16 results
Search Results
Now showing 1 - 10 of 16
- ItemStrong out-of-plane magnetic anisotropy in ion irradiated anatase TiO2 thin films(New York, NY : American Inst. of Physics, 2016) Stiller, M.; Barzola-Quiquia, J.; Esquinazi, P.; Spemann, D.; Meijer, J.; Lorenz, M.; Grundmann, M.The temperature and field dependence of the magnetization of epitaxial, undoped anatase TiO2 thin films on SrTiO3 substrates was investigated. Low-energy ion irradiation was used to modify the surface of the films within a few nanometers, yet with high enough energy to produce oxygen and titanium vacancies. The as-prepared thin film shows ferromagnetism which increases after irradiation with low-energy ions. An optimal and clear magnetic anisotropy was observed after the first irradiation, opposite to the expected form anisotropy. Taking into account the experimental parameters, titanium vacancies as di-Frenkel pairs appear to be responsible for the enhanced ferromagnetism and the strong anisotropy observed in our films. The magnetic impurities concentrations was measured by particle-induced X-ray emission with ppm resolution. They are ruled out as a source of the observed ferromagnetism before and after irradiation.
- ItemApplication of the transferred matrix method to a unified evaluation of the cathodic electron emission(New York, NY : American Inst. of Physics, 2018) Baeva, M.The work is concerned with the Transfer Matrix Method for solving the steady-state Schrödinger equation applied for a unified evaluation of the emission current density from non-refractory cathodes. The method is applicable to arbitrary shapes of the potential barrier and its transmission probability is obtained without any analytical approximations. The Fermi-Dirac distribution for the free electrons in the metal is considered as a supply function. The results, obtained for a work function of the cathode material of 4.5 eV over a wide range of values of the surface temperature and the electric field strength, clearly show a growing deviation from those obtained by the classical Jeffreys-Wentzel-Kramers-Brillouin approximation with the increase of the electric field strength. Preliminary results are obtained to demonstrate the applicability of the Transfer Matrix method to the evaluation of the ion-assisted electron emission. A significant local enhancement of the emission current density is obtained as a result of the presence of an ion at a fixed position near the metal surface. The effect becomes very strongly pronounced at an appropriate value of the electric field strength, for which a resonant ion contribution appears.
- ItemResistive switching in polycrystalline YMnO3 thin films(New York, NY : American Inst. of Physics, 2014) Bogusz, A.; Müller, A.D.; Blaschke, D.; Skorupa, I.; Bürger, D.; Scholz, A.; Schmidt, O.G.; Schmidt, H.We report a unipolar, nonvolatile resistive switching in polycrystalline YMnO3 thin films grown by pulsed laser deposition and sandwiched between Au top and Ti/Pt bottom electrodes. The ratio of the resistance in the OFF and ON state is larger than 103. The observed phenomena can be attributed to the formation and rupture of conductive filaments within the multiferroic YMnO3 film. The generation of conductive paths under applied electric field is discussed in terms of the presence of grain boundaries and charged domain walls inherently formed in hexagonal YMnO3. Our findings suggest that engineering of the ferroelectric domains might be a promising route for designing and fabrication of novel resistive switching devices.
- ItemNovel concept for VCSEL enhanced silicon photonic coherent transceiver(New York, NY : American Inst. of Physics, 2019) Seiler, Pascal M.; Ronniger, Gregor; Troppenz, Ute; Sigmund, Ariane; Moehrle, Martin; Peczek, Anna; Zimmermann, LarsWe present a novel concept for an integrated silicon photonic coherent transceiver using vertical-emitting laser sources at 1550 nm. In a state of the art external modulation configuration, we deploy a VCSEL on the transmit and a HCSEL on the receive side. We demonstrate the feasibility of this approach by externally modulating the VCSEL with QPSK at up to 28 Gbaud. We also perform experiments with the VCSEL-HCSEL configuration in a slave-master optical injection locking setup for future data center interconnects. The results show stable locking conditions and the VCSEL is detuned to perform predominant phase modulation. To the best of our knowledge, this is the first time direct phase modulation of a VCSEL under optical injection locking was demonstrated using two vertically emitting laser sources as master - and slave laser. © 2019 Author(s).
- ItemHigh-temperature high-sensitivity AlN-on-SOI Lamb wave resonant strain sensor(New York, NY : American Inst. of Physics, 2018) Dou, Shaoxu; Qi, Mengke; Chen, Cong; Zhou, Hong; Wang, Yong; Shang, Zhengguo; Yang, Jing; Wang, Dengpan; Mu, XiaojingA piezoelectric AlN-on-SOI structured MEMS Lamb wave resonator (LWR) is presented for high-temperature strain measurement. The LWR has a composite membrane of a 1 μm thick AlN film and a 30 μm thick device silicon layer. The excited acoustic waves include Rayleigh wave and Lamb waves. A tensile strain sensor has been prepared with one LWR mounted on a uniaxial tensile plate, and its temperature characteristics from 15.4°C to 250°C and tensile strain behaviors from 0 μϵ to 400 μϵ of Rayleigh wave and S4 mode Lamb wave were tested. The temperature test verifies the adaptability of the tensile strain sensor to temperature up to 250°C, and S4 mode Lamb wave and Rayleigh wave represent almost the same temperature characteristics. The strain test demonstrates that S4 mode Lamb wave shows much higher strain sensitivity (-0.48 ppm/μϵ) than Rayleigh wave (0.05 ppm/μϵ) and confirms its advantage of strain sensitivity. Finally, for this one-LWR strain sensor, a method of beat frequency between S4 mode Lamb wave and Rayleigh wave is proposed for temperature compensation and high-sensitivity strain readout.
- ItemTopography evolution of germanium thin films synthesized by pulsed laser deposition(New York, NY : American Inst. of Physics, 2017) Schumacher, P.; Mayr, S.G.; Rauschenbach, B.Germanium thin films were deposited by Pulsed Laser Deposition (PLD) onto single crystal Ge (100) and Si (100) substrates with a native oxide film on the surface. The topography of the surface was investigated by Atomic Force Microscopy (AFM) to evaluate the scaling behavior of the surface roughness of amorphous and polycrystalline Ge films grown on substrates with different roughnesses. Roughness evolution was interpreted within the framework of stochastic rate equations for thin film growth. Here the Kardar-Parisi-Zhang equation was used to describe the smoothening process. Additionally, a roughening regime was observed in which 3-dimensional growth occurred. Diffusion of the deposited Ge adatoms controlled the growth of the amorphous Ge thin films. The growth of polycrystalline thin Ge films was dominated by diffusion processes only in the initial stage of the growth.
- ItemGraphene Q-switched Yb:KYW planar waveguide laser(New York, NY : American Inst. of Physics, 2015) Kim, Jun Wan; Young Choi, Sun; Aravazhi, Shanmugam; Pollnau, Markus; Griebner, Uwe; Petrov, Valentin; Bae, Sukang; Jun Ahn, Kwang; Yeom, Dong-Il; Rotermund, FabianA diode-pumped Yb:KYW planar waveguide laser, single-mode Q-switched by evanescent-field interaction with graphene, is demonstrated for the first time. Few-layer graphene grown by chemical vapor deposition is transferred onto the top of a guiding layer, which initiates stable Q-switched operation in a 2.4-cm-long waveguide laser operating near 1027 nm. Average output powers up to 34 mW and pulse durations as short as 349 ns are achieved. The measured output beam profile, clearly exhibiting a single mode, agrees well with the theoretically calculated mode intensity distribution inside the waveguide. As the pump power is increased, the repetition rate and pulse energy increase from 191 to 607 kHz and from 7.4 to 58.6 nJ, respectively, whereas the pulse duration decreases from 2.09 μs to 349 ns.
- ItemPhotoluminescence at room temperature of liquid-phase crystallized silicon on glass(New York, NY : American Inst. of Physics, 2016) Vetter, Michael; Schwuchow, Anka; Andrä, GudrunThe room temperature photoluminescence (PL) spectrum due band-to-band recombination in an only 8 μm thick liquid-phase crystallized silicon on glass solar cell absorber is measured over 3 orders of magnitude with a thin 400 μm thick optical fiber directly coupled to the spectrometer. High PL signal is achieved by the possibility to capture the PL spectrum very near to the silicon surface. The spectra measured within microcrystals of the absorber present the same features as spectra of crystalline silicon wafers without showing defect luminescence indicating the high electronic material quality of the liquid-phase multi-crystalline layer after hydrogen plasma treatment.
- ItemPhotoluminescence lineshape of ZnO(New York, NY : American Inst. of Physics, 2014) Ullrich, B.; Singh, A.K.; Bhowmick, M.; Barik, P.; Ariza-Flores, D.; Xi, H.; Tomm, J.W.The merger of the absorption coefficient dispersion, retrieved from transmission by the modified Urbach rule introduced by Ullrich and Bouchenaki [Jpn. J. Appl. Phys. 30, L1285, 1991], with the extended Roosbroeck-Shockley relation reveals that the optical absorption in ZnO distinctively determines the photoluminescence lineshape. Additionally, the ab initio principles employed enable the accurate determination of the carrier lifetime without further specific probing techniques.
- ItemImproving the efficiency of copper indium gallium (Di-)selenide (CIGS) solar cells through integration of a moth-eye textured resist with a refractive index similar to aluminum doped zinc oxide(New York, NY : American Inst. of Physics, 2014) Burghoorn, M.; Kniknie, B.; van Deelen, J.; Xu, M.; Vroon, Z.; van Ee, R.; van de Belt, R.; Buskens, P.Textured transparent conductors are widely used in thin-film silicon solar cells. They lower the reflectivity at interfaces between different layers in the cell and/or cause an increase in the path length of photons in the Si absorber layer, which both result in an increase in the number of absorbed photons and, consequently, an increase in short-circuit current density (Jsc) and cell efficiency. Through optical simulations, we recently obtained strong indications that texturing of the transparent conductor in copper indium gallium (di-)selenide (CIGS) solar cells is also optically advantageous. Here, we experimentally demonstrate that the Jsc and efficiency of CIGS solar cells with an absorber layer thickness (dCIGS) of 0.85 μm, 1.00 μm and 2.00 μm increase through application of a moth-eye textured resist with a refractive index that is sufficiently similar to AZO (nresist = 1.792 vs. nAZO = 1.913-at 633 nm) to avoid large optical losses at the resist-AZO interface. On average, Jsc increases by 7.2%, which matches the average reduction in reflection of 7.0%. The average relative increase in efficiency is slightly lower (6.0%). No trend towards a larger relative increase in Jsc with decreasing dCIGS was observed. Ergo, the increase in Jsc can be fully explained by the reduction in reflection, and we did not observe any increase in Jsc based on an increased photon path length. © 2014 Author(s).