Substrate-mediated control of the microstructure of YBa2Cu3O7−δ thin films

Journanl of

AHD C ~ ~ J H [ : ~ Journal of Alloys and Compounds 251 (1997) 23-26

ELSEVIER

Substrate-mediated control of the microstructure of YBa2Cu307_8 thin films T. Haage*, J. Zegenhagen, M. Cardona, H.-U. H a b e r m c i e r Max-Planc'<-lnstitut fiir Festkiirperforschung, Heisenbergstr. i, D-70569 Stuttgart, Germany

Abstract We have studied the surface of vicinal SrTiO~(001) substrates and thereon deposited YBa,Cu~O~_~ (YBCO) thin films by UHV-scanning tunneling microscopy (STM). We discuss the influenceof the uniformity of substrate terraces on film growth. STM images indicate, that a combination of island and step-flow growth occurs on i.2° miscut SrTiOr In particular, a regular nanoscale step structure of a 10° miscut SrTiO~ surface generates an almost periodic surface structure of the YBCO films. Our findings indicate that a substrate-mediated control of the surface structure of YBCO films can be achieved. Keywords: Microstructure: YBa:Cu~O, a thin lilms

discuss the impact of the substrate surface structure on film growth in terms of its unilbrmity and terrace size.

1. Introduction The surface, here of SrTiOd001 ), is the starting place for heteroepitaxy of high-temperature superconducting oxides. On a well-oriented substrate surface, YBa2Cu307 o~ (YBCO) thin films grow in the Stranski-Krastanov mode, i.e. a number of two-dimensional layers grow before threedimensional islands are formed. The growth mode is generally governed by a variety of experimental i'actors (deposition rate, substrate temperature, etc.), as well as the lattice mismatch between the SrTiO~ substrate and the high-?~ material. Initial scanning tunneling microscopy (STM) studies detected a suppression of spiral growth on miscut substrates [1~-3l. However, a systematic characterio zation of vicinal substrate surfaces and thereon deposited fihns is needed to ascertain the effect of the substrate surface structure on the growth mechanism. Recently, we have generated a terraced surface structure of YBCO films grown on 10° miscut SrTiO3 substrates [4]. A combined STM and transmission electron microscopy (TEM) study revealed the presence of a defect microstructure which is aligned along the direction of regular film step edges. Moreover, we observed an anisotropic flux pinning and an enhanced critical current density of our vicinally grown films. In this proceeding, we present the results of UHV-STM studies of vicinal SrTiO3(001) surfaces with a miscut of 1.2°, 3° and 10° and thereon deposited YBCO films. We *Phone: -4.49-I 1-689- i 333; fax: haage@servix,mpi-stuttgart,mpg,de

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2. Experimental YBCO films were grown by pulsed laser deposition (PLD) as described in detail in i51. The SrTiO~ substrates were cut and polished 1.2°, 3°, and l0 ° off the (001) plane towards 10101. The substrates were annealed in UHV ( ~ 10 ='~ cabar) at a temperature of 950 °C for 2 h which produced stepped and clean surfaces. The loss of oxygen during UHV annealing results in notype conductivity which enables us to perform STM on such prepared SrTiO+ surfaces. All STM images were acquired in the constant current mode with tunneling cut+cuts of 0.2-0.3 nA and a positive sample bias of 0.75=1.9 V. After the surface characterization, the substrates were transferred to the PLD chamber for the growth of YBCO thin films. At the end of the PLD process and cooli,lg in oxygen the de~sition chamber was pumped to a pressure of less than 10= mbar and the samples were transferred to the STM stage without exposure to air.

3. Results and discussion The vicinai SrTiOd001) substrates exhibit different types of surt~ce structure. As shown in Fig. l a. the surface of the 1.2° miscut substrate consists of terraces of irregular

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7" Haage el al. I JtJurmd q/'Aih~.vs aml Compemmts 25l (19~7) 23-2b

(a)

(b)

(¢) t:iL~, I, STM ittlage~ of vi¢inal SrTiO, ~urfil¢cs after UHVoannealing: (a) 350 milx 350 nm im~ge of a 1.2° mi,~ul substrate, (b) 150 tun x 150 nnt image of a ,t~' miseut ~ubslralc. (¢) I(X) am× I(X) nm image of a I0" substrale. Crystallographic directions arc marked by arn~ws,

width, Most of the steps are spaced ~, 18 nm apart. They exhibit a height of ~0.39 nm which corresponds to the lattice constant of SrTiO,. Additionally, we observe some ~37 rim wide terraces along with double unitocell (~0.78 nm) high steps. The continuous and straight step edges run parallel to l lOOl. The surface structure of a Y~ miscut S~=TiO~ substrate shown in Fig. i b appears to be even more non=unit~rm than that of the 1.2° miscut substnlte, It contains straight step edges pai'allel to I lOOI, as well as some kinks in the IOlOI direction which result from an additional miscut of 0,2" toward [100l, us determined by Laueodiffraction. We observe a similar number of single

(~0.39 ran) ,~nd triple (~1.17 rim) unit-cell-high steps separating terraces of varying width (7-22 nm), The remarkably regular terrace structure of the I O° miscut SrTiO~ surface, visible in Fig, Ic, differs from that shown in Fig, la and Fig, lb. in the STM image, what appears like rows are in fact 2,3 nm wide SrTiOdOOI ) terraces which a~ separated by unitocell high (~0.39 nm) steps in the I0101 direction. The STM images shown in Fig, 2 demonstrate the effect of the individual step structures of the vicinal substrate surf'aces on YBCO lilm growth, Several features can be seen from the STM image of a I00 unit-cell ( = ! 2 0 nm)

7". Haage et al. I Jounml of AUoyv and Compounds 251 (1997) 23-26

2.5

~

k (a)

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

(e) Fig. 2. STM images of I00 unit-cell (~-~120 nm) thick YBCO lUres grown on vicinal SrTiO~ substrates. (a) and (b) arc 350 nm×350 nm images of YBCO on (a) 1.2° miscut and (b) 3 ° miscut SrTiO,. The black arrow shown in (a) marks an island (see text). (c) displays a I I0 nmX 110 nm image of YBCO on I0 ° miscut Sr'l'iOv

thick film grown on the 1.2° miscut SrTiO~ (see Fig. 2a). We observe 100-130 nm wide terraces which are about six times larger than those of the pristine substratc (see FiB. l a for comparison). The step height of ~ 1.2 mn corresponds I'airly well to the unit-cell length of YBCO along its c-axis. Some steps are spaced ~ 1.8 nm apart. This terrace size was frequently observed on the pristine substrate. Moreover, the large terraces, visible in Fig. 2a, are partially covered by non-unit-cell layers (i.e. layers which exhibit growth steps smaller than the YBCO unit-cell length c~ 1.2 nm) with an irregular shape. Similar morphological

features have recently been observed on the surface of coaxis oriented YBCO films Brown on wellooriented SrTiO~ and LaSrAIO~ substrates 15]. FiB. 2a also displays a 45 nmX75 nm large island which has formed on a 130 nm wide terrace. Even growth islands consisting of stacks of terraces were detected in different surface regions of this YBCO lilm. The surface of a IO0 unit~cell thick YBCO film grown on a 3° miscut substrate contains irregular, 52-73 nm wide terraces which are mostly separated by unit-cell (~ 1.2 nm) high steps (see Fig. 2b). The kink structure of the pristine

26

T. /hinge ,'t al. / Jounud o[ Alloys aml Compoumi.~" 251 t 1997) 23-26

substrate persists to the surface of this relatively thick tilm, But the lihn step edges are less continuous and welloriented than that of the miscut snbstrate. As can be seen from the STM image shown in Fig. 2c, the 100 unit-cell thick YBCO film grown on the 10° miscut substrate displays very regular nanoscale terraces with straight step edges along [100l. But this array of steps is less periodic than that of the vicinal SrTiO 3 surface: A detailed line profile analysis of STM images reveals that most of the terraces are about three times wider than that of the substrate. We observe growth steps of 0.4 nm and 0.6 nm which are shorter than the YBCO unit-cell height c==1.2 nm. Since x-ray diffraction measurements reveal that the film under investigation is c-axis-oriented (c-axis parallel to 10011, i.e. normal to the surface of the terraces), these findings imply that the YBCO unit cells grown on the upper and lower terrace are often shifted vertically, along 10011, forming a translational boundary. it is generally believed that step edges can act as energetically preferred adsorption sites for deposited species, as demonstrated by recent transmission electron microscopy studies 161. YBCO layers will nucleate at the step edges and propagate laterally across the surface as they accommodate species, if the width of the substrate terraces is much smaller than the diffusion length L~>of the deposited species on the substrate surface. As we know from our previous results 171, a variable stacking mode at the interface allows a partial overgrowth of SrTiO~ steps. Therefore, the YBCO terraces become wider than that of the pristine substrate as growth proceeds. Moreover, our findings imply a correlation between the ~rfection of the terrace size distribution of the substrate and YBCO film surfilce: the irregularity of the ~tep ,~pacing which c~m be seen on the 1.20 and 3~ miscut suhstl'ates persists t~, the film surface, whereas the periodic ~tep structure of the 10° mi~cul ~ubstrate leads to a very regular terrace structure of the YBCO filnl grown thereon, As mentioned above, the fact that the nanoscale terraces of the I ~ unit~.cell thick YBCO tilm on 10° miscut SrTiO~ are separated by sub=unit=cell high steps indicates the presence of translational boundaries. Cross-sectional transmission electron microscopy (TEM) measurements confirm this conclusion. They reveal that translational boundaries are frequently generated and healed by stacking faults as growth proceeds I41, Moreover, XRD measure-

ments indicate, that the translational boundaries are aligned with the straight fihn step edges. As a consequence of the variable stacking mode at the interface the size of the YBCO terraces increases as film growth proceeds. We suggest that the width of the terraces on which we detect the island formation is close to the diffusion length Lt,. Thus, adsorbed particles can reach a supersaturation sufficient for nucleation of islands on the terrace. Note, that in the picture of step-flow growth step edges act as a continuous sink for adsorbed particles, if the step spacing is smaller than L o [8]. Therefore, we estimate the surface diffusion length LD----100 nm under the conditions in which our films are grown. In conclusion, we have shown that vicinal SrTiO3(001) surfaces exhibit different types of surface structure. The terraced morphology of our YBCO films originates from a step-flow growth mechanism, it is strongly influenced by the uniformity of the substrate surface structure. We observed a transition from step flow to island growth on 1.2° miscut SrTiO.a. Moreover, we have generated an almost periodic surface structure of c-axis-oriented YBCO films on 10° miscut SrTiO.~. This morphology is linked to a defect microstructure which is aligned along the direction of the straight film step edges. Our results indicate that a control of the surface and defect microstructure of YBCO lilms can be acllieved on tailored substrate surfaces. As described in a separate communication, the resultant film structure strongly influences the electronic properties of the highoT,, material 141.

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