量子点场效应单光子探测器二维电子气载流子浓度研究

设计了一种基于场效应晶体管的量子点场效应单光子探测器(quantum dot field effect transistor,QDFET),建立了二维电子气(two-dimensional electron gas,2DEG)的薛定谔方程和泊松方程,通过对薛定谔方程和泊松方程的自洽求解,对2DEG的载流子浓度进行了模拟。模拟结果显示,AlGaAs的Al组分、δ掺杂层的掺杂浓度以及隔离层的厚度对于2DEG的载流子浓度均有影响。为了使2DEG具有较高的载流子浓度,AlGaAs的Al组分应为0.2~0.4,δ掺杂浓度应为6~8×10~(13)/cm~2,隔离层厚度应在50nm以下。通过对2DEG的载流子浓度进行研究,可以掌握2DEG载流子浓度的影响因素,从而通过优化QDFET结构,可提高2DEG的载流子浓度。这对于高灵敏度QDFET的制备具有重要的意义和应用价值。     

Influence of a drain field plate on the forward blocking characteristics of an AlGaN/GaN high electron mobility transistor

In this paper,the influence of a drain field plate(FP)on the forward blocking characteristics of an AlGaN/GaN high electron mobility transistor(HEMT)is investigated.The HEMT with only a gate FP is optimized,and breakdown voltage VBRis saturated at 1085 V for gate–drain spacing LGD≥8μm.On the basis of the HEMT with a gate FP,a drain FP is added with LGD=10μm.For the length of the drain FP LDF≤2μm,VBRis almost kept at 1085 V,showing no degradation.When LDFexceeds 2μm,VBRdecreases obviously as LDFincreases.Moreover,the larger the LDF,the larger the decrease of VBR.It is concluded that the distance between the gate edge and the drain FP edge should be larger than a certain value to prevent the drain FP from affecting the forward blocking voltage and the value should be equal to the LGDat which VBR begins to saturate in the first structure.The electric field and potential distribution are simulated and analyzed to account for the decrease of VBR.     

Enhancement of terahertz coupling efficiency by improved antenna design in GaN/AlGaN high electron mobility transistor detectors

An optimized micro-gated terahertz detector with novel triple resonant antenna is presented.The novel resonant antenna operates at room temperature and shows more than a 700% increase in photocurrent response compared to the conventional bowtie antenna.In finite-difference-time-domain simulations,we found the performance of the self-mixing GaN/AlGaN high electron mobility transistor detector is mainly dependent on the parameters L gs(the gap between the gate and the source/drain antenna) and L w(the gap between the source and drain antenna).With the improved triple resonant antenna,an optimized micrometer-sized AlGaN/GaN high electron mobility transistor detector can achieve a high responsivity of 9.45×102 V/W at a frequency of 903 GHz at room temperature.     

单根In掺杂ZnO纳米带场效应管的电学性质

采用化学气相沉积法合成了In掺杂ZnO纳米带,并对其进行了X射线衍射、光致发光及透射电镜表征.基于单根纳米带,采用廉价微栅模板法制备了背栅场效应管,利用半导体参数测试仪测量了场效应管的输出(Ids-Vds)和转移(Ids-Vgs)特性,得出相关电学参数,其中迁移率值为622 cm2·V-1·s-1,该值明显优于包括ZnO在内的大多数材料;讨论了迁移率提高的可能原因.     

Light scattering by closely packed clusters: Shielding of particles by each other in the near field

Equations of the theory of light scattering by clusters (aggregates) of spherical particles are analyzed, and peculiarities of interaction of scatterers in the near field are discussed. It is shown that one of the manifestations of the near field is a mutual shielding of particles. For simple clusters consisting of two identical spherical particles (bisphere), the mutual shielding leads to a decrease in the intensity of light scattered along the axis of the bisphere. If the bispheres are small as compared to the wavelength, shielding is caused by electrostatic interaction of charges induced in the particles by the external field. Calculations of the intensity of light scattered by randomly oriented clusters of spherical particles show that for the model ignoring the near field the intensity is significantly larger than for the model with the near field taken into account.     

Kinetic parameters,collision rates,energy exchanges and transport coefficients of non-thermal electrons in premixed flames at sub-breakdown electric field strengths

The effects of an electric field on the collision rates, energy exchanges and transport properties of electrons in premixed flames are investigated via solutions to the Boltzmann kinetic equation. The case of high electric field strength, which results in high-energy, non-thermal electrons, is analysed in detail at sub-breakdown conditions. The rates of inelastic collisions and the energy exchange between electrons and neutrals in the reaction zone of the flame are characterised quantitatively. The analysis includes attachment, ionisation, impact dissociation, and vibrational and electronic excitation processes. Our results suggest that Townsend breakdown occurs for E/N = 140 Td. Vibrational excitation is the dominant process up to breakdown, despite important rates of electronic excitation of CO, CO₂ and N₂ as well as impact dissociation of O₂ being apparent from 50 Td onwards. Ohmic heating in the reaction zone is found to be negligible (less than 2% of peak heat release rate) up to breakdown field strengths for realistic electron densities equal to 10¹⁰ cm^?3. The observed trends are largely independent of equivalence ratio. In the non-thermal regime, electron transport coefficients are insensitive to mixture composition and approximately constant across the flame, but are highly dependent on the electric field strength. In the thermal limit, kinetic parameters and transport coefficients vary substantially across the flame due to the spatially inhomogeneous concentration of water vapour. A practical approach for identifying the plasma regime (thermal versus non-thermal) in studies of electric field effects on flames is proposed.     

Theoretical studies on the near field properties of non-concentric core-shell nanoparticle dimers

The concentric core-shell nanoparticle dimers have “hot spots” with enhanced electric fields in their junctions, which can be used in the surface enhanced Raman spectra analysis. Here the non-concentric core-shell nanoparticle dimers are proposed by introducing a shift between the dielectric core and the metal shell. By using the three dimensional finite difference time domain method, the plasmon resonances and the near field properties of the core-shell nanoparticle dimers affected by the non-concentric shift, dimer separation, excitation wavelength and polarization are analyzed in detail. The results show that the local near fields of the non-concentric core-shell nanoparticle dimers can be much more enhanced than those of the concentric ones. Also the plasmon resonance wavelengths of the dimers can be effectively tuned by the non-concentric shifts between the core and shell. The proposed nanostructures can have great potential in various near field applications.     

Terahertz sensing and imaging based on nanostructured semiconductors and carbon materials

The advantageous properties of terahertz (THz) waves, such as permeability through objects that are opaque for visible light and the energy spectrum in the microelectron‐volt range that are important in materials research, allow their potential use in various applications of sensing and imaging. However, since the THz region is located between the electronic and photonic bands, even the basic components such as detectors and sources have not been fully developed, unlike in other frequency regions. THz technology also has the problem of low imaging resolution, which results from a considerably longer wavelength than that of the visible light. However, the utilization of nanostructured electronic devices has recently opened up new horizons for THz sensing and imaging. This paper provides an overview of the THz detector and imaging techniques and tracks their recent progress. Specifically, two cutting‐edge techniques, namely, frequency‐selective THz‐photon detection and integrated near‐field THz imaging, are discussed in detail. Finally, the studies of superconductors and semiconductors with high‐resolution THz imaging are described.     

Mapping an on-chip terahertz antenna by a scanning near-field probe and a fixed field-effect transistor

In the terahertz(THz) regime,the active region for a solid-state detector usually needs to be implemented accurately in the near-field region of an on-chip antenna.Mapping of the near-field strength could allow for rapid verification and optimization of new antenna/detector designs.Here,we report a proof-of-concept experiment in which the field mapping is realized by a scanning metallic probe and a fixed AlGaN/GaN field-effect transistor.Experiment results agree well with the electromagnetic-wave simulations.The results also suggest a field-effect THz detector combined with a probe tip could serve as a high sensitivity THz near-field sensor.     

氮化镓基感光栅极高电子迁移率晶体管器件设计与制备

GaN基高电子迁移率晶体管(HEMT)作为栅控器件,具有AlGaN/GaN异质结处高浓度的二维电子气(2DEG)及对表面态敏感等特性,在栅位置处与感光功能薄膜的结合是光探测器领域重要的研究方向之一.本文首先提出在GaN基HEMT栅电极上引入光敏材料锆钛酸铅(PZT),将具有光伏效应的铁电薄膜PZT与HEMT栅极结合,提出一种新的"金属/铁电薄膜/金属/半导体(M/F/M/S)"结构;然后在以蓝宝石为衬底的AlGaN/GaN外延片上制备感光栅极HEMT器件.最后,通过PZT的光伏效应来调控沟道中的载流子浓度和通过源漏电流的变化来实现对可见光和紫外光的探测.在365 nm紫外光和普通可见光条件下,对比测试有/无感光栅极的HEMT器件,在较小V_(gs)电压时,可见光下测得前者较后者的饱和漏源电流I_(ds)的增幅不下降,紫外光下前者较后者的I_(ds)增幅大5.2 mA,由此可知,感光栅PZT在可见光及紫外光下可作用于栅极GaN基HEMT器件并可调控沟道电流.     

Electric field driven plasmon dispersion in AlGaN/GaN high electron mobility transistors

We present a theoretical study on the electric field driven plasmon dispersion of the two-dimensional electron gas(2DEG)in AlGaN/GaN high electron mobility transistors(HEMTs).By introducing a drifted Fermi–Dirac distribution,we calculate the transport properties of the 2DEG in the AlGaN/GaN interface by employing the balance-equation approach based on the Boltzmann equation.Then,the nonequilibrium Fermi–Dirac function is obtained by applying the calculated electron drift velocity and electron temperature.Under random phase approximation(RPA),the electric field driven plasmon dispersion is investigated.The calculated results indicate that the plasmon frequency is dominated by both the electric field and the angle between wavevector and electric field.Importantly,the plasmon frequency could be tuned by the applied source–drain bias voltage besides the gate voltage(change of the electron density).     

掺Fe高阻GaN缓冲层特性及其对AlGaN/GaN高电子迁移率晶体管器件的影响研究

利用金属有机物化学气相沉积技术在蓝宝石衬底上制备了掺Fe高阻Ga N以及Al Ga N/Ga N高电子迁移率晶体管(HEMT)结构.对Cp_2Fe流量不同的高阻Ga N特性进行了研究.研究结果表明,Fe杂质在Ga N材料中引入的Fe~(3+/2+)深受主能级能够补偿背景载流子浓度从而实现高阻,Fe杂质在Ga N材料中引入更多起受主作用的刃位错,也在一定程度上补偿了背景载流子浓度.在一定范围内,Ga N材料方块电阻随Cp_2Fe流量增加而增加,Cp_2Fe流量为100 sccm(1 sccm 1mL min)时,方块电阻增加不再明显;另外增加Cp_2Fe流量也会导致材料质量下降,表面更加粗糙.因此,优选Cp_2Fe流量为75 sccm,相应方块电阻高达×10?/,外延了不同掺Fe层厚度的Al Ga N/Ga N HEMT结构,并制备成器件.HEMT器件均具有良好的夹断以及栅控特性,并且增加掺Fe层厚度使得HEMT器件的击穿电压提高了39.3%,同时对器件的转移特性影响较小.     

Simultaneous near field imaging of electric and magnetic field in photonic crystal nanocavities

The insertion of a metal-coated tip on the surface of a photonic crystal microcavity is used for simultaneous near field imaging of electric and magnetic fields in photonic crystal nanocavities, via the radiative emission of embedded semiconductor quantum dots (QD). The photoluminescence intensity map directly gives the electric field distribution, to which the electric dipole of the QD is coupled. The magnetic field generates, via Faraday's law, a circular current in the apex of the metallized probe that can be schematized as a ring. The resulting magnetic perturbation of the photonic modes induces a blue shift, which can be used to map the magnetic field, within a single near-field scan.     

Trapping of microparticles in the near field of an ultrasonic transducer

We are investigating means of handling microparticles in microfluidic systems, in particular localized acoustic trapping of microparticles in a flow-through device. Standing ultrasonic waves were generated across a microfluidic channel by ultrasonic microtransducers integrated in one of the channel walls. Particles in a fluid passing a transducer were drawn to pressure minima in the acoustic field, thereby being trapped and confined at the lateral position of the transducer. The spatial distribution of trapped particles was evaluated and compared with calculated acoustic intensity distributions. The particle trapping was found to be strongly affected by near field pressure variations due to diffraction effects associated with the finite sized transducer element. Since laterally confining radiation forces are proportional to gradients in the acoustic energy density, these near field pressure variations may be used to get strong trapping forces, thus increasing the lateral trapping efficiency of the device. In the experiments, particles were successfully trapped in linear fluid flow rates up to 1mm/s. It is anticipated that acoustic trapping using integrated transducers can be exploited in miniaturised total chemical analysis systems (microTAS), where e.g. microbeads with immobilised antibodies can be trapped in arrays and subjected to minute amounts of sample followed by a reaction, detected using fluorescence.     

Molecular scale electronic devices using single molecules and molecular monolayers

Molecular electronic devices that utilize single molecules or molecular monolayers as active electronic components represent a promising approach in the ongoing miniaturization and integration of electronic devices. Rapid advances in technology have enabled us to engineer molecular electronic devices with diverse functionalities. Significant progress has been made in understanding charge transport in molecular systems at the single-molecule level, and concomitantly, new device concepts have emerged. This review article focuses on experimental aspects of electronic devices made with single molecules or molecular monolayers, with a primary focus on the characterization and manipulation of charge transport.     

Far field detection of terahertz near field enhancement of sub-wavelength slits using Kirchhoff integral formalism

We demonstrate that the near field enhancement of sub-wavelength slits can be experimentally determined in the far field by using a reference aperture. Our simple model derived from the Kirchhoff integral formalism shows that enhancement of the near field at a slit exit with respect to the incident wave can be read in the transmitted amplitude through the slit attached on a reference aperture, normalized by the transmitted amplitude through the reference aperture. Furthermore, the near field enhancement obtained in such a way is essentially independent of the reference aperture size. By performing terahertz time domain spectroscopy we experimentally confirm the inverse frequency dependence of the near field enhancement of extremely narrow slits and measure the maximum field enhancement reaching 200 at 0.1 THz for a 500-nm-width slit.     

X射线光场成像技术研究

X射线三维成像技术是目前国内外X射线成像研究领域的一个研究热点.但针对一些特殊成像目标,传统X射线计算层析(CT)成像模式易出现投影信息缺失等问题,影响CT重建的图像质量,使得CT成像的应用受到一定的限制.本文主要研究了基于光场成像理论的X射线三维立体成像技术.首先从同步辐射光源模型出发,对X射线光场成像进行建模;然后,基于光场成像数字重聚焦理论,对成像目标场在深度方向上进行切片重建.结果表明:该方法可以实现对成像目标任一视角下任一深度的内部切片重建,但是由于光学聚焦过程中的离焦现象,会引入较为严重的背景噪声.当对其原始数据进行滤波后,再进行X射线光场重聚焦,可以有效消除重建伪影,提高图像的重建质量.本研究既有算法理论意义,又可应用于工业、医疗等较复杂目标的快速检测,具有较大的应用价值.     

用杂化CIS-DFT方法研究外电场对乙烯激发态的影响

采用杂化CIS-DFT方法研究了外电场对乙烯分子基态和激发态性质的影响,结果表明外电场对分子几何结构,总能量,偶极矩,极化率,振子强度和激发能有显著影响.CIS-DFT的优点在于能确定外场下分子的对称性,给出正确的激发顺序以及分子轨道的电子组态,由此导出乙烯分子的激发态,结果与实验一致.首次研究了乙烯分子的外电场效应.与其他从头算方法相比,杂化CIS-DFT方法计算精确和效率相对较高,可用于研究大分子体系.     

Magnetic field effect in photodetachment from negative ion in electric field near metal surface

Based on the closed-orbit theory, the magnetic field effect in the photodetachment of negative ion in the electric field near a metal surface is studied for the first time. The results show that the magnetic field can produce a significant effect on the photodetachment of negative ion near a metal surface. Besides the closed orbits previously found by Du et al. for the H in the electric field near a metal surface (J. Phys. B 43 035002 (2010)), some additional closed orbits are produced due to the effect of magnetic field. For a given ion-surface distance and an electric field strength, the cross section depends sensitively on the magnetic field strength. As the magnetic field strength is very small, its influence can be neglected. With the increase of the magnetic field strength, the number of the closed orbits increases greatly and the oscillation in the cross section becomes much more complex. Therefore we can control the photodetachment cross section of the negative ion by changing the magnetic field strength. We hope that our results may guide future experimental studies for the photodetachment process of negative ion in the presence of external fields and surfaces.     

Nano-image profiles transferred by near field phase-shifting lithography precisely simulated by finite element method and fabricated

The operating frequency of the SAW filters is limited by the gap width but not the line width. The narrow gap width is required for the high operating frequency SAW filters. Therefore, in this study, high precision nano-image profiles transferred by near field phase-shifting mask (NFPSM) lithography at various exposure-energy-intensities (EEIs) are simulated by the finite element method (FEM). The transferred energy-intensity distribution (TEID) in the photoresist during the NFPSM process (at the wavelength of 248 nm) can be accurately simulated by the FEM. The TEID at the interface between the mask and the photoresist is also simulated. The fabricated pattern widths clearly match the simulation results. The study of the simulated image profiles shows that they are dependent on the EEI. The greater the EEI is, the narrower the width and the shorter the height of the image profile. The nano-linewidth of 60 nm is simulated and fabricated. The fabricated nano-imaging profile precisely fits the simulation results. Therefore, any expected nano-image profile can easily be fabricated by way of the simulation.