High-Mobility Oxide Films by MBE

Susanne StemmerMaterials Department

University of California, Santa Barbara

19th International Conference on Molecular-Beam EpitaxyMontpelier, FranceSeptember 6, 2016

Acknowledgements

Graduate students and postdocs:Adam Kajdos (now at Sila Nanotech.)Timo SchumannSantosh RaghavanKaveh AhadiHonggyu Kim

Former students/postdocs: Bharat Jalan (now at U. Minnesota), Roman Engel-Herbert (now at Penn State), Tyler Cain (now at Carbonics)

Funding

Outline

3

Challenges in MBE of complex oxides: stoichiometry controlMBE of high mobility complex oxide films

Perovskite titanatesPerovskite stannates

Molecular Beam Epitaxy of Complex Oxides

4

High purityLayer-by-layer controlLow energetic deposition

Challenges: stoichiometry controlWithout a MBE growth window, stoichiometry control requires precise flux control → only possible to 0.1 - 1 %Corresponds to defect concentrations of 1020-1021 cm-3

Perovskite structure A-site cation

B-site cation

Oxygen

Stoichiometry Control: Growth Window

5

Temperature (1000/K)

Gas

Pre

ssur

e (T

orr)

GaAs(s) ⇔ Ga(l) + As(g)

4 As(s) ⇔ As4(g)

MBE Growth Window

Temperature (°C)

4As(s) ⇔ As4(g)

GaAs(s) ⇔ Ga(l) + As(g)

MBE Growth Window

Below this line, solid As will not precipitate → excess As desorbs in the chamber

Above this line, As will condense on a Ga-rich GaAs surface

104

10-1

10-6

10-11

1.05 1.10 1.15 1.20 1.25

700 650 600 550 500

C.D. Theis et al., Thin Solid Films 325, 107 (1998).

A wide MBE growth window is largely responsible for the ease and success of III-V MBE**J. Tsao, Materials Fundamentals of Molecular Beam Epitaxy.

No need for precise flux control

GaAs

Stoichiometry Control: Growth Window

6

No practical growth window in MBE using only solid sourcesRely on flux control?Alternate solution: volatile, metal-organic sources

1500 K

SrO(s)⇔ SrO(g)

SrO(g) + TiO2(s) ⇔ SrTiO3(s)

Below this line, solid SrO will not precipitate: SrO will desorb in the chamber*

Above this line, SrO will condense on a TiO-rich SrTiO3 surface*

10-6

10-12

10-18

10-24

SrO

Gas

Pre

ssur

e (T

orr)

0.6 0.7 0.8 0.9 1.0Temperature (1000/K)

MBE Growth Window

SrTiO3

Ti has a sticking coefficient of 1*

*C.D. Theis et al., Thin Solid Films 325, 107 (1998).

Hybrid Oxide MBE

7

~

Gd effusion cell

Sr effusion cell

Oxygen plasma source

Metal organic sourceBayard-Alpert

ion gauge

Electron gunfor RHEED

Resiudal gasanalyzer

Substratemanipulator

Manipulator forsample transfer

RHEEDScreen

Titanium tetra iso-propoxide

Ti

B. Jalan, R. Engel-Herbert, N. J. Wright, S. Stemmer, J. Vac. Sci. and Technol. A 27, 461 (2009). B. Jalan, P. Moetakef, S. Stemmer, Appl. Phys. Lett. 95, 032906 (2009).

Hybrid MBE = use both metal-organic and solid sources for the metalsIn

tens

ity (a

.u.)

4003002001000Time (s)

SrTiO3

Hybrid and Metal-Organic Oxide MBE

8

A. P. Kajdos, S. Stemmer, Hybrid molecular beam epitaxy for the growth of complex oxide materials, in: Epitaxial Growth of Complex Metal Oxides, G. Koster, M. Huijben and G. Rijnders (Eds.), Woodhead Publishing, Amsterdam (2015).

... but does it improve film stoichiometry control?

Metal-organic precursors used in oxide MBE

9

oxygen plasma sourceSr source TTIP

Use of a metalorganic Ti source (TTIP) leads to a growth window at practical substrate temperatures and fluxes. Excellent stoichometry controlSupplies extra oxygenHigh growth rates

Hybrid Oxide MBE: SrTiO3

B. Jalan, et al., J. Vac. Sci. and Technol. A 27, 461 (2009). B. Jalan, P. Moetakef, S. Stemmer, Appl. Phys. Lett. 95, 032906 (2009).

SrTiO3

SrTiO3

MBE growth window

10

B. Jalan, et al., J. Vac. Sci. and Technol. A 27, 461 (2009). B. Jalan, P. Moetakef, S. Stemmer, Appl. Phys. Lett. 95, 032906 (2009).

MBE growth window

Growth Window

Inte

nsity

(a.u

.)

46.846.446.045.62θ (°)

Ti-rich

Ti-rich

Sr-rich

Sr-rich

Are x-ray lattice parameter measurements sufficiently accurate?

11

RHEED patterns for samples grown within the XRD growth window

Change in surface reconstruction within the XRD growth window:

(1 × 1) → (2 × 1) → c(4 × 4)→ increasing TTIP flux

Fine-tuning Stoichiometry with RHEED

A. P. Kajdos, and S. Stemmer, Appl. Phys. Lett. 105, 191901 (2014).

Tsub = 810 °C

SrTiO3

SrTiO3

TTIP/S

r

12

Consistent trend in surface reconstructions for different substrate temperaturesReconstructions observed outside the growth window similar to those reported in solid-source MBE

Fine-tuning Stoichiometry with RHEEDSrTiO3

SrTiO3

A. P. Kajdos, and S. Stemmer, Appl. Phys. Lett. 105, 191901 (2014).

13

Within the XRD growth window, surface reconstructions indicate transition to complete TiO2 coverage with increasing TTIP/Sr flux ratio

Reconstructions Within the XRD Growth Window

A. P. Kajdos, and S. Stemmer, Appl. Phys. Lett. 105, 191901 (2014).

(1×1)mixed SrO and TiO2 termination

(4×4)TiO2

termination

M. Castell, Surf. Sci. 505, 1 (2002).

14

Incomplete TiO2 surface saturation: phase shift in oscillations from specular reflection

Shift may be related to boundaries between SrO- and TiO2-terminated regions

Fine-tuning Stoichiometry with RHEED

A. P. Kajdos, and S. Stemmer, Appl. Phys. Lett. 105, 191901 (2014).

SrTiO3

SrTiO3

15

Implications of Stoichiometry Control

A. P. Kajdos, and S. Stemmer, Appl. Phys. Lett. 105, 191901 (2014).

(1×1) surface indicative of mixed SrO and TiO2 termination

c(4×4) indicates a TiO2-saturated surface

Desorption-regulated growth relies on excess of the volatile component (TTIP)

Transition to c(4×4) indicates boundary to truly adsorption controlled regime

Overlap of c(4 × 4) reconstruction AND XRD growth window is the best indicator of stoichiometry: highest mobility films are grown in this regime

16

100

101

102

103

104

105

Mob

ility

(cm

2 V-1

s-1)

2 4 6 810

2 4 6 8100

2

Temperature (K)

8×1017

cm-3

2×1018

cm-3

4×1018

cm-3

9×1018

cm-3

4×1019

cm-3

9×1019

cm-3

2×1020

cm-3

J. Son, P. Moetakef, B. Jalan, O. Bierwagen, N. J. Wright, R. Engel-Herbert, S. Stemmer, Nat. Mater. 9, 482 (2010).T. A. Cain, A. P. Kajdos, and S. Stemmer, Appl. Phys. Lett. 102, 182101 (2013).

Electron Mobilities in SrTiO3

Electron Mobilities in SrTiO3

SrTiO3

La:SrTiO3

SrTiO3 films doped with LaHighest mobility films are grown in regime of c(4×4) reconstructionMobilities of > 50,000 cm2V-1s-1

17

100

101

102

103

104

105

Mob

ility

(cm

2 V-1

s-1)

2 4 6 810

2 4 6 8100

2

Temperature (K)

8×1017

cm-3

2×1018

cm-3

4×1018

cm-3

9×1018

cm-3

4×1019

cm-3

9×1019

cm-3

2×1020

cm-3

Single CrystalsBest PLD films*

*PLD data: Y. Kozuka et al., Appl. Phys. Lett. 97, 012107 (2010).

SrTiO3

La:SrTiO3Hybrid Oxide MBE: SrTiO3

Electron Mobilities in SrTiO3

J. Son, P. Moetakef, B. Jalan, O. Bierwagen, N. J. Wright, R. Engel-Herbert, S. Stemmer, Nat. Mater. 9, 482 (2010).T. A. Cain, A. P. Kajdos, and S. Stemmer, Appl. Phys. Lett. 102, 182101 (2013).

SrTiO3 films doped with LaHighest mobility films are grown in regime of c(4×4) reconstructionMobilities of > 50,000 cm2V-1s-1

Higher mobility than PLD films or single crystals

18

Hybrid Oxide MBE: (Ba,Sr)TiO3

Dielectric Quality Factor (Q) for (BaxSr1-x)TiO3

(Ba,Sr)TiO3 films have a tunable dielectric constant that is of interest for microwave applicationsHigh dielectric losses have prevented wide-spread applicationQ = (tanδ)-1 of MBE-grown films is higher than single crystals

SrTiO3

(Ba,Sr)TiO3

Pt

Q = (tanδ)-1

E. Mikheev, A. P. Kajdos, A. J. Hauser, S. Stemmer, Appl. Phys. Lett. 101, 252906 (2012).

Outline

19

Why MBE of Oxides?Challenges in oxide MBE: stoichiometry controlAdvanced materials by oxide MBE:

Perovskite titanatesPerovskite stannates

Perovskite Stannate: BaSnO3

La:BaSnO3 Single Crystal

300 cm2/Vs

n (1020 cm-3)

µ (c

m-2

/Vs)

Perovskite structureHigh room temperature mobility: 300 cm2/Vs

Compare with SrTiO3: 10 cm2/Vs

Higher mobilities than any other semiconductor at these carrier densitiesWide band gap (~3 eV): transparent conductorPower electronics?Epitaxial integration with functional perovskites

H. J. Kim, et al., Appl. Phys. Expr. 5, 061102 (2012).X. Luo, et al., Appl. Phys. Lett. 100, 172112 (2012).

20

Substrate

Perovskite

BaSnO3

Ultra-high permittivity oxides (BST, SrTiO3)Ferroelectrics (negative capacitance devices)Metal-Insulator Transitions

All-epitaxial perovskite oxide device structures with high mobility channels

BaSnO3 Thin Films

150

100

50

0

µ (c

m2 /V

s)

0.12 3 4 5 6

12 3 4 5 6

10

n3D (1020

cm-3

)

Lee et al.Kim et al. Ganguly et al.Wadekar et al.

PLD and sputtered BaSnO3 Thin Films

Kim et al., Appl. Phys. Exp. 5, 061102 (2012)*Lee et al., Appl. Phys. Lett. 108, 082105 (2016): BaSnO3 substratesWadekar et al., Appl. Phys. Lett. 105,052104 (2014)Ganguly et al., APL Mater. 3, 062509 (2015)**Z. Lebens-Higgins et al., Phys. Rev. Lett. 116, 027602 (2016): TbScO3 substrates (smaller mismatch than SrTiO3)

Best film mobilities ~100 cm2/Vs, even on BaSnO3 substrates*MBE films** with mobilities of 81 cm2/Vs on TbScO3

21

High mobility BaSnO3 thin films have been challenging to growCarrier densities are highLikely a number of challenges

Challenges in MBE of BaSnO3

~

Sn cell

Ba effusion cell

Oxygen plasma source

SrTiO3 Substrate 2 min 30 min 60 min

550 ºCVolatile SnO consumes all the oxygen: only Sn is left behindAt higher substrate temperatures nothing growsOxygen pressures in MBE are limited

Sn Cell

S. Raghavan, T. Schumann, H. Kim, J. Y. Zhang, T. A. Cain, and S. Stemmer, APL Mater. 4, 016106 (2016).

Sn droplets

MBE of BaSnO3 Using a SnO2 Source

SrTiO3 Substrate 2 min 30 min 60 min

800 ºC

Streaky RHEED throughout: smooth filmsHigh growth temperatures possible

~

SnO2 cell

Ba effusion cell

Oxygen plasma source

Replaced Sn source with SnO2 source

SnO2 Cell

Inte

nsity

(arb

. uni

ts)

46454443422Θ (deg)

PrScO3 022

BaSnO3 002

S. Raghavan, T. Schumann, H. Kim, J. Y. Zhang, T. A. Cain, and S. Stemmer, APL Mater. 4, 016106 (2016).

MBE of BaSnO3 Using a SnO2 Source

Higher mobility than films grown by any other methodMobility limited by mismatch with the substrate

on BaSnO3

Room temperature electron mobilities

SrTiO3: More than 5% mismatch

PrScO3: More than 2% mismatch

All films are fully relaxed.

150

100

50

0

µ (c

m2 /V

s)

0.12 4 6 8

12 4 6 8

10 n3D (1020 cm-3)

Lee et al.Kim et al. Ganguly et al.Wadekar et al.

MBE - SrTiO3MBE - PrScO3

S. Raghavan, T. Schumann, H. Kim, J. Y. Zhang, T. A. Cain, S. Stemmer, APL Mater. 4, 016106 (2016).

MBE of BaSnO3 Using a SnO2 Source

Higher mobility than films grown by any other methodMobility limited by mismatch with the substrate

Room temperature electron mobilities

SrTiO3: More than 5% mismatch

PrScO3: More than 2% mismatch

All films are fully relaxed.

S. Raghavan, T. Schumann, H. Kim, J. Y. Zhang, T. A. Cain, S. Stemmer, APL Mater. 4, 016106 (2016).

150

100

50

0

µ (c

m2 /V

s)

0.12 4 6 8

12 4 6 8

10 n3D (1020 cm-3)

MBE - SrTiO3MBE - PrScO3

Reduced lattice mismatch Increased film

thickness

MBE of BaSnO3 Using a SnO2 Source

Mobility limited by mismatch with the substrate ?

LaA

lO3

GdS

cO3

DyS

cO3

LSAT

3.7

Å

4.0

Å

Films

Substrates

BaS

nO3

PrS

cO3

SrT

iO3

26

−5.4% −2.2%

MBE of (Ba,Sr)SnO3

27

LaA

lO3

GdS

cO3

DyS

cO3

LSAT

3.7

Å

4.0

Å

Films

Substrates

BaS

nO3

PrS

cO3

SrT

iO3

SrS

nO3

PrScO3

SrSnO3

Inte

nsity

(arb

. uni

ts)

4746454443422Θ (º)

SrTiO3 substrate PrScO3 substrate

220 SrSnO3

220 PrScO3

002 SrTiO3

4.6

4.4

4.2

4.0

3.8

3.6

3.4

3.2

Dire

ct b

and

gap

(eV

)

1.00.80.60.40.20.0x

insulating n = 1.9 × 10

19 cm

-3

n = 1.5 × 1020

cm-3

Smaller lattice mismatch with substrates allows for high-quality SrSnO3 films(Ba,Sr)SnO3 films allow for band gap tuning

(BaxSr1-x)SnO3 films

T. Schumann, S. Raghavan, K. Ahadi, H. Kim, and S. Stemmer, J. Vac. Sci. Technol. A 34, 050601 (2016).

Burstein–Moss shift

SrSnO3 films

Summary

28

Oxide MBE grown films consistently show superior performance than oxide films grown by other methods

Electron mobilitiesDielectric lossesThermopower

Requires new approaches to address stoichiometry control issuesDifferent solutions for different materials families

Titanates: required a volatile cation source: use metal-organic sourceStannates: replace metal Sn source with SnO2

Need lattice matched substrates for BaSnO3

29

Thank you