DOI: 10.1021/cg100474x
Microwave Plasma Nitridation of SrTiO3: A Quantitative EELS, TEM, and STEM-HAADF Analysis of the SrTiO3-xNy Growth and the Structural Evolution
2010, Vol. 10 3562–3567
Myriam H. Aguirre,* Andrey Shkabko, and Anke Weidenkaff Solid State Chemistry and Catalysis, Empa-Swiss Federal Laboratories for Materials Science and Technology, Uberlandstrasse 129, CH-8600, Switzerland Received April 9, 2010; Revised Manuscript Received May 31, 2010
ABSTRACT: The structural and microstructural evolution of microwave plasma nitrided SrTiO3 was studied by scanning, high-resolution transmission electron microscopy (STEM, HRTEM), electron energy loss spectroscopy, and energy dispersion X-ray spectroscopy (EELS/EDX) combined with elemental mapping at the nanometer scale. EELS/EDX mappings show the SrTiO3-xNy single crystal formation as well as the TiN1-xOy nanograin layer formation on the surface, which depends on the plasma treatments. Stacking faults were the most common defects found in single crystal SrTiO3-xNy, either with SrO planeexcess or TiO2 plane-excess. The excess of SrO was integrated as Ruddlesden-Popper planar faults, showing an arrangement of the three-dimensional nanostructure, and the excess of TiO2-xNy was integrated as crystallographic shear defects. Nitrogen incorporation into the structure is not homogeneous at the atomic level, and a high concentration of nitrogen was found in Sr-deficient planar defects.
Introduction The electrical properties of perovskite-type materials with general formula ABO3 are highly sensitive to the deviations from the ideal composition or the nonstoichiometry. Accordingly, this prompted much research in the field of phase stability, solid solutions, and defect structures. Most of the studies on perovskite-type oxides have focused on cationic composition modifications,1 while anionic substitutions are barely examined.2 However, even a small change in the anion content (oxygen) of certain perovskites can induce a metal-to-insulator transition which is significant for field-effect transistors and transparent electronics.3-5 SrTiO3 (STO) is considered as a model material for composition studies,6 and the complexity of its surface and defect structure varies depending on the preparation conditions.7 In STO, the Ti4þ transition metal ion is octahedrally coordinated by O2-, and the Sr2þ cation is 12-fold coordinated in cuboctahedral sites (space group Pm3m). STO is a 3d0 band insulator with an indirect band gap of 3.25 eV. The valence band and the empty conduction band are preferentially formed by an O 2p orbital and a Ti 3d orbital,8 respectively. The anionic substitution provides an additional alternative route to tune the material properties.9,10 Replacement of oxygen by nitrogen atoms in oxides is possible due to the similar ionic radii and electronic structures of these elements.11 As an important concomitant effect, O2- substitution by N3- ions in STO to form oxynitride perovskites is accompanied by a charge compensation mechanism such as hole doping (h• = oger-Vink notation) and oxygen vacancies. O2--N3- in Kr€ The oxygen vacancies, VO•• in the structure are compensated by the generation of Ti 3d electrons, e0 = Ti3þ-Ti4þ. Experimentally, the charge compensation mechanisms are difficult to control insofar as it is not possible to control the localization of defects.12 For example, oxygen deficiency in STO, even *Corresponding author. E-mail:
[email protected]. pubs.acs.org/crystal
Published on Web 06/15/2010
near the surface, can induce metallic conductivity and also superconductivity;13 however, the required arrangement of the oxygen vacancies in the superconducting phase still remains unknown. Therefore, the defect nanostructure largely depends on the synthesis methods and the heat-treatment history14 of the materials. It was reported15,16 that thermal treatments under reducing or oxidizing conditions develop chemical inhomogeneities in STO. For example, SrOx appears during annealing at 800-1000 C in an oxidizing atmosphere and could form Ruddlesden-Popper (RP) phases in the first STO nanolayer, while under reduction conditions Ti-rich phases can growth near the STO surface.17 One novel method to introduce N atoms into the STO perovskite structure is the nitriding treatment in a microwave induced NH3 plasma.18 Previous works have shown that microwave induced plasma treatment is a suitable tool for the synthesis of bulk metal oxides and oxynitrides.19,20 Microwaves possess several advantages over conventional thermal ammonolysis methods, one of which is rapid and selective heating.20 Additionally, the microwaves serve to ignite the reactive plasma, and the physical-chemical reactions between plasma and the surface are dependent on the time of processing and the gas flux. In this work, we studied the microwave plasma nitridation of an STO single crystal as a function of ammonia gas flow and time. The microstructure of the bulk sample was evaluated by electron microscopy. STEM combined with EDX and with EELS are powerful tools to study the local composition and defect-structure in nanoscale regions because of the higher spatial resolution that can be achieved in imaging and spectroscopy compared to other techniques.21,22 High-angle annular dark-field (HAADF) imaging using STEM provides sensitive imaging of heavy elements due to Z-contrast, while EELS allows examining the local chemical composition and bonding around light elements. The defects and nonstoichiometry that control the electrical properties of the nitrided STO r 2010 American Chemical Society
Article
Crystal Growth & Design, Vol. 10, No. 8, 2010
are studied extensively in order to obtain the best experimental parameters for the plasma nitridation of STO. Experimental Section The plasma-solid reaction was carried out in a modified domestic microwave oven18 operated at 2.45 GHz with a power of 700 W. Samples were placed in the active glow discharge region23 in a quartz tube. The first set of samples, SrTiO3 (111) single crystals from CRYSTEC cut into rectangular bars of 1 0.5 10 mm3, was prepared at a constant NH3 flow of 100 mL/min and varying the treatment times between 2 and 130 min (see Table 1). In the second study, STO (100) single crystals were cut into rectangular bars of 2 0.5 10 mm3, and treated at a constant pressure of 10 mbar for 10 min at NH3 flow rates varying from 50 to 175 mL/min.10,19,23 The maximum measured temperature depended on the NH3 flow (see Table 2). HRTEM analysis was performed with a Philips CM30 microscope. Scanning transmission in dark field mode (STEM-HAADF) and EDX-EEL spectroscopy analysis were carried out using a Jeol JEM 2200 FS in-column filter and a Tecnai G2 F20 X-Twin postcolumn GIF filter. The high resolution STEM-HAADF image was acquired with a Jeol 2200 FS in-column filter with 0.2 nm of spot size. The heterostructures were thinned in cross-sectional geometry by the tripod method and with a dimple grinder (Gatan Model 656). The final electron transparency was achieved with an ion-beam milling system (RES 101 from Baltec). The EELS energy resolution was 1 eV with probe currents in the range 0.6-1 nA. In STEM mode, the electron probe sizes of 0.2, 0.5, and 0.7 nm were used. The EELS spectra were acquired with a dispersion of 0.2 eV/channel in order to simultaneously record the N K-, Ti L-, and O K-edges, while the condition of 0.06 eV/channel was chosen for a good resolution of the Ti L3,2 peaks. Spectrum acquisition and processing were done with Digital Micrograph software. The background for each spectrum was subtracted with a power law function fit to the pre-edge region.24 The EELS spectra energy scale was calibrated by setting the N K threshold energy at 397 eV; the energy shifts measured are relative to this value. The chemical composition at the near-surface (0-10 nm) was studied by the XPS method. A detailed experimental description of the method can be found in refs 10, 19, and 25.
Results and Discussion The results of the plasma treatment mainly depended on the NH3 flow. Samples prepared at 50 and 100 mL/min flow rates showed a nanocrystalline (polycrystalline) layer irrespective of the single crystal orientation (STO (111) or STO (001)) and a form TiNxO1-y/SrTiO3-zNδ layered structure.10 Samples treated at 125, 150, and 175 mL/min flow rates produced SrTiO3-zNδ single crystals devoid of TiNxO1-y layers.19 STO pristine material studies (see the Supporting Information) show a perfect crystal structure prior to the nitridation process, and by establishing the comparison with the nitrided samples, it is possible to observe different defects appearing Table 1. Parameter Summary for the Treatment of STO (111) Samples at a Constant NH3 Flow of 100 mL/min Leading to a TiNxO1-x Layer on STON Single Crystals sample no.
1
2
3
4
5
6
7
time (min) 2 10 30 40 53 120 130 30 50 120 150 150 thickness of TiNxO1-x (nm) 10 20 N/Ti ratio (0-10 nm) 0.6 0.86 0.72 0.87 0.85 1.1 0.6
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near the surface, and their amount decreases with increasing flow rates. A. Samples Prepared at NH3 Flow Rates of e100 mL/min. Figure 1 shows a TEM micrograph at medium magnification of the sample treated at a 100 mL/min NH3 flow rate for 2 min (a) and 10 min (b), respectively. The sample that underwent short time treatment reveals unconnected TiNxO1-y nanocrystals with a size
