Materials Chemistry and Physics 91 (2005) 243–246

Materials science communication

Electronic structures near surfaces of perovskite type oxides

Toru Hara∗

R&D Center, Taiyo Yuden Co., Ltd., 1037 Oazasasoh, Shinji-Town, Yatsuka-Gun 699 0406, Japan

Received 17 September 2004; accepted 22 November 2004

Abstract

This work is intended to draw attention to the origin of the electronic structures near surfaces of perovskite type oxides. Deep stateswere observed by ultraviolet photoelectron spectroscopic measurements. The film thickness dependent electronic structures near surfacesof (Ba0.5Sr0.5)TiO3 thin films were observed. As for the 117–308 nm thick (Ba0.5Sr0.5)TiO3 films, deep states were lying at 0.20, 0.55, and0.85 eV below the quasi-fermi level, respectively. However, as for the 40 nm thick (Ba0.5Sr0.5)TiO3 film, the states were overlapped. The A-sitedoping affected electronic structures near surfaces of SrTiO3 single crystals. No evolution of deep states in non-doped SrTiO3 single crystalwas observed. However, the evolution of deep states in La-doped SrTiO3 single crystal was observed.© 2004 Elsevier B.V. All rights reserved.

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eywords:Perovskite; Electronic structure; SrTiO3

. Introduction

Much attention has been focused on perovskite type ox-des thin films due to their potential application as dynamicandom-access memories (DRAMs), and as tunable mi-rowave devices. Although the effects of electronic structuresf perovskite type oxides thin films on their electrical charac-

eristics have been discussed, real physical mechanisms seemo be not fully understood probably due to the insufficient un-erstanding of the origins of electronic structures.

There are several experimental and theoretical re-ults discussing the electronic structures of SrTiO3 and/orBa,Sr)TiO3 single crystals and/or thin films[1–11], and dis-ussing the existing of shallow donor states (0.04, 0.08, and.15 eV below the conduction band minimum) induced byxygen vacancies[5–9], deep states (0.5, 0.8, 1.2, 1.3, 1.5,nd 1.6 eV below the conduction band minimum) inducedy the oxygen vacancies clustering[1–3,11], and deep-lyingcceptor-like surface state (2.4–2.9 eV below the conductionand minimum)[1,2,9–11].

In this work, we supply the experimental results for the

thin films as a function of film thickness, which are invtigated by ultraviolet photoelectron spectroscopy in air.advantage of this study is that the measurements were dair. Sample surfaces are covered with oxygen in air. Elecnear surfaces of (Ba,Sr)TiO3 thin films can be captured boxygen. Therefore, electron depletion occurs near surof these films due to oxygen in air. Previously, we relathe electronic structure of free surfaces of (Ba,Sr)TiO3 thinfilms to the relaxation currents of Pt/(Ba,Sr)TiO3/Pt thin filmcapacitors, and concluded that the relaxation currents arto the electron detrapping from deep states in the band[12,15].

In this work, we investigate the effect of La-dopingthe electronic structures at near surfaces of SrTiO3 singlecrystals, in which the concentration of oxygen vacancieassumed to be less than that of sputtered thin film. The Ametals are ionic rather than B-site metals, which havevalent character. Therefore, it is assumed that the La-docan distort the TiO6 octahedron due to the smaller radof La (0.117 nm) in comparison with that of Sr (0.132 nThis can cause the tensile strain in the SrTiO3 single

lectronic structures near surfaces of sputtered (Ba,Sr)TiO3

∗ Tel.: +81 852 66 2735; fax: +81 852 66 2735.E-mail address:thara@m6.dion.ne.jp.

crystals.We hope that the discussion in this paper will be useful

for understanding of the physical nature of perovskite typeoxides thin films.

d.

254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserveoi:10.1016/j.matchemphys.2004.11.032

244 T. Hara / Materials Chemistry and Physics 91 (2005) 243–246

2. Experimental

The (Ba0.5Sr0.5)TiO3a(308–40 nm)/Pt(100 nm) spec-imens were fabricated on TiO2(2 nm)/SiO2(80 nm)/Sisubstrates for ultraviolet photoelectron spectroscopicmeasurements. The Pt bottom electrodes were depositedby dc sputtering in an ambient of argon (Ar) gases at aconstant pressure of 0.7 Pa, at a constant substrate tem-perature of 250◦C, and at a fixed dc power of 100 W.

The (Ba0.5Sr0.5)TiO3 films were deposited on the bottomPt electrodes by rf magnetron sputtering in an ambientof oxygen (O2) and Ar gases with O2/Ar ratio of 5/5, ata constant pressure of 0.035 Pa, at a constant substratetemperature of 600◦C, and at a fixed rf power of 1 kWand a fixed dc power of 200 W. The Pt top electrodes weredeposited with a diameter of 0.5 mm on the (Ba0.5Sr0.5)TiO3films by electron beam evaporation at 120◦C, througha metal shadow mask. After a series of depositions, the

Fa

ig. 1. The results of ultraviolet photoelectron spectroscopic measurementnd (c) 40 nm.

s for (Ba0.5Sr0.5)TiO3(308–40 nm)/Pt(100 nm) specimen. (a) 308 nm, (b) 117 nm

T. Hara / Materials Chemistry and Physics 91 (2005) 243–246 245

capacitors were annealed at 600◦C in oxygen ambient for30 min.

Single crystals of non-doped, La-doped (0.73 wt.%)SrTiO3 were obtained from Earth Chemical Co., Ltd. Sam-ples were cut in the shape of rectangular plates having di-mensions in the order of 10 mm× 10 mm× 1 mm, and hav-ing major faces parallel to the (1 0 0) crystallographic plane.The sample surfaces were mechanically polished.

The ultraviolet photoelectron spectroscopic measure-ments were performed using a model AC-2 ultraviolet photo-electron spectrometer (RIKEN KEIKI Co., Ltd.). The valueof each measuring time was 10 s. The ultraviolet photoelec-tron spectroscopic measurements were performed at 25◦C inair with an incident angle of 30◦.

3. Results and discussion

Fig. 1(a)–(c) shows the results of ultraviolet photo-electron spectroscopic measurements for (Ba0.5Sr0.5)TiO3(308–40 nm)/Pt(100 nm) specimens. As shown inFig. 1(a)–(c), deep states in the band gap of (Ba0.5Sr0.5)TiO3thin film were observed. They were lying at 0.20 eVbelow the quasi-fermi level (1.60 eV below the conductionband minimum), at 0.55 eV below the quasi-fermi level(1.95 eV below the conduction band minimum), at 0.85 eVb tion

band minimum), respectively. Recalling the upward bandbending of 0.7 eV at the free surface[13] and the downwardbending of quasi-fermi level in the depletion layer, itis reasonable to consider that the electrons trapped bythe states lying at 0–1.4 eV below the conduction bandminimum in the depletion layer are swept out. As reportedpreviously [14], two kinds of activation energies for therelaxation behavior of bismuth doped SrTiO3 (0.13–0.28and 0.59–0.78 eV assigned as the electron detrapping fromdeep states induced by oxygen vacancies) were observed.Assuming the overlapping of the state lying at 0.45–0.60 eVand the state lying at 0.80–0.90 eV below the quasi-fermilevel, these results are in good agreement with our resultsof ultraviolet photoelectron spectroscopic measurements.However, as for the (Ba0.5Sr0.5)TiO3(40 nm)/Pt(100 nm)specimen, the states were overlapped. This may becaused by the insufficient relaxation of tensile straindue to the thermal expansion coefficient mismatch be-tween the 40 nm thick (Ba0.5Sr0.5)TiO3 film and the Sisubstrate.

Fig. 2(a) and (b) shows the results of ultraviolet photo-electron spectroscopic measurement for non-doped or La-doped SrTiO3 single crystals. No evolution of deep statesin non-doped SrTiO3 single crystal was observed, as shownin Fig. 2(a). However, the evolution of deep states in La-doped SrTiO single crystal was observed, as shown inF e to

elow the quasi-fermi level (2.25 eV below the conduc

Fig. 2. The results of ultraviolet photoelectron spectroscopic meas

3ig. 2(b). This may be induced by the tensile strain du

urements for SrTiO3 single crystals. (a) non-doped and (b) La-doped.

246 T. Hara / Materials Chemistry and Physics 91 (2005) 243–246

the smaller radius of La (0.117 nm) in comparison with thatof Sr (0.132 nm).

Why the discrepancy of electronic structure occurs be-tween the 40 nm thick (Ba0.5Sr0.5)TiO3 film and the La-dopedSrTiO3 single crystal? The conceivable reasons are the fol-lowings: (1) The sputtered films are multi-orientated [(1 0 0),(1 1 0), and (1 1 1)], and the orientation axes are declinedirregularly. And the sputtered films have grain boundaries.These characteristics may cause the dispersion of energy lev-els for deep states in (Ba0.5Sr0.5)TiO3 films. It is assumed thatthe tensile strain exert the different influence on each orien-tation. (2) The thermal expansion coefficient mismatch be-tween the (Ba0.5Sr0.5)TiO3 film and the Si substrate causethe two-dimensional tensile strain in the (Ba0.5Sr0.5)TiO3film. However, La-doping cause the three-dimensional ten-sile strain in the SrTiO3 single crystal. It is assumed that thetwo-dimensional strain can cause the energy level dispersionbetween Ti-3dxyorbital component and Ti-3dyzor Ti-3dzxor-bital component, which are the components of deep states,rather than that the three-dimensional strain can cause.

4. Conclusions

Deep states were observed by ultraviolet photoelectronspectroscopic measurements in air. The film thickness depen-dt lec-

tronic structures near surfaces of SrTiO3 single crystals. Anadvantage of this study is that the measurements were done inair. Sample surfaces are covered with oxygen in air. Electronsat near surfaces of perovskite type oxides can be captured byoxygen. Therefore, electron depletion occurs near surfacesof these films due to oxygen in air, as if the metal electrodeformed the Schottky barrier.

References

[1] H. Tanaka, T. Matsumoto, T. Kawai, S. Kawai, Jpn. J. Appl. Phys.32 (1993) 1405.

[2] Y. Aiura, I. Hase, H. Bando, T. Yasue, T. Saitoh, D.S. Dessau, Surf.Sci. 515 (2002) 61.

[3] N. Shanthi, D.D. Sarma, Phys. Rev. B 57 (1998) 2153.[4] A. Ohtomo, H.Y. Hwang, Nature 427 (2004) 423.[5] O.N. Tufte, P.W. Chapman, Phys. Rev. 155 (1967) 155.[6] J.D. Baniecki, R.B. Laibowitz, T.M. Shaw, C. Parks, J. Lian, H. Xu,

Q.Y. Ma, J. Appl. Phys. 89 (2001) 2873.[7] G.M. Choi, H.L. Tuller, D. Goldschmidt, Phys. Rev. B 34 (1986)

6972.[8] C.N. Berglund, W.S. Baer, Phys. Rev. 157 (1967) 358.[9] C. Lee, J. Yaiha, J.L. Brebner, Phys. Rev. B 3 (1971) 2525.

[10] R.L. Wild, E.M. Rocker, J.C. Smith, Phys. Rev. B 8 (1973) 3828.[11] E. Longo, E. Orhan, F.M. Pontes, C.D. Pinheiro, E.R. Leite, J.A.

Varela, P.S. Pizani, T.M. Boschi, F. Lanciotti Jr., A. Beltran, J. An-dres, Phys. Rev. B 69 (2004) 125115.

[12] T. Hara, Solid State Commun. 132 (2004) 109.[[[

ent electronic structures near surfaces of (Ba0.5Sr0.5)TiO3hin films were observed, and the A-site doping affected e

13] A. Ohtomo, H.Y. Hwang, Appl. Phys. Lett. 84 (2004) 1716.14] C. Ang, Z. Yu, L.E. Cross, Phys. Rev. B 62 (2000) 228.15] T. Hara, Microelectron. Eng. 75 (2004) 316.