Lead titanate ferroelectric films on single-crystal silicon

This paper reports on the results of investigations into the phase transformations observed in Pb/Ti/Si and Ti/Pb/Si thin-film heterostructures upon layer-by-layer magnetron sputtering of lead and titanium onto a single-crystal silicon substrate and subsequent annealing in an oxygen atmosphere. It is shown that the dielectric properties of lead titanate films depend on the order of sputtering of lead and titanium metal layers onto the surface of single-crystal silicon. The ferroelectric properties are revealed in 3000-nm-thick lead titanate films prepared by two-stage annealing of the Pb/Ti/Si thin-film heterostructure (with the upper lead layer) at T ₁=473 K and T ₂=973 K for 10 min. These films are characterized by the coercive field E _c=4.8 kV/cm and the spontaneous polarization P _s=16.8 μC/cm². The lead titanate films produced by annealing of the Ti/Pb/Si thin-film heterostructure (with the upper titanium layer) do not possess ferroelectric properties but exhibit properties of a conventional dielectric. __________ Translated from Fizika Tverdogo Tela, Vol. 44, No. 4, 2002, pp. 745–749. Original Russian Text Copyright ? 2002 by Sidorkin, Sigov, Khoviv, Yatsenko, Logacheva.     

Direct formation of graphene layers on top of SiC during the carburization of Si substrate

We grow graphene film on silicon substrates having various orientations by simple heating in the presence of carbon source gas. We observed that a 3C-SiC (111) film would form upon carburizing silicon with carbon deposited from a carbon source because it is well lattice-matched with Si (110) (less than 2%). Graphene grew on the buffer layer of 3C-SiC (111). The surface consists of hexagonal arrays that can act as a template for graphene growth. This simple and inexpensive method of forming graphene on silicon wafer in situ is compatible with silicon technology.     

Electrical behaviors of ZnO/graphene/Si and ZnO/graphene/PS composite structures obtained by spraying different volumes of graphene solution

In this paper, a novel ZnO/graphene/porous silicon hybrid device is fabricated and its electrical behaviors are studied along with a ZnO/graphene/silicon device. Graphene (G) is prepared by exfoliation of graphite foil in aqueous solution of inorganic salt. Porous silicon (PS) is fabricated by electrochemical etching of p-type silicon (Si). Graphene is deposited on the surface of Si and PS substrates by thermal spray pyrolysis method. ZnO rods are grown on the samples by using catalyst-free chemical vapor transport and condensation method. The current–voltage relationships of ZnO/G/Si and ZnO/G/PS devices are studied under different volumes of graphene solution. The results reveal the distinctive features of the I–V characteristics of the two devices for different volumes of graphene solution under room light as well as UV illumination.     

金属衬底上高质量大面积石墨烯的插层及其机制

石墨烯作为一种新型二维材料,因其优异的性质,在科学和应用领域具有非常重要的意义.而其超高的载流子迁移率、室温量子霍尔效应等,使其在信息器件领域备受关注.如何获得高质量并且与当代硅基工艺兼容的石墨烯功能器件,是未来将石墨烯应用于电子学领域的关键.近年来,研究人员发展了一种在外延石墨烯和金属衬底之间实现硅插层的技术,将金属表面外延石墨烯高质量、大面积的特点与当代硅基工艺结合起来,实现了无需转移且无损地将高质量石墨烯置于半导体之上.通过系统的实验研究并结合理论计算,揭示了插层过程包含四个主要阶段:诱导产生缺陷、异质原子插层、石墨烯自我修复和异质原子扩散成膜,并证实了这一插层机制的普适性.拉曼和角分辨光电子能谱实验结果表明,插层后的石墨烯恢复了本征特性,接近自由状态.此外,还实现了多种单质元素的插层.不同种类的原子形成不同的插层结构,从而构成了多种石墨烯/插层异质结.这为调控石墨烯的性质提供了实验基础,也展现了该插层技术的普适性.     

Enhanced Raman scattering of graphene on Ag nanoislands

The effect of Ag nanoislands on the Raman of graphene was investigated in this work. Compared with that on the bare silicon wafer, Raman enhancement was observed in the graphene film that covered on Ag/Si surface with nanoscale Ag islands, which would be induced by the localized plasmon resonance in Ag nanostructures. The interaction between the graphene sheet and Ag/Si substrate was further studied. The peak shift and line shape of Raman spectroscopy indicated a nonuniform strain distribution in the Ag/Si supported graphene film.     

Low-temperature preparation of SrxBi2+yTa2O9 ferroelectric thin film by pulsed laser deposition and its application to a metal–ferroelectric–nitride–oxide–semiconductor structure

Preferentially (105)-oriented Sr_xBi_2+yTa₂O₉ (SBT) thin films on SiN/SiO₂/p-Si(100) prepared by the pulsed laser deposition (PLD) method at a temperature as low as 400 °C, which is the lowest process temperature for growing SBT ferroelectric thin films on a silicon nitride film. Excess Bi promotes crystallization of the SBT film. A metal-ferroelectric-nitride-oxide-semiconductor (MFNOS) structure, which is very important in ferroelectric gate memory FET, has been fabricated by depositing the SBT film on silicon nitride-oxide-silicon. The MFNOS structures show capacitance-voltage (C-V) hysteresis corresponding to ferroelectric hysteresis. A memory window of the C-V hysteresis is improved, to be as high as 3.5 V in the SBT(400 nm)/SiN_x(7 nm)/SiO₂(18 nm)/Si compared with the window of 2.7 V in the SBT(400 nm)/SiO₂(27 nm)/Si (MFOS), where the thicknesses of their insulator layers are nearly the same. Little degradation is induced in the C-V characteristics of the SiN_x/SiO₂/p-Si structure when depositing the SBT film by PLD at low temperature. It is also found that the SiN_x layer acts as a diffusion barrier against component atoms in the SBT film during its deposition. Finally, the MFNOS structure prepared at the low temperature is very promising for a next-generation ferroelectric gate memory FET.     

硅功能化石墨烯热导率的分子动力学模拟

采用Tersoff势函数与Lennard-Jones势函数,结合速度形式的Verlet算法和Fourier定律,对单层和两层硅功能化石墨烯沿长度方向的导热性能进行了正向非平衡态分子动力学模拟.通过模拟发现,硅原子的加入改变了石墨烯声子的模式、平均自由程和移动速度,使得单层硅功能化石墨烯模型的热导率随着硅原子数目的增加而急剧地减小.在300 K至1000 K温度变化范围内,单层硅功能化石墨烯的热导率呈下降趋势,具有明显的温度效应.对双层硅功能化石墨烯而言,少量的硅原子嵌入,起到了提高热导率的作用,但当硅原子数目达到一定数量后,材料的导热性能下降.     

Structural and electrical properties of reactive magnetron sputtered yttrium-doped HfO_2 films

Hafnium oxide thin films doped with different concentrations of yttrium are prepared on Si(100) substrates at room temperature using a reactive magnetron sputtering system.The effects of Y content on the bonding structure,crystallographic structure,and electrical properties of Y-doped HfO_2 films are investigated.The x-ray photoelectron spectrum(XPS) indicates that the core level peak positions of Hf 4 f and O 1 s shift toward lower energy due to the structure change after Y doping.The depth profiling of XPS shows that the surface of the film is completely oxidized while the oxygen deficiency emerges after the stripping depths have increased.The x-ray diffraction and high resolution transmission electron microscopy(HRTEM) analyses reveal the evolution from monoclinic HfO_2 phase towards stabilized cubic HfO_2 phase and the preferred orientation of(111) appears with increasing Y content,while pure HfO_2 shows the monoclinic phase only.The leakage current and permittivity are determined as a function of the Y content.The best combination of low leakage current of 10-7 A/cm~2 at 1 V and a highest permittivity value of 29 is achieved when the doping ratio of Y increases to 9 mol%.A correlation among Y content,phase evolution and electrical properties of Y-doped HfO_2 ultra-thin film is investigated.     

Intercalation synthesis of cobalt silicide under a graphene layer

The silicon intercalation under single-layer graphene formed on the surface of an epitaxial Co(0001) film was investigated. The experiments were performed under conditions of ultra-high vacuum. The thickness of silicon films was varied within the range of up to 1 nm, and the temperature of their annealing was 500°C. The characterization of the samples was carried out in situ by the methods of low-energy electron diffraction, high-energy-resolution photoelectron spectroscopy using synchrotron radiation, and magnetic linear dichroism in photoemission of Co 3p electrons. New data were obtained on the evolution of the atomic and electronic structure, as well as on the magnetic properties of the system with an increase in the amount of intercalated silicon. It was shown that the intercalation under a graphene layer is accompanied by the synthesis of surface silicide Co₂Si and a solid solution of silicon in cobalt.     

Sensitivity enhancement of graphene coated surface plasmon resonance biosensor

In this paper, a highly sensitive surface plasmon resonance biosensor is presented using angular interrogation. Due to low sensitivity of conventional biosensor, graphene/two-dimensional transition metal are used in surface plasmon resonance biosensor to improve the sensitivity. Here, we propose a seven layer model of biosensor which shows by incorporating silicon layer in addition of transition metal dichalcogenides MoS₂ and graphene, the sensitivity of the proposed SPR biosensor can be greatly enhanced than the conventional gold film SPR sensors. It is observed that the highest sensitivity can be obtained by optimizing the structure with 8 nm thickness of silicon layer, one layer of MoS₂ and one layer of graphene. The highest sensitivity of our proposed sensor is 210°/RIU.     

Dependence of the solar cell performance on nanocarbon/Si heterojunctions

Solar cells that combine single-crystalline silicon(Si) with graphene(G) have been widely researched in order to develop next-generation photovoltaic devices. However, the power conversion efficiency(PCE) of G/Si solar cell without chemical doping is commonly low due to the relatively high resistance of graphene. In this work, through combining graphene with carbon nanotube(CNT) networks, we fabricated three kinds of hybrid nanocarbon film/Si heterojunction solar cells in order to increase the PCE of the graphene based Si solar cell. We investigated the characteristics of different nanocarbon film/Si solar cells and found that their performance depends on the heterojunctions. Specifically, a doping-free G-CNT/Si solar cell demonstrated a high PCE of 7.9%, which is nearly equal to the combined value of two individuals(G/Si and CNT/Si). This high efficiency is attributed to the synergistic effect of graphene and CNTs, and can be further increased to 9.1% after applying a PMMA antireflection coating. This study provides a potential way to further improve the Si based heterojunction solar cells.     

Au/PZT/BIT/p-Si异质结的制备与性能研究

采用脉冲激光沉积(PLD)工艺,制备了以Bi₄Ti₃O₁₂(BIT)为过渡阻挡层的Au/PZT/BIT/p-Si异质结.研究了BIT铁电层对Pb(Zr_0.52Ti_0.48)O₃(PZT)薄膜晶相结构、铁电及介电性能的影响,对Au/PZT/BIT/p-Si异质结的导电机制进行了讨论.氧气氛530℃淀积的PZT为多晶铁电薄膜,与直接淀积在Si基片上相比,加入BIT铁电层后PZT铁 关键词： 铁电薄膜 异质结构 脉冲激光沉积(PLD)     

Electrons diffusion study on the nitrogen-doped nanocrystalline diamond film grown by MPECVD method

Nitrogen-doped nanocrystalline diamond (NNCD) films were deposited onto p-type silicon substrates with three different layer structures: (i) directly onto the silicon substrate (NNCD/Si), (ii) silicon with undoped nanocrystalline diamond layer which was deposited in the same way as the above mentioned NNCD by the recipe Ar/CH₄/H₂ with a ratio of 98%/1%/1% (NNCD/NCD/Si), and (iii) silicon wafer with 100 nm thickness SiO₂ layer (NNCD/SiO₂/Si). Atomic force microscopy (AFM), X-ray diffraction (XRD) and Raman spectroscopy were employed to characterize the morphology and microstructure of the as-grown nitrogen-doped diamond films. Silver colloid/silver contacts were made at to measure the current-voltage (I-V) characteristics for the three different structures. Electrons from a CVD reactor hydrogen plasma diffuse toward the p-type silicon substrate during a deposition process under the high temperature (∼800 °C). The study concluded that the SiO₂ layer could effectively prevents the diffusion of electrons.     

Physical and mechanical properties of silicon near the SiO2/Si interface

The influence of an oxide coating on the strength characteristics of single-crystal silicon surface layers is investigated by the microindentation method. It is shown experimentally that a strengthened layer with a thickness of 0.2–0.4 μm and a microhardness of 20–35 GPa, which is two or three times as much as the microhardness of bulk single-crystal silicon, is present near the SiO₂/Si interface. The thickness and microhardness of this layer depends on the growth conditions of the oxide. The formation of this layer is most probably caused by interstitial silicon atoms formed near the SiO₂/Si interface during silicon oxidation.     

Ferroelectric characteristics of MFIS structure with P(VDF–TrFE)/BaTiO3 nanocomposite as ferroelectric layer

A metal–ferroelectric–insulator–semiconductor (MFIS) structure has been made using poly(vinylidene difluoride–trifluoroethylene)/barium titanate [P(VDF–TrFE)/BaTiO₃] nanocomposite as ferroelectric layer, on silicon/silicon dioxide (Si/SiO₂) substrate. Different concentrations of BaTiO₃ were added to P(VDF–TrFE) polymer using bath sonication method, and the films were prepared using spin coating method. The structure was annealed to 120 °C for 2 h and then the top aluminium electrode was deposited by thermal evaporation method. Capacitance–voltage shows an increase in accumulation capacitance as the BaTiO₃ nanoparticle concentrations increases. Dielectric constant was estimated from the capacitance voltage (C–V) characteristics and found to be changing as the concentration of BaTiO₃ is varied. Polarization–electric field analyses show hysteresis behaviour of the nanocomposite. A comparison of MFIS and metal–ferroelectric–semiconductor structures was done with varying ferroelectric film thicknesses. All these results suggest that this polymer nanocomposite can be a promising material which can be used in non-volatile memory devices.     

Modification of the electronic structure of graphene by intercalation of iron and silicon atoms

The ab initio calculations of the electronic structure of low-dimensional graphene–iron–nickel and graphene–silicon–iron systems were carried out using the density functional theory. For the graphene–Fe–Ni(111) system, band structures for different spin projections and total densities of valence electrons are determined. The energy position of the Dirac cone caused by the p _z states of graphene depends weakly on the number of iron layers intercalated into the interlayer gap between nickel and graphene. For the graphene–Si–Fe(111) system, the most advantageous positions of silicon atoms on iron are determined. The intercalation of silicon under graphene leads to a sharp decrease in the interaction of carbon atoms with the substrate and largely restores the electronic properties of free graphene.

     

SiO2/Si衬底上石墨烯的制备与结构表征

在分子束外延(MBE)设备中,利用直接沉积C原子的方法在覆盖有SiO₂的Si衬底(SiO₂/Si)上生长石墨烯,并通过Raman光谱和近边X射线吸收精细结构谱等实验技术对不同衬底温度(500℃,600℃,700℃,900℃,1100℃,1200℃)生长的薄膜进行结构表征.实验结果表明,在衬底温度较低时生长的薄膜是无定形碳,在衬底温度高于700℃时薄膜具有石墨烯的特征,而且石墨烯的结晶质量随着衬底温度的升高而改善,但过高的衬底温度会使石墨烯质量降低.衬底温度为1100℃时结晶质量最好.衬底温度较低时C原子活性较低,难以形成有序的C-sp²六方环.而衬底温度过高时(1200℃),衬底表面部分SiO₂分解,C原子与表面的Si原子或者O原子结合而阻止石墨烯的形成,并产生表面缺陷导致石墨烯结晶变差.     

基于graphene条带的硅纳米结构

采用分子动力学模拟方法研究了graphene条带上生长硅纳米结构的过程,分析了不同温度下硅原子在graphene条带边沿生成的新型纳米结构.研究表明,随机分布的硅原子吸附到锯齿型graphene条带边沿在不同的温度T下可生成不同类型的硅纳米结构：300K≤T<2000K时形成无规则的团簇,2000K≤T≤2800K时形成单原子链结构,2800K<T<3900K时形成含缺陷的硅链结构,T≥3900K时硅原子逐渐替代条带边沿的碳原子直至graphene条带破坏.而硅原子吸附到扶手椅型graphene条带边沿在300K≤T<3000 K内仅能形成非链状的不定型的硅纳米结构. 关键词： graphene 硅 纳米结构 分子动力学模拟     

基于双尺度4分离层优化的石墨烯太赫兹宽带吸波结构

为实现对未来远程太赫兹雷达的高效对抗与隐身,针对典型太赫兹雷达工作频率设计了一种石墨烯太赫兹宽带吸波结构。宽带吸波结构以表层金属层/石墨烯层/介质层/底层金属层为基本吸波结构单元,利用遗传算法对双尺度基本吸波结构单元进行4分离层优化设计,确定宽带吸波结构的各层结构参数。仿真结果表明:宽带吸波结构在0.138 THz～2 THz频率范围内吸收效率优于80%,在0.157 THz～2 THz频率范围内吸收效率优于97.46%,典型太赫兹雷达工作频率处吸收效率均优于92.27%,满足太赫兹雷达对抗与隐身要求。     

Electronic properties of silicene in BN/silicene van der Waals heterostructures

Silicene is a promising 2D Dirac material as a building block for van der Waals heterostructures(vd WHs). Here we investigate the electronic properties of hexagonal boron nitride/silicene(BN/Si) vd WHs using first-principles calculations.We calculate the energy band structures of BN/Si/BN heterostructures with different rotation angles and find that the electronic properties of silicene are retained and protected robustly by the BN layers. In BN/Si/BN/Si/BN heterostructure, we find that the band structure near the Fermi energy is sensitive to the stacking configurations of the silicene layers due to interlayer coupling. The coupling is reduced by increasing the number of BN layers between the silicene layers and becomes negligible in BN/Si/(BN)_3/Si/BN. In(BN)_n/Si superlattices, the band structure undergoes a conversion from Dirac lines to Dirac points by increasing the number of BN layers between the silicene layers. Calculations of silicene sandwiched by other 2D materials reveal that silicene sandwiched by low-carbon-doped boron nitride or HfO_2 is semiconducting.