high-mobility oxide films by mbe - mrl.ucsb.edustemmer/main_pdfs/imbe aug 16.pdf · high-mobility...
TRANSCRIPT
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