R.S. Yu

Positronium time-of-flight measurements of porous low-k films

We demonstrate that positronium (Ps) time-of-flight (TOF) spectroscopy provides a powerful technique for probing the structure of porous low-dielectric constant (low-k) films. The emission of ortho-positronium (o-Ps) from the surface of a series of nano-porous silsesquioxane films was studied with a newly developed TOF spectrometer. Clear Ps emission peaks were observed from the two high porosity films (F38 and KI31), indicating their higher open porosity in comparison with other films. We also observed that the energy of o-Ps emission from F38 is much higher than that from KI31. It was concluded that the open pores in the former film are less tortuous, and o-Ps loses significantly less energy emerging from its surface than from that of latter film.

Effects of coexistent pores and paramagnetic defects on positron annihilation in silicon oxide thin films

We measured the Doppler broadening of positron 2γ annihilation and the probability of positron 3γ annihilation (I3γ) for paramagnetic defects (E′ and Pb) containing SiOx (1.5<x<2) films with different porosities, which were prepared by radio-frequency cosputtering of Si and SiO2 at different Ar pressures. The observed increase of I3γ with increasing Ar pressure was attributed to the increased open porosity of the film. Under a condition of constant paramagnetic defect concentration, Doppler broadening S parameter increases with increasing I3γ and open porosity of the film. This was caused by enhancement of a relative contribution from p-Ps to the 511 keV 2γ annihilation peak as a result of reduced probability of o-Ps pickoff annihilation. The relationship between the S parameter and I3γ can be affected strongly by the paramagnetic defects in the sense that film with a higher open porosity does not necessarily present a higher S parameter. A low porosity film with higher concentration Pb centers exhibited ...

Pore interconnectivity of nanoclustering silica porous films as studied by positronium time-of-flight spectroscopy

Positronium time-of-flight spectroscopy with improved stability and signal-to-noise ratio, achieved by a developed off-line digital data analysis, was applied to the characterization of three types of nanoclustering silica porous films with different relative dielectric constants and refractive indices. The emission of triplet ortho-positronium (o‐Ps) from the film surface was examined as a function of incident positron energy (Ein). It was found that the o‐Ps emission peak energies from two highly porous films with similar total porosities decrease similarly to each other with increasing Ein up to 1.50keV. On the other hand, o‐Ps emission intensities from the two films differed considerably in the range between 0.5keV<Ein<4keV, which reflects a difference in pore interconnectivity between the two films with different mean secondary particle sizes. Some interconnected pores are expected to be closed by the necking at the particle contacts as calcination proceeds, possibly leading to more necks in the pore...

Positron annihilation in (Ga, Mn)N: A study of vacancy-type defects

The vacancy-type defects in (Ga,Mn)N films grown by metal organic chemical vapor deposition were studied by positron annihilation technique. Doppler broadening spectra were measured for the films. Compared to the undoped GaN film, the positron trapping defects in the (Ga,Mn)N films have been changed to a new type defects and its concentration increases with the increasing Mn concentration. By analyzing the S-W correlation plots and our previous results, we identify this type defects in the (Ga,Mn)N as VN-MnGa complex. This type of defects should be considered when understand the magnetic properties in a real (Ga,Mn)N system.

Propagation of magnetically controllable lasers and magneto-optic dual switching using nitrogen-vacancy centers in diamond

We study coherent laser-induced optical behaviors in weak twin-light propagation through strained diamond nitrogen-vacancy (NV) centers via switching on and off an external magnetic field. By numerically solving the coupled Bloch-Maxwell equations for NV center and field simultaneously in space and time, we address dynamic control of pulse propagation and magneto-optic dual switching in such a laser-driven NV center system. The proposed scheme may have applications in the design of magneto-optic switching and magneto-optic storage devices.

An undergraduate experiment to test relativistic kinematics using in flight positron annihilation

The basic theoretical tenants of special relativity can be presented in introductory physics courses. However, because special relativistic effects only become apparent at velocities close to the speed of light, there are few experiments at the undergraduate level that involve special relativistic effects. We show that it is possible to experimentally verify the relation E2=p2c2+m02c4 in an undergraduate student laboratory using commercially available equipment. The experiment uses the annihilation of positrons in flight.

Correlation between the Doppler Broadening S Parameter and Ortho-Positronium Formation Probability in Polymers

Various kinds of polymers were systematically studied by the combination of positron lifetime and coincident Doppler broadening spectroscopy. The correlation between the S parameter and ortho-positronium (o-Ps) fraction (Io-Ps), strongly dependent on the chemical element that the polymers contain, enabled us to separately extract the Doppler broadening profiles of positrons and positronium.

Positron and Positronium Annihilation in Low-Dielectric-Constant Films Studied by a Pulsed Positron Beam

Positron and positronium annihilation in porous low-dielectric-constant (low-k) films deposited by plasma-enhanced chemical vapor deposition (PECVD) and spin-on dielectric (SOD) have been investigated by means of positron annihilation lifetime spectroscopy (PALS) and age-momentum correlation (AMOC) spectroscopy with a pulsed slow positron beam. The ortho-positronium (o-Ps) lifetime strongly depends on the deposition condition. In general, PECVD low-k films have shorter o-Ps lifetimes than SOD low-k films, indicating PECVD low-k films have smaller pores. Since o-Ps diffusion and escaping from the surface occurs in most of porous SOD films, three-gamma annihilation measurement is important. To investigate o-Ps behavior in SOD films, we have carried out two-dimensional (2D) PALS measurement, which measures annihilation time and pulse-height of the scintillation detector simultaneously. Monte-Carlo simulation of the o-Ps diffusion and escaping in porous films has been carried out to simulate the 2D-PALS results.

Luminescence and Defect Study of Ce3+-Doped Lu3Al5O12 Phosphors

Cerium doped lutetium aluminium garnet (Lu3Al5O12:Ce, LuAG:Ce) displays particularly promising performance and has a great potential to be used in PET and SPECT. LuAG:Ce phosphors was synthesized with a simple sol-gel process. The structure was identified with XRD. Variations of luminescent properties and defects of LuAG:Ce phosphors with different heat-treatment temperatures and Ce3+-dope concentrations have been investigated, with the aid of emission spectra and positron annihilation lifetime measurements. The influence of defects on the luminescence properties was discussed.

Cu Precipitates in Fe Ion Irradiated Fe-Cu Alloys Studied Using Positron Techniques

In this paper, we summarized our recent experimental results on Fe-Cu model alloys irradiated by Fe ion. Two kinds of Fe-Cu alloys with 0.3%Cu and 0.6%Cu were prepared and irradiated by 2.5 MeV Fe ion at 573 K. Irradiation dose is 0.1 dpa and 1.2 dpa for each type alloy respectively. Positron annihilation techniques of slow positron beam were used to investigate the irradiation induced defects. Results show that the S parameters are higher in the specimens with high irradiation dose, but the intensity of Cu peaks of CDB is lower. It indicates that the precipitation of Cu atoms formed easily as lower irradiation dose.