A photoluminescence study of

Cd, In and Sn in ZnO using radioisotopes

Joseph Cullen, Martin Henry, Enda McGlynn

Dublin City University

Karl Johnston

Universitat des Saarlandes and CERN/ISOLDE

III IV V VI

OIB IIB

Zn

The nature of ZnO

ZnO - semiconductor at room temperature, energy gap ~ 3.4 eV

- universally n-type as-grown - p-type conduction can be

obtained, but not readily

Difficult to purify for growth of large single crystal boules

Questions: Origin(s) of dominant n-type conduction

Understanding impurities / defects

III IV V VI

B OIB IIB Al

Zn GaInTl

Group III impurity on Zn site should:

• provide one excess electron

• act as donor: n-type conductivity

• provide binding centre for e-h pairs under optical excitation

III IV V VI

N OIB IIB P

Zn AsSbBi

Group V impurity on O site should:

• create a free hole

• act as acceptor: p-type conductivity

BUT these prefer to occupy Zn sites and/or form complex defects

The neighbourhood of ZnO

• Proof of common donor impurity identifications

• Wider study of Zn-site and O-site impurities

Principal experimental technique:

• Photoluminescence at low temperatures

- in conjunction with other techniques/partners in ISOLDE collaboration

Our research programme:

PL

inte

nsity

Photon energy (eV)3.34 3.36 3.38

Multiplicity of lines

- Various impurities

- Various transition typesI-lines

D0X – neutral donor-bound excitons

D+X – ionised donor bound excitons

Also - DAP, eA, A0X

III IV V VI

B OIB IIB Al

Zn GaInTl

Proof of identity of common donor impurities

IIIA IVA VA VIAB C N O

IB IIB Al Si P SCu Zn Ga Ge As SeAg Cd In Sn Sb TeAu Hg Ti Pb Bi Po

IIIA IVA VA VIAB C N O

IB IIB Al Si P SCu Zn Ga Ge As SeAg Cd In Sn Sb TeAu Hg Ti Pb Bi Po

IIIA IVA VA VIAB C N O

IB IIB Al Si P SCu Zn Ga Ge As SeAg Cd In Sn Sb TeAu Hg Ti Pb Bi Po

72Zn 72Ga

72Ge 73Ga

-

-

-46.5 hr

14.1 hr

4.86 hr

73Ge

ZnO:Ga I-line identification

ZnO:73As → 73Ge decay

Half-life: 80.3 days

3.3550 3.3575 3.3600 3.3625 3.3650

1.2

1.6

2.0

2.4

Lum

inesc

ence

inte

nsi

ty (

a.u

)

I6

Energy (eV)

I8

10 100

0.1

1

10

73Ga __ I8 __72Zn72Ga

I1 : 73Ga+

Inte

grat

ed I

nten

sity

(a.

u.)

Time (hours)

Decay of Ga-related I8 (and I1) Growth of Ge-related DD2

New results from Ga → Ge decay

• Ge-related luminescence observed for the first time• Large spectral binding energy compared to III impurities• Low thermal binding energy• Similar to I-lines under stress

III IV V VI

B OIB IIB Al Si

Zn Ga GeInTl

Nature of Ge-related luminescence?

Ge on Zn site:

• two extra electrons per Ge atom

• several electron-hole recombination paths are possible

• we are pursuing this using Zeeman/stress

Theory:

Ge and Si should act as shallow double-donors

Lyons et al (2009)

ZnO:In I-line identification

III IV V VI

B OIB IIB Al

Zn Ga GeCd In Sn

Tl

Muller at al – APL (2007)

I-9 identified with In D0X

No evidence for D+X line

Possible Cd-related weak band

Is I-9 the DoX for In ?

Any evidence for D+X line ?

Also: does Sn behave like Ge?

111In → 111Cd

We examined the reverse decay path

III IV V VI

B C N OIB IIB Al Si P S

Cu Zn Ga Ge As SeAg Cd In Sn Sb TeAu Hg Ti Pb Bi Po

117Ag 72 s

117Cd 3 h

117In 43 m

3.350 3.355 3.360 3.3650

20000

40000

60000

80000

100000

120000

140000I2

I7/6

I10

I9

Energy (eV)

2h52 7h15 18h45

3.360 3.365 3.370 3.375 3.3800

500

1000

1500

2000

2500

3000

3500

4000I2

I7/6

I10

I9

Energy (eV)

10

0.01

0.1

1

10

100

5

I9: 117In

Inte

gra

ted

Inte

nsi

ty (

a.u

.)

Time (hours)

I2: 117In+

I-9 line

I-2 line

Confirm Muller et al result for I-9

New results:

• I-2 is D+X for In

• No Cd or Sn signals observed

ZnO:117Cd/In/Sn

Isotope Fitted τ½ Tabulated τ½

72Ga 12.7 ± 1.5 h 14.1 h

73Ga 4.9 ± 0.2 h 4.76 h

73As 78 ± 3 d 80.3 d

117In 43 ± 2 m 43.2 m

Fits to experimental data

III IV V VI

B C OIB IIB Al Si

Zn Ga GeAg Cd In Sn

Hg Tl Pb

Summary

Lines I-1 and I-8 due to Ga

Lines I-2 and I-9 due to In

New PL line due to Ge

For Zn-site impurities in ZnO

Not observed: Sn counterpart of Ge lineCd counterpart of Hg line (Agne et al 2003)

Thank you!

ENSAR

Zn-site impurities

R. J. Mendelsberg et al. J. Vac. Sci. Technol. B 27(3) (2009)

Pb in ZnO….