University of Oulu

X-ray diffraction studies of VO₂ thin films

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Author: Timonen, Tero1
Organizations: 1University of Oulu, Faculty of Information Technology and Electrical Engineering, Department of Electrical Engineering, Electrical Engineering
Format: ebook
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 3.5 MB)
Persistent link: http://urn.fi/URN:NBN:fi:oulu-201410281952
Language: English
Published: Oulu : T. Timonen, 2014
Publish Date: 2014-11-03
Physical Description: 68 p.
Thesis type: Master's thesis (tech)
Tutor: Lappalainen, Jyrki
Reviewer: Kangaspuoskari, Matti
Lappalainen, Jyrki
Description:
The aim of this work is to investigate the effect of strain in vanadium dioxide (VO₂) thin films. Vanadium dioxide undergoes a metal-insulator transition involving radical changes in the electrical conductivity, and strain is one of the parameters which can influence the transition characteristics. In form of a thin film, vanadium dioxide is subjected to strain which arises from the lattice mismatch between the substrate and the film. To study the effect of strain, a set of vanadium dioxide thin films with thickness from 20 to 240 nm were grown using pulsed laser deposition method, and the structure of the films were characterized using X-ray diffraction. The surface morphology of the thin films was also examined using atomic force microscopy. In the insulating state, the films were composed of both M₁ and M₂ phases of vanadium dioxide. The transition was observed to be controlled by M₂ phase in films which were thinner than 100 nm and by M₁ phase in thicker films. The presence of M₂ phase was caused by strain. The metal-insulator transition occurred at much wider temperature range in thinner films which were more strained than the thicker films showing more abrupt transitions. The relaxation of the strain in thicker films resulted in misorientation of the crystallites and caused the surface roughness to increase with the film thickness.
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Tämän työn tarkoituksena on tutkia venymän vaikutusta vanadiinidioksidi (VO₂) ohutkalvoissa. Vanadiinidioksidissa ilmenevä metalli-eriste transitio aiheuttaa suuria muutoksia aineen sähkönjohtavuudessa ja venymän on huomattu vaikuttavan transition ominaisuuksiin. Ohutkalvomuodossa vanadiinidioksidiin kohdistuu venymä, joka aiheutuu kalvon ja substraatin hilavakioiden epäsovituksesta. Venymästä aiheutuvien vaikutusten selvittämiseksi eri paksuisia (20–240 nm) vanadiinidioksidi ohutkalvoja valmistettiin pulssilaserkasvatuksella ja niiden rakennetta karakterisoitiin röntgendiffraktiolla. Myös ohutkalvojen pinnanmuotoa tutkittiin atomivoimamikroskopian avulla. Eristetilassa kalvot koostuivat vanadiinioksidin M₁ ja M₂ faaseista, ja M₂ faasin huomattiin kontrolloivan transitiota kalvoissa, jotka olivat ohuempia kuin 100 nm, kun taas paksuimmissa kalvoissa transitiota kontrolloi M₁ faasi. M₂ faasin mukanaolo johtui venymästä. Metalli-eriste transitio tapahtui laajemmalla lämpötila-alueella ohuimmissa kalvoissa, joissa venyminen oli suurempaa kuin paksummissa kalvoissa. Paksuimmissa kalvoissa transitio olikin paljon voimakkaampi. Venymän relaksoituminen paksuimmissa kalvoissa johti kiteiden kallistumiseen aiheuttaen sen, että pinnan karheus kasvoi kalvon paksuuden mukana.
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Copyright information: © Tero Timonen, 2014. This publication is copyrighted. You may download, display and print it for your own personal use. Commercial use is prohibited.