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HydrogenHydrogen as a shallow center in as a shallow center in semiconductors and oxidessemiconductors and oxides
Chris G. Van de WallePalo Alto Research Center
International Workshop on Hydrogen in Materials and Vacuum systems
Jefferson Lab, Newport News, VA, November 11-13, 2002
AcknowledgmentsAcknowledgments
• Computations and DiscussionsS. LimpijumnongA. Janotti, S. Wei, S. ZhangJ. McCaldin, J. Neugebauer
• SupportAFOSR Alexander von Humboldt FoundationFritz-Haber-Institut and Paul-Drude-Institut, Berlin, Germany
Hydrogen…Hydrogen…
•• The most abundant elementThe most abundant element~90% of the universe, by weight~90% of the universe, by weight
•• Omnipresent impurityOmnipresent impurity–– Present in most materialsPresent in most materials
»» intentionally or notintentionally or not
–– Major effect on materials propertiesMajor effect on materials properties–– “Prototype Impurity”“Prototype Impurity”
»» simplesimple»» genericgeneric }} Not really!Not really!
• Electronics– Integrated circuits
» passivate defects at Si/SiO2 interface– Amorphous and polycrystalline silicon– Silicon-on-insulator (Smart Cut)– Semiconductor growth: carrier gas, …
• Power– Fuel cells
» Hydrogen storage» Proton exchange membranes
R. Griessen, Amsterdam
• Optics– switchable mirrors– proton-exchanged waveguides
Hydrogen in technologyHydrogen in technology
Hydrogen interactions …Hydrogen interactions …… with the perfect lattice
» interstitial H can create large rearrangements of host atoms
… with defects» passivation of dangling bonds
… with impurities» neutralization of donors or acceptors
… with itself» interstitial H2 molecules
Interstitial hydrogenInterstitial hydrogen
• Understanding interstitial hydrogen ⇒ foundation for understanding
interactions with defects and impurities• Hydrogen is electrically active
– H0, but also H+ and H-» H+ seeks out regions of high electronic charge density
interacts mainly with anions» H- seeks out regions of low electronic charge density
interacts mainly with cations– relative stability depends on Fermi level
• Eform: formation energy Concentration:
C = Nsites exp [− Eform/kT]
• Example: Hydrogen in GaNEform(H+) = Etot(GaN:H+) − Etot(GaN) − µH + EF
µH: energy of hydrogen in reservoir, i.e., H chemical potentialEF: energy of electron in its reservoir, i.e., the Fermi level
FormalismFormalism
• First-principles calculations:– Density-functional theory, local density approximation (LDA)– Pseudopotentials; Coulomb potential for H– Atomic relaxation– Supercell geometry (96 atoms); plane waves
fhi98md: M. Bockstedte et al., Comp. Phys. Commun. 207, 187 (1997)
“ Negative U ”H0 never stable
AmphotericH+ favorable in p-typeH- favorable in n-type
0.0 0.5 1.0 1.5 2.0 2.5 3.0
0
1
2
3
4
5ε(+/-)
ε(+/0)ε(0/-)
-
H0
HH+
Form
atio
n En
ergy
(eV)
EF (eV)
Eform(H+) = Etot(H+) − Etot(GaN) − µH + EF
µH: Hydrogen chemical potentialEF: Fermi level
PRL 75, 4452 (1995)
Example: interstitial H inExample: interstitial H in GaNGaN
CB
VB
ε(+/0) = εD
ε(0/-) = εA
ε(+/-)
EF
⇒ Interaction with impurities
Generic behavior of interstitial HGeneric behavior of interstitial H
Amphoteric– always counteracts
prevailing conductivityApplies to:– Si, Ge,…– GaAs, AlAs, AlN, …– ZnSe, …
What about ZnO?…0.0 0.5 1.0 1.5 2.0 2.5 3.0
0
1
2
3
4
5ε(+/-)
ε(+/0)ε(0/-)
-
H0
HH+
Form
atio
n En
ergy
(eV)
EF (eV)
Hydrogen in ZnOHydrogen in ZnO
H+ is the only stable charge statePRL 85, 1012 (2000)
0.0 0.5 1.0 1.5 2.0 2.5 3.0
-2
-1
0
1
2
H+
Form
atio
n en
ergy
(eV)
EF (eV)
Acknowledgement: J. McCaldin
Applications of zinc oxide
Cao et al., Northwestern U.
ZnO crystalsnanocrystalsnanowiresbulk!
Applications of zinc oxideElectronics
• Varistors (surge protectors)• Transducers
Chemistry• catalysis• sensors!
Optoelectronics• Nonlinear optics • Blue/UV LEDs, lasers,
photodetectorsDirect band gap: 3.4 eV!
Huang et al., Science 292, 1897 (2001)
Devices: Control of conductivity required!
ExperimentExperiment
• Exposure to H2– Mollwo, Z. Phys. 138, 478 (1954)– Thomas and Lander, 1956– increase in conductivity
• Muon spin rotation– Muonium: pseudo-isotope of hydrogen
» Cox et al., PRL 86, 2601 (2001)
• Electron paramagnetic resonance + ENDOR» Hofmann et al., PRL 88, 045504 (2002)
• Hydrogen as an unintentional dopant:– vapor-phase transport, hydrothermal growth – MOCVD (sources, carrier gas), MBE (H2O residual gas) – laser ablation, sputtering (H2 atmosphere)
Why isWhy is ZnOZnO different?different?
H+
H-H+
• Position of ε(+/-) in the band gap
GaN ZnO• Question:
– Why is ε(+/-) so much higher in energy in ZnO?
CB
VB
ε(+/-)
EFCB
VB
ε(+/-)EF
Why isWhy is ZnOZnO different?different?
• Band lineups!
GaN ZnO
CB
VB
ε(+/-)
Band lineupsBand lineups
-10
Use natural band lineups to align band structures[PRB 39, 1871 (1989)]
-8
-6
-4
-2
0E (eV)
GaN
GaSbGaAs
ZnO
ZnSe
GeSi
SiC
AlN
InN
SiO2
-10
Lineup of Lineup of εε(+/(+/--) level) level
-8
-6
-4
-2
0E (eV)
GaN
GaSbGaAs
ZnO
ZnSe
GeSi
SiC
AlN
InN
SiO2
ε(+/-) level
ZnO
ZnSe
SiO2
SiO2
-10
Lineup of Lineup of εε(+/(+/--) level) level
-8
-6
-4
-2
0E (eV)
GaN
GaSbGaAs
GeSi
SiC
AlN
InN
ε(+/-) level
P. Blöchl, PRB 62, 6158 (2000)
ZnO
ZnSe
SiO2
ZnO
-10
Lineup of Lineup of εε(+/(+/--) level) level
-8
-6
-4
-2
0E (eV)
GaN
GaSbGaAs
GeSi
SiC
AlN
InN
ε(+/-) level
InN
-10
Lineup of Lineup of εε(+/(+/--) level) level
-8
-6
-4
-2
0E (eV)
GaN
GaSbGaAs
ZnO
ZnSe
GeSi
SiC
AlN
SiO2InN: H exclusively a donor
Hydrogen in nitridesHydrogen in nitrides
• S. Limpijumnong et al., phys. stat. sol. (b) 228, 303 (2001)
• Experiment: D. C. Look et al., Appl. Phys. Lett. 80, 258 (2002)
0 1 2 3 4 5−1
0
1
2
3
4
0 1 2 3 0 1EF (eV) EF (eV) EF (eV)
For
mat
ion
Ene
rgy
(eV
)
H+
H−
H0
H+
H−
H0
H0
H−
H+
ε(0/−) ε(+/0) ε(0/−) ε(0/−)ε(+/−) ε(+/−)(a) (b) (c)
AlN GaN InN
--) level) level
GaN
H exclusively a
-10
Lineup of Lineup of εε(+/(+/
-8
-6
-4
-2
0E (eV)
GaSbGaAs
ZnO
ZnSe
GeSi
SiC
AlN
InN
SiO2
InGaAsN: donor
0.0 0.5 1.0-0.5
0.0
0.5
1.0
1.5
VBM CBM
Form
atio
n en
ergy
(eV
)
εF
H in GaAsN
BCN–
BCN0
BCN+ (+/–)
» A. Janotti, S. Zhang, S. Wei, and C. G. Van de Walle,Phys. Rev. Lett. (in press)
(a) BCN+ (b) ABN
+
N H As
Ga
(c) α-H2*(N)
NH(1)
Ga
(d) β-H2*(N)
H(2)
Monohydride complexes
Dihydride complexes
H in (In)H in (In)GaAsNGaAsN
GeSi
Ge: H acceptor ?-10
Lineup of Lineup of εε(+/(+/--) level) level
-8
-6
-4
-2
0E (eV)
GaN
GaSbGaAs
ZnO
ZnSeSiC
AlN
InN
SiO2
GaSbGaAs
ZnO
ZnSe
ely an acceptor
-10
Lineup of Lineup of εε(+/(+/--) level) level
-8
-6
-4
-2
0E (eV)
GaN
GeSi
SiC
AlN
InN
SiO2
GaSb: H exclusiv
ConclusionsConclusions
• Hydrogen exhibits a fascinating array of behaviors in semiconductors and oxides
• Harnessing this behavior provides opportunities for defect and impurity engineering:– Doping:
» suppressing compensation» increasing solubility
• Understanding interstitial hydrogen provides basis for understanding more complex interactions
• Band lineups + electronic behavior of H:– Predictive model for hydrogen across a range of materials