Oxides as Semiconductors
Chris G. Van de Walle
Materials Department, University of California, Santa Barbara
Acknowledgments:
A. Janotti, J. Varley (UCSB)
A. Singh (UCSB, now Rice U.); M. Scheffler (UCSB and Fritz Haber Instititute, Berlin)
P. Reunchan, S. Limpijumnong (Suranaree U., Thailand)
J. Neugebauer (MPI Düsseldorf)
J. Speck (USCB)
NSF MRSEC
SSLEC
© C. Van de Walle 2008
Van de Walle Computational Materials Group
www.mrl.ucsb.edu/~vandewalle
First-principles calculations
Density functional theory
Nitrides
Ga
-2
-3
-4
Cu O
2
NiO
TiO2
ZrO2
Oxides
SnO2
In2O3
ZnO
Ga2O3
SiO2
-10
GaN
-9
AlN
-8
InN
-7
SiC
-6
GaAs
-5
Si
Hydrogen storage
• Kinetics
• NaAlH4
• metal
hydrides
-1
N
Energy (eV)
N
0
Ge
• Electronic
structure
of nitride
surfaces
Novel channel
materials for CMOS
A new look at oxides
Transparent Conductors
Wide gap + conducting
Contacts for
• LEDs
• Solar cells
• Smart windows
Optoelectronics
• Direct band gap: 3.4 eV!
• Photodetectors
• LEDs, lasers
Sensors&Actuators
• Piezoelectricity
• Magnetic impurities
• Chemical sensors
Electronics
• Transistors
• HEMTs
• FETs
• Transparent displays
Bulk crystalsÆ substrates
Vision
• Multifunctional materials
Huang et al., Science 292, 1897 (2001)
– Novel high-k dielectrics/Nonlinear optics
– Ferroelectricity/Chemical sensors/Nanotechnology
• Reach new levels of performance
– Conventional preparation methods (sputtering, laser ablation):
» levels of stoichiometry and purity on the order of 0.1 – 1% (~ 1020 cm-3)
» Still: high mobility, low resistivity!
– Semiconductor standards of purity and crystalline quality:
» impurity and point defect concentrations in ppm range (< 1017 cm-3)
• Semiconducting binary oxides
– ZnO, SnO2, In2O3 (and ITO), Ga2O3, TiO2, …
• Vision:
– Enhanced control over impurities and defects will enable
unprecedented performance and new science,
leading to new applications
Motivation: ZnO
• Widely studied, but still major gaps in knowledge
– Typically n-type. Source? How to control?
– p-type doping possible?
– Interfaces:
» A. Janotti and C. G. Van de Walle, Phys. Rev. B 75, 121201 (2007).
• Control of conductivity essential!
– Many oxides: as-grown typically n-type
– Cause: heavily debated
– Still widely attributed to oxygen vacancies
• Approach:
– First-principles calculations
– Theoretical framework
– Defect and impurity engineering
Formalism
• Eform: formation energy
Concentration of defects or impurities:
C = Nsites exp [− Eform/kT]
• Example: oxygen vacancy in ZnO
Eform(VO2+) = Etot(VO2+) − Etot(bulk) + μO + 2 EF
μO: energy of oxygen in reservoir, i.e., oxygen chemical potential
EF: energy of electron in its reservoir, i.e., the Fermi level
• First-principles calculations:
– Density-functional theory (DFT), local density approximation (LDA)
– Supercell geometry (96 atoms); pseudopotentials; plane waves
Review: Van de Walle & Neugebauer, J. Appl. Phys. 95, 3851 (2004).
– Overcoming the DFT-LDA band-gap problem: “LDA+U” approach
– A. Janotti and C. G. Van de Walle, Appl. Phys. Lett. 87, 122102 (2005).
– A. Janotti, D. Segev, and C. G. Van de Walle, Phys. Rev. B 74, 045202
(2006).
Native point defects in ZnO
Zn-rich
• VO, VZn dominate
– A. Janotti and C. G. Van de Walle,
Appl. Phys. Lett. 87, 122102 (2005).
– S. B. Zhang et al., Phys. Rev. B 63,
075205 (2001).
– F. Oba et al., Phys. Rev. B 77,
245202 (2008).
• VO: deep donor
– Also high formation energy in
n-type ZnO
• VZn: deep acceptor
– Cause of green luminescence
– A. F. Kohan, G. Ceder,
D. Morgan, C. G. Van de Walle,
Phys. Rev. B 61, 15019 (2000)
Oxygen vacancy in ZnO
VO0
CBM
VBM
VO+
VO2+
VO: Comparison with experiment
Vlasenko & Watkins, Phys. Rev. B 71, 125210 (2005).
A. Janotti and C. G. Van de Walle, Appl. Phys. Lett. 87, 122102 (2005).
EF=Ev
VVOO00 +
+ hh→
→ VVOO++
Need to create VO
by irradiation!
No VO observed in
as-grown material.
Consistent with high
formation energy.
VO: Comparison with experiment
Evans, Giles, Halliburton & Kappers, J. Appl. Phys. 103, 043710 (2008).
A. Janotti and C. G. Van de Walle, Appl. Phys. Lett. 87, 122102 (2005).
EF=Ec
VO created by
irradiation
2.1 eV treshold for
VO0 Æ VO+ + e
Diffusion of point defects
Top
View
• Relevant for …
– growth
» Defects ‘frozen in’ or not
– Ion implantation
» Anneal damage
Side
View
– Degradation
– Irradiation
• Zinc interstitial:
– Em=0.57 eV
Annealing temperature of point defects
⎛ Eb ⎞
Γ = Γ0 exp ⎜ −
⎟
⎝ kT ⎠
Γ0 ≈ 1013 s −1
Γ ≈ 1s −1
Eb (eV)
T annealing (K)
Zni2+
0.57
219
VZn2-
1.40
539
VO2+
1.70
655
VO 0
2.36
909
Oi0(split)
0.87
335
Oi2-(oct)
1.14
439
A. Janotti and C. G. Van de Walle, Phys. Rev. B 76, 165202 (2007).
Native defects vs. impurities
• Native defects cannot explain n-type doping
• Impurities: donors?
Interstitial Hydrogen in ZnO
Formation energy (eV)
3
2
1
H
+
0
-1
0.0
0.5
1.0
1.5 2.0
EF (eV)
2.5
3.0
H+ is the only stable charge state Î hydrogen acts as shallow donor
Unexpected! In other semiconductors hydrogen reduces the conductivity
C. G. Van de Walle, Phys. Rev. Lett. 85, 1012 (2000).
Hydrogen is a likely candidate for unintentional incorporation
• But: highly mobile
M. G. Wardle, J. P. Goss and P. R. Briddon, Phys. Rev. Lett. 96, 205504 (2006).
Æ unstable at temperatures where n-type conductivity is known to
persist (>500oC)
Also cannot explain dependence of conductivity on oxygen partial pressure…
Substitutional hydrogen in ZnO
• Forced to reconsider the role of
hydrogen...
– … and in the process some interesting
new physics/chemistry emerged!
VO
• Substitutional hydrogen
– Hydrogen on a substitutional oxygen site
– Formation energy: low
– Ionization energy: small; shallow donor
HO
Hi
n-type conductivity on oxygen partial
pressure
Zn
O
H
log[X]
• Consistently explains dependence of
[n]
[HO]
[Hi]
[VO]
1/2
pO2
Diffusion of substitutional hydrogen
• How does HO move?
• Dissociation:
HO+ → Hi+ + VO0: costs 3.8 eV!
• Migration:
– Concerted exchange of H and
neighboring O
– Barrier: 2.5 eV
⇒ becomes mobile above 500oC
• Consistent with experimental
observations
– G. A. Shi et al., Phys. Rev. B 72,
195211 (2005)
– S. J. Jokela and M. D. McCluskey,
Phys. Rev. B 72, 113201 (2005)
Hydrogen multicenter bonds
• Hydrogen equally bonds to four atoms
• Truly multicoordinated configuration
Zn
O
A. Janotti and C. G. Van de Walle, Nature Mater. 6, 44 (2007).
H
Density of states of HO in ZnO
Zn d
Os
Op
gap
Zn s
Zn s
Op
Zn d
H
Hydrogen multicenter bond in zb-ZnO
Conductivity in SnO2
• Rutile structure; band gap: 3.6 eV
– Sensors
– Transparent conductor
c
u
a
• n-type conductivity:
not due to intrinsic point defects
– VO high formation energy, deep donor
– Sni, SnO: high formation energy
• Impurities?
• Hydrogen
HO+
Hi+
– Interstitial hydrogen:
Shallow donor, Low diffusion barrier
– Substitutional hydrogen:
Shallow donor, Diffusion barrier: 2.2 eV
A. K. Singh, A. Janotti, M. Scheffler, and C. G. Van de Walle,
Phys. Rev. Lett. 101, 055502 (2008).
Fermi Energy (eV)
Conductivity in SnO2
• p-type doping
– Difficult in ZnO
» N: high formation energy
» Group-I on Zn site:
deep acceptors, or self-compensation
– Potentially more feasible in SnO2:
» Group-III on Sn site
• Acceptors
– Al, Ga, In on Sn site
– Low ionization energy
– Modest formation
energy
• Complexes
– Al-H, Ga-H, In-H
c
u
a
Hydrogen multicenter bonds in other oxides
ZnO wurtzite
5-center bond
MgO rocksalt
7-center bond
SrTiO3 perovskite
3-center bond
Zn
O
H
H
Sr
H
Mg
Ti
SnO2 rutile
4-center bond
In2O3
5-center bond
TiO2 rutile
4-center bond
H
Sn
In
H
Ti
H
Conclusions
• Laying the groundwork for oxide-based
materials and device technology
– First-principles methods
• Doping
– Understanding and
controlling n-type doping
– Role of hydrogen
– Solid foundation for
tackling p-type doping