Surface Physics with Positrons

AES – Auger Electron Spectroscopy

Auger electron spectroscopy (AES) is an established method in solid state physics for surface sensitive analysis. Conventionally an incoming electron/photon with high energy (a few keV) ionises the inner shell of an atom, i.e. the K-shell. The hole of the exited atom is filled with an electron of an outer shell and the further relaxation is realized either by the emission of a photon with characteristic energy or the available energy is transferred to an electron of a higher energy level, that leaves the atom with a characteristic energy. This radiationless transition is called Auger-transition. Since the emitted electrons have a sharp and for each material typical energy, the analysis of the electrons provides the information of the analysed element in the material.

A disadvantage of conventional EAES is the high secondary electron background and the high penetration depth of the electrons, both due to the high energetic primary electron beam.
To overcome these facts, in 1988 A. WEISS et al. performed for the first time Positron annihilation induced Auger electron spectroscopy (PAES).

PAES – Positron Annihilation Induced AES

The energy of the positrons can be very low, typically a few 10 eV. Due to this, the secondary electron background at higher energies is non existent and the damage at the surface is negligible. The positrons are thermalised within a few 100 nm and diffuse back to the surface, where they are trapped in a surface state. From the surface they are either emitted with a characteristic energy (due to the positive positron work function) or annihilate with a electron or they annihilate with an inner shell electron and induce the Auger-effect. The probability for the Auger transition is in the range of a few percent per positron annihilation.

The big advantages of PAES over EAES are the higher surface sensitivity, the low secondary electron background and the fact, that this technique is non-destructive.
The only disadvantage of PAES, the very low positron intensity of conventional 22Na based beams, is reduced at our spectrometer, since we are working with the high intensity positron source NEPOMUC at the FRM II.
At this facility a positron intensity of 107 e+/s with an energy of 15 eV is available.
With this high intensity, the measurement times for one spectrum has been reduced to typically a few hours compared to a few days for a spectrum recorded with a lab source.

TRHEPD – Total-Reflection High-Energy Positron Diffraction

Total-Reflection High-Energy Positron Diffraction (TRHEPD) demonstrated to be an ideal technique to determine the crystalline structure of topmost and subsurface atomic layers. TRHEPD is the positron counterpart of RHEED and was suggested by Ichimiya in 1992. As for RHEED, the surface sensitivity of TRHEPD primarily originates from the large cross section for inelastic scattering. However, in contrast to electrons, incident positrons are repelled from the surface as the crystal potential is repulsive. Due to the reduced penetration depth of positrons, the interaction with the solid is less complex, which typically improves the agreement between theory and experiment. For further reading, see Review Positrons in surface physics.

Until now, there has been just a single TRHEPD experiment in operation worldwide. In order to benefit from the high beam intensity available at the neutron induced positron source Munich (NEPOMUC), we recently installed a new positron diffractometer at the positron beam line at MLZ.