Magnetiliselt ja elektriliselt polariseeruvate õhukeste tahkiskilede aatomkihtsadestamine ja mikroskoopiline analüüs

Väitekirja elektrooniline versioon ei sisalda publikatsiooneTehnoloogia kiire areng on olnud võimalik tänu uute materjalide loomisele ja tehnoloogiliste meetodite rakendamisele. Selleks, et see areng elektroonika ja infotehnoloogia valdkonnas ka edaspidi jätkuda saaks, püsib pidevalt vajadus luua uudseid spetsiifiliste omadustega materjale, mida loodavates seadmetes kasutada saaks. Muuhulgas soovitakse saada selliseid õhukesi materjale, mis oleksid oma olemuselt multiferroidsete omadustega. Sellised materjalid on nii ferroelektriliste kui ka ferromagnetiliste omadustega ehk need materjalid polariseeruvad nii välise elektri- kui ka magnetvälja toimel ning on võimelised polarisatsiooni säilitama ka siis kui neile pidevalt välist elektri- või magnetvälja peale ei rakendata. Selliseid materjale soovitakse kasutada muuhulgas uue põlvkonna mäluseadmetes, kuna andmete salvestamiseks ja lugemiseks saaks sellisel juhul kasutada nii elektri kui ka magnetvälja. Sellest tulenevalt sadestati ja karakteriseeriti doktoritöös selliseid uudseid materjalikombinatsioone, mis võiksid olla toatemperatuuril magnetiliselt ja elektriliselt polariseeruvad. Selleks loodi tehislikult aatomkihtsadestamise nimelist meetodit kasutades mitmekihilised õhukesed nanomaterjalid (nanolaminaadid) ja segukihid (ka segukiled) kuhu liideti kokku potentsiaalselt ferromagnetiliste ja ferroelektriliste omadustega materjalid (peamiselt metalloksiidid). Töös kasutati materjalide valmistamiseks aatomkihtsadestamise meetodit, kuna see meetod on laialdaselt kasutuses muuhulgas mikroelektroonika valdkonnas ning võimaldab sadestada kõrge kvaliteedi ja kontrollitava paksusega õhukesi materjale, mida nimetatakse ka tahkiskihtideks. Doktoritööst selgus, et sadestatud materjalide omadused sõltusid lisaks valitud materjalikombinatsioonidele ootuspäraselt nii faasikoostisest kui ka sellest, kas tegu oli kihilise objektiga või segukilega. Oma positiivsete omaduste poolest paistsid silma mitmete metalloksiidide metastabiilsed faasid, mis mõjutasid sadestatud kilede magnetilisi ja elektrilisi omadusi, ning mille domineerimine faasikoostises oli kohati võimalik tänu sellele, et tegu oli kihiliste objektidega. Enamus sadestatud materjalidest näitasid ferromagnetilistele materjalidele omast käitumist polariseerudes välise magnetvälja mõjul ning osad materjalid käitusid ferroelektrilisele materjalile omaselt polariseerudes välise elektrivälja mõjul.The constant development of technology has been possible thanks to the development of new materials and technological methods. However, there remains a continual demand for new novel materials or material combinations with new or improved functionalities for electronics and information technology development to continue. One such attractive type of material is thin solid film material with multiferroic properties. Such materials have both ferroelectric and ferromagnetic properties, i.e. they are polarized by external electric and magnetic fields and can maintain polarization even if they are not continuously subjected to an external electric or magnetic field. Multiferroic materials are also attractive because they could be used in new generation memory devices since electric and magnetic fields could be used to store and read data. Therefore, novel material combinations that were expected to show magnetic and electric polarization under the influence of external field at room temperature were created and characterized in the Thesis. For this, potentially ferromagnetic and ferroelectric materials (mainly metal oxides) were combined into multilayer structures and mixed films using the atomic layer deposition method. The atomic layer deposition method was used to fabricate thin solid films because it is already widely used in the field of microelectronics and allows to deposit films with high quality and controlled thickness. The results showed that, in addition to the material combinations, the properties of the materials expectedly depended on the phase composition and deposition recipe. The metastable phases of several metal oxides were successfully stabilized, which affected the magnetic and electrical properties of the deposited films. Most of the created materials showed ferromagnetic-like behaviour by demonstrating non-linear magnetization and hysteresis under the influence of an external magnetic field. Few materials behaved similarly to the ferroelectric material under the influence of an external electric field.https://www.ester.ee/record=b544942

Magnetiliste metalloksiidkilede ja nende korrusstruktuuride mikroskoopia ja struktuuranalüüs

http://www.ester.ee/record=b4689531*es

Aatomkihtsadestatud alumiiniumoksiidi mõju mikrotoomitera lõikevõimele

http://www.ester.ee/record=b448402

Graphene functionalised by laser ablated V2O5 as highly sensitive NH3 sensor

Graphene has been recognized as a promising gas sensing material. The response of graphene-based sensors can be radically improved by introducing defects in graphene using, e. g., metal or metal oxide nanoparticles. We have functionalised CVD grown, single layer graphene by applying pulsed laser deposition (PLD) of V2O5 which resulted in a thin V2O5 layer on graphene with average thickness of ~0.6 nm. According to Raman analysis, PLD process also induced defects in graphene. Compared to unmodified graphene, the obtained chemiresistive sensor showed considerable improvement of sensing ammonia at room temperature. In addition, also the response time, sensitivity and reversibility were essentially enhanced due to graphene functionalisation by laser deposited V2O5. This can be explained by increased surface density of gas adsorption sites introduced by high energy atoms in laser ablation plasma and formation of nanophase boundaries between deposited V2O5 and graphene.Comment: 22 pages, 6 figure

Optical and mechanical properties of nanolaminates of zirconium and hafnium oxides grown by atomic layer deposition

Nanolaminates of ZrO2 and HfO2 were grown by atomic layer deposition, using metal halides and water as precursors, on silicon and fused quartz substrates at 300 degrees C. The crystalline phase composition, optical refraction, and mechanical performance of the multilayers were influenced by the relative contents of the constituent metal oxides. The crystal growth in as-deposited HfO2 dominantly led to the monoclinic phase, whereas ZrO2 was partially crystallized as its metastable and hard tetragonal polymorph. The hardness and elasticity of the nanolaminate structures could be modified by varying the amounts of either oxide contributing to the crystallographic order formed in the solid films. The refractive indexes depended on the nanolaminate structure.Peer reviewe

Behavior of nanocomposite consisting of manganese ferrite particles and atomic layer deposited bismuth oxide chloride film

Nanocomposites of manganese ferrite and bismuth oxide chloride were synthesized. The composites consisted of 10 nm thick nanocrystalline bismuth oxide chloride thin film grown by atomic layer deposition on spinel MnFe2O4 nanoparticles prepared by wet chemical synthesis. The composite layers exhibited nonlinear polarization behavior in both magnetic and electric fields at room temperature. The magnetic coercive force, HC, was 30 – 40 Oe at room temperature. The width of electrical charge – voltage hysteresis loop reached 3.6 MV/cm.Peer reviewe

Electric and Magnetic Properties of Atomic Layer Deposited ZrO2-HfO2 Thin Films

Atomic layer deposition method was employed to deposit thin films consisting of ZrO2 and HfO2. Zirconia films were doped with hafnia and vice versa, and also nanolaminates were formed. All depositions were carried out at 300 degrees C. Most films were crystalline in their as-deposited state. Zirconia exhibited the metastable cubic and tetragonal phases by a large majority, whereas hafnia was mostly in its stable monoclinic phase. Magnetic and electrical properties of the films were assessed. Un-doped zirconia was ferromagnetic and this property diminished with increasing the amount of hafnia in a film. All films exhibited ferroelectric-like behavior and the polarization curves also changed with respect to the film composition. (C) The Author(s) 2018. Published by ECS.Peer reviewe

Properties of Atomic Layer Deposited Nanolaminates of Zirconium and Cobalt Oxides

Five-layer crystalline thin film structures were formed, consisting of ZrO2 and Co3O4 alternately grown on Si(100) substrates by atomic layer deposition at 300 degrees C using ZrCl4 and Co(acac)(3) as the metal precursors and ozone as the oxygen precursor. The performance of the laminate films was dependent on the relative content of constituent oxide layers. The magnetization in these films was nonlinear, saturative, and with very weak coercive fields. Electrical measurements revealed the formation of significant polarization versus external field loops and implied some tendency toward memristive behavior. (C) The Author(s) 2018. Published by ECS.Peer reviewe

Magnetic and Electrical Performance of Atomic Layer Deposited Iron Erbium Oxide Thin Films

Mixed films of a high-permittivity oxide, Er2O3, and a magnetic material, Fe2O3, were grown by atomic layer deposition on silicon and titanium nitride at 375 degrees C using erbium diketonate, ferrocene, and ozone as precursors. Crystalline phases of erbium and iron oxides were formed. Growth into three-dimensional trenched structures was demonstrated. A structure deposited using tens to hundreds subsequent cycles for both constituent metal oxide layers promoted both charge polarization and saturative magnetization compared to those in the more homogeneously mixed films.Peer reviewe