Analytical electron microscopy of a reaction-bonded Si3N4-based ceramic containing oxide additions

The microstructure of a silicon nitride (Si₃N₄)-based ceramic, prepared by a process combining direct nitridation and reactive liquid phase sintering of silicon/ceramic oxide powder compacts, has been characterised using analytical transmission electron microscopy. The presence of the reactive liquid phase, promoted by the addition of oxides from the CaO-Al₂O₃-SiO₂ ternary system, resulted in an as-fired microstructure containing a mixture of crystalline phases based on -Si₃N₄, β-Si₃N₄ and Si₂ N₂O, and distinct amorphous regions rich in Si, Al and Ca. X-ray microanalysis revealed the calcium to be wholly partitioned to the glassy phase, while significant concentrations of aluminium were detected in both β-Si₃N₄ and Si₂N₂O. The observed compositions of these phases, together with measured lattice parameters systematically in excess of those of the pure compounds, imply that they are in fact β- and O-sialons respectively. Semi-quantitative energy dispersive X-ray spectroscopy, using an ultra-thin window detector, is demonstrated to be capable of distinguishing clearly between these phases according to their oxygen content and of determining the aluminium content of both phases to within ± 1 equ.%, even at concentration levels of <5 equ.%.     

Absorption coefficients of O3 at CO2 laser wavelengths

The O₃ absorption coefficients for the rotational lines P(12)–P(28) of the 9.4 μm emission band of the CO₂ laser are presented. Measurements were made in O₃–air dilute mixtures (20–600 ppm) at 25°C and a total pressure of 1013.25 h Pa using a frequency stabilized cw CO₂ laser and values have been determined with greater precision than in previously reported studies.     

Photo-operated two-photon absorption effects in the GeSe2–Ga2S3–PbI2 glasses

The possibility to operate the two-photon absorption (TPA) of newly synthesized GeSe₂–Ga₂S₃–PbI₂ glasses using the CO laser beam (λ=5.5 μm) as a photoinducing one has been demonstrated. As the fundamental laser beam we have used the illumination of 10.6 μm passively modulated 0.5 ns CO₂ laser with a rate repetition of about 10 Hz. We have established that the maximal photoinduced TPA is observed for the 8% doped samples (up to 14 cm/GW), which is achieved at a pump CO laser pump power density equal to about 0.6 GW/cm². The undoped PbI₂ samples show the TPA maximum at a pump power density of about 0.2 cm/GW. The minimal TPA values were observed for the samples with 5% of PbI₂. The obtained results show that these materials can be used as effective optically operated optical limiters.     

Rigorous reflectance performance analysis of Si3N4 self-suspended subwavelength gratings

In this paper, we make an investigation into the reflectance performance of the Si₃N₄ self-suspended subwavelength grating (SSG) via changing several parameters including the incident wavelength, the grating period, the grating thickness and the grating filling ratio. For TM polarization of incidence, numerical results by the rigorous coupled-wave method reveal that the reflectance spectrum of the Si₃N₄ SSG has a narrow linewidth (less than 1 nm), high peak reflectance (100%), and low side bands (less than 0.5%) simultaneously in the optical telecommunication wavelength region. By using a micro-electromechanical-systems actuator, it is believed that the Si₃N₄ SSG should serve as tunable optical filters in optical telecommunication systems.     

The influence of different fabrication processes on characteristics of excess Bi2O3-doped 0.95 (Na0.5Bi0.5)TiO3–0.05 BaTiO3 ceramics

In this study, we will develop the influences of the excess x wt% (x=0, 1, 2, and 3) Bi₂O₃-doped and the different fabricating process on the sintering and dielectric characteristics of 0.95 (Na_0.5Bi_0.5)TiO₃–0.05 BaTiO₃ ferroelectric ceramics with the aid of SEM and X-ray diffraction patterns, and dielectric–temperature curves. The 0.95 (Na_0.5Bi_0.5)TiO₃–0.05 BaTiO₃+x wt% Bi₂O₃ ceramics are fabricated by two different processes. The first process is that (Na_0.5Bi_0.5)TiO₃ composition is calcined at 850 °C and BaTiO₃ composition is calcined at 1100 °C, then the calcined (Na_0.5Bi_0.5)TiO₃ and BaTiO₃ powders are mixed in according to 0.95 (Na_0.5Bi_0.5)TiO₃–0.05 BaTiO₃+x wt% Bi₂O₃ compositions. The second process is that the raw materials are mixed in accordance to the 0.95 (Na_0.5Bi_0.5)TiO₃–0.05 BaTiO₃+x wt% Bi₂O₃ compositions and then calcining at 900 °C. The sintering process is carried out in air for 2 h from 1120 to 1240 °C. After sintering, the effects of process parameters on the dielectric characteristics will be developed by the dielectric–temperature curves. Dielectric–temperature properties are also investigated at the temperatures of 30–350 °C and at the frequencies of 10 kHz–1 MHz.     

InGaAs/Si3N4 interface obtained in ultrahigh vacuum multipolar plasma: In-situ control by ellipsometry and electrical characterization

A multipolar plasma passivation scheme controlled by in-situ ellipsometry has been developed to produce a high electrical quality InGaAs/Si₃N₄ interface. We have demonstrated the possibility to remove all native oxides at the InGaAs surface by heating the sample at 240°C, then using the action of a multipolar H₂ plasma at 185°C, without optical degradation of the surface. The passivation by a native nitride layer is then performed using a N₂ plasma, and Si₃N₄ is deposited. The treatment induces a reduction of the density of interface states N_ss(E), and also a change of the nature of these interface states as shown by a reduction in the capture cross section σ_n(E).     

电荷俘获存储器的过擦现象

基于密度泛函理论的第一性原理平面波超软赝势方法和VASP软件对电荷俘获存储器过擦现象进行了分析研究.通过形成能的计算,确定了含有氮空位缺陷的Si3N4和含有间隙氧缺陷的Hf O2作为研究的对象;俘获能的计算结果表明两种体系对电子的俘获能力比对空穴的大,因而对两体系擦写载流子确定为电子.分别计算了Hf O2和Si3N4擦写前后的能量、擦写前后电荷分布变化、吸附能和态密度,以说明过擦的微观机理.对能量和擦写电荷变化的研究,表明Si3N4相比于Hf O2,其可靠性较差,且Si3N4作为俘获层,在一个擦写周期后,晶胞中电子出现减少现象;界面吸附能的研究表明,Si3N4相比于Hf O2在缺陷处更容易与氧进行电子交换;最后,通过对态密度的分析表明Si3N4和Hf O2在对应的缺陷中均有缺陷能级俘获电子,前者为浅能级俘获,后者为深能级俘获.综上分析表明,Si3N4在氮空位的作用下,缺陷附近原子对电子的局域作用变弱,使得Si3N4作为俘获层时,材料本身的电子被擦出,使得擦操作时的平带偏移电压增大,导致存储器发生过擦.本文的研究结果揭示了过擦的本质,对提高电荷俘获存储器的可靠性以及存储特性有着重要的指导意义.     

Effects of substitution of Ti for Fe in BiFeO3 films prepared by sol–gel process

Ti substituted BiFe_1−xTi_xO_3+δ films have been prepared on indium–tin oxide (ITO)/glass substrates by the sol–gel process. The films with x=0.00–0.20 were prepared at an annealing temperature of 600 °C. X-ray diffraction patterns indicate that all films adopt R3m structure and the films with x=0 and 0.10 show pure perovskite phase. Cross-section scanning shows the thickness of the films is about 300 nm. Through 0.05 Ti substitution, the 2P_r increases to 8.30 μC/cm² from 2.12 μC/cm² of the un-substituted BiFeO₃ film and show enhanced ferroelectricity at room temperature. The 2P_r values are 2.63 and 0.44 μC/cm² for the films with x=0.01 and 0.2, respectively. Moreover, the films with x=0.05 and 0.10 show enhanced dielectric property since the permittivity increases near 150 at the same measuring frequency. Through the substitution of Ti, the leakage conduction is reduced for the films with x=0.05–0.20.     

Green and red up-conversion emissions of Er–Yb Co-doped TiO2 nanocrystals prepared by sol–gel method

In this paper, green and red up-conversion emissions of Er³⁺–Yb³⁺ co-doped TiO₂ nanocrystals were reported. The phase structure, particle size and optical properties of Er³⁺–Yb³⁺ co-doped TiO₂ nanocrystals samples were characterized by using X-ray diffraction (XRD), transmission electron microscopy (TEM), UV–vis–NIR absorption spectra and photoluminescence (PL) spectra. Green and red up-conversion emissions in the range of 520–570 nm (²H_11/2, ⁴S_3/2→⁴I_15/2) and 640–690 nm (⁴F_9/2→⁴I_15/2) were observed for the Er³⁺–Yb³⁺ co-doped TiO₂ nanocrystals. The visible up-conversion mechanism and temperature dependence of up-conversion emission for Er³⁺ in TiO₂ nanocrystals were discussed in detail.     

Growth and magnetism of Ni nanoparticles in Ni/Al2O3/Si or Si3N4 multilayers

We report on the fabrication of Ni/Al₂O₃/Si and textured Ni/Al₂O₃/Si₃N₄ multilayers containing Ni nanoparticles that exhibit significantly improved results. The secondary phases arising from thermal reaction between Ni and Si can be remarkably suppressed with increasing layers of Al₂O₃ and deposition of Ni/Al₂O₃ multilayers on Si₃N₄ substrates. Atomic force microscopy shows the formation of large as well as nanoclusters of Ni when grown on Si, whereas textured Ni nanoparticles are formed on Si₃N₄ substrates. The magnetization measurements on Ni/Al₂O₃/Si containing a single buffer layer of Al₂O₃ shows higher coercivity field with magnetic nanowire-like behavior, whereas with several Al₂O₃ alternate layers almost a superparamagnetic-like behavior is observed. However, significantly improved magnetic hysteresis was observed in textured Ni/Al₂O₃/Si₃N₄ multilayers due to preferred alignment of Ni nanocrystallites.     

Effect of WO3 on the spectroscopic properties in Er/Yb co-doped bismuth–borate glasses

The spectroscopic properties of Er³⁺/Yb³⁺ co-doped Bi₂O₃–B₂O₃–WO₃ (BBW) glasses were analyzed and discussed. The effect of WO₃ content on the absorption spectra, the Judd–Ofelt parameters Ω_t (t=2, 4, 6), emission spectra and the lifetime of the ⁴I_13/2 level and the quantum efficiency of Er³⁺:⁴I_13/2→⁴I_15/2 transition were also investigated. With the substitution of WO₃ for B₂O₃, the measured lifetime of the ⁴I_13/2 level and the quantum efficiency of Er³⁺:⁴I_13/2→⁴I_15/2 transition increase from 0.98 to 1.31 ms and from 38.2% to 49.2%, respectively. The effective width of emission band and the emission cross-section both decrease slightly. And the emission spectra is analyzed via the different curve (σ_e−σ_a) of BBW glasses, the influence of OH⁻is also discussed.     

紫外波段SiO2/Si3N4介质膜分布式布拉格反射镜的制备与研究

采用光学传递矩阵方法设计了紫外波段SiO₂/Si₃N₄介质膜分布式布拉格反射镜, 并利用等离子体增强化学气相沉积技术在蓝宝石(0001)衬底上制备了SiO₂/Si₃N₄介质膜分布式布拉格反射镜. 光反射测试表明, 样品反射谱的峰值波长仅与理论模拟谱线相差10 nm, 并随着反射镜周期数的增加而蓝移. 由于SiO₂与Si₃N₄具有相对较大的折射率比, 因而制备的周期数为13的样品反射谱的峰值反射率就已大于99%. 样品反射谱的中心波长为333 nm, 谱峰的半高宽为58 nm. 样品截面的扫描电子显微镜和表面的原子力显微镜测量结果表明, 样品反射谱的中心波长蓝移是由子层的层厚和界面粗糙度的变化引起的. X射线反射谱表明,子层界面过渡层对于反射率的影响较小, 并且SiO₂膜的质量比Si₃N₄差, 也是造成反射率低于理论值的原因之一.     

Structural and electrical properties of crystalline (1−x)Ta2O5−xTiO2 thin films fabricated by metalorganic decomposition

Polycrystalline (1−x)Ta₂O₅−xTiO₂ thin films were formed on Si by metalorganic decomposition (MOD) and annealed at various temperatures. As-deposited films were in the amorphous state and were completely transformed to crystalline after annealing above 600 °C. During crystallization, a thin interfacial SiO₂ layer was formed at the (1−x)Ta₂O₅−xTiO₂/Si interface. Thin films with 0.92Ta₂O₅–0.08TiO₂ composition exhibited superior insulating properties. The measured dielectric constant and dissipation factor at 1 MHz were 9 and 0.015, respectively, for films annealed at 900 °C. The interface trap density was 2.5×10¹¹ cm⁻² eV⁻¹, and flatband voltage was −0.38 V. A charge storage density of 22.8 fC/μm² was obtained at an applied electric field of 3 MV/cm. The leakage current density was lower than 4×10⁻⁹ A/cm² up to an applied electric field of 6 MV/cm.     

Study on the structure dependent ultrafast third-order optical nonlinearity of GeS2–In2S3 chalcogenide glasses

The results of the femtosecond time-resolved optical Kerr at 820 nm in GeS₂–In₂S₃ chalcogenide glasses indicate that the response time in GeS₂–In₂S₃ glasses is subpicosecond, which is predominantly due to the distortion of the electron cloud. The value of χ⁽³⁾ in 0.95GeS₂–0.05In₂S₃ glass is also as large as 2.7 × 10⁻¹³ esu, and it reduces with the addition of In₂S₃, which may be ascribed to the microstructure evolution of GeS₂–In₂S₃ glasses. It is deduced that the intrinsic [Ge(In)S₄] tetrahedral structure units that possess the high hyperpolarizability may do great contribution to the enhancement of third-order optical nonlinearity while [S₃Ge–GeS₃] ethane-like molecular units make no considerable contribution to that in femtosecond time scale. These GeS₂–In₂S₃ and GeS₂–In₂S₃-based chalcogenide glasses would be expected to be the promising materials for all-optical switching devices.     

Enhanced luminescence from transparent Ni-doped MgO–Al2O3–SiO2 glass ceramics by Ga2O3 addition

Transparent Ni²⁺-doped MgO–Al₂O₃–SiO₂ glass ceramics without and with Ga₂O₃ were synthetized. The precipitation of spinel nanocrystals, which was identified as solid solutions in the glass ceramics, could be favored by Ga₂O₃ addition and their sizes were about 7.6 nm in diameter. The luminescent intensity of the Ni²⁺-doped glass ceramics was largely enhanced by Ga₂O₃ addition which could mainly be caused by increasing of Ni²⁺ in the octahedral sites and the reduction of the mean frequency of phonon density of states in the spinel nanocrystals of solid solutions. The full width at half maximum (FWHM) of emissions for the glass ceramics with different Ga₂O₃ content was all more than 200 nm. The emission lifetime increased with the Ga₂O₃ content and the longest lifetime is about 250 μs. The Ni²⁺-doped transparent glass ceramics with Ga₂O₃ addition have potential application as broadband optical amplifier and laser materials.     

RF-sputtered CrB2 diffusion barrier for Ni/Au Ohmic contacts on p-CuCrO2

Ohmic contacts to p-type CuCrO₂ using Ni/Au/CrB₂/Ti/Au contact metallurgy are reported. The samples were annealed in the 200–700 °C range for 60 s in flowing oxygen ambient. A minimum specific contact resistance of 2 × 10⁻⁵ Ω cm² was obtained after annealing at 400 °C. Further increase in the annealing temperature (>400 °C) resulted in the degradation of contact resistance. Auger Electron Spectroscopy (AES) depth profiling showed that out-diffusion of Ti to the surface of the contact stacks was evident by 400 °C, followed by Cr at higher temperature. The CrB₂ diffusion barrier decreases the specific contact resistance by almost two orders of magnitude relative to Ni/Au alone.     

Magnetic properties of Lu2Co17−xSix single crystals

The Co-sublattice anisotropy in Lu₂Co₁₇ consists of four competitive contributions from Co atoms at crystallographically different sites in the Th₂Ni₁₇-type of crystal structure, which result in the appearance of a spontaneous spin-reorientation transition (SRT) from the easy plane to the easy axis at elevated temperatures. In order to investigate this SRT in detail and to study the influence of Si substitution for Co on the magnetic anisotropy, magnetization measurements were performed on single crystals of Lu₂Co_17−xSi_x (x=0−3.4) grown by the Czochralski method. The SRT in Lu₂Co₁₇ was found to consist of two second-order spin reorientations, “easy-plane”–“easy-cone” at T_SR1≈680 K and “easy-cone”–“easy-axis” at T_SR2≈730 K. Upon Si substitution for Co, both SRTs shift toward the lower temperatures in Lu₂Co₁₆Si (T_SR1≈75 K and T_SR2≈130 K) with the further onset of the uniaxial type of magnetic anisotropy in the whole range of magnetic ordering for Lu₂Co_17−xSi_x compounds with x>1 due to a weakening of the easy-plane contribution from the Co atoms at the 6g and 12k sites to the total anisotropy.     

Na2TiGeO5: Crystal structure stability at low temperature and high pressure

The temperature evolution of the lattice parameters measured from 295 to 125 K exhibits a small instability below T_c≈278 K, indicating ferroelastic properties of Na₂TiGeO₅. The behavior is related to the specific crystal structure built of polyhedral layers with shared TiO₅ pyramids and GeO₄ tetrahedra, alternating with layers of Na⁺ cations. Antiparallel alignment of the short apical titanyl bond in adjacent rows of the polyhedral layer gives rise to spontaneous strain, when a distortion of the TiO₅ groups occurs. Single-crystal structures determined at room temperature and 120 K suggest that {1 1 0} domains, developing below T_c, entail a tetragonal-to-orthorhombic symmetry change. The mechanism is attributed to a shortening of the O–O distance between the polyhedral layers, and to minor shifts of the positions of the Ti atoms and the correlated oxygen atoms along the c-axis. The structure distortion, however, is too small to allow any unambiguous determination of the symmetry-breaking effects. The bulk modulus and its pressure derivative have been determined as B₀=89(2) GPa and . A pressure-induced phase transformation takes place at P_c≈12.5 GPa, presumably to an orthorhombic structure. The pressure effect on the transition temperature is given by ΔT_c/ΔP≈1.76 K/GPa.     

Effects of precursors on nucleation in atomic layer deposition of HfO2

Atomic layer deposition of hafnium dioxide (HfO₂) on silicon substrates was studied. It was revealed that due to low adsorption probability of HfCl₄ on silicon substrates at higher temperatures (450–600 °C) the growth was non-uniform and markedly hindered in the initial stage of the HfCl₄–H₂O process. In the HfI₄–H₂O and HfI₄–O₂ processes, uniform growth with acceptable rate was obtained from the beginning of deposition. As a result, the HfI₄–H₂O and HfI₄–O₂ processes allowed deposition of smoother, more homogeneous and denser films than the HfCl₄–H₂O process did. The crystal structure developed, however, faster at the beginning of the HfCl₄–H₂O process.     

特殊构型Si_2N_2分子团簇电致激发特性的密度泛函理论研究

以在可见光区有吸收峰的Cs构型的Si2N2分子团簇为研究对象,利用密度泛函B3LYP方法,在aug-ccpVTZ基组水平下优化得到了处于不同外电场中的Si2N2分子团簇的稳定结构.分析发现:在不同的外电场中,Si2N2分子构型对称性没有发生改变,均为Cs对称性,且都有6种振动模式;随着外电场强度的逐渐增大,Si2N2分子振动频率较低的前三种振动模式的频率略有减小,而后三种振动模式的频率逐渐增加;随着外电场强度的逐渐增大,在一定电场范围内最高占据分子轨道与最低空分子轨道的能隙值出现振荡,之后能隙值随着外电场强度的增大而减小.在此基础上,采用含时密度泛函TD-B3LYP方法研究了外电场对Si2N2分子吸收谱的影响规律.计算得到的吸收谱范围在紫外-可见光区,这与实验值相符合.随着外电场强度的逐渐增大,在可见光区吸收谱发生红移,最大跃迁振子强度逐渐增大.结果表明,施加外电场有利于Si2N2分子在可见光区的吸收,也有利于操控分子特定激发态的电子状态,进而调节相应的跃迁光谱特性,可达到获得所需特定波长的要求.