Paramagnetic Defects in Variously Processed SrTiO3/LaAlO3 Thin Film Structures

Duane McCrory1

Co-authors: Patrick Lenahan1, Alexander Demkov2, Andy O’Hara2, Miri Choi2, Agham Posadas2

1Pennsylvania State University 2The University of Texas at Austin

n-type SrTiO3/LaAlO3 Interface

• Alternating (LaO)- and (AlO2)+ layers within the LaAlO2

•Alternating (SrO) and (TiO2) layers within the SrTiO2

Lee, J., & Demkov, A. (2008). Charge origin and localization at the n-type SrTiO3/LaAlO3 interface. Physical Review B, 78(19), 193104. doi:10.1103/PhysRevB.78.193104

n-type SrTiO3/LaAlO3 Interface

•What are the defects? •Do oxygen vacancies play a dominating role? •Are other possibly extrinsic defects important? •Electron Paramagnetic Resonance (EPR) is unrivaled in its analytical power to determine the physical and chemical nature of point defects.

Electron Paramagnetic Resonance (EPR)

• Conventional EPR sensitivity ≈ 1010 spins.

• EPR is sensitive to defects throughout the sample.

Microwave Bridge

Computer (Data Acquisition and Field Control)

Lock-in Amplifier

Electromagnetic Modulation Coil

E

E=0 H

H = 0

E = hν

absorption

E-1/2=gβH

E+1/2=gβH

ΔE = hν=gβH

Bres

β = Bohr magneton. ν = microwave frequency. h = Planck’s constant, g = 2.0023 for a free electron

C.J. Cochrance, P.M. Lenahan, Magnetic Resonance, (2013)

• In the simplest case, an isolated electron, resonance occurs when hν = geβH. (ge = 2.0023219)

• Deviation from this simple expression due to spin orbit coupling, hyperfine interactions, and fine structure provide its analytical power.

Deviations from Simple Resonance Condition

•Spin Orbit Coupling: g factor becomes a second rank tensor. Tensor affected by orbital angular momentum, nuclear charge, defect symmetry, occupied and unoccupied energy levels. •Hyperfine Interactions: Some nuclei have magnetic moments. If a magnetic moment interacts with a paramagnetic site, the magnetic field it generates will alter the resonance condition. •In high spin sites, often associated with transition metal impurities, fine structure can have an extremely large effect on the resonance condition.

Deviations from Simple Resonance Condition • g tensor: ge becomes a tensor (spin orbit coupling)

• Hyperfine: A tensor: electron interacts with nearby magnetic nuclei (simplest case)

• Fine Structure: high spin site (general case)

Hgh

AMHgh I

]sin2cos3)1cos3(][2/)12[( 22 EDMHH o

]9)1(4)1(24][2sincos)2cos)[(2/(sin 22222 SSMMEEDHo

}2sincos4)]cos1(2cossin){[8/1( 222222 EEDHo

]3)1(6)1(2[ MMSS

Ikeya, M. “New Applications of Electron Spin Resonance” (1993)

TE104 Microwave Double Cavity

Sample

Standard

Sample Geometry (Various Processing Conditions)

SrTiO3 1 mm LaAlO3 1 mm

SrTiO3

SrTiO3

1 mm

40 nm

LaAlO3

SrTiO3

1 mm

40 nm

Structure List of Impurities from MTI Crystal

SrTiO3 Ca 20 ppm, Zn 10 ppm, Fe 20 ppm, Mg 20 ppm, Al 20 ppm

LaAlO3 Fe 2 ppm, Na 0.02 ppm, K 0.03 ppm, Zn 0.04 ppm, Si 34 ppm, Cl 76 ppm, Ca 11 ppm, Cu 2 ppm, Pb 7.5 ppm, Pr 7.5 ppm, Sm 7.5 ppm, P 0.02 ppm, Ti 0.2 ppm, Ni 0.02 ppm, Ga 0.03 ppm, Nd 7.5 ppm

Relevant EPR Studies on STO and LAO in the Literature

• Badalyan, a G., Azzoni, C. B., Galinetto, P., Mozzati, M. C., Trepakov, V. a, Savinov, M., … Rosa, J. (2007). Impurity centers and host microstructure in weakly doped SrTiO 3 :Mn crystals: new findings. Journal of Physics: Conference Series, 93, 012012. doi:10.1088/1742-6596/93/1/012012

• Badalyan, a. G., Azamat, D., Babunts, R. a., Neverova, E. V., Dejneka, a., Trepakov, V. a., & Jastrabik, L. (2013). EPR study of charge compensation of chromium centers in the strontium titanate crystal. Physics of the Solid State, 55(7), 1454–1458. doi:10.1134/S1063783413070044

• Dobrov, W.I., Vieth, R.F, Browne, M. E. (1959). Electron Paramagnetic Resonance in SrTiO3. Physical Review, 115(1), 1–2.

• Ivanova, T. A., Petrashen, V. E., Chezhina, N. V, & Yablokov, Y. V. (2002). Jahn – Teller Effect in Low-Spin Ni 3 + in the LaAlO 3 Ceramic, 44(8), 1468–1470.

• Meierijnq, H. D. (1971). Charge Compensation by O2- Vacancies in Cr3+ Doped SrTiO3, 191, 191–197.

• Muller, K. A. (1959). Electron Paramagnetic Resonance of Manganese IV in SrTiO3. Physical Review Letters, 2(8), 3–5.

• Yamasaka, D., Tamagawa, K., & Ohki, Y. (2011). Effects of ultraviolet photon irradiation on the transition metal impurities in LaAlO3. Journal of Applied Physics, 110(7), 074103. doi:10.1063/1.3641974

• Zvanut, M. E., Jeddy, S., Towett, E., Janowski, G. M., Brooks, C., & Schlom, D. (2008). An annealing study of an oxygen vacancy related defect in SrTiO[sub 3] substrates. Journal of Applied Physics, 104(6), 064122. doi:10.1063/1.2986244

SrTiO3 and SrTiO3/ SrTiO3

0 1000 2000 3000 4000 5000 6000 7000-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

Magnetic Field (Gauss)

EP

R A

mplit

ude (

A.U

)

STO

STO/STO

Fe3+- Vo Fe3+- Vo Fe3+

Cr3+

Fe3+- Vo Fe3+- Vo

Fe3+

Cr3+

Zvanut, M. E., Jeddy, S., Towett, E., Janowski, G. M., Brooks, C., & Schlom, D. (2008). An annealing study of an oxygen vacancy related defect in SrTiO[sub 3] substrates. Journal of Applied Physics, 104(6), 064122. doi:10.1063/1.2986244

4 x1016 defects/cm3

2 x1017 defects/cm3

∆Defects = 1.6 x1017 defects/cm3

STO

STO/STO

LaAlO3 and SrTiO3/ LaAlO3

• Tentative Identification: Mostly transition metal impurity coupled sites with long spin-lattice relaxation times.

0 1000 2000 3000 4000 5000 6000 7000-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

EP

R A

mplit

ude (

A.U

.)

Magnetic Field (Gauss)

1x1017 defects/cm3

LAO 8 x1017 defects/cm3

100nm STO/LAO

∆Defects = 7 x1017 defects/cm3

SrTiO3/LaAlO3 - Vacuum Anneal and No Vacuum Anneal

0 1000 2000 3000 4000 5000 6000 7000-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

Magnetic Field (Gauss)

EP

R A

mplit

ude (

A.U

.)

3% electron activation

11% electron activation

Vacuum Anneal

No Vacuum Anneal

4 x1017 defects/cm3

1 x1017 defects/cm3

Conclusion

• Large number of extrinsic defects within the structure, as evidence of fine structure suggesting that most of them involve transition metal impurity sites with relatively long spin-lattice relaxation times.

• The density of these extrinsic defects is tremendously altered by processing conditions involved in deposition of SrTiO3.

• Any discussion of the role of the addition of dopants, introduction of simple point defects such as oxygen vacancies or oxygen interstitials should take into account the large number of extrinsic processing dependent point defects.