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.     

Tailoring optical properties of TiO2 in silica glass for limiting applications

We present the linear and nonlinear optical studies on TiO2-SiO2 nanocomposites with varying compositions. Opti- cal band gap of the material is found to vary with the amount of SiO2 in the composite. The phenomenon of two-photon absorption （TPA） in TiO2/SiO2 nanocomposites has been studied using open aperture Z-scan technique. The nanocom- posites show better nonlinear optical properties than pure TiO2, which can be attributed to the surface states and weak dielectric confinement of TiO2 nanoparticles by SiO2 matrix. The nanocomposites are thermally treated and similar studies are performed. The anatase form of TiO2 in the nanocomposites shows superior properties relative to the amorphous and rutile counterpart. The involved mechanism is explained by rendering the dominant role played by the excitons in the TiO2 nanoparticles.     

Near Infrared Photoluminescence from Yb,Al Co-implanted SiO2 Films on Silicon

Intense room-temperature near infrared （NIR） photoluminescence （980 nm and 1032 nm） is observed from Yb,Al co-implanted SiO2 films on silicon. The optical transitions occur between the ^2F5/2 and ^2F7/2 levels of Yb^3＋ in SiO2. The additional Al-implantation into SiO2 films can effectively improve the concentration quenching effect of Yb^3＋ in SiO2. Photoluminescence excitation spectroscopy shows that the NIR photoluminescence is due to the non-radiative energy transfer from Al-implantation-induced non-bridging oxygen hole defects in SiO2 to Yb^3＋ in the Yb-related luminescent complexes. It is believed that the defect-mediated luminescence of rare-earth ions in SiO2 is very effective.     

Strong Green Light Emission from Low-Temperature Grown a-SiNx:H Film after Different Oxidation Routes

Room-temperature deposited amorphous silicon nitride （a-SiNx ：H） films exhibit intense green light emission after post-treated by plasma oxidation, thermal oxidation and natural oxidation, respectively. All the photoluminescence （PL） spectra are peaked at around 500nm, independent of oxidation method and excitation wavelength. Compared with the PL results from oxidized a-Si：H and as-deposited a-SiNx：H samples, it is indicated that not only oxygen but also nitrogen is of an important role in enhancing light emission from the oxidized a-SiNx：H. Combining the PL results with the analyses of the bonding configurations as well as chemical compositions of the films, the strong green light emission is suggested to be from radiative recombination in luminescent centres related to N Si-O bonds.     

Defects in Si-Rich SiO2 Films Prepared by Radio-Frequency Magnetron Co-sputtering Using Variable Energy Positron Annihilation Spectroscopy

Si-rich SiO2 films prepared by rf magnetron co-sputtering method are studied by slow positron beams. The nega- tively charge point defects （probably Pb centres or peroxy radicals） at the silicon nanocluster （nc-Si）/SiO2 interface are observed by Doppler broadening spectra. Coincidence Doppler-broadening spectra show that positrons have a higher annihilation probability with core electrons nearby oxygen atoms than silicon atoms. The formation of N-related bonds may be the reason for the prevention of the migration reaction of Si and 0 atoms, hence nc-Si formation is inhibited by annealing in nitrogen compared to in vacuum.     

Optimization of a GaN-based irregular multiple quantum well structure for a dichromatic white LED

GaN-based irregular multiple quantum well(IMQW) structures assembled two different types of QWs emitting complementary wavelengths for dichromatic white light-emitting diodes(LEDs) are optimized in order to obtain near white light emissions.The hole distributions and spontaneous emission spectra of the IMQW structures are analysed in detail by fully considering the effects of strain,well-coupling,valence band-mixing and polarization effect through employing a newly developed theoretical model from the k · p theory.Several structure parameters such as well material component,well width,layout of the wells and the thickness of barrier between different types of QWs are employed to analyse how these parameters together with the polarization effect influence the electronic and the optical properties of IMQW structure.Numerical results show that uniform hole distributions in different types of QWs are obtained when the number of the QWs emitting blue light is two,the number of the QWs emitting yellow light is one and the barrier between different types of QWs is 8nm in thickness.The near white light emission is realized using GaN-based IMQW structure with appropriate design parameters and injection level.     

Optical behavior of self-assembled high-density Ge nanoislands embedded in SiO2

The radio frequency magnetron sputtering method is used to prepare well-dispersed pyramidal-shaped Ge nanoislands embedded in amorphous SiO2 sublayers of various thicknesses. The estimated size and number density of Ge nanoislands in SiO2 sublayer thicknesses beyond 30 nm are approximately 15 nm and 1011 cm-2, respectively. Atomic force microscopy （AFM） reveals root mean square （RMS） roughness sensitivity as the SiO2 sublayer thickness varies from 30 to 40 nm. The formation of nanoislands with high aspect ratios is attributed to the higher rate of surface reactions between Ge adatoms and nucleated Ge islands than reactions associated with SiO2 and Ge. The Ge nanoisland polyorientation on SiO2 （50-nm thickness） is revealed by X-ray diffraction （XRD） patterns. Photoluminescence （PL） peaks of 2.9 and 1.65 eV observed at room temperature （RT） are attributed to the radiative recombination of electrons and holes from the Ge nanoislands/SiO2 and Si02/Si interfaces, respectively. The mean island sizes are determined by fitting the experimental Raman profile to two models, namely, the phonon confinement model and the size distribution combined with phonon confinement model. The latter model yields the best fit to the experimental data. We confirm that SiO2 matrix thickness variations play a significant role in the formation of Ge nanoislands mediated via the minimization of interfacial and strain energies. OCIS codes： 250.5230, 170.5660.     

Enhanced Raman scattering from nano-SnO2 grains

We present the Raman spectra of nano-SnO_{2} grains with sizes from 4nm to 80nm excited by 532nm and 1.06μm lines. The enhanced Raman scattering of the nanograins is observed for both exciting lines when the grain size is less than 8nm. The less the grain size is, the more intensely the Raman scattering is enhanced. According to our results, the enhancements of the Raman intensity are a few tenfolds and different for different exciting lines when the grain size is 4nm. It can be attributed to enhanced Raman scattering by electron－hole pair excitations in the nanograins that originate from sub-microscopic (10nm) size and other defect- and surface-related features. A critical size that divides respective predominance of bulk properties and the defect-, surface-, and size-related features can be determined to be about 8nm.     

Stimulated photoluminescence emission and trap states in Si/SiO2 interface formed by irradiation of laser

Stimulated photoluminescence （PL） emission has been observed from an oxide structure of silicon when optically excited by a radiation of 514nm laser. Sharp twin peaks at 694 and 692nm are dominated by stimulated emission, which can be demonstrated by its threshold behaviour and linear transition of emission intensity as a function of pump power. The oxide structure is formed by laser irradiation on silicon and its annealing treatment. A model for explaining the stimulated emission is proposed, in which the trap states of the interface between an oxide of silicon and porous nanocrystal play an important role.     

Electronic Properties of Nanocrystalline-Si Embedded in Asymmetric Ultrathin SiO2 by In-Situ Fabrication Technique

Structures of nanocrystalline-Si (nc-Si) sandwiched between two asymmetric ultrathin SiO2 layers were fabricated. The nc-Si (dot density of 10^11 cm^-2) was formed by decomposition of hydrogen-diluted silane and the ultrathin SiO2 layers (about 2 nm) were prepared by plasma oxidation at a lower temperature (250℃). The whole fabrication processes were completed in situ in a plasma-enhanced chemical vapour deposition system. By using the capacitance-voltage ( C-V) and conductance-voltage ( G-V) spectroscopy, we studied the electronic properties of the annealed samples. The experimental results show that there are distinct capacitance peaks and conductance plateau or peaks for annealed samples at room temperature, which can be explained by direct tunnelling of electrons into the nc-Si. At the same time, Coulomb blockade plays an important role in the electronic transport in the nc-Si. The effect of thermal annealing in N2 ambient on the electronic properties was studied and the results indicate that high temperature (1000℃) annealing can improve the size uniformity of the nc-Si prepared by decomposition of hydrogen-diluted silane.     

Electroluminescence of Si Nanocrystal-Doped SiO2

We perform a comparative st udy on the electroluminescence （EL） and photoluminescence （PL） of Si nanocrystaldoped SiO2 （nc-Si：SiO2） and SiO2, and clarify whether the contribution from Si nanocrystals in the EL of nc-Si：SiO2 truly exists. The results unambiguously indicate the presence of EL of Si nanocrystals. The difference of peak positions between the EL and PL spectra are discussed. It is found that the normal method of passivation to enhance the PL of Si nanocrystals is not equally effective for the EL, hence new methods need to be explored to promote the EL of Si nanocrystals.[第一段]     

Physical and Chemical Properties of TiOxNy Prepared by Low-Temperature Oxidation of Ultrathin Metal Nitride Directly Deposited on SiO2

Physical and chemical properties of titanium oxynitride （TiOxNy ） formed by low-temperature oxidation of titanium nitride （TIN） layer are investigated for advanced metal-oxide-semiconductor （MOS） gate dielectric application. TiOxNy exhibits polycrystalline properties after the standard thermal process for MOS device fabrication, showing the preferred orientation at [200]. Superior electrical properties of TiOxNy can be maintained before and after the annealing, probably due to the nitrogen incorporation in the oxide bulk and at the interface. Naturally formed transition layer between TiOxNy and SiO2 is also confirmed.     

Electroluminescence from Multilayered Diamond/CeF3/SiO2 Films

We report a thin film electroluminescent device with a three-layer structure （diamond/CeF3/SiO2 films）, which has a luminance of 1.5 cd/m^2 at dc voltage 215 V. The electroluminescence spectrum at room temperature shows that the main peaks locate at 527 and 593nm, which are attributed to isolated emission centers of Ce^3＋ ions.     

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.     

High-Temperature Dielectric Response and Multiscale Mechanism of SiO2/Si3N4 Nanocomposites

The high-temperature dielectric properties of SiO₂/Si₃N₄ nanocomposites are investigated theoretically and experimentally. Its permittivities and loss tangents at the temperature ranging from room temperature to 1300°C at 9.0GHz are measured by the resonant cavity method. The SiO_2/Si₃N₄ nanocomposites show complex dielectric behaviour at elevated temperature, and a multi-scale model is proposed to describe the dependence of the dielectric properties in the SiO₂/Si₃N₄ on its compositional variations. Such a theory is needed so that the available property measurements could be extrapolated to other operating frequencies and temperatures.     

Si nanocrystallites in SiO2 with intense visible photoluminescence synthesized from SiOx films deposited by laser ablation

We observed very intense and highly reproducible photoluminescence (PL) spectra for SiO_x films obtained by laser ablation of Si targets in 50-mTorr oxygen gas followed by proper annealing. It was found that the PL peak continuously changes from 1.4 eV at the center of the samples to 1.8 eV at the sample edge. The optimum values of the oxygen component in SiO_x was x=1.3-1.4 and the optimum annealing temperature was 1000 °C for intense PL. From transmission electron microscopy images of annealed films, Si nanocrystallites are found to be formed in the matrix of SiO₂ grown from the SiO_x and have diameters of 2-3 nm. These indicate that a high density of Si nanocrystallites with diameters of 2-3 nm in the SiO₂ phase are probably responsible for the PL and that the Si nanostructure is well formed from the as-deposited, metastable SiO_x (x=1.3-1.4) films by annealing at 1000 °C.     

Large Storage Window in a-SiNx/nc-Si/a-SiNx Sandwiched Structure for Nanocrystalline Silicon Floating Gate Memory Application

An a-SiNx/nanocrystalline silicon [（nc-Si）/a-SiNx] sandwiched structure is fabricated in a plasma enhanced chemical vapour deposition （PECVD） system at low temperature （250℃）. The nc-Si layer is fabricated from a hydrogen-diluted silane mixture gas by using a layer-by-layer deposition technique. Atom force microscopy measurement shows that the density of nc-Si is about 2 ×10^11 cm^-2. By the pretreatment of plasma nitridation, low density of interface states and high-quality interface between the Si substrate and a-SiNs insulator layer are obtained. The density of interface state at the midgap is calculated to be 1 ×10^10 cm^-2eV^-1 from the quasistatic and high frequency C - V data. The charging and discharging property of nc-Si quantum dots is studied by capacitance-voltage （C- V） measurement at room temperature. An ultra-large hysteresis is observed in the C - V characteristics, which is attributed to storage of the electrons and holes into the nc-Si dots. The long-term charge-loss process is studied and ascribed to low density of interface states at SiNx/Si substrate.     

Electronic Structure and Optical Properties of La-Doped SrTiO3 and Sr2TiO4 by Density Function Theory

The effect of La doping on the electronic structure and optical properties of SrTiO₃ and Sr₂TiO₄ is investigated by the first-principles calculation of plane wave ultrasoft pseudopotential based on the density function theory (DFT). The calculated results reveal that the electron doping in the case of Sr_0.875La_0.125TiO₃ and Sr_1.875La_0.125TiO₄ can be described within the rigid band model. The La³⁺ ions fully acts as electron donors in Sr_0.875La_0.125TiO₃ and Sr_1.875La_0.125TiO₄ systems and the Fermi level shifts further into the conduction bands (CBs) for Sr_1.875La_0.125TiO₄ compared to Sr_0.875La_0.125TiO₃. The two systems exhibit n-type degenerate semiconductor features. At the same time, the density of states (DOS) of the two systems shift towards low energies and the optical band gaps are broadened. The Sr_1.875La_0.125TiO₄ is highly transparent with the transmittance about 90% in the visible range, which is larger than that of Sr_0.875La_0.125TiO₃(85%). The wide band gap, small transition probability and weak absorption due to the low partial density of states (PDOS) of impurity in the Fermi level result in the optical transparency of the films...     

高压下SiO_2的第一性原理计算

基于密度泛函理论(DFT)的第一性原理,采用Hartree-Fork(HF)方法,分别计算了Si O2的α-石英结构、金红石结构以及氯化钙结构的总能量随体积的变化关系。利用Murnaghan状态方程,通过能量和体积拟合,得到了3种结构的体变模量及其对压强的一阶导数。计算结果表明,随着压强的增加,Si O2会从α-石英结构转变为金红石结构,与实验结果和其它理论结果一致;金红石结构与氯化钙结构之间不存在相变,可以共存。此外,对具有α-石英结构的Si O2的晶格常数、电子态密度和带隙随压强的变化关系进行了计算和分析,结果表明:加压作用下,能带向高能方向移动,Si─O键缩短,电子数转移增加,带隙展宽,电荷发生重新分布。