Litton C.W., . Reynolds D.C., Collins T.C. Zinc Oxide Materials for Electronic and Optoelectronic Device Applications
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264 Growth and Characterization
10
Room Temperature Stimulated Emission
and ZnO-Based Lasers
D.M. Bagnall
University of Southampton, Southampton, UK
10.1 Introduction
The fundamental emission properties of semiconductors under high excitation conditions
are well described by Klingshirn.
[1]
In general, increasing the excitation levels of
semiconductors results in increased interaction between excitonic species and carriers,
and these interactions can lead to the observation of nonlinear spontaneous emission bands
with quadratic dependence on excitation intensity. At low temperatures and intermediate
density regimes, biexcitonic, exciton–exciton or exciton–carrier emissions can be
expected, though small biexciton binding energies only allow biexcitonic emissions to
be observed at very low temperature. At higher temperatures exciton ionization increases
free carrier densities and exciton–carrier emissions are more likely to occur, then at very
high excitation intensities exciton densities reach such levels that they overlap. Beyond
this “Mott density”, phase space filling and Coulomb interactions cause excitons to lose
their individual character and an “electron–hole plasma” results.
Some of these spont aneous emission bands, under the right conditions, can provide a
mechanism for stimulated emission and in the right circumstances, optical gain. Stimu-
lated emission is identified by a strongly super-linear increase of the optical luminescence
output as a function of excitation power above a certain threshold and a spectral narrowing
of the emission. If these effects are accompanied by a suitable optical cavity then laser
action or “lasing” can be confirmed by the occurrence of Fabry–Perot modes or spatially
directed emission.
[1]
Zinc Oxide Materials for Electronic and Optoelectronic Device Applications, First Edition.
Edited by Cole W. Litton, Donald C. Reynolds and Thomas C. Collins.
© 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. ISBN: 978-0-470-51971-4
All of the effects outlined above have been observed in many semiconductor systems,
however the nature of emission for given temperature ranges and excitation levels are
strongly dependent upon both the fundamental material properties and the actual material
quality of a given sample. For a long time ZnO was known to have rather desirable
fundamental properties, with a wide band gap and a large exciton binding energy that
ought to ensure violet and blue excitonic emissions at room temperature. However,
deposition of suitable ZnO layers with low defect densities and low impurity concentra-
tions has remained problematic. So, as with preceding developments of other semicon-
ductor lasers the main aim of researchers wishing to realize low-threshold high-brightness
blue and violet laser diodes based on ZnO was to first find techniques by which suitably
perfect ZnO layers could be deposited. The second aim would be to optimize band-
engineering by alloying with Al or Cd to allow the formation of double-heterostructures,
quantum wells or superlattices and thereby enhance carrier densi ties and gain. The final
requirement is for the realisation of p-type ZnO and thereby formation of diodes and
carrier injection.
10.2 Emission Mechanisms
In the 1970s Klingshirn, Hvam and co-workers
[2–9]
carried out thorough studies of high
excitation effects in ZnO. In perha ps the most complete work, Klingshirn
[5]
studied one-
and two-photon excitation studies in the range 2–300 K. Emission bands were attributed to
the interactions of free excitons with phonons (E
x
–mLO), free excitons with free electrons
(E
ex
–E
el
), excitons with excitons (E
x
E
x
), bound excitons with phonons, and free excitons
and bound excitons.
Above 150 K only three intrinsic high-excitation emission mechanisms are observed.
During exciton–exciton scattering, one exciton decays to create a photon and scatters
another exciton from the lowest (n ¼1 ) excitation state into higher excitation states.
Emissions due to these processes are labelled the “P” band, and individual emission lines
are labelled P
n
(where n ¼1, 2, 3,...,1 and represents the excitation state of the remaining
exciton). The energy of the emitted photon (P
n
) can be readily calculated according to:
P
n
¼ E
ex
E
ex
b
1
1
n
2
3
2
kT ð10:1Þ
where E
ex
is the free exciton emission energy, E
ex
b
is the exciton binding energy and kT is
the thermal energy.
A second important band is due to exciton–electron scattering.
[7]
A feature of this
emission band is a strong shift to lower energy with increasing temperature. The emission
energy for this process is approximated by:
hv
E
ex
E
el
max
¼ E
ex
7:74 kT ð10:2Þ
The “N” band, a final source of stimulated emission at room temperature, is due to
electron–hole plasma recombination. Excitons begin to overlap at the Mott density
(10
18
cm
3
). A distinctive feature of the N band is that, becau se of band-gap renorma-
lization, it moves to lower energy with increasing excitation.
[1]
266 Room Temperature Stimulated Emission and ZnO-Based Lasers
Although much of the literature that reports stimulated emission and lasing in ZnO
assigns emissions to either P
n
or N bands, Klingshirn et al.
[10–12]
have recently argued that
these assignments have been made too readily and often with relatively weak evidence. In
particular it is felt that although excitonic processes are likely at low temperatures and low
excitation densities, at room temperature and with high excitation and exciton densities
close to the Mott density there are more likely mechanisms including those based upon
exciton–phonon and exciton–electron processes.
10.3 Stimulated Emission
10.3.1 Bulk ZnO
Bulk ZnO, pumped by electron beam,
[13–15]
was shown to produce stimulated emission at
cryogenic temperatures during the 1960s. Later studies with optical pumping also
produced stimulated emission at cryogenic temperatures.
[3,6–8,16]
Throughout this time
stimulated emission at room temperature had proved illusive. The first, brief, report of
stimulated emission in ZnO crystals at room temperature was by Koch et al.
[7]
in 1978 and
then Reynolds et al.
[17]
were the first to report optically pumped lasing in bulk ZnO as late
as 1997.
10.3.2 Epitaxial Layers
Although ZnO thin films were regularly studied for application as a transparent conductor
and as a piezoelectric material, up until 1996 relatively little attention had been paid to the
ZnO system for its potential use in the fabrication of short wavelength laser diodes.
However, a number of developments, particularly the remarkable success of the GaN-
based system, caused a number of researchers to look at ZnO in more detail.
In 1996 Yu et al.
[18]
reported room temperature stimulated emission from ZnO epitaxial
layers. The samples, produced at the Tokyo Institute of Technology, were hexagonal ZnO
microcrystalites grown on sapphire substrates by a technique they called laser molecular
beam epitaxy (laser-MBE). High excitation effects were studied using the frequency-tripled
output (355 nm, 15 ps) of a Nd:YAG laser. Two spontaneous emission mechanisms and two
stimulated emission mechanisms were identified at room temperature. Initially at low
excitation a single broad spontaneous luminescence peak, attributed to free exciton recom-
bination, was observed at 3.3 eV. At intermediate excitation levels the P
2
band, attributed to
exciton–exciton scattering process in which one exciton is scattered into the n ¼2state,
emerged 70 meV below the free exciton band in good agreement with Equation (10.1). Above
a threshold of 24 kW cm
2
the much sharper “P” band was observed 100 meV below the free-
exciton emission band. With increasing excitation the P band showed a superlinear increase
in intensity, and was seen to suppress the spontaneous emission background, conditions
highly indicative of stimulated emission. Beyond 50 kW cm
2
the appearance of a second
narrow band (N), with its peek moving to lower energies with increasing excitation, was
attributed to stimulated emission due to electron–hole plasma recombination. Figure 10.1
shows the development of the P and N lines with increasing excitation.
[18]
These remarkable
results represented a considerable improvement on all earlier works and even now, there are
few reports of lower threshold intensities for ZnO epilayers.
Stimulated Emission 267
The dramatic improvement in performance can be attributed to a number of features;
most significantly, the excellent crystallinity and purity of the ZnO microcrystals produced
by the laser-MBE process. High quality material leads to longer carrier lifetimes, reduced
losses by nonradiatative processes, narrow spectral features and lowers absorption. Each of
these effects reduces the threshold for stimulated emission and it is for these reasons
research efforts towards improved optical devices in any semiconductor system are
predominately based around the perfection of growth systems and processes. Perhaps
the most important fundamental aspect was the excitonic nature of the stimulated
emission. The exciton through its stability in ZnO even at room temperature and its
narrow energy distribution provides for low thresholds. In addition the exciton–exciton
scattering process which puts the emission peak 100 meV below the fundamental
absorption edge is of additional benefit as self-absorption is significantly reduced.
ZnO
RT
ZnO
RT
Emission Intensity (a.u.)Emission Intensity (a.u.)
x0.2x0.2
x0.4x0.4
x0.4x0.4
x0.5x0.5
3.053.05 3.153.15
Photon Ener
gy
(eV)Photon Ener
gy
(eV)
NN
PP
2.731
th
2.731
th
2.181
th
2.181
th
1.791
th
1.791
th
1.581
th
1.581
th
1.441
th
1.441
th
1.211
th
1.211
th
3.253.25 3.353.35
Figure 10.1 Lasing spectra of the ZnO microcrystallite film pumped using the frequency-
tripled output of a mode-locked Nd:YAG laser at various pumping intensities. Reprinted from Z.
K. Tang, et al., Room-temperature ultraviolet laser emission from self-assembled ZnO micro-
crystallite thin films, Appl. Phys. Lett. 72, 3270. Copyright (1998) with permission from
American Institute of Physics
268 Room Temperature Stimulated Emission and ZnO-Based Lasers
In a later work on the same samples,
[19]
the stimulated emission threshold is shown to be
a strong function of crystallite size with the lowest threshold occurring at a film thickness
of 55 nm. So, it was shown for the first time in the case of ZnO that the spatial confinement
of excitons can have the expected beneficial effect of driving down threshold values.
Shortly after the first report of stimulated emission at room temperature, the first room
temperature ZnO lasers were reported, by workers at Tohoku University
[20]
and at the
Tokyo Institute of Technology.
[19,21,22]
Workers at Tohoku University had been studying the growth of ZnO by plasma-
enhanced MBE on sapphire substrates.
[23,24]
Under relatively high excitation by the
frequency-tripled Nd:YAG laser, regions of single crystal epilayers had been found to
exhibit stimulated emission and, after cavities were formed by cleaving, some localized
regions were seen to lase. The stimulated emission and lasing thresholds were found to be
in the range 200–240 kW cm
2
. In further studies at higher temperatures stimulated
emission due to exciton–exciton scattering and electron–hole plasma were clearly resolved
(Figure 10.2).
[25]
At room temperature the threshold for stimulated emission at the “P”
free exciton (emission)
exciton-exciton scattering
Transition energy (eV)
electron hole plasma
300 350
400
3.30
3.25
3.20
3.15
3.10
3.05
3.00
2.95
2.90
2.85
Temperature (K)
450
500 550
Figure 10.2 Temperature dependence of peak energy positions. Reprinted from D.M. Bagnall,
et al., High temperature excitonic stimulated emission from ZnO epitaxial layers, Appl. Phys.
Lett. 73, 1038. Copyright (1998) with permission from American Institute of Physics
Stimulated Emission 269
band was 400 kW cm
2
while the “N” band had a threshold of 800 kW cm
2
. Even at 550 K
both features, with peaks at 3.05 eV and 2.9 eV, were still observed with thresholds of 1.2
and 1.9 MW cm
2
, respectively.
In further works on ZnO produced by the Tokyo Institute of Technology
[26–29]
and
Tohoku University,
[30,31]
further inform ation on lasing and stimulated emission have been
described. Two publications in particular have provided detailed analysis of gain spectra
produced by the variable stripe length method of Shaklee.
[32]
Yu et al.
[19]
describe the lasing and gain features of the microcrystalline samples of the
Tokyo Institiute of Technology. Gain spectra are plotted for three excitation fluencies; at
2.1 mJcm
2
the peak in the gain spect rum of 50 cm
1
is at 3.19 eV, at 3.1 mJcm
2
the peak is
165 cm
1
at 3.175 eV and at 3.4 mJcm
2
it is 220 cm
1
at 3.17 eV. A maximum gain of
280 cm
1
is found at a fluence of 3.8 mJcm
2
whereupon further increases in excitation lead
to free carrier densities close to the Mott transition and, as a consequence, electron–hole
plasma formation. The maximum values of gain are in reasonably good agreement with
the values of 320 cm
1
(at 3.0 mJcm
2
) and 300 cm
1
determined by the same authors for
the same samples by analysis of lasing spectra.
[29]
In the second work specifically dedicated to studies of gain spectra, Chen et al.
[30]
have
studied ZnO produced by plasma-MBE at Tohoku University. Once again, values are
determined by the stripe length method at room temperature. At 180 kW cm
2
a peak gain
of about 40 cm
1
at 3.17 eV is attributed to exciton–exciton scattering, at 220 kW cm
2
the
mechanism produces symmetric excitonic gain spectra with a maximum of 177 cm
1
with
a red-shift of 6 meV (Figure 10.3). At 300 kW cm
2
a gain value in excess of 550 cm
1
is
reached at 3.125 eV but this gain is attributed to the electron–hole plasma recombination
mechanism.
10.3.3 Quantum wells and Superlattices
The formation of quantum well structures within a semiconductor matrix can allow tuning
of emission wavelengths, the realization of enhanced exciton binding energies, enhanced
carrier confinement and thereby enhanced gain. Alloying to provide material such as
ZnMgO with a larger band gap can be used to surround a ZnO quantum well, while
alloying to provide a material such as ZnCdO with narrower band gap can provide
quantum wells within a ZnO matrix.
10.3.4 ZnMgO/ZnO Structures
The ZnMgO alloy system allows for band-gap engineering from 3.37 eV (ZnO) to around
4 eV (Zn
0.67
Mg
0.33
O) at room temperature
[33]
and photoluminescence studies of ZnO
quantum wells within ZnMgO and superlattices have shown tuning of room temperature
emission wavelengths from 3.36 to around 3.87 eV.
[34,35]
Relatively few researchers have described stimulated emission from ZnO-based quan-
tum well samples. This lack of success reflects the difficulties associated with the growth
of these structures, in particular avoiding the precipitation of cubic MgO and the
maintenance of excellent structural quality. Success in this regard seems to be enhanced
by growth on lattice matched ScAlMgO
4
substrates rather than sapphire, although some
recent reports indicate increasing success with growth on sapphire.
[36–38]
270 Room Temperature Stimulated Emission and ZnO-Based Lasers
Workers at the Tokyo Institute of Technology and their collaborators have performed the
most detailed studies of ZnO-based superlattice systems.
[36–39]
H. D. Sun et al.
[40]
provided one of the earliest detailed studies of stimulated emission
from a range of ZnO/Zn
1-x
Mg
x
O multiple quantum well (MQW) samples grown on
ScAlMgO
4
substrates by laser-MBE. The magnesium content of the barrier layers was
kept at x ¼0.12 to provide a barrier of around 0.2 eV. Barrier thickness was kept at 5 nm,
while the quantum wells ranged from 0.69 to 4.65 nm. Optical excitation was carried out
using an excimer laser and edge emission was detected. At low excitation intensities a
ZNO/Al
2
O
3
T = 297 K
I
Pump
= 300 kW/Cm
2
I
Pump
= 220 kW/Cm
2
I
PUMP
= 180 kW/Cm
2
600
500
400
300
200
100
0
–100
150
100
50
0
-50
40
20
0
–20
–40
–60
-80
3.05 3.10 3.15
(c)
(b)
(a)
Optical Gain (cm
–1
)
Photon Ener
g
y (eV)
3.20 3.25 3.30
Figure 10.3 Optical gain spectrum of a ZnO epilayer at excitation density of (a) 180 kW cm
2
,
(b) 220 kW cm
2
and (c) 300 kW cm
2
at room temperature. Reprinted from Y. Chen, N.T.
Tuan, Y. Segawa, H. Ko, S. Hong and T. Yao, Stimulated emission and optical gain in ZnO
epilayers grown by plasma-assisted molecular-beam epitaxy with buffers, Appl. Phys. Lett. 78,
1469. Copyright (2001) with permission from American Institute of Physics
Stimulated Emission 271
spontaneous emission peak due to localized excitons is observed but as excitation levels
were increased this peak was found to saturate before a stimulated emission peak due to
exciton–exciton scattering emerged. Exciton–exciton scattering was found to be the
main stimulated emission mechanism for all temperatures from 5 K through to room
temperature. Although at higher temperatures, and higher excitation levels, stimulated
emission due to electron–hole plasma recombination was simultaneously observed, this
would probably be from localized regions of the material. Exciton binding energies were
readily deduced and found to be enhanced for all quantum well samples, increasing with
decreasing well width to a maximum value of 86 meV in the case of 1.75 nm wide
quantum wells. Thresholds for stimulated emission at 5 and 300 K were determined at
around 220 and 480 kW cm
2
, respectively, values an order of magnitude greater
than those observed for the best ZnO epilayers, though the authors suggest that shorter
excitation pulses and subtraction of reflected and transm itted light might reduce
these values.
Ohtomo et al.
[36]
have produced 10-period superlattices of ZnO/Zn
0.88
Mg
0.12
O and
ZnO/Zn
0.74
Mg
0.26
O on ScAlMgO
4
substrates using combinatorial laser-MBE. ZnO
quantum well thicknesses were varied from 0.7 to 4.7 nm while barrier layers were kept
at 5 nm. Excitation was carried out using a frequency-tripled Nd:YAG laser with 15 ps
pulses, and edge emission was studied in the temperature range 294–380 K. At room
temperature all of the ZnO/Zn
0.88
Mg
0.12
O superlattices were found to exhibit stimulated
emission at energies in the range 3.2–3.4 eV with thresholds below 22 kW cm
2
. The
lowest threshold of 11 kW cm
2
, reported for the sample with 4.7 nm wells, compares
favorably with typically optical pumping thresholds reported for high quality ZnSe- and
GaN-based systems. The stimulated emission mechanism is tentatively assigned to
exciton–exciton scattering. Clear threshold characteristics are provided for temperatures
up to 377 K. and the characteristic temperature is determined at 87 K.
In other work of note, J. W. Sun et al.
[41]
have observed room temperature stimulated
emission at 3.33 eV and excitation intensities of around 200 kW cm
2
, from ZnO/
Mg
0.2
Zn
0.8
O MQW structures grown by plasma-assisted MBE on sapphire. The emission
mechanism is assigned to exciton–exciton scattering and the enhanced exciton binding
energy calculated to be 122 meV in good agreement with other similar work.
[38,39]
10.3.5 ZnO/ZnCdO Structures
Early studies of Zn
y
Cd
1-y
O alloy films were carried out by the Tokyo Institute of
Technology,
[42–44]
where it was found that a band gap of 3.0 eV at room temperature
could be achieved with y ¼0.07, when layers were grown on ScAlMgO
4
(0001)
substrates by pulsed laser deposition.
[44]
More recent work by Sadofev et al.
[45]
has
shown that this range can be extended to as far as 2.15 eV and emission in the yellow
spectral range at Cd compositions as high as y ¼0.32. Strong band-gap-related emission
was reported for both epilayers and quantum wells grown using MB E at remarkably low
growth temperatures.
The success of this MBE approach has recently yielded the first evidence of room
temperature stimulated emission and lasing in ZnCdO quantum well samples.
[46]
Samples
containing multiple Zn
y
Cd
1-y
O quantum wells with y ¼0.1 and widths of 3.2 nm were
found to lase at 5 K with a threshold of 60 kW cm
2
and with the emission peak at 2.77 eV.
272 Room Temperature Stimulated Emission and ZnO-Based Lasers