Atomic-scale manipulation of potential barriers at SrTiO3 grain boundaries

Pradyumna Prabhumirashi, Vinayak P. Dravid, Andrew R. Lupini, Matthew F. Chisholm, Stephen J. Pennycook
2005 Applied Physics Letters  
In oxide grain boundaries ͑GBs͒, oxygen ions and their vacancies serve as a common denominator in controlling properties such as GB barrier height and capacitance. Therefore, it is critical to analyze, control and manipulate oxygen and vacancies at oxide interfaces as most of the practical devices are almost always influenced by the presence of electrostatic potential barriers at interfaces. Here, we report adjustment of a single GB potential barrier via manipulation of oxygen vacancy
more » ... ion using simple oxidation and reduction treatments. We validate our analysis with aberration-corrected HAADF imaging and column-by-column EELS coupled with macroscopic transport measurements of isolated GBs to gain important insight into the physical attributes of GB potential barriers. It is well-known that oxygen ions and their vacancies serve as a common denominator in oxides for controlling their technologically important properties. 1,2 Recent reports 3,4 have elegantly demonstrated the ability to control, manipulate and analyze oxygen and vacancies at the nanoscale, albeit in the bulk crystals. However, in addition to control of the bulk structure and stoichiometry, it is equally important to atomically "engineer" interfaces in oxide systems because almost all practical devices are greatly influenced by the presence of electrically active interfaces. In fact, the trapping and flow of mobile charge carriers across and at interfaces form the basis for numerous electronic, transport, and capacitive properties of interfacial systems in oxides. Thus, the key to understand, manipulate and realize the full functionality of oxides is to engineer oxygen and vacancy content at individual interfaces by suitable doping and heat treatment procedures. Although the utility and study of GB doping has an extensive history, 6-8 only few reports exist on structureproperty co-relationship of single SrTiO 3 GBs. 5, [9] [10] [11] We oriented bicrystals of SrTiO 3 as a model system. The bicrystals are used in order to avoid the issue of geometric randomness in polycrystalline materials. Bulk donor ͑Nb͒ doped SrTiO 3 bicrystals exhibit nonlinear I-V behavior with resistivity almost 4 orders of magnitude higher than their single crystal counterparts. 5 This behavior can be significantly changed by simple thermal oxidation and reduction treatments, which alter the electrostatic potential barriers at the GBs, owing to change in oxygen and vacancy content and configuration. In this letter, we investigate the effects of oxygen and oxygen vacancy concentration on the GB potential barrier. Symmetrical tilt bicrystals of SrTiO 3 , 24°͓001͔, were obtained from CrysTec GmbH ͑Berlin, Germany͒. In order to manipulate the oxygen vacancy concentration at the GB, one set of samples was reduced ͓at 800°C with a constant flow of CO-CO 2 ͑5% CO by volume͒ for 30 min͔, while another set was oxidized ͑at 1000°C with a constant flow of highpurity oxygen for 2 h͒. These were chosen simply as two diverse conditions in order to get different oxygen vacancy concentration at the GB. Self-supporting TEM specimens were prepared using standard TEM specimen preparation techniques. The electrical characterization was carried out using a computer interface controlled current source and a multimeter. 12 The detailed atomic structure of the GB, including local chemistry and valence changes were studied using scanning transmission electron microscope ͑STEM͒ equipped with a spherical aberration ͑C s ͒ corrector ͑Nion Co., Kirkland WA, USA͒. 13 C s corrected STEM offers unprecedented opportunity to probe the details of atomic structure at grain boundaries with a sub-Å electron probe 14,15 using high angle annular dark field ͑HAADF͒ or so-called Z-contrast and phase contrast bright-field ͑BF͒ STEM imaging, coupled with atomic-scale electron energy loss spectroscopy ͑EELS͒. The electrons form the HAADF image in which the intensity of each atomic column, to a good approximation, is directly proportional to the atomic number ϳZ 2 . 16 For this study, HAADF imaging was performed using a C s corrected STEM with a probe size close to 0.6 Å ͑VG HB603 U͒, 14 while HAADF imaging with EELS was done using a C s corrected STEM with a probe size close to 1 Å ͑VG HB501 UX͒. Four probe dc I-V measurements were carried out on pristine, reduced, and oxidized SrTiO 3 bicrystals ͑Fig. 1͒. As shown in the figure, a strong nonlinear characteristic is observed, clearly marking the critical voltage ͑V C ͒ above which the current abruptly begins to flow. The V C varies considerably with the processing conditions: ϳ0.12 V for the reduced sample, ϳ0.24 V for the pristine sample, and ϳ0.7 V for the oxidized sample. The decrease in the V C for the reduced sample may be due to the decrease in the number of trap states at the GB or an increase in the number of majority charge carriers ͑electrons͒, while the increased V C for the oxidized sample may be due to an increased number of traps at the GB. By looking at V C , together with prebreakdown resistance ͑see inset of Fig. 1͒ it can be inferred that as compared to the pristine samples, reduced samples showed a decrease in the GB barrier height, while oxidized samples showed an increase in the GB barrier height. 17 In order to probe the atomic-scale origin of this differing behavior, we imaged the reduced and oxidized 24°symmet-a͒ Electronic
doi:10.1063/1.2046734 fatcat:pqef2m2zcbhyti2k4jqdvimzhm