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Structural and Optical Properties of SrTiO 3 on Different Substrates

Structural and Optical Properties of SrTiO 3 on Different Substrates. N. Samarasingha, Cesar Rodriguez , Jaime Moya, Stefan Zollner , Nalin Fernando Department of Physics, New Mexico State University, Las Cruces, NM Sudeshna Chattopadhyay, Indian Institute of Technology Indore, Indore, India

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Structural and Optical Properties of SrTiO 3 on Different Substrates

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  1. Structural and Optical Properties of SrTiO3 on Different Substrates N. Samarasingha, Cesar Rodriguez, Jaime Moya, Stefan Zollner, Nalin FernandoDepartment of Physics, New Mexico State University, Las Cruces, NM Sudeshna Chattopadhyay, Indian Institute of Technology Indore, Indore, India Patrick Ponath, Kristy J. Kormondy, Alex A. Demkov, University of Texas at Austin 2016 Lawrence Symposium on Epitaxy 20-24 February 2016, Scottsdale, AZ 20 nm SrTiO3 SrTiO3 LaAlO3 Si Light NIR/VIS/QUV ellipsometry: 190 to 2500 nm, 77 to 800 K Excitons at Interfaces NSF: DMR-1505172

  2. Graduate Students: Lina Abdallah, Travis Willett-Gies, Nalin Fernando, Tarek Tawalbeh (Theory), Nuwanjula Samarasingha, Nathan NunleyUndergraduate Students: Cesar Rodriguez, Khadijih Mitchell, Cayla Nelson, Jaime Moya, Jackie Cooke, Maria Spies Flat & uniform films, at least 5 by 5 mm2, low surface roughness, films on single-side polished substrateEmail: zollner@nmsu.edu 2 New Mexico State University http://ellipsometry.nmsu.edu

  3. Biography Motorola (Mesa, Tempe) Arizona, 1997-2005 Regensburg Germany Motorola, Freescale Texas, 2005-2007 Freescale, IBM New York, 91-92;07-10 Las Cruces, NM Since 2010 3 New Mexico State University 3

  4. Epitaxy at Motorola (1997-2007) • First SiGeepi reactor in CMOS production fab (Mesa). • First HR-XRD in Si wafer fab. • First six-inch GaAsfab. • Si:C alloys for NMOS (ASM) • AlN on Si (NCSU, Texas Tech) • Jaguar: SrTiO3 on Si • Hobby: Ge1-xSnx on Si Optical spectrum SiC:P SiC:P InGaP HBT epi stack Si:C NMOS Stefan Zollner, 06/09/2012, Conf. for Undergrad. Women in Physical Sciences 4 4

  5. SiGe:C Metrology for the semiconductor industry E1Exciton SiGe:C base High-resolution XRD Si cap (emitter) Si substrate Spectroscopic Ellipsometry 5 SZ, Hildreth, Liu, Zaumseil, Weidner, Tillack, J. Appl. Phys. 88, 4102 (2000)

  6. Optical and X-Ray Characterization of Epitaxial Films Do the properties of SrTiO3 epilayers depend on the substrates? SrTiO3 Bulk SrTiO3 Monochromator LaAlO3 SrTiO3 • What x-ray diffraction reveals: • Distances between atoms • Lattice mismatch (strain) • Grain size • What x-ray reflectivity reveals: • Thickness (5 Åto 1000 Å) • Surface and interface roughness • Electron density profile • What spectroscopic ellipsometryreveals: • Thickness (100 to 10000 A) • Excitonic absorption • Refractive index detector polarizer SrTiO3 SrTiO3 SrTiO3 Ge Si analyzer Φ X-ray beam Thickness sample Roughness atoms New Mexico State University Excitons at Interfaces 6

  7. Ellipsometry of SrTiO3 on Si (Jaguar data) How do the properties of SrTiO3 epilayers on Si depend on thickness? Monochromator • What spectroscopic ellipsometry reveals: • Thickness (100 to 10000 A) • Excitonic absorption • Refractive index detector polarizer SrTiO3 SrTiO3 Si analyzer Φ sample Thin SrTiO3 epilayers on Si absorb less light than thick or bulk SrTiO3. Lower refractive index. 7 New Mexico State University SZ, Demkov, Liu, Curless, Yu, Ramdani, Droopad; Spring 2000 MRS Meeting

  8. Old Explanation of Ellipsometry Data for SrTiO3 on Si (wrong) How do the properties of SrTiO3 epilayers on Si depend on thickness? Monochromator SrTiO3 SiO2 • What spectroscopic ellipsometry reveals: • Thickness (100 to 10000 A) • Excitonic absorption • Refractive index detector polarizer Si SrTiO3 SrTiO3 Si analyzer Φ Thin SrTiO3 epilayers on Si absorb less light and have a lower refractive index (Kramers-Kroning). Why? SiO2 interfacial oxide layer contributes more to thin SrTiO3epilayer measurements. sample 8 New Mexico State University SZ, Demkov, Liu, Curless, Yu, Ramdani, Droopad; Spring 2000 MRS Meeting

  9. Ellipsometry Data for (111)-textured CVD SrTiO3 on Pt How do the properties of SrTiO3 layers depend on deposition technique? Prepared by CVD on 1000 A Pt films (Si substrate). Pt aligned along (111), 350 A grain size. SrTiO3 lattice constant close to that of Pt. Our best SrTiO3 films are strongly textured along Pt (111) and cannot be seen in XRD. Only SrTiO3 with weak texture observed in XRD. Texture controlled by deposition conditions. SrTiO3 Pt Poor texture and oxygen deficiency lead to broad critical points, low refractive index, and an Urbach tail. Textured polycrystyallineSrTiO3 layers on Pt absorb less light than epitaxial SrTiO3 on Si. Lower refractive index. 9 New Mexico State University SZ, Demkov, Liu, Curless, Yu, Ramdani, Droopad; Spring 2000 MRS Meeting

  10. Fast Forward to 2010: Thick poly-SrTiO3 on Si by liquid deposition How do the properties of SrTiO3 epilayers on Si depend on thickness? Monochromator • What spectroscopic ellipsometry reveals: • Thickness (100 to 10000 A) • Excitonic absorption • Refractive index detector polarizer SrTiO3 SrTiO3 Si outstanding dielectric (better than bulk) analyzer Φ Photon Energy (eV) sample Thick polycrystalline SrTiO3 layers have lower absorption and refractive index than bulk SrTiO3. Why? Back in 2010, we suspected low density (wrong). 10 New Mexico State University Weiss, Zhang, Spies, Abdallah, SZ, Cole, Alpay; J. Appl. Phys. 111, 054108 (2012).

  11. New in 2014: X-ray reflectivity (XRR) XRR measures density, thickness, roughness Critical angle:Electron Density Film Thickness Period of oscillation - Film Thickness = 17nm Q= Amplitude of oscillation- Contrast of Electron Density Roughness 2θ New Mexico State University ARO W911NF-14-1-0072 11

  12. Combine X-ray reflectivity (XRR) and Ellipsometry (SE) XRR: (Si)(SiO2) (SrTiO3) large Ellipsometry: a(SrTiO3)a(SiO2) a(Si) large Monochromator SrTiO3 SrTiO3 SiO2 SiO2 • What x-ray reflectivity reveals: • Thickness (5 Åto 1000 Å) • Surface and interface roughness • Electron density profile • What spectroscopic ellipsometryreveals: • Thickness (100 to 10000 A) • Excitonic absorption • Refractive index detector polarizer Si Si analyzer Φ Thickness sample Roughness New Mexico State University 12

  13. Combine X-ray reflectivity (XRR) and Ellipsometry (SE) If we only have ellipsometry, we assume that the low absorption of SrTiO3 is because of a thick SiO2 interfacial layer. We use XRR to determine the thickness of the SrTiO3 layer separately. Since XRR and SE find the same SrTiO3 thickness, the interfacial SiO2 layer is thin. Ellipsometry: a(SrTiO3)a(SiO2) a(Si) large Monochromator SrTiO3 SiO2 • What spectroscopic ellipsometryreveals: • Thickness (100 to 10000 A) • Excitonic absorption • Refractive index detector polarizer Si analyzer Φ sample Old explanation was wrong! n and a vary with STO thickness New Mexico State University 13

  14. Optical and X-Ray Characterization of Epitaxial Films Do the properties of SrTiO3 layers depend on the substrates? SrTiO3 Bulk SrTiO3 LaAlO3 SrTiO3 • What x-ray diffraction reveals: • Distances between atoms • Lattice mismatch (strain) • Grain size • What x-ray reflectivity reveals: • Thickness (5 Åto 1000 Å) • Surface and interface roughness • Electron density profile • What spectroscopic ellipsometryreveals: • Thickness (100 to 10000 A) • Excitonic absorption • Refractive index Grown by MBE at UT Austin. Thickness: 20 nm. SrTiO3 SrTiO3 SrTiO3 Si Ge X-ray beam Monochromator detector polarizer analyzer Φ Thickness Roughness atoms sample New Mexico State University Epi process adjusted for substrate 14

  15. X-ray reflectivity measurement of SrTiO3 on Ge Bad sample Bad sample Q= SrTiO3 GeO2 Ge STO Ge GeO2 STO New Mexico State University 15

  16. X-ray reflectivity measurement of SrTiO3 on various substrates • We determined electron density profile versus depth for all layers. • XRR fitting is sensitive to surface roughness and interfacial layers. • Even SrTiO3 layer on SrTiO3 substrate shows some contrast. • SrTiO3 on Ge was not stable (deteriorated over several months). Bad sample New Mexico State University 16

  17. X-Ray Diffraction: Lattice constant, strain, grain size Grain Size = 168Å Vertical Strain= -0.15% FWHM=0.80º Grain Size = 161Å Vertical Strain = -0.18% FWHM=0.94º Vertical strain-0.15% FWHM=0.02º ω scan ω-2θ scan (zoomed) ω-2θ scan 17

  18. X-Ray Diffraction: Lattice constant, strain, grain size /2 SrTiO3 on Si and Ge:Tensile stress (thermal expansion mismatch). Almost fully relaxed. Large mosaic spread. SrTiO3 on bulk SrTiO3: Slight lattice mismatch (perhaps non-stoichiometric). SrTiO3 on LaAlO3: Nearly pseudomorphic, but still large rocking curve width. (Substrate twinned.) STO LAO Si,Ge Ge Si Rocking Curves LAO STO 18

  19. Ellipsometry measures thickness and optical constants Ellipsometric angles (,) are fitted to determine dielectric function e. SrTiO3 on Si/Ge: Low absorption SrTiO3 on LaAlO3: High absorption Exciton (de)confinement! 19

  20. Comparison of Ellipsometry and XRR Density Not a good correlation between ellipsometry and XRR density. Absorption varies much more than XRR density. Ellipsometry Density Aged sample XRR Density 20

  21. Mechanism for optical absorption in SrTiO3 • Excitonic effects in bulk SrTiO3 have been observed (Gogoi & Schmidt). • Details of band structure not well known (localized d electrons). • Elliot theory for excitonic absorption: Assume this works like in GaAs Free-carrier absorption prop. to # carriers Sommerfeld enhancement (excitonic effects) Si SrTiO3 SrTiO3 LaAlO3 confinement(Type I) deconfinement (spatially indirect) 21

  22. Tanguy formalism for excitonic absorption Assume this works like in GaAs Sommerfeld enhancement (excitonic effects) Free-carrier absorption Dipole matrix element: C. Tanguy, Phys. Rev. Lett. 75, 4090 (1995). 22

  23. Tanguy formalism for excitonic absorption Built into WVASE software. excitonic enhancement Assume this works like in GaAs excitonic enhancement New Mexico State University Tanguy (1995). 23

  24. We see a similar reduction in ZnO layers on Si • ZnO grown on Si by ALD (35ºC or 200ºC). • Thickness varies from 5 to 70 nm. • r almost constant, but e drops very fast in thin films. • We also see a small blueshift (due to confinement). Dielectric function XRR reflectance r and evs thickness Pal, Mathur, Singh, Dutta, Singhal, SZ, Chattopadhyay, ICSE-6 (Berlin), submitted 24

  25. What is responsible for the reduction? Amplitude pre-factor: • Excitonic screening? • Overlap reduction? • Clue: • SrTiO3 on Si or Ge: Low absorption. • SrTiO3 on LaAlO3: High absorption. P: Overlap of electron and hole Lineshape Si SrTiO3 LaAlO3 SrTiO3 Sommerfeldenhancement Free-carrier term confinement(Type I) deconfinement (spatially indirect) 25

  26. Summary Do the properties of SrTiO3 epilayers depend on the substrate? SrTiO3 Bulk SrTiO3 LaAlO3 SrTiO3 • SrTiO3epilayers (20 nm) were characterized to determine structural and optical properties (XRD, XRR, ellipsometry). • If the substrate band gap (LaAlO3) is larger, the SrTiO3exciton is confined and the absorption increases (compare GaAs quantum dot lasers). • Thin widebandgap oxide epilayers (SrTiO3, ZnO) on a narrow-gap substrate (Si, Ge) experience excitondeconfinement(lower absorption, refractive index). • Polycrystalline STO: Probably an increase of the broadening. SrTiO3 SrTiO3 SrTiO3 Ge Si New Mexico State University

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