Magnetism and Epitaxy

Transcription

1 Integration of Functional Oxides and Semiconductors: Magnetism and Epitaxy Alex Demkov The University of Texas at Austin Texas A&M University, Commerce, November 2013

2 People involved: Hosung Seo Miri Choi Patrick Ponath Kristy Kormondy Agham Posadas Chandrima Mitra Chungwei Lin Richard Hatch

3 Outline of the talk Introduction Magnetism in Oxides Molecular Beam Epitaxy COX LaCoO 3 on Si Conclusions

4 Advances in Oxide Epitaxy

5 Epitaxial oxide on semiconductors BaTiO 3 on Ge SrTiO 3 on Si Model Experiment R. McKee, F. Walker, M. Chisholm, PRL (1998) R. McKee, F. Walker, M. Chisholm, Science 293, 468 (2001)

6 SrTiO 3 /LaAlO 3 heterostructure: H. W. Hwang et al., Nature 427, 423 (2004) H. W. Hwang, Science 313, 1895 (2006)

7 N. Reyren et al., Science (2007) A. Brinkman et al., Nature Materials 6, 493 (2007)

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9 Conceptual structure of the 3-D heterogeneous optoelectronic integrated system-on-silicon for an intelligent vehicle system s variable signal-processing functions depending on the moving speed of the car. K.-W. Lee, A. Noriki, K. Kiyoyama, T. Fukushima, T. Tanaka, and M. Koyanagi, IEEE Trans. Electron Dev. 58, 748 (2011).

10 Diverse Accessible Heterogeneous Integration (DAHI): Compound Semiconductor Materials on Si, Electronic-photonic heterogeneous integration

11 Transition metals A transition metal is one which forms one or more stable ions which have incompletely filled d orbitals. Erwin Schrödinger [Ar] = 1s 2 2s 2 2p 6 3s 2 3p 6 [Ti] = [Ar]3d 2 4s 2 [V] = [Ar]3d 3 4s 2

12 Perovskite oxides ABO 3 z A - Ligand B O TM y x CaTiO 3, BaTiO 3, SrHfO 3, MO 6 - Count Lev Alekseevich Perovski Octahedral symmetry (O h ): Energy 10 Dq e g High spin Low spin Fe 3+ (d 5 ) t 2g Ligand field theory E S -E T =2J

13 Molecular Orbital Theory M 3d e g * 10 Dq t 2g * e g 10 Dq t 2g O 2p t 2g M 3d e g c O 2p Important energies: crystal filed splitting 10Dq exchange energy J charge transfer energy c

14 Ferroelectricity BaTiO 3 V.L. Ginzburg L.D. Landau B.M. Vul

15 Molecular Beam Epitaxy Epitaxy: ordered growth on a monocrystalline substrate From two Greek words: epi -above and taxis -in ordered manner

16 MBE was invented in the late 1960s at Bell Laboratories by J. R. Arthur and Alfred Y. Cho

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18 Making Nothing: Vacuum Pumps Torr Torr Torr Torr

19 Vacuum Chamber Flanges Manipulators Vacuum gauges Transfer rods

20 Knudsen Cell Martin Hans Christian Knudsen ( ) E-gun evaporator

21 Quartz Crystal Monitor Jacques Curie Pierre Curie

22 RHEED

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24 Materials Physics Laboratory

25 Theoretical methods i i i i i x m F R E F = = ) ( ) ( ) ( r r r V m i i i ψ ψ = ε + ] [ ] [ ] [ ] [ ] [. n E E n E n E n E H n E XC ion ion ion elec Hartree E K KS = Ψ Ψ = ) ( ) ( ) ( r r r V m i i i KS ψ ψ = ε + ) ( ) ( ) ( ) ( r V dr r r r n r V r V XC ext KS + + = i xc i i i i i xc i i i i V Z V E E Φ Σ Φ + Φ Σ Φ + = ) ( ) ( ε ε ε

26 SrTiO 3 /LaAlO 3 heterostructure: 10 Average POT (ev) vacuum LAO STO LAO vacuum Distance along the 001 (A) 5 4 LAO LAO LAO LAO LAO STO STO STO STO STO LAO LAO LAO LAO LAO STO Ti-d orbital 1st interface layer 3 Energy (ev) LAO LAO E F E gsto =3.2eV DOS J.K. Lee and AAD, Phys. Rev. B 78, (2008) Energy (ev)

27 COX: Crystalline oxide on semiconductor BaTiO 3 on Ge SrTiO 3 on Si Model Experiment R. McKee, F. Walker, M. Chisholm, PRL (1998) R. McKee, F. Walker, M. Chisholm, Science 293, 468 (2001)

28 A. Geometry: Silicon Si and STO are very different! 45 rotation ABO 3 A-layer B-layer a Si /(2) 0.5 =3.84 Å a STO =3.905 Å B. Chemistry:?

29 Zintl intermetallics : SrAl 2 -e Sr Al Al Al Si P Zintl Alchemy Edward Zintl ( ) fcc Al metal SrAl 2 structure ti10 SrAl 4 structure

30 SrTiO 3 deposition on Si Sr-assisted SiO 2 desorption Y. Wei et al., J. Vac. Sci. Technol. B 20, 1402 (2002). B. K. Moon and H. Ishiwara, Jpn. J. Appl. Phys., Part 2 33, L472 (1994). ½ monolayer Sr on Si (Zintl template layer) Edward Zintl Initial amorphous SrTiO 3 seed layer at 200 C (4 unit cells) Crystallize at 550 C Main SrTiO 3 deposition 4x10-7 torr O 2 at 550 C Co-evaporation of Sr and Ti at 1 monolayer per minute 20 unit cells (fully relaxed) STO <110> STO <100>

31 D. Smith, Arizona State 5.98 nm 1.72 nm J. Bruley and C. Dubourdieu, IBM

32 Integrating ferromagnets on Si (001) 7.0x x10-5 Magnetic moment (emu) 5.0x x x x x Temperature (K) Magnetic moment (µ B /Co) Magnetic field (koe)

33 Properties and applications La 1-x Sr x CoO 3 Properties Co 3+ : 3d ev gap semiconductor Non-magnetic at low temperature but paramagnetic at room temperature Epitaxial strain induces ferromagnetism* Spin state transitions Low, intermediate, high-spin Metal-insulator transition when doped Possible applications Electrode (Sr-doped) Cathode material for solid oxide fuel cells Epitaxial oxide electrode for perovskite multilayers Gas sensors / catalysis Magnetic semiconductor Spintronics t 2g e g Fuchs et al., PRB 75, (2007) Rondinelli&Spaldin, PRB 79, (2009) NO Gupta&Mahadevan, PRB 79, (2009) YES

34 LaCoO 3 Low spin (LS) S = 0 Intermediate spin (IS) S = 1 High spin (HS) S = 2 t 2g * (W 1.5 ev) E F e g * (W 4 ev) *Low spin (LS); S = 0

35 Energy vs. strain DOS (a.u.) Energy (ev/cell) d(yz) / d(xz) t 2g* e g * NM (a - a - c 0 ) NM (a 0 a 0 c - ) NM (a - a - c - ) Homo. IS Strain (%) Energy (ev) d(yz) / d(xz) IS e g * t 2g * Half-metallic IS is stabilized beyond 3.8%. Experimentally, strained LCO on STO is insulating. Experimental critical strain is less than 3.8%.

36 Issues related to MBE growth of LCO on Si Direct deposition of La, Co on Si in oxygen at high temperature will form CoSi 2 and SiO 2 Incommensurate or amorphous Prevents epitaxy Phase formation range of LaCoO 3 requires both high oxygen chemical potential and high temperature Typical MBE growth conditions using molecular oxygen (10-6 torr) results in Co 2+ oxidation state To overcome these difficulties we will use an SrTiO 3 /Si pseudo substrate Use an epitaxial template layer SrTiO 3 on Si Use activated oxygen atomic oxygen from rf plasma source

37 Growth of LaCoO 3 on STO/silicon Atomic oxygen 300 W rf power 1x10-5 torr background oxygen pressure Substrate temperature 750 C Co-deposition of La and Co with matched fluxes 2 unit cells per minute rate Slow cooling in oxygen 10 C per minute to 100 C LCO <110> LCO <100>

38 Cross-section TEM LaCoO 3 8 nm SrTiO 3 6 nm SiO 2 Si

39 X-ray diffraction 30 nm LCO/8 nm STO/Si Core level spectra (XPS) La 3d 3/2 La 3d 5/2 O 1s Co 2p La 3d Intensity (a.u.) Co 2p 1/2 Co 2p 3/2 O 1s No secondary phases (La 4 Co 3 O 10,La 2 CoO 4, CoO) LaCoO 3 lattice parameters (bulk a = 3.80 Å) c = 3.77 Å a = 3.89 Å Strained to SrTiO 3 (a = 3.90 Å) Binding energy (ev) No Co metal detected in XPS Spectra consistent with literature data for single crystal

40 Magnetization vs. temperature 7.0x x10-5 H = 1 koe Field cooled Magnetic moment (emu) 5.0x x x x x T C = 85 K Temperature (K)

41 Magnetization vs. field T = 10 K Magnetic moment (µ B /Co) Magnetic field (koe) Posadas, et al., Appl. Phys. Lett. 98, , (2011).

42 Energy vs. strain DOS (a.u.) Energy (ev/cell) d(yz) / d(xz) t 2g* e g * NM (a - a - c 0 ) NM (a 0 a 0 c - ) NM (a - a - c - ) Homo. IS Strain (%) Energy (ev) d(yz) / d(xz) IS e g * t 2g * Half-metallic IS is stabilized beyond 3.8%. Experimentally, strained LCO on STO is insulating. Experimental critical strain is less than 3.8%.

43 Supercells independent Co sites 2 in-plane, 2 out-of-plane Identical site - 8 independent Co sites 2 in-plane, 4 out-of-plane - 8 independent Co sites 4 in-plane, 2 out-of-plane

44 Energy vs. strain: HS/LS mixed states Energy (ev/cell) 1.2 Non-magnetic Homo. IS HS/LS (1:3) HS/LS (1:1) Strain (%) Seo, et al., Phys. Rev. B 86, (2012). 3.8% 2.5%

45 Band gap change as a function of strain Energy (ev) strain (%) d x 2 -y 2 d 3z 2 -r 2 d xy d yz, d xz D 4h Compressive Cubic, O h D 4h Tensile d 3z 2 -r d 2 x 2 -y 2 d yz, d xz d xy

46 Strain accommodation LaCoO 3 SrTiO 3 Corner-sharing octahedral network with relatively rigid CoO 6 units under epitaxial stress c a b out b -3 in -6-9 ( bin bout ) TD = -4 b + b / in out Δ TD (%) strain (%)

47 Bond lengths and angles Δ TD (%) NM HS site LS site strain (%) θ in ( ) θ 0 = NM HS/LS strain (%) θ out ( ) NM HS/LS θ 0 = strain (%)

48 Voltage-switchable magnetoresistance in LaCoO 3 SEM image of device Normally nonmagnetic LaCoO 3 becomes ferromagnetic below 85 K under tensile strain No magnetoresistance above T C for both voltage polarities Magnetoresistance observed only below T C and for only positive voltage Critical voltage needed to observe magentoresistance In collaboration with Ed Yu, UT Austin

49 Summary First demonstration of epitaxial growth of magnetic LaCoO 3 on silicon. High quality crystalline LaCoO 3 layer epitaxially strained to underlying SrTiO 3 buffer (XRD, TEM, XPS), T C ~ 85 K (SQUID) Beyond biaxial tensile strain of 2.5% local magnetic moments, originating from HS (S=2) states of Co 3+ ions, emerge in the LS Co 3+ matrix. The HS/LS state is insulating. The stabilization of the FM state is attributed to increased compliance of LCO when it has higher concentration of HS Co 3+ ions. Despite the energy cost to excite LS Co 3+ to HS state, LCO chooses this option and gains energy above tensile strain of 2.5% owing to the softness of the HS CoO 6 clusters. In contrast, compressive strain could not produce a magnetic state in LCO.