Development of membrane-based calorimeters to measure phase transitions at the nanoscale

Consultable des del TDXTítol obtingut de la portada digitalitzadaLa nanocalorimetria abre la posibilidad de realizar medidas calorimétricas en capas finas o ultra finas debido al substancial aumento de sensibilidad que presenta respecto a los sistemas comerciales. Basándonos en esa premisa, este trabajo de investigación se ha dedicado al desarrollo de nanocalorimetros basados en membranas que incorporan calentadores y termómetros en capa fina, que permiten reducir la capacidad calorífica de la celda calorimétrica y por tanto al aumento den sensibilidad. En la primera parte de la tesis describimos las técnicas de procesado de semiconductores que se usan para fabricar los microdispositivos con una especial atención a su estabilidad térmica a alta temperatura. Se muestra que usando como actuador una la combinación metálica de Pt/Ti cubierta por Al2O3, se puede utilizar con gran reproducibilidad para calentar/sensar hasta temperaturas de alrededor de 1200K. La nanocalorimetria adiabática se presenta en el capitulo 4. La técnica trabaja a ritmos de calentamiento alrededor de 104 K/s. A estos ritmos de calentamiento se pueden estudiar transformaciones de fase en capas ultrafinas con una sensitividad en energía inferior al nJ. El ruido pico a pico asociado a las medidas de capacidad calorífica es de alrededor a 20 pJ/K, para transformaciones reversibles. La dependencia con el tamaño del punto de fusión y de la entalpía de transformación para capas finas de In se han analizado como estudio preliminar. También presentamos una nueva metodología para poder evaluar la potencia de pérdidas a altas temperaturas. Empleando esta metodología se ha determinado la capacidad calorífica de capas muy finas de Ni alrededor de la transición de curie. Se presencia un estudio en el que se evidencia como los efectos de tamaño tienen un rol fundamental en la transición. La última parte de este capítulo presenta el análisis de capas ultrafinas de Ge encapsuladas entre capas de SiO2 cuando son sometidas a calentamientos ultrarápidos hasta 1200K. Se describen la transformación de amorfo a líquido así como la dependencia con el tamaño de la fusión y el sobreenfriamiento de nanocristales de Ge. En el capítulo 5 presentamos un nuevo sistema de control digital que hemos desarrollado para trabajar con los nanocalorímetros en modo compensación de potencia a ritmos de calentamiento que se expanden desde 0.1 hasta 103 K/s. Este sistema ha permitido analizar muestras de microgramos con sensitividades energéticas de µJ. Este nuevo desarrollo abre la posibilidad al estudio de transformaciones cinéticamente limitadas que típicamente necesitan de ritmos de calentamiento bajos, como por ejemplo para analizar los procesos de RTA (rápidos recocidos térmicos) usados en la industria microelectrónica. Finalmente, en los apéndices tratamos la teoría de control calorimetría y la cristalización de capas finas de Ge de diferentes espesores.Thin film calorimetry opens the possibility to perform calorimetric measurements on ultra-thin or thin films due to the substantial increase in sensitivity compared to commercial systems. Based on this premise, the present research work deals with the development of membrane-based nanocalorimeters incorporating thin film heaters and thermometers which can work with high sensitivity because of their very low thermal mass. In the first part we describe semiconductor processing techniques that are used to fabricate the microdevices with a special care devoted to their high temperature thermomechanical stability. It is shown that alumina coated Pt/Ti resistive elements can be reproducibly used for heating/sensing up to temperatures around 1200 K. Quasi-adiabatic nanocalorimetry is presented in Chapter 4. The technique works at heating rates above 104 K/s. At these rates phase transitions in ultra-thin films can be measured with energy sensitivity in the nJ range. The associated noise in heat capacity is around 20 pJ/K, for reversible transitions. The size-dependent melting point and enthalpy of ultra-thin films of In is analyzed as a case study. A new methodology to account for power losses at high temperatures is presented in this chapter. By employing this methodology the heat capacity of very thin films of Ni at the Curie transition is determined. It is shown that size effects also play a key role in this transition. The last part of this chapter is devoted to the analysis of ultra-thin films of Ge embedded within SiO2 layers during ultrafast heating up to 1200K. The melting of the amorphous phase along with the size-dependent melting and supercooling of Ge nanocrystals is also described. In Chapter 5 we present a new digital-based control system which has been developed to work in power compensation mode at heating rates spanning from 0.1 to 103 K/s. It permits to analyze samples in the microgram range with a energy sensitivity around the µJ. This new development opens the possibility to study kinetically limited transformations that typically need for lower rates or to mimic real conditions similar to those achieved in rapid thermal processing in the microelectronic industry. Finally, several appendix dealing with control theory, calorimetry and the crystallization of Ge films of different thickness are also presented

Nanocalorimetric analysis of the ferromagnetic transition in ultrathin films of nickel

We report on in situheat capacity measurements (370-800K) using quasiadiabatic ultrafast differential scanning nanocalorimetry in thin films(1-200nm) of Nigrown by electron beam evaporation. The heat capacity shows a broad peak with a rounded maximum that is attributed to the decrease of long-range interactions in the ferromagnetic to paramagnetic phase transition of Ni. The calorimetric data exhibit a reduction of the Curie temperature as the thickness of the films (or the average grain size) decreases. The magnitude of the jump in specific heat at TC scales with the number of surface or interface atoms

Growth and characterization of CoO ultra thin films

In this report we present the growth process of the cobalt oxide system using reactive electron beam deposition. In that technique, a target of metallic cobalt is evaporated and its atoms are in-flight oxidized in an oxygen rich reactive atmosphere before reaching the surface of the substrate. With a trial and error procedure the deposition parameters have been optimized to obtain the correct stoichiometry and crystalline phase. The evaporation conditions to achieve the correct cobalt oxide salt rock structure, when evaporating over amorphous silicon nitride, are: 525 K of substrate temperature, 2.5·10-4 mbar of oxygen partial pressure and 1 Å/s of evaporation rate. Once the parameters were optimized a set of ultra thin film ranging from samples of 1 nm of nominal thickness to 20nm thick and bulk samples were grown. With the aim to characterize the samples and study their microstructure and morphology, X-ray diffraction, transmission electron microscopy, electron diffraction, energy dispersive X-ray spectroscopy and quasi-adiabatic nanocalorimetry techniques are utilised. The final results show a size dependent effect of the antiferromagnetic transition. Its Néel temperature becomes depressed as the size of the grains forming the layer decreases.Un profund estudi s'ha realitzat sobre el sistema d'òxid de cobalt. El coneixement de la seva microestructura ens permetrà establir la relació amb les propietats magnètiques. Les mostres són preparades mitjançant un evaporador de feix d'electrons seguint un procediment d'assaig i error. Un blanc de cobalt metàl·lic és evaporat i els àtoms són oxidats durant el seu desplaçament dins d'una atmosfera reactiva d'oxigen. Diverses mostres son estudiades amb gruixos que van de 1 fins 20 nm i mostres massives. Les tècniques emprades són difracció de raigs X i microscòpia electrònica de transmissió per a la caracterització morfològica i estructural i nanocalorimetría per a la caracterització magnètica

Evidence of thermal transport anisotropy in stable glasses of vapour deposited organic molecules

Vapour-deposited organic glasses are currently in use in many optoelectronic devices. Their operation temperature is limited by the glass transition temperature of the organic layers and thermal management strategies become increasingly important to improve the lifetime of the device. Here we report the unusual finding that molecular orientation heavily influences heat flow propagation in glassy films of small molecule organic semiconductors. The thermal conductivity of vapour-deposited thin-film semiconductor glasses is anisotropic and controlled by the deposition temperature. We compare our data with extensive molecular dynamics simulations to disentangle the role of density and molecular orientation on heat propagation. Simulations do support the view that thermal transport along the backbone of the organic molecule is strongly preferred with respect to the perpendicular direction. This is due to the anisotropy of the molecular interaction strength that limit the transport of atomic vibrations. This approach could be used in future developments to implement small molecule glassy films in thermoelectric or other organic electronic devices.Comment: main manuscript: 17 pages and 7 figures; supplementary material: 6 pages and 7 figure

Multiple glass transitions in vapor-deposited orientational glasses of the most fragile plastic crystal Freon 113

We investigate by fast-scanning nanocalorimetry the formation of Freon 113 films from the vapor phase at deposition temperatures ranging from 50 to 120 K, that is, spanning above and below the transition temperature of the glassy crystal to the plastic crystal (Tgc = 72 K). Analysis of the heat capacity curves indicates that vapor deposition at T < Tgc of the highly fragile Freon 113 yields structural and orientational glasses in the as-deposited state depending on the temperature range of deposition. Interestingly, growing above Tgc produces plastic crystals with a conformational ratio C1/Cs that changes with Tdep above and below 110–120 K, the temperature at which previous works have identified the arrest of the transformations between the C1 and Cs conformers.Peer ReviewedPostprint (author's final draft

Formation of Pd2Si on single-crystalline Si (100) at ultrafast heating rates : an in-situ analysis by nanocalorimetry

The kinetics of intermediate phase formation between ultrathin films of Pd (12 nm) and single-crystalline Si (100) is monitored by in-situ nanocalorimetry at ultrafast heating rates. The heat capacity curves show an exothermic peak related to the formation of Pd2Si. A kinetic model which goes beyond the conventional linear-parabolic growth to consider independent nucleation and lateral growth of Pd2Si along the interface and vertical growth mechanisms is developed to fit the calorimetric curves. The model is used to extract the effective interfacial nucleation/growth and diffusion coefficients at the unusually high temperatures of silicide formation achieved at very fast heating rates

Regulating oxygen ion transport at the nanoscale to enable highly cyclable magneto-ionic control of magnetism

Altres ajuts: acords transformatius de la UABMagneto-ionics refers to the control of magnetic properties of materials through voltage-driven ion motion. To generate effective electric fields, either solid or liquid electrolytes are utilized, which also serve as ion reservoirs. Thin solid electrolytes have difficulties to (i) withstand high electric fields without electric pinholes and (ii) maintain stable ion transport during long-term actuation. In turn, the use of liquid electrolytes can result in poor cyclability, thus limiting their applicability. Here we propose a nanoscale-engineered magneto-ionic architecture (comprising a thin solid electrolyte in contact with a liquid electrolyte), that drastically enhances cyclability while preserving sufficiently high electric fields to trigger ion motion. Specifically, we show that the insertion of a highly nanostructured (amorphous-like) Ta layer (with suitable thickness and electric resistivity) between a magneto-ionic target material (i.e., Co3O4) and the liquid electrolyte, increases magneto-ionic cyclability from < 30 cycles (when no Ta is inserted) to more than 800 cycles. Transmission electron microscopy together with variable energy positron annihilation spectroscopy reveal the crucial role of the generated TaOx interlayer as a solid-electrolyte (i.e., ionic conductor) that improves magneto-ionic endurance by proper tuning of the types of voltage-driven structural defects. The Ta layer is very effective in trapping oxygen and hindering O2- ions from moving into the liquid electrolyte, thus keeping O2- motion mainly restricted between Co3O4 and Ta when voltage of alternating polarity is applied. We demonstrate that this approach provides a suitable strategy to boost magneto-ionics by combining the benefits of solid and liquid electrolytes in a synergetic manner

Magneto-ionics in single-layer transition metal nitrides

Altres ajuts: Acord transformatiu CRUE-CSICMagneto-ionics allows for tunable control of magnetism by voltage-driven transport of ions, traditionally oxygen or lithium and, more recently, hydrogen, fluorine, or nitrogen. Here, magneto-ionic effects in single-layer iron nitride films are demonstrated, and their performance is evaluated at room temperature and compared with previously studied cobalt nitrides. Iron nitrides require increased activation energy and, under high bias, exhibit more modest rates of magneto-ionic motion than cobalt nitrides. Ab initio calculations reveal that, based on the atomic bonding strength, the critical field required to induce nitrogen-ion motion is higher in iron nitrides (≈6.6 V nm -1) than in cobalt nitrides (≈5.3 V nm -1). Nonetheless, under large bias (i.e., well above the magneto-ionic onset and, thus, when magneto-ionics is fully activated), iron nitride films exhibit enhanced coercivity and larger generated saturation magnetization, surpassing many of the features of cobalt nitrides. The microstructural effects responsible for these enhanced magneto-ionic effects are discussed. These results open up the potential integration of magneto-ionics in existing nitride semiconductor materials in view of advanced memory system architectures

Kinetics of silicide formation over a wide range of heating rates spanning six orders of magnitude

Kinetic processes involving intermediate phase formation are often assumed to follow an Arrhenius temperature dependence. This behavior is usually inferred from limited data over narrow temperature intervals, where the exponential dependence is generally fully satisfied. However, direct evidence over wide temperature intervals is experimentally challenging and data are scarce. Here, we report a study of silicide formation between a 12 nm film of palladium and 15 nm of amorphous silicon in a wide range of heating rates, spanning six orders of magnitude, from 0.1 to 105 K/s, or equivalently more than 300 K of variation in reaction temperature. The calorimetric traces exhibit several distinct exothermic events related to interdiffusion, nucleation of Pd2Si, crystallization of amorphous silicon, and vertical growth of Pd2Si. Interestingly, the thickness of the initial nucleation layer depends on the heating rate revealing enhanced mass diffusion at the fastest heating rates during the initial stages of the reaction. In spite of this, the formation of the silicide strictly follows an Arrhenius temperature dependence over the whole temperature interval explored. A kinetic model is used to fit the calorimetric data over the complete heating rate range. Calorimetry is complemented by structural analysis through transmission electron microscopy and both standard and in-situ synchrotron X-ray diffraction

Boosting room-temperature magneto-ionics in a non-magnetic oxide semiconductor

Voltage control of magnetism through electric field-induced oxygen motion (magneto-ionics) could represent a significant breakthrough in the pursuit for new strategies to enhance energy efficiency in magnetically actuated devices. Boosting the induced changes in magnetization, magneto-ionic rates and cyclability continue to be key challenges to turn magneto-ionics into real applications. Here, it is demonstrated that room-temperature magneto-ionic effects in electrolyte-gated paramagnetic Co3O4 films can be largely increased both in terms of generated magnetization (6 times larger) and speed (35 times faster) if the electric field is applied using an electrochemical capacitor configuration (utilizing an underlying conducting buffer layer) instead of placing the electric contacts at the side of the semiconductor (electric-double-layer transistor-like configuration). This is due to the greater uniformity and strength of the electric field in the capacitor design. These results are appealing to widen the use of ion migration in technological applications such as neuromorphic computing or iontronics in general