Electrically pumped waveguide lasing from
ZnO nanowires
Sheng Chu
1
†
, Guoping Wang
1
†
, Weihang Zhou
2
, Yuqing Lin
3
, Leonid Cherny ak
3
, Jianze Zhao
1,4
,
Jieying Kong
1
,LinLi
1
,JingjianRen
1
and Jianlin Liu
1
*
Ultraviolet semiconductor lasers are widely used for applications
in photonics, information storage, biology and medical thera-
peutics. Although the performance of gallium nitride ultraviolet
lasers has improved significantly over the past decade, demand
for lower costs, higher powers and shorter wavelengths has
motivated interest in zinc oxide (ZnO), which has a wide direct
bandgap and a large exciton binding energy
1–6
.ZnO-based
random lasing has been demonstrated with both optical and
electrical pumping
7–10
, but random lasers suffer from reduced
output powers, unstable emission spectra and beam divergence.
Here, we demonstrate electrically pumped Fabry–Perot type
waveguide lasing from laser diodes that consist of Sb-doped
p-type ZnO nanowires and n-type ZnO thin films. The diodes
exhibit highly stable lasing at room temperature, and can be
modelled with finite-difference time-domain methods.
Single-crystalline semiconductor nanowires have long been
considered an excellent means by which to realize small and cost-
effective Fabry–Perot (FP) type lasers, because of the optical
feedbacks provided by the naturally formed flat facets in the ends
of nanowires. Although optically pumped nanowire lasers have
been reported widely
1–5
, only a single cadmium sulphide (CdS)
nanowire/silicon heterojunction laser has been demonstrated in an
electrically driven configuration
11
, and there is still a lack of high-
efficiency homojunction lasers. When ZnO material is considered,
this lack of efficient laser devices arises mainly because of the
difficulty in achieving controllable p-type doping
12–14
. As the
development of reliable p-type doping of ZnO progresses, more
nanowire-based optoelectronic device will certainly emerge; for
example, p-n homojunction nanowire light-emitting diodes
(LEDs)
14
and photodiodes
15
have recently been realized. In this
Letter, we report a homojunction diode that consists of p-type
Sb-doped ZnO nanowires on a high-quality n-type ZnO film.
Evident FP-type UV lasing was demonstrated, and the gain/feedback
mechanisms and laser emission profile were studied in detail.
Figure 1 presents a schematic (Fig. 1a) and a photograph (Fig. 1b)
of the device. Growth of the p-type ZnO nanowire/n-type ZnO film
diode structure was carried out by means of a seed-assisted growth
scheme
16,17
. A 1,050-nm-thick high-quality n-type ZnO seed film
was grown on a c-plane sapphire substrate by plasma-assisted
molecular beam epitaxy (MBE) (Supplementary Figs S1 and S2).
Sb-doped p-type nanowires were then grown on top of the film by
chemical vapour deposition (CVD). The c-axis of the ZnO nanowires
perfectly follows the growth direction of the underlying film,
resulting in a highly oriented vertical nanowire array (Fig. 1c,
Supplementary Fig. S3). The length and diameter of the nanowires
are, on average, 3.2 mm and 200 nm, respectively. Subsequent
device fabrication details are given in the Methods.
The major merit of this device structure lies in the integration
of the advantages of MBE and CVD. MBE results in the growth
of high-quality thin films, but is not practical for the growth of
nanowires because of its relatively low growth rate. CVD, on the
other hand, results in the synthesis of high-quality nanowires with
a fast growth speed, but cannot satisfactorily achieve the controllable
growth of multi-segment nanowires with different conductivity
types. Therefore the approach of growing p-type nanowires using
CVD on high-quality n-type films grown by MBE can solve this
dilemma and produce controllable p-n homojunctions. The single-
crystalline nature of ZnO nanowires was confirmed by high-
resolution transmission electron microscopy (TEM) imaging
analysis (Supplementary Fig. S4). The incorporation of Sb dopant
was demonstrated by X-ray photoelectron spectroscopy (XPS)
(Fig. 1d), which showed a clear Sb 3d
3/2
peak at 539.5 eV. This
peak position suggests that the Sb atoms substitute Zn atoms
(Sb
Zn
)
18
. Sb distribution along the nanowires was also proved by
Auger electron spectroscopy (AES), as shown in Supplementary
Figs S5 and S6.
Lasing of the nanowires was first demonstrated using optical
pumping. Figure 1e shows the lasing spectra at different pumping
powers. Equidistant peaks with a separation of 2.4 nm can be
observed (solid arrows). At higher pumping powers, additional
modes (indicated by dashed arrow) also begin to emerge as a
result of the excitation of adjacent nanowires with slightly different
lengths. The threshold power (P
th
) was found to be 180 kW cm
22
from a plot of the intensity as a function of pumping power (Fig. 1e,
inset). The density of electron–hole pairs (n
p
) produced by the
optical pumping can be calculated as n
p
¼ I
exc
t
/h
v
l (ref. 19),
where I
exc
is the excitation power,
t
is the spontaneous emission
lifetime (
t
varies
20
and is assumed to be 300 ps, ref. 21) and l is
the diffusion length (2 mm, ref. 22, from top excitation). The
analysis gives an n
p
of 5.1 × 10
17
cm
23
at threshold.
In Fig. 2a, the current–voltage (I–V) characteristics show a rec-
tifying diode behaviour. However, the large reverse current is
related to the formation of an indium tin oxide (ITO)/Sb-doped
ZnO nanowire metal–semiconductor junction, which is in series
with the ZnO p-n homojunction. The formation of the ZnO homo-
junction between the nanowires and film was investigated by
electron-beam-induced current (EBIC) profiling
23,24
. To facilitate
the EBIC experiment, silver paste was used to contact the top
ends of the nanowires. Figure 2b shows the EBIC profile superim-
posed on a cross-sectional scanning electron microscopy (SEM)
image. The EBIC signal forms a peak across the nanowire/film junc-
tion due to the drift of electron-beam generated electrons and holes
under the influence of the built-in electric field, indicating the
formation of a p-n junction. The second peak on the right side is
1
Quantum Structures Laboratory, Department of Electri cal Engineering, University of California at Riverside, Riverside, CA 92521 U SA ,
2
Laboratory
of Advanced Materials, Departmen t of Physics, Fudan University, Shanghai, 200433 C hina,
3
Departm ent of Physics, University of Central Florida,
Orlando, FL 32816 USA,
4
School of Physics and O ptoelectronic Engineering, Dalian Universi ty of Technology, Dalian, 116024 China;
†
These authors
contributed equally to this work.
*
e-mail: jianlin@ee.ucr.edu
LETTERS
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related to the additional non-equilibrium electron collection from
the ZnO/ITO/silver paste contacts. The p-type conductivity of
the ZnO nanowires can be demonstrated by field-effect measure-
ment (Supplementary Figs S7 and S8). A hole concentration of
4.5 × 10
17
to 2.5 × 10
18
cm
23
is extracted, and the electrical proper-
ties are comparable to those of p-type ZnO nanogenerators
12
.
Moreover, these carriers seem to be degenerate
25
(Supplementary
Fig. S9), and can potentially give rise to efficient hole injection for
lasing. In addition, low-temperature photoluminescence studies
reveal acceptor-associated emission features in the Sb-doped ZnO
nanowires (Supplementary Fig. S10).
Electroluminescence characterizations were performed to
demonstrate the lasing action. Fig. 3a presents electroluminescence
spectra under an injection current of between 20 mA and 70 mA.
Under low injection currents (from 20 to 40 mA), only free-
exciton spontaneous emissions centred at 385 nm are observed.
As the pump current reaches a threshold of 50 mA, drastic
sharp emissions with line-widths as narrow as 0.5 nm emerge
from the single broad emission around 385 nm, which indicates
that the gain is now large enough to enable the cavity mode to
start lasing. A further increase in the injection current stimulates
the onset of lasing in additional nanowires with slightly different
lengths, resulting in an increase in the number of lasing peaks. A
stable, quasi-equidistant pattern of lasing peaks can be extracted
with reasonable wavelength deviation. The average spacing
between modes (D
l
) is 2.52 nm for the selected peaks denoted by
arrows in Fig. 3a, which is close to the spacing in optical
pumping obtained from Fig. 1e (2.4 nm). Spacing D
l
for a FP
cavity is given by the expression D
l
=
l
2
2L(n −
l
(dn/d
l
))
[]
−1
(ref. 11), where n ¼ 2.5 is the refractive index of ZnO and
dn/d
l
¼ –0.015 nm
21
denotes the dispersion relation for the
refractive index. For a 4.2 mm cavity between the top end of
the ZnO nanowire and the bottom ZnO film/sapphire interface
(determined by SEM imaging), D
l
is calculated to be 2.95 nm,
which is in close agreement with the observed experimental
values. Another device with a longer cavity length of 10 mmwas
fabricated, and the line spacing was also found to be consistent
with the value calculated from the above formula (Supplementary
Figs S11 and S12).
In addition to the spectral features, the far-field microscope
images in Fig. 3b show direct evidence of nanowire FP-type
lasing. A side view of a ZnO nanowire array can be seen in the
centre of the first image. The illumination lamp was switched off
and emission light was recorded for the biased device with excitation
b
d
530 535 540
Binding energy (eV)
Sb 3d
3/2
O 1s
Count (a.u.)
Wavelength (nm)
Intensity (a.u.)
Intensity (a.u.)
370 380 390 400 410 420
0 200
~403−
46
kW cm
–2
Power (kW cm
–2
)
400
P
th
e
c
ITO glass
a
Au/Ti contact
Substrate
n-type film
p-type nanowires
Figure 1 | Structure and material properties of the ZnO nanowire/film
laser device. a, Schematic of the laser device, which consists of an n-type
ZnO thin film on a c-sapphire substrate, p-type vertically aligned ZnO
nanowir es, ITO contact and Au/T i contact. b, Photo-image of the device.
c, Side-view SEM image of the device structur e showing the ZnO thin film
and nanowires. Scale bar , 1
m
m. d, XPS spectrum of the Sb-doped ZnO
nanowir es arra y. e, Room-temper ature optically pumped lasing spectra fr om
46 kW cm
22
to 403 kW cm
22
with aver ag e steps of 20 kW cm
22
. Solid
arro ws denote equidistant lasing peaks, and a spacing of 2.4 nm is extracted.
Inset: integrated spectra intensity as a function of pumping power density.
Solid lines represent threshold P
th
(180 kW cm
22
).
−20 −10 0
Voltage (V)
10 20
−5
0
5
10
Current (mA)
15
20
a
b
3.5
3.0
2.5
2.0
0123456
Depth (μm)
EBIC signal (nA)
n-film
p-nanowire
Figure 2 | I–V properties and evidence of the formation of a ZnO
nanowire/film p-n junction. a, I–V characteristic of the IT O/ZnO
nanowir e/ZnO film laser device. Positive bias is applied on the ITO side.
b, EBIC profile superimposed on the side-view SEM image of the
cleave d device.
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currents from 30 mA to 70 mA. Under excitation with 30 mA, dis-
tinct light emission can be observed, which forms a stripe close
to the bottom of the nanowires/thin film interface, indicating
that electrically pumped light emission starts near the p-n junction
active region rather than at the ITO/ZnO nanowire interface.
With an increase in the injection current, light emerges from
the top ends of the nanowires. As the pumping current
increases to 60 or 70 mA, this behaviour becomes very striking,
with bright light spot pairs at the two ends of the nanowires. This
phenomenon comprises strong proof of longitudinal lasing modes
in a waveguide that has been constantly observed in optically
pumped nanowire lasing
3,11,26
.
The integrated lasing spectrum intensity is plotted against injec-
tion current in Fig. 4a. The dashed line is a guide to the eye, showing
an evident threshold current of 48 mA. The gain/feedback mech-
anism of this nanowire/thin film FP laser is shown in Fig. 4b. It can
be inferred that the gain length is determined by the minority
carrier diffusion lengths in the p-type nanowire (L
n
≈ 2 mm;
ref. 22) and n-type ZnO film (L
p
≈ 200 nm; ref. 27), as well as the
width of the space charge region (,100 nm). The total gain
length is therefore 2.3 mm. In a FP laser, the threshold gain G
th
is given as
26
G
th
=
1
2L
ln
1
R
1
× R
2
where R
1
and R
2
are the reflectivities on the two ends of the cavity
(0.04 for sapphire/ZnO and 0.09 for ZnO/ITO/glass), and L is
the gain length. A calculated value of G
th
gives 1.2 × 10
4
cm
21
for this laser. However, optically pumped lasing (Fig. 1e) needs a
reduced G
th
of 5.6 × 10
3
cm
21
, mainly due to the longer L
(4.2 mm) and larger reflectivity at the ZnO/ITO/air interface
(0.2). An excited carrier density of 5.1 × 10
17
cm
23
was estimated
at threshold for optically pumped lasing in the previous paragraph,
so it is reasonable to assume that higher carrier densities, for
example .1.0 × 10
18
cm
23
, are needed in the electrically pumped
case because of the larger G
th
. These numbers are around or
larger than the calculated Mott density of ZnO (ref. 19, 28) so elec-
tron–hole plasma (EHP) rather than an exciton–exciton interaction
360 390 420
Wavelength (nm)
450
20 mA
a
b
30 mA
40
mA
50
mA
Electroluminescence
intensity (a.u.)
Electroluminescence
intensity (a.u.)
Electroluminescence
intensity (a.u.)
Electroluminescence
intensity (a.u.)
Electroluminescence
intensity (a.u.)
Electroluminescence
intensity (a.u.)
60 mA
70
mA
ZnO nanowires
No current
Figure 3 | Laser emission charac terizations. a, Electroluminescence spectra of the laser device operated between 20 mA and 70 mA. Above 50 mA, lasing
characteristics are clearly observed. Arrows in the 70 mA spectrum represent quasi-equidistant peaks. b, Side-view optical microscope images of the lasing
device, corresponding to the electroluminescence spectra in a. The first image was taken with lamp illumination and without current injection.
LETTERS
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may dominate the lasing process. Using n
p
¼ I
th
t
/eV
gain
(ref. 19),
where I
th
is the threshold current and V
gain
¼ L × S (S, nanowire
cross-sectional area) is the volume of the gain region, threshold
current in each lasing nanowire, I
th
, is determined to be .39 mA.
This is comparable with the threshold current of 200 mA measured
in a CdS nanowire laser
11
. This situation can be readily achieved due
to the fact that the initial current crowding effect will allow a handful
of nanowires among those tightly connected with ITO/glass con-
tacts to meet the threshold gain and lase, with a further increase
in injection current causing more nanowires to lase. The laser
diode is quite stable, and can produce FP-stimulated lasing with
only slightly degraded output power even six and seven months
after the first test (Supplementary Figs S13, S14 and S15).
Under injection currents corresponding to the red dots in
Fig. 4, lasing images were taken from the nanowire length direction
(Fig. 4, inset). Blue-purple light becomes significantly brighter as
the pumping increases above threshold. The evolution of the spatial
distribution of the emissions was studied by finite-difference
time-domain (FDTD) simulations (see Methods). The simulation/
measurement environment is schematically illustrated in Fig. 5a.
The simulated spatial distribution of the emission is presented in
Fig. 5b, and the far-field emission intensity as a function of angle
with respect to the nanowire length direction in Fig. 5c. The results
suggest that the nanowire laser device emits intense light close to
the nanowire length direction, and the light spreads in a concentrated
conic shape with angular oscillation. The simulated pattern (orange
line) closely follows the results of previous nanowire laser
studies
3,26,29,30
. The experimental far-field emission values (blue
squares) in Fig. 5c are in close agreement with the simulated data
of the far-field pattern, further proving the waveguide mode emission.
In conclusion, we have achieved the fabrication of electrically
pumped ZnO nanowire diode lasers using p-type Sb-doped ZnO
nanowires and n-type ZnO film. FP-type UV lasing was demonstrated
at room temperature with good stability. Work on ZnO UV lasing may
facilitate many potential applications. Future work is needed to further
optimize laser performance. For example, the top contact with the
p-type nanowire might be engineered to offer both good optical trans-
parency and low electrical resistivity. Also, heterojunction nanowire
diode structures may be used to achieve stronger power output.
Methods
ZnO thin film growth. The n-type ZnO film was grown on a 2-inch c-plane
sapphire substrate using plasma-assisted MBE. Growth began with a 1 min
growth of MgO to improve subsequent ZnO film quality, followed by a regular ZnO
buffer layer growth at 550 8C for 8 min. The main ZnO film was then grown at
010203040506070
Injection current (mA)
Integrated intensity (a.u.)
ITO/glass
a
b
Sapphire
80 90 100 110
Gain region
L
n
L
p
Figure 4 | Lasing threshold gain/feedback properties. a, Integrated spectrum intensity as a function of injection current. Dashed line is a guide to the eye.
Inset: camer a images corresponding to the emission pattern along the nanowir e length direction at each injection current. b, Gain feedback diagram of the
ZnO nanowire/thin-film laser ca vity. The laser gain area (r ed) is defined by the diffusion length L
n
and L
p
.
200 nm
θ
4.2 μm
a
b
2
1
0
−1
−2
−3
−4
−5
−60 −40 −20 0 20
θ (deg)
40 60
0.2
0.4
Normalized intensity (a.u.)
0.6
0.8
1.0
c
Simulation
Experiment
Figure 5 | Far -field pattern of light emission. a, Schematic of the FDTD
simulation/measurement environment (area, 9 × 10
m
m
2
). b,Simulated
spatial distribution of the light (385 nm) intensity. c, Angle distribution of
the far-field emission patterns (the x-axis represents the emission angle
u
with respect to the nanowire growth direction and the y-axis represents
the normalized emission intensity). Orange curve, simulation results; blue
squares, results from electroluminescence measurements when rotating
the device with respect to the nanowire length direction.
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700 8C for 5 h, yielding a total film thickness of 1,050 nm. The Zn effusion cell
temperature was kept at 360 8C with a beam flux on the order of 1 × 10
27
torr.
Oxygen plasma was generated by a radiofrequency system and the flow rate of
oxygen was 5 s.c.c.m.
ZnO nanowire growth. The c-axis oriented ZnO thin film acted both as a seed layer
for ZnO nanowire growth and also as an n-type component of the p-n junction
light-emitting device. The ZnO/c-sapphire sample was subsequently transferred to a
CVD furnace for vapour–solid growth of Sb-doped ZnO nanowires. The sample was
partially covered during nanowire growth to expose the ZnO film for n-type
contact deposition.
The ZnO nanowires were grown using a quartz tube furnace system (Thermal
Scientific Inc.). Zinc powder (99.999% Sigma Aldrich) in a glass bottle was placed in the
centre of the quartz tube. Sb powder (99.99% Sigma Aldrich) was put into an open glass
boat. The boat was placed 5 cm upstream of the zinc source. The ZnO film samplewas
kept 10 cm away from the zinc source on the downstream side. A flow of 1,000 s.c.c.m.
nitrogen was passed continuously through the furnace. The sources and sample were
then heated to 650 8C at a ramp rate of 30 8Cmin
21
. Once the desired temperature was
reached, 200 s.c.c.m. of a mixture of argon/oxygen (99.5:0.5) was introduced to the
quartz tube for ZnO nanowire growth. Growth was maintained for 15 min.
Device fabrication. For the n-type contact, a Au/Ti (100 nm/10 nm) contact
was deposit ed on the n-type ZnO films (the n-contact area was intentionally
covered during nanowire growth). For the top ITO contact, following ZnO
nanowire formation, polymethyl methacrylate (PMMA) was sp un onto the
sample to separate the bottom ZnO film and subsequent ITO top contact.
The sp in rate was 2,000 r.p.m. for 30 s, and this process was repeated five
times. After drying, the sample was placed in a d.c. m agnetron sputtering
system. The ITO target (99.99%) was acquired from Sigma Aldrich. The
growth was carried ou t at room temperature and th e pressure maintained at
1 × 10
22
torr. The sputtering power and time were 180 W and 10 min,
respectively. The ITO glass slides (15–25 V/sq) for the reliable current
feedthrough we re acquired from Sigma Aldrich. The fabricated device had an
area of 5 × 10 mm
2
.
EBIC measurement. EBIC profile measurements were conducted on the ZnO
nanowire/ZnO film cross-sectional structure by cleaving the sample. Measurement
was carried out in a Philips XL30 SEM under a 30 kV electron-beam accelerating
voltage. EBIC signal line scans were recorded using homemade software. A Stanford
Research System low-noise current amplifier and a Keithley 2000 digital multimeter
were used as the digitizer.
Photo- and electroluminesence measurements. The system consisted of an
Oriel monochromator and a lock-in amplifier with a chopper. A 325 nm
He–Cd laser was used as the excitation source. A photomultiplier tube was used
to detect device light emission, which emanated from the ITO/glass electrode
from the nanowires. The scan step for the photo- and electroluminescence was
0.3 nm. An external HP E3630A d.c. power supply was used to input current
for electroluminescence measurement. To record the far-field lasing pattern, a
Nikon Eclipse L200 microscope was equipped with a Sony DXC 970 charge-
coupled device (CCD) camera. For the optically pumped lasing demonstration,
the system was built by using a UV enhanced objective (×40) and Princeton
Instrument monochromator equipped with a silicon CCD. The laser excitation
source was a Nd:YAG pulse laser with an output wavelength of 355 nm (3 ns pulse).
Scanning AES measurement. A PHI 700 Scanning Auger Nanoprobe system was
used for Sb-dopant profile distribution characterization. A reference sample grown
under the same conditions was used for characterization.
FDTD simulation. FDTD solution 6.5 (Lumerical Inc.) was used for simulation.
The pumping source was a point transverse electric wave (380 nm to 400 nm)
source located between the ZnO nanowire (centre one) and the ZnO film,
which corresponds to the p-n junction active area. The diameter and length of
the nanowire were 200 nm and 3.2 mm, respectively. The distance between
nanowires was 400 nm. The thickness of the ZnO film was 1.05 mm. A frequency-
domain power monitor was used to record the emission profile over the
simulation region.
Received 30 March 2011; accepted 24 May 2011;
published online 3 July 2011
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Acknowledgements
The authors would like to thank K.N. Bozhilov for assistance in TEM imaging, Z.H. Chen
for guidance in optical pumping measurements and D. Paul for AES measurement and
analysis. The work on the ZnO device was in part supported by Army Research Office
Young Investigator Program (grant no. W911NF-08-1-0432) and by the National Science
Foundation (grant no. ECCS-0900978). The work on p-type ZnO was supported by the
Department of Energy (DE-FG02-08ER46520).
Author contributions
S.C., G.W. and J.L. conceived and designed the experiments. S.C., G.W. and J.Z. carried
out the experiments. Y.L. and L.C. performed and analysed the EBIC experiment. W.Z.
performed the lasing measurement by optical pumping. S.C. and J.K. carried out theoretical
simulations. J.R. and L.L. contributed material analysis. S.C., G.W. and J.L. co-wrote
the paper. J.L. supervised the project.
Additional information
The authors declare no competing financial interests. Supplementary information
accompanies this paper at www.nature.com/naturenanotechnology.
Reprints and
permission information is available online at http://www.nature.com/reprints.
Correspondence and requests for materials should be addressed to J.L.
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NATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2011.97
NATURE NANOTECHNOLOGY | VOL 6 | AUGUST 2011 | www.nature.com/naturenanotechnology510
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