Comments on the originof the two dimensional electron gas at the polar/non polar oxide interfaces

Transcription

1 Comments on the originof the two dimensional electron gas at the polar/non polar oxide interfaces U. Scotti di Uccio S. Amoruso, C. Aruta, R. Bruzzese, E. Di Gennaro, A. Gadaleta, D. Maccariello, D. Marrè, L. Marrucci, F. Miletto Granozio, I. Pallecchi, D. Paparo, P. Perna, M. Radovic, Mohamed Riaz, A. Rubano, A. Sambri, X. Wang R. Di Capua, Z. Ristic, M. Salluzzo, R. Vaglio, I. Maggio-Aprile* CNR-SPIN & University FEDERICO II, Napoli (Italy) C. Cantoni, S. J. Pennycook, M. Varela Oak Ridge National Laboratory *Dép. de Physique de la Matière Condensée, University of Geneva 1

2 Polar-non polar interfaces Where do electrons come from? Electronic reconstruction P o = ½e/S donors STO + + LAO Donor defects + + Cation intermixing σ b = ½e/S CB e - STO LAO STO - LAO + VB VB - Oxygen vacancies This talk: focus on oxygen vacancies 2

3 2DEG at STO interfaces Polar-non polar heterostructures A. Brinkman, et al., Nature Mat Fabrication conditions: 10-6 mbar Oxygen vacancies dominate! MODEL A quantum well at STO surface/interfaces is formed Different types of donors can provide mobile carriers Questions: Can we demonstrate a system where the formation of a 2DEG in STO as exclusively due to oxygen vacancies? Can we present 2DEG in STO where the role of oxygen vacancies appears as negligible? 3

4 2DEG at STO surfaces Deliberately introducing oxygen vacancies Investigating the electronic properties of the STO surface 1 µm 1 µm AFM STM R. Di Capua, M. Radovic, G. M. De Luca, I. Maggio-Aprile, F. Miletto Granozio, N. C. Plumb, Z. Ristic, U. Scotti di Uccio, R. Vaglio, M. Salluzzo Submitted to Phys. Rev. B

5 Thermal treatment (01) Annealing in oxygen at 850 C Highly insulating Surface charging: no LEED pattern Sample A (-10) (0-1) (10) Annealing in UHV at C (01) Bulk insulating (transparent) 1 1 reconstruction (-10) (10) (0-1) Sample B Annealing in UHV at 900 C Bulk conducting (shiny black) 2 1 reconstruction P 5 o = mbar

6 Scanning Tunneling Spectroscopy tip STO UHV STM tip tip STO STO surface Average on grid of points Regulation points V = -1.5 V, I = 0.1 na V = -1.5 V, I = 0.7 na Agreement with STS Surf. Sci. 542, 177 (2003) Agreement with ARPES Nature 469, 189 (2010) 6

7 2DEG at STO surfaces XPS Photoelectrons mfp nm Shallow angle emission: surface probe 250 C XPS: Ti 3p doublet 900 C Results on 2DEG Spectroscopic signature of 2DEG: Ti 3+ states Confinement: nm scale 7

8 2DEG at STO interfaces Polar-non polar heterostructures P. Perna, et al, APL 2010 LAO STO LGO STO Lanthanum Gallate Lanthanum Aluminate 8

9 EELS in the aberration corrected microscope Spectroscopy with atomic-scale resolution Conducting sample Abrupt interface C. Cantoni, et al. ADV. MAT

10 Oxygen vacancy stoichiometry Integrated O-K Simulation δ 10-3 mbar but 2% maximum error 0.12 e - /cell experiment C. Cantoni, et al. ADV. MAT O-K ELNES Intensity (a.u.) 10

11 Further limiting the role of oxygen vacancies Deposition at high oxygen pressure Drawback The plasma plume is stopped by the buffer gas The energy of impinging species is reduced The surface mobility of adatoms drops Energetic particles can favor 2D growth of oxide thin films through an island break-up mechanism with prompt insertion, at very low coverage, and enhanced surface diffusion, above 50% monolayer coverage growth conditions may be bad! 11

12 Plume analysis and film mbar Plume fast photography Time Resolved Emission Spectroscopy Growth at 10-1 mbar AFM 2D growth LGO/STO A. Aruta, et al, APL 2010 Insulating samples! 12

13 More on mbar Shock wave regime mm Internal energy mm Interesting regime Low K ( ev) High U High µ(o2) 13

14 More on mbar 14 R (Ω / )

15 Conclusions 1. Oxygen vacancies can determine the formation of a 2DEG at the STO free surface Local V(O) 2DEG 2. STO interfaces 10-3 mbar : V(O) = 0 (EELS) 10-1 mbar: conductive interfaces 2DEG Local V(O) 15

16 Conclusions 1. Oxygen vacancies can determine the formation of a 2DEG at the STO free surface Local V(O) 2DEG 2. STO interfaces 10-3 mbar : V(O) = 0 (EELS) 10-1 mbar: conductive interfaces 2DEG Local V(O) Message: Different donor states lead to essentially the same 2DEG at the surface and interfaces of SrTiO 3 16

17 MODA LAB: an integrated laboratory for fabrication of oxide films and for surface analyses 17