ELSEVIER Surface Science 369 (1996) 313-320

surface science

Atomic force microscopy study of (001) SrTiO3 surfaces

B. St/iuble-POmpin, B. Ilge, V.C. Matijasevic 1 P.M.L.O. Scholte, A.J. Steinfort, F. Tuinstra

Department of Applied Physics, Solid State Physics, TU Delft, Lorentzweg 1, 2628 C J, Delft, The Netherlands

Received 17 March 1996; accepted for publication 27 June 1996

Abstract

By means of atomic force microscopy, we have investigated the surfaces of SrTiO3(001) substrates (used for the growth of high-T~ superconducting films) after different heat treatments. These treatments were (i) annealing in Oa, (ii) annealing in 02, (iii) annealing in Oa followed by deposition of SrTiO3, and (iv) annealing in UHV. Our results suggest that, independently of the type of annealing, the surfaces of the polished SrTiO 3 substrates rearrange, either by diffusion or desorption of atoms, until reaching the energetically most favourable surface structure. For vicinal (001) substrates, this equilibrium structure corresponds to atomically fiat terraces separated by smooth steps of one unit-cell in height, with the orientation and width of the terraces being deter- mined by the miscut of the substrate. However, the kinetics involved in the reconstruction of the SrTiOa surface were observed to be strongly dependent on the annealing conditions. In particular, the use of O 3 instead of 02 or UHV was found to accelerate the reconstruction of the SrTiOa surface. Finally, this study provides an accurate characterisation of the defects present on SrTiO3 substrates.

Keywords: Atomic force microscopy; Strontium titanate; Surface defects; Surface structure, morphology, roughness, and topography; Vicinal single crystal surfaces

1. Introduction

W i t h recent years, the unde r s t and ing of the mechan i sms invo lved in the nuc lea t ion and g rowth of th in films of cupra te supe rconduc to r s has p ro - gressed rapidly . In par t i cu la r , i t is now accepted tha t the surface s t ruc ture of a subs t ra te can s t rong ly influence the m o r p h o l o g y of the subse- quent ly depos i t ed high-T~ film. Therefore , an accu- ra te cha rac te r i sa t ion and unde r s t and ing of the surface of subs t ra tes is essent ial to g row high- qua l i ty th in and u l t r a th in films.

1 Present address: Conductus, 969 West Maude Avenue, Sunnyvale, CA 94086, USA.

The surface of s ingle-crysta l SrTiO3 (which is used extensively as a subs t ra te for the epi taxia l g rowth of YBa2Cu3Or and re la ted high-T~ superconduc tors ) has recent ly been the subject of b o t h theore t ica l and exper imenta l s tudies I -1-7] . However , due to the var ie ty of different surface t r ea tmen t s app l ied to the inves t iga ted SrTiOa crys- tals, the pub l i shed results are difficult to c ompa re and often seem conflicting.

T h e a im of the a tomic force mic roscopy ( A F M ) s tudy presented here was to invest igate and com- pare the surface m o r p h o l o g y of SrTiO3(001) after different t rea tments . These t r ea tments were: (i) annea l ing in 03, (ii) annea l ing in 02, (iii) annea l ing

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314 B. Stdiuble-Piimpin et al./Surface Science 369 (1996) 313-320

in 03 followed by deposition of SrTiO3, and (iv) operating in the tapping mode with a silicon-type annealing in UHV. cantilever, under ambient conditions.

2. Experimental

SrTiO3 is a widely used substrate to grow epitax- ial RBa2Cu30 r ( R = Y or lanthanide) films. The substrates investigated in our study are commer- dally available (Crystal GmbH, Berlin) polished single-crystal substrates with a nominal (001) ori- entation. In order to simplify the comparison, most of the AFM results shown here (Figs. 1-5) were obtained from the same lcm x lcm x lmm sub- strate which was cut into four parts. Comparable measurements were performed on other SrTiO3 substrates and gave consistent results.

Annealing in 03 (ozone) was performed in an UHV-MBE system adapted for reactive coevapor- ation, with a background pressure of 5 x 10 .6 mbar. The ozone was generated with a commercial

generator and then purified in a home-made still. The ozone concentration was between 50 and 100%. The incidence rate of the ozone beam was 1016 molecules cm -2 S -1.

Annealing in 02 was performed in flowing oxygen, in a furnace at ambient pressure.

The deposition of SrTiO3 was done by coevapor- ating Sr and Ti at 750°C in an ozone beam in the reactive MBE system. By adequately timing the deposition process, a total of 10 monolayers of SrTiO 3 were grown on top of the previously annealed substrates. This was confirmed by the analysis (after film growth) of the RHEED oscilla- tions of the specular spot.

The miscut angles of the substrates were deter- mined by optically aligning the sample with a laser and then measuring the position of two different crystal reflections using a standard four-circle diffractometer. The direction of the surface normal with respect to the crystal lattice was characterised by the angles e (the vicinal angle between the surface normal and the (001) direction) and fl (the angle between the (100) direction and the pro- jection of the surface normal into the (001) plane).

All images were obtained on a Nanoscope III AFM (Digital Instruments, Santa Barbara, CA)

3. Results

3.1. Annealing in 03

The first type of heat treatment used was annea- ling of the substrate for 15-25 min in 03 at a temperature of 700, 750 or 900°C. The first two temperatures are typical for the growth of cuprate films in our reactive MBE system. Within the investigated range, the treated SrTiO3 surface was not found to depend significantly on the annealing time or temperature. Typically, the substrates were warmed to the desired annealing temperature within 5-10 min and cooled down within approxi- mately 30 min.

As shown in Fig. 1; the resulting surface mor- phology of the SrTiO3 is characterised by a series of parallel steps indicating the existence of a slight miscut of the substrate with respect to its nominal (001) orientation. The height of the steps is 0.39 __ 0.01 nm (i.e. one lattice constant of SrTiO3) and the terrace width is equal to 85_+ 5 nm. From independent X-ray diffraction measurements, the vicinal angle e of the same substrate was found to be equal to 0.27___0.05 °, which corresponds to a terrace width of 85__15nm, in good agreement with the above AFM results. The step ledges are perpendicular to the projection of the surface normal into the (001) plane. Their orientation with respect to the crystallographic lattice is character- ised by the miscut angle fi (see definition in Section 2). As this angle fi can take on any value (for the sample shown in Fig. 1, fi=17__5°), we would expect that on a microscopic scale, due to the orientation dependence of the step energy, the step ledges should zigzag along well-defined crys- tallographic directions. However, within the reso- lution of the measurements performed here, the observed step ledges are in general smooth, i.e. only a limited amount of kink bunching is observedl

In addition, the surface of SrTiO3 was character- ised by different types of defects. Dislocations(see

B. Stgiuble-Pfimpin et al./Surface Science 369 (1996) 313-320 315

(a)

, 0 0 i 3 . 0 r ~

1 . 5 r'm

O . 0 1 ~

1. ( iO

(b)

~o o.zo o.~0 o.~o 03,0 IIN

A A'

Fig. 1. (a) AFM image of an SrTiOs(001) substrate after annea- ling in Oa at 750°C for 15 rain. The black arrow points to a dislocation. The vertical profile taken along the line A-A' is shown in (b). The profile was averaged over a width of 200 nm. The observed step height is equal to one unit cell of SrTiO3.

arrow in Fig. 1) were observed with a density of 6 + 1 x 107 cm -2. Such dislocations could, by inher- itance, lead to the formation of spiral'shaped islands in the subsequently deposited high-T~ superconductor film. Furthermore, line-shaped defects running along the (100) and (010) directions of the substrate were found. The length of these one-dimensional arrays of defects was observed to vary between a few hundred nm and a few #m. Typically, a height difference of one lattice constant of SrTiO3 is observed across such a defect. In some cases, two linear defects cross each other, resulting in the cross-shaped defect displayed in Fig. 2. Similar cross-shaped defects have been reported by Sum et al. [5] . As shown in Fig. 2, step ledges are occasionally found to have a very wavy, almost dendritic structure. Finally, holes with a diameter of 10 -20nm

were observed with a density of approximately 2 + 1 x 109 cm -2.

3.2. Annealing in Oe

Figs. 3 and 4 display an SrTiO3 surface after annealing for 1 h at 750°C in 02. In order to minimise thermal strain resulting from a possible temperature gradient over the sample, the substrate was warmed up to the desired temperature within 8 h and cooled down again to room temperature within approximately 10 h.

The surface morphology shown in Fig. 3a is characterised by irregularly shaped terraces. Although the step ledges appear to be wavy due to step bunching, they are locally (on a scale of 10-100 nm) parallel to crystallographic directions, such as for example, (100), (110), (120) and (130). The height of the steps typically varies between 0.5, 1, 1.5, 2 and 2.5 unit cells of SrTiO3 (see, e.g., Fig. 3b), indicating the presence of different termination layers. To exclude calibration errors, the z-calibration of the AFM was checked immediately before and after the recordings of the AFM images shown in Figs. 3 and 4.

As in the case of 03 annealing, line-shaped defects and holes were also observed after heat treatment in 02 (see Fig. 4). Typically the holes have the form of inverted pyramids with the sides (10 -20nm long) aligned along the (110) direc- tions. Larger holes with the sides aligned along the (100) directions were also observed. Comparable square holes have been observed by Jiang et al. [6] after annealing in hydrogen as well as by Kawasaki et al. [3] after etching the surface of their samples. While some of t h e holes on our substrates were distributed randomly on the SrTiO3 surface (approximately 2 x 10 s cm-2), most of them were found to be grouped into chains parallel to the (100) and (010) directions (see Fig. 4).

Finally, annealing a substrate first in O2 and then in 03, or vice-versa, resulted in a surface comparable to that described in Section 3.1.

3.3. Annealing in 03followed by the deposition o f SrTi03

We attempted to examine how the SrTiO3 deposition affected the surface morphology and in

316 B. Stgiuble-Pfimpin et al./Surface Science 369 (1996) 313-320

.z,oo

~onN

0 nM

~! .00

0 nM

1.00 2 . 0 0

Fig. 2. A F M image of the same sample as displayed in Fig. 1, showing two line-shaped defects crossing each other. The linear defects are parallel to the (100) and (010) directions.

particular whether it would improve its smooth- ness. Therefore, one of the treatments investigated was annealing of the substrate in 03 as described in Section 3.1 followed by the deposition of 10 layers of SrTiO3 in the reactive MBE system. The resulting surface is shown in Fig. 5.

As for all SrTiO3 substrates with 03 annealing, no step bunching was observed. Although line- shaped defects were still present, they were observed to heal as additional material was depos- ited. No holes could be resolved. Furthermore, the surface roughness was found to have increased due to the formation of islands nucleating on the terraces, in between two steps. These islands are typically 0.4 nm high, which suggests that they are SrTiO3 unit cells.

3.4. Annealing in U H V

Fig. 6 displays the surface of a SrTiO3 substrate which was annealed for 20 min at 750°C in UHV (background pressure of 10 -7 mbar). The warming and cooling procedures for this sample are compa- rable to those used in 03 annealing described in Section 3.1.

When imaging this sample with AFM, steps on

its surface can generally be recognised. However, unlike the atomically fiat substrates annealed in 03, this UHV annealed substrate displays rough step ledges and small clusters are clearly seen on top of its terraces. This is consistent with results by Jiang et al. 1-7] showing that higher temper- atures and longer annealing times are required to obtain an atomically fiat SrTiO3 surface by annealing in UHV.

4. Discussion

Two possible models could explain the pro- nounced differences in the surface morphology of SrTiO3 substrates after annealing at 750°C in 03 (Fig. 1) or in O2 (Fig. 3).

Model I. Each observed surface reflects a thermo- dynamically stable state for a given annealing pressure and temperature. When cooling down to room temperature, this state is frozen in.

Model H. Before annealing, the surface is disor- dered, probably due to the polishing of the sub- strate. While annealing the sample, adatoms are desorbed and/or diffuse on the surface. Different annealing conditions (e.g. pressure and temper-

B. Stgiuble-Pfimpin et al./Surface Science 369 (1996) 313-320 317

(a)

A

0 0 . 2 5 0 . 5 0 0 . 7 5 1 . 0 0 m,

(b) ~ ] ~ v.. ,',mnc.

0 0 . ~ o , ~ 0 . ~

A ~ A'

Fig. 3. (a) AFM image of an SrTiO3(001) substrate after annea- ling in 02 at 750°C for 1 h. The open arrows show step ledges which are parallel to crystallographic directions (see text for more details). The vertical profile taken along the (110) direc- tion (line A-A') is shown in (b). The markers show steps with heights of 1 and 2.5 unit cells of SrTiO3, respectively.

ature) correspond to different kinetics all leading to the same end state, the surface structure with lowest (surface) energy.

In order to test which of these two models can best explain our experimental results, we checked whether the obtained surface morphologies were reversible under subsequent annealings. A set of substrates was first annealed in 03 (as described in Section 3.1) and then in 02 (as described in Section 3.2). Another sample was first annealed in O2 (as described in Section 3.2) and then in 03 (as described in Section 3.1). In both cases, the surface structure after two consecutive annealings was always characterised by parallel steps with a step height of one unit-cell of SrTiO3. The surface displayed in Fig. 3, which was obtained after a single annealing in 02 at 750°C, is therefore not a reversible structure. As a consequence, model I cannot explain our experimental data.

Model II, however, provides a consistent expla- nation of the results discussed so far. It should be pointed out that the idea of a disordered SrWiO 3 surface before annealing is in agreement with the results of Ref. [3] . Using i o n scattering spectro- scopy, Kawasaki et al. [3] showed that for com- mercially available SrTiO3, the terminating atomic layer was 5-25% SrO and 95-75% TiO2. Annealing the substrates under appropriate conditions allows the SrTiO3 surfaces to reconstruct and reach the energetically most favourable surface structure. For a vicinal (001) SrTiO3 substrate, this corresponds to atomically flat terraces separated by smooth steps of one unit-cell in height, running parallel to each other. Our results are consistent with those of Sum et al. [2,5], who showed that after 30 min annealing in 0 2 at 800°C, the SrTiO3 surface was characterised by parallel steps with rather curved step ledges, while after 30 min at 900°C, these step ledges were much straighter.

A comparison between Figs. 1 and 3 and the results of Refs. [2,5] clearly shows that the time scale for surface reconstruction is much slower for the annealing process in Oz than for the annealing process in 03. In particular, the observation of steps of half a unit-cell in height (indicative for different termination layers) strongly suggests that either the annealing time is too short or the annealing temperature too low.

It is likely that the lower background pressure used for the 03 annealing and the reactiveness of the 03 are responsible for a higher desorption rate and a stronger interaction with the SrTiO3 surface, respectively. In order to investigate the importance of the low background pressure and the role of the ozone, a substrate was heated to 750°C in U H V (see Section 3.4). A comparison between Fig. 1 (annealing in 03) and Fig. 6 (annealing in UHV) clearly shows that 03 plays an important role in accelerating the reconstruction of the SrTiO 3 surface.

Because of the reactiveness of the 03 molecules, the existence of a s tronger interaction between SrTiO3 and 03 than between SrTiO3 and 02 is in principle not surprising: it is well known that much lower temperatures are required for an 03 molecule to give up an O atom than for an 02 mole- cule. Furthermore, high-resolution transmission

318 B. Stgiuble-Piimpin et al./Surface Science 369 (1996) 313-320

.50

5.0 nN

2 . 5 nH

0,0 n M

.2S

0 0 . 2 5 0o 50 0 ,75

IJN

Fig. 4. AFM image of the same sample as displayed in Fig. 3, showing two chains of holes both parallel to the (100) direction. The black arrow points to a square-shaped hole. The open arrows show step ledges which are parallel to crystallographic directions.

.3.1111]

.2.(11)

,0 nK

.0 nM

,0 n M

-1.011

o 1 .oo z.bo 3.bo °

Fig. 5. AFM image of an SrTiO3(00l) substrate which was first annealed in 03 followed by the deposition of Ten unit-cell layers of SrTiO3. Note that the linear defect shown in this image is healing. Furthermore, the surface roughness has increased due to the nucleation of SrTiO3 islands.

electron microscopy results have shown that the terminating layer of substrates having the equilib- rium surface structure is TiO2 [8]. One could

therefore speculate that during the annealing, oxygen atoms react with the uppermost SrO atomic layer, leading to its dissolution. For com-

B. Stgiuble-Pf#npin et al./Surfaee Science 369 (1996) 313-320

1,0 rim

319

.~,0 nH

~250 1.0 nN

.... ~ 0 0 250 500

nl<

Fig. 6. A F M image of an SrTiO3(001 ) substrate after annealing in U H V at 750°C for 20 min. The terraces are not atomically flat. The open arrows show step ledges which are parallel to crystallographic directions.

pleteness, it is interesting to mention that SrTiO3 substrates with an equilibrium surface structure were obtained by Kawasaki et al. [3] without annealing the substrates, but by etching them with a pH-controlled NH4F-HF (BHF) solution. Using ion scattering spectroscopy, Kawasaki et al. [3] observed that the terminating atomic layer is TiOz with a coverage factor of 100%, and con- cluded that the BHF solution selectively dissolves the SrO atomic plane.

The remaining part of this discussion will con- centrate on the origin of the defects observed on most substrates. As described in Section 3, the most striking features are one-dimensional arrays of defects parallel to the (100) and (010~ direc- tions. Because such defects were also observed after the very slow annealing in O2 (see Section 3.2 it is unlikely that they are the result of thermal strain induced by a temperature gradient over the samples. The fact that these linear defects follow a crystallographic orientation indicates that they can be related to dislocation lines already present in the single-crystalline SrTiO3 before it was cut and polished. In Ref. [5], Sum et al. suggest that line- shaped defects result from the relaxation of such

dislocations during the annealing of the substrate. Such a relaxation can occur by glide of the disloca- tion. However, the chains of holes described in Section 3.2 indicate that another relaxation mecha- nism might also play a role in the case of SrTiO3 substrates. Due to the stress field induced by dislocations below the surface of the substrates, favourable desorption sites for adatoms can align along crystallographic axes. During the annealing, material preferentially evaporates from these desorption sites, leading to the formation of chains of holes. As soon as enough material is evaporated, the dislocations within the SrTiO3 could relax, forming the one-dimensional arrays of defects described in Section 3.1.

An attempt to smoothen the surface of the substrates by first annealing them in 03 and then depositing a few layers of SrTiO3 was only partially successful. It was found that the SrTiO3 was prefer- entially deposited along steps, such as, for example, those of the line-shaped defects. As a consequence, these defects were observed to heal. However, SrTiO3 islands were also found to nucleate on the existing terraces, resulting in an increased rough- ness of the surface of the substrates.

320 B. Stiiuble-Piimpin et al./Surface Science 369 (1996) 313-320

5. Conclusions

This A F M study emphasises the importance of an appropriate pretreatment of SrTiO3 substrates in order to obtain smooth and well-defined sur- faces. The results presented here strongly suggest that, at high temperatures, the surface of the pol- ished SrTiO3 substrate rearranges itself, either by diffusion or desorption of atoms, until reaching the energetically most favourable surface structure. For vicinal (001) substrates, this equilibrium struc- ture corresponds to series of smooth, one unit-cell high steps running parallel to each other, with the terrace width being defined by the magnitude of the vicinal angle.

The time needed to reach this equifibrium struc- ture was found to depend strongly on the annealing conditions. For instance, it was observed that, for the same temperature (750°C), annealing a sub- strate in 03 resulted in a smoother, better defined surface than annealing (even for longer times) in O2 or in UHV. In terms of rearrangement of the SrTiO3 surface, annealing in 03 is more efficient than annealing in Oz or in UHV. Based on our results, it is likely that the reactiveness of 03 plays an important role in accelerating the reconstruc- tion of the SrTiO3 surface.

Finally, several types of defects were observed on the substrates investigated, such as holes (diam- eter 10-20nm), (screw-) dislocations (density 6-t-1 x 10 7 c m - 2 ) and line-shaped defects parallel to crystallographic orientations. The later were

found to heal after an O3 annealing followed by the deposition of a few layers of SrTiO3. However, after the deposition of SrTiO3, the surface rough- ness increased due to the nucleation of SrTiO3 islands.

Acknowledgements

We are very grateful to J. van Wingerden for helpful and stimulating discussions leading to this work. One of us (B.S.-P.) acknowledges financial support from DIMES. This work was supported by F O M and the Dutch National Research Program (NOP) for High-T~ Superconductors.

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