Ion implanted dopants in GaN and AlN: Lattice sites, annealing behavior, and defect recovery Defect formation and annealing behaviors of fluorine-implanted, unintentionally doped GaN layers were studied by positron annihilation spectroscopy ͑PAS͒. Single Ga vacancies ͑V Ga ͒ were identified as the main vacancy-type defects detected by PAS after fluorine implantation at 180 keV with a dose of 1 ϫ 10 15 cm −2 . Implantation-induced V Ga tend to aggregate and form vacancy clusters after postimplantation annealing in N 2 ambient at 600°C. Fluorine ions tend to form F-vacancy complexes quickly after thermal annealing, which is consistent with the proposed diffusion model that predicts the behaviors of fluorine in GaN.

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.+e, 78.70.Bj Positron annihilation lifetime spectroscopy has been used to study the formation of positronium in high density polyethylene as a function of temperature over the range 30 K-350 K. It is observed that the thermal history of the sample, while having no influence on the positronium lifetime, has a strong effect on the formation of positronium. A hysteresis is seen in the positronium formation probability in cooling and heating cycles. This is explained on a two channel formation model, the first channel being through "blob" formation and the second through the pick-up of shallow trapped electrons.1 Introduction Positron annihilation lifetime spectroscopy (PALS) is one of the most popular and useful methods for studying the characteristics of polymers. The positron source, however, which is required in conventional PALS, has sufficient energy ( ~200 keV per e + in case of 22 Na) to induce cumulative radiation effects in the polymer thus making it more difficult to investigate the polymer structure. Such effects depend on the type of polymer, the radiation dosage and temperature. It is important to fully understand these effects if the PALS technique is to attain its full potential in polymer research.Ps formation tends to decrease with source dosage at room temperature [1, 2] while at low temperatures an increase is observed [3]. The high temperature effect, which appears to be irreversible, has been tentatively attributed to a restructuring of the polymer due to increased cross-linking [2] while the low temperature effect is commonly attributed to Ps formation from shallow trapped electrons [4][5][6][7]. The present work seeks to ratify and further discuss some of these findings. It also extends previous work by investigating the effect of cooling speed on the o-Ps formation.

The Richardson-Lucy, Maximum Entropy and Huber regularization methods are popularly used in solving ill-posed inverse problems. This paper considers the use of these three methods in the deconvoluting DBARS (Doppler Broadening of Annihilation Radiation Spectroscopy) data. As DBARS data have a constant background on the high-energy side and a long exponential tail on the low-energy side, we check the different deconvolution schemes paying specific attention to the quality of the deconvolution at the peak and tail positions. Comparison of the three methods is made by testing on Monte-Carlo simulated data both in terms of the deconvoluted quality and computational resources required. Finally, we apply these methods to experimental DBARS data taken on polycrystalline metal samples.

Modulations of energy band and polarization field by fluorine ions in fluorine plasma treated AlGaN/GaN heterostructures were revealed by positron annihilation spectroscopy (PAS). It is found that the annihilation probability is mainly governed by the electric field in the AlGaN/GaN heterostructure, which could be modulated by charged ions, opposite to what was first expected from the large number of plasma-induced defects such as Ga-vacancies. The modulation of electric field is successfully observed through the opposite changes in the S parameters on the two sides of the hetero-interface after fluorine plasma treatment due to the opposite E-field directions. Fluorine is experimentally proved to be negatively charged in GaN related materials, which is consistent with the operation principle of enhancement-mode AlGaN/GaN HEMT fabricated by fluorine plasma treatment. It is also suggested that PAS is a useful tool to probe the intrinsic electric field in AlGaN/GaN system. Figure 4 (online color at: www.pss-a.com) Illustration of the modification of conduction band by negatively charged fluorine ions. The fluorine profile in the AlGaN/GaN heterostructure detected by SIMS after 150S treatment is also shown.