Contact Formation Mechanism Research Articles

Investigation of In contacts on atomically clean GaAs(110) surfaces

We have examined In–GaAs(110) contacts fabricated in situ both by depositing In at room temperature followed with annealing and by depositing In on heated surfaces [n- and p-type GaAs(110) surfaces (250 and 300 °C, respectively) using synchrotron radiation photoemission. It is found that In deposition on GaAs(110) surfaces with temperature below room temperature does not induce measurable reaction, whereas reaction takes place with InAs as product when the room-temperature-formed interface is annealed or when In is deposited on the heated substrate. However, there is no evidence of InxGa1−xAs graded heterojunction formation at the interface in either case. Outdiffusion of the excess As from the substrate is suggested to provide a source for InAs formation instead of the previously proposed In–GaAs replacement reaction. Electrical measurements were also performed on in situ In/n-GaAs(110) contacts before and after annealing. Reproducible high-barrier heights were recorded (1.02±0.02 eV) with good ideality factors (1.05±0.01) for the unannealed contacts, which is significantly different from the previously published values (0.76 and 0.78 eV). No dramatic change has been recorded in terms of barrier height upon annealing up to 375 °C although the ideality factor increases to around 1.2±0.1. The implication of this study to ohmic contact formation mechanisms will be discussed.

Atomistic Mechanisms and Dynamics of Adhesion, Nanoindentation, and Fracture

Molecular dynamics simulations and atomic force microscopy are used to investigate the atomistic mechanisms of adhesion, contact formation, nanoindentation, separation, and fracture that occur when a nickel tip interacts with a gold surface. The theoretically predicted and experimentally measured hysteresis in the force versus tip-to-sample distance relationship, found upon approach and subsequent separation of the tip from the sample, is related to inelastic deformation of the sample surface characterized by adhesion of gold atoms to the nickel tip and formation of a connective neck of atoms. At small tipsample distances, mechanical instability causes the tip and surface to jump-to-contact, which in turn leads to adhesion-induced wetting of the nickel tip by gold atoms. Subsequent indentation of the substrate results in the onset of plastic deformation of the gold surface. The atomic-scale mechanisms underlying the formation and elongation of a connective neck, which forms upon separation, consist of structural transformations involving elastic and yielding stages.

On the Ohmic Contact Formation Mechanism in the Au/Te/N-GaAs System

ABSTRACTAlloyed Au/Te/n–GaAs ohmic contacts, with contact resistivities comparable to those of the AuGe device standard, have been developed and studied by Mässbauer spectroscopy, Raman scattering and X-Ray Diffraction. The formation of Au-doped Ga2Te3 crystallites, grown epitaxially on a defectively GaAs surface was observed. No evidence for the formation of an n++–GaAs surface layer could be derived. The interpretation of all experimental results leads to a description of the ohmic conduction mechanism based on a resonant tunneling process assisted by defect/impurity related deep levels through low barrier metal/Te/(Au)Ga2Te3/GaAs interfaces

Planar Ge/Pd and alloyed Au-Ge-Ni ohmic contacts to n-AlxGa1−xAs (0≤x≤0.3)

Specific contact resistivity ρc of planar Ge/Pd ohmic contacts to n-type AlxGa1−xAs is measured as a function of AlAs mole fraction x and anneal temperature Tann. The functional dependence of ρc on Tann is the same for all x, decreasing to a minimum at 275–325 °C. This indicates that the ohmic contact formation mechanism is independent of x(0≤x≤0.3) as verified by MeV Rutherford backscattering spectrometry and Read camera glancing angle x-ray diffraction. Decomposition of an epitaxial Pd-AlxGa1−xAs phase is correlated with the onset of ohmic behavior and may result in a thin solid phase regrown interfacial AlxGa1−xAs layer. An undoped 20 nm GaAs cap layer reduces ρc by about one order of magnitude. Ge/Pd contacts display greater dependence of ρc on x and much smoother surface morphology compared with those of standard Au-Ge-Ni contacts on AlxGa1−xAs (0≤x≤0.3).

Ohmic Contact Formation Mechanism in the Ge/Pd/N-GaAs System

ABSTRACTAnnealed Ge/Pd/n-GaAs samples utilizing substrates with superlattice marker layers have been analyzed using high resolution backside secondary ion mass spectrometry and cross-sectional transmission electron microscopy. Interfacial compositional and microstructural changes have been correlated with changes in contact resistivity. The onset of good ohmic behavior is correlated with the decomposition of an intermediate epitaxial Pd4(GaAs,Ge2) phase and solid-phase regrowth of Ge-incorporated GaAs followed by growth of a thin Ge epitaxial layer.

Polymorphic behavior of gram-negative bacteria membranes

Freeze-fracture and ultrathin section electron microscopy as well as31P-NMR spectroscopy and light scattering ofEscherichia coli andPseudomonas putida cells under conditions promoting the ability of cells to take up exogenous DNA's (high concentrations of divalent cations and a specific temperature regime) reveal the extensive polymorphic changes and the formation of various structural defects in cellular membranes. Polymorphic changes occur during the heat shock at 42 to 44°C of the cells preincubated at 0°C in the presence of high concentration of Ca2+ or Ba2+ cations and include the formation of various vesicle- and tube-like structures, intermembrane and intercellular contacts followed by membrane fusion and sometimes even by cell fusion. The results obtained suggest the occurrence of phospholipid-enriched zones in the outer leaflet ofE. coli outer membrane. This suggestion is verified and confirmed with the help of phospholipase C, a specific phospholipid binding and digesting enzyme. The presented experimental evidence directly supports the suggestion of Ahkong et al. (Nature253:194–195, 1975) on the identity of the mechanisms of membrane contact formation and membrane fusion in model and cellular membranes. The biological relevance of the polymorphic changes observed is shortly discussed.

Initial stages of the Pd-GaAs reaction: Formation and decomposition of ternary phases

Abstract Investigations of the metallurgical reactions occuring at metal-compound semiconductor interfaces are essential for the understanding of ohmic contact and Schottky barrier formation mechanisms. The resulting film and interface morphologies are also important for the performance of discrete devices and integrated circuits. In this study the reactions occurring during thermal annealing of the Pd-GaAs system are investigated. Reported here are the initial stages of the Pd-GaAs reactions as studied by high resolution transmission electron microscopy, electron diffraction and energy-dispersive analysis of X-rays. The first Pd-GaAs reaction product, observed in the low temperature range (less than about 315°C), is the hexagonal ternary phase Pd 5 (GaAs) 2 (phase I with a 0 = 0.673 nm and c 0 = 0.338 nm ). The second stage of the reaction leads to the formation of a second hexagonal ternary phase, Pd 4 GaAs (phase II with a 0 = 0.92 nm and c 0 = 0.370 nm ), and the binary phases Pd 2 Ga and Pd 2 As. The concurrent application of energy-dispersive X-ray analysis and high resolution cross-sectional transmission electron microscopy leads to the unambiguous identification of the ternary Pd 5 (GaAs) 2 and Pd 4 GaAs phases. X-ray diffraction data alone could erroneously lead to the identification of Pd 5 (GaAs) 2 as PdGa and of Pd 4 GaAs as PdAs 2 . In view of this source of ambiguity in previous work, the sequence of reactions in the Pd-GaAs system is re-evaluated and the evolution of the film and interface morphologies is discussed.

Metallic contact formation and growth by dislocation glide

Abstract The theoretical objections to glide dislocation nucleation during metallic contact formation (sintering) are discussed, and their limitations at small particle sizes and under certain geometrical conditions are pointed out. A mechanism for contact formation by the direct conversion of surface free energy to glide dislocation strain energy is presented and evaluated with respect to the activation energy for nucleation and the limiting extent of the process for sphere-flat plate contacts. Finally, some diverse experimental observations from the literature are interpreted in terms of the proposed mechanism.

Mechanism of metallic contact formation in SAP type alloys

1. The rupture of a hard film on particles of a ductile metal powder during compaction is accomplished in the course of plastic deformation as a result of the spreading of the particle metal, i. e., increase of the particle surface. 2. During the simultaneous plastic deformation of powder particle contact surfaces, symmetrical rupture of the films takes place. 3. At a certain magnitude of plastic deformation, junction bridges separated by film fragments are formed between the particles. The coupling strength is determined by the area of symmetrical cracks. To achieve a strong bond, it is necessary to select a suitable particle shape and the optimum relative film thickness. 4. The results obtained are generally applicable to different ductile metal powders whose particles are coated with hard surface films. The mechanism of rupture of the hard films is identical, and differences may arise only in details and in values of specific pressure. 5. The theory of coupling of metals during simultaneous deformation, as developed in [2, 3, 8], applies to the conditions prevailing during powder compaction.

Differences between the electrical charge carried by normal and homologous tumour cells.

DIFFERENCES between the behaviour of normal fibroblasts and sarcoma cells have been observed in tissue culture by Abercrombie and Heaysman1. Normal fibroblasts affect each other's movements by contact inhibition, whereas sarcoma cells do not show inhibition either with respect to each other or to normal fibroblasts. Time-lapse colour films taken with the interference microscope (shown at the Bristol meeting (1955) of the British Association by E. J. Ambrose and M. Abercrombie) have shown that this difference in behaviour is due to differences in the mechanism of contact formation in the two cases, being dependent upon a loss of adhesiveness of the cell surface of the tumour cells. This reduction in the adhesiveness of the tumour cell suggests that the electrical properties of the surface may have altered during the malignant transformation. These properties may be investigated by electrophoretic measurements of cellular mobility. For example, studies of bacteria during growth, in the presence of bacteriostatic agents, have revealed marked changes in the nature of the cell surface, changes which are passed on to the progeny during subsequent growth, either in the presence or the absence of the drug. Thus the growth of Aerobacter aerogenes in the presence of proflavine2 or crystal violet3 gives rise to new populations of cells, which, although biologically indistinguishable from the original strain, have a distinctive electrometric behaviour.