Surface characterization of oxygen deficient SrTiO 3

Transcription

1 U. Scotti di Uccio Surface characterization of oxygen deficient SrTiO 3 films and single crystals Coherentia-CNR-INFM Napoli, Italy Prof. R. Vaglio R. Di Capua, G. De Luca, M. Radovic, N. Lampis, P. Perna, M. Salluzzo, A. Sambri and F. Miletto Granozio In my talk I will present very recent results achieved by the Coherentia group in Napoli, regarding the surface characterization of oxygen deficient Strontium Titanate films and crystals. 1

2 Coherentia Labs. Napoli UHV base P < mbar Modular system for Oxide Deposition and Analyses NEW PLD chamber AFM STM SPA-LEED XPS RHEED This is a picture of our lab. It shows the MODA system, that is a Modular system for Oxide Deposition and Analysis. It mainly consists of a chamber devoted to pulsed laser deposition, that is connected to several analysis chambers, that is, X ray photoemission Spectroscopy, Low Energy Electron Diffraction, Scanning Tunneling Microscopy. 2

3 PLD chamber Base pressure P = 5 x 10-9 mbar Radiative heater T max = 1000 C in air Quadrupole Residual gas determination High pressure RHEED Fast photography setup for analysis of the plume expansion Here is the sketch of the PLD chamber. I haven t got time to get into the details. Briefly, it is equipped with a heater that can reach up to one thousend degrees in air, a quadrupole for residual gas analysis, a high pressure RHEED and a fast photography setup for analysis of the plume. 3

4 Research activity on SrTiO 3 Motivation: SrTiO 3 is the first brick in many Oxide Electronics devices PI-07 Room-temperature blue-light emission in electron-doped SrTiO 3 PII-24 PLD propagation dynamics of the plasma plume PII-49 Surface structure of SrTiO 3 by GXID Other oxides: PI-32, PII-14, PII-18 This presentation: Focus on 1. fundamental properties of SrTiO 3 surface at conditions of typical UHV deposition environments treatments up to 900 C - NO ion cleaning procedure 2. growth mode of thin films The motivation of this work is that STO is the very first brick in many oxide electronics devices. We have a widespread activity on STO both aimed to fundamental physics and to applications. We present several posters at this meeting that are related to this material. The first one is devoted to blue light fluorescence. The second is a presentation of our investigations on plume dynamics during PLD. The third reports on our grazing angle diffraction measurements performed in synchrotron light. The focus of this talk is instead on fundamental properties of the surface of samples that are treated at moderate temperatures, without resorting to any ion cleaning procedure, and on growth modes of thin films. 4

5 Outline LEED, STM 1. Single crystal (100) SrTiO 3 with TiO 2 termination plane This is the outline. 5

6 Outline LEED, STM 1. Single crystal (100) SrTiO 3 with TiO 2 termination plane Selective ethcing procedure Un. Twente, APL 73, 2920 (1998) I will first talk about single termination STO crystals that we got from the Twente University. 6

7 Outline LEED, STM 1. Single crystal (100) SrTiO 3 with TiO 2 termination plane 2. PLD of epitaxial (100) SrTiO 3 and TiO 2 films FILM DEPOSITION CONDITIONS: Eccimer laser KrF - λ = 248 nm Target substrate 41 mm P(O 2 ) = 0.1 mbar SrTiO 3 TiO 2 Effective Fluence: 80 mj /2.6 mm 2 Repetition rate: 2 Hz Deposition temperature: 890 C Effective Fluence: : 100 mj /2.6 mm 2 Repetition rate: 1 Hz Deposition temperature: 800 C Then I will show you results concerning STO and TiO2 films deposited in MODA on those substrates. 7

8 Outline LEED, STM 1. Single crystal (100) SrTiO 3 with TiO 2 termination plane 2. PLD of epitaxial (100) SrTiO 3 and TiO 2 films 3. (110) SrTiO 3 single crystals and films Finally I will briefly show results for the 110 crystal orientation 8

9 (01) 1. Single crystal (100) SrTiO 3 - TiO 2 termination 2h UHV P < 10-9 Torr T = 910 C (-10) (10) (0-1) E = 22 C Robust 1 1 lattice No reconstruction This is a map of the intensity of diffracted electrons in LEED. In this case, the sample was prepared by annealing in vacuum at about nine hundred degrees. After cooling we observe a clear diffracted pattern, but no trace of reconstruction. 9

10 1. Single crystal (100) SrTiO 3 - TiO 2 termination 2h UHV annealing P < 10-9 Torr T = 800 C (-10) (10) Cps (0 0) (1 /2 0) (1 0) K // / K B Spot Analysis Profile LEED (0-1) E = C 2 1 lattice double domain On the contrary, samples that have been annealed at 8 hundred degrees do show a reconstruction. As we see from the map, it is the 2 by 1 reconstruction with two perpendicular domains. On the right, I am showing a scan along the 1 0 direction, that is easily obtained by our Spot Profile Analysis LEED 10

11 1. Single crystal (100) SrTiO 3 - TiO 2 termination Is O diffusion detrimental to the stability of reconstruction? diffusion from the bulk: low activation energy Surface BULK emission to the vacuum: high activation energy O vacancies start at step edges and move through colunar defects SrTiO 3 (001) (2 1) reconstructions: First-principles calculations K. Johnston, M. R. Castell, A. T. Paxton, and M. W. Finnis PRB 70, (2004) Switching the electrical resistance K. Szot, et al., Nature Materials 5, 312 (2006) The reconstructions that we find are generally ascribed to oxygen loss due to emission toward the external environmente. However, the surface can also exchange oxygen with the bulk. The bulk diffusion of oxygen is very fast in oxygen depleted STO. The question is, is this diffusion detrimental to the stability of reconstruction? 11

12 1. Single crystal (100) SrTiO 3 - TiO 2 termination Is O diffusion detrimental to the stability of reconstruction? diffusion from the bulk: low activation energy Surface BULK emission to the vacuum: high activation energy 800 C annealing in UHV Experiment Investigate the stability of the reconstructions against low temperature annealing Room temperature C annealings in UHV To check this, we designed an experiment in which a sample was first prepared at 8 hundred degrees, then cooled down to room temperature, and again heated at moderate temperature to promote bulk diffusion. 12

13 1. Single crystal (100) SrTiO 3 - TiO 2 termination (100) SrTiO 3 K 4 (010) Two domains One domain No reconstr. T = 23 C T = 255 C T = 280 C Line of O vacancies Reconstruction domain O diffusion from the bulk is filling vacancies The sample is a vicinal cut STO crystal. These are the results. At room temperature we get the usual 2x1 reconstruction with two domains. At about 2 hundred fifty degrees, one domain of reconstruction disappears, and at about 3 hundred degrees there is no more reconstruction. So we see that oxygen diffusion really seems to kill some vacancies at the surface. 13

14 1. Single crystal (100) SrTiO 3 - TiO 2 termination Summary and interpretation 300 C ideal Models of Reconstruction: (2 1) domain (a) 800 C Ti 2 O 3 average stoichiometry filling Top views of the Ti-O Surface lattice (2 1) domain (b) 900 C disorder SrTiO 3 (001) (2 1) reconstructions: First-principles calculations K. Johnston, M. R. Castell, A. T. Paxton, and M. W. Finnis PRB 70, (2004) So what is the interpretation of these results? Also in the framework of the current literature, we believe that at 8 hundred degrees we get the reconstruction of oxygen vacancies at the surface. Heating up to 9 hundred degrees the vacancy content increases and the ordered lattice is destroyed. On the contrary oxygen diffusion from the bulk fills up surface vacancies and can destroy reconstructions. 14

15 2. PLD homoepitaxial (100) STO film room temperature STM 2D island growth mode 10 pa 2 V Typical 10 pa 2 V T d = 890 C 1 µm 1 µm 1 µm 1 µm (100) Single crystal STO 1h UHV 900 C 20 nm thick (100) STO film 1h UHV 800 C Now let me show some results on STO films. Here I compare the morphology of a typical substrate and of a film grown at about 9 hundred degrees and treated in UHV at 8 hundred degrees. At these measurement conditions the STM reveals more features than usual AFM maps. We see that our STO films have a 2 dimensional growth mode. Islands are at an advanced level of coalescence in this picture. Probably, the step flow growth mode is not achieved in this case because of the huge width of substrate terraces. 15

16 2. PLD homoepitaxial (100) STO film Possible Reconstructions of films are different from crystals (01) indication of Sr at the surface Surface structure of SrTiO 3 (1 0 0) T. Kubo, H. Nozoye Surf. Sci. 542 (2003) (-10) (10) (0-1) c (2 2) reconstruction P reconstructions O vacancies C reconstructions Sr adatoms Test : TiO 2 films It is quite interesting to note that our STO films have a centered 2 by 2 reconstruction, that is different from the 2 by 1 reconstruction observed for single crystals. We think that this is possibly related to the presence of some strontium adatoms, and we tentatively propose that as a general rule in our experiments, the p reconstructions are due to oxygen vacancies, while c reconstructions are due to strontium adatoms. To test this idea, we investigated Titanum dioxide films, that should not be affected by presence of strontium at the surface. 16

17 2. PLD epitaxial (100) TiO 2 film Room T STM 200 x 200 nm 2 Anatase TiO 2 fits (100) SrTiO 3 10 nm thick TiO 2 film Flat terraces, sharp step edges 800 C UHV annealing Here are the results. The anatase phase of titanum dioxide is epitaxial on STO and the films show sharp step edges separating different growth domains. However the morphology is not worse than polished crystals, and can be considered very good for this material. 17

18 2. PLD epitaxial (100) TiO 2 film Evidence of superstructure d 1.6 nm Local Fourier analysis global 80 x 80 nm 2 Correlation filtering More interestingly, STM provides evidence of a surface reconstruction. This is clear in Fourier analyses that show reconstruction spots. We see 4 spots in the global analysis of the picture, and only 2 in the analysis performed on a smaller area. This indicates two domains of reconstruction. The reconstruction is also clear in real space and the filtered map performed on a smaller scale demonstrates it clearly. The distance between stripes is about 1 dot 6 nanometers. 18

19 2. PLD epitaxial (100) TiO 2 film Close up STM STM LEED Quantitative agreement double domain 4 1 lattice No Sr adatoms p reconstruction STM results are in quantitative agreement with LEED data. We observe a 4 by one lattice, confirming our hypothesis that when Sr adatoms don t play a role, titanum oxide planes show p reconstructions. 19

20 3. (110) STO single crystals and films 800 C UHV annealing (6 x 4) reconstruction 200 x 200 nm 2 ( ) Structure and electronic states on reduced SrTiO3 (110) H. Bando, et al. J. Vac. Sci. Technol. B 13, 1150, (1995) Finally I come to 110 STO. Both in the case of crystals and of films the samples show the 6 by 4 reconstruction or similar variants. The STM pictures show some features that could be related to the reconstruction, but are not as clear. The 6 by 4 reconstruction is generally ascribed to titanum dioxide chains at the surface or even to microfaceting. 20

21 3. (110) STO 400 x 400 nm C 500 x 500 nm C 500 x 500 nm C UHV annealing STM shows increasing disorder LEED pattern is preserved: The reconstruction is not due to O vacancies Log ( I / I o ) 550 C 20 C SPA LEED Scans along [ 110 ] k x (a. u.) In this slide I show the effect of moderate annealing on the surface. The STM shows that roughning of step edges takes place at relatively low temperature. Above 500 C the surface is rougher. The white dots are probably Strontium oxide precipitates. Quite interestingly, the LEED pattern is stable under these cycles. This is probably related to the type of reconstruction, that doesn t seem to be due to the oxygen vacancies. 21

22 Summary We grew high quality SrTiO 3 and TiO 2 films We investigated the structure and morphology of the SrTiO 3 surface prepared at relatively low temperatures 800 C UHV annealings TiO 2 terminated (100) STO crystals (100) STO homoepitaxial films (100) TiO 2 films (110) STO films & crystals P type 2 x 1 C type 2 x 2 P type 4 x 1 P type 6 x 4 Suggested interpretations O vacancies Sr adatoms O vacancies TiO 2 stripes So I come to my conclusions. We grew high quality STO and TiO2 epitaxial films, and we investigated the structure and morphology of STO surface. In the table below I summarize the the observed reconstructions and the suggested interpretations. Thank you for your attention. 22

23 Instrumental Setup SPA LEED Spot Profile Analysis LEED Omicron 23

24 variable temperature STM/AFM Omicron 24

25 STM su Si (111) 25

26 High pressure RHEED 35 KeV gun STAIB Screen TSST Double differential pumping 0,5 Torr 50 kev 26

27 References Selective ethcing procedure Stability of SrTiO 3 reconstructions Diffusion of oxygen in bulk SrTiO 3 Role of Sr adatoms TiO SrTiO 3 reconstructions Un. Twente, APL 73, 2920 (1998) K. Johnston, M. R. Castell, A. T. Paxton, and M. W. Finnis PRB 70, (2004) K. Szot, et al., Nature Materials 5, 312 (2006) T. Kubo, et al., Surf. Sci. 542 (2003) Q. Jiang, ej. Zegenhagen, Surf. Sci. Lett. 367, L42 (1996) U. Diebold, Surf. Sci. Rep. 48, 53 (2003) Yong Liang, et al., PRB 63, (2001) H. Bando, et al. J. Vac. Sci. Technol. B 13, 1150, (1995) 27