Resistive switching in Titanium-Aluminium thin films

Käesolevas magistritöös uuriti takistuslülituste toimumist aatomkihtsadestatud titaanalumiiniumoksiid-kiledes. Uuritud struktuuride alumine elektrood oli RuO2 ja ülemine elektrood elektronkiiraurustamisega sadestatud Pt. Eesmärgiks oli välja selgitada, kas takistuslülitused toimuvad antud struktuurides ja kui toimuvad, siis milline on TiO2-l põhinevasse dielektrikku lisatud Al2O3 sisaldavate kihtide mõju lülitumistele. Lisaks sellele töötati välja metoodika takistuslülituste tuvastamiseks ja elektriliseks karakteriseerimiseks. Selgitati välja, et kõikides uuritud struktuurides toimuvad takistuslikud lülitused. Al2O3 sisaldava kihi paksuse suurendamine suurendas lülitumiste efektiivsust ja nende kihtide erinev paigutus mõjutas oluliselt juhtivuse sõltuvust ümberlülitamiseks kasutatavast pingest, eriti lülitumisel suure takistusega režiimi. Lisaks kirjeldati käesolevas töös takistuslülituste iseärasusi ja sellest lähtuvalt pakuti välja, millised mehhanismid võisid takistuslülitumisi antud struktuurides põhjustada

Structure and behavior of ZrO2-graphene-ZrO2 stacks

Producción CientíficaZrO2-graphene-ZrO2 layered structures were built and their crystallinity was characterized before resistive switching measurements. Thin nanocrystalline ZrO2 dielectric films were grown by atomic layer deposition on chemical vapor deposited graphene. Graphene was transferred, prior to the growth of the ZrO2 overlayer, to the ZrO2 film pre-grown on titanium nitride. Nucleation and growth of the top ZrO2 layer was improved after growing an amorphous Al2O3 interface layer on graphene at lowered temperatures. Studies on resistive switching in such structures revealed that the exploitation of graphene interlayers could modify the operational voltage ranges and somewhat increase the ratio between high and low resistance states.Fondo Europeo de Desarrollo Regional (project TK134)Estonian Research Agency (grants PRG753 and PRG4)Ministerio de Economía, Industria y Competitividad (grant TEC2017-84321-C4-2-R

Software for characterization of electrical memory cells with a probe station

Käesolev töö kirjeldab mäluelementide elektriliste mõõtmiste tegemiseks loodud tarkvara, mis on välja töötatud Tartu Ülikooli Füüsika Instituudi kiletehnoloogia labori sondijaama CascadeMicrotech EPS150TRIAX kasutamiseks koos mõõteseadmetega Agilent E4980A ja Keithley 2636A. Töö heidab pilgu probleemidele, millega seisab silmitsi elektroonsete arvutimälude tööstus tänapäeval ja selgitab miks ja mida on tarvis probleemide lahendamiseks välja töötada, uurida ja mõõta. Töös on analüüsitud nõudeid sellisteks mõõtmisteks vajalikule tarkvarale ning kirjeldatud valminud tarkvara. Antud tarkvara arenduseks ja kontrollimiseks viidi läbi näidismõõtmised kiletehnoloogia laboris valmistatud mäluelementidel. Tarkvara rakendati ka GaAs jõudioodide karakteriseerimise

Bipolar Resistive Switching in Hafnium Oxide-Based Nanostructures with and without Nickel Nanoparticles

As research into additives and intentionally introduced impurities in dielectric thin film for enhancing the resistive switching based random access memories (RRAM) continues to gain momentum, the aim of the study was to evaluate the effects of chemically presynthesised Ni nanoparticles (NPs) embedded in a dielectric layer to the overall structure and resistive switching properties. HfO2-based thin films embedded with Ni NPs were produced by atomic layer deposition (ALD) from tetrakis(ethylmethylamino)hafnium (TEMAH) and the O2 plasma ALD process onto a TiN/Si substrate. The Ni NPs were separately synthesised through a continuous flow chemistry process and dispersed on the dielectric layer between the two stages of preparing the HfO2 layer. The nanodevices’ morphology and composition were analysed with physical characterisation methods and were found to be uniformly dispersed across the sample, within an amorphous HfO2 layer deposited around them. When comparing the resistive switching properties of otherwise identical samples with and without Ni NPs, the ILRS/IHRS ratio rose from around a 4 to 9 at 0.2 V reading voltage, the switching voltage dropped from ~2 V to ~1.5 V, and a distinct increase in the endurance characteristics could be seen with the addition of the nanoparticles

Structure and Electrical Properties of Zirconium-Aluminum-Oxide Films Engineered by Atomic Layer Deposition

Thin films containing either multilayer ZrO2:Al2O3 structures or ZrO2 deposited on ZrxAlyOz buffer layers were characterized. The films were grown by atomic layer deposition (ALD) at 300 °C from ZrCl4, Al(CH3)3, and H2O. The multilayer ZrO2:Al2O3 structures were grown repeating different combinations of ZrO2 and Al2O3 ALD cycles while the ZrxAlyOz layers were obtained in a novel process using ALD cycles based on successive adsorption of ZrCl4 and Al(CH3)3, followed by surface reaction with H2O. The films were grown on TiN electrodes, and supplied with Ti top electrodes, whereby ZrxAlyOz films were exploited as thin buffer layers between TiN and ZrO2. The as-deposited ZrO2 films and ZrO2:Al2O3 structures with sufficiently low concentrations of Al2O3 were crystallized in the form of cubic or tetragonal ZrO2 polymorph possessing relative permittivities reaching 35. Notably, multilayered ZrO2:Al2O3 films could exhibit resistive switching behavior with ratios between low- and high-resistive-state current values, extending up to five orders of magnitude. Implications of multilevel switching were recorded. In the double-layered ZrxAlyOz-ZrO2 stacks, the ON/OFF current ratios remained below 40, but the endurance could become extended over 3000 cycles. Remarkably, instabilities, when detected in endurance behavior expressed by reduction in an ON/OFF current ratio could be compensated and the current values restored by real time readjustment of the programming voltage amplitude

Nanostructures Stacked on Hafnium Oxide Films Interfacing Graphene and Silicon Oxide Layers as Resistive Switching Media

SiO2 films were grown to thicknesses below 15 nm by ozone-assisted atomic layer deposition. The graphene was a chemical vapor deposited on copper foil and transferred wet-chemically to the SiO2 films. On the top of the graphene layer, either continuous HfO2 or SiO2 films were grown by plasma-assisted atomic layer deposition or by electron beam evaporation, respectively. Micro-Raman spectroscopy confirmed the integrity of the graphene after the deposition processes of both the HfO2 and SiO2. Stacked nanostructures with graphene layers intermediating the SiO2 and either the SiO2 or HfO2 insulator layers were devised as the resistive switching media between the top Ti and bottom TiN electrodes. The behavior of the devices was studied comparatively with and without graphene interlayers. The switching processes were attained in the devices supplied with graphene interlayers, whereas in the media consisting of the SiO2-HfO2 double layers only, the switching effect was not observed. In addition, the endurance characteristics were improved after the insertion of graphene between the wide band gap dielectric layers. Pre-annealing the Si/TiN/SiO2 substrates before transferring the graphene further improved the performance