Voss, Tobias

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Voss, Tobias
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Voss, Tobias

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    Item-typ:Veröffentlichung,
    Strukturierung von Zinkoxid und Galliumnitrid mit Femtosekundenlaserpulsen : Ablation, Oberflächenstrukturen und optische Eigenschaften
    (2015-04-24)
    Schneider, Andreas
    ;
    Rosenauer, Andreas
    ;
    Voß, Tobias
    ;
    Voß, Tobias
    Structuring the surface of semiconductor devices with femtosecond laser pulses is a promising method for enhancing the device performance while keeping thermal damage as small as possible. The surface processing with femtosecond pulses provides two main opportunities that are hard to realize with other processing techniques. First femtosecond laser pulses can generate a large number of different self-organized surface morphologies with characteristic sizes ranging from nanometers to micrometers. They make it possible to create subwavelength structures not available by laser with longer pulses. Thus they offer the possibility to roughen the surface of semiconductors at different spatial scales and therefore allow one to adapt the laser process to the desired functionality of the device surface. Second femtosecond pulses of sufficient intensity allow to trap a very large amount of dopants within a small depth from the surface. In this way it is possible to create highly doped surfaces that can be used to increase the efficiency of solar cells or photodetectors. In this thesis the author examines the femtosecond laser pulse structuring of gallium nitride and zinc oxide. Both are technologically important wide band gap semiconductors. The first part of the thesis explores the ablation and surface modification of both semiconductors. For zinc oxide it is found that surface is ablated at a laser fluence of 0.5 J/cm^2. For gallium nitride the measured single laser pulse threshold lies at 0.6 J/cm^2. The single pulse threshold values agree with what is expected for an electrostatic ablation process. The ablation threshold depends strongly on the number of structuring laser pulses. Crossectional energy dispersive x-ray measurements show that it is possible to incorporate antimony into the zinc oxide surface layer at level of 1-2 atomic percent. In the second part of the thesis laser-induced self-organized surface structures are analyzed. The presented studies focus on so called LIPSS (laser induced periodic surface structures) on the c-plane zinc oxide and gallium nitride. By varying laser parameters such as laser fluence and the angle of incidence the formation mechanism is studied. It is observed that the periodicity of the surface structures increases for larger incidence angles in a way that can be explained by surface scattering of the incident laser light. When lowering the applied laser fluence a transition from wavelength-sized LIPSS (650 nm) to subwavelength-sized LIPSS (200 nm) is observed indicating a pronounced transient change of the optical properties of the surface layer. This alteration allows the excitation of high frequency surface plasmons that might well explain the observed LIPSS. The LIPSS behave similar on both semiconductor surfaces. In the third part of the thesis the optical properties of femtosecond laser processed zinc oxide are studied by Raman spectroscopy, photoluminescence spectroscopy and absorption spectroscopy. High above the ablation threshold of zinc oxide a low number of laser pulses will cause large stress on the surface layer that leads to surface cracking and a delamination of crack tiles. This cracking does not fully relax the surface layer and a residual strain of up to 1.8 % is detected. The observed strain pattern supports an origin by thermal stress generated by the cooling following the laser-matter-interaction. For samples structured with a larger number of laser pulses, a phonon confinement induced broadening and shift of the E2(high) mode is observed. Simultaneously the intensity of the A1(LO) mode is enhanced and polar surface phonons are observed pointing to a nanocrystalline surface layer rich in defects. Photoluminescence measurements confirm this and provide evidence for an increase in the zinc interstitial density. As a consequence the light absorption increases drastically from the near-UV to near-IR spectral region. The laser processing allows to control the optical absorption of zinc oxide over a wide range not known from other processing techniques.
    Dissertation
      351  163
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    Item-typ:Veröffentlichung,
    ZnO nanowires: surface functionalization with colloidal semiconductor quantum dots and excitation-intensity-dependent photoluminescence properties
    (2012-10-29)
    Hou, Dongchao
    ;
    Rosenauer, Andreas
    ;
    Voss, Tobias
    ;
    Voss, Tobias
    Contemporary nanotechnology has great interest in the assembly and study of hybrid structures composed of different materials that can offer enhanced properties or achieve new functions through the interaction between different constituents. ZnO nanowires, with the wide direct band gap and large exciton binding energy of the material, have attracted intense research interests for their potential optoelectronic applications in the near ultraviolet spectral region. They have also been studied for use in energy conversion and storage. They are promising candidates as photoanodes in nanostructured photovoltaic devices. Their high electron mobility favors a quick collection of the photogenerated charge carriers. Semiconductor quantum dots represent another research focus due to their striking size-tunable band gap, which provides a convenient approach to tune the spectral region of absorption and emission of the materials. In this work, a hybrid nanostructure is built by surface functionalization of ZnO nanowire arrays with colloidal CdSe quantum dots (QDs), which has potential applications in photovoltaics and sensing applications. The QDs are synthesized by a wet-chemical method and stabilized with bifunctional 3-mercaptopropionic acid (MPA) molecules. The growth kinetics and optical properties of the QDs are studied. The average QD size can be tuned in the range of 1.4 - 2.5 nm by adjusting the growth time. The photoluminescence of the dry QD powder is found to be very sensitive to the ambient environment, which is attributed to oxygen-related surface effects. Adsorption of oxygen molecules can passivate the surface defects of the QDs which otherwise act as trap centers during photo-excitation and induce nonradiative and fast Auger recombination processes in the QDs. The attachment of the CdSe QDs on the ZnO nanowire surface is achieved by using the stabilizers of the QDs as molecular linkers, which further favors the charge transfer between these two systems. The photoconductivity of the nanowire/quantum-dot hybrid structure is studied under selective photoexcitation of the QDs. An enhancement of of the photoconductivity up to 10 times is observed in air. The dynamics is further found to strongly depend on the gas environment. Desorption of surface oxygen from the ZnO nanowires, activated by charge tunnelling between the nanowires and the QDs, is found to be the dominating process for the photoconductivity enhancement. The gas environment influences the charge relaxation in the QDs through oxygen-related surface passivation, which impacts the charge tunnelling between the nanowires and the QDs and, hence, the photoconductivity dynamics. Defects in ZnO materials can significantly influence their physical properties such as the electrical conductivity and luminescence spectra. Photoluminescence spectroscopy is a convenient, non-destructive technique for studying the crystal quality and defect states of semiconductors. ZnO generally shows an ultraviolet emission band due to the near-band-edge recombination processes and several defect-related emission bands in the visible spectral region. The dependence of the photoluminescence properties on the excitation intensity of ZnO nanowires and bulk wafers is studied. It is found that the relative strength of the band-edge emission and the defect-related emission dramatically varies with the excitation intensity. The increase of these two emission bands with excitation intensity further exhibits sample-specific behaviors, which depends on the defect species and concentrations and the microscopic origins of the defect-related emission processes. Low-temperature measurements further reveal that biexcitons or even an electron-hole plasma (EHP) may be formed in ZnO nanowires for excitation intensities >10^3 W/cm^2, which causes the broadening of the donor-bound exciton emission peak and the superlinear increase of the band-edge emission with excitation intensity.
    Dissertation
      271  127
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    Item-typ:Veröffentlichung,
    Optische Eigenschaften von ZnO-Nanodrähten:Einfluss von Oberflächenbehandlungen und hohen Anregungsdichten
    (2010-08-13)
    Richters, Jan-Peter
    ;
    Ronning, Carsten
    ;
    Gutowski, Jürgen
    ;
    Voß, Tobias
    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.
    Dissertation
      273  140