http://www.recercat.cat:80/handle/2072/48796 Fri, 24 May 2013 09:37:40 GMT 2013-05-24T09:37:40Z http://www.recercat.cat:80/bitstream/id/34138/ http://www.recercat.cat:80/handle/2072/48796 http://www.recercat.cat:80/handle/2072/207640 Readout schemes for low noise single-photon avalanche diodes fabricated in conventional HV-CMOS technologies Vilella Figueras, Eva; Diéguez Barrientos, Àngel Three different pixels based on single-photon avalanche diodes for triggered applications, such as fluorescence lifetime measurements and high energy physics experiments, are presented. Each pixel consists of a 20µm x 100µm (width x length) single photon avalanche diode and a monolithically integrated readout circuit. The sensors are operated in the gated mode of acquisition to reduce the probability to detect noise counts interferring with real radiation events. Each pixel includes a different readout circuit that allows to use low reverse bias overvoltages. Experimental results demonstrate that the three pixels present a similar behaviour. The pixels get rid of afterpulses and present a reduced dark count probability by applying the gated operation. Noise figures are further improved by using low reverse bias overvoltages. The detectors exhibit an input dynamic range of 13.35 bits with short gated"on" periods of 10ns and a reverse bias overvoltage of 0.5V. The three pixels have been fabricated in a standard HV-CMOS process. http://www.recercat.cat:80/handle/2072/207640 http://www.recercat.cat:80/handle/2072/207215 Enhancement of the photoelectrochemical properties of Cl-doped ZnO nanowires by tuning their coaxial doping profile Fan, Jiandong; Güell Vilà, Frank; Fábrega, Cristian; Shavel, Alexey; Carreté Bello, Àlex; Andreu Arbella, Teresa; Morante i Lleonart, Joan Ramon; Cabot i Codina, Andreu Arrays of vertically aligned ZnO:Cl/ZnO core-shell nanowires were used to demonstrate that the control of the coaxial doping profile in homojunction nanostructures can improve their surface charge carrier transfer while conserving potentially excellent transport properties. It is experimentally shown that the presence of a ZnO shell enhances the photoelectrochemical properties of ZnO:Cl nanowires up to a factor 5. Likewise, the ZnO shell promotes the visible photoluminescence band in highly conducting ZnO:Cl nanowires. These lines of evidence are associated with the increase of the nanowires" surface depletion layer http://www.recercat.cat:80/handle/2072/207215 http://www.recercat.cat:80/handle/2072/206919 Recombination dynamics in ZnO nanowires: Surfaces states versus mode quality factor Reparaz, J. S.; Güell Vilà, Frank; Wagner, M. R.; Callsen, G.; Kirste, R.; Claramunt Ruiz, Sergi; Morante i Lleonart, Joan Ramon; Hoffmann, A. In this work, we investigate the influence of finite size on the recombinations dynamics of ZnO nanowires. We demonstrate that diameter as well as lenght of nanowires determine the lifetime of the neutral donor bound excitons. Our findings suggest that while the length is mainly responsible for different mode quality factors of the cavity-like nanowires, the diameter determines the influence of surface states as alternative recombinations channels for the optical modes trapped in the nanocavity. In addition, comparing nanowires grown using different catalyst we show that the surfaces states strongly depend on each precursor characteristics. http://www.recercat.cat:80/handle/2072/206919 http://www.recercat.cat:80/handle/2072/206918 Size-dependent recombination dynamics in ZnO nanowires. Reparaz, J. S.; Güell Vilà, Frank; Wagner, M. R.; Hoffmann, A.; Cornet i Calveras, Albert; Morante i Lleonart, Joan Ramon A deep understanding of the recombination dynamics of ZnO nanowires NWs is a natural step for a precise design of on-demand nanostructures based on this material system. In this work we investigate the influence of finite-size on the recombination dynamics of the neutral bound exciton around 3.365 eV for ZnO NWs with different diameters. We demonstrate that the lifetime of this excitonic transition decreases with increasing the surface-to-volume ratio due to a surface induced recombination process. Furthermore, we have observed two broad transitions around 3.341 and 3.314 eV, which were identified as surface states by studying the dependence of their life time and intensitiy with the NWs dimensions. http://www.recercat.cat:80/handle/2072/206918 http://www.recercat.cat:80/handle/2072/206917 Làsers d"upconversion: llum làser visible a partir de llum infraroja. Güell Vilà, Frank Els làsers aporten solucions a una gran varietat de problemes de la vida diària ... http://www.recercat.cat:80/handle/2072/206917 http://www.recercat.cat:80/handle/2072/206531 1.84 micron emission of Tm3+ sensitized by Yb3+ ions in monoclinic KGd(WO4)2 single crystals Güell Vilà, Frank; Massons, Jaume; Gavaldà, Josefina; Pujol Baigés, Maria Cinta; Aguiló Díaz, Magdalena; Díaz, Francesc (Díaz González) By exciting at 940 nm, we have characterized the 1.84 m near infrared emission of trivalent thulium ions in Yb3+, Tm3+:KGd WO4 2 single crystals as a function of the dopant concentration and temperature, from 10 K to room temperature. An overall 3H6 Stark splitting of 470 cm−1 for the Tm3+ ions in the Yb3+, Tm3+:KGd WO4 2 was obtained. We also studied the blue emission at 476 nm Tm3+ and the near infrared emissions at 1.48 m Tm3+ and 1 m Yb3+ as a function of the dopant concentration. Experimental decay times of the 1G4, 3H4, and 3F4 Tm3+ and 2F5/2 Yb3+ excited states have been measured as a function of Yb3+ and Tm3+ ion concentrations. For the 3F4 →3H6 transition of Tm3+ ions, we used the reciprocity method to calculate the maximum emission cross section of 3.07 10−20 cm2 at 1.84 m for the polarization parallel to the Nm principal optical direction. http://www.recercat.cat:80/handle/2072/206531 http://www.recercat.cat:80/handle/2072/206529 Green luminescence of Er3+ in stoichiometric KYb(WO4)2 single crystals Mateos Ferré, Xavier; Güell Vilà, Frank; Pujol Baigés, Maria Cinta; Bursukova, M.; Solé i Cartañá, Rosa Ma.; Gavaldà, Jna.; Aguiló Díaz, Magdalena; Díaz, Francesc (Díaz Gonaález); Massons, Jaume We grew good-optical-quality KYb~WO4)2 single crystals doped with erbium ions by the top seeded solution growth slow cooling method. Optical absorption of erbium was performed at room temperature ~RT! and at 6 K. Green photoluminescence of erbium was achieved at RT and 6 K after selective excitation of ytterbium ions at 940 nm (10 638 cm21). The splitting of all found excited energy levels and the ground energy level of erbium in KYb~WO4)2 is presented derived from the accurate absorption and emission measurements, respectively. The lifetime of the Stokes and the anti-Stokes green emissions of erbium were measured after excitation at 488 nm (20 490 cm21) and 940 nm(10 638 cm21), respectively. We propose applying the up-conversion model to the observed green emission. http://www.recercat.cat:80/handle/2072/206529 http://www.recercat.cat:80/handle/2072/206530 1.48 and 1.84 µm thulium emissions in monoclinic KGd(WO4)2 single crystals Güell Vilà, Frank; Gavaldà, Josefina; Solé i Cartañá, Rosa Ma.; Aguiló Díaz, Magdalena; Díaz, Francesc (Díaz González); Galan, M.; Massons, Jaume By exciting at 788 nm, we have characterized the near infrared emissions of trivalent thulium ions in monoclinic KGd(WO4)2 single crystals at 1.48 and 1.84 mm as a function of dopant concentration from 0.1% to 10% and temperature from 10 K to room temperature. We used the reciprocity method to calculate the maximum emission cross-section of 3.0310220 cm2 at 1.838 mm for the polarization parallel to the Nm principal optical direction. These results agrees well with the experimental data. Experimental decay times of the 3H4!3F4 and 3F4!3H6 transitions have been measured as a function of thulium concentration. http://www.recercat.cat:80/handle/2072/206530 http://www.recercat.cat:80/handle/2072/206528 Large Magnetoresistance in Fe/MgO/FeCo(001) Epitaxial Tunnel-Junctions on GaAs(001). Bowen, M.; Cros, V.; Petroff, F.; Fert, Albert, 1938-; Martínez Boubeta, José Carlos; Costa Krämer, José Luis; Anguita, José Virgilio; Cebollada, Alfonso; Briones Fernández-Pola, Fernando; Teresa, J. M. de; Morellon, L.; Ibarra, M. R.; Güell Vilà, Frank; Peiró Martínez, Francisca; Cornet i Calveras, Albert We present tunneling experiments on Fe~001!/MgO~20 Å!/FeCo~001! single-crystal epitaxial junctions of high quality grown by sputtering and laser ablation. Tunnel magnetoresistance measurements give 60% at 30 K, to be compared with 13% obtained recently on ~001!-oriented Fe/amorphous-Al2O3 /FeCo tunnel junctions. This difference demonstrates that the spin polarization of tunneling electrons is not directly related to the density of states of the free metal surface Fe~001! in this case but depends on the actual electronic structure of the entire electrode/barrier system. http://www.recercat.cat:80/handle/2072/206528 http://www.recercat.cat:80/handle/2072/205075 Limit to the erbium ions emission in silicon-rich oxide films by erbium ion clustering Prtljaga, Nikola; Navarro Urrios, Daniel; Tengattini, Andrea; Anopchenko, Aleksei; Ramirez Ramírez, Joan Manel; Rebled Corsellas, José Manuel; Estradé Albiol, Sònia; Colonna, Jean-Philippe; Fedeli, Jean-Marc; Garrido Fernández, Blas; Pavesi, Lorenzo We have fabricated a series of thin (~50 nm) erbium-doped (by ion implantation) silicon-rich oxide films in the configuration that mitigates previously proposed mechanisms for loss of light emission capability of erbium ions. By combining the methods of optical, structural and electrical analysis, we identify the erbium ion clustering as a driving mechanism to low optical performance of this material. Experimental findings in this work clearly evidence inadequacy of the commonly employed optimization procedure when optical amplification is considered. We reveal that the significantly lower erbium ion concentrations are to be used in order to fully exploit the potential of this approach and achieve net optical gain. http://www.recercat.cat:80/handle/2072/205075 http://www.recercat.cat:80/handle/2072/204983 Dynamic electrostatic force microscopy in liquid media Gramse, Georg; Edwards, M. A.; Fumagalli, Laura; Gomila Lluch, Gabriel We present the implementation of dynamic electrostatic force microscopy in liquid media. This implementation enables the quantitative imaging of local dielectric properties of materials in electrolyte solutions with nanoscale spatial resolution. Local imaging capabilities are obtained by probing the frequency-dependent and ionic concentration-dependent electrostatic forces at high frequency (&1 MHz), while quantification of the interaction forces is obtained with finite-element numerical calculations. The results presented open a wide range of possibilities in a number of fields where the dielectric properties of materials need to be probed at the nanoscale and in a liquid environment. http://www.recercat.cat:80/handle/2072/204983 http://www.recercat.cat:80/handle/2072/204758 Ferromagnetism in transparent thin films of MgO Martínez Boubeta, José Carlos; Beltrán, J. I.; Balcells i Argemí, Lluís; Konstantinović, Zorica; Valencia, S.; Schmitz, D.; Arbiol i Cobos, Jordi; Estradé Albiol, Sònia; Cornil, J.; Martínez Perea, Benjamin We show both theoretical and experimental evidences of the appearance of ferromagnetism in MgO thin films. First-principles calculations allow predicting the possibility of the formation of a local moment in MgO, provided the existence of Mg vacancies which create holes on acceptor levels near the O 2p-dominated valence band. Magnetic measurements evidence of the existence of room-temperature ferromagnetism in MgO thin films. High-resolution transmission electron microscopy demonstrates the existence of cation vacancies in our samples. Finally, by applying the element specificity of the x-ray magnetic circular dichroism technique, we also demonstrate that the magnetic moments of the system arise from the spin polarization of the 2p electrons of oxygen atoms surrounding Mg vacancies. http://www.recercat.cat:80/handle/2072/204758 http://www.recercat.cat:80/handle/2072/204584 SiC-Based MIS gas sensor for high water vapor environments Casals Guillen, Olga; Becker, Thomas; Godignon, Philippe; Romano Rodríguez, Alberto In this work we will prove that SiC-based MIS capacitors can work in environments with extremely high concentrations of water vapor and still be sensitive to hydrogen, CO and hydrocarbons, making these devices suitable for monitoring the exhaust gases of hydrogen or hydrocarbons based fuel cells. Under the harshest conditions (45% of water vapor by volume ratio to nitrogen), Pt/TaOx/SiO2/SiC MIS capacitors are able to detect the presence of 1 ppm of hydrogen, 2 ppm of CO, 100 ppm of ethane or 20 ppm of ethene, concentrations that are far below the legal permissible exposure limits. http://www.recercat.cat:80/handle/2072/204584 http://www.recercat.cat:80/handle/2072/204392 Structural and optical properties of high quality zinc-blende/wurtzite GaAs nanowire heterostructures Spirkoska, D.; Arbiol i Cobos, Jordi; Gustafsson, A.; Conesa Boj, Sònia; Glas, F.; Zardo, I.; Heigoldt, M.; Gass, M. H.; Bleloch, A. L.; Estradé Albiol, Sònia; Kaniber, M.; Rossler, J.; Peiro Martínez, Francisca; Morante i Lleonart, Joan Ramon; Abstreiter, G.; Samuelson, L.; Fontcuberta i Morral, A. The structural and optical properties of three different kinds of GaAs nanowires with 100% zinc-blende structure and with an average of 30% and 70% wurtzite are presented. A variety of shorter and longer segments of zinc-blende or wurtzite crystal phases are observed by transmission electron microscopy in the nanowires. Sharp photoluminescence lines are observed with emission energies tuned from 1.515 eV down to 1.43 eV when the percentage of wurtzite is increased. The downward shift of the emission peaks can be understood by carrier confinement at the interfaces, in quantum wells and in random short period superlattices existent in these nanowires, assuming a staggered band offset between wurtzite and zinc-blende GaAs. The latter is confirmed also by time-resolved measurements. The extremely local nature of these optical transitions is evidenced also by cathodoluminescence measurements. Raman spectroscopy on single wires shows different strain conditions, depending on the wurtzite content which affects also the band alignments. Finally, the occurrence of the two crystallographic phases is discussed in thermodynamic terms. http://www.recercat.cat:80/handle/2072/204392 http://www.recercat.cat:80/handle/2072/204391 Long-range order of Ni2+ and Mn4+ and ferromagnetism in multiferroic (Bi0.9La0.1)2NiMnO6 thin films Langenberg, E.; Rebled Corsellas, José Manuel; Estradé Albiol, Sònia; Daumont, C. J. M.; Ventura, J.; Coy, L. E.; Polo Trasancos, Ma. del Carmen; García-Cuenca Varona, María Victoria; Ferrater Martorell, Cèsar; Noheda, B.; Peiró Martínez, Francisca; Varela Fernández, Manuel, 1956-; Fontcuberta i Griñó, Josep Epitaxial thin films of biferroic Bi1−xLax 2NiMnO6 have been grown on SrTiO3 001 substrates. High resolution electron microscopy, energy-loss spectroscopy and synchrotron radiation have been used to demonstrate that, under appropriate growth conditions, stoichiometric, and fully oxidized thin films with long-range order of Ni2+ and Mn4+ ions can be obtained, despite the presence of randomly distributed dissimilar cations Bi, La at the A-site. This ordering leads to Ni2+&br&O&br&Mn4+ ferromagnetic interactions and its preservation in thin films is key for implementation of these biferroic materials in practical devices. http://www.recercat.cat:80/handle/2072/204391 http://www.recercat.cat:80/handle/2072/203828 High quality InAlN single layers lattice-matched to GaN grown by molecular beam epitaxy Gacevic, Z.; Fernández-Garrido, S.; Rebled Corsellas, José Manuel; Estradé Albiol, Sònia; Peiró Martínez, Francisca; Calleja Pardo, Enrique We report on properties of high quality 60 nm thick InAlN layers nearly in-plane lattice-matched to GaN, grown on c-plane GaN-on-sapphire templates by plasma-assisted molecular beam epitaxy. Excellent crystalline quality and low surface roughness are confirmed by X-ray diffraction, transmission electron microscopy, and atomic force microscopy. High annular dark field observations reveal a periodic in-plane indium content variation (8 nm period), whereas optical measurements evidence certain residual absorption below the band-gap. The indium fluctuation is estimated to be 61.2% around the nominal 17% indium content via plasmon energy oscillations assessed by electron energy loss spectroscopy with sub-nanometric spatial resolution. http://www.recercat.cat:80/handle/2072/203828 http://www.recercat.cat:80/handle/2072/203827 Effects of thickness on the cation segregation in epitaxial (001) and (110) La2/3Ca1/3MnO3 thin films Estradé Albiol, Sònia; Rebled Corsellas, José Manuel; Arbiol i Cobos, Jordi; Peiró Martínez, Francisca; Infante, I. C.; Herranz, G.; Sánchez, F.; Fontcuberta i Griñó, Josep; Córdoba, R.; Mendis, B. G.; Bleloch, A. L. Electron-energy-loss spectroscopy is used to map composition and electronic states in epitaxial La2/3Ca1/3MnO3 LCMO films of various thicknesses grown on SrTiO3 001 and 110 substrates. For relatively thick films 20 nm , epitaxial tensile strain in 001 films promotes a compositional La/Ca gradient across the film thickness, being the interface La rich, while the relaxed 110 films are chemically homogeneous. In contrast, much thinner 001 and 110 LCMO films display a different La/Ca distribution, being La rich at the free surface. The observed distinct thickness-dependent composition gradient behavior reflects a balance between strain-induced elastic energy minimization and kinetic effects during growth. http://www.recercat.cat:80/handle/2072/203827 http://www.recercat.cat:80/handle/2072/203754 Growth and magnetic characterization of Co nanoparticles obtained by femtosecond pulsed laser deposition. Cebollada, Alfonso; García Martín, J. M.; Clavero, C.; Balcells i Argemí, Lluís; Estradé Albiol, Sònia; Arbiol i Cobos, Jordi; Peiró Martínez, Francisca; Smith, C.; Clarke, R.; Martínez, L.; Huttel, Y.; Román, E.; Telling, N. D.; Van der Laan, G. We present a detailed study on the morphology and magnetic properties of Co nanostructures deposited onto oxidized Si substrates by femtosecond pulsed laser deposition. Generally, Co disks of nanometric dimensions are obtained just above the ablation threshold, with a size distribution characterized by an increasingly larger number of disks as their size diminishes, and with a maximum disk size that depends on the laser power density. In Au/Co/Au structures, in-plane magnetic anisotropy is observed in all cases, with no indication of superparamagnetism regardless of the amount of material or the laser power density. Magnetic force microscopy observations show coexistence of single-domain and vortex states for the magnetic domain structure of the disks. Superconducting quantum interference device magnetometry and x-ray magnetic circular dichroism measurements point to saturation magnetization values lower than the bulk, probably due to partial oxidation of the Co resulting from incomplete coverage by the Au capping layer. http://www.recercat.cat:80/handle/2072/203754 http://www.recercat.cat:80/handle/2072/203753 Bulk silica-based luminescent materials by sol-gel processing of non-conventional precursors Rodríguez, J. A.; Fernández-Sánchez, C.; Domínguez, Carlos (Domínguez Horna); Hernández Márquez, Sergi; Berencén Ramírez, Yonder Antonio The sol-gel synthesis of bulk silica-based luminescent materials using innocuous hexaethoxydisilane and hexamethoxydisilane monomers, followed by one hour thermal annealing in an inert atmosphere at 950oC&br&1150oC, is reported. As-synthesized hexamethoxydisilane-derived samples exhibit an intense blue photoluminescence band, whereas thermally treated ones emit stronger photoluminescence radiation peaking below 600 nm. For hexaethoxydisilane-based material, annealed at or above 1000oC, a less intense photoluminescence band, peaking between 780 nm and 850 nm that is attributed to nanocrystalline silicon is observed. Mixtures of both precursors lead to composed spectra, thus envisaging the possibility of obtaining pre-designed spectral behaviors by varying the mixture composition. http://www.recercat.cat:80/handle/2072/203753 http://www.recercat.cat:80/handle/2072/203109 Polarization strategies to improve the emission of a Si-based light source emitting at 1.55 um Ramírez Ramírez, Joan Manel; Jambois, Olivier; Berencén Ramírez, Yonder Antonio; Navarro Urrios, Daniel; Anopchenko, Aleksei; Marconi, A.; Prtljaga, Nikola; Daldosso, Nicola; Pavesi, Lorenzo; Colonna, J. P.; Fedeli, J. M.; Garrido Fernández, Blas We present a electroluminescence (EL) study of the Si-rich silicon oxide (SRSO) LEDs with and without Er3+ ions under different polarization schemes: direct current (DC) and pulsed voltage (PV). The power efficiency of the devices and their main optical limitations are presented. We show that under PV polarization scheme, the devices achieve one order of magnitude superior performance in comparison with DC. Time-resolved measurements have shown that this enhancement is met only for active layers in which annealing temperature is high enough (&1000 ◦C) for silicon nanocrystal (Si-nc) formation. Modeling of the system with rate equations has been done and excitation cross-sections for both Si-nc and Er3+ ions have been extracted. http://www.recercat.cat:80/handle/2072/203109 http://www.recercat.cat:80/handle/2072/202745 Distinct magnetism in ultrathin epitaxial NiFe2O4 films on MgAl2O4 and SrTiO3single crystalline substrates Foerster, Michael; Rebled Corsellas, José Manuel; Estradé Albiol, Sònia; Sánchez Barrera, Florencio; Peiró Martínez, Francisca; Fontcuberta i Griñó, Josep Spinel ferrites are being considered for advanced spintronic applications. Here, we report on the magnetic properties of ultrathin (3&br&37 nm) epitaxial films of NiFe2O4 (NFO) on MgAl2O4 (MAO) and SrTiO3 (STO) single crystalline substrates. It is found that NFO films on STO display superparamagnetic response down to 50 K, whereas films grown on MAO display ferrimagnetic response up to room temperature. Microstructural information indicates that this distinct response can be attributed to the different growth mechanisms of the spinel ferrite on the isostructural MAO substrate (two-dimensional growth) and the perovskite STO (Volmer-Weber three-dimensional growth). We discuss the reasons for this distinct behavior and its relevance for the integration of ferrites in epitaxial heterostructures for tunnel devices. http://www.recercat.cat:80/handle/2072/202745 http://www.recercat.cat:80/handle/2072/202744 Effect of the capping on the local Mn oxidation state in buried (001) and (110) SrTiO3/La2/3Ca1/3MnO3 interfaces Estradé Albiol, Sònia; Rebled Corsellas, José Manuel; Walls, M.; Peña, F. de la; Colliex, C.; Córdoba, R.; Infante, I. C.; Herranz Casabona, Gervasi; Sánchez Barrera, Florencio; Fontcuberta i Griñó, Josep; Peiró Martínez, Francisca The electronic stability of (001) and (110) surfaces of La2/3Ca1/3MnO3 (LCMO) capped with nanometric SrTiO3 (STO) layers in epitaxial heterostructures is addressed by (S)TEM electron energy loss spectroscopy. It is found that growth of STO on (001)LCMO promotes a significant electron doping of LCMO that extends a few nanometers deep into the film. In contrast, (110)LCMO appears to be electronically more robust and no electronic reordering has been observed. These results are in clear correlation with the distinct magnetic properties observed in those interfaces and illustrate that complex chemical phenomena take place at interfaces among multivalent oxides. VC 2011 American Institute of Physics. http://www.recercat.cat:80/handle/2072/202744 http://www.recercat.cat:80/handle/2072/202743 Correlation between charge transport and electroluminescence properties of Si-rich oxide/nitride/oxide-based light emitting capacitors. Berencén Ramírez, Yonder Antonio; Ramírez Ramírez, Joan Manel; Jambois, Olivier; Domínguez, Carlos (Domínguez Horna); Rodríguez, J. A.; Garrido Fernández, Blas The electrical and electroluminescence (EL) properties at room and high temperatures of oxide/ nitride/oxide (ONO)-based light emitting capacitors are studied. The ONO multidielectric layer is enriched with silicon by means of ion implantation. The exceeding silicon distribution follows a Gaussian profile with a maximum of 19%, centered close to the lower oxide/nitride interface. The electrical measurements performed at room and high temperatures allowed to unambiguously identify variable range hopping (VRH) as the dominant electrical conduction mechanism at low voltages, whereas at moderate and high voltages, a hybrid conduction formed by means of variable range hopping and space charge-limited current enhanced by Poole-Frenkel effect predominates. The EL spectra at different temperatures are also recorded, and the correlation between charge transport mechanisms and EL properties is discussed. http://www.recercat.cat:80/handle/2072/202743 http://www.recercat.cat:80/handle/2072/202742 Cationic and charge segregation in La2/3Ca1/3MnO3 thin films grown on (001) and (110) SrTiO3. Estradé Albiol, Sònia; Arbiol i Cobos, Jordi; Peiró Martínez, Francisca; Infante, I. C.; Sánchez Barrera, Florencio; Fontcuberta i Griñó, Josep; Peña, F. de la; Walls, M.; Colliex, C. Electron energy-loss spectroscopy is used to map composition and electronic states in epitaxial La2/3Ca1/3MnO3 films grown on SrTiO3 001 and 110 substrates. It is found that in partially relaxed 110 films cationic composition and valence state of Mn3+/4+ ions are preserved across the film thickness. In contrast, in fully strained 001 films, the Ca/La ratio gradually changes across the film, being La rich at film/substrate interface and La depleted at free surface; Mn valence state changes accordingly. These observations suggest that a strongly orientation-dependent adaptative composition mechanism dominates stress accommodation in manganite films and provides microscopic understanding of their dissimilar magnetic properties. http://www.recercat.cat:80/handle/2072/202742 http://www.recercat.cat:80/handle/2072/202574 Micro-Raman study of stress distribution in local isolation structures and correlation with transmission electron microscopy De Wolf, I.; Vanhellemont, J.; Romano Rodríguez, Alberto; Norström, H.; Maes, H.E. Stress in local isolation structures is studied by micro‐Raman spectroscopy. The results are correlated with predictions of an analytical model for the stress distribution and with cross‐sectional transmission electron microscopy observations. The measurements are performed on structures on which the Si3N4 oxidation mask is still present. The influence of the pitch of the periodic local isolation pattern, consisting of parallel lines, the thickness of the mask, and the length of the bird"s beak on the stress distribution are studied. It is found that compressive stress is present in the Si substrate under the center of the oxidation mask lines, with a magnitude dependent on the width of the lines. Large tensile stress is concentrated under the bird"s beak and is found to increase with decreasing length of the bird"s beak and with increasing thickness of the Si3N4 film. http://www.recercat.cat:80/handle/2072/202574 http://www.recercat.cat:80/handle/2072/202575 Alloy inhomogeneities in InAlAs strained layers grown by MBE Peiró Martínez, Francisca; Cornet i Calveras, Albert; Morante i Lleonart, Joan Ramon; Clark, S. A.; Williams, R. H. Transmission electron microscopy studies have been performed to characterize InxAl1−xAs layers grown by molecular beam epitaxy on (100) InP substrates. The first observations of compositional nonuniformities in strained InAlAs layers are reported. The coarse quasiperiodic structure present in each sample has been found to be dependent upon the growth parameters and the sample characteristics. http://www.recercat.cat:80/handle/2072/202575 http://www.recercat.cat:80/handle/2072/202576 Raman scattering and photoluminescence analysis of silicon on insulator structures obtained by single and multiple oxygen implants Pérez Rodríguez, Alejandro; Cornet i Calveras, Albert; Morante i Lleonart, Joan Ramon; Jiménez, J.; Hemment, Peter L. F.; Homewood, K. P. An analysis of silicon on insulator structures obtained by single and multiple implants by means of Raman scattering and photoluminescence spectroscopy is reported. The Raman spectra obtained with different excitation powers and wavelengths indicate the presence of a tensile strain in the top silicon layer of the structures. The comparison between the spectra measured in both kinds of samples points out the existence in the multiple implant material of a lower strain for a penetration depth about 300 nm and a higher strain for higher penetration depths. These results have been correlated with transmission electron microscopy observations, which have allowed to associate the higher strain to the presence of SiO2 precipitates in the top silicon layer, close to the buried oxide. The found lower strain is in agreement with the better quality expected for this material, which is corroborated by the photoluminescence data. http://www.recercat.cat:80/handle/2072/202576 http://www.recercat.cat:80/handle/2072/202571 Majority carrier capture cross section determination in the large deep trap concentration cases Morante i Lleonart, Joan Ramon; Samitier i Martí, Josep; Cornet i Calveras, Albert; Herms Berenguer, Atilà; Cartujo Estébanez, Pedro A method to determine the thermal cross section of a deep level from capacitance measurements is reported. The results enable us to explain the nonexponential behavior of the capacitance versus capture time when the trap concentration is not negligible with respect to that of the shallow one, and the Debye tail effects are taken into account. A figure of merit for the nonexponential behavior of the capture process is shown and discussed for different situations of doping and applied bias. We have also considered the influence of the position of the trap level"s energy on the nonexponentiality of the capture transient. The experimental results are given for the gold acceptor level in silicon and for the DX center in Al0.55 Ga0.45As, which are in good agreement with the developed theory. http://www.recercat.cat:80/handle/2072/202571 http://www.recercat.cat:80/handle/2072/202573 On the artificial creation of the EL2 center by means of boron implantation in gallium arsenide Morante i Lleonart, Joan Ramon; Pérez Rodríguez, Alejandro; Samitier i Martí, Josep; Romano Rodríguez, Alberto In the present work, an analysis of the dark and optical capacitance transients obtained from Schottky Au:GaAs barriers implanted with boron has been carried out by means of the isothermal transient spectroscopy (ITS) and differential and optical ITS techniques. Unlike deep level transient spectroscopy, the use of these techniques allows one to easily distinguish contributions to the transients different from those of the usual deep trap emission kinetics. The results obtained show the artificial creation of the EL2, EL6, and EL5 defects by the boron implantation process. Moreover, the interaction mechanism between the EL2 and other defects, which gives rise to the U band, has been analyzed. The existence of a reorganization process of the defects involved has been observed, which prevents the interaction as the temperature increases. The activation energy of this process has been found to be dependent on the temperature of the annealing treatment after implantation, with values of 0.51 and 0.26 eV for the as‐implanted and 400 °C annealed samples, respectively. The analysis of the optical data has corroborated the existence of such interactions involving all the observed defects that affect their optical parameters http://www.recercat.cat:80/handle/2072/202573 http://www.recercat.cat:80/handle/2072/202572 Analysis of the near-intrinsic and extrinsic photocapacitance due to the EL2 level in boron implanted GaAs Morante i Lleonart, Joan Ramon; Samitier i Martí, Josep; Pérez Rodríguez, Alejandro; Altelarrea Soria, Hermenegildo; Gourrier, S. A detailed analysis of the photocapacitance signal at the near‐band and extrinsic energetic ranges in Schottky barriers obtained on horizontal Bridgman GaAs wafers, which were implanted with boron at different doses and annealed at several temperatures, has been carried out by using the optical isothermal transient spectroscopy, OITS. The optical cross sections have been determined as well as the quenching efficiency of the EL2 level which has been found to be independent of the annealing temperature. Moreover, the quenching relaxation presents two significant features: (i) a strong increase of the quenching efficiency from 1.35 eV on and (ii) a diminution of the quenching transient amplitude in relation with that shown by the fundamental EL2 level. In order to explain this behavior, different cases are discussed assuming the presence of several energy levels, the existence of an optical recuperation, or the association of the EL2 trap with two levels located, respectively, at Ev+0.45 eV and Ec−0.75 eV. The theoretical simulation, taking into account these two last cases, is in agreement with the experimental photocapacitance data at low temperature, as well as at room temperature where the EL2 filling phototransient shows an anomalous behavior. Moreover, unlike the previous data reported for the EL2 electron optical cross section, the values found using our experimental technique are in agreement with the behavior deduced from the theoretical calculation. The utilization of the OITS method has also allowed the determination of another level, whose faster optical contribution is often added to that of the EL2 level when the DLOS or standard photocapacitance is used. http://www.recercat.cat:80/handle/2072/202572