Growth of Al2O3 stripes in NiA(001)

surface science ELSEVIER

Surface Science 396 (1998) 176-188

Growth of A1203 stripes in NiA(001 ) Ralf-Peter Blum *, Dirk Ahlbehrendt, Horst Niehus Humboldt-Universitgit zu Berlin, Institut fiir Physik, OberflSchenphysik und Atomstoflprozesse, [nvalidenstrafie 110, D 10115 Berlin, Germany

Received 6 March 1997; accepted for publication 13 August 1997

Abstract

Structure and growth mechanism of ultra-thin A1203 films on NiAI(001 ) have been investigated with 180°-neutral impact collision ion scattering spectroscopy, high resolution spot profile analysis of low energy electron diffraction, and scanning tunnelling microscopy (STM). Ordered A1203 films have been prepared by oxygen exposure at room temperature and subsequent annealing at T= 1200 K. The formation of epitaxial oxide films is unaffected by the initial composition or reconstruction of the substrate. The oxidized surface being covered by A1203 appears to be microscopically rough, reversible stepped with step heights of about 3 A and consists of a network of elongated equally distributed oxide stripes along (100) and (010) directions of the NiAl(001 ) surface. The lateral anisotropy of the oxide is probably caused by the build up of internal stress in the growth process. After oxygen saturation exposure at room temperature and subsequent annealing, regions with rather regular arranged parallel oxide stripes show up in the STM images (mean width ~ 27 A, period ~ 54 A). The corresponding low energy electron diffraction pattern shows the existence of orthogonal (2 x 1) antiphase domains with a (9 x 1) superstructure (period 26.02 A) close to the mean width of oxide stripes. Next to the oxide stripes, regrowing NiA1 terraces and thin layers of amorphous A1203 have been found. © 1998 Elsevier Science B.V. Keywords: Aluminum; Aluminum oxide; Scanning tunneling microscopy; Surface relaxation and reconstruction

1. Introduction

M e t a l oxides i n c l u d i n g u l t r a - t h i n epitaxial oxide films p l a y a n i m p o r t a n t role in all k i n d s o f m a t e r i a l research investigations, displaying metal-, s e m i c o n d u c t o r - a n d i n s u l a t o r - l i k e p r o p e r t i e s [1,2]. I n the p r e s e n t w o r k the g r o w t h a n d m o r p h o l o g y o f A l / O 3 films f o r m e d b y direct o x i d a t i o n o f an NiAl(001) surface has been investigated. Previously, t w o - d i m e n s i o n a l film g r o w t h o f oxides has been f o u n d b y o x i d a t i o n o f N i A I ( l l 0 ) . Such e p i t a x i a l oxide films c a n be u s e d as s u p p o r t for e v a p o r a t e d m e t a l clusters in c a t a l y t i c systems * Corresponding author. Tel: (+ 49) 30 2093 7921; fax: (+ 49) 30 2093 7737.

[3 5]. A s i d e o f catalysis, m e t a l oxide substrates i n c l u d i n g o x i d e / m e t a l interfaces m a y well serve as specimens for the s t u d y o f m a g n e t i c p r o p e r t i e s in m a t e r i a l s o f r e d u c e d d i m e n s i o n a l i t y . This also covers the r e p o r t e d effects o f m a r k e d l y increased m a g n e t i c m o m e n t s o f the F e / M g O ( 0 0 1 ) interface due to the r e d u c t i o n o f the d i m e n s i o n a l i t y as c o m p a r e d with b u l k p r o p e r t i e s [6,7]. P r e p a r a t i o n o f well-defined m e t a l oxide surfaces are a critical prerequisite for studies o f the m a c r o s c o p i c system properties. T h i n epitaxial oxide films with a w e l l - c o n t r o l l e d thickness c a n be expected for the g r o w t h o f u l t r a thin A1203 films o n l o w - i n d e x e d NiA1 surfaces [8-11 ]. D i r e c t o x i d a t i o n seems to be a simple w a y to p r e p a r e suitable oxide films. T h e bimetallic

0039-6028/98/$19.00 © 1998 Published by Elsevier Science B.V. All rights reserved. PII S0039-6028 (97) 00667-5

R.-P. Blum et al. /Surface Science 396 (1998) 176-188

compound NiAI contains A1 as a natural source for alumina formation by surface segregation during the oxidation process. A1203 exhibits a considerably lower surface free energy than metallic NiA1. Also, the heat of formation for aluminium oxide is seven times higher than the corresponding value for NiO ( - 240.8 kJ mol - ~). These features favour thermodynamically the formation of a stable A1203 film. In the literature, several A1203 phase are reported (for a compilation see e.g. Ref. [9]), the most stable ones are the c~ (korund), 7 and 0 modifications. All phases are marked by a sublattice of closed packed oxygen ions. The phase differences arise from the stacking sequence of the individual 0 2 . planes (hcp vs. fcc) and the position of A13+ ions in octahedral and/or tetrahedral sites. K o r u n d shows a hcp oxygen lattice with A1 in octahedral sites only, whereas for both the 7 and 0 modifications oxygen forms a fcc stacking with A1 in octahedral and tetrahedral sites and results actually in a very similar oxide lattice. Their distinction arises basically from a difference in the A13 ÷ site occupation. As a result, a straight discrimination between the V and 0 phase, in particular for thin films, is difficult to achieve. The oxide grows with the (110) plane of the oxygen fcc sublattice perpendicular to the NiAI(001) surface and parallel to the NiAI (100) direction (or N i A L (010) direction for the other domain). This epitaxial relation for the 7- or 0-AtzO3/NiA1 (001 ) system corresponds to the classical Bain orientation relationship between bcc (NiA~)and fee (°xygen sublattice)structures and results in a small lattice mismatch of 2.42% (2aNi~ = 5.78 * ; ao-Aho3 = 5.65 A) in one direction and only 1% (bNiAl=2.89A; bo-AlzO3 =2.91 A) in the perpendicular direction for monoclinic 0-A1203. The cubic v-like oxide has a much greater mismatch of 13.7% (aT_Alzo~ = 7.89 A) in both directions. The rather small lattice mismatch might stabilize an ordered 0-A1203 film on large areas on NiAI(001). F r o m an experimental point of view the temperature stability of NiA1 up to 1911 K might also be helpful, because it is assumed to allow oxidation without drastic kinetic restrictions in the growth mechanisms. Considering arguments from thermodynamics, geometry and

177

kinetics a nearly perfect two-dimensional growth of a well-ordered oxide film on NiAI(001) has been proposed and supported by high resolution electron energy loss spectrometry ( H R E E L S ) , low energy electron diffraction ( L E E D ) and Auger electron spectrometry (AES) measurements [8,9]. On the other hand, it will be demonstrated in the following by electron diffraction, ion scattering and scanning tunnelling microscopy (STM) data that a closer look at this oxide system shows threedimensional asymmetric oxide islands and a more complex behaviour of the film growth.

2. Experimental procedures The experiments were carried out in two separate U H V systems. The investigations of surface composition, crystallography and defect structure performed by 180°-neutral impact collision ion scattering spectroscopy (NICISS) and high resolution spot profile analysis of low energy electron diffraction (SPALEED) were done in one U H V system (A) [12]. The investigation of imaging the surface morphology directly by STM were carried out in a second chamber (B) [13]. The two U H V systems are equipped with known standard tools for sample preparation and characterization such as noble gas ion sputtering, high temperature annealing for in situ sample treatment, AES, L E E D and a quadrupole mass analyser for monitoring the residual gas composition. The sample temperature during preparation was determined by the same infrared pyrometer in both chambers. All reported NICISS, SPALEED and STM measurements were carried out after cooling the sample to room temperature. The surface composition and crystallography was determined by 180 ° NICISS with a pulsed low intensity (average current of 2 0 0 p A ) He + ion beam at a primary energy of 3 keV. The energy spectra of 180 ° backscattered neutralized He projectiles were measured via a time-of-flight ( T O F ) technique. An analysis of chemical elements is obtained by comparing the observed T O F spectra with those calculated from elastic losses on energy and momentum conservation laws. Structural analysis can be extracted from the angular depen-

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dence of the backscattered particles. In addition, ISS spectra could be also evaluated by comparison with data obtained by the computer simulation package FAN [14]. Techniques and experimental details have been described elsewhere [ 15,16]. The SPALEED measurements were carried out using a commercial system which is based on the design of Henzler et al. [17]. The equipment was used in the channeltron mode, where the diffraction pattern is electrostatically scanned across the channeltron aperture. An electron beam current of less than 10 gA has been used. The instrument is characterized by a transfer width of about

reported below it turned out that the start-up condition of a clean NiAI(001) surface prior to oxidation does not effect the oxygen adsorption at room temperature and neither the nucleation nor oxidation behaviour at elevated temperatures. Therefore usually the most stable Al-terminated surface with a ( c ( ~ x 3~/2)R45 °) superstructure was chosen as a starting point for the oxidation experiments. After oxygen exposure at room temperature the sample was subsequently annealed at the temperatures indicated ( 1000-1400 K ) in order to form epitaxial crystalline oxide films.

lOOOX. In the present work a Beetle-type scanning tunnelling microscope [13] was used to determine details of the structural parameters of AlzO3NiAI(001). Constant current topographies were recorded for sample bias voltages b e t w e e n - 5 V and + 5 V with a tunnelling current in the range of 1 nA to 5 nA (typically 1 nA). The stable tunnelling conditions with low bias voltages (<1 V) at the clean metallic NiAI(001) surface have to be changed to higher bias voltages at the oxidized surface. The increased isolating character of the thin A1203 film makes tunnelling at low bias highly unstable. The bias dependence described has also been used as an indicator for tunnelling over metallic NiA1 or A1 oxide surfaces, respectively, even without measuring strictly in the scanning tunnelling spectroscopic mode. All STM images are recorded in a differential mode to ensure image reproduction of different topographic heights at reasonable grey levels. No Fourier transform based filtering has been applied. The NiAI(001) substrate was cut by spark erosion from a single crystal rod. The surface was polished mechanically and the crystal orientation had an accuracy better than 0.05 °. The main impurities (C, O) were removed in U H V by standard procedures of several cycles of Ar + ion sputtering (1 gA, 1.5 keV) and subsequent annealing. This procedure was repeated until no O or C contamination was detectable by AES. The sample preparation was found to influence most sensitively the chemical composition of the NiA1 surface. Details are reported in Ref. [12]. In the course of oxidation experiments at the NiAI(001) surface

3. Results 3.1. L o w energy electron diffraction ( S P A L E E D )

The surface structure of the fully oxidized NiAI(001) surface was analysed using SPALEED. Starting from the oxygen-saturated surface at room temperature, the oxide film was prepared by subsequent annealing at T = 1200 K. The proceeding is quite similar to that described previously by Gassmann et al. [9] where the growth of the 0-A1203 phase has been proposed. It should be noted, however, that a proper discrimination between the 0- and the 7-oxide modifications is at present not well founded basically because of the already-mentioned similarities in structure of the two phases. Anyhow, the proposed conclusions on structure and growth mechanism will not be effected much because of the structural similarities. According to the structural model of crystalline 0-A1203 which has been proposed by Gassmann et al. [9] it is expected to form a (2 x 1) superlattice at the oxide surface. The corresponding superstructure has also been observed in the conventional L E E D pattern in this investigation and is shown in Fig. la for a primary electron energy Eo= 127 eV. The intense but slightly broadened (1 x 1) substrate spots in combination with some background intensity seem to be quite common for binary alloys or compounds. These features are generally attributed to partial disorder in the interface region and to oxygen-induced A1 segregation during oxidation. It can be seen in the

R.-P. Blum et al. / Surface Science 396 (1998) 176 188

127 eV

55 eV Fig. 1. SurveyLEED pattern of the oxidizedNiAI(001) surface (1000 L oxygen exposure at room temperature and subsequent annealing to T=1200K) at different electron energies: (a) /[o=127 eV; (b) E0=55 eV.

179

L E E D pattern that the half-order spots are clearly elliptically shaped. If the electron energy is lowered this asymmetry converts into a splitting of the half-order L E E D spots and streaking along the <100) and <010) directions (Fig. lb). The spot splitting appears exactly at positions of multiples of 1/9 of the Brillouin zone as related to the NiA1 lattice constant. Backtransformation from reciprocal into real space results in a characteristic length of 26.01 A. This length shows reasonable agreement with previously proposed surface domains at NiA1 with a mean size of about 30 f~ [91. The second L E E D feature streaks is usually assigned to a loss of ordering in the corresponding surface direction. Here, streaking could not be completely removed even after different annealing times (note the high sensitivity of SPALEED). The surfaces investigated in the present work correspond to L E E D patterns in which streaking was of minor importance. The intensity line scans along a <100) direction, shown in Fig. 2 at different electron energies, indicate a typical intensity modulation which in fact suggests a different explanation rather than loss of ordering. The SPALEED data are plotted as a function of the in-plane scattering vector kEi, and the size given on the x-axis in percentage (%/%z) of the total width of the [100] Brillouin zone. The intensity modulation in the line scans shows peaks or shoulders (A) which fit onto a grid showing an interspacing of AAkil with AA= 1/9 of the width of the [100] Brillouin zone. The position of the (A) spots are schematically inserted in a bar line in the upper part of Fig. 2. The position of the expected (2 × 1) spots is also indicated. The line shape analysis of the line scans (Gaussian line shapes for all peaks) requires one to consider additional energy-dependent intensities (B) next to all full-order spots for the best fit. These intensities behave as satellites (marked only near the (0, 0) spot as grey area in Fig. 2) in addition to the broadened (A) spots. Their kll distances do not fit into the AAkil screen of the A spots. The occurrence of four distinct satellites in the streaks (two in each direction) in contrast to a recently measured concentric ring structure around (0, 0) ("Henzler ring" [18]) indicates that either the A1203 island shapes are not circular or

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R.-P. Blum et al. / Surface Science 396 (1998) 176 188

-1

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0

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"t ....

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100 eV i

-100

I

-50

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50

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Fig. 2. Line scan of the diffraction intensity in the <100> direction of completely oxidized NiA1 (001) at different electron energies. Similar line scans can be obtained along the <010) direction.

the islands are not randomly orientated but quite asymmetric with respect to the <100> and <010> directions. Both configurations, the arrangement of the islands on a long-range scale or an individual asymmetric island shape might produce the satellite structures in LEED. It will be shown below from STM data that in the case of the NiAI(001) surface the asymmetric island size itself gives rise to satellite formation in the electron diffraction data. In addition, the intensity dependence on Eo is obviously completely different for the two classes of spots, A and B (cf. line scans at Eo = 55 eV and E0 = 100 eV). It follows directly from the L E E D data in Fig. 2 that the absolute intensity of the features in the streaks reacts sensitively to variation of Eo while the position of the diffraction peaks in k space did not vary with the electron beam energy.

Finally, it can be observed that in a normal survey L E E D pattern (e.g. at Eo = 100 eV) the broadening of the mentioned structures might appear only as a "pseudo 2 × 1" structure in connection with streaks in the L E E D images. The energy-dependent intensity oscillations of the central spot compared with the satellite intensities can be explained by the electron scattering at a stepped surface. The (0, 0) spot intensity dominates at values of k± ( ~ 1 / ~ 0 ) for which constructive interference is expected to occur from diffraction at terraces separated by a monatomic step. On the other hand, the satellite intensities should dominate at values of k± for which destructive interference occurs. Owing to the fixed positions of the satellites on variation of Eo the explanation of the intensity variation by surface facetting can be excluded. Recently, Henzler and co-workers [18,19] have demonstrated that the development of satellites in L E E D might indicate nucleation and growth of islands on terraces during epitaxy. The line shape of the satellites includes information on the lateral distribution of the grown islands. With the assumption that the centre-to-centre distance distribution behaves as a P distribution [20], a mean separation between islands of 54 4- 5 A can be calculated related with a quite large variance of 12 + 2 A. With the SPALEED data the vertical roughness can also be determined. In order to quantify the roughness (i.e. to establish the step height of A1203 islands), the central spike profile was analysed with the help of computer simulations based upon kinematic theory. The oscillations of the half width and intensity of the (0, 0) spot was measured as a function of E 0. An evaluation procedure suggested by Wollschlfiger [21,22] has been followed. In brief, on rough reversibly stepped (steps up and down) surfaces the measured half-width 6k of a diffraction spot is connected via the corresponding Fourier transforms with the vertical lateral length distributions of islands. Hence, an analysis of the spot half width allows the determination of the ratio between step height d and the mean island width according to the expression [21,221 2 a ~ k -
R.-P. Blum et aL / Surface Science 396 (1998) 176-188

The fraction of double- to mono-steps is given by a and the lattice parameter by a. The oscillation of the (normalized) intensity of the central spot vs. Eo is determined only by the vertical roughness of the surface. The normalized central spike intensity was approximated by a Gaussian function G(S) ~ e x p { - [172(27zcSSZ]}. c~S gives the deviation of the scattering phase S from the next integer value of S and V represents the root mean square value of the surface roughness. The step height could be derived from the distance AS between subsequent in-plane conditions according to d = 2re AS/k±. Following the valuation scheme described, a set of parameters ( ( F ) , or, 17, d) was determined and consequently used in the fitting procedures to reproduce the experimentally determined half-width and intensity oscillations. The corresponding measured half width oscillation is shown by open squares in Fig. 3a and the measured normalized intensities are plotted by open circles in Fig. 3b. Both quantities are plotted as a function of energy. With the best fit conditions

2.5 @ t~

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20

40

60

80

100

120

140

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Fig. 3, (a) Half-width and (b) normalized intensity G(S) oscillations of the LEED (0, 0) spot recorded as a function of the energy Eo. A12OJNiAI(001) surface as prepared from the oxygen saturation coverage.

181

the solid lines in Fig. 3 have been constructed. From this parameter set the following island properties can be extracted: a mean island width £ F ) = 28_+3 A, a vertical roughness given by a step height of d = 2 . 8 _ 0 . 1 A (rms value V =0.7) with a relatively low probability for double steps o-= 0.11. The asymmetry in the rectangular island shape manifests itself in the small width of about 28 in combination with a much larger length ("oxide stripes"). In order to determine the lateral distribution of the islands, the line scans of Fig. 2 have to be discussed in more detail. The diffraction peaks of the above-mentioned class A might be explained by coherent scattering at ordered arrangements of antiphase domains. The domains organize in a pattern of an overall (9 x 1) periodicity which in fact may be correlated with a regular arrangement of aluminium oxide domains ( A 1 2 0 3 stripes) on the NiAI(001) substrate. Each domain itself as a subunit of the configuration is composed of aluminium oxide exposing a surface structure with a (2 x 1) periodicity. By application of the kinematic rules of diffraction, the coherent interference of beams scattered at a grid which is composed of two subgrids (stripes of individual (2 x 1) superstructure which form a (9 x 1) stripe lattice) will finally result in an intensity pattern with a (9 x 1) periodicity and an intensity variation with kit as determined by the intensity envelope of the (2 x 1) subunits. The relative height of the individual spot can be expressed by Hi=l,2[sin2(iTzhn)/sin2(~zn)] with n = ( 0 , 1/9, 2/9 . . . . , 1) [23]. n is the order of the main maxima and t~ describes the number of scatterers. Consequently, only those ninth-order spots close to half-order positions will have strong intensities. This explanation can completely describe in detail the measured spot profiles in L E E D of Fig. 2. Only in a rough inspection of L E E D patterns it might appear that the half-order spots are elongated or split into pairs. Based on the observations of ordered A1203 films by other authors [11,24], the measured L E E D patterns from such oxide films are expected to reflect primarily the ordering effect of the O zions in the superlattice even though the A13 + ions in the lattice may be the stronger electron scatterers. This is because the formation of the regular

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R.-P. Blum et al. / Surface Science 396 (1998) 176-188

structure within the oxide film is presumably determined by the 0 2. skeleton while the A13+ ions are more likely to be randomly distributed among the octahedral and tetrahedral sites in the different A1203 structures.

3.2. Ion scattering (NICISS) The low energy ion backscattering method NICISS has previously been described in detail [16]. In particular with He scattering at grazing angles of incidence the composition and the structure of the top surface layer can be determined directly [14,15]. In a recent publication three different surface structures of clean NiAI(001) have been identified predominantly by the surface crystallographic information of NICISS [12]. A low temperature annealed (1 x 1) surface was shown to be terminated by A1, another high temperature annealed (1 x 1) surface by Ni and a surface with a (c(1/2x 3~v/2)R45 °) superstructure obtained upon medium temperature annealing by A1 in the topmost layer in combination with a missing row structure. The information about the surface composition can be extracted directly from the ion scattering angular intensity patterns obtained with He 180 ° backscattered at either Ni or A1 atoms of the topmost layer. Depending on the surface configuration, a steep increase of backscattering intensity will appear at grazing angles of incidence ~ provided the corresponding surface scatterer is located in the first surface layer, whereas the intensity increase will be shifted towards larger angles of incidence ~ for scattering in deeper layers [12]. For the NICISS investigation of the oxidized surface, the clean NiAI(001 ) was prepared initially and consequently exposed to oxygen at room temperature up to saturation [1000 Langmuir (L) oxygen]. Irrespective of the clean surface start-up condition the oxygen exposure at 300 K directly induces segregation of A1 into the top layer which immediately shows up in the NICISS data. After annealing at T = 1200 K the fully oxidized structure was obtained as already discussed in Section 3.1. 3 keV He scattering has been performed at the oxidized surface; the intensities of He scattered off

Ni, A1 and O as functions of the grazing angle of incidence ~ (as measured with respect to the surface plane) are shown in Fig. 4. The NICISS geometry has been set up in such a way that the scattering plane normal to the surface includes a (110) azimuth direction in the NiAI(001) surface plane. The lines plotted are extracted from a set of experimental T O F spectra after appropriate background subtraction (cf. Ref. [12]) and not corrected for any difference in the He scattering cross-sections for the three elements. First of all, it can be noted that all three lines start to increase at very low angles of incidence (about 4 ° as referred to the surface plane). As a consequence, the surface has to be composed of all three elements, i.e. O, A1 and Ni. Such a finding is rather surprising, because the fully oxidized surface has been predicted to consist exclusively of (7-) 0-A1203, i.e. no nickel component is expected in the top layers. The absolute intensity maximum of the Ni line is less than that of the A1 line (cf. Fig. 4) in spite of a larger cross-section for He scattering at Ni as compared with A1. Hence, the Ni content in the surface is obviously small. Because of the already

5 .o

c-

i

0°

10 °

20 °

30 °

grazing a n g l e o f i n c i d e n c e

Fig. 4. Measured intensities"of He backscattered at A1, Ni and O as a function of the angle of incidence ~ from order AI203 in (110) scattering geometry (see text). Base lines are shifted for clarity.

R.-P. Blum et al. / Surface Science 396 (1998) 176 188

known surface roughness of the oxide, one might also consider scattering of He from Ni to arise in fact f r o m the second layer of Ni atoms. In the case of a reversibly stepped surface there are m a n y expected channels open for head on collisions at grazing angles of incidence with second-layer atoms owing to the number of direct points of entrance at step edges. Another feature in the N I C I S S spectra can be seen for He at Ni scattering as an additional peak appearing at 7t about 20 °. This feature is indicative for scattering at deeper layers in a well-ordered crystallographic structure, i.e. in the mentioned N I C I S S investigation of the clean surface the same structure occurs in the corresponding N I C I S S pattern of the clean (1 x 1) Ni terminated surface and has been attributed to He Ni scattering in the third layer (cf. Figure 9c of ref. [12]). A similar structure is hardly visible in Fig. 4 for He scattering at A1, although a peak with lower intensity can be also expected by comparison with the data from the clean surface (cf. also Figure 9c of Ref. [12]). Based on these N I C I S S observations, the following model of an oxidized NiA1 surface can be proposed: because of the high intensity of the A1 intensity maximum, the majority of the surface is proposed to be covered by A1 and/or A1103. A considerable amount of disorder in the occupation of A1 positions in the oxide layer (probably a fraction of amorphous A1203) leads to the obscuration of the second layer A1 peak, which on the other hand is only expected to appear for wellordered structures. In addition areas of proper NiA1 crystallography probably covered by a monolayer of A1 have been identified.

183

®

®

Fig. 5. STM images of AIzO3/NiAI(001) after 10L oxygen exposure and subsequent annealing at 1200 K. (a) survey showing the initial oxide formation in "oxide stripes"; Utlp=- 1.24 V. (b) high resolution STM image showing the (2 x 1) periodicity on the oxide stripes, areas with A1terminated NiAl(001)-(c(X/2x31/2)R45 °) in the upper left and amorphous A1203 in the lower right; Utlp= - 1.24 V.

3.3. S c a n n i n g tunnelling microscopy

Oxide growth on the NiAI(001) surface was also monitored using STM. The initial formation of the oxide island can best be seen at surfaces with low A1203 coverage. In Fig. 5 such a low oxide coverage surface is shown, which has been prepared by 10 L oxygen adsorption at r o o m temperature and subsequent annealing at T = 1200 K. The different oxidation state changes the overall surface morphology but has no importance of the properties of individual oxide islands. The corre-

sponding survey L E E D pattern shows a (1 x 1) with a weak (c(1/2 x 3X/2)R45 °) superstructure. As proposed in Section 3.1, highly asymmetric oxide island formation can be recognized immediately in Fig. 5a. On the plain substrate surface white stripes with different length, width and spacing running along the (100) and (010) can be seen which in fact have been formed upon the oxidation (Fig. 5a). The density of stripes is too low for the formation of a long-range order by self-organization compared with the fully oxidized surface.

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From the influence of different tunnelling parameters, i.e. the variation of the sample bias voltage, it can be seen that the regions between the stripes often show metallic behaviour like the clean NiAI(001) surface (stable tunnelling conditions at less than 1 V sample bias). Usually on an oxygenfree clean NiAI(001) metallic sample the surface crystallography can be imaged with atomic resolution. A discussion of the surface configuration of the clean NiAI(001) surface revealed by STM data will be presented elsewhere [25]. Indeed, the metallic-like surface as shown in parts of Fig. 5a and at higher magnification in Fig. 5b at and close by the oxide stripes turned out to expose two different kinds of composition. In the upper left corner of Fig. 5b the STM features reflect the single-crystal surface structure by lines 45 ° rotated with respect to the indicated {010) direction. This part of the surface area looks in fact identical (aside of some defects) to STM data measured at the clean NiAI(001) surface exposing the (c(V~ x 3~v/2)R45 °) superstructure [25]. Another part on the surface shows a more disordered - non-crystalline - structure (see Fig. 5b, the area on the lower right) which might be caused in the preparation process by formation of amorphous A1203. Upon initial oxygen adsorption at room temperature the NICISS data have demonstrated that oxygen induces the segregation of A1 to the surface. Thus, even at 300 K the mobility of A1 in the NiA1 lattice is high enough to enrich the surface layer with A1. Thus in contrast with oxygen chemisorption at an A1 single crystal [26], in case of NiA1 oxygen chemisorbs probably in a sort of a precursor state and will be surrounded immediately by A1 atoms in such a manner that it creates right away the chemically more favourable configuration of an A1 "oxide cluster". Because of the immobility of such an "oxide cluster" at 300 K the entire surface is probably in a state of incomplete crystalline oxide formation including many defects in the oxide layer. Subsequent sample annealing might still leave the surface with some areas of segregated A1 to form the (c(V~ x 3V~)R45 °) superstructure in combination with an imperfect ordering of the oxide between the stripes of ordered A1203.

In the high resolution image of the oxide stripes (Fig. 5b) one can clearly recognize the areas on the oxide stripes to be composed of small lines running along (010). The interspacing between the lines can be measured to 6 A whereas along these lines the resolution is not high enough to discriminate between single atoms. The interspacing of 6 A is in complete agreement with a superstructure periodicity of two (2 x 2.89 A,) perpendicular to the oxide stripes. Along the rows the distance between atoms is probably smaller (not resolved with STM; 1 x 2.89 A). As a result, such a (2 x 1) superstructure on oxide stripes confirms directly the SPALEED data presented above. The height of the oxide stripes appears largely independent of the sample bias voltage and is measured to 3 A. The also visible stronger corrugations (high protrusions) on the oxide stripes in Fig. 5b appear to depend much on the annealing temperatures during the oxidation and are believed to indicate the onset of thicker oxide formation. Even in the low coverage state occasionally it seems that new material has flowed around edges of the oxide stripes and sometimes the stripes appear to be fully embedded in a NiA1 or amorphous oxide matrix (upper part of Fig. 5a). The oxide stripe in this area shows an apparent height of only 0.2 A with respect to the upper part filled with the new material. Similar finding will be shown in the case of the highly oxidized surface. Following the same preparation procedure as described above but increasing the initial oxygen coverage leads to surfaces covered more or less densely by a network of stripes. All stripes are running along the (100) and ~010) directions of the NiAI(001) surface with an equal probability for the orthogonal domains. The increasing density of stripes in combination with their decreasing mean length, width and spacing scales with oxygen exposure (not shown here). It has been found that oxide stripes may coalesce but even after oxygen saturation they will never cross each other. The impossibility to cross also determines the final length scale of the stripes as limited by their geometric arrangement. Comparing the tunnelling conditions on the stripes (Ubias > 1.2 V) with those on regions between the stripes (Ubia+< 1.0 V) at low oxygen exposure the stripes can be clearly

R.-P. Blum et aL / Surface Science 396 (1998) 176-188

identified as oxidized islands, probably A1203. Fig. 6a shows the fully oxidized NiAI(001 ) surface (oxygen exposure 1000 L, T = 1200 K). Upon saturation exposure the width and spacing (centre-tocentre distance) of the oxide stripes converge towards discrete values. The width is shown by open circles in Fig. 6b. The majority of oxide stripes show a width of about 27 A. A line scan in the (100) direction is shown in Fig. 6c. The axis of the scan direction is divided in a grid of 54 A. The spacing of parallel oxide stripes fit over a length of about 1000 A into this grid. The surface

®

185

is completely covered by a dense network of stripes resulting in a reversibly stepped surface exposing predominantly single steps (3A) and a small amount of double steps. The overall height variation is below 10 A. A closer look to the fully oxidized surface reveals also any "sea"-like structures surrounded by oxide stripes with an apparent height of 0.25 A. Because the isolating oxide and the overall roughness hinder a better resolution with STM, details of this structure cannot be seen clearly. Similar structures are found at low coverage oxide structures

©

40F 0

c ~. ~.C.C.c,

20 30 Stripe Width [ A ]

coooocc,

40 "

0

540 1018'0' ' ' Scan Distance [ A ]

Fig. 6. (a) 3D STM images of AI2OJNiAI(001 ) after saturation exposure (I000L) and subsequent annealing to 1200K; Utip = - 3 . 8 5 V. (b) Distribution of the width of oxide stripes. (c) Profile of corrugation along section indicated in (a).

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with better resolution (Fig. 7). Indeed the surface structure of the "seas" looks quite the same as shown above for the areas between low coverage oxide structures in Fig. 5. This coincidence strongly suggests the identification of the "sea" plateaus with the same type of surface as a mixture of NiAI(001)-(c(V~ x 3~v/2)R45 °) and amorphous A120 3. The high density of steps up and down is also in complete agreement with the NICISS measurements showing the appearance of the three species oxygen, aluminium and nickel in the surface layer.

4. Discussion As a result of SPALEED, NICISS and STM measurements the main mechanisms for the complex three-dimensional growth mode of A1 oxide on NiAI(001) are based on the experimental findings of a network of oxide stripes and the tendency to embed the growing oxide islands in a mixture of amorphous oxide and a NiA1 matrix by regrowth of new NiA1 terraces close the oxide stripes. In the SPALEED and the STM data the pronounced anisotropic growth of the oxide

®

Fig. 7. (a) STM image of oxidized NiA1 covered by amorphous and crystalline A1 oxide. (b) Inset shows a plateau region which is completely filled by amorphous A1203. After 10 L oxygenexposure at 300 K and annealing at 1200K; Utip= -- 1.4 V.

R.-P. Blum et al. /Surface Science 396 (1998) 176-188

islands has been identified. The evaluation of oxide stripes in (100) or (010) directions with extreme aspect ratios suggest a growth mechanism based on lattice mismatch in one direction in combination with compressive or tensile stress relaxation processes. The direct epitaxial relation between A1203 (either in the y or 0 phase) and NiAI(001) according the classical Bain orientation relationship between bcc (NiA1) and fee (°xygen sublattice) structures results in a lattice mismatch of 2.42% (tensile stress) along the a-axis and of 1% (compressive stress) along the b-axis of the oxide unit cell with respect to the lattice parameter o f NiA1 (ao=2.89 A). It should be noted that for a variety of ordered ultra-thin A1203 films on different substrates an enlarged 0 2 - - 0 2 . interionic spacing has been observed. Sometimes a lattice dilation as high as 9% with respect to bulk A1203 occurred. For example, ordered A1203 films on T a ( l l 0 ) show a lattice constant of 3.05 ~ [27], and an even larger lattice constant of 3.07 A has been reported for A1203 on NiA I ( l l 0) [11]. Accordingly, an expanded O 2 - - O 2- interionic separation appears often in thin films, whereas a compression of the oxygen sublattice has never been observed even though the reason for this behaviour is still unclear. Assuming a slightly expanded interionic oxygen separation for A1203 on the NiAI(001) substrate relieves the tensile stress along the a-axis completely and presumably results in extremely wellexpressed epitaxial growth in one of the equivalent (010) or (100) directions. On the other hand, even the small compression of 1% in the lattice parameter of A1203 in the perpendicular b-direction will probably add unfavourable stress to the lattice so that the lateral growth stops after a few lattice distances. As a result, the growth of A1203 manifests itself in highly anisotropic islands, i.e. the "oxide stripes". The azimuthal orientation of the stripes is determined by the strong difference in the behaviour of growth along the a- or the baxis of the oxide. Two main orientations of stripes may appear in either the (100) or the (010) direction of the NiAI(001) surface lattice depending on the initial oxide nucleation with the A1203 a-axis along NiAI(001) ~100) or (010), respectively.

187

Finally, the observed embedding of oxide islands in the mixed matrix of NiA1 and amorphous oxide might be plausible because of thermodynamics. The applied high temperature for oxide ordering on the NiAI(001) surface allows a growth mode without harsh kinetic restrictions. The growth of the A12Oa film on an oxygen-covered NiA1 surface occurs generally by a complicated process of solidstate diffusion transport of the reactants. The two main processes for solid-state transport in the growing oxide film as well as in the substrate are diffusion either through the lattice or along grain boundaries or stacking faults. Neither mechanism is expected to occur exclusively in the covered range of annealing temperatures either in the NiA1 alloy or in the oxide. At the oxidation temperature of about 1200 K the overall lattice diffusion is expected to be high enough to achieve thermodynamic equilibrium at the external surfaces (oxide and alloy) as well as at the internal surface, the A1203/NiAI(001) interface. Hence, the oxide growth may occur in a steady-state condition under the influence of the gradient in chemical potential and electric field, which may develop during oxide growth by separation of chemical elements and electrical charge. On the other hand, incomplete formation of crystalline A1203 gives rise to the formation of defect structures in the growing oxide (amorphous A1203) which in turn might serve as lively channels for fast A1 diffusion. Thus the transport of A1 under the influence of chemical and electrical gradients in the solvage region might be quite effective. Providing such an enhanced presentation of A1 atoms at the surface is not completely balanced by a formation of ordered oxide, the locally changed chemical potential might trigger additional Ni segregation to the surface region. Consequently at the annealing temperature of 1200K the regrowth of new NiA1 terraces will set in. The effect is supposed to appear particularly at prominent defect structures such as the edges of ordered oxide stripes (cf. Figs. 5 and 7). These new NiA1 areas will be stabilized by the coverage of an ultra-thin layer of amorphous Al-oxide or similar as in case of the clean NiAI(001) surface by 0.75 monolayers of plain A1 which is well known to form a missing

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r o w structure finally resulting in the ( c ( V 2 x 3 V ~ ) R 4 5 °) s u p e r s t r u c t u r e [12].

LEED

5. Conclusions SPALEED, NICISS and STM measurements were used to d e t e r m i n e details o f the f o r m a t i o n a n d structure o f a thin A1203 film o n N i A I ( 0 0 1 ) . T h e oxide f o r m a t i o n is unaffected b y the initial c o m p o s i t i o n ( p r e p a r a t i o n - d e p e n d e n t A1 o r N i t e r m i n a t i o n ) o r r e c o n s t r u c t i o n o f the substrate. T h e p a r t o f the N i A I ( 0 0 1 ) surface c o v e r e d b y 0o r ~-A120 3 exhibits a m i c r o s c o p i c a l l y r o u g h reversible s t e p p e d surface with a n e t w o r k o f e l o n g a t e d oxide islands a l o n g ( 1 0 0 ) a n d ( 0 1 0 ) directions o f the N i A I ( 0 0 1 ) surface with an equal p r o b a b i l i t y for o r t h o g o n a l d o m a i n s . I s l a n d length, w i d t h a n d distance scales with the o x y g e n e x p o s u r e u p to s a t u r a t i o n . A t s a t u r a t i o n oxide stripes occur with an a v e r a g e w i d t h o f 27 A_ a n d a centre-to-centre d i s t a n c e o f 45 A. T h e self-organized a r r a n g e m e n t o f p a r a l l e l oxide stripes results in a (9 x 1) superstructure. T h e oxide stripes themselves expose a (2 x 1 ) superstructure. A s a sum it yields in coexisting a n t i p h a s e d o m a i n s with a (9 x 1) superstructure c o n t a i n i n g (2 x 1 ) subunits. T h e overall height v a r i a t i o n is b e l o w 1 0 A with d o m i n a n t l y single steps o f a b o u t 3 A height. I n a d d i t i o n to the crystalline A120 3 stripes, r e g r o w n NiA1 terraces close to the o r d e r e d oxide islands have been identified. These new NiA1 areas are stabilized b y the c o v e r a g e with a m i x t u r e o f a n u l t r a - t h i n layer o f amorphous Al-oxide and plain A1 in a ( c ( V 2 x 3 X / 2 ) R 4 5 °) missing r o w structure. This indicates a t h r e e - d i m e n s i o n a l g r o w t h m o d e with a c o m p l i c a t e d interface structure a n d a thicker oxide film then the visible surface r o u g h n e s s o f 10 A.

Acknowledgements W e a c k n o w l e d g e with p l e a s u r e the s u p p o r t o f the D e u t s c h F o r s c h u n g s g e m e i n s c h a f t for the w o r k as a p a r t o f the SFB290.

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