arXiv subjects

O. Conde

Publications and source records attributed to O. Conde.

15 recordsLinked to original sources

arXiv2017

Resistive switching in MoSe$_{2}$/BaTiO$_{3}$ hybrid structures

Here we study the resistive switching (RS) effect that emerges when ferroelectric BaTiO$_{3}$ (BTO) and few-layers MoSe$_{2}$ are combined in one single structure. The C-V loops reveal the ferroelectric nature of both Al/Si/SiO$_{x}$/BTO/Au and Al/Si/SiO$_{x}$/MoSe$_{2}$/BTO/Au structures and the high quality of the SiO$_{x}$/MoSe$_{2}$ interface in the Al/Si/SiOx/MoSe$_{2}$/Au structure. Al/Si/SiO$_{x}$/MoSe$_{2}$/BTO/Au hybrid structures show the electroforming free resistive switching that is explained on the basis of the modulation of the potential distribution at the MoSe$_{2}$/BTO interface via ferroelectric polarization flipping. This structure shows promising resistive switching characteristics with switching ratio of $\approx{}$10$^{2}$ and a stable memory window, which are highly required for memory applications.

physics.app-ph↗

arXiv2016

Argon assisted chemical vapor deposition of CrO$_2$: an efficient process leading to high quality epitaxial films

A comparative study of the structural, microstructural and magnetic properties of CrO$_2$ thin films grown onto (110) and (100) TiO$_2$ rutile single crystal substrates by chemical vapor deposition (CVD), using CrO$_3$ as chromium precursor and either oxygen or argon as carrier gas is presented. Our results show that growth under argon carrier gas leads to high quality CrO$_2$ epilayers with structural and magnetic properties similar to those obtained using the more standard oxygen carrier gas. Furthermore, we interpret the larger magnetic coercivity observed for the (110) oriented films in terms of their microstructure, in particular of the highest strain and edge roughness of the building structures of the CrO$_2$ epilayers, which are settled by the substrate crystallographic orientation.

cond-mat.mtrl-sci↗

arXiv2016

Electrical Switching of Magnetization in Films of alpha-Iron with Naturally Hydroxidized Surface

Control of the magnetization vector in ferromagnetic films and heterostructures by using electric tools instead of external magnetic fields can lead to low-power memory devices. We observe the robust changes in magnetization states of a thin (about 30 nm) film of alpha-Fe covered by the naturally formed layer ( about 6 nm in thickness) of iron ohyhydroxides (FeOOH) under discharging a capacitor through the film. Strikingly, the magnetization vector is switchable by the discharge even with no any biasing field at room temperatures. In this electrically induced magnetization switching (EIMS) we reveal the key role of the FeOOH layer. We demonstrate experimental evidences that not the discharge current itself but the electric field (of the order of 10 kV/m) generated by this current is responsible for EIMS. The results reported here provide a plausible explanation of the observed phenomenon in terms of electric-field-induced weak ferromagnetism in the FeOOH layer and its coupling with the underlying alpha-Fe.

cond-mat.mtrl-sci↗

arXiv2014

Very high thermoelectric power factor in a Fe3O4/SiO2/p-type Si(100)heterostructure

The thermoelectric and transport properties of a Fe3O4/SiO2/p-Si(100) heterostructure have been investigated between 100 and 300 K. Both Hall and Seebeck coefficients change sign from negative to positive with increasing temperature while the resistivity drops sharply due to tunneling of carriers into the p-Si(100). The low resistivity and large Seebeck coefficient of Si give a very high thermoelectric power factor of 25.5mW/K2m at 260K which is an underestimated, lower limit value and is related to the density of states and difference in the work functions of Fe3O4 and Si(100) that create an accumulation of majority holes at the p-Si/SiO2 interface

cond-mat.mtrl-sci↗

arXiv2014

Current transport and thermoelectric properties of very high power factor Fe3O4 / SiO2 / p-type Si (001) devices

The current transport and thermoelectric properties of Fe3O4 / SiO2 / p-type Si(001) heterostructures with Fe3O4 thicknesses of 150, 200, and 350 nm have been investigated between 100 and 300 K. We observe a sharp drop of the in-plane resistivity at 200K due to the onset of conduction along the Si / SiO2 interface related to tunneling of electrons from the Fe3O4 into the accumulation layer of holes at the Si / SiO2 interface, whose existence was confirmed by capacitance-voltage measurements and a two band analysis of the Hall effect. This is accompanied by a large increase of the Seebeck coefficient reaching +1000 μV/K at 300K that is related to holes in the p-type Si(001) and gives a power factor of 70 mW/K2m when the Fe3O4 layer thickness is reduced down to 150 nm. We show that most of the current flows in the Fe3O4 layer at 300 K, while the Fe3O4 / SiO2 / p-type Si(001) heterostructures behave like tunneling p-n junctions in the transverse direction.

cond-mat.mtrl-sci↗

arXiv2013

Structural, electrical and magnetic studies of Co:SnO2 and (Co,Mo):SnO2 films prepared by pulsed laser deposition

Here we report on the structural, optical, electrical and magnetic properties of Co-doped and (Co,Mo)-codoped SnO2 thin films deposited on r-cut sapphire substrates by pulsed laser deposition. Substrate temperature during deposition was kept at 500 C. X-ray diffraction analysis showed that the undoped and doped films are crystalline with predominant orientation along the [101] direction regardless of the doping concentration and doping element. Optical studies revealed that the presence of Mo reverts the blue shift trend observed for the Co-doped films. For the Co and Mo doping concentrations studied, the incorporation of Mo did not contribute to increase the conductivity of the films or to enhance the ferromagnetic order of the Co-doped films.

cond-mat.mtrl-sci↗

arXiv2011

Cr2O3 thin films grown at room temperature by low pressure laser chemical vapour deposition

Chromia (Cr2O3) has been extensively explored for the purpose of developing widespread industrial applications, owing to the convergence of a variety of mechanical, physical and chemical properties in one single oxide material. Various methods have been used for large area synthesis of Cr2O3 films. However, for selective area growth and growth on thermally sensitive materials, laser-assisted chemical vapour deposition (LCVD) can be applied advantageously. Here we report on the growth of single layers of pure Cr2O3 onto sapphire substrates at room temperature by low pressure photolytic LCVD, using UV laser radiation and Cr(CO)6 as chromium precursor. The feasibility of the LCVD technique to access selective area deposition of chromia thin films is demonstrated. Best results were obtained for a laser fluence of 120 mJ cm-2 and a partial pressure ratio of O2 to Cr(CO)6 of 1.0. Samples grown with these experimental parameters are polycrystalline and their microstructure is characterised by a high density of particles whose size follows a lognormal distribution. Deposition rates of 0.1 nm s-1 and mean particle sizes of 1.85 μm were measured for these films.

cond-mat.mtrl-sci↗

arXiv2007

CVD of CrO2: towards a lower temperature deposition process

We report on the synthesis of highly oriented a-axis CrO2 films onto (0001) sapphire by atmospheric pressure CVD from CrO3 precursor, at growth temperatures down to 330 degree Celsius, i.e. close to 70 degrees lower than in published data for the same chemical system. The films keep the high quality magnetic behaviour as those deposited at higher temperature, which can be looked as a promising result in view of their use with thermally sensitive materials, e.g. narrow band gap semiconductors.

cond-mat.mtrl-sci↗

arXiv2006

CVD of CrO2 Thin Films: Influence of the Deposition Parameters on their Structural and Magnetic Properties

This work reports on the synthesis of CrO2 thin films by atmospheric pressure CVD using chromium trioxide (CrO3) and oxygen. Highly oriented (100) CrO2 films containing highly oriented (0001) Cr2O3 were grown onto Al2O3(0001) substrates. Films display a sharp magnetic transition at 375 K and a saturation magnetization of 1.92 Bohr magnetons per f.u., close to the bulk value of 2 Bohr magnetons per f.u. for the CrO2. Keywords: Chromium dioxide (CrO2), Atmospheric pressure CVD, Spintronics.

cond-mat.mtrl-sci↗

arXiv2006

KrF pulsed laser deposition of chromium oxide thin films from Cr8O21 targets

Chromium oxides, CrxOy, are of great interest due to the wide variety of their technological applications. Among them, CrO2 has been extensively investigated in recent years because it is an attractive compound to be used in spintronic heterostructures. However, its synthesis at low temperatures has been a difficult task due to the metastable nature of this oxide. This is indeed essential to ensure interface quality and the ability to coat thermal-sensitive materials such as those envisaged in spintronic devices. Pulsed Laser Deposition (PLD) is a technique that has the potential to meet the requirements stated above. In this work, we describe our efforts to grow chromium oxide thin films by PLD from Cr8O21 targets, using a KrF excimer laser. The as-deposited films were investigated by X-ray diffraction and Rutherford backscattering spectrometry. Structural and chemical composition studies showed that the films consist of a mixture of amorphous chromium oxides exhibiting different stoichiometries depending on the processing parameters, where nanocrystals of mainly Cr2O3 are dispersed. The analyses do not exclude the possibility of co-deposition of Cr2O3 and a low fraction of CrO2. PACS: 81.15.Fg, 75.50.Dd

cond-mat.mtrl-sci↗

arXiv2006

Morphological and Structural Characterization of Cro2/Cr2o3 Films Grown by Laser-CVD

This work reports on the synthesis of chromium (III, IV) oxides films by KrF laser-assisted CVD. Films were deposited onto sapphire substrates at room temperature by photodissociation of Cr(CO)6 in dynamic atmospheres containing oxygen and argon. A study of the processing parameters has shown that partial pressure ratio of O2 to Cr(CO)6 and laser fluence are the prominent parameters that have to be accurately controlled in order to co-deposit both crystalline oxide phases. Films consistent with such a two-phase system were synthesised for a laser fluence of 75 mJ cm-2 and a partial pressure ratio about 1. PACS: 81.15.Fg, 81.15.Kk, 81.05.Je

cond-mat.mtrl-sci↗

arXiv2002

TiN Films Deposited by Laser CVD: A Growth Kinetics Study

Results on the chemical composition, structure and growth kinetics of titanium nitride (TiN) films deposited on mild steel substrates by pyrolytic laser-induced chemical vapour deposition (LCVD) are presented. Golden coloured lines of TiN were deposited from a reactive gas mixture of TiCl4, N2 and H2 using a continuous wave TEM00 CO2 laser beam as heat source. The chemical composition and structure of the films were determined by electron probe microanalysis (EPMA) and glancing incidence X-ray diffraction (GIXRD). A non-contact laser profilometer was used to measure the thickness profiles of the films. Using the data obtained in the steady-state region of the TiN laser-written lines, growth rates in the range 3.7 to 6.9 micrometers per second were deduced. The Arrhenius relation between the deposition rate and the deposition temperature yields an apparent activation energy of 46.9(+-)3.8 kJ.mol-1. This result enabled us to conclude that under our deposition conditions the LCVD of TiN is controlled by mass transport in the vapour phase. Keywords: Coatings; Titanium nitride (TiN); Laser chemical vapour deposition (LCVD); Growth mechanisms.

cond-mat.mtrl-sci↗

arXiv2002

Laser-Assisted Deposition of r-B4C Coatings Using Ethylene as Carbon Precursor

Rhombohedral B4C coatings were synthesised on fused silica substrates by CO2 laser-assisted chemical vapour deposition (LCVD), using a dynamic reactive atmosphere of BCl3, C2H4 and H2. Films with carbon content from 15 to 22 at.% were grown at deposition rates as high as 0.12 micrometers per second. The kinetics of the reactive system used to deposit the B4C films and the influence of growth conditions on the structure and morphology of the deposits were investigated. Keywords: Rhombohedral boron carbide (r-B4C), Laser-CVD, growth kinetics.

cond-mat.mtrl-sci↗

arXiv2002

The Role of Carbon Precursor on Boron Carbide Synthesis by Laser-CVD

This paper focuses on the synthesis of rhombohedral B4C (r-B4C) coatings by CO2 laser-assisted chemical vapour deposition (LCVD), using a dynamic reactive atmosphere of BCl3, H2 and CH4 or C2H4. The influence of the carbon precursor on the deposition kinetics is discussed. The use of ethylene as carbon precursor presents several advantages over the use of methane, which is the conventional carbon precursor in CVD processes. These advantages are mainly related to its high absorption coefficient at the laser wavelength and a higher sticking coefficient, which enables to attain higher deposition rates and film thickness control at lower carbon precursor concentration. Films with carbon content from 15 to 22 at.% were grown at a deposition rate as high as 0.12 micrometers per second. Keywords: Rhombohedral boron carbide (r-B4C), Laser CVD.

cond-mat.mtrl-sci↗

arXiv2002

Influence of Carbon Content on the Crystallographic Structure of Boron Carbide Films

Boron carbide thin films were synthesised by laser-assisted chemical vapour deposition (LCVD), using a CO2 laser beam and boron trichloride and methane as precursors. Boron and carbon contents were measured by electron probe microanalysis (EPMA). Microstructural analysis was carried out by Raman microspectroscopy and glancing incidence X-ray diffraction (GIXRD) was used to study the crystallographic structure and to determine the lattice parameters of the polycrystalline films. The rhombohedral-hexagonal boron carbide crystal lattice constants were plotted as a function of the carbon content, and the non-linear observed behaviour is interpreted on the basis of the complex structure of boron carbide. Keywords: Boron carbide; Laser CVD; crystallographic structure; micro-Raman spectroscopy.

cond-mat.mtrl-sci↗