F. A. Selim

Identification of defects in Y3Al5O12 crystals by positron annihilation spectroscopy

Positron annihilation, thermoluminescence, and optical absorption measurements were applied with the aid of several annealing and diffusion procedures to investigate the nature of point defects in Y3Al5O12 (YAG) single crystals. By annealing at 1500°C in air or O, and diffusing Al into a Ce doped YAG single crystal, a reduction of nearly two orders of magnitude in vacancy concentration was observed. Scintillation measurements showed a significant improvement in energy resolution after Al diffusion. This study revealed the presence of vacancy-defect complexes, most likely associated with cation antisites in YAG crystals.

On the optical properties of undoped and rare-earth-doped yttrium aluminium garnet single crystals

Optical absorption and photoluminescence (PL) measurements were performed on single crystals of undoped Y3Al5O12 (YAG) and a number of rare-earth-doped YAG to study the effect of dopant type and concentration, growth atmosphere, post-growth annealing and UV irradiation on the optical properties of YAG crystals. The presence of hydrogen in the growth atmosphere was found to be essential for enhancing the incorporation of Ce ions in the Ce3+ state in Ce-doped YAG (Ce : YAG). Annealing in air was shown to have no effect on the PL emission of Ce : YAG crystals. An absorption peak around 256 nm was observed in the undoped YAG and Ce : YAG crystals after air anneal at 1200 °C. Optical absorption and annealing experiments support the association of the 256 nm peak with Fe impurities and oxygen ions. UV irradiation modifies the valency of impurities and generates electronic defects leading to an increase in the optical density of YAG crystals. Optimizing the growth and annealing conditions is critical in order to develop Ce : YAG single crystals as efficient scintillators.

Strong visible and near infrared luminescence in undoped YAG single crystals

Strong luminescence peaks were observed at 700 and 800 nm in undoped yttrium aluminum garnet (YAG) single crystals. They were attributed to low level of iron impurities as confirmed by Glow Discharge Mass Spectrometry analysis. The 800 nm was only excited by high energy band at 270 nm; the reason behind that was discussed. Photoluminescence measurements revealed a large number of luminescence peaks in all YAG crystals regardless of the growth conditions due to native defects and low-level impurities. These luminescence centers have significant effects on the optical properties of rare-earth doped YAG crystals and their performance in laser and scintillation applications. Excitons released in the lattice may be easily captured by iron ions instead of Ce3+ ions and the scintillation output is substantially decreased. Nevertheless, Undoped YAG crystals may have the potential to be developed into efficient scintillators.

Energy levels of exciton traps in yttrium aluminum garnet single crystals

Electronic defects and exciton traps were studied in yttrium aluminum garnet (YAG) single crystals by wavelength and temperature resolved thermoluminescence (TL). Measurements were carried out from room temperature to 400 °C on a number of rare earth (RE) doped and undoped YAG crystals, and the trap parameters were determined. Although the TL emission spectrum is characteristic of the RE ion, the main trap levels are found to be characteristic of the undoped host crystal. Nevertheless, the thermal activation energies of the traps are slightly modified by doping. The trap kinetics are found to be of the first order in both undoped and Ce doped YAG single crystals indicating the absence of retrapping, which suggests that the traps and recombination centers exhibit a close spatial correlation. The effect of annealing on TL response suggests that some of the major traps are associated with oxygen vacancies.

Donor characterization in ZnO by thermally stimulated luminescence

Low temperature thermo-luminescence (TL) has been applied to measurements of the ionization energy of donors in ZnO. Three hydrogen-related donors were characterized with ionization energies of 36, 47, and 55 meV—values that are in complete agreement with previous reports. The donor types can be “switched” by relevant thermal treatments. This work shows that TL can be used to measure the donor energies in luminescent semiconductors in general. This approach can be particularly useful for thin-film investigations when the results of Hall-effect measurements are obscured by contributions from conductive interfaces or substrates.

Cu-Doping of ZnO by Nuclear Transmutation

Zinc oxide single crystals were doped with copper acceptors by means of the nuclear transmutation doping method, which gives highly uniform dopant distributions and has a much higher probability of controlling the dopant locations in the lattice. The Cu doping was confirmed by the infrared absorption signature of Cu2+ at 5780 cm−1. Hall-effect measurements were performed to study the effect of CuZn on the electrical properties of ZnO. These measurements indicated that the Cu acceptor level lies 0.160 eV below the conduction-band minimum.

ZnO Luminescence and scintillation studied via photoexcitation, X-ray excitation, and gamma-induced positron spectroscopy

The luminescence and scintillation properties of ZnO single crystals were studied by photoluminescence and X-ray-induced luminescence (XRIL) techniques. XRIL allowed a direct comparison to be made between the near-band emission (NBE) and trap emissions providing insight into the carrier recombination efficiency in the ZnO crystals. It also provided bulk luminescence measurements that were not affected by surface states. The origin of a green emission, the dominant trap emission in ZnO, was then investigated by gamma-induced positron spectroscopy (GIPS) - a unique defect spectroscopy method that enables positron lifetime measurements to be made for a sample without contributions from positron annihilation in the source materials. The measurements showed a single positron decay curve with a 175 ps lifetime component that was attributed to Zn vacancies passivated by hydrogen. Both oxygen vacancies and hydrogen-decorated Zn vacancies were suggested to contribute to the green emission. By combining scintillation measurements with XRIL, the fast scintillation in ZnO crystals was found to be strongly correlated with the ratio between the defect luminescence and NBE. This study reports the first application of GIPS to semiconductors, and it reveals the great benefits of the XRIL technique for the study of emission and scintillation properties of materials.

Study of exciton dynamics in garnets by low temperature thermo-luminescence

Shallow traps that affect exciton dynamics in undoped and Ce doped yttrium aluminum garnet (CeYAG) single crystals were studied by low temperature thermo-luminescence spectroscopy. The nature of traps was identified and their energy levels in the band gap were measured. Three types of traps were found: two very shallow traps with activation energy of less than 0.2 eV associated with electronic defects, one trap with activation energy of 0.5 eV associated with oxygen vacancies, and one relatively deeper trap with activation energy of 0.7 eV associated with hydrogen impurities. The association of oxygen vacancies with shallow traps as evident from this work and with deep traps as reported by Varney et al. [J. Appl. Phys. 111, 063505 (2012)] suggest their presence in the lattice in more than one charge state.

Development of accelerator-based γ-ray-induced positron annihilation spectroscopy technique

Accelerator-based γ-ray-induced positron annihilation spectroscopy performs positron annihilation spectroscopy by utilizing MeV bremsstrahlung radiation generated from an accelerator (We have named the technique “accelerator-based γ-ray-induced PAS,” even though “bremsstrahlung” is more correct here than “γ rays”. The reason for that is to make the name of the technique more general, since PAS may be performed by utilizing MeV γ rays emitted from nuclei through the use of accelerators as described later in this article and as in the case of positron lifetime spectroscopy [F.A. Selim, D.P. Wells, and J.F. Harmon, Rev. Sci. Instrum. 76, 033905 (2005)].) instead of using positrons from radioactive sources or positron beams. MeV γ rays create positrons inside the materials by pair production. The induced positrons annihilate with the material electrons emitting a 511-keV annihilation radiation. Doppler broadening spectroscopy of the 511-keV radiation provides information about open-volume defects and plastic ...

Doppler broadening measurements of positron annihilation using bremsstrahlung radiation

Abstract The first system to measure Doppler broadening of positron annihilation based on during electron-pulse bremsstrahlung radiation has been constructed and demonstrated. No photon-induced activation or positron emitters are involved in the process. The collimated bremsstrahlung radiation from a small electron accelerator, which exhibits excellent penetrability, is used to generate positrons inside the sample via pair production. The annihilation photons are recorded by a HPGe detector. The line-shape parameters of Doppler broadening can be used to identify defects in pure metals and alloys. The dependence of these parameters on different elements has been measured and shows promise as a probe of momentum of electronic wave-functions in pure and composite materials. This method also shows promise as an additional tool for measuring elemental composition, when used in conjunction with accelerator-based X-ray fluorescence.