998 resultados para 111 SI
Resumo:
载流子传输一直是有机半导体研究中最核心的问题,通过弱外延生长方法可制备高取向、大面积连续的平面酞菁化合物薄膜,其迁移率可达到单晶水平。本论文中应用建立起来的有机半导体开尔文探针力显微镜(KPFM)表征方法,从微观水平研究了这种弱取向外延薄膜和常规的多晶有机半导体薄膜中结构和电特性的关系。 1. 建立了有机半导体的KPFM表征方法。KPFM用微探针方法同时获得样品的表面形貌和电势,本文第二章中论述了KPFM的工作原理、操作方法和成像机制,解释了相关的有机半导体表面态的问题。并且定量验证我们所建立的这种方法的可靠性。 2. 运用KPFM研究了多晶态的有机半导体CuPc和F16CuPc,直接观察到晶界势垒的存在,说明CuPc晶界处存在类施主的缺陷能级,F16CuPc晶界处存在类受主的缺陷能级,这两两种缺陷态分别俘获空穴和电子,使晶界周围的载流子耗尽而形成空间电荷区,限制了载流子的传输,我们从实验的角度证明了有机半导体中晶界限制载流子传输的理论。另外得到多晶CuPc高能量分辨的局域功函图像,越是π电子暴露的表面其功函越高。 3. 在导电的H-Si(111)衬底上制备出6P诱导弱取向生长的CuPc和F16CuPc薄膜,比较了和SiO2衬底上的弱外延生长薄膜的特征。KPFM观察到在CuPc界面处的能带弯曲和空穴累积,这大大降低了载流子传输沟道内CuPc晶界的势垒,从而其迁移率得到很大提高,这说明6P除了具有在结构上诱导使酞菁取向的作用外,还具有电子结构上的效应。另外研究了6P弱外延生长的亚单层F16CuPc薄膜,直接观察到在6P-F16CuPc界面存在强烈的电荷累积,这从微观上说明了有机异质结的电特性。
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单晶Si由于它的禁带宽度(1.1ev)与太阳光谱相匹配,作为光活性电极材料被广泛研究。但它作为光阳极不稳定,易腐蚀。所以提高单晶Si作为光阳极时的稳定性就成了关键问题。我们所采用的是在n-Si表面沉积保护膜的方法。首先用电沉积法在n-Si表面沉积一层Pd,光电流得到了很大的提高。从未沉积Pd前的2mA/cm~2增加到29.1mA/cm~2(电极电位在1.5V vs,SCE),并且稳定性增强,能在光电解水体系中稳定工作4小时,但4小时后光电流衰减,所以在n-Si表面沉积Pd有一定的保护作用,但保护作用是有限的。我们在n-Si/Pd电极上用化学沉积法再沉积一层锰的氧化物膜,大大提高了电极的稳定性,在0.5MKOH溶液中,电位控制在0.4V(vs.SCE)时,可以稳定工作110小时。化学沉积锰氧化物膜的烧结温度为250℃(在N_2或Ar保护下),得到的锰氧化物膜经X-射线光电子能谱确定锰的价态为+3价。RuO_2在n-Si/Pd/Mn_2O_3表面的沉积对氧的析出有催化作用,与未沉积前相比不电流起始电位负移了0.15V,比氧析出可逆电位负80mV [氧析出中逆电位在0.5M KOH溶液中(pH=13.7)为0.18v(vs.SCE)],沉积RuO_2的n-Si/Pd/Mn_2O_3/RuO_2电极,也具有同样好的稳定性,连续工作112小时光电流无明显变化。由交流阻抗法求得它的平带电位为-0.5V vs.SCE (0.5M KOH溶液)和0.0V vs.SCE(0.5M K_2SO_4溶液),并由此进一步得出它在0.5M KOH溶液中的能级结构。据此分析了n-Si电极在不加偏压下不能实现水的光电化学分解的原因。由光谱响应实验结果得到。沉积保护膜后的n-Si电极在整个可见光及近红外区内仍然有较好的光吸收。由光电流起始波长1130nm计算出的禁带宽度为1.1eV,与文献值一致。
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In this report we present the effects of 1 MeV-electron irradiation on i a-Si:H films and solar cells. It is observed that in the dose range of 1.4-8.4 x 10(15) cm(-2) the defect creation has not reached its saturation level and the metastable defects caused by the irradiation cannot be completely removed by a two hour annealing at 200 degrees C for i a-Si:H films or at 130 degrees C for a-Si:H solar cells. The results may be understood in terms of a model based on two kinds of metastable defects created by 1 MeV-electron irradiation.
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20-period strained-layer superlattices of nominal composition and width Ge0.2Si0.8 (5 nm)/Si(25 nm) and Ge0.5Si0.5 (5 nm)/Si(25 nm) were studied by double-crystal X-ray diffraction. The Ge content x was determined by computer simulation of the diffraction features from the superlattice. This method is shown to be independent of the relaxation of the superlattice. Alternatively, x can be obtained from the measured difference DELTAa/a in lattice spacing perpendicular to the growth plane. It is sensitive to the relaxation. Comparing the results obtained in these two different ways, information about the relaxation of the superlattices can be obtained.
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The lattice damage accumulation in GaAs and Al0.3Ga0.7As/GaAs superlattices by 1 MeV Si+ irradiation at room temperature and 350-degrees-C has been studied. For irradiations at 350-degrees-C, at lower doses the samples were almost defect-free after irradiation, while a large density of accumulated defects was induced at a higher dose. The critical dose above which the damage accumulation is more efficient is estimated to be 2 x 10(15) Si/cm2 for GaAs, and is 5 x 10(15) Si/cm2 for Al0.8Ga0.7As/GaAs superlattice for implantation with 1.0 MeV Si ions at 350-degrees-C. The damage accumulation rate for 1 MeV Si ion implantation in Al0.3Ga0.7As/GaAs superlattice is less than that in GaAs.
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A new method developed for detecting possible subtle changes in Si-H bonds of a-Si:H after light soaking is described in detail. The method promises a sensitivity orders of magnitude higher than that reached by the usual IR spectrometer. Some preliminary results on phosphorus doped a-Si:H are given.
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By using the technique of elastic recoil detection (ERD), we have measured the hydrogen profiles in a-Si:H/a-Si structure samples annealed at various temperatures with and without electrical bias, and investigated the influence of electrical bias on hydrogen diffusion. The results show that hydrogen diffusion in a-Si is significantly enhanced by the action of electrical bias. The existence of the excess carriers, which are introduced by electrical injection, is considered to be responsible for the enhancement of hydrogen diffusion, and the microprocess of hydrogen transport has been exploited.
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Intervalley GAMMA - X deformation potential constants (IVDP's) have been calculated by first principle pseudopotential method for the III-V zincblende semiconductors AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs and InSb. As a prototype crystal we have also carried out calculations on Si. When comparing the calculated IVDP's of LA phonon for GaP, InP and InAs and LO phonon for AlAs, AlSb, GaAs, GaSb and InSb with a previous calculation by EPM in rigid approximation, good agreements are found. However, our ab initio pseudopotential results of LA phonon for AlAs, AlSb, GaAs, GaSb and InSb and LO phonon for GaP, InP and InAs are about one order of magnitude smaller than those obtained by EPM calculations, which indicate that the electron redistributions upon the phonon deformations may be important in affecting GAMMA - X intervalley shatterings for these phonon modes when the anions are being displaced. In our calculations the phonon modes of LA and LO at X point have been evaluated in frozen phonon approximation. We have obtained, at the same time, the LAX and LOX phonon frequencies for these materials from total energy calculations. The calculated phonon frequencies agree very well with experimental values for these semiconductors.
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We have studied the photovoltaic effects in Si doping superlattices (nipi) under different excitation conditions with and without additional cw optical biasing using a He-Ne laser. On the basis of the photovoltaic theory of carrier spatial separation in superlattices, we propose the concept of spatial fixity of the photovoltage polarity in type-II superlattices and examine the experimental results. The photovoltaic effect in Si nipi is found mainly from the direct transitions related with shallow impurities in real space, not the electron-hole band-to-band process as in GaAs nipi.
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A theoretical model for the electronic structure of porous Si is presented. Three geometries of porous Si (wire with square cross section, pore with square cross section, and pore with circular cross section) along both the [001] and [110] directions are considered. It is found that the confinement geometry affects decisively the ordering of conduction-band states. Due to the quantum confinement effect, there is a mixing between the bulk X and GAMMA states, resulting in finite optical transition matrix elements, but smaller than the usual direct transition matrix elements by a factor of 10(-3). We found that the strengths of optical transitions are sensitive to the geometry of the structure. For (001) porous Si the structure with circular pores has much stronger optical transitions compared to the other two structures and it may play an important role in the observed luminescence. For this structure the energy difference between the direct and the indirect conduction-band minima is very small. Thus it is possible to observe photoluminescence from the indirect minimum at room temperature. For (110) porous Si of similar size of cross section the energy gap is smaller than that of (001) porous Si. The optical transitions for all three structures of (110) porous Si tend to be much stronger along the axis than perpendicular to the axis.
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Electrical measurements were combined with surface techniques to study the Pt/Si interfaces at various silicide formation temperatures. Effects of deep centers on the Schottky barrier heights were studied. Hydrogen plasma treatment was used to passivate the impurity/defect centers at the interfaces, and the effects of hydrogenation on the Schottky barrier heights were also examined. Combining our previous study on the Pt/Si interfacial reaction, factors influencing the PtSi/Si Schottky barrier diode are discussed.
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Two samples of nominal 20-period Ge0.20Si0.80(5 nm)/Si(25 nm) and Ge0.5Si0.5(5 nm)/Si(25 nm) strained-layer superlattices (SLSs) were studied by the double-crystal X-ray diffraction method. It is convenient to define the perpendicular strains relative to the average crystal. Computer simulations of the rocking curves were performed using a kinematical step model. An excellent agreement between the measured and simulated satellite patterns is achieved. The dependence of the sensitivity of the rocking curves to the structural parameters of the SLS, such as the alloying concentration x and the layer thicknesses and the L component of the reflection g = (HKL), are clearly demonstrated.
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A matrix formulation has been developed and applied to simulate large-angle convergent-beam electron diffraction (LACBED) patterns from the Si/GexSi1-x strained layer superlattice (SLS). Good quantitative agreement has been achieved between experimental and simulated patterns. By utilizing dynamical HOLZ line patterns, we demonstrate that an accuracy of better than 0.1% can be achieved in the determination of the averaged lattice constant of a SLS, and the averaged number of layers of atoms within one period of SLS can be determined up to a single monolayer.
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The Pb-doped BiSrCaCuO superconducting films were grown by the single source mixed evaporation technique. The microbridges of dimensions 50 mum x 40 mum were fabricated by standard photolithography technologies. Si films with a thickness of 2500 angstrom were deposited on the microbridge area surfaces of BiPbSrCaCuO superconducting films by rf-magnetron sputtering. A greatly lowered zero resistance temperature of the microbridge area of the BiPbSrCaCuO film after Si sputtering was found. A non-linear effect of the current-voltage (I-V) characteristics at 78 K was shown. The high-frequency capacitance-voltage (C-V) curve of this structure at 78 K was symmetrical with the maximum capacitance at V = 0, and the capacitance decreased with increasing applied bias voltage. Afl experimental results are discussed.
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The interfacial reactions between thin films of cobalt and silicon and (100)-oriented GaAs substrates in two configurations, Co/Si/GaAs and Si/Co/GaAs, were studied using a variety of techniques including Auger electron spectroscopy, x-ray diffraction, and transmission electron microscopy. The annealing conditions were 200, 300, 400, 600-degrees-C for 30 min, and rapid thermal annealing for 15 s. It was found that Si layer in the Co/Si/GaAs system acts as a barrier at the interface between Co and GaAs when annealed up to 600-degrees-C. The interfacial reaction between Co and Si is faster than that between Co and GaAs in the system of Si/Co/GaAs. The sequence of compound formation for the two metallizations studied (Co/Si/GaAs and Si/Co/GaAs) depends strongly on the sample configuration as well as the layer thickness of Si and Co (Co/Si atomic ratio). From our results, it is promising to utilize Co/Si/GaAs multilayer film structure to make a CoSi2/GaAs contact, and this CoSi2 may offer an alternative to the commonly used W silicides as improved gate metallurgies in self-aligned metal-semiconductor field effect transistor (MESFET) technologies.