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ZnO(0 0 0 1) interface: Formation and thermal stability
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Applied Surface Science 254 (2008) 2309–2318 www.elsevier.com/locate/apsusc
The Fe/ZnO(0 0 0 1) interface: Formation and thermal stability D. Wett a,*, A. Demund a, H. Schmidt b, R. Szargan a a
Wilhelm-Ostwald-Institut fu¨r Physikalische und Theoretische Chemie, Universita¨t Leipzig, Linne´strasse 2, 04103 Leipzig, Germany b Institut fu¨r Experimentelle Physik II, Halbleiterphysik Universita¨t Leipzig, Linne´strasse 5, 04103 Leipzig, Germany Received 18 May 2007; received in revised form 9 September 2007; accepted 9 September 2007 Available online 14 September 2007
Abstract The room temperature growth mode and the interface reaction of Fe films on single crystalline ZnO(0 0 0 1) substrates prepared in ultra high vacuum (UHV) has been investigated by means of X-ray photoelectron and Auger electron spectroscopy (XPS, AES), low energy electron diffraction (LEED) and low energy ion scattering spectroscopy (LEIS). The results show that Fe grows in the pseudo layer-by-layer mode. At ambient temperature the deposited Fe film reduces the underlying ZnO single crystal resulting in FeO at the interface and metallic Zn, which partially diffuses into the remaining Fe overlayer. Annealing leads to a stepwise oxidation of the Fe to FeO (670 K) and Fe2O3 (820 K). The Fe2O3 mixes with the substrate resulting in two (1 1 1) oriented textures of a spinel phase found by electron backscatter diffraction analysis (EBSD). Febased spin-injection may play a vital role for ZnO-based spintronic devices. # 2007 Elsevier B.V. All rights reserved. Keywords: Zinc oxide; ZnO; Iron; Fe; Iron oxide; FeO; Fe2O3; Interface; Epitaxy; Growth; X-ray photoelectron spectroscopy; XPS; Low energy ion scattering; LEIS; Low energy electron diffraction; LEED; Electron backscatter diffraction; EBSD
1. Introduction The interactions at metal/oxide interfaces are strongly linked to chemical, electronic as well as mechanical properties and thus play a fundamental role for the application of such materials in catalysis, sensor and semiconductor technology or microelectronics [1]. Therefore, in the last years again many metal/oxide interfaces have been studied extensively. For example, 10 ML Fe were found to grow uniformly on a 10 ML predeposited NiO(0 0 1). An evidence for the Ni2+ reduction was found [2]. Later the same authors reported about the formation of a Fe–Ni mixed body-centered cubic (bcc) phase due to the diffusion of the reduced Ni into the Fe overlayer [3]. On a TiO2(1 1 0) substrate Fe was found to grow in the Stransky–Krastanov mode. The formation of Ti3+ states at the early state of the deposition was reported in that case [4]. Al2O3(0 0 0 1) and Fe form a rather stable interface, the appearance of Fe2+ states was observed, but no reduced Al states were found. Annealing at 870 K leads to Ostwald ripening of the Fe film, Fe2+ states disappear and metallic Al is
* Corresponding author. Tel.: +49 341 9736500; fax: +49 341 9736399. E-mail address: [email protected] (D. Wett). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.09.020
found [5]. Similar thermally activated processes were also observed in the case of Cu deposited on ZnO(0 0 0 1) as well as ZnO(0 0 0 1) [6–8]. During the deposition of thin Cr films on ZnO(0 0 0 1) metallic Zn was formed and segregated to the surface. Annealing at 830 K resulted in the formation of the spinel ZnCr2O4. Only octahedral sites, occupied by Cr3+, were exposed at the surface [9]. ZnO is a low cost material with excellent optical, electronic and dielectric properties [10]. ZnO can be used as transparent, conductive material for solar cell caps. MgxZn1xO alloys from wurtzite-type ZnO (Egap = 3.37 eV) and rock-salt-type MgO (Egap = 7.5 eV), which can be prepared under conservation of the hexagonal as well as the cubic lattice depending on the Mg concentration [11] and demonstrate the tunability of the direct band-gap of in MgxZn1xO alloys over a wide wavelength range. Furthermore, MgxZn1xO alloys could be used as new materials for tunneling barriers in magnetic tunneling junction (MTJ) devices. The composition and the thermal stability of the metal/barrier interfaces strongly influence the functionality of such devices. In the case of an Fe/MgO/Fe MTJ structure during the MgO deposition a small part of oxygen is placed into the first atom layer of the Fe. Coherent growth of the top Fe electrode of the MTJ structure was observed only for Fe deposition in ambient oxygen atmosphere leading to a
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symmetric MTJ structure characterized by FeO layers at both interfaces. Calculations indicate large positive tunnel magnetoresistance (TMR) values in such symmetric junctions [12]. So far, no information exists on the growth mode, thermal stability, reactions and transport properties of the Fe/ZnO interface. ZnO crystallizes in the wurtzite structure (space group: ˚ and c = 5.213 A ˚ P63mc) with the lattice constants a = 3.252 A [13]. Each Zn atom is nearly tetrahedrally surrounded by four O atoms and vice versa (lattice parameter u = 0.380). Along its [0 0 0 1] direction (c axis), Zn and O layers are alternatively arranged. By cutting a ZnO crystal perpendicular to its c axis two different polar surfaces are formed indexed as ZnO(0 0 0 1) and ZnO(0 0 0 1) and alternatively called +c and c plane. The (0 0 0 1) plane is terminated by Zn atoms, the (0 0 0 1) by O atoms. One should consider that termination does not mean that there are exclusively Zn or O atoms on the surface. Depending on the type of the plane one can find triangular pits or hexagonal islands or terraces, whose corners and edges consist of both atom types [14]. These reconstructions result in a considerable reduction of the surface polarity and a gain of the surface stability in comparison to the energetic unfavorable flat (1 1) terminated crystal. Studies of the solid state reaction between ZnO and a-Fe2O3 showed, that the system tends to form a spinel [15]. The magnetisation of these compounds depends on the preparation method as well as on the particle size. Using standard preparation methods, where ZnO and a-Fe2O3 are mixed and calcinated at 900 8C, Zn2+ ions were found to occupy only tetrahedral sites. On the other hand, rapid quenching of ZnO and a-Fe2O3 as well as the preparation of ultrafine particles by the co-precipitation method results in spinels with a certain degree of inversion, leading to a considerable magnetisation due to A–B coupling of the Fe3+ spins [16–19]. Rectifying and non-rectifying metal contacts on ZnO may be prepared after the metal deposition using standard photolithography techniques. The work presented so far clearly shows how deviations from the expected Fe/ZnO band structure can be traced back to interface properties and chemical reactions. Our detailed investigation of the Fe/ZnO(0 0 0 1) interface after different annealing steps contributes towards elucidating new properties also in this context. 2. Experimental and data processing The XPS, AES, LEED and LEIS experiments have been carried out at an ESCALAB 220iXL ultra-high vacuum (UHV) system with a base pressure of 3.0 109 mbar. We used a Mg/Al dual mode X-ray anode, a hemispherical analyser equipped with six channeltrons, a differentially pumped ion source EX05 for sputtering and LEIS experiments and an RVL900 LEED optic. The axis of the lens assembly of the analyser is normal to the sample surface. X-ray and ion source axis are tilted with respect to the analyser axis by an angle of 458. For each deposition or annealing step the samples had to be transferred from the analysis chamber to the preparation chamber. The samples were mounted on a Mo sample holder covered by a Mo mask. A chromel–alumel thermocouple with
an estimated uncertainty of 15 8C attached close to the sample. The ZnO(0 0 0 1) single crystals were obtained from CrysTek, Berlin. The deposition of the Fe thin films was carried out with a water cooled electron beam evaporator (EFM3, OMICRON Vakuumphysik/Focus GmbH). The Fe (purity: 99.995%), obtained from Alfa Aesar, was evaporated from a tantalum crucible. If the emission current is kept constant, exact deposition rate monitoring by the evaporator control unit during the evaporation is possible [20]. We derived the thickness of the fabricated Fe films on ZnO(0 0 0 1) from the deposition time and the rate found for the reference system Fe/Ni(1 1 1), where Fe is known to grow layer-by-layer up to several angstroms [21,22]. Fitting the Ni 2p XPS intensity versus time by an ˚ /h exponential function resulted in a growth rate of 12 A ˚ working with a flux of 0.5 mA. A thickness of 2 A corresponds to one atom layer Fe, if the spacing between two (1 1 0) planes in bcc(Fe) is used for the estimation. During the deposition the evaporator was operated at a flux of 0.5 mA ˚ /h) and 2.0 mA for thick films (i.e. for thin films (i.e. <12 A ˚ >48 A/h). Because the Fe growth mode on ZnO may differ from that on Ni(1 1 1), in this paper all values for Fe film thickness are thickness equivalents. Annealing was carried out by backside electron bombardment. XP spectra were recorded using non-monochromatic Mg Ka radiation (hn = 1253.6 eV) and a pass energy of 50 eV. The energy calibration of the spectrometer was done by measuring the Cu 2p (932.6 0.1 eV), Ag 3d (368.2 0.1 eV) and Au 4f (84.1 0.1 eV) photoemission lines. The spectral resolution for the Ag 3d doublet was 1.4 eV at 50 eV pass energy. For the qualitative and quantitative analysis of the XP spectra the software UNIFIT 2007 [23–25] was used. XP spectra of a sputtered (5 kV Ar+ ions) Zn metal foil (purity: 99%, from Alfa ˚ thick metallic Fe film deposited at Aesar) and a 240 A 9 3 10 mbar on a ZnO(0 0 0 1) substrate were measured for reference. Furthermore, FeO powder (purity: 99.95%, from Alfa Aesar) was measured after cleaning the surface and reducing a small fraction of Fe3+ to Fe2+ by sputtering with 3 kVAr+. Fe2O3 powder (purity: 99.95%, form Alfa Aesar) was analysed without sputter cleaning in order to avoid the reduction of trivalent Fe. The peak shifts from sample charging were corrected by setting the C 1s peak to 285 eV. During the work function measurements the sample was biased by a 9 V accumulator. Using a pass energy of 2 eV, the maximum value of the first derivation of the secondary electron emission onset under Mg Ka irradiation was corrected by the bias voltage and used as work function. The LEIS measurements were carried out using the sputter ion gun EX05 operated with 1 keV He+ ions. The scattering angle was 1308 with the sample normal in the axis of the analyser lens tube. In order to prevent sputter damage, the LEIS were measured very fast (1 min per measurement) and with a very low dose of He-ions (sample current during LEIS measurements 5 nA). The pass energy of the analyser was set to 400 eV. The XPS data analysis comprises the X-ray satellite substraction and fitting procedure. The Fe 2p spectrum could only be analysed below 732 eV because of the disturbance caused by the OKLL Auger peak at 740 eV. This might result
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Table 1 Parameters used to model the Fe 2p XP spectra
Ebind. DS 2p1/2:2p3/2 Gauss FWHM Lorentz FWHM
Fe
FeO
FeO sat.
Fe2O3
Fe2O3 sat.
707.0 0.2 eV 13.2 0.1 eV 0.45–0.5 1.2 1.2
709.9 0.2 eV 13.2 0.1 eV 0.45–0.5 2.4 2.4
715.8 0.2 eV 13.2 0.1 eV 0.45–0.5 2.4 2.4
711.4 0.2 eV 13.7 0.l eV 0.45–0.5 2.4 2.4
719.6 0.2 eV 13.7 0.1 eV 0.45–0.5 2.4 2.4
in an underestimation of the Fe 2p peak area but should not influence the relative concentration of the different Fe0, Fe2+ and Fe3+ features. A mixture of a Shirley- and a polynomial background was included in the fitting process. All peaks were treated as convoluted Gaussian and Lorentzian functions. Fit parameters found for the whole set of measurements are shown in Table 1. Binding energies (Ebind.), full width half maxima values (FWHM), 2p1:2p3 intensity ratios and doublet splittings (DS) as well as the asymmetry parameter for the Fe 2p lines of the metal were determined by fitting the reference substances ˚ thick Fe film. During the FeO, Fe2O3 (powders) and the 240 A fitting process only the background, the position of the metallic Fe 2p peak and the relative intensities were allowed to be optimised. The quality of the fit is given by the residuum function R(E) = [S(E) M(E)]/[M(E)]1/2, where S(E) is the normalized sum curve and M(E) is the measured curve. For quantitative analysis the concentrations cx (given in at.%, with x = Zn, O or Fe) where determined from the Zn 2p3/2, O 1s and Fe 2p peak areas Ix using the formulas cx ¼ I 0x =ðI 0Zn þ I 0O þ I 0Fe Þ and I 0x ¼ I x =s x lx T, with s – Scofield photoionisation cross sections [26], l – inelastic mean free path of the photoelectrons from l = 0.103 E0.745 [25] and T – the energy transmission function of the spectrometer [25]. The surface of the untreated ZnO samples has been found to be covered by hydrocarbons and water. Thus, all samples had to
be cleaned by repeated cycles of 3.5 kV Ar+ sputtering at ambient temperature and annealing at 870 K for 30 min until no carbon contaminants and almost no OH groups were detectable by XPS (Fig. 1). This cleaning method has been found to be effective for the investigated ZnO single crystals resulting in clean surfaces with a very good crystallinity and excellent (1 1) LEED patterns. AFM images of a sputtered and annealed ZnO single crystal revealed that the surface is very ˚ . During the smooth giving an average roughness Ra = 3 A cleaning procedure the Zn:O XPS atom concentration ratio changed from 1:1 to 1:0.67 caused by the preferential sputtering of oxygen. Annealing under an oxygen atmosphere of 5 106 mbar did not reverse this effect. The XPS binding energy values for Zn 2p3/2 and O 1s are 1021.8 eV and 530.8 eV, respectively. The shoulder at the high energy side of the O 1s peak at 532.3 eV is attributed to chemisorbed hydroxyl groups bound to the surface. The overall concentration of this feature is less than 5% and the amount of the hydroxyl groups should therefore lie below one monolayer. The LEIS data were evaluated as follows. All peaks found in the LEIS spectra (for example: Zn at 820 eV final energy on the energy loss scale, O at 435 eV and Fe at 780 eV) were fitted by Gaussian functions in order to estimate the relative peak areas. The changes of these relative areas indicate changes of Zn:O atomic ratios. For clean ZnO we obtained a Zn:O peak area ratio of 6
Fig. 1. XP and Auger spectra of ZnO(0 0 0 1) after cleaning by several sputtering/annealing cycles (3 kV Ar+ ions, 870 K). Upper left: Overview spectra (inset: region of ZnL3M45M45, C 1s and O 1s).
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and 3 in case of the Zn-terminated (0 0 0 1) and O-terminated (0 0 0 1) surfaces, respectively. The roughness has been determined from 5 mm 2.5 mm scans using an atomic force microscope (AFM) DI3100 from Veeco Instruments. Information about the crystallographic structure and orientation of the phases formed during annealing was obtained by electron backscatter diffraction (EBSD) using a dual beam microscope Nanolab 200. The microscope was operated with an electron beam of 20 kV and a beam current set to 2.4 nA. EBSD, also known as backscatter Kikuchi diffraction (BKD) is a technique which allows to obtain the crystallographic texture and orientation of crystalline or polycrystalline materials using a scanning electron microscope (SEM). In EBSD a 708 from the horizontal tilted sample is hit by a stationary electron beam. The atoms inelastically scatter a part of the electrons with a small loss of energy forming a divergent source of electrons close to the surface. Some of these electrons are incident on atomic planes satisfying the Bragg equation being diffracted and forming a set of paired large angle cones corresponding to each diffracting plane resulting in a characteristic pattern (also named Kikuchi pattern) on a fluorescent screen. Regions of enhanced electron intensity between those cones produce characteristic bands (also named Kikuchi bands) on the screen. Bands correspond to crystal planes, intersections of bands to crystal directions. From the pattern the crystal structure and orientation is calculated. If a large number of points is measured the resulting map reveals the structure and the preferred orientation of a large sample area with a high spatial resolution. 3. Results 3.1. Growth of Fe on ZnO(0 0 0 1) The Fe deposition was monitored by measuring the Fe 2p, Zn 2p3/2, O 1s XPS and ZnL3M45M45 AES signals as well as LEED, LEIS and the work function. Fig. 2 shows the Fe 2p XPS and ZnL3M45M45 Auger lines after deposition of 4, 24 ˚ Fe on ZnO(0 0 0 1). At an Fe coverage of 4 A ˚ the and 60 A metal is found to appear in the metallic (Ebind. 707 eV) as well as the oxidised state +2 (Ebind. 710 eV). This is a first indication for an oxidation/reduction process between the ˚ Fe coverage the deposited metal and the substrate. At 24 A metallic component in the Fe 2p spectra becomes dominant, suppressing the FeO band to about 10% of the overall Fe 2p ˚ Fe coverage the FeO related components in spectrum. At 60 A the Fe 2p XPS are almost completely suppressed with a remaining amount of around 5–7% Fe2+ states attributed to weakly bound, chemisorbed hydroxyl groups (OH) on the surface of the Fe film. The ZnL3M45M45 Auger spectrum ˚ Fe coverage is composed of two lines measured at 4 A positioned at energies of about 262.2 eV and 265.6 eV which ˚ are attributed to ZnO [27]. For an Fe coverage larger than 16 A the ZnL3M45M45 spectrum is composed of two line pairs originating from (i) substrate Zn–O bonds, becoming weaker and (ii) a second growing line pair at 257.9 eV and 261.4 eV
Fig. 2. (a) Fe 2p XPS and (b) ZnL3M45M45 AES, measured after the deposition of different Fe films on ZnO(0 0 0 1).
˚ thick which can be attributed to metallic Zn [27]. For a 60 A 0 Fe film the Zn related peaks dominate the ZnL3M45M45 Auger spectrum. We assume an oxidation process taking place at the Fe/ZnO(0 0 0 1) interface and a diffusion of metallic Zn into the Fe overlayer. Thus, the main interface reaction can be summarized as Fe0 + Zn2+ ! Zn0 + Fe2+. The O 1s spectra (not shown in Fig. 2) are composed of a main component attributed to Zn–O groups of the substrate and a weak component at about 1.4 eV higher binding energy attributed to chemisorbed OH groups. In the case of the Zn 2p3/2 spectra one cannot distinguish between Zn2+ states and Zn0 states of the reduced substrate because of a very low binding energy shift. In order to reveal the electronic processes during the formation of the Fe/ZnO(0 0 0 1) interface and the growth of Fe, in Fig. 3 the change of the work function and the relative shift of the Zn 2p3/2 and O 1s binding energies are plotted versus the nominal Fe film thickness. After the deposition of ˚ Fe (0.5 Fe layers) the binding energies shift to higher 1A values. This shift reflects a reduction of the upward band bending with respect to the clean ZnO or even a downward band bending caused by the electronic charge donation from Fe to ZnO in this early state of the interface formation resulting in a positive charge build up at the surface and a negatively charged space charge layer in the ZnO. This shift is a direct probe of the interaction between semiconductor and metal and is linked to the observed chemical reaction. For thicker Fe films, where the Fe film exhibits properties similar to the bulk, the downward band bending is more and more
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Fig. 3. (a) relative change of the binding energy and (b) absolute work function change during the growth of Fe on ZnO(0 0 0 1).
compensated by the metal induced upward band bending related to the alignment of the Fermi level in the growing Fe ˚ thick Fe this film and the ZnO single crystal. For 24 A process has still not finished, the finally measured shift of the Zn 2p3/2 peak amounts to 0.8 eV compared to an uncovered, sputtered, and annealed surface. The work function of 3.6 eV found for the uncovered substrate is close to the average value of 3.75 eV found for sputtered and annealed or cleaved and annealed ZnO(0 0 0 1) single crystals [28–30]. ˚ thick Fe film the measured work After depositing a 16 A function was 4.7 eV. Because the work function has not ˚ thick Fe films are not bulk-like. reached a constant value, 20 A Extrapolating exponentially the work function curve results in a value of 4.82 0.07 eV, comparable to that found for the (1 0 0) surface of face-centered cubic fcc-Fe on Cu3Au [31], for example. Plotting the normalised substrate XPS intensities (O 1s and Zn 2p3/2) on a logarithmic scale versus the deposition time (Fig. 4) results in a nearly linear function with ˚ larger absolute values of the slope for films smaller than 2 A thickness (1 ML) verifying a growth mode change from 2D to 3D. Fitting an exponential function of the type Ix = I0,x exp(d/lx) to the curves (with Ix – peak areas, d – thickness and l – IMFP ˚ /h values) leads to an average deposition rate of 10.7(3.0) A which is smaller than the value found for the reference system
Fig. 4. Change of the normalised XPS intensity during the growth of Fe films on ZnO(0 0 0 1) single crystals on a logarithmic scale vs. film thickness d.
˚ /h. Fig. 5 shows LEED images, recorded Fe/Ni(1 1 1) giving 12 A ˚ after the deposition of different Fe amounts. At a thickness of 8 A there are still reflexes originating from the uncovered areas of the substrate. No LEED spots were detected on ZnO ˚ thick Fe film indicating a disordering in the covered with a 24 A ˚ Fe washed-out spots Fe film. After the deposition of 60 A corresponding to the (1 1 1) surface of fcc-Fe appear revealing that for a larger film thickness Fe grows in this metastable structure with the orientation Fe(1 1 1) jj ZnO (0 0 0 1) and Fe(1 1 0) jj ZnO(11–20). Helpful information was gained from the LEIS measurements. Due to its extreme surface sensitivity LEIS is a powerful complementary technique to XPS. The LEIS data shown in Fig. 6 indicate a fast increase of the surface Fe fraction together with a decrease of both O and Zn up to a nominal thickness of ˚ , which is roughly 0.5 atom layer. The slope of these first 1A parts of the curves would give a zero-intensity of either O or Zn ˚ Fe. At the (i.e. a closed Fe layer) at a nominal thickness of 2 A point of 50% coverage the film begins to grow in vertical direction without changing the atomic fractions at the surfaces ˚ . A diffusion of Zn atoms towards the up to a thickness of 6 A surface of these thin films may contribute also to the LEIS signals. The further decrease (increase) of the Zn (Fe) LEIS ˚ may be caused by the lateral signals beyond a thickness of 6 A extension of the islands and thus a simultaneous filling of the areas of uncovered ZnO and also by a reduced diffusion of metallic Zn. The remarkable variation of the O intensity may also be understood. In the beginning the O fraction decreases like the Zn fraction due to Fe deposition and the formation of ˚ OHmainly Fe2+. At a Fe film thickness of more than 1.5 A groups originating from the residual gas in the chamber are adsorbed on the now dominating metallic Fe sites present at the surface. Therefore, the O and Fe signal intensities for films ˚ thickness behave so similar. The combination larger than 1.5 A of XPS with LEIS and LEED data elucidates the kinetic inhibition of the thermodynamically preferred 3D growth at low temperatures giving the 2D-island (‘‘pseudo layer-by-layer’’) growth mode [32] as the most favorable for the Fe/ZnO(0 0 0 1) system. Only by means of LEIS and LEED data the growing of the first atomic layer of the deposit up to a critical coverage followed by a layer-by-layer thickening and a slow lateral increasing of the particular patches could be detected confirming the suggestion of the pseudo 2D growth mode.
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˚ ) and after (8, 24 and 60 A ˚ ) the deposition of a Fe film on ZnO(0 0 0 1). Primary energy 74 eV for 0, 8 and 24 A ˚ and Fig. 5. Change of the LEED pattern before (0 A ˚. 64 eV for 60 A
3.2. Annealing experiments The first part of the annealing experiments at W = 150– 600 8C (30 min for each temperature in 50 K steps) was carried ˚ Fe films (for XPS, out without breaking the vacuum using 24 A LEIS and LEED analysis) deposited at room temperature on ZnO(0 0 0 1). Up to a temperature of 300 8C no XPS shifts and no XPS and LEIS intensity changes could be observed. Weak spots reappearing from the substrate in the LEED pattern (Fig. 7) at 300 8C indicate the started rearrangement of the Fe films due to the increased mobility of the Fe atoms. This agglomeration process is verified by an increase of the XPS Zn 2p3/2 and O 1s intensities accompanied by a decreasing Fe 2p intensity at 350 8C (Fig. 8). The 2D Fe islands further thicken by Ostwald ripening exposing a previously covered interfacial Fe layer. This almost completely closed and stable first Fe monolayer beneath and between the
Fig. 6. Change of the O, Fe and Zn LEIS intensities during the growth of an ˚ thick Fe film on a ZnO(0 0 0 1) single crystal. 24 A
islands is not affected by the ripening due to the direct bonding to the substrate and may explain the missing Zn LEIS signal. Since the He+ angle of incidence during a LEIS measurement is around 508 also shadowing effects by neighbouring Fe clusters may lead to the absence of Zn signals in the measured LEIS (Fig. 9) for samples annealed below 400 8C. Further heating forces a two step oxidation of Fe to FeO and thereafter to Fe2O3. The first step is roughly completed at a temperature of around 400 8C (see Fig. 10), where the Zn 2p3/2 intensity in the XPS was found to drop to a local minimum (see Fig. 8) and a very sharp LEED pattern (Fig. 7) was obtained. We suggest the formation of monocrystalline FeO completely covering the substrate. One should note here that the temperature region where only FeO was found is quite narrow around 370 8C in the limits of an uncertainty of 15 8C resulting from three independent annealing series. At higher temperatures the FeO tends to be further oxidised to Fe2O3 giving a complete oxidation at 550 8C. This oxidation may correlate with the diffusion of Fe3+ and thus with the mixing of Fe2O3 with ZnO. The LEIS data (Fig. 9) confirm this two-step oxidation process. In the temperature range from 300–400 8C an oxygen enrichment at the surface due to the FeO formation is observed, no Zn signal is present in the spectra. At higher temperatures a Zn signal appears in the LEIS spectra while the oxygen signal intensity keeps constant. This appearance of Zn may indicate the mixing of Fe2O3 with ZnO, giving Zn lattice sites terminating the surface or the flanks of newly formed islands or cracks. At 600 8C the surface of the formed ZnO–Fe2O3 mixture gives a sharp LEED pattern of hexagonal symmetry (see Fig. 7) similar to that found for the (1 1 1) surface of Fe3O4 [33]. A second part of the annealing experiments was carried out for further investigation of the annealing products FeO (from ˚ Fe/ZnO(0 0 0 1) annealed at 400 8C for 1 h) and Fe2O3– 60 A
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˚ thick Fe film deposited on a ZnO(0 0 0 1) substrate, primary energy was set to 74 eV. Fig. 7. LEED pattern obtained after annealing of a 24 A
˚ Fe/ZnO(0 0 0 1) annealed at 640 8C for 1 h) ZnO (from 240 A by means of XPS quantification, SEM, AFM and EBSD. Annealing of the first sample led to the formation of FeO leaving a small Fe component (Fig. 11). Quantification using the Fe2+ component of the Fe 2p spectrum and the oxidic component of the O 1s spectrum leads to an Fe2+:O ratio of ˚ Fe/ZnO(0 0 0 1) film 1:1.1, close to that of FeO. For the 240 A annealed at 640 8C for 1 h we obtained a Zn:Fe:O ratio of
Fig. 8. Normalised (peak area divided by background) XPS intensities for Fe ˚ thick Fe film on 2p, O 1s and Zn 2p3/2 peaks during the annealing of a 24 A ZnO(0 0 0 1).
1:1.7:4. This ratio lies close to that of ZnFe2O4. The smaller value found for Fe in both samples may be caused by the partial overlapping of the Fe 2p and the OKLL spectra (see Section 2) resulting in a systematic underestimation of the Fe fraction. No OH component could be found in the O 1s spectrum of the film annealed at 640 8C, giving evidence that the presence of hydroxyl groups (chemisorbed OH) is linked to the presence of metallic Fe. In the AFM and SEM images (Fig. 12) for the samples annealed at 400 8C one can clearly see the metallic Fe islands (bright parts) resulting from incomplete oxidation and ripening of the metallic film. In the space between these islands the surface is rather smooth comparable to that of the substrate (see section lines). In the AFM and SEM images of the samples ˚ annealed at 640 8C ditches appear with a depth up to 100 A ˚ resulting in an average roughness of Ra = 34 A covering a fraction of around 10% of the surface. The roughness found for
Fig. 9. Relative LEIS intensities for Fe, O and Zn during the annealing of an ˚ thick Fe film on ZnO(0 0 0 1). 24 A
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˚ thick Fe film on Fig. 10. Temperature dependence of the relative intensities of the different components in the Fe 2p spectra (left) during the annealing of a 24 A ZnO(0 0 0 1). Fe 2p XPS (right) of the same sample (a) room temperature, (b) after annealing at 400 8C and (c) after annealing at 600 8C.
˚ Fe/ZnO(0 0 0 1) annealed at 400 8C for 1 h giving Fig. 11. Fe 2p XP spectra and XPS quantification derived from the Fe 2p, O 1s and Zn 2p3/2 XPS peaks for (a) 60 A ˚ Fe/ZnO(0 0 0 1) annealed at 640 8C for 1 h giving the ZnO–Fe2O3 mixture. nearly stoichiometric FeO and (b) 240 A
Fig. 12. First row: SEM (diameters: 3.8 mm 3.8 mm), second row: AFM (5.0 mm 2.5 mm) images of samples prepared as follows: (a) sputtered and annealed ˚ Fe/ZnO(0 0 0 1) annealed at 400 8C for 1 h and (c) 240 A ˚ Fe/ZnO(0 0 0 1) annealed at 640 8C for 1 h. Graphs below the AFM images are ZnO(0 0 0 1), (b) 60 A section lines measured in the center, all measurements were carried out ex situ.
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Fig. 13. Left: SEM image of a 3.8 8 mm2 large area (tilted sample) in the roughened Fe2O3–ZnO region. Right: EBSD mapping of same area, dark and light grey parts correspond to the two different (1 1 1) textures.
˚ . EBSD analysis showed the clean ZnO(0 0 0 1) substrate is 3 A that the formed Kikuchi pattern does fit to the spinell structure ˚ , comparable to that of with a lattice constant close to 8.4 A magnetite (Fe3O4). For a closer insight, an EBSD mapping was carried out (Fig. 13). We found two (1 1 1) oriented textures of this spinell phase, rotated by 1808 with respect to each other around the (1 1 1) direction.
which may be accompanied by the formation of ZnFe2O4. The ˚ Fe/ZnO(0 0 0 1) sample annealed at 640 8C consists of 240 A two (1 1 1)-oriented spinel phases rotated by 1808 relative to each other and clearly confirms the reported change of crystallinity, composition, and surface morphology.
4. Conclusion
This work was financed by DFG FOR 404-SZ58/15. For the assistance in the measurements at ESCALAB we thank R. Hesse. Furthermore, we are grateful to Dr. I. Konovalov for the SEM and EBSD analysis.
Fe grows on ZnO(0 0 0 1) single crystals in a pseudo layerby-layer mode giving large islands. Fe phases with fcc structure ˚ Fe(1 1 1) jj ZnO(0 0 0 1) and were found to build up 60 A Fe(110) jj ZnO(11–20) films. A chemical reaction at the interface takes place at the initial period of the room temperature deposition, forming metallic Zn and FeO. The metallic Zn was found to diffuse into the Fe layer if the deposition continues. On metallic Fe at the surface always chemisorbed OH groups could be detected by XPS. The chemical reaction leads to the formation of interface states as well as to electron charge donation into the depletion layer of ˚ thick Fe film results in a work the ZnO. Depositing a 16 A function of 4.7 eV. The extrapolated value of the work function for a bulky Fe film is 4.82 0.07 eV. Annealing leads initially to an agglomeration of the metallic Fe which is hindered by a continuing stepwise oxidation. At 400 8C the metallic Fe has almost completely reacted to FeO under evaporation of most of the reduced Zn species. The Fe:O ratio ˚ thick Fe layer found after oxidation of a nominally 60 A on ZnO(0 0 0 1) at 400 8C is 1:1.1. The LEED pattern of the FeO layer indicates a FeO(1 1 1) orientation. Above 400 8C FeO is further oxidized giving Fe2O3. The oxidation is finished at 550–600 8C. During the oxidation a roughening of the surface and a diffusion and mixing of ZnO and Fe2O3 occurs
Acknowledgements
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Applied Surface Science 254 (2008) 2309–2318 www.elsevier.com/locate/apsusc
The Fe/ZnO(0 0 0 1) interface: Formation and thermal stability D. Wett a,*, A. Demund a, H. Schmidt b, R. Szargan a a
Wilhelm-Ostwald-Institut fu¨r Physikalische und Theoretische Chemie, Universita¨t Leipzig, Linne´strasse 2, 04103 Leipzig, Germany b Institut fu¨r Experimentelle Physik II, Halbleiterphysik Universita¨t Leipzig, Linne´strasse 5, 04103 Leipzig, Germany Received 18 May 2007; received in revised form 9 September 2007; accepted 9 September 2007 Available online 14 September 2007
Abstract The room temperature growth mode and the interface reaction of Fe films on single crystalline ZnO(0 0 0 1) substrates prepared in ultra high vacuum (UHV) has been investigated by means of X-ray photoelectron and Auger electron spectroscopy (XPS, AES), low energy electron diffraction (LEED) and low energy ion scattering spectroscopy (LEIS). The results show that Fe grows in the pseudo layer-by-layer mode. At ambient temperature the deposited Fe film reduces the underlying ZnO single crystal resulting in FeO at the interface and metallic Zn, which partially diffuses into the remaining Fe overlayer. Annealing leads to a stepwise oxidation of the Fe to FeO (670 K) and Fe2O3 (820 K). The Fe2O3 mixes with the substrate resulting in two (1 1 1) oriented textures of a spinel phase found by electron backscatter diffraction analysis (EBSD). Febased spin-injection may play a vital role for ZnO-based spintronic devices. # 2007 Elsevier B.V. All rights reserved. Keywords: Zinc oxide; ZnO; Iron; Fe; Iron oxide; FeO; Fe2O3; Interface; Epitaxy; Growth; X-ray photoelectron spectroscopy; XPS; Low energy ion scattering; LEIS; Low energy electron diffraction; LEED; Electron backscatter diffraction; EBSD
1. Introduction The interactions at metal/oxide interfaces are strongly linked to chemical, electronic as well as mechanical properties and thus play a fundamental role for the application of such materials in catalysis, sensor and semiconductor technology or microelectronics [1]. Therefore, in the last years again many metal/oxide interfaces have been studied extensively. For example, 10 ML Fe were found to grow uniformly on a 10 ML predeposited NiO(0 0 1). An evidence for the Ni2+ reduction was found [2]. Later the same authors reported about the formation of a Fe–Ni mixed body-centered cubic (bcc) phase due to the diffusion of the reduced Ni into the Fe overlayer [3]. On a TiO2(1 1 0) substrate Fe was found to grow in the Stransky–Krastanov mode. The formation of Ti3+ states at the early state of the deposition was reported in that case [4]. Al2O3(0 0 0 1) and Fe form a rather stable interface, the appearance of Fe2+ states was observed, but no reduced Al states were found. Annealing at 870 K leads to Ostwald ripening of the Fe film, Fe2+ states disappear and metallic Al is
* Corresponding author. Tel.: +49 341 9736500; fax: +49 341 9736399. E-mail address: [email protected] (D. Wett). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.09.020
found [5]. Similar thermally activated processes were also observed in the case of Cu deposited on ZnO(0 0 0 1) as well as ZnO(0 0 0 1) [6–8]. During the deposition of thin Cr films on ZnO(0 0 0 1) metallic Zn was formed and segregated to the surface. Annealing at 830 K resulted in the formation of the spinel ZnCr2O4. Only octahedral sites, occupied by Cr3+, were exposed at the surface [9]. ZnO is a low cost material with excellent optical, electronic and dielectric properties [10]. ZnO can be used as transparent, conductive material for solar cell caps. MgxZn1xO alloys from wurtzite-type ZnO (Egap = 3.37 eV) and rock-salt-type MgO (Egap = 7.5 eV), which can be prepared under conservation of the hexagonal as well as the cubic lattice depending on the Mg concentration [11] and demonstrate the tunability of the direct band-gap of in MgxZn1xO alloys over a wide wavelength range. Furthermore, MgxZn1xO alloys could be used as new materials for tunneling barriers in magnetic tunneling junction (MTJ) devices. The composition and the thermal stability of the metal/barrier interfaces strongly influence the functionality of such devices. In the case of an Fe/MgO/Fe MTJ structure during the MgO deposition a small part of oxygen is placed into the first atom layer of the Fe. Coherent growth of the top Fe electrode of the MTJ structure was observed only for Fe deposition in ambient oxygen atmosphere leading to a
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symmetric MTJ structure characterized by FeO layers at both interfaces. Calculations indicate large positive tunnel magnetoresistance (TMR) values in such symmetric junctions [12]. So far, no information exists on the growth mode, thermal stability, reactions and transport properties of the Fe/ZnO interface. ZnO crystallizes in the wurtzite structure (space group: ˚ and c = 5.213 A ˚ P63mc) with the lattice constants a = 3.252 A [13]. Each Zn atom is nearly tetrahedrally surrounded by four O atoms and vice versa (lattice parameter u = 0.380). Along its [0 0 0 1] direction (c axis), Zn and O layers are alternatively arranged. By cutting a ZnO crystal perpendicular to its c axis two different polar surfaces are formed indexed as ZnO(0 0 0 1) and ZnO(0 0 0 1) and alternatively called +c and c plane. The (0 0 0 1) plane is terminated by Zn atoms, the (0 0 0 1) by O atoms. One should consider that termination does not mean that there are exclusively Zn or O atoms on the surface. Depending on the type of the plane one can find triangular pits or hexagonal islands or terraces, whose corners and edges consist of both atom types [14]. These reconstructions result in a considerable reduction of the surface polarity and a gain of the surface stability in comparison to the energetic unfavorable flat (1 1) terminated crystal. Studies of the solid state reaction between ZnO and a-Fe2O3 showed, that the system tends to form a spinel [15]. The magnetisation of these compounds depends on the preparation method as well as on the particle size. Using standard preparation methods, where ZnO and a-Fe2O3 are mixed and calcinated at 900 8C, Zn2+ ions were found to occupy only tetrahedral sites. On the other hand, rapid quenching of ZnO and a-Fe2O3 as well as the preparation of ultrafine particles by the co-precipitation method results in spinels with a certain degree of inversion, leading to a considerable magnetisation due to A–B coupling of the Fe3+ spins [16–19]. Rectifying and non-rectifying metal contacts on ZnO may be prepared after the metal deposition using standard photolithography techniques. The work presented so far clearly shows how deviations from the expected Fe/ZnO band structure can be traced back to interface properties and chemical reactions. Our detailed investigation of the Fe/ZnO(0 0 0 1) interface after different annealing steps contributes towards elucidating new properties also in this context. 2. Experimental and data processing The XPS, AES, LEED and LEIS experiments have been carried out at an ESCALAB 220iXL ultra-high vacuum (UHV) system with a base pressure of 3.0 109 mbar. We used a Mg/Al dual mode X-ray anode, a hemispherical analyser equipped with six channeltrons, a differentially pumped ion source EX05 for sputtering and LEIS experiments and an RVL900 LEED optic. The axis of the lens assembly of the analyser is normal to the sample surface. X-ray and ion source axis are tilted with respect to the analyser axis by an angle of 458. For each deposition or annealing step the samples had to be transferred from the analysis chamber to the preparation chamber. The samples were mounted on a Mo sample holder covered by a Mo mask. A chromel–alumel thermocouple with
an estimated uncertainty of 15 8C attached close to the sample. The ZnO(0 0 0 1) single crystals were obtained from CrysTek, Berlin. The deposition of the Fe thin films was carried out with a water cooled electron beam evaporator (EFM3, OMICRON Vakuumphysik/Focus GmbH). The Fe (purity: 99.995%), obtained from Alfa Aesar, was evaporated from a tantalum crucible. If the emission current is kept constant, exact deposition rate monitoring by the evaporator control unit during the evaporation is possible [20]. We derived the thickness of the fabricated Fe films on ZnO(0 0 0 1) from the deposition time and the rate found for the reference system Fe/Ni(1 1 1), where Fe is known to grow layer-by-layer up to several angstroms [21,22]. Fitting the Ni 2p XPS intensity versus time by an ˚ /h exponential function resulted in a growth rate of 12 A ˚ working with a flux of 0.5 mA. A thickness of 2 A corresponds to one atom layer Fe, if the spacing between two (1 1 0) planes in bcc(Fe) is used for the estimation. During the deposition the evaporator was operated at a flux of 0.5 mA ˚ /h) and 2.0 mA for thick films (i.e. for thin films (i.e. <12 A ˚ >48 A/h). Because the Fe growth mode on ZnO may differ from that on Ni(1 1 1), in this paper all values for Fe film thickness are thickness equivalents. Annealing was carried out by backside electron bombardment. XP spectra were recorded using non-monochromatic Mg Ka radiation (hn = 1253.6 eV) and a pass energy of 50 eV. The energy calibration of the spectrometer was done by measuring the Cu 2p (932.6 0.1 eV), Ag 3d (368.2 0.1 eV) and Au 4f (84.1 0.1 eV) photoemission lines. The spectral resolution for the Ag 3d doublet was 1.4 eV at 50 eV pass energy. For the qualitative and quantitative analysis of the XP spectra the software UNIFIT 2007 [23–25] was used. XP spectra of a sputtered (5 kV Ar+ ions) Zn metal foil (purity: 99%, from Alfa ˚ thick metallic Fe film deposited at Aesar) and a 240 A 9 3 10 mbar on a ZnO(0 0 0 1) substrate were measured for reference. Furthermore, FeO powder (purity: 99.95%, from Alfa Aesar) was measured after cleaning the surface and reducing a small fraction of Fe3+ to Fe2+ by sputtering with 3 kVAr+. Fe2O3 powder (purity: 99.95%, form Alfa Aesar) was analysed without sputter cleaning in order to avoid the reduction of trivalent Fe. The peak shifts from sample charging were corrected by setting the C 1s peak to 285 eV. During the work function measurements the sample was biased by a 9 V accumulator. Using a pass energy of 2 eV, the maximum value of the first derivation of the secondary electron emission onset under Mg Ka irradiation was corrected by the bias voltage and used as work function. The LEIS measurements were carried out using the sputter ion gun EX05 operated with 1 keV He+ ions. The scattering angle was 1308 with the sample normal in the axis of the analyser lens tube. In order to prevent sputter damage, the LEIS were measured very fast (1 min per measurement) and with a very low dose of He-ions (sample current during LEIS measurements 5 nA). The pass energy of the analyser was set to 400 eV. The XPS data analysis comprises the X-ray satellite substraction and fitting procedure. The Fe 2p spectrum could only be analysed below 732 eV because of the disturbance caused by the OKLL Auger peak at 740 eV. This might result
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Table 1 Parameters used to model the Fe 2p XP spectra
Ebind. DS 2p1/2:2p3/2 Gauss FWHM Lorentz FWHM
Fe
FeO
FeO sat.
Fe2O3
Fe2O3 sat.
707.0 0.2 eV 13.2 0.1 eV 0.45–0.5 1.2 1.2
709.9 0.2 eV 13.2 0.1 eV 0.45–0.5 2.4 2.4
715.8 0.2 eV 13.2 0.1 eV 0.45–0.5 2.4 2.4
711.4 0.2 eV 13.7 0.l eV 0.45–0.5 2.4 2.4
719.6 0.2 eV 13.7 0.1 eV 0.45–0.5 2.4 2.4
in an underestimation of the Fe 2p peak area but should not influence the relative concentration of the different Fe0, Fe2+ and Fe3+ features. A mixture of a Shirley- and a polynomial background was included in the fitting process. All peaks were treated as convoluted Gaussian and Lorentzian functions. Fit parameters found for the whole set of measurements are shown in Table 1. Binding energies (Ebind.), full width half maxima values (FWHM), 2p1:2p3 intensity ratios and doublet splittings (DS) as well as the asymmetry parameter for the Fe 2p lines of the metal were determined by fitting the reference substances ˚ thick Fe film. During the FeO, Fe2O3 (powders) and the 240 A fitting process only the background, the position of the metallic Fe 2p peak and the relative intensities were allowed to be optimised. The quality of the fit is given by the residuum function R(E) = [S(E) M(E)]/[M(E)]1/2, where S(E) is the normalized sum curve and M(E) is the measured curve. For quantitative analysis the concentrations cx (given in at.%, with x = Zn, O or Fe) where determined from the Zn 2p3/2, O 1s and Fe 2p peak areas Ix using the formulas cx ¼ I 0x =ðI 0Zn þ I 0O þ I 0Fe Þ and I 0x ¼ I x =s x lx T, with s – Scofield photoionisation cross sections [26], l – inelastic mean free path of the photoelectrons from l = 0.103 E0.745 [25] and T – the energy transmission function of the spectrometer [25]. The surface of the untreated ZnO samples has been found to be covered by hydrocarbons and water. Thus, all samples had to
be cleaned by repeated cycles of 3.5 kV Ar+ sputtering at ambient temperature and annealing at 870 K for 30 min until no carbon contaminants and almost no OH groups were detectable by XPS (Fig. 1). This cleaning method has been found to be effective for the investigated ZnO single crystals resulting in clean surfaces with a very good crystallinity and excellent (1 1) LEED patterns. AFM images of a sputtered and annealed ZnO single crystal revealed that the surface is very ˚ . During the smooth giving an average roughness Ra = 3 A cleaning procedure the Zn:O XPS atom concentration ratio changed from 1:1 to 1:0.67 caused by the preferential sputtering of oxygen. Annealing under an oxygen atmosphere of 5 106 mbar did not reverse this effect. The XPS binding energy values for Zn 2p3/2 and O 1s are 1021.8 eV and 530.8 eV, respectively. The shoulder at the high energy side of the O 1s peak at 532.3 eV is attributed to chemisorbed hydroxyl groups bound to the surface. The overall concentration of this feature is less than 5% and the amount of the hydroxyl groups should therefore lie below one monolayer. The LEIS data were evaluated as follows. All peaks found in the LEIS spectra (for example: Zn at 820 eV final energy on the energy loss scale, O at 435 eV and Fe at 780 eV) were fitted by Gaussian functions in order to estimate the relative peak areas. The changes of these relative areas indicate changes of Zn:O atomic ratios. For clean ZnO we obtained a Zn:O peak area ratio of 6
Fig. 1. XP and Auger spectra of ZnO(0 0 0 1) after cleaning by several sputtering/annealing cycles (3 kV Ar+ ions, 870 K). Upper left: Overview spectra (inset: region of ZnL3M45M45, C 1s and O 1s).
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and 3 in case of the Zn-terminated (0 0 0 1) and O-terminated (0 0 0 1) surfaces, respectively. The roughness has been determined from 5 mm 2.5 mm scans using an atomic force microscope (AFM) DI3100 from Veeco Instruments. Information about the crystallographic structure and orientation of the phases formed during annealing was obtained by electron backscatter diffraction (EBSD) using a dual beam microscope Nanolab 200. The microscope was operated with an electron beam of 20 kV and a beam current set to 2.4 nA. EBSD, also known as backscatter Kikuchi diffraction (BKD) is a technique which allows to obtain the crystallographic texture and orientation of crystalline or polycrystalline materials using a scanning electron microscope (SEM). In EBSD a 708 from the horizontal tilted sample is hit by a stationary electron beam. The atoms inelastically scatter a part of the electrons with a small loss of energy forming a divergent source of electrons close to the surface. Some of these electrons are incident on atomic planes satisfying the Bragg equation being diffracted and forming a set of paired large angle cones corresponding to each diffracting plane resulting in a characteristic pattern (also named Kikuchi pattern) on a fluorescent screen. Regions of enhanced electron intensity between those cones produce characteristic bands (also named Kikuchi bands) on the screen. Bands correspond to crystal planes, intersections of bands to crystal directions. From the pattern the crystal structure and orientation is calculated. If a large number of points is measured the resulting map reveals the structure and the preferred orientation of a large sample area with a high spatial resolution. 3. Results 3.1. Growth of Fe on ZnO(0 0 0 1) The Fe deposition was monitored by measuring the Fe 2p, Zn 2p3/2, O 1s XPS and ZnL3M45M45 AES signals as well as LEED, LEIS and the work function. Fig. 2 shows the Fe 2p XPS and ZnL3M45M45 Auger lines after deposition of 4, 24 ˚ Fe on ZnO(0 0 0 1). At an Fe coverage of 4 A ˚ the and 60 A metal is found to appear in the metallic (Ebind. 707 eV) as well as the oxidised state +2 (Ebind. 710 eV). This is a first indication for an oxidation/reduction process between the ˚ Fe coverage the deposited metal and the substrate. At 24 A metallic component in the Fe 2p spectra becomes dominant, suppressing the FeO band to about 10% of the overall Fe 2p ˚ Fe coverage the FeO related components in spectrum. At 60 A the Fe 2p XPS are almost completely suppressed with a remaining amount of around 5–7% Fe2+ states attributed to weakly bound, chemisorbed hydroxyl groups (OH) on the surface of the Fe film. The ZnL3M45M45 Auger spectrum ˚ Fe coverage is composed of two lines measured at 4 A positioned at energies of about 262.2 eV and 265.6 eV which ˚ are attributed to ZnO [27]. For an Fe coverage larger than 16 A the ZnL3M45M45 spectrum is composed of two line pairs originating from (i) substrate Zn–O bonds, becoming weaker and (ii) a second growing line pair at 257.9 eV and 261.4 eV
Fig. 2. (a) Fe 2p XPS and (b) ZnL3M45M45 AES, measured after the deposition of different Fe films on ZnO(0 0 0 1).
˚ thick which can be attributed to metallic Zn [27]. For a 60 A 0 Fe film the Zn related peaks dominate the ZnL3M45M45 Auger spectrum. We assume an oxidation process taking place at the Fe/ZnO(0 0 0 1) interface and a diffusion of metallic Zn into the Fe overlayer. Thus, the main interface reaction can be summarized as Fe0 + Zn2+ ! Zn0 + Fe2+. The O 1s spectra (not shown in Fig. 2) are composed of a main component attributed to Zn–O groups of the substrate and a weak component at about 1.4 eV higher binding energy attributed to chemisorbed OH groups. In the case of the Zn 2p3/2 spectra one cannot distinguish between Zn2+ states and Zn0 states of the reduced substrate because of a very low binding energy shift. In order to reveal the electronic processes during the formation of the Fe/ZnO(0 0 0 1) interface and the growth of Fe, in Fig. 3 the change of the work function and the relative shift of the Zn 2p3/2 and O 1s binding energies are plotted versus the nominal Fe film thickness. After the deposition of ˚ Fe (0.5 Fe layers) the binding energies shift to higher 1A values. This shift reflects a reduction of the upward band bending with respect to the clean ZnO or even a downward band bending caused by the electronic charge donation from Fe to ZnO in this early state of the interface formation resulting in a positive charge build up at the surface and a negatively charged space charge layer in the ZnO. This shift is a direct probe of the interaction between semiconductor and metal and is linked to the observed chemical reaction. For thicker Fe films, where the Fe film exhibits properties similar to the bulk, the downward band bending is more and more
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Fig. 3. (a) relative change of the binding energy and (b) absolute work function change during the growth of Fe on ZnO(0 0 0 1).
compensated by the metal induced upward band bending related to the alignment of the Fermi level in the growing Fe ˚ thick Fe this film and the ZnO single crystal. For 24 A process has still not finished, the finally measured shift of the Zn 2p3/2 peak amounts to 0.8 eV compared to an uncovered, sputtered, and annealed surface. The work function of 3.6 eV found for the uncovered substrate is close to the average value of 3.75 eV found for sputtered and annealed or cleaved and annealed ZnO(0 0 0 1) single crystals [28–30]. ˚ thick Fe film the measured work After depositing a 16 A function was 4.7 eV. Because the work function has not ˚ thick Fe films are not bulk-like. reached a constant value, 20 A Extrapolating exponentially the work function curve results in a value of 4.82 0.07 eV, comparable to that found for the (1 0 0) surface of face-centered cubic fcc-Fe on Cu3Au [31], for example. Plotting the normalised substrate XPS intensities (O 1s and Zn 2p3/2) on a logarithmic scale versus the deposition time (Fig. 4) results in a nearly linear function with ˚ larger absolute values of the slope for films smaller than 2 A thickness (1 ML) verifying a growth mode change from 2D to 3D. Fitting an exponential function of the type Ix = I0,x exp(d/lx) to the curves (with Ix – peak areas, d – thickness and l – IMFP ˚ /h values) leads to an average deposition rate of 10.7(3.0) A which is smaller than the value found for the reference system
Fig. 4. Change of the normalised XPS intensity during the growth of Fe films on ZnO(0 0 0 1) single crystals on a logarithmic scale vs. film thickness d.
˚ /h. Fig. 5 shows LEED images, recorded Fe/Ni(1 1 1) giving 12 A ˚ after the deposition of different Fe amounts. At a thickness of 8 A there are still reflexes originating from the uncovered areas of the substrate. No LEED spots were detected on ZnO ˚ thick Fe film indicating a disordering in the covered with a 24 A ˚ Fe washed-out spots Fe film. After the deposition of 60 A corresponding to the (1 1 1) surface of fcc-Fe appear revealing that for a larger film thickness Fe grows in this metastable structure with the orientation Fe(1 1 1) jj ZnO (0 0 0 1) and Fe(1 1 0) jj ZnO(11–20). Helpful information was gained from the LEIS measurements. Due to its extreme surface sensitivity LEIS is a powerful complementary technique to XPS. The LEIS data shown in Fig. 6 indicate a fast increase of the surface Fe fraction together with a decrease of both O and Zn up to a nominal thickness of ˚ , which is roughly 0.5 atom layer. The slope of these first 1A parts of the curves would give a zero-intensity of either O or Zn ˚ Fe. At the (i.e. a closed Fe layer) at a nominal thickness of 2 A point of 50% coverage the film begins to grow in vertical direction without changing the atomic fractions at the surfaces ˚ . A diffusion of Zn atoms towards the up to a thickness of 6 A surface of these thin films may contribute also to the LEIS signals. The further decrease (increase) of the Zn (Fe) LEIS ˚ may be caused by the lateral signals beyond a thickness of 6 A extension of the islands and thus a simultaneous filling of the areas of uncovered ZnO and also by a reduced diffusion of metallic Zn. The remarkable variation of the O intensity may also be understood. In the beginning the O fraction decreases like the Zn fraction due to Fe deposition and the formation of ˚ OHmainly Fe2+. At a Fe film thickness of more than 1.5 A groups originating from the residual gas in the chamber are adsorbed on the now dominating metallic Fe sites present at the surface. Therefore, the O and Fe signal intensities for films ˚ thickness behave so similar. The combination larger than 1.5 A of XPS with LEIS and LEED data elucidates the kinetic inhibition of the thermodynamically preferred 3D growth at low temperatures giving the 2D-island (‘‘pseudo layer-by-layer’’) growth mode [32] as the most favorable for the Fe/ZnO(0 0 0 1) system. Only by means of LEIS and LEED data the growing of the first atomic layer of the deposit up to a critical coverage followed by a layer-by-layer thickening and a slow lateral increasing of the particular patches could be detected confirming the suggestion of the pseudo 2D growth mode.
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˚ ) and after (8, 24 and 60 A ˚ ) the deposition of a Fe film on ZnO(0 0 0 1). Primary energy 74 eV for 0, 8 and 24 A ˚ and Fig. 5. Change of the LEED pattern before (0 A ˚. 64 eV for 60 A
3.2. Annealing experiments The first part of the annealing experiments at W = 150– 600 8C (30 min for each temperature in 50 K steps) was carried ˚ Fe films (for XPS, out without breaking the vacuum using 24 A LEIS and LEED analysis) deposited at room temperature on ZnO(0 0 0 1). Up to a temperature of 300 8C no XPS shifts and no XPS and LEIS intensity changes could be observed. Weak spots reappearing from the substrate in the LEED pattern (Fig. 7) at 300 8C indicate the started rearrangement of the Fe films due to the increased mobility of the Fe atoms. This agglomeration process is verified by an increase of the XPS Zn 2p3/2 and O 1s intensities accompanied by a decreasing Fe 2p intensity at 350 8C (Fig. 8). The 2D Fe islands further thicken by Ostwald ripening exposing a previously covered interfacial Fe layer. This almost completely closed and stable first Fe monolayer beneath and between the
Fig. 6. Change of the O, Fe and Zn LEIS intensities during the growth of an ˚ thick Fe film on a ZnO(0 0 0 1) single crystal. 24 A
islands is not affected by the ripening due to the direct bonding to the substrate and may explain the missing Zn LEIS signal. Since the He+ angle of incidence during a LEIS measurement is around 508 also shadowing effects by neighbouring Fe clusters may lead to the absence of Zn signals in the measured LEIS (Fig. 9) for samples annealed below 400 8C. Further heating forces a two step oxidation of Fe to FeO and thereafter to Fe2O3. The first step is roughly completed at a temperature of around 400 8C (see Fig. 10), where the Zn 2p3/2 intensity in the XPS was found to drop to a local minimum (see Fig. 8) and a very sharp LEED pattern (Fig. 7) was obtained. We suggest the formation of monocrystalline FeO completely covering the substrate. One should note here that the temperature region where only FeO was found is quite narrow around 370 8C in the limits of an uncertainty of 15 8C resulting from three independent annealing series. At higher temperatures the FeO tends to be further oxidised to Fe2O3 giving a complete oxidation at 550 8C. This oxidation may correlate with the diffusion of Fe3+ and thus with the mixing of Fe2O3 with ZnO. The LEIS data (Fig. 9) confirm this two-step oxidation process. In the temperature range from 300–400 8C an oxygen enrichment at the surface due to the FeO formation is observed, no Zn signal is present in the spectra. At higher temperatures a Zn signal appears in the LEIS spectra while the oxygen signal intensity keeps constant. This appearance of Zn may indicate the mixing of Fe2O3 with ZnO, giving Zn lattice sites terminating the surface or the flanks of newly formed islands or cracks. At 600 8C the surface of the formed ZnO–Fe2O3 mixture gives a sharp LEED pattern of hexagonal symmetry (see Fig. 7) similar to that found for the (1 1 1) surface of Fe3O4 [33]. A second part of the annealing experiments was carried out for further investigation of the annealing products FeO (from ˚ Fe/ZnO(0 0 0 1) annealed at 400 8C for 1 h) and Fe2O3– 60 A
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˚ thick Fe film deposited on a ZnO(0 0 0 1) substrate, primary energy was set to 74 eV. Fig. 7. LEED pattern obtained after annealing of a 24 A
˚ Fe/ZnO(0 0 0 1) annealed at 640 8C for 1 h) ZnO (from 240 A by means of XPS quantification, SEM, AFM and EBSD. Annealing of the first sample led to the formation of FeO leaving a small Fe component (Fig. 11). Quantification using the Fe2+ component of the Fe 2p spectrum and the oxidic component of the O 1s spectrum leads to an Fe2+:O ratio of ˚ Fe/ZnO(0 0 0 1) film 1:1.1, close to that of FeO. For the 240 A annealed at 640 8C for 1 h we obtained a Zn:Fe:O ratio of
Fig. 8. Normalised (peak area divided by background) XPS intensities for Fe ˚ thick Fe film on 2p, O 1s and Zn 2p3/2 peaks during the annealing of a 24 A ZnO(0 0 0 1).
1:1.7:4. This ratio lies close to that of ZnFe2O4. The smaller value found for Fe in both samples may be caused by the partial overlapping of the Fe 2p and the OKLL spectra (see Section 2) resulting in a systematic underestimation of the Fe fraction. No OH component could be found in the O 1s spectrum of the film annealed at 640 8C, giving evidence that the presence of hydroxyl groups (chemisorbed OH) is linked to the presence of metallic Fe. In the AFM and SEM images (Fig. 12) for the samples annealed at 400 8C one can clearly see the metallic Fe islands (bright parts) resulting from incomplete oxidation and ripening of the metallic film. In the space between these islands the surface is rather smooth comparable to that of the substrate (see section lines). In the AFM and SEM images of the samples ˚ annealed at 640 8C ditches appear with a depth up to 100 A ˚ resulting in an average roughness of Ra = 34 A covering a fraction of around 10% of the surface. The roughness found for
Fig. 9. Relative LEIS intensities for Fe, O and Zn during the annealing of an ˚ thick Fe film on ZnO(0 0 0 1). 24 A
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˚ thick Fe film on Fig. 10. Temperature dependence of the relative intensities of the different components in the Fe 2p spectra (left) during the annealing of a 24 A ZnO(0 0 0 1). Fe 2p XPS (right) of the same sample (a) room temperature, (b) after annealing at 400 8C and (c) after annealing at 600 8C.
˚ Fe/ZnO(0 0 0 1) annealed at 400 8C for 1 h giving Fig. 11. Fe 2p XP spectra and XPS quantification derived from the Fe 2p, O 1s and Zn 2p3/2 XPS peaks for (a) 60 A ˚ Fe/ZnO(0 0 0 1) annealed at 640 8C for 1 h giving the ZnO–Fe2O3 mixture. nearly stoichiometric FeO and (b) 240 A
Fig. 12. First row: SEM (diameters: 3.8 mm 3.8 mm), second row: AFM (5.0 mm 2.5 mm) images of samples prepared as follows: (a) sputtered and annealed ˚ Fe/ZnO(0 0 0 1) annealed at 400 8C for 1 h and (c) 240 A ˚ Fe/ZnO(0 0 0 1) annealed at 640 8C for 1 h. Graphs below the AFM images are ZnO(0 0 0 1), (b) 60 A section lines measured in the center, all measurements were carried out ex situ.
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Fig. 13. Left: SEM image of a 3.8 8 mm2 large area (tilted sample) in the roughened Fe2O3–ZnO region. Right: EBSD mapping of same area, dark and light grey parts correspond to the two different (1 1 1) textures.
˚ . EBSD analysis showed the clean ZnO(0 0 0 1) substrate is 3 A that the formed Kikuchi pattern does fit to the spinell structure ˚ , comparable to that of with a lattice constant close to 8.4 A magnetite (Fe3O4). For a closer insight, an EBSD mapping was carried out (Fig. 13). We found two (1 1 1) oriented textures of this spinell phase, rotated by 1808 with respect to each other around the (1 1 1) direction.
which may be accompanied by the formation of ZnFe2O4. The ˚ Fe/ZnO(0 0 0 1) sample annealed at 640 8C consists of 240 A two (1 1 1)-oriented spinel phases rotated by 1808 relative to each other and clearly confirms the reported change of crystallinity, composition, and surface morphology.
4. Conclusion
This work was financed by DFG FOR 404-SZ58/15. For the assistance in the measurements at ESCALAB we thank R. Hesse. Furthermore, we are grateful to Dr. I. Konovalov for the SEM and EBSD analysis.
Fe grows on ZnO(0 0 0 1) single crystals in a pseudo layerby-layer mode giving large islands. Fe phases with fcc structure ˚ Fe(1 1 1) jj ZnO(0 0 0 1) and were found to build up 60 A Fe(110) jj ZnO(11–20) films. A chemical reaction at the interface takes place at the initial period of the room temperature deposition, forming metallic Zn and FeO. The metallic Zn was found to diffuse into the Fe layer if the deposition continues. On metallic Fe at the surface always chemisorbed OH groups could be detected by XPS. The chemical reaction leads to the formation of interface states as well as to electron charge donation into the depletion layer of ˚ thick Fe film results in a work the ZnO. Depositing a 16 A function of 4.7 eV. The extrapolated value of the work function for a bulky Fe film is 4.82 0.07 eV. Annealing leads initially to an agglomeration of the metallic Fe which is hindered by a continuing stepwise oxidation. At 400 8C the metallic Fe has almost completely reacted to FeO under evaporation of most of the reduced Zn species. The Fe:O ratio ˚ thick Fe layer found after oxidation of a nominally 60 A on ZnO(0 0 0 1) at 400 8C is 1:1.1. The LEED pattern of the FeO layer indicates a FeO(1 1 1) orientation. Above 400 8C FeO is further oxidized giving Fe2O3. The oxidation is finished at 550–600 8C. During the oxidation a roughening of the surface and a diffusion and mixing of ZnO and Fe2O3 occurs
Acknowledgements
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