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The electronic properties of sputtered chromium and iron oxide films
Corrosion Science 46 (2004) 1479–1499 www.elsevier.com/locate/corsci
The electronic properties of sputtered chromium and iron oxide films G. Goodlet a, S. Faty a,1, S. Cardoso b, P.P. Freitas b,c, A.M.P. Sim~ oes a, M.G.S. Ferreira a,d,*, M. Da Cunha Belo
a
a
d
Department of Chemical Engineering, Instituto Superior T ecnico, Av. Rovisco Pais, Lisboa 1049-001, Portugal b INESC Microsystems and Nanotechnologies, R. Alves Redol 9, Lisboa 1000 , Portugal c Department of Physics, Instituto Superior Tecnico, Av. Rovisco Pais, Lisboa 1049-001, Portugal Department of Ceramics and Glass Engineering, University of Aveiro, Campus de Santiago, Aveiro 3810-193, Portugal Received 31 March 2003; accepted 16 September 2003
Abstract The semiconducting properties and electrochemical behaviour of thin chromium and iron oxide films produced by sputtering were investigated by capacitance and photoelectrochemical measurements. The films were deposited onto various substrates and submitted in some cases, to a thermal treatment. It appears that sputtered chromium oxide on iron substrate reveals both types of semiconductivity (p and n) and that the thermal treatment enhances the n-type character. Finally, a study is developed using a duplex film formed by sputtered layers of chromium and iron oxides. This system allows for the discussion of the problem of the electronic structure of the heterojunction created by the two kinds of oxides. Comparison is made with the case of passive films on stainless steel. 2003 Elsevier Ltd. All rights reserved. Keywords: A. Sputtered films; B. Capacitance and photoelectrochemistry; C. Passivity
*
Corresponding author. Address: Department of Chemical Engineering, Instituto Superior Tecnico, Av. Rovisco Pais, Lisboa 1049-001, Portugal. Tel.: +351-234370354; fax: +351-218404589. E-mail address: [email protected] (M.G.S. Ferreira). 1 Permanent address: Faculte des Sciences et Techniques, UCAD, Dakar, Senegal. 0010-938X/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2003.09.022
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1. Introduction Although the presence of chromium oxides in the passive films formed on Fe–Cr stainless steels play the major role on corrosion resistance of these materials, the electrochemical properties of the films are also strongly influenced by the presence of iron oxides. There are in fact, throughout the film thickness, regions of approximately pure chromium oxide, pure iron oxide and transition regions of mixed oxides. Chromium oxides are generally considered to have p-type behaviour [1]. Possible doping species are cationic and anionic vacancies or cationic interstitials. When chromium interstitials or oxygen vacancies are the predominating defects, the chromium oxide is an n-type semiconductor and when chromium vacancies are the predominating defects the chromium oxide is a p-type semiconductor. For the electronic characterization of chromium oxides, no well defined values of the band gap exist, but instead several values have been reported and assumed to be related to the different degrees of hydration [2–4]. On the other hand, because chromium and iron oxides reveal distinct semiconducting properties, an important characteristic of the electronic structure of passive films on stainless steels is the development of an interface, inside the film, with a distribution of charge carriers similar to that of a p–n heterojunction [5,6]. Simultaneous ionic and electronic transport and high doping levels make the study of the passive films very complex. The study of well-defined oxide layers could help in interpreting results concerning passivation. In this work the electronic properties of chromium and iron oxide films produced by sputtering were investigated in borate buffer solution. The central part of the research concerns the possible effects of the applied potential on the electronic structure of the n–p junction formed by sputtered chromium and iron oxide layers. In this junction only electronic processes take place when submitted to external polarization. In fact, ionic transport throughout the junction during electrochemical and photoelectrochemical measurements can be excluded. Only dissolution processes at film-electrolyte interface may occur. The methods used in this study are the photocurrent spectroscopy in order to obtain band gap values and the Mott–Schottky approach based on capacitance measurements. Both methods have been used in semiconductor electrochemistry and can also be considered as good tools for characterization of semiconducting passive films. Capacitance measurements have been carried out in the present investigation using the frequency values equivalent to that used in previous research on passive films. This procedure helps in making the results comparable.
2. Experimental Chromium and iron oxide films were sputtered onto various substrates (glassy carbon, tantalum and iron). They were fabricated by reactive ion beam sputtering from Cr and Fe metallic targets, respectively, in the presence of a mixed Ar–O2 beam coming from the 20 cm diameter assist gun (15.02% O2 in Ar). A dual source
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Nordiko 3000 ion beam system was used [7,8]. The substrate holder was water cooled. The deposition source was running with the standard conditions for the metallic film deposition (acceleration voltage of +1450 V, deceleration voltage of )300 V, 33 mA Xe beam, 1 sccm Xe, 100 W RF plasma), while an Ar-O2 beam was created by accelerating the plasma inside the assist gun with +200 V (18 mA beam, 10 sccm Ar–O2 , 60W RF plasma) and using a deceleration voltage of )100 V. A deficient content of O2 (low acceleration voltage) would cause the film to be metallic, instead of oxide. Also, too high acceleration voltages will reduce the deposition rate, since the deposited material would be sputtered by the Ar–O2 beam, instead of reacting with metallic ions coming from the target. The pressure during the process is 1.2 · 104 Torr. A filamentless neutraliser (40 mA, 3 sccm Ar– O2 ) beside to assist the ion source avoids surface charging during the dielectric deposition. During the oxide deposition the substrate table is at 80, and rotates at 15 rpm. A thermal treatment at 300 C for 8 h, was applied to some of the films. The film on pure chromium and pure iron (Goodfellow 99.9%) were grown at 450 C for 2 h in a furnace. Prior to growth the chromium was polished with silicon carbide paper of various grades and finished with 1 lm aluminium oxide. Structural information on the films was obtained by glancing XRD measurements, using a Siemens X-ray D 500 diffractometer. Atomic force microscopy (AFM) was carried out by using a Topometrix 2010 ‘‘Discover’’ working in non-contact mode with a silicon nitride tip. In the photoelectrochemical study the working electrode was illuminated through a quartz window with monochromatic light (beam area ¼ 0.12 cm2 ). The optical instrumentation consisted of a 150 W Xe lamp, a 250 mm f18 monochromator, a stepper motor to control the wavelength and a mechanical chopper. The grating was changed at 340 nm for maximum flux and a filter was used above 450 nm to eliminate any second order effects. All photocurrent measurements were normalized with respect to the flux recorded by a silicon photodiode. The photocurrent was obtained by connecting the current output of the potentiostat (EG&G 273) to a lock-in amplifier (Brookdeal EG&G 5210) and recording the voltage output due to the signal at 19 Hz (the chopping frequency). The recorded values were then worked out to calculate the photocurrent and the quantum efficiency values using a spreadsheet software package. For the capacitance–potential measurements the lock-in was used to produce a sinusoidal signal, 10 mV peak-to-peak at 1 kHz, which was superimposed on the potential applied by the potentiostat. The output of the potentiostat was then fed to the lock-in. A three-electrode cell was used, consisting of a Pt wire counter electrode, a saturated calomel reference electrode and the oxide film on the substrate was the working electrode. All measurements were carried out in sodium borate/boric acid buffer solution (pH 9.1). The effect of pH was observed by using 0.5 M sulfuric acid and a 0.01 M solution of the same acid was prepared for the profiling experiments. All the reagents were of an analytical grade.
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3. Results 3.1. AFM observations and XRD measurements The AFM images obtained with sputtered iron and chromium oxide films on tantalum are shown in Fig. 1. Both films can be considered compact but the roughness morphology of the iron oxide film (Fig. 1a) is very different from that of the chromium oxide film (Fig. 1b). It is known that the AFM technique allows us to observe surface morphology but does not give, in the experimental conditions used, direct information about the crystallographic structure. Nevertheless, nucleation aspects of the film growth may relate to a larger grain size of the chromium oxide relatively to that of the iron oxide. This analysis is supported by XRD results, which show that the annealed chromium oxide (8 h at 300 C) depicted any Cr2 O3 peaks, where a peak corresponding to the (1 1 0) plane was observed. Although the other films appeared to be amorphous or nanocrystalline due to a lack of peaks, the film sputtered onto Ta displayed an increased response in the spectrum of the 1000 A film. This response may be caused by nano30–40 region compared to the 50 A crystalline material. There is, thus, evidence to suggest that the films are not completely amorphous. Marcus et al. [9] have reported the presence of nanocrystallites
on tantalum. Fig. 1. AFM images of sputtered: (a) chromium and (b) iron oxide films of 500 A
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(<3 nm) of Cr2 O3 -like oxide anodically grown on chromium. Therefore some shortrange order can be considered within these films.
3.2. Current–potential curves Fig. 2 allow us to compare the dc electrochemical behaviour of the sputtered films (Fig. 2a) to that of anodic passive films formed on chromium and on iron (Fig. 2b). In both cases the iron oxides are more stable than the chromium oxides in the more anodic potential region. Conversely, the passive films on chromium and sputtered chromium oxide are more stable at lower potentials. The question is, which processes are associated with the rise in cathodic and anodic current. In spite of the fact that, from thermodynamic considerations, the passivation potential of chromium is )1.09 V/SCE (at pH 8.2) the cathodic currents for chromium increases at potentials that are more positive than most of the values proposed for the equilibrium potential of passivation. However, at these potentials the chromium oxide cannot be reduced. The only thermodynamic feasible process is the reduction of the iron oxide and that of hydrogen evolution.
and (b) Fig. 2. Current–potential curves: (a) for sputtered chromium and iron oxides (thickness of 500 A) for passive films formed on chromium and iron.
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3.3. Photocurrent measurements on chromium oxides The fundamental optical transitions (band gap) were determined from work produced by G€ artner [10] and for a semiconductor/electrolyte junction by Bulter [11]. The photoresponse near the band gap can be described by: g¼
Iph ðhm Eg Þ ¼ qAW U hm
n
ð1Þ
where g is the quantum efficiency, Iph is the photocurrent, U is the photon flux of energy hm, q is the electronic charge, A is a constant, W is the thickness of the space charge layer and Eg the band gap energy. The value of n depends on the type of the transition between the valence band and the conduction band. The analysis of the photocurrent spectra for passive films shows that n ¼ 2 is the most appropriate value to represent the photoelectrochemical behaviour observed. This value corresponds to indirect transitions in the crystalline band model or non-direct transitions in amorphous films [12]. The optical band gap determined for the films formed under different conditions (Fig. 3) ranges from 2.7 to 2.9 eV, and can be associated with the chromium oxide films. In the literature values between 2.4 and 3.55 eV have been reported [1–4]. 3.4. Capacitance–potential measurements The properties of a semiconductor/electrolyte interface can be described by Mott– Schottky theory [13], according to which the space charge capacitance of a p-type or n-type semiconductor is given by:
Fig. 3. Photocurrent as a function of the energy of the incident light for different chromium oxide films thick oxide film sputtered on polarized at 0.1 V (1) oxide film air grown at 450 C on chromium; (2) 1000 A thick oxide film sputtered on tantalum. tantalum; (3) 50 A
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1 2 kT ¼ U U fb 2 qNq ee0 e CSC
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ð2Þ
where e is the relative permittivity of the oxide (12 for the bulk Cr2 O3 [14]), e0 is the vacuum permittivity, q is the elementary charge (+e for electrons and )e for holes), Nq is the acceptor or donor concentration, U is the applied potential and Ufb is the flat band potential. The technique is based on the fact that in the case of a Schottky contact the space charge layer gives rise to a space charge capacitance, CSC to which corresponds an 1 effective complex impedance Rþ ðixCSC Þ . Both the ohmic parts of R and the capacitance CSC can be determined by superimposing an ac voltage of frequency x on a dc bias applied across the junction. Lock-in detection at frequency x allows the separation of CSC and R. Theoretically, in a highly doped oxide film in contact with an electrolyte the classic Mott–Schottky relationship must be modified, because the space charge capacitance and charge density cannot be calculated on the basis of the Boltzmann function. However, it has been demonstrated [15] that even using a more general Fermi distribution a linear relationship between C 2 and the applied potential is always found whatever the position of the Fermi level and of the doping donor level are. Consequently, highly doped and degenerate semiconductors may by characterized by the Mott–Schottky method. 3.4.1. Capacitance of the chromium oxide Fig. 4 shows capacitance–potential measurements in the form of Mott–Schottky plots for different chromium oxide films. In each plot there is a linear region with a negative slope, which indicates that the films reveal p-type semiconductivity. These films formed on metallic substrates exhibit flat band potentials that were in agree film sputtered onto Ta and the air grown ment with previous results [5]. The 1000 A
Fig. 4. Mott–Schottky plots obtained with chromium oxide films (1) formed on chromium at 450 C for 2 thick (3) 50 A thick. h; sputtered on tantalum (2) 1000 A
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Table 1 The effect of pH on the electronic properties of chromium oxide films Film type
pH
Band gap/eV
Ufb vs. SCE/V
Na 1019 atoms/cm3
Air grown
9.2 0.6
2.87 2.94
0.46 0.95
13 31
CrOx on Ta 1000 A
9.2 0.6
2.76 2.84
0.54 1.25
6 3
film showed an increase in capacitance (decrease in 1=C 2 ) for potentials below )0.5 V. Particularly relevant are the high doping densities, especially those exhibited by sputtered films. According to this high doping the thickness of the space the 50 A charge region extends across a relatively short distance into the film. For example, film on Ta, the doping density leads to a calculated space charge for the 1000 A The doping densities obtained in this work are approximately one thickness of 40 A. order of magnitude below those reported by B€ ohni [16]. Table 1 summarizes the results of measurements made in both alkaline basic (borate buffer) and acidic (0.5 M sulphuric acid) conditions for the air grown film and a sputtered film. The band gaps are practically unchanged whereas the flat band potentials change practically in accordance with the Nernst equation (approximately 0.060 V per pH unit). It should be noted that the straight lines in the Mott–Schottky representation are relatively short. This can be interpreted as a consequence of the low thickness of the films. Note that, highly doped bulk crystalline semiconductors also exhibit the same feature [17]. 3.4.2. Capacitance of the iron oxide In the case of the iron oxide film sputtered on tantalum (Fig. 5a) the Mott– Schottky plot displays a positive slope in the region from 0 to )0.5 V vs. SCE, in the same way as the film formed on pure iron by air oxidation (Fig. 5b). This can be attributed to the n-semiconductive behaviour of Fe2 O3 . The capacitance behaviour of iron oxide films sputtered onto the various substrates was also similar. The doping densities were all very high (1021 cm3 ) and the photocurrent was much lower than that in air grown films on pure iron and on stainless steel. In the XRD analysis only iron oxide film on silicon displayed any peaks and these could not be the 500 A clearly identified as Fe2 O3 or Fe3 O4 . The change in slope of the C 2 ¼ f ðU Þ plot observed at about 0 V (Fig. 5) may be explained assuming that a deep donor level is localized in the octahedral sites of the spinel oxide [18,19]. 3.4.3. Chromium oxide sputtered on iron substrate (effect of annealing) The capacitance behaviour of both thermally and anodically grown films (passive films) on stainless steel gives Mott–Schottky plots with positive (n-type) and negative (p-type) slopes [5]. This same type of behaviour is observed in as-deposited sputtered
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sputtered iron oxide film on tantalum and (b) oxide film formed Fig. 5. Mott–Schottky plots of: (a) 500 A on pure iron at 450 C for 2 h.
chromium oxide films on pure iron (Fig. 6). Such behaviour was not observed in the case of sputtered chromium oxide film on glassy carbon. The effect of the heat sputtered film was minor (Fig. treatment on the Mott–Schottky plot of the 500 A 6a). The n-type character ()0.6 to )0.25 V vs. SCE) did not show any enhancement, the linear region in the plot is the same and even the capacitance values are only slightly changed. However, when a similar procedure was implemented on the 50 A chromium oxide film, the Mott–Schottky plot changed radically (Fig. 6b). The overall capacitance changed, the n-type region became wider, extending from 0.5 to about )0.5 V, and the p-type region below )0.5 V practically vanished. The photocurrent response (at 0.1 V vs. SCE) also changed markedly with the thermal treatment (Fig. 7). The as-deposited film showed a response similar to stainless steel and iron oxide [5], although tailing obscured a definite single band gap. After annealing the response became shifted to lower photon energies (longer wavelengths), with Eg ¼ 2:0 eV, a value characteristic of iron oxides. Accompanying this shift there was a change in the current and in the photocurrent, specially for the film. Fig. 8 shows (curve 1) that the current–potential curve is relatively similar 50 A to that of passive chromium (Fig. 2b), whereas the annealed film (curve 2) was almost identical to that of the oxide passive film formed on pure iron (Fig. 2b). The photocurrent response (inset of Fig. 8) also suggests that the surfaces of the films
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Fig. 6. Mott–Schottky plots of chromium oxide films sputtered on iron (1) as deposited and (2) after thick (b) 50 A thick. heating at 300 C for 8 h. (a) 500 A;
behave as chromium oxide and iron oxide-like for the as-deposited and annealed films, respectively. 3.4.4. Chemical profiling on duplex film formed by chromium and iron sputtered oxides Chemical profiling was used to assess the way in which the Mott–Schottky plots change when the surface layer of iron oxide was removed. This technique is based on the commonly used doping density profiling technique used in the semiconductor iron oxide on 500 A chromium oxide sputtered onto Ta industry. A sample of 500 A was immersed in a solution 1 M H2 SO4 . Capacitance-potential measurements were then obtained at several time intervals (Fig. 9a) and show a transition. Immediately after immersion there is a sharp increase of 1=C 2 in the region below )0.5 V. As the chemical etching proceeds, the positive slope (characteristic of n-type semi-
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Fig. 7. Photocurrent as a function of the energy of the incident light of sputtered chromium oxide (1) as thick (b) 50 A thick. deposited and (2) thermally treated. (a) 500 A
conductivity) disappears and a negative slope appears instead. This response is similar to that observed for sputtered chromium oxide films on iron. The point of inflection at about )0.2 V gradually vanished with time, which suggests that there is no abrupt change from one material to the other, but that some inter-mixing occurs. It is important to note that in the duplex film, iron oxide is placed in degeneracy condition at potentials more negative than the flat band of this oxide. This means that the capacity of the iron oxide layer is similar to that of a metallic conductor. Thus, the low capacity observed in capacitance measurements must be interpreted as being due to the inner chromium oxide layer of the duplex. Fig. 9b shows a schematic band structure model of the duplex when polarized at potentials either higher than the iron oxide flat band potential or lower than this potential. According to these models the space charge of the iron oxide controls the capacitance response in the anodic potential region and the p-side of the n–p junction controls the capacitance response in the cathodic potential region.
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thick) on iron obtained in borate Fig. 8. Current–potential curves of sputtered chromium oxide film (50 A buffer solution (curve (1) as deposited, curve (2) after thermal treatment). The inset represents photo chromium oxide on iron (curve 1) and annealed (curve 2) current–voltage curves for sputtered 50 A (photocurrent obtained at 350 nm, 2 mV s1 ).
4. Discussion 4.1. Oxide dissolution processes In the study of the solid state properties of the sputtered films by electrochemical methods one have to take into account that some oxide dissolution occurs during polarization. Dissolution processes depend on the nature of the solution but also on the solid-state properties of the oxide. Their predominantly ionic or covalent conductivity as well as their stoichiometric and crystallographic parameters can be of considerable importance [20]. For a p-type semiconductor, increasing cation dissolution occurs by polarization at potentials lower than the flat band potential (anodic decomposition). Degeneracy,
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iron oxide film over 500 A chromium oxide film sputtered onto Fig. 9. (a) Mott–Schottky plots of a 500 A tantalum and etched in 0.01 M sulphuric acid solution for different periods of time; (b) schematic band structure models which represent the film–electrolyte interface at potentials below and above the flat band potential.
promoted by the accumulation of holes, occurs at potentials higher than this potential, i.e., in the transpassive potential region of chromium oxides. For an n-type semiconductor, cation dissolution occurs by polarization at potentials higher than the flat band potential (cathodic decomposition). In the case of the iron oxide the flat band potential is situated at )0.5 V/SCE and degeneracy occurs when the Fermi level reaches the conduction band energy by cathodic polarization.
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The situation, which may be controversial, is that of the capacitance measurements made in the cathodic potential region where the dissolution current of the iron oxide may be relatively high, i.e., at potentials lower than the flat band potential of the iron oxide ()0.5 V). Based on electrochemical concepts, the removal rate for each ion of an oxide electrode is given by [21]: i ¼ nk exp
azF W RT
ð3Þ
in which n is the number of ions/cm2 , a the transfer coefficient, z the ionic charge, k a constant and W the double layer potential. Eq. (3) indicates that a small change in the double layer potential corresponds to a rather large change in the composition of the contacting phase. A corollary of this large change of surface composition with potential is that of a possible change of the capacitance. It is important to think about this capacitance by taking into account the influence of the applied potential on both the double layer potential and the space charge potential in the oxide film. Strong changes in the double layer potential only become important at potentials situated in the cathodic potential region where the hydrogen discharge takes place and the capacitance of the double layer increases with decreasing potential [22]. It is likely that most of the applied potential is developed in the space charge layer in the semiconducting oxides and band structure changes are predominantly responsible for the observed changes in capacitance. 4.1.1. Thermodynamic considerations The Mott–Schottky plots of the sputtered chromium oxides onto Ta appear similar to that of the chromium oxide obtained by direct oxidation of chromium. In both cases the films behave as p-type semiconductors. However, interpretation of the capacitance behaviour of the sputtered thin chromium oxide film onto iron requires consideration of regions of distinct semiconductivity in the films. This may occur if for instance both chromium vacancies and oxygen vacancies exist in the oxide and if close to the surface the number of oxygen vacancies is higher than that of the chromium vacancies. In this case the oxide film can become n-type at the surface with a p–n junction between the surface layer and the p-type bulk. The transport processes across the oxide is regulated by the gradient of electric potential as well as by the gradient of the chemical potential. For a species denoted as i, the chemical potential is given by: li ¼ l0i þ RT ln½Xi
ð4Þ
where li , l0i and [Xi ] are the actual chemical potential, the standard chemical potential and the activity of the species, respectively. Since the electrochemical potential g of each ionic species is constant throughout the system metal–film–electrolyte, the following relationship holds [23]:
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giðsÞ ¼ giðbÞ
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ð5Þ
in which, the subscripts s and b denote surface and bulk, respectively. On the other hand g and l for each species are related by: gi ¼ li þ zi qW
ð6Þ
where zi is the valence of the species, q the elementary charge and W the electric potential at the location of the species in the system. So, taking into account Eqs. (4) and (5), it comes: liðsÞ þ zi qWs ¼ liðbÞ þ zi qWb
ð7Þ
l0i þ RT ln½Xi s þ zi qWs ¼ l0i þ RT ln½xi b þ zi qWb
ð8Þ
which gives RT ln
½Xi s ¼ zi qðWb Ws Þ ½Xi b
ð9Þ
Wb Ws ¼ u is the potential difference between the bulk and the surface, then it follows: ½Xi s ¼ ½Xi b expðzi qu=kT Þ
ð10Þ
Applying this equation to the defect concentrations the following relationship is found for divalent cation vacancies VM (negatively charged zi ¼ 2) and oxygen vacancies V0 (positively charged zi ¼ þ2): 2uq ½VM s ¼ ½VM b exp ð11Þ kT and
½V0 s ¼ ½V0 b exp
þ2uq kT
ð12Þ
Inside the film, ½VM b ¼ ½V0 b and, as a consequence of the Schottky equilibrium, the concentration of the two types of vacancies vary in a symmetrical way [24]. According to capacitance results the transition from n-type to p-type behaviour occurs at potential of )0.2 V (Fig. 6). Thus, it can be assumed that at lower potentials the concentration of oxygen vacancies is higher than that of chromium vacancies and the film behaves as an n-type semiconductor (positive slopes of the Mott–Schottky plots). At potentials higher than )0.2 V, the film behaves as a ptype semiconductor (negative slopes of the Mott–Schottky plots). The thermal treatment does not have a marked influence on the capacitance (Fig. 6a). However, when this behaviour of the chromium oxide film with 500 A treatment is applied to the very thin film of 50 A (Fig. 6b) its capacitance behaviour is strongly modified. The film manifests only n-type semiconductivity at potentials higher than the flat band potential situated at about )0.5 V. Because the latter is the flat band potential of the iron oxide it can be assumed that the thermal treatment
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promotes iron diffusion across the very thin chromium oxide film. This interpretation is also supported by photoelectrochemical results, which can be related to the formation of oxide layers close to the surface with band gap energy close to 2.3 eV, which is the band gap value of the iron oxide. This chemical evolution is illustrated by the current–potential curves obtained in a borate buffer solution of pH 9.2. Indeed, the electrochemical behaviour of the sputtered chromium oxide is similar to that of the passive film formed on chromium. After thermal treatment the chromium oxide becomes more stable at anodic potential, resembling more the behaviour of the iron oxide film. In the solution used the stability of the passive film formed on pure chromium goes down to lower potentials than that of the iron passive film. Fig. 9 represents the capacitance behaviour of a duplex film obtained by sputtering and composed of two oxides i.e. an inner chromium oxide layer and an outer iron oxide layer. Mott–Schottky plots reveal the existence of straight lines of negative and positive slopes separated by a potential situated at about )0.5 V, which corresponds to the flat band potential of the iron oxide for the pH used in this study. These results support the electronic band structure model proposed for passive films thick) formed on stainless steel in borate buffer solution [5]. This model (30 A assumes that the capacitance response of a film composed of an inner chromium oxide layer and an outer iron oxide layer behaves as a p-type and n-type semiconductor at potentials lower and higher than about )0.5 V, respectively. Further, to interpret the evolution of the capacitance when the iron oxide is progressively removed by dissolution in the solution of 0.01 M H2 SO4 , it is useful to introduce the thermodynamic concepts relative to the equilibrium potential between two phases. It is assumed that when two phases are in equilibrium, the potential will not depend on the ratio of the quantities of the phases and remain constant until one of the phases is consumed. In this system the potential is now defined by the ratio of difference between the free enthalpies of formation of each phase to the difference between the degree of oxidation of each phase [24]. 4.2. The properties of the sputtered oxide films as compared to that of passive films When the electrochemical properties of the passive films formed on stainless steels are discussed the fact that the ionic and electronic structure results from reactions at metal/film and film/electrolyte interfaces has to be taken into account. The migration of the various species occurs under the combined influence of the concentration gradient and the electric field. Under an applied potential, the total electric potential results not only from the charge at the surface but also from the space charge contribution. The driving force is now mainly dependent of the electric field, E, developed in this part of the film, which according to the Mott–Schottky approach is given by: 1=2 1=2 2qNq KT E¼ U Ufb ð13Þ e e e0 where Nq is the doping density (donor or acceptor concentrations).
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The particle transport in the case of the sputtered oxide films is essentially electronic. Ionic transport throughout the film must be assumed to be completely inhibited. Because the film is not formed on its parent metal, there is also electronic transfer of charge at the interface metal/film. At the film/electrolyte interface the reaction O2 + 2Hþ fi H2 O is however possible. This means that for both passive films and sputtered films the electrode potential depends in a similar way of the thermodynamic properties of the outermost oxide layer situated at the oxide/electrolyte interface. This explains why the flat band potential of the mixed iron/chromium oxides is always situated at )0.5 V. A characteristic feature of the electronic structure of the passive films formed on stainless steels is the presence of the heterojunction developed inside the film [5,25]. Fig. 10a shows the schematic band structure model proposed to explain the capacitance behaviour of the passive film formed on 18Cr–10Ni stainless steel in the borate buffer solution of pH 9.2. The heterojunction is situated between the p-type chromium oxide inner layer and the n-type iron oxide outer layer. Assuming that the Fermi energy is maintained constant across the film, then the electronic equilibrium is reached when the doping densities of the n-side and p-side of the heterojunction become equivalents (Na Nd ). Such equilibrium is obtained in an ionic transport during film growth. The situation is very different when the heterojunction is formed by sputtered deposition (Fig. 10b). The development of the heterojunction results in this case from the displacement of the Fermi level in order to establish the electronic equilibrium. There is in this case only electronic transfer. Further, the doping density of the chromium and iron constitutive oxides of the heterojunction formed by sputtering may be very different. In fact, as in the case of a classical heterojunction the electronic properties depend on the band gap and the doping densities of the p-type and n-type regions among other factors. If the changes in both sides of the junction resulting from the donor Nd and acceptor Na doping levels are equated one has at equilibrium: Nd Wn ¼ Na Wp
ð14Þ
where Wn and Wp are the lengths of the depletion regions in the n-side and p-side respectively. At this point of the discussion it is important to remark that the Mott– Schottky method used gives direct information concerning only the capacitance of the film–solution interface. However, it can be assumed that the charge concentration at this interface will equilibrate the charge concentration generated at the metal–film interface. This means that the charge concentration inside the film is related to the exchange processes at both interfaces. When the influence of polarization on the electronic structure of passive films and sputtered films is examined, one has to consider that the capacitance response can be related to the capacitance of the film–electrolyte interface and also to that of the n-side or p-side of the heterojunction at potentials higher or lower than the flat band potential, respectively.
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Fig. 10. Schematic band structure models which represent the metal–film and film–electrolyte interface and the p–n heterojunction: (a) case of the passive film formed on Fe–Cr stainless steel and (b) case of the duplex oxide film obtained by sputtering.
In the schematic representation of the variation of the capacitance with potential (C 2 vs. U ) the existence of two important potentials has to be considered (Fig. 11); the flat band potential Ufb situated at )0.5 V where predominant electronic defects establish the transition between p-type and n-type semiconductivity, and the
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Fig. 11. Schematic representation of the electronic structure of the interface film–electrolyte at potential below and above the flat band potential Ufb . The variation with the distance to the interface film–electrolyte of the electrostatic potential below and above the transition potential Uo is also indicated.
potential situated at about )0.2 V where the transition from positive to negative slopes can be related to the change in the predominant ionic defects.
5. Conclusions The present investigation shows that the electronic structure of chromium oxide films obtained by sputtering on an iron-containing substrate may be strongly modified close to the surface where the neutrality of charges is not maintained. It appears that the capacitance behaviour (Mott–Schottky representation) of this kind of films is qualitatively similar to that of passive films formed by direct oxidation of Fe–Cr alloys or stainless steels. In both cases the capacitance study reveals the existence of p-type and n-type semiconductivity. Annealing treatment of the films raises the n-type semiconductor character. The effect of this treatment could be two folds. It could cause the iron, used as a support, to diffuse into the chromium oxide and it could also cause the chromium oxide to become n-type semiconductor in the outer most atomic layers of the film.
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The electronic band structure model proposed previously for passive films formed on stainless steels can describe the results. Furthermore, the potential regions situated between Mott–Schottky plots of negative and positive slopes can be interpreted as transitions potential regions where either electronic or ionic predominant transport of charges takes place. Finally the use of a duplex film composed of a chromium oxide inner layer and an iron oxide outer layer gives valuable information about the heterojunction formed in the bulk of the film. By examining the electronic properties of the sputtered films, information can be obtained relating to the electronic properties of passive films formed by direct oxidation of the metallic substrate.
Acknowledgements The authors would like to thank PRAXIS for proving funds for Dr. G. Goodlet. Also the authors would like to express their gratitude to Dr. M.L. Param^es for her help with the X-Ray diffraction technique and to Dr. P. Brogueira for useful discussion.
References [1] C. Sunseri, S. Piazza, F. Di Quarto, J. Electrochem. Soc. 137 (1990) 2411. [2] J.-S. Kim, H.-S. Kwon, in: The Eighth International Symposium on Passivity of Metals and Semiconductors, Canada, 1998. [3] S. Fujimoto, O. Chihara, T. Shibata, Materials Science Forum 289–292 (1998) 989. [4] P.C. Searson, R.M. Latanision, Electrochim. Acta 35 (1990) 445. [5] N.E. Hakiki, S. Boundin, B. Rondot, M. Da Cunha Belo, Corros. Sci. 37 (1995) 1809. [6] N.E. Hakiki, B. Rondot, M. Da Cunha Belo, C. R. Acad. Sci. Ser. II, Paris 317 (1993) 457. [7] S. Cardoso, V. Gehanno, R. Ferreira, P.P. Freitas, IEEE Trans. Magn. 35 (1999) 2952. [8] M. Fraune, U. Rudiger, G. Guntherodt, S. Cardoso, P.P. Freitas, Appl. Phys. Lett. 77 (2000) 3815. [9] P. Marcus, V. Maurice, W.P. Young, J. Electrochem. Soc. 141 (1994) 3017. [10] W.W. G€ artner, Physc. Rev. 116 (1959) 84. [11] M.A. Bulter, J. Appl. Phys. 48 (1977) 1914. [12] N.F. Mott, E.A. Davies, Electronic Processes in Non-Crystalline Materials, Clarendon, Oxford, 1971. [13] S.R. Morrisson, Electrochemistry at Semiconductor and Oxidized Metal Electrodes, Plenum Press, London, 1980 (Chapter 4). [14] B. Lovrecek, J. Sefaja, Electrochim. Acta 17 (1972) 1151. [15] N.E. Hakiki, M. Da Cunha Belo, C. R. Acad. Sci. Ser. II, Paris T320 (1995) 463. [16] H. Bohni, P. Schmuki, S. Virtanen, P. Vuoristo, T. Mantyla, J. Electrochem. Soc. 142 (1995) 3967. [17] F. Cardon, W.P. Gomes, J. Phys. 11 (1978) L63. [18] A.M.P. Sim~ oes, M.G.S. Ferreira, G. Lorang, M. Da Cunha Belo, Electrochim. Acta 36 (1991) 315. [19] J.H. Kennedy, K.W. Frese, J. Electrochem. Soc. 125 (1978) 723. [20] S.M. Ahamed, in: John W. Diggle (Ed.), Oxides and Oxide Films, vol. I, Marcel Dekker, New York, 1972, p. 319. [21] D.A. Vermilyea, J. Electrochem. Soc. 113 (1966) 1067. [22] L.D. Locker, P.L. de Bruyn, J. Electrochem. Soc. 116 (1969) 1659. [23] F.A. Kroger, The Chemistry of Imperfect Crystals, vol. 2, North-Holland Publishing Company, New York, 1974.
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[24] K.J. Vetter, J. Electrochem. Soc. 110 (1963) 597. [25] N.E. Hakiki, M. Da Cunha Belo, A.M.P. Sim~ oes, M.G.S. Ferreira, J. Electrochem. Soc. 145 (1998) 3821; N.E. Hakiki, M. Da Cunha Belo, A.M.P. Sim~ oes, M.G.S. Ferreira, J. Electrochem. Soc. 146 (1999) 807.
The electronic properties of sputtered chromium and iron oxide films G. Goodlet a, S. Faty a,1, S. Cardoso b, P.P. Freitas b,c, A.M.P. Sim~ oes a, M.G.S. Ferreira a,d,*, M. Da Cunha Belo
a
a
d
Department of Chemical Engineering, Instituto Superior T ecnico, Av. Rovisco Pais, Lisboa 1049-001, Portugal b INESC Microsystems and Nanotechnologies, R. Alves Redol 9, Lisboa 1000 , Portugal c Department of Physics, Instituto Superior Tecnico, Av. Rovisco Pais, Lisboa 1049-001, Portugal Department of Ceramics and Glass Engineering, University of Aveiro, Campus de Santiago, Aveiro 3810-193, Portugal Received 31 March 2003; accepted 16 September 2003
Abstract The semiconducting properties and electrochemical behaviour of thin chromium and iron oxide films produced by sputtering were investigated by capacitance and photoelectrochemical measurements. The films were deposited onto various substrates and submitted in some cases, to a thermal treatment. It appears that sputtered chromium oxide on iron substrate reveals both types of semiconductivity (p and n) and that the thermal treatment enhances the n-type character. Finally, a study is developed using a duplex film formed by sputtered layers of chromium and iron oxides. This system allows for the discussion of the problem of the electronic structure of the heterojunction created by the two kinds of oxides. Comparison is made with the case of passive films on stainless steel. 2003 Elsevier Ltd. All rights reserved. Keywords: A. Sputtered films; B. Capacitance and photoelectrochemistry; C. Passivity
*
Corresponding author. Address: Department of Chemical Engineering, Instituto Superior Tecnico, Av. Rovisco Pais, Lisboa 1049-001, Portugal. Tel.: +351-234370354; fax: +351-218404589. E-mail address: [email protected] (M.G.S. Ferreira). 1 Permanent address: Faculte des Sciences et Techniques, UCAD, Dakar, Senegal. 0010-938X/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2003.09.022
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1. Introduction Although the presence of chromium oxides in the passive films formed on Fe–Cr stainless steels play the major role on corrosion resistance of these materials, the electrochemical properties of the films are also strongly influenced by the presence of iron oxides. There are in fact, throughout the film thickness, regions of approximately pure chromium oxide, pure iron oxide and transition regions of mixed oxides. Chromium oxides are generally considered to have p-type behaviour [1]. Possible doping species are cationic and anionic vacancies or cationic interstitials. When chromium interstitials or oxygen vacancies are the predominating defects, the chromium oxide is an n-type semiconductor and when chromium vacancies are the predominating defects the chromium oxide is a p-type semiconductor. For the electronic characterization of chromium oxides, no well defined values of the band gap exist, but instead several values have been reported and assumed to be related to the different degrees of hydration [2–4]. On the other hand, because chromium and iron oxides reveal distinct semiconducting properties, an important characteristic of the electronic structure of passive films on stainless steels is the development of an interface, inside the film, with a distribution of charge carriers similar to that of a p–n heterojunction [5,6]. Simultaneous ionic and electronic transport and high doping levels make the study of the passive films very complex. The study of well-defined oxide layers could help in interpreting results concerning passivation. In this work the electronic properties of chromium and iron oxide films produced by sputtering were investigated in borate buffer solution. The central part of the research concerns the possible effects of the applied potential on the electronic structure of the n–p junction formed by sputtered chromium and iron oxide layers. In this junction only electronic processes take place when submitted to external polarization. In fact, ionic transport throughout the junction during electrochemical and photoelectrochemical measurements can be excluded. Only dissolution processes at film-electrolyte interface may occur. The methods used in this study are the photocurrent spectroscopy in order to obtain band gap values and the Mott–Schottky approach based on capacitance measurements. Both methods have been used in semiconductor electrochemistry and can also be considered as good tools for characterization of semiconducting passive films. Capacitance measurements have been carried out in the present investigation using the frequency values equivalent to that used in previous research on passive films. This procedure helps in making the results comparable.
2. Experimental Chromium and iron oxide films were sputtered onto various substrates (glassy carbon, tantalum and iron). They were fabricated by reactive ion beam sputtering from Cr and Fe metallic targets, respectively, in the presence of a mixed Ar–O2 beam coming from the 20 cm diameter assist gun (15.02% O2 in Ar). A dual source
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Nordiko 3000 ion beam system was used [7,8]. The substrate holder was water cooled. The deposition source was running with the standard conditions for the metallic film deposition (acceleration voltage of +1450 V, deceleration voltage of )300 V, 33 mA Xe beam, 1 sccm Xe, 100 W RF plasma), while an Ar-O2 beam was created by accelerating the plasma inside the assist gun with +200 V (18 mA beam, 10 sccm Ar–O2 , 60W RF plasma) and using a deceleration voltage of )100 V. A deficient content of O2 (low acceleration voltage) would cause the film to be metallic, instead of oxide. Also, too high acceleration voltages will reduce the deposition rate, since the deposited material would be sputtered by the Ar–O2 beam, instead of reacting with metallic ions coming from the target. The pressure during the process is 1.2 · 104 Torr. A filamentless neutraliser (40 mA, 3 sccm Ar– O2 ) beside to assist the ion source avoids surface charging during the dielectric deposition. During the oxide deposition the substrate table is at 80, and rotates at 15 rpm. A thermal treatment at 300 C for 8 h, was applied to some of the films. The film on pure chromium and pure iron (Goodfellow 99.9%) were grown at 450 C for 2 h in a furnace. Prior to growth the chromium was polished with silicon carbide paper of various grades and finished with 1 lm aluminium oxide. Structural information on the films was obtained by glancing XRD measurements, using a Siemens X-ray D 500 diffractometer. Atomic force microscopy (AFM) was carried out by using a Topometrix 2010 ‘‘Discover’’ working in non-contact mode with a silicon nitride tip. In the photoelectrochemical study the working electrode was illuminated through a quartz window with monochromatic light (beam area ¼ 0.12 cm2 ). The optical instrumentation consisted of a 150 W Xe lamp, a 250 mm f18 monochromator, a stepper motor to control the wavelength and a mechanical chopper. The grating was changed at 340 nm for maximum flux and a filter was used above 450 nm to eliminate any second order effects. All photocurrent measurements were normalized with respect to the flux recorded by a silicon photodiode. The photocurrent was obtained by connecting the current output of the potentiostat (EG&G 273) to a lock-in amplifier (Brookdeal EG&G 5210) and recording the voltage output due to the signal at 19 Hz (the chopping frequency). The recorded values were then worked out to calculate the photocurrent and the quantum efficiency values using a spreadsheet software package. For the capacitance–potential measurements the lock-in was used to produce a sinusoidal signal, 10 mV peak-to-peak at 1 kHz, which was superimposed on the potential applied by the potentiostat. The output of the potentiostat was then fed to the lock-in. A three-electrode cell was used, consisting of a Pt wire counter electrode, a saturated calomel reference electrode and the oxide film on the substrate was the working electrode. All measurements were carried out in sodium borate/boric acid buffer solution (pH 9.1). The effect of pH was observed by using 0.5 M sulfuric acid and a 0.01 M solution of the same acid was prepared for the profiling experiments. All the reagents were of an analytical grade.
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3. Results 3.1. AFM observations and XRD measurements The AFM images obtained with sputtered iron and chromium oxide films on tantalum are shown in Fig. 1. Both films can be considered compact but the roughness morphology of the iron oxide film (Fig. 1a) is very different from that of the chromium oxide film (Fig. 1b). It is known that the AFM technique allows us to observe surface morphology but does not give, in the experimental conditions used, direct information about the crystallographic structure. Nevertheless, nucleation aspects of the film growth may relate to a larger grain size of the chromium oxide relatively to that of the iron oxide. This analysis is supported by XRD results, which show that the annealed chromium oxide (8 h at 300 C) depicted any Cr2 O3 peaks, where a peak corresponding to the (1 1 0) plane was observed. Although the other films appeared to be amorphous or nanocrystalline due to a lack of peaks, the film sputtered onto Ta displayed an increased response in the spectrum of the 1000 A film. This response may be caused by nano30–40 region compared to the 50 A crystalline material. There is, thus, evidence to suggest that the films are not completely amorphous. Marcus et al. [9] have reported the presence of nanocrystallites
on tantalum. Fig. 1. AFM images of sputtered: (a) chromium and (b) iron oxide films of 500 A
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(<3 nm) of Cr2 O3 -like oxide anodically grown on chromium. Therefore some shortrange order can be considered within these films.
3.2. Current–potential curves Fig. 2 allow us to compare the dc electrochemical behaviour of the sputtered films (Fig. 2a) to that of anodic passive films formed on chromium and on iron (Fig. 2b). In both cases the iron oxides are more stable than the chromium oxides in the more anodic potential region. Conversely, the passive films on chromium and sputtered chromium oxide are more stable at lower potentials. The question is, which processes are associated with the rise in cathodic and anodic current. In spite of the fact that, from thermodynamic considerations, the passivation potential of chromium is )1.09 V/SCE (at pH 8.2) the cathodic currents for chromium increases at potentials that are more positive than most of the values proposed for the equilibrium potential of passivation. However, at these potentials the chromium oxide cannot be reduced. The only thermodynamic feasible process is the reduction of the iron oxide and that of hydrogen evolution.
and (b) Fig. 2. Current–potential curves: (a) for sputtered chromium and iron oxides (thickness of 500 A) for passive films formed on chromium and iron.
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3.3. Photocurrent measurements on chromium oxides The fundamental optical transitions (band gap) were determined from work produced by G€ artner [10] and for a semiconductor/electrolyte junction by Bulter [11]. The photoresponse near the band gap can be described by: g¼
Iph ðhm Eg Þ ¼ qAW U hm
n
ð1Þ
where g is the quantum efficiency, Iph is the photocurrent, U is the photon flux of energy hm, q is the electronic charge, A is a constant, W is the thickness of the space charge layer and Eg the band gap energy. The value of n depends on the type of the transition between the valence band and the conduction band. The analysis of the photocurrent spectra for passive films shows that n ¼ 2 is the most appropriate value to represent the photoelectrochemical behaviour observed. This value corresponds to indirect transitions in the crystalline band model or non-direct transitions in amorphous films [12]. The optical band gap determined for the films formed under different conditions (Fig. 3) ranges from 2.7 to 2.9 eV, and can be associated with the chromium oxide films. In the literature values between 2.4 and 3.55 eV have been reported [1–4]. 3.4. Capacitance–potential measurements The properties of a semiconductor/electrolyte interface can be described by Mott– Schottky theory [13], according to which the space charge capacitance of a p-type or n-type semiconductor is given by:
Fig. 3. Photocurrent as a function of the energy of the incident light for different chromium oxide films thick oxide film sputtered on polarized at 0.1 V (1) oxide film air grown at 450 C on chromium; (2) 1000 A thick oxide film sputtered on tantalum. tantalum; (3) 50 A
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1 2 kT ¼ U U fb 2 qNq ee0 e CSC
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ð2Þ
where e is the relative permittivity of the oxide (12 for the bulk Cr2 O3 [14]), e0 is the vacuum permittivity, q is the elementary charge (+e for electrons and )e for holes), Nq is the acceptor or donor concentration, U is the applied potential and Ufb is the flat band potential. The technique is based on the fact that in the case of a Schottky contact the space charge layer gives rise to a space charge capacitance, CSC to which corresponds an 1 effective complex impedance Rþ ðixCSC Þ . Both the ohmic parts of R and the capacitance CSC can be determined by superimposing an ac voltage of frequency x on a dc bias applied across the junction. Lock-in detection at frequency x allows the separation of CSC and R. Theoretically, in a highly doped oxide film in contact with an electrolyte the classic Mott–Schottky relationship must be modified, because the space charge capacitance and charge density cannot be calculated on the basis of the Boltzmann function. However, it has been demonstrated [15] that even using a more general Fermi distribution a linear relationship between C 2 and the applied potential is always found whatever the position of the Fermi level and of the doping donor level are. Consequently, highly doped and degenerate semiconductors may by characterized by the Mott–Schottky method. 3.4.1. Capacitance of the chromium oxide Fig. 4 shows capacitance–potential measurements in the form of Mott–Schottky plots for different chromium oxide films. In each plot there is a linear region with a negative slope, which indicates that the films reveal p-type semiconductivity. These films formed on metallic substrates exhibit flat band potentials that were in agree film sputtered onto Ta and the air grown ment with previous results [5]. The 1000 A
Fig. 4. Mott–Schottky plots obtained with chromium oxide films (1) formed on chromium at 450 C for 2 thick (3) 50 A thick. h; sputtered on tantalum (2) 1000 A
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Table 1 The effect of pH on the electronic properties of chromium oxide films Film type
pH
Band gap/eV
Ufb vs. SCE/V
Na 1019 atoms/cm3
Air grown
9.2 0.6
2.87 2.94
0.46 0.95
13 31
CrOx on Ta 1000 A
9.2 0.6
2.76 2.84
0.54 1.25
6 3
film showed an increase in capacitance (decrease in 1=C 2 ) for potentials below )0.5 V. Particularly relevant are the high doping densities, especially those exhibited by sputtered films. According to this high doping the thickness of the space the 50 A charge region extends across a relatively short distance into the film. For example, film on Ta, the doping density leads to a calculated space charge for the 1000 A The doping densities obtained in this work are approximately one thickness of 40 A. order of magnitude below those reported by B€ ohni [16]. Table 1 summarizes the results of measurements made in both alkaline basic (borate buffer) and acidic (0.5 M sulphuric acid) conditions for the air grown film and a sputtered film. The band gaps are practically unchanged whereas the flat band potentials change practically in accordance with the Nernst equation (approximately 0.060 V per pH unit). It should be noted that the straight lines in the Mott–Schottky representation are relatively short. This can be interpreted as a consequence of the low thickness of the films. Note that, highly doped bulk crystalline semiconductors also exhibit the same feature [17]. 3.4.2. Capacitance of the iron oxide In the case of the iron oxide film sputtered on tantalum (Fig. 5a) the Mott– Schottky plot displays a positive slope in the region from 0 to )0.5 V vs. SCE, in the same way as the film formed on pure iron by air oxidation (Fig. 5b). This can be attributed to the n-semiconductive behaviour of Fe2 O3 . The capacitance behaviour of iron oxide films sputtered onto the various substrates was also similar. The doping densities were all very high (1021 cm3 ) and the photocurrent was much lower than that in air grown films on pure iron and on stainless steel. In the XRD analysis only iron oxide film on silicon displayed any peaks and these could not be the 500 A clearly identified as Fe2 O3 or Fe3 O4 . The change in slope of the C 2 ¼ f ðU Þ plot observed at about 0 V (Fig. 5) may be explained assuming that a deep donor level is localized in the octahedral sites of the spinel oxide [18,19]. 3.4.3. Chromium oxide sputtered on iron substrate (effect of annealing) The capacitance behaviour of both thermally and anodically grown films (passive films) on stainless steel gives Mott–Schottky plots with positive (n-type) and negative (p-type) slopes [5]. This same type of behaviour is observed in as-deposited sputtered
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sputtered iron oxide film on tantalum and (b) oxide film formed Fig. 5. Mott–Schottky plots of: (a) 500 A on pure iron at 450 C for 2 h.
chromium oxide films on pure iron (Fig. 6). Such behaviour was not observed in the case of sputtered chromium oxide film on glassy carbon. The effect of the heat sputtered film was minor (Fig. treatment on the Mott–Schottky plot of the 500 A 6a). The n-type character ()0.6 to )0.25 V vs. SCE) did not show any enhancement, the linear region in the plot is the same and even the capacitance values are only slightly changed. However, when a similar procedure was implemented on the 50 A chromium oxide film, the Mott–Schottky plot changed radically (Fig. 6b). The overall capacitance changed, the n-type region became wider, extending from 0.5 to about )0.5 V, and the p-type region below )0.5 V practically vanished. The photocurrent response (at 0.1 V vs. SCE) also changed markedly with the thermal treatment (Fig. 7). The as-deposited film showed a response similar to stainless steel and iron oxide [5], although tailing obscured a definite single band gap. After annealing the response became shifted to lower photon energies (longer wavelengths), with Eg ¼ 2:0 eV, a value characteristic of iron oxides. Accompanying this shift there was a change in the current and in the photocurrent, specially for the film. Fig. 8 shows (curve 1) that the current–potential curve is relatively similar 50 A to that of passive chromium (Fig. 2b), whereas the annealed film (curve 2) was almost identical to that of the oxide passive film formed on pure iron (Fig. 2b). The photocurrent response (inset of Fig. 8) also suggests that the surfaces of the films
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Fig. 6. Mott–Schottky plots of chromium oxide films sputtered on iron (1) as deposited and (2) after thick (b) 50 A thick. heating at 300 C for 8 h. (a) 500 A;
behave as chromium oxide and iron oxide-like for the as-deposited and annealed films, respectively. 3.4.4. Chemical profiling on duplex film formed by chromium and iron sputtered oxides Chemical profiling was used to assess the way in which the Mott–Schottky plots change when the surface layer of iron oxide was removed. This technique is based on the commonly used doping density profiling technique used in the semiconductor iron oxide on 500 A chromium oxide sputtered onto Ta industry. A sample of 500 A was immersed in a solution 1 M H2 SO4 . Capacitance-potential measurements were then obtained at several time intervals (Fig. 9a) and show a transition. Immediately after immersion there is a sharp increase of 1=C 2 in the region below )0.5 V. As the chemical etching proceeds, the positive slope (characteristic of n-type semi-
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Fig. 7. Photocurrent as a function of the energy of the incident light of sputtered chromium oxide (1) as thick (b) 50 A thick. deposited and (2) thermally treated. (a) 500 A
conductivity) disappears and a negative slope appears instead. This response is similar to that observed for sputtered chromium oxide films on iron. The point of inflection at about )0.2 V gradually vanished with time, which suggests that there is no abrupt change from one material to the other, but that some inter-mixing occurs. It is important to note that in the duplex film, iron oxide is placed in degeneracy condition at potentials more negative than the flat band of this oxide. This means that the capacity of the iron oxide layer is similar to that of a metallic conductor. Thus, the low capacity observed in capacitance measurements must be interpreted as being due to the inner chromium oxide layer of the duplex. Fig. 9b shows a schematic band structure model of the duplex when polarized at potentials either higher than the iron oxide flat band potential or lower than this potential. According to these models the space charge of the iron oxide controls the capacitance response in the anodic potential region and the p-side of the n–p junction controls the capacitance response in the cathodic potential region.
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thick) on iron obtained in borate Fig. 8. Current–potential curves of sputtered chromium oxide film (50 A buffer solution (curve (1) as deposited, curve (2) after thermal treatment). The inset represents photo chromium oxide on iron (curve 1) and annealed (curve 2) current–voltage curves for sputtered 50 A (photocurrent obtained at 350 nm, 2 mV s1 ).
4. Discussion 4.1. Oxide dissolution processes In the study of the solid state properties of the sputtered films by electrochemical methods one have to take into account that some oxide dissolution occurs during polarization. Dissolution processes depend on the nature of the solution but also on the solid-state properties of the oxide. Their predominantly ionic or covalent conductivity as well as their stoichiometric and crystallographic parameters can be of considerable importance [20]. For a p-type semiconductor, increasing cation dissolution occurs by polarization at potentials lower than the flat band potential (anodic decomposition). Degeneracy,
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iron oxide film over 500 A chromium oxide film sputtered onto Fig. 9. (a) Mott–Schottky plots of a 500 A tantalum and etched in 0.01 M sulphuric acid solution for different periods of time; (b) schematic band structure models which represent the film–electrolyte interface at potentials below and above the flat band potential.
promoted by the accumulation of holes, occurs at potentials higher than this potential, i.e., in the transpassive potential region of chromium oxides. For an n-type semiconductor, cation dissolution occurs by polarization at potentials higher than the flat band potential (cathodic decomposition). In the case of the iron oxide the flat band potential is situated at )0.5 V/SCE and degeneracy occurs when the Fermi level reaches the conduction band energy by cathodic polarization.
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The situation, which may be controversial, is that of the capacitance measurements made in the cathodic potential region where the dissolution current of the iron oxide may be relatively high, i.e., at potentials lower than the flat band potential of the iron oxide ()0.5 V). Based on electrochemical concepts, the removal rate for each ion of an oxide electrode is given by [21]: i ¼ nk exp
azF W RT
ð3Þ
in which n is the number of ions/cm2 , a the transfer coefficient, z the ionic charge, k a constant and W the double layer potential. Eq. (3) indicates that a small change in the double layer potential corresponds to a rather large change in the composition of the contacting phase. A corollary of this large change of surface composition with potential is that of a possible change of the capacitance. It is important to think about this capacitance by taking into account the influence of the applied potential on both the double layer potential and the space charge potential in the oxide film. Strong changes in the double layer potential only become important at potentials situated in the cathodic potential region where the hydrogen discharge takes place and the capacitance of the double layer increases with decreasing potential [22]. It is likely that most of the applied potential is developed in the space charge layer in the semiconducting oxides and band structure changes are predominantly responsible for the observed changes in capacitance. 4.1.1. Thermodynamic considerations The Mott–Schottky plots of the sputtered chromium oxides onto Ta appear similar to that of the chromium oxide obtained by direct oxidation of chromium. In both cases the films behave as p-type semiconductors. However, interpretation of the capacitance behaviour of the sputtered thin chromium oxide film onto iron requires consideration of regions of distinct semiconductivity in the films. This may occur if for instance both chromium vacancies and oxygen vacancies exist in the oxide and if close to the surface the number of oxygen vacancies is higher than that of the chromium vacancies. In this case the oxide film can become n-type at the surface with a p–n junction between the surface layer and the p-type bulk. The transport processes across the oxide is regulated by the gradient of electric potential as well as by the gradient of the chemical potential. For a species denoted as i, the chemical potential is given by: li ¼ l0i þ RT ln½Xi
ð4Þ
where li , l0i and [Xi ] are the actual chemical potential, the standard chemical potential and the activity of the species, respectively. Since the electrochemical potential g of each ionic species is constant throughout the system metal–film–electrolyte, the following relationship holds [23]:
G. Goodlet et al. / Corrosion Science 46 (2004) 1479–1499
giðsÞ ¼ giðbÞ
1493
ð5Þ
in which, the subscripts s and b denote surface and bulk, respectively. On the other hand g and l for each species are related by: gi ¼ li þ zi qW
ð6Þ
where zi is the valence of the species, q the elementary charge and W the electric potential at the location of the species in the system. So, taking into account Eqs. (4) and (5), it comes: liðsÞ þ zi qWs ¼ liðbÞ þ zi qWb
ð7Þ
l0i þ RT ln½Xi s þ zi qWs ¼ l0i þ RT ln½xi b þ zi qWb
ð8Þ
which gives RT ln
½Xi s ¼ zi qðWb Ws Þ ½Xi b
ð9Þ
Wb Ws ¼ u is the potential difference between the bulk and the surface, then it follows: ½Xi s ¼ ½Xi b expðzi qu=kT Þ
ð10Þ
Applying this equation to the defect concentrations the following relationship is found for divalent cation vacancies VM (negatively charged zi ¼ 2) and oxygen vacancies V0 (positively charged zi ¼ þ2): 2uq ½VM s ¼ ½VM b exp ð11Þ kT and
½V0 s ¼ ½V0 b exp
þ2uq kT
ð12Þ
Inside the film, ½VM b ¼ ½V0 b and, as a consequence of the Schottky equilibrium, the concentration of the two types of vacancies vary in a symmetrical way [24]. According to capacitance results the transition from n-type to p-type behaviour occurs at potential of )0.2 V (Fig. 6). Thus, it can be assumed that at lower potentials the concentration of oxygen vacancies is higher than that of chromium vacancies and the film behaves as an n-type semiconductor (positive slopes of the Mott–Schottky plots). At potentials higher than )0.2 V, the film behaves as a ptype semiconductor (negative slopes of the Mott–Schottky plots). The thermal treatment does not have a marked influence on the capacitance (Fig. 6a). However, when this behaviour of the chromium oxide film with 500 A treatment is applied to the very thin film of 50 A (Fig. 6b) its capacitance behaviour is strongly modified. The film manifests only n-type semiconductivity at potentials higher than the flat band potential situated at about )0.5 V. Because the latter is the flat band potential of the iron oxide it can be assumed that the thermal treatment
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promotes iron diffusion across the very thin chromium oxide film. This interpretation is also supported by photoelectrochemical results, which can be related to the formation of oxide layers close to the surface with band gap energy close to 2.3 eV, which is the band gap value of the iron oxide. This chemical evolution is illustrated by the current–potential curves obtained in a borate buffer solution of pH 9.2. Indeed, the electrochemical behaviour of the sputtered chromium oxide is similar to that of the passive film formed on chromium. After thermal treatment the chromium oxide becomes more stable at anodic potential, resembling more the behaviour of the iron oxide film. In the solution used the stability of the passive film formed on pure chromium goes down to lower potentials than that of the iron passive film. Fig. 9 represents the capacitance behaviour of a duplex film obtained by sputtering and composed of two oxides i.e. an inner chromium oxide layer and an outer iron oxide layer. Mott–Schottky plots reveal the existence of straight lines of negative and positive slopes separated by a potential situated at about )0.5 V, which corresponds to the flat band potential of the iron oxide for the pH used in this study. These results support the electronic band structure model proposed for passive films thick) formed on stainless steel in borate buffer solution [5]. This model (30 A assumes that the capacitance response of a film composed of an inner chromium oxide layer and an outer iron oxide layer behaves as a p-type and n-type semiconductor at potentials lower and higher than about )0.5 V, respectively. Further, to interpret the evolution of the capacitance when the iron oxide is progressively removed by dissolution in the solution of 0.01 M H2 SO4 , it is useful to introduce the thermodynamic concepts relative to the equilibrium potential between two phases. It is assumed that when two phases are in equilibrium, the potential will not depend on the ratio of the quantities of the phases and remain constant until one of the phases is consumed. In this system the potential is now defined by the ratio of difference between the free enthalpies of formation of each phase to the difference between the degree of oxidation of each phase [24]. 4.2. The properties of the sputtered oxide films as compared to that of passive films When the electrochemical properties of the passive films formed on stainless steels are discussed the fact that the ionic and electronic structure results from reactions at metal/film and film/electrolyte interfaces has to be taken into account. The migration of the various species occurs under the combined influence of the concentration gradient and the electric field. Under an applied potential, the total electric potential results not only from the charge at the surface but also from the space charge contribution. The driving force is now mainly dependent of the electric field, E, developed in this part of the film, which according to the Mott–Schottky approach is given by: 1=2 1=2 2qNq KT E¼ U Ufb ð13Þ e e e0 where Nq is the doping density (donor or acceptor concentrations).
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The particle transport in the case of the sputtered oxide films is essentially electronic. Ionic transport throughout the film must be assumed to be completely inhibited. Because the film is not formed on its parent metal, there is also electronic transfer of charge at the interface metal/film. At the film/electrolyte interface the reaction O2 + 2Hþ fi H2 O is however possible. This means that for both passive films and sputtered films the electrode potential depends in a similar way of the thermodynamic properties of the outermost oxide layer situated at the oxide/electrolyte interface. This explains why the flat band potential of the mixed iron/chromium oxides is always situated at )0.5 V. A characteristic feature of the electronic structure of the passive films formed on stainless steels is the presence of the heterojunction developed inside the film [5,25]. Fig. 10a shows the schematic band structure model proposed to explain the capacitance behaviour of the passive film formed on 18Cr–10Ni stainless steel in the borate buffer solution of pH 9.2. The heterojunction is situated between the p-type chromium oxide inner layer and the n-type iron oxide outer layer. Assuming that the Fermi energy is maintained constant across the film, then the electronic equilibrium is reached when the doping densities of the n-side and p-side of the heterojunction become equivalents (Na Nd ). Such equilibrium is obtained in an ionic transport during film growth. The situation is very different when the heterojunction is formed by sputtered deposition (Fig. 10b). The development of the heterojunction results in this case from the displacement of the Fermi level in order to establish the electronic equilibrium. There is in this case only electronic transfer. Further, the doping density of the chromium and iron constitutive oxides of the heterojunction formed by sputtering may be very different. In fact, as in the case of a classical heterojunction the electronic properties depend on the band gap and the doping densities of the p-type and n-type regions among other factors. If the changes in both sides of the junction resulting from the donor Nd and acceptor Na doping levels are equated one has at equilibrium: Nd Wn ¼ Na Wp
ð14Þ
where Wn and Wp are the lengths of the depletion regions in the n-side and p-side respectively. At this point of the discussion it is important to remark that the Mott– Schottky method used gives direct information concerning only the capacitance of the film–solution interface. However, it can be assumed that the charge concentration at this interface will equilibrate the charge concentration generated at the metal–film interface. This means that the charge concentration inside the film is related to the exchange processes at both interfaces. When the influence of polarization on the electronic structure of passive films and sputtered films is examined, one has to consider that the capacitance response can be related to the capacitance of the film–electrolyte interface and also to that of the n-side or p-side of the heterojunction at potentials higher or lower than the flat band potential, respectively.
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Fig. 10. Schematic band structure models which represent the metal–film and film–electrolyte interface and the p–n heterojunction: (a) case of the passive film formed on Fe–Cr stainless steel and (b) case of the duplex oxide film obtained by sputtering.
In the schematic representation of the variation of the capacitance with potential (C 2 vs. U ) the existence of two important potentials has to be considered (Fig. 11); the flat band potential Ufb situated at )0.5 V where predominant electronic defects establish the transition between p-type and n-type semiconductivity, and the
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Fig. 11. Schematic representation of the electronic structure of the interface film–electrolyte at potential below and above the flat band potential Ufb . The variation with the distance to the interface film–electrolyte of the electrostatic potential below and above the transition potential Uo is also indicated.
potential situated at about )0.2 V where the transition from positive to negative slopes can be related to the change in the predominant ionic defects.
5. Conclusions The present investigation shows that the electronic structure of chromium oxide films obtained by sputtering on an iron-containing substrate may be strongly modified close to the surface where the neutrality of charges is not maintained. It appears that the capacitance behaviour (Mott–Schottky representation) of this kind of films is qualitatively similar to that of passive films formed by direct oxidation of Fe–Cr alloys or stainless steels. In both cases the capacitance study reveals the existence of p-type and n-type semiconductivity. Annealing treatment of the films raises the n-type semiconductor character. The effect of this treatment could be two folds. It could cause the iron, used as a support, to diffuse into the chromium oxide and it could also cause the chromium oxide to become n-type semiconductor in the outer most atomic layers of the film.
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The electronic band structure model proposed previously for passive films formed on stainless steels can describe the results. Furthermore, the potential regions situated between Mott–Schottky plots of negative and positive slopes can be interpreted as transitions potential regions where either electronic or ionic predominant transport of charges takes place. Finally the use of a duplex film composed of a chromium oxide inner layer and an iron oxide outer layer gives valuable information about the heterojunction formed in the bulk of the film. By examining the electronic properties of the sputtered films, information can be obtained relating to the electronic properties of passive films formed by direct oxidation of the metallic substrate.
Acknowledgements The authors would like to thank PRAXIS for proving funds for Dr. G. Goodlet. Also the authors would like to express their gratitude to Dr. M.L. Param^es for her help with the X-Ray diffraction technique and to Dr. P. Brogueira for useful discussion.
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