镶嵌在氢化氮化硅中纳米非晶硅粒子光吸收的模拟

采用量子限制效应模型对镶嵌有纳米非晶硅粒子的氢化氮化硅薄膜的光吸收进行了理论模拟,探讨了由吸收谱分析给出该结构薄膜光学参数的方法,并通过对不同氮含量样品的讨论给出了量子限制效应和纳米硅粒子表面的结构无序对薄膜光吸收特性的影响规律。分析结果表明,随氮含量的增加,薄膜有效光学带隙增大,该结果与薄膜中纳米硅粒子平均尺寸的减小引起的量子限制效应的增强相关,而小粒度纳米硅粒子比例增加所引入的较高微观结构无序度和较多缺陷将会导致薄膜低能吸收区吸收系数增加。     

Chemical tuning of metal-semiconductor interfaces

We report a study of the Schottky barrier for Pb films grown on Si surfaces terminated by various metals (Ag, In, Au, and Pb) to explore the atomic-scale physics of the interface barrier and a means to control the barrier height. Electronic confinement by the Schottky barrier results in quantum well states in the Pb films, which are measured by angle-resolved photoemission. The barrier height is determined from the atomic-layer-resolved energy levels and the line widths. A calculation based on the known interface chemistry and the electronegativity yields predicted barrier heights in good agreement with the experiment.     

Interface effects on the quantum well states of Pb thin films

Using scanning tunneling spectroscopy, we have studied the interface effect on quantum well states of Pb thin films grown on various metal-terminated (Pb, Ag, and Au) n-type Si(111) surfaces and on two different p-type Si(111) surfaces. The dispersion relation E(k) of the electrons of the Pb film and the phase shift at the substrate interface were determined by applying the quantization rule to the measured energy positions of the quantum well states. Characteristic features in the phase shift versus energy curves were identified and were correlated to the directional conduction band of the silicon substrate and to the Schottky barrier formed between the metal film and the semiconductor. A model involving the band structure of the substrate, the Schottky barrier, and the effective thickness of the interface was introduced to qualitatively but comprehensively explain all the observed features of the phase shift at the substrate interface. Our physical understanding of the phase shift is critically important for using interface modification to control the quantum well states.     

Performance Improvement of GaN Based Schottky Barrier Ultraviolet Photodetector by Adding a Thin A1GaN Window Layer

We propose a new structure of GaN based Schottky barrier ultraviolet photodetector, in which a thin n-type A1GaN window layer is added on the conventional n^--GaN/n^＋-GaN device structure. The performance of the Schottky barrier ultraviolet photodetector is found to be improved by the new structure. The simulation result shows that the new structure can reduce the negative effect of surface states on the performance of Schottky barrier GaN photodetectors, improving the quantum efficiency and decreasing the dark current. The investigations suggest that the new photodetector can exhibit a better responsivity by choosing a suitably high carrier concentration and thin thickness for the A1GaN window layer.     

结构参数对GaN肖特基紫外探测器性能的影响及器件设计

研究了GaN肖特基结构(n^--GaN /n⁺-GaN)紫外探测器的结构参数对器件性能的影响机理。模拟计算结果表明:提高肖特基势垒高度和减小表面复合速率,不仅可以增加器件的量子效率,而且可以极大地减小器件的暗电流;适当地增加n^--GaN层厚度和载流子浓度可以提高器件的量子效率,但减小n^--GaN层的载流子浓度却有利于减小器件的暗电流。我们针对实际应用的需要,提出了一个优化器件结构参数的设计方案,特别是如果实际应用中对器件的量子效率和暗电流都有较高的要求,肖特基势垒高度应该≥0.8 eV,n^--GaN层的厚度≥200 nm,载流子浓度1×10¹⁷ cm^-3 左右,表面复合速率＜1×10⁷ cm/s。     

Optimum Quantum Yield of the Light Emission from 2 to 10 nm Hydrosilylated Silicon Quantum Dots

Optimizing the light‐emitting efficiency of silicon quantum dots (Si QDs) has been recently intensified by the demand of the practical use of Si QDs in a variety of fields such as optoelectronics, photovoltaics, and bioimaging. It is imperative that an understanding of the optimum light‐emitting efficiency of Si QDs should be obtained to guide the design of the synthesis and processing of Si QDs. Here an investigation is presented on the characteristics of the photoluminescence (PL) from hydrosilylated Si QDs in a rather broad size region (≈2–10 nm), which enables an effective mass approximation model to be developed, which can very well describe the dependence of the PL energy on the QD size for Si QDs in the whole quantum‐confinement regime, and demonstrates that an optimum PL quantum yield (QY) appears at a specific QD size for Si QDs. The optimum PL QY results from the interplay between quantum‐confinement effect and surface effect. The current work has important implications for the surface engineering of Si QDs. To optimize the light‐emission efficiency of Si QDs, the surface of Si QDs must be engineered to minimize the formation of defects such as dangling bonds at the QD surface and build an energy barrier that can effectively prevent carriers in Si QDs from tunneling out.     

纳米硅结构薄膜光致发光的温度依赖特性

综合考虑纳米硅结构薄膜的特殊性质,如量子限制效应、光学带隙和光跃迁振子强度对纳米硅粒径的依赖特性以及光学带隙和光辐射的温度依赖特性等,给出了一个解析表达式来分析具有一定粒径分布的纳米硅结构薄膜的光致发光(PL)强度分布,其中选取了两种纳米硅的粒径分布,即高斯分布和对数正态分布。结果表明,随着平均粒径和粒径分布偏差的减小,纳米硅薄膜的PL谱峰蓝移。随着环境温度的升高,纳米硅结构薄膜的PL谱峰红移且相对发光强度减弱。纳米硅结构薄膜光辐射拟合的结果与实验数据的比较分析表明,该模型能够很好地解释纳米硅结构薄膜在不同温度下的PL特性。     

Barrier height and tunneling current in Schottky diodes with embedded layers of quantum dots

Electrical characteristics of silicon Schottky diodes containing Ge quantum dot (QD) arrays are investigated. It has been found that the potential barrier height at the metal-semiconductor contact can be controlled by introducing dense QD layers, which is a consequence of the formation of a planar electrostatic potential of charged QDs. When the applied voltage is varied, the ideality factors of Schottky barriers exhibit oscillations due to the tunneling of holes through discrete levels in quantum dots.     

Quantum confinement effects and source-to-drain tunneling in ultra-scaled double-gate silicon n-MOSFETs

By using the linear combination of bulk band (LCBB) method incorporated with the top of the barrier splitting (TBS) model, we present a comprehensive study on the quantum confinement effects and the source-to-drain tunneling in the ultra-scaled double-gate (DG) metal-oxide-semiconductor field-effect transistors (MOSFETs). A critical body thickness value of 5 nm is found, below which severe valley splittings among different X valleys for the occupied charge density and the current contributions occur in ultra-thin silicon body structures. It is also found that the tunneling current could be nearly 100% with an ultra-scaled channel length. Different from the previous simulation results, it is found that the source-to-drain tunneling could be effectively suppressed in the ultra-thin body thickness (2.0 nm and below) by the quantum confinement and the tunneling could be suppressed down to below 5% when the channel length approaches 16 nm regardless of the body thickness.     

SiC Schottky结反向特性的研究

对Ti/6H-SiC Schottky结的反向特性进行了测试和理论分析，提出了一种综合的包括SiC Schottky结主要反向漏电流产生机理的反向隧穿电流模型，该模型考虑了Schottky势垒不均匀性、Ti/SiC界面层电压降和镜像力对SiC Schottky结反向特性的影响，模拟结果和测量值的相符说明了以上所考虑因素是引起SiC Schottky结反向漏电流高于常规计算值的主要原因.分析结果表明在一般工作条件下SiC Schottky结的反向特性主要是由场发射和热电子场发射电流决定的.     

InAs量子点在肖特基势垒二极管输运特性中的影响

在77到292K的范围内,系统研究了含InAs自组装量子点的金属-半导体-金属双肖特基势垒二极管的输运特性.随着温度上升,量子点的存储效应引起的电流回路逐渐减小.在测试温度范围内,通过量子点的共振隧穿过程在电流电压(I-V)曲线中造成台阶结构,且使电流回路随温度的上升急剧减小.根据肖特基势垒的反向I-V曲线,计算了势垒的反向饱和电流密度和平均理想因子.发现共振随穿效应使肖特基势垒在更大的程度上偏离了理想情况,而量子点的电子存储效应主要改变了肖特基势垒的有效势垒高度,从而影响了势垒的反向饱和电流密度 关键词： 自组装量子点 肖特基势垒 电流-电压特性     

Dielectric and relaxation properties of thermally evaporated nanostructured bismuth sulfide thin films

Nanostructured bismuth sulfide thin films were prepared onto glass substrates with particle size of 21 nm by thermal evaporation using readily prepared bismuth sulfide nanocrystallite powder. The X-ray diffraction pattern revealed that bismuth sulfide thin films exhibit orthorhombic structure. The existence of quantum confinement effect was confirmed from the observed band gap energy of 1.86 eV. AC and DC electrical conductivity of Al/BiSnc/Al structures was investigated in the frequency range 0.5-100 kHz at different temperatures (303-463 K) under vacuum. The AC conductivity (σ_ac) is found to be proportional to angular frequency (ω^s). The obtained experimental result of the AC conductivity showed that the correlated barrier hopping model is the appropriate mechanism for the electron transport in the nanostructured bismuth sulfide thin films. DC conduction mechanism in these films was studied and possible conduction mechanism in the bismuth sulfide thin films was discussed.     

Phonon confinement effects in Raman spectra of porous silicon at non‐resonant excitation condition

Light emitting porous silicon samples with different porosities, i.e. crystalline sizes, were produced from the low level doped p‐type silicon wafers by the anodization process. The effects of strong phonon confinement, redshift and broadening, were found on the O(Γ) phonon mode of the Raman spectra recorded at non‐resonant excitation condition using a near infrared 1064 nm laser excitation wavelength. Similarly, the blueshift of the photoluminescence peak was observed by reducing the crystalline sizes. Vibrational and optical findings were analysed within the existing models of confinement on the vibrational and electronic states of silicon nanocrystals. Since the energy of the photoluminescence peak of small nanocrystals also depends on the oxygen content on the surface of nanocrystals, the surface oxidation states were examined using infrared and energy dispersive spectroscopy. The partial coverage of the surface of nanocrystals was found due to the sample exposure to air. As a consequence, the photoluminescence energy did not increase as would be expected from the quantum confinement model. These results further indicate that the oxygen passivation along with the quantum confinement determines the electronic states of the silicon nanocrystals in porous silicon. Copyright © 2014 John Wiley & Sons, Ltd.     

垒层厚度对InGaN/GaN多量子阱电注入发光性能的影响及机理

研究了不同垒厚对InGaN/GaN多量子阱电注入发光性能的影响及机理。实验发现,当GaN垒层的厚度从6 nm增大到24 nm时,垒厚的样品发光强度更强,而且当注入电流增加时,适当增加垒厚,可以更显著增加发光强度。进一步结合发光峰位和光谱宽度的研究表明,由于应力和极化效应的存在,当垒层厚度在6~24 nm范围内时,适当增加垒层厚度不仅会使得能带的倾斜加剧,减少电子泄露,而且也会增加InGaN阱层的局域态深度,从而改善量子阱的发光性能。     

Tunnelling current in Schottky diodes containing InAs quantum dots

Current transport mechanism in Schottky diode containing InAs quantum dots (QDs) is investigated using temperature-varying current-voltage characteristics. We found that the tunnelling emission has obvious effects on the I-V characteristics. The I-V-T measurements revealed clear effects of QDs on the overall current flow. Field emission (FE, pure tunnelling effect) was observed at low temperature and low voltages bias region. The zero-bias barrier height decreases and the ideality factor increases with decreasing temperature, and the ideality factor was found to follow the T₀-effect. When the reverse bias is varied, the ideality factors of Schottky barriers exhibit oscillations due to the tunnelling of electrons through discrete levels in quantum dots. The traps distributed within InAlAs layer can also act as a transition step for reverse bias defect-assisted tunnelling current which can phenomenologically explain the decrease of the effective barrier height with measurement temperature.     

The Spectrum of Deep Levels in the InP Schottky Barrier

The energy spectrum of deep levels in the space-charge region of the Schottky barrier of InP field-effect transistors is determined by the method of tunneling spectroscopy. It has been established that for the most part the space-charge region of the Schottky barrier has electron traps with large capture cross sections that control the tunneling current in the Schottky barrier.     

Density-functional theory of inhomogeneous electron systems in thin quantum wires

Motivated by current interest in strongly correlated quasi-one-dimensional (1D) Luttinger liquids subject to axial confinement, we present a novel density-functional study of few-electron systems confined by power-low external potentials inside a short portion of a thin quantum wire. The theory employs the 1D homogeneous Coulomb liquid as the reference system for a Kohn-Sham treatment and transfers the Luttinger ground-state correlations to the inhomogeneous electron system by means of a suitable local-density approximation (LDA) to the exchange-correlation energy functional. We show that such 1D-adapted LDA is appropriate for fluid-like states at weak coupling, but fails to account for the transition to a “Wigner molecules” regime of electron localization as observed in thin quantum wires at very strong coupling. A detailed analyzes is given for the two-electron problem under axial harmonic confinement.     

Leakage currents in thin ferroelectric films

The basic mechanisms of leakage current components of thin lead zirconate titanate (PZT) ferroelectric films grown by the sol-gel method have been studied. Characteristic regions of current-voltage characteristics with different charge transport mechanisms have been determined. It has been shown that there is an intermediate region which separates such regions. In one of them, the leakage current depends on properties of the contact of electrodes with PZT film at low voltages; in the other, the leakage current is controlled by intrinsic properties of the PZT film bulk, and the basic mechanism of charge transport is Poole-Frenkel emission. In the intermediate region, a stepwise change in the current has been observed, which is caused by relaxing breakdown of the Schottky barrier. Time dependences of the leakage currents have been determined. It has been shown that the leakage current decreases with increasing delay time before the Schottky barrier breakdown, and the dependence becomes opposite in character after the breakdown.     

Quantum mechanical tunneling phenomena in metal–semiconductor junctions

The impact of the quantum mechanical tunneling effect on the operation of MESFET device structure has been investigated. Due to the presence of a Schottky barrier in a highly doped semiconductor, the depletion region is so narrow that electrons can tunnel through the barrier and contribute to the gate leakage current. This, in turn, facilitates current gain of the Schottky junction transistor (SJT) in the subthreshold region. In a simulation of a SJT we have used 2D Monte Carlo particle-based simulations. Quantum mechanical tunneling effects have been accounted for by using the Airy function transfer matrix approach, valid for piecewise linear potential barriers.