Shallow carrier traps in hydrothermal ZnO crystals
C Ton-That
1
, L L C Lem
1
, M R Phillips
1
, F Reisdorffer
2
, J Mevellec
2
,
T-P Nguyen
2
, C Nenstiel
3
and A Hoffmann
3
1
School of Physics and Advanced Materials, University of Technology Sydney, PO Box 123,
Broadway, NSW 2007, Australia
2
Institut des Matériaux Jean Rouxel, Universite de Nantes, F-44322 Nantes Cedes 03, France
3
Institut für Festkörperphysik, Technische Universität Berlin, Hardenbergstr. 36 D-10623
Berlin, Germany
E-mail: [email protected]
Received 25 March 2014, revised 2 July 2014
Accepted for publication 7 July 2014
Published 26 August 2014
New Journal of Physics 16 (2014) 083040
doi:10.1088/1367-2630/16/8/083040
Abstract
Native and hydrogen-plasma induced shallow traps in hydrothermally grown
ZnO crystals have been investigated by charge-based deep level transient
spectroscopy, photoluminescence and cathodoluminescence microanalysis. The
as-grown ZnO exhibits a trap state at 23 meV, while H-doped ZnO produced by
plasma doping shows two levels at 22 meV and 11 meV below the conduction
band. As-grown ZnO displays the expected thermal decay of bound excitons
with increasing temperature from 7 K, while we observed an anomalous beha-
viour of the excitonic emission in H-doped ZnO, in which its intensity increases
with increasing temperature in the range 140–300 K. Based on a multitude of
optical results, a qualitative model is developed which explains the Yline
structural defects, which act as an electron trap with an activation energy of
11 meV, being responsible for the anomalous temperature-dependent cath-
odoluminescence of H-doped ZnO.
Keywords: ZnO, DLTS, cathodoluminescence
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New Journal of Physics 16 (2014) 083040
1367-2630/14/083040+12$33.00 © 2014 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft
1. Introduction
Nominally undoped ZnO exhibits n-type conductivity due to the existence of shallow intrinsic
defects and the high solubility of extrinsic donor-like impurities such as hydrogen [1,2]. It has
been established that hydrogen strongly affects the electronic properties of ZnO. Calculations
based on density-functional theory (DFT) by Van de Walle et al [3,4] indicated that interstitial
hydrogen (H
i
) and hydrogen trapped at an oxygen vacancy (H
O
) can act as two shallow donors,
which can be a cause of n-type conductivity in ZnO. These theoretical results have been
supported by experiments, which showed that the n-type conductivity and the near-band-edge
(NBE) luminescence efficiency of ZnO can be enhanced at the expense of defect-related
emissions via hydrogen incorporation [5–10]. Although the origins of defect-related emissions
in ZnO are still not thoroughly understood, the quenching effect suggests that hydrogen
interacts with native defects in some way. There have been reports of a stable hydrogen-related
complex formed by zinc vacancy (V
Zn
) acceptor and two H atoms [11]. As a reactive and
common impurity, understanding the interaction of hydrogen with native defects in ZnO is not
only of fundamental interest but also of technological importance.
Deep defect levels in ZnO have been previously investigated by the capacitance-based
deep level transient spectroscopy (C-DLTS) technique as complementary investigations to
optical studies [12–16]. Deep traps below the conduction band minimum such as E
1
(0.12 eV),
E
2
(0.10 eV), E
3
(0.29–0.30 eV) and E
4
(0.53 eV) were reported for ZnO grown by different
methods; these traps were assigned to significant impurities or point defects in the bulk
[12,16–18]. Surface traps in ZnO have previously been investigated by C-DLTS [14]; however,
C-DLTS signals can be severely distorted as the trapped charge can be varied as a function of
applied bias. While the conventional C-DLTS is a powerful technique to investigate the
behaviour of traps, it is not capable of identifying shallow trap states due to carrier freeze-out at
low temperatures and immeasurably small capacitance of ZnO junctions [19,20]. Electrically
active shallow defects play a pivotal role in determining the electrical and optical properties of
ZnO; however, controlling the behaviour of these defects remains a major challenge.
Conversely, the charge-based deep level transient spectroscopy (Q-DLTS) used in this work has
been specifically developed to facilitate probing of shallow trapping states and accordingly has
been applied to the investigation of these defect centers in ZnO crystals. In contrast to C-DLTS,
Q-DLTS is an isothermal technique in which the transient process of trapped charge (not
capacitance) is measured after voltage stimulation. In this work, combined results from Q-
DLTS and temperature-resolved photoluminescence (PL) and cathodoluminescence (CL)
spectroscopy allow quantitative evaluation of shallow level defects in the near-surface region of
the as-grown and H-doped ZnO crystals.
Traps in the near-surface region of a semiconductor can be investigated through the use of
Q-DLTS because the sensitivity of charge transient detection is enhanced for surface states in
metal–semiconductor junctions [21]. In the Q-DLTS method, cyclic bias pulses are applied to a
Schottky barrier junction to excite carrier traps; the electron occupation of the traps is monitored
by measuring the associated charge transients as the junction returns to thermal equilibrium.
The charge transient is measured at two times (t
1
and t
2
from the beginning of discharge) and
the charge ΔQflowed through the circuit during the period t
1
→t
2
is measured as a function of
rate window
τ
=−tt tt()ln()
21 21
. For electrons, the gate timed charge difference is [22,23]:
2
New J. Phys. 16 (2014) 083040 C Ton-That et al
Δ=−= −−−
[]
Q Qt Qt Q et et( ) ( ) exp ( ) exp ( ) , (1)
on n21 2 1
where Qo is the total charge trapped during the filling pulse. The thermal emission rate e
n
,
according to Maxwell–Boltzmann statistics, can be expressed as [21,22]:
σΓ=−
⎜⎟
⎛
⎝
⎞
⎠
e
TE
kT
exp , (2)
nn
a
2
where σis the capture cross section, E
a
the activation energy, and Γ
n
is a constant associated
with the electron effective mass. The equations for holes are analogous. A common
experimental approach is to keep the ratio α=
t
t
21 constant; this leads to the function
Δ
τQ()
being maximum when the rate window is equal to the emission rate of the trap, i.e.
τ
==
α
α
−
−
e
m
ln
tn
1
(1)
1
. The activation energy of a trap and its cross section can therefore be
obtained from an Arrhenius plot of equation [2], whereas the trap density can be calculated from
the maximum
Δ
Q
max
[24].
2. Experimental details
Identical samples from a single a-plane ZnO wafer (grown hydrothermally by the MTI, USA,
0.5 mm thick) were used in this study. The crystal was polished both sides to 1 nm surface
roughness. The samples of 5 mm × 5 mm square sections were cleaned in acetone and ethanol
then rinsed in deionised water. One sample was exposed to radio-frequency plasma in hydrogen
atmosphere (power 15 mW for 1 min with the sample kept at 200 °C); this doping method leads
to a near-surface hydrogen rich region as described in our previous work [25,26]. No changes
to the crystal structure were detected by Raman spectroscopy or x-ray diffraction. The Hall
effect characterization revealed that the as-grown and H-doped crystals have a carrier
concentration of 4.4 × 10
15
to 7.1 × 10
17
cm
−3
, respectively. The as-grown sample was also
plasma cleaned by mild oxygen plasma to improve the reliability of electrical contacts according
to the procedure as described by other workers [27]. A gold Schottky contact pad with a thickness
of 150 nm was deposited by evaporation on one face of the crystal through a shadow mask, while
In ohmic contact was established on the opposite face. Gold was used for the Schottky contact to
prevent surface oxidation and to facilitate wire bonding. Current–voltage and isothermal Q-DLTS
measurements were conducted in vacuum using the ASMEC-02 system, supplied by InOmTech
CL was performed using a FEI Quanta 200 SEM equipped with a liquid helium cold stage. The
light emission was collected by a parabolic mirror and detected by a high-resolution Hamamatsu
S7011-1007 CCD image sensor. PL measurements were performed in a liquid helium bath
cryostat at 7 K. The samples were excited by the 325 nm emission line of a HeCd laser and the
emitted light was dispersed by a Spex-1404 double monochromator (spectral resolution 50 μeV)
and detected by a bi-alkali photodetector.
3. Results and discussion
3.1. Shallow carrier traps investigated by Q-DLTS
A typical I–Vcharacteristic of the ZnO/Au junction, showing a rectifying behaviour, is
presented in figure 1. The forward bias was obtained with a positive bias applied to the Au
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New J. Phys. 16 (2014) 083040 C Ton-That et al
electrode. Similar rectifying junctions on c-plane ZnO using Au electrodes have been reported
previously [28]. The bulk resistivity of the crystal is 15.6 Ωcm at room temperature. The sign of
rectifying voltage (positive on the Au electrode) indicates the presence of n-type band bending
with a depletion layer for electrons at the ZnO/Au interface. The relatively large reverse current,
characteristic of Au Schottky contacts to ZnO, has been attributed to tunnelling
conduction [14].
Figure 2(a) shows representative Q-DLTS traces from the as-grown crystal at temperatures
ranging from 250 to 290 K. The Q-DLTS signal of the as-grown ZnO became negligible at
temperatures below 250 K. As expected, the peak shifts gradually from a longer emission rate to
a shorter emission rate with increasing temperature. The amount of charge emitted from the as-
grown ZnO crystal increases with temperature, which is attributable to the addition of charge
being trapped as the free carrier concentration increases with rising temperature. Figure 2(b)
shows an Arrhenius plot derived from the data. Analysis of the Arrhenius plot using the
effective mass m
e
* = 0.24 m
e
[1] reveals a trap position of 23 ± 2 meV and a capture cross
section, σ= 2.6 × 10
−17
cm
2
.
Selective Q-DLTS spectra recorded on the H-doped crystal at temperatures between 130 K
and 290 K are presented in figure 3(a), with the corresponding Arrhenius plot shown in
figure 3(b). The Arrhenius plot shows the presence of two linear regions with different slopes,
with a distinct break occurring at ∼220 K (approximately the same temperature at which the Q-
DLTS signal in the as-grown crystal completely disappears). Two defect states can be
determined from the slopes with activation energies of 22 ± 4 meV and 11 ± 2 meV and capture
cross sections of σ= 4.2 × 10
−17
and 3.8 × 10
−17
cm
2
, respectively. To index Q-DLTS peaks, the
subscript of Eis used to indicate the energy level of a trap in meV. The difference in the
activation energy of the H-doped E
22
and as-grown E
23
traps are within the experimental error
of the Q-DLTS measurements. These two states, having similar activation energies and thermal
emission rates, can be assigned to the same trap. We attribute the additional, lower energy state
Figure 1. I–Vcharacteristics of Au Schottky contacts deposited on the H-doped ZnO
crystal, measured at 300 K.
4
New J. Phys. 16 (2014) 083040 C Ton-That et al
at 11 meV in the H-doped ZnO to a sub-surface state induced by the hydrogen plasma. While
Q-DLTS itself cannot determine whether carriers in this trap are thermally activated to the
valence band or conduction band, the coincidence of the H-related shallow state formation and
the increase in electron density as revealed by the Hall effect measurement (from 4.4 × 10
15
to
7.1 × 10
17
cm
−3
upon hydrogen doping) confirms that a new donor state is responsible for the
increased n-type conductivity. It was not possible to resolve the Q-DLTS peak of the H-doped
crystal into components due to the small energy difference between the traps and inherent peak
widths of the Q-DLTS spectra. Consequently, their trap energies could only be analyzed using
Figure 2. (a) Q-DLTS spectra obtained from the as-grown ZnO in the temperature range
250–300 K using charging voltage ΔV= 1 V and charging time t
C
= 1 s. (b) Arrhenius
plot derived from the data in (a) indicating a state with an average activation energy of
23 ± 2 meV.
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New J. Phys. 16 (2014) 083040 C Ton-That et al
the Arrhenius plot. It is expected that the Q-DLTS signal in the H-doped ZnO at temperatures
below 220 K arises predominantly from the 11 meV trap since the charge released from the E
22
trap is negligibly small over this temperature range (figure 2(a)).
Figure 3 (a) Q-DLTS spectra obtained from H-doped ZnO using charging voltage
ΔV= 1 V and charging time t
C
= 1 s. The spectra shown were recorded within the
temperature range 130–290 K. (b) Arrhenius plot derived from the data in (a) indicating
two states with energies of 22 ± 4 meV and 11 ± 2 meV. Hydrogen doping generates a
new defect state, which becomes activated at ∼150 K.
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New J. Phys. 16 (2014) 083040 C Ton-That et al
Figure 4. Evolution of CL spectra in the NBE region with temperature for (a) as-grown
ZnO and (b) H-doped ZnO crystals (e-beam parameters E
B
=15keV, I
B
= 5.7 nA). The
LO-phonon replicas of FX and BX transitions are indicated by arrows. With increasing
temperature the BX transition turns into the FX transition. (c) Integrated intensities of the
NBE emission plotted as a function of temperature for the as-grown and H-doped ZnO.
Solid curves are fits to the data providing the activation energies for the luminescence
quenching, which are distinctly dependent on sample type and temperature range.
7
New J. Phys. 16 (2014) 083040 C Ton-That et al
3.2. Luminescence properties
Figures 4(a) and (b) display representative CL spectra of the as-grown and H-doped ZnO
crystals at various temperatures from 10 K to 300 K, together with the assignments for the
spectral features. These spectra, collected with the same spectral resolution and the SEM
operated at 15 kV corresponding to an excitation depth of 480 nm [25], display similar features
in the NBE region. Notable spectral features are a bound exciton (BX) emission peak at 3.35 eV
at 10 K and the phonon replicas of free exciton (FX) emission. The absence of the zero phonon
FX emission in the NBE spectra is due to preferential binding of excitons to defects and
impurities in ZnO. The strong sub-band gap FX-nlongitudinal optical (LO) signals in the CL
spectra also suggest efficient self-absorption of FX photons in the crystal [25]. The phonon
replica peaks (FX-1LO and FX-2LO) are separated by the LO phonon energy E
LO
=72eV[29].
This energy separation is insensitive to temperature over the measured temperature range but
the well-resolved FX phonon replicas at low temperatures become thermally broadened with
increasing temperature. The asymmetric shape of the NBE spectra with a long low-energy tail is
due to the contribution from the higher-order phonon replicas. Comparison of the CL spectra of
the as-grown and H-doped ZnO at 10 K reveals an increase in the BX intensity by
approximately 50% after hydrogen incorporation, while the FX remains virtually unchanged for
the CL spectra recorded at low temperatures (< 60 K), indicating the formation of additional
recombination channels via neutral-donor-bound excitons (D°X).
The general trend displayed in figures 4(a) and (b) is that the rapid thermal quenching of
BX occurs with increasing temperature up to 150 K due to the delocalization of BXs, while the
FX replica intensities are largely unaffected. With a further increase in temperature, the BX
emission is not detectable while the FX-1LO becomes the strongest feature in the spectral
region. It is interesting to observe that the phonon-assisted FX recombination dominates the
NBE emission in both the as-grown and H-doped ZnO at temperatures above 150 K. As the
average kinetic energy of FXs increases with rising temperature, the thermal redistribution leads
to a smaller number of FXs near the center of the Brillouin zone, thus enhancing the
participation of LO phonons in the radiative recombination. The strong LO-phonon-exciton
coupling is consistent with the PL spectral evolution of ZnO with increasing temperatures
[30,31], with the coupling strength found to be four times greater than that of GaN [32]. While
the overall features of the CL spectra are similar for the as-grown and H-doped ZnO at
temperatures below 140 K, hydrogen has a striking effect on the FX and its phonon replicas:
their intensities continues to rise in intensity with increasing temperature up to 320 K. This
intriguing phenomenon is clearly seen in figure 4(b), which shows the BX and FX-1LO peaks
exhibiting an opposite dependence on temperature.
The enhancement of the NBE due to H doping is more evident in figure 4(c), which shows
the temperature dependence of the integrated intensity of the NBE emission. For both as-grown
and H-doped ZnO, pronounced decreases of intensity with temperature over the range 10–140 K
is typical of a CL spectrum that is dominated by shallow BXs. The activation energy for the
luminescence quenching, determined from Arrhenius plots, are 11.6 and 6.5 meV for the as-
grown and H-doped ZnO, respectively. The activation energy (11.6 meV) of the as-grown NBE
is within the range of reported localization energies for D°X (10–28 meV) [1] and indicates that
the thermal decay of BXs is the main mechanism for the luminescence quenching. On the other
hand, the activation energy (6.5 eV) for the H-doped ZnO is significant smaller the localization
energies, indicating excited states of excitons are involved [33] (see resolved peaks in the
8
New J. Phys. 16 (2014) 083040 C Ton-That et al
high-resolution PL spectra below). The NBE emission of H-doped ZnO exhibits an anomalous
behaviour, in which its intensity increases with increasing temperature over the range
140–300 K. The delocalization and thermalization of excitons cannot explain this peculiar
temperature-dependent behaviour. At elevated temperatures (>300 K), the CL quenching of H-
doped ZnO was observed with an apparent activation energy of 18.7 meV, significantly smaller
than the FX binding energy (60 meV). This is not unexpected given the fact that the de-trapping
of carriers from traps occurs over this temperature range (as demonstrated by Q-DLTS) and
carriers might take part in thermally activated non-radiative recombination processes.
To gain further insight into the enhanced optical and electrical effects in the H-doped ZnO,
high-resolution PL was conducted. Figure 5(a) shows low temperature (7 K) PL spectra of the
as-grown and H-doped ZnO. Within the region of 3.14–3.34 eV, the notable spectral change
after hydrogen plasma is the considerable enhancement of the peak at 3.336 eV, attributed to Y
line [34,35]. The Yline could not be resolved in the CL spectra, possibly due to the fact that the
analysis depth (the CL generation depth is 490 nm at 15 kV) is significantly greater than the
depth profile of surface extended defects induced by hydrogen plasma. The phonon replicas and
two-electron satellite (TES) transition of the BX and FX are also observed in this spectral range.
In an enlargement of the BX region (inset of figure 5(a)), the most intense lines in the as-grown
ZnO are I
4
and I
6
, which have been attributed to H and Al shallow donors [1]. Neutral-donor-
BX transition I
9
at 3.357 eV, which was recently verified by time-of-flight PL [36], appears as a
shoulder with intensity about an order of magnitude weaker than those of I
4
and I
6
. The
presence of these impurities in the as-grown crystal is unsurprising since they are commonly
present during the hydrothermal growth. For H-doped ZnO, the dominant peak at 3.362 eV
(labelled I
4a
) has previously attributed to an excited state of the H-related neutral donor BX I
4
[33,37]. Analysis of the BX spectral region reveals that I
4a
intensity increases in comparison
Figure 5. (a) High resolution PL spectra taken at 7 K for the as-grown and H-doped
ZnO crystals, showing the Yline and phonon replicas of FX and D
o
X. Several Ilines in
the D
o
X region after normalization is displayed in the inset. The enhancement of Ilines,
especially the excited states I
4a
, results in the broadening of the D
o
X peak. (b) PL
spectra of the Yline and TES I
4
emissions for the H-doped ZnO as a function of
temperature between 7 and 75 K.
9
New J. Phys. 16 (2014) 083040 C Ton-That et al
with that of I
4
after hydrogen incorporation suggests that hydrogen dopants introduced by the
plasma is in a different chemical state to those incorporated during the hydrothermal growth.
Figure 5(b) displays the temperature-resolved PL spectra of the Yline and TES I
4
emission in
the range of 7–75 K. The Yline emission cannot be resolved at temperatures above 75 K due to
its overlap with the dominant D
o
X peak. Analysis of the thermalization behaviour of the Yline
emission yields a thermal activation energy of 11 meV. Although the absolute energy of the Y
line is slightly shifted compared with those in Cermet and EaglePicher ZnO samples [35],
possibly due to strain, this energy is in excellent agreement with previously reported activation
values of 10–12 meV for the Yline [34,35]. The Yline recombination has been found in ion
implanted ZnO crystals and attributed to extended structural defects [35]; our results here
indicate that similar structural defects could be introduced in ZnO by hydrogen plasma.
4. Discussion
The dominant electron trap E
11
in the H-doped ZnO could be associated with either hydrogen
dopants or Yline structural defects induced by the plasma process. However, the shallow E
11
trap is unlikely to be a H-induced donor state, which has a much larger ionization energy
(E
i
(H) = 47–53 meV [38,39]). The nature of the shallow E
11
strap is currently undetermined in
the literature, with its activation energy being significantly smaller than that of the shallowest
traps detected previously by C-DLTS in hydrothermal ZnO (55 meV) [40] or in ZnO thin films
deposited by pulsed laser deposition (31 meV) [41]. The trap depth of E
11
is consistent with the
thermal activation energy of the Yline (see section 3.2 above). The Yline luminescence exhibits
an unusual behaviour with increasing temperature—despite possessing a large localization
energy of 38 meV, the thermal activation energy of the Yline is only 11 meV. Such thermal
behaviour of the Yline can be well explained if its thermal activation energy corresponds to the
detachment of an electron from the E
11
shallow trap, rather than the detachment of an entire
exciton, as suggested by Wagner et al [35]. Furthermore, the Q-DLTS peak intensity of the E
11
trap does not saturate with increasing pulse-width in the range of 0.1–5 ms, which is highly
characteristic of traps associated with extended defects that can trap multiple charges [42]. This
result further supports the contention that the E
11
trap is associated with the Yline structural
defects.
In Q-DLTS, the maximum depth of the detectable trap volume is the width of the depletion
layer [19]. From the standard calculation of the depletion layer at the applied bias, we can
estimate that the E
11
trap arises from depths up to 60 nm. Based on the above discussion, the E
11
trap is therefore be attributable to Yline structural defects in the near-surface region that are
induced by the plasma process. Our previous depth-resolved CL microanalysis showed the
hydrogen plasma causes the greatest changes to the defect structure in the top 1.5 μm layer of
the crystal [25]. A possible model that embodies the observed temperature-dependent
luminescence and defect properties could be based on the release of electrons from Yline
structural defects at temperatures above 140 K, leading to the generation of additional excitons.
At temperatures below 140 K, the Yline defects act like electrically active shallow traps with an
electron binding energy of 11 meV. Furthermore, the fact that the activation energy of extended
defects is significantly smaller than the localization energy of the Yline emission suggests that Y
line excitons do not bind to the trap as a whole quasiparticle, but rather as a separate electron
and hole bound by weak Coulombic interaction.
10
New J. Phys. 16 (2014) 083040 C Ton-That et al
The origin of the 23 meV electron trap in the as-grown ZnO is unknown at present but
hydrogen and aluminium are prime candidates because their emission lines are most clearly
detected by PL at low temperatures. A shallow donor level at 20 meV in hydrothermally grown
ZnO, close to the E
23
trap, has been previously reported by other workers using thermal
admittance spectroscopy [40], but this trap state was not detectable by C-DLTS, probably due to
carrier freeze-out at low temperatures. The slight increase in the cross section of the E
23
trap
after the hydrogen plasma might be due to reduction in the number of competitive traps as the
ZnO surface is passivated by hydrogen.
5. Summary
In summary, Q-DLTS has been applied successfully to ZnO crystals to reveal shallow traps at
11 and 23 meV below the conduction band edge, which have not been previously detected by
C-DLTS. Hydrogen doping by plasma causes significant enhancement of the Yline emission at
3.336 eV and the anomalous behaviour of the excitonic emission in H-doped ZnO over the
temperature range 140–300 K. Based on the presented experimental results, the unusual
temperature-dependent luminescence of H-doped ZnO is explained by a model in which
generation of additional excitons occurs as a result of electron release from the 11 meV trap.
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