride. The samples were thermally treated in an inert atmosphere for 1 h at 500 °C for dehydrogenation. Room-temperature photoluminescence (RT-PL) and optical transmission in different ranges were used to evaluate the optical properties of the structures. UV-VIS absorption spectra present two band edges. These band edges are well fitted by the Tauc model typically used for amorphous materials. RT-PL spectra are characterized by strong broad bands, which have a blue shift as a function of the deposition time of the silicon layer, even for as-grown samples. The broad luminescence could be associated with the confinement effect in the silicon clusters. After annealing of the samples, the PL bands red shift. This is probably due to the thermal decomposition of N-H bonds with further effusion of hydrogen and better nitrogen passivation of the nc-Si/SiN_x interfaces.     

Design of a nanoscale silicon laser

The recent observation of optical gain from silicon nanocrystals embedded in SiO₂ opens an opportunity to develop a nanoscale silicon-based laser. However, the challenge remains to design and develop a laser architecture using CMOS-compatible materials. In this paper we present two designs for a waveguide laser in which silicon nanocrystals embedded in SiO₂ are used as the optical gain media. One design employs a SiO₂ membrane containing encapsulated Si nanocrystals. Preliminary calculations given here show that a highly resonant laser cavity can be produced in a SiO₂ membrane using sub-wavelength structures. This photonic crystal architecture, used to guide and contain the light, can be combined with a gain medium of optically active Si nanocrystals synthesized in the SiO₂ membrane using ion implantation/thermal annealing to produce a Si-based laser. The laser cavity dimensions can be matched to the near-infrared wavelengths where optical gain has been observed from Si nanocrystals. The second design utilizes silicon nanocrystals embedded in a distributed-feedback laser cavity fabricated in SiO₂. Lasing action over a broad wavelength range centered at ∼770 nm should be possible in both of these configurations. Received: 20 December 2002 / Accepted: 7 January 2003 / Published online: 11 April 2003 RID="*" ID="*"Corresponding author. Fax: +1-434/982-2037, E-mail: supriya@virginia.edu     

Size control of Si nanocrystals by two-step rapid thermal annealing of sputtered Si-rich oxide/SiO2 superlattice

Controllable size of silicon (Si) nanocrystals can be achieved by a two-step rapid thermal annealing technique consisting of rapid annealing at 1000°C in nitrogen ambient and rapid oxidation at 600–800°C of a radio frequency magnetron co-sputtered Si-rich oxide/SiO₂ superlattice structure. The photoluminescence (PL) spectra related to Si nanocrystals were observed in the visible range (600–900 nm). After rapid oxidation, the size of the nanocrystals was reduced and the quality of the Si nanocrystal/SiO₂ interface was improved, resulting in a blue shift and an increase of the PL peak intensity. Finally, annealing in air increases the PL intensity further.     

Pulsed electroluminescence in silicon nanocrystals-based devices fabricated by PECVD

Fully compatible CMOS capacitive devices have been developed in order to obtain electrically stimulated luminescence. By high-temperature annealing in N₂ atmosphere PECVD non-stoichiometric silica layers, silicon nanocrystals were formed. Photoluminescence, as well as structural studies, were carried out on these layers to decide the best material composition, which lies next to 17% of silicon excess. Under pulsed electrical stimulation, devices show sharp, narrow, less than 5 μs and pulse-frequency-independent, luminescence peaks at the end of the stimulation pulse. Current analysis on those capacities show hole injection at the beginning and electron injection at the end of the stimulation pulses. It is seen that no positive pulses are needed for attaining bipolar charge injection. Electroluminescence is detected when biasing with negative pulses at about 15 V and increasing up to 50 V. The electroluminescence spectrum matches photoluminescence one, allowing assigning both luminescent radiation to the same emission mechanism, that is, electron–hole recombination within the silicon nanocrystals.     

Ge nanoclusters in PECVD-deposited glass after heat treatment and electron-beam irradiation

This paper reports the formation of Ge nanoclusters in silica glass thin films deposited by plasma-enhanced chemical vapor deposition (PECVD). We studied the samples by transmission electron microscopy (TEM) and Raman spectroscopy after annealing. TEM investigation shows that the Ge nanoclusters at two areas were formed by different mechanisms. The Ge nanoclusters formed in a single row along the interface of a silicon substrate and the silica glass film by annealing during high-temperature heat treatment. Ge nanoclusters did not initially form in the bulk of the film but could be subsequently formed by the electron-beam irradiation. The interface between the silicon substrate and the silica glass film was investigated by Raman spectroscopy. The shift of the Raman peaks around 286.8 cm^-1 and 495 cm^-1 suggests that the interface is a Si_1-xGe_x alloy film and that the composition x varies along the film growth direction. PACS 81.07.Bc; 78.66.Jg; 42.65.Wi     

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.     

Controlled SnO2 nanocrystal growth in SiO2–SnO2 glass‐ceramic monoliths

Crack‐free (100–x) SiO₂–x SnO₂ glass‐ceramic monoliths have been prepared by the sol–gel method obtaining for the first time SnO₂ concentrations of 20% with annealing at 1100 °C. Heat‐treatment resulted in the formation and growth of SnO₂ nanocrystals within the silica matrices. Combined use of Fourier transform–Raman spectroscopy and in situ high‐temperature X‐Ray diffraction shows that SnO₂ particles begin to crystallize in the cassiterite‐type phase at 80 °C and that their average apparent size remains around 7 nm, even after annealing at 1100 °C. Nanocrystal sizes and size distributions determined by low‐wavenumber Raman are in good agreement with those obtained from transmission electron microscopy measurements. Results indicate that the formation and the growth of SnO₂ nanocrystals impose a residual porosity in the silica matrix. Copyright © 2011 John Wiley & Sons, Ltd.