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Title: Enhancement of spin-lattice coupling in nanoengineered oxide films and heterostructures by laser MBE

Abstract

The objective of the proposed research is to investigate nanoengineered oxide films and multilayer structures that are predicted to show desirable properties. The main focus of the project is an atomic layer-by-layer laser MBE (ALL-Laser MBE ) technique that is superior to the conventional laser MBE in broadening the conditions for the synthesis of high quality nanoscale oxides and new designer materials. In ALL-Laser MBE, separate oxide targets are used instead of one compound target in the conventional laser MBE. The targets are switched back and forth in front of a UV laser beam as they are alternately ablated. The oxide film is thus constructed one atomic layer at a time. The growth of each atomic layer is monitored and controlled by the reflection high energy electron diffraction (RHEED). The intensity of the diffraction spots increases or decreases depending on the chemistry of each atomic layer as well as the surface roughness. This allows us to determine whether the chemical ratio of the different elements in the films meets the desired value and whether each atomic layer is complete. ALL-Laser MBE is versatile: it works for non-polar film on non-polar substrate, polar film on polar substrate, and polar film onmore » non-polar substrate. (In a polar material, each atomic layer is charged whereas in a non-polar material the atomic layers are charge neutral.) It allows one to push the thermodynamic boundary further in stabilizing new phases than reactive MBE and PLD, two of the most successful techniques for oxide thin films. For example, La 5Ni 4O 13, the Ruddlesden-Popper phase with n = 4, has never been reported in the literature because it needs atomic layer-by-layer growth at high oxygen pressures, not possible with other growth techniques. ALL-Laser MBE makes it possible. We have studied the interfacial 2-dimensional electron gas in the LaAlO 3/SrTiO 3 system, whose mechanism has been a subject of controversy. According to the most prevailing electronic reconstruction mechanism, a positive diverging electric potential is built up in the polar LaAlO 3 film when it is grown on a TiO 2-terminated SrTiO 3 substrate, which is non-polar. This leads to the transfer of half of an electron from the LaAlO 3 film surface to SrTiO 3 when the LaAlO 3 layer is thicker than 4 unit cells, creating a 2D electron gas at the interface with a sheet carrier density of 3.3×10 14/cm 2 for sufficiently thick LaAlO 3. A serious inconsistency with this mechanism is that the carrier densities reported experimentally are invariably lower than the expected value. The most likely reason is that the SrTiO 3 substrate is oxygen difficient due to the low oxygen pressures (< 10 mTorr) during growth, and post-growth annealing in oxygen is often used to remove the oxygen vacancies. People cannot grow the LaAlO 3 film in higher oxygen pressures - it results in insulating samples or 3D island growth. Because we grow the LaAlO 3 film one atomic layer at a time, we were able to grow conducting LaAlO 3/SrTiO 3 interfaces at a high oxygen pressure with ALL-Laser MBE, as high as 37 mTorr. The high oxygen pressure helps to prevent the possible oxygen reduction in SrTiO 3, ensure that the LaAlO 3 films are sufficiently oxygenated. Measurements of x-ray linear dichroism (XLD) and x-ray magnetic circular dichroism (XMCD) both show that the spectra of our films are similar to those of well oxygenated samples. In the LaAlO 3/SrTiO 3 interfaces grown by ALL-Laser MBE at 37 mTorr oxygen pressure, a quantitative agreement between our experimental result and the theoretical prediction was observed, which provides a strong support to the electronic reconstruction mechanism. The key differences between our result and the previous reports are the high oxygen pressure during the film growth and the high film crystallinity. The high oxygen pressure suppresses the likelihood of oxygen vacancies in SrTiO 3. Well oxygenated samples produced during film growth can avoid possible defects when sufficient oxygen is provided only after the growth by annealing. Using ALL-Laser MBE, we also synthesized high-quality singlec-rystalline CaMnO 3 films. The systematic increase of the oxygen vacancy content in CaMnO 3 as a function of applied in-plane strain is observed and confirmed experimentally using high-resolution soft x-ray XAS and hard x-ray photoemission spectroscopy (HAXPES). The relevant defect states in the densities of states are identified and the vacancy content in the films quantified using the combination of first-principles theory and core-hole multiplet calculations with holistic fitting. The strain-induced oxygen-vacancy formation and ordering are a promising avenue for designing and controlling new functionalities in complex transition-metal oxides.« less

Authors:
 [1]
  1. Temple Univ., Philidelphia, PA (United States)
Publication Date:
Research Org.:
Temple Univ., Philidelphia, PA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1362040
Report Number(s):
Final Report: DOE-Temple
DOE Contract Number:  
SC0004764
Resource Type:
Technical Report
Resource Relation:
Related Information: 1. Guozhen Liu, Qingyu Lei, Matthäus A. Wolak, Qun Li, Long-Qing Chen, Christopher Winkler, Jennifer Sloppy, Mitra L. Taheri, and Xiaoxing Xi, Epitaxial strain and its relaxation at the LaAlO3/SrTiO3 interface, J. Appl. Phys. 120, 085302 (2016).2. Ravini U. Chandrasena, Weibing Yang, Qingyu Lei, Mario U. Delgado-Jaime, Kanishka D. Wijesekara, Maryam Golalikhani, Bruce A. Davidson, Elke Arenholz, Keisuke Kobayashi, Masaaki Kobata, Frank M. F. de Groot, Ulrich Aschauer, Nicola A. Spaldin, Xiaoxing Xi,and Alexander X. Gray, Strain-Engineered Oxygen Vacancies in CaMnO3 Thin Films, Nano Lett. 17, 794 (2017).3. Qingyu Lei, Maryam Golalikhani, Bruce A. Davidson, Guozhen Liu, Darrell G. Schlom, Qiao Qiao, Yimei Zhu, Ravini U. Chandrasena, Weibing Yang, Alexander X. Gray, Elke Arenholz, Andrew K. Farrar, Dmitri A. Tenne, Minhui Hu, Jiandong Guo, Rakesh K Singh, X. X. Xi, Constructing oxide interfaces and heterostructures by atomic layer-by-layer laser molecular beam epitaxy, npj Quant. Mater. 2, 10 (2017).
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE

Citation Formats

Xi, Xiaoxing. Enhancement of spin-lattice coupling in nanoengineered oxide films and heterostructures by laser MBE. United States: N. p., 2017. Web. doi:10.2172/1362040.
Xi, Xiaoxing. Enhancement of spin-lattice coupling in nanoengineered oxide films and heterostructures by laser MBE. United States. doi:10.2172/1362040.
Xi, Xiaoxing. Thu . "Enhancement of spin-lattice coupling in nanoengineered oxide films and heterostructures by laser MBE". United States. doi:10.2172/1362040. https://www.osti.gov/servlets/purl/1362040.
@article{osti_1362040,
title = {Enhancement of spin-lattice coupling in nanoengineered oxide films and heterostructures by laser MBE},
author = {Xi, Xiaoxing},
abstractNote = {The objective of the proposed research is to investigate nanoengineered oxide films and multilayer structures that are predicted to show desirable properties. The main focus of the project is an atomic layer-by-layer laser MBE (ALL-Laser MBE ) technique that is superior to the conventional laser MBE in broadening the conditions for the synthesis of high quality nanoscale oxides and new designer materials. In ALL-Laser MBE, separate oxide targets are used instead of one compound target in the conventional laser MBE. The targets are switched back and forth in front of a UV laser beam as they are alternately ablated. The oxide film is thus constructed one atomic layer at a time. The growth of each atomic layer is monitored and controlled by the reflection high energy electron diffraction (RHEED). The intensity of the diffraction spots increases or decreases depending on the chemistry of each atomic layer as well as the surface roughness. This allows us to determine whether the chemical ratio of the different elements in the films meets the desired value and whether each atomic layer is complete. ALL-Laser MBE is versatile: it works for non-polar film on non-polar substrate, polar film on polar substrate, and polar film on non-polar substrate. (In a polar material, each atomic layer is charged whereas in a non-polar material the atomic layers are charge neutral.) It allows one to push the thermodynamic boundary further in stabilizing new phases than reactive MBE and PLD, two of the most successful techniques for oxide thin films. For example, La5Ni4O13, the Ruddlesden-Popper phase with n = 4, has never been reported in the literature because it needs atomic layer-by-layer growth at high oxygen pressures, not possible with other growth techniques. ALL-Laser MBE makes it possible. We have studied the interfacial 2-dimensional electron gas in the LaAlO3/SrTiO3 system, whose mechanism has been a subject of controversy. According to the most prevailing electronic reconstruction mechanism, a positive diverging electric potential is built up in the polar LaAlO3 film when it is grown on a TiO2-terminated SrTiO3 substrate, which is non-polar. This leads to the transfer of half of an electron from the LaAlO3 film surface to SrTiO3 when the LaAlO3 layer is thicker than 4 unit cells, creating a 2D electron gas at the interface with a sheet carrier density of 3.3×1014/cm2 for sufficiently thick LaAlO3. A serious inconsistency with this mechanism is that the carrier densities reported experimentally are invariably lower than the expected value. The most likely reason is that the SrTiO3 substrate is oxygen difficient due to the low oxygen pressures (< 10 mTorr) during growth, and post-growth annealing in oxygen is often used to remove the oxygen vacancies. People cannot grow the LaAlO3 film in higher oxygen pressures - it results in insulating samples or 3D island growth. Because we grow the LaAlO3 film one atomic layer at a time, we were able to grow conducting LaAlO3/SrTiO3 interfaces at a high oxygen pressure with ALL-Laser MBE, as high as 37 mTorr. The high oxygen pressure helps to prevent the possible oxygen reduction in SrTiO3, ensure that the LaAlO3 films are sufficiently oxygenated. Measurements of x-ray linear dichroism (XLD) and x-ray magnetic circular dichroism (XMCD) both show that the spectra of our films are similar to those of well oxygenated samples. In the LaAlO3/SrTiO3 interfaces grown by ALL-Laser MBE at 37 mTorr oxygen pressure, a quantitative agreement between our experimental result and the theoretical prediction was observed, which provides a strong support to the electronic reconstruction mechanism. The key differences between our result and the previous reports are the high oxygen pressure during the film growth and the high film crystallinity. The high oxygen pressure suppresses the likelihood of oxygen vacancies in SrTiO3. Well oxygenated samples produced during film growth can avoid possible defects when sufficient oxygen is provided only after the growth by annealing. Using ALL-Laser MBE, we also synthesized high-quality singlec-rystalline CaMnO3 films. The systematic increase of the oxygen vacancy content in CaMnO3 as a function of applied in-plane strain is observed and confirmed experimentally using high-resolution soft x-ray XAS and hard x-ray photoemission spectroscopy (HAXPES). The relevant defect states in the densities of states are identified and the vacancy content in the films quantified using the combination of first-principles theory and core-hole multiplet calculations with holistic fitting. The strain-induced oxygen-vacancy formation and ordering are a promising avenue for designing and controlling new functionalities in complex transition-metal oxides.},
doi = {10.2172/1362040},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Thu Jun 08 00:00:00 EDT 2017},
month = {Thu Jun 08 00:00:00 EDT 2017}
}

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