Geraldo Cristian Vásquez

• Mayandi, Jeyanthinath; Finstad, Terje Gunnar; Stange, Marit Synnøve Sæverud; Vasquez, Geraldo Cristian; Sunding, Martin Fleissner & Løvvik, Ole Martin [Show all 8 contributors for this article] (2021). Controlling the Electrical Properties of Reactively Sputtered High Entropy Alloy CrFeNiCoCu Films. Journal of Electronic Materials. ISSN 0361-5235. 51, p. 803–812. doi: 10.1007/s11664-021-09343-3. Full text in Research Archive Show summary

  Oxide-containing films were made by reactively sputtering a high-entropy alloy target of CrFeCoNiCu. We report on a wide range of changes to the electrical properties made by different heat treatments in oxidizing and reducing atmospheres, respectively. We combine temperature-dependent Hall effect measurements down to 10 K to study the transport mechanisms and correlate that with structural measurements by x-ray diffraction and scanning electron microscopy. The measured/effective resistivity could be varied between 1.3 × 10−4 Ω cm and 1.2 × 10−3 Ω cm by post-deposition processing. The temperature coefficient of resistivity could be varied between − 1.2 × 10−3 K−1 through 0 and to + 0.7 × 10−3 K−1. The key to the variation is controlling the morphology and topology of the film. The conduction of charge carriers is dominated by the relative contribution of weak localization and alloy scattering by varying the degree of disorder in the metallic high-entropy alloy and its topology.

• Nyborg, Martin; Kolevatov, Ilia; Vasquez, Geraldo Cristian; Bergum, K. & Monakhov, E. (2021). Dominant defects and carrier transport in single crystalline cuprous oxide: A new attribution of optical transitions. Journal of Applied Physics. ISSN 0021-8979. 130(17). doi: 10.1063/5.0059406. Full text in Research Archive Show summary

  Electronic properties of single crystal (111) Cu2O wafers have been investigated using a number of complementary techniques. Secondary ion mass spectrometry has shown significant presence of hydrogen and nitrogen. Cathodoluminescence measurements reveal strong near-band emission indicating the good electronic quality of the wafers. Two deep emission lines are observed at 1.3 and 1.7 eV. Temperature-dependent Hall effect measurements reveal electronic levels at around EV + 0.16 eV, EV + 0.22 eV, and ∼EV + 0.4 eV, where EV is the valence band edge. The discussion on the identity of the electronic centers calls for a revision of the traditional assignments of the 1.3-eV and 1.7-eV lines in order to take into account independent theoretical predictions. The temperature dependence of carrier mobility shows that the mechanism limiting the mobility can be described by scattering on neutral and ionized defect centers.

• Abad, Sara; Vasquez, Geraldo Cristian; Vines, Lasse & Ranchal, Rocío (2020). Use of Ga2O3[1 0 0] monocrystals as substrates for the synthesis of GaFeO3 thin films. Materials Letters. ISSN 0167-577X. 261. doi: 10.1016/j.matlet.2019.126949. Full text in Research Archive
• Vasquez, Geraldo Cristian; Johansen, Klaus Magnus H; Galeckas, Augustinas; Vines, Lasse & Svensson, Bengt Gunnar (2020). Optical signatures of single ion tracks in ZnO. Nanoscale Advances. 2(2), p. 724–733. doi: 10.1039/C9NA00677J. Full text in Research Archive Show summary

  The optical properties of single ion tracks have been studied in ZnO implanted with Ge by combining depth-resolved hyperspectral cathodoluminescence (CL) and photoluminescence (PL) spectroscopy techniques. The results indicate that ZnO is susceptible to implantation doses as low as 108 to 109 cm−2. We demonstrate that the intensity ratio of ionized and neutral donor bound exciton emissions [D+X/D0X] can be used as a tracer for a local band bending both at the surface as well as in the crystal bulk along the ion tracks. The hyperspectral CL imaging performed at 80 K with 50 nm resolution over the regions with single ion tracks permitted direct assessment of the minority carrier diffusion length. The radii of distortion and space charge surrounding single ion tracks were estimated from the 2D distributions of defect-related green emission (GE) and excitonic D+X emission, both normalized with regard to neutral D0X emission, i.e., from the [GE/D0X] and [D+X/D0X] ratio maps. Our results indicate that single ion tracks in ZnO can be resolved up to ion doses of the order of 5 × 109 cm−2, in which defect aggregation along the extended defects obstructs signatures of individual tracks.

• Chandrasekaran, Abinaya; Bethke, Kevin; Andrei, Virgil; Baumann, Jonas; Pollakowski-Herrmann, Beatrix & Kanngießer, Birgit [Show all 11 contributors for this article] (2020). The effect of post-deposition annealing conditions on structural and thermoelectric properties of sputtered copper oxide films. RSC Advances. 10(49), p. 29394–29401. doi: 10.1039/D0RA03906C. Full text in Research Archive Show summary

  The development of thin-film thermoelectric applications in sensing and energy harvesting can benefit largely from suitable deposition methods for earth-abundant materials. In this study, p-type copper oxide thin films have been prepared on soda lime silicate glass by direct current (DC) magnetron sputtering at room temperature from a pure copper metallic target in an argon atmosphere, followed by subsequent annealing steps at 300 °C under various atmospheres, namely air (CuO:air), nitrogen (CuO:N) and oxygen (CuO:O). The resultant films have been studied to understand the influence of various annealing atmospheres on the structural, spectroscopic and thermoelectric properties. X-ray diffraction (XRD) patterns of the films showed reflexes that could be assigned to those of crystalline CuO with a thin mixed Cu(I)Cu(II) oxide, which was also observed by near edge X-ray absorption fine structure spectroscopy (NEXAFS). The positive Seebeck coefficient (S) reached values of up to 204 μV K−1, confirming the p-type behavior of the films. Annealing under oxygen provided a significant improvement in the electrical conductivity up to 50 S m−1, resulting in a power factor of 2 μW m−1 K−2. The results reveal the interplay between the intrinsic composition and the thermoelectric performance of mixed copper oxide thin films, which can be finely adjusted by simply varying the annealing atmosphere.

• Vasquez, Geraldo Cristian; Bathen, Marianne Etzelmüller; Galeckas, Augustinas; Bazioti, Kalliopi; Johansen, Klaus Magnus H & Maestre, D. [Show all 11 contributors for this article] (2020). Strain Modulation of Si Vacancy Emission from SiC Micro- and Nanoparticles. Nano Letters. ISSN 1530-6984. 20(12), p. 8689–8695. doi: 10.1021/acs.nanolett.0c03472. Full text in Research Archive Show summary

  Single-photon emitting point defects in semiconductors have emerged as strong candidates for future quantum technology devices. In the present work, we exploit crystalline particles to investigate relevant defect localizations, emission shifting, and waveguiding. Specifically, emission from 6H-SiC micro- and nanoparticles ranging from 100 nm to 5 μm in size is collected using cathodoluminescence (CL), and we monitor signals attributed to the Si vacancy (VSi) as a function of its location. Clear shifts in the emission wavelength are found for emitters localized in the particle center and at the edges. By comparing spatial CL maps with strain analysis carried out in transmission electron microscopy, we attribute the emission shifts to compressive strain of 2–3% along the particle a-direction. Thus, embedding VSi qubit defects within SiC nanoparticles offers an interesting and versatile opportunity to tune single-photon emission energies while simultaneously ensuring ease of addressability via a self-assembled SiC nanoparticle matrix.

• Kumar, Raj; Nordseth, Ørnulf; Vasquez, Geraldo Cristian; Foss, Sean Erik; Monakhov, Eduard & Svensson, Bengt Gunnar (2019). Investigation of n-Al:ZnO/p-Cu2O heterojunction for c-Si tandem heterojunction solar cell applications. AIP Conference Proceedings. ISSN 0094-243X. 2147(1). doi: 10.1063/1.5123885. Full text in Research Archive Show summary

  In this work, an in-situ growth approach has been employed to fabricate Al:ZnO/Cu2O/Cu and Al:ZnO/ZnO/Cu2O/Cu heterojunctions using direct current (DC) and radio frequency (RF) magnetron sputtering technique in a controlled atmospheric condition. The effect of ZnO buffer layer thickness (30 and 50 nm) as well as in-situ Cu2O annealing at 600 °C in low vacuum (~ 10−6 Torr) prior to Al:ZnO deposition were studied. The carrier density of Al:ZnO was ~2 × 1020 cm3 with mobility ~8 cm2/V·s and resistivity ~1 × 10−3. Ω·cm, while the carrier density of Cu2O was 1 × 1015 cm3 with mobility ~19 cm2/V·s and resistivity ~200 .Ω·cm. The heterojunctions were investigated by depth resolved Cathodoluminescence (CL) spectroscopy at 80 K to analyze the influence of defects at the interface. The two emissions at 1.51 eV (Vo+) and 1.69 eV (Vo2+) dominate in all CL spectra related to oxygen vacancy defects in Cu2O. The relative intensity of the defect luminescence band Vo+ respect to Vo2+ is greater at the interface compared to bulk Cu2O, while the incorporation of the ZnO layer reduces significantly both radiative recombination and the Vo2+ related emission at the interface.

• Vasquez, Geraldo Cristian; Maestre, David; Cremades, Ana; Ramírez-Castellanos, Julio; Magnano, Elena & Nappini, Silvia [Show all 7 contributors for this article] (2018). Understanding the effects of Cr doping in rutile TiO2 by DFT calculations and X-ray spectroscopy. Scientific Reports. 8(1). doi: 10.1038/s41598-018-26728-3. Full text in Research Archive
• Vasquez, Geraldo Cristian; Taeno, Maria; Maestre, David & Cremades, Ana (2018). Synthesis and Characterization of TiO2 Micro‐ and Nano‐Structures with Variable Morphology and Aluminum Content. Physica Status Solidi (a) applications and materials science. ISSN 1862-6300. 215(19). doi: 10.1002/pssa.201800249. Full text in Research Archive
• Garcia-Tecedor, M.; Karazhanov, Smagul; Vasquez, Geraldo Cristian; Haug, Halvard; Maestre, D. & Cremades, A. [Show all 12 contributors for this article] (2018). Silicon surface passivation by PEDOT: PSS functionalized by SnO2 and TiO2 nanoparticles. Nanotechnology. ISSN 0957-4484. 29(3). doi: 10.1088/1361-6528/aa9c9e. Full text in Research Archive

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• Kumar, Raj; Nordseth, Ørnulf; Foss, Sean Erik; Vasquez, Geraldo Cristian; Monakhov, Eduard & Svensson, Bengt Gunnar (2019). Investigation of n-Al:ZnO/p-Cu2O Heterojunction for c-Si Tandem Heterojunction Solar Cell Applications. Full text in Research Archive
• Vasquez, Geraldo Cristian; Galeckas, Augustinas; Johansen, Klaus Magnus H; Vines, Lasse & Svensson, Bengt Gunnar (2019). Optical and electrical properties of single ion-tracks in crystalline SiC. Full text in Research Archive
• Abad, Sara; Prados, Alicia; Muñoz-Noval, Alvaro; Vasquez, Geraldo Cristian; Vines, Lasse & Ranchal, Rocío (2019). Use of electrodeposition and thermal oxidation for the synthesis of Ga-Fe-O thin films. Full text in Research Archive
• Vasquez, Geraldo Cristian; Bazioti, Kalliopi; Galeckas, Augustinas; Johansen, Klaus Magnus H; Granerød, Cecilie Skjold & Nguyen, Phuong Dan [Show all 8 contributors for this article] (2019). Luminescent properties of Zn2GeO4 nanoparticles embedded in ZnO. Full text in Research Archive
• Vasquez, Geraldo Cristian; Johansen, Klaus Magnus H; Galeckas, Augustinas; Vines, Lasse; Svensson, Bengt Gunnar & Maestre, David (2018). Single ion-tracks in crystalline ZnO and their influence on the near band edge luminescence. Full text in Research Archive
• Johansen, Klaus Magnus H & Vasquez, Geraldo Cristian (2017). Cathodoluminescence characterization of semiconducting oxides. Full text in Research Archive

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