Deep level defects in a nitrogen-implanted ZnO homogeneous p-n junction
Q. L. Gu,
1
C. C. Ling,
1,a兲
G. Brauer,
2
W. Anwand,
2
W. Skorupa,
2
Y. F. Hsu,
1
A. B. Djurišić,
1
C. Y. Zhu,
1
S. Fung,
1
and L. W. Lu
1
1
Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong,
People’s Republic of China
2
Institut für Ionenstrahlphysik und Materialforschung, Forschungszentrum Dresden-Rossendorf,
Postfach 510119, D-01314 Dresden, Germany
共Received 15 January 2008; accepted 13 May 2008; published online 5 June 2008兲
Nitrogen ions were implanted into undoped melt grown ZnO single crystals. A light-emitting
p-n junction was subsequently formed by postimplantation annealing in air. Deep level transient
spectroscopy was used to investigate deep level defects induced by N
+
implantation and
the effect of air annealing. The N
+
implantation enhanced the electron trap at E
C
−共0.31⫾0.01兲 eV 共E3兲 and introduced another one at E
C
−共0.95⫾0.02兲 eV 共D1兲, which were
removed after annealing at 900 and 750 °C, respectively. Another trap D2 共E
a
=0.17⫾0.01 eV兲 was
formed after the 750 °C annealing and persisted at 1200 °C. © 2008 American Institute of Physics.
关DOI: 10.1063/1.2940204兴
ZnO has recently received attention because of its appli-
cations in optoelectronics and spintronics.
1,2
Undoped ZnO is
of n type. Asymmetry difficulty in p-type doping of ZnO is a
major obstacle for device applications, although progress has
recently been made with N, As, P doping, and codoping with
group III and V elements.
2
Ion implantation is useful for a
selective region doping, but it inevitably introduces undesir-
able defect. This is particularly important for ZnO as the
compensating intrinsic donors have low formation energy
and are energetically stable.
3,4
There have been only a few
reports of p-ZnO induced by ion implantation 关N
+
共Refs.
5–8兲 and As
+
implantations 共Ref. 9兲兴 and the ion-
implantation induced defects were poorly understood. Be-
cause of difficulties in making a junction, deep level tran-
sient spectroscopy 共DLTS兲 studies have also been very
few.
8,10–16
In this study, N
+
ions were implanted into the n-type
melt grown 共MG兲 ZnO single crystal and no rectifying prop-
erty was observed in the as-implanted sample. A homoge-
neous p-n junction which emitted light at room temperature
共RT兲 was formed after the postimplantation annealing. Deep
level defects in the structure and also their thermal evolution
were studied by DLTS.
The starting material was one-side polished undoped
n-type MG ZnO 共0001兲 substrate 共n=5⫻10
16
cm
−3
and
=203 cm
2
V
−1
s
−1
兲 from Cermet Inc. N
+
ions were im-
planted into the polished front side 共F-side兲 at 300 °C with
energy of 150 keV 共fluence of 10
14
cm
−2
兲. This corresponds
to a N-implantation profile peaking at ⬃270 nm 共calculated
by the program TRIM 共Ref. 17兲 and measured by secondary-
ion-mass spectroscopy兲. Ohmic contacts were fabricated by
evaporating 50 nm Al film onto the samples. The postim-
plantation annealing was performed in air for 30 min. C-V,
I-V, and DLTS measurements were made using a HP4155A
semiconductor parameter analyzer and a Sula Technologies
DLTS system, respectively. The DLTS measurements em-
ployed a reverse bias and a filling pulse voltage of V
R
=
−2 V and V
P
=0 V. The activation energy E
a
, capture cross
section
, and concentration N
T
of the traps were calculated
by the Arrhenius plot.
18
To fabricate a light-emitting diode
共LED兲 for electroluminescence 共EL兲 measurement, four-
folded N
+
implantations with energies of 80, 180, 310, and
500 keV 共with fluences of ⬃10
14
cm
−2
兲 were carried out on
the same raw ZnO sample. This resulted in a ⬃ 1
m deep
box-shaped region with N concentration ⬃6⫻ 10
18
cm
−3
.
Then a 750 ° C air annealing was performed. A ⬃50 nm
thick ITO film was evaporated on the implanted side using
an electron beam evaporator while keeping the sample at
250 °C.
19
The EL spectra were collected using a monochro-
mator 共Acton SpectraPro 500i兲 and a Peltier-cooled photo-
multiplier detector 共Hamamatsu R636-10兲.
I-V data in Fig. 1 showed that the as-implanted sample
has no rectifying property. However, rectifying behavior
was observed for samples annealed at 650, 750, 900, and
1200 °C in air, which indicated the formation of p layer in
the N
+
-implanting region. Their leakage currents at −1.5 V
were 1270, 43, 12, and 140 nA, respectively. Referring to the
C-V measurement scheme in Ref. 20 the hole concentrations
of the p-type layers were estimated to be ⬃10
17
cm
−3
. The
thermoelectric probe measurements performed on the an-
nealed N
+
-implanted samples also confirmed the p-layer
formation.
a兲
Author to whom correspondence should be addressed. Electronic mail:
ccling@hku.hk.
FIG. 1. 共Color online兲 I-V data of the N
+
-implanted undoped MG ZnO
samples with different postimplantation annealing temperatures in air. The
I-V measurements were performed across the Al共F兲-Al共B兲 connection.
APPLIED PHYSICS LETTERS 92, 222109 共2008兲
0003-6951/2008/92共22兲/222109/3/$23.00 © 2008 American Institute of Physics92, 222109-1
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The RT EL spectra of the forward biased LED 共Fig. 2兲
shows two broad peaks at ⬃530 nm 共⬃2.34 eV兲 and
⬃740 nm 共⬃1.68 eV兲 . Their peak intensities increase with
forward bias voltage. The observed light emission clearly
demonstrated the formation of a p-n junction in the annealed
N-implanted ZnO samples.
To carry out DLTS study on the nonrectifying as-
implanted and nonimplanted samples, Au rectifying contacts
were fabricated on the F-side of the samples pretreated with
H
2
O
2
.
21
For the as-grown sample DLTS spectrum, a majority
peak 共i.e., e
−
trap兲 with E
a
=0.31 eV,
n
⬃10
−16
cm
−2
, and
N
T
⬃10
15
cm
−3
关usually referred to E3 共Refs. 8 and 10–16兲兴
and another one 共E
a
=0.10 eV,
n
⬃10
−17
cm
−2
, and N
T
⬃10
13
cm
−3
兲 with a much weaker intensity 共less than 20
times of E3兲 were identified. The E3 intensity dropped with
annealing temperature, but persisted at 1200 °C annealing.
The E
a
=0.10 eV level anneals at 900 °C. The DLTS spec-
trum of the as-N
+
-implanted sample is shown in Fig. 3 共the
as-grown sample spectrum also included for reference兲,
which shows that N implantation enhances the E3 intensity
and introduces another e
−
trap at ⬃360 K 共D1兲 having E
a
=0.94 eV and
n
⬃10
−14
cm
−2
.
Figure 3 shows the DLTS spectra of the annealed
N
+
-implanted p-n junctions. Biasing at V
R
=−2 V during the
DLTS measurements corresponded to a depletion of ⬃30 and
100 nm extending to the p and n sides, respectively. It is thus
difficult to determine the nature 共i.e., e
−
trap or h
+
trap兲 of the
signals identified in these spectra. However, the two majority
peaks at ⬃160 and 360 K in these annealed sample spectra
have E
a
and
n
共as shown in Table I兲 agreeing well to the E3
and D1 already identified in the as-grown and the as-
implanted samples, and are thus attributed to E3 and D1. The
E3 concentration decreases with annealing temperature and
reduces by a factor of ⬃100 after the 900 °C annealing.
D1 annealed out at 750 °C. Referring to Fig. 3, another ma-
jority peak 共D2兲 at ⬃120 K with E
a
=0.17 eV and
n
⬃10
−16
cm
−2
was introduced after 750 °C annealing. How-
ever, it is not certain as to whether this is an e
−
or a h
+
trap.
Its concentration drops with annealing temperature, but per-
sists at 1200 °C.
Although DLTS itself cannot offer definitive information
about the defect microstructure, it would still be useful to
explore the possible origins of the identified defects E3, D1,
and D2. E3 is commonly observed in as-grown ZnO single
crystals.
10
However, its exact microstructure is still contro-
versial and has been attributed to V
O
and V
Zn
-related
defects.
10–12,16
In the N
+
-implanted sample 共Fig. 3兲,E3be-
came undetectable upon 1200 °C annealing. However, in
the nonimplanted sample, E3 persisted 共concentration
⬃10
15
cm
−3
兲 upon 1200 °C annealing. Thus, the mechanism
of the E3 annealing process in the N
+
-implanted sample
probably involves another defect created by the N
+
implan-
tation.
DLTS measurements were performed also at the Au
Schottky contacts fabricated on the as-C
+
-implanted and the
as-As
+
-implanted samples 共fluence=10
14
cm
−2
each兲 using
FIG. 3. 共Color online兲 DLTS spectra of the N
+
-implanted undoped MG ZnO
samples annealed at different temperatures. The DLTS spectra of the as-
grown ZnO and the as-C
+
-implanted ZnO samples are included for refer-
ence. The measurements were taken with the rate window, the reverse bias
voltage, the filling pulse width, and the filling pulse voltage equal to ⌬t
=21.5 ms, V
R
=−2 V, t
P
=1 ms, V
P
=0 V.
FIG. 2. 共Color online兲 EL spectra of the N-implanted ZnO homogeneous
p-n junction taken at RT. The IV character of the p-n junction is shown in
the inset at the top right corner. The device schematic diagram and the LED
photo are shown at the top left corner.
TABLE I. Activation energies E
a
, trap concentrations N
T
, and capture cross sections
T
of the deep level defects
introduced by the N implantation.
Unimplanted As implanted 650 ° C 750 ° C 900 ° C 1200 ° C
E3 E
a
=0.31 eV,
N
T
⬃10
15
cm
−3
,
⬃10
−16
cm
2
E
a
=0.31 eV,
N
T
⬃10
16
cm
−3
,
⬃10
−15
cm
2
E
a
=0.32 eV,
N
T
⬃10
16
cm
−3
,
⬃10
−15
cm
2
E
a
=0.31 eV,
N
T
⬃10
15
cm
−3
,
⬃10
15
cm
2
E
a
=0.31 eV,
N
T
⬃10
14
cm
−3
,
⬃10
15
cm
2
Annealed out
D1 ¯ E
a
=0.94 eV,
N
T
⬃10
16
cm
−3
,
⬃10
−14
cm
2
E
a
=0.98 eV,
N
T
⬃10
16
cm
−3
,
⬃10
−14
cm
2
Annealed out ¯¯
D2 ¯¯E
a
=0.18 eV,
N
T
⬃10
15
cm
−3
,
⬃10
−16
cm
2
E
a
=0.17 eV,
N
T
⬃10
16
cm
−3
,
⬃10
−16
cm
2
E
a
=0.17 eV,
N
T
⬃10
16
cm
−3
,
⬃10
−16
cm
2
E
a
=0.17 eV,
N
T
⬃10
15
cm
−3
,
⬃10
16
cm
2
222109-2 Gu et al. Appl. Phys. Lett. 92, 222109 共2008兲
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the same raw materials. D1 is also observed in the
as-C
+
-implanted and the as-As
+
-implanted samples 共the
former shown in Fig. 3兲, hence indicative of being an intrin-
sic defect. D1’s intensity and peak position dependences on
t
p
共10
−6
–0.1 s兲 were also studied while fixing ⌬t =43 ms.
The D1 intensity increases with t
p
and saturates at t
p
=0.01 s. Its peak position is independent of t
p
, which implies
D1 is a point defect. D1 is close to the calculated energy
state 共2+/ 0兲 of V
O
at E
C
–1.0 eV, which possesses negative
U behavior.
22
This theoretical result also allowed a detailed
and consistent interpretation of the optically detected elec-
tron paramagnetic resonance data.
23
We thus tentatively at-
tribute D1 to the 共2+/ 0兲 state of V
O
.
D2 was only formed in the N
+
-implanted samples after
750 °C annealing but not in the nonimplanted sample. The
formation process of D2 in the N
+
-implanted samples thus
probably involves other defects created by the implantation.
D2 has E
a
close to that of a hole trap located at E
V
=0.15–0.16 eV identified in particle irradiated ZnO
materials.
8,15,24,25
However, further effort is needed to clarify
the defect identity.
In conclusion, a ZnO homogeneous p-n junction was
formed by N implantation followed by postimplantation
air annealing. DLTS study showed that the N
+
implanta-
tion enhanced the E3 共E
C
=0.31 eV兲 intensity and induced
the level D1 共E
C
=0.95 eV兲 , which were removed after
900 and 750 °C annealing, respectively. Another trap
D2 共E
a
=0.17 eV兲 was formed after the 750 °C
post-N
+
-implantation annealing and it persisted after the
1200 °C annealing.
This work was supported by the RGC, HKSAR 共7037/
06P and GគHK026/07兲, and the UDG, HKU. The authors
thank Professor K. W. Cheah 共Hong Kong Baptist Univer-
sity兲 for the EL measurements.
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222109-3 Gu et al. Appl. Phys. Lett. 92, 222109 共2008兲
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