101 research outputs found
Effect of impurities on morphology and growth mode of (111) and (001) epitaxial-like ScN films
ScN material is an emerging semiconductor with an indirect bandgap. It has
attracted attention for its thermoelectric properties, use as seed layers, and
for alloys for piezoelectric application. ScN or other transition metal nitride
semiconductors used for their interesting electrical properties are sensitive
to contaminants, such as oxygen or fluorine. In this present article, the
influence of depositions conditions on the amount of oxygen contaminants
incorporated in ScN films were investigated and their effects on the electrical
properties (electrical resistivity and Seebeck coefficient) were studied. The
epitaxial-like films of thickness 125 +-5 nm to 155 +-5 nm were deposited by
D.C.-magnetron sputtering on c-plane Al2O3, MgO(111) and r-plane Al2O3 at a
substrate temperature ranging from 700 to 950 degree C. The amount of oxygen
contaminants presents in the film, dissolved into ScN or as an oxide, was
related to the adatom mobility during growth, which is affected by the
deposition temperature and the presence of twin domain growth. The lowest
values of electrical resistivity of 50 micro-ohm cm were obtained on
ScN(111)/MgO(111) and on ScN(001)/r-plane Al2O3 grown at 950 degree C with no
twin domains and the lowest amount of oxygen contaminant. At the best, the
films exhibited an electrical resistivity of 50 micro-ohm cm with Seebeck
coefficient values maintained at -40 microV K-1, thus a power factor estimated
at 3.2 10-3 W m-1 K-2 (at room temperature)
An upgraded ultra-high vacuum magnetron-sputtering system for high-versatility and software-controlled deposition
Magnetron sputtering is a widely used physical vapor deposition technique.
Reactive sputtering is used for the deposition of, e.g, oxides, nitrides and
carbides. In fundamental research, versatility is essential when designing or
upgrading a deposition chamber. Furthermore, automated deposition systems are
the norm in industrial production, but relatively uncommon in laboratory-scale
systems used primarily for fundamental research. Combining automatization and
computerized control with the required versatility for fundamental research
constitutes a challenge in designing, developing, and upgrading laboratory
deposition systems. The present article provides a detailed description of the
design of a lab-scale deposition chamber for magnetron sputtering used for the
deposition of metallic, oxide, nitride and oxynitride films with automated
controls, dc or pulsed bias, and combined with a coil to enhance the plasma
density near the substrate. LabVIEW software (provided as Supplementary
Information) has been developed for a high degree of computerized or automated
control of hardware and processes control and logging of process details.Comment: 17 pages, 8 figure
Thermoelectric properties and electronic structure of Cr(Mo,V)Nx thin films studied by synchrotron and lab-based X-ray spectroscopy
Chromium-based nitrides are used in hard, resilient coatings, and show
promise for thermoelectric applications due to their combination of structural,
thermal, and electronic properties. Here, we investigated the electronic
structures and chemical bonding correlated to the thermoelectric properties of
epitaxially grown chromium-based multicomponent nitride Cr(Mo,V)Nx thin films.
Due to minuscule N vacancies, finite population of Cr 3d and N 2p states appear
at the Fermi level and diminishes the band opening for Cr0.51N0.49.
Incorporating holes by alloying V in N deficient CrN matrix results in enhanced
thermoelectric power factor with marginal change in the charge transfer of Cr
to N compared to Cr0.51N0.49. Further alloying Mo isoelectronic to Cr increases
the density of states across the Fermi level due to hybridization of the (Cr,
V) 3d and Mo 4d-N 2p states in Cr(Mo,V)Nx. The hybridization effect with
reduced N 2p states off from stoichiometry drives the system towards metal like
electrical resistivity and reduction in Seebeck coefficient compensating the
overall power factor still comparable to Cr0.51N0.49. The N deficiency also
depicts a critical role in reduction of the charge transfer from metal to N
site. The present work envisages ways for enhancing thermoelectric properties
through electronic band engineering by alloying and competing effects of N
vacancies.Comment: 27 pages, 11 figures, 2 table
Study of ferroelectric/dielectric multilayers for tunable stub resonator applications at microwaves
International audienceTunable coplanar waveguide stub resonators deposited on various ferroelectric/dielectric heterostructures are studied in the 10-GHz band. A frequency tunability of up to ~ 45% is achieved under a moderate biasing field (Ebias < 100 kV/cm) when the resonator is printed on KTa0.5Nb0.5O3 (KTN) ferroelectric thin film alone: this comes from the large permittivity agility of the KTN material (εr(KTN) varies from ~ 700 to ~ 200). Nevertheless this also leads to significant insertion loss due to the dielectric loss of the ferroelectric material itself (tanδr(KTN) ≈ 0.15-0.30 at 10 GHz). In this paper, an original route has been considered to reduce the device loss while keeping up a high frequency tunability. It consists in associating the KTN film with a dielectric film to elaborate ferroelectric/dielectric multilayers. The Bi1.5Zn0.9Nb1.5O7−δ (BZN) oxide material is selected here for two main reasons, namely its low dielectric loss (tanδr(BZN) ≈ 0.005-0.0075) and its moderate relative permittivity (εr(BZN) ≈ 95-125) at 12.5 GHz. The relevance of this approach is studied numerically and experimentally. We compare numerically two different heterostructures for which the ferroelectric film is grown on the dielectric film (KTN/BZN), or vice versa (BZN/KTN). A stub resonator printed on the most relevant heterostructure has been fabricated, and experimental data are discussed and compared to the numerical results
Loss reduction technique in ferroelectric tunable devices by laser micro-etching. Application to a CPW stub resonator in X-band
International audienceFerroelectric materials are known to be lossy at microwaves. A local microetching technique based on laser ablation is implemented here to reduce the insertion loss of highly tunable devices fabricated on KTa1-xNbxO3 (KTN) ferroelectric thin films. The relevance of this approach is studied in X-band by comparing numerically and experimentally the performance of a frequency-tunable coplanar waveguide stub resonator before and after KTN microetching. The experimental data demonstrate a large loss reduction (by a factor 3.3), while keeping a high-frequency tunability (47%) under a moderate biasing static electric field (80 kV/cm). This approach paves the way for the design of ferroelectric reconfigurable devices with attractive performance in X-band and even beyond
Cистемна організація уваги та загальні принципи її корекції як засіб оптимізації учбової та професійної діяльності
У статті викладено результати літературних та експериментальних досліджень, у яких розкриваються основні напрямки, котрі стосуються феномени уваги:механізми її виникнення, типи уваги, функції контролю та управління. Розглянуто ряд методів, котрі можуть бути використані для тренінгів, з метою збільшення об’єму уваги, ступеня її концентрації, швидкості, ступеня зосередження, переключення і розподілу
Valence electron concentration- and N vacancy-induced elasticity in cubic early transition metal nitrides
Motivated by frequently reported deviations from stoichiometry in cubic
transition metal nitride (TMNx) thin films, the effect of N-vacancy
concentration on the elastic properties of cubic TiNx, ZrNx, VNx, NbNx, and
MoNx (0.72<x<1.00) is systematically studied by density functional theory (DFT)
calculations. The predictions are validated experimentally for VNx
(0.77<x<0.97). The DFT results indicate that the elastic behavior of the TMNx
depends on both the N-vacancy concentration and the valence electron
concentration (VEC) of the transition metal: While TiNx and ZrNx exhibit
vacancy-induced reductions in elastic modulus, VNx and NbNx show an increase.
These trends can be rationalized by considering vacancy-induced changes in
elastic anisotropy and bonding. While introduction of N-vacancies in TiNx
results in a significant reduction of elastic modulus along all directions and
a lower average bond strength of Ti-N, the vacancy-induced reduction in [001]
direction of VNx is overcompensated by the higher stiffness along [011] and
[111] directions, resulting in a higher average bond strength of V-N. To
validate the predicted vacancy-induced changes in elasticity experimentally,
close-to-single-crystal VNx (0.77<x<0.97) are grown on MgO(001) substrates. As
the N-content is reduced, the relaxed lattice parameter a0, as probed by X-ray
diffraction, decreases from 4.128 A to 4.096 A. This reduction in lattice
parameter is accompanied by an anomalous 11% increase in elastic modulus, as
determined by nanoindentation. As the experimental data agree with the
predictions, the elasticity enhancement in VNx upon N-vacancy formation can be
understood based on the concomitant changes in elastic anisotropy and bonding.Comment: 30 pages, 8 figures in the manuscript, 1 figure in supplementary
material
Intercomparison of Permittivity Measurement Techniques for Ferroelectric Thin Layers
International audienceThe dielectric properties of a KTa0.65Nb0.35O3 (KTN) ferroelectric composition for a submicronic thin layer were measured in the microwave domain using different electromagnetic characterization methods. Complementary experimental techniques (broadband methods versus resonant techniques, waveguide versus transmission line) and complementary data processing procedures (quasi-static theoretical approaches versus full-wave analysis) were selected to investigate the best way to characterize ferroelectric thin films. The measured data obtained from the cylindrical resonant cavity method, the experimental method that showed the least sources of uncertainty, were taken as reference values for comparisons with results obtained using broadband techniques. The error analysis on the methods used is discussed with regard to the respective domains of validity for each method; this enabled us to identify the best experimental approach for obtaining an accurate determination of the microwave dielectric properties of ferroelectric thin layers
Formation mechanism and thermoelectric properties of CaMnO3 thin films synthesized by annealing of Ca0.5Mn0.5O films
International audienceA two-step synthesis approach was utilized to grow CaMnO 3 on M-, R-and C-plane sapphire substrates. Radio-frequency reactive magnetron sputtering was used to grow rock-salt-structured (Ca, Mn)O followed by a 3-h annealing step at 800 °C in oxygen flow to form the distorted perovskite phase CaMnO 3. The effect of temperature in the post-annealing step was investigated using x-ray diffraction. The phase transformation to CaMnO 3 started at 450 °C and was completed at 550 °C. Films grown on R-and C-plane sapphire showed similar structure with a mixed orientation, whereas the film grown on M-plane sapphire was epitaxially grown with an out-of-plane orientation in the [202] direction. The thermoelectric characterization showed that the film grown on M-plane sapphire has about 3.5 times lower resistivity compared to the other films with a resistivity of 0.077 Xcm at 500 °C. The difference in resistivity is a result from difference in crystal structure, single orientation for M-plane sapphire compared to mixed for R-and C-plane sapphire. The highest absolute Seebeck coefficient value is-350 lV K-1 for all films and is decreasing with temperature
Phase formation in CrFeCoNi nitride thin films
As a single-phase alloy, CrFeCoNi is a face centered cubic (fcc) material
related to the archetypical high-entropy Cantor alloy CrFeCoNiMn. For thin
films, CrFeCoNi of approximately equimolar composition tends to assume an fcc
structure when grown at room temperature by magnetron sputtering. However, the
single-phase solid solution state is typically not achieved for thin films
grown at higher temperatures. The same holds true for Cantor alloy-based
ceramics (nitrides and oxides), where phase formation is extremely sensitive to
process parameters such as the amount of reactive gas. This study combines
theoretical and experimental methods to understand the phase formation in
nitrogen-containing CrFeCoNi thin films. Density functional theory calculations
considering three competing phases (CrN, Fe-Ni and Co) show that the free
energy of mixing, delta G of (CrFeCoNi)1-xNx solid solutions has a maximum at x
= 0.20-0.25, and delta G becomes lower when x less than 0.20, greater than
0.25. Thin films of (CrFeCoNi)1-xNx (x = 0.14-0.41) grown by magnetron
sputtering show stabilization of the metallic fcc when x lesser than or equal
to 0.22 and the stabilization of the NaCl B1 structure when x is greater than
0.33, consistent with the theoretical prediction. In contrast, films with
intermediate amounts of nitrogen (x = 0.22) grown at higher temperatures show
segregation into multiple phases of CrN, Fe-Ni-rich and Co. These results offer
an explanation for the requirement of kinetically limited growth conditions at
low temperature for obtaining single-phase CrFeCoNi Cantor-like
nitrogen-containing thin films and are of importance for understanding the
phase-formation mechanisms in multicomponent ceramics
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