Positron annihilation spectroscopy for the determination of thickness and defect profile in thin semiconductor layers

We present a method, based on positron annihilation spectroscopy, to obtain information on the defect depth profile of layers grown over high-quality substrates. We have applied the method to the case of ZnO layers grown on sapphire, but the method can be very easily generalized to other heterostructures (homostructures) where the positron mean diffusion length is small enough. Applying the method to the ratio of W and S parameters obtained from Doppler broadening measurements, W∕S plots, it is possible to determine the thickness of the layer and the defect profile in the layer, when mainly one defect trapping positron is contributing to positron trapping at the measurement temperature. Indeed, the quality of such characterization is very important for potential technological applications of the layer.Peer reviewe

Positron annihilation lifetime spectroscopy of ZnO bulk samples

In order to gain a further insight into the knowledge of point defects of ZnO, positron annihilation lifetime spectroscopy was performed on bulk samples annealed under different atmospheres. The samples were characterized at temperatures ranging from 10 to 500 K. Due to difficulties in the conventional fitting of the lifetime spectra caused by the low intensity of the defect signals, we have used an alternative method as a solution to overcome these difficulties and resolve all the lifetime components present in the spectra. Two different vacancy-type defects are identified in the samples: Zn vacancy complexes (VZn−X) and vacancy clusters consisting of up to five missing Zn-O pairs. In addition to the vacancies, we observe negative-ion-type defects, which are tentatively attributed to intrinsic defects in the Zn sublattice. The effect of the annealing on the observed defects is discussed. The concentrations of the VZn−X complexes and negative-ion-type defects are in the 0.2–2 ppm range, while the cluster concentrations are 1–2 orders of magnitude lower.Peer reviewe

Combining Lattice QCD Results with Regge Phenomenology in a Description of Quark Distribution Functions

The most striking feature of quark distribution functions transformed to the longitudinal distance representation is the recognizable separation of small and large longitudinal distances. While the former are responsible for the average properties of parton distributions, the latter can be shown to determine specifically their small-$x$ behavior. In this paper we demonstrate how the distribution at intermediate longitudinal distances can be approximated by taking into account constraints which follow from the general properties of parton densities, such as their support and behavior at $x \to 1$. We show that the combined description of small, intermediate, and large longitudinal distances allows a good approximation of both shape and magnitude of parton distribution functions. As an application we have calculated low-virtuality C even and odd (valence) u and d quark parton densities of the nucleon and the C-even transversity distribution $h_1(x)$, combining recent QCD sum rules and lattice QCD results with phenomenological information about their small-$x$ behavior.Comment: LaTeX, 17 pages including 7 figures, shorter version will appear in Phys. Lett.

Hadron Structure Functions in a Chiral Quark Model: Regularization, Scaling and Sum Rules

We provide a consistent regularization procedure for calculating hadron structure functions in a chiral quark model. The structure functions are extracted from the absorptive part of the forward Compton amplitude in the Bjorken limit. Since this amplitude is obtained as a time-ordered correlation function its regularization is consistently determined from the regularization of the bosonized action. We find that the Pauli-Villars regularization scheme is most suitable because it preserves both the anomaly structure of QCD and the leading scaling behavior of hadron structure functions in the Bjorken limit. We show that this procedure yields the correct pion structure function. In order to render the sum rules of the regularized polarized nucleon structure functions consistent with their corresponding axial charges we find it mandatory to further specify the regularization procedure. This specification goes beyond the double subtraction scheme commonly employed when studying static hadron properties in this model. In particular the present approach serves to determine the regularization prescription for structure functions whose leading moments are not given by matrix elements of local operators. In this regard we conclude somewhat surprisingly that in this model the Gottfried sum rule does not undergo regularization.Comment: 42 pages LaTex, 5 figures included via epsfi

On the interplay of point defects and Cd in non-polar ZnCdO films

Non-polar ZnCdO films, grown over m- and r-sapphire with a Cd concentration ranging between 0.8% and 5%, have been studied by means of slow positron annihilation spectroscopy (PAS) combined with chemical depth profiling by secondary ion mass spectroscopy and Rutherford back-scattering. Vacancy clusters and Zn vacancies with concentrations up to 10exp17 cm−3 and 10exp18 cm−3, respectively, have been measured inside the films. Secondary ion mass spectroscopy results show that most Cd stays inside the ZnCdO film but the diffused atoms can penetrate up to 1.3 μm inside the ZnO buffer. PAS results give an insight to the structure of the meta-stable ZnCdO above the thermodynamical solubility limit of 2%. A correlation between the concentration of vacancy clusters and Cd has been measured. The concentration of Zn vacancies is one order of magnitude larger than in as-grown non-polar ZnO films and the vacancy cluster are, at least partly, created by the aggregation of smaller Zn vacancy related defects. The Zn vacancy related defects and the vacancy clusters accumulate around the Cd atoms as a way to release the strain induced by the substitutional CdZn in the ZnO crystal.Peer reviewe

Association of antiretroviral therapy with high-risk human papillomavirus, cervical intraepithelial neoplasia, and invasive cervical cancer in women living with HIV: a systematic review and meta-analysis.

BACKGROUND: The interactions between antiretroviral therapy (ART) and high-risk human papillomavirus (HPV) and cervical lesions in women living with HIV are poorly understood. We reviewed the association of ART with these outcomes. METHODS: We did a systematic review and meta-analysis by searching MEDLINE and Embase databases for cross-sectional or cohort studies published in English between Jan 1, 1996, and May 6, 2017, which reported the association of ART with prevalence of high-risk HPV or prevalence, incidence, progression, or regression of histological or cytological cervical abnormalities, or incidence of invasive cervcal cancer. Studies were eligible if they reported the association of combination ART or highly active ART use with the following outcomes: high-risk HPV prevalence; squamous intraepithelial lesion (SIL) or cervical intraepithelial neoplasia (CIN) prevalence, incidence, progression, or regression; and invasive cervical cancer incidence among women living with HIV. We did random-effects meta-analyses to estimate summary statistics. We examined heterogeneity with the I2 statistic. This review is registered on the PROSPERO database at the Centre of Reviews and Dissemination, University of York, York, UK (registration number CRD42016039546). FINDINGS: We identified 31 studies of the association of ART with prevalence of high-risk HPV (6537 women living with HIV) and high grade cervical lesions (HSIL-CIN2+; 9288 women living with HIV). Women living with HIV on ART had lower prevalence of high-risk HPV than did those not on ART (adjusted odds ratio [aOR] 0·83, 95% CI 0·70-0·99; I2=51%, adjusted for CD4 cell count and ART duration), and there was some evidence of association with HSIL-CIN2+ (0·65, 0·40-1·06; I2=30%). 17 studies reported the association of ART with longitudinal cervical lesion outcomes. ART was associated with a decreased risk of HSIL-CIN2+ incidence among 1830 women living with HIV (0·59, 0·40-0·87; I2=0%), SIL progression among 6212 women living with HIV (adjusted hazard ratio [aHR] 0·64, 95% CI 0·54-0·75; I2=18%), and increased likelihood of SIL or CIN regression among 5261 women living with HIV (1·54, 1·30-1·82; I2=0%). In three studies among 15 846 women living with HIV, ART was associated with a reduction in invasive cervical cancer incidence (crude HR 0·40, 95% CI 0·18-0·87, I2=33%). INTERPRETATION: Early ART initiation and sustained adherence is likely to reduce incidence and progression of SIL and CIN and ultimately incidence of invasive cervical cancer. Future cohort studies should aim to confirm this possible effect. FUNDING: UK Medical Research Council

Positron annihilation lifetime spectroscopy of ZnO bulk samples

In order to gain a further insight into the knowledge of point defects of ZnO, positron annihilation lifetime spectroscopy was performed on bulk samples annealed under different atmospheres. The samples were characterized at temperatures ranging from 10 to 500 K. Due to difficulties in the conventional fitting of the lifetime spectra caused by the low intensity of the defect signals, we have used an alternative method as a solution to overcome these difficulties and resolve all the lifetime components present in the spectra. Two different vacancy-type defects are identified in the samples: Zn vacancy complexes (VZn−X) and vacancy clusters consisting of up to five missing Zn-O pairs. In addition to the vacancies, we observe negative-ion-type defects, which are tentatively attributed to intrinsic defects in the Zn sublattice. The effect of the annealing on the observed defects is discussed. The concentrations of the VZn−X complexes and negative-ion-type defects are in the 0.2–2 ppm range, while the cluster concentrations are 1–2 orders of magnitude lower.Peer reviewe

Charge Symmetry Breaking in the Valence Quark Distributions of the Nucleon

Using a quark model, we study the effect of charge symmetry breaking on the valence quark distributions of the nucleon. The effect due to quark mass differences and the Coulomb interaction of the electrically charged quarks is calculated and, in contrast to recent claims, found to be small. In addition, we investigate the effect of charge symmetry breaking in the confining interaction, and in the perturbative evolution equations used to relate the quark model distributions to experiment. We find that both these effects are small, and that the strong charge symmetry breaking effect included in the scalar confining interactions may be distinguishable from that generated by quark mass differences.Comment: 10 pages, LaTEX, 5 Postscript figure

SP-0489: HPV-transformation in the cervix and at non-cervical sites

Pla general d'un dels panells horitzontals sobre espais verds de Barcelona a l'exposició Ciutat. Barcelona projecta a l'Edifici Fòrum. Exposició sobre la planificació urbanística i arquitectònica de Barcelon

Does lowering the screening age for cervical cancer in The Netherlands make sense?\ud

Recommendations for the age to initiate cervical cancer screening should be directed towards maximum detection of early cervical cancer. However, the screening programme should do more good than harm. The aim of this analysis was to determine whether the target age for cervical cancer screening should be lowered in view of apparent increases in new cases of invasive cancer below age 30 and in age group 30–44 years in The Netherlands. Therefore, all cervical cancer cases diagnosed between January 1, 1989 and December 31, 2003 were selected from the nationwide population-based Netherlands Cancer Registry. For age group 25–39 years, incidence data were also available for 2004 and 2005. To describe trends, the estimated annual percentage of change and joinpoint analysis were used. Between ages 25 and 28 years, the absolute number of new cases of cervical cancer annually has varied between 0 and 9 per age. Significantly decreasing trends in incidence were observed for age groups 35–39 and 45–49 (p < 0.0001 and p = 0.01, respectively). The annual number of deaths fluctuated with a decreasing trend for age groups 30–34 and 35–39 years (p = 0.01 and p = 0.03, respectively). Because the incidence and mortality rates for cervical cancer among women younger than 30 are low and not increasing, lowering the age for cervical cancer screening is not useful at this time. Although the number of years of life gained is high for every case of cervical cancer prevented, the disadvantages of lowering the screening age would be very large and even become disproportionate compared to the potential advantage