This document discusses evidence that hydrogen is involved in the formation of the E3 defect center in zinc oxide (ZnO). Proton implantation into hydrothermally grown ZnO samples was found to significantly increase the concentration of E3 centers, as measured by capacitance-voltage profiling and deep level transient spectroscopy. The concentration of E3 centers increased by over an order of magnitude in samples implanted with protons compared to unimplanted control samples. Implantation with helium ions did not produce a similar increase in E3 centers. This provides strong evidence that hydrogen plays a role in the formation of E3 centers in ZnO.

1. The E3 center in zinc oxide: Evidence for involvement of hydrogen
A. Hupfer, C. Bhoodoo, L. Vines, and B. G. Svensson
Citation: Applied Physics Letters 104, 092111 (2014); doi: 10.1063/1.4867908
View online: http://dx.doi.org/10.1063/1.4867908
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2. The E3 center in zinc oxide: Evidence for involvement of hydrogen
A. Hupfer, C. Bhoodoo, L. Vines, and B. G. Svensson
Physics Department/Center for Materials Science and Nanotechnology, University of Oslo,
P.O. Box 1048 Blindern, Oslo N-0316, Norway
(Received 5 February 2014; accepted 19 February 2014; published online 5 March 2014)
Proton implantation is shown to increase the concentration of the so called and commonly
observed E3 defect level in zinc oxide (ZnO). Box and single profiles of protons with doses
ranging from 6 Â 1010
cmÀ2
to 4:3 Â 1012
cmÀ2
were implanted into hydrothermally grown ZnO
samples with original concentrations of E3 below 5 Â 1014
cmÀ3
. Capacitance-Voltage profiling
and junction spectroscopy measurements showed that the charge carrier concentration and absolute
concentration of E3 centers increase by more than one order of magnitude compared to the
as-grown samples as well as control samples implanted with He ions. The results provide strong
evidence for the involvement of H in the formation of the E3 center, and a complex involving
interstitial H and an oxygen sub-lattice primary defect are discussed. VC 2014 AIP Publishing LLC.
[http://dx.doi.org/10.1063/1.4867908]
Zinc oxide (ZnO) is a wide band gap semiconductor
(Eg % 3.4 eV) that has received considerable attention in the
past few years and is presently used in many diverse prod-
ucts, notably piezoelectric transducers, varistors, and trans-
parent conductive films. While classical semiconductor
applications, like photovoltaics and UV-light emitting diodes
and lasers, are certainly attractive for ZnO, it has also been
proposed as a host material for spintronics and quantum
computing due to its low spin-orbit coupling.1
A prerequisite
for spintronic devices is a fundamental understanding of
deep level defects serving as a base for spin-manipulations
with a very weak coupling to the environment. In ZnO, a
prominent defect level is found around Ec À 0.3 eV (Ec
denotes the conduction band edge), normally labeled E3, and
it occurs in most ZnO materials irrespective of the growth
method used. E3 was first reported by Simpson and Cordaro2
and later characterized in more detail and labeled E3 by
Auret et al.,3,4
who showed that E3 is not influenced by MeV
electron or proton irradiation. Furthermore, the concentration
of E3 is also not affected by implantation of self-ions, i.e., O
and Zn,5,6
while seemingly different results are reported on
the dependence on the annealing ambience.7–9
Quemener
et al.8
found variations in the concentration of E3 by about
one order of magnitude between nominally identical hydro-
thermal samples, implying that the growth conditions have a
crucial impact on the E3 formation. However, post-growth
consecutive heat treatments of these samples in atmospheres
of Ar, Zn, and O2 or Ar, O2, and Zn at 1100
C did not affect
the E3 concentration which remained constant. Mtangi
et al.9
have undertaken extensive studies of the evolution
of deep-level defects in melt grown samples during
post-growth annealing at temperatures typically between 600
and 900
C in Ar and O2 ambient. A variation in the E3 peak
intensity by a factor of Շ3 was found but it was not clear if
this variation originated from a true change in the E3 con-
centration or a variation in the net doping concentration.
Ultimately, the origin of the E3 defect is indeed a subject of
ongoing debate and speculations include a relation to inter-
stitial zinc,10
the oxygen vacancy, or even transition met-
als.11
However, as discussed in Ref. 3, the very low (if any)
generation rate of E3 in irradiated samples does not favor an
assignment to a primary intrinsic defect like interstitial Zn or
the oxygen vacancy, and in general, the results from the
different post-growth heat treatments support rather the
involvement of an abundant impurity. Finally, it has also
been reported that the defect signature around Ec À 0.3 eV in
hydrothermally grown samples may contain two closely
spaced levels, where the additional level is labelled E3’, and
it was speculated that oxygen vacancies and zinc interstitials
were responsible.12
In this study, we show that hydrogen implantation
drastically increases the E3 concentration. Box (multiple
energies) profiles as well as single energy profiles of protons
were implanted into hydrothermally grown ZnO samples
with an original concentration of E3 below 5 Â 1014
cmÀ3
.
The charge carrier concentration and the absolute E3 concen-
tration after implantation exhibit an increase of more than
one order of magnitude compared to the non-implanted
samples. The results provide strong evidence that hydrogen
is involved in the formation of E3.
Wafers of hydrothermally grown n-type ZnO (HT-ZnO)
purchased from Tokyo Denpa were cut into 5 Â 5 mm2
pieces. The samples were cleaned in acetone and ethanol.
After a 40 s treatment in boiling H2O2 (31%), 100 nm thick
Pd Schottky contacts were deposited on the Zn-polar face
using electron-beam evaporation. The Schottky contacts
displayed a rectification of almost two orders of magnitude
between forward and reverse bias (2 V and À2 V). The
samples were then implanted at room temperature (RT)
either with (a) 325 keV protons, with an expected projected
range of %2.0 lm, as estimated by Monte Carlo simulations
using the SRIM code,13
and doses between 6 Â 1010
and
3 Â 1011
cmÀ2
, or (b) protons having three different energies,
forming a box-like depth profile with a total dose between
%1 Â 1011
and %4 Â 1012
cmÀ2
, of 1344, 1366, and
1400 keV moderated through a 21 lm thick aluminum foil.
After implantation, the samples were stored in a freezer
(À20
C) until measured. In addition, a reference implanta-
tion using Heþ
ions with an energy of 800 keV and a dose of
4 Â 1011
cmÀ2
was conducted to unveil the effect of ion
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APPLIED PHYSICS LETTERS 104, 092111 (2014)
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3. induced defects only. Deep level transient spectroscopy
(DLTS) was carried out while scanning the sample tempera-
ture from 100 to 250 K using a refined version of a setup
described in Ref. 14.
A reverse bias of À5 V was applied with a filling pulse
of 5 V and 50 ms duration. The bulk net carrier concentration
was determined through capacitance-voltage (CV) measure-
ments (1 MHz probing frequency) in the scanned tempera-
ture range. For defect concentration versus depth profiling, a
single rate window was applied, and the temperature was
held constant at the maximum of the studied DLTS peak.
The steady-state bias voltage was kept constant with increas-
ing amplitude of the filling pulse, and the depth profile was
subsequently extracted from the dependence of the DLTS
signal on the pulse amplitude.
Figure 1 shows the charge carrier concentration (Nd)
versus depth extracted from CV measurements for samples
implanted with (a) 325 keV protons and (b) multiple energy
protons. Results for the respective unimplanted control
samples are also included in Fig. 1. Prior to the implantation,
the samples showed an almost uniform carrier concentration
of %8 Â 1014
cmÀ3
for (a) and %1 Â 1015
cmÀ3
for (b) at
depths տ0:8 lm while closer to the surface an increase
occurred. The calculated profiles in Fig. 1 were obtained by
SRIM simulations13
(dashed lines) and correspond to the
highest doses used. Figure 1 shows that the implantation
causes an increase in the charge carrier concentration which
largely follows the implantation profile. Because of this
increase in Nd and the limit of the maximum applicable
reverse bias voltage, the probe region becomes more shallow
with dose and all the profiles cannot be monitored to the
same depth. The increase in Nd is, indeed, attributed to the
implanted H atoms which can act as a shallow donor but
also exhibit a strong reactivity with impurities/defects and
efficiently passivate acceptors.15
No corresponding effect on
Nd was observed after He implantation.
Figure 2 shows DLTS spectra for the box-profile
implanted samples compared with those for the unimplanted
reference sample and the He-implanted control sample. Two
levels are observed, labeled E3 and E4. The pronounced
peak at $160 K (E3) has an energy position of Ec À 0.3 eV,
while E4 occurs at Ec À 0.57 eV. The concentration of E4
increases with the H dose, in accordance with previous
reports for electron and proton irradiated samples.3
It has
also been shown that E4 is enhanced after treatment in
Zn-ambient and suppressed in O-rich ambient;16
this behav-
ior is reversible and an association with a Zn-rich defect like
the oxygen vacancy (VO) appears to be plausible.17
Further,
also in the single profile samples the E4 intensity is enhanced
after implantation and the increase is proportional to the ion
dose (not shown).
E3 is the most prominent peak in Fig. 2 and presents in
the as-grown samples with concentrations of %40% of Nd.
Interestingly, the concentration of E3 increases by about one
order of magnitude in the high dose box profile sample, as
compared to the unimplanted reference sample. Moreover, a
high energy resolution weighting function of GS4-type18
did
not reveal multiple levels and the peak position as well
as the apparent capture cross section ($2 Â 10À15
cm2
)
remained the same in all the samples, indicating no other
contribution than the E3 center. As further shown in Fig. 2,
implantating He ions does not give rise to an increase in the
E3 concentration which stays identical to that in the unim-
planted control sample. Hence, the presence of H is a neces-
sary condition and ion-induced defects only are not sufficient
to enhance the formation of E3, as corroborated by results
FIG. 1. Influence of the hydrogen dose (per cm2
) on the charge carrier pro-
file for single profile (a) and box-profile implantations (b). Results are also
shown for the unimplanted reference samples. The calculated profiles corre-
spond to the highest implantation doses, respectively, and are obtained using
the SRIM-code.13
FIG. 2. DLTS spectra from (i) box-profile samples with different implanta-
tion doses, (ii) unimplanted reference sample, and (iii) sample before and af-
ter He implantation. Rate window ¼ 640 msÀ1
.
092111-2 Hupfer et al. Appl. Phys. Lett. 104, 092111 (2014)
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4. reported previously in the literature for self-ions,5
elec-
trons,19
and swift light ions.3
Here, it should be noted that a quantitative conversion
of the DLTS peak amplitude to concentration is strictly valid
for uniform defect profiles only. Thus, depth profiling meas-
urements are highly appropriate for the studied samples and
the obtained results are shown in Fig. 3. Before implantation,
the E3 distribution is almost uniform while after implanta-
tion it increases with depth having a shape resembling that of
the simulated H-profiles, Fig. 1. The absolute concentration
of E3 grows strongly with the H-dose, consistent with the
data in Fig. 2, and at a depth of %0.9 lm the growth is more
than a factor 20 in the high-dose box-implanted sample rela-
tive to unimplanted control one, Fig. 3(b). Similar to that for
the carrier-versus-depth profiles in Fig. 1, the probing depth
for E3 decreases with the H-dose because of the concurrent
increase in Nd and limited applicable maximum reverse bias
voltage. Accordingly, it was not possible to monitor the com-
plete profile of E3 for the high dose samples in Fig. 3.
Figure 4 shows the evolution of the E3 and Nd profiles
in the low dose box profile sample, 1 Â 1011
cmÀ2
, after
consecutive DLTS measurements performed up to RT and
storage at RT for a few hours at zero bias voltage. Both E3
and Nd decrease in concentration within a few hours and this
holds irrespective of the implantation dose used (see inset of
Fig. 4). Eventually, both E3 and Nd return to their original
concentration values recorded prior to implantation.
Interstitial hydrogen (Hi) is reported to be mobile at RT with
a migration energy (Em) in the range of %0.5–0.9 eV,20,21
and putting Em ¼ 0.7 eV with a pre-exponential factor of
%3 Â 10À2
cm2
sÀ1
for the diffusivity,20
a diffusion length of
%0.5 lm is obtained after 12 h at 295 K. This length is of the
same order of magnitude as the redistribution observed in
Fig. 4 and indicates that Hi plays a vital role for the enhance-
ment of both Nd and E3. Indeed, Hi, in a bond-centered con-
figuration, is well-established to act as a shallow donor15,22
contributing to Nd whilst previous reports showing an associ-
ation between Hi (or H in general) and E3 are scarce in the
literature. Here, it should be noted that implantation of H
under the dilute (low dose) conditions used in the present
study gives rise to a defect concentration of the same order
of magnitude as that of the implanted H ions; for instance, a
proton with an energy of 325 keV (cf. our single implant,
Figs. 1 and 3) generates on average 7–8 Frenkel pairs in total
(Zn interstitials (Zni) and Zn vacancies (VZn) plus the corre-
sponding pairs for oxygen (Oi, VO)) according to TRIM
simulations assuming a displacement energy threshold of
43 eV and 68 eV for Zn and O, respectively.23
In these simu-
lations, dynamic annealing is not considered and taking into
account that only a few percent of the Frenkel pairs survive
immediate recombination,24
the resulting defect formation
becomes less than 1 Frenkel pair per proton. This estimate of
the surviving fraction is based on data for silicon24
and is
probably an upper limit for ZnO which is known as a
radiation-hard material with strong dynamic annealing
effects.3
Hence, a major fraction of the H atoms is
FIG. 3. Influence of the implantation dose (per cm2
) on the concentration
versus depth profiles of E3; (a) single-profile samples and (b) box-profile
samples.
FIG. 4. Decay of (a) E3 concentration and (b) charge carrier concentration
at RT for the low-dose box-profile implanted sample, consecutive days of
measurement after the implantation. The insets show the data for all the
other box profile implanted samples after storage for 3 days at RT.
092111-3 Hupfer et al. Appl. Phys. Lett. 104, 092111 (2014)
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5. anticipated to reside on interstitial sites directly after implan-
tation, consistent with the large increase in Nd prior to the
subsequent diffusion/redistribution of Hi. Moreover, the
close resemblance between the evolution of E3 and Nd as a
function of (i) ion dose (albeit a weaker relative increase of
E3), (ii) depth, and (iii) post-implant time, implies strongly
that Hi is also decisive for the formation of E3. In addition,
based on the fact that the E3 depth profile peaks directly after
implantation with a minimum of Hi migration and that the
estimated concentration of the generated Frenkel pairs is in
the range of the increase in the E3 concentration (or some-
what higher), it is tempting to associate E3 with a defect con-
figuration invoking Hi and a primary intrinsic defect like VO,
VZn, or Oi. A center involving Zni is regarded as less likely
since both Zni and Hi are expected to be positively charged
in the present samples and Coulomb repulsion will suppress
the formation of such a center.
Generally, E3 is considered to be donor-like in the eval-
uation of data from Hall-effect measurements25
but DLTS
studies have not revealed any clear dependence of the elec-
tron emission rate from the E3 level on the electric field,16
as
expected for a well-behaved donor-like center in n-type ma-
terial. However, the data in Fig. 1 provide, indeed, evidence
for E3 as a donor-like center since no anomalous peak occurs
in the Nd profiles, which is an unambiguous feature of deep
acceptor-like traps with non-uniform depth distribution and
sufficient concentration to affect the C-V profiles in n-type
material.26,27
Accordingly, VO and Oi remain as the most
plausible intrinsic candidates to be involved in E3, whilst a
complex between VZn and Hi is anticipated to be
acceptor-like. Finally, the redistribution/migration of E3 at
RT may seem surprising, since E3 is known to survive
annealing up to 1100
C7
in as grown samples. However, it
has been shown for other H-related centers like the OH-LiZn
complex28
that they may dissociate at relatively low temper-
ature but display an apparent high temperature stability
caused by re-capturing of H during sample cooling. A similar
scenario appears also conceivable for E3 and the concentra-
tion measured in as-grown samples reflects a “dynamic equi-
librium” between capturing and dissociation of H.
In summary, hydrothermally grown n-type ZnO samples
have been implanted with low doses of protons and the gen-
eration of electrically active defects has been studied by
DLTS. An increase in both the E3 center and the charge car-
rier concentration is observed and attributed to the implanted
H. Further, the excess of both E3 and charge carriers in the
implanted region are shown to redistribute at RT, which indi-
cates the involvement of a rapidly migrating species, like Hi.
Arguments for an assignment of the E3 center to a defect
configuration involving Hi and a primary oxygen sublattice
defect (VO or Oi) are presented.
This work was supported by the Norwegian Research
Council through the FriPro Program (WEDD Project) and
Norwegian Ph.D. Network on Nanotechnology for
Microsystems. Enlightening discussions with Dr. Frank
Herklotz are highly appreciated.
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092111-4 Hupfer et al. Appl. Phys. Lett. 104, 092111 (2014)
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