Enhancement of the room temperature oxidation of silicon by very thin predeposited gold layers

The oxidation of Si(111) surfaces covered with very thin layers of gold is studied by Auger and electron energy loss spectroscopies under ultra high vacuum conditions. It is found that by exposing the Au covered surface to an oxidizing atmosphere, formation of silicon dioxide occurs at room temperature on top of the substrate and the presence of SiO₄ tetrahedra is clearly seen on electron energy loss spectra. In contrast, oxidation under the same conditions of a clean Si(111) surface leads to the formation of an oxygen monolayer and no structure corresponding to Si-O bonds in SiO₄ tetrahedra are observed. This enhancement of the oxidation is attributed to a change in the hybridization state of Si atoms in a gold environment.     

Catalytic action of gold atoms on the oxidation of Si(111) surfaces

Auger spectroscopy, electron energy loss spectroscopy and ion depth profiling techniques, under ultra high vacuum conditions, have been used in a comparative study of the oxidation of clean and gold precovered silicon (111) surfaces. Exposure of a Si surface covered by a few Au monolayers to an oxygen partial pressure induces the formation of SiO₄ tetrahedra even at room temperature. In contrast, oxidation under the same conditions of a clean Si(111) surface leads to the well known formation of a chemisorbed oxygen monolayer. In the case of the Au covered surfaces, the enhancement of the oxide growth is attributed to the presence of an AuSi alloy where the hybridization state of silicon atoms is modified as compared to bulk silicon. This Au catalytic action has been investigated with various parameters as the substrate temperature, oxygen partial pressure and Au coverage. The conclusions are two fold. At low temperature (T < 400°C), gold atoms enhance considerably the oxidation process. SiO₄ tetrahedra are readily formed even at room temperature. Nevertheless, the SiO₂ thickness saturates at about one monolayer, this effect being attributed to the lack of Si atoms alloyed with gold in the reaction area. By increasing the temperature (from 20°C to ～400°C), silicon diffusion towards the surface is promoted and a thicker SiO₂ layer can be grown on top of the substrate. In the case of the oxidation performed at temperature higher than 400°C, the results are similar to the one obtained on a clean surface. At these temperatures, the metallic film agglomerates into tridimensional crystallites on top of a very thin AuSi alloyed layer. The fact that the latter has no influence on the oxidation is attributed to the different local arrangement of atoms at the sample surface.     

THE INTERACTION OF Au AND THE Si/SiO2 INTERFACE DEFECT Hit(0.494)

The defects at the Si/SiO₂ interface have been studied by the deep-level transient spectroscopy (DLTS) technique in p-type MOS structures with and without gold diffusion. The experimental results show that the interaction of gold and Si/SiO₂ interface defect,H_it(0.494), results in the formation of a new interface de-fect, Au-H_it(0.445). Just like the interface defect, H_it(0.494), the new interface defect possesses a few interesting properties, for example, when the gate voltage applied across the MOS structure reduces the energy interval between Fermi-level and Si valence band of the Si surface to values smaller than the hole ionization Gibbs free energy of the defect, a sharp DLTS peak is still observable; and the hole apparent activation energy increases with the decrease of the Si surface potential barrier height. These properties can be successfully explained with the transition energy band model of the Si/SiO₂ interface.     

用可调探深的电子能量损失谱研究Sn/Si界面

用可调探测深度的电子能量损失谱辅以俄歇电子能谱和低能电子衍射,研究Sn/Si系统的界面反应。结果表明:当Sn蒸镀量大于两个原子单层,退火温度由400℃到700℃,在Sn/Si(111)界面Sn与Si发生互混,形成几个原子层厚的Sn/Si互混层,该互混层的特征体峰在15.5eV。在相同温度范围退火,Sn/Si(001)界面无可察觉的互混,仍有Sn岛存在,长时间在550℃退火低能电子衍射图形上出现(113)小晶面的衍射斑。 关键词：     

THE INTERACTION OF Au AND THE Si/SiO2 INTERFACE DEFECT Hit(0.494)

The defects at the Si/SiO₂ interface have been studied by the deep-level transient spectroscopy (DLTS) technique in p-type MOS structures with and without gold diffusion. The experimental results show that the interaction of gold and Si/SiO₂ interface defect,H_it(0.494), results in the formation of a new interface de-fect, Au-H_it(0.445). Just like the interface defect, H_it(0.494), the new interface defect possesses a few interesting properties, for example, when the gate voltage applied across the MOS structure reduces the energy interval between Fermi-level and Si valence band of the Si surface to values smaller than the hole ionization Gibbs free energy of the defect, a sharp DLTS peak is still observable; and the hole apparent activation energy increases with the decrease of the Si surface potential barrier height. These properties can be successfully explained with the transition energy band model of the Si/SiO₂ interface.     

Comparative AES and RHEED study of the formation of Pd silicide on clean and oxide covered Si(100) and (111) surfaces

The formation and epitaxial orientation of Pd silicide on clean and native oxide covered Si(100) and (111) surfaces was studied by Auger electron spectroscopy (AES) and reflection high energy electron diffraction (RHEED). Pd was vapor deposited in UHV on to the substrates up to thicknesses of about 6 nm. On clean Si substrates, ultra-thin Pd deposits reacted to form Pd₂Si already at room temperature, as detected by a characteristic splitting of the Si LVV Auger peak. However, a polycrystalline structure with very small crystallite sizes was indicated by diffuse ring patterns in RHEED. When the initial thickness of the Pd deposit exceeded about 3 nm, the diffraction ring pattern of unreacted metal developed. During annealing of room temperature deposits of Pd, the (100) and (111) substrates behaved differently. Larger crystallites formed on Si(100), but the films remained polycrystalline, though textured. On Si(111), virtually perfect epitaxial re-orientation of the silicide was found. When the substrates were initially covered with native oxide of about 2 nm thickness, silicide formation started at about 200°C, resulting in polycrystalline, but strongly textured Pd₂Si. Upon further annealing at temperatures up to 600°C, an additional phase of epitaxially oriented Pd₂Si developed on Si(111), similar to that on clean Si(100). In all experiments, extended annealing at temperatures above 250°C caused segregation of Si to the surface. This was accompanied by the development of an additional peak in the Auger electron spectra at about 313 eV, which we assign to a plasmon loss of δE = 17 eV in the Si overlayer, being excited by Pd Auger electrons of energy 330 eV.     

Penetration of electronic states from silicon substrate into silicon oxide

The depth profiling of O 1s energy loss in silicon oxide near the SiO₂/Si interface was performed using extremely small probing depth. As a result, the energy loss of O 1s photoelectrons with threshold energy of 3.5 eV was found. This value of 3.5 eV is much smaller than the SiO₂ bandgap of 9.0 eV, but quite close to direct interband transition at Γ point in energy band structure of silicon. This can be explained by considering the penetration of electronic states from silicon substrate into silicon oxide up to 0.6 nm from the interface. In addition, the penetrating depth is larger than the thickness of the compositional transition layer.     

第一性原理研究厚度和空位缺陷对Si/SiO2界面电子结构与光学性质的影响

采用基于密度泛函理论的平面波超软赝势方法,在局域密度近似（LDA）下研究了Si纳米层厚度和O空位缺陷对Si/SiO2界面电子结构及光学性质的影响．电子结构计算结果表明：在0.815~2.580nm的Si层厚度范围内,Si/SiO2界面结构的能隙随着厚度减小而逐渐增大,表现出明显的量子尺寸效应,这与实验以及其他理论计算结果一致;三种不同的O空位缺陷的存在均使得Si/SiO2界面能隙中出现了缺陷态,费米能级向高能量方向移动,且带隙有微弱增加．光学性质计算结果表明：随着Si纳米层厚度的减小,Si/SiO2界面吸收系数产生了蓝移;O空位缺陷引入后,界面光学性质的变化主要集中在低能区,即低能区的吸收系数和光电导率显著增加．可见,改变厚度和引入缺陷能够有效地调控Si/SiO2界面体系的电子和光学性质,上述研究结果为Si/SiO2界面材料的设计与应用提供了一定的理论依据．     

第一性原理研究厚度和空位缺陷对Si/SiO2界面电子结构与光学性质的影响

采用基于密度泛函理论的平面波超软赝势方法,在局域密度近似（ LDA）下研究了Si纳米层厚度和O空位缺陷对Si／SiO2界面电子结构及光学性质的影响．电子结构计算结果表明：在0.815～2.580nm的Si层厚度范围内, Si／SiO2界面结构的能隙随着厚度减小而逐渐增大,表现出明显的量子尺寸效应,这与实验以及其他理论计算结果一致;三种不同的O空位缺陷的存在均使得Si／SiO2界面能隙中出现了缺陷态,费米能级向高能量方向移动,且带隙有微弱增加．光学性质计算结果表明：随着Si纳米层厚度的减小, Si／SiO2界面吸收系数产生了蓝移; O空位缺陷引入后,界面光学性质的变化主要集中在低能区,即低能区的吸收系数和光电导率显著增加．可见,改变厚度和引入缺陷能够有效地调控Si／SiO2界面体系的电子和光学性质,上述研究结果为Si／SiO2界面材料的设计与应用提供了一定的理论依据．     

AES and ELS study of gold deposition on top of hydrogen saturated Si(1 1 1) surfaces

Gold was deposited on top of hydrogen saturated Si(1 1 1) clean surfaces. The electronic nature and atomic intermixing between gold overlayer and silicon substrate were studied by AES and ELS. It is claimed that there is a critical thickness (～ 2 ML), where 1 ML = 7.8 × 10¹⁴ atoms cm^?2 for Si(1 1 1), for gold to induce alloyed metallic overlayer formation on the surface of a specimen due to intermixing reaction independently whether a surface of Si(1 1 1) substrate is saturated with hydrogen atoms or not.     

Passivation of Si (111) surfaces by ordered metal layers

The properties of √3 × √3 ordered gold and silver monolayers on a Si(111) substrate have been investigated by Auger, low energy electron diffraction and photo-emission analysis. It has been found that oxygen adsorption on these surfaces is considerably weaker than on clean Si surfaces. This new result clearly emphasizes the correlation between the oxidation properties of Si atoms and their local environment. A comparison is made with previous results concerning Au-Si amorphous metallic alloys where gold atoms act as a catalyst for the oxidation.     

Capping layers for extreme-ultraviolet multilayer interference coatings

The reflectivity of extreme-ultraviolet thin-film multilayer (ML) interference coatings, terminated by a native oxide or other capping layer (CL), is critically dependent on the thickness of the final deposited layer of the top period. We show in this numerical study that, for a molybdenum-silicon ML, a high reflectivity loss may be incurred if the final Si layer is not of optimum thickness. For maximum reflectivity the thickness of the final Si layer must be controlled such that the node of the standing wave lies within the absorptive CL.The final Si layer may be replaced, at the expense of reflectivity, by SiC and capped with another inert material for improved protection of the ML.     

Application of high-energy synchrotron-radiation XPS to determine the thickness of SiO2 thin films on Si(100)

The escape depth of photoelectrons depends on their kinetic energy. We apply this relationship to measure film thicknesses from X-ray photoelectron spectroscopy (XPS) measurements with tunable-energy synchrotron radiation (SR). For this purpose, a “high-energy SR-XPS” instrument has been constructed and used to characterize thermally oxidized thin films on Si(100) single crystals. In order to observe photoelectrons emitted from deeper regions than with conventional XPS, Si 1s photoelectrons with an energy up to 4000 eV were measured with X-ray energies up to 5800 eV. The oxide thickness was estimated from measurements of the relative Si intensities from the oxide and the substrate at various photon energies. Our results suggest that the SR-XPS system is useful for measuring the thickness of thin films.     

Probing Lattice Vibrations at SiO_2/Si Surface and Interface with Nanometer Resolution

Recent advances in monochromatic aberration corrected electron microscopy make it possible to detect the lattice vibrations with both high-energy resolution and high spatial resolution. Here, we use sub-10 meV electron energy loss spectroscopy to investigate the local vibrational properties of the SiO_2/Si surface and interface. The energy of the surface mode is thickness dependent, showing a blue shift as z-thickness(parallel to the fast electron beam)of SiO_2 film increases, while the energy of the bulk mode and the interface mode keeps constant. The intensity of the surface mode is well-described by a Bessel function of the second kind. The mechanism of the observed spatially dependent vibrational behavior is discussed and compared with dielectric response theory analysis. Our nanometer scale measurements provide useful information on the bonding conditions at the surface and interface.     

Edge effect and interface confinement modulated strain distribution and interface adhesion energy in graphene/Si system

In order to clarify the edge and interface effect on the adhesion energy between graphene(Gr)and its substrate,a theoretical model is proposed to study the interaction and strain distribution of Gr/Si system in terms of continuum medium mechanics and nanothermodynamics.We find that the interface separation and adhesion energy are determined by the thickness of Gr and substrate.The disturbed interaction and redistributed strain in the Gr/Si system induced by the effect of surface and interface can make the interface adhesion energy decrease with increasing thickness of Gr and diminishing thickness of Si.Moreover,our results show that the smaller area of Gr is more likely to adhere to the substrate since the edge effect improves the active energy and strain energy.Our predictions can be expected to be a guide for designing high performance of Grbased electronic devices.     

Inelastic electron scattering observation using energy filtered transmission electron microscopy for silicon -- germanium nanostructures imaging

This paper presents a new technique using energy filtered TEM (EFTEM) for inelastic electron scattering contrast imaging of Germanium distribution in Si-SiGe nanostructures. Comparing electron energy loss spectra (EELS) obtained in both SiGe and Si single crystals, we found a spectrum area strongly sensitive to the presence of Ge in the range [50-100 eV]. In this energy loss window, EELS spectrum shows a smooth steeply shaped background strongly depending on Ge concentration. Germanium mapping inside SiGe can thus be performed through imaging of the EELS background slope variation, obtained by processing the ratio of two energy filtered TEM images, respectively, acquired at 90 and 60 eV. This technique gives contrasted images strongly similar to those obtained using STEM Z-contrast, but presenting some advantages: elastic interaction (diffraction) is eliminated, and contrast is insensitive to polycrystalline grains orientation or specimen thickness. Moreover, since the extracted signal is a spectral signature (inelastic energy loss) we demonstrate that it can be used for observation and quantification of Ge concentration depth profile of SiGe buried layers.     

High-resolution absorption measurements in gold doped Si/Ge alloys

High-resolution studies of a deep impurity in Si/Ge alloys are presented. It is shown that gold forms at least two different centers, a single substitutional defect and a gold pair. The energy structure and internal transitions of these defects were studied for different alloy compositions and implications of possible strain and alloying effects are briefly discussed.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

Band-structure engineering of gold atomic wires on silicon by controlled doping

We report on the systematic tuning of the electronic band structure of atomic wires by controlling the density of impurity atoms. The atomic wires are self-assembled on Si(111) by substitutional gold adsorbates and extra silicon atoms are deposited as the impurity dopants. The one-dimensional electronic band of gold atomic wires, measured by angle-resolved photoemission, changes from a fully metallic to semiconducting one with its band gap increasing above 0.3 eV along with an energy shift as a linear function of the Si dopant density. The gap opening mechanism is suggested to be related to the ordering of the impurities.     

超短激光与金薄膜靶相互作用产生的质子能谱

在能量11 mJ、波长744 nm、脉宽120 fs、功率密度61016 W/cm2的超短脉冲装置上,开展了超短脉冲激光与2.1 m和5.0 m金薄膜靶相互作用产生质子束的实验研究。利用Thomson谱仪测量了产生的质子能谱,发现利用2.1 m金薄膜靶时,质子能谱由于质子源数量不足而在74 keV附近出现单能峰,5.0 m的金薄膜靶产生的质子计数和能谱均比2.1 m的金薄膜靶产生的低,主要原因是超热电子穿过薄膜靶时出现的能量损失和几何倾斜降低了电子回流所致。     

Long range surface plasmon polariton waveguides at 1.31 and 1.55 μm wavelengths

We investigate characteristics of gold metal strip waveguides based on long range surface plasmon polaritons (LRSPPs) along thin metal strips embedded in a polymer for practical applications at the telecommunication wavelengths of 1.31 and 1.55 μm. Guiding properties of the gold strip waveguides are theoretically and experimentally evaluated with the limited thickness and width up to ∼20 nm and ∼10 μm, respectively. The lowest propagation loss of ∼1.4 dB/cm is obtained with a 14.5-nm-thick and 2-μm-wide gold strip at 1.55 μm. With a single-mode fiber, the lowest coupling loss of ∼0.4 dB/facet is achieved with a 14.5-nm-thick and 5-μm-wide gold strip at 1.55 μm. The lowest insertion losses are obtained 8-9 dB with 1.5 cm-long gold strips of a limited thickness and width at both the wavelengths. We demonstrate a 10 Gbps optical signal transmission via the LRSPP waveguide with a 14 nm-thick, 2.5 μm-wide, and 4 cm-long gold strip. These LRSPP waveguides have potential applications for optical interconnects and communications.