Positrons in surface physics

Abstract

Within the last decade powerful methods have been developed to study surfaces using bright low-energy positron beams. These novel analysis tools exploit the unique properties of positron interaction with surfaces, which comprise the absence of exchange interaction, repulsive crystal potential and positron trapping in delocalized surface states at low energies. By applying reflection high-energy positron diffraction (RHEPD) one can benefit from the phenomenon of total reflection below a critical angle that is not present in electron surface diffraction. Therefore, RHEPD allows the determination of the atom positions of (reconstructed) surfaces with outstanding accuracy. The main advantages of positron annihilation induced Auger-electron spectroscopy (PAES) are the missing secondary electron background in the energy region of Auger-transitions and its topmost layer sensitivity for elemental analysis. In order to enable the investigation of the electron polarization at surfaces low-energy spin-polarized positrons are used to probe the outermost electrons of the surface. Furthermore, in fundamental research the preparation of well defined surfaces tailored for the production of bound leptonic systems plays an outstanding role. In this report, it is envisaged to cover both the fundamental aspects of positron surface interaction and the present status of surface studies using modern positron beam techniques.

Introduction

In surface science the structure and elemental composition ideally of the topmost atomic layer of a solid and all kinds of phenomena related to the surface are subject of most research activities. For this a zoo of well-known standard analysis tools based on spectroscopic, microscopic, scattering and diffraction methods are commonly applied.

In the 1980s, positron diffraction experiments [1], [2] as well as measurements of positron annihilation induced Auger electrons [3] have been carried out for the first time. Several techniques using low-energy positrons were shown to exhibit, among other features, outstanding surface sensitivity. In contrast, complementary methods, such as electron diffraction, classical Auger-electron and photo-electron spectroscopy, suffer from additional signals originating from layers below. For this reason, open questions with regard to the elemental analysis, atom positions and electron polarization at the surface have been successfully addressed by positron beam experiments. The application of the positron as a surface probe, however, has become established only slowly mainly due to the comparable low intensities provided by positron beam systems built in several laboratories around the world. Therefore, great efforts have been made to improve both the beam intensity of low-energy positrons and the efficient detection of the messenger particles, i.e. positrons, electrons or photons.

In the last decades remarkable advances have been made in the production of evermore intense positron beams. A great step forward has been the development of strong positron sources at large-scale research facilities yielding intensities in the order of ${10}^8–{10}^9$ moderated positrons per second. Currently, two positron beam systems operated at an electron linac (KEK, Japan) [4], [5], [6] and at a research reactor (NEPOMUC, Germany) [7], [8] are dedicated to surface experiments. In the light of their great potential novel positron beam techniques have been developed successfully such as (total) reflection high-energy positron diffraction ((T)RHEPD) [9] and time dependent positron annihilation induced Auger-electron spectroscopy (PAES) [10]. On the other hand the measurement times for surface studies could be reduced considerably by improving detection efficiencies, e.g. for recording electron spectra, and by the development of new techniques such as the combination of the time-of-flight method and adiabatic magnetic beam guiding systems [11], [12].

In solid state physics and material science positron annihilation spectroscopy (PAS) has become an established non-destructive technique for defect spectroscopy of bulk materials since lattice defects such as vacancies, dislocations or precipitates determine the mechanical, optical and electronic properties of materials to a large extent. The most widespread methods are positron annihilation lifetime spectroscopy (PALS) and Doppler-broadening spectroscopy (DBS) of the positron electron annihilation line where one profits from the positrons high affinity to open-volume defects. Coincident DBS (CDBS) provides additional information on the chemical surrounding of open volume defects or the presence of precipitates. Since the positron typically diffuses over hundreds of lattice constants prior to annihilation, the positron can be regarded as an extremely sensitive nano-probe to detect e.g. single vacancy concentrations as low as 10⁻⁷ vacancies per atom. On the other hand, in defect-free crystals the electronic structure can be revealed by the measurement of the angular correlation of annihilation radiation (ACAR). For PAS in solid state physics refer to standard references such as [13], [14].

With the advent of monoenergetic positron beams, the near surface region of samples became object of research. By variation of the positron energy up to 30 keV the mean positron implantation depth can be chosen between a few nm and several μm. The according probed volume is hence defined by the positron implantation profile blurred by the positron diffusive motion, which is in the order of 100 nm for defect-free crystals, and by the positron beam diameter which typically ranges between 10 μm and a few mm. Thus, the investigation of the type and the concentration of open volume defects as a function of positron implantation depth, and, provided that a sufficiently small beam diameter is achieved, spatially resolved PAS have become feasible. Therefore, since many years numerous positron beam experiments have been performed addressing a wide field of scientific issues with regard to the structure and defects in the near surface region, thin films, multi-layers, and interfaces. Such depth dependent PAS studies comprise: (i) element selective analysis of metallic clusters and layers buried in Al with CDBS [15], [16], and, using ACAR, the detection of semi-coherently Li clusters embedded in single crystalline MgO, which have been formed after Li ion implantation [17], (ii) in oxide systems, the characterization of different types of metallic vacancies present in pulsed laser deposited thin film perovskites (see e.g. [18]), and imaging of the spatial variation of the oxygen deficiency, and hence the local variation of the transition temperature for superconductivity, in single crystalline YBa₂Cu₃${\mathrm{O}}_{7-\delta }$ thin films [19], (iii) in polymer films, the determination of the free volume [20] and the open volume in bioadhesive [21], (iv) thin film annealing and alloying at a Au/Cu interface [22] as well as interdiffusion in epitaxial single-crystalline Au/Ag thin films [23], (v) irradiation induced defects e.g. in Mg [24] and in silica [25], [26] as well as defect annealing after He implantation in InN and GaN [27] and H induced defects in Pd films [28], (vi) defect imaging of the fatigue damage in Cu [29], around a scratch on a GaAs sample [30], and in Al samples after plastic deformation [31]. For further details of near-surface studies with positron refer to appropriate reviews (see e.g. [32], [33], [34]).

Within the scope of this review it is intended to focus on positron studies of the topmost atomic layers, i.e. positron surface investigations par excellence. Thus, low-energy positron beams with a kinetic energy perpendicular to the surface of typically a few eV have to be applied in order to examine the features of reconstructed surfaces, adsorbates, single adsorbed layers and their spacing to the substrate as well as cover layers with a nominal thickness in the (sub-)monolayer (ML) range. The studies presented here comprise investigations of the elemental composition, the atom positions and the electron polarization in the outermost atomic layer.

The knowledge of slight displacements of the atomic positions in the topmost surface layers is crucial for the correct description of the electronic structure and magnetic properties of crystal surfaces. For this reason, (total) reflection high-energy positron diffraction ((T)RHEPD) was shown to be highly beneficial since the intrinsic features of this technique, i.e. missing exchange correlation and repulsive scattering potential, give rise to an outstanding accuracy in the determination of the atom positions at the surface. In contrast to its electron counterpart, positron diffraction patterns are easier to calculate due to the opposite sign of the scattering potential, and total reflexion is only present in the positron case. Other diffraction techniques such as small angle X-ray or neutron scattering are barely sensitive to the topmost atomic layer since even at grazing incidence the penetration depth of X-rays still amounts to a few ML. Surface scattering of He atom beams exhibits high sensitivity to the surface, but precise calculations are more complicated and information on the subsurface interspacings is not accessible. In order to gain information on the surface composition, element sensitive techniques such as electron or X-ray induced Auger-electron spectroscopy (EAES or XAES) are applied. Positron annihilation induced AES, however, exhibits intrinsically major advantages such as suppressed secondary electron background in the energy range of Auger-transitions, topmost layer sensitivity and less damage of the probed surface or adsorbed molecules due to the only thermal energy of the annihilating positron trapped at the surface. Thanks to the availability of high intensity low-energy positron beams time dependent PAES could be successfully applied to observe surface segregation in situ. Moreover, surface studies using spin-polarized positrons have been demonstrated to be particularly suitable for the observation of spin phenomena at surfaces. Finally, tailored surfaces for the efficient creation and release of free positronium (Ps) and the positronium negative ion ${\mathrm{Ps}}^{-}$ play a major role in fundamental research. In particular, a high yield of cold Ps interacting with anti-protons is required to produce larger amounts of anti-hydrogen atoms. Apart from a well-defined surface structure, a high positron density is mandatory to pave the way for the prospective creation of a Ps Bose–Einstein condensate.

In this paper, I will review the current status of state-of-the art positron beam setups, physical basics for surface experiments with positrons as well as past and present surface studies with particular emphasis on diffraction, Auger-electron, polarization and fundamental experiments using low-energy positrons. Accordingly, this review is organized in three parts: in the first more methodical part (Section 2), a concise review is given on the generation of low-energy positron beams and the experimental setups realized in laboratories as well as at large-scale facilities. The second part (Section 3) deals with the positron׳s fate in matter and at the surface. In particular, the energetics at the surface is explained giving rise to the high sensitivity of the positron as surface probing particle. The third and most comprehensive part comprises four sections of positron beam studies including introductory explanations of the basic principles and the applied techniques. First, fundamental experiments benefiting from well-defined surfaces are presented (Section 4), followed by an overview of positron diffraction experiments reported so far (Section 5). Then PAES studies for the determination of the elemental composition at the surface (Section 6), and experiments using spin-polarized positrons are presented (Section 7).

The paper is concluded by giving an outlook with regard to ongoing and future developments of novel techniques and planned experiments. A table of most relevant positron emitting isotopes, an overview of positron beam systems at large-scale facilities, and calculated core-annihilation probability compiled from the literature can be found in the Appendix.