The origin of blue photoluminescence from nc-Si/SiO2 multilayers

Intensive blue photoluminescence （PL） was observed at room temperature from the nanocrystalline-Si/SiO2 （nc-Si/SiO2） multilayers （MLs） obtained by thermal annealing of SiO/SiO2 MLs for the first time. By controlling the size of nc-Si formed in SiO sublayer from 3.5 to 1.5 nm, the PL peak blueshifts from 457 to 411 nm. Combining the analysis of TEM, Raman and absorption measurement, this paper attributes the blue PL to multiple luminescent centres at the interface of nc-Si and SiO2.     

Pseudo nanocrystal silicon induced .luminescence enhancement in a-Si/SiO2 multilayers

Enhanced photoluminescence （PL） at room temperature from thermally annealed a-Si：H/SiO2 multilayers is observed through the step-by-step thermal post-treatment. The correlation between the PL and the crystallization process is studied using temperature-dependent PL, Raman, cross section high-resolution transmission electron microscopy （XHRTEM） and x-ray diffraction （XRD） techniques. An intensified PL band around 820 nm is discovered from the sample annealed near the crystallization onset temperature, which is composed of two peaks centred at 773 nm and 863 nm, respectively. It is found that the PL band centred at 863 nm is related to the pseudo nanocrystal （p-nc-Si） silicon, and the PL band centred at 773 nm is attributed to Si = O bonds stabilized in the p-nc-Si surface.     

Effects of hydrogen plasma annealing on the luminescence from a-Si:H/SiO2 and nc-Si/SiO2 multilayers

Effects of post-hydrogen plasma annealing (HPA) on a-Si:H/SiO₂ and nc-Si/SiO₂ multilayers have been investigated and compared. It is found that photoluminescence (PL) from hydrogen-passivated samples was improved due to the reduction of non-radiative recombination defects. Some interesting difference is that during HPA, atomic hydrogen can directly passivate defects of a-Si:H/SiO₂, which results in the reappearance of luminescence band at 760 nm, while for nc-Si/SiO₂, hydrogen passivation requires additional thermal annealing after nc-Si/SiO₂ multilayer was treated by HPA. It is indicated that higher atomic mobility is needed to passivate defects at nc-Si/SiO₂ interface compared with a-Si:H/SiO₂ interface.     

The origin of photoluminescence in Ge-implanted SiO2 layers

Ge ions were implanted at 100 keV with 3×10¹⁶ cm⁻² into a 300  nm thick SiO₂ layer on Si. Visible photoluminescence (PL) around 2.1 eV from an as-implanted sample is observed, and faded out by subsequent annealing at 900°C for 2 h. However, PL shows up again after annealing above 900°C at the same peak position. Compared with the as-implanted sample, significant increase of Ge–Ge bonds is measured in X-ray photoelectron spectroscopy, and the formation of Ge nanocrystals with a diameter of 5 nm are observed in transmission electron microscopy from the sample annealed at 1100°C. We conclude that the PL peak from the sample annealed above 900°C is caused by the quantum confinement effects from Ge nanocrystals, while the luminescence from the as-implanted sample is due to some radiative defects formed by Ge implantation.     

nc-Si/SiO2多层膜的制备及蓝光发射

在等离子体增强化学气相淀积(PECVD)系统中，采用a-Si∶H层淀积与原位等离子体氧化相结合的逐层生长的方法成功制备出a-Si∶H/SiO2多层膜 (ML)；利用限制性结晶原理通过两步退火处理使a-Si∶H层晶化获得尺寸可控的nc-Si/SiO2 ML，并观察到室温下的蓝光发射；结合Raman散射和剖面透射电子显微镜技术分析了nc-Si/SiO2 ML的结构特性；通过对晶化样品光致发光谱和紫外-可见光吸收谱的研究，探讨了蓝光发射的起源. 关键词： 纳米硅多层膜 等离子体氧化 蓝光发射 热退火     

发光纳米硅/二氧化硅多层膜的特性与氢气氛退火的影响

利用等离子体增强化学气相沉积法制备了氢化非晶硅/二氧化硅多层膜，通过两步热退火的方法获得了尺寸可控的纳米硅/二氧化硅多层结构，晶粒尺寸约为4nm，在室温下观察到了较强的光致可见发光，其发光峰位于750nm.在此基础上，发现合适的氢气氛退火能有效地提高材料的发光强度.电子顺磁共振实验表明氢气氛退火有效地降低了纳米硅中的非辐射复合中心而导致发光效率的提高.     

Substrate temperature dependence of the photoluminescence efficiency of co-sputtered Si/SiO2 layers

Si particles embedded in an SiO₂ matrix were obtained by co-sputtering of Si and SiO₂ at various deposition temperatures T_d (200–700°C) and annealing at different temperatures T_a (900–1100°C). The systems were characterized by X-ray photoelectron, Raman scattering, infrared absorption and photoluminescence spectroscopy techniques. The results show that the photoluminescence efficiency is strongly dependent on the degree of phase separation between the Si nanocrystals and the SiO₂ matrix. This is likely connected with the Si/SiO₂ interface characteristics, together with the features indicating the involvement of quantum confinement.     

Si/SiO2 multilayers: synthesis by reactive magnetron sputtering and photoluminescence emission

This paper relates a complete study of Si/SiO₂ multilayer (ML) structures. First, we suggest an original way of synthesis based on reactive magnetron sputtering of a pure silica target. The photoluminescence spectra of these MLs consist of two Gaussian bands in the visible-near infrared spectral region. The stronger one (I band) is fixed at about 780 nm and probably due to interface states. The weaker one (Q band) is tuneable with the Si sublayer thickness and originates from a radiative recombination within the nanosized Si layers. For this latter band the peak position is a function of the Si sublayer thickness and shows a discontinuity at 30 Å. This corresponds to an Si phase change. For thicknesses above 30 Å, the sublayers are composed of nanocrystalline silicon whereas below 30 Å the sublayers are made of amorphous silicon. We develop a model based on a quantum well to which we have added an interfacial region between Si and SiO₂. It is characterised by an interfacial potential of 0.3 eV. This model depicts the simultaneous behaviour of Q and I bands for an Si sublayer thickness below 30 Å.     

Micro-Raman and photoluminescence spectra of Sm-doped β-BaB2O4 crystal lines written by YAG laser irradiation in glass

Nonlinear optical β-BaB₂O₄ (β-BBO) crystal straight lines are written in 10Sm₂O₃·40BaO·50B₂O₃ glass by irradiation of a continuous wave Nd:YAG laser operating at 1064 nm. The linearly polarized micro-Raman scattering spectra for β-BBO crystal lines are consistent with those for commercially available y-cut β-BBO bulk single-crystals, supporting that β-BBO crystal straight lines with the c-axis orientation along the YAG laser scanning direction might be single-crystals. The photoluminescence spectra with large intensities and Stark splitting are observed for β-BBO crystal lines, and it is concluded that some amounts of Sm³⁺ ions in the glass are incorporated into β-BBO crystal lines. Two-dimensional β-BBO crystal curved lines with a bending angle of 30° are successfully written in 10Sm₂O₃·42BaO·48B₂O₃ glass.     

Influence of the SiO thickness on the photoluminescence properties of Er-doped SiO/SiO2 multilayers

Er-doped SiO single layer and Er-doped SiO/SiO₂ multilayers with different SiO thicknesses were prepared by evaporation. In the as-deposited samples, the erbium ions exhibit a very weak photoluminescence emission at 1.54 μm. This luminescence is strongly enhanced after annealing treatments between 500 and 1050 °C, with an optimal annealing temperature which is dependent from the SiO thickness. For the SiO single layer, this optimal temperature is around 700 °C while it is shifted at highest temperature for the multilayers. The origin of the higher luminescence intensity in the SiO layer is also discussed.     

Optical properties from SRSO/SiO2 multilayers in planar microcavities

We report on the fabrication and characterization of an efficient and CMOS compatible Si-based light source. Silicon-rich silicon oxide (SRSO)/SiO₂ superlattices-doped planar optical microcavities have been grown by reactive magnetron sputtering. The devices were studied using reflectance and photoluminescence measurements. The reflectivity of the resonator is above 99.9% and the quality factor is reaching a value of 100. Modification of silicon nanocrystals emission is clearly observed in the visible and reveals narrow, intense and highly directional emission properties.     

Raman spectroscopic analysis of sulphur-doped TiO2 by co-grinding with TiS2

Doping sulphur into titania has been tried using TiS₂ as a doper based on the mechanically induced solid-state reaction between TiO₂ and TiS₂. The prepared samples have been characterized by X-ray diffraction (XRD), Raman spectroscopy and UV-Vis reflectance spectroscopy. Raman analysis, particularly has been proved to be effective in assessing the sulphur doping by correlating the oxygen deficiency of the doped oxide with the change of active Eg mode of rutile phase.     

Splitting of X-ray diffraction peak in (Ge:SiO2)/SiO2 multilayers

(Ge:SiO₂)/SiO₂ multilayers were fabricated for exploring the influence of the stress on the structure of Ge nanocrystals. When annealed at 800 °C, the multilayers show a clear splitting (fine structure) of the Ge (220) X-ray diffraction peak and have a preferred orientation. Similar effects cannot take place in the multilayers annealed at higher or lower temperature. Analyses of Raman scattering, X-ray diffraction spectroscopy, and transmission electron microscope observations suggest that the observed phenomena arise from compressive stress exerted on Ge nanocrystals, which is induced by the confinement of both the SiO₂ matrix in the cosputtered layer and neighboring SiO₂ layers. The stress may cause an orthorhombic distortion of the diamond structure of bulk Ge. This will lead to the disappearance of the (111) and (311) diffraction peaks and the splitting of the (220) peak. This kind of (Ge:SiO₂)/SiO₂ multilayers enables us to control the sizes of the Ge crystallites and enhance the stress, and is thus promising in forming new nanocrystal structures.     

Ferromagnetic resonance studies in Co/SiO2 multilayers

The uniaxial in-plane and out-of-plane anisotropies of [Co/SiO₂] × 10 multilayers have been studied by ferromagnetic resonance, magnetometry and transmission electron microscopy. The surface and volume anisotropy constants are in the range of values typical for multilayers with Co and transition metals of the iron group. The influence of the intermixed Co-SiO₂ region is discussed.     

纳米Si/SiO2多层膜的结构表征及发光特性

采用等离子体化学气相沉积系统生长非晶硅薄膜并用原位等离子体氧化的方法制备出具有不同子层厚度的非晶Si/SiO₂多层膜,然后利用限制性晶化原理使非晶硅层晶化生成纳米硅。利用Raman、TEM等手段对薄膜结构进行了系统表征,在室温下观测到了光致发光信号,其发光峰峰位在750nm附近。进而在样品上下表面蒸镀电极,构建了电致发光原型器件并观测到了室温下的电致发光谱,开启电压约为6V,有两个明显的发光带,分别位于在650nm和520nm处。初步探讨了纳米硅及纳米硅/二氧化硅界面态对发光特性的影响。     

Improvement of photoluminescence mechanism of CTA-treated Si-implanted SiO2 films by using RTA

In this paper, a shift in the photoluminescence (PL) peak from blue to near-infrared region was observed in the Si⁺-implanted 400-nm-thick SiO₂ films with the rapid thermal annealing (RTA) method only. As the Si⁺-fluence was 1×10¹⁶ ions/cm², a blue band was observed in the films after RTA at 1050 °C for 5 s in dry-N₂ atmosphere; then, the band shifted from blue to orange upon increasing the holding temperature of RTA to 1250 °C in the films after the isochronal RTA in dry N₂. Furthermore, while the fluence was increased to 3×110¹⁶ ions/cm² and the holding temperature was at the same range between 1050 and 1250 °C, the PL peak occurred between red and near-infrared regions. Although the RTA and conventional thermal annealing (CTA) methods produce a similar mechanism, the CTA method needs a much longer annealing-time and a higher Si⁺-implanted dose than the RTA method for producing the same shift and intensity of PL peak from the as-implanted sample. Therefore, the RTA method can produce the mechanism in the Si⁺-implanted sample with the PL energy between blue and near-infrared band in place of the CTA method.     

Energy of excitons and acceptor-exciton complexes to explain the origin of ultraviolet photoluminescence in ZnO quantum dots embedded in a SiO2 matrix

Assuming finite depth and within the effective mass approximation, the energies of exciton states and of the acceptor-exciton complexes confined in spherical ZnO quantum dots (QDs) embedded in a SiO₂ matrix are calculated using a matrix procedure, including a three-dimensional confinement of carrier in the QDs. This theoretical model has been designed to illustrate the two emission bands in the UV region observed in our experimental Photoluminescence spectrum (PL), with the first emission band observed at 3.04 eV and attributed to the bound ionized acceptor-exciton complexes, and the second one located at 3.5 and assigned to the free exciton. Our calculations have revealed a good agreement between the matrix element calculation method and the experimental results.     

Dose dependence of room temperature photoluminescence from Si implanted SiO2

Photoluminescence from Si implanted silica is studied as a function of Si fluence and Si concentration profile in order to assess the effect of particle size and size distribution on emission spectra. Peaked (skewed Gaussian) concentration profiles were produced by implanting with 400 keV Si ions and uniform Si profiles were produced by a multi-energy implant sequences. Both as-implanted and annealed samples are shown to exhibit a distinct maximum in the emission intensity as a function of ion fluence, with the intensity increasing with fluence up to the maximum and then decreasing at higher fluences. Samples with a uniform Si profile are also shown to produce emission which is significantly red-shifted relative to that of samples with a peaked Si profile. This is consistent with the fact that such samples are expected to have a narrower particle size distribution (i.e. a greater fraction of larger particles).     

Silicon nanoparticles formation in annealed SiO/SiO2 multilayers

We present a study on amorphous SiO/SiO₂ superlattice performed by grazing-incidence small-angle X-ray scattering (GISAXS). Amorphous SiO/SiO₂ superlattices were prepared by high-vacuum evaporation of 3 nm thin films of SiO and SiO₂ (10 layers each) onto Si(1 0 0) substrate. After the deposition, samples were annealed at 1100 °C for 1 h in vacuum, yielding to Si nanocrystals formation. Using a Guinier approximation, the shape and the size of the crystals were obtained. The size of the growing nanoparticles in the direction perpendicular to the film surface is well controlled by the bilayer thickness. However, their size varies more significantly in the direction parallel to the film surface.     

Anneal temperature dependence of Si/SiO2 superlattices photoluminescence

Photoluminescence spectroscopy, Fourier transform infrared spectroscopy, X-ray reflectometry and high resolution electron microscopy have been used to interpret the photoluminescence properties of annealed (3/19 nm) Si/SiO₂ multilayers grown by reactive magnetron sputtering. The multilayers show an emission in the visible and near-infrared range after heat treatment from 900°C which tends to decrease from 1200°C. Three different origins for the photoluminescence activity have been found. An anneal temperature of 1200°C is necessary to optimise the silicon crystallisation within the silicon sublayers.