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134 Native Point Defects and Doping in ZnO
6
Spectral Identification of Impurities
and Native Defects in ZnO
B.K. Meyer
1
, D.M. Hofmann
1
, J. Stehr
1
and A. Hoffmann
2
1
Physics Institute, Justus Liebig University Giessen, Giessen, Germany
2
Institute of Solid State Physics, Technical University Berlin, Berlin, Germany
6.1 Introduction
This chapter deals with the spectral identification of impurities and defects in ZnO.
High resolution luminescence and magnetic resonance spectroscopy are employed to
study the role of extrinsic and intrinsic defects in ZnO. The neutral donor bound exciton
transitions are studied and identified. The most common donors stem from the group-III
elements Al, Ga and In. Hydrogen as a shallow donor – quite uncommon in compound
semiconductors – has the lowest binding energy. The doping with nitrogen and arsenic is
discussed and the different roles of the acceptors in the optical recombinations are
presented. Magnetic resonance adds valuable information to the role of the shallow
donors in ZnO and often allows for an unambiguous identification. The situation is more
controversial for the deep, intrinsic centers such as the oxygen and zinc vacancy, their
contribution to the deep luminescence bands in the green yellow spectral range is
discussed. For the acceptors N, Li and Na magnetic resonance gives clear evidence that
they act as deep centers, and there is up to now no evidence for shallow acceptors, at least
from magnetic resonance.
Zinc Oxide Materials for Electronic and Optoelectronic Device Applications, First Edition.
Edited by Cole W. Litton, Donald C. Reynolds and Thomas C. Collins.
© 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. ISBN: 978-0-470-51971-4
6.2 Optical Spectroscopy
6.2.1 Excitons Bound to Shallow Donors
ZnO is a direct band gap semiconductor which crystallizes in the wurtzite symmetry. The
valence band is split by crystal field and spin orbit interaction into three states named A, B
and C. The symmetry of the upper valence subband (A-subband) in ZnO has been
the subject of controversy (G
9
or G
7
character) for more than 40 years.
[1–11]
Based on the
polarization properties of the free exciton transitions, most of the authors assume that
the symmetry of the A-valence subband is G
7
.
[1,2,4–8,12–14]
At low cryogenic temperatures
bound exciton emission is the dominant radiative channel, whereas at higher temperatures
free exciton emission usually takes over. Up to eleven excitonic recombinations where
excitons bind to ionized, neutral donors and/or acceptors have been observed.
[1,2,15–17]
As
there was no definite assignment of the bound exciton recombinations to the chemical
nature of the donors or acceptors, they are numbered I
0
to I
11
in the early work of Reynolds
et al.
[15]
There was considerable dispute on these assignments. Based on magneto-
photoluminescence experiments, Gutowski et al .
[12]
attributed all recombinations from
I
5
to I
10
to acceptor bound excitons in conflict with the early magneto-absorption data of
Loose et al.
[14]
and with recently pu blished experiments
[17]
which showed that I
4
to I
8
are
neutral donor bound excitons.
The luminescence from high quality bulk ZnO crystals extends from the band edge to
the green/orange spectral range (an overview spectrum is shown in Figure 6.1). Very
common is a broad band centered around 2.45 eV extending from the blue into the green
range, where different impurities and defects contribute (see below). The lines dominating
originate from bound exciton (BE) recombinations [excitons bound to ionized (D
þ
X),
3.43.33.23.13.02.92.82.72.62.52.42.32.22.1
E
LO
DA
x
P-2xE
LO
DA
x
P-E
LO
DA
x
P
PL-intensity (arb. units)
Ener
g
y (eV)
T = 4.2 K
Figure 6.1 Photoluminescence spectrum of bulk ZnO showing excitonic, donor acceptor pair
(DAP) and deep level emission. The corresponding phonon replica with longitudinal optical
phonons (LO) are indicated. Reprinted from B.K. Meyer, et al., Bound exciton and donor–
acceptor pair recombinations in ZnO, phys. stat. sol. (b) 241, 231–260. Copyright (2004) with
permission from WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
136 Spectral Identification of Impurities and Native Defects in ZnO
neutral donors (D
o
X) and/or acceptors (A
o
X)] followed by longitudinal optical (LO)
phonon replicas with an energy separation of 72 meV. In some samples a donor-acceptor-
pair (DAP) transition is found, the chemical identity of the acceptor A
x
is unknown. The
transition energy is around 3.22 eV again followed by phonon replicas (see Figure 6.1).
The prominent lines in the bulk ZnO are the bound excitons positioned at 3.3628, 3.3608
and 3.3598 eV (labeled in Figure 6.2 as I
4
,I
6
..
8
). At 3.357 eV another bound exciton (I
9
)
can be observed. At lower energies from 3.34 to 3.31 eV further recombination lines
appear. It is the region where the two-electron satellite (TES) recombinatio n lines of the
neutral donor bound excitons show up.
[18,19]
During the recombination of an exciton
bound to a neutral donor the donor final state can be the 1s state (normal D
o
X line) or the
2s, 2p state (TES line). The energetic distance between the D
o
X and its TES is
consequently the difference between the donor energies in the 1s and 2p states, which
is three-quarters of the donor binding energy (E
D
) in the hydrogenic effective mass
approach (EMA). Therefore, by determining the position of the TES the related donor
binding energy is obtained with high precision.
From luminescence experiments on ZnO the localiza tion energies E
L
are determined as
the difference between the recombination energy of the respective bound exciton lines and
the free transversal exciton line (see Figure 6.1). In many semiconductors the localization
energies E
L
of excitons bound to ionized (D
þ
) and neutral shallow donors (D
0
) and acceptors
(A
0
) follow a certain ordering e.g. E
L
(D
þ
X) G E
L
(D
0
X) G E
L
(A
0
X) as found in CdS
[20]
and
is the case for ZnO
[21]
or E
L
(D
0
X) G E
L
(D
þ
X) G E
L
(A
0
X) as found in ZnSe
[22]
and
proposed for GaN.
[23]
This sequence can be understood as a result of the electron to hole
mass ratio s ¼m
e
/m
h
which has a characteristic value for each semiconductor. From this
mass ratio one can theoretically predict the localization energies of the bound excitons as a
function of the binding energies of the shallow donors E
D
and acceptors E
A
. In addition it
also answers the question on the stability of the ionized donor bound exciton com plexes.
Magnetic resonance showed that one of the shallow donors (I
4
) is hydrogen. Based on
electron nuclear double resonance (ENDOR) experiments the hyperfine interaction with a
3.383.373.363.353.34
T = 4.2 K
I
9
I
6...8
I
4
FX
A
PL-intensity (arb. units)
Energy (eV)
Figure 6.2 Photoluminescence spectrum in the excitonic range. FX
A
is the transversal free
exciton, I
4
–I
9
are bound exciton lines
Optical Spectroscopy 137
single H-nucleus was resolved.
[24]
Experiments showed that I
4
annealed out completely at
temperatures between 650
C and 800
C depending on the annealing time. Electron
paramagnetic resonance (EPR) showed a distinct reduction in the signal amplitude of the
neutral donor resonance togethe r with a disappearance of the hydrogen-related ENDOR
signal. I
4
and the hydrogen donor are typical for ZnO bulk crystals grown by the
hydrothermal and seeded vapor transport methods. It is, however, absent in crystals
grown from the vapor phase.
As to I
6/6a
we can refer to the implantation studies of Schilling et al.
[25]
They showed (see
Figure 6.3) that upon implantation of Al and successive annealing the I
6/6a
line gained in
intensity and was the strongest for Al concentrations above 9 10
16
cm
3
. Aluminum seems
to be an omnipresent impurity in vapor grown ZnO as already outlined by Gonzales et al.
[26]
In Ga-doped epitaxial films, the prominent excitonic recombinantion is I
8
. There has
been a report published on Ga-doped ZnO by the Sendai group.
[27]
In agreement with the
findings in Meyer et al.
[17]
and Reuss et al.
[28]
they showed that the exciton bound to
neutral Ga-donor recombination occurs at 3.359 eV. I
8
always dominates in epitaxial films
on GaN templates. Secondary ion mass spectrometry (SIMS) experiments showed a severe
interdiffusion of Ga from the GaN template into the ZnO epitaxial film.
I
9
could be identified with the donor In based on diffusion
[17]
and implantation
experiments.
[29]
M
€
uller et al. implanted radioactive
111
In into Zn bulk single crystals,
and In occupies substitutional Zn lattice sites after annealing at 700
C. They monitored
the photoluminescence while the donor In decayed into the stable and isoe lectronic Cd;
[29]
the decay of the I
9
line is shown in Figure 6.4.
For the donors which are responsible for the I
4
to I
10
recombinations we can deduce the
1s to 2p transition energy from the location of the two-electron satellites. Transitions to
higher excited states e.g. n ¼3, n ¼4, ... were not observed (see Figure 6.5).
In a simple hydrogen-like EMA the energy separation between n ¼1 and n ¼2 states
would be equal to three-quar ters of the donor binding energy R
. A short range chemical
potential of the impurity would affect only the states of the S symmetry thus leading to the
chemical shift of the real donor binding energy E
D
from the effective mass value R
and to
the chemical shift of the 2S state. In this case , the donor energy E
D
can be estimated as
(E
2p
E
1s
) þ
1
/
4
R
. However, the 2S and 2P states in polar hexagonal semiconductors are
additionally split due to the effects of anisotropy (into 2S, 2P
z
and 2P
x,y
states, where the
hexagonal axis c is directed along z) and the polar interaction with optical phono ns. The
latter is different for the states of S and P symme try and may also modify the chemical shift
corrections on all states. The results of numerical calculations on the binding energies of
the different donors are given in Meyer et al.
[17]
It is interesting to test whether the donor binding energies E
D
show a linear relationship
to the bound exciton localization energies E
loc
¼(E
FX
E
0
DX
) known as Haynes rule,
E
loc
¼aE
D
.
[30]
More generally, E
loc
is given by:
E
loc
¼ A
0
þB
0
E
D
and A
0
and B
0
must be determined by experiment.
The localization energies can be measured with rather high precision, whereas the
binding energies could be in error of 5% depending on the choice of the band parameters
(electron effective masses, dielectric constants including the anisotropy of both). The
138 Spectral Identification of Impurities and Native Defects in ZnO
Zn0 : Al
sample 3
T=4 K
3670 3680 3690 Å
N
A1
= 9-10
18
cm
–3
N
A1
= 9-10
17
cm
–3
N
A1
= 9-10
16
cm
–3
N
A1
= 9-10
15
cm
–3
N
A1
= 0
cm
–3
annealing: T
A
= 800ºC
t = 30 min
I
516
I
516
I
g
I
g
x 1
x 10
x 1
x 0.5
x 0.66
Figure 6.3 Luminescence spectra as a function of wavelength of Al-implanted ZnO taken after
implantation and annealing at 800
C for 30 min. Reprinted from M. Schilling, R. Helbig, and G.
Pensl, Bound exciton luminescence of Ar- and Al-implanted ZnO, Journal of Luminescence 33,
201–212. Copyright (1985) with permission from Elsevier
Optical Spectroscopy 139
fitting of the data in Figure 6.6 gives A
0
¼3:8 meV; B
0
¼ 0:365 (note that the Haynes
rule actually does not demand for A
0
¼ 0). The good linear relationship gives further
support that I
10
is also donor related.
We demonstrated that the localization energies of the neutral and ionized donor bound
excitons scale with the respective donor binding energies.
[21]
Figure 6.7 shows the
Figure 6.4 Luminescence spectra of the bound exciton range of radioactive
111
In-implanted
ZnO (a) 31 h, (b) 95 h and (c) 243 h after the implantation and annealing process. Reprinted
from S. Mu
¨
ller, D. Stichtenoth, M. Uhrmacher, H. Hofsa¨ss, C. Ronning, and J. Ro¨der,
Unambiguous identification of the PL-I9 line in zinc oxide, App. Phys. Lett. 90, 012107.
Copyright (2007) American Institute of Physics
3.3253.3203.3153.310
2s
2s
2p
2p
TES (I
6/6a
)
TES (I
8
)
PL-intensity (arb. units)
Ener
g
y (eV)
Figure 6.5 Two-electron-satellite transitions of different donor bound excitons. The splitting of
the excited states into the 2s and 2p states is indicated. Reprinted from B.K. Meyer, et al., Bound
exciton and donor–acceptor pair recombinations in ZnO, phys. stat. sol. (b) 241, 231–260.
Copyright (2004) with permission from WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
140 Spectral Identification of Impurities and Native Defects in ZnO
7060504030
0
5
10
15
20
25
30
D
+
X
D
o
X
Exciton localization energy (meV)
E
D
(meV)
Figure 6.6 Localization energies of neutral D
0
and ionized D
þ
bound excitons in ZnO as a
function of the donor binding energies E
D
. Reprinted from B.K. Meyer, J. Sann, S. Lautens-
chlaeger, M.R. Wagner, and A. Hoffmann, Ionized and neutral donor-bound excitons in ZnO,
Phys. Rev.B 76, 184120. Copyright (2007) with permission from American Physical Society
3.35 3.36 3.37 3.38
Intensity (arb. units)
Ener
gy
(
eV
)
I
0
I
1
I
9
I
8
I
4
I
6a
A
T
A
L
I
2
Figure 6.7 Photoluminescence spectrum of a homoepitaxial ZnO thin film taken at T ¼4.2 K
showing neutral donor bound exciton recombination I
6a
,I
8
and I
9
and the corresponding
ionized donor bound exciton lines I
0
,I
1
and I
2
.A
T
and A
L
are the transversal and longitudinal
free A-exciton recombinations. Reprinted from B.K. Meyer, J. Sann, S. Lautenschlaeger, M.R.
Wagner, and A. Hoffmann, Ionized and neutral donor-bound excitons in ZnO, Phys. Rev.B 76,
184120. Copyright (2007) with permission from American Physical Society
Optical Spectroscopy 141