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Structural characterization of NiO films on Al2O3(0 0 0 1)
Journal of Magnetism and Magnetic Materials 211 (2000) 283}290
Structural characterization of NiO "lms on Al O (0 0 0 1) 2 3 C. Mocuta*, A. Barbier, G. Renaud, Y. Samson, M. Noblet CEA-Grenoble, De& partement de Recherche Fondamentale sur la Matie% re Condense& e, Se& rvice de Physique des Mate& riaux et Microstructures, 17, rue des Martyrs, 38054 Grenoble Ce& dex 9, France
Abstract NiO "lms on clean a-Al O (0 0 0 1) were prepared by molecular beam epitaxy. Two parameters were considered: the 2 3 growth temperature (320}7003C) and the "lm thickness (29}200 nm). Low energy electron di!raction (LEED) patterns of the deposits show a six-fold symmetry. The samples were ex situ characterized by atomic force microscopy (AFM) and by grazing incidence X-ray di!raction. For all samples, NiO islands with di!erent sizes and shapes were observed. A layer of pyramidal NiO islands, with triangular facets is present. Depending on the sample, also hexagonal or square islands were observed. The angles of the triangular facets correspond to (1 0 0) surfaces of the NiO(1 1 1). X-ray di!raction evidences that the NiO "lm grows with the (1 1 1) plane parallel to the (0 0 0 1)Al O one. Twinned and not-twinned face centered 2 3 cubic (FCC) NiO(1 1 1) stacking are present in approximately equal quantities. The crystallographic quality of the NiO layers was investigated. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: Atomic force microscopy (AFM); Crystallographic structure; Epitaxy; Grazing incidence X-ray di!raction (GIXD); Growth; Nickel oxide
1. Introduction NiO is a promising candidate as a pinning antiferromagnetic (AF) layer in spin-valves magnetoresistive devices [1] because, compared to FeMn [2,3], it reduces the functioning noise of such devices. Moreover, its high resistance to corrosion, insulating character and high NeH el temperature (523 K, which also implies a high blocking temperature for the NiO-based spin-valves) are all favorable properties in this context. For the spin-valve fabrication process, good sensitivities and a reduction in the heat generation due to current shunting
* Corresponding author. Tel.: #33-0-476-88-95-40; fax: #33-0-476-88-51-38. E-mail address: [email protected] (C. Mocuta)
when using NiO pinning layers are also features supporting its use [1,2,4]. Up to now most studies were performed on NiO "lms prepared by sputtering [5}12]. Sputtering techniques generally give polycrystalline textured "lms. The in#uence of the crystalline quality on the magnetic properties for NiO "lms was investigated in previous studies [6}9,13}16]. However, the crystallographic quantitative analysis of such "lms can be quite di$cult. In contrast, the use of MBE prepared "lms allows for a quantitative characterization by X-rays because of the higher crystallographic quality. It seemed to be an interesting alternative approach. Here, we propose a study of the structure of NiO "lms supported by aAl O (0 0 0 1) substrates. Our "lms were prepared 2 3 by molecular beam epitaxy (MBE) at di!erent temperatures.
0304-8853/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 9 ) 0 0 7 4 8 - 9
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2. Experiment NiO "lms of di!erent thickness were prepared by MBE on sapphire substrates. Several samples were prepared, at four di!erent deposition temperatures: 320, 400, 500 and 7003C. During Ni evaporation, the oxygen pressure was maintained constant at 2]10~4 Pa, as well as the deposition #ux of the Ni source (0.2 nm/min, as calibrated using a quartz microbalance). Thicknesses in the range of 200 nm were achieved and checked by X-ray re#ectivity for all preparation temperatures. Three additional samples were prepared with di!erent thickness. For the 400 and 7003C deposits, thinner samples were prepared and investigated: 150 nm at 4003C and 30 and 170 nm at 7003C. Polished and cleaned Al O (0 0 0 1) substrates were used. All our sub2 3 strates were annealed at 1170 K under an oxygen pressure p(O )"5]10~3 Pa for 10 min and then 2 checked by low energy electron di!raction (LEED) and by X-rays: well crystallized samples were obtained, with a near-surface mosaic spread of 0.004}0.0093. LEED images were recorded for each sample at the end of the deposition whereas tapping mode AFM images were taken ex situ, in air. The crystallographic structure of the samples was investigated by grazing incidence X-ray di!raction (GIXD). It is not a!ected by the insulating character of the "lm, so the whole structure of thick NiO "lms could be investigated. The measurements were performed using the 4 circle GMT goniometer on the French CRG-Interfaces BM32 beamline, at ESRF (European Synchrotron Radiation Facility, Grenoble, France) [17]. In order to reduce the absorption of X-rays by air, the samples were mounted in a small vacuum chamber (pressure in the range of 10~3 Pa). In GIXD, the samples are illuminated by the well-collimated X-ray beam under an incidence angle of the order of the critical angle for total external re#ection of the material. The penetration depth can easily be varied from a few nm up to tens of nm by increasing the incidence angle. For all our measurements, a 18 keV incident beam (size: 350 lm horizontal]250 lm vertical FWHM) was used. An incident angle of 0.33, larger than both the critical angle for total external re#ection of NiO (0.1653) and Al O (0.1283) was used, in order to 2 3
get signal from the whole NiO "lm, even for thickness of tens of nm. In this case, signal from the a-Al O substrate was also detected. 2 3
3. Results and discussion The main epitaxial relationships one can expect are represented in Fig. 1. The surface scattering extend in the reciprocal space along directions perpendicular to the surface plane: the so-called crystal truncation rods (CTRs) [18]. The reciprocal inplane unit cell of a-Al O and relaxed NiO(1 1 1) 2 3 are shown in Fig. 1. Di!erent types of CTRs are also evidenced (see note Fig. 1). Due to the particular stacking along the [0 0 0 1] sapphire direction and [1 1 1]NiO one, Bragg peaks appear along the CTRs at precise positions. They allow to unambiguously identify the presence of di!erent NiO in-plane orientational relationships between NiO and a-Al O . 2 3 LEED di!raction patterns were taken in situ before and after oxygen annealing of the sapphire substrate, as well as once the desired NiO thickness was achieved. Two typical images are shown in Fig. 2 for the same sample. The reciprocal unit vectors of the substrate and of the NiO "lm are rotated by 303 and a six-fold symmetry is found for the NiO "lm. Comparing with the possible epitaxial relationships (Fig. 1), the LEED patterns correspond to NiO(1 1 1)EAl O (0 0 0 1) with the NiO 2 3 in-plane cell rotated by 303 with respect to the sapphire (denominated in the following by FCC variant, R303: [h h 0] E[h 0 0 0] ), with N*O 4!11)*3% twins (i.e. rotated by 903 with respect to Al O , 2 3 denominated as twinned-FCC, R903: [h h 0] E N*O [2h h 0 0] ). 4!11)*3% At the end of the preparation, the thickness of all samples was checked by X-ray re#ectivity. The re#ectivity signal damps rapidly, showing that the prepared surfaces are rough. AFM images were taken in order to determine the morphology of the NiO "lms. Some of these situations are shown in Figs. 3a}f. For all our samples, a background layer made of triangular pyramids, which have di!erent dimensions, depending on the preparation conditions is found.
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Fig. 1. In-plane view of the reciprocal space of the NiO/Al O (0 0 0 1) interface. Di!erent epitaxial relationships are represented. 2 3 Represented reciprocal unit cells: Al O (0 0 0 1) (OABC circles, continuous line); a possible orientation of NiO(0 0 1) (gray squares); 2 3 NiO(1 1 1)R03 (ODEF, gray hexagonal cell with (1 1 1) E(0 0 0 1) [h 0 0] E[h 0 0 0] ); NiO(1 1 1)R303 (OGHI, dashed N*O 4!11)*3% N*O 4!11)*3% line, (1 1 1) E(0 0 0 1) [h h 0] E[h 0 0 0] . For the sake of clarity, twined orientations are not shown. They can be easily N*O 4!11)*3% N*O 4!11)*3% obtained by rotating the reciprocal unit cells of NiO by 603. Symbols represent di!erent types of NiO CTRs: (1 1 l)-like rods ("lled symbols), with Bragg peaks at l positions which can be expressed as l"3]m; (1 0 l)-like rods (open symbols), with Bragg peaks at l"3]m#1 and (0 1 l)-like rods (gray symbols), with Bragg peaks at l"3]m#2. All indexes are expressed in the reciprocal lattice units of NiO.
Fig. 2. LEED patterns for (a) the oxygen annealed substrate and (b) after NiO deposition at 7003C. The reciprocal unit cells are also shown. Peaks appear at di!erent energies after NiO deposition.
The roughness of the NiO "lms depends on the thickness and deposition temperature. The smallest roughness is found for the thinner NiO layer deposited at 7003C, probably because the NiO islands are
just formed. The 200 nm thickness samples are very rough; the height of the NiO islands can be of several tens of nanometers. The general trend is an increase of the roughness of the "lm with
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Fig. 3. AFM images (tapping mode) for di!erent samples. The thickness of the NiO "lm and preparation temperature (¹ ) are $%104*5 shown on each image, as well as the scanned size. (a) and (b): real heights grayscale maps; (c): (1 0 0) faceted pyramids (FCC and twinned-FCC, marked by arrows; the arrow on the right-side of the "gure shows two neighboring islands rotated one with respect to the other by 603), square (1 0 0) facets (*) and square corner truncated (L) islands; (d) the angle measured along the OM direction is 54$43 and AO) B"120$103; (e) hexagonal-shaped islands (*) on the triangular pyramids background layer (f) triangular pyramids (FCC and twinned-FCC) having (1 0 0) facets tilted by 54$43 (e.g. along OM direction) and square (1 0 0) facets with A@O) B@"90$73.
temperatures above 4003C; the sample prepared at 4003C has the smallest roughness. Figs. 3a and b show the morphology of two situations: the 200 nm NiO deposit at 320 and 4003C. The images
obtained for the other samples are quite similar. The shape and dimensions of the islands may change as seen by AFM. Note that the color scale used here (Figs. 3c}f ) does not represent the real
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measured heights but the variation of the amplitude of the vibrating AFM tip. In this case, the contrast is enhanced for small details in the morphology. Several features are visible. On images like Fig. 3d the angle of the facets with respect to the surface plane can easily be measured. The quantitative measurement was made on real height images; these facets are found to form an angle of 54$43. In addition, the angles between the corners of the triangular facets were found &1203$103 (threefold symmetry). Indeed, the angle between the (1 1 1) and (1 0 0) planes is 54.73 and the pyramid has a three-fold symmetry axis perpendicular to the (1 1 1) plane; a top view will show angles between corners of facets of 1203. The pyramids thus expose (1 0 0) facets of NiO(1 1 1). (1 1 0) facets (expected angle of 35.263) were not found. The shape of the islands also strongly depends on the preparation conditions. We "rst notice #at hexagonal-like forms for the 4003C prepared samples, which can be assigned to the (1 1 1)NiO surface plane. AFM images show &12$53 tilt between the surface of these hexagons and the basal surface plane, along directions having again a three-fold symmetry. Square-shaped islands, probably (1 0 0) facets, are also present for the extreme preparation conditions (200 nm at 3403C and samples at 7003C). This supports the NiO(1 0 0)EAl O (0 0 0 1) epitaxial rela2 3 tionship. The in-plane orientation cannot be addressed by AFM measurements. Some of these (1 0 0) facets have corners truncated by (1 1 1) facets (Figs. 3c and f). In Fig. 3c, two types of islands, with facets rotated by 603 one with respect to the other can be seen. On the right-hand side of the image, an arrow shows two neighboring triangular islands having their basis rotated by 603. The morphological analysis thus allows to evidence the presence of twinned NiO(1 1 1) on the prepared layers. If the AFM images allow the determination of the morphology of the NiO "lm and some crystallographic features (e.g. facets planes, possibly presence of twins } see note of Fig. 3), the epitaxial relationships mentioned in Fig. 1 cannot be distinguished. Moreover, only a very small surface fraction is investigated in AFM. To fully characterize the NiO "lms, X-ray di!raction was used to probe their crystallographic structure in conditions where the whole NiO "lm, the interface and the substrate
287
are probed. The a-Al O unit-cell parameters used 2 3 were a"b"4.754 As , c"12.99 As , a"b"903 and c"1203. Since LEED patterns support the (1 1 1) epitaxy of NiO (Fig. 1), we will describe NiO by a triangular unit cell related to the cubic one by: a"a (!1, 1, 0), b"a (0, !1, 1), 2 2 #6"% 2 2 #6"% c"a (1, 1, 1) with a "4.177 As , so the unit cell #6"% #6"% vectors will be a"b"2.95 As and c"7.235 As . The in-plane cell parameter mis"t is thus of (4.754!2.95)/4.754"37.9%. Fig. 1 illustrates how to distinguish the di!erent possible crystallographic structures of the NiO layer. Along a CTR of the NiO, Bragg peaks layout of the plane of the surface and the two CTRs represented by open and gray symbols can be deduced one from another by a shift along the [0 0 l] direction. The positions of the Bragg peaks depend on the stacking perpendicular to the surface. The stacking perpendicular to the surface can be extracted from the specular [0 0 l] rod. The characteristic inter-plane distances present in the samples are extracted from the peak positions. Particular expected out-of-plane distances are: 7.235 As (c-axis of the NiO(1 1 1)EAl O (0 0 0 1) FCC and twinned 2 3 FCC stacking, giving peaks at l"5.4]m expressed in the Al O reciprocal lattice units (r.l.u.), with 2 3 m being an integer) and 4.177 As (c-axis of the NiO(1 0 0)EAl O (0 0 0 1), giving FCC character2 3 istic peaks at l"2]3.1]m Al O r.l.u., m integer). 2 3 However, the in-plane orientation of the (1 1 1) or (1 0 0)NiO plane with respect to the substrate cannot be assessed from [0 0 l] measurements. For that purpose, large in-plane rocking scans (in which the value of the q is kept constant) and in-plane , scans along [h 0 0] or [h h 0] directions are necessary. The two sets of measurements were performed for all samples. In the following, if not other speci"ed, the a-Al O r.l.u. are used. 2 3 In all cases, along the [h 0 0 0] direction of sapphire, Bragg peaks were found at h" 1.61J3"2.78 r.l.u. evidencing an in-plane NiO unit cell rotated by 303 with respect to the sapphire one, con"rming that the LEED result is not due to the surface layers. The NiO rod (at 2.78/3"0.93 r.l.u. [0.93, 0.93, l] at point I in Fig. 1) crosses Bragg peaks which are distinct for FFC (l"(3]m#1)]1.8), and twinned-FCC stacking (l"(3]m#2)]1.8, with m integer). Rocking
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Fig. 4. Rocking scans (obtained by keeping the detector position "xed and scanning the sample around the perpendicular direction to its surface; in this way, the value of the in-plane transfer momentum is maintained constant) at q "2.78 , a-Al O r.l.u.. Peaks denoted by NiO(1 1 1)R303 and 2 3 NiO(1 1 1)R03 correspond to H and E points in Fig. 1.
scans at FCC and twinned-FCC positions allow estimating the quantity of both stacking in the "lm. For all our prepared "lms, the FCC and twinned-FCC NiO(1 1 1) structures were found to be present in about the same proportion, with a small preference for the twin orientation (NiO(1 1 1)R903). Large in-plane rocking scans at q "1.61J3"2.78 r.l.u. evidence un-rotated , NiO(1 1 1)R03. Twins are also present (R603, see Fig. 4). Integrated intensities show that less than 0.9% of the NiO layer adopts this un-rotated structure for the samples prepared in extreme conditions (3203C and 7003C deposit). This is also con"rmed by in-plane [h h 0] scans, where the NiO(1 1 1)R03 peak is expected at (1.61, 1.61, 0) in reciprocal space. Moreover, scanning along the [h h 0] inplane direction, a small peak is present at h" k"1.61J3"0.93 r.l.u. (Denoted by I in Fig. 1: it is the intersection of a NiO(1 1 1)R303CTR with the surface plane), showing the presence of the surface of FCC NiO(1 1 1). All the quantitative results were deduced from integrated intensities of rocking scans at characteristic positions of each structure, after background, geometrical factors, Lorentz and polarization corrections.
Fig. 5. X-ray intensities measured along di!erent directions in the reciprocal space: (a) [0, 0, l]¹ "3203C; $%104*5 (b) [0.93, 0.93, l]¹ "7003C; (c) [h, 0, 0] and [h, h, 0] di$%104*5 rections; ¹ "7003C. These scans were selected from di!er$%104*5 ent samples in order to highlight the possible NiO crystallographic structure with respect to the sapphire. Positions where scattering is expected are marked.
Fig. 5 shows examples of measured intensities along di!erent directions in the reciprocal space. The positions where scattering is expected for the di!erent structures are shown. The quality of di!erent NiO variants of the "lm can be addressed by analyzing the peaks widths at di!erent in-plane q positions. Then the mosaic spread and a charac, teristic size of the di!racting domains are deduced. The square of the corrected peak width should vary linearly with (1/q )2. The intersection with the ordi, nate axis yields the mosaic spread of the "lm, while the slope is related to a characteristic di!racting domain size. The NiO(1 1 1) part of the "lm (either
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FCC or twinned-FCC) has a similar quality for all prepared "lms, between 0.8 and 1.73 mosaic spread and a di!racting domain size generally larger than 10 nm. The NiO(1 0 0) variant, when present, is of poor crystallographic quality; the intensity of the signal is spread out over several degrees (5}83). The NiO(1 0 0) has no preferential in-plane orientation with respect to the sapphire unit cell. It is likely that the observed signal on the specular rod comes from stacking faults. Let us now discuss these results with respect to other NiO "lms. Sputtered NiO(1 1 1) "lms have already been used as pinning AF layers to elaborate spin-valve read heads [1,2,19]. For industrial applications, this preparation method is faster and more cost e!ective than MBE, and the obtained "lms are smoother. Relatively high deposition rates lead to sandwiches of several well-de"ned layers without pin-holes in between allowing for the elaboration of functioning devices. However, this method has also drawbacks, the NiO target crystallizes easily and become rapidly unusable. It has also not allowed to completely understand the structure of the interfaces during growth nor the role of each parameter (roughness, texture, di!usion at interface, etc.). Indeed, some of the reported results are in contradiction with each other. Generally, the exchange coupling is believed to be an interface e!ect [20] although some studies report di!erent assumptions [21]. Some authors claim, in contradiction with the theory [6,7,13,22,23], that the NiO texture, for the spin compensated as well as for the uncompensated planes, does not to make a great di!erence when exchange coupling is considered. On the other hand, other studies report increases or decreases of the coupling when the uncompensated (1 1 1) surface of the NiO is used [24] ([9]). Our results are comparable with similar studies of sputtered NiO "lms or NiO}CoO multilayers elaborated on sapphire [11,12], where the presence of twins was explained as induced by steps on the surface of the substrate. For very thick NiO "lms the main phase is NiO(1 1 1), with two variants: the R303 and R903 with respect to the a-Al O (0 0 0 1) 2 3 substrate. MBE grown "lms stand for an alternative approach in which "lms of good crystallographic quality can be expected at smaller deposition rates.
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One can also expect to better control each parameter. Unfortunately, our NiO "lms are much too rough for being used as AF substrates in a spinvalve device. Their magnetic properties for sensors are thus not accessible. In the range of explored parameters, the 4003C prepared "lm is the smoother one but the problem of depositing a continuous F "lm persists. The present study aimed at determining, if they exist, the conditions in which well characterized MBE "lms could be produced to get usable substrates for spin-valve preparation.
4. Conclusions Our GIXD data show that the growth of NiO on sapphire is epitaxial within a twinned-FCC scheme. The NiO unit cells are rotated by 303 and 903, for the FCC and twinned-FCC stacking, respectively, with respect to the Al O one. The low-temper2 3 ature growth show the additional presence of small quantities of NiO(1 1 1)R03. The unit-cell parameter evolution does not evidence strain in the NiO "lms and only the NiO composition was found. Quantitative measurements allowed estimating the fraction of each structure as well as the corresponding crystallographic quality. The complementary use of LEED patterns, AFM images and X-ray di!raction showed that the crystallographic quality of the MBE grown "lms is comparable to the one obtained for sputtered "lms [11]. However, our "lms are rougher in the whole range of the parameters we have investigated. The small deposition rate in our case (a few As per minute) is favorable for island formation during the growth with complex morphologies. Conditions in which the NiO "lm is perfectly 2D were not found. From these facts we do not expect a di!erent magnetic behavior from our "lms with respect to sputtered ones and since spin-valve elaboration needs #at interfaces it seems not straightforward at all to build epitaxial spin-valves on a-Al O (0 0 0 1). The 2 3 elaboration of single crystalline spin-valves remains a challenge that seems to be di$cult to tackle with NiO "lms on sapphire. The present results support a growth mode in which at the beginning, a layer made of micropyramids is formed. Depending on the growth
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conditions these pyramids grow more or less homogeneously. It is suggested that the polar nature of the NiO(1 1 1) surface prevents the growth without NiO(1 0 0) facets. References [1] Y. Hamakawa, H. Hoshiya, T. Kawabe et al., IEEE Trans. Magn. 32 (1996) 149. [2] S.L. Burkett, S. Kora, J.L. Bresowar et al., J. Appl. Phys. 81 (1997) 4912. [3] W.C. Cain, W.H. Meiklejohn, M.H. Kryder, J. Appl. Phys. 61 (1987) 4170. [4] H. Yamane, M. Kobayashi, J. Appl. Phys. 83 (1998) 4862. [5] D.G. Hwang, C.M. Park, S.S. Lee, J. Magn. Magn. Mater. 186 (1998) 265. [6] S.-S. Lee, D.-G. Hwang, C.M. Park et al., J. Appl. Phys. 81 (1997) 5298. [7] D.-H. Han, J.-G. Zhu, J.H. Judy et al., J. Appl. Phys. 81 (1997) 340. [8] M.-H. Lee, S. Lee, K. Sin, Thin Solid Films 320 (1998) 298. [9] J.X. Shen, M.T. Kief, J. Appl. Phys. 79 (1996) 5008. [10] C.-H. Lai, T.C. Anthony, E. Iwamura et al., IEEE Trans. Magn. 32 (1996) 3419.
[11] M.J. Carey, F.E. Spada, A.E. Berkowitz et al., J. Mater. Res. 6 (1991) 2680. [12] W. Cao, G. Thomas, M.J. Carey et al., Scripta Metal. Mater. 25 (1991) 2633. [13] D.-H. Han, J.-G. Zhu, J.H. Judy, J. Appl. Phys. 81 (1997) 4996. [14] C.-H. Lai, H. Matsuyama, R.L. White, IEEE Trans. Magn. 31 (1995) 2609. [15] C.-H. Lai, T.J. Regan, R.L. White et al., J. Appl. Phys. 81 (1997) 3989. [16] R.P. Michel, A. Chaiken, C.T. Wang et al., Phys. Rev. B 58 (1998) 8566. [17] http://www.esrf.fr. [18] I.K. Robinson, Phys. Rev. B 33 (1986) 3830. [19] K. Nakamoto, Y. Kawato, Y. Suzuki et al., IEEE Trans. Magn. 32 (1996) 3374. [20] C. Tsang, N. Heiman, K. Lee, J. Appl. Phys. 81 (1981) 2471. [21] N.J. GoK kemeijer, T. Ambrose, C.L. Chien et al., J. Appl. Phys. 81 (1997) 4999. [22] G. Chern, D.S. Lee, H.C. Chang et al., J. Magn. Magn. Mater. 193 (1999) 497. [23] K. Takano, R.H. Kodama, A.E. Berkowitz et al., Phys. Rev. Lett. 79 (1997) 1130. [24] O. Kitakami, H. Takashima, Y. Shimada, J. Magn. Magn. Mater. 164 (1996) 43.
Structural characterization of NiO "lms on Al O (0 0 0 1) 2 3 C. Mocuta*, A. Barbier, G. Renaud, Y. Samson, M. Noblet CEA-Grenoble, De& partement de Recherche Fondamentale sur la Matie% re Condense& e, Se& rvice de Physique des Mate& riaux et Microstructures, 17, rue des Martyrs, 38054 Grenoble Ce& dex 9, France
Abstract NiO "lms on clean a-Al O (0 0 0 1) were prepared by molecular beam epitaxy. Two parameters were considered: the 2 3 growth temperature (320}7003C) and the "lm thickness (29}200 nm). Low energy electron di!raction (LEED) patterns of the deposits show a six-fold symmetry. The samples were ex situ characterized by atomic force microscopy (AFM) and by grazing incidence X-ray di!raction. For all samples, NiO islands with di!erent sizes and shapes were observed. A layer of pyramidal NiO islands, with triangular facets is present. Depending on the sample, also hexagonal or square islands were observed. The angles of the triangular facets correspond to (1 0 0) surfaces of the NiO(1 1 1). X-ray di!raction evidences that the NiO "lm grows with the (1 1 1) plane parallel to the (0 0 0 1)Al O one. Twinned and not-twinned face centered 2 3 cubic (FCC) NiO(1 1 1) stacking are present in approximately equal quantities. The crystallographic quality of the NiO layers was investigated. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: Atomic force microscopy (AFM); Crystallographic structure; Epitaxy; Grazing incidence X-ray di!raction (GIXD); Growth; Nickel oxide
1. Introduction NiO is a promising candidate as a pinning antiferromagnetic (AF) layer in spin-valves magnetoresistive devices [1] because, compared to FeMn [2,3], it reduces the functioning noise of such devices. Moreover, its high resistance to corrosion, insulating character and high NeH el temperature (523 K, which also implies a high blocking temperature for the NiO-based spin-valves) are all favorable properties in this context. For the spin-valve fabrication process, good sensitivities and a reduction in the heat generation due to current shunting
* Corresponding author. Tel.: #33-0-476-88-95-40; fax: #33-0-476-88-51-38. E-mail address: [email protected] (C. Mocuta)
when using NiO pinning layers are also features supporting its use [1,2,4]. Up to now most studies were performed on NiO "lms prepared by sputtering [5}12]. Sputtering techniques generally give polycrystalline textured "lms. The in#uence of the crystalline quality on the magnetic properties for NiO "lms was investigated in previous studies [6}9,13}16]. However, the crystallographic quantitative analysis of such "lms can be quite di$cult. In contrast, the use of MBE prepared "lms allows for a quantitative characterization by X-rays because of the higher crystallographic quality. It seemed to be an interesting alternative approach. Here, we propose a study of the structure of NiO "lms supported by aAl O (0 0 0 1) substrates. Our "lms were prepared 2 3 by molecular beam epitaxy (MBE) at di!erent temperatures.
0304-8853/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 9 ) 0 0 7 4 8 - 9
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2. Experiment NiO "lms of di!erent thickness were prepared by MBE on sapphire substrates. Several samples were prepared, at four di!erent deposition temperatures: 320, 400, 500 and 7003C. During Ni evaporation, the oxygen pressure was maintained constant at 2]10~4 Pa, as well as the deposition #ux of the Ni source (0.2 nm/min, as calibrated using a quartz microbalance). Thicknesses in the range of 200 nm were achieved and checked by X-ray re#ectivity for all preparation temperatures. Three additional samples were prepared with di!erent thickness. For the 400 and 7003C deposits, thinner samples were prepared and investigated: 150 nm at 4003C and 30 and 170 nm at 7003C. Polished and cleaned Al O (0 0 0 1) substrates were used. All our sub2 3 strates were annealed at 1170 K under an oxygen pressure p(O )"5]10~3 Pa for 10 min and then 2 checked by low energy electron di!raction (LEED) and by X-rays: well crystallized samples were obtained, with a near-surface mosaic spread of 0.004}0.0093. LEED images were recorded for each sample at the end of the deposition whereas tapping mode AFM images were taken ex situ, in air. The crystallographic structure of the samples was investigated by grazing incidence X-ray di!raction (GIXD). It is not a!ected by the insulating character of the "lm, so the whole structure of thick NiO "lms could be investigated. The measurements were performed using the 4 circle GMT goniometer on the French CRG-Interfaces BM32 beamline, at ESRF (European Synchrotron Radiation Facility, Grenoble, France) [17]. In order to reduce the absorption of X-rays by air, the samples were mounted in a small vacuum chamber (pressure in the range of 10~3 Pa). In GIXD, the samples are illuminated by the well-collimated X-ray beam under an incidence angle of the order of the critical angle for total external re#ection of the material. The penetration depth can easily be varied from a few nm up to tens of nm by increasing the incidence angle. For all our measurements, a 18 keV incident beam (size: 350 lm horizontal]250 lm vertical FWHM) was used. An incident angle of 0.33, larger than both the critical angle for total external re#ection of NiO (0.1653) and Al O (0.1283) was used, in order to 2 3
get signal from the whole NiO "lm, even for thickness of tens of nm. In this case, signal from the a-Al O substrate was also detected. 2 3
3. Results and discussion The main epitaxial relationships one can expect are represented in Fig. 1. The surface scattering extend in the reciprocal space along directions perpendicular to the surface plane: the so-called crystal truncation rods (CTRs) [18]. The reciprocal inplane unit cell of a-Al O and relaxed NiO(1 1 1) 2 3 are shown in Fig. 1. Di!erent types of CTRs are also evidenced (see note Fig. 1). Due to the particular stacking along the [0 0 0 1] sapphire direction and [1 1 1]NiO one, Bragg peaks appear along the CTRs at precise positions. They allow to unambiguously identify the presence of di!erent NiO in-plane orientational relationships between NiO and a-Al O . 2 3 LEED di!raction patterns were taken in situ before and after oxygen annealing of the sapphire substrate, as well as once the desired NiO thickness was achieved. Two typical images are shown in Fig. 2 for the same sample. The reciprocal unit vectors of the substrate and of the NiO "lm are rotated by 303 and a six-fold symmetry is found for the NiO "lm. Comparing with the possible epitaxial relationships (Fig. 1), the LEED patterns correspond to NiO(1 1 1)EAl O (0 0 0 1) with the NiO 2 3 in-plane cell rotated by 303 with respect to the sapphire (denominated in the following by FCC variant, R303: [h h 0] E[h 0 0 0] ), with N*O 4!11)*3% twins (i.e. rotated by 903 with respect to Al O , 2 3 denominated as twinned-FCC, R903: [h h 0] E N*O [2h h 0 0] ). 4!11)*3% At the end of the preparation, the thickness of all samples was checked by X-ray re#ectivity. The re#ectivity signal damps rapidly, showing that the prepared surfaces are rough. AFM images were taken in order to determine the morphology of the NiO "lms. Some of these situations are shown in Figs. 3a}f. For all our samples, a background layer made of triangular pyramids, which have di!erent dimensions, depending on the preparation conditions is found.
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Fig. 1. In-plane view of the reciprocal space of the NiO/Al O (0 0 0 1) interface. Di!erent epitaxial relationships are represented. 2 3 Represented reciprocal unit cells: Al O (0 0 0 1) (OABC circles, continuous line); a possible orientation of NiO(0 0 1) (gray squares); 2 3 NiO(1 1 1)R03 (ODEF, gray hexagonal cell with (1 1 1) E(0 0 0 1) [h 0 0] E[h 0 0 0] ); NiO(1 1 1)R303 (OGHI, dashed N*O 4!11)*3% N*O 4!11)*3% line, (1 1 1) E(0 0 0 1) [h h 0] E[h 0 0 0] . For the sake of clarity, twined orientations are not shown. They can be easily N*O 4!11)*3% N*O 4!11)*3% obtained by rotating the reciprocal unit cells of NiO by 603. Symbols represent di!erent types of NiO CTRs: (1 1 l)-like rods ("lled symbols), with Bragg peaks at l positions which can be expressed as l"3]m; (1 0 l)-like rods (open symbols), with Bragg peaks at l"3]m#1 and (0 1 l)-like rods (gray symbols), with Bragg peaks at l"3]m#2. All indexes are expressed in the reciprocal lattice units of NiO.
Fig. 2. LEED patterns for (a) the oxygen annealed substrate and (b) after NiO deposition at 7003C. The reciprocal unit cells are also shown. Peaks appear at di!erent energies after NiO deposition.
The roughness of the NiO "lms depends on the thickness and deposition temperature. The smallest roughness is found for the thinner NiO layer deposited at 7003C, probably because the NiO islands are
just formed. The 200 nm thickness samples are very rough; the height of the NiO islands can be of several tens of nanometers. The general trend is an increase of the roughness of the "lm with
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Fig. 3. AFM images (tapping mode) for di!erent samples. The thickness of the NiO "lm and preparation temperature (¹ ) are $%104*5 shown on each image, as well as the scanned size. (a) and (b): real heights grayscale maps; (c): (1 0 0) faceted pyramids (FCC and twinned-FCC, marked by arrows; the arrow on the right-side of the "gure shows two neighboring islands rotated one with respect to the other by 603), square (1 0 0) facets (*) and square corner truncated (L) islands; (d) the angle measured along the OM direction is 54$43 and AO) B"120$103; (e) hexagonal-shaped islands (*) on the triangular pyramids background layer (f) triangular pyramids (FCC and twinned-FCC) having (1 0 0) facets tilted by 54$43 (e.g. along OM direction) and square (1 0 0) facets with A@O) B@"90$73.
temperatures above 4003C; the sample prepared at 4003C has the smallest roughness. Figs. 3a and b show the morphology of two situations: the 200 nm NiO deposit at 320 and 4003C. The images
obtained for the other samples are quite similar. The shape and dimensions of the islands may change as seen by AFM. Note that the color scale used here (Figs. 3c}f ) does not represent the real
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measured heights but the variation of the amplitude of the vibrating AFM tip. In this case, the contrast is enhanced for small details in the morphology. Several features are visible. On images like Fig. 3d the angle of the facets with respect to the surface plane can easily be measured. The quantitative measurement was made on real height images; these facets are found to form an angle of 54$43. In addition, the angles between the corners of the triangular facets were found &1203$103 (threefold symmetry). Indeed, the angle between the (1 1 1) and (1 0 0) planes is 54.73 and the pyramid has a three-fold symmetry axis perpendicular to the (1 1 1) plane; a top view will show angles between corners of facets of 1203. The pyramids thus expose (1 0 0) facets of NiO(1 1 1). (1 1 0) facets (expected angle of 35.263) were not found. The shape of the islands also strongly depends on the preparation conditions. We "rst notice #at hexagonal-like forms for the 4003C prepared samples, which can be assigned to the (1 1 1)NiO surface plane. AFM images show &12$53 tilt between the surface of these hexagons and the basal surface plane, along directions having again a three-fold symmetry. Square-shaped islands, probably (1 0 0) facets, are also present for the extreme preparation conditions (200 nm at 3403C and samples at 7003C). This supports the NiO(1 0 0)EAl O (0 0 0 1) epitaxial rela2 3 tionship. The in-plane orientation cannot be addressed by AFM measurements. Some of these (1 0 0) facets have corners truncated by (1 1 1) facets (Figs. 3c and f). In Fig. 3c, two types of islands, with facets rotated by 603 one with respect to the other can be seen. On the right-hand side of the image, an arrow shows two neighboring triangular islands having their basis rotated by 603. The morphological analysis thus allows to evidence the presence of twinned NiO(1 1 1) on the prepared layers. If the AFM images allow the determination of the morphology of the NiO "lm and some crystallographic features (e.g. facets planes, possibly presence of twins } see note of Fig. 3), the epitaxial relationships mentioned in Fig. 1 cannot be distinguished. Moreover, only a very small surface fraction is investigated in AFM. To fully characterize the NiO "lms, X-ray di!raction was used to probe their crystallographic structure in conditions where the whole NiO "lm, the interface and the substrate
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are probed. The a-Al O unit-cell parameters used 2 3 were a"b"4.754 As , c"12.99 As , a"b"903 and c"1203. Since LEED patterns support the (1 1 1) epitaxy of NiO (Fig. 1), we will describe NiO by a triangular unit cell related to the cubic one by: a"a (!1, 1, 0), b"a (0, !1, 1), 2 2 #6"% 2 2 #6"% c"a (1, 1, 1) with a "4.177 As , so the unit cell #6"% #6"% vectors will be a"b"2.95 As and c"7.235 As . The in-plane cell parameter mis"t is thus of (4.754!2.95)/4.754"37.9%. Fig. 1 illustrates how to distinguish the di!erent possible crystallographic structures of the NiO layer. Along a CTR of the NiO, Bragg peaks layout of the plane of the surface and the two CTRs represented by open and gray symbols can be deduced one from another by a shift along the [0 0 l] direction. The positions of the Bragg peaks depend on the stacking perpendicular to the surface. The stacking perpendicular to the surface can be extracted from the specular [0 0 l] rod. The characteristic inter-plane distances present in the samples are extracted from the peak positions. Particular expected out-of-plane distances are: 7.235 As (c-axis of the NiO(1 1 1)EAl O (0 0 0 1) FCC and twinned 2 3 FCC stacking, giving peaks at l"5.4]m expressed in the Al O reciprocal lattice units (r.l.u.), with 2 3 m being an integer) and 4.177 As (c-axis of the NiO(1 0 0)EAl O (0 0 0 1), giving FCC character2 3 istic peaks at l"2]3.1]m Al O r.l.u., m integer). 2 3 However, the in-plane orientation of the (1 1 1) or (1 0 0)NiO plane with respect to the substrate cannot be assessed from [0 0 l] measurements. For that purpose, large in-plane rocking scans (in which the value of the q is kept constant) and in-plane , scans along [h 0 0] or [h h 0] directions are necessary. The two sets of measurements were performed for all samples. In the following, if not other speci"ed, the a-Al O r.l.u. are used. 2 3 In all cases, along the [h 0 0 0] direction of sapphire, Bragg peaks were found at h" 1.61J3"2.78 r.l.u. evidencing an in-plane NiO unit cell rotated by 303 with respect to the sapphire one, con"rming that the LEED result is not due to the surface layers. The NiO rod (at 2.78/3"0.93 r.l.u. [0.93, 0.93, l] at point I in Fig. 1) crosses Bragg peaks which are distinct for FFC (l"(3]m#1)]1.8), and twinned-FCC stacking (l"(3]m#2)]1.8, with m integer). Rocking
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Fig. 4. Rocking scans (obtained by keeping the detector position "xed and scanning the sample around the perpendicular direction to its surface; in this way, the value of the in-plane transfer momentum is maintained constant) at q "2.78 , a-Al O r.l.u.. Peaks denoted by NiO(1 1 1)R303 and 2 3 NiO(1 1 1)R03 correspond to H and E points in Fig. 1.
scans at FCC and twinned-FCC positions allow estimating the quantity of both stacking in the "lm. For all our prepared "lms, the FCC and twinned-FCC NiO(1 1 1) structures were found to be present in about the same proportion, with a small preference for the twin orientation (NiO(1 1 1)R903). Large in-plane rocking scans at q "1.61J3"2.78 r.l.u. evidence un-rotated , NiO(1 1 1)R03. Twins are also present (R603, see Fig. 4). Integrated intensities show that less than 0.9% of the NiO layer adopts this un-rotated structure for the samples prepared in extreme conditions (3203C and 7003C deposit). This is also con"rmed by in-plane [h h 0] scans, where the NiO(1 1 1)R03 peak is expected at (1.61, 1.61, 0) in reciprocal space. Moreover, scanning along the [h h 0] inplane direction, a small peak is present at h" k"1.61J3"0.93 r.l.u. (Denoted by I in Fig. 1: it is the intersection of a NiO(1 1 1)R303CTR with the surface plane), showing the presence of the surface of FCC NiO(1 1 1). All the quantitative results were deduced from integrated intensities of rocking scans at characteristic positions of each structure, after background, geometrical factors, Lorentz and polarization corrections.
Fig. 5. X-ray intensities measured along di!erent directions in the reciprocal space: (a) [0, 0, l]¹ "3203C; $%104*5 (b) [0.93, 0.93, l]¹ "7003C; (c) [h, 0, 0] and [h, h, 0] di$%104*5 rections; ¹ "7003C. These scans were selected from di!er$%104*5 ent samples in order to highlight the possible NiO crystallographic structure with respect to the sapphire. Positions where scattering is expected are marked.
Fig. 5 shows examples of measured intensities along di!erent directions in the reciprocal space. The positions where scattering is expected for the di!erent structures are shown. The quality of di!erent NiO variants of the "lm can be addressed by analyzing the peaks widths at di!erent in-plane q positions. Then the mosaic spread and a charac, teristic size of the di!racting domains are deduced. The square of the corrected peak width should vary linearly with (1/q )2. The intersection with the ordi, nate axis yields the mosaic spread of the "lm, while the slope is related to a characteristic di!racting domain size. The NiO(1 1 1) part of the "lm (either
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FCC or twinned-FCC) has a similar quality for all prepared "lms, between 0.8 and 1.73 mosaic spread and a di!racting domain size generally larger than 10 nm. The NiO(1 0 0) variant, when present, is of poor crystallographic quality; the intensity of the signal is spread out over several degrees (5}83). The NiO(1 0 0) has no preferential in-plane orientation with respect to the sapphire unit cell. It is likely that the observed signal on the specular rod comes from stacking faults. Let us now discuss these results with respect to other NiO "lms. Sputtered NiO(1 1 1) "lms have already been used as pinning AF layers to elaborate spin-valve read heads [1,2,19]. For industrial applications, this preparation method is faster and more cost e!ective than MBE, and the obtained "lms are smoother. Relatively high deposition rates lead to sandwiches of several well-de"ned layers without pin-holes in between allowing for the elaboration of functioning devices. However, this method has also drawbacks, the NiO target crystallizes easily and become rapidly unusable. It has also not allowed to completely understand the structure of the interfaces during growth nor the role of each parameter (roughness, texture, di!usion at interface, etc.). Indeed, some of the reported results are in contradiction with each other. Generally, the exchange coupling is believed to be an interface e!ect [20] although some studies report di!erent assumptions [21]. Some authors claim, in contradiction with the theory [6,7,13,22,23], that the NiO texture, for the spin compensated as well as for the uncompensated planes, does not to make a great di!erence when exchange coupling is considered. On the other hand, other studies report increases or decreases of the coupling when the uncompensated (1 1 1) surface of the NiO is used [24] ([9]). Our results are comparable with similar studies of sputtered NiO "lms or NiO}CoO multilayers elaborated on sapphire [11,12], where the presence of twins was explained as induced by steps on the surface of the substrate. For very thick NiO "lms the main phase is NiO(1 1 1), with two variants: the R303 and R903 with respect to the a-Al O (0 0 0 1) 2 3 substrate. MBE grown "lms stand for an alternative approach in which "lms of good crystallographic quality can be expected at smaller deposition rates.
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One can also expect to better control each parameter. Unfortunately, our NiO "lms are much too rough for being used as AF substrates in a spinvalve device. Their magnetic properties for sensors are thus not accessible. In the range of explored parameters, the 4003C prepared "lm is the smoother one but the problem of depositing a continuous F "lm persists. The present study aimed at determining, if they exist, the conditions in which well characterized MBE "lms could be produced to get usable substrates for spin-valve preparation.
4. Conclusions Our GIXD data show that the growth of NiO on sapphire is epitaxial within a twinned-FCC scheme. The NiO unit cells are rotated by 303 and 903, for the FCC and twinned-FCC stacking, respectively, with respect to the Al O one. The low-temper2 3 ature growth show the additional presence of small quantities of NiO(1 1 1)R03. The unit-cell parameter evolution does not evidence strain in the NiO "lms and only the NiO composition was found. Quantitative measurements allowed estimating the fraction of each structure as well as the corresponding crystallographic quality. The complementary use of LEED patterns, AFM images and X-ray di!raction showed that the crystallographic quality of the MBE grown "lms is comparable to the one obtained for sputtered "lms [11]. However, our "lms are rougher in the whole range of the parameters we have investigated. The small deposition rate in our case (a few As per minute) is favorable for island formation during the growth with complex morphologies. Conditions in which the NiO "lm is perfectly 2D were not found. From these facts we do not expect a di!erent magnetic behavior from our "lms with respect to sputtered ones and since spin-valve elaboration needs #at interfaces it seems not straightforward at all to build epitaxial spin-valves on a-Al O (0 0 0 1). The 2 3 elaboration of single crystalline spin-valves remains a challenge that seems to be di$cult to tackle with NiO "lms on sapphire. The present results support a growth mode in which at the beginning, a layer made of micropyramids is formed. Depending on the growth
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conditions these pyramids grow more or less homogeneously. It is suggested that the polar nature of the NiO(1 1 1) surface prevents the growth without NiO(1 0 0) facets. References [1] Y. Hamakawa, H. Hoshiya, T. Kawabe et al., IEEE Trans. Magn. 32 (1996) 149. [2] S.L. Burkett, S. Kora, J.L. Bresowar et al., J. Appl. Phys. 81 (1997) 4912. [3] W.C. Cain, W.H. Meiklejohn, M.H. Kryder, J. Appl. Phys. 61 (1987) 4170. [4] H. Yamane, M. Kobayashi, J. Appl. Phys. 83 (1998) 4862. [5] D.G. Hwang, C.M. Park, S.S. Lee, J. Magn. Magn. Mater. 186 (1998) 265. [6] S.-S. Lee, D.-G. Hwang, C.M. Park et al., J. Appl. Phys. 81 (1997) 5298. [7] D.-H. Han, J.-G. Zhu, J.H. Judy et al., J. Appl. Phys. 81 (1997) 340. [8] M.-H. Lee, S. Lee, K. Sin, Thin Solid Films 320 (1998) 298. [9] J.X. Shen, M.T. Kief, J. Appl. Phys. 79 (1996) 5008. [10] C.-H. Lai, T.C. Anthony, E. Iwamura et al., IEEE Trans. Magn. 32 (1996) 3419.
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