Article pubs.acs.org/JPCC
Enhanced Depth Profiling of Perovskite Oxide: Low Defect Levels Induced in SrTiO3 by Argon Cluster Sputtering Karl Ridier,†,‡ Damien Aureau,‡ Bruno Bérini,† Yves Dumont,† Niels Keller,† Jackie Vigneron,‡ Arnaud Etcheberry,‡ and Arnaud Fouchet*,† Groupe d’Étude de la Matière Condensée (UMR 8635), Université de Versailles Saint-Quentin-en-YvelinesCNRSUniversité Paris-Saclay, 45 Avenue des États-Unis 78035 Versailles, France ‡ Institut Lavoisier de Versailles (UMR 8180), Université de Versailles Saint-Quentin-en-YvelinesCNRSUniversité Paris-Saclay, 45 Avenue des États-Unis 78035 Versailles, France †
S Supporting Information *
ABSTRACT: We have studied the influence of the argon cluster ion sputtering technique in the X-ray photoelectron spectroscopy (XPS) depth-profiling analysis of a strontium titanate SrTiO3 (STO) substrate, chosen as a prototype perovskite-type oxide material. Unlike “standard” sputtering technique using monatomic ions, a gentle digging through STO (without inducing a large amount of defects) has been evidenced. Several improvements are evidenced by using this low-energy abrasion process: (i) the absence of argon implantation, (ii) the creation of very few oxygen vacancies which lead in the classical way to the lowering of oxidation states of titanium and the appearance of in-gap electronic states, and (iii) the preservation of the cationic stoichiometry. In addition, electrical measurements confirm that no metal−insulator transition is evidenced using the cluster ion source, unlike the case of the monatomic ion etching. Furthermore, for the latter case, a relaxation effect of the Ar+ ion induced electronic properties has been evidenced by combining XPS and transport measurements. Due to its unique depth-profiling features, the cluster ion sputtering technique offers new insights in the chemical and physical analysis of sensitive oxide surfaces and buried interfaces as in oxide heterostructures and superlattices.
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INTRODUCTION Synthesis and study of oxide surfaces, interfaces, and heterostructures have generated growing interest in the field of oxide-based electronics since spectacular and fascinating properties not present in constitutive bulk semiconductors such as superconductivity or multiferroicity can be found.1 The exact origin of the physical and electronic properties that emerge at the surface2 or at the buried interface3 between insulating complex oxides are strongly linked to the chemical properties of the system, and a better understanding requires a complete knowledge of surface and interface chemistries. X-ray photoelectron spectroscopy (XPS) is a powerful tool to investigate the chemistry of surfaces. This technique gives valuable information about both the elemental composition and the chemical state and environment of atoms located near the surface. Due to the photoelectron effective attenuation length (EAL) of the photoemitted electrons, the XPS analysis is strongly limited in depth, and it is difficult (if not impossible) to probe the chemistry of a buried interface located at several nanometers deep from the surface. One way to overcome this difficulty consists of bombarding the studied surface using noble gas ions with a dual aim: (1) removing the surface contaminants and (2) realizing depth profiling of the material to acquire vertical resolution in the analysis. Nevertheless, ion bombardment of oxide surfaces causes important modification of its atomic structure (amorphization), chemical properties © XXXX American Chemical Society
(implantation, defects, and vacancies), and electronic structure (creation of new electronic states).4,5 In this context, the use of an argon clusters (composed of several thousand ions) beam instead of monatomic ions is very promising. The main advantage is that the average energy per argon atom is considerably lower (only a few electronvolts per atom) and the penetration length noticeably smaller into the material. The Ar cluster sputtering technique has already been used on organic material6,7 and as a cleaning tool to prepare oxide surfaces8 but very rarely as a depth-profiling tool on oxides. Due to its dielectric and electrical properties, strontium titanate, SrTiO3 (STO), is an important and promising perovskite-type material for the emerging field of oxide-based electronics. STO is favorably lattice-matched to many other complex oxides and is therefore a widely used substrate for epitaxial oxide growth and oxide heterostructures synthesis. In its stoichiometric state, STO is a transparent nonmagnetic band insulator (band gap ≈ 3.2 eV). However, STO becomes an ntype conductor with high mobility9−11 by doping cations such as Nb5+, La3+, and Y3+ or by oxygen vacancies creation. It can also become a superconductor at very low temperature with Received: April 20, 2016 Revised: August 26, 2016
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DOI: 10.1021/acs.jpcc.6b04007 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 1. Argon XPS spectra. (a and b) XPS spectra of the Ar 2p binding-energy region at different times of the A1 and A2 ion bombardments, respectively. (c) Time evolution of the argon XPS atomic percentage during the two etchings.
exceptionally low carrier densities.12,13 The interest in STO is further increased by the unique electronic, magnetic, and superconducting properties of the quasi-two-dimensional electron gas formed at the interface between STO and other oxides such as LaAlO3.3,14,15 Effects of ion sputtering on STO single crystals have been subject to many studies. It has been demonstrated that ion beam etching can drastically modify the properties of STO substrates, and so far, ion sputtering has essentially been used to make surface engineering. In particular, it was shown that argon-ion-bombarded STO surfaces exhibit high-mobility conduction16−21 in contrast to the insulating behavior of stoichiometric STO single crystals. This conductivity is known to result from the generation of a layer of oxygen vacancies near the surface (each of them ideally donating two conducting electrons) owing to the “high-energetic” (usually >50 eV) Ar ion bombardment. Many efforts have been devoted to elucidating the properties of the induced areal electron gas and its spatial extension for different energies and current densities of the incident ion beams.17,19,22 Due to different Ar ion penetration lengths and temperature-dependent relaxation processes18,20 (diffusion of vacancy defects deeply into the crystal), various results were obtained for the spatial extension of the electron gas ranging from a few or several tens of nanometers16,20,22,23 to several hundreds of nanometers24 or even a few micrometers.19 Moreover, argon ion STO bombardment (at 300 eV) underlines a blue-light emission at room temperature,16 opening the way for the possibility of oxide-based optoelectronic devices using photoluminescence and generated by Ar etching. In the present study, we have explored the capabilities of the Ar cluster ion source in performing depth profiling (i.e., depthresolved chemical analysis) of STO, considered as a prototype complex oxide material. For this, combining XPS and electrical measurements, the detailed time evolution of the chemical and electronic properties under argon ion bombardments is investigated.
1s core lines are presented in the Supporting Information). In all XPS measurements, an electron flood gun is used to compensate for the charges created by the photoelectron emission. Because of unavoidable charging effects and resulting uncertainties in the absolute binding energies, all XPS spectra have been aligned considering the main O 1s core line at 530.7 eV as reported in the literature.26,27 Argon ion bombardments were performed using the MAGCIS dual beam ion source which enables depth profiling with both argon monatomic and cluster ions. In the “monoatomic” mode, the sample was irradiated with Ar+ ions accelerated at 4000 eV. In the “cluster” mode, the average kinetic energy of each cluster (≈2000 atoms per cluster) was fixed to 4000 eV. In both cases, the incoming argon ion projectiles reach the sample with an angle of 30° from the surface normal and the ion current density is fixed to ≈15 μA/ cm2. To compare the bombardment effects during depth profiling, two different abrasions were carried out. In the first one (hereafter denoted A1), a pure cluster ion etching of 2720 s is performed. In the second one, we carried out a 250 s monatomic etching (4000 eV), immediately followed by a 2100 s cluster ion sputtering. Since the monatomic ion sputtering causes strong modification of the STO surface, the cluster abrasion was used to dig through this altered layer. This “monoatomic + cluster” depth profile is denoted A2. Sheet resistance measurements of the bombarded areas were performed using the two-point probes method with Al wires. The typical distance between the probes was ≈500 ± 100 μm. Topography and roughness of the irradiated substrates have been investigated by atomic force microscopy (AFM; Bruker Dimension 3100) in tapping mode using commercial tips with 300 kHz resonant frequency and 40 N/m spring constant.
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RESULTS AND DISCUSSION Argon Peaks. First, implantation of argon was investigated by XPS during both A1 and A2 etchings. Panels a and b of Figure 1 show the Ar 2p binding-energy region (241−249 eV) at different times of the A1 and A2 bombardments. The argon XPS atomic percentage based on the analysis of the Ar 2p doublet is displayed in Figure 1c as a function of etching time. No signal of argon is detected with the cluster ion source (in the XPS detection limit) even after 2720 s etching showing that the low energy per atom (∼2 eV) with the cluster ion source does not generate XPS measurable implantation (
