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Deep ultraviolet emitting polarization induced nanowire light emitting diodes with Alx Ga1−x N
active regions
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2014 Nanotechnology 25 455201
(http://iopscience.iop.org/0957-4484/25/45/455201)
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Nanotechnology
Nanotechnology 25 (2014) 455201 (6pp)
doi:10.1088/0957-4484/25/45/455201
Deep ultraviolet emitting polarization
induced nanowire light emitting diodes with
AlxGa1−xN active regions
Thomas F Kent1, Santino D Carnevale2, A T M Sarwar2, Patrick J Phillips3,
Robert F Klie3 and Roberto C Myers1,2
1
Department of Materials Science and Engineering, The Ohio State University, Columbus, OH, USA
Department of Electrical and Computer Engineering, The Ohio State University, Columbus, OH, USA
3
Department of Physics, University of Illinois at Chicago, Chicago, IL 60607, USA
2
E-mail: [email protected]
Received 31 July 2014, revised 25 September 2014
Accepted for publication 26 September 2014
Published 20 October 2014
Abstract
In this report, we demonstrate band gap tuning of the active region emission wavelength from
365 nm to 250 nm in light emitting diodes fashioned from catalyst-free III-nitride nanowires.
Optical characteristics of the nanowire heterostructures and fabricated devices are studied via
electroluminescence (EL) and photoluminescence spectroscopy over a wide range of active
region compositions. It is observed that for typical nanowire plasma assisted molecular beam
epitaxy growth conditions, tuning of emission to wavelengths shorter than 300 nm is hampered
by the presence of an optically active defect level. We show that by increasing the AlGaN
nanowire growth temperatures this defect emission can be suppressed. These findings are applied
to growth of the active region of a nanowire light emitting diode, resulting in a polarizationinduced nanowire light emitting diode with peak EL at 250 nm.
Keywords: nanowire, light emitting diode, ultraviolet, semiconductor, AlGaN, wide band gap
(Some figures may appear in colour only in the online journal)
1. Introduction
the large ionization energy of Mg acceptors in this material.
Second, it is well known that AlGaN which is tensilely
strained (as is the case when grown on sapphire and most
common planar substrates) exhibits a switch from TE
polarized emission with propagation vector along the c-axis
to TM polarization with propagation vector perpendicular to
the c-axis [6, 7]. This transition from TE to TM emission
greatly reduces light extraction efficiency for c-axis oriented
planar film devices.
In this paper we describe a unique nanowire-based device
heterostructure capable of composition tunable deep ultraviolet light emission peaks below the 270 nm threshold for
UVC. We have previously demonstrated that the intrinsically
polar nature of the III-Nitrides can be exploited to enhance, or
even substitute for, impurity doping of compound semiconductors in compositionally graded AlGaN nanowire heterostructures, called polarization induced nanowire light
emitting diodes (PINLEDs) [8–10]. In our original report, this
The field of optoelectronic devices operating in the deep
ultraviolet or UVC spectral range has recently attracted
much research activity [1–4]. This has been driven in part
by successes in development of commercial ultraviolet light
emitting diodes which operate at sub 270 nm wavelengths.
However, important technological applications such as chemical agent detection, biological disinfection and ultraviolet
curing of adhesives require more efficient emitters at shorter
wavelengths than are currently available [2]. Pushing emission deeper into the ultraviolet has resulted in a number of
materials related problems associated with high Al composition AlGaN. Consequently, much of the effort in this area
has been centered on thin film growth techniques for
improving material quality of high composition AlGaN.
First and foremost is the difficulty of achieving efficient hole
doping in high Al content AlGaN layers [5]. This is due to
0957-4484/14/455201+06$33.00
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© 2014 IOP Publishing Ltd Printed in the UK
T F Kent et al
Nanotechnology 25 (2014) 455201
Figure 1. Polarization induced nanowire light emitting diodes. (a) Band diagram showing p and n type regions as well as the AlGaN QW
active region. (b) HAADF mode STEM micrograph showing individual heterostructure nanowire. Insert shows heterostructure schematic and
device schematic.
2. Heterostructure design and growth
technique was used to fashion GaN active region light emitting diodes with bright emission at 365 nm. We have subsequently investigated the potential for using the PINLED
heterostructure as a host for rare earth (Gd) phosphor devices
which has led to devices with narrow emission lines at
318 nm [10]. In this study, we explore a wide composition
range of AlGaN active regions to yield deep ultraviolet light.
The technique of polarization doping has much promise
for overcoming the poor hole conductivity in high composition AlGaN. Since the gradient in polarization gives rise to a
secondary driving force for activation of impurity dopants,
polarization graded layers make a good candidate for
enhancing hole concentration in AlGaN layers while potentially reducing the amount of Mg impurity atoms required to
achieve a sufficiently high conductivity [11–15]. High Mg
concentrations currently required for even marginally conductive AlGaN present a major roadblock toward developing
electrically driven deep ultraviolet laser diodes, since Mg
gives rise to an optically active absorption band in the bluevisible, which causes significant optical loss. In recent years,
multiple groups have demonstrated the promise for using
polarization graded heterostructures to improve hole conductivity in p-AlGaN layers with the application of deep UV
LEDs in mind [8, 16, 17]. Most of these efforts are focused on
thin film heterostructures, which contain the benefit of planar,
uniform samples. Magnitudes of composition gradients in
planar geometries are inherently limited by strain considerations, which therefore limits the magnitude of polarization
doping possible.
The well-known strain accommodating nature of semiconductor nanowires [18] offers an ideal system for realization of optoelectronic devices based on polarization enhanced
doping. This is primarily due to the fact that the entire
compositional range of AlGaN can be accessed in a linear
gradient of composition from GaN to AlN resulting in higher
carrier densities over larger regions than possible in thin film
devices. Additionally, UVB devices based on self-assembled
III-nitride nanowire heterostructures have been demonstrated
with tunable emission to 340 nm [19]. Utilizing polarization
gradients in nanowire heterostructures, we previously have
demonstrated the concept for the PINLED. These heterostructures are prepared using a Vecco GEN 930 MBE system
with a RF plasma nitrogen source on Si (111) substrates. In
order to suppress SiNx formation on the Si during plasma
lighting and tuning, the substrate is rotated away from the
sources until the beginning of the growth. After thermal
desorption of the native oxide at high temperature, GaN wires
are nucleated at 710 °C for a set duration under nitrogen rich
conditions to fix a density and wire aspect ratio. Growth of the
heterostructure then proceeds at high temperature (790 °C)
which suppresses further nucleation. The heterostructure
consists of a linear grade from GaN to AlN over 100 nm to
form a p-type region (assuming N-polar wires), active regions
consisting of one or more quantum wells and a linear grade
from AlN to GaN over 100 nm to form an n-type region
(again assuming N-polar wires). The band diagram, calculated using a self-consistent Schrodinger–Poisson solver [20],
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T F Kent et al
Nanotechnology 25 (2014) 455201
is shown in figure 1(a). The as-grown sample morphology,
studied by scanning transmission electron microscopy, was
investigated using a JEOL JEM-ARM200CF microscope,
operated at 200 kV in high angle annular dark field (HAADF)
mode, which yields z-contrast and is shown in figure 1(b).
Back and forth linear grades in composition can be observed
from z-contrast as well as the active region.
The benefits of such a heterostructure for deep ultraviolet
light emitting diodes are manifold. Firstly, low resistance
contacts can be made to the conductive p-Si substrate (using
pressed on In) and the very thin layer of n-GaN at the top of
the device (using a e-beam deposited 10 nm Ti/20 nm Au
stack) as shown in figure 1(b)(insert). This removes the difficulty of fashioning Ohmic contacts to p-AlGaN or using
large lossy regions of p-GaN as the contact layer, which is
currently the industry standard for DUV LEDs. Secondly, pconductive graded AlGaN layers readily provide holes to the
active region of the device (figure 1(a)), allowing for reduced
Mg concentrations and low reflective losses. Thirdly, the
graded regions terminate at AlN cladding layers (6 eV band
gap), meaning that any composition in the AlInGaN system
can be used as the quantum well active region, provided it can
be grown with sufficient optical quality and radiative efficiency. It is likely that the decreased strain in the crystal
structure of the nanowire may give rise to a higher onset of
the TE to TM emission polarization transition in these devices, enhancing their vertically directed optical output power.
Lastly, the ability to arbitrarily tune the nanowire composition
without concern for strain relaxation (at least in the AlGaN
nanowire heterostructures) would allow polarization and band
gap tuning of the active region potential, recently suggested
as a method to reduce non-radiative Auger processes in
nitride LEDs [21].
Figure 2. Demonstration of potential for PINLEDs as hosts for many
active region compositions. Electroluminescence spectra for a
number of PINLEDs with increasing %Al in the active region of the
device.
diffraction grating equipped with a UV–vis CCD (PI PIXIS
BR100).
All devices exhibit similar PL spectra, with the emission
dominated by two peaks, a high energy emission that initially
blue shifts with composition and a lower energy composition
invariant peak at 3 eV, likely attributed to an optically active
trap in the heterostructure. Analysis of the peak position with
increasing Al composition (figure 3(b)) shows a saturating
behavior toward 4 eV for the highest energy peak and a rather
composition invariant behavior for the broad emission at
3 eV. Moreover, the intensity of the PL signal from the high
energy peak is strongly quenched as the composition is
increased (figure 3(c)). This behavior is indicative of the
presence of a deep level in the active region of the device
which is dominating over radiative band-to-band recombination processes.
In order to study the optical properties of the active
region without simultaneously exciting PL in the rest of the
structure, two-step AlGaN nanowires (figure 4(c), insert) were
prepared under identical nucleation and growth conditions to
the PINLED heterostructures. The two step method of Carnevale, et al [22] allows for identical nucleation conditions,
which are important for maintaining nanowire density and
morphology, across samples with varying heterostructures
and active region growth conditions. An array of GaN/AlGaN
nanowire samples are prepared with varying %Al and growth
temperatures. We observe that when the temperature is held
constant and the Al composition is varied, the dominant
emission occurs around 290 nm as shown in figure 4(a). This
peak location correlates with the pinning behavior observed in
figure 3. The slight blue shift in emission, 310–290 nm with
increasing composition, can be explained by a change in the
overall band gap of the material, shifting the deep level
transition energy slightly. Peaks consistent with the target
3. Optical characterization of active regions for many
compositions
As part of this study, we have explored the composition and
emission space for this heterostructure by growing a wide
variety of AlGaN composition active regions in these devices,
the results of which are shown in figure 2. As expected,
increasing %Al in the active region of the PINLED causes the
electroluminescence (EL) to blue shift and does so in a
monotonic fashion from 365 nm to 300 nm. In this composition region, the EL intensity stays relatively constant indicating that the external quantum efficiency of the devices is
relatively composition independent. However, as %Al
exceeds 40%, EL intensity begins to drop and becomes pinned at 290 nm. To further elucidate the cause of this pinning,
we performed photoluminescence (PL) spectroscopy on
unprocessed samples, the results of which are shown in
figure 3. Spectra are collected using the third harmonic of a
tunable, pulsed Ti:Sapphire oscillator (Coherent Chameleon
Ultra II) as the excitation source. Fluorescence is collected by
a 0.5NA 36x reflective microscope objective and free space
coupled to an 0.5 m spectrometer with a 1200 g mm−1
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T F Kent et al
Nanotechnology 25 (2014) 455201
Figure 3. Pinning of emission at 300 nm in high Al composition active regions. (a) Photoluminescence spectra with 250 nm. Excitation
showing initial shift in emission of the active region at low %Al and a clear decrease in intensity and pinning of emission at 300 nm as %Al
exceeds 50%. (b) Plot of peak energy versus target active region composition showing initial shift then saturation of QW emission. (c) Plot of
emission intensity versus target composition showing droop in intensity at pinned emission of QW.
Figure 4. Photoluminescence of bulk AlGaN nanowires. (a) PL spectra for AlGaN nanowire samples grown at three compositions showing
dominance of defect emission in the 300 nm region. (b) PL for samples of fixed composition and varying substrate temperature showing
suppression of defect emission with higher growth temperature. (c) Cross section SEM images of samples showing that nanowire
morphology is maintained despite higher temperatures.
Additionally, the low mobility of Al adatoms on the nanowire
surface at typical growth temperatures may lead to composition fluctuations and reduction of crystalline quality.
Increasing growth temperature will enhance Al mobility and
therefore enhance Al and Ga miscibility, leading, in principle,
to reduction of composition fluctuations in the AlGaN
regions.
In principle, increased substrate temperatures should
limit the incorporation of oxygen and other high vapor
composition in the 250–270 nm range are, however, not
observed.
Similar behavior has been recently observed in other IIINitride nanowires studied by Daudin and coworkers [23],
who examined catalyst-free nanowires grown by PAMBE
over a large portion of the composition range of AlGaN. AlN
and AlGaN are well known to have a very high affinity for
oxygen incorporation during thin film growth, leading to
below bandgap optically active defect levels [24].
4
T F Kent et al
Nanotechnology 25 (2014) 455201
Figure 5. Deep ultraviolet emission from PINLED grown with the high temperature active region. (a) Device I–V curve showing 6 V turn on
and rectification. Insert shows an image of the active device collected with an ultraviolet microscope. (b) Electroluminescence spectra of the
high temperature active region device showing evolution of EL with increasing current density. Insert shows peak power versus current
density.
pressure impurities [25, 26], as well as enhance composition
uniformity. The use of increased substrate temperature to
reduce oxygen incorporation was well established in AlGaAs
grown by MBE and essential in the development of the first
bright red LEDs [27]. PL spectra for samples grown at a fixed
target composition of 80% Al and increasing substrate temperature are shown in figure 4(b). It can be readily observed
that as the substrate temperature increases, the once dominant
emission peak at 300 nm is readily suppressed and dominant
emission shifts to 260 nm. This may suggest that higher
temperature growth suppresses formation of an impurity
related deep level responsible for the 300 nm emission, for
example by increased oxygen desorption rate (effectively
reducing the sticking coefficient for oxygen). However, it
should be noted that the observed highest energy peak is still
lower in energy than expected for the target composition,
suggesting that emission may have shifted to a slightly
shallower defect state rather than true band to band recombination. Samples grown at all three temperatures exhibit
similar morphology, indicating that growth at higher temperatures did not induce a change in growth mode as can be
observed from the SEM micrographs, collected using a FEI
Sirion, in figure 4(c).
are deposited on the tops of the nanowires via photolithography. Device IV’s (figure 5(a)) show rectification and
turn on at 6 V, approximately the bandgap of AlN, as
expected from the PINLED band diagram shown in
figure 1(a). EL spectra are collected on a custom built deep
ultraviolet optical probe station equipped with a Keithley
2604B SMU. Light is collected by a fused silica objective
lens and free space coupled to a 0.15 m imaging spectrometer
equipped a 1200 g mm−1 diffraction grating and a large area
UV-CCD (PI PIXIS BR400).
Unlike previous PINLED devices with AlGaN active
regions, which exhibited emission pinned at 290 nm at similar
compositions, the device with high temperature grown active
regions exhibits EL peaks at 250 nm as shown in figure 5(b).
At low current densities, emission at 250 nm is dominant.
However, as injection level increases, a 280 nm peak begins
to grow, eventually equaling the intensity of the high energy
emission at 250 nm at 368 A cm−2, the highest value studied.
This behavior suggests that some carrier overflow or ‘overshoot’. Peak power was measured to be 90 nW @ 400 mA in
dc, yielding an EQE of approximately 2 μ% at dc. Although
this is a very small efficiency, it is emphasized that the device
geometry for such devices is far from optimized, causing a
low percentage of nanowires to be contacted and requiring
thick, absorbing contact layers to form coalesced tops that
contact all nanowires in parallel, suggesting that significant
improvements can be made with future process development.
Additionally, it is noted that fully optimized commercial
devices emitting in this spectral range are around 0.2% efficient [26]. In pulsed mode, which offers the ability to neglect
self-heating effects in the measurement, (5% duty cycle,
15 ms period) a peak power of 0.22 μW is measured at
400 mA for the DUV nanowire LED. This is to our knowledge the shortest operating wavelength nanowire LED yet
reported.
In principle, employing this technique at increasingly
higher substrate temperatures should allow formation of high
optical quality DUV nanowire LEDs with emission below
4. Device fabrication and characteristics
Using this higher substrate temperature growth (840 °C) for
the active region only, a new high %Al PINLED device
heterostructure is prepared. The substrate temperature is
ramped toward the end of the p-type graded layer, removing
the necessity of a growth interruption around the active region
of the device. A three period 5 nm Al0.8Ga0.2N multiple
quantum well with 5 nm UID AlN inserts is deposited at
840 °C. The rest of the structure is grown normally at 790 °C
with the temperature ramp down occurring at the end of the
active region without a growth interruption. Contacts are
made to the p-Si substrate and 350 μm × 350 μm gridded pads
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T F Kent et al
Nanotechnology 25 (2014) 455201
250 nm. However, at higher growth temperatures Ga flux will
need to be increase substantially to achieve stoichiometric
incorporation due to growth far in the decomposition regime.
Additionally, a number of challenges still exist with achieving
deeper emission such as the presence of additional higher
energy optically active deep levels and increased optical loss
in the graded regions and top contacts.
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5. Conclusion
In summary, the PINLED heterostructure takes advantage of a
number of unique properties of nitride semiconductors and
nanowires which work in concert to house wide bandgap
AlGaN active regions for production of deep ultraviolet light.
In these devices, it is possible to reproducibly control AlGaN
compositions and resulting EL across a wide range of emission wavelengths. Furthermore, when growing high composition AlGaN regions in nanowire devices, a substantial
increase in substrate temperature is required in order to produce material with dominant emission below 290 nm. Using
both control of AlGaN composition and high temperature
growth conditions, we are able to demonstrate a nanowire
light emitting diode operating in the technologically important UV-C band at 250 nm electrically integrated on silicon
wafers.
Acknowledgments
This work is supported by the Army Research Office
(W911NF-13-1-0329) and by the National Science Foundation CAREER award (DMR-1055164). S D Carnevale
acknowledges support from the National Science Foundation
Graduate Research Fellowship Program (2011101708).
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