Selective Area Chemical Vapor Deposition of Chromium Oxides
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SELECTIVE AREA CHEMICAL VAPOR DEPOSITION OF CHROMIUM OXIDES Ruihua Cheng, C.N. Borca, and P.A. Dowben Department of Physics and Astronomy and the Center for Materials Research and Analysis (CMRA), Behlen Laboratory of Physics, University of Nebraska-Lincoln, NE 68588-0111, USA, [email protected] ABSTRACT We demonstrate that two-phase CrO2 and Cr2O3 thin films can be grown by using selective organometallic chemical vapor deposition through the oxidation of Cr(CO)6 in an oxygen environment. While the magnetization measurements show that both chromium oxides are present, the relative weight of each phase depends on the oxygen partial pressure. Changes of the Curie temperature, Tc, and the saturation magnetization field may be possible by controlling the stoichiometry. INTRODUCTION Chromium oxides have been seriously considered as spin-polarized electron injectors to spin-tunnel junctions [1] and other magnetoresistive devices [2,3]. The insulating antiferromagnetic chromium oxide Cr2O3 has a Neél temperature of 307K and is suitable as a tunnel junction barrier [3] both below and above the Neél temperature. The ferromagnetic chromium oxide CrO2 with Tc of 397k [4], has been predicted half-metallic (metallic for one spin direction while insulating for the other spin direction) by band structure calculations [5-9], though Kulatov and Mazin found CrO2 to be insulating in both spin directions [10]. Evidence of nearly 100% polarization, consistent with the half metallic character of CrO2, were observed in spin-polarized photoemission [11], vacuum tunneling [12], and Andreev scattering [13], though some discrepancies from perfect half metallic character remain unresolved. The high electron polarization, in addition to the half metallic character of the surface [9] (and by extension, one hopes the interface as well) makes CrO2 an attractive material for spin-polarized electron tunneling, and very large tunneling magnetoresistance (TMR) is expected. It is difficult to fabricate CrO2 films using conventional methods because CrO2 is metastable. This is not altogether bad, as the two-phase CrO2/Cr2O3 system exhibits higher magnetoresistance than the pure material [3]. The oxidation of the organometallic complex hexacarbonyl Cr(CO)6 has the potential for selective deposition of CrO2 [14-17]. These studies have established that chromium oxides are the thermodynamic sinks of chromium hexacorbonyl decomposition [16], and the oxidation is further aided by the presence of an ambient oxygen background [14,15]. By modifying this organometallic chemical vapor deposition (OMCVD) procedure, we have been able to fabricate the ferromagnetic and antiferromagnetic chromium oxides. Here we describe both the growth and the magnetic properties of these films. EXPERIMENTAL APPROACH Film growth was carried out on Si (111) substrates in an ultra high vacuum chamber with maintained base pressure of 1.0-9 Torr. This chamber was designed for laser-initiated chemical vapor deposition, as described elsewhere [18-19]. The prevalent source compound wa
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