Chromium-Doped MgO Thin Films: Morphology, Electronic Structure

Article pubs.acs.org/JPCC

Chromium-Doped MgO Thin Films: Morphology, Electronic Structure, and Segregation Effects Stefania Benedetti,*,† Niklas Nilius,‡ and Sergio Valeri†,§ †

CNR, Istituto Nanoscienze, Via G. Campi 213/a, 41125 Modena, Italy Carl von Ossietzky Universität Oldenburg, Institut für Physik, D-26111 Oldenburg, Germany § Dipartimento di Scienze Fisiche, Informatiche e Matematiche, Università di Modena e Reggio Emilia, via Campi 213/a, 41125 Modena, Italy ‡

ABSTRACT: Incorporation of Cr into a crystalline MgO(001) thin film has been investigated by means of scanning tunneling microscopy, X-ray photoelectron spectroscopy, and diffraction. For this purpose, samples of different Cr content (0−33 at %) and postannealing treatments (300−1050 K) have been analyzed. The Cr impurities mainly adopt a 3+ oxidation state, which renders the formation of compensating Mg vacancies necessary to maintain charge neutrality. Only at 33 at % doping amount are Cr6+ species detected in the as-grown films, indicating the development of a metastable CrO3 phase. At low doping level, the Cr ions fully dissolve in the MgO lattice, while segregation toward the surface is observed for Cr-rich films. In the latter case, a new surface oxide develops that is characterized by a lower binding energy of the Mg 1s and Cr 2p core levels and a (2 × 1) superstructure with respect to weakly doped MgO. In combination with a distinct 4-fold symmetric XPD pattern, the new surface phase resembles Mg chromite (MgCr2O4), growing on top of the heavily doped MgO film. Our study provides insight into the technologically relevant transition region between doped binary and ternary oxides.

1. INTRODUCTION

Even a magnetic response might be generated if the dopants energy levels are populated by unpaired electrons.11 Despite this relevance in key technologies, ranging from heterogeneous catalysis, microelectronics, to optics and photovoltaics, systematic studies of doped oxides are scarce. This is particularly true when it comes to experiments performed with high energy and spatial resolution on well-defined crystalline materials. The main goal of such experiments is to determine the lattice position of the impurities as well as their charge state with respect to the native ions.12,13 Moreover, the interplay between dopants and intrinsic lattice defects is in the focus of research, as aliovalent impurities tend to get neutralized by charge-compensating vacancies.14,15 Typical neutralizing defects are oxygen and cationic vacancies for low- and high-valence dopants, respectively. Although the overall compensation mechanism is well understood, the geometric correlation and the formation energy of the respective defect pairs are often only known from theoretical studies, while experimental data are missing. Our study focuses on Cr-doped MgO, a model system for the exploration of the optical behavior of doped oxides.16−18 The material is prepared as thin crystalline film on a Mo(001) sample and therefore accessible to conventional surface-science

Doping has shown to be a promising pathway to tune the properties of oxide materials toward the demands of different electronic, optical and chemical applications.1,2 The dopants thereby fulfill a variety of functions. They locally perturb the lattice structure, for instance, by breaking bonds to adjacent atoms.3 As a result, interatomic coupling is weakened and atoms become susceptible to desorption, a relevant process for Mars−van-Krevelen type of reactions.4 The dopants also generate electronic states in the oxide band gap, giving rise to new optical transitions. 5−7 Dopant-related emission or absorption lines are often atomically sharp and tunable via external electric or magnetic fields, which renders them interesting for laser applications. If the dopants adopt charge states different from those of the native ions, redox centers are formed in the oxide matrix, being able to exchange electrons with surface species.1,2 While high-valence dopants may transfer electrons toward adsorbates, reflecting the donor character introduced to the material, low-valence impurities act as electron acceptors. These electron-transfer processes often constitute the initial step for a chemical reaction, for example, for the dissociation or activation of molecular species.8 Finally, doping is a flexible means to tune electrical and magnetic properties of the host oxide.9,10 For impurity states in the vicinity of the band edges, carriers can be thermally excited into the bulk bands to alter the intrinsic conductivity of the material. © 2015 American Chemical Society

Received: September 16, 2015 Revised: October 21, 2015 Published: October 21, 2015 25469

DOI: 10.1021/acs.jpcc.5b09033 J. Phys. Chem. C 2015, 119, 25469−25475

Article

The Journal of Physical Chemistry C techniques without charging effects. While oxidation state and electronic properties of the dopants are determined from X-ray photoelectron spectroscopy (XPS), structure and morphology of the doped oxide are deduced from photoelectron diffraction (XPD) and scanning tunneling microscopy (STM). The evolution of the oxide behavior is systematically studied as a function of dopant concentration and annealing temperature. Our investigation illuminates the transition from weakly doped binary to ternary oxides with the original dopants playing the role of the main cationic species.

2. EXPERIMENTAL SECTION The experiments have been performed in an ultrahigh vacuum chamber (1 × 10−10 mbar base pressure) equipped with an XPS setup comprising a nonmonochromatized Al−Kα source and a hemispherical electrostatic analyzer of 8° acceptance angle, a LEED system, and a room-temperature STM. Binding energies (BEs) were determined via Voigt (O 1s, Mg 1s) or DoniachSunjic (Cr 2p, Mo 3d) fitting after removing a Shirley background. Photoelectron diffraction (XPD) maps were recorded by rotating the sample around the surface normal (azimuthal angle ϕ: 0−90°) and tilting it against the surface normal (polar angle θ: 0−70°). The angular acceptance of the analyzer was reduced to ±1° in this case.19 The samples were prepared by Mg and Cr codeposition in 5 × 10−7 mbar of oxygen at 300 K onto a sputtered and annealed Mo(001) single crystal.20,21 Deposition rates were set to 4 and 0.02 to 0.7 Å/ min for Mg and Cr, respectively. Subsequent annealing to 1050 K was applied to convert the initially amorphous to a crystalline film, displaying a square LEED pattern representative for the MgO(001) termination (Figure 1a). The nominal MgO thickness was set to 30 ML, as determined with a quartz microbalance and by evaluating the thickness-dependent LEED and XPS response of the film. In total, samples with five different Cr concentrations (0, 3, 6, 12, and 33 atom percent at %) have been prepared. Additionally, a 10 ML pristine MgO buffer was inserted below the 30 ML thick MgOCr film to suppress interfacial diffusion processes. To determine the effective Cr concentration and the thickness of Cr-rich surface oxides, we developed an exponential attenuation model using bulk-like XP spectra as reference. The latter were obtained by growing thick MgO and CrOx films on the Mo(001) support, using similar conditions as for the doped films.

Figure 1. Low-energy electron diffraction (E = 90 eV) of (a) weakly and (b) strongly doped MgOCr films annealed to 1050 K. The (2 × 1) superstructure in panel b indicates the formation of a mixed Mg−Cr oxide at the surface of heavily doped films. (c) Two-layer spinel structure (big dots; interfacial layer: Mg atoms; surface layer: Cr−O plane) on MgO (small dots). The (2 × 1) unit cell of the Cr−O surface layer is depicted by a dashed line.

3. RESULTS AND DISCUSSION 3.1. STM Measurements and Photoelectron Spectra. Figure 2 shows a series of empty-state STM images of 15 ML thick, annealed MgO films with increasing Cr concertation. In the upper two panels, the doping level amounts to (a) 1.5 and (b) 3 at %, while preparations with 12 and 30 at % are displayed in the lower panels c and d. The weakly doped MgO films are governed by wide terraces delimited by a straight [100]oriented dislocation network. The main defects in this case are circular depression of ∼0.5 nm diameter being homogeneously distributed on the surface. At 3 at % Cr in the lattice, small protrusions start to appear along the dislocation lines in addition to the minima previously described (Figure 2b). Their size continuously increases with Cr concentration until roughly half of the MgO surface is covered by large roundish islands at 12 at % Cr content. Although no atomic resolution has been obtained, the deviating contrast of the ad-islands suggests a different composition with respect to clean MgO. At the final

Figure 2. STM topographic images of 15 ML MgO films doped with (a) 1.5, (b) 3, (c) 12 and (d) 30 at % Cr. The top and bottom images are (50 × 50 nm2) and (100 × 100 nm2) in size, respectively, and have been measured at 3.5 V sample bias. Note the gradual transition from a defective MgO surface (a,b) to an entirely new morphology in panels c and d.

25470

DOI: 10.1021/acs.jpcc.5b09033 J. Phys. Chem. C 2015, 119, 25469−25475

Article

The Journal of Physical Chemistry C Cr concentration of 30 at %, the entire film is covered with a thick ad-layer of rippled morphology (Figure 2d). The described evolution of the surface appearance is accompanied by a characteristic change in the LEED periodicity that evolves from a simple (1 × 1) to a (2 × 1) pattern when going from weakly to strongly doped MgO films (Figure 1). Figure 3 shows a series of Cr 2p and Mg 1s core-level spectra measured as a function of the dopant concentration, the

Figure 4. Cr 2p/Mg 1s intensity ratio measured as a function of (a) the Cr concentration, (b) the emission angle, and (c) the annealing temperature at 3 at % Cr. Black triangles and red dots are for as-grown and annealed samples, respectively, and the green squares denote samples that contain a 10 ML pristine MgO buffer between MgOCr and the Mo support. The size of the markers is indicative for the error bars. The lines in panel b depict the results of an XPS intensity simulation.

1s intensity ratio rises with doping level, reflecting the larger Cr content in the matrix (Figure 4a). Upon annealing, the ratio decreases again and reaches ∼30% of its initial value at 1050 K, the highest temperature investigated here (Figure 4c). Interestingly, the reduction of the Cr signal cannot be prevented by putting a 10 ML MgO buffer layer in between MgOCr and the Mo support (Figure 4c). This finding suggests that the decrease is not caused by Cr migration toward the interface followed by dissolution in the metal substrate. Angleresolved measurements of the Cr 2p/Mg 1s ratio rather indicate Cr enrichment in the surface region of the oxide (Figure 4b). In fact, the ratio almost doubles if probed at grazing instead of normal emission, a behavior that is found for as-grown and annealed films. Apparently, the annealing causes part of the Cr to diffuse into the MgO, while another part evaporates into the gas phase, resulting in the observed intensity loss (Figure 4c). We note that a slight increase in the Cr 2p/Mg 1s intensity ratio would be expected at high emission angles because of the different inelastic mean free paths, however, much smaller than the observed effect.24 3.2. Charge State of the Cr Dopants. From the reported XPS data, detailed insight into the chemical state and lattice position of Cr ions in the MgO matrix is extracted. According to the BE of the Cr 2p doublet, the Cr dopants primarily occur in the 3+ charge state. This is a surprising result at first glance, as, for charge neutrality reasons, Cr on Mg substitutional sites should adopt the intrinsic 2+ charge state of cations in the rocksalt lattice. However, previous luminescence and paramagnetic resonance data on MgOCr single crystals are in line with this observation and have identified Cr3+ as the exclusive dopant species as well.16,25 Rationalization for this finding comes from DFT calculations that revealed a number of Cr 3d crystal field states in the band gap of MgO.26 The Cr2+ ground state thereby corresponds to a t2g3eg1 high-spin configuration with three electrons in the low-lying t2g and one electron in the high-lying eg manifold. In particular, the latter electron sits in an

Figure 3. Normalized XP spectra of (a) Cr2p and (c) Mg 1s levels as a function of the Cr content, measured for as-grown samples at normal emission. (b) Cr 2p spectra for Cr-rich preparations measured at normal (continuous) and grazing emission (dashed) for as-grown (black) and annealed samples (red). (d) Mg 1s spectra for annealed samples measured at normal emission.

emission angle, and the annealing temperature after deposition. At low Cr content, the shape of the Cr 2p peak corresponds to a simple doublet with ∼578 eV BE and ∼3 eV line width of the Cr 2p3/2 component (Figure 3a). The two values are compatible with a Cr3+ species embedded in the MgO matrix.22 With increasing doping concentration, the BE slightly downshift to 577 eV, while the fwhm of the peak decreases to 2.5 eV. Only at the maximum Cr content of 33 at % does a high-energy shoulder become visible next to the Cr 2p3/2 peak, reaching ∼10% of its intensity (Figure 3b). Its BE position of 579 eV matches the one of Cr6+ ions as found in CrO3. Interestingly, the new feature is strongest in room temperature preparations and at grazing emission angle and disappears after annealing to 1050 K. Both observations are compatible with a surface Cr oxide that develops above the solubility limit of Cr in MgO (15 at %) and gets reduced at elevated temperature.23 Also, the Mg 1s undergoes a distinct BE shift as a function of Cr exposure, which points, however, to different directions for as-grown and annealed films (Figure 3c,d). In the first case, the Mg 1s position shifts to lower BE with increasing Cr content, while an initial upshift followed by a BE decrease is observed after annealing to 1050 K. The different trends relate to a Crinduced band bending in combination with a chemical reorganization of the oxide film, as explained later in the text. Furthermore, we have analyzed the intensity behavior of the different XPS peaks to obtain information on the spatial distribution of Cr in the MgO film. As expected, the Cr 2p/Mg 25471

DOI: 10.1021/acs.jpcc.5b09033 J. Phys. Chem. C 2015, 119, 25469−25475

Article

The Journal of Physical Chemistry C

positive and negative sides to the MgO film and the Mo metal, respectively. This dipole orientation stabilizes the oxide electronic states and leads to a systematic increase in their BEs. The scenario accounts for the situation in annealed films that have perfectly accommodated the Cr impurities in the rocksalt structure and makes charge compensation for the Cr3+ ions necessary. Similar trends have been observed for Modoped CaO films in previous measurements.28 The shift is not evident for the Cr 2p binding energy, as the stabilizing effect of the interface dipole is overridden by a chemical shift of the Cr states due to changes in the local environment at increasing doping level (Figure 5b). The BE shift goes in the opposite direction for as-grown films, an effect that can be linked to their reduced crystallographic quality before annealing. As-grown MgOCr likely exhibits an abundance of intrinsic defects, suitable for trapping the extra electrons of the high-valence dopants. As a result, the incentive for charge transfer into the support and therefore for the BE increase disappears. Conversely, charges may flow in opposite direction, hence into the film, for the following reason. Some of the anions in as-grown films might not have the required number of cationic partners and therefore develop hole states in their O 2p orbitals. This electron deficiency is possibly healed by an electron transfer from the substrate, which reverses the interface dipole and explains the reduction of the Mg 1s and O 1s BEs. Apart from such electrostatically driven shifts, there might be chemical contributions as well. In different Mg−Cr bulk compounds, for example, in MgCr2O4, the Mg 1s core level has lower BE than in the weakly doped MgOCr films studied here.22 Apparently, the formation of Cr− Mg−O units at high doping concertation leads to a systematic decrease in the respective BEs, explaining the trend depicted in Figure 5a,b. We will elaborate on this point when discussing the development of ternary oxides in the following chapter. 3.4. Formation of a Ternary Oxide. Information on the spatial distribution of Cr ions in the MgO lattice and the possible formation of a mixed Cr−Mg oxide at high doping level is obtained from angle-resolved XPS data (Figure 4b). The measurements reveal an increasing Cr 2p/Mg 1s intensity ratio at grazing emission, indicative of Cr accumulation at the surface. This conclusion is supported by the STM data (Figure 2b,c) that display the development of distinct surface aggregates on top of the highly doped films. To quantitatively reproduce this behavior, we have calculated the XPS intensity ratios with an exponential attenuation model, considering a homogeneously doped MgOCr layer at the interface and a Crrich oxide layer at the surface. Using the absolute Cr content from quartz microbalance data and the measured Cr 2p/Mg 1s and Mo 3d/Mg 1s intensity ratios as input parameters, the amount of diluted Cr as well as the thickness of a hypothetical surface Cr oxide could be derived from the calculation (Figure 6). The obtained XPS intensity course thereby follows the angle-resolved experimental data (Figure 4b) with good accuracy. At low doping content, the simulation predicts a homogeneous Cr distribution in the oxide lattice, whereby the effective concentration rises linearly with the exposed material. Apparently, most of the Cr dissolves in the MgO at high temperature, following the bulk phase diagram of Cr−Mg spinel.23 Deviations become discernible only at 10 (15) at % doping for as-grown (annealed films), when Cr does not fully dissolve in the oxide film and accumulates at the surface, producing the higher Cr 2p signal at grazing emission. This result suggests that the Cr solubility limit of MgO (15 at %) can

energetically unfavorable position just below the MgO conduction band edge and shows high propensity to be transferred into more favorable energy levels. While no suitable acceptor states are available in the ideal MgO lattice, those might be generated by inserting electron-trapping defects into the rocksalt structure. The common acceptor-type defect in MgO is a Mg vacancy that creates a deficiency of two electrons in neighboring oxygen ions.6 These hole states spontaneously fill up with the topmost Cr electron, putting the dopant into a 3+ oxidation state. The associated energy gain is substantial, as revealed from the calculated formation energy of a single cation defect in pristine (8 eV) and Cr-doped (1 eV) MgO.26 Not surprisingly, the spontaneous insertion of charge-compensating Mg vacancies combined with the oxidation of essentially all Cr ions is thermodynamically favorable and explains the exclusive observation of 3+ species in our XPS data. The associated cationic defects have even been identified in corresponding STM images of weakly doped MgO films. We associate the faint depressions visible in Figure 2a to individual Mg vacancies, while a bright contrast would be expected for substitutional Cr ions in the surface.21 Note that also the dislocation lines, emerging in MgO/Mo films for reasons of strain relaxation, offer an abundance of electron traps and will thus be involved in the oxidation of the Cr impurities.20,27 3.3. Analysis of the Binding Energies. Insight into bandbending and charge-transfer processes in Cr-doped MgO films comes from the analysis of the Cr 2p and Mg 1s BEs, as summarized for different Cr concentrations in Figure 5. At low

Figure 5. BEs for (a) Mg 1s and (b) Cr 2p3/2 as a function of the Cr doping level. All peak positions have been corrected for shifts in the Mo0 3d5/2 peak of the support to eliminate possible charging effects. Black and red symbols are for as-grown and annealed films, respectively; dashed lines report the bulk data from ref 22.

doping level the Mg 1s experiences opposite shifts for as-grown (1.0 eV downward) and annealed films (0.5 eV upward) (Figure 5a). The O 1s core levels follow this trend, indicating that the BE shift results from the electrostatic response of the oxide to the presence of the dopants. Conversely, the Cr 2p states show a systematic downshift independent of the thermal treatment, suggesting a decisive role of chemical effects in this case. Let us discuss the upward shift of the MgO states in annealed films first. As previously mentioned, Cr impurities in a 3+ charge state require donation of one high-lying Cr 3d electron to the environment. If the excess electron gets trapped by a compensating Mg vacancy nearby, no systematic energy shift is expected as charges are displaced only over short distances. The situation changes, however, for near-interface dopants, for which the Mo metal below the film takes the role of the electron acceptor. The donation of Cr electrons to the support generates an interface dipole that aligns with its 25472

DOI: 10.1021/acs.jpcc.5b09033 J. Phys. Chem. C 2015, 119, 25469−25475

Article

The Journal of Physical Chemistry C

both phases is likely gradual, as no indication for a sharp boundary is obtained from the experimental data. Input on the structure of the new phase comes from LEED measurements that revealed a transition from a (1 × 1) to a (2 × 1) superstructure when increasing the Cr content above 15 at % (Figure 1). Apparently, the Cr-rich surface oxide keeps the square symmetry of the rocksalt MgO but doubles its unit-cell size in one direction. This observation disfavors a potential Cr2O3 phase, which would be the thermodynamically preferred binary oxide but mainly occurs in hexagonal configuration. More likely is the development of a mixed phase, for example, of MgCr2O4. Mg chromite crystallizes in cubic spinel structure with alternating planes of Mg2+ ions in tetrahedral coordination and Cr3+ ions in octahedral coordination with the O species (Figure 1c).29 Its bulk lattice parameter amounts to 8.3 Å, matching the doubled MgO lattice with