Chien-Neng Liao

Observation of atomic diffusion at twin-modified grain boundaries in copper.

Grain boundaries affect the migration of atoms and electrons in polycrystalline solids, thus influencing many of the mechanical and electrical properties. By introducing nanometer-scale twin defects into copper grains, we show that we can change the grain-boundary structure and atomic-diffusion behavior along the boundary. Using in situ ultrahigh-vacuum and high-resolution transmission electron microscopy, we observed electromigration-induced atomic diffusion in the twin-modified grain boundaries. The triple point where a twin boundary meets a grain boundary was found to slow down grain-boundary and surface electromigration by one order of magnitude. We propose that this occurs because of the incubation time of nucleation of a new step at the triple points. The long incubation time slows down the overall rate of atomic transport.

Enhancement of thermoelectric properties of sputtered Bi–Sb–Te thin films by electric current stressing

Both electrical conductivity and Seebeck coefficient of sputtered Bi–Sb–Te films were enhanced by introducing a high density of electric current through the films during thermal annealing. The electrically stressed Bi–Sb–Te films were found to have lower carrier concentration but much higher mobility than the films thermally treated at the same temperatures. A mechanism based on electromigration-induced preferential Sb diffusion is proposed to explain the observed electrical transport properties and precipitation of Sb-rich phase in the electrically stressed Bi–Sb–Te films. The study shall lead to an effective strategy of improving thermoelectric properties of Bi–Sb–Te films by electric current stressing.

Effect of ball milling and post treatment on crystal defects and transport properties of Bi2(Se,Te)3 compounds

Ionized point defects that act as electron donors or acceptors can strongly affect thermoelectrictransport properties of bismuth telluride compounds. These crystal imperfections including antisite defects and vacancies can be generated or annihilated during material processing. Here, we reported the effect of ball milling, thermal annealing, and electrical stressing on defect population in the Bi2(Se,Te)3 prepared by powder metallurgy. The milling process can modulate the relative portion of antisite defects and vacancies in crystal, and the Bi2(Se,Te)3 made of the powders with extended milling time showed high electron concentration and poor carrier mobility. The crystal defects, Te vacancies in particular, were eliminated to some degree during subsequent thermal treatment. An electric-current assisted thermal treatment was found to be very efficient in eliminating crystal defects, which can improve transport properties of Bi2(Se,Te)3 without causing re-evaporation of volatile Te and Se elements during high-temperature annealing.

Thermal transport properties of nanocrystalline Bi-Sb-Te thin films prepared by sputter deposition

Grain-size dependent thermal conductivity of sputtered nanocrystalline Bi–Sb–Te thin films was measured by a 3ω method. By changing deposition temperature from 100 °C to room temperature, the mean grain size of the Bi–Sb–Te films decreased from 83 to 26 nm and the lattice thermal conductivity reduced from 0.79 to 0.45 W/mK proportionally. The effect of grain boundary on lattice thermal conductivity can be described by an effective thermal boundary resistance that was determined in the range of 0.56–1.8×10−8 m2K/W for the nanocrystalline Bi–Sb–Te thin films studied.

Direct observation of electromigration-induced surface atomic steps in Cu lines by in situ transmission electron microscopy

Surface atomic steps in unpassivated copper lines under electromigration (EM) have been directly observed in ultrahigh vacuum by in situ transmission electron microscopy (in situ TEM). The combination of {111} planes and ⟨110⟩ directions for crystalline Cu were found to be the most favored EM paths. The in situ TEM study of EM-induced evolution of Cu surface structures provides a sound basis for understanding the dependence of EM-induced atomic migration mechanism on crystal orientation of crystalline Cu. The understanding shall lead to the effective strategy of using appropriate passivation layer to suppress the electromigration.

Electric current enhanced defect elimination in thermally annealed Bi–Sb–Te and Bi–Se–Te thermoelectric thin films

An electric-current assisted thermal treatment is demonstrated to be an effective process for eliminating crystal lattice defects and improving thermoelectric properties of both Bi–Sb–Te and Bi–Se–Te nanocrystalline thin films. A model based on electromigration-induced preferential Sb and Te diffusion is proposed to explain the observed Sb-rich and Te precipitation as well as the enhancement of Seebeck coefficient and electrical conductivity of the electrically stressed thin films. Owing to anisotropic diffusion and electrical transport properties, charged lattice defects are preferentially eliminated in the direction parallel to the basal plane of bismuth telluride crystal under electric current stressing. The presented current assisted annealing approach can be an efficient postdeposition treatment that prevents from gross grain growth and evaporation of volatile constituents in Bi–Te based nanocrystalline thin films during high-temperature annealing process.

Enhancement of carrier transport properties of BixSb2−xTe3 compounds by electrical sintering process

BixSb2−xTe3 (x=0.4 and 0.5) powders were pressed and sintered by passing a high-density electric current under nitrogen ambient. The electrically sintered BixSb2−xTe3 was found to have a marked increase in Hall mobility but a moderate reduction in carrier density. We propose that electric-current induced atomic diffusion plays an important role in modulating crystal defects in BixSb2−xTe3 compounds. The hot-pressed Bi0.4Sb1.6Te3 shows a threefold increase in ZT after electrical sintering process. With the electrical sintering approach we can preserve fine-grain microstructure, and hence low thermal conductivity of BixSb2−xTe3 with reasonable electrical resistivity and Seebeck coefficient.

Stability of nanoscale twins in copper under electric current stressing

Migration of {112} incoherent twin boundary (ITB) in nanotwinned Cu under electric current stressing has been observed using in situ high-resolution transmission electron microscopy. The current-driven ITB migration is found to be four orders of magnitude faster than that driven thermally. We propose that electric current plays a role of shuffling Cu atoms at ITB/coherent twin boundary junctions, which enhances nucleation of {112} steps and facilitates twin boundary migration in Cu. By understanding how twin boundaries respond to electric current force we shall be able to trace the property change in nanotwinned Cu under electric current stressing, which would be an essential assessment of interconnect reliability.

Surface roughness reduction in nanocrystalline Cu thin films by electrical stressing treatment

Electromigration-induced surface morphological evolution of nanocrystalline Cu thin films is reported. When applying a high-density current (106 A/cm2), the Cu films showed reduced surface roughness from 7.5 to 1.4 nm after electrical stressing at the temperature below 100 °C. It is suggested that preferential surface diffusion on Cu(111) planes leads to thinning of extruded grains in the electrically stressed Cu film, as evidenced by the weakening (111) texture of the Cu film after electrical stressing. The electrical stressing process shall help reduce roughness of Cu metallization after post thermal treatment.

Chemical reactivity of twin-modified copper nanowire surfaces

Effect of twin boundary (TB) spacing on atomic surface structure and chemical reactivity of nanotwinned Cu nanowires (NWs) is investigated. Post-etching surface structure and wire diameter of Cu NWs were examined by transmission electron microscopy. When TB spacing is less than 10 nm, the Cu NWs remain almost intact after chemical attack and show faceted surface structures with low atomic step density. A mechanism based on surface tension torque acting on TB/surface triple junctions is proposed to explain the faceted structure formation and enhanced corrosion resistance of nanotwinned Cu NWs.