Determination of the valence band offset at cubic CdSe/ZnTe type II heterojunctions: A combined experimental and theoretical approach

We present a combined experimental and theoretical approach for the determination of the low-temperature valence band offset (VBO) at CdSe/ZnTe heterojunctions with underlying zincblende crystal structure. On the experimental side, the optical transition of the type II interface allows for a precise measurement of the type II band gap. We show how the excitation-power dependent shift of this photoluminescence (PL) signal can be used for any type II system for a precise determination of the VBO. On the theoretical side, we use a refined empirical tight-binding parametrization in order to accurately reproduce the band structure and density of states around the band gap region of cubic CdSe and ZnTe and then calculate the branch point energy (also known as charge neutrality level) for both materials. Because of the cubic crystal structure and the small lattice mismatch across the interface, the VBO for the material system under consideration can then be obtained from a charge neutrality condition, in good agreement with the PL measurements.Comment: 11 pages, 5 figure

Optical properties of ZnO nanowires: influence of surface treatments and high excitation densities

ZnO nanowires are promising building blocks for optoelectronic applications. Lasers andlight-emitting diodes fabricated from ZnO nanowires will emit in the blue and near-UVspectral region due to the wide band gap of ZnO. Their large refractive index makes themuseful as active waveguides in optoelectronic devices. In addition to the near UV emission,ZnO can show several defect related emission bands in the visible spectral region. By carefullydoping the material, these defect bands can be tailored to generate a white-light spectrumwhich will be beneficial for applications as light emitting diodes. Much research interestfocused on the various defect bands in the past years. Previous work of the author on ZnOnanowires embedded in polymer matrices shows a strong decrease of the defect luminescencefor the embedded samples, combined with a stronger surface exciton emission band.In this work, the dependence of the defect and near-band-edge luminescence of ZnO nanowireson surrounding materials necessary to fabricate nanowire-LED devices is investigated. First,coating of the nanowires with a dielectric shell of amorphous Al2O3 is found to stronglysuppress the defect luminescence while enhancing the surface excitonic emission. This effectis similar to the previously investigated polymer samples and is explained by a model basedon the dielectric properties of the material. A dielectric coating shields the surface states andreduces the band bending typically observed for the ZnO nanowire surfaces. The band bendinginfluences the activation mechanism of deep centers which cause the defect luminescence andredistributes the excitons near the surface in favour of the surface excitonic recombinationmechanism. As a next step, the influence of metallic coatings on the optical properties ofZnO nanowires is investigated and found to result in an opposite behaviour compared to thedielectric coating. An increased defect luminescence and a reduced surface emission bandare observed, completely independent of the work function of the metal. These findings areexplained by the formation of metal-induced gap states at the nanowire surface, which trapexcitons close to the surface.The interaction between plasmonic resonances of metal nanoparticles and excited states ofsemiconductors has attracted much attention in the research community. The use of a plasmasputter-coater is one method to deposit such metal nanoparticles. In this work, experimentalevidence is presented which shows that the plasma of such a system itself changes theoptical properties of ZnO nanowires by implantation of hydrogen into the crystal. The defectluminescence is weakened while the near-UV emission is enhanced, but does not show astronger surface excitonic emission.The lifetime of the charge carriers excited by a fs laser is investigated in pump-and-probeexperiments on ZnO bulk surfaces. Even at excitation densities below the lasing threshold, anelectron-hole-plasma forms with lifetimes of several tens of picoseconds. In order to investigatethe lasing properties of single ZnO nanowires, a setup based on the variable-stripe-lengthmethod is developed and used to determine the modal gain of single nanowires. When excitingwith a fs laser, modal gain values of up to 4900 cm−1 are found which are in good agreementwith theoretical predictions. The gain is found to depend on the nanowire diameter

Optische Eigenschaften von ZnO-Nanodrähten:Einfluss von Oberflächenbehandlungen und hohen Anregungsdichten

ZnO nanowires are promising building blocks for optoelectronic applications. Lasers andlight-emitting diodes fabricated from ZnO nanowires will emit in the blue and near-UVspectral region due to the wide band gap of ZnO. Their large refractive index makes themuseful as active waveguides in optoelectronic devices. In addition to the near UV emission,ZnO can show several defect related emission bands in the visible spectral region. By carefullydoping the material, these defect bands can be tailored to generate a white-light spectrumwhich will be beneficial for applications as light emitting diodes. Much research interestfocused on the various defect bands in the past years. Previous work of the author on ZnOnanowires embedded in polymer matrices shows a strong decrease of the defect luminescencefor the embedded samples, combined with a stronger surface exciton emission band.In this work, the dependence of the defect and near-band-edge luminescence of ZnO nanowireson surrounding materials necessary to fabricate nanowire-LED devices is investigated. First,coating of the nanowires with a dielectric shell of amorphous Al2O3 is found to stronglysuppress the defect luminescence while enhancing the surface excitonic emission. This effectis similar to the previously investigated polymer samples and is explained by a model basedon the dielectric properties of the material. A dielectric coating shields the surface states andreduces the band bending typically observed for the ZnO nanowire surfaces. The band bendinginfluences the activation mechanism of deep centers which cause the defect luminescence andredistributes the excitons near the surface in favour of the surface excitonic recombinationmechanism. As a next step, the influence of metallic coatings on the optical properties ofZnO nanowires is investigated and found to result in an opposite behaviour compared to thedielectric coating. An increased defect luminescence and a reduced surface emission bandare observed, completely independent of the work function of the metal. These findings areexplained by the formation of metal-induced gap states at the nanowire surface, which trapexcitons close to the surface.The interaction between plasmonic resonances of metal nanoparticles and excited states ofsemiconductors has attracted much attention in the research community. The use of a plasmasputter-coater is one method to deposit such metal nanoparticles. In this work, experimentalevidence is presented which shows that the plasma of such a system itself changes theoptical properties of ZnO nanowires by implantation of hydrogen into the crystal. The defectluminescence is weakened while the near-UV emission is enhanced, but does not show astronger surface excitonic emission.The lifetime of the charge carriers excited by a fs laser is investigated in pump-and-probeexperiments on ZnO bulk surfaces. Even at excitation densities below the lasing threshold, anelectron-hole-plasma forms with lifetimes of several tens of picoseconds. In order to investigatethe lasing properties of single ZnO nanowires, a setup based on the variable-stripe-lengthmethod is developed and used to determine the modal gain of single nanowires. When excitingwith a fs laser, modal gain values of up to 4900 cm−1 are found which are in good agreementwith theoretical predictions. The gain is found to depend on the nanowire diameter

CdSe/ZnTe superlattices optimized as absorbers for photovoltaics

International audienceWe have grown, by MBE, CdSe/ZnTe superlattices with different periods, to evaluate a new concept of solar cells

CdSe/ZnTe superlattices optimized as absorbers for photovoltaics

International audienceWe have grown, by MBE, CdSe/ZnTe superlattices with different periods, to evaluate a new concept of solar cells

Tight binding simulation of type-II superlattice absorption

International audienceWe perform accurate tight binding simulations to design type II short-period CdSe/ZnTe superlattices suited for photovoltaic applications. Absorption calculations demonstrate a very good agreement with photoluminescence results with threshold strongly depending on the chemical species near interfaces

Atomistic simulations of the optical absorption of type II CdSe/ZnTe superlattices

International audienceWe perform accurate tight binding simulations to design type-II short-period CdSe/ZnTe superlattices suited for photovoltaic applications. Absorption calculations demonstrate a very good agreement with optical results with threshold strongly depending on the chemical species near interfaces

Cervical screening program and the psychological impact of an abnormal Pap smear: a self-assessment questionnaire study of 590 patients

Invasive cervical cancer is today the fourth most common cancer of women in western civilization. Screening programs have led to a continuously decrease. Nevertheless, both screening and a positive test result are known to be associated with a negative psychological impact. Screening programs in European countries differ and thus psychological impact might as well. The aim of this study was to evaluate the psychological impact of women with an abnormal Pap smear in a German cohort. Between July 2013 and May 2014, a self-assessment questionnaire was distributed to 595 patients that were referred to a special clinic for cervical dysplasia for further evaluation of an abnormal Pap smear. Patients were recruited in five different centers. Most patients (45.9 %) were informed about the test result via phone call by their doctor. 68.8 % of the patients felt anxious and 26.3 % even felt panic. After having talked to their physician, 51.4 % of our cohort still felt worried and only 24.4 % felt reassured. Concerning disease management, 48.4 % underwent a control Pap smear in 6 months. The preferred information source was the physician (63.9 %). Compared to the results in other European countries, our study cohort showed differences concerning age distribution, patients living in a partnership, number of children and especially disease management. Cancer screening itself and abnormal test results have an impact on patient's feelings. To reduce the psychological impact, patients need to be better informed about the risks and benefits of cancer screening programs and in case of cervical cancer screening about the meaning of an abnormal test result. Our results underline the importance of a trustful physician-patient relationship in that matter