R. Wesche

Isolated indium atoms on copper surfaces: A perturbed γ-γ angular correlation study

Abstract III In hyperfine probe atoms with a concentration of about 10 −4 ML have been utilized as nuclear observers on various copper surfaces. These isolated III In probe atoms are highly sensitive to the local atomic structure via the electric-field-gradient tensor acting at the probe nuclear site; this quantity has been detected using the perturbed γ — γ angular correlation (PAC) method. The experiments deal with the localization of indium on substitutional terrace sites for Cu(100), Cu(111) and Cu(110) surfaces. Step sites have been observed in addition on vicinal Cu(111) and from thermal conversion between different sites diffusion data have been deduced. The electric field gradient has been investigated on Cu(110) for III In deposited at various depths from the surface, revealing the extreme short-range character of the electric field gradient. Finally depositions (few percent of a ML) of natural indium have allowed to study formation and thermal decomposition of In-In configurations on copper surfaces.

Studies of compound formation at CuIn, AgIn and AuIn interfaces with perturbed γ-γ angular correlations

Abstract The thermal behaviour of the thin metal film couples Cu/In, Ag/In and Au/In was studied using the perturbed γ-γ angular correlation method. Formation of interface compounds CuIn 2 , AgIn 2 and AuIn 2 was observed above temperatures T of 240 K, 220 K and 260 K respectively. The CuIn 2 and AgIn 2 compounds start to decompose above temperatures of 440 K and 460 K, though for AuIn 2 no decomposition was observed up to 550 K. The interface compounds are characterized by the following electric field gradient parameters (at T = 77 K): CuIn 2 V zz = 4.3(7) × 10 17 V cm −2 η = 0.57(1) AgIn 2 V zz = 3.7(6) × 10 17 V cm −2 η = 0.16(2) AuIn 2 V zz = 0 η = 0 Isothermal annealing of the film couples yielded a square root of time behaviour for the average interface compound thickness, which supports the assumption of a diffusion-controlled growth mechanism. Diffusion constants D 0 and activation energies E a were evaluated from an Arrhenius plot as follows: CuIn 2 D 0 = 1.0(3) × 10 −6 cm 2 S −1 E a = 0.42(2) eV AgIn 2 D 0 = 4(3) × 10 −5 cm 2 S −1 E a = 0.46(3) eV AuIn 2 D 0 = 2(1) × 10 −9 cm 2 S −1 E a = 0.31(4) eV

Microscopic Observation of Step and Terrace Diffusion of Indium Atoms on Cu(111) Surfaces

The perturbed γγ angular correlation (PAC) method has been applied to study isolate 111In probe atoms on vicinal Cu(111) surfaces. The properties of the observed electric-field gradients and the annealing behaviour suggest the existence of indium as step adatoms up to 140 K, above this temperature these adatoms convert into substitutional step atoms. At 220 K the indium probes finally start to diffuse into substitutional terrace sites. Desorption and diffusion into bulk was also detected. From the thermal conversion between the different sites activation energies have been estimated.

Step-correlated diffusion of in atoms on Ag(100) and Ag(111) surfaces

Abstract Diffusion processes on vicinally cut Ag(100) and Ag(111) surfaces have been studied utilizing the perturbed-angular-correlation (PAC) spectroscopy with 111 In probes at very low concentration (10 −4 ML). The occupation of step and terrace sites by PAC probes at the Ag surfaces is detected via the electric-field-gradient tensor and is observed as a function of annealing temperature. From the thermal conversion between the different 111 In surface sites activation energies for microscopic diffusion steps have been estimated.

Compound formation in Ni/In thin film systems

Interface compound formation in Ni/In film couples has been studied by means of the PAC method using radioactive111In probe atoms. Subsequent occurrence of the compounds Ni10In27, Ni2In3, NiIn and Ni3In has been observed after isochronal annealing. Interdiffusion was found to start at temperatures about 250K whereby Ni propagates into the In film.

The electric field gradient for single indium atoms on low-index silver surfaces

The perturbed gamma gamma angular correlation (PAC) method has been applied to study isolated 111In atoms on low index silver surfaces. Strong, axial-symmetric electric field gradients are observed on the highly symmetric (111) and (100) surfaces, whereas a non-axial-symmetric field gradient is found for the (110) orientation. Strength and symmetry of the electric field gradients have been studied as a function of measurement temperature. The influence of impurities on the field gradient distributions is discussed. Finally, a comprehensive comparison is given to results reported previously for 111In on the respective copper surfaces. The observed behaviour reflects well known features of the bulk metals.

Hyperfine-interaction studies of surfaces

Applications of hyperfine-interaction techniques, like NMR, PAC and Mößbauer spectroscopy, to well-characterized surfaces are discussed and the present knowledge of surface hyperfine fields is reviewed. Measurements of nuclear spin relaxation permit to extract the local density of electron states at the Fermi level of adsorbed alkali atoms. From the observed electric-field-gradient properties surface probe sites and diffusion processes can be inferred; the experimentally determined magnetic hyperfine fields give access to the electron-spin behaviour at magnetic surfaces.

Investigations of Ag(100)In and Ag(111)In interfaces with local probes

Abstract The structure of Ag(100)In and Ag(111)In interfaces and the formation of AgIn 2 were studied with the perturbed γ-γ angular correlation method using radioactive 111 In probe atoms. At the Ag(111)In interface the probe atoms experience a strong electric field gradient with the z principal axis perpendicular to the interface, which reflects an ordered interface structure. This can be explained by epitaxial growth of the very thin indium film on the Ag(111) substrate. In both systems, interdiffusion starts at around 240 K and leads to the formation of the interfacial alloy AgIn 2 . Since the same behaviour has been observed for polycrystalline AgIn thin film couples, this result rules out grain-boundary-assisted diffusion of indium in silver as the mechanism for AgIn 2 compound formation. Further experiments indicate that the AgIn 2 Ag interface profile is rather uneven. This result may be explained by grain-boundary-assisted diffusion of silver in indium at the onset of AgIn 2 compound formation.

Electric field gradient at111In on Cu (100) surfaces and its utilization for indium surface diffusion

The electric field gradient at111In probe atoms on Cu (100) surfaces was studied. At clean surfaces all probes are exposed to a well-defined surface field gradient. This is used to investigate indium surface diffusion, where the applied PAC method allows to observe diffusion steps on an atomistic scale. The jump rate for indium on Cu (100) was found to be in the order of 10−3 Hz at 200 K.

SURFACE INVESTIGATIONS WITH PAC

The present status of surface electric field gradients measured with perturbed γγ-angular correlations (PAC) is reviewed. Experiments concentrate on well-characterized, free surfaces of metals and applications to detection of probe sites and diffusion phenomena are discussed.