Investigation of a 4H-SiC metal-insulation-semiconductor structure with an Al2O3/SiO2 stacked dielectric

Atomic layer deposited(ALD) Al2O3 /dry-oxidized ultrathin SiO2 films as a high-k gate dielectric grown on 8°off-axis 4H-SiC(0001) epitaxial wafers are investigated in this paper.The metal-insulation-semiconductor(MIS) capacitors,respectively with different gate dielectric stacks(Al2O3/SiO2,Al2O3,and SiO2) are fabricated and compared with each other.The I-V measurements show that the Al2O3/SiO2 stack has a high breakdown field(≥12 MV/cm) comparable to SiO2,and a relatively low gate leakage current of1×10-7A/cm2 at an electric field of4 MV/cm comparable to Al2O3.The 1-MHz high frequency C-V measurements exhibit that the Al2O3/SiO2 stack has a smaller positive flat-band voltage shift and hysteresis voltage,indicating a less effective charge and slow-trap density near the interface.     

Microbiological identification by surface-enhanced Raman spectroscopy

New methods for pathogens identification are of growing interest in clinical and food sectors. The challenge remains to develop rapid methods that are more simple, reliable, and specific. Surface-enhanced Raman spectroscopy (SERS) appears to be a promising tool to compete with current untargeted identification methods. This article presents the intensive research devoted to the use of SERS for bacterial identification, from the first to the very recent published results. Compared to normal Raman spectroscopy, the introduction of nanoparticles for SERS acquisition introduces a new degree of complexity. Bacterial Raman fingerprints, which are already subject to high spectral variability for a given strain, become then very dependent on numerous experimental parameters. To overcome these limitations, several approaches have been proposed to prepare the sample, from the microbiological culture conditions to the analysis of the spectrum including the coupling of nanoparticles on the bacterial membrane. Main strategies proposed over the last 20 years are examined here and discussed in the perspective of a protocol transfer towards industry.     

Annealing effects on the optical and structural properties of Al2O3/SiO2 films as UV antireflection coatings on 4H-SiC substrates

Al₂O₃/SiO₂ films have been prepared by electron-beam evaporation as ultraviolet (UV) antireflection coatings on 4H-SiC substrates and annealed at different temperatures. The films were characterized by reflection spectra, ellipsometer system, atomic force microscopy (AFM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. As the annealing temperature increased, the minimum reflectance of the films moved to the shorter wavelength for the variation of refractive indices and the reduction of film thicknesses. The surface grains appeared to get larger in size and the root mean square (RMS) roughness of the annealed films increased with the annealing temperature but was less than that of the as-deposited. The Al₂O₃/SiO₂ films maintained amorphous in microstructure with the increase of the temperature. Meanwhile, the transition and diffusion in film component were found in XPS measurement. These results provided the important references for Al₂O₃/SiO₂ films annealed at reasonable temperatures and prepared as fine antireflection coatings on 4H-SiC-based UV optoelectronic devices.     

Determination of the transport properties in 4H-SiC wafers by Raman scattering measurement

The free carrier density and mobility in n-type 4H-SiC substrates and epilayers were determined by accurately analysing the frequency shift and the full-shape of the longitudinal optic phonon--plasmon coupled (LOPC) modes, and compared with those determined by Hall-effect measurement and that provided by the vendors. The transport properties of thick and thin 4H-SiC epilayers grown in both vertical and horizontal reactors were also studied. The free carrier density ranges between 2×10¹⁸ cm^-3 and 8×10¹⁸ cm^-3 with a carrier mobility of 30--55 cm²/(V·s) for n-type 4H-SiC substrates and 1×10¹⁶--3×10¹⁶ cm^-3 with mobility of 290--490 cm²/(V·s) for both thick and thin 4H-SiC epilayers grown in a horizontal reactor, while thick 4H-SiC epilayers grown in vertical reactor have a slightly higher carrier concentration of around 8.1×10¹⁶ cm^-3 with mobility of 380 cm²/(V·s). It was shown that Raman spectroscopy is a potential technique for determining the transport properties of 4H-SiC wafers with the advantage of being able to probe very small volumes and also being non-destructive. This is especially useful for future mass production of 4H-SiC epi-wafers.     

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.     

N型4H-SiC低温拉曼光谱特性

利用拉曼散射技术对N型4H-SiC单晶材料进行了30～300 K温度范围的光谱测量。实验结果表明,随着温度的升高,N型4H-SiC单晶材料的拉曼峰峰位向低波数方向移动,峰宽逐渐增宽。分析认为,晶格振动随着温度的升高而随之加剧,其振动恢复力会逐渐减小,使振动频率降低;原子相对运动会随温度的升高而加剧,使得原子之间及晶胞之间的相互作用减弱,致使声学模和光学模皆出现红移现象。随着温度的升高,峰宽逐渐增宽。这是由于随着温度的升高声子数逐渐增加,增加的声子进一步增加了散射概率,从而降低了声子的平均寿命,而声子的平均寿命与峰宽成反比,因此随着温度的升高峰宽逐渐增宽。声子模强度随温度升高呈现不同规律,E₂(LA),E₂(TA),E₁(TA)和A₁(LA)声子模随着温度升高强度单调增加,而E₂(TO),E₁(TO)和A₁(LO)声子模强度出现了先增后减的明显变化,在138 K强度出现极大值。分析认为造成原因是由于当温度高于138 K时,高能量的声子分裂成多个具有更低能量的声子所致。     

Evaluation of Si3N4/Si interface by UV Raman spectroscopy

The stresses at Si₃N₄/Si (1 0 0), (1 1 1) and (1 1 0) interfaces were measured by UV Raman spectroscopy with a 364 nm excitation laser whose penetration depth into the Si substrate was estimated to be 5 nm. The Si₃N₄ films were formed on Si (1 0 0), (1 1 1) and (1 1 0) using nitrogen-hydrogen (NH) radicals produced in microwave-excited high-density Xe/NH₃ mixture plasma. The localized stress detected from Raman peak shift was compressive at the (1 0 0) interface, and tensile at the (1 1 1) and (1 1 0) interfaces. The results showed that stress had strong correlation with the total density of subnitrides at the Si₃N₄/Si interface, and also with the full-width at half-maximum (FWHM) of Si the 2p_3/2 photoemission spectrum arising from the substrate. We believe that the localized stress affected subnitride formation because the amount of subnitride and the FWHM of Si 2p_3/2 decreased while the interface stress shifted in the tensile direction.     

Nitridation of thin SiO2 films in N2 and NH3 plasmas

A low-temperature (700°C) plasma-enhanced nitridation process which improves the dielectric breakdown of thin silicon dioxide (SiO₂) layers is presented. It uses a new, production compatible, parallel plate plasma reactor working at low RF frequencies. Nitrided oxides produce less charge trapping under high field stress, higher breakdown charge and a tighter distribution of breakdown fields than pure SiO₂. More nitrogen is incorporated in films treated in a NH₃ plasma than in a N₂ plasma. However, the latter present better electrical properties.     

SiO2 surface defect centers studied by AES

A theoretical model is proposed on how a Si dangling bond associated with an oxygen vacancy on a SiO₂ surface (E_s′ center) should be observed by Auger electron spectroscopy (AES). The Auger electron distribution N_A(E) for the L₂₃VV transition band is calculated for a stoichiometric SiO₂ surface, and for a SiO_x surface containing Si-(e^?O₃) coordinations. The latter is characterized by an additional L₂₃VD transition band, where D is the energy level of the unpaired electron e^?. The theoretical N_A(E) spectra are compared with experimental N(E) spectra for a pristine, and for an electron radiation damaged quartz surface. Agreement with the theoretical model is obtained if D is assumed to lie ≈2 eV below the conduction band edge. This result shows that AES is uniquely useful in revealing the absolute energy level of localized, occupied surface defect states. As the L₂₃VD transition band (main peak at 86 eV) cannot unambiguously be distinguished from a SiSi₄ coordination L₂₃VV spectrum (main peak at 88 eV), supporting evidence is presented as to why we exclude a SiSi₄ coordination for our particular experimental example. Application of the Si-(e^?O₃) model to the interpretation of SiO₂Si interface Auger spectra is also discussed.     

NH4H2PO4和ND4D2PO4晶体微结构的拉曼光谱研究

本文利用偏振拉曼光谱和第一性原理, 对磷酸二氢铵(NH₄H₂PO₄, ADP)和不同氘含量磷酸二氢铵DADP晶体的晶格振动模式进行了研究. 实验测得了不同几何配置、200–4000 cm^-1范围的偏振拉曼光谱, 分析在不同氘含量条件下921 cm^-1和3000 cm^-1附近拉曼峰的变化. 在ADP晶体中, 基于基本结构单元NH₄⁺ 和H₂PO₄^-基团的振动模, 用第一性原理进行了数值模拟, 进一步明确拉曼峰与晶体中原子振动的对应关系; 通过洛伦兹拟合不同氘含量DADP晶体的拉曼光谱中2000–2600 cm^-1处各峰的变化讨论了DADP 晶体的氘化过程, 结果表明氘化顺序是先NH₄⁺ 基团后H₂PO₄^-基团, 研究结果为今后此类材料的生长和性能优化奠定了基础.     

Al2O3/SiO2 films prepared by electron-beam evaporation as UV antireflection coatings on 4H-SiC

Al₂O₃/SiO₂ films have been deposited as UV antireflection coatings on 4H-SiC by electron-beam evaporation and characterized by reflection spectrum, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The reflectance of the Al₂O₃/SiO₂ films is 0.33% and 10 times lower than that of a thermally grown SiO₂ single layer at 276 nm. The films are amorphous in microstructure and characterize good adhesion to 4H-SiC substrate. XPS results indicate an abrupt interface between evaporated SiO₂ and 4H-SiC substrate free of Si-suboxides. These results make the possibility for 4H-SiC based high performance UV optoelectronic devices with Al₂O₃/SiO₂ films as antireflection coatings.     

Study of the interface Si-nc/SiO2 by infrared spectroscopic ellipsometry and X-ray photoelectron spectroscopy

SiO_x films (1<x<2), 0.5 μm thick, have been elaborated by electron-gun evaporation. A thermal annealing of these films induced a phase separation leading to the formation of Si nanocrystals embedded in a SiO₂ matrix. These films have been studied by infrared spectroscopic ellipsometry and by X-ray photoelectron spectroscopy (XPS). The effective dielectric function of the thin films has been extracted in the 600–5000 cm⁻¹ range which allowed us to deduce the dielectric function of the matrix surrounding the Si-nc. A study of the Transverse Optical (TO) vibration mode has revealed the presence of SiO_x into the matrix. Before XPS measurements, the films have been etched in fluorhydric acid to remove the superficial SiO₂ layer formed during air exposure. The Si 2p core-level emission has been recorded. The decomposition of the Si 2p peak into contributions of the usual five tetrahedrons Si-(Si_4−nO_n) (n=0–4) has also revealed the presence of a SiO_x phase. Consistency between infra-red and XPS results is discussed.     

Self-assembly 2D plasmonic nanorice film for surface-enhanced Raman spectroscopy

As an ultrasensitive sensing technology, the application of surface enhanced Raman spectroscopy (SERS) is one interesting topic of nano-optics, which has huge application prospectives in plenty of research fields. In recent years, the bottleneck in SERS application could be the fabrication of SERS substrate with excellent enhancement. In this work, a two-dimensional (2D) Ag nanorice film is fabricated by self-assembly method as a SERS substrate. The collected SERS spectra of various molecules on this 2D plasmonic film demonstrate quantitative detection could be performed on this SERS substrate. The experiment data also demonstrate this 2D plasmonic film consisted of anisotropic nanostructures has no obvious SERS polarization dependence. The simulated electric field distribution points out the SERS enhancement comes from the surface plasmon coupling between nanorices. And the SERS signals is dominated by molecules adsorbed at different regions of nanorice surface at various wavelengths, which could be a good near IR SERS substrate for bioanalysis. Our work not only enlarges the surface plasmon properties of metal nanostructure, but also exhibits the good application prospect in SERS related fields.     

复合Ag/SiO_2正弦光栅基底SERS特性分析

当前微流控表面增强拉曼散射(SERS)检测领域常用的贵金属纳米颗粒溶胶单位体积内热点区域数量有限且热点区域范围较小,而贵金属纳米三维阵列结构加工时间长,成本高昂并存在"记忆效应"。本文提出了集成到微流道的复合Ag/SiO_2正弦光栅SERS基底结构,可以利用激光干涉光刻技术进行制备,无需预制掩膜版,可实现大面积、低成本SERS基底简易快速制备。利用严格耦合波分析方法(RCWA)建立了复合正弦光栅表面电场增强数学评估模型,推导了表面等离子体共振(SPP)耦合吸收率数学模型,分析了入射光、复合正弦光栅结构与外界环境介电常数的优化匹配关系,得到了入射光785 nm条件下的最佳复合正弦光栅结构。通过制备加工并实验验证了复合正弦光栅的SERS性能,SERS增强因子(EF)能够达到10~4。     

压力对纳米CoFe2O4/SiO2复合材料结构的影响

 采用溶胶-凝胶工艺和高温高压实验技术,制备了纳米CoFe₂O₄/SiO₂复合材料。利用X射线衍射仪、扫描电子显微镜和振动样品磁强计,对样品的结构、微观形貌和磁性进行了研究,并对CoFe₂O₄中阳离子的占位情况进行了讨论。结果表明,随着处理压力的升高,样品的晶粒尺寸增大,晶格常数减小,比饱和磁化强度增大。通过计算结果可以推断,压力的升高导致CoFe₂O₄中的部分Fe³⁺从A位移向了B位,而部分Co²⁺则从B位移向了A位。     

Phonon confinement effect on nanocrystalline LiCoO2 studied with Raman spectroscopy

A systematic investigation on nanocrystalline LiCoO₂ has been carried out using Raman spectroscopy. We synthesized nanocrystalline LiCoO₂ (ca. 20-50 nm) through a combination of rapid thermal annealing at various annealing temperatures and a sol-gel method assisted with a triblock copolymer surfactant. Powder X-ray diffraction measurements revealed the formation of LiCoO₂. The crystallite size of LiCoO₂ from the Scherrer equation strongly depended on the annealing temperature. The crystallite size was confirmed by SEM and TEM measurements. Raman shifts of the A_1g and E_g modes for nanocrystalline LiCoO₂ exhibited a broadening and a frequency shift according to the crystallite size. While the frequency shift could be ascribed to a structural strain at the surface, the broadening was due to the phonon confinement effect produced by narrow crystal boundaries.     

A facile synthesis of monodisperse CoFe2O4/SiO2 nanoparticles

Aminated-CoFe₂O₄/SiO₂ magnetic nanoparticles (NPs) were prepared from primary silica particles using modified StÖber method. By optimizing the preparation conditions, monodisperse CoFe₂O₄/SiO₂ NPs with high amino groups’ density were obtained, which is necessary for enzyme immobilization. TEM confirm that the sample is a core/shell structure. These aminated-CoFe₂O₄/SiO₂ NPs have narrow size distributions with a mean size of about 60 nm. Moreover, the aminated-CoFe₂O₄/SiO₂ NPs can be easily dispersed in aqueous medium. The experimental results also show that the NPs have superparamagnetism, indicating that the aminated-CoFe₂O₄/SiO₂ NPs can be used as an effective carrier for the enzyme immobilization.     

Detection of fentanyl in binary mixtures with cocaine by use of surface-enhanced Raman spectroscopy

Abstract

The ongoing epidemic pertaining to overdose deaths has been attributed to the synthetic opioid fentanyl due to its use as an adulterant in other, less potent drugs of abuse. Detection of low quantities of fentanyl would, therefore, be extremely useful in a forensic science laboratory. While Raman spectroscopy is particularly effective at distinguishing between classes of drugs, weak signatures can prove difficult when dealing with microscopic samples. Surface-enhanced Raman scattering spectroscopy provides the enhancement necessary to make Raman a viable approach for the detection of small amounts of fentanyl. This work explores the use of a paper-based substrate loaded with silver nanoparticles for the recovery of small quantities of fentanyl in cocaine, where it was identified at a lower limit of 500?ng (～65?ppm) in mixtures. Linear relationships were investigated between intensity and concentration for diagnostic peaks associated with fentanyl and cocaine, which in turn sheds light on the attenuation of the enhancement intensity as a result of competitive binding to silver nanoparticles. This work demonstrates a potentially simple and qualitative pathway for the forensic analysis of fentanyl as an adulterant in cocaine.     

Propene-V2O2 interactions studied by infrared emission spectroscopy

Infrared emission spectroscopy was used to investigate the interactions between propene and vanadium hemipentoxide. The spectrum obtained for V₂O₅ alone was very similar to the spectrum given by KBr disc transmission method. Reaction of propene on V₂O₅ was performed between 110 and 250°C; IR spectra were recorded in situ. Reduction of the oxide occurred and its bands were strongly altered. The bands attributed to the terminal oxygen (1018 cm^?1) and to the doubly bridged oxygen (820 cm^?1) were mainly affected by the reaction with propene. For a reaction temperature of 250°C, the recorded spectrum was close to that given by hydrogen reduction at 230°C. It can be concluded that V₂O₅ was reduced by propene with the formation of a superficial suboxide. By oxygen treatment, the reduced form was restored to the initial V₂O₅ sample. Infrared emission spectroscopy appears as a very suitable method for studying the interactions of the reactants with the catalysts.     

Band alignment of Ga2O3/6H-SiC heterojunction

A high-quality Ga₂O₃ thin film is deposited on an SiC substrate to form a heterojunction structure. The band alignment of the Ga₂O₃/6H-SiC heterojunction is studied by using synchrotron radiation photoelectron spectroscopy. The energy band diagram of the Ga₂O₃/6H-SiC heterojunction is obtained by analysing the binding energies of Ga 3d and Si 2p at the surface and the interface of the heterojunction. The valence band offset is experimentally determined to be 2.8 eV and the conduction band offset is calculated to be 0.89 eV, which indicate a type-II band alignment. This provides useful guidance for the application of Ga₂O₃/6H-SiC electronic devices.