CVD of CrO₂: Towards a Lower Temperature Deposition Process

Abstract

The ability to manipulate and amplify currents of different spin-type is the basis of an important and rapidly developing field of fundamental and applied research known as spintronics. Among the magnetic materials being actively investigated for their potential use in spintronic devices, chromium dioxide (CrO 2 ) is one of the most attractive because it is a half-metal, fully spin polarized at the Fermi level, with a Curie temperature above room temperature (T c = 393 K) and a calculated magnetic moment of 2 l B per f.u. (forming unit) [1][2][3][4] Therefore, much effort has been put into developing efficient and controlled methods for preparing CrO 2 films. Nevertheless, the synthesis of CrO 2 films has been a difficult task because it lacks a thermodynamic stable plateau at atmospheric pressure and, if heated, easily decomposes into the insulating antiferromagnetic Cr 2 O 3 phase which is the most stable chromium oxide under ambient conditions. Although efforts to grow CrO 2 thin films have involved several techniques, [5][6][7][8][9][10][11] until now CVD seems to be the most successful for producing these films. Highly oriented thin films of CrO 2 are currently grown on TiO 2 rutile phase and Al 2 O 3 (0001) by thermal CVD using CrO 3 as the chromium precursor. [12][13][14][15] In this work we report on the deposition of highly oriented a-axis CrO 2 films onto Al 2 O 3 (0001) by atmospheric pressure (AP)CVD at temperatures as low as 330°C. Our experiments were based on the method described by Ishibashi et al. [12] and already modified by Watts and coworkers. [16,19] Therefore, a quartz tube placed inside a singlezone furnace and an independent control of the substrate temperature were used. However, the reactor was designed with a configuration that attempts to minimize the turbulence along the tube, guaranteeing processing conditions close to the ideal for laminar flux over the substrate surface, and increasing the reactor efficiency (see Experimental). CrO 3 powder was used as the precursor and O 2 as the carrier gas. The powder was loaded into a stainless-steel boat aligned with the substrate holder, 15 cm apart from it. Films were grown for deposition times (t dep ) of 4 and 8 h, by using an oxygen flow rate (f O2 ) of 50 sccm, a precursor temperature (T p ) of 260°C and substrate temperatures (T s ) ranging from 330 to 410°C. All the films were produced on 10 mm × 10 mm substrates, are homogeneous, and exhibit a good adherence, and are the black shiny metallic color characteristic of chromium dioxide. Surface morphology and roughness (rms) depend mainly on film thickness, the lowest value for the rms, ∼7 nm, was measured over an area of 20 lm × 20 lm by atomic force microscopy (AFM) of 200 nm thick films. Film thickness strongly depends on substrate temperature, values of (1424 ± 49) nm and (57 ± 13) nm having been measured for films deposited over 4 h at T s = 410°C and T s = 330°C, respectively. Therefore, CrO 2 deposition rates (r CrO2 ) varying between 59.3 and 2.4 Å min -1 were calculated, the hig...

Anomalous transport in half-metallic ferromagnetic CrO2

“…Growth on sapphire actually starts with Cr 2 O 3 and then changes to the required CrO 2 . 21,22 Grains are aligned at 60 • to each other with sixfold rotational symmetry of hexagonal crystal structure of underlying sapphire substrate. We earlier reported in some detail on the film growth, the morphology of the films, and also on the magnetization of the films.…”

Coexistence of Universal and Topological Anomalous Hall Effects in MetalCrO2Thin Films in the Dirty Limit

“…16 The grain size measured by XRD increased with deposition temperature from 33nm at 330°C to 80nm at 380°C. 16 Films grown by this method have been extensively characterized [15][16][17][18] , point contact Andreev reflection measurements on the 595, 251 and 62nm films in this study were published previously, 17 confirming bulk spin polarisation of the transport carriers (P t ) close to 100% and no strong correlation with morphology.…”

Influence of Growth Temperature and Carrier Flux on the Structure and Transport Properties of Highly Oriented CrO2 on Al2O3 (0001)

“…In relation to the latter reactor configuration, we added two more significant modifications of the deposition geometry which lead to improved reactor efficiency and a better quality of the grown films. The details of this set-up were described in a previous paper [29].…”

“…The quality of the CrO 2 films has been claimed to depend critically on the substrate temperature, the best re-sults having been obtained at temperatures between 390 and 400°C. [19,22,25] Recently, we have shown that highly oriented a-axis CrO 2 films can be synthesized onto (0001) Al 2 O 3 by APCVD from CrO 3 , at temperatures as low as 330°C [29] i.e., 70°C lower than usually reported. Most important, the samples grown at low temperatures keep the same room temperature magnetic behavior and high degree of spin polarization as the films grown at higher temperatures.…”