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Phys. Status Solidi A 207, No. 1, 67–72 (2010) / DOI 10.1002/pssa.200925393
status
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applications and materials science
Electro-optical and
Editor’s Choice
cathodoluminescence properties of
low temperature grown ZnO nanorods/p-GaN
white light emitting diodes
,1 1 1 1 1 2 2
S. Kishwar* , K. ul Hasan , G. Tzamalis , O. Nur , M. Willander , H. S. Kwack , and D. Le Si Dang
1
Department of Science and Technology, Linköping University, Campus Norrköping, 601 74 Norrköping, Sweden
2
CEA-CNRS Group ‘Nanophysique et Semiconducteurs’, Institut Néel, CNRS and Université Joseph Fourier,
38042 Grenoble, France
Received 28 July 2009, accepted 16 September 2009
Published online 22 October 2009
PACS 61.46.Km, 78.55.Hx, 78.60.Hk, 85.60.Jb
* Corresponding author: e-mail kissu@itn.liu.se, Phone: þ46 11 36 3119, Fax: þ46 11 363270
Vertically aligned ZnO nanorods (NRs) with a diameter in the bands centered at 450 nm and a second broad deep level defect
range of 160–200 nm were grown on p-GaN/sapphire substrates related emission centered at 630 nm and extending from 500 nm
by aqueous chemical growth technique and white light emitting and up to over 700 nm. Moreover, the room temperature PL
diodes (LEDs) are fabricated. The properties of this LED were spectrum of the ZnO NRs/p-GaN reveals an extra peak at the
investigated by parameter analyzer, cathodoluminescnce (CL), green color wavelength centered at 550 nm. Comparison of the
electroluminescence (EL), and photoluminescence (PL). PL, CL, and EL data suggest that the blue and near red emissions
The I–V characteristics of the fabricated ZnO/GaN hetero- in the EL spectra are originating from Mg acceptor levels in
junction revealed rectifying behavior and the LED emits visible the p-GaN and from the deep levels defects present in the
EL when bias is applied. From the CL it was confirmed that both ZnO NRs, respectively. The mixture of high and low energy
the ZnO NRs and the p-GaN are contributing to the observed colors, i.e., blue, green, and red, has led to the white observed
peaks. The observed EL measurements showed two emission luminescence.
ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Zinc oxide (ZnO) is a unique material decreases the carrier injection efficiency. This problem can
with semiconducting and piezoelectric dual properties. It is be resolved by making a lot of nanosized junctions because
turning out to be a very important material due to its wide the carrier injection rate increases considerably for nano-
variety of potential applications in everyday life like junctions and thus increasing the device performance [4].
sunscreens, miniaturized lasers, light sources, sensors, Valuable properties of ZnO can still be utilized by making a
piezoelectric elements for power nanogenerators, transpar- heterostructure of ZnO nanorods (NRs) with p-GaN, using
ent electrodes, etc. [1]. ZnO is gaining much research ZnO NRs as an active layer. ZnO and GaN are analogous
attention due to many advantageous properties like direct materials with many similar properties. Both have wurtzite
band gap of 3.37 eV and large exciton binding energy of crystal structure, a very little lattice mismatch of 1.8%, and
60 meV at room temperature and deep level defects band gaps of 3.37 and 3.4 eV, respectively [5].
emissions that cover the whole visible range. Due to these The hetero-epitaxial combination of n-ZnO thin film/
properties it is regarded as a very promising material for p-GaN light emitting diodes (LEDs) has been reported
many optoelectronic devices [2]. previously [5]. In this work, p-type GaN doped with Mg
Stable and reproducible p-type doping for ZnO is still a having a thickness of 1 mm was grown on sapphire by
problem, that is, hindering the possibility of a ZnO p–n molecular beam epitaxy. N-type ZnO NRs were then grown
homojunction devices [3]. However, the p–n heterojunction using a chemical vapor deposition method to a thickness of
devices show a lower efficiency than homojunction devices 1.0 mm and its electro-optical properties were measured.
because an energy barrier is created at the junction which Their electroluminescence (EL) data in the forward bias
ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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68 S. Kishwar et al.: EL and PL properties of low-T grown ZnO nanorods/p-GaN white light emitting diodes
indicates that the holes are injected from p-GaN into the ZnO 2 Experimental Magnesium doped p-type GaN of
NRs under forward bias. From the EL of the fabricated ZnO thickness 4 mm, grown on sapphire substrates was commer-
NRs/p-type GaN thin films, only emission due to the GaN cially bought from TDI Inc. USA. Due to the wide bandgap
was observed, no emission-related peak due to ZnO NRs was nature of GaN, high work function metal schemes were
observed [5], probably due to the high density of interface proposed for the contact. On one-third part of the sample
states at the heterojunction. (p-type GaN) Ni/Au are deposited to form ohmic contact [21,
In general NRs offer a number of potential advantages 22]. This contact was annealed at 500 8C in air for 1 min. ZnO
over conventional thin films. The ZnO NRs array can be NRs arrays were then grown on the rest of the GaN substrate
grown on p-GaN thin film directly by a low temperature and using a seed layer deposition followed by aqueous chemical
low-cost process for making functional p–n heterojunctions. growth technique described in Ref. [23]. Scanning electron
In addition NRs with a large surface area to volume ratio and microscopy (SEM) images have revealed the general
having small footprint are expected to grow with a better morphology of vertically well-aligned ZnO NR arrays. The
interface quality as the stress/strain due to lattice mismatch mean diameter of the NRs was in the range of 160–200 nm.
can be released more efficiently in the case of NRs when Hexagonal face of the well-aligned ZnO NRs is visible from
compared to thin films. Moreover, vertical NRs are like the SEM micrographs in Fig. 1.
natural wave guiding cavities for making the emitted light to The ZnO NRs/p-GaN samples were used to process
travel to the top of the device minimizing partial leakage and LEDs. An insulating layer was deposited in the gaps between
thus enhancing the light extraction efficiency from the device the NRs in order to isolate the NRs contact from the p-GaN.
surface [6]. Due to this, ZnO NRs have a wide range of For that purpose, Shipley photoresist (S1805) was spin-
options for the growth regarding the choice of the substrate. coated. After that, the tips of the ZnO NRs were exposed by
ZnO nanostructures have been successfully grown on very oxygen reactive ion etching (RIE) technique. Finally, Al and
cheap substrates, e.g., glass, plastic, etc. [7–10]. Hetero- Pt were evaporated on the NR tips through a shadow mask,
junction LEDs of ZnO NRs arrays have been fabricated on resulting in a circular contact layer on the ZnO NRs [24].
p-type GaN thin films previously [6, 11–20]. Figure 2 shows a schematic diagram of the fabricated LED.
In this paper, n-ZnO NRs/p-GaN heterostructure was
achieved using low temperature aqueous chemical growth 3 Results and discussion A rectifying behavior was
for the ZnO NRs. The structure was used to fabricate expected from the I–V characteristic of the n-ZnO/p-GaN
heterojunction LEDs. Cathodoluminescnce (CL) was com- heterostructure [22]. This is in agreement with the exper-
plemented by photoluminescence (PL) and EL to character- iment results obtained and shown in Fig. 3. It also shows a
ize the optical and electro-optical properties. rectifying behavior with threshold voltage of 6 V. This
implies that good ohmic contacts were formed at both metal/
Kishwar Sultana earned her n-ZnO and metal/p-GaN interfaces.
M.Sc. in Physics from ‘‘The Islamia Figure 4 shows CL spectra at room temperature for ZnO
University of Bahawal Pur’’ Paki- NRs grown on GaN with spot size around 0.2 mm in diameter
stan in 1990. She then served as a and at two different areas a and b for various accelerating
Physics lecturer at Hazara Univer-
sity Mansehra NWFP, Pakistan
until 2007. In 2007, she joined
the Department of Science and
Technology, Campus Norrköping,
Linköping University, Sweden. Her
research interest is in the field of nanotechnology, with
special focus on ZnO-based nanostructures for techno-
logic and medical applications.
Magnus Willander holds a full
professorship at the Department of
Science and Technology (ITN)
Campus Norrköping, at Linköping
University, Sweden. His research
focuses on materials and devices,
and he combines experimental and
theoretical research in these areas.
He has published around 850 papers
and seven books.
Figure 1 SEM image of ZnO NRs grown on p-type GaN.
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Editor’s
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Phys. Status Solidi A 207, No. 1 (2010) 69
Figure 2 (online color at: www.pss-a.com) Schematic illustration
of the fabricated ZnO nanorods-based LED.
-4
2.0x10
-4
1.5x10
Current (A)
-4
1.0x10
Figure 4 (online color at: www.pss-a.com) Cathodoluminescnce
5.0x10
-5 (CL) at two areas (a) and (b) for various acceleration voltages; (c)
Emission from GaN only, and (d) emissions from both GaN and ZnO.
0.0 peaks. First PL peak was observed at 360 nm, which is
-15 -10 -5 0 5 10 15
attributed to the recombination of free excitons, i.e., band
Voltage (V) edge emission of GaN. The second dominant emission
Figure 3 (online color at: www.pss-a.com) Typical I–V curve of around 446 nm was also observed which is commonly
the ZnO/GaN heterojunction light emitting diode. observed in Mg doped GaN due to dominant band to acceptor
transitions [6]. Figure 6b shows the PL spectrum for ZnO
NRs measured at room temperature. Laser line of wave-
voltages. Area (a) portrays emission from GaN only and area length 266 nm from a diode laser (Coherent Verdi) pumped
(b) describes emissions from both GaN and ZnO, the latter resonant frequency doubling unit (MBD 266) was used as an
being at the surface. From area (a), two dominant CL peaks excitation source. The PL spectrum shows three peaks. First
were observed at 3.36 eV (370 nm) and 2.37 (523 nm) at dominant PL peak was observed at 378 nm, which was
10 kV and the dominant CL peak of GaN corresponds to band
edge emission at 3.36 eV (370 nm).
From area (b), two dominant CL peaks were observed at
3.41 eV (365 nm) with a shoulder at 3.27 eV (380 nm)
and 1.67 eV (630 nm). This result seemed to be a bit
ambiguous due to superposition of CL emission from ZnO
and GaN. For clarification, the surface of the NRs was
scratched to expose full NRs. Figure 5 shows another view
for comparison of CL at scratched (a) and (b) areas with ZnO
NRs clearly exposed and (c) CL spectra of these exposed
NWs areas. Two CL peaks were observed and verified to be
emitted from ZnO NRs. The strong and sharp CL peak at
3.25 eV (380 nm) corresponds to the band gap emission of
ZnO whereas the broad visible emission at about 1.97 eV
(630 nm) is due to local levels of the ZnO energy gap
resulting from defects or impurities like oxygen vacancies or Figure 5 (online color at: www.pss-a.com) Another view for
interstitial zinc vacancies [25]. comparison of cathodoluminescence at scratched areas (a) upper
Figure 6a shows the PL spectrum for GaN epitaxy left (b) lower left with ZnO NWs clearly exposed, and (c) right side
measured at room temperature. The PL spectrum shows two CL spectra of these exposed NWs areas.
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70 S. Kishwar et al.: EL and PL properties of low-T grown ZnO nanorods/p-GaN white light emitting diodes
a) 4.5x10
3
446nm 4 450 nm
1.0x10
4.0x10
3 650nm
3
3.5x10 3
8.0x10
Intensity (arb. unit)
3
44 V
Intensity (arb. units)
3.0x10
3 3
2.5x10 6.0x10
3
2.0x10
3
4.0x10
34 V
3
1.5x10 365 nm
3
1.0x10
3
2.0x10
2
5.0x10
24 V
0.0
0.0
300 350 400 450 500 550
Wavelength (nm)
300 400 500 600 700 800 900
b) wavelength(nm)
378nm
3
6.0x10
Figure 7 (online color at: www.pss-a.com) The electrolumines-
cence (EL) spectrum of the fabricated heterojunction light emitting
Intensity (arb. units)
diode under various forward bias voltages.
3
3.0x10
third broad peak centered around 600 nm is the yellow peak
700nm
of ZnO attributed to oxygen interstitial defects [6]. In another
550nm result the weak UV emission centered at 365 nm corresponds
to the NBE emission of GaN and the stronger peaks at
0.0
470 nm and 526 nm corresponds to the emission of InGaN
quantum wells and oxygen vacancies in ZnO [17]. The
400 600 800 1000
stronger intensity of green emission relative to the UV
Wavelength (nm)
emission suggests the non-stochiometric composition of
Figure 6 (a) Room temperature PL spectrum of a bare p-GaN ZnO [17]. But in our case a dominant PL peak was observed
substrate using Nd:YAG laser (330 nm) with optical parameter at 378 nm and a broad defect band emission centered at
oscillator (OPO) and frequency doublers and (b) PL spectrum for 700 nm in the red spectral range extends to yellow, orange,
ZnO NRs measured at room temperature, using a diode laser and a shoulder in green band 550 nm. Many researchers have
(Coherent Verdi) pumped resonant frequency doubling unit
proposed that the emission near the yellow region is due to
(MBD 266) as an excitation source.
the presence of OH groups which are found mostly in ZnO
rods prepared by using aqueous chemical growth [28, 19]. PL
spectrum shows that there are significant crystal defects in
attributed to the recombination of free excitons, i.e., band the NRs, the most probable cause of which is an excess of
edge emission of ZnO NRs [26]. The second green emission oxygen and hydroxyl species in the structures grown by our
550 nm was also observed which may be due to the aqueous chemical growth technique [28, 19].
recombination of electrons at the conduction band with Under forward bias, the EL spectrum measured at
holes trapped in oxygen-related defects [15]. The third peak different voltages (Fig. 7) showed two emission peaks at
is a broad defect band emission centered in the red spectral different voltages. Although the absolute emission intensity
range with center at 700 nm (1.77 eV). The red emission peak was not measured but the light emission was quite strong
can be attributed to interstitial oxygen defects [27]. The from our sample to the extent that it was possible to observe it
related energy levels are thought to be deep donor and with the naked eye. In our present work, the EL spectrum
acceptor centers with strong electron–phonon interaction. from the ZnO NRs/p-GaN heterojunction LED fabricated in
The transition from the donor to the acceptor center is thus this work was taken out at very low injection currents of
assumed to be responsible for the red emission centered at 0.5–2 mA under the forward bias voltages of 20–45 V. These
700 nm [16]. forward voltages required for EL emission are very high as
The PL results obtained from similar structures showed compared with conventional LEDs. The cause for these high
two emission peaks, one intense emission band centered at voltage values can be attributed to the low doped ZnO NRs
380 nm and the other broad emission band centered at about with high resistivity and unstable metal contacts. In addition,
570 nm previously reported [15]. In other similar PL spectra the formation of point defects acting as carrier trapping and
showed three peaks [6]. The first peak at around 380 nm scattering centers is more favorable in NRs grown by
corresponds to interband transitions in ZnO material. The solution techniques than those gown by other vacuum-based
second peak at around 430 nm corresponds to p-GaN and growth techniques such as MOCVD and thermal CVD
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Phys. Status Solidi A 207, No. 1 (2010) 71
accompanying high growth temperature [15]. EL emission interstitial to zinc vacancies. The second is a broad and
from our sample at such low current values is also an emissions near 650 nm are credited to be effected by
encouraging sign. impurity emissions of the ZnO. Increased temperature of
The EL spectrum consists of two peaks, a blue-violet the device with applied voltage can be responsible for the red
emission peak at 450 nm that comes from two possible shift of the emission related to a shallow Mg acceptor in EL
sources. This first is the transmission from the conduction [31]. CL, PL, and EL result of our LED comprehend each
band to a shallow Mg acceptor in the p-GaN [5]. The other other very well.
possible source of this blue-violet emission line is the
combination of zinc interstitial level to valence band
4 Summary and conclusions In conclusion, we
combined with emission due to transmission from the zinc
have fabricated n-ZnO NRs/p-GaN heterojunction LED on
interstitial to zinc vacancies [29]. The second peak (Fig. 7) is
a sapphire substrate by the aqueous chemical growth
a broad luminescence band centered at 650 nm related
technique. The resulting devices exhibited diode like current
to the orange-red band of ZnO, that is, associated with
voltage behavior and white EL visible to human eye with an
transition from the conduction band of ZnO to oxygen onset at forward bias of 24 V. The white EL is observed from
interstitials combined with recombination from zinc inter- the ZnO NRs grown by aqueous chemical growth on p-GaN
stitials to oxygen interstitials [29]. Defect levels at the ZnO/
with high structural defect density at the n-ZnO NRs/p-GaN
GaN interface influence the recombination radiation spec-
interface. The observed EL is owing to the superposition of
trum which results in the increased spectral width of the
the two bright spectral lines, a blue-violet emission peak at
emission band with center at 650 nm. The intensities of both
450 nm that comes from the transmission from the conduc-
the emission lines are comparable, thus the superposition of
tion band of GaN to shallow Mg acceptor combined with zinc
strong narrow emission line at 450 nm and strong broad
interstitials related transitions and the other is a broad
emission line at 650 nm is responsible for the white light
luminescence band centered at 650 nm related to the
emission from our LED. The ZnO growth conditions actually
orange-red band of ZnO, that is, associated with oxygen deep
are responsible for the state of the defect levels at the ZnO/
level related defects. Thus, we propose a simple and
GaN interface and thus affect the observed radiative inexpensive technique for the growth of ZnO NR-based
spectrum. Previously, good quality ZnO thin films grown white LEDs. However, there are advantages and disadvan-
on GaN showed a dominant blue EL band in the spectrum,
tages of the technical route used to fabricate such LEDs.
while thick and amorphous films demonstrated a set of
Requisition for the use of an expensive GaN substrate is a
several spectral lines [30]. It is important to note that the
main disadvantage which needs further experimentation in
blue emission line at 450 nm is only related to the p-GaN
order to be replaced by other optimized cheap substrate. The
region and independent of the ZnO growth conditions. Thus
advantages include the aqueous chemical growth process for
we can modify the second near yellow emission band by
the growth of well-aligned ZnO NRs and possibility of
changing the ZnO interface conditions. The EL results
technical control over the emission spectrum with the
obtained from similar structures shows two clear emission
possibility to develop white LED technology. We believe
bands, one centered at 415 nm and a broad emission band
that the outcomes of this work are an important step towards
covering the range from 485 to 750 nm at higher injection the fabrication of nanomaterials-based optical devices by
currents of 40 and 50 mA under forward bias [15]. In simple low temperature growth processes applicable on a
another results the EL dominating peak is centered at round
large scale.
about 410 nm and was attributed to acceptor to band
transitions in the p-GaN [6]. In our case the EL spectrum is
dominated by a blue-violet emission peak at 450 nm and the References
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