The document summarizes research on growing chromium dioxide (CrO2) thin films at low temperatures using chemical vapor deposition. Key findings include: 1) High quality CrO2 thin films with the desired structural and magnetic properties could be grown at as low as 330°C, significantly lower than previous methods allowed. 2) Films grown at lower temperatures had thinner intermediate chromium oxide layers and higher spin polarization near 100%, desirable for spintronic devices. 3) Growth temperature, oxygen flow rate, and film thickness were found to influence the structural, magnetic, and electrical properties of the resulting CrO2 thin films.

1. CrO 2 – low temperature thin film growth, structural and physical properties Pedro Miguel F. D. Sousa Jornadas do ICEMS , January 2009

2. Rua Alves Redol, 9 1000-029 Lisboa Portugal Silicon and all-polymer thin films Microelectromechanical Systems - MEMS Nanoelectromechanical Systems - NEMS Source: Sandia National Laboratory Source: Sandia National Laboratory - Motion (10 nm) detector iPhone - Apple [email_address] http://www.inesc-mn.pt/ INESC MN Microsistemas & Nanotecnologias

5. Motivation Spin-Valve Spin Polarized Field Effect Transistor (Spin-FET) Source: NIMS Magnetoresistive Random Access Memory (MRAM) μ s FM1 NM FM2 H > H s μ s FM1 NM FM2 H = 0 Low resistance High resistance

6. Materials Half-metallic ferromagnets - source of currents with high spin -polarization CrO 2 spin-polarization DOS (A. Gupta et . al .) * Measured by PCAR Cr 4+ : [Ar] 3d 2 (t 2g 2 ) ↑ Half-metals *P n (%) T C (K) NiMnSb 44 - 58 730 Co 2 MnSi 45 - 55 600 Sr 2 FeMoO 6 60 - 75 420 La 0.7 Sr 0.3 MnO 3 78 - 80 350 CrO 2 90 - 98.4 395

8. Chemical Vapour Deposition Atmospheric pressure CVD of CrO 2 Ishibashi method S. Ishibashi, et al . Japn. J. of Appl. Physics, 17 (1978) 249. S. Ishibashi, et al . Mater. Research Bulletin, 14 (1979) 51. 260 ºC 390 ºC CrO 3 Δ + O 2 CrO 2 260 T p , CrO 3 (ºC) Parameter value Substrate T S (ºC) 390 - 400 Tube diameter,  (cm) 3 Tube length, L (cm) 90 Cold zone (cm) n.a. Source  to  substrate distance (cm) n.a. Oxygen flow rate,  O 2 (sccm) 500 Deposition time (hours) n.a.

14. Interfacial layer thickness Cr 2 O 3 (0001) CrO 2 (100) Al 2 O 3 (0001) Cr 2 O 3 interlayer thickness ? - Extended face imperfect crystal model B. E. Warren, “X-Ray Diffraction”, Dover, inc., New York, 1990, page 44 F (hkl) , n a , m s Proposed films layered structure

18. Deposition Kinetics surface chemical reaction kinetics rate-limiting mechanism Mass transport kinetics rate-limiting mechanism 0.1 nm s -1 surface chemical reaction kinetics rate-limiting mechanism  O 2 E a (kJ mol -1 ) 50 121 ± 4 100 146 ± 7 200 166 ± 7

24. Pedro Miguel Sousa, “Chromium dioxide – low temperature thin film growth, structural and physical properties”, Tese de Doutoramento em Física, ramo de Física da Matéria Condensada, Universidade de Lisboa, Faculdade de Ciências, 2008. P. M. Sousa, S. A. Dias, O. Conde, A. J. Silvestre, W. R. Branford, B. Morris, K. A. Yates, and L. F. Cohen, “Influence of growth temperature and carrier flux on the structure and transport properties of highly-oriented CrO 2 on Al 2 O 3 (0001)”, Chemical Vapour Deposition, 13 (2007) 537-545. M. I. Ortiz, P. M. Sousa, C. Ballesteros, A. J. Silvestre, L. F. Cohen and O. Conde, “Structural and microanalytical studies of CrO 2 thin films on c-sapphire by high resolution electron microscopy methods”, Microscopy and Microanalysis, 14 (2008) 47. References