B. Luckscheiter

Growth and melting behaviour of thin in films on Ge(100)

Growth and melting behaviour of thin indium films on Ge(100) have been investigated by Auger-electron spectroscopy (AES), atomic force microscopy (AFM) and perturbed γγ angular correlation (PAC) spectroscopy, respectively. At room temperature inidium is found to grow in three-dimensional islands even at submonolayer coverages. A very rough film surface is observed for thicknesses up to 230 ML. The melting behaviour of such films has been studied by PAC. A reduction of the melting temperature Tm as well as a strong supercooling of the films is observed. The electric field gradient for 111In(111Cd) in the indium islands is determined as a function of temperature and is used to monitor the local crystalline order of the films up to temperatures just below the melting point.

Surface andinterface studies with perturbed angular correlations

The present status of surface, interface andthin film studies with perturbed γγ-angular correlation (PAC) spectroscopy is reviewed. Applications include adsorbate diffusion andstructural phase transitions on stepped metal surfaces, surface andinterface magnetism as well as compound formation at reactive metal/metal interfaces. First applications to semiconductor surfaces andmetal/semiconductor interfaces are discussed.

Indium adsorption on silicon surfaces: a PAC study

Abstract The perturbed γγ-angular correlation (PAC) technique has been used to study the hyperfine parameters for 111 In 111 Cd on different adsorption sites on Si(111)7 × 7, Si(111)2 × 1 and Si(100)2 × 1, respectively. Unique, strong electric field gradients are detected and studied as a function of substrate temperature. Possible microscopic adsorption geometries are discussed and compared to the results obtained earlier by different techniques. For Si(111)7 × 7 the mobility of the probe atoms is determined from isochronal annealing cycles. In case of Si(100) two different indium superstructures are identified and an irreversible phase transition is observed on successive annealing. The results are compared to earlier investigations of metal surfaces.

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.

Microscopic Observation of a Superstructure Phase Transition: In/Si(100)

The PAC method has been used to study the (2 × 2) In → (4 × 3) In superstructure phase transition on Si(100) 2 × 1. About 0.05 ML natural In doped by a small amount of the radioactive probe isotope 111In was deposited on a Si(100) 2 × 1 surface. Two different electric-field gradient situations are found and attributed to the low-concentration precursors of the (2 × 2) In and (4 × 3) In superstructures, respectively. The (2 × 2) In → (4 × 3) In phase transition has been studied as a function of annealing temperature and time, revealing a critical temperature of 610(20) K in contrast to earlier investigations. The results are compared to recent STM investigations of the low-concentration adsorption behaviour of In on Si(100) 2 × 1.

Magnetic hyperfine fields at uncovered ultrathin Ni films on Cu(100) substrates and at single-crystal Ni surfaces

Abstract Magnetic and electric hyperfine fields were measured at 111 In( 111 Cd) probe atoms in the topmost atomic layer of Ni(111) and Ni(100) single-crystal surfaces as well as in ultrathin Ni films epitaxially grown on Cu(100), utilizing the perturbed γγ-angular correlation (PAC) method. The behaviour of the magnetic hyperfine fields as a function of temperature were studied for both Ni surfaces as well as for different Ni(100) films with thicknesses ranging from 2 up to 10 monolayers. It was found that strength and orientation of the magnetic hyperfine fields are quite different for these systems. The critical temperatures are strongly reduced for thin nickel films and approach the bulk value with increasing film thickness, whereas at the surfaces no change in the Curie temperatures was detected.

Interface compound formation and dependence on In‐layer thickness in Ni/In thin‐film systems

Interdiffusion and interface compound formation has been observed at the system Ni/In by using thin‐film couples as well as thin In films on low index Ni single‐crystal substrates. The method applied was the perturbed γγ‐angular correlation technique, which is very sensitive to local structures and their changes around probe atoms. The successive occurrence of different Ni/In compounds could be observed on isochronal annealing above 230 K. A correlation between the appearance of compounds and In film thickness has been found.

Magnetic hyperfine fields in ultrathin Ni films on Cu(100)

The magnetic hyperfine field was measured at 111In(111Cd) probe atoms in ultrathin Ni films epitaxially grown on Cu(100) utilizing the perturbed γγ-angular correlation (PAC) method. The behaviour of the hyperfine field as a function of temperature was studied for different film thicknesses ranging from 2 up to 10 monolayers. It was found that the strength of the hyperfine fields as well as the critical temperatures are strongly reduced for thin nickel films and approach the bulk value with increasing film thickness. The orientation of the hyperfine field is discussed.

Indium adsorption sites at Pd(100) surfaces studied by PAC spectroscopy

Perturbed gamma gamma -angular correlation (PAC) experiments have been performed to study the occupation of different surface sites by 111In probe atoms at vicinal Pd surfaces with (100) terrace orientation. Within an isochronal annealing sequence In probes are observed to dwell in adatomic and substitutional step and terrace sites, in agreement with earlier experiments on Cu, Ag and Ni surfaces. The temperature dependence of the electric field gradient for 111In atoms occupying substitutional terrace sites deviates from the behaviour detected on Cu(100), Ag(100) and Ni(100) surfaces. The observed deviation in the low-temperature regime is attributed to the adsorption of hydrogen.

Nuclear probes for surface characterization

Recent applications of the perturbed γγ-angular correlation (PAC) technique to free surfaces and interfaces have revealed numerous interesting results. The microscopic sensitivity of the method permits to study problems like binding and mobility of isolated adsorbates and two-dimensional structural phase transitions. In addition interface reactions and compound formation have been characterized by the electromagnetic hyperfine fields present at suitable probe nuclei.