基于共面传输线的太赫兹片上系统的研究

太赫兹片上系统是将太赫兹的产生、传输和探测都集中在一个几平方厘米的基片上,线宽及间距达到微米量级,具有集成化程度高、系统尺寸小、稳定性好以及操作简便的特点,有利于与微量样品检测技术相互结合。研究采用HFSS软件对共面波导和共面带状线两种太赫兹共面传输线进行了仿真计算,通过优化传输线的宽度、长度、基底的厚度等关键几何参数确定出最佳的传输线结构;研究工作重点对太赫兹波段的共面带状线结构进行了设计和优化,实现了在介电常数很小的BCB基底上的低损耗传输。所得到的最佳结构参数为线宽及间距均为20μm,此结构为后续片上系统实物芯片的制作提供了参数依据。     

基于LT-GaAs外延片的THz片上系统

太赫兹（THz）波在物质检测方面发挥着巨大的作用,是一种非常有潜力的生化传感工具。但是传统的太赫兹时域光谱系统（TDS）结构复杂,系统的集成度低,占用空间较大。所以,如何对THz波进行有效引导、实现集成化传输并得到高质量光谱就成为太赫兹光谱系统的研究热点。太赫兹片上系统是将THz的产生、传输以及探测都集成到同一芯片上,然后通过相干探测的方法获得THz时域光谱。它可以实现对多种样品的检测,尤其在对难于取样的微量样品探测方面具有广泛的应用价值。它无需光路准直,操作简便,成品率高。两个研究工作都是基于低温砷化镓(LT-GaAs)外延片开展的。首先将一根直径为200 μm的铜线固定在LT-GaAs外延片的上方,通过真空蒸镀的方法制备出天线电极,同时得到天线间隙,研制出基于LT-GaAs外延片的THz天线。利用波长为800 nm的飞秒激光对其进行测试,得到了质量较高的THz信号,验证了天线的实用性。然后在另一外延片上利用光刻微加工工艺制作出传输线和微电极,得到了集成的THz片上系统。使用波长为1 550 nm的飞秒激光分别激发片上系统的太赫兹产生天线和探测天线,天线产生的太赫兹波在传输线上传播,在探测端同样得到了质量较高的THz时域信号,证实了THz片上系统的可行性。该方法省去了腐蚀牺牲层以及LT-GaAs薄膜的转移、键合等步骤,极大地提高了片上系统的成品率,避免了薄膜转移过程中易破碎及腐蚀液存在毒性的问题。最后,研究了外加电压对从片上系统中获得的THz波性能的影响,结果为电压越高,THz波的信号强度越强;另外,通过在传输线上方垂直放置铜箔的方法验证了THz波沿着传输线传播的事实。该研究中采用的基于LT-GaAs外延片的片上系统的制备方法简单,制作周期短,制作过程安全,应用领域广泛,这为将来与微流控芯片相结合实现对液体样品的探测打下了基础。     

Atomic-Scale Study of Ge-Induced Incommensurate Phases on Si（111）

Two Ge-induced incommensurate phases, γ and β, on Si（111） are observed and studied by {／it in situ} scanning tunneling microscopy. The γ phase consists of aligned triangular domains whose stacking sequence is faulted with respect to the Si（111）-1×1 surface. The β phase consists of two kinds of triangular domains whose stacking sequences are faulted and unfaulted with respect to the Si（111）-1×1 surface, respectively. In the β phase, two types of domain walls, zigzag＇＇ and face-to-face＇＇, form to release the strain. The triangular domains all exhibit a quasi-1×1 hexagonal close-packed structure. By studying the structural evolution from magic clusters to incommensurate structures, the structure models for γ and β phases are proposed.     

In situ electronic structural study of VO_2 thin film across the metal–insulator transition

The in situ valence band photoemission spectrum （PES） and X-ray absorption spectrum （XAS） at V LⅡ-LⅢ edges of the VO2 thin film, which is prepared by pulsed laser deposition, are measured across the metal–insulator transition （MIT） temperature （TMIT=67 ℃）. The spectra show evidence for changes in the electronic structure depending on temperature. Across the TMIT, pure V 3d characteristic d‖ and O 2p-V 3d hybridization characteristic πpd, σpd bands vary in binding energy position and density of state distributions. The XAS reveals a temperature-dependent reversible energy shift at the V LⅢ-edge. The PES and XAS results imply a synergetic energy position shift of occupied valence bands and unoccupied conduction band states across the phase transition. A joint inspection of the PES and XAS results shows that the MIT is not a one-step process, instead it is a process in which a semiconductor phase appears as an intermediate state. The final metallic phase from insulating state is reached through insulator–semiconductor, semiconductor–metal processes, and vice versa. The conventional MIT at around the TMIT=67 ℃ is actually a semiconductor–insulator transformation point.     

Electronic states and molecular orientation of ITIC film

ITIC is the milestone of non-fullerene small molecule acceptors used in organic solar cells. We study the electronic states and molecular orientation of ITIC film using photoelectron spectroscopy and x-ray absorption spectroscopy. The negative integer charge transfer energy level is determined to be 4.00 ± 0.05 eV below the vacuum level, and the ionization potential is 5.75 ± 0.10 eV. The molecules predominantly have the face-on orientation on inert substrates as long as the surfaces of the substrates are not too rough. These results provide the physical understanding of the high performance of ITIC-based solar cells, which also afford implications to design more advanced photovoltaic small molecules.     

Temperature effect on the electronic structure of Nb:SrTiO3 (100) surface

The effect of temperature on the electronic structure of Nb-doped SrTiO_3(100) surface is investigated by highresolution synchrotron radiation photoemission spectroscopy.According to the x-ray photoemission spectroscopy(XPS)results,at an annealing temperature of less than 700 ℃,the adsorbed carbon and hydroxyl on the STO surface could be removed,to expose the fresh intrinsic surface with a constant ratio of Ti/O.It is obvious that the STO would be doped by Ca~+ impurities of bulks and O vacancies in the surface after annealing at 920 ℃ for one hour.     

Photoelectric characteristics of silicon P–N junction with nanopillar texture:Analysis of X-ray photoelectron spectroscopy

Silicon nanopillars are fabricated by inductively coupled plasma(ICP) dry etching with the cesium chloride(CsCl)islands as masks originally from self-assembly. Wafers with nanopillar texture or planar surface are subjected to phosphorus(P) diffusion by liquid dopant source(POCl3) at 870℃ to form P–N junctions with a depth of 300 nm. The X-ray photoelectron spectroscopy(XPS) is used to measure the Si 2p core levels of P–N junction wafer with nanopillar texture and planar surface. With a visible light excitation, the P–N junction produces a new electric potential for photoelectric characteristic, which causes the Si 2p core level to have a energy shift compared with the spectrum without the visible light.The energy shift of the Si 2p core level is-0.27 eV for the planar P–N junction and-0.18 eV for the nanopillar one. The difference in Si 2p energy shift is due to more space lattice defects and chemical bond breaks for nanopillar compared with the planar one.     

Observation of selective surface element substitution in FeTe_(0.5)Se_(0.5) superconductor thin film exposed to ambient air by synchrotron radiation spectroscopy

A systematic investigation of oxidation on a superconductive Fe Te_(0.5)Se_(0.5)thin film,which was grown on Nb-doped SrTiO_3(001) by pulsed laser deposition,has been carried out.The sample was exposed to ambient air for one month for oxidation.Macroscopically,the exposed specimen lost its superconductivity due to oxidation.The specimen was subjected to in situ synchrotron radiation photoelectron spectroscopy(PES) and x-ray absorption spectroscopy(XAS) measurements following cycles of annealing and argon ion etching treatments to unravel what happened in the electronic structure and composition after exposure to air.By the spectroscopic measurements,we found that the as-grown FeTe_(0.5)Se_(0.5)superconductive thin film experienced an element selective substitution reaction.The oxidation preferentially proceeds through pumping out the Te and forming Fe–O bonds by O substitution of Te.In addition,our results certify that in situ vacuum annealing and low-energy argon ion etching methods combined with spectroscopy are suitable for depth element and valence analysis of layered structure superconductor materials.