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Si (1 0 0) substrate
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Journal of Crystal Growth 290 (2006) 653–659 www.elsevier.com/locate/jcrysgro
CrO2 (1 0 0) and TiO2 (1 0 0) film heteroepitaxy on a BaF2 (1 1 1)/Si (1 0 0) substrate L.D. Doucettea,, T.M. Christensenb, W.J. DeSistoa, R.J. Lada a
Laboratory for Surface Science and Technology, University of Maine, Orono, ME 04469-5708, USA Department of Physics and Energy Science, University of Colorado at Colorado Springs, Colorado Springs, CO 80933-7150, USA
b
Received 21 October 2005; received in revised form 25 January 2006; accepted 26 January 2006 Communicated by P. Rudolph Available online 10 March 2006
Abstract Multi-domained heteroepitaxial rutile-phase TiO2 (1 0 0)-oriented films were grown on Si (1 0 0) substrates by using a 30-nm-thick BaF2 (1 1 1) buffer layer at the TiO2–Si interface. The 50 nm TiO2 films were grown by electron cyclotron resonance oxygen plasmaassisted electron beam evaporation of a titanium source, and the growth temperature was varied from 300 to 600 1C. At an optimal temperature of 500 1C, X-ray diffraction measurements show that rutile phase TiO2 films are produced. Pole figure analysis indicates that the TiO2 layer follows the symmetry of the BaF2 surface mesh, and consists of six (1 0 0)-oriented domains separated by 301 in-plane ¯ or rotations about the TiO2 [1 0 0] axis. The in-plane alignment between the TiO2 and BaF2 films is oriented as [0 0 1] TiO2 || BaF2 ½1 0 1 ¯ [0 0 1] TiO2 || BaF2 ½1 1 2. Rocking curve and STM analyses suggest that the TiO2 films are more finely grained than the BaF2 film. STM imaging also reveals that the TiO2 surface has morphological features consistent with the BaF2 surface mesh symmetry. One of the optimally grown TiO2 (1 0 0) films was used to template a CrO2 (1 0 0) film which was grown via chemical vapor deposition. Point contact Andreev reflection measurements indicate that the CrO2 film was approximately 70% spin polarized. r 2006 Elsevier B.V. All rights reserved. PACS: 61.10.Nz; 68.35.–p; 68.55.–a; 68.55.Jk; 68.65.Ac Keywords: A3. Oxide heteroepitaxy; B1. BaF2 on silicon; B1. CrO2 film; B1. TiO2 rutile film
1. Introduction Thin films of crystalline TiO2 are of considerable interest due to their potential applications arising from a large dielectric constant, high refractive index and highly catalytic surface. Over the past few years, both rutileand anatase-phase TiO2 films have been grown on several different substrates using a variety of methods. In this study, we have successfully grown a (1 0 0)oriented rutile-phase TiO2 film on a silicon (1 0 0) substrate by using a BaF2 (1 1 1) thin film buffer layer. The insulating BaF2 layer plays an important role in influencing the rutile phase stability and heteroepitaxial orientation. One potential application for rutile-phase TiO2 films is to use them as Corresponding author.
E-mail address: [email protected] (L.D. Doucette). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.01.040
a template for CrO2 films (or other transition metal oxides crystallizing in the rutile structure). CrO2 is a promising material with a wide variety of applications due to the theoretical 100% spin polarization of its conduction band electrons [1]. TiO2/BaF2/Si substrates potentially allow the integration of rutile transition metal oxide films with Sibased electronics. Rutile CrO2(1 0 0)-oriented films can be grown on bulk (rutile) TiO2 single crystal (1 0 0) surfaces using chemical vapor deposition (CVD) growth methods [2–6]. Lattice matching of the (1 0 0) surfaces stimulates the CrO2 phase, and suppresses the more energetically favorable hexagonal phase Cr2O3 that typically forms during thin film growth on other substrates; Cr2O3 films do not exhibit spinpolarized behavior. For rutile TiO2, the (1 1 0) surface has the highest atomic density and the lowest surface energy. This orientation of rutile TiO2 tends to form during thin
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film growth unless other energy terms compete with the surface energy. Substrate epitaxy, ion bombardment during diffusion and energetic deposition processes can all contribute additional energies to the film growth process. (1 0 0)-oriented rutile phase TiO2 film growth has been reported on several different substrates. Heteroepitaxial growth has been achieved on the ð1 1 2¯ 0Þ and (0 0 0 1) surfaces of sapphire a-(Al2O3) by MOCVD [7,8]. Reactive sputtering of Ti in an oxygen background has also been used to produce TiO2 (1 0 0) films on MgO (1 1 1) and on sapphire (0 0 0 1) and ð1 0 1¯ 0Þsurfaces [9,10]. Fujii et al. [9] activated the oxygen using an RF plasma and found that the rutile structure grows best at oxygen pressures greater than 2 104 Torr. Evaporation of Ti in an oxygen background has produced ultrathin films of epitaxial TiO2 (1 0 0) on Mo (1 0 0) surfaces [11]. Other recent studies of TiO2 films include pulsed laser deposition of rutile TiO2 (1 0 0) on sapphire (0 0 0 1) [12], mixed orientations of rutile TiO2 from thermal oxidation of sputtered Ti films [13] and RF sputtering of sintered TiO2 targets followed by annealing in oxygen to produce rutile TiO2 films with (1 1 0) or mixed orientations [14,15]. Two groups have reported growth of rutile-phase TiO2 (1 0 0) films directly on Si (1 0 0) substrates. Zhang et al. [16] used ion beam-assisted evaporation of Ti in an oxygen environment onto carbon, quartz, Si (1 0 0) and Si (1 1 1) surfaces at 250 1C. They were able to produce a (1 0 0)oriented structure for a narrow range of Xe ion flux. Increasing the arrival rate of ions relative to deposited atoms at a constant 40 keV energy increased the (1 0 0) character of the films up to some level. Further increases in the ion flux resulted in some (1 1 0)-oriented crystals being grown. The results were independent of the type of substrate indicating that epitaxial growth was not necessarily occurring. X-ray diffraction (XRD) rocking curves of the TiO2 (2 0 0) peak exhibit a FWHM of 3.31 indicating that slightly misoriented crystallites were formed. Dai et al. [17] produced TiO2(1 0 0) films on a 100 nm amorphous oxide interlayer containing Ti, Si and O on Si (1 0 0) by electron beam evaporation of Ti followed by oxidation of the metal in a tube furnace at 500–700 1C. The resultant films exhibit crystalline structure with TiO2 (1 0 0) parallel to the Si (1 0 0) surface based on XRD measurements. The rocking curve width for the TiO2 (1 0 0) peak is 2.321 which suggests that these films contains dislocations, stacking faults and other defects. Pole figure analysis indicates that the TiO2 [1 1 0] direction is along Si [1 1 0]. Other rutile TiO2 film orientations have also been produced on Si substrates. Hiratani et al. [14,15] prepared rutile TiO2 on Si by RF magnetron sputtering and a post deposition 800 1C anneal and oxidation. XRD indicated a polycrystalline film with several different reflections observed. RF magnetron sputtering and 1000 1C annealing were used to produce highly oriented (1 1 0) polycrystalline rutile films on Si (1 0 0) [18]. To produce TiO2 (1 0 0)-oriented surfaces on Si (1 0 0) substrates, our approach is to use a highly structured BaF2
(1 1 1) buffer layer at the Si/TiO2 interface in order to control the subsequent TiO2 growth structure. BaF2 thin films have been shown to form well-defined layers on Si (1 0 0) [19,20] which possess excellent insulating qualities for semiconductor-on-insulator (SOI) [21] and metal–insulator–semiconductor (MIS) [22] device applications. The use of a 30 nm BaF2 (1 1 1) buffer layer film for growth of oxide films on Si (1 0 0) has also been examined for the WO3 system [23]. In that case, chemical reactions occur within the buffer layer leading to barium tungstate (BaWO4) formation at the interface. The results presented here demonstrate that heteroepitaxial TiO2 (1 0 0) films can be integrated with a Si substrate using this BaF2 buffer layer approach, and in this case, the layers do not intermix and have abrupt interfaces. Finally, we show that a (1 0 0)oriented CrO2 film can be grown on a TiO2 (1 0 0)/BaF2 (1 1 1)/Si (1 0 0) substrate via CVD, which demonstrates that CrO2 films can be integrated onto buffered silicon substrates. 2. Experimental methods The BaF2 buffer layers were grown on Si (1 0 0) substrates using very low rate MBE at the Naval Surface Warfare Center Dahlgren Division, Dahlgren, VA. Details of the BaF2/Si growth process and characterization of the BaF2 film are discussed elsewhere [19]. For this study, the BaF2 buffer layer was 30 nm thick as determined by Rutherford backscattering measurements. The TiO2 films were deposited at the University of Maine using a versatile thin film deposition, processing and characterization facility [24]. A four-pocket Telemark electron beam evaporator was used to evaporate Ti slugs of 99.995% purity. The depositions were assisted by a Wavemat ECR oxygen plasma source, which provided a relatively low energy (about 20 eV) flux of oxygen ions (O+ 2 ) and excited oxygen radicals (O*). Film deposition rates were determined in situ from a quartz crystal monitor that was calibrated against ex situ film thickness profilometry measurements. Sample heating was accomplished via a boron nitride-encased graphite heater placed in contact with the back of the sample holder. Deposition temperature was measured using an S-type thermocouple located between the heater and the back of the sample holder, which was calibrated against surface temperature measured with an infrared pyrometer. BaF2 (1 1 1)/Si (1 0 0) wafers were mounted onto tantalum carriers by spot-welding 8 mil tantalum wire across the sample corners. A 200 W ECR argon plasma operating at 104 Torr for 20 min was used to clean the growth surface of the mounted wafers prior to film growth. This sample processing eliminates adventitious carbon and oxygen species without affecting the BaF2 composition and structure as determined by XPS and XRD. TiO2 growth temperatures were varied between 300 and 600 1C; at temperatures of 650 1C and greater, BaF2 has been shown to dissociate and react with the Si substrate to form a BaSi2
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interfacial reaction product [19]. All other deposition parameters remained fixed for the different TiO2 film growths, including deposition rate (0.02 nm/s), ECR plasma power (200 W), oxygen ECR plasma pressure (105 Torr) and TiO2 film thickness (50 nm). Some of the TiO2/BaF2/Si samples were transferred under vacuum to a scanning probe microscopy chamber where images of the surface were acquired with a JEOL variable temperature STM/AFM. The microstructure of the TiO2 films was analyzed by ex situ XRD with a fourcircle Scintag diffractometer. The diffractometer was operated with a 1.8 kW line focus achromatic Cu Ka source and a solid-state detector. XRD y2y measurements were performed with the Bragg–Brentano geometry; pole figures were acquired using the Schulz geometry for 01of o 3601 and 01owo601, with 11 step sizes for both angular parameters. X-ray rocking curve measurements were performed with source and detector slits of 0.5 and 0.1 mm, respectively. These slit sizes were effective at minimizing instrumental/source peak broadening as determined in a separate experiment that compared the Si (1 0 0) full-width half-maximum (FWHM) obtained with and without a Bartels-type four-bounce monochromator attached to the source. The 200-nm-thick CrO2 film was grown on a TiO2/BaF2/ Si substrate in a separate CVD chamber. The CVD process utilized a chromyl chloride (CrO2Cl2) precursor and highpurity oxygen as the carrier gas [25]. The gas flow into the reactor (a 350 1C tube furnace) was between 20 and 40 sccm. The resulting CrO2 film was characterized using XRD. Also, point contact Andreev reflection measurements were conducted at the Naval Research Laboratory (Washington, DC) to determine the amount of spin polarization within the CrO2 film [26]. 3. Results and analysis 3.1. Structure of the BaF2 layer on Si (1 0 0) BaF2 has the face-centered cubic fluorite crystal structure (space group Fm3m) with a bulk lattice constant a ¼ 0.620 nm [27]. In a previous study [23] we characterized the structure and orientation of (1 1 1)-oriented BaF2 films on Si (1 0 0). Here, we briefly review the results that are relevant to the present study. XRD pole figure measurements indicate that the BaF2 film layer consists of four distinct (1 1 1) cubic domains differentiated by 301 rotations about the BaF2 [1 1 1] axis. LEED and RHEED measurements also confirm this film orientation of the BaF2 film. AFM images of the BaF2 surface show evidence of the BaF2 grains, where relatively smooth 500-nm-wide regions are separated by well-defined grain boundaries. The fine structure observed within the AFM images has a root mean square (rms) roughness of 0.8 nm. When truncated along the (1 1 1) plane, the four cubic orientations in three dimensions reduce to two distinct hexagonal meshes oriented 301 with respect to each other.
655
Si(100) Substrate [010] 5.43 Å _ [001]
_ [011]
2D Unit Cell
_ [112]
_ [101]
8.77 Å
a √2
BaF2(111) Surface Mesh Orientations Fig. 1. Top view model of the BaF2 structure relative to the Si (1 0 0) substrate. Two distinct BaF2 (1 1 1) domains, oriented 301 with respect to each other, exist on the square Si (1 0 0) substrate.
Fig. 1 shows the relative orientations of these two hexagonal surface meshes with respect to the underlying ¯ || BaF2 Si (1 0 0) surface; one mesh is oriented with Si ½0 1 1 ¯ ¯ ¯ ½1 0 1, and the other is oriented with Si ½0 1 1 || BaF2 ½1 1 2. Santiago et al. [19,20] have shown that a BaSi2 compliance layer of approximately 1–2 monolayers forms at the BaF2 (1 1 1)–Si (1 0 0) interface, and this compliance layer likely plays a role in producing the (1 1 1) rather than the (0 0 1) BaF2 orientation for this system. 3.2. Heteroepitaxial TiO2 (1 0 0) film growth on BaF2/Si substrates Rutile TiO2 has a tetragonal crystal structure (space group P42/mnm) with lattice constants a ¼ 0.459 nm, c ¼ 0.296 nm [27]. For the rutile phase of TiO2, the atomic arrangement consists of six oxygen atoms octahedrally coordinated about each titanium atom. In general, the octahedral arrangement for the rutile structure is slightly irregular, having four of the oxygen atoms at a slightly different distance from the other two. Crystalline TiO2 also stabilizes in two other polymorphs, anatase (tetragonal) and brookite (orthorhombic). Anatase TiO2 often forms during thin film TiO2 growth on several different substrates [28]. However, only rutile TiO2 was formed for all the growth conditions used in this study. Fig. 2 shows a compilation of XRD y2y measurements acquired from 50-nm-thick TiO2 films grown on a BaF2 (1 1 1)/Si (1 0 0) substrate, which illustrates the effect of substrate temperature on the film crystallographic texture. For substrate temperatures between 400–500 1C, the predominant peak occurs at 39.21, which corresponds to a (2 0 0) reflection from the formation of a (1 0 0)-oriented rutile TiO2 component within the film matrix. Growth temperatures below and above this temperature range do not produce any measurable crystallinity within the TiO2
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BaF2 (111)
TiO2 (110)
TiO2 (200)
2 = 25.5° = TiO2(110) Poles at χ = 45° φ = 0˚
600°C Log Intensity
_ Si(111) 550°C 500°C ° 45 χ=
450°C
Si(111)
400°C 300°C 24
26
28 38
40
__ Si(111)
2θ Fig. 2. XRD y2y scans from TiO2 films on BaF2/Si, which show the effects of growth temperature upon film structure.
film. Also observed is the formation of a (1 1 0)-oriented rutile TiO2 component at 450 1C, as evidenced by the smaller peak occurring at 27.51 in Fig. 2. For rutile TiO2, the scattering intensity of the (2 0 0) reflection is comparatively weaker than other low-index planes, whereas the (1 0 0) reflection is completely forbidden. The weak coherent scattering associated with the (2 0 0) planes is primarily due to the partial incoherence between the scattered wave intensities of the titanium sublattice and the oxygen sublattice. By comparison, the scattering intensity for the (1 1 0) reflection is larger since in this orientation scattered waves from all of the titanium atoms and half of the oxygen atoms are in phase [29]. Powder diffraction measurements indicate that the peak height ratio of the (1 1 0) and (2 0 0) reflections can be as large as 10 to 1 [30]. For the film grown at 450 1C (Fig. 2) the (2 0 0) raw peak height is slightly larger than then (1 1 0) raw peak height on a log scale, and an intensity-corrected peak comparison between the (2 0 0) and (1 1 0) peaks suggests that this film contains approximately 93% (2 0 0)oriented TiO2 crystallites. Fig. 2 also shows a peak at 24.91, which corresponds to the (1 1 1) planes from the 30-nm-thick BaF2 film layer. This peak intensity does not appreciably change over the entire growth temperature range, suggesting that the BaF2 film layer remains stable at these elevated growth temperatures and does not chemically react with the deposited TiO2 layer. This stability is in contrast to WO3 films grown on the same BaF2 (1 1 1)/Si (1 0 0) substrates, which were shown to react with the BaF2 film layer and form an interfacial barium tungstate (BaWO4) reaction product for substrate temperatures X400 1C [23]. For TiO2 films grown within the 400–500 1C temperature range, XRD film texture analysis indicates a distinct heteroepitaxial relationship between the TiO2 film and
_ Si(111)
180° Fig. 3. XRD (1 1 0) pole figure measured from the rutile TiO2 (1 0 0)oriented film layer. Four Si (1 1 1) poles at w ¼ 54:71 are also observed due to a broad radiation shoulder associated with these strong reflections.
the BaF2/Si substrate. Fig. 3 shows the XRD pole figure from a TiO2 film that was grown at 500 1C. For this measurement, the (1 1 0) poles associated with the (1 0 0)oriented TiO2 film layer were measured by setting 2y ¼ 27.51 and rotating the sample about the f and w axes. Twelve peaks, equally spaced about the azimuthal f angle, were measured at a polar angle of w ¼ 451, which is the expected angle between the (1 0 0) and (1 1 0) planes in rutile TiO2. Similar pole figure results (not shown) were obtained for TiO2 films grown at 400 1C and 450 1C. Also indicated in the pole figure are four Si (1 1 1) poles at w ¼ 54:71, which is the expected polar angle between the (1 0 0) and (1 1 1) planes in the single crystal Si substrate. Despite the fact that the difference in the 2y parameters between Si (1 1 1) and TiO2 (1 1 0) is nearly 11 (Si (1 1 1) ¼ 28.41), the broad shoulder associated with the intense Si (1 1 1) peak from the achromatic Cu Ka radiation still makes this reflection observable in the pole figure. 3.3. Grain size and morphology of TiO2 (1 0 0)-oriented films X-ray rocking curve measurements and STM imaging were used to characterize the grain size and morphology of the TiO2 films. Fig. 4 shows rocking curve measurements that were acquired from the bare BaF2/Si substrate as well as a TiO2 film (500 1C growth temperature) grown on the
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BaF2(111)/Si(100)
Intensity
TiO2(200)/BaF2(111)/Si(100)
Intensity 11 (a)
657
12
13
14
ω (degrees)
17
18
19 20 ω (degrees)
(b)
21
22
Fig. 4. XRD rocking curves (a) from the bare BaF2 (1 1 1)/Si (1 0 0) substrate and (b) the (2 0 0) reflection from the heteroepitaxial TiO2 (1 0 0) film.
Log Intensity
BaF2(111)
CrO2(200) TiO2(200)
23
24
25
26
38 2θ
39
40
41
42
Fig. 6. XRD y2y scan from a CVD grown CrO2 film on top of the TiO2/ BaF2/Si heteroepitaxial multilayer structure. Fig. 5. 200 nm 200 nm STM image acquired in situ from a 50-nm-thick rutile TiO2 film.
BaF2/Si substrate. A Voigt function peak fit yields FWHM’s of 0.511 and 2.251 for the BaF2 (1 1 1) and TiO2 (200) peaks, respectively. The wider FWHM of the TiO2 (2 0 0) peak indicates that the TiO2 film is more finely grained than the BaF2 film composition, and likely contains a larger defect density and mosaic spread. Evidence of these smaller TiO2 grain sizes was corroborated by STM imaging. In Fig. 5, a 200 nm 200 nm STM image shows a granular TiO2 film surface morphology with an rms roughness of approximately 2 nm. Most of the apparent grain features in the image are elongated with dimensions of approximately 5 nm 25 nm, which is much less than the approximated 500 nm diameter BaF2 grain
sizes observed by AFM [23]. TiO2 grain features in the STM image are also elongated in directions consistent with the hexagonal symmetry of the BaF2 (1 1 1) surface mesh. 3.4. Heteroepitaxial CrO2 (1 0 0) film growth on a TiO2/ BaF2/Si substrate CrO2 is identical in crystal structure as rutile TiO2, and has nearly identical lattice constants of a ¼ 0.442 nm, c ¼ 0.292 nm [31]. A 200-nm-thick CrO2 film was grown by CVD on a TiO2 (1 0 0)/BaF2 (1 1 1)/Si (1 0 0) substrate, and the XRD y2y scan in Fig. 6 shows the corresponding CrO2 film and substrate film structures. The peak at 41.81 corresponds to the (2 0 0) reflection from the formation of a (1 0 0)-oriented CrO2 layer, while the underlying substrate film layer structures are evidenced by the peaks occurring
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at 24.91 for BaF2 (1 1 1) and 39.21 for TiO2 (2 0 0). A pole figure measurement of the CrO2 (1 1 0) poles (not shown) resulted in 12 equally spaced peaks about the azimuthal angle, identical to the pole figure that was acquired for the TiO2 film layer (see Fig. 3). A point contact Andreev reflection measurement of the CrO2 film showed that the film achieved a spin polarization of approximately 70%. Based on theory [1] and measurement [3], CrO2 is expected to have 100% spin polarization. Our measurement of 70% is likely due to the mosaic spread, crystalline defects and grain boundary effects in the CrO2 film, which would partially disrupt electron spin orientation within the conduction band.
4. Discussion For heteroepitaxial growth of a true single crystal TiO2 (1 0 0) layer, only two (1 1 0)-TiO2 poles are expected in an XRD pole figure measurement at w ¼ 451, and these two poles are separated by a 1801 rotation about the f-axis (i.e. the TiO2 [1 0 0] axis). In Fig. 3, however, 12 poles are observed, indicating that there are six distinct crystalline domains of rutile TiO2 separated by 301 rotations about the TiO2 [1 0 0] axis. These six domains result from the fact that the underlying BaF2 (1 1 1) layer is comprised of two equivalent six-fold symmetric surface meshes offset by a 301 in-plane rotation. Therefore, the subsequently grown heteroepitaxial TiO2 film becomes multidomained and mirrors the same overall symmetry as the BaF2 film. In addition, the heteroepitaxial relationship between the templated CrO2 (1 0 0) film and the TiO2 (1 0 0) film contains the same six equivalent multidomained texture within the CrO2 layer. The presence of multiple crystalline domain orientations creates crystallite boundaries which presumably are responsible for the less than 100% spin polarization measured for this CrO2 film. The relative alignment of the TiO2 (1 1 0) and Si (1 1 1) poles in the pole figure can be used to resolve the six TiO2 (2 0 0) in-plane orientations (and similarly the six CrO2 (2 0 0) in-plane orientations) with respect to the Si (1 0 0) substrate and the BaF2 (1 1 1) surface meshes (Fig. 7). In particular, the TiO2 (1 1 0) and Si (1 1 1) poles that are ¯ coincident in f indicate a [0 0 1] TiO2 || Si ½0 1 1
orientation. Using the model shown in Fig. 1, we deduce the in-plane orientation between the TiO2 and BaF2 layers ¯ (orientation 1) or [0 0 1] to be [0 0 1] TiO2 || BaF2 ½1 0 1 ¯ TiO2 || BaF2 ½1 1 2 (orientation 2). Because of the high degree of symmetry, the distinction between orientations 1 and 2 cannot be resolved from pole figure data. In-plane lattice misfit calculations over an integral number of lattice spacings for the TiO2 and BaF2 film layers result in mismatches of 4.5%, +2.6% and 1.2%, +10.3% for orientations 1 and 2, respectively. Although orientation 1 has a smaller overall misfit, metal oxide systems having larger misfits similar to orientation 2 have been reported to lead to heteroepitaxial growth despite the discrepancy [28]. Therefore, it is likely that both in-plane orientations 1 and 2 shown in Fig. 7 form within TiO2 films grown on these substrates. 5. Conclusions A high-quality BaF2 (1 1 1) buffer on single crystal Si (1 0 0) can be used to stabilize rutile phase TiO2 films with (1 0 0) heteroepitaxy. These substrates can then be used to template a CrO2 film, which demonstrates the integration of a rutile metal oxide material with unique electrical/ magnetic properties with a silicon substrate. XRD pole figure analyses reveal distinct heteroepitaxial relationships between all layers in the CrO2 (1 0 0)/TiO2 (1 0 0)/BaF2 (1 1 1)/Si (1 0 0) system. The symmetry of the BaF2 surface induces similar symmetry within the CrO2/TiO2 film layers. The resultant CrO2 and TiO2 films each consist of six distinct crystalline domains differentiated by 301 rotations about [1 0 0] axis. Acknowledgements We gratefully acknowledge the assistance from the following individuals: Francisco Santiago (Naval Surface Warfare Center) for providing the BaF2/Si substrates; Young-Nam Cho (University of Maine) for growing the CrO2 film; Steven Smallwood (University of Maine) for acquiring the STM image and Konrad Bussmann (Naval Research Lab) for performing the Andreev reflection measurement. References
BaF2(111) Surface [001] a
[001] a c
c (1)
2D Unit Cell _ [110]
_ [101] _ [112]
(2)
TiO2(100) Domains Fig. 7. Model of the two possible TiO2 (1 0 0) domain orientations on the BaF2 (1 1 1) surface. See text for details.
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Journal of Crystal Growth 290 (2006) 653–659 www.elsevier.com/locate/jcrysgro
CrO2 (1 0 0) and TiO2 (1 0 0) film heteroepitaxy on a BaF2 (1 1 1)/Si (1 0 0) substrate L.D. Doucettea,, T.M. Christensenb, W.J. DeSistoa, R.J. Lada a
Laboratory for Surface Science and Technology, University of Maine, Orono, ME 04469-5708, USA Department of Physics and Energy Science, University of Colorado at Colorado Springs, Colorado Springs, CO 80933-7150, USA
b
Received 21 October 2005; received in revised form 25 January 2006; accepted 26 January 2006 Communicated by P. Rudolph Available online 10 March 2006
Abstract Multi-domained heteroepitaxial rutile-phase TiO2 (1 0 0)-oriented films were grown on Si (1 0 0) substrates by using a 30-nm-thick BaF2 (1 1 1) buffer layer at the TiO2–Si interface. The 50 nm TiO2 films were grown by electron cyclotron resonance oxygen plasmaassisted electron beam evaporation of a titanium source, and the growth temperature was varied from 300 to 600 1C. At an optimal temperature of 500 1C, X-ray diffraction measurements show that rutile phase TiO2 films are produced. Pole figure analysis indicates that the TiO2 layer follows the symmetry of the BaF2 surface mesh, and consists of six (1 0 0)-oriented domains separated by 301 in-plane ¯ or rotations about the TiO2 [1 0 0] axis. The in-plane alignment between the TiO2 and BaF2 films is oriented as [0 0 1] TiO2 || BaF2 ½1 0 1 ¯ [0 0 1] TiO2 || BaF2 ½1 1 2. Rocking curve and STM analyses suggest that the TiO2 films are more finely grained than the BaF2 film. STM imaging also reveals that the TiO2 surface has morphological features consistent with the BaF2 surface mesh symmetry. One of the optimally grown TiO2 (1 0 0) films was used to template a CrO2 (1 0 0) film which was grown via chemical vapor deposition. Point contact Andreev reflection measurements indicate that the CrO2 film was approximately 70% spin polarized. r 2006 Elsevier B.V. All rights reserved. PACS: 61.10.Nz; 68.35.–p; 68.55.–a; 68.55.Jk; 68.65.Ac Keywords: A3. Oxide heteroepitaxy; B1. BaF2 on silicon; B1. CrO2 film; B1. TiO2 rutile film
1. Introduction Thin films of crystalline TiO2 are of considerable interest due to their potential applications arising from a large dielectric constant, high refractive index and highly catalytic surface. Over the past few years, both rutileand anatase-phase TiO2 films have been grown on several different substrates using a variety of methods. In this study, we have successfully grown a (1 0 0)oriented rutile-phase TiO2 film on a silicon (1 0 0) substrate by using a BaF2 (1 1 1) thin film buffer layer. The insulating BaF2 layer plays an important role in influencing the rutile phase stability and heteroepitaxial orientation. One potential application for rutile-phase TiO2 films is to use them as Corresponding author.
E-mail address: [email protected] (L.D. Doucette). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.01.040
a template for CrO2 films (or other transition metal oxides crystallizing in the rutile structure). CrO2 is a promising material with a wide variety of applications due to the theoretical 100% spin polarization of its conduction band electrons [1]. TiO2/BaF2/Si substrates potentially allow the integration of rutile transition metal oxide films with Sibased electronics. Rutile CrO2(1 0 0)-oriented films can be grown on bulk (rutile) TiO2 single crystal (1 0 0) surfaces using chemical vapor deposition (CVD) growth methods [2–6]. Lattice matching of the (1 0 0) surfaces stimulates the CrO2 phase, and suppresses the more energetically favorable hexagonal phase Cr2O3 that typically forms during thin film growth on other substrates; Cr2O3 films do not exhibit spinpolarized behavior. For rutile TiO2, the (1 1 0) surface has the highest atomic density and the lowest surface energy. This orientation of rutile TiO2 tends to form during thin
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film growth unless other energy terms compete with the surface energy. Substrate epitaxy, ion bombardment during diffusion and energetic deposition processes can all contribute additional energies to the film growth process. (1 0 0)-oriented rutile phase TiO2 film growth has been reported on several different substrates. Heteroepitaxial growth has been achieved on the ð1 1 2¯ 0Þ and (0 0 0 1) surfaces of sapphire a-(Al2O3) by MOCVD [7,8]. Reactive sputtering of Ti in an oxygen background has also been used to produce TiO2 (1 0 0) films on MgO (1 1 1) and on sapphire (0 0 0 1) and ð1 0 1¯ 0Þsurfaces [9,10]. Fujii et al. [9] activated the oxygen using an RF plasma and found that the rutile structure grows best at oxygen pressures greater than 2 104 Torr. Evaporation of Ti in an oxygen background has produced ultrathin films of epitaxial TiO2 (1 0 0) on Mo (1 0 0) surfaces [11]. Other recent studies of TiO2 films include pulsed laser deposition of rutile TiO2 (1 0 0) on sapphire (0 0 0 1) [12], mixed orientations of rutile TiO2 from thermal oxidation of sputtered Ti films [13] and RF sputtering of sintered TiO2 targets followed by annealing in oxygen to produce rutile TiO2 films with (1 1 0) or mixed orientations [14,15]. Two groups have reported growth of rutile-phase TiO2 (1 0 0) films directly on Si (1 0 0) substrates. Zhang et al. [16] used ion beam-assisted evaporation of Ti in an oxygen environment onto carbon, quartz, Si (1 0 0) and Si (1 1 1) surfaces at 250 1C. They were able to produce a (1 0 0)oriented structure for a narrow range of Xe ion flux. Increasing the arrival rate of ions relative to deposited atoms at a constant 40 keV energy increased the (1 0 0) character of the films up to some level. Further increases in the ion flux resulted in some (1 1 0)-oriented crystals being grown. The results were independent of the type of substrate indicating that epitaxial growth was not necessarily occurring. X-ray diffraction (XRD) rocking curves of the TiO2 (2 0 0) peak exhibit a FWHM of 3.31 indicating that slightly misoriented crystallites were formed. Dai et al. [17] produced TiO2(1 0 0) films on a 100 nm amorphous oxide interlayer containing Ti, Si and O on Si (1 0 0) by electron beam evaporation of Ti followed by oxidation of the metal in a tube furnace at 500–700 1C. The resultant films exhibit crystalline structure with TiO2 (1 0 0) parallel to the Si (1 0 0) surface based on XRD measurements. The rocking curve width for the TiO2 (1 0 0) peak is 2.321 which suggests that these films contains dislocations, stacking faults and other defects. Pole figure analysis indicates that the TiO2 [1 1 0] direction is along Si [1 1 0]. Other rutile TiO2 film orientations have also been produced on Si substrates. Hiratani et al. [14,15] prepared rutile TiO2 on Si by RF magnetron sputtering and a post deposition 800 1C anneal and oxidation. XRD indicated a polycrystalline film with several different reflections observed. RF magnetron sputtering and 1000 1C annealing were used to produce highly oriented (1 1 0) polycrystalline rutile films on Si (1 0 0) [18]. To produce TiO2 (1 0 0)-oriented surfaces on Si (1 0 0) substrates, our approach is to use a highly structured BaF2
(1 1 1) buffer layer at the Si/TiO2 interface in order to control the subsequent TiO2 growth structure. BaF2 thin films have been shown to form well-defined layers on Si (1 0 0) [19,20] which possess excellent insulating qualities for semiconductor-on-insulator (SOI) [21] and metal–insulator–semiconductor (MIS) [22] device applications. The use of a 30 nm BaF2 (1 1 1) buffer layer film for growth of oxide films on Si (1 0 0) has also been examined for the WO3 system [23]. In that case, chemical reactions occur within the buffer layer leading to barium tungstate (BaWO4) formation at the interface. The results presented here demonstrate that heteroepitaxial TiO2 (1 0 0) films can be integrated with a Si substrate using this BaF2 buffer layer approach, and in this case, the layers do not intermix and have abrupt interfaces. Finally, we show that a (1 0 0)oriented CrO2 film can be grown on a TiO2 (1 0 0)/BaF2 (1 1 1)/Si (1 0 0) substrate via CVD, which demonstrates that CrO2 films can be integrated onto buffered silicon substrates. 2. Experimental methods The BaF2 buffer layers were grown on Si (1 0 0) substrates using very low rate MBE at the Naval Surface Warfare Center Dahlgren Division, Dahlgren, VA. Details of the BaF2/Si growth process and characterization of the BaF2 film are discussed elsewhere [19]. For this study, the BaF2 buffer layer was 30 nm thick as determined by Rutherford backscattering measurements. The TiO2 films were deposited at the University of Maine using a versatile thin film deposition, processing and characterization facility [24]. A four-pocket Telemark electron beam evaporator was used to evaporate Ti slugs of 99.995% purity. The depositions were assisted by a Wavemat ECR oxygen plasma source, which provided a relatively low energy (about 20 eV) flux of oxygen ions (O+ 2 ) and excited oxygen radicals (O*). Film deposition rates were determined in situ from a quartz crystal monitor that was calibrated against ex situ film thickness profilometry measurements. Sample heating was accomplished via a boron nitride-encased graphite heater placed in contact with the back of the sample holder. Deposition temperature was measured using an S-type thermocouple located between the heater and the back of the sample holder, which was calibrated against surface temperature measured with an infrared pyrometer. BaF2 (1 1 1)/Si (1 0 0) wafers were mounted onto tantalum carriers by spot-welding 8 mil tantalum wire across the sample corners. A 200 W ECR argon plasma operating at 104 Torr for 20 min was used to clean the growth surface of the mounted wafers prior to film growth. This sample processing eliminates adventitious carbon and oxygen species without affecting the BaF2 composition and structure as determined by XPS and XRD. TiO2 growth temperatures were varied between 300 and 600 1C; at temperatures of 650 1C and greater, BaF2 has been shown to dissociate and react with the Si substrate to form a BaSi2
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interfacial reaction product [19]. All other deposition parameters remained fixed for the different TiO2 film growths, including deposition rate (0.02 nm/s), ECR plasma power (200 W), oxygen ECR plasma pressure (105 Torr) and TiO2 film thickness (50 nm). Some of the TiO2/BaF2/Si samples were transferred under vacuum to a scanning probe microscopy chamber where images of the surface were acquired with a JEOL variable temperature STM/AFM. The microstructure of the TiO2 films was analyzed by ex situ XRD with a fourcircle Scintag diffractometer. The diffractometer was operated with a 1.8 kW line focus achromatic Cu Ka source and a solid-state detector. XRD y2y measurements were performed with the Bragg–Brentano geometry; pole figures were acquired using the Schulz geometry for 01of o 3601 and 01owo601, with 11 step sizes for both angular parameters. X-ray rocking curve measurements were performed with source and detector slits of 0.5 and 0.1 mm, respectively. These slit sizes were effective at minimizing instrumental/source peak broadening as determined in a separate experiment that compared the Si (1 0 0) full-width half-maximum (FWHM) obtained with and without a Bartels-type four-bounce monochromator attached to the source. The 200-nm-thick CrO2 film was grown on a TiO2/BaF2/ Si substrate in a separate CVD chamber. The CVD process utilized a chromyl chloride (CrO2Cl2) precursor and highpurity oxygen as the carrier gas [25]. The gas flow into the reactor (a 350 1C tube furnace) was between 20 and 40 sccm. The resulting CrO2 film was characterized using XRD. Also, point contact Andreev reflection measurements were conducted at the Naval Research Laboratory (Washington, DC) to determine the amount of spin polarization within the CrO2 film [26]. 3. Results and analysis 3.1. Structure of the BaF2 layer on Si (1 0 0) BaF2 has the face-centered cubic fluorite crystal structure (space group Fm3m) with a bulk lattice constant a ¼ 0.620 nm [27]. In a previous study [23] we characterized the structure and orientation of (1 1 1)-oriented BaF2 films on Si (1 0 0). Here, we briefly review the results that are relevant to the present study. XRD pole figure measurements indicate that the BaF2 film layer consists of four distinct (1 1 1) cubic domains differentiated by 301 rotations about the BaF2 [1 1 1] axis. LEED and RHEED measurements also confirm this film orientation of the BaF2 film. AFM images of the BaF2 surface show evidence of the BaF2 grains, where relatively smooth 500-nm-wide regions are separated by well-defined grain boundaries. The fine structure observed within the AFM images has a root mean square (rms) roughness of 0.8 nm. When truncated along the (1 1 1) plane, the four cubic orientations in three dimensions reduce to two distinct hexagonal meshes oriented 301 with respect to each other.
655
Si(100) Substrate [010] 5.43 Å _ [001]
_ [011]
2D Unit Cell
_ [112]
_ [101]
8.77 Å
a √2
BaF2(111) Surface Mesh Orientations Fig. 1. Top view model of the BaF2 structure relative to the Si (1 0 0) substrate. Two distinct BaF2 (1 1 1) domains, oriented 301 with respect to each other, exist on the square Si (1 0 0) substrate.
Fig. 1 shows the relative orientations of these two hexagonal surface meshes with respect to the underlying ¯ || BaF2 Si (1 0 0) surface; one mesh is oriented with Si ½0 1 1 ¯ ¯ ¯ ½1 0 1, and the other is oriented with Si ½0 1 1 || BaF2 ½1 1 2. Santiago et al. [19,20] have shown that a BaSi2 compliance layer of approximately 1–2 monolayers forms at the BaF2 (1 1 1)–Si (1 0 0) interface, and this compliance layer likely plays a role in producing the (1 1 1) rather than the (0 0 1) BaF2 orientation for this system. 3.2. Heteroepitaxial TiO2 (1 0 0) film growth on BaF2/Si substrates Rutile TiO2 has a tetragonal crystal structure (space group P42/mnm) with lattice constants a ¼ 0.459 nm, c ¼ 0.296 nm [27]. For the rutile phase of TiO2, the atomic arrangement consists of six oxygen atoms octahedrally coordinated about each titanium atom. In general, the octahedral arrangement for the rutile structure is slightly irregular, having four of the oxygen atoms at a slightly different distance from the other two. Crystalline TiO2 also stabilizes in two other polymorphs, anatase (tetragonal) and brookite (orthorhombic). Anatase TiO2 often forms during thin film TiO2 growth on several different substrates [28]. However, only rutile TiO2 was formed for all the growth conditions used in this study. Fig. 2 shows a compilation of XRD y2y measurements acquired from 50-nm-thick TiO2 films grown on a BaF2 (1 1 1)/Si (1 0 0) substrate, which illustrates the effect of substrate temperature on the film crystallographic texture. For substrate temperatures between 400–500 1C, the predominant peak occurs at 39.21, which corresponds to a (2 0 0) reflection from the formation of a (1 0 0)-oriented rutile TiO2 component within the film matrix. Growth temperatures below and above this temperature range do not produce any measurable crystallinity within the TiO2
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BaF2 (111)
TiO2 (110)
TiO2 (200)
2 = 25.5° = TiO2(110) Poles at χ = 45° φ = 0˚
600°C Log Intensity
_ Si(111) 550°C 500°C ° 45 χ=
450°C
Si(111)
400°C 300°C 24
26
28 38
40
__ Si(111)
2θ Fig. 2. XRD y2y scans from TiO2 films on BaF2/Si, which show the effects of growth temperature upon film structure.
film. Also observed is the formation of a (1 1 0)-oriented rutile TiO2 component at 450 1C, as evidenced by the smaller peak occurring at 27.51 in Fig. 2. For rutile TiO2, the scattering intensity of the (2 0 0) reflection is comparatively weaker than other low-index planes, whereas the (1 0 0) reflection is completely forbidden. The weak coherent scattering associated with the (2 0 0) planes is primarily due to the partial incoherence between the scattered wave intensities of the titanium sublattice and the oxygen sublattice. By comparison, the scattering intensity for the (1 1 0) reflection is larger since in this orientation scattered waves from all of the titanium atoms and half of the oxygen atoms are in phase [29]. Powder diffraction measurements indicate that the peak height ratio of the (1 1 0) and (2 0 0) reflections can be as large as 10 to 1 [30]. For the film grown at 450 1C (Fig. 2) the (2 0 0) raw peak height is slightly larger than then (1 1 0) raw peak height on a log scale, and an intensity-corrected peak comparison between the (2 0 0) and (1 1 0) peaks suggests that this film contains approximately 93% (2 0 0)oriented TiO2 crystallites. Fig. 2 also shows a peak at 24.91, which corresponds to the (1 1 1) planes from the 30-nm-thick BaF2 film layer. This peak intensity does not appreciably change over the entire growth temperature range, suggesting that the BaF2 film layer remains stable at these elevated growth temperatures and does not chemically react with the deposited TiO2 layer. This stability is in contrast to WO3 films grown on the same BaF2 (1 1 1)/Si (1 0 0) substrates, which were shown to react with the BaF2 film layer and form an interfacial barium tungstate (BaWO4) reaction product for substrate temperatures X400 1C [23]. For TiO2 films grown within the 400–500 1C temperature range, XRD film texture analysis indicates a distinct heteroepitaxial relationship between the TiO2 film and
_ Si(111)
180° Fig. 3. XRD (1 1 0) pole figure measured from the rutile TiO2 (1 0 0)oriented film layer. Four Si (1 1 1) poles at w ¼ 54:71 are also observed due to a broad radiation shoulder associated with these strong reflections.
the BaF2/Si substrate. Fig. 3 shows the XRD pole figure from a TiO2 film that was grown at 500 1C. For this measurement, the (1 1 0) poles associated with the (1 0 0)oriented TiO2 film layer were measured by setting 2y ¼ 27.51 and rotating the sample about the f and w axes. Twelve peaks, equally spaced about the azimuthal f angle, were measured at a polar angle of w ¼ 451, which is the expected angle between the (1 0 0) and (1 1 0) planes in rutile TiO2. Similar pole figure results (not shown) were obtained for TiO2 films grown at 400 1C and 450 1C. Also indicated in the pole figure are four Si (1 1 1) poles at w ¼ 54:71, which is the expected polar angle between the (1 0 0) and (1 1 1) planes in the single crystal Si substrate. Despite the fact that the difference in the 2y parameters between Si (1 1 1) and TiO2 (1 1 0) is nearly 11 (Si (1 1 1) ¼ 28.41), the broad shoulder associated with the intense Si (1 1 1) peak from the achromatic Cu Ka radiation still makes this reflection observable in the pole figure. 3.3. Grain size and morphology of TiO2 (1 0 0)-oriented films X-ray rocking curve measurements and STM imaging were used to characterize the grain size and morphology of the TiO2 films. Fig. 4 shows rocking curve measurements that were acquired from the bare BaF2/Si substrate as well as a TiO2 film (500 1C growth temperature) grown on the
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BaF2(111)/Si(100)
Intensity
TiO2(200)/BaF2(111)/Si(100)
Intensity 11 (a)
657
12
13
14
ω (degrees)
17
18
19 20 ω (degrees)
(b)
21
22
Fig. 4. XRD rocking curves (a) from the bare BaF2 (1 1 1)/Si (1 0 0) substrate and (b) the (2 0 0) reflection from the heteroepitaxial TiO2 (1 0 0) film.
Log Intensity
BaF2(111)
CrO2(200) TiO2(200)
23
24
25
26
38 2θ
39
40
41
42
Fig. 6. XRD y2y scan from a CVD grown CrO2 film on top of the TiO2/ BaF2/Si heteroepitaxial multilayer structure. Fig. 5. 200 nm 200 nm STM image acquired in situ from a 50-nm-thick rutile TiO2 film.
BaF2/Si substrate. A Voigt function peak fit yields FWHM’s of 0.511 and 2.251 for the BaF2 (1 1 1) and TiO2 (200) peaks, respectively. The wider FWHM of the TiO2 (2 0 0) peak indicates that the TiO2 film is more finely grained than the BaF2 film composition, and likely contains a larger defect density and mosaic spread. Evidence of these smaller TiO2 grain sizes was corroborated by STM imaging. In Fig. 5, a 200 nm 200 nm STM image shows a granular TiO2 film surface morphology with an rms roughness of approximately 2 nm. Most of the apparent grain features in the image are elongated with dimensions of approximately 5 nm 25 nm, which is much less than the approximated 500 nm diameter BaF2 grain
sizes observed by AFM [23]. TiO2 grain features in the STM image are also elongated in directions consistent with the hexagonal symmetry of the BaF2 (1 1 1) surface mesh. 3.4. Heteroepitaxial CrO2 (1 0 0) film growth on a TiO2/ BaF2/Si substrate CrO2 is identical in crystal structure as rutile TiO2, and has nearly identical lattice constants of a ¼ 0.442 nm, c ¼ 0.292 nm [31]. A 200-nm-thick CrO2 film was grown by CVD on a TiO2 (1 0 0)/BaF2 (1 1 1)/Si (1 0 0) substrate, and the XRD y2y scan in Fig. 6 shows the corresponding CrO2 film and substrate film structures. The peak at 41.81 corresponds to the (2 0 0) reflection from the formation of a (1 0 0)-oriented CrO2 layer, while the underlying substrate film layer structures are evidenced by the peaks occurring
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at 24.91 for BaF2 (1 1 1) and 39.21 for TiO2 (2 0 0). A pole figure measurement of the CrO2 (1 1 0) poles (not shown) resulted in 12 equally spaced peaks about the azimuthal angle, identical to the pole figure that was acquired for the TiO2 film layer (see Fig. 3). A point contact Andreev reflection measurement of the CrO2 film showed that the film achieved a spin polarization of approximately 70%. Based on theory [1] and measurement [3], CrO2 is expected to have 100% spin polarization. Our measurement of 70% is likely due to the mosaic spread, crystalline defects and grain boundary effects in the CrO2 film, which would partially disrupt electron spin orientation within the conduction band.
4. Discussion For heteroepitaxial growth of a true single crystal TiO2 (1 0 0) layer, only two (1 1 0)-TiO2 poles are expected in an XRD pole figure measurement at w ¼ 451, and these two poles are separated by a 1801 rotation about the f-axis (i.e. the TiO2 [1 0 0] axis). In Fig. 3, however, 12 poles are observed, indicating that there are six distinct crystalline domains of rutile TiO2 separated by 301 rotations about the TiO2 [1 0 0] axis. These six domains result from the fact that the underlying BaF2 (1 1 1) layer is comprised of two equivalent six-fold symmetric surface meshes offset by a 301 in-plane rotation. Therefore, the subsequently grown heteroepitaxial TiO2 film becomes multidomained and mirrors the same overall symmetry as the BaF2 film. In addition, the heteroepitaxial relationship between the templated CrO2 (1 0 0) film and the TiO2 (1 0 0) film contains the same six equivalent multidomained texture within the CrO2 layer. The presence of multiple crystalline domain orientations creates crystallite boundaries which presumably are responsible for the less than 100% spin polarization measured for this CrO2 film. The relative alignment of the TiO2 (1 1 0) and Si (1 1 1) poles in the pole figure can be used to resolve the six TiO2 (2 0 0) in-plane orientations (and similarly the six CrO2 (2 0 0) in-plane orientations) with respect to the Si (1 0 0) substrate and the BaF2 (1 1 1) surface meshes (Fig. 7). In particular, the TiO2 (1 1 0) and Si (1 1 1) poles that are ¯ coincident in f indicate a [0 0 1] TiO2 || Si ½0 1 1
orientation. Using the model shown in Fig. 1, we deduce the in-plane orientation between the TiO2 and BaF2 layers ¯ (orientation 1) or [0 0 1] to be [0 0 1] TiO2 || BaF2 ½1 0 1 ¯ TiO2 || BaF2 ½1 1 2 (orientation 2). Because of the high degree of symmetry, the distinction between orientations 1 and 2 cannot be resolved from pole figure data. In-plane lattice misfit calculations over an integral number of lattice spacings for the TiO2 and BaF2 film layers result in mismatches of 4.5%, +2.6% and 1.2%, +10.3% for orientations 1 and 2, respectively. Although orientation 1 has a smaller overall misfit, metal oxide systems having larger misfits similar to orientation 2 have been reported to lead to heteroepitaxial growth despite the discrepancy [28]. Therefore, it is likely that both in-plane orientations 1 and 2 shown in Fig. 7 form within TiO2 films grown on these substrates. 5. Conclusions A high-quality BaF2 (1 1 1) buffer on single crystal Si (1 0 0) can be used to stabilize rutile phase TiO2 films with (1 0 0) heteroepitaxy. These substrates can then be used to template a CrO2 film, which demonstrates the integration of a rutile metal oxide material with unique electrical/ magnetic properties with a silicon substrate. XRD pole figure analyses reveal distinct heteroepitaxial relationships between all layers in the CrO2 (1 0 0)/TiO2 (1 0 0)/BaF2 (1 1 1)/Si (1 0 0) system. The symmetry of the BaF2 surface induces similar symmetry within the CrO2/TiO2 film layers. The resultant CrO2 and TiO2 films each consist of six distinct crystalline domains differentiated by 301 rotations about [1 0 0] axis. Acknowledgements We gratefully acknowledge the assistance from the following individuals: Francisco Santiago (Naval Surface Warfare Center) for providing the BaF2/Si substrates; Young-Nam Cho (University of Maine) for growing the CrO2 film; Steven Smallwood (University of Maine) for acquiring the STM image and Konrad Bussmann (Naval Research Lab) for performing the Andreev reflection measurement. References
BaF2(111) Surface [001] a
[001] a c
c (1)
2D Unit Cell _ [110]
_ [101] _ [112]
(2)
TiO2(100) Domains Fig. 7. Model of the two possible TiO2 (1 0 0) domain orientations on the BaF2 (1 1 1) surface. See text for details.
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