The Electronic Properties of Extended Defects in SrTiO3—A Case Study of a Real Bicrystal Boundary

: This study investigates the impact of extended defects such as dislocations on the electronic properties of SrTiO3 by using a 36.8° bicrystal as a model system. In order to evaluate the hypothesis that dislocations can serve as preferential reduction sites, which has been proposed in the literature on the basis of ab initio simulations, as well as on experiments employing local-conductivity atomic force microscopy (LC-AFM), detailed investigations of the bicrystal boundary are conducted. In addition to LC-AFM, fluorescence lifetime imaging microscopy (FLIM) is applied herein as a complementary method for mapping the local electronic properties on the microscale. Both techniques confirm that the electronic structure and electronic transport in dislocation-rich regions significantly differ from those of undistorted SrTiO 3 . Upon thermal reduction, a further confinement of conductivity to the bicrystal boundary region was found, indicating that extended defects can indeed be regarded as the origin of filament formation. This leads to the evolution of inhomogeneous properties of defective SrTiO 3 on the nano- and microscales. measurements were conducted after the reduction in out-of-plane geometry using pasted indium electrodes with a diameter of 500 gm. The resistance was measured using a remote preamplifier (6430, Keithley, Solon, OH, USA). Simulations of the conductivity were performed using an electronic circuit simulator (Micro-Cap 6 , Spectrum Software,

highly inhom ogeneous in the first stage of reduction w hen a solid oxide such as SrTiO 3 is reduced by either therm al annealing under low oxygen partial pressure or by applying an electric field, leading to stoichiom etric polarization [13]. As the regions close to the dislocations are reduced at first, conducting paths evolve, w hich are called filam ents [14]. Those filam ents are of particular im portance for resistive sw itch in g p h en om en a, as th eir resistan ce can b e sw itch ed b etw een a h ig h ly co n d u ctin g O N and an in su latin g O FF state, w h ich can be u sed for d ata storage [9 , 15] [16].
T hese in v estig atio n s hav e d em o n strated th a t in the first stage o f red u ctio n , o n ly a few, sp atially highly confined cond u ctive spots are detectable, w hich serves as a strong indicator of the presence of localized conducting paths [15]. To directly investigate the electronic properties of dislocation-rich grain bo u n d aries, h erein w e in v estigate co m m ercially a v ailab le large-an g le SrT iO 3 b icry stals. In o rd er to determ ine the electronic structure by optical m eans, w e analyse the photolum inescence of the bicrystals in different reduction states. It has been show n that there is a close correlation betw een the presence of oxygen vacancies and lum inescence [17][18][19]. In order to m ap the differences betw een the SrTiO 3 m atrix and bicry stal boundary, w e em ploy FLIM , w h ich has b een sh ow n to b e a p ow erful tool for d etecting sm all variations in the concentration of electronic charge carriers in SrTiO 3 and has been em ployed to in vestigate inhom og eneities in N b-and L a-d oped SrT iO 3 [20,21]

M aterials and M ethods
SrT iO 3  Tokyo, Jap an ) eq u ip p ed w ith P t/Ir-coated tips (P P P -C o n tP t, N an o sen sors, N eu ch atel, Sw itzerlan d ).
The sam ples w ere positioned on a heating stage, w hich allow ed the perform ance of AFM m easurem ents at 260 °C in situ. Therm al reduction of the crystals w as conducted by annealing at 800 °C for one hour under vacuum conditions (p < 1 X 1 0 -6 mbar) using a glass tube furnace for those sam ples investigated b y electrical m easurem ents. T he redu ced sam ples in vestigated b y FL IM w ere an n ealed at 850 °C for 0.5 h under a 300 m bar deuterium atm osphere. Som e crystals w ere etched using buffered hydrofluoric acid, resulting in the evolution of etch pits at the exits of the dislocations [23]. A fter etching, the sam ples w ere subsequently carefully rinsed w ith w ater in order to avoid contam inants on the surface influencing the electrical m easurem ents. M acroscopic resistance m easurem ents w ere conducted after the reduction in o u t-o f-p lan e g eo m etry u sin g pasted in d iu m electrod es w ith a d iam eter o f 500 gm . T he resistance w as m easu red u sin g a rem ote p ream p lifier (6430, K eithley, So lon , O H , U SA ). S im u lation s o f the con d u ctivity w ere perform ed u sin g an electronic circu it sim ulator (M icro-C ap 6 , Sp ectru m Softw are, Su n n yv ale, C A , U SA ). F L IM m easu rem en ts w ere p erfo rm ed o n an u p rig h t scan n in g flu orescen ce m icroscope (A1R, N ikon, A m sterdam , The N etherlands) using a pulsed high repetition rate Ti:Sa-laser  [21,24,25]. D ue to the h ig h in ten sity o f the laser ligh t, m u lti-p h o to n ab so rp tio n b y SrTiO 3 w as enabled to a significant degree and hence photolum inescence could be observed, although the single photon energy w as sm aller than the band gap of SrT iO 3.

Structure o f the B icrystal B oundary
For this study, w e selected bicry stals w ith a large tilt angle o f 36 .8°, p rovidin g a grain bou n d ary w ith a high dislocation density. In order to produce such tilt bicrystals, a single crystal is cut at an angle o f 18.4° w ith resp ect to the < 1 0 0 > axes. Th e tw o p ieces are th en jo in ed to g eth er b y p ressin g a t h ig h tem perature [26][27][28]. In our study, this preparation w as perform ed by the m anufacturers. In the ideal case, this approach should result in an energetically favorable, sym m etric E 5 boundary that has a regular arran g em en t o f d islocation s alth o u g h this is, how ever, d ifficu lt to ach iev e in reality [  Before focusing on the investigation of the electrical and optical properties of the SrTiO 3 bicrystals, w e w ould lige to com m ent on the quality pf com m ercially available bicrystals. As elaborated in deTail in o u r p reviou s review [14], a w ell-o rd ered a rra n g em en t o f d islo catio n s a t the E 5 b o u n d a ry is n o t alw ays observed. U sing the etch-pit technique to m ark the exits of dislocations, it has been noted thaf com m ercially available S rT iC l bicry stals one far from perfect [14]. /On exam ple is show n in Figure 2a .
at the bicrystal boundary, bu t a m osaic structure of m any sm all grains em erging from the ju n ction can also be identified, each grain itself possessing dislocation-rich grain boundaries. As both the front-and backsid e of the bicry stal show a com p arable pattern of etch pits. The blue coloration of tha images results (rom tho optimization of the differential interference contreat. Adapted from Szot et al . [14] . W hereas, in the latter case, the bound ary appears to have an increased but inhom ogeneous conductivity, in the other case, the boundary itself appears to be insulating but w ith a zone of increased conductivity form ed beside it. This illustrates that in a "re a l" bicrystal, a variety of electrically conducting paths can tie present, w hich m o st likely relate to the im perfections in bicrystal fabrication, as described above.  By extracting line profiles perpen d icu lar to the boundary, these differences are illustrated (Figure 5c).
In the first case, the w idth of the w ell-conducting area at the boundary w as in the range of 100 nm , w hile in the second, the inhom ogeneously conducting band had a w id th of m ore than 1 gm . This difference m ig h t ag ain relate to the im p erfectio ns in b icry stal p ro d u ctio n ; how ever, th e m ain resu lt from this m easurem ent is that the region of increased con-ductivity is m uch m ore confined to the boundary after th erm al red u ctio n th an p rio r (Figu re 3 ). It m u st b e n oted th at the m easu rem en ts sh o w n in Figure 5 w ere p erfo rm ed ex situ after the exp o su re o f the sam p le to a m b ien t co n d itio n s. It has b een sh ow n previously that the therm ally reduced SrTiC>3 surface is highly conductive under in situ conditions [18] bu t that the m ajority of the surface becom es passivated upon eaposure to oxygen at room tem perature, th u s b eco m in g in su la ting ag ain [16 ]. O n ly a few co n d u ctin g spots w ere fou n d to rem ain h ig h lg

Investigation o f M acroscopic C onductivity
To transform this m odel w ith varying layer thicknesses into a quadiatic grid of resistors, w e scaled the single resistor elem ents accordingly. fulfilled [42,4 3 ]. This m echanism is further supported by a rearrangem ent and bundling of dislocations occurring due to the high-tem perature annealing [14]. In consequence, this im plies that the form ation of conducting filam ents can be influenced by the introduction of dislocations e.g., by form ing a bicrystal bound ary or by m echanical polishing of the surface. This could offer prom ising opportunities to tune the electrical properties of SrTiO 3 b y m ech an ical m ethods.

HF-etched SrTi03
As-received SrTi03 Thermally-reduced SrTi03 Figure 9. Illustration of the model of a hierarchical tree of dislocations acting as preferentially conducting paths in tOe surface layer of SrTiO3. The three sample states, HF-etched, as-received and thermally reduced, are depicted.  As discussed above, the dislocation density of the surface layer is m uch higher than that of the bulk [36] .

Investigation o f P hotolum inescence
H ence, this could indicate that the presence of dislocation correlates to a sm aller lum inescence lifetim e, w hich w ould be in agreem ent w ith the conclusions derived from the lifetim e analysis of the boundary. For instance, oxygen vacancies can serve as localized traps for electrons, contribu ting to the observed ex ten sion o f p h o to lu m in escen ce lifetim es [48]. T his effect cou ld also b e related to self-trap p in g of polarons, e.g., at the Ti site next to an isolated oxygen vacancy, w hich w as found to have a significant im p act on the electronic structure an d the lu m inescen ce [53] . N everth eless, in ou r data, n o t o n ly the m agnitude of the fast decay com ponent bu t also the lifetim e of the slow decay com ponent has changed dram atically, bein g about 10 tim es longer com pared to the as-received SrTiO 3 crystal.
It is also know n that w ith the therm ally induced reduction of SrTiO 3, the surface region becom es Ti-rich and eventually, titanium suboxides form [54,5 5 ]. Hence, the incipient phase transform ation could lead to clustering effects on the nanoscale, resulting in the observed inhom ogeneous photolum inescence lifetim e d istribu tion. R eg ard in g the d ecay cu rves sh ow n fo r tw o selected spots a t the b o u n d ary and su rround in g su rface (Figure 12b ), it can be seen that the fast com p on en t at the b ou n d ary had a m uch higher am plitude than in the su rrou nd in g su rface. In consequen ce, the am plitud e-w eighted average lifetim e at the bou nd ary w as also m uch shorter than in the su rroun d in g surface. can easily be transported to recom bination centers located at the dislocations. O ur results show that the detailed physical nature of the photolum inescence decay close to dislocations is com plex, as one has to consider the electronic structure of SrTiO3, the electronic structure of the dislocations, and furtherm ore potentiaf e a sy m ig ra tio n paths of the oxcited charge carriers.

C onclusions
The