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Structure of chromium oxide ultrathin films on Ag(1 1 1)
Surface Science 578 (2005) 149–161 www.elsevier.com/locate/susc
Structure of chromium oxide ultrathin films on Ag(1 1 1) W.A.A. Priyantha, G.D. Waddill
*
Department of Physics, University of Missouri-Rolla, 1870 Miner Circle, Rolla, MO 65409, USA Received 1 May 2004; accepted for publication 19 January 2005 Available online 25 January 2005
Abstract Epitaxial chromium oxide films are grown on Ag(1 1 1) by serial deposition. This method is repeated sequences of the deposition of sub-monolayer Cr films (typically 60.5 ML) followed by oxidation. The procedure is repeated until the desired oxide film thickness is achieved. The oxide films are characterized using low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and X-ray photoelectron diffraction (XPD). Chromium oxide films with ˚ show a p(2 · 2) LEED pattern consistent with Cr3O4(1 1 1) films. XPD results, thickness less than approximately 5 A however, suggest that the low coverage chromium oxide films may not be Cr3O4, and definitive identification of the low coverage oxide structure requires further work. Chromium oxide films with thickness greater than approximately ˚ show a (p 3 · p 3)R30 LEED pattern consistent with a-Cr2O3(0 0 0 1). XPD results confirm this and further iden12 A tify the surface termination as a single Cr layer with an inward relaxation of 50%. 2005 Elsevier B.V. All rights reserved. Keywords: Electron–solid diffraction; Low energy electron diffraction (LEED); Photoelectron diffraction measurement; Growth; Oxidation; Interfaces; Single crystal epitaxy
1. Introduction The importance of metal oxides in many technological fields has fueled an increase in interest in surface science investigations of these materials. The ability to produce ordered metal oxide surfaces with well defined structure and chemical *
Corresponding author. Tel.: +1 573 341 4797; fax: +1 573 341 4715. E-mail address: [email protected] (G.D. Waddill).
composition is critical to the success of these investigations. Recent investigations have established that it is possible to grow some oriented single crystal transition metal oxide thin films with well defined stoichiometry [1–6]. These films have been grown in a variety of ways including oxidation of the surface region of single crystal metals, molecular beam epitaxy in an oxygen or other reactive gas atmosphere, oxygen plasma assisted molecular beam epitaxy, and metal deposition on suitable substrates followed by oxidation of the resulting
0039-6028/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2005.01.028
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metal film. For the last method substrates for metal-oxide growth have generally been Al2O3 or MgO making performance of electron based surface spectroscopies on the resulting films an imposing experimental challenge. More recently it has been demonstrated that it is possible to stabilize thin transition metal oxide single crystal films by oxidizing metal films grown on single crystal metal substrates [3–5]. This eliminates charging difficulties encountered in electron or ion spectroscopies on insulating surfaces, and also opens the possibility of isolating new oxide structures not encountered in bulk form. The surfaces of chromium and chromium oxides are interesting for a variety of reasons. Cr is one of several transition metals predicted to show strong magnetic moment enhancement in ultrathin films, and single monolayer films may exhibit ferromagnetic behavior [7,8]. One oxide of chromium, CrO2, is believed to be a half metallic ferromagnet with a metallic band structure for one spin and an insulating band structure for the other [9,10]. CrO2 is used in magnetic recording media because of its corrosion resistance and high coercivity. Above about 280 C, CrO2 decomposes to the most stable oxide of chromium, Cr2O3. Cr2O3 has the corundum structure, is an important polymerization catalyst, and is used in passivating stainless steel. Above 1600 C the stable oxide of chromium is a tetragonally distorted Cr3O4 spinel phase. The formation of c-Cr2O3, a cubic spinel phase, has also been reported for certain thin film growth conditions. Ordered single-crystal samples of any of these chromium oxides are unavailable making their thin film preparation more important. Most previous studies of chromium oxidation have involved the oxidation of the Cr(1 1 0) surface [2,11–14]. There are also isolated examples of oxidation of the (1 0 0)and (1 1 1) surfaces of chromium [15]. More recently, Diebold et al. [3,16] have succeeded in growing Cr2O3 and perhaps cubic spinel Cr3O4 films on the Pt(1 1 1) surface. In this study we have characterized the growth and structure of Cr oxide films on Ag(1 1 1) using X-ray photoelectron spectroscopy (XPS), X-ray photoelectron diffraction (XPD), and low-energy electron diffraction (LEED). We have grown the oxide films in two different ways. The first involves
deposition of a multilayer Cr film followed by oxidation of that film at approximately 240 C. For ˚ all Cr coverages investigated p (2–20 p A prior to oxidation) this results in a ( 3 · 3)R30 LEED pattern expected for Cr2O3(0 0 0 1) films. The Cr film prior to oxidation is most likely Cr(1 1 0). This is based on observations of Fe(1 1 0) growth on Ag(1 1 1) [17], and because our LEED patterns for the Cr films are a diffuse (1 · 1) structure consistent with Cr(1 1 0) growth. Previous studies of the oxidation of Cr(1 1 0) generally find ordered Cr2O3(0 0 0 1) growth [2,11–14] so our results are consistent with those findings. The second method grows the chromium oxide film by repeated cycles of sub-monolayer Cr film deposition (typically 0.5 ML or lower) followed by exposure to an oxy˚ ) films, a gen atmosphere at 240 C. For thin (65 A p(2 · 2) LEED pattern is observed. This LEED pattern is consistent with the formation of Cr3O4(1 1 1) films. p Oxide films thicker than about ˚ exhibit a ( 3 · p 3)R30 LEED pattern, 12 A ˚ have and films with thickness between 5 and 12 A LEED patternsp thatpare a superposition of the p(2 · 2) and ( 3 · 3)R30 patterns consistent with the presence of both Cr3O4 and Cr2O3 in this coverage regime. This paper will discuss exclusively Cr oxide films produced by the sequential deposition method.
2. Experimental procedure Experiments were performed in an ultra-high vacuum chamber at base pressures of 61 · 1010 Torr [18]. The Ag single crystal was cleaned by cycles of Ar ion sputtering followed by annealing to 450 C. XPS showed no contamination within detectable limits. LEED showed a sharp (1 · 1) pattern. Chromium thin films were deposited at room temperature by electron beam evaporation at pressures below 1 · 109 Torr. A quartz crystal thickness monitor and XPS Ag 3d5/2 and Cr 2p1/2 intensity variations were used to monitor the chromium coverage. Chromium evaporation ˚ /min. Oxide thickness rates were typically 0.5–1.0 A was determined using XPS core level intensity variations. When using XPS to determine film thickness we compared both the Ag 3d5/2 attenuation
W.A.A. Priyantha, G.D. Waddill / Surface Science 578 (2005) 149–161
and the Cr 2p1/2 to Ag 3d5/2 intensity ratio for a given film. In converting these to Cr or Cr-oxide thickness we used electron mean free paths calculated according to the method given in Ref. [19]. Chromium oxide films were grown by sequential deposition of submonolayer Cr films on the Ag substrate with each submonolayer metal deposition followed by an oxidation step. The oxidation step involved backfilling the chamber with O2 to a pressure of 105 Torr for 5 min while the sample was held at an elevated temperature-generally 240 C. This sequence was repeated until the desired oxide film thickness was achieved. During the deposition and oxidation the substrate was maintained at 240 C. Following the final cycle, the sample was annealed at 420 C for 30 min. Photoelectron diffraction data was taken using MgKa radiation (hm = 1254 eV) and a hemispherical energy analyzer of mean radius 125 mm and angular acceptance of ±1. The sample was mounted in a holder capable of 360 polar rotation, h, and ±100 azimuthal rotation, /. Both polar and azimuthal rotations had an angular resolution of better than ±0.5. Angular scans were obtained for Cr 2p1/2 (binding energy = 584 eV), Ag 3d5/2 (binding energy = 368 eV), and O 1s (binding energy = 531 eV) core levels. The Cr 2p3/2 peak which normally would be used for the XPD studies could not be used here as it overlaps with the Ag 3p3/2 peak. The integrated area of these peaks, after background subtraction, was used to generate the photoelectron diffraction curves. XPD and LEED studies were performed on chromium oxide films with thickness ranging from <1 ML up to >20 ML.
3. Results and discussion 3.1. LEED results Fig. 1 shows the LEED patterns observed for clean Ag(1 1 1) and for three different chromium oxide films. The clean Ag(1 1 1) shows a sharp (1 · 1) pattern (Fig. 1(a)) with low background intensity. For chromium oxide films with thickness ˚ a p(2 · 2) pattern is observed. An less than 5 A ˚ chroexample of this is shown Fig. 1(b) for a 4.5 A
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mium oxide film. For chromium oxide films thicker ˚ a (p 3 · p 3)R30 pattern than approximately 12 A ˚ chrois observed. An example for a 12 A mium oxide film is shown in Fig. 1(d). For films ˚ a complicated with thickness between 5 and 12 A LEED pattern is observed. This is shown in Fig. ˚ oxide film. Closer inspection of this 1(c) for a 10 A LEED pattern reveals that it is a superposition of the low and high coverage LEED patterns observed for chromium oxide films on Ag(1 1 1). These LEED patterns were recorded at primary electron energy 70 eV after annealing the samples at 420 C for 30 min. Similar results have been reported for Cr oxide films on Pt(1 1 1) [3] where the p(2 · 2) pattern p waspattributed to Cr3O4(1 1 1) films and the ( 3 · 3)R30 pattern to aCr2O3(0 0 0 1) films. The hexagonal unit cell of the ˚. Ag(1 1 1) surface has a lattice constant of 2.89 A The Cr3O4(1 1 1) surface has a hexagonal unit cell ˚ and an oxygen with a lattice constant of 5.72 A ˚ . a-Cr2O3(0 0 0 1) interatomic distance of 2.85 A has a hexagonal unit cell with a lattice constant ˚ and an interatomic distance of 2.86 A ˚ of 4.95 A within the close-packed oxygen planes. The close lattice match between the Ag substrate and the chromium oxide films leads to the epitaxial relationships reflected in the observed LEED patterns. In order to further test the structural nature of the chromium oxide films we now turn to X-ray photoelectron diffraction (XPD) studies of the high and low coverage oxide structures. 3.2. XPD results ˚ Fig. 2 shows azimuthal XPD data for a 4.5 A chromium oxide film for both Cr 2p1/2 (left) and O 1s (right). The data are for a polar scattering angle of 55 measured from the sample normal and corresponding to a body diagonal for the cubic oxide structure. We observe 60 periodicity confirming the 6-fold symmetry observed in the LEED pattern. Figs. 3–5 show polar XPD data for scattering planes coinciding with the Agð0 1 1Þ; Agð1 2 1Þ; and Agð1 1 0Þ respectively. In each figure the Cr 2p1/2 data are shown in the left panel and the O 1s data in the right panel. The experimental data is the top curve in each figure, and these in general are complex and not readily
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˚ ) chromium oxide, (c) mixed phase (10 A ˚ ) chromium oxide, Fig. 1. LEED patterns for (a) clean Ag(1 1 1), and for (b) low coverage (5 A ˚ ) chromium oxide using an electron energy of 70 eV. and (d) high coverage (12 A
O 1s
Intensity (Arb. Units)
Intensity (Arb. Units)
Cr 2p1/2
0
40
80
120
Phi (Deg.)
0
40
80
120
Phi (Deg.)
Fig. 2. Azimuthal angle XPD curves for Cr 2p (left) and O 1s (right) for a polar scattering angle of 55.
interpreted using simple forward scattering arguments due in large part to the complicated nature of the oxide structure. To explore the possible
oxide film structures further we compared our experimental results with results of multiple scattering simulations [20] for a number of possible
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Cr 2p
153
O 1s Exp.
Exp. COCO OCOC COCO COCC OCOC
Intensity (Arb. Units)
Intensity (Arb. Units)
OCCC
COCC
OCCC
CCCO
CCCO CCOC
CCOC bwCOCO bwCOCO oxCOCC oxCOCC
COCC + cbCOCC
COCC + cbCOCC 0
10
20
30
40
50
60
Theta (Deg.)
0
10
20
30
40
50
60
Theta (Deg.)
Fig. 3. Polar angle XPD curves for the low coverage chromium oxide films. Experimental (circles) and multiple scattering simulations (diamonds) for Cr 2p (left) and O 1s (right) are for a scattering plane corresponding to the Agð0 1 1Þ plane. The different surface terminations and reconstructions examined are indicated to the left of each curve.
Cr3O4(1 1 1) surface structures. The results of those simulations are also shown in Figs. 3–5. In order to understand the different surface structures that we simulated we show a side view of Cr3O4(1 1 1) in Fig. 6. The structure is made up of a closepacked fcc oxygen sublattice with tetrahedral interstitials occupied by Cr3+ ions and octahedral interstitials occupied by equal numbers of Cr2+ and Cr3+ ions. From Fig. 6 we see that there are
six possible bulk terminations of Cr3O4(1 1 1): CrtetOCroctO (COCO), CroctOCrtetCroct (COCC), CroctCrtetOCroct (CCOC), CrtetCroctCrtetO (CCCO), OCrtetCroctCrtet (OCCC), OCroctOCrtet (OCOC). We have examined all these surface terminations using the multiple scattering XPD calculations. In addition we have also modeled different reconstructed Cr3O4(1 1 1) surfaces suggested in LEED [21] and STM [22] studies of Fe3O4(1 1 1) films
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O 1s
Cr 2p
Exp.
Exp. COCO
COCO
OCOC
COCC OCOC
OCCC
Intensity (Arb. Units)
Intensity (Arb. Units)
COCC OCCC
CCCO
CCCO CCOC
CCOC
bwCOCO bwCOCO oxCOCC oxCOCC
COCC + cbCOCC
COCC + cbCOCC
0
10
20
30
40
50
60
Theta (Deg.)
0
10
20
30
40
50
60
Theta (Deg.)
Fig. 4. Polar angle XPD curves for the low coverage chromium oxide films. Experimental (circles) and multiple scattering simulations (diamonds) for Cr 2p (left) and O 1s (right) are for a scattering plane corresponding to the Agð1 2 1Þ plane. The different surface terminations and reconstructions examined are indicated to the left of each curve.
since Cr3O4 and Fe3O4 have the same structure. The LEED study by Barbieri et al. [21] arrived at an Fe3O4(1 1 1) surface structure based on a relaxation of termination (COCO). We will refer to this structure as bwCOCO. In that study, the distance between the uppermost Fe layer and the oxygen layer below was found to be reduced from 0.63 ˚ . That oxygen layer exhibits some buckto 0.40 A ling with the oxygen ions not bonded to Fe in
˚ below O ions that are bonded the top layer 0.42 A to Fe in the top layer. The distance between the top oxygen and second iron layers is decreased ˚ , and the distance between from 1.19 to 0.83 A the second iron layer and the second oxygen layer ˚ . We adjusted the is increased from 1.19 to 1.42 A Cr oxide parameters according to the same ratios. The STM study [22] actually identified the presence of two surface terminations. The first is
W.A.A. Priyantha, G.D. Waddill / Surface Science 578 (2005) 149–161
Cr 2p
155
O 1s
Exp.
Exp. COCO COCO
OCOC
COCC
COCC
OCCC Intensity (Arb. Units)
Intensity (Arb. Units)
OCOC
OCCC CCCO
CCCO
CCOC
CCOC
bwCOCO
bwCOCO oxCOCC
COCC + cbCOCC
oxCOCC COCC + cbCOCC 0
10
20
30
40
50
60
Theta (Deg.)
0
10
20
30
40
50
60
Theta (Deg.)
Fig. 5. Polar angle XPD curves for the low coverage chromium oxide films. Experimental (circles) and multiple scattering simulations (diamonds) for Cr 2p (left) and O 1s (right) are for a scattering plane corresponding to the Agð1 1 0Þ plane. The different surface terminations and reconstructions examined are indicated to the left of each curve.
CCOC, and the second is based on COCC, but with 1/4 ML oxygen ions occupying three-fold sites above the top iron layer. We will refer to this reconstructed surface as OxCOCC. In addition, all of these structures can show rotational twinning due to the random occurrence of ABCABC and ACBACB stacking sequences expected for these fcc-like structures. We will refer to the second stacking sequence with the addition of cb prior to the termination identification, for example cbCOCC. Finally, since we were unable to distin-
guish Cr2+ and Cr3+ in our XPS data we have included contributions from both cations in our multiple scattering calculations. The results of our comparison between experiment and simulation are summarized in Table 1. There we present summed R-factor analysis for the multiple scattering simulations of the different surface terminations compared to our experimental XPD results. P 2
ðv vexp Þ The R-factor used was R ¼ Pðvcalc . The sum 2 þv2 Þ calc
exp
includes O 1s and Cr 2p1/2 results for all three
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Fig. 6. Side view of the Cr3O4(1 1 1) structure.
Table 1 R-factors for the different structures proposed for the low coverage chromium oxide phase Surface termination
R-factor
COCO OCOC COCC OCCC CCCO CCOC bwCOCO oxCOCC COCC + cbCOCC
0.674 1.261 0.715 0.644 0.827 0.822 1.042 0.843 0.960
scattering planes. The lowest R-factors are for the OCCC and COCO bulk terminations. Both of these surface structures have a non-zero dipole moment perpendicular to the surface and ionic models predict that they will be unstable. However, a covalent bonding picture generally associates surface stability with surface dangling bond minimization. This is achieved for Cr3O4(1 1 1) for the COCO and COCC bulk terminations. Another useful model for predicting surface terminations, particularly for covalent materials, is
the surface autocompensation model [23]. In this model, the most stable surfaces are those for which the anion-derived dangling bonds are completely filled and the cation-derived dangling bonds are completely empty. For Cr3O4(1 1 1) the CCOC termination is autocompensated provided the topmost Cr layer is composed of a 3:1 ratio of Cr2+:Cr3+ rather than the bulk 1:1 ratio, and if the second Cr layer is Cr2+ rather than Cr3+. In addition, the COCO termination is autocompensated when the top Cr layer is changed from all Cr3+ to a 3:1 mixture of Cr2+:Cr3+, and if one oxygen vacancy per unit cell is introduced in the topmost oxygen layer. R-factor analyses for these structures are indistinguishable from the unmodified CCOC and COCO terminations, and therefore do not present any improvement in the agreement between experimental XPD results and our MSCD simulations. Based on these arguments and the results of our XPD simulations it may be that the low coverage oxide films we observe are due to an autocompensated COCO termination of Cr3O4(1 1 1), but the overall agreement between experiment and simulation for all the surface structures considered is rather poor. Thus it seems likely that, as was concluded for low coverage Cr oxide films on Pt(1 1 1), the low coverage chromium oxide films here are not Cr3O4(1 1 1), but perhaps either c-Cr2O3 or reconstructed a-Cr2O3(0 0 0 1) [16]. We now turn to a discussion p pof the XPD results for the high coverage ( 3 · 3)R30 chromium
Fig. 7. Side view of first five atomic planes of a-Cr2O3(0 0 0 1).
W.A.A. Priyantha, G.D. Waddill / Surface Science 578 (2005) 149–161
oxide films. As previously stated this pattern is expected for a-Cr2O3(0 0 0 1). a-Cr2O3 has the corundum structure with a rhombohedral primitive cell containing 10 atoms [24,25]. The structure consists of close-packed oxygen layers with hcp layer stacking (abab) each separated by two closely spaced Cr planes that follow fcc stacking (ABCABC). The Cr ions occupy two-thirds of the octahedral sites between the oxygen planes. The structure of aCr2O3(0 0 0 1) is shown in a side view in Fig. 7.
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From this figure we see that there are three possible bulk terminations for a-Cr2O3(0 0 0 1). These are single layer chromium termination referred to as AaBCb (here the upper case letters refer to Cr and the lower case letters to O with the sequence indicating the stacking of the layers), double layer chromium termination referred to as BCbAB, and oxygen termination referred to as aBCbA. Figs. 8–10 show our experimental XPD results for scattering planes coinciding with the Agð01 1Þ;
Exp. Cr 2p
O 1s
Exp.
AaB relaxed
AaB relaxed
Intensity (Arb. Units)
Intensity (Arb. Units)
AaB
aBC
AaB
BCb aBC
BCb
0
10
20
30
40
Theta (Deg.)
50
60
0
10
20
30
40
50
60
Theta (Deg.)
Fig. 8. Polar angle XPD curves for the high coverage chromium oxide films. Experimental (circles) and multiple scattering simulations (diamonds) for Cr 2p (left) and O 1s (right) are for a scattering plane corresponding to the Agð0 1 1Þ plane. The different surface terminations and reconstructions examined are indicated to the left of each curve.
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Exp.
Cr 2p
O 1s
Exp. AaB relaxed
AaB relaxed
Intensity (Arb. Units)
Intensity (Arb. Units)
AaB
aBC
AaB
aBC
BCb
BCb
0
10
20
30
40
50
60
Theta (Deg.)
0
10
20
30
40
50
60
Theta (Deg.)
Fig. 9. Polar angle XPD for the high coverage chromium oxide films. Experimental (circles) and multiple scattering simulations (diamonds) for Cr 2p (left) and O 1s (right) are for a scattering plane corresponding to the Agð1 2 1Þ plane. The different surface terminations and reconstructions examined are indicated to the left of each curve.
Agð 12 1Þ; and Agð 110Þ respectively. In each figure the Cr 2p1/2 data are shown in the left panel and the O 1s data in the right panel. These figures also show the results of multiple scattering simulations for the three bulk surface terminations of a-Cr2O3(0 0 0 1). Again we have used R-factor analysis to facilitate comparison between our experimental data and the results of our simulations for the possible surface terminations. The results are given in Table 2. Here we see a slight preference for the surface terminated by a single chromium
layer (AaBCb). This finding is consistent with studies of the surface of a-Al2O3(0 0 0 1) [25–27] and MBE grown a-Fe2O3(0 0 0 1) [28] with the same corundum structure as a-Cr2O3(0 0 0 1). This surface termination for a-Cr2O3(0 0 0 1) is the only termination that is autocompensated, and is therefore expected to be the most stable. In addition, studies find that the a-Al2O3(0 0 0 1) surface undergoes a significant relaxation involving a large first interlayer contraction between 50% and 90% that results in a reduction of the surface energy
W.A.A. Priyantha, G.D. Waddill / Surface Science 578 (2005) 149–161
159
Exp. Cr 2p
O 1s
Exp. AaB relaxed
AaB relaxed
Intensity (Arb. Units)
Intensity (Arb. Units)
AaB
aBC
AaB
aBC
BCb
BCb
0
10
20
30
40
50
60
Theta (Deg.)
0
20
40
60
Theta (Deg.)
Fig. 10. Polar angle XPD for the high coverage chromium oxide films. Experimental (circles) and multiple scattering simulations (diamonds) for Cr 2p (left) and O 1s (right) are for a scattering plane corresponding to the Agð1 1 0Þ plane. The different surface terminations and reconstructions examined are indicated to the left of each curve. Table 2 R-factors for the different surface terminations for aCr2O3(0 0 0 1) and for the single chromium terminated surface with an inward relaxation of 50% Surface termination
R-factor
AaB aBC BCb AaB-relaxed
0.653 0.768 0.681 0.379
by approximately a factor of 2. We have explored this possibility for the high coverage chromium
oxide films grown on Ag(1 1 1). The results of an R-factor analysis for the single layer Cr terminated surface as a function of the first interlayer spacing is shown in Fig. 11. We see a pronounced minimum ˚ in the R-factor at an interlayer separation of 0.47 A which is a relaxation of 50%. This is consistent with the findings for a-Al2O3(0 0 0 1), and we conclude that the most likely structure for our high coverage chromium oxide films is a Cr terminated a-Cr2O3(0 0 0 1) surface with the first interlayer spacing contracted by 50%.
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Cr layer with the first interlayer spacing reduced from its bulk value by 50%. Similar attempts to better identify the surface structure for the low coverage chromium oxide films using XPD resulted in less satisfactory results. They suggest that the surface may be Cr3O4(1 1 1) terminated by CrtetOCroctO or a modified CroctOCrtetCroct, however the poor fit between experiment and simulation suggest that the low coverage chromium oxide films may not be Cr3O4(1 1 1). Conclusive identification of the low coverage structure clearly requires additional studies probably using other surface structural determination techniques.
Acknowledgment This work was supported by the University of Missouri Research Board.
References
Fig. 11. R-factor variation as a function of distance between the top chromium layer and the underlying oxygen layer.
4. Summary We have shown that thin well-ordered chromium oxide films can be grown on Ag(1 1 1) using a sequential deposition technique. For films less ˚ thick a p(2 · 2) LEED pattern consistent than 5 A with the structure of Cr3O4(1 1 1) is observed. For ˚ a oxide films thicker than approximately 12 A p p ( 3 · 3)R30 LEED pattern is observed. This is consistent with the structure of a-Cr2O3 ˚ in thickness (0 0 0 1). For films between 5 and 12 A a LEED pattern p that p is the superposition of the p(2 · 2) and ( 3 · 3)R30 patterns is seen. Our XPD results compared to multiple scattering simulations for a number of possible surface structures and terminations for the high coverage chromium oxide films confirm that the most likely structure is a-Cr2O3(0 0 0 1) terminated by a single
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Structure of chromium oxide ultrathin films on Ag(1 1 1) W.A.A. Priyantha, G.D. Waddill
*
Department of Physics, University of Missouri-Rolla, 1870 Miner Circle, Rolla, MO 65409, USA Received 1 May 2004; accepted for publication 19 January 2005 Available online 25 January 2005
Abstract Epitaxial chromium oxide films are grown on Ag(1 1 1) by serial deposition. This method is repeated sequences of the deposition of sub-monolayer Cr films (typically 60.5 ML) followed by oxidation. The procedure is repeated until the desired oxide film thickness is achieved. The oxide films are characterized using low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and X-ray photoelectron diffraction (XPD). Chromium oxide films with ˚ show a p(2 · 2) LEED pattern consistent with Cr3O4(1 1 1) films. XPD results, thickness less than approximately 5 A however, suggest that the low coverage chromium oxide films may not be Cr3O4, and definitive identification of the low coverage oxide structure requires further work. Chromium oxide films with thickness greater than approximately ˚ show a (p 3 · p 3)R30 LEED pattern consistent with a-Cr2O3(0 0 0 1). XPD results confirm this and further iden12 A tify the surface termination as a single Cr layer with an inward relaxation of 50%. 2005 Elsevier B.V. All rights reserved. Keywords: Electron–solid diffraction; Low energy electron diffraction (LEED); Photoelectron diffraction measurement; Growth; Oxidation; Interfaces; Single crystal epitaxy
1. Introduction The importance of metal oxides in many technological fields has fueled an increase in interest in surface science investigations of these materials. The ability to produce ordered metal oxide surfaces with well defined structure and chemical *
Corresponding author. Tel.: +1 573 341 4797; fax: +1 573 341 4715. E-mail address: [email protected] (G.D. Waddill).
composition is critical to the success of these investigations. Recent investigations have established that it is possible to grow some oriented single crystal transition metal oxide thin films with well defined stoichiometry [1–6]. These films have been grown in a variety of ways including oxidation of the surface region of single crystal metals, molecular beam epitaxy in an oxygen or other reactive gas atmosphere, oxygen plasma assisted molecular beam epitaxy, and metal deposition on suitable substrates followed by oxidation of the resulting
0039-6028/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2005.01.028
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metal film. For the last method substrates for metal-oxide growth have generally been Al2O3 or MgO making performance of electron based surface spectroscopies on the resulting films an imposing experimental challenge. More recently it has been demonstrated that it is possible to stabilize thin transition metal oxide single crystal films by oxidizing metal films grown on single crystal metal substrates [3–5]. This eliminates charging difficulties encountered in electron or ion spectroscopies on insulating surfaces, and also opens the possibility of isolating new oxide structures not encountered in bulk form. The surfaces of chromium and chromium oxides are interesting for a variety of reasons. Cr is one of several transition metals predicted to show strong magnetic moment enhancement in ultrathin films, and single monolayer films may exhibit ferromagnetic behavior [7,8]. One oxide of chromium, CrO2, is believed to be a half metallic ferromagnet with a metallic band structure for one spin and an insulating band structure for the other [9,10]. CrO2 is used in magnetic recording media because of its corrosion resistance and high coercivity. Above about 280 C, CrO2 decomposes to the most stable oxide of chromium, Cr2O3. Cr2O3 has the corundum structure, is an important polymerization catalyst, and is used in passivating stainless steel. Above 1600 C the stable oxide of chromium is a tetragonally distorted Cr3O4 spinel phase. The formation of c-Cr2O3, a cubic spinel phase, has also been reported for certain thin film growth conditions. Ordered single-crystal samples of any of these chromium oxides are unavailable making their thin film preparation more important. Most previous studies of chromium oxidation have involved the oxidation of the Cr(1 1 0) surface [2,11–14]. There are also isolated examples of oxidation of the (1 0 0)and (1 1 1) surfaces of chromium [15]. More recently, Diebold et al. [3,16] have succeeded in growing Cr2O3 and perhaps cubic spinel Cr3O4 films on the Pt(1 1 1) surface. In this study we have characterized the growth and structure of Cr oxide films on Ag(1 1 1) using X-ray photoelectron spectroscopy (XPS), X-ray photoelectron diffraction (XPD), and low-energy electron diffraction (LEED). We have grown the oxide films in two different ways. The first involves
deposition of a multilayer Cr film followed by oxidation of that film at approximately 240 C. For ˚ all Cr coverages investigated p (2–20 p A prior to oxidation) this results in a ( 3 · 3)R30 LEED pattern expected for Cr2O3(0 0 0 1) films. The Cr film prior to oxidation is most likely Cr(1 1 0). This is based on observations of Fe(1 1 0) growth on Ag(1 1 1) [17], and because our LEED patterns for the Cr films are a diffuse (1 · 1) structure consistent with Cr(1 1 0) growth. Previous studies of the oxidation of Cr(1 1 0) generally find ordered Cr2O3(0 0 0 1) growth [2,11–14] so our results are consistent with those findings. The second method grows the chromium oxide film by repeated cycles of sub-monolayer Cr film deposition (typically 0.5 ML or lower) followed by exposure to an oxy˚ ) films, a gen atmosphere at 240 C. For thin (65 A p(2 · 2) LEED pattern is observed. This LEED pattern is consistent with the formation of Cr3O4(1 1 1) films. p Oxide films thicker than about ˚ exhibit a ( 3 · p 3)R30 LEED pattern, 12 A ˚ have and films with thickness between 5 and 12 A LEED patternsp thatpare a superposition of the p(2 · 2) and ( 3 · 3)R30 patterns consistent with the presence of both Cr3O4 and Cr2O3 in this coverage regime. This paper will discuss exclusively Cr oxide films produced by the sequential deposition method.
2. Experimental procedure Experiments were performed in an ultra-high vacuum chamber at base pressures of 61 · 1010 Torr [18]. The Ag single crystal was cleaned by cycles of Ar ion sputtering followed by annealing to 450 C. XPS showed no contamination within detectable limits. LEED showed a sharp (1 · 1) pattern. Chromium thin films were deposited at room temperature by electron beam evaporation at pressures below 1 · 109 Torr. A quartz crystal thickness monitor and XPS Ag 3d5/2 and Cr 2p1/2 intensity variations were used to monitor the chromium coverage. Chromium evaporation ˚ /min. Oxide thickness rates were typically 0.5–1.0 A was determined using XPS core level intensity variations. When using XPS to determine film thickness we compared both the Ag 3d5/2 attenuation
W.A.A. Priyantha, G.D. Waddill / Surface Science 578 (2005) 149–161
and the Cr 2p1/2 to Ag 3d5/2 intensity ratio for a given film. In converting these to Cr or Cr-oxide thickness we used electron mean free paths calculated according to the method given in Ref. [19]. Chromium oxide films were grown by sequential deposition of submonolayer Cr films on the Ag substrate with each submonolayer metal deposition followed by an oxidation step. The oxidation step involved backfilling the chamber with O2 to a pressure of 105 Torr for 5 min while the sample was held at an elevated temperature-generally 240 C. This sequence was repeated until the desired oxide film thickness was achieved. During the deposition and oxidation the substrate was maintained at 240 C. Following the final cycle, the sample was annealed at 420 C for 30 min. Photoelectron diffraction data was taken using MgKa radiation (hm = 1254 eV) and a hemispherical energy analyzer of mean radius 125 mm and angular acceptance of ±1. The sample was mounted in a holder capable of 360 polar rotation, h, and ±100 azimuthal rotation, /. Both polar and azimuthal rotations had an angular resolution of better than ±0.5. Angular scans were obtained for Cr 2p1/2 (binding energy = 584 eV), Ag 3d5/2 (binding energy = 368 eV), and O 1s (binding energy = 531 eV) core levels. The Cr 2p3/2 peak which normally would be used for the XPD studies could not be used here as it overlaps with the Ag 3p3/2 peak. The integrated area of these peaks, after background subtraction, was used to generate the photoelectron diffraction curves. XPD and LEED studies were performed on chromium oxide films with thickness ranging from <1 ML up to >20 ML.
3. Results and discussion 3.1. LEED results Fig. 1 shows the LEED patterns observed for clean Ag(1 1 1) and for three different chromium oxide films. The clean Ag(1 1 1) shows a sharp (1 · 1) pattern (Fig. 1(a)) with low background intensity. For chromium oxide films with thickness ˚ a p(2 · 2) pattern is observed. An less than 5 A ˚ chroexample of this is shown Fig. 1(b) for a 4.5 A
151
mium oxide film. For chromium oxide films thicker ˚ a (p 3 · p 3)R30 pattern than approximately 12 A ˚ chrois observed. An example for a 12 A mium oxide film is shown in Fig. 1(d). For films ˚ a complicated with thickness between 5 and 12 A LEED pattern is observed. This is shown in Fig. ˚ oxide film. Closer inspection of this 1(c) for a 10 A LEED pattern reveals that it is a superposition of the low and high coverage LEED patterns observed for chromium oxide films on Ag(1 1 1). These LEED patterns were recorded at primary electron energy 70 eV after annealing the samples at 420 C for 30 min. Similar results have been reported for Cr oxide films on Pt(1 1 1) [3] where the p(2 · 2) pattern p waspattributed to Cr3O4(1 1 1) films and the ( 3 · 3)R30 pattern to aCr2O3(0 0 0 1) films. The hexagonal unit cell of the ˚. Ag(1 1 1) surface has a lattice constant of 2.89 A The Cr3O4(1 1 1) surface has a hexagonal unit cell ˚ and an oxygen with a lattice constant of 5.72 A ˚ . a-Cr2O3(0 0 0 1) interatomic distance of 2.85 A has a hexagonal unit cell with a lattice constant ˚ and an interatomic distance of 2.86 A ˚ of 4.95 A within the close-packed oxygen planes. The close lattice match between the Ag substrate and the chromium oxide films leads to the epitaxial relationships reflected in the observed LEED patterns. In order to further test the structural nature of the chromium oxide films we now turn to X-ray photoelectron diffraction (XPD) studies of the high and low coverage oxide structures. 3.2. XPD results ˚ Fig. 2 shows azimuthal XPD data for a 4.5 A chromium oxide film for both Cr 2p1/2 (left) and O 1s (right). The data are for a polar scattering angle of 55 measured from the sample normal and corresponding to a body diagonal for the cubic oxide structure. We observe 60 periodicity confirming the 6-fold symmetry observed in the LEED pattern. Figs. 3–5 show polar XPD data for scattering planes coinciding with the Agð0 1 1Þ; Agð1 2 1Þ; and Agð1 1 0Þ respectively. In each figure the Cr 2p1/2 data are shown in the left panel and the O 1s data in the right panel. The experimental data is the top curve in each figure, and these in general are complex and not readily
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˚ ) chromium oxide, (c) mixed phase (10 A ˚ ) chromium oxide, Fig. 1. LEED patterns for (a) clean Ag(1 1 1), and for (b) low coverage (5 A ˚ ) chromium oxide using an electron energy of 70 eV. and (d) high coverage (12 A
O 1s
Intensity (Arb. Units)
Intensity (Arb. Units)
Cr 2p1/2
0
40
80
120
Phi (Deg.)
0
40
80
120
Phi (Deg.)
Fig. 2. Azimuthal angle XPD curves for Cr 2p (left) and O 1s (right) for a polar scattering angle of 55.
interpreted using simple forward scattering arguments due in large part to the complicated nature of the oxide structure. To explore the possible
oxide film structures further we compared our experimental results with results of multiple scattering simulations [20] for a number of possible
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Cr 2p
153
O 1s Exp.
Exp. COCO OCOC COCO COCC OCOC
Intensity (Arb. Units)
Intensity (Arb. Units)
OCCC
COCC
OCCC
CCCO
CCCO CCOC
CCOC bwCOCO bwCOCO oxCOCC oxCOCC
COCC + cbCOCC
COCC + cbCOCC 0
10
20
30
40
50
60
Theta (Deg.)
0
10
20
30
40
50
60
Theta (Deg.)
Fig. 3. Polar angle XPD curves for the low coverage chromium oxide films. Experimental (circles) and multiple scattering simulations (diamonds) for Cr 2p (left) and O 1s (right) are for a scattering plane corresponding to the Agð0 1 1Þ plane. The different surface terminations and reconstructions examined are indicated to the left of each curve.
Cr3O4(1 1 1) surface structures. The results of those simulations are also shown in Figs. 3–5. In order to understand the different surface structures that we simulated we show a side view of Cr3O4(1 1 1) in Fig. 6. The structure is made up of a closepacked fcc oxygen sublattice with tetrahedral interstitials occupied by Cr3+ ions and octahedral interstitials occupied by equal numbers of Cr2+ and Cr3+ ions. From Fig. 6 we see that there are
six possible bulk terminations of Cr3O4(1 1 1): CrtetOCroctO (COCO), CroctOCrtetCroct (COCC), CroctCrtetOCroct (CCOC), CrtetCroctCrtetO (CCCO), OCrtetCroctCrtet (OCCC), OCroctOCrtet (OCOC). We have examined all these surface terminations using the multiple scattering XPD calculations. In addition we have also modeled different reconstructed Cr3O4(1 1 1) surfaces suggested in LEED [21] and STM [22] studies of Fe3O4(1 1 1) films
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O 1s
Cr 2p
Exp.
Exp. COCO
COCO
OCOC
COCC OCOC
OCCC
Intensity (Arb. Units)
Intensity (Arb. Units)
COCC OCCC
CCCO
CCCO CCOC
CCOC
bwCOCO bwCOCO oxCOCC oxCOCC
COCC + cbCOCC
COCC + cbCOCC
0
10
20
30
40
50
60
Theta (Deg.)
0
10
20
30
40
50
60
Theta (Deg.)
Fig. 4. Polar angle XPD curves for the low coverage chromium oxide films. Experimental (circles) and multiple scattering simulations (diamonds) for Cr 2p (left) and O 1s (right) are for a scattering plane corresponding to the Agð1 2 1Þ plane. The different surface terminations and reconstructions examined are indicated to the left of each curve.
since Cr3O4 and Fe3O4 have the same structure. The LEED study by Barbieri et al. [21] arrived at an Fe3O4(1 1 1) surface structure based on a relaxation of termination (COCO). We will refer to this structure as bwCOCO. In that study, the distance between the uppermost Fe layer and the oxygen layer below was found to be reduced from 0.63 ˚ . That oxygen layer exhibits some buckto 0.40 A ling with the oxygen ions not bonded to Fe in
˚ below O ions that are bonded the top layer 0.42 A to Fe in the top layer. The distance between the top oxygen and second iron layers is decreased ˚ , and the distance between from 1.19 to 0.83 A the second iron layer and the second oxygen layer ˚ . We adjusted the is increased from 1.19 to 1.42 A Cr oxide parameters according to the same ratios. The STM study [22] actually identified the presence of two surface terminations. The first is
W.A.A. Priyantha, G.D. Waddill / Surface Science 578 (2005) 149–161
Cr 2p
155
O 1s
Exp.
Exp. COCO COCO
OCOC
COCC
COCC
OCCC Intensity (Arb. Units)
Intensity (Arb. Units)
OCOC
OCCC CCCO
CCCO
CCOC
CCOC
bwCOCO
bwCOCO oxCOCC
COCC + cbCOCC
oxCOCC COCC + cbCOCC 0
10
20
30
40
50
60
Theta (Deg.)
0
10
20
30
40
50
60
Theta (Deg.)
Fig. 5. Polar angle XPD curves for the low coverage chromium oxide films. Experimental (circles) and multiple scattering simulations (diamonds) for Cr 2p (left) and O 1s (right) are for a scattering plane corresponding to the Agð1 1 0Þ plane. The different surface terminations and reconstructions examined are indicated to the left of each curve.
CCOC, and the second is based on COCC, but with 1/4 ML oxygen ions occupying three-fold sites above the top iron layer. We will refer to this reconstructed surface as OxCOCC. In addition, all of these structures can show rotational twinning due to the random occurrence of ABCABC and ACBACB stacking sequences expected for these fcc-like structures. We will refer to the second stacking sequence with the addition of cb prior to the termination identification, for example cbCOCC. Finally, since we were unable to distin-
guish Cr2+ and Cr3+ in our XPS data we have included contributions from both cations in our multiple scattering calculations. The results of our comparison between experiment and simulation are summarized in Table 1. There we present summed R-factor analysis for the multiple scattering simulations of the different surface terminations compared to our experimental XPD results. P 2
ðv vexp Þ The R-factor used was R ¼ Pðvcalc . The sum 2 þv2 Þ calc
exp
includes O 1s and Cr 2p1/2 results for all three
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Fig. 6. Side view of the Cr3O4(1 1 1) structure.
Table 1 R-factors for the different structures proposed for the low coverage chromium oxide phase Surface termination
R-factor
COCO OCOC COCC OCCC CCCO CCOC bwCOCO oxCOCC COCC + cbCOCC
0.674 1.261 0.715 0.644 0.827 0.822 1.042 0.843 0.960
scattering planes. The lowest R-factors are for the OCCC and COCO bulk terminations. Both of these surface structures have a non-zero dipole moment perpendicular to the surface and ionic models predict that they will be unstable. However, a covalent bonding picture generally associates surface stability with surface dangling bond minimization. This is achieved for Cr3O4(1 1 1) for the COCO and COCC bulk terminations. Another useful model for predicting surface terminations, particularly for covalent materials, is
the surface autocompensation model [23]. In this model, the most stable surfaces are those for which the anion-derived dangling bonds are completely filled and the cation-derived dangling bonds are completely empty. For Cr3O4(1 1 1) the CCOC termination is autocompensated provided the topmost Cr layer is composed of a 3:1 ratio of Cr2+:Cr3+ rather than the bulk 1:1 ratio, and if the second Cr layer is Cr2+ rather than Cr3+. In addition, the COCO termination is autocompensated when the top Cr layer is changed from all Cr3+ to a 3:1 mixture of Cr2+:Cr3+, and if one oxygen vacancy per unit cell is introduced in the topmost oxygen layer. R-factor analyses for these structures are indistinguishable from the unmodified CCOC and COCO terminations, and therefore do not present any improvement in the agreement between experimental XPD results and our MSCD simulations. Based on these arguments and the results of our XPD simulations it may be that the low coverage oxide films we observe are due to an autocompensated COCO termination of Cr3O4(1 1 1), but the overall agreement between experiment and simulation for all the surface structures considered is rather poor. Thus it seems likely that, as was concluded for low coverage Cr oxide films on Pt(1 1 1), the low coverage chromium oxide films here are not Cr3O4(1 1 1), but perhaps either c-Cr2O3 or reconstructed a-Cr2O3(0 0 0 1) [16]. We now turn to a discussion p pof the XPD results for the high coverage ( 3 · 3)R30 chromium
Fig. 7. Side view of first five atomic planes of a-Cr2O3(0 0 0 1).
W.A.A. Priyantha, G.D. Waddill / Surface Science 578 (2005) 149–161
oxide films. As previously stated this pattern is expected for a-Cr2O3(0 0 0 1). a-Cr2O3 has the corundum structure with a rhombohedral primitive cell containing 10 atoms [24,25]. The structure consists of close-packed oxygen layers with hcp layer stacking (abab) each separated by two closely spaced Cr planes that follow fcc stacking (ABCABC). The Cr ions occupy two-thirds of the octahedral sites between the oxygen planes. The structure of aCr2O3(0 0 0 1) is shown in a side view in Fig. 7.
157
From this figure we see that there are three possible bulk terminations for a-Cr2O3(0 0 0 1). These are single layer chromium termination referred to as AaBCb (here the upper case letters refer to Cr and the lower case letters to O with the sequence indicating the stacking of the layers), double layer chromium termination referred to as BCbAB, and oxygen termination referred to as aBCbA. Figs. 8–10 show our experimental XPD results for scattering planes coinciding with the Agð01 1Þ;
Exp. Cr 2p
O 1s
Exp.
AaB relaxed
AaB relaxed
Intensity (Arb. Units)
Intensity (Arb. Units)
AaB
aBC
AaB
BCb aBC
BCb
0
10
20
30
40
Theta (Deg.)
50
60
0
10
20
30
40
50
60
Theta (Deg.)
Fig. 8. Polar angle XPD curves for the high coverage chromium oxide films. Experimental (circles) and multiple scattering simulations (diamonds) for Cr 2p (left) and O 1s (right) are for a scattering plane corresponding to the Agð0 1 1Þ plane. The different surface terminations and reconstructions examined are indicated to the left of each curve.
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Exp.
Cr 2p
O 1s
Exp. AaB relaxed
AaB relaxed
Intensity (Arb. Units)
Intensity (Arb. Units)
AaB
aBC
AaB
aBC
BCb
BCb
0
10
20
30
40
50
60
Theta (Deg.)
0
10
20
30
40
50
60
Theta (Deg.)
Fig. 9. Polar angle XPD for the high coverage chromium oxide films. Experimental (circles) and multiple scattering simulations (diamonds) for Cr 2p (left) and O 1s (right) are for a scattering plane corresponding to the Agð1 2 1Þ plane. The different surface terminations and reconstructions examined are indicated to the left of each curve.
Agð 12 1Þ; and Agð 110Þ respectively. In each figure the Cr 2p1/2 data are shown in the left panel and the O 1s data in the right panel. These figures also show the results of multiple scattering simulations for the three bulk surface terminations of a-Cr2O3(0 0 0 1). Again we have used R-factor analysis to facilitate comparison between our experimental data and the results of our simulations for the possible surface terminations. The results are given in Table 2. Here we see a slight preference for the surface terminated by a single chromium
layer (AaBCb). This finding is consistent with studies of the surface of a-Al2O3(0 0 0 1) [25–27] and MBE grown a-Fe2O3(0 0 0 1) [28] with the same corundum structure as a-Cr2O3(0 0 0 1). This surface termination for a-Cr2O3(0 0 0 1) is the only termination that is autocompensated, and is therefore expected to be the most stable. In addition, studies find that the a-Al2O3(0 0 0 1) surface undergoes a significant relaxation involving a large first interlayer contraction between 50% and 90% that results in a reduction of the surface energy
W.A.A. Priyantha, G.D. Waddill / Surface Science 578 (2005) 149–161
159
Exp. Cr 2p
O 1s
Exp. AaB relaxed
AaB relaxed
Intensity (Arb. Units)
Intensity (Arb. Units)
AaB
aBC
AaB
aBC
BCb
BCb
0
10
20
30
40
50
60
Theta (Deg.)
0
20
40
60
Theta (Deg.)
Fig. 10. Polar angle XPD for the high coverage chromium oxide films. Experimental (circles) and multiple scattering simulations (diamonds) for Cr 2p (left) and O 1s (right) are for a scattering plane corresponding to the Agð1 1 0Þ plane. The different surface terminations and reconstructions examined are indicated to the left of each curve. Table 2 R-factors for the different surface terminations for aCr2O3(0 0 0 1) and for the single chromium terminated surface with an inward relaxation of 50% Surface termination
R-factor
AaB aBC BCb AaB-relaxed
0.653 0.768 0.681 0.379
by approximately a factor of 2. We have explored this possibility for the high coverage chromium
oxide films grown on Ag(1 1 1). The results of an R-factor analysis for the single layer Cr terminated surface as a function of the first interlayer spacing is shown in Fig. 11. We see a pronounced minimum ˚ in the R-factor at an interlayer separation of 0.47 A which is a relaxation of 50%. This is consistent with the findings for a-Al2O3(0 0 0 1), and we conclude that the most likely structure for our high coverage chromium oxide films is a Cr terminated a-Cr2O3(0 0 0 1) surface with the first interlayer spacing contracted by 50%.
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Cr layer with the first interlayer spacing reduced from its bulk value by 50%. Similar attempts to better identify the surface structure for the low coverage chromium oxide films using XPD resulted in less satisfactory results. They suggest that the surface may be Cr3O4(1 1 1) terminated by CrtetOCroctO or a modified CroctOCrtetCroct, however the poor fit between experiment and simulation suggest that the low coverage chromium oxide films may not be Cr3O4(1 1 1). Conclusive identification of the low coverage structure clearly requires additional studies probably using other surface structural determination techniques.
Acknowledgment This work was supported by the University of Missouri Research Board.
References
Fig. 11. R-factor variation as a function of distance between the top chromium layer and the underlying oxygen layer.
4. Summary We have shown that thin well-ordered chromium oxide films can be grown on Ag(1 1 1) using a sequential deposition technique. For films less ˚ thick a p(2 · 2) LEED pattern consistent than 5 A with the structure of Cr3O4(1 1 1) is observed. For ˚ a oxide films thicker than approximately 12 A p p ( 3 · 3)R30 LEED pattern is observed. This is consistent with the structure of a-Cr2O3 ˚ in thickness (0 0 0 1). For films between 5 and 12 A a LEED pattern p that p is the superposition of the p(2 · 2) and ( 3 · 3)R30 patterns is seen. Our XPD results compared to multiple scattering simulations for a number of possible surface structures and terminations for the high coverage chromium oxide films confirm that the most likely structure is a-Cr2O3(0 0 0 1) terminated by a single
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