Diffusion dynamics during the nucleation and growth of Ge/Si nanostructures on Si(111)

We report a low energy electron microscopy study of the relation between self-organized Ge/Si(111)nanostructures and their local environment. By comparison with Monte Carlo simulations, three-dimensional islands are shown to display a substantial tendency towards self-ordering. This tendency may result from the diffusive nature of the nucleation processes. The size of individual nanostructures does not significantly correlate with the distance between neighboring islands. Thus energetic factors are thought to govern the competition among coexisting nanostructures to capture the deposited mass.     

Simulation of range profiles for boron implantation into SiO2/Si and Si3N4/SiO2/Si targets

The simple method of profile combination is shown to be applicable to the simulation of boron profiles in SiO₂/Si and Si₃N₄/Si layered targets. This is demonstrated by comparison with range distributions calculated by more sophisticated theoretical methods, i.e. TRIM Monte Carlo simulations and the algorithm of Christel et al., and with experimental data. The method of profile combination can also be applied to layered targets with a crystalline silicon substrate.     

Formation of pits during growth of Si/Ge nanostructures

Alternating deposition of Ge and Si in the step-flow growth regime using Bi acting as a surfactant can lead to a spontaneous formation of one atomic layer deep pits in the area of surface covered by Ge. During Si growth Ge atoms of the epitaxial 2D Ge layer move to Si step edges where stronger bonds with Si atoms are formed. Appropriate growth conditions can suppress or enhance the pit formation effect and consequently a new type of self-organized nanostructures can be formed.     

Ferroelectric SrxBa1-xNb2O6 optical waveguiding thin films on SiO2-coated Si(100) substrates

x Ba_1-xNb₂O₆ (x=0.5) films (abbreviated as SBN:0.5) on SiO₂-coated Si substrates are potential components for the application of integrated electro-optics devices. SBN:0.5 optical waveguiding thin films on SiO₂-coated Si substrates with a very thin MgO diffusion buffer have been successfully prepared by pulsed laser deposition. The as-grown films have a refractive index of 2.28, which is close to that of bulk SBN. X-ray analysis showed that the as-grown films have a single-phase tetragonal tungsten bronze structure. The SBN:0.5 thin films prepared by PLD exhibit favorable ferroelectric and optical waveguiding properties. The composition and the morphology of the films were also examined by XPS and by SEM, respectively. Ferroelectric SBN:0.5 optical waveguiding thin films on SiO₂-coated Si substrates are expected to be used in integrated electro-optic devices. Received: 27 February 1997/Accepted: 17 October 1997     

Transformation behavior of Ni-Mn-Ga/Si(100) thin film composites with different film thicknesses

A series of Ni_51.4Mn_28.3Ga_20.3/Si(100) thin film composites with different film thicknesses varying from 0.1 to 5 μm have been prepared by magnetron sputtering and subsequently annealed. X-ray powder diffraction patterns of the films show the features associated with the lattice-modulated martensitic phase and/or cubic austenite at room temperature. 220-fiber texture was confirmed by the X-rays measurements made at 150 °C. While the Curie temperature is almost film thickness independent, the martensitic transformation temperature shows a strong descended dependence in the submicron range. The substrate curvature measurements demonstrate that the forward and reverse martensitic transformation in the films is accompanied by the reversible relaxation and accumulation of residual stress, originally created by the thermal treatment due to the difference in thermal expansion of the film and substrate. The values of residual stresses measured by both substrate curvature and X-rays diffraction methods at constant temperatures are found to be dependent on the film thickness. This behavior appears in correlation with the thickness dependence of the transformation temperature.     

Surfactant-mediated growth of CoSi2 on Si(100)

Thin co-deposited CoSi₂ films grown at 500–600°C on Si(100) are studied by scanning tunneling microscopy (STM). With a Si rich deposition we observe initially the formation of elongated three-dimensional CoSi₂ islands. The use of one preadsorbed atomic layer of As as a surfactant results in a drastic increase of the island density. This effect appears to be a consequence of a decreased rate of surface diffusion of Co and Si. At higher coverages the roughness of the CoSi₂ film is reduced considerably by the surfactant. The results are discussed with regard to the method of allotaxy which allows the fabrication of buried silicide layers. Here, the requirements for small precipitates and high growth temperature can possibly be met more efficiently using As as a surfactant.     

ZnSe/SiO2复合薄膜光学常数与荧光光谱的研究

采用溶胶-凝胶工艺与原位生长技术,制备了ZnSe/SiO2复合薄膜.X射线衍射分析表明薄膜中ZnSe晶体呈立方闪锌矿结构.X射线荧光分析结果显示薄膜中Zn与Se摩尔比为1:1.01-1:1.19.利用场发射扫描电子显微镜观察了复合薄膜的表面形貌,结果表明复合薄膜表面既存在尺寸约为400nm的ZnSe晶粒,也存在尺寸小于100nm的ZnSe晶粒.利用椭偏仪测量了薄膜椭偏角Ψ,Δ与波长λ的关系,采用Maxwell-Garnett有效介质理论对薄膜的光学常数、厚度、气孔率、ZnSe的浓度进行了数据拟合.利用荧光光谱分析了薄膜的光致发光,结果表明在波长为395nm的激发光下,487nm的发射峰对应着闪锌矿型ZnSe的带边发射,同时也观测到薄膜中ZnSe晶体增强的自由激子发射及伴随着ZnSe晶体缺陷而产生的辐射发光.     

SiO production from Si(100) and (111) surfaces by reaction with O2 beams

Desorption kinetics of SiO in the reaction of O₂ with Si(100) and (111) surfaces were investigated at surface temperatures between 1000 and 1300 K by using a pulsed molecular beam technique. The gaseous SiO product was detected by a mass spectrometer above 1000 K. At temperatures lower than 1050 K, the SiO signal appeared with an induction time after the O₂ beam irradiation onto the surface, which means that the desorption occurs via sequential steps. The rate constants for two steps were obtained by a computer simulation of the relaxation waveforms of the SiO signal. The values obtained are in the same range as those of D'Evelyn et al. However, they are one order of magnitude larger than those of Yu and Eldridge. The activation energies for the two steps are 2.8 and 2.4 eV for both Si(100) and (111) surfaces. No significant difference between the two kinds of surfaces was found from these values. However, many etch pits were observed on the Si(100) surface after the reaction, while no such etch pits were formed on the Si(111) surface. The planes of the etch pits formed on the (100) surfaces consisted mostly of the {111} facets.     

Quantum-chemical simulation of adsorption of B+ ions on a SiO2/Si(100) interface

Quantum-chemical simulation of adsorption and migration of B⁺ ions on a SiO₂/Si(100) interface has been performed. The dependences of the total energy of the cluster-B⁺ ion system on the reaction coordinate and geometric and electronic characteristics of the equilibrium states of a cluster with an adsorbed boron ion have been calculated.     

Optical second-harmonic investigations of the isothermal desorption of SiO from the Si(100) and Si(111) surfaces

The isothermal desorption of SiO from the Si(100) and Si(111) surfaces was investigated by means of optical second-harmonic generation (SHG). Due to the high adsorbate sensitivity of this method, desorption rates could be measured over a wide range from 10⁻¹ to 10⁻⁶ ML s⁻¹. From their temperature dependence between 780 and 1000 K, activation energies of E_A=3.4±0.2 eV and E_A=4.0±0.3 eV and pre-exponential factors of ν₀=10^16±1 s⁻¹ and ν₀=10^20±1 s⁻¹ for SiO desorption were obtained for Si(100) and Si(111), respectively. In the case of the Si(100) surface, a pronounced decrease of the first-order rate constants was observed upon increasing the initial coverage from 0.02 to 0.6 ML. The results are interpreted in terms of coverage-dependent oxygen-binding configurations, which influence the stability of the oxide layer.     

Photoacoustic spectroscopy of thin SiO2 films grown on (100) crystalline Si substrates

Photoacoustic Spectroscopy (PAS) has been used to measure the thickness of thin SiO₂ films grown on (100) Si wafers. The data are in reasonable agreement with a simple theoretical model. It suggests that photoacoustic Spectroscopy is complementary to optical interferometry, in that it is capable of giving quantitative estimates of thin transparent films on opaque substrates of low reflectivity via the transmitted fraction of the optical energy incident on the sample. Both theoretical and experimental results indicate that PAS can be very useful in the measurement of thin films on substrates of low reflectivity.     

XANES investigations of interatomic interactions in multilayered nanostructures (Co45Fe45Zr10/a-Si)40 and (Co45Fe45Zr10/SiO2)32

The electronic structure and phase composition of amorphous multilayered nanostructures (Co₄₅Fe₄₅Zr₁₀/a-Si)₄₀ and (Co₄₅Fe₄₅Zr₁₀/SiO₂)₃₂ have been investigated by means of the X-ray absorption near-edge structure (XANES) technique, which is the most sensitive and useful in investigation of the chemical environment of elements in multicomponent nanostructures. The fact of interatomic interactions leading to the formation of composite “nanoferrite”-like FeO · Fe₂O₃ · ZrO₂(CoO) was established. Also it was shown that in the mentioned nanoferrite there is an exchange interaction which involves not only two- and three-charged ions of iron (Fe²⁺ and Fe³⁺) but also ions like Zr⁴⁺ and, partially, Co²⁺. The transformation of the thin structure of L _2,3-ranges for the iron component of multilayered nanostructures in XANES spectra reflects on the change of the ratio of di- and trivalent ions in iron oxides as a part of the composite “nanoferrite.”     

Annealing Induced Degradation of Thermal SiO2 On (100)Si: Point Defect Generation

The structural degradation of thermal SiO₂ on (100)Si under isochronal post oxidation vacuum annealing (POVA) has been probed by electron spin resonance (ESR). The degradation process, studied in the range T_an = 950–1250 °C, is established as intense point defect generation including E′_γ, E′_δ, EX and the elusive predominant degradation center S. Thermally activated generation is revealed over broad T_an ranges for the two most populous defects, S and E′_γ, with a common activation energy ～1.6eV. Depth profiling after heating at 1200°C shows that the S centers predominantly reside near the oxide borders, generally in anticorrelation with the E′_γ distribution. The S center susceptibility has been inferred from conventional ESR signal intensity monitoring as well as from revealed anisotropic demagnetisation effects. It is found Curie-Weiss type with critical temperature of ～ 1.3 K. Newly observed weak hyperfine structure may comply with the S center being an E′-like defect.