Oxides as SemiconductorsOxides as Semiconductors

Chris G. Van de WalleChris G. Van de WalleMaterials 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 MRSECSSLEC

© C. Van de Walle 2008

Van de Walle Computational Materials GroupVan de Walle Computational Materials Groupwww.mrl.ucsb.edu/~vandewallewww.mrl.ucsb.edu/~vandewalle

Nitrides• Electronic

structure of nitride surfaces

Ga

N N

First-principles calculationsDensity functional theory

Novel channel materials for CMOS

-10-9-8-7-6-5-4-3-2-10

Si G

eSiC

AlNG

aNInN

GaAs

SiO2

ZnOG

a2 O

3

In2 O

3

SnO2

Cu

2 ONiO

TiO2 ZrO

2

Ener

gy (e

V)

Oxides

Hydrogen storage• Kinetics• NaAlH4• metal

hydrides

Optoelectronics• Direct band gap: 3.4 eV!• Photodetectors• LEDs, lasers

A new look at oxidesA new look at oxidesTransparent Conductors

Wide gap + conductingContacts for

• LEDs• Solar cells• Smart windows

Sensors&Actuators• Piezoelectricity• Magnetic impurities• Chemical sensors

Electronics• Transistors

• HEMTs• FETs

• Transparent displays

Bulk crystals substrates

VisionVision• Multifunctional materials

– 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

Huang et al., Science 292, 1897 (2001)

• 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

Motivation: ZnOMotivation: ZnO

FormalismFormalism• Eform: formation energy

Concentration of defects or impurities:C = Nsites exp [− Eform/kT]

• Example: oxygen vacancy in ZnOEform(VO

2+) = Etot(VO2+) − Etot(bulk) + μO + 2 EF

μO: energy of oxygen in reservoir, i.e., oxygen chemical potentialEF: 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 ZnONative point defects in ZnO• 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)

Zn-rich

VO0

Oxygen vacancy in ZnOOxygen vacancy in ZnO

VBM

CBM

VO+ VO

2+

VVOO: Comparison with experiment: Comparison with experimentVlasenko & Watkins, Phys. Rev. B 71, 125210 (2005).A. Janotti and C. G. Van de Walle, Appl. Phys. Lett. 87, 122102 (2005).

EF=Ev

VO0 + h→ VO

+VO0 + h→ VO

+

Need to create VOby irradiation! No VO observed in as-grown material. Consistent with high formation energy.

VVOO: Comparison with experiment: Comparison with experimentEvans, Giles, Halliburton & Kappers, J. Appl. Phys. 103, 043710 (2008).A. Janotti and C. G. Van de Walle, Appl. Phys. Lett. 87, 122102 (2005).

VO created by irradiation

2.1 eV treshold for VO

0 VO+ + e

EF=Ec

Diffusion of point defectsDiffusion of point defects

Side View

Top View

• Relevant for …– growth

» Defects ‘frozen in’ or not

– Ion implantation» Anneal damage

– Degradation– Irradiation

• Zinc interstitial:– Em=0.57 eV

Annealing temperature of point defectsAnnealing temperature of point defects

4391.14Oi2-(oct)

3350.87Oi0(split)

9092.36VO0

6551.70VO2+

5391.40VZn2-

2190.57Zni2+

T annealing (K)Eb (eV)

⎟⎠⎞

⎜⎝⎛−Γ=Γ

kTEbexp0

1130 s10 −≈Γ 1s1 −≈Γ

A. Janotti and C. G. Van de Walle, Phys. Rev. B 76, 165202 (2007).

Native defects Native defects vsvs. impurities. impurities

• Native defects cannot explain n-type doping• Impurities: donors?

Interstitial Hydrogen in ZnOInterstitial Hydrogen in ZnO

H+ is the only stable charge state hydrogen acts as shallow donorUnexpected! 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).

unstable at temperatures where n-type conductivity is known to persist (>500oC)Also cannot explain dependence of conductivity on oxygen partial pressure…

0.0 0.5 1.0 1.5 2.0 2.5 3.0

-1

0

1

2

3

+H

Form

atio

n en

ergy

(eV)

EF (eV)

Substitutional hydrogen in ZnOSubstitutional hydrogen in ZnO• Forced to reconsider the role of

hydrogen...– … and in the process some interesting

new physics/chemistry emerged!• Substitutional hydrogen

– Hydrogen on a substitutional oxygen site– Formation energy: low– Ionization energy: small; shallow donor

• Consistently explains dependence of n-type conductivity on oxygen partial pressure

VO HO

Hi

Zn

HO [Hi]

log[

X]

pO2

[VO]

[HO]

[n]

1/2

[Hi][Hi]

log[

X]

pO2

[VO][VO]

[HO][HO]

[n][n]

1/2

Diffusion of substitutional hydrogenDiffusion 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 bondsHydrogen multicenter bonds• Hydrogen equally bonds to four atoms• Truly multicoordinated configuration Zn

HO

A. Janotti and C. G. Van de Walle, Nature Mater. 6, 44 (2007).

Density of states of HDensity of states of HOO in ZnOin ZnOO s

O p

Zn d

Zn sgap

HZn d

O p

Zn s

Hydrogen multicenter bond in Hydrogen multicenter bond in zbzb--ZnOZnO

Fermi Energy (eV)

Conductivity in SnOConductivity in SnO22

• Rutile structure; band gap: 3.6 eV– Sensors– Transparent conductor

• n-type conductivity: not due to intrinsic point defects– VO high formation energy, deep donor– Sni, SnO: high formation energy

• Impurities?• Hydrogen

– Interstitial hydrogen:Shallow donor, Low diffusion barrier

– Substitutional hydrogen:Shallow donor, Diffusion barrier: 2.2 eV

a

c

u

Hi+

HO+

A. K. Singh, A. Janotti, M. Scheffler, and C. G. Van de Walle, Phys. Rev. Lett. 101, 055502 (2008).

Conductivity in SnOConductivity in SnO22

• 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

a

c

u

Hydrogen multicenter bonds Hydrogen multicenter bonds in other oxidesin other oxides

Zn

O H

ZnO wurtzite 5-center bond

SrTiO3 perovskite 3-center bond

HSr

Ti

H

Mg

MgO rocksalt7-center bond

SnO2 rutile 4-center bond

SnH

In2O35-center bond

InH

TiO2 rutile 4-center bond

Ti H

ConclusionsConclusions• 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