C.L. Bauer

Interfacial reactions between gold thin films and GaAs substrates

Abstract Interfacial reactions between polycrystalline thin films of gold, either 30 or 60 nm in thickness, and monocrystalline substrates of GaAs have been investigated as a function of deposition temperature and subsequent annealing conditions by transmission electron microscopy, incorporating a special cross-sectioning technique. For depositions of gold onto GaAs{100} substrates at ambient temperature, interfacial reactions proceed in four separate stages during isothermal annealing at temperatures ranging between room temperature and 350 °C: (1) elongated pyramidal pits, bounded by GaAs{111} planes and the original AuGaAs{100} interface, form during annealing for 30 min at 100 °C; (2) precipitates of Au2Ga form in GaAs near the AuGaAs interface during annealing for 30 min at 200 °C or during annealing for about 6 months at room temperature; (3) these precipitates redissolve during annealing for 30 min at 320 °C; (4) the remaining (unalloyed) gold agglomerates in monocrystalline form at the pyramidal pits by solid state surface diffusion during annealing for 30 min at 330 °C. The orientation relationships between the monocrystalline gold and GaAs are {111}Au// {111}GaAs and 〈211〉Au // 〈110〉GaAs. For depositions of gold onto {100} substrates of GaAs at 280 °C and subsequent annealing at 330 °C, pyramidal pits bounded by GaAs{111} planes and agglomerated monocrystalline gold form, containing a reaction product of a monocrystalline hexagonal AuGa phase. The orientation relationships between the monocrystalline gold, hexagonal AuGa phase and GaAs are {111} Au // {01 1 0} AuGa // {111} GaAs and 〈110〉Au // 〈0001〉AuGa // 〈110〉GaAs. The formation (and dissolution) of these phases, the resultant morphology and the corresponding orientation relationships are interpreted in terms of both thermodynamic and crystallographic concepts.

Determination of the oxidation states of tin by Auger electron spectroscopy

Abstract The oxidation states of tin in pure foils and compressed powders of tin oxides were determined from shifts in the principal Auger transitions of 3 ± 1 eV for tin in SnO and 7 ± 1 eV for tin in SnO 2 . Moreover, SnO powder is reduced to tin and oxygen during prolonged periods of bombardment with argon ions whereas similar effects are not observed for SnO 2 , and examination of the oxidation states of tin in pure tin foils exposed to dry oxygen at 200°C reveals that SnO is uniquely formed at the external surface during early stages of annealing and gradually decreases in concentration as a function of increasing depth. The last results are confirmed by transmission electron microscopy, which also indicates that both SnO and SnO 2 are formed during prolonged annealing at 200°C.

Investigation of interfacial reactions in thin film couples of aluminum and copper by measurement of low temperature contact resistance

Abstract Interfacial reactions in bimetallic thin film couples of aluminum and copper were investigated by measurement of contact resistance between 4.2 and 300 K. Results for as-deposited couples indicate that electrical properties, such as resistance and temperature coefficient of resistance, correspond closely to those computed from a simple model involving current flow parallel to the interface. These properties change appreciably after step annealing at elevated temperatures. Results in this case indicate that interfacial reactions commence by nucleation and growth of Al2Cu along grain boundaries at 145°C, whereas significant solid solution alloying precedes nucleation and growth of intermetallic compounds at 250°C. Homogenization of these couples proceeds, sequentially, by (1) rapid diffusion of aluminum along grain boundaries in copper, (2) rapid diffusion of copper along grain boundaries in aluminum and (3) slower diffusion of each species into grain interiors of the other until a uniform composition, controlled by relative thickness of the constituent films, is achieved. This interpretation of the evolution of contact resistance is entirely consistent with complementary microstructural characterization by optical and transmission electron microscopy and chemical characterization by Auger electron spectroscopy.

Evolution of hole size and shape in {100}, {110} and {111} monocrystalline thin films of gold

Abstract Monocrystalline thin films of gold, containing controlled distributions of small holes, were produced by an epitaxial flash deposition process on heated {100}, {110} and {111} monocrystalline substrates of sodium chloride. These films, ranging from 10 to 20 nm in thickness, were then removed from their substrates, annealed for various periods at temperatures ranging from 180 to 290 °C and subsequently examined by transmission electron microscopy in order to record the evolution of hole size and shape as a function of crystallographic orientation and annealing conditions. During annealing, these holes either grow or shrink, depending on the ratio of hole diameter to film thickness, with growing holes developing clearly defined crystallographic facets aligned normal to the film surface. The evolution of hole size is in satisfactory agreement with a kinetic analysis based on atomic surface mobility, whereas the evolution of hole shape is consistent with anisotropy of the surface energy, as computed from a nearest neighbor bond model.

Characterization of r.f.-sputtered ZnO thin films by X-ray diffraction and scanning electron microscopy

Abstract Thin films of ZnO, ranging in thickness from 0.08 to 6 μm, have been prepared by r.f. sputtering on substrates of either quartz or glass under various deposition conditions and subsequently characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results from XRD indicate that the grain size increases from 0.01 to 0.5 μm as the film thickness is increased from 0.08 to 6 μm for deposition at 65 °C and increases from 0.1 to 0.3 μm as the deposition temperature is increased from 65 to 480 °C for a constant film thickness of 2 μm, whereas the lattice strain and dislocation density decrease slightly under similar conditions. Results from SEM indicate that the particle size parallel to the plane of the film is approximately equal to the mean grain size perpendicular to the plane of the film, suggesting that growth proceeds by the nucleation of new grains rather than by the elongation of columnar grains in the growth direction. General observations indicate that microstructural parameters, such as grain size, grain shape and lattice strain, depend sensitively on the exact nature of the deposition conditions.

Investigation of interfacial reactions in monocrystalline and polycrystalline thin film couples of gold and silver by Auger depth profiling, transmission electron microscopy and measurement of contact resistance

Abstract Interfacial reactions in monocrystalline and polycrystalline thin film couples of gold and silver were investigated by Auger depth profiling (ADP), transmission electron microscopy (TEM) and measurement of the contact resistance at temperatures ranging from 25 to 418°C in order to determine the effect of grain boundaries on concomitant kinetic processes. Results from both ADP and measurement of the contact resistance indicate that interfacial reactions are controlled by bulk interdiffusion, characterized by a mean activation energy of 1.45±0.10 eV, in monocrystalline couples and by grain boundary diffusion, characterized by an activation energy of 0.50±0.02 eV, in polycrystalline couples. Results from TEM reveal the formation of an interfacial network of dislocations in monocrystalline couples and the expansion of this network during subsequent interdiffusion. It is concluded that characterization of both chemical composition and microstructure, combined with measurement of concomitant electrical properties, provides a reliable description of degradation processes in thin film couples.

The effect of the electron bombardment of NaCl substrates on the epitaxial growth of gold and silver films

Abstract The effect of the electron bombardment of NaCl substrates on the epitaxial growth of gold and silver films was investigated by transmission electron microscopy and Auger electron spectroscopy. The results indicate that anion surface vacancies are created during low energy electron bombardment; these vacancies, in turn, promote nucleation and subsequent epitaxial growth of vapor-deposited films. Nucleation is interpreted in terms of the attraction of metal atoms to the surface vacancies and the subsequent formation of a rigid cluster in an epitaxial orientation. These clusters then grow by the accretion of deposited atoms to form epitaxial islands which eventually coalesce with near-perfect register to form a continuous film characterized by a high degree of epitaxy and low defect densities.

Segregation of solute atoms at dislocations in low-angle grain boundaries

Abstract Bicrystalline thin films of gold, containing 4 and 16° [001] tilt boundaries, have been implanted at room temperature with cobalt, chromium, lead and gold at an energy of 40 keV and integrated fluxes ranging from 1 to 100 × 10 13 ion/cm 2 and subsequently examined by transmission electron microscopy. It is observed that the discrete strain-contrast pattern associated with individual dislocations in low-angle boundaries is obliterated by implantation with cobalt or chromium, but not by implantation with lead or self-implantation with gold at integrated fluxes ranging from 1 to 10 × 10 13 ion/cm 2 . It is concluded that cobalt and chromium segregate at grain boundaries, thereby distorting the lattice and obliterating the discrete strain-contrast pattern, whereas lead, being totally immiscible in gold, segregates at free surfaces. This interpretation is consistent with a model wherein the width of dislocation cores is dependent on the gradient of grain boundary energy as a function of misorientation angle. These results suggest that segregation of impurities at low-angle grain boundaries alters grain boundary structure in much the same manner that adsorption of impurities alters surface structure.

Low temperature resistivity of thin film and bulk samples of CuAl2 and Cu9Al4

Abstract The resistivities of thin film and bulk samples of CuAl2 (θ phase) and Cu9Al4(γ2 phase) were measured at temperatures between 4.2 and 300 K in order to characterize more fully their electrical behavior. The structures of both thin films and bulk samples were identified and the corresponding lattice parameters were determined by precision X-ray diffraction. The measurements indicate that CuAl2 behaves as an ordered electron compound whereas Cu9Al4 behaves as a disordered alloy phase and that the thin films in general are characterized by a greater amount of defect structure than the corresponding bulk samples are. The results are compared with previous measurements on low temperature transport properties in thin film couples of copper and aluminum.

Estimation of grain boundary torques in bicrystalline specimens

Abstract A method for estimation of grain boundary torques τ(φ), based on displacement of a boundary in bicrystalline specimens, has been described and results obtained for certain 〈110〉 symmetric tilt boundaries in 99.998 a/o aluminum bicrystals have been presented. The (111) and (113) twin boundaries are characterized by large values of τ(φ) at all temperatures investigated, whereas the (112) twin boundary is characterized by a large value only at relatively low temperatures. A qualitative ranking of the magnitude of normalized values of τ(φ) suggests that the largest value is associated with the (111) twin boundary followed by, in order of decreasing magnitude, the (113), (112) and (221) twin boundaries and the 10° low-angle boundary. This ranking is in accord with progressively decreasing degrees of atomic fit at the boundary as predicted from coincidence-lattice theory. Magnitude of the normalized grain boundary torques is proportional to the depth of the cusp in the energy-inclination plots. Such cusps are consistent with the significant degree of dependence of grain boundary structure on inclination.