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 Patrick Ponath, Kristy J. Kormondy, Alex A. Demkov, University of Texas at Austin NSF: DMR Si SrTiO 3 LaAlO 3 SrTiO 3 Light Excitons at Interfaces 2016 Lawrence Symposium on Epitaxy February 2016, Scottsdale, AZ 20 nm NIR/VIS/QUV ellipsometry: 190 to 2500 nm, 77 to 800 K

New Mexico State University Flat & uniform films, at least 5 by 5 mm 2, low surface roughness, films on single-side polished substrate Graduate Students: Lina Abdallah, Travis Willett-Gies, Nalin Fernando, Tarek Tawalbeh (Theory), Nuwanjula Samarasingha, Nathan Nunley Undergraduate Students: Cesar Rodriguez, Khadijih Mitchell, Cayla Nelson, Jaime Moya, Jackie Cooke, Maria Spies 2

New Mexico State University Biography Regensburg Germany Las Cruces, NM Since Motorola (Mesa, Tempe) Arizona, Motorola, Freescale Texas, Freescale, IBM New York, 91-92;

Epitaxy at Motorola ( ) First SiGe epi reactor in CMOS production fab (Mesa). First HR-XRD in Si wafer fab. First six-inch GaAs fab. Si:C alloys for NMOS (ASM) AlN on Si (NCSU, Texas Tech) Jaguar: SrTiO 3 on Si Hobby: Ge 1-x Sn x on Si Stefan Zollner, 06/09/2012, Conf. for Undergrad. Women in Physical Sciences 4 InGaP HBT epi stack SiC:P Si:C NMOS 4 Optical spectrum

SiGe:C Metrology for the semiconductor industry 5 Si cap (emitter) SiGe:C base Si substrate High-resolution XRD Spectroscopic Ellipsometry SZ, Hildreth, Liu, Zaumseil, Weidner, Tillack, J. Appl. Phys. 88, 4102 (2000) E 1 Exciton

New Mexico State University Optical and X-Ray Characterization of Epitaxial Films Bulk SrTiO 3 Do the properties of SrTiO 3 epilayers depend on the substrates? What x-ray diffraction reveals: Distances between atoms Lattice mismatch (strain) Grain size Ge SrTiO 3 Si SrTiO 3 What spectroscopic ellipsometry reveals: Thickness (100 to A) Excitonic absorption Refractive index sample Monochromator polarizer analyzer detector Φ X-ray beam atoms What x-ray reflectivity reveals: Thickness (5 Å to 1000 Å) Surface and interface roughness Electron density profile Thickness Roughness 6 LaAlO 3 SrTiO 3 Excitons at Interfaces

New Mexico State University Ellipsometry of SrTiO 3 on Si (Jaguar data) SrTiO 3 How do the properties of SrTiO 3 epilayers on Si depend on thickness? What spectroscopic ellipsometry reveals: Thickness (100 to A) Excitonic absorption Refractive index Si SrTiO 3 sample Monochromator polarizer analyzer detector Φ 7 Thin SrTiO 3 epilayers on Si absorb less light than thick or bulk SrTiO 3. Lower refractive index. SZ, Demkov, Liu, Curless, Yu, Ramdani, Droopad; Spring 2000 MRS Meeting

New Mexico State University Old Explanation of Ellipsometry Data for SrTiO 3 on Si (wrong) SrTiO 3 How do the properties of SrTiO 3 epilayers on Si depend on thickness? What spectroscopic ellipsometry reveals: Thickness (100 to A) Excitonic absorption Refractive index Si SrTiO 3 sample Monochromator polarizer analyzer detector Φ 8 Thin SrTiO 3 epilayers on Si absorb less light and have a lower refractive index (Kramers-Kroning). Why? SiO 2 interfacial oxide layer contributes more to thin SrTiO 3 epilayer measurements. Thin SrTiO 3 epilayers on Si absorb less light and have a lower refractive index (Kramers-Kroning). Why? SiO 2 interfacial oxide layer contributes more to thin SrTiO 3 epilayer measurements. Si SiO 2 SrTiO 3 SZ, Demkov, Liu, Curless, Yu, Ramdani, Droopad; Spring 2000 MRS Meeting

New Mexico State University Ellipsometry Data for (111)-textured CVD SrTiO 3 on Pt How do the properties of SrTiO 3 layers depend on deposition technique? 9 SZ, Demkov, Liu, Curless, Yu, Ramdani, Droopad; Spring 2000 MRS Meeting Poor texture and oxygen deficiency lead to broad critical points, low refractive index, and an Urbach tail. Prepared by CVD on 1000 A Pt films (Si substrate). Pt aligned along (111), 350 A grain size. SrTiO 3 lattice constant close to that of Pt. Our best SrTiO 3 films are strongly textured along Pt (111) and cannot be seen in XRD. Only SrTiO 3 with weak texture observed in XRD. Texture controlled by deposition conditions. Pt SrTiO 3 Textured polycrystyalline SrTiO 3 layers on Pt absorb less light than epitaxial SrTiO 3 on Si. Lower refractive index.

New Mexico State University Fast Forward to 2010: Thick poly-SrTiO 3 on Si by liquid deposition SrTiO 3 How do the properties of SrTiO 3 epilayers on Si depend on thickness? What spectroscopic ellipsometry reveals: Thickness (100 to A) Excitonic absorption Refractive index Si SrTiO 3 sample Monochromator polarizer analyzer detector Φ 10 Thick polycrystalline SrTiO 3 layers have lower absorption and refractive index than bulk SrTiO 3. Why? Back in 2010, we suspected low density (wrong). Thick polycrystalline SrTiO 3 layers have lower absorption and refractive index than bulk SrTiO 3. Why? Back in 2010, we suspected low density (wrong). Weiss, Zhang, Spies, Abdallah, SZ, Cole, Alpay; J. Appl. Phys. 111, (2012). Photon Energy (eV) outstanding dielectric (better than bulk)

New Mexico State University New in 2014: X-ray reflectivity (XRR) Film Thickness Amplitude of oscillation- Contrast of Electron Density Roughness Period of oscillation - Film Thickness = 17nm 2θ2θ 11 ARO W911NF Critical angle: Electron Density XRR measures density, thickness, roughness

New Mexico State University Combine X-ray reflectivity (XRR) and Ellipsometry (SE) 12 What spectroscopic ellipsometry reveals: Thickness (100 to A) Excitonic absorption Refractive index sample Monochromator polarizer analyzer detector Φ What x-ray reflectivity reveals: Thickness (5 Å to 1000 Å) Surface and interface roughness Electron density profile Thickness Roughness Si SiO 2 SrTiO 3 Si SiO 2 SrTiO 3 XRR:  (Si)  (SiO 2 )  (SrTiO 3 ) large Ellipsometry:  (SrTiO 3 )  (SiO 2 )  (Si) large

New Mexico State University Combine X-ray reflectivity (XRR) and Ellipsometry (SE) 13 What spectroscopic ellipsometry reveals: Thickness (100 to A) Excitonic absorption Refractive index sample Monochromator polarizer analyzer detector Φ Si SiO 2 SrTiO 3 Ellipsometry:  (SrTiO 3 )  (SiO 2 )  (Si) large If we only have ellipsometry, we assume that the low absorption of SrTiO 3 is because of a thick SiO 2 interfacial layer. We use XRR to determine the thickness of the SrTiO 3 layer separately. Since XRR and SE find the same SrTiO 3 thickness, the interfacial SiO 2 layer is thin. Old explanation was wrong! n and  vary with STO thickness

New Mexico State University Optical and X-Ray Characterization of Epitaxial Films Bulk SrTiO 3 Do the properties of SrTiO 3 layers depend on the substrates? What x-ray diffraction reveals: Distances between atoms Lattice mismatch (strain) Grain size Ge SrTiO 3 Si SrTiO 3 What spectroscopic ellipsometry reveals: Thickness (100 to A) Excitonic absorption Refractive index sample Monochromator polarizer analyzer detector Φ X-ray beam atoms What x-ray reflectivity reveals: Thickness (5 Å to 1000 Å) Surface and interface roughness Electron density profile Thickness Roughness 14 LaAlO 3 SrTiO 3 Grown by MBE at UT Austin. Thickness: 20 nm. Epi process adjusted for substrate

New Mexico State University X-ray reflectivity measurement of SrTiO 3 on Ge 15 STO GeO 2 STOGe GeO 2 SrTiO 3 Bad sample

New Mexico State University X-ray reflectivity measurement of SrTiO 3 on various substrates 16 We determined electron density profile versus depth for all layers. XRR fitting is sensitive to surface roughness and interfacial layers. Even SrTiO 3 layer on SrTiO 3 substrate shows some contrast. SrTiO 3 on Ge was not stable (deteriorated over several months). Bad sample

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

X-Ray Diffraction: Lattice constant, strain, grain size 18 SrTiO 3 on Si and Ge: Tensile stress (thermal expansion mismatch). Almost fully relaxed. Large mosaic spread. SrTiO3 on bulk SrTiO 3 : Slight lattice mismatch (perhaps non-stoichiometric). SrTiO 3 on LaAlO 3 : Nearly pseudomorphic, but still large rocking curve width. (Substrate twinned.) Rocking Curves  /2  LAO Si,Ge STO LAOSTO Ge Si

Ellipsometry measures thickness and optical constants 19 Ellipsometric angles ( ,  ) are fitted to determine dielectric function . SrTiO 3 on Si/Ge: Low absorption SrTiO 3 on LaAlO 3 : High absorption Exciton (de)confinement!

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

Mechanism for optical absorption in SrTiO 3 Excitonic effects in bulk SrTiO 3 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 21 Free-carrier absorption prop. to # carriers Sommerfeld enhancement (excitonic effects) SrTiO 3 LaAlO 3 SrTiO 3 Si confinement (Type I) deconfinement (spatially indirect)

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

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

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.  almost constant, but  drops very fast in thin films. We also see a small blueshift (due to confinement). Dielectric functionXRR reflectance  and  vs thickness Pal, Mathur, Singh, Dutta, Singhal, SZ, Chattopadhyay, ICSE-6 (Berlin), submitted 24

What is responsible for the reduction? Excitonic screening? Overlap reduction? Clue: –SrTiO 3 on Si or Ge: Low absorption. –SrTiO 3 on LaAlO 3 : High absorption. Amplitude pre-factor: Lineshape Sommerfeld enhancement Free-carrier term P: Overlap of electron and hole 25 SrTiO 3 LaAlO 3 confinement (Type I) deconfinement (spatially indirect) SrTiO 3 Si

New Mexico State University Summary SrTiO 3 epilayers (20 nm) were characterized to determine structural and optical properties (XRD, XRR, ellipsometry). If the substrate band gap (LaAlO 3 ) is larger, the SrTiO 3 exciton is confined and the absorption increases (compare GaAs quantum dot lasers). Thin wide bandgap oxide epilayers (SrTiO 3, ZnO) on a narrow- gap substrate (Si, Ge) experience exciton deconfinement (lower absorption, refractive index). Polycrystalline STO: Probably an increase of the broadening. Bulk SrTiO 3 Do the properties of SrTiO 3 epilayers depend on the substrate? Ge SrTiO 3 Si SrTiO 3 LaAlO 3 SrTiO 3