Preparation of thin GaAs suspended membranes for gas micro-sensors using plasma etching

The GaAs/AlGaAs heterostructure layer system grown by MBE on GaAs substrate was designed to be used for micro-mechanical structure fabrication. In the first step, double-side aligned photolithography is carried out to define the etching masks on both sides of the substrate. After this, highly selective reactive ion etching (SRIE) of GaAs from the front side is used to determine the lateral dimensions of the membrane structure. The vertical dimension is defined by deep backside SRIE through a 300 μm thick GaAs substrate to the AlGaAs etch stop layer, hence the structure thickness is precisely determined by the thickness of MBE-grown GaAs layer over this etch stop layer. The last step is selective etching of the AlGaAs etch stop layer. The thermal resistance value of the membrane structure as high as 21 K/mW is achieved.     

CF3 etching silicon surface: A molecular dynamics study

In this study, a molecular dynamics simulation method has been employed to investigate CF₃⁺ ions bombarding Si surface with the energy of 100, 200, 300 and 400 eV and an incident angle of 45° with respect to the normal. The simulation results show that when CF₃⁺ ions approach the Si surface they are broken up into small fragments. Some fragments deposit on the surface to form a “fluorocarbosilyl” layer. The erosion of Si is dominated by formation of SiF₃ followed by SiF₂ species and in minority species SiF.     

Chemical/mechanical polishing of diamond films assisted by molten mixture of LiNO3 and KNO3

Chemical/mechanical polishing can be used to polish the rough surface of diamond films prepared by chemical vapor deposition (CVD). In this paper, a mixture of oxidizing agents (LiNO₃ + KNO₃) has been introduced to improve the material removal rate and the surface roughness in chemical/mechanical polishing because of its lower melting point. It had been shown that by using this mixture the surface roughness R_a (arithmetic average roughness) could be reduced from 8-17 to 0.4 μm in 3 h of polishing, and the material removal rate can reach 1.7-2.3 mg/cm²/h at the temperature of 623 K. Pure aluminium is compared with cast iron as the contact disk material in the polishing. Although the material removal rate of aluminiumdisk is lower than that of cast iron, it can eliminate the carbon contamination from the contact disk to the surface of diamond films, and facilitate the analysis of the status of diamond in the chemical/mechanical polishing. The surface character and material removal rate of diamond films under different polishing pressure and rotating speed have also been studied. Graphite and amorphous carbon were detected on the surface of polished diamond films by Raman spectroscopy. It has been found that the oxidization and graphitization combined with mechanical cracking account for the high material removal rate in chemical/mechanical polishing of diamond films.     

Simulation of Si and SiO2 etching in CF4 plasma

The reactive ion etching (RIE) of Si and SiO₂ in CF₄ plasma is considered. The dependences of RIE rates of Si and SiO₂ on pressure have maxima due to the presence of single-atom vacancies. The RIE rates approach the maximum values at different pressures but at the same concentration of SiF and SiOF molecules in the adsorbed layer. Using the obtained results Si/SiO₂ etching selectivity is investigated.     

Critical cooling rate for the glass formation of ferromagnetic Fe40Ni40P14B6 alloy

Bulk ferromagnetic amorphous Fe-Ni-P-B alloys in rod shape were formed by a rapid solidification technique. The largest amorphous specimen prepared had a diameter of ∼ 2.5 mm and the corresponding cooling rate for the glass formation of this alloy system in our experiment can be estimated to be around 492.4 K/s by the method of finite-difference numerical calculation. This value is on the same order of magnitude as the critical cooling rate R_c of Fe₄₀Ni₄₀P₁₄B₆ alloy estimated by the method of constructing the continuous-cooling-transformation (CCT) curve. It is indicated that the heterophase impurities have been eliminated well in our experiment.     

Molecular dynamics simulations of amorphisation in Al and Ni3Al

Using Molecular Dynamics and an Embedded Atom Method (EAM) potential derived by Voter and Chen [1] for the Ni---Al binary alloy system, simulations of the crystalline to amorphous transition have been performed in fcc Al and L1₂ Ni₃Al lattices containing approximately 10 000 atoms under shock loading conditions. With tensile stress simulations, at 300 K Al turned amorphous after copious emission of dislocations, while at 10 K it did not become amorphous. The regions of amorphisation were found to be well correlated to high levels of local volume change (ΔV/V = 10%). For the Ni₃Al simulations, only shear stresses were used, and in this case the onset of amorphisation appeared to be triggered by copious dislocation multiplication. Both homogeneous and heterogeneous nucleations were observed and the propagation pattern of the amorphisation transformation very much resembled a percolation process.     

Influence of activated polymer on the etching rate of silicon in CF4+H2 plasma

The reactive ion etching (RIE) of silicon in CF₄+H₂ plasma is considered. The influence of activated polymer on the RIE rate of silicon in CF₄+H₂ plasma is determined by extrapolation of experimentally measured kinetics of the etching rate. It is found that increased adsorption of CF₂ radicals suppresses the RIE rate of silicon in CF₄+H₂ plasma during the initial stages of the etching process. The formation of activated polymer becomes pronounced when adsorbed CF₂ radicals are slowly activated. The activated polymer intensifies the etching reaction and enhances the etching rate. C atoms, produced during the reaction, contribute to the formation of polymer on the surface. The increased concentration of the polymer suppresses the RIE rate of silicon in CF₄+H₂ plasma at later stages of the etching process.     

Ag-Al2O3粉体复合材料形成过程分子动力学模拟

采用分子动力学的方法研究了Al-Ag合金粉末在氧化气氛进行热处理的物相转变与扩散行为,着重对物相扩散速率、转变温度、Ag-Al2O3物相转变过程进行了研究。结果表明：O原子与Al原子的相互作用为Ag原子的扩散提供动力,Ag的析出与Al2O3致密氧化膜的形成相互冲突,在600 K氧化气氛下合金粉末中Ag的析出速率大,内部...     

XPS study of surface chemistry of epiready GaAs(1 0 0) surface after (NH4)2Sx passivation

The results of systematic XPS studies of wet sulfide passivation capabilities to remove the native oxides and excess arsenic at the epiready GaAs(1 0 0) native surfaces are presented. Different procedures of dipping an epitaxy-ready sample (at room and elevated temperatures) in an ammonium polysulfide (NH₄)₂S_x solution combined with a UHV flash annealing were used. The surface chemistry after each processing step was investigated by the inspection of the XPS As3d and Ga3d spectra taken using an enhanced surface sensitivity mode. The analysis revealed that the procedure of sulfidation itself removes native oxides and that both As-S and Ga-S bondings are created, and when combined with subsequent UHV annealing, diminishes excess As efficiently. Moreover, the results are in agreement with our previous work on the surface Fermi level position of (NH₄)₂S_x-treated samples.     

Plasma polymerization of fluorocarbon thin films on high temperature substrate and its application to low-k films

Amorphous fluorinated carbon films have been prepared by plasma enhanced chemical vapor deposition method using C₆F₆ at substrate temperature of 400 °C. Thermal stability up to 400 °C is automatically achieved. Dielectric constant of the films is 2. Infrared absorption spectra of the films have indicated that the films contain benzene rings, and F/C ratio is unity as seen in monomer molecules. No film deposition has been observed if C₅F₈ is used as a source monomer at this substrate temperature. In situ Fourier transform infrared spectroscopy of C₆F₆ plasma has indicated production of C₆F₅ because C₁₂F₁₀ has been observed. This result and inclusion of benzene rings in the films suggest that a possible deposition precursor is C₆F₅.     

Growth conditions of the cubic phase cBN in boron nitride films

The nucleation and the subsequent coalescence period of the cubic phase cBN in sputter deposited BN-films is characterized by a shrinking of the film thickness. This is due to the transition of hBN into the denser cBN-phase which occurs inside a highly textured hBN base layer. The corresponding variation of the film thickness with the deposition time is described by a quantitative model. Full BN-stoichiometry in the hBN base layer is shown to be a mandatory condition for the nucleation process and the following growth of the cubic BN-phase. An increase of the substrate temperature fosters the incorporation of nitrogen into the growing film and, thus, the achievement of the stoichiometry condition.     

Comparison of models for silicon etching in CF4 + O2 plasma

The plasma chemical etching (PCE) of Si in CF₄ + O₂ plasma is considered. The concentrations of plasma components are calculated using values extrapolated from experimental data. Resulting calculations of plasma components are used for the calculation of Si etching rates. The concentrations of the adsorbed layer and surface components, obtained from analysis of PCE of silicon, are used for the comparison of site-balance and adsorbed-layer models. It is found that adsorbed-layer model predicts higher concentration of SiO₂ molecules on the surface than site-balance model. The difference in SiO₂ concentration is important during ion-beam-assisted etching and reactive ion etching processes as the models predict different etching rates due to different sputtering yields of Si atoms and SiO₂ molecules.     

Impedance studies on plasticized PMMA-LiX [X: CF3SO3, N(CF3SO2)2−] polymer electrolytes

Plasticized polymer electrolytes composed of poly(methylmethacrylate) (PMMA) as the host, propylene carbonate (PC) or ethylene carbonate (EC) as a plasticizer and LiX (X: CF₃SO₃⁻ or N(CF₃SO₂)₂⁻) as a salt were prepared by the solution cast technique. Impedance spectroscopy was performed in the temperature range between 303 and 383 K. In this paper, we report the electrical properties of polymer electrolytes with different lithium salts and plasticizers. The polymer electrolytes investigated exhibited high ionic conductivity at room temperature in the range of 10^− 6 to 10^− 4 S cm^− 1. The temperature dependence studies showed that the samples were ionic conductors and seemed to obey the Vogel-Tamman-Fulcher (VTF) rule. FTIR spectroscopy studies confirmed the polymer-salt interaction.     

Plasma-enhanced chemical vapor deposition of carbon films using dibromoadamantane

Carbon thin films are prepared from adamantane and dibromoadamantane by using a plasma-enhanced chemical vapor deposition method. Deposition rate for dibromoadamantane is approximately two times higher than that for pure adamantane. Infrared spectra of the films indicate that adamantane units are incorporated in the films, although higher electron temperature results in disorder in the films. The films prepared from dibromoadamantane have higher thermal stability, higher hardness and Young modulus than those from pure adamantane. Permittivity (= 3-4) of the dibromoadamantane films is higher than that (= 2-3) of pure adamantane films, which is regarded as a result of incorporation of bromine atoms and C=C bonds having higher polarizability according to the structural analysis of the films. Possible solution methods are proposed for reducing inclusion of such unfavorable chemical species and chemical bonds.     

Electrical properties of Pb(Mg1/3Nb2/3)O3-PbTiO3 ceramics modified with WO3

Ferroelectrics 0.67Pb (Mg_1/3Nb_2/3)O₃-0.33PbTiO₃ (PMN-PT) + x mol% WO₃ (x=0.1, 0.5, 1, 2) were prepared by columbite precursor method. Electrical properties of WO₃-modified ferroelectrics were investigated. X-ray diffraction (XRD) was used to identify crystal structure, and pyrochlore phase were observed in 0.67Pb (Mg_1/3Nb_2/3)O₃-0.33PbTiO₃+2 mol% WO₃. Dielectric peak temperature decreased with WO₃ doping, indicating that W⁶⁺ incorporated into PMN-PT lattice. Lattice constant, pyrochlore phase and grain size contribute to the variation of K_max. Both piezoelectric constant (d₃₃) and electromechanical coupling factors (k_p) were enhanced by doping 0.1 mol% WO₃, which results from the introduction of “soft” characteristics into PMN-PT, while further WO₃ addition was detrimental. We consider that the two factors, introduction of “soft” characteristics and the formation of pyrochlore phase, appear to act together to cause the variation of piezoelectric properties of 0.67PMN-0.33PT ceramics doping with WO₃.     

Magnetic coupling in La0.67Ca0.33MnO3/La0.67Sr0.33CoO3/La0.67Ca0.33MnO3 sandwiches

La_0.67Ca_0.33MnO₃ (LCMO)/La_0.67Sr_0.33CoO₃ (LSCO)/LCMO trilayer films are fabricated on single-crystal substrates NdGaO₃ (110) and the interlayer coupling are investigated. Compared with LCMO single layer, sandwiches showed the enhanced metal-insulator transition temperature of LCMO layers. The magnetoresistance is dependent on spacer thickness and the peak value dramatically decreases when LSCO layer is thick enough because of shorting by the LSCO layer. The magnetic coercivity H_C shows a nonmonotonic behavior with changing spacer layer thickness and the waist-like hysteresis indicates that there is an indirect exchange coupling between the top and bottom LCMO layers across the spacer layer.     

Direct diffusion bonding of Ti3SiC2 and Ti3AlC2

Two typical layered ternary compounds, Ti₃SiC₂ and Ti₃AlC₂, were joined directly by solid-state diffusion bonding method. By various bonding tests at 1100-1300 °C for 30-120 min under 10-30 MPa, and characterizing the microstructure and diffusion reactive phases of the joints by scanning electron microscopy (SEM), energy dispersive X-ray spectrometer (EDS), transmission electron microscopy (TEM) and X-ray diffraction (XRD), the optimal condition for direct joining of Ti₃SiC₂ and Ti₃AlC₂ was obtained. Strong joints of Ti₃SiC₂/Ti₃AlC₂ can be achieved via diffusion bonding, which is attributed to remarkable interdiffusion of Si and Al at the joint interface. The shear strength of the Ti₃SiC₂/Ti₃AlC₂ joints was determined.     

Electrical properties of sintered EuZrO3  EuNbO3 and EuNbO3

Electrical resistivity of two-phase products [yEuZrO₃ + (1 ? y) EuNbO₃] increased continuously with y, and a transition from a metallic to semiconducting characteristic occured at y = 0.14. The resistivity varied almost linearly with temperaure in the range y = 0 to y = 0.24, and thermal coefficients of resistivity at 300 K for the products decreased from +5.9 × 10^?4 K^?1 to ?7.4 × 10^?4 K^?1 according to the value of y. At y = 0.14, the thermal coefficient was almost zero. Thermal coefficients of electrical resistivity for the niobates with various oxygen contents were all positive in the range $\text{2.55 <}\text{O}\text{Nb}\text{< 3.24}$ and exhibited a sharp minimum at $\text{O}\text{Nb}\text{= 2.92}$. In all these niobates, Eu_xNbO₃ was a major phase and Eu₃NbO₆ or EuNbO₂ was detected as a second phase in the range $\text{O}\text{Nb}\text{> 3}$ or $\text{O}\text{Nb}\text{< 3}$ respectively. Peaks in the resistivity curves were correlated with a magnetic ordering temperature for samples with an overall ratio $\text{O}\text{Nb}\text{> 3}$.     

Diffuse transition and piezoelectric properties of Pb[(Zr1−xTix)0.74(Mg1/3Nb2/3)0.20(Zn1/3Nb2/3)0.06]O3 Ceramics

In this work, the piezoelectric ceramic system of Pb[(Zr_1−xTi_x)_0.74(Mg_1/3Nb_2/3)_0.20(Zn_1/3Nb_2/3)_0.06]O₃, 0.47≤x≤0.57, with composition close to the morphotropic phase boundary, was studied. From the results of X-ray diffraction and piezoelectric measurement, ceramics near x=0.51 were found at the morphotropic phase boundary (MPB) between the tetragonal and pseudocubic perovskite. The planar coupling factor (k_p=0.72) is high at compositions near the MPB, but the mechanical quality factor (Q_m=75) is low. The calculation of the diffuseness of phase transition shows that the region of phase coexistence of this system is broader than that of the ternary system.     

Molecular dynamics simulations of energetic aluminum cluster deposition

We investigated energetic aluminum cluster deposition using a classical molecular dynamics (MD) simulation and the Metropolis Monte Carlo (MC) simulation. When local area reached melting state on the surface around impact point of an energetic aluminum cluster during a few ps, intermixing was easily achieved and a good epitaxial film with optimum bulk density was formed. For excellent film growth using cluster impact, it is necessary to make local area temperature higher than melting temperature on the surface around the impact point of an energetic cluster.