Chromium dioxide-low temperature thin film growth, structural and physical properties

Tese de doutoramento em Física, apresentada à Universidade de Lisboa através da Faculdade de Ciências, 2008Magnetic materials exhibiting a high degree of spin polarization are being actively investigated for their potential use in spintronic devices. Among them, CrO2 is very attractive because it is a half-metal fully spin polarized at the Fermi level with a magnetic moment of 2 µB/f.u. and Curie temperature well above room temperature (RT). Therefore, much effort has been put into developing efficient and controlled methods for preparing CrO2 films which nevertherless has been a difficult task due to the CrO2 metastability. Results on the deposition of CrO2 thin films by laser-assisted Chemical Vapour Deposition (LCVD) and by Chemical Vapour Deposition (CVD) are presented in this thesis. The LCVD experiments were carried out in a high vacuum chamber at working pressures of 10-5 - 10-1 mbar and using different geometries. Films were mainly grown at RT onto sapphire substrates by photodissociation of Cr(CO)6 using a KrF excimer laser, in dynamic atmospheres containing oxygen and Argon. The structural characterization showed that some CrO2 was deposited at RT in a narrow window of experimental parameters at 0.5 mbar, although together with a significant amount of Cr2O3. Precursors partial pressures ratio and laser fluence are the prominent parameters that have to be accurately controlled in order to co-deposit both phases. A highly reproducible CVD setup consisting of a single-zone furnace with independent control of the substrate temperature was developed. CrO3 powder was used as precursor and oxygen as carrier gas. Films were grown within a broad range of deposition temperatures from 320 to 410 ºC, and O2 carrier gas flow ratesof 50 500 sccm, showing that it is viable to grow highly oriented a-axis CrO2 films at temperatures as low as 330 ºC, i.e. ~70 ºC lower than in published data for the same chemical system. Deposition parameters were correlated with the structure and microstructure, deposition kinetics, magnetic and electrical transport properties, and spin polarization of the deposited material. The growth of a Cr2O3 interfacial layer as an intrinsic feature of the deposition process is studied and discussed. Films synthesised at 330 ºC keep the high quality magnetic and transport properties as those deposited at higher temperatures.Fundação para a Ciência e a Tecnologia (FCT), (BD16567/2004