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Effect of Gadolinium Doping on the Electronic Band Structure of Europium Oxide

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Effect of Gadolinium Doping on the Electronic Band Structure of Europium Oxide University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Peter Dowben Publications Research Papers in Physics and Astronomy 2012 Effect of gadolinium doping on the electronic band structure of europium oxide Juan Colon Santana University of Nebraska-Lincoln Joonhee Michael An University of Nebraska-Lincoln Ning Wu University of Nebraska-Lincoln Kirill D. Belashchenko University of Nebraska-Lincoln, [email protected] Xianjie Wang University of Wyoming, Laramie See next page for additional authors Follow this and additional works at: https://digitalcommons.unl.edu/physicsdowben Part of the Physics Commons Colon Santana, Juan; Michael An, Joonhee; Wu, Ning; Belashchenko, Kirill D.; Wang, Xianjie; Liu, Pan; Tang, Jinke; Losovyj, Yaroslav B.; Yakovkin, I. N.; and Dowben, Peter A., "Effect of gadolinium doping on the electronic band structure of europium oxide" (2012). Peter Dowben Publications. 248. https://digitalcommons.unl.edu/physicsdowben/248 This Article is brought to you for free and open access by the Research Papers in Physics and Astronomy at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Peter Dowben Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Authors Juan Colon Santana, Joonhee Michael An, Ning Wu, Kirill D. Belashchenko, Xianjie Wang, Pan Liu, Jinke Tang, Yaroslav B. Losovyj, I. N. Yakovkin, and Peter A. Dowben This article is available at DigitalCommons@University of Nebraska - Lincoln: https://digitalcommons.unl.edu/ physicsdowben/248 PHYSICAL REVIEW B 85, 014406 (2012) Effect of gadolinium doping on the electronic band structure of europium oxide Juan A. Colon´ Santana,1 Joonhee M. An,2 Ning Wu,2 Kirill D. Belashchenko,2 Xianjie Wang,3 Pan Liu,3 Jinke Tang,3 Yaroslav Losovyj,4 I. N. Yakovkin,5 and P. A. Dowben2 1Department of Electrical Engineering, W. Scott Engineering Center, North 16th Street, University of Nebraska-Lincoln, Lincoln, Nebraska 68588-0656, USA 2Department of Physics and Astronomy and Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, Theodore Jorgensen Hall, 855 North 16th Street, Lincoln, Nebraska 68588-0299, USA 3Department of Physics & Astronomy, University of Wyoming, Laramie, Wyoming 82071, USA 4Center for Advanced Microstructures and Devices, Louisiana State University, 6980 Jefferson Highway, Baton Rouge, Louisiana 70806, USA 5Institute of Physics, National Academy of Sciences of Ukraine, Prospect Nauki 46, Kyiv 680028, Ukraine (Received 2 August 2011; revised manuscript received 30 November 2011; published 5 January 2012) High quality films of EuO and Eu0.96Gd0.04O were grown on p-type Si(100) via pulsed laser deposition. X-ray- diffraction results show that the addition of Gd changes the growth texture from [001] to [111]. Angular-resolved photoemission spectroscopy reveals electron pockets around the X points in Gd-doped EuO, indicating that the band gap in EuO is indirect. Combined photoemission and inverse photoemission measurements show an apparent transition from n-type to p-type behavior, which is likely due to band bending near the polar (111) surface. DOI: 10.1103/PhysRevB.85.014406 PACS number(s): 75.50.Pp, 75.70.Ak, 79.60.−i I. INTRODUCTION at the Si/EuO interface.3,13 Methods for preparing EuO films Europium oxide is a well-known ferromagnetic semicon- reported so far include reactive thermal evaporation of Eu and 1,2 molecular beam epitaxy (MBE) under ultrahigh vacuum in the ductor and a candidate for spin filter barrier materials. 3,6,13,17,18 Stoichiometric EuO has a Curie temperature (T )of69K, presence of oxygen gas. Here we used pulsed laser C deposition (PLD) for the growth of EuO and Gd-doped EuO which is strongly enhanced by electron doping via rare-earth 8 substitution3–8 or oxygen vacancies.4,7–11 Furthermore, such on Si(100), shown previously to be viable. Hydrofluoric acid (HF) and acetone were used to clean the silicon wafers. Before doping can tune the conductivity of EuO to match that of ◦ 3,12,13 the deposition, the silicon wafers were annealed at 750 Cin silicon. Epitaxial or very strongly textured EuO(100) −5 films can be grown on Si(100) wafers with a high quality vacuum, at a pressure of 10 Torr of pure H2 gas to reduce EuO/Si interface.8,14 the native SiO2 surface layer from the wafers. Both EuO and Both oxygen deficiency and Gd doping are expected to Gd-doped EuO films were grown on these wafers using PLD introduce n-type donors in EuO, but their effects may be at room temperature. The targets used in the PLD process were either Eu (99.9%) metal or a mixture of Eu (99.9%) and Gd somewhat different. While the TC of Gd-doped unreduced 8 7,8 (99.9%) metals, as described previously. We chose the Gd Eu0.96Gd0.04O is 120 K, it increases to 145 K when the 8 substitution level of 4% (Eu0.96Gd0.04O), which was reported Gd-doped sample is oxygen deficient (Eu0.96Gd0.04O1−x ). For 6 6 optimal. Eu0.96Gd0.04OaTC as high as 170 K has been reported. The combination of Gd doping and oxygen vacancies or The texture growth orientation for both EuO and Gd-doped EuO films grown in this way was determined by x-ray sufficient doping of either brings Eu0.96Gd0.04O across the metal-insulator transition. EuO (and Gd-doped EuO) is a 4f diffraction (XRD) with Cu-Kα radiation obtained using a system whose electronic structure consists of two subsystems: Philips X’Pert diffractometer. The undoped stoichiometric a localized f system and an itinerant spd system.15,16 Rare- EuO films adopt the expected rocksalt structure with a lattice constant of 5.131 A˚ and exhibit (100) growth texture. A strong earth dopants have large local moments and also provide ◦ carriers to populate the bottom of the conduction band. (200) reflection is seen at the 2θ angle of 35.3 ,asshownin 8 In the present work, we show that the addition of Gd leads to Fig. 1 and reported elsewhere. The slightly reduced, oxygen- the filling of electron pockets, revealing the conduction-band deficient EuO film has a lattice constant of 5.106 A˚ [Fig. 1(a)]. minimum at the X point. We also report that the surface of The Eu0.96Gd0.04O films, in contrast, adopts textured growth along the [111] direction, and a strong (111) Bragg peak is seen Gd-doped EuO may appear either p type or n type depending ◦ on surface preparation. The Gd doping also results in a change at the 2θ angle of 30.3 . This peak yields the lattice spacing of the texture growth from (100) to (111), so that the surface is of 2.95 A˚ [Fig. 1(b)] and a slightly larger lattice constant of 8 nominally a polar surface. We argue that this variability may 5.11 A˚ compared to undoped EuO films. We find no evidence be explained by band bending, which screens the electrostatic of Eu metal either as bulk precipitates or at the surface in either field of the polar surface. XRD or x-ray photoemission in any of our samples. Combined ultraviolet photoemission (UPS) and inverse photoemission (IPES) spectroscopies were carried out, as has II. EXPERIMENTAL DETAILS been done elsewhere for both main group element compound There are known complications to the growth of EuO on semiconductors19–21 and other rare-earth oxides.22 Some UPS silicon. Key among the problems is that the presence of a and IPES spectra were taken in a single ultrahigh vacuum high oxygen partial pressure at the initial stages of the EuO chamber to study the placement of both occupied and unoccu- 3+ film growth leads to formation of Eu (indicative of Eu2O3) pied states under the same conditions. The IPES spectra were 1098-0121/2012/85(1)/014406(9)014406-1 ©2012 American Physical Society JUAN A. COLON´ SANTANA et al. PHYSICAL REVIEW B 85, 014406 (2012) common in the study of dielectric oxide materials of any appreciable thin-film thickness.19–21 Segregation and formation of a “surface” Gd2O3 layer at the surface of the Gd-doped EuO films can be excluded as the combined photoemission and inverse photoemission are inconsistent with such an insulating (almost certainly n-type) surface oxide.33,34 The photoemission spectra for EuO and Gd-doped EuO differ only slightly. The resonant photoemission intensity is weak, even compared to similarly 33–35 36 Gd-doped HfO2, consistent with a more metallic oxide, and the photovoltaic charging seen with Gd2O3 and Gd-doped HfO2 (Ref. 32) is not observed here, also consistent with the absence of a dielectric “surface” Gd2O3 layer. III. VALENCE-BAND ELECTRONIC STRUCTURE The valence-band photoemission (ARPES) spectra for both doped and undoped EuO films are shown in Fig. 2. These spectra exhibit photoemission features attributable to the Eu 4f states near the Fermi level and the O 2p states at about 4–6 eV below the Fermi level. The binding energy of the O FIG. 1. (Color online) X-ray-diffraction pattern for PLD-grown states is consistent with GW calculations,37 while LSDA or (a) EuO and (b) Gd-doped EuO films on Si(100). LSDA + U calculations place the binding energy of the O states significantly closer to the Fermi level.11,15,16,37–42 The binding energies of Eu 4f and oxygen 2p orbitals are also consistent with previous photoemission studies of undoped EuO films.38–45 The splitting of the Eu 4f states in the obtained using variable kinetic-energy incident electrons while photoemission spectra has been previously reported,42 and in detecting the emitted photons at a fixed energy (9.7 eV) with a those high-resolution photoemission studies, the splitting was Geiger-Muller¨ detector.19–24 The IPES resolution was limited clearly resolved at the surface Brillouin-zone center ()for by an instrumental linewidth of ∼400 meV, as described undoped EuO(100).
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