Considerable Intermixing Research Articles

A comparative study of Ti and Cr based p-ohmic contacts on high power GaAs laser diodes

The present work involves a comparison of sequentially evaporated Ti/Pt/Au and Cr/Au p-ohmic contacts for high power laser diode fabrication. Subsequently, the contacts were annealed at 440 °C in furnace annealing under N2 ambient and their surfaces were evaluated by optical and electron microscopy. The specific contact resistivity of the contacts was determined by circular transfer length method (c-TLM). Secondary ion mass spectroscopy (SIMS) analysis of the annealed contacts indicates considerable intermixing of the metal and Ga atoms across the interface for Cr/Au contact. On the other hand, insignificant intermixing of atoms was observed in Ti based contacts. The electrical and optical characteristics of the laser diodes formed with Ti/Pt/Au ohmic contact suggest superior performance compared to the diode formed with Cr/Au contact.

Electrical and structural properties of surfaces and interfaces in Ti/Al/Ni Ohmic contacts to p-type implanted 4H-SiC

In this work, the electrical and structural properties of Ti/Al/Ni Ohmic contacts to p-type implanted silicon carbide (4H-SiC) were studied employing different techniques. With increasing the annealing temperature, an improvement of the electrical properties of the contacts is highlighted, until an Ohmic behavior is obtained at 950°C, with a specific contact resistance ρc=2.3×10−4Ωcm2. A considerable intermixing of the metal layers occurred upon annealing, as a consequence of the formation of different phases, both in the uppermost part of the stack (mainly Al3Ni2) and at the interface with SiC, where the formation of preferentially aligned TiC is observed. The formation of an Ohmic contact was associated with the occurrence of the reaction and the disorder at the interface, where the current transport is dominated by the thermionic field emission mechanism with a barrier height of 0.56eV.

High dose ion irradiation effects on immiscible AlN/TiN nano-scaled multilayers

The effects of high dose Ar ion irradiation on immiscible AlN/TiN multilayered structures were studied. The structures with 30 alternate layers of a total thickness of ~260nm were deposited by reactive sputtering on (100) Si wafers. Individual layer thickness was ~8nm AlN and ~9.3nm TiN. Irradiation was done with 180keV Ar+ ions to 1×1016–8×1016ions/cm2, with the projected range around mid-depth of the structures. It was found that the highest applied dose induced a considerable intermixing, where the growing TiN grains consume the adjacent AlN layers, transforming partly to (TiAl)N phase. Intermixing occurs due to a high contribution of collision cascades, which was not compensated in demixing by chemical driving forces. However, a multilayered structure with relatively flat surface and interfaces is still preserved, with measured nano-hardness value above the level for the as-deposited sample. The results are compared to other systems and discussed in the light of the existing ion beam mixing models. They can be interesting towards better understanding of the processes involved and to development of radiation tolerant coatings.

<i>In Situ</i> X-Ray Diffraction Investigations during Diffusion Anneals of Ni-Cu Thin Film Diffusion Couples

The stress evolution during diffusion annealing of Ni-Cu bilayers (individual layer thicknesses of 50 nm) was investigated employing ex-situ and in-situ X-ray diffraction measurements. Annealing at relatively low homologous temperatures (about 0.3 - 0.4 Tm) for durations up to about 100 hours results in considerable diffusional intermixing, as demonstrated by Auger-electron spectroscopy investigations (in combination with sputter-depth profiling). In addition to thermal stresses due to differences of the coefficients of thermal expansion of layers and substrate, tensile stress con-tributions in the sublayers arise during the diffusion anneals. The obtained stress data have been discussed in terms of possible mechanisms of stress generation. The influence of diffusion on stress development in the sublayers of the diffusion couple during heating and isothermal annealing was investigated by comparing stress changes in the bilayer system with corresponding results obtained under identical conditions for single layers of the components in the bilayer system. The specific residual stresses that emerge due to diffusion between the (sub)layers in the bilayer could thereby be identified.

Interdiffusion and stress development in Ni-Cu thin film diffusion couples

Thin film Ni-Cu diffusion couples (individual layer thicknesses: 50 mn) have been prepared by direct-current magnetron sputtering on silicon substrates. The microstructural development and the stress evolution during diffusion annealing have been investigated employing ex-situ and in-situ X-ray diffraction, transmission electron microscopy and Auger-electron spectroscopy (in combination with sputter-depth profiling). Annealing at relatively low temperatures (175 degrees C to 350 degrees C) for durations up to about 100 hours results in considerable diffusional intermixing. In addition to thermal stresses due to mismatch of the coefficients of thermal expansion of layers and substrate, tensile stress contributions in the sublayers arise during diffusion anneals. The obtained stress data are discussed in terms of possible mechanisms of stress generation.

Mechanisms of interdiffusion in Pd–Cu thin film diffusion couples

Interdiffusion in sputter-deposited polycrystalline Pd–Cu bilayers (thickness of each sublayer: 50 nm) was studied in the temperature range 175 °C–250 °C by sputter–depth profiling in combination with Auger electron spectroscopy. X-ray diffraction and transmission electron microscopy investigations revealed that the layers are polycrystalline, consisting of columnar grains separated by grain boundaries oriented more or less perpendicularly to the film surface. Considerable diffusional intermixing occurred in the studied temperature range, which was accompanied by the sequential formation of (ordered) phases Cu 3Pd and CuPd. Volume interdiffusion coefficients were determined using the so-called ‘centre-gradient’ and ‘plateau-rise’ methods. Grain-boundary diffusion coefficients of Pd through Cu grain boundaries were determined by the Whipple–Le Claire method and grain-boundary diffusion coefficients of Cu through Pd grain boundaries were determined by the Hwang–Balluffi method. It was found that both volume and grain-boundary diffusion coefficients decreased roughly exponentially with annealing time. Activation energies were determined which pertain to the same (defect) microstructure at each temperature. The differences with literature results for macroscopic diffusion couples were discussed.

Interdiffusion, phase formation, and stress development in Cu–Pd thin-film diffusion couples: Interface thermodynamics and mechanisms

Cu–Pd thin-film diffusion couples (individual layer thicknesses of 50nm) have been prepared by dc-magnetron sputtering on silicon substrates coated with a thin amorphous Si3N4 layer. Stress evolution, microstructural development, and phase formation during interdiffusion have been investigated employing Auger-electron spectroscopy (in combination with sputter-depth profiling), x-ray diffraction, wafer curvature measurements and transmission electron microscopy. Upon annealing at relatively low temperatures (175–250°C) for durations up to 10h, considerable diffusional intermixing occurs. Interdiffusion is accompanied by sequential formation of new phases. First, Cu3Pd forms; subsequently, CuPd forms and grows at the expense of Cu3Pd, which has been interpreted as a consequence of interface thermodynamics. Annealing leads to a slight sharpening of the pre-existing {111}-fiber textures and a little increase in the average grain size. A combination of ex situ (x-ray diffraction) and in situ (wafer curvature) stress measurements revealed that tensile stresses are generated during annealing. The obtained stress results have been discussed in the light of possible mechanisms of stress generation, and the dominant sources of stress buildup and relaxation have been identified.

Interdiffusion and stress development in Cu–Pd thin film diffusion couples

Thin film Cu–Pd diffusion couples (individual sublayer thicknesses: 50 nm) were prepared by direct current magnetron sputtering on silicon substrates coated with amorphous diffusion/reaction barriers (Si 3N 4 on top of SiO 2). The microstructural variations, phase formation and stress evolution during diffusion annealing were investigated employing Auger-electron spectroscopy in combination with sputter-depth profiling, transmission electron microscopy and in-situ X-ray diffraction. Upon annealing at relatively low temperatures (185 °C) for durations up to 17 h, considerable diffusional intermixing occurs. Interdiffusion is accompanied by the formation of a compound phase (ordered Cu 3Pd (α′)). The stress changes in the bilayer system during heating and isothermal annealing were investigated employing in-situ X-ray diffraction stress measurements and were compared to corresponding results obtained for single layers of the components in the Cu–Pd bilayer system, produced under conditions identical to those employed for the sublayers of the diffusion couple. The specific residual stresses that emerge due to diffusion (and reactions) between the (sub)layers in the bilayer could then be identified by comparing the stress evolutions upon annealing of the single layers with those recorded for the sublayers of the bilayer.

Interdiffusion and Stress Development in Thin Film Diffusion Couples

Thin film diffusion couples (Pd-Cu, individual layer thicknesses: 50 nm) have been prepared by DC-magnetron sputtering on silicon substrates coated with amorphous inter-layers (Si3N4 on top of SiO2). The microstructural development, phase formation and the stress evolution during diffusion annealing have been investigated employing Auger-electron spectroscopy in combination with sputter depth profiling, ex-situ and, in particular, in-situ X-ray diffraction measurements. Upon annealing at relatively low temperatures (175°C to 250°C) for durations up to 100 hours, considerable diffusional intermixing occurs. Interdiffusion is accompanied by the sequential formation of a new phase (Cu3Pd). The detected stress changes are discussed in terms of possible mechanisms of stress generation.

Comparative study of magnetism and interface composition in Fe/GaAs(1 0 0) and Fe/InAs(1 0 0)

Thin layers of Fe are deposited at 100 °C on GaAs(1 0 0) and InAs(1 0 0) substrates by molecular beam epitaxy. These structures are characterized by photoelectron spectroscopy (PES), in order to derive their chemical composition and interface reactivity, and by X-ray magnetic circular dichroism (XMCD) recorded at room temperature in remanence mode, in order to derive the atomic magnetic moment of Fe. As previously reported, Fe/GaAs(1 0 0) exhibits a strong and complicated interface reactivity, with considerable intermixing of Fe with Ga and As. Unlike the above structure, Fe/InAs(1 0 0) exhibits a much lower interface reactivity, whereas the Fe magnetic moment increase is much more pronounced as function of Fe thickness. The total atomic ferromagnetic moment reaches at room temperature 2.0 Bohr magnetons by about 1 nm Fe effective thickness, which is almost the Fe bcc bulk value (2.2 Bohr magnetons). The novelty of the present study is a direct comparison of the two interfaces prepared and measured in the same experimental arrangements.

Photon Emission Spectroscopy of Single Oxide-Supported Ag-Au Alloy Clusters

The alloying of Ag and Au has been investigated on the level of individual clusters by analyzing the light emission excited by electron injection from an STM tip. Different Ag-Au alloy and shell-core clusters have been prepared at room temperature on a thin ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ film on NiAl(110) by simultaneous and successive deposition of both noble metals. For simultaneous deposition, one Mie-plasmon resonance has been detected with a wavelength position shifting from the pure Au to the Ag value with increasing Ag content. The results are in agreement with calculations based on Mie theory indicating a complete mixing of both materials. For successive deposition, two Mie resonances have been observed, attributed to plasmon excitations in the shell and core of the clusters. Comparing these results to model calculations, a considerable intermixing of the core and shell materials is concluded, which is especially strong in Au shell-Ag core clusters.

Film thickness degradation of Au/GaN Schottky contact characteristics

Electrical characteristics of Au/n-GaN Schottky contacts with different Au film thicknesses up to 1300 Å, have been investigated using current–voltage ( I– V) and capacitance–voltage ( C– V) techniques. Results show a steady decrease in the quality of the Schottky diodes for increasing Au film thickness. I– V measurements indicate that thin (<500 Å) film Schottky diodes show significantly better rectification characteristics than those of thick Au films (>500 Å). Depth profiling Auger electron spectroscopy (AES) shows that the width of the Au/GaN junction interface increases with increasing Au thickness, suggesting considerable inter-mixing of Au, Ga and N. The results have been interpreted in terms of Ga out-diffusion from the GaN giving rise to gallium vacancies that in turn act as sites for electron–hole pair generation within the depletion region. The study supports the recent suggestion that gallium vacancies associated with threaded dislocations are playing an important role in junction breakdown.

Self-Consistent Brownian Dynamics Simulation of Bimodal Polymer Brushes under Shear

The method of self-consistent Brownian dynamics developed recently has been adapted to simulate the properties of bimodal polymer brushes under shear. Simulations of three systems with different compositions have been carried out for a range of shear rate values. In the absence of shear, the free ends of short and long chains are mostly found in different sublayers inside the brush. Similar to the collapse of monodisperse brushes, we observed under shear a collapse of the outer sublayer, which is composed of long chains. Short chains are not stretched and inclined if the shear is not strong enough to cause a considerable intermixing of their free ends with the free ends of long chains in the direction perpendicular to the grafting surface. When the shear becomes very strong such intermixing occurs, and all chains are stretched and inclined in the shear direction. The number of chains, directly affected by shear, is very small. However, the shear stress is transferred into the brush below the flow penetration level owing to the rotation of the free chain ends.

Origin and mobility of humic colloids in the Gorleben aquifer system

The origin and mobility of humic colloids in the Gorleben aquifer system have been examined. For this purpose, the distribution of humic colloids and relevant hydrological and geochemical parameters were examined. An investigation area was selected where sediments have been disturbed by salt dome uplift and glacial events. It is shown that, on a local scale, considerable groundwater movement and intermixing takes place from the surface down to the salt dome. Consequently no effective separation of groundwater layers occurs. Two different humic colloid sources are identified: influx from the humus horizon with recharge water and continuous in situ generation via mineralization of sedimentary organic carbon (SOC). The in situ generation leads to groundwaters with humic colloid concentrations approaching 0.4 g/L, compared to concentrations of less than 0.005 g/L in recharge waters. Young groundwaters (no 14C decay detected) between approximately 50 and 200 m depth exhibit these highly elevated humic colloid concentrations. At greater depth, salt brines are found with low humic colloid concentrations. This can be attributed to precipitation of humic acid and/or hampering of the in situ generation process due to the high salt content. There is no indication of retention or decomposition of humic colloids. The fate of in situ generated humic colloids cannot be precisely evaluated due to analytical limitations and insufficient understanding of groundwater movement.

A medium energy ion scattering study of the structure of Sb overlayers on Cu(111)

Using 100 keV H + ions, medium energy ion scattering (MEIS) has been applied to a study of Sb overlayers on Cu(111). Upon deposition of 10 ML of Sb at room temperature, considerable intermixing of Sb and Cu occurs in thick (100 Å) overlayers, forming a partially ordered alloy phase, of composition Cu 3Sb. Heating to 200°C produces a second ordered alloy phase with reduced Sb content, whilst heating to 305°C diffuses excess Sb into the bulk and produces an ordered Cu(111) ( 3 × 3 ) R30°Sb surface phase with 0.33 ML Sb. Comparison of blocking curves obtained from this phase with the results of simulations for a range of model structures leads to the conclusion that the Sb atoms substitute outermost layer Cu atoms, but this surface alloy layer is laterally displaced so that all constituent atoms occupy hcp hollow sites relative to the underlying Cu(111) substrate. In this stacking faulted surface layer, the layer spacings of the Sb and Cu atoms relative to the outermost underlying Cu surface layer have been determined to be 2.52±0.13 Å and 2.05±0.09 Å respectively. Some aspects of the methodology for the application of MEIS to surface structure determination and structural parameter precision estimation are addressed.

Differential progenitor dispersal and the spatial origin of early neurons can explain the predominance of single-phenotype clones in the chick hindbrain.

Clonal analysis of the chick embryo hindbrain has shown that during the first 48 hr of neurogenesis the large majority of neural progenitor cells generate clones of neurons of only a single major phenotype or of only closely related phenotypes. This is despite considerable spatial intermixing of diverse neuronal phenotypes at these stages of development and suggests that phenotype may be decided early in mitotic precursors and remembered through several subsequent rounds of division and dispersal (Lumsden et al. [1994] Development 120:1581-1589). Here we have used fate-mapping and clonal analysis to study neuroepithelial cell dispersal and mixing in the early hindbrain and discuss this data in relation to the generation of single phenotype neuronal clones. We find that dispersal is not uniform throughout the dorsoventral axis of the neural tube, but is highly dependent on position along that axis. Neuronal identity is related to the spatial origin and, hence, environment of the cell, and the spatial intermixing of diverse neuronal phenotypes at HH stage 20 is largely the result of circumferential neuronal migration as medially born branchial motor neurons migrate laterally while the more laterally born mlf neurons migrate medially. Constraints on the dispersal of clonally related progenitors, in particular those that lie adjacent to the floor plate, may serve to restrict the fate of these cells to the generation of only one major neuronal phenotype, i.e., motor neurons.

X-ray reflectometry study of interdiffusion in Si/Ge heterostructures

Interdiffusion in molecular-beam-epitaxy-grown Si-Ge heterostructures has been investigated by grazing incidence x-ray reflectometry. Measurements on Si1−xGex/Si superlattices reveal that the thickness of the Si1−xGex layers increases when intermixing is induced by high-temperature annealing. This results from a strong composition dependence of the Ge diffusion coefficient in Si1−xGex. Experiments on coherently strained superlattices show that the diffusion rate is faster in the early stage of annealing, before the strain is relieved. This suggests that strain enhances interdiffusion at the interfaces. Similar experiments performed on (SimGen)p atomic layer superlattices show that considerable intermixing occurs in these heterostructures even after moderate annealing treatments (i.e., 20 s at 700 °C). Due to the very low diffusion rate of Ge in Si-rich Si1−xGex alloys, interdiffusion is more pronounced in structures of smaller periodicity or in which the interfaces are initially intermixed.

Phase formation in the interfacial reactions of ultrahigh vacuum deposited titanium thin films on (111)Si

Phase formation in the interfacial reactions of ultrahigh vacuum deposited Ti thin films on (111)Si has been studied by in situ reflected high energy electron diffraction (RHEED) and transmission electron microscopy (TEM). In situ RHEED and high resolution TEM data showed unambiguously that there is considerable intermixing of Ti and Si atoms during Ti deposition. Ti5Si3 was found to be the first nucleated phase followed by Ti5Si4 and TiSi. Ti5Si3, Ti5Si4, TiSi, and C49-TiSi2 along with a-interlayer were found to form in samples annealed at 475 °C for 30 and 60 min. Ti5Si4 was the first silicide phase to disappear followed by Ti5Si3 then TiSi. In samples annealed at 700 °C for 10 min, C54-TiSi2 started to appear. To understand the thermodynamic origin of the phase formation, metastable free energy diagrams at 450–600 °C have been constructed. Ti5Si3 and C49-TiSi2 were found to have the highest and lowest driving force, respectively. On the other hand, Ti5Si4 has the lowest interface energy. The sequence of formation and disappearance of silicide phases is discussed accordingly.

On the use of H + and Ar + ions for high spatial resolution depth profiling

We report on the destructive sputtering of NiC multilayer coatings which have a periodicity of 2.7 nm, using H + as well as Ar + ions of 750 eV energy. The influence of ion bombardment on interface roughness, interface mixing and density has been investigated by monitoring this process in situ using a soft X-ray reflection system. Ion bombardment has been demonstrated to decrease the roughness of a sputtered surface while a considerable intermixing of the interface below the sputtered layer occurs. Changes in the film density have not been observed.

Interfacial reactions at copper surfaces coated with polyimide films prepared from poly(amide–acid) precursors

Thin films of polyimide were prepared by spin coating the poly(amide–acid) precursor onto copper and aluminum substrates, followed by the usual heat treatment to promote imidization (curing) of the film. Films prepared on aluminum substrates were completely cured during the heat treatment, as shown by x-ray photoelectron spectroscopy and infrared measurements. On copper substrates, the thinnest films (2000 Å or less) showed considerable intermixing of copper ions in the polymer layer. This prevented the films from curing completely during heat treatment. The formation of a copper carboxylate at the acid site in the polyimide precursor is postulated.