Defect versus nanocrystal luminescence emitted from room temperature and hot-implanted SiO2 layers

Silicon nanocrystals have been synthesized in SiO₂ matrix using Si ion implantation. Si ions were implanted into 300-nm-thick SiO₂ films grown on crystalline Si at energies of 30–55 keV, and with doses of 5×10¹⁵, 3×10¹⁶, and 1×10¹⁷ cm⁻². Implanted samples were subsequently annealed in an N₂ ambient at 500–1100°C during various periods. Photoluminescence spectra for the sample implanted with 1×10¹⁷ cm⁻² at 55 keV show that red luminescence (750 nm) related to Si-nanocrystals clearly increases with annealing temperature and time in intensity, and that weak orange luminescence (600 nm) is observed after annealing at low temperatures of 500°C and 800°C. The luminescence around 600 nm becomes very intense when a thin SiO₂ sample is implanted at a substrate temperature of 400°C with an energy of 30 keV and a low dose of 5×10¹⁵ cm⁻². It vanishes after annealing at 800°C for 30 min. We conclude that this luminescence observed around 600 nm is caused by some radiative defects formed in Si-implanted SiO₂.     

Photoluminescence and electroluminescence investigations at Ge-rich SiO2 layers

Strong blue and violet photo (PL) and electroluminescence (EL) at room temperature was obtained from SiO₂-films grown on crystalline Si, which were either single (SI) or double implanted (DI) with Ge ions and annealed at different temperatures. The PL spectra of Ge-rich layers reach a maximum after annealing at 500–700°C for DI layers or 900–1000°C for SI layers, respectively. Both, PL and EL of 500 nm thick Ge-rich layers are easily visible by the naked eye at ambient light due to their high intensity. Based on excitation spectra we tentatively interpret the blue PL as due to the oxygen vacancy in silicon dioxide.

The EL spectrum of the Ge-implanted oxide correlates very well with the PL one and shows a linear dependence on the injected current over three orders of magnitude. For DI layers much higher injection currents than for SI layers can be achieved. An EL efficiency in the order of 10⁻⁴ for Ge⁺-implanted silicon dioxide was determined.     

Visible light emission from Si/SiO2 superlattices in optical microcavities

Bright quantum confined luminescence due to band-to-band recombination can be obtained from Si/SiO₂ superlattices. Placing them in a one-dimensional optical microcavity results in a pronounced modulation of the photoluminescence (PL) intensity with emission wavelength, as a consequence of the standing wave set up between the substrate and top interfaces. For a Si substrate, absorption of light reduces the PL efficiency, but for an Al-coated glass substrate the PL intensity is twice that of a quartz substrate case. The addition of a broad-band high reflector to the superlattice surface results in enhanced narrow-band emission. These results show that a suitably designed planar microcavity can not only considerably increase the external efficiency of luminescence in Si/SiO₂ superlattices but can also be used to decrease the bandwidth and selectively tune the peak wavelength.     

Size-dependent visible photoluminescence from ZnO nanoparticles prepared via SiO2 network

This study reports a simple method for the synthesis of different size of wurtzite ZnO nanoparticles in assistance of tetraethyl orthosilicate (TEOS). With the increase of the amount of TEOS added, the average size of ZnO nanoparticles was found decreased from ∼14.6 to ∼1.9 nm by characterization of X-ray diffraction (XRD) and high-resolution electron microscopy (HRTEM). The growth of ZnO nanoparticles is proposed to be controlled by the density of the SiO₂ chain mesh which is determined by TEOS amount in precursor. Ultraviolet–visible (UV–VIS) absorption and photoluminescence (PL) spectra show both shift to higher energy in cut-off edge and in visible emission bands respectively. The electron transition process in the mechanism of the visible emission shift was described and related to quantum size effect in ZnO nanoparticles.     

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.     

Continuous photoluminescence, time resolved photoluminescence and optically detected magnetic resonance measurements of PbI2 nanometer-sized particles, embedded in SiO2 films

The nanometer sized particles of PbI₂ were embedded in SiO₂ films. X-ray diffraction and the TEM pictures showed the preservation of the bulk layered structure and symmetry. The PL spectrum of the nano-particles exhibited a pronounced blue shift of the exciton band due to quantum size effect. The Lead Iodide represents an exceptionally small exciton Bohr radius (a_B = 19 Å) and a special case in which m_e m_h. The prepared samples contained particles with mean radii, a, in the range a_B < a < 3a_B. Within this limit (with m_e m_h), the experimental results suggest that the electron is localized nearly at the center of the particle, enabling the hole to move around it. Thus, the size confinement permits the creation of an acceptor-like exciton. The PL spectrum revealed additional states, associated with stoichiometric defects either at the interior or surface sites of the nano-particles. These defects act either as donor or acceptor states. The dynamics of the various recombination processes has been investigated by measuring the time resolved PL spectra. The results show a multiexponential behavior of the various recombination emission bands, indicating the occurrence of trapping and detrapping processes. Analysis of these results suggests that the existence of surface states give rise to these complex radiative decay processes. The correlation between donor-acceptor recombination emission bands in the aforementioned samples and lattice imperfections was examined, utilizing optically detected magnetic resonance (ODMR) spectroscopy. The results identified the following imperfection sites: an acceptor site associated with an isotropic Lead vacancy defect, [V⁻]_pb²⁺ and a donor site, associated with an anisotropic Iodine vacancy, [V⁰]_Iodine.     

低密度Cu掺杂SiO2复合气凝胶的制备及表征

分别以正硅酸甲酯、醋酸铜为硅源和铜源,通过溶胶凝胶法及CO2超临界干燥技术制备了一系列Cu掺杂SiO2复合气凝胶。采用红外谱仪（FTIR）、扫描电镜（SEM）、X射线能谱仪（EDS）、比表面积和孔隙度分析仪（BET）、动态热机械分析仪（DMA）对样品进行了表征。结果表明随着铜含量的增加,复合气凝胶密度增加,比表面积降低,平均孔径增加,实际掺杂比与理论掺杂比对应增加。该复合气凝胶具有三维网状结构,密度低（＜40 mg/cm3）,比表面积高（＞390 m2/g）,成型性较好,有望应用于惯性约束聚变（ICF）实验。     

Tunable photoluminescence and electroluminescence of size-controlled silicon nanocrystals in nanocrystalline-Si/SiO2 superlattices

We present photoluminescence and electroluminescence of silicon nanocrystals deposited by plasma-enhanced chemical vapor deposition (PECVD) using nanocrystalline silicon/silicon dioxide (nc-Si/SiO₂) superlattice approach. This approach allows us to tune the nanocrystal emission wavelength by varying the thickness of the Si layers. We fabricate light emitting devices (LEDs) with transparent indium tin oxide (ITO) contacts using these superlattice materials. The current-voltage characteristics of the LEDs are measured and compared to Frenkel-Poole and Fowler-Nordheim models for conduction. The EL properties of the superlattice material are studied, and tuning, similar to that of the PL spectra, is shown for the EL spectra. Finally, we observe the output power and calculate the quantum efficiency and power conversion efficiency for each of the devices.     

A comparative study of electroluminescence from Ge／SiO2 and Si／SiO2 films

Ge/SiO_2 and Si/SiO_2 films were deposited using the two-target alternation magnetron sputtering technique. The Au/Ge/SiO_2/p-Si and Au/Si/SiO_2/p-Si structures were fabricated and their electroluminescence (EL) characteristics were comparatively studied. Both Au/Ge/SiO_2/p-Si and Au/Si/SiO_2/p-Si structures have rectifying property. All the EL spectra from the two types of the structure have peak positions around 650－660 nm. The EL mechanisms of the structures are discussed.     

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

Intensive blue photoluminescence (PL) was observed at room temperature from the nanocrystalline-Si/SiO$_{2}$ (nc-Si/SiO$_{2})$ multilayers (MLs) obtained by thermal annealing of SiO/SiO$_{2}$\,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 SiO$_{2}$.     

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.     

Characterization of SiO2 layers thermally grown on 4H-SiC using high energy photoelectron spectroscopy

Si 2p and C 1s core level spectra recorded at different electron emission angles from SiO₂/SiC samples using a photon energy of 3.0 keV show two components. These are identified as originating from SiO₂ and SiC for Si 2p while for C 1s they are identified to originate from graphite like carbon and SiC. The relative intensity of these components are extracted and compared to calculated intensity variations assuming different models for the elemental distribution in the surface region. For both samples investigated best agreement between experimental and calculated intensity variations with emission angle is obtained when assuming a graphite like layer on top of the oxide layer. Contribution from carbon at the SiC/SiO₂ interface could not be identified.     

超低密度SiO2气凝胶的制备研究

超低密度的SiO2气凝胶是一种经典的三维网状纳米多孔材料,已经广泛应用于如保温隔热、吸附等多种领域。以四甲氧基硅烷(TMOS)为硅源,采用酸碱两步法,利用乙醇超临界干燥技术制备了超低密度的SiO2气凝胶,分别利用SEM\TEM\BET等表征手段对该气凝胶进行了一系列的研究,发现当其密度为0.6 mg/cm3时,气凝胶拥有最佳的综合性能。该种气凝胶具有超低密度、高比表面积、加工成型性好、制备周期短等优点,有望在激光惯性约束聚变实验中作为冷冻靶发挥巨大的作用。     

SiO2薄膜的液相沉积及特性

将基片浸入到低温SiO₂过饱和的六氟硅酸(H₂SiF₆)溶液中,在其表面上沉积SiO₂薄膜。这种新的生长工艺称之为液相沉积(LPD)。本文着重介绍LPD工艺及LPD SiO₂薄膜的特性。     

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根据垂直扫描干涉技术结合样品折射率与密度间的依赖关系,建立了低密度SiO2泡沫微球密度的检测方法,用Lorentz-Lorenz和Gladstone-Dale公式分析了SiO2泡沫微球折射率与密度间的依赖关系。实验结果和测量误差估计表明,利用垂直扫描干涉技术并结合Gladstone-Dale分析方法,可实现对低密度SiO2泡沫微球密度的精密检测,其测量误差好于5%。     

表面活性剂辅助二氧化硅纳米颗粒在非水基液中的分散研究

研究了粒径10~100 nm的二氧化硅纳米颗粒在非水基液中的表面活性剂辅助分散。结合颗粒表面特定官能团结构,针对性选择了合适的表面活性剂,氢键桥梁作用和长链分子空间位阻作用抑制了颗粒团聚行为。当表面活性剂体积分数6%的时候,动态光散射测试结果表明颗粒中位尺度30.2 nm,与透射电镜测试结果吻合,展现了良好的分散性。     

基于Ni电极和ZrO_2/SiO_2/ZrO_2介质的MIM电容的导电机理研究

研究了基于Ni电极和原子层淀积的ZrO_2/SiO_2/ZrO_2对称叠层介质金属-绝缘体-金属(MIM)电容的电学性能.当叠层介质的厚度固定在14nm时,随着SiO_2层厚度从0增加到2nm,所得电容密度从13.1 fF/μm~2逐渐减小到9.3fF/μm~2,耗散因子从0.025逐渐减小到0.02.比较MIM电容的电流-电压(I-V)曲线,发现在高压下电流密度随着SiO_2厚度的增加而减小,在低压下电流密度的变化不明显,还观察到电容在正、负偏压下表现出完全不同的导电特性,在正偏压下表现出不同的高、低场I-V特性,而在负偏压下则以单一的I-V特性为主导.进一步对该电容在高、低场下以及电子顶部和底部注入时的导电机理进行了研究.结果表明,当电子从底部注入时,在高场和低场下分别表现出普尔-法兰克(PF)发射和陷阱辅助隧穿(TAT)的导电机理;当电子从顶部注入时,在高、低场下均表现出TAT导电机理.主要原因在于底电极Ni与ZrO_2之间存在镍的氧化层(NiO_x),且ZrO_2介质层中含有深浅两种能级陷阱(分别为0.9和2.3 eV),当电子注入的模式和外电场不同时,不同能级的陷阱对电子的传导产生作用.     

TiO2/SiO2多层膜的带隙结构及光催化性能

利用传输矩阵法设计了由SiO2、TiO2组成的多层膜高透射率光子晶体结构,并分析了其透射谱特性,根据等效层原理改变多层膜一维光子晶体的自身结构来提高通带内特征波长附近的透射率,获得了最佳结构参数。研究结果表明,当晶格参数为150nm,填充比为0.346,周期数为6时,400nm波长附近吸收带处的透射率最低也可达96.5%,并且不论是TM模式还是TE模式,入射角在0°～45°范围内仍保持高的透射率,该结构可望用于空气净化装置以提高SiO2、TiO2光催化剂的光催化效率。     

Annealing effects on structure and laser-induced damage threshold of Ta2O5/SiO2 dielectric mirrors

The effects of annealing on structure and laser-induced damage threshold (LIDT) of Ta₂O₅/SiO₂ dielectric mirrors were investigated. Ta₂O₅/SiO₂ multilayer was prepared by ion beam sputtering (IBS), then annealed in air under the temperature from 100 to 400 °C. Microstructure of the samples was characterized by X-ray diffraction (XRD). Absorption of the multilayer was measured by surface thermal lensing (STL) technique. The laser-induced damage threshold was assessed using 1064 nm free pulsed laser at a pulse length of 220 μs.

It was found that the center wavelength shifted to long wavelength gradually as the annealing temperature increased, and kept its non-crystalline structure even after annealing. The absorbance of the reflectors decreased after annealing. A remarkable increase of the laser-induced damage threshold was found when the annealing temperature was above 250 °C.     

二氧化硅对稀土掺杂二氧化钛薄膜形貌与发光性能的影响

为提高稀土掺杂TiO₂薄膜的上转换效率,采用溶胶-凝胶法和旋涂镀膜工艺制备了Yb³⁺-Er³⁺共掺杂SiO₂/TiO₂上转换光致发光薄膜,研究了SiO₂对TiO₂薄膜形貌以及发光性能的影响。利用FE-SEM观察了薄膜的表面形貌,利用分光光度计测试了薄膜在近红外光区域的透射率的变化,并用荧光光谱仪测试了薄膜的上转换发光光谱。结果表明：SiO₂的掺杂导致TiO₂颗粒尺寸显著减小,TiO₂薄膜在近红外的透射率也有所下降。在980 nm红外光激发下,SiO₂/TiO₂薄膜在630~670 nm处获得了明显的上转换红光发射,在516~537 nm和537~570 nm处获得了较弱的上转换绿光发射。由上转换发光强度与激光泵浦功率的关系推知,绿色和红色上转换发光均为双光子吸收发射过程。