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Elemental steps in the growth of thin β-Ga2O3 films on CoGa(100 )
Surface Science 470 (2001) 337±346
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Elemental steps in the growth of thin b-Ga2O3 ®lms on CoGa(1 0 0) R. Franchy *, M. Eumann 1, G. Schmitz Institut fur Grenz¯ achenforschung und Vakuumphysik Forschungszentrum J ulich, FZ-J ulich, D-52425 J ulich, Germany Received 19 July 2000; accepted for publication 26 September 2000
Abstract The oxidation of CoGa(1 0 0) at 700 K was studied by means of high resolution electron energy loss spectroscopy (EELS), scanning tunneling microscopy, low energy electron diraction and Auger electron spectroscopy (AES). At 700 K, thin well-ordered b-Ga2 O3 ®lms grow on CoGa(1 0 0). The EEL spectrum of the Ga-oxide ®lms exhibit Fuchs± Kliewer phonons at 305, 455, 645, and 785 cmÿ1 . For low oxygen exposure (<0.2 L), the growth of oxide-islands starts at step edges and on defects. The oxide ®lms have the shape of long, rectangular islands and are oriented in the [1 0 0] and [0 1 0] directions of the substrate. For higher oxygen exposure, islands of b-Ga2 O3 are found also on the terraces. After an exposure of 200 L O2 at 700 K, the CoGa(1 0 0) surface is homogeneously covered with a thin ®lm of bGa2 O3 . Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Electron energy loss spectroscopy (EELS); Scanning tunneling microscopy; Auger electron spectroscopy; Low energy electron diraction (LEED); Alloys; Oxidation; Cobalt; Gallium
1. Introduction In general, oxidation of metals and metal alloy surfaces is of considerable intrinsic interest [1,2] and also important for numerous applications ranging from microelectronics, gas sensors and heterogeneous catalysis to the nuclear power industry [3]. Recently, this subject has attracted much interest, since ultra-thin oxide layers may be used in the future in permanent RAM devices as
* Corresponding author. Tel.: +49-2461-613469; fax: +492461-613907. E-mail address: [email protected] (R. Franchy). 1 Present address: Max Planck Institut f ur Eisenforschung, D usseldorf, Germany.
insulating barrier between two ferromagnetic materials, based on the tunneling magneto-resistance (TMR) eect [4]. Thin, well-ordered ®lms of oxides, nitrides and oxynitrides can be prepared on the basis of intermetallic alloys, e.g., NiAl and CoGa [5,6]. In the oxidation process, Al (Ga) atoms segregate to the surface and react with the adsorbed oxygen. At room temperature, the oxide layers are amorphous and order at elevated temperature. In previous studies on CoGa(1 0 0) we reported on the formation of amorphous Ga-oxide [7] and well-ordered, homogeneous b-Ga2 O3 ®lms [8]. At 700 K, oxidation with 1 L O2 leads to the growth of long, rectangular islands of b-Ga2 O3 [9]. A similar situation also occurs on NiAl(1 0 0): Niehus and coworkers [10,11] studied the adsorption of oxygen and the initial stages of oxidation
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on NiAl(1 0 0) by means of 180°-neutral impact collision ion scattering spectroscopy (NICISS), spot pro®le analysis of low energy electron diffraction (SPA-LEED) and scanning tunneling microscopy (STM). After adsorption of oxygen and annealing at 1000 K the STM images show the formation of Al-oxide islands along the [1 0 0] and [0 1 0] directions of the NiAl(1 0 0) surface with two domains oriented perpendicular to each other. Ga-oxide has attracted much interest recently, because it can be used as an oxygen sensor at high temperatures [12]. Ga-oxides are formed by the oxidation of GaAs surfaces [13,14]. In contrast to the many dierent alumina phases, Ga-oxide is known to have only two: the metastable a-and the stable b-phase which is isomorphic to h-Al2 O3 . In this paper we report on the elemental steps of the growth of thin, crystalline Ga2 O3 ®lms on CoGa(1 0 0). The oxidation was performed with oxygen at 700 K in a wide exposure range up to saturation. The growth of oxide layers was studied by means of STM, high resolution electron energy loss spectroscopy (EELS), Auger electron spectroscopy (AES), and low energy electron diraction (LEED). With STM the structure and topography of the surface can be determined on an atomic scale, while the chemistry of the surface can easily be understood by analyzing the vibration properties via EELS. CoGa orders in a CsCl-type The structure with a lattice constant of 2.878 A. (1 0 0) layers have ABAB... stacking sequence. The clean CoGa(0 0 1) surface shows a c 2 4 reconstruction in two domains perpendicularly oriented with respect to each other [8,15]. The paper is organized as follows: Section 2 deals with the experimental methods and the cleaning procedure. The experimental results are presented in Section 3 and are discussed in Section 4. A summary is given in Section 5. 2. Experimental The experiments were performed in an UHV chamber in which STM, EELS, LEED, and AES are in situ combined. The base pressure of the chamber was about 5 10ÿ11 mbar. A detailed description of the apparatus can be found in Ref.
[16]. The scanning tunneling microscope is a modi®ed beetle-type which was originally developed by Besocke [17]. For the STM measurements constant current topographies were recorded. The computer-controlled EEL spectrometer is based on optimized 127° cylindrical de¯ectors [18]. The CoGa(1 0 0) single crystal was cut by spark erosion and polished mechanically. It was oriented with an accuracy of about 0.5°. The main impurities of the CoGa(1 0 0) sample are carbon, oxygen and sulfur. Heating the sample for 2 min in an oxygen atmosphere ( pO2 1 10ÿ6 mbar) at 800 K leads to the oxidation of the surface and of the carbon impurities. The surface oxides formed in this way desorb by annealing at 1070 K. Several repeated cycles of this cleaning procedure create an oxygen, carbon, and sulfur depleted zone in the surface region. For the clean CoGa(1 0 0) surface, the AES peak-to-peak (p-to-p) ratio of the Ga transition at 1070 eV and the Co transition at 775 eV amounts to 0.588. 3. Results Fig. 1 shows a set of EEL spectra taken as a function of oxygen exposure. The oxidation was performed with an O2 -partial pressure of 10ÿ9 mbar at 700 K surface temperature, while the EELS spectra were measured at room temperature. The EEL spectrum of the clean CoGa(1 0 0) surface (not shown in Fig. 1) does not exhibit any losses showing that no contamination is present in the vicinity of the surface. Already at an exposure of 0.075 L of O2 losses around 305, 455 and 750 cmÿ1 are found. With increasing oxygen exposure the intensity of these losses strongly increases and a new loss at 645 cmÿ1 occurs. For an exposure of 10 L O2 the loss originally found at 750 cmÿ1 is shifted to 785 cmÿ1 . The four losses at 305, 455, 645, and 785 cmÿ1 for the completely oxidized surface of CoGa(1 0 0) [8] were already reported before. They are typical for Fuchs±Kliewer (FK) modes of b-Ga2 O3 . Therefore, at 700 K the Gaoxide starts to grow at very low oxygen exposure. The losses at 910 455 455 and 1240 cmÿ1 455 785, observed in spectrum (e), are combination losses. From the EELS experiment shown
R. Franchy et al. / Surface Science 470 (2001) 337±346
θ∇ θ
Fig. 1. EEL spectra of the oxidized CoGa(1 0 0) surface. The oxidation was performed with oxygen partial pressure between 2 10ÿ9 and 5 10ÿ8 mbar at 700 K and the EEL spectra were measured at room temperature.
in Fig. 1 we conclude that the Ga-oxide starts already to grow at very low oxygen exposure. Fig. 2a±d show STM images with a scan width 2 as a function of oxygen expoof 2560 2560 A sure at 700 K. Fig. 2a obtained for an exposure of 0.05 L O2 shows three terraces (T1±T3) which are separated by step edges. The areas marked with Ox represent oxide islands which start to grow from the step edges. At this oxygen-exposure, no oxide islands are found on the terraces. With increasing oxygen exposure: Fig. 2b (0.2 L), Fig. 2c (0.6 L) and 2d (1.0 L) some islands grow also on the terraces. The oxide-islands, which are located only on the terraces, are denoted with T. In contrast to Ox, A are islands which are located in the terrace but are also connected to Ox. Some characteristics of these islands are obvious: · The islands are preferentially oriented in the [1 0 0] and [0 1 0] directions of the substrate.
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· The islands have a long rectangular shape. · The islands start mainly at the step edges of the terraces. · No crossing of the rectangular islands can be observed. Fig. 3a±d, show STM images for oxygen exposure: (a) 2.0 L, (b) 3.0 L, (c) 5.0 L and (d) 10 L. 2 . With inThe scanned area is 2560 2560 A creasing oxygen exposure the density of oxideislands on the terraces increases, e.g., for an exposure of 3.0 L, 40% of the oxide-islands are on the terraces. With increasing exposure of oxygen it becomes more and more dicult to discriminate between the dierent terraces of the original CoGa(1 0 0) surface. As a guide for the eyes some traces of step edges are marked with the character S, while the oxide-free areas of the CoGa(1 0 0) are marked with E. In these images some domains of the clean surface, denoted with W, can be observed which remain unoxidized between two parallel oxide islands. Fig. 4 shows a STM image with a scan width of 2 taken after oxidation of the surface 640 640 A with 10 L of O2 at 700 K. The islands grow in two directions: the [1 0 0] and [0 1 0] directions of the substrate. If the domains of the islands are parallel to each other they can grow together without a mismatch. One example for this is shown in the ®gure where the two islands are in part separated by the domains W of clean CoGa(1 0 0). Two possibilities occur for the coalescence of domains of oxide islands which grow perpendicular to each other. One possibility is to build boundaries of 45°, denoted in the ®gure by Ab, and another possibility is to coalesce with a boundary of 90° which are marked in the ®gure by a circle. In the later case these boundaries are oriented in the [1 0 0] and [0 1 0] directions of the substrate, respectively. Most of the Ga oxide islands appear as trenches in the STM images. As a consequence the areas of the clean unoxidized CoGa(1 0 0) surface appear as protrusions (see Figs. 3 and 4). For exposure above 200 L of O2 at 700 K, the CoGa(1 0 0) surface is homogeneously covered with a thin ®lm of b-Ga2 O3 [8]. The oxide ®lm consists in very ¯at and large domains which grow epitaxially on CoGa(1 0 0). The domains are oriented along
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2 of the CoGa(1 0 0) surface as a function of oxygen exposure: (a) 0.05 L, (b) Fig. 2. STM images with a scan width of 2560 2560 A 0.2 L, (c) 0.6 L, and (d) 1.0 L, at 700 K. (It 0:7±0.9 nA, Ut 0:8±1.4 eV.)
the [1 0 0] and [0 1 0] directions of the substrate. In the case of the homogeneously covered CoGa(1 0 0) surface with b-Ga2 O3 the AES p-to-p ratio of the Ga transition at 1070 eV and the Co transition at 775 eV amounts to 0.62. 4. Discussion The four losses observed in the EEL spectra at 305 (m1 ), 455 (m2 ), 645 (m3 ), and 785 cmÿ1 (m4 ) are FK modes [19,20] of the b-Ga2 O3 ®lm grown on CoGa(1 0 0). In a previous paper we have shown that, the growth of the Ga-oxide on CoGa(1 0 0)
can be concluded unequivocally by the excellent agreement between calculated EEL spectra (on base of dielectric theory) for a thin ®lm of Ga2 O3 on a metallic substrate and the experimental spectra [8]. All the spectra in Fig. 1 show already the characteristic FK-modes of b-Ga2 O3 . Therefore, the rectangular islands shown in Fig. 2a±d and Fig. 3a±d represent islands of b-Ga2 O3 . The frequency shift of the m4 mode from originally at 750 ! 785 cmÿ1 could be a result of a dipole± dipole interaction of dierent islands on the surface or due to the growing of the oxide area with increasing oxygen exposure. From the view of thermodynamics the formation of Ga-oxide is also
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2 of the CoGa(1 0 0) surface as a function of oxygen exposure: (a) 2.0 L, (b) 3.0 Fig. 3. STM images with a scan width of 2560 2560 A L, (c) 5.0 L, and (d) 10.0 L, at 700 K. (It 0:7±0.9 nA, Ut 0:8±1.4 eV.)
favored over the formation of Co-oxide, because the heat of formation of Ga2 O3 (DHf 1815 kJ molÿ1 ) is much larger than the value of Co3 O4 (DHf 905 kJ molÿ1 ) [21]. This growth model is consistent with our AES measurements: The p-top ratio between the Ga transition at 1070 eV and the Co transition at 775 eV increases during oxidation 0:588 ! 0:62. This shows that the Ga concentration in the vicinity of the surface increases due to the oxidation. The b-Ga2 O3 islands grow in two domains oriented in the [1 0 0] and [0 1 0] directions of the substrate. The rectangular islands are characterized by equidistant streaks (see Fig. 4). The dis-
tance between the streaks is determined to be The streaks are always oriented b 5:8 0:1 A. parallel to the preferential orientations of the rectangular islands. A structure model for one domain of b-Ga2 O3 on CoGa(1 0 0) is shown in Fig. 5. The b-Ga2 O3 has a monoclinic structure consisting of a close-packed oxygen f.c.c-sublattice in which Ga cations occupy tetrahedral and octahedral vacancies. Because of the symmetry of the system, a second domain is expected which is rotated by 90°. The monoclinic structure of b-Ga2 O3 is determined by the occupation probability of the tetrahedral and octahedral vacancies. For the sake of clarity, only the octahedral site is depicted in
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2 of Fig. 4. STM image with a scan width of 640 640 A CoGa(1 0 0) oxidized at 700 K with 10 L of O2 . The coalescence of oxide islands leads to 45° and 90° boundaries, respectively.
Fig. 5. Model of monoclinic b-Ga2 O3 on CoGa(1 0 0).
Fig. 5. As shown in the model, the (1 0 0) plane of the oxygen f.c.c-sublattice is perpendicular to the
CoGa(1 0 0) surface and parallel to the [1 0 0] direction (or the [0 1 0] direction for the second domain). This is known as the classical Bain orientation relationship between fcc and bcc structures. The distance between the streaks in Fig. 4 is in very good agreement with the lattice parameter b of b-Ga2 O3 . In each oxygen layer of b-Ga2 O3 the Ga-ions form chains that have a Thus, the streaks in the distance of b 5:80 A. STM image are connected with the gallium chains (or with the chains where Ga is missing). In the present study the tip had a positive potential with respect to the sample. Thus, tunneling occurs from occupied states of the oxide into unoccupied states of the tip. Via oxidation, charge transfer from Ga atoms to O atoms occurs leading to a depletion of charge in the Ga-rows. Therefore, in our STM measurements it is more likely that the streaks in Fig. 4 are due to the rows where Ga is missing. The square lattice of the oxygen ions in b-Ga2 O3 has a In a lattice constant of a b-Ga2 O3 3:04 A. previous STM study [9] we determined this lattice which agrees very well parameter to be 2:9 0:1 A with the predicted value. The lattice constants of the b-Ga2 O3 oxide grown on CoGa(1 0 0) agree very well with a 2 1 structure with respect to the substrate. The mismatch only amount to 4.6 ab-Ga O 2:9 % in one direction (aCoGa 2:878 A, 2 3 A) and even only 0.68% (2aCoGa 5:76 A, bb-Ga2 O3 5:8 A) in the other direction. The lattice constants determined by STM appear to ®t better to the CoGa(1 0 0) substrate than to the lattice constants of the ideal b-Ga2 O3 . At least in one direction the oxide lattice seems to be compressed measured experimentally by STM and 3.04 (2.9 A A for the ideal oxide) due to the in¯uence of the CoGa substrate. For an exposure of 0.05 L of O2 all of the islands start at the step edges between two terraces. After an exposure of 0.2 L about 95% of the islands start from the step edges and only few are located on a terrace. The mean lengths of the is and about 2.8% of the surlands is about 250 A, face is occupied by the oxides. The area which is occupied by the oxide islands increases with increasing exposure: 3.6% for 0.6 L and 8.5% for 1 L. In Table 1, the dependence of the oxide-covered area versus the oxygen exposure is represented.
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Table 1 Some parameters of the growth of b-Ga2 O3 on CoGa(1 0 0) at 700 K O2 -exposure (L)
Mean width of the islands (A)
Mean length of the islands (A)
Oxide-covered area (%)
Proportion of step edges in the [1 0 0] and [0 1 0] directions (%)
Proportion of islands on step edges (%)
0.05 0.2 0.6 1.0 2.0 3.0 5.0 10.0
11.6 11.6 17.4 17.4 29.0 29.0 40.6 69.6
250 250 500 500 ± ± ± ±
1.0 2.8 3.6 8.5 13.5 23.3 36.4 61.4
31 68 94 >99 >99 >99 ± ±
>99 95 95 85 70 60 ± ±
The data shown in Table 1 represent averages which were determined from many STM images taken for one certain oxygen-exposure. In Table 1 we have also represented the area on the CoGa(1 0 0) surface which is occupied by the oxide islands as a function of oxygen exposure. These areas were also determined by AES. A comparison between the areas covered by the oxide determined by STM and AES is shown in Fig. 6. The AES data in Fig. 6 are obtained by normalizing the pto-p ratio IO 503 eV =ICo 775 eV for a given exposure to the p-to-p ratio obtained for saturationcoverage IO 503 eV =ICo 775 eV (sat). The two depen-
dences agree very well and show a linear function which suggests that the growth of b-Ga2 O3 on CoGa(1 0 0) is two-dimensional. This assumption is also supported by the fact that in the STM study no steps of oxides are found on the oxide islands. The insert in Fig. 6 show the p-to-p ratio (IO 503 eV =ICo 775 eV ) as a function of oxygen exposure at a surface temperature of 700 K. During the adsorption steps the oxygen partial pressure was adjusted between 2 10ÿ9 and 5 10ÿ8 mbar. After a high initial oxidation rate the growth drops to a very low rates and ends in an eventual saturation. The saturation level is reached at
Fig. 6. The area covered by b-Ga2 O3 islands determined by STM and AES as a function of oxygen exposure. The AES peak-to-peak ratio IO 503 eV=ICo 775 eV is normalized to the ratio for saturation coverage.
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Fig. 7. The distribution of the average length of the oxide islands for (a) 0.05 L, (b) 0.2 L, (c) 0.6 L, (d) 1 L, (e) 2 L, (f) 3 L, (g) 5 L and (h) 10 L oxygen.
about 200 L with IO 503 1.35.
eV =ICo 775 eV (sat)
equal to
The average width of the oxide islands is found This correto be, in general, a multiple of 5.8 A.
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Fig. 8. Schematic of the coalescence of oxide islands showing the formation of 45° and 90° boundaries, respectively.
sponds to the lattice constant b of the 2 1 oxide structure. It is equivalent to the distance between the Ga-rows or the rows where Ga atoms are missing. From this ®nding we conclude, that the oxide grows preferentially in units of an unit cell or multiples of it. Fig. 7 shows the distribution of the width of the oxide islands for dierent oxygen exposures. With increasing exposure the center of at 0.05 L the distribution shifts from 2 b (11.6 A) to 3 b (17.4 A) at 0.6 L, and to a broad distri at 5.0 L bution centered around 7 b (40.6 A) oxygen, respectively. A similar distribution was found by Brune et al. [22] for oxygen islands on Al(1 1 1). The mean length of the islands is more dicult to determine, because for higher oxygen exposure, the oxide islands exceed the area which was scanned by STM. Therefore, the mean lengths shown in Table 1 represent lower limits; for exposures >2 L no mean length could be determined. Fig. 8 shows a schematic of the coalescence of the oxide islands. The boundaries between the two domains are represented by solid lines. Fig. 4 shows that the two oxide domains can coalesce with a boundary of 45° and 90°, respectively. In both cases of coalescence the sublattice of oxygen (shown as a square) grows periodically, without defects, while for the chains of Ga-atoms a
dierent situation can occur. The 90° boundary between two rectangular islands suggests that one of them started to grow before the second one started. The 45° boundary seems to occur when two perpendicular islands start to grow simultaneously. The reason for the shape of the b-Ga2 O3 island is yet not clari®ed. Maybe the oxide grows with an elongated shape due to the rows of the Ga-atoms which are closed-packed parallel to the [1 0 0] aligned islands. This would stabilize the oxide islands in the [1 0 0] direction and lead to the observed island shapes [9]. 5. Summary The elemental steps of the oxidation of Gaoxide on CoGa(1 0 0) was studied at 700 K with EELS, AES, LEED, and STM. For low exposure, the oxidation starts of the step edges and on defects. For higher exposure the oxidation on the terraces is also found. The oxide islands have a long rectangular shape oriented in the [1 0 0] and [0 1 0] directions of the substrate. The EEL spectra of the oxide islands exhibit losses which are characteristic for b-Ga2 O3 .
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References [1] A. Atkinson, Rev. Mod. Phys. 57 (1985) 437. [2] F.P. Fehlner, Low-Temperature Oxidation. Wiley, New York, 1986. [3] IAEA-TECDOC-684, IAEA, Vienna, 1993. [4] S. Moodera, L.R. Kinder, T.M. Wong, R. Meservey, Phys. Rev. Lett. 75 (1998) 3273. [5] R. Franchy, G. Schmitz, P. Gassmann, F. Bartolucci, Appl. Phys. A 65 (1997) 551. [6] R. Franchy, Surf. Sci. Rep. 38 (6±8) (2000) 195. [7] G. Schmitz, P. Gassmann, R. Franchy, Surf. Sci. 397 (1998) 339. [8] G. Schmitz, P. Gassmann, R. Franchy, J. Appl. Phys. 83 (1998) 2533. [9] M. Eumann, G. Schmitz, R. Franchy, Appl. Phys. Lett. 72 (1998) 3440. [10] R.-P. Blum, D. Ahlberendt, H. Niehus, Surf. Sci. 396 (1998) 176.
[11] R.-P. Blum, H. Niehus, Appl. Phys. A66 (1998) 529. [12] M. Fleischer, H. Meixner, Sensors Actuat. B 5 (1992) 115. [13] E.D. Palik, N. Ginsburg, R.T. Holm, J.W. Gibson, J. Vac. Sci. Technol. 15 (1978) 1488. [14] C. Huang, A. Ludviksson, R.M. Martin, Surf. Sci. 265 (1992) 314. [15] F.M. Pan, C. P¯itsch, R. David, L.K. Verheij, R. Franchy, Phys. Rev. B, in press. [16] P. Gassmann, Aachen: RWTH Aachen, 1996. [17] K. Besocke, Surf. Sci. 181 (1987) 145. [18] H. Ibach, Electron Energy Loss Spectrometers ± The Technology of High Performance, Berlin, Springer, 1991. [19] K.L. Kliewer, R. Fuchs, Phys. Rev. 144 (1966) 495. [20] K.L. Kliewer, R. Fuchs, Adv. Chem. Phys. 27 (1974) 355. [21] R.C. Weast, D.R. Lide (Eds.), Handbook of Chemistry and Physics, vols. D-64 and D-67, CRC press, Boca Raton, 1989±1990 (70th edition: edited by R.C. Weast, D.R. Lide). [22] H. Brune, J. Wintterlin, J. Trost, G. Ertl, J. Wiechers, R.J. Behm, J. Chem. Phys. 99 (1993) 2128.
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Elemental steps in the growth of thin b-Ga2O3 ®lms on CoGa(1 0 0) R. Franchy *, M. Eumann 1, G. Schmitz Institut fur Grenz¯ achenforschung und Vakuumphysik Forschungszentrum J ulich, FZ-J ulich, D-52425 J ulich, Germany Received 19 July 2000; accepted for publication 26 September 2000
Abstract The oxidation of CoGa(1 0 0) at 700 K was studied by means of high resolution electron energy loss spectroscopy (EELS), scanning tunneling microscopy, low energy electron diraction and Auger electron spectroscopy (AES). At 700 K, thin well-ordered b-Ga2 O3 ®lms grow on CoGa(1 0 0). The EEL spectrum of the Ga-oxide ®lms exhibit Fuchs± Kliewer phonons at 305, 455, 645, and 785 cmÿ1 . For low oxygen exposure (<0.2 L), the growth of oxide-islands starts at step edges and on defects. The oxide ®lms have the shape of long, rectangular islands and are oriented in the [1 0 0] and [0 1 0] directions of the substrate. For higher oxygen exposure, islands of b-Ga2 O3 are found also on the terraces. After an exposure of 200 L O2 at 700 K, the CoGa(1 0 0) surface is homogeneously covered with a thin ®lm of bGa2 O3 . Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Electron energy loss spectroscopy (EELS); Scanning tunneling microscopy; Auger electron spectroscopy; Low energy electron diraction (LEED); Alloys; Oxidation; Cobalt; Gallium
1. Introduction In general, oxidation of metals and metal alloy surfaces is of considerable intrinsic interest [1,2] and also important for numerous applications ranging from microelectronics, gas sensors and heterogeneous catalysis to the nuclear power industry [3]. Recently, this subject has attracted much interest, since ultra-thin oxide layers may be used in the future in permanent RAM devices as
* Corresponding author. Tel.: +49-2461-613469; fax: +492461-613907. E-mail address: [email protected] (R. Franchy). 1 Present address: Max Planck Institut f ur Eisenforschung, D usseldorf, Germany.
insulating barrier between two ferromagnetic materials, based on the tunneling magneto-resistance (TMR) eect [4]. Thin, well-ordered ®lms of oxides, nitrides and oxynitrides can be prepared on the basis of intermetallic alloys, e.g., NiAl and CoGa [5,6]. In the oxidation process, Al (Ga) atoms segregate to the surface and react with the adsorbed oxygen. At room temperature, the oxide layers are amorphous and order at elevated temperature. In previous studies on CoGa(1 0 0) we reported on the formation of amorphous Ga-oxide [7] and well-ordered, homogeneous b-Ga2 O3 ®lms [8]. At 700 K, oxidation with 1 L O2 leads to the growth of long, rectangular islands of b-Ga2 O3 [9]. A similar situation also occurs on NiAl(1 0 0): Niehus and coworkers [10,11] studied the adsorption of oxygen and the initial stages of oxidation
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on NiAl(1 0 0) by means of 180°-neutral impact collision ion scattering spectroscopy (NICISS), spot pro®le analysis of low energy electron diffraction (SPA-LEED) and scanning tunneling microscopy (STM). After adsorption of oxygen and annealing at 1000 K the STM images show the formation of Al-oxide islands along the [1 0 0] and [0 1 0] directions of the NiAl(1 0 0) surface with two domains oriented perpendicular to each other. Ga-oxide has attracted much interest recently, because it can be used as an oxygen sensor at high temperatures [12]. Ga-oxides are formed by the oxidation of GaAs surfaces [13,14]. In contrast to the many dierent alumina phases, Ga-oxide is known to have only two: the metastable a-and the stable b-phase which is isomorphic to h-Al2 O3 . In this paper we report on the elemental steps of the growth of thin, crystalline Ga2 O3 ®lms on CoGa(1 0 0). The oxidation was performed with oxygen at 700 K in a wide exposure range up to saturation. The growth of oxide layers was studied by means of STM, high resolution electron energy loss spectroscopy (EELS), Auger electron spectroscopy (AES), and low energy electron diraction (LEED). With STM the structure and topography of the surface can be determined on an atomic scale, while the chemistry of the surface can easily be understood by analyzing the vibration properties via EELS. CoGa orders in a CsCl-type The structure with a lattice constant of 2.878 A. (1 0 0) layers have ABAB... stacking sequence. The clean CoGa(0 0 1) surface shows a c 2 4 reconstruction in two domains perpendicularly oriented with respect to each other [8,15]. The paper is organized as follows: Section 2 deals with the experimental methods and the cleaning procedure. The experimental results are presented in Section 3 and are discussed in Section 4. A summary is given in Section 5. 2. Experimental The experiments were performed in an UHV chamber in which STM, EELS, LEED, and AES are in situ combined. The base pressure of the chamber was about 5 10ÿ11 mbar. A detailed description of the apparatus can be found in Ref.
[16]. The scanning tunneling microscope is a modi®ed beetle-type which was originally developed by Besocke [17]. For the STM measurements constant current topographies were recorded. The computer-controlled EEL spectrometer is based on optimized 127° cylindrical de¯ectors [18]. The CoGa(1 0 0) single crystal was cut by spark erosion and polished mechanically. It was oriented with an accuracy of about 0.5°. The main impurities of the CoGa(1 0 0) sample are carbon, oxygen and sulfur. Heating the sample for 2 min in an oxygen atmosphere ( pO2 1 10ÿ6 mbar) at 800 K leads to the oxidation of the surface and of the carbon impurities. The surface oxides formed in this way desorb by annealing at 1070 K. Several repeated cycles of this cleaning procedure create an oxygen, carbon, and sulfur depleted zone in the surface region. For the clean CoGa(1 0 0) surface, the AES peak-to-peak (p-to-p) ratio of the Ga transition at 1070 eV and the Co transition at 775 eV amounts to 0.588. 3. Results Fig. 1 shows a set of EEL spectra taken as a function of oxygen exposure. The oxidation was performed with an O2 -partial pressure of 10ÿ9 mbar at 700 K surface temperature, while the EELS spectra were measured at room temperature. The EEL spectrum of the clean CoGa(1 0 0) surface (not shown in Fig. 1) does not exhibit any losses showing that no contamination is present in the vicinity of the surface. Already at an exposure of 0.075 L of O2 losses around 305, 455 and 750 cmÿ1 are found. With increasing oxygen exposure the intensity of these losses strongly increases and a new loss at 645 cmÿ1 occurs. For an exposure of 10 L O2 the loss originally found at 750 cmÿ1 is shifted to 785 cmÿ1 . The four losses at 305, 455, 645, and 785 cmÿ1 for the completely oxidized surface of CoGa(1 0 0) [8] were already reported before. They are typical for Fuchs±Kliewer (FK) modes of b-Ga2 O3 . Therefore, at 700 K the Gaoxide starts to grow at very low oxygen exposure. The losses at 910 455 455 and 1240 cmÿ1 455 785, observed in spectrum (e), are combination losses. From the EELS experiment shown
R. Franchy et al. / Surface Science 470 (2001) 337±346
θ∇ θ
Fig. 1. EEL spectra of the oxidized CoGa(1 0 0) surface. The oxidation was performed with oxygen partial pressure between 2 10ÿ9 and 5 10ÿ8 mbar at 700 K and the EEL spectra were measured at room temperature.
in Fig. 1 we conclude that the Ga-oxide starts already to grow at very low oxygen exposure. Fig. 2a±d show STM images with a scan width 2 as a function of oxygen expoof 2560 2560 A sure at 700 K. Fig. 2a obtained for an exposure of 0.05 L O2 shows three terraces (T1±T3) which are separated by step edges. The areas marked with Ox represent oxide islands which start to grow from the step edges. At this oxygen-exposure, no oxide islands are found on the terraces. With increasing oxygen exposure: Fig. 2b (0.2 L), Fig. 2c (0.6 L) and 2d (1.0 L) some islands grow also on the terraces. The oxide-islands, which are located only on the terraces, are denoted with T. In contrast to Ox, A are islands which are located in the terrace but are also connected to Ox. Some characteristics of these islands are obvious: · The islands are preferentially oriented in the [1 0 0] and [0 1 0] directions of the substrate.
339
· The islands have a long rectangular shape. · The islands start mainly at the step edges of the terraces. · No crossing of the rectangular islands can be observed. Fig. 3a±d, show STM images for oxygen exposure: (a) 2.0 L, (b) 3.0 L, (c) 5.0 L and (d) 10 L. 2 . With inThe scanned area is 2560 2560 A creasing oxygen exposure the density of oxideislands on the terraces increases, e.g., for an exposure of 3.0 L, 40% of the oxide-islands are on the terraces. With increasing exposure of oxygen it becomes more and more dicult to discriminate between the dierent terraces of the original CoGa(1 0 0) surface. As a guide for the eyes some traces of step edges are marked with the character S, while the oxide-free areas of the CoGa(1 0 0) are marked with E. In these images some domains of the clean surface, denoted with W, can be observed which remain unoxidized between two parallel oxide islands. Fig. 4 shows a STM image with a scan width of 2 taken after oxidation of the surface 640 640 A with 10 L of O2 at 700 K. The islands grow in two directions: the [1 0 0] and [0 1 0] directions of the substrate. If the domains of the islands are parallel to each other they can grow together without a mismatch. One example for this is shown in the ®gure where the two islands are in part separated by the domains W of clean CoGa(1 0 0). Two possibilities occur for the coalescence of domains of oxide islands which grow perpendicular to each other. One possibility is to build boundaries of 45°, denoted in the ®gure by Ab, and another possibility is to coalesce with a boundary of 90° which are marked in the ®gure by a circle. In the later case these boundaries are oriented in the [1 0 0] and [0 1 0] directions of the substrate, respectively. Most of the Ga oxide islands appear as trenches in the STM images. As a consequence the areas of the clean unoxidized CoGa(1 0 0) surface appear as protrusions (see Figs. 3 and 4). For exposure above 200 L of O2 at 700 K, the CoGa(1 0 0) surface is homogeneously covered with a thin ®lm of b-Ga2 O3 [8]. The oxide ®lm consists in very ¯at and large domains which grow epitaxially on CoGa(1 0 0). The domains are oriented along
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2 of the CoGa(1 0 0) surface as a function of oxygen exposure: (a) 0.05 L, (b) Fig. 2. STM images with a scan width of 2560 2560 A 0.2 L, (c) 0.6 L, and (d) 1.0 L, at 700 K. (It 0:7±0.9 nA, Ut 0:8±1.4 eV.)
the [1 0 0] and [0 1 0] directions of the substrate. In the case of the homogeneously covered CoGa(1 0 0) surface with b-Ga2 O3 the AES p-to-p ratio of the Ga transition at 1070 eV and the Co transition at 775 eV amounts to 0.62. 4. Discussion The four losses observed in the EEL spectra at 305 (m1 ), 455 (m2 ), 645 (m3 ), and 785 cmÿ1 (m4 ) are FK modes [19,20] of the b-Ga2 O3 ®lm grown on CoGa(1 0 0). In a previous paper we have shown that, the growth of the Ga-oxide on CoGa(1 0 0)
can be concluded unequivocally by the excellent agreement between calculated EEL spectra (on base of dielectric theory) for a thin ®lm of Ga2 O3 on a metallic substrate and the experimental spectra [8]. All the spectra in Fig. 1 show already the characteristic FK-modes of b-Ga2 O3 . Therefore, the rectangular islands shown in Fig. 2a±d and Fig. 3a±d represent islands of b-Ga2 O3 . The frequency shift of the m4 mode from originally at 750 ! 785 cmÿ1 could be a result of a dipole± dipole interaction of dierent islands on the surface or due to the growing of the oxide area with increasing oxygen exposure. From the view of thermodynamics the formation of Ga-oxide is also
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2 of the CoGa(1 0 0) surface as a function of oxygen exposure: (a) 2.0 L, (b) 3.0 Fig. 3. STM images with a scan width of 2560 2560 A L, (c) 5.0 L, and (d) 10.0 L, at 700 K. (It 0:7±0.9 nA, Ut 0:8±1.4 eV.)
favored over the formation of Co-oxide, because the heat of formation of Ga2 O3 (DHf 1815 kJ molÿ1 ) is much larger than the value of Co3 O4 (DHf 905 kJ molÿ1 ) [21]. This growth model is consistent with our AES measurements: The p-top ratio between the Ga transition at 1070 eV and the Co transition at 775 eV increases during oxidation 0:588 ! 0:62. This shows that the Ga concentration in the vicinity of the surface increases due to the oxidation. The b-Ga2 O3 islands grow in two domains oriented in the [1 0 0] and [0 1 0] directions of the substrate. The rectangular islands are characterized by equidistant streaks (see Fig. 4). The dis-
tance between the streaks is determined to be The streaks are always oriented b 5:8 0:1 A. parallel to the preferential orientations of the rectangular islands. A structure model for one domain of b-Ga2 O3 on CoGa(1 0 0) is shown in Fig. 5. The b-Ga2 O3 has a monoclinic structure consisting of a close-packed oxygen f.c.c-sublattice in which Ga cations occupy tetrahedral and octahedral vacancies. Because of the symmetry of the system, a second domain is expected which is rotated by 90°. The monoclinic structure of b-Ga2 O3 is determined by the occupation probability of the tetrahedral and octahedral vacancies. For the sake of clarity, only the octahedral site is depicted in
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2 of Fig. 4. STM image with a scan width of 640 640 A CoGa(1 0 0) oxidized at 700 K with 10 L of O2 . The coalescence of oxide islands leads to 45° and 90° boundaries, respectively.
Fig. 5. Model of monoclinic b-Ga2 O3 on CoGa(1 0 0).
Fig. 5. As shown in the model, the (1 0 0) plane of the oxygen f.c.c-sublattice is perpendicular to the
CoGa(1 0 0) surface and parallel to the [1 0 0] direction (or the [0 1 0] direction for the second domain). This is known as the classical Bain orientation relationship between fcc and bcc structures. The distance between the streaks in Fig. 4 is in very good agreement with the lattice parameter b of b-Ga2 O3 . In each oxygen layer of b-Ga2 O3 the Ga-ions form chains that have a Thus, the streaks in the distance of b 5:80 A. STM image are connected with the gallium chains (or with the chains where Ga is missing). In the present study the tip had a positive potential with respect to the sample. Thus, tunneling occurs from occupied states of the oxide into unoccupied states of the tip. Via oxidation, charge transfer from Ga atoms to O atoms occurs leading to a depletion of charge in the Ga-rows. Therefore, in our STM measurements it is more likely that the streaks in Fig. 4 are due to the rows where Ga is missing. The square lattice of the oxygen ions in b-Ga2 O3 has a In a lattice constant of a b-Ga2 O3 3:04 A. previous STM study [9] we determined this lattice which agrees very well parameter to be 2:9 0:1 A with the predicted value. The lattice constants of the b-Ga2 O3 oxide grown on CoGa(1 0 0) agree very well with a 2 1 structure with respect to the substrate. The mismatch only amount to 4.6 ab-Ga O 2:9 % in one direction (aCoGa 2:878 A, 2 3 A) and even only 0.68% (2aCoGa 5:76 A, bb-Ga2 O3 5:8 A) in the other direction. The lattice constants determined by STM appear to ®t better to the CoGa(1 0 0) substrate than to the lattice constants of the ideal b-Ga2 O3 . At least in one direction the oxide lattice seems to be compressed measured experimentally by STM and 3.04 (2.9 A A for the ideal oxide) due to the in¯uence of the CoGa substrate. For an exposure of 0.05 L of O2 all of the islands start at the step edges between two terraces. After an exposure of 0.2 L about 95% of the islands start from the step edges and only few are located on a terrace. The mean lengths of the is and about 2.8% of the surlands is about 250 A, face is occupied by the oxides. The area which is occupied by the oxide islands increases with increasing exposure: 3.6% for 0.6 L and 8.5% for 1 L. In Table 1, the dependence of the oxide-covered area versus the oxygen exposure is represented.
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Table 1 Some parameters of the growth of b-Ga2 O3 on CoGa(1 0 0) at 700 K O2 -exposure (L)
Mean width of the islands (A)
Mean length of the islands (A)
Oxide-covered area (%)
Proportion of step edges in the [1 0 0] and [0 1 0] directions (%)
Proportion of islands on step edges (%)
0.05 0.2 0.6 1.0 2.0 3.0 5.0 10.0
11.6 11.6 17.4 17.4 29.0 29.0 40.6 69.6
250 250 500 500 ± ± ± ±
1.0 2.8 3.6 8.5 13.5 23.3 36.4 61.4
31 68 94 >99 >99 >99 ± ±
>99 95 95 85 70 60 ± ±
The data shown in Table 1 represent averages which were determined from many STM images taken for one certain oxygen-exposure. In Table 1 we have also represented the area on the CoGa(1 0 0) surface which is occupied by the oxide islands as a function of oxygen exposure. These areas were also determined by AES. A comparison between the areas covered by the oxide determined by STM and AES is shown in Fig. 6. The AES data in Fig. 6 are obtained by normalizing the pto-p ratio IO 503 eV =ICo 775 eV for a given exposure to the p-to-p ratio obtained for saturationcoverage IO 503 eV =ICo 775 eV (sat). The two depen-
dences agree very well and show a linear function which suggests that the growth of b-Ga2 O3 on CoGa(1 0 0) is two-dimensional. This assumption is also supported by the fact that in the STM study no steps of oxides are found on the oxide islands. The insert in Fig. 6 show the p-to-p ratio (IO 503 eV =ICo 775 eV ) as a function of oxygen exposure at a surface temperature of 700 K. During the adsorption steps the oxygen partial pressure was adjusted between 2 10ÿ9 and 5 10ÿ8 mbar. After a high initial oxidation rate the growth drops to a very low rates and ends in an eventual saturation. The saturation level is reached at
Fig. 6. The area covered by b-Ga2 O3 islands determined by STM and AES as a function of oxygen exposure. The AES peak-to-peak ratio IO 503 eV=ICo 775 eV is normalized to the ratio for saturation coverage.
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Fig. 7. The distribution of the average length of the oxide islands for (a) 0.05 L, (b) 0.2 L, (c) 0.6 L, (d) 1 L, (e) 2 L, (f) 3 L, (g) 5 L and (h) 10 L oxygen.
about 200 L with IO 503 1.35.
eV =ICo 775 eV (sat)
equal to
The average width of the oxide islands is found This correto be, in general, a multiple of 5.8 A.
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Fig. 8. Schematic of the coalescence of oxide islands showing the formation of 45° and 90° boundaries, respectively.
sponds to the lattice constant b of the 2 1 oxide structure. It is equivalent to the distance between the Ga-rows or the rows where Ga atoms are missing. From this ®nding we conclude, that the oxide grows preferentially in units of an unit cell or multiples of it. Fig. 7 shows the distribution of the width of the oxide islands for dierent oxygen exposures. With increasing exposure the center of at 0.05 L the distribution shifts from 2 b (11.6 A) to 3 b (17.4 A) at 0.6 L, and to a broad distri at 5.0 L bution centered around 7 b (40.6 A) oxygen, respectively. A similar distribution was found by Brune et al. [22] for oxygen islands on Al(1 1 1). The mean length of the islands is more dicult to determine, because for higher oxygen exposure, the oxide islands exceed the area which was scanned by STM. Therefore, the mean lengths shown in Table 1 represent lower limits; for exposures >2 L no mean length could be determined. Fig. 8 shows a schematic of the coalescence of the oxide islands. The boundaries between the two domains are represented by solid lines. Fig. 4 shows that the two oxide domains can coalesce with a boundary of 45° and 90°, respectively. In both cases of coalescence the sublattice of oxygen (shown as a square) grows periodically, without defects, while for the chains of Ga-atoms a
dierent situation can occur. The 90° boundary between two rectangular islands suggests that one of them started to grow before the second one started. The 45° boundary seems to occur when two perpendicular islands start to grow simultaneously. The reason for the shape of the b-Ga2 O3 island is yet not clari®ed. Maybe the oxide grows with an elongated shape due to the rows of the Ga-atoms which are closed-packed parallel to the [1 0 0] aligned islands. This would stabilize the oxide islands in the [1 0 0] direction and lead to the observed island shapes [9]. 5. Summary The elemental steps of the oxidation of Gaoxide on CoGa(1 0 0) was studied at 700 K with EELS, AES, LEED, and STM. For low exposure, the oxidation starts of the step edges and on defects. For higher exposure the oxidation on the terraces is also found. The oxide islands have a long rectangular shape oriented in the [1 0 0] and [0 1 0] directions of the substrate. The EEL spectra of the oxide islands exhibit losses which are characteristic for b-Ga2 O3 .
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