Impulsive fracture of silicon by elastic surface pulses with shocks

During nonlinear evolution of surface acoustic waves (SAWs) stress increases with propagation, and may cause fracture of brittle materials. This effect was used to evaluate the strength of crystalline silicon with respect to impulsive load in the nanosecond time scale without using seed cracks. Short SAW pulses propagating in the [11(macro)2] direction on the Si(111) plane induce fracture at significantly lower SAW amplitudes than the mirror symmetric wave propagating in the [112(macro)] direction. This effect is explained by the differences in elastic nonlinearity of the two propagation directions.     

Ideal shear strength under compression and tension in C, Si, Ge, and cubic SiC: an ab initio density functional theory study

Ideal shear strength under superimposed normal stress of cubic covalent crystals (C, Si, Ge, and SiC) is evaluated by ab initio density functional theory calculation. Shear directions in [112] and [110] on the (111) plane are examined. The critical shear stress along the former direction is lower than that along the latter in all the crystals unless the hydrostatic tension is extremely high. In both the [112]-shear and [110]-shear, critical shear stress is increased by compression in C but is decreased in the other crystals. The different response of the critical shear stress to normal stress is due to the strength of the bond-order term, i.e., dependence of the short-range interatomic attraction on the bond-angle.     

Si衬底上分子束外延Ge,Si时的反射式高能电子衍射强度振荡观察

本文观察了在Si(100)和Si(111)衬底上分子束外延Si,Ge时的反射式高能电子衍射(RHEED)强度振荡现象。其振荡特性表明,外延一定厚度的缓冲层可以改善表面的平整性,较慢的生长速率或中断生长一段时间有利于外延膜晶体质量的提高。Si(100)上外延Si或Ge时,沿[100]和[110]方位观测到的振荡特性均为单原子模式,起因于表面存在双畴(2×1)再构;而Si(111)上外延Ge时,[112]方位观测到的振荡为双原子层模式,但在[110]方位观察到不均匀周期的强度振荡行为。两种衬底上保持RHEED     

Interaction of a Surface Acoustic Wave with a Two-dimensional Electron Gas

When a surface acoustic wave （SAW） propagates on the surface of a GaAs semiconductor, coupling between electrons in the two-dimensional electron gas beneath the interface and the elastic host crystal through piezoelectric interaction will attenuate the SAW. The coupling coemcient is ~alculated for the SAW propagating along an arbitrary direction. It is found that the coupling strength is strongly dependent on the propagating direction. When the SAW propagates along the [011] direction, the coupling becomes quite weak.     

Interaction of a Surface Acoustic Wave with a Two-dimensional Electron Gas

When a surface acoustic wave (SAW) propagates on the surface of a GaAs semiconductor, coupling between electrons in the two-dimensional electron gas beneath the interface and the elastic host crystal through piezoelectric interaction will attenuate the SAW. The coupling coefficient is calculated for the SAW propagating along an arbitrary direction. It is found that the coupling strength is strongly dependent on the propagating direction. When the SAW propagates along the [011] direction, the coupling becomes quite weak.     

Angular dependence of surface acoustic wave characteristics in AlN thin films on a-plane sapphire substrates

AlN thin films have been grown on a-plane sapphire (Al₂O₃(112̄0)) substrates. X-ray diffraction measurements indicate the films are fully c-plane (0001) oriented with a full width at half maximum of the AlN(0002) rocking curves of 0.92. The epitaxial growth relationships have been determined by the reflection high energy electron diffraction analysis as AlN[11̄00]//Al₂O₃[0001] and AlN[112̄0]//Al₂O₃[11̄00]. Angular dependence of important surface acoustic wave (SAW) characteristics, such as the phase velocity and electromechanical coupling coefficient, has been investigated on the AlN(0001)/Al₂O₃(112̄0) structure. While the SAW is excited at all propagation angles with an angular dispersion of the phase velocity in the range of 5503–6045 m/s, a higher velocity shear-horizontal (SH) mode is observed only at 0°, 105° and 180° off the reference Al₂O₃[11̄00] over a 180° angular period. The phase velocity of the SH mode shows dispersion (6089–6132 m/s) as a function of the SAW wavelength. Temperature coefficients of frequency are also demonstrated for both modes. PACS 81.15.Hi; 77.84.-s; 77.65.Dq     

第一性原理研究[112]硅锗异质结纳米线的电子结构与光学性质

基于密度泛函理论体系下的广义梯度近似,本文利用第一性原理方法着重研究了[112]晶向硅锗异质结纳米线的电子结构与光学性质.能带结构计算表明:随着锗原子数的增加,[112]晶向硅锗纳米线的带隙逐渐减小;对Si_(36)Ge_(24)H_(32)纳米线施加单轴应变,其能量带隙随拉应变的增加而单调减小.光学性质计算则表明:随着锗原子数的增加,[112]硅锗纳米线介电函数的峰位和吸收谱的吸收边均向低能量区移动;而随着拉应变的增大,吸收系数峰值呈现出逐渐减小的趋势,且峰位不断向低能量区移动,上述结果说明锗原子数的增加与施加拉应变均导致[112]硅锗纳米线的吸收谱产生红移.本文的研究为硅锗异质结纳米线光电器件研究与设计提供一定的理论参考.     

气体吸附对硅锗异质结纳米线电子结构与光学性质的影响

基于密度泛函理论体系下的广义梯度近似(GGA),采用第一性原理方法探讨了沿[112]晶向的硅锗异质结纳米线作为气体传感器检测CO,CO₂和Cl₂的能力,着重计算了其吸附气体分子前后的吸附能、能带结构与光学性质.几何结构优化计算表明：不同硅锗组分的[112]晶向的硅锗纳米线对CO,CO₂和Cl₂分子的吸附能的绝对值在0.001 eV至1.36 eV之间,其中Si₂₄Ge₃₆H₃₂对CO₂气体的吸附能最大,气敏性能最好.能带结构计算表明：吸附CO和CO₂分子的[112]晶向硅锗纳米线能带的简并度明显减小,带隙变化较小;而吸附Cl₂分子后的价带顶与导带底之间产生了杂质能级使其带隙减小.光学性质计算表明：Si₂₄Ge₃₆H₃₂纳米线吸附CO, CO₂和Cl₂分子后的光学...     

SAW characteristics of AlN films sputtered on silicon substrates

This article is focused on the analysis of the electroacoustic response of surface acoustic wave (SAW) filters made of aluminium nitride (AlN) thin films on various types of Si wafers. AlN films with (00.2) orientation were deposited by RF reactive sputtering of an Al target in Ar and N(2) admixtures on Si(100) and (111) wafers with resistivities ranging between 10 and 2000 Omega cm. The electroacoustic response of SAW filters with an acoustic wavelength of 40 microm was analysed by measuring the Sij parameters with a network analyser. We have determined that the out-of-band loss is directly related to the Si substrate resistivity, varying from 26 dB for 10 Omega cm to 55 dB for 2000 Omega cm. The SAW velocity depends on the orientation of the Si wafer, being approximately 4700 m/s for Si(111) and 5100 m/s for Si(100). The electroacoustic responses of the SAW filters were fitted by computations based on a simple circuital model that takes into account parasitic effects such as airborne electromagnetic coupling and conduction through the substrate. This procedure provides accurate values of the electromechanical coupling factor k2 even for devices with poor characteristics. Good quality SAW filters of AlN on high resistivity Si(100) wafers with k2 larger than 0.12% are demonstrated.     

MOCVD法生长SAWF用ZnO/Diamond/Si多层结构

使用等离子体辅助MOCVD系统在金刚石,硅衬底上成功地制备了氧化锌多层薄膜材料,通过两步生长法对薄膜质量进行了优化。XRD测试显示优化后的样品具有c轴的择优取向生长,PL谱测试表明样品经优化后不仅深能级发射峰消失,同时紫外发射峰增强。对优化后的样品的表面测试显示出较低的表面粗糙度。比较氧化锌多层薄膜结构的声表面波频散曲线,ZnO薄膜声表面滤波器受膜厚和衬底材料的影响较大。当ZnO薄膜较薄时,在它上面的传播速度将与衬底上的传播速度接近,与其他衬底上生长的薄膜相比,以金刚石这种快声速材料为衬底的ZnO多层薄膜结构,声表面波滤波器的中心频率将提高1倍左右。     

国产纯铁的轧制与再结晶织构

用极固与金相研究工业纯铁的轧制与再结晶织构和组织。热轧后的试样经过两种冷轧方法：(1)压下率为98.8%,与(2)压下率为64.5%,中间700℃熟炼;二次冷轧和压下率63.5%。试样在氢气中分别于(a)650°和(b)1000℃熟炼。第一类材料的轧制织构经测定为(100)[011]+(112)[110]+(111)[112].试样在a与γ区域熟炼后的主要取向为(100)[011]和(111)[112]。第二类材料的轧制织构与第一类相似,惟偏离角度较大。表面与内部织构不同。第二类材料熟炼后的再结晶织构基本上相似,金相组织显出第二次再结晶现象。     

Transition from overlayer growth to alloying growth of Ga on Si(111)-alpha-(sqrt[3]xsqrt[3])-Au

Atomic depth distribution and growth modes of Ga on an Si(111)-alpha-(sqrt[3]xsqrt[3])-Au surface at room temperature were studied after each monolayer deposition of Ga via reflection high-energy electron diffraction and characteristic x-ray spectroscopy measurements as functions of glancing angle theta(g) of the incident electron beam. One monolayer of Ga grew on the Au layer, and the sqrt[3]xsqrt[3] periodicity was conserved below the Ga overlayer. Above a critical Ga coverage of about one monolayer, this growth mode drastically changed; i.e., Au atoms dissociated from the sqrt[3]xsqrt[3] structure and Ga grew into islands of Ga-Au alloy.     

Nonlinear surface acoustic waves: silicon strength in phonon-focusing directions

The anisotropy of the elastic properties of single-crystal silicon manifests itself in features of both the linear and nonlinear surface acoustic wave (SAW) propagation. Directions showing the phonon-focusing effect and strong nonlinearity were employed in contact-free and notch-free laser-based fracture experiments, yielding the intrinsic strength of silicon. The critical tensile stress values vary between 2.5 GPa and 7 GPa for the different crystallographic planes and directions of SAW propagation investigated.     

High-K band structures in 164Er

High-spin states of the doubly-odd ¹¹²Sb were studied by in-beam spectroscopy using the ⁸⁸Sr (²⁸Si, p3n) and ⁸⁹Y (²⁹Si, α2n) fusion-evaporation reactions at beam energies of 120 and 108 MeV, respectively. γ?γ, charged particle-γ?γ coincidences, and γ?γ angular correlation analyses were employed for determining the level scheme of ¹¹²Sb. In the present work, all the levels except for low-lying states in ¹¹²Sb were newly established. Two ΔI = 1 strongly coupled bands were observed; one is a negative-parity band that is similar to those observed in the neighboring doubly-odd Sb isotopes and the other is a positive-parity band that has a new type structure not observed in the other isotopes. From the similarity of the properties of these ΔI = 1 bands to the bands built on 9/2⁺ 2p?1h states in the odd-A Sb isotopes, we suggest that these two ΔI = 1 bands should be associated with the [π(g_9/2)^?1 ? νh_11/2] and [π(g_9/2)^?1νg_7/2] configurations, respectively.     

Fabrication and characteristics of a surface acoustic wave UV sensor based on ZnO thin films grown on a polycrystalline 3C-SiC buffer layer

Zinc oxide (ZnO) thin films were deposited onto a polycrystalline (poly) 3C-SiC buffer layer for surface acoustic wave (SAW) ultraviolet (UV) sensing using a magnetron sputtering system. X-ray diffraction (XRD) and photoluminescence (PL) spectra showed that the ZnO film grown on 3C-SiC/Si had a dominant c-axis orientation, a lower residual stress, and higher intensity of luminescence at 380 nm of ZnO thin film. The SAW resonator UV detector were fabricated on ZnO/Si structures with a 3C-SiC buffer layer. The SAW resonator exposed under UV illumination had a linear response with sensitivity of 85 Hz/(μW/cm²) in ZnO/3C-SiC/Si structures, as compared to 25 Hz/(μW/cm²) in ZnO/Si structures with UV intensity varied until 600 μW/cm².     

Characteristics of SAW UV sensors based on a ZnO/Si structure using third harmonic mode

This paper describes the characteristics of surface acoustic wave (SAW) ultraviolet (UV) sensors fabricated from a ZnO thin film using the third harmonic mode. A ZnO thin film was used as an active layer for UV detection, and a piezoelectric layer was sputtered using magnetron sputtering. The X-ray diffraction (XRD) and photoluminescence (PL) spectra showed that the ZnO sputtered onto Si(100) was highly (002)-oriented and had good optical properties. The two-port SAW resonator was based on an inter-digital transducer (IDT)/ZnO/Si structure and was fabricated and exposed under UV light at a wavelength of 380 nm. As a result, under a UV intensity of 3 mW/cm², the SAW UV sensor was greatly shifted by 400 kHz at the third harmonic mode compared to a frequency shift of 10 kHz in the fundamental mode.     

Vibration amplification, damping, and self-oscillations in micromechanical resonators induced by optomechanical coupling through carrier excitation

Carrier-induced dynamic backaction in micromechanical resonators is demonstrated. Thermal vibration of an n-GaAs/i-GaAs bilayer cantilever is amplified by optical band-gap excitation, and for the excitation power above a critical value, self-oscillations are induced. These phenomena are found in the [1[over ˉ]10]-oriented cantilever, whereas the damping (deamplification) is observed in the [1[over ˉ]10] orientation. This optomechanical coupling does not require any optical cavities but is instead based on the piezoelectric effect that is generated by photoinduced carriers.     

Search for doubly charged Higgs bosons with lepton-flavor-violating decays involving tau leptons

We search for pair production of doubly charged Higgs particles (H+/- +/-) followed by decays into electron-tau (etau) and muon-tau (mutau) pairs using data (350 pb(-1) collected from [over]pp collisions at sqrt[s]=1.96 TeV by the CDF II experiment. We search separately for cases where three or four final-state leptons are detected, and combine results for exclusive decays to left-handed etau (mutau) pairs. We set an H+/- +/- lower mass limit of 114(112) GeV/c(2) at the 95% confidence level.     

Disproportionation phenomena on free and strained Sn/Ge(111) and Sn/Si(111) surfaces

Distortions of the sqrt[3]x sqrt[3] Sn/Ge(111) and Sn/Si(111) surfaces are shown to reflect a disproportionation of an integer pseudocharge, Q, related to the surface band occupancy. A novel understanding of the (3 x 3)-1U ("1 up, 2 down") and 2U ("2 up, 1 down") distortions of Sn/Ge(111) is obtained by a theoretical study of the phase diagram under strain. Positive strain keeps the unstrained value Q=3 but removes distortions. Negative strain attracts pseudocharge from the valence band causing first a (3 x 3)-2U distortion (Q=4) on both Sn/Ge and Sn/Si, and eventually a (sqrt[3] x sqrt[3])-3U ("all up") state with Q=6. The possibility of a fluctuating phase in unstrained Sn/Si(111) is discussed.     

Some improvement in Fe/Si multilayered neutron polarizor by modified sputtering geometry

Aperture geometry of magnetron sputtering unit was modified by insertion of longitudinal [B] and transverse [A] slits with respect to the motion of substrate. Fe-, Si-mono and Fe/Si multilayer [Fe (70 Å)/Si (50 Å)]₁₀ (called ML70) were grown with and without modified apertures. Maximum stress anisotropy was found in samples prepared with transverse modified aperture [A]. X-ray reflectometry (XRR) results showed a marked difference in roughness values of interdiffusion layers Fe-_Si and Si-_Fe. Magnetic measurements showed coercive field strength of 50 Oe in ML70 prepared by modified aperture geometry Fe[A]/Si[O]. A significant improvement in the neutron polarization efficiency is observed in multilayer ML70 with the use of modified aperture geometry as compared to similar sample prepared by normal aperture.