CPTF1, a transcription factor with significant homology to ATF/CREB bZIP factors, was identified during an expressed sequence tag (EST) analysis of in planta-expressed genes of the phytopathogen ...Claviceps purpurea. Using a gene-replacement approach, deletion mutants of cptf1 were created. Expression studies in axenic culture showed that the H2O2-inducible gene cpcat1 (encoding a secreted catalase) had a reduced basal expression level and no longer responded to oxidative stress in the deltacptf1 mutant. Biochemical analyses indicated that CPTF1 is a general regulator of catalase activity. deltacptf1 mutants showed significantly reduced virulence on rye. Electron microscopical in situ localization revealed significant amounts of H2O2 in deltacptf1-infected rye epidermal tissues, indicating that the plant tissue displayed an oxidative burst-like reaction, an event not detected in wild-type infections. These data indicate that CPTF1 is involved not only in oxidative stress response in the fungus but also in modulation of the plant's defense reactions.
Different types of scaling in epitaxial growth Brendel, L.; Schindler, A.; von den Driesch, M. ...
Computer physics communications,
08/2002, Volume:
147, Issue:
1
Journal Article, Conference Proceeding
Peer reviewed
Simulation results for (a) interfacial alloying and (b) the Villain instability are presented. Both phenomena are important for the interface structure of epitaxial heterolayers and involve new ...scaling laws. For (a) a mechanism is discussed, which in a limiting case leads to a concentration profile decaying like (distance from the interface)
−2. In (b) the critical film thickness is estimated, at which the Villain instability becomes important. Its scaling with the diffusion constant and the Schwoebel length can be combined with results on the strain dependence of these parameters in order to predict what kind of strains enhance or suppress the instability.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Large-scale optoelectronics integration is limited by the inability of Si to emit light efficiently, because Si and the chemically well-matched Ge are indirect-bandgap semiconductors. To overcome ...this drawback, several routes have been pursued, such as the all-optical Si Raman laser and the heterogeneous integration of direct-bandgap III-V lasers on Si. Here, we report lasing in a direct-bandgap group IV system created by alloying Ge with Sn without mechanically introducing strain. Strong enhancement of photoluminescence emerging from the direct transition with decreasing temperature is the signature of a fundamental direct-bandgap semiconductor. For T ≤ 90 K, the observation of a threshold in emitted intensity with increasing incident optical power, together with strong linewidth narrowing and a consistent longitudinal cavity mode pattern, highlight unambiguous laser action. Direct-bandgap group IV materials may thus represent a pathway towards the monolithic integration of Si-photonic circuitry and complementary metal-oxide-semiconductor (CMOS) technology.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, SBMB, UL, UM, UPUK
Group IV semiconductor optoelectronic devices are now possible by using strain-free direct band gap GeSn alloys grown on a Ge/Si virtual substrate with Sn contents above 9%. Here, we demonstrate the ...growth of Ge/GeSn core/shell nanowire arrays with Sn incorporation up to 13% and without the formation of Sn clusters. The nanowire geometry promotes strain relaxation in the Ge0.87Sn0.13 shell and limits the formation of structural defects. This results in room-temperature photoluminescence centered at 0.465 eV and enhanced absorption above 98%. Therefore, direct band gap GeSn grown in a nanowire geometry holds promise as a low-cost and high-efficiency material for photodetectors operating in the short-wave infrared and thermal imaging devices.
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The recent observation of a fundamental direct bandgap for GeSn group IV alloys and the demonstration of low temperature lasing provide new perspectives on the fabrication of Si photonic circuits. ...This work addresses the progress in GeSn alloy epitaxy aiming at room temperature GeSn lasing. Chemical vapor deposition of direct bandgap GeSn alloys with a high Γ- to L-valley energy separation and large thicknesses for efficient optical mode confinement is presented and discussed. Up to 1 μm thick GeSn layers with Sn contents up to 14 at. % were grown on thick relaxed Ge buffers, using Ge2H6 and SnCl4 precursors. Strong strain relaxation (up to 81%) at 12.5 at. % Sn concentration, translating into an increased separation between Γ- and L-valleys of about 60 meV, have been obtained without crystalline structure degradation, as revealed by Rutherford backscattering spectroscopy/ion channeling and transmission electron microscopy. Room temperature reflectance and photoluminescence measurements were performed to probe the optical properties of these alloys. The emission/absorption limit of GeSn alloys can be extended up to 3.5 μm (0.35 eV), making those alloys ideal candidates for optoelectronics in the mid-infrared region. Theoretical net gain calculations indicate that large room temperature laser gains should be reachable even without additional doping.
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We report on the fabrication and characterization of an optimized comb-drive actuator design for strain-dependent transport measurements on suspended graphene. We fabricate devices from highly ...p-doped silicon using deep reactive ion etching with a chromium mask. Crucially, we implement a gold layer to reduce the device resistance from 51.6 k to 236 at room temperature in order to allow for strain-dependent transport measurements. The graphene is integrated by mechanically transferring it directly onto the actuator using a polymethylmethacrylate membrane. Importantly, the integrated graphene can be nanostructured afterwards to optimize device functionality. The minimum feature size of the structured suspended graphene is 30 nm, which allows for interesting device concepts such as mechanically-tunable nanoconstrictions. Finally, we characterize the fabricated devices by measuring the Raman spectrum as well as the a mechanical resonance frequency of an integrated graphene sheet for different strain values
Interfacial mixing in heteroepitaxial growth Bierwald, Boris; von den Driesch, Michael; Farkas, Zéno ...
Physical review. E, Statistical, nonlinear, and soft matter physics,
08/2004, Volume:
70, Issue:
2 Pt 1
Journal Article
Open access
We investigate the growth of a film of some element B on a substrate made of another substance A in a model of molecular beam epitaxy. A vertical exchange mechanism (partial surfactant behavior) ...allows the A atoms to stay on the growing surface with a certain probability. Using kinetic Monte Carlo simulations as well as scaling arguments, the incorporation of the A 's into the growing B layer is investigated. Moreover, we develop a rate equation theory for this process. The concentration of A impurities decays in the B -film like (distance from the interface)(-1-beta), where beta approximately 0.5 for two-dimensional surfaces, approximately 0.8 in the one-dimensional case, and 1 in mean-field approximation. The power law is cut off exponentially at a characteristic thickness of the interdiffusion zone that depends on the rate of exchange of a B adatom with an A atom in the surface and on the diffusion length. Under certain conditions the interdiffusion zone is predicted to become narrower, if the growth temperature is increased.
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We present results on CVD growth and electro-optical characterization of Ge(0.92)Sn(0.08)/Ge p-i-n heterostructure diodes. The suitability of Ge as barriers for direct bandgap GeSn active layers in ...different LED geometries, such as double heterostructures and multi quantum wells is discussed based on electroluminescence data. Theoretical calculations by effective mass and 6 band k∙p method reveal low barrier heights for this specific structure. Best configurations offer only a maximum barrier height for electrons of about 40 meV at the Γ point at room temperature (e.g. 300 K), evidently insufficient for proper light emitting devices. An alternative solution using SiGeSn as barrier material is introduced, which provides appropriate band alignment for both electrons and holes resulting in efficient confinement in direct bandgap GeSn wells. Finally, epitaxial growth of such a complete SiGeSn/GeSn/SiGeSn double heterostructure including doping is shown.
By successive oxygen treatments of graphene nonlocal spin-valve devices we achieve a gradual increase of the contact-resistance-area products (R sub(c)A) of Co/MgO spin injection and detection ...electrodes and a transition from linear to nonlinear characteristics in the respective differential dV-dI curves. With this manipulation of the contacts, both spin lifetime and the amplitude of the spin signal can significantly be increased by a factor of seven in the same device. This demonstrates that contact-induced spin dephasing is the bottleneck for spin transport in graphene devices with small R sub(c)A values. With increasing R sub(c)A values, we furthermore observe the appearance of a second charge neutrality point (CNR) in gate-dependent resistance measurements. Simultaneously, we observe a decrease of the gate voltage separation between the two CNPs. The strong enhancement of the spin-transport properties as well as the changes in charge transport are explained by a gradual suppression of a Co-graphene interaction by improving the oxide barrier during oxygen treatment.
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