hydrogen as a shallow center in semiconductors and oxides · muon spin rotation – muonium:...

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