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Author: Schroeder, T. × DDC: 620 ×

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    Ba termination of Ge(001) studied with STM
    (Bristol : IOP Publishing, 2015) Koczorowski, W.; Grzela, T.; Radny, M.W.; Schofield, S.R.; Capellini, G.; Czajka, R.; Schroeder, T.; Curson, N.J.
    We use controlled annealing to tune the interfacial properties of a sub-monolayer and monolayer coverages of Ba atoms deposited on Ge(001), enabling the generation of either of two fundamentally distinct interfacial phases, as revealed by scanning tunneling microscopy. Firstly we identify the two key structural phases associated with this adsorption system, namely on-top adsorption and surface alloy formation, by performing a deposition and annealing experiment at a coverage low enough (~0.15 ML) that isolated Ba-related features can be individually resolved. Subsequently we investigate the monolayer coverage case, of interest for passivation schemes of future Ge based devices, for which we find that the thermal evaporation of Ba onto a Ge(001) surface at room temperature results in on-top adsorption. This separation (lack of intermixing) between Ba and Ge layers is retained through successive annealing steps to temperatures of 470, 570, 670 and 770 K although a gradual ordering of the Ba layer is observed at 570 K and above, accompanied by a decrease in Ba layer density. Annealing above 770 K produces the 2D surface alloy phase accompanied by strain relief through monolayer height trench formation. An annealing temperature of 1070 K sees a further change in surface morphology but retention of the 2D surface alloy characteristic. These results are discussed in view of their possible implications for future semiconductor integrated circuit technology.
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    Modeling of edge-emitting lasers based on tensile strained germanium microstrips
    (New York, NY : IEEE, 2015) Peschka, D.; Thomas, M.; Glitzky, A.; Nürnberg, R.; Gärtner, K.; Virgilio, M.; Guha, S.; Schroeder, T.; Capellini, G.; Koprucki, Th.
    In this paper, we present a thorough modeling of an edge-emitting laser based on strained germanium (Ge) microstrips. The full-band structure of the tensile strained Ge layer enters the calculation of optical properties. Material gain for strained Ge is used in the 2D simulation of the carrier transport and of the optical field within a cross section of the microstrips orthogonal to the optical cavity. We study optoelectronic properties of the device for two different designs. The simulation results are very promising as they show feasible ways toward Ge emitter devices with lower threshold currents and higher efficiency as published insofar.
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    Understanding the growth mechanism of graphene on Ge/Si(001) surfaces
    (London : Nature Publishing Group, 2016) Dabrowski, J.; Lippert, G.; Avila, J.; Baringhaus, J.; Colambo, I.; Dedkov, Yu S.; Herziger, F.; Lupina, G.; Maultzsch, J.; Schaffus, T.; Schroeder, T.; Kot, M.; Tegenkamp, C.; Vignaud, D.; Asensio, M.-C.
    The practical difficulties to use graphene in microelectronics and optoelectronics is that the available methods to grow graphene are not easily integrated in the mainstream technologies. A growth method that could overcome at least some of these problems is chemical vapour deposition (CVD) of graphene directly on semiconducting (Si or Ge) substrates. Here we report on the comparison of the CVD and molecular beam epitaxy (MBE) growth of graphene on the technologically relevant Ge(001)/Si(001) substrate from ethene (C2H4) precursor and describe the physical properties of the films as well as we discuss the surface reaction and diffusion processes that may be responsible for the observed behavior. Using nano angle resolved photoemission (nanoARPES) complemented by transport studies and Raman spectroscopy as well as density functional theory (DFT) calculations, we report the direct observation of massless Dirac particles in monolayer graphene, providing a comprehensive mapping of their low-hole doped Dirac electron bands. The micrometric graphene flakes are oriented along two predominant directions rotated by 30° with respect to each other. The growth mode is attributed to the mechanism when small graphene “molecules” nucleate on the Ge(001) surface and it is found that hydrogen plays a significant role in this process.