Core to surface excitations on GaAs(110)

We have carried out ab initio calculations of surface core excitations on finite cluster models of the GaAs(110) surface. For the Ga core excitation we find a localized excited state involving excitation into the empty Ga-4p orbital and bound with respect to the conduction band minimum (CBM) by 0.7 eV. This is in reasonable agreement with experiment (binding energy >~0.8 eV). This transition, which is not analogous to bulk core excitations, is termed a core surfaston to emphasize the character of the state. We find that the As core surfaston is above the CBM by 1.0 eV and hence should be difficult to observe

Chemisorption of Al and Ga on the GaAs (110) surface

We have studied the initial stages of the chemisorption of Al and Ga on the clean GaAs (110) surface by applying quantum chemical methods to small clusters representing Al or Ga on GaAs (110). These calculations suggest that at smallest coverages Al or Ga bind to a surface Ga atom; for higher coverages Al and the surface Ga interchange positions. We have obtained the binding energy, the chemical shifts of the Ga–3d, As–3d and Al–2p states, and the microscopic dipole associated with chemisorption of Al or Ga on GaAs (110). These results are compared to experimental values and further experiments are suggested

Awareness, perceived risk and practices related to cervical cancer and Pap smear screening: A crosssectional study among HIV-positive women attending an urban HIV clinic in Johannesburg, South Africa

Background. Cervical cancer is a major cause of cancer-related deaths, especially in the context of the HIV epidemic.Objective. To examine awareness, perceived risk and practices related to cervical cancer screening among HIV-positive women.Methods. Interviewer-administered structured questionnaires were administered to HIV-positive women (aged ≥18 years) enrolled in a cervical cancer screening study at the Themba Lethu Clinic, Johannesburg, South Africa, from November 2009 to December 2011. Modified Poisson regression with robust standard errors was used to identify factors at enrolment associated with awareness, perceived risk and adequate practice related to cervical screening. Adjusted relative risks (aRRs) with 95% confidence intervals (CIs) are presented.Results. Of the 1 202 women enrolled, 71.3% and 18.2% were aware of the Pap smear and HPV, respectively. Of the 1 192 participants with data evaluated, 76.5% were worried and 23.5% were not worried about cervical cancer; 28.6% of the women had adequate screening practice. Older age (40 - 49 years or ≥50 years v. 18 - 29 years) (aRR 1.63, 95% CI 1.12 - 2.37; aRR 2.22, 95% CI 1.44 - 3.41), higher education (tertiary v. less than grade 10) (aRR 1.39, 95% CI 1.00 - 1.93), initiation on combination antiretroviral therapy (aRR 1.36, 95% CI 1.00 - 1.85) and awareness of Pap smear screening (aRR 16.18, 95% CI 7.69 - 34.01) were associated with adequate screening practice.Conclusions. High levels of Pap smear awareness and low levels of Pap smear screening uptake were observed. However, Pap smear awareness was associated with adequate screening practice. More research into effective health education programmes to address these gaps is needed

Geometry of the abrupt (110) Ge/GaAs interface

We have studied the structure relaxation at the abrupt (110) Ge/GaAs interface by applying quantum chemical methods to clusters modeling this interface. Application of this model to bulk Ge and to bulk GaAs leads to theoretical Ge–Ge and Ga–As bond distances of 2.452 and 2.451 Å, respectively, in good agreement with the experimental values of 2.450 and 2.448 Å, respectively. Application of the model to the Ge/GaAs (110) interface indicates that this interface is nearly ideal. We find a very slight reconstruction at the interface leading to a Ge–Ga bond distance which is 0.04 Å longer than the Ge–As bond distance of 2.430 Å. The calculated spacing of the interface layer is 2.3% greater than that of bulk Ge or bulk GaAs

Theoretical studies of the reconstruction of the (110) surface of III–V and II–VI semiconductor compounds

We have studied the reconstruction of the (110) surface of various III–V semiconductor compounds (GaAs, GaP, GaN, AlAs, AlP, AlN, BAs, BP, BN) by applying quantum chemical methods to small clusters representative of these surfaces. Application of these techniques to GaAs (110) leads to a surface shear (0.67 Å) in excellent agreement with experimental values (0.65–0.70 Å). The results lead to trends in the surface distortions and reconstruction consistent with those predicted from local valence considerations. Possibilities for the electronic structure of II–VI semiconductor compounds are also considered

Bulk vacancies in CdxHg1–xTe

We report the first theoretical study of vacancies in CdxHg1–xTe alloys. The study employs the tight-binding method for obtaining the Hamiltonian. The Slater–Koster Green's function method is used to obtain the electronic states that result from removing a cation or anion from the virtual crystal used to model the alloys. The primary results are that the anion vacancy levels are far into the conduction band and hence are not likely to produce levels in the gap. In contrast, the cation vacancy is found to produce levels near the valence band edge. We find that spatially these states are very localized on the atoms nearest the vacancy

Oxidation of silicon surfaces

We have carried out theoretical studies (generalized valence bond) for chemisorbed O atom and O2 molecule on Si(111) surfaces using clusters of atoms to model the surface. For the perfect surface we find that O2 molecule binds to a single surface Si with an Si–O bond length of 1.68 Å (bulk SiO2RSi–O = 1.61 Å), an Si–O–O angle of 116 °, and an O–O bond distance of 1.32 Å. We have also calculated the shifts in the Si(2p) and O(1s) core level energies for chemisorbed O and O2 on the surface. The Si(2p) shift is +1.1 eV (higher binding energy) for the O atom and +1.5 eV for O2, in agreement with the shifts observed for low oxygen exposure by Spicer and co-workers