SUPPORTING INFORMATION
Electrically pumped near-ultraviolet lasing from ZnO/MgO
core/shell nanowires
C. Y. Liu,1 H. Y. Xu,1,a) J. G. Ma,1 X. H. Li,1 X. T. Zhang,1 Y. C. Liu,1,a) and R. Mu2,a)
1Center
for Advanced Functional Optoelectronic Materials Research, Key Laboratory for UV
Light-Emitting Materials and Technology of the Ministry of Education, Northeast Normal
University, Changchun 130024, China
2Department
of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235-1807, USA
a)Author
to whom correspondence should be addressed. Electronic mail: hyxu@nenu.edu.cn, ycliu@nenu.edu.cn, and
richard.mu@vanderbilt.edu
1
Experimental Section
CS-NW LD fabrication
A ZnO seed layer was first prepared on the pre-cleaned ITO surface through a sol-gel
process. After a calcination treatment in air at 450 oC for 30 mins to obtain a transparent and
crystallized ZnO film, the substrate was then placed into a Teflon-lined autoclave which contains
an aqueous solution of zinc acetate hydrate (25 mM) and hexamethylenediamine (25 mM). The
autoclave was sealed and heated to 95 oC, and the reaction continued for 2.5 h. Then, the sample
was rinsed with deionized water to remove particles and dried in the vacuum oven. A ~70 nmthick MgO thin layer was deposited on the as-grown ZnO NWs by e-beam evaporation of bulk
MgO crystals at room temperature. Au electrodes were thermally evaporated on top of the MgO
layer and patterned into 1 mm-diameter circular pad by a shadow mask to form the designed CSNW LD, as shown in a schematic diagram below.
FIG. S1. A schematic diagram of Au/MgO/ZnO-NWs/ITO heterostructure.
2
Planar device fabrication
ZnO and MgO films were sequentially grown on pre-cleaned ITO glass by pulsed laser
deposition (PLD) technique. A Nd:YAG (yttrium aluminum garnet) pulsed laser (355nm, 5 ns,
10Hz) was employed to ablate the ceramic targets. Before the deposition, the growth chamber
was evacuated to a base pressure below 5×10−5 Pa with a turbo molecular pump. Then, ultrapure
O2 gas was introduced into the chamber, the growth pressure and substrate temperature was set at
20 Pa and 450 oC. The growth durations for ZnO and MgO films are 120 and 60 mins,
respectively. In similar manner as described above, Au electrodes were thermally evaporated and
patterned into 1 mm circular pad as the top electrodes.
Characterizations and measurements
The morphological and structural characterizations of CS-NWs were performed by SEM
(Hitachi S4800) and HRTEM (JEM-2100F). The structural, optical, and electrical properties
were examined with XRD (Rigaku D/max 2500), PL (Jobin-Yvon HR800), and I-V
characteristic (Agilent B1500A), respectively. EL spectra were collected from the top electrode
side using a fluorescent spectrometer (Shimadzu RF-5301pc). A continuous power source was
used to apply current bias. All the measurements were carried out at room temperature.
3
XRD pattern
FIG. S2. (a) XRD pattern of ZnO/MgO CS-NWs.
The X-ray diffraction analysis indicates that the MgO film is well crystallized in cubic phase and
oriented along [200] direction. All the other diffraction peaks can be indexed to wurtzite ZnO.
4
Calculation of Q factor for F-P cavity
The Q factor for an F-P cavity mode can be expressed as the following equation:
Q
2nL
, where R1 and R2 are the reflectivities of the two boundaries, λ is the lasing
 (1  R1R2 )
wavelength, n the reflective index, and L the cavity length.1 For our NW, L is equal to the
distance between the two opposite end facets (~1 μm). In normal incidence condition, the
reflectivity can be determined by R 
(n  1) 2
. For the wavelength of 400 nm, the corresponding
(n  1) 2
reflective index could be obtained from the database: n(MgO) = 1.762, n(ITO) = 2.115, and n(ZnO) =
2.213. So the calculated reflectivities at two end facets are: R(ZnO/ITO) = 0.051%, R(ZnO/MgO) =
1.289%. For our CS-NW LD, if the laser originated from an F-P cavity, the deduced Q factor
would be about 35, which is much smaller than the experimental value. This indicates that the
lasing action is not an F-P resonator mode.
Calculation of Q factor for WGM cavity
As to the WGM laser mode, it has been reported that: only for an individual microcavity,
the lasing spectrum presents a group of distinct modes; when multiple different-sized microrods
are pumped simultaneously, the lasing modes will overlap each other to form a broad emission
band.2 For our CS-NW LD, there are a large amount of different-sized NWs under one electrode.
When they are pumped together, a broad emission band should appear in theory, but distinct
emission spikes are observed in our experiment, which is different from the characteristic of the
WGM laser mode. Furthermore, the Q factor for a hexagonal WGM cavity can be determined by
5
the equation, Q 
3 3DnR 3 / 2
, where D and R are the length of the diagonal (~75 nm) and the
2 (1  R 3 )
reflectivity at the ZnO/air boundary, respectively.3 It has been reported that in ZnO microcavity
the reflectivity is around 85% for WGM or quasi-WGM laser.4 So the deduced Q factor for a
WGM cavity is only about 7, which is far less than the experiment value, thus ruling out the
possibility of the WGM mode.
REFERENCE
[1] P. L. Knight, and A. Miller, Vertical-Cavity Surface Emitting Laser, Springer, Berlin, (1999)
[2] J. Dai, C. X. Xu, R. Ding, K. Zheng, Z. L. Shi, C. G. Lv, and Y. P. Cui, Appl. Phys. Lett. 95,
191117 (2009)
[3] A. K. Bhowmik, Appl. Opt. 39, 3071 (2000)
[4] G. P. Zhu, C. X. Xu, J. Zhu, C. G. Lv, and Y. P. Cui, Appl. Phys. Lett. 94, 051106 (2009)
6
Random lasing from ZnO film LD
A planar device with the same MIS structure was also fabricated for comparison. EL spectra
were recorded under the same configuration. Almost no emission can be detected up to 30 mA
forward bias. With increasing current to 40 mA, near-UV emission with distinct sharp peaks
emerges in the spectrum. Further increase of the current leads to a rapid emission enhancement
and much sharper modes. The threshold current density is determined to be 4.8 A/cm2 from the
relationship of emission intensity vs. current density. To determine the laser mode of the film
LD, an angle-dependent measurement is carried out. Figure S3(b) shows EL spectra collected
from different angles of 0°, 30° and 45°. Distinct lasing spikes are observed at all the angles and
vary randomly, which demonstrates a random lasing behavior.
7
FIG. S3. (a) EL spectra of the film LD under different injection currents showing obvious lasing
behavior. The left and right insets display a schematic diagram of device structure and the
dependence of the integrated emission intensity on current density. (b) EL spectra of the film LD
taken from different angles. The recording configuration is drafted in the inset.
8