The mechanisms of formation of ohmic contacts to AlGaAs: A microstructural,elemental diffusion and electrical investigation

Interfacial microstructure and phase composition of PtTiGePd ohmic contacts to heavily C-doped AlGaAs were investigated as a function of annealing temperatures. Results of the material analyses were used to explain the specific contact resistances measured for each thermal treatment. Evidence of interdiffusion and compound formation between AlGaAs and Pd was visible in a Ga-rich Pd-Ga-As reaction zone prior to heat treatment. As the annealing temperature was raised from 530 to 600°C, As began to out-diffuse. At 600°C, this As out-diffusion, which is critical to the formation of good p-type ohmic contacts, contributed to the creation and development of the laterally continous two-phase interfacial region, TiAs/Pd₁₂Ga₂Ge₅, overlying the AlGaAs substrate. The minimum specific contact resistance was also achieved at this temperature. As the annealing temperature was elevated to 650°C, the specific contact resistance degraded in response to intensive chemical diffusion and development of a broad, nonuniform multiphased interfacial region.     

电子显微镜在金属-半导体纳米线界面原位合金化中的应用

利用原位电子显微方法研究了金属-ZnSe半导体纳米线界面合金化过程,结果表明:在脉冲电流的作用下,ZnSe纳米线可以产生明显的焦耳热效应,导致金属-ZnSe纳米线界面发生合金化反应.通过调整纳米线与金属电极的接触面积,可将焦耳热效应准确控制在界面内,进而实现纳米尺度的合金化反应.     

A study of electrical charge at head-disk interface

The electrical charge at head-disk interface (HDI) of disk drives becomes increasingly important as head-disk spacing drops below 10 nm range. In this study, a new method of measuring electrical charge at HDI is presented. It involves measuring magnetic read-back signals (i.e. PW50), while the flying height (FH) is lowered by electrostatic force. Typical HDI charges are in the range of –0.2 to –0.9 V, depending on individual head–disk combination. Experiments were also conducted to eliminate the HDI charge by using an ionizer and surface treatment of magnetic heads. It was found that the HDI charge can be effectively eliminated by treating the magnetic head with a fluorinated carbon coating. The mechanisms of HDI charge generation and elimination are discussed.     

Modelling of frictional and adhesive contacts at silicon/polymer interface for nanotechnological applications

Abstract

The minimisation of friction and adhesion during sliding contacts is crucial for the industrial fabrication of many micro/nanodevices (e.g. MEMS/NEMS), as well as in nanotechnological processes, e.g. in nanoimprint lithography where a silicon mould is used to fabricate polymeric nanostructures by imprinting. We have conducted intensive research on the contact between the mould and PMMA polymeric resist film via advanced modelling and computer simulations. The properties of the contacting surfaces have been identified with the atomic force microscope and nanoindentation, as well as wettability tester applied for the identification of the surface free energy. A model of contact has been elaborated and adequate original software was used to calculate the frictional and adhesive forces in particular at the silicon mould/polymeric resist interface.     

On the thermal elastohydrodynamic lubrication in opposite sliding circular contacts

Cases of elastohydrodynamic lubrication (EHL) point contacts running under opposite sliding conditions have been studied with consideration of the thermal effect for various loads and entrainment velocities. A thermal EHL solver has been developed and proven to be able to deal even with extreme cases under an infinite slide–roll ratio. Results show that film profiles featuring a dimple can be formed in the contact zone when the slide–roll ratio exceeds a certain level. Moreover, the present study provides theoretical evidence for the lubricating film build up in the case where the two bounding surfaces run with equal but opposite velocities. An effective lubricating film under zero entrainment speeds was experimentally proven by Dyson and Wilson [1] (Proc Instn Mech Engrs, 1968–1969 183(3P) 81) in the 1960's, which, however, cannot be explained by the isothermal EHL theory.     

A class of slipline field solutions for metal machining with slipping and sticking contact at the chip-tool interface

In the present investigation slipline field solutions for orthogonal machining are presented when the plastically stressed region in the chip/tool contact length consists of both slipping (τk) and sticking (τ=k) zones. The interface friction in the slipping region is assumed to obey Coulomb's law and the fields are analysed using the linear approximation to the above non-linear boundary value problem as suggested by Dewhurst. The range of validity of the above slipline fields is examined from the consideration of overstressing of rigid vertices in the assumed rigid regions. Results are presented for variation of cutting forces, cutting ratio, chip curl radius and contact length with variation in rake angle and interface friction coefficient. Solutions incorporating elastic effects are obtained by the method suggested by Childs. Results from the theoretical analysis are compared with experimental values reported in literature.     

线接触曲面加工方法的研究

针对高效率加工曲面的生产要求,研究了一种线接触加工曲面的方法。通过对现有数控机床结构形式的分析,提出一种可以使刀具侧切削刃与工件理论廓形接触,同时主轴座的回转模拟车削加工的机床结构形式。并采用Denarit-Hartenbarg方法,对该刀具机构的运动进行了分析。以此为基础,结合曲面特征,研究刀位轨迹。最后,将五坐标点接触加工方法和五坐标线接触加工方法与本方法的效率和精度进行对比,得到了该方法优于以上两种方法的结论。     

A correlation of the process zone properties in complete, incomplete and almost complete fretting contacts

A careful analytical comparison is made of the size and shape of the process zones present in a range of fretting fatigue experiments. These include the conventional Hertzian contact, the ‘flat and rounded’ pad problem and the classical square-ended pad geometries. The results clearly display the load and geometry regimes where the ‘flat and rounded’ pad performance may be matched with either of the limiting forms. This should permit a unifying re-interpretation of fatigue data already obtained from each of the three sources.     

Modelling and evaluation of the fretting fatigue cracking risk in smooth spherical contacts

A numerical model for the calculation of fretting fatigue crack initiation is presented and compared with experiments. The model is focused on smooth sphere-on-plane contact in partial and gross slip conditions. It is based on Hamilton’s explicit stress equations and the multi-axial Dang Van and Findley fatigue criteria enhanced with a statistical size factor concept. Promising correlation was found between the model and the experimental results with quenched and tempered steel 34CrNiMo6. The model assumptions, limitations and general application are also discussed.     

Preliminary investigation of the effect of dimple size on friction in line contacts

The scale of surface texture is becoming an important issue of surface texture design, particularly for the condition of low speed and high load. Experiments were carried out to investigate the effect of dimple size on friction under line contact condition. The patterns of dimples distributed as square array were fabricated on the surface of brass disks. Each pattern has the same area density of 7%, the same depth over diameter ratio h/d of 0.03, and dimple diameter d varying from 20 to 60 μm. The frictional tests of the brass disk sliding against a stationary cylindrical surface of bearing roller were conducted. It was found that the pattern with dimple diameter of 20 μm presented the effect of friction reduction. For the further understanding of the effect of dimple size under line contact condition, numerical simulations were also carried out to evaluate the hydrodynamic pressure within the contact of cylindrical and plane surfaces. The effects of dimple size and radius of the cylinder on the load carrying capacity were evaluated and discussed.     

Feasibility of using digital speckle correlation in the study of seal contacts

This paper presents studies of the contact between a soft rubber specimen and glass counterface using the Digital Speckle Correlation method, which provides information of displacements and structural similarities between recorded images. The setup is designed with a real contact and changes in the contact can be varied. Microscopic images using laser light illumination for different displacements are recorded and correlated. The results show that the contact area can be identified both for dry and lubricated contacts. The method can be applied on different geometries, surface roughness and lubricants. Influences of scars and contaminations, e.g. wear particles, may also be analysed. Copyright © 2009 John Wiley & Sons, Ltd.     

Determination of diffusion controlled reaction rates at a solid/liquid interface using scanning electron microscopy

A high‐resolution method has been developed for the determination of localized values of interfacial reaction rate and mass transfer coefficient in aqueous solution. Scanning electron microscopy has been successfully applied to this problem through the measurement of electroplated film thickness formed under limiting current conditions. The method involves the calculation of local values of reaction rate via Faraday's laws and subsequent conversion of the data to absolute values of mass transfer coefficient. The technique has been verified in an undisturbed, turbulent flow regime (rotating cylinder electrode) through the use of Sherwood group dimensionless analysis. The resulting relationship shows comparable accuracy relative to electrochemical measurements. Favourable comparison has also been made with the generally accepted rotating cylinder correlation of Eisenberg, Tobias and Wilke. Differential rates of mass transfer to a single surface under conditions of disturbed flow have also been examined at a high spatial resolution using the stepped rotating cylinder electrode geometry. In this case, reaction rates have been measured as a function of circumferential distance within a recirculation zone situated immediately downstream of a backward‐facing step.     

On the influence of test parameters on friction and wear of ceramics in oscillating sliding contacts

The tribological behaviour of different ceramics in contact with steel was studied for the case of oscillating sliding motion with a ball-on-disc apparatus. The influence of several test condition parameters was investigated by a systematic variation of the stroke, frequency, and normal load at room temperature in laboratory air at different levels of relative humidity. Each of the four parameters was varied in three stages. While the coefficient of friction was only mildly influenced by the operational variables, the coefficient of wear showed great variations and depended strongly on the humidity of the surrounding air. The effect of the operational variables and of the humidity on friction and wear varied for the different materials under investigation.     

A contour integral method to compute the generalized stress intensity factor in complete contacts under sliding conditions

In complete contact fretting problems under global sliding conditions, the stress state at the corner of the contact zone is usually singular (assuming elastic behaviour). This stress state is characterized by two parameters: the order of singularity and the generalized stress intensity factor (GSIF). The former can be analytically calculated for a given problem. However, the GSIF is usually obtained by means of numerical procedures. One of the most used is the application of the stress extrapolation technique in combination with a FE analysis. In this work, a path-independent contour integral is defined which enables the GSIF calculation. Using this novel technique, a much more accurate estimation of the GSIF is obtained for a given discretization. In addition, a refined mesh around the singular point is not needed, because the contour integral can be applied along paths far from the singularity dominated zone due to its path independence.     

Chemical reactions and morphological stability at the Cu/Al2O3 interface

The microstructures of diffusion‐bonded Cu/(0001)Al₂O₃ bicrystals annealed at 1000 °C at oxygen partial pressures of 0.02 or 32 Pa have been studied with various microscopy techniques ranging from optical microscopy to high‐resolution transmission electron microscopy. The studies revealed that for both oxygen partial pressures a 20–35 nm thick interfacial CuAlO₂ layer formed, which crystallises in the rhombohedral structure. However, the CuAlO₂ layer is not continuous, but interrupted by many pores. In the samples annealed in the higher oxygen partial pressure an additional reaction phase with a needle‐like structure was observed. The needles are several millimetres long, ～10 µm wide and ～1 µm thick. They consist of CuAlO₂ with alternating rhombohedral and hexagonal structures. Solid‐state contact angle measurements were performed to derive values for the work of adhesion. The results show that the adhesion is twice as good for the annealed specimen compared to the as‐bonded sample.     

A new method to simulate the elastic interactions in the tribosystems of a multi‐bar mechanism

This paper presents a new method and criterion for predicting the occurrence of the interruption of a kinematic joint contact in a multi‐bar mechanism. The method is based on the elastic interactions in the tribosystems of the joints with imposed clearances. The method was used in a four‐bar mechanism with clearance in the joint formed from the coupler and the rocker. The analytical results were experimentally confirmed by measurement of the acceleration of the rocker.     

Numerical modeling of processes of mass transfer in tribological contacts by the method of movable cellular automata

Wear is viewed as a stochastic process in the surface layers of contacting bodies; it comprises the processes of spalling of wear particles and their stochastic transportation and nanobonding. These processes can be described within the framework of the macroscopic phenomenological theory [1]; however, the parameters of the macroscopic theory can be found only by microscopic modeling of the tribological interface. In the present paper, the dynamic processes are modeled by means of the method of movable cellular automata in the interface of a tribological couple. The results allow us to identify the parameters of the macroscopic model based on the modeling of processes at the nanolevel; this enables us to extrapolate from the nano- to the macrolevel.     

Influence of non-ideal circumferential contacts on errors in the measurements of the resistivity of layers using the van der Pauw method

The present work deals with practical consequences of non-point character of circumferential contacts on samples tested using van der Pauw (vdP) method. Apart from the measurement method itself, the fundamental paper of van der Pauw deals also with an estimate of the error due to contact of finite length at the circumference of a circular sample on assumption of equipotential contact region. In general, this is true only in case of vanishing contact resistivity. The present study discusses the consequences of non-fulfilment of this assumption in practice and demonstrates it on real samples made from thin copper foil. The circumferential contact surfaces on these samples are made of the same material and the same thickness as measured sample. It was shown that, in principle, there is no experimental error in the resistivity measurements caused by finite size of contact surfaces. This is a consequence of the trivial fact that the contact surfaces modify only the shape of the sample, which – in the sense of the vdP methodology – can be arbitrary. This conclusion can be further generalised as follows. A non-point contact of lower resistivity than the measured sample results in negative measurement error, whereas the contact of higher resistivity results in positive error. So the magnitude and sign of the error depends not only on the relation between the contact length and sample diameter but also on the relation between the resistivities of the contact and the sample.