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….