CrO2Cl2 for CrO2 CVD
Chromyl chloride CrO2Cl2 was applied as precursor for the growth of CrO2 and Cr2O3 thin layers on SiO2 glass, Si(001), Al2O3(0001), TiO2(001) by photo-assisted CVD using photolytic decomposition of precursor molecules by UV-visible light at wavelengths less than 565 nm and UV energy was in a range of 5–20 mJ/cm2; photolytic deposition times 5-20 min were used (i.e., 3000–12000 laser pulses). Two patterns obtained under these conditions in a 2D or 3D configuration are shown in Fig.a,b. [873]
PHYSICAL REVIEW B, VOLUME 64, 180408~R!
http://www.micromagnetics.com/pdfs/Anguelouch_01_near_complete_spin_polarization.pdf
Single crystalline CrO2 films were grown by CVD with chromyl chloride CrO2Cl2 as a liquid precursor. Atomically smooth films with a rms roughness of less than 5 Å for 1000 Å thick films were obtained, having a spin polarization of P=98.4%, as determined by point-contact Andreev reflection. Magnetization and resistivity of the films are in good agreement with those grown with the CrO3 solid precursor. The process using the liquid precursor is superior to other methods of preparation of single crystal single- and multilayers containing CrO2, especially for the magnetic tunnel junctions structures. [874]
Phase selective growth of CrO2 film was performed using chromyl chloride by CVD at 320–380 °C. Phase selectivity was driven in part by a templating effect of the substrate: thin films of metastable CrO2 in (100), (001), (110) orientations have been stabilized by (100), (001), (110) TiO2 single crystal substrates, respectively. In contrast, thin oriented films of the more stable Cr2O3 in (006) orientation were grown on Al2O3(0001) single crystals. This phase selectivity appears to be unique to the chromyl chloride precursor, as CVD with CrO3 precursor does not result in phase-selective CVD between Al2O3 and TiO2 substrates. The CrO2 films were shiny, black, and highly conductive. [875]
Solid CrO3 precursor and liquid CrO2Cl2 precursor were used for the preparation of epitaxial CrO2 thin films on TiO2(100) single crystal substrates by CVD. Magnetic properties of the films were studied by the Ferromagnetic resonance (FMR) technique.[865]
In the films grown using CrO3 precursor (Fig.), strong dependence of magnetic anisotropies on the CrO2 thickness was observed; the reason is magnetoelastic anisotropy in the plane of CrO2 film caused by the lattice mismatch with the TiO2 substrate.
On the contrary, CrO2Cl2-grown CrO2 films demonstrate very weak variation of anisotropy parameters with film thickness. According to AFM studies, different behaviour is related to the different morphologies of the films prepared with CrO3 and CrO2Cl2 precursors,. In the case of CrO2Cl2, the individual CrO2 grains are growing epitaxially on the TiO2 substrate and forming very regular rectangular-shaped blocks with the lateral size of ~150nm (Fig “5”); breaks in the substrate-neighboring layer of CrO2 film result in quick relaxation of of the tensile stress arising from the lattice mismatch between CrO2 and TiO2.
As an opposite, the thinnest films grown using CrO3 precursor were consisting of spherical interconected grains rather than reguar shaped blocks.
The roughness of the films was quickly decreasing for the thicker films with both precursors; the grain structure was not clearly observable in the AFM scans of the thicker films.
The comparison of the anisotropy factors in the films deposited in CrO3 and CrO2Cl2 precursors is presented in the Table .
Liquid CrO2Cl2 precursor was used for the growth of CrO2 films on TiO2 (100) and Al2O3 (0001) substrates by hot-wall CVD , the susbtrate temperature was ~400°C. CrO2Cl2 bubbler temperature was 0°C; high purity O2 or Ar (20 sccm) was used as carrier gas. ,growth time was ~1 h. [[i]] Films grown at ~400°C on Al2O3 were green and insulating. Layers grown on TiO2 substrates at 450°C were composed of a mixture of CrO2 and Cr2O3, while no change in properties was observed in the films grown on Al2O3.
Shiny black conducting CrO2 films were deposited on TiO2 substrates (~200nm thick, growth rate ~3.3nm/min, rms roughness 3,5-6,0nm (5µm2 area), granular microstructure with grain size 0.5-2µm).. According to XRD, films were fully CrO2 (100) oriented with no impurities of Cr2O3 (Fig.1); the rocking curves of the (200) reflection had ~0.1° FWHM.; the φ-scans of CrO2 (110) reflection presented 2-fold symmetry, with no evidence of misaligned material (Fig.2). The measured lattice constants were a = 4.395 A, b = 4.443 A, c = 2.916 A, showing that despite films being under compresion in the growth direction (~0.5%), the in-plane constant expended by 0.5%, while c was the same as the bulk value of (100) CrO2. The electrical resistivity of the 200nm CrO2 film was 240 µΩcm at RT and decreased to 10 µΩcm at 5K, consistent with metallic behaviour (Fig.3). A high resistivity ratio (ρ298K / ρ5K = 24) confirms high epitaxial quality of the films. The magnetic propeties (hysteresis) of the films were measured by SQUID magnetoemter. Large magnetic field (H > 4kOe) was needed to saturate the magnetization; relatively small coercitivity (Hc < 100 Oe) observed. The saturation magnetisation was 670 and 365 emu/cm2 at 5 and 298 K, respectively. The classic temperature dependence of magnetisation using an applied field of 500 Oe is shown in Fig. 5. The Curie temperature of ghe film was 395 K, in agreement with spin polarised CrO2. Normalised conductunce as a function of bias voltage is presented in Fig.6. The spin polarisation of CrO2 films (81+-3%) was measured by the point contact Andreev reflection; the barrier strength Z range was 0.6-1.3
[i] W.J. Desito: US Patent 613385 Sep2003
CrO2Cl2 in combination with MeOH as oxygen source was applied as precursor for the ALD growth of Cr2O3 thin films on α-Al2O3(001) (c -sapphire) substrates at 420°C. Epitaxial films (RHEED) with 4-400 nm thicknesses (XRR) having O/Cr atomic ratio close to 1.5 with negligible amounts of residual impurities (electron probe microanalysis) were grown, the rms roughness was ≤1 nm according to AFM and X-ray reflection. Films were strained (tensile in the c direction and compressive in the c plane) according to XRD and Raman scattering. The elastic properties of the films differed only slightly from those for single-crystal bulk chromia based on surface Brillouin scattering measurements. A moderate response to CO in air with the tens of seconds transition times was observed for the films which surface was activated with Pt nanoislands. [876]
CrO2Cl2 and MeOH were used as precursors for the deposition of epitaxial α-Cr2O3 films on (11¯02) oriented α-Al2O3 by ALD at 375 °C. Film thickness was 10-310 nm (XRR), average growth rate was 0.1 nm/ cycle. The epitaxial relationship was (11¯02)[110]Cr2O3 || (11¯02)[110]Al2O3 in thinner films according to XRD; (1¯102) became the preferred growth plane at the thicknesses >40 nm (RHEED, XRD). This change was interpreted as the appearance of an asymmetric rhombohedral twin with the orientation relationship between the layers (1¯102)[110]top||(11¯02)[110]bottom and (1¯102)[11¯1]top||(11¯02)[1¯11]bottom. The structural model of the twin interface was used for characterisation of the match of the anion and cation sublattices of both layers. [877]
Thin epitax. α-Cr2O3(001) films were prepared by ALD at 420°C (reactor press. Ca. 10mbar), using CrO2Cl2 and MeOH precursors (evaporated at –20°C ?); each ALD cycle consisted of: 0.2s CrO2Cl2 exposure, first 2s N2 purge, 2s MeOH exposure, second 2 s N2 purge. The resistive response of the films covered with Pt nanoislands to H2 and CO in air was fast at 250°C, but slowed at 450°C. [878]
Ultrathin Cr2O3 layers were grown on GaAs (100), (110) substrates by molecular layering (ALD) using sequential exposures of the substrates to the vapors of CrO2Cl2 precursor and H2O (neutral hydroxylation agent) or MeOH (reducing hydroxylation agent). The effect of process conditions on the layer composition and growth mechanism was studied; layer dielectric properties and dielectric-semiconductor interface quality were evaluated. [879, 880]
CrO2Cl2 for Cr2-xTixO3, Cr2-xTixO3Cly, CrxTi1-xO2 CVD
Thin Cr2−xTixO3Cly films(≤3 µm) (x = 0.01-0.4; y = 0.1−0.3) were deposited by atmospheric pressure CVD from CrO2Cl2 and TiCl4 or Ti(OiPr)4 as precursors in the presence of an oxygen source (O2, H2O, ethyl acetate, MeOH, EtOH) on prefabricated sensor substrates. Film annealing at 600 °C/1 h under a flow of 5% H2 in N2 led to single phase Cr2−xTixO3 films (x = 0.01-0.4). Film were characterised by SEM, Raman spectroscopy, EDXA, XPS and the determination of the activation energy of conductance. The gas sensor response of the Cr2−xTixO3 films to ethanol was studied.[881]
Mixed chromia/ titania CrxTi1-xO2 powders with large solid solution range were deposited by plasma-enhanced CVD (with rf Ar-O2 plasma) using CrO2Cl2 as Cr precursor. The prepared mixed oxides consisted of 0.01-0.15 µm spherical particles. Metastable anatase phase obtained with 0% Cr; increase of Cr concentration resulted in increase of the rutile phase content and of stacking faults in the rutile structure. [882]
CrO2Cl2 for Cr2-xAlxO3 CVD
A mixture of CrO2Cl2 and AlCl3 was used for thermal plasma CVD preparation of a complex oxide consisted of chromia and alumina (Cr2-xAlxO3), using rf Ar-O2 plasma. The products were powders, generally consisting of spherical 0.01 - 0.15µm particles; chromia particles (α-chromia as major phase) often had hexagonal platelet shape up to 0.4µm diameter. The solid solution of Al in theα-chromia phase was below the detection limit of 10%, while the maximum degree of solid solution of Cr in theδ-alumina was ~6%. [883]
Chromium oxyselenide Cr2O3−xSex (x = 0-2.15) solid–solutions were prepared by the atmospheric pressure CVD of CrO2Cl2 and Et2Se at 600 °C. Gas-phase molar flows of reagents were directly influencing Se incorporation. Chromium oxyselenide with x = 0–0.2 crystallises in the Cr2O3 structure, whereas at x = 0.3–2.15 it adopts the Cr2Se3 structure type. Cr2O3−xSex were antiferromagnetic materials with Néel temperature varying with composition with a minimum temperature of 11 K observed for x = 0.7. [884]