Visit the VEPAS laboratory

Politecnico di Milano - L-NESS (Como, ITALY)                                            VEPAS: Variable Energy Positron Annihilation Spectroscopy

The speciality of the VEPAS laboratory is the analysis of lattice defects in structural or functional materials. The main techniques adopted in this laboratory are based on positron annihilation spectroscopy (PAS), in two variants: lifetime spectroscopy (LS) and Doppler broadening spectroscopy (DBS).

LS gives information on the time of survival of positrons implanted into the sample. This can be directly related to the electron density at the annihilation site, making LS an ideal tool for the study of the concentration and of the size of open volume defects present in the sample.

DBS gives information on the momentum distribution of the electron-positron pair at the annihilation site, which contains contributions coming from annihilations with valence and core electrons. When a positron is trapped at a lattice defect, the core electron contribution allows us to reconstruct the chemical composition of the environment of the defect itself.

L-NESS building

Research

  • Defects in semiconductor thin layers
  • Positronium formation in porous materials
  • Precipitation hardening of light alloys

 Defects in semiconductor thin layers

Positrons are sensitive to defects associated with threading dislocations generated during the epitaxial growth of semiconductor materials on a substrate with a different lattice parameter. A reduction of the threading dislocation density is expected for thin SiGe layers deposited on top of an even thinner silicon layer, bonded on silicon oxide (silicon-on-insulator, SOI), which insulates it both mechanically and electrically from the thick Si wafer mechanically sustaining the layer stack. Defect concentration might be reduced by the elastic deformation of the silicon substrate or by the migration of dislocations from the interface to the oxide layer. Our results show that an increment of the relaxation degree in the overlayer is accompanied by the formation of Ge-rich point defects in the SiGe layer.

Our interests also comprise the study of GaN layers grown on sapphire substrates. Positrons have been recently used to identify the gallium vacancy as the main defect responsible for the observed GaN yellow luminescence. Defects in such materials act as charge compensating centers and are closely related to impurities or dopants introduced into the layers. Our purpose is to investigate the influence of the growth conditions on impurity incorporation and vacancy formation.

(see some works about defects in semiconductors)

Positronium formation in porous materials

Positronium is an “atom” made of a positron and an electron. There is no nucleus, but the two particles rotate one around the other. Positronium is formed with very high efficiency in soft matter and insulators and it is used nowadays in the study of porous materials for microelectronics (low-kdielectrics), of phase transformations in polymers and in general in the growing field of nano-structured materials. Our efforts are concentrated on the selection of an appropriate material to be used as a positronium converter in the AEgIS (Antimatter Experiment: Gravity, Interferometry, Spectroscopy) experiment at CERN. The ambitious goal of the aforementioned experiment is to measure the gravitational acceleration of antihydrogen atoms formed by the change exchange reaction between a positronium atom and an antiproton produced by CERN's antiproton decelerator.

(see someworks about Positronium formation in Porous Materials)


Precipitation hardening of light alloys

Hardening in selected aluminum and magnesium alloys (2000/7000 series aluminum alloys and WE Mg-Rare earths magnesium alloys) is accomplished by controlling the precipitation kinetics of small particles which impede dislocation motion during plastic deformation. Precipitation mechanisms often involve solute interactions with vacancies during aging: positrons can thus be used to monitor the formation of precipitate precursors made of a few atoms at a such an early stage that hardly any other technique can give the same reliable information. Positrons are also sensitive to open spaces created by the loss of coherence between the precipitates and the host matrix at later stages of the precipitation process. DBS has been used to identify the chemical species of the atoms which surround the vacancies and to study their evolution during aging. Complementary information has been sought by performing small angle X-ray scattering (SAXS) experiments at international synchrotron facilities.

(see some works about precipitation and defects in light alloys)

 

Facilities

  • Slow positron beam
  • Coincidence Doppler broadening
  • Fast positron spectrometers

Slow positron beam

Slow positron measurements are performed by moderating positrons emitted by an 50 mCi sealed radioactive source and then implanting them into the sample with a kinetic energy ranging from few eV to 20 keV. The positron implantation depth is a few microns, depending on the sample density.
Slow positron beam. 1. Radioactive source; 2. Electrostatic optics; 3. Sample chamber; 4. HpGe detectors; 5. Cryostat; 6. High voltage protection cage; 7. Power suppliers; 8. Detector electronics.

The positron beam is equipped with HpGe detectors for momentum distribution measurements, which can be operated in both single and coincidence mode. Samples are kept in high vacuum (~10-9 mbar) and their temperature can be varied from 10 to 1000 K. The slow positron beam has been calibrated for positronium fraction measurements.

(more information about the Positron beam)

Coincidence Doppler Broadening





(after Asoka-Kumar et al. Phys. Rev. Lett. 77 (1996) 2097)

What is Coincidence Doppler broadening? CDB is the one technique developed by Kelvin Lynn and collaborators [1,2] giving information on the chemical composition of solute aggregates containing open volume defects (vacancies or misfit regions). This is a very important information, since vacancies help the transport of the solute in the solid matrix and affect the stability of the aggregates by relieving the local stress due to different atomic sizes. The method for obtaining the full quantitative analysis has been proposed and improved by our group.










Block diagram of the CDB system.                                                

This is the bi-dimensional energy spectrum of annihilation photon pairs. The diagonal marked with red arrows corresponding to energy conservation (2m0c2). The central peak is elongated along the diagonal due to the Doppler effect.













This is the momentum spectrum obtained by cutting the bi-dimensional energy spectrum along the red diagonal. The high-momentum tails are due to annihilations with fast core electrons and carry information on the atomic species. It is possible to obtain the S and W parameters to characterize the annihilation peak. The Positron Laboratory is equipped with a CDB spectrometer.

The high-momentum details are enhanced when shown in terms of relative difference to a reference spectrum. The spectrum for an Al-Zn-Mg-Cu alloy as-quenched (open symbols green, left side frame) is reproduced by a linear combination (dashed green line) of the spectra measured for pure metals saturated of defects (right-side frame). For more details see Refs. 3 and 4 (and references therein).

[1] J. R. MacDonald, R. A. Boie, L. C. Feldman, M. F. Robbins, P. Mauger and K. G. Lynn, Bull. Am. Phys. Soc. 24 (1975) 580.
[2] K. G. Lynn, J. R. MacDonald, R. A. Boie, L. C. Feldman, J. D. Gable and M. F. Robbins,Phys. Rev. Lett. 38 (1977) 241.
[3] A. Dupasquier, R. Ferragut, M. M. Iglesias, M. Massazza, G. Riontino, P. Mengucci, G. Barucca, C. E. Macchi, and A. Somoza, Phil. Mag. 87 (2007) 3297.
[4] R. Ferragut, A. Dupasquier, C. E. Macchi, A. Somoza, R. N. Lumley, and I. J. Polmear,Scripta Mater. 60 (2009) 137.

 

Fast positron spectrometers

 

Fast positrons are obtained by the radioactive decay of a 22Na sealed radioactive source sandwiched between two identical samples. Positrons are emitted from the source with an high energy (22Na has a positron endpoint energy of 540 keV) and sampled several hundred microns from the surface of the specimen.

Our laboratory is equipped with two analog gamma ray spectrometers for PL measurements with a time resolution of about 240 ps. The gamma spectrometers are fast scintillators (BaF and plastic scintillators) coupled with photomultipliers. DBS is performed by means of two HpGe detectors with an energy resolution of about 1.2 keV on the annihilation line. The two detectors are coupled with a multi-parametric pulse analyzer which allows for coincidence measurements at a very high signal to noise ratio. Samples can be measured at room temperature or at liquid nitrogen temperature. High vacuum measurements can also be performed.



Positron lifetime spectrometer. 1. Scintillators and photomultipliers; 2. Sample; 3. Liquid nitrogen Dewar; 4. Detector electronics.


Former group members