Spin-based electronic devices constitute an intriguing area in the development of the future nanoelectronics. Recently, 3D topological insulators (TI), when in contact with ferromagnets (FM), play a central role in the context of enhancing the spin-to-charge conversion efficiency in FM/TI heterostructures. The main subject of this thesis is the study of the chemical-physical interactions between the granular and epitaxial Sb2Te3 3D-TI with Fe and Co thin films by means of X-ray Diffraction/Reflectivity, Ferromagnetic Resonance spectroscopy (FMR) and Spin Pumping-FMR. Beside the optimization of the materials properties, particular care was taken on the industrial impact of the presented results, thus large-scale deposition processes such as Metal Organic Chemical Vapor Deposition (MOCVD) and Atomic Layer Deposition (ALD) were adopted for the growth of the Sb2Te3 3D-TI and part of the FM thin films respectively. A thorough chemical, structural and magnetic characterization of the Fe/granular Sb2Te3 interface evidenced a marked intermixing between the materials and a general bonding mechanism between Fe atoms and the chalcogen element in chalcogenide-based TIs. Through rapid and mild thermal treatments performed on the granular Sb2Te3 substrate prior to Fe deposition, the Fe/granular-Sb2Te3 interface turned out to be sharper and chemically stable. The study of ALD-grown Co thin films deposited on top of the granular-Sb2Te3 allowed the production of high-quality Co/granular-Sb2Te3interfaces, with also the possibility to tune the magneto-structural properties of the Co layer through a proper substrate selection. In order to improve the structural properties of the Sb2Te3, specific thermal treatments were performed on the as deposited granular Sb2Te3, achieving highly oriented films with a nearly epitaxial fashion. The latter substrates were used to produce Au/Co/epitaxial-Sb2Te3 and Au/Co/Au/epitaxial-Sb2Te3 and the dynamic of the magnetization in these structures was investigated studying their FMR response. The FMR data for the Au/Co/Sb2Te3 samples were interpreted considering the presence of a dominant contribution attributed to the Two Magnon Scattering (TMS), likely due to the presence of an unwanted magnetic roughness at the Co/epitaxial-Sb2Te3 interface. The introduction of a Au interlayer to avoid the direct contact between Co and Sb2Te3 layers was shown to be beneficial for the total suppression of the TMS effect. SP-FMR measurements were conducted on the optimized Au/Co/Au/epitaxial-Sb2Te3 structure, highlighting the role played by the epitaxial Sb2Te3substrate in the SP process. The SP signals for the Au/Co/Au/Si(111) and Co/Au/Si(111) reference samples were measured and used to determine the effective spin-to-charge conversion efficiency achieved with the introduction of the epitaxial Sb2Te3 layer. The extracted SCC efficiency was calculated interpreting the SP-FMR data using the Inverse Edelstein effect and Inverse Spin-Hall effect models, which demonstrated that the Sb2Te3 3D-TI is a promising candidate to be employed in the next generation of spintronic devices.

I dispositivi elettronici che sfruttano proprietà legate allo spin elettronico costituiscono un settore molto promettente per lo sviluppo della nanoelettronica del futuro. Recentemente, gli isolanti topologici tridimensionali (IT-3D), quando posti a contatto con materiali ferromagnetici (FM), giocano un ruolo centrale nel contesto del miglioramento dell’efficienza di conversione tra spin e carica elettronici in eterostrutture di tipo FM/TI. L’oggetto principale di questa tesi è lo studio delle interazioni chimico-fisiche tra l’IT-3D Sb2Te3, nelle sue forme granulare ed epitassiale, con film di Fe e Co attraverso l’uso di tecniche di Diffrazione/Riflettività di raggi-X, spettroscopia di risonanza ferromagnetica (FMR) e pompaggio di spin in risonanza ferromagnetica (SP-FMR). In concomitanza con l’ottimizzazione delle proprietà dei materiali, un particolare interesse è stato rivolto verso l’impatto industriale della ricerca presentata. Per questo motivo, per la produzione di Sb2Te3 e di alcuni dei FM impiegati, sono state impiegate tecniche di deposizione di materiali su larga scala ( 4 pollici), quali la Metal Organic Chemical Vapor Deposition (MOCVD) e l’Atomic Layer Deposition (ALD) rispettivamente. Una approfondita caratterizzazione chimica, strutturale e magnetica dell’interfaccia Fe/ Sb2Te3-granulare ha evidenziato un marcato intermixing tra i materiali e una generale tendenza degli atomi di Fe nel legare con l’elemento calcogenuro quando presente in un IT. Attraverso trattamenti termici rapidi e a bassa temperatura sottoposti sui film di Sb2Te3 granulare prima della crescita del Fe, l’interfaccia Fe/Sb2Te3-granulare è risultata morfologicamente più netta e chimicamente stabile. Lo studio di film sottili di Co cresciuti attraverso ALD su Sb2Te3 granulare ha permesso la produzione di interfacce Co/Sb2Te3-granulare di alta qualità, con la possibilità inoltre di modificare le proprietà magneto-strutturali dei film di Co attraverso una selezione appropriata di substrati. Con l’obbiettivo di migliorare le proprietà dei film di Sb2Te3, dei trattamenti termici specifici sono stati condotti su Sb2Te3 granulare appena cresciuto, ottenendo film di Sb2Te3 altamente orientati con una qualità cristallina vicina al cristallo singolo di tipo epitassiale. Questi substrati di Sb2Te3 sono stati utilizzati per produrre eterostrutture di Au/Co/Sb2Te3-epitassiale e Au/Co/Au/Sb2Te3-epitassiale per studiare la loro risposta di FMR. I dati di FMR per il campione Au/Co/Sb2Te3-epitassiale sono stati interpretati considerando un contributo di Two Magnon Scattering (TMS) dominante, verosimilmente a causa della presenza di rugosità magnetica all’interfaccia Co/Sb2Te3-epitassiale. L’introduzione di un interlayer di Au per evitare il contatto diretto tra Co e Sb2Te3 si è dimostrato vantaggioso per la totale eliminazione del contributo di TMS. Misure di SP-FMR sono state condotte sulla struttura ottimizzata Au/Co/Au/Sb2Te3-epitassiale, sottolineando il ruolo giocato dallo strato di Sb2Te3-epitassiale nel processo di SP. I segnali di SP ricavati da campioni di Au/Co/Au/Si(111) e Co/Au/Si(111) sono stati utilizzati per determinare l’efficienza di conversione spin-carica ottenuta dall’introduzione dello strato di Sb2Te3. L’efficienza estratta è stata calcolata interpretando i dati di SP-FMR attraverso i modelli di effetto Edelstein inverso ed effetto di Spin-Hall inverso, i quali hanno dimostrato che l’IT-3D Sb2Te3 è un candidato promettente per essere impiegato nella prossima generazione di dispositivi spintronici.

(2021). HETEROSTRUCTURES BASED ON THE LARGE-AREA Sb2Te3 TOPOLOGICAL INSULATOR FOR SPIN-CHARGE CONVERSION. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2021).

HETEROSTRUCTURES BASED ON THE LARGE-AREA Sb2Te3 TOPOLOGICAL INSULATOR FOR SPIN-CHARGE CONVERSION

LONGO, EMANUELE MARIA
2021

Abstract

Spin-based electronic devices constitute an intriguing area in the development of the future nanoelectronics. Recently, 3D topological insulators (TI), when in contact with ferromagnets (FM), play a central role in the context of enhancing the spin-to-charge conversion efficiency in FM/TI heterostructures. The main subject of this thesis is the study of the chemical-physical interactions between the granular and epitaxial Sb2Te3 3D-TI with Fe and Co thin films by means of X-ray Diffraction/Reflectivity, Ferromagnetic Resonance spectroscopy (FMR) and Spin Pumping-FMR. Beside the optimization of the materials properties, particular care was taken on the industrial impact of the presented results, thus large-scale deposition processes such as Metal Organic Chemical Vapor Deposition (MOCVD) and Atomic Layer Deposition (ALD) were adopted for the growth of the Sb2Te3 3D-TI and part of the FM thin films respectively. A thorough chemical, structural and magnetic characterization of the Fe/granular Sb2Te3 interface evidenced a marked intermixing between the materials and a general bonding mechanism between Fe atoms and the chalcogen element in chalcogenide-based TIs. Through rapid and mild thermal treatments performed on the granular Sb2Te3 substrate prior to Fe deposition, the Fe/granular-Sb2Te3 interface turned out to be sharper and chemically stable. The study of ALD-grown Co thin films deposited on top of the granular-Sb2Te3 allowed the production of high-quality Co/granular-Sb2Te3interfaces, with also the possibility to tune the magneto-structural properties of the Co layer through a proper substrate selection. In order to improve the structural properties of the Sb2Te3, specific thermal treatments were performed on the as deposited granular Sb2Te3, achieving highly oriented films with a nearly epitaxial fashion. The latter substrates were used to produce Au/Co/epitaxial-Sb2Te3 and Au/Co/Au/epitaxial-Sb2Te3 and the dynamic of the magnetization in these structures was investigated studying their FMR response. The FMR data for the Au/Co/Sb2Te3 samples were interpreted considering the presence of a dominant contribution attributed to the Two Magnon Scattering (TMS), likely due to the presence of an unwanted magnetic roughness at the Co/epitaxial-Sb2Te3 interface. The introduction of a Au interlayer to avoid the direct contact between Co and Sb2Te3 layers was shown to be beneficial for the total suppression of the TMS effect. SP-FMR measurements were conducted on the optimized Au/Co/Au/epitaxial-Sb2Te3 structure, highlighting the role played by the epitaxial Sb2Te3substrate in the SP process. The SP signals for the Au/Co/Au/Si(111) and Co/Au/Si(111) reference samples were measured and used to determine the effective spin-to-charge conversion efficiency achieved with the introduction of the epitaxial Sb2Te3 layer. The extracted SCC efficiency was calculated interpreting the SP-FMR data using the Inverse Edelstein effect and Inverse Spin-Hall effect models, which demonstrated that the Sb2Te3 3D-TI is a promising candidate to be employed in the next generation of spintronic devices.
FANCIULLI, MARCO
MANTOVAN, ROBERTO
spintronica; isolanti topologici; spin pumping; ALD; FMR
spintronics; topologicalinsulator; spin pumping; ALD; FMR
FIS/01 - FISICA SPERIMENTALE
Italian
15-apr-2021
SCIENZA E NANOTECNOLOGIA DEI MATERIALI
33
2019/2020
open
(2021). HETEROSTRUCTURES BASED ON THE LARGE-AREA Sb2Te3 TOPOLOGICAL INSULATOR FOR SPIN-CHARGE CONVERSION. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2021).
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10281/311358
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