C. M. Aldao

Influence of particle size on the conductance of SnO2 thick films

Abstract The electrical response of SnO2 thick films was found to be highly dependent on the grain size. This result can be explained as the consequence of the type of intergranular potential barriers developed at intergrains. Specifically, samples with smaller particle size have overlapped potential barriers and samples with higher particle size have separated potential barriers. Also, the influence of gas transport phenomena on the resistance of the gas sensor was investigated. From an analysis of the possible mechanisms involved in the oxygen diffusion, the Knudsen diffusion is suggested as the controlling step.

Ag and Co cluster deposition on GaAs(110): Fermi level pinning in the absence of metal‐induced gap states and defects

A novel method of forming interfaces makes it possible to produce abrupt, defect‐free metal/semiconductor boundaries. The procedure involves the growth of metal clusters on Xe buffer layers condensed on clean surfaces, and Xe sublimation brings the clusters into contact with the pristine surface. Clusters of Co and Ag produce unique, nearly coverage‐independent Fermi level positions ∼0.32 and 1.0 eV below the conduction‐band minimum for n‐ and p‐GaAs (110), respectively. Detailed line shape analysis of the photoemission spectra shows no evidence for cluster‐induced disruption or conventional defect formation. Instead, the EF position appears to be related to intrinsic surface states swept into the gap as a result of surface unrelaxation around the metal clusters. The results are contrasted to those for metal atom deposition at 60 and 300 K. For Co atom‐by‐atom deposition, substantial temperature dependences are observed in the Schottky barrier evolution, despite nearly equivalent adatom‐induced surface di...

Temperature‐dependent interface morphology and Schottky barrier evolution for Au/InP(110)

Results of extensive high‐resolution synchrotron radiation photoemission studies reveal significant differences between Au/InP(110) interfaces formed at 300 and at 60 K. At 300 K, Au atom deposition results in substantial substrate disruption and, for coverages ≥2 A, a thickening Au overlayer containing intermixed In and P atoms. For atom deposition at 60 K, the amount of substrate disruption is not significantly changed, but surface segregation of the released In and P atoms is largely, though not entirely, inhibited. These reults show that the distinctly different temperature‐dependent Schottky barrier evolution on n‐ and p‐type InP(110) cannot be attributed to thermally controlled reaction since there is little such control. Low‐temperature changes in Fermi‐level pinning for Au coverages at 2–4 A appear related to the development of metallic character in the overlayer.

On the origin of metal film work function changes under electrochemical modification

Abstract The catalytic activity and selectivity of metals can be reversibly altered by supplying promoters electrochemically. This has been achieved by using a catalyst as the working electrode in a solid electrolyte electrochemical cell. In this letter we review the accepted explanation for the relationship between the catalyst work function ( eφ W ) and the potential difference between the working and reference electrodes of the electrochemical cell ( V WR ). We show that this explanation is not correct. A theoretical derivation for the relationship found between eφ W and V WR in some experiments is not presently available. For the moment, it must be considered only an experimental finding.

CdTe(110) interface formation with reactive and nonreactive overlayers: Al, Ti, Pd, Ag, Au, In, and Ce

High resolution synchrotron radiation photoemission has been used to investigate room temperature interface formation involving CdTe(110) and transition metals (Ti), noble and near‐noble metals (Ag, Au, Pd), lanthanide metals (Ce), and simple metals (Al, In). Core level and valence band results reveal substrate interactions with quite different chemical behavior and morphologies. Deposition of Ag and In at 300 K produces a nondisrupted substrate on which clusters form spontaneously. Deposition of Al, Ti, and Ce produces metal–anion reactions and substrate disruption at low coverage, with metal clusters that grow on the reacted layer after reaction has been curtailed by kinetic considerations. For Pd and Au, there is substrate disruption, even though the heats of formation for Pd and Au tellurides are smaller than CdTe, and there is segregation to the surface for both Cd and Te atoms. Studies of Au overlayers formed on reacted Ti interlayers on CdTe show changes in Cd atom distribution and give insight int...

Monte Carlo simulations of monatomic steps dynamics on Si(100)-(2 × 1)

Abstract By means of Monte Carlo simulations the frozen pattern of a B -type step edge in the Si(100)-(2 × 1) surface is simulated. Sets of two dimers are attached and detached from the step according to the number of nearest and next-nearest neighbors. Average kink length, average kink separation and correlation length from experiments and modeling are presented in order to check the quality of our results. Experimental results are well reproduced by using measured experimental rates. The characteristics of the resulting step profile are also analyzed as a function of the step width.

A 20–350 K variable‐temperature sample holder with sample interchangeability

A simple design is presented for an ultrahigh‐vacuum‐compatible sample holder capable of maintaining temperatures between 20 K and ∼350 K. The design involves a copper sample holder (tank) mounted on a closed‐cycle He refrigerator. Mechanical, thermal, and electrical contact between the sample and the tank is obtained by melting and resolidifying Ga in a recess in the tank. Temperature regulation is achieved through simultaneous action of the cold stage and a filament heater attached to the tank. This allows in situ sample exchange without compromise of ultimate vacuum or temperature.

A Monte Carlo simulation of the NO+CO reaction on Na-promoted platinum

An irreversible surface reaction lattice-gas model for the reaction of carbon monoxide and nitric oxide on an Na-modified Pt(111) surface is presented. The eVect of Na adatoms on the reaction kinetics is examined in terms of a ‘‘local’’ (i.e. short-range) model for alkali-promoted NO dissociation. Within this framework we also study the eVects of extending the range of alkali-induced NO dissociation from nearest neighbour to next nearest neighbour sites. The influence of varying amounts of sodium on overall catalytic activity, reaction kinetics, and on the N 2 /N 2 O selectivity are investigated by means of Monte Carlo simulations. It is found that the principal features of recent experimental data involving electrochemical promotion by Na of the CO+NO reaction over Pt are satisfactorily accounted for in terms of this model.

A steady-state approach to determine diffusion coefficients: the migration of silicon on the Si(100) surface

We introduce a steady-state method to analytically determine diffusion coefficients for a particle in a strip with multiple inequivalent adsorption sites in the unit cell. This approach is applied to a variety of models showing its simplicity and usefulness. In particular, the diffusion coefficient of a Si adatom on the Si(100) for a model based on the Stillinger-Weber potential is computed. Comparisons with stochastic kinetic simulations are presented.

Calculated photocurrents and surface barrier heights

This article solves the equations that dictate band flattening due to photo‐induced electron–hole pair generation in the depletion region of a semiconductor. The calculations show the influence of the barrier height, the optical properties of the substrate, and the minority carrier properties. The calculations are compared to typical temperature‐dependent photoemission results obtained after 10 A of Ge were deposited onto p‐GaAs(110). The results show significant band flattening when a lightly doped sample is brightly illuminated, even at room temperature.