Hot electron generation by aluminum oligomers
in plasmonic ultraviolet photodetectors
Arash Ahmadivand,* Raju Sinha, Phani Kiran Vabbina, Mustafa Karabiyik,
Serkan Kaya, and Nezih Pala
Department of Electrical and Computer Engineering, Florida International University, 10555 W Flagler St., Miami,
Florida 33174, USA
*
[email protected]
Abstract: We report on an integrated plasmonic ultraviolet (UV)
photodetector composed of aluminum Fano-resonant heptamer
nanoantennas deposited on a Gallium Nitride (GaN) active layer which is
grown on a sapphire substrate to generate significant photocurrent via
formation of hot electrons by nanoclusters upon the decay of
nonequilibrium plasmons. Using the plasmon hybridization theory and
finite-difference time-domain (FDTD) method, it is shown that the
generation of hot carriers by metallic clusters illuminated by UV beam leads
to a large photocurrent. The induced Fano resonance (FR) minimum across
the UV spectrum allows for noticeable enhancement in the absorption of
optical power yielding a plasmonic UV photodetector with a high
responsivity. It is also shown that varying the thickness of the oxide layer
(Al2O3) around the nanodisks (tox) in a heptamer assembly adjusted the
generated photocurrent and responsivity. The proposed plasmonic structure
opens new horizons for designing and fabricating efficient opto-electronics
devices with high gain and responsivity.
©2016 Optical Society of America
OCIS codes: (040.5160) Photodetectors; (250.5403) Plasmonics; (250.0250) Optoelectronics.
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1. Introduction
In recent years, there has been growing interest for plasmonic photodetectors for a wide
spectral range covering terahertz (THz) to visible frequencies [1–3]. In all these works,
several strategies have been used to enhance the absorption, responsivity, and quantum
efficiency of the photodetectors. For instance, graphene plasmonics has been introduced as a
promising platform to enhance the absorption of metal-semiconductor-metal (MSM)
photodetectors including Schottky contacts at the optical and THz frequencies [4]. Moreover,
plasmonic nanoparticles with absorptive characteristics (ohmic losses) have widely been
utilized to improve the spectral response of detectors [5]. In the previously reported works,
several techniques were used to enhance the detection performance of plasmonic
photodetectors such as enhancing of the Schottky barrier height at the metal-semiconductor
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13 Jun 2016 | Vol. 24, No. 12 | DOI:10.1364/OE.24.013665 | OPTICS EXPRESS 13667
interface, which provides a wider depletion region [6,7], and excitation of surface plasmon
resonances based on collective, coherent hot electron oscillations [8,9]. The ultraviolet (UV)
detectors are very useful for applications in UV astronomy, environmental monitoring, missile
warning, and biotechnology and medicine. However, in spite of the extensive researches, the
UV photodetectors suffer from dissipative losses, large dark currents, limited responsivity and
quantum efficiency [10,11]. To address these challenges and improve the performance of the
UV detectors, two major methods have been proposed: (1) Avalanche multiplication [12], and
(2) photoconductive gain [13]. However, high responsive GaN-based avalanche detectors
suffer from an increased noise [14]. On the other hand, the photoconductive UV detectors are
slow and noisy [15]. As another solution, GaN-based UV photodetectors with silver (Ag)
plasmonic nanoparticles have been introduced to enhance the responsivity [6,16,17]. The
major problem correlating with this method is the performance of utilized metals for UV
bandwidth. The plasmon resonances in the subwavelength structures based on conventional
noble metals (e.g. Au, Ag, and Cu) can be tuned across the visible wavelengths to the near
infrared region (NIR). However, extending these plasmonic properties into the UV spectrum
is highly challenging due to the intrinsic limitations in the chemical characteristics of the used
metals. For instance, silver shows a dramatic degradation in plasmonic properties because of
rapid oxidation and gold suffers from the interband transitions in the UV band [18]. Lately,
Aluminum (Al), Rhodium (Rh), Gallium (Ga), Chromium (Cr), and Indium (In) have been
introduced as potential plasmonic materials for the UV spectrum [19]. Aluminum has widely
been employed in designing light harvesting devices, nanoantennas, cathodoluminescence
spectroscopy, and antireflective surfaces [18–21], in spite of the inherent and rapid oxidation.
Aluminum also shows significant EM field localization because of its low screening ( ε ∞ ≈ 1)
in comparison to gold ( ε ∞ ≈ 9) and silver ( ε ∞ ≈ 5). In addition, aluminum has high electron
density since a single aluminum atom contributes three electrons compared to a single
electron per atom for gold and silver [22]. Due to the negligible influence of interband
transitions in aluminum across the UV spectrum, therefore, the geometry of nanoscale
structure plays a major role in decaying plasmons and generation of photoexcited hot carriers
during light-matter interactions.
Closely packed and strongly coupled plasmonic nanoparticle assemblies in symmetric and
antisymmetric orientations, known as plasmonic oligomers, can be tailored to support strong
resonances across the visible to the NIR [23]. These nanoparticle clusters show significant
absorption cross-sections in the visible and NIR ranges, including strong plasmon resonance
hybridization in the offset gaps between proximal particles [24]. Depending on their shape
and orientation, nanoparticle clusters are able to show unique spectral lineshapes, called
“Fano resonances (FR)” that can be characterized by narrow spectral windows, where
scattering maxima are suppressed and absorption peaks are enhanced [23–25]. The physical
mechanism behind formation of the plasmonic FR is a weak and destructive coupling between
a spectrally broad superradiant mode and a narrow subradiant mode. When a plasmonic
oligomer is excited at the frequency of the bonding mode, the incident light directly couples
into the bonding mode via light-matter interaction resulting a robust indirect excitation of the
antibonding resonant mode. In the nonretarded limit, the antibonding mode is dark without a
net dipole moment and, hence, cannot be coupled directly to the incident beam. In contrast, in
the retarded limit, the bonding mode becomes bright and a weak coupling mediated by the
strong near-field coupling gives rise to the interaction between bonding and antibonding
resonant modes, inducing a FR dip in the bonding continuum at the energy level of the dark
mode [26]. Excitation of a plasmonic FR mode leads to significant absorption compared to
excitation of usual bright resonant mode which could be used for hot electron generation. This
feature of plasmonic FR mode can be exploited to enhance the photocurrent in plasmonic
photodetectors [27]. However, inducing FR dips in the UV band is challenging due to
limitations correlating with the chemical properties of conventional noble metals at this
domain. Recently, it is proved that Al/Al2O3 nanodisk clusters in symmetric and
antisymmetric orientations are able to support strong FR modes with excellent absorption
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Received 17 Mar 2016; revised 21 May 2016; accepted 25 May 2016; published 10 Jun 2016
13 Jun 2016 | Vol. 24, No. 12 | DOI:10.1364/OE.24.013665 | OPTICS EXPRESS 13668
coefficient at the UV spectrum [28]. Low-cost, CMOS compatibility, and supporting strong
plasmon resonances at the UV spectrum are some of unique features of aluminum-based
molecular clusters that make these nanoscale assemblies suitable for designing efficient
nanoplasmonic devices.
In this paper, we propose a novel device based on plasmonic Al/Al2O3 nanoparticle
assemblies integrated into a GaN UV photodetector. To this end, we utilized seven-member
heptamers with the symmetry of a benzene molecule as Fano-resonant plasmonic
nanoclusters. All of the aluminum particles are deposited between Ni/Au fingers on a GaN
active layer grown on a sapphire substrate. The presented results show that nanoplasmonic
aluminum assemblies could generate hot electrons to enhance the absorption via inducing the
FR modes across the UV spectrum. The proposed structure could realize the UV
photodetectors with a significantly improved responsivity.
2. The proposed device
Radiative and non-radiative excitation of plasmons in metallic components and their decay
leads to generation of hot carriers at the metal-semiconductor interfaces [6,29–31]. The
surface modes are important to achieve the plasmon resonant behavior and hot carrier
distribution at the metal-semiconductor interfaces [8,36,37]. Plasmonic photovoltaic devices
[32,33], and photodetectors [34,35] employ the decay of plasmons to generate hot carriers.
Aluminum with the ability of generating continuous energy distribution for electrons is one of
the most suitable metals for this process [6]. In our proposed system, the confined plasmons
also lead to the hotspot formation with extremely intense local fields in the capacitive regions
between the proximal particles. Assuming electrons have an isotropic momentum distribution,
approximately half of the photoexcited electrons are expected to be transported to the
aluminum-GaN interface. Due to the continuous distribution of highly energetic hot electrons
in the aluminum nanostructures [6,38], hence, we expect large number of charges to reach the
metal/semiconductor interface compared to the conventional noble metals. Figure 1(a) shows
a three-dimensional schematic of the proposed plasmonic UV detector (not to scale). The
device comprises arrays of Al/Al2O3 heptamer antennas between two Ni/Au fingers
(electrodes) deposited on an undoped n-type GaN epilayer with the thickness of 4 μm which
is grown on a sapphire substrate. The inset figure shows the geometry of the heptamer
assembly. The space between two neighboring heptamers is set to 250 nm to prevent any
destructive optical interference between the scattered fields associating with hybridized
modes arising from the nearby antennas. In Fig. 1(b), we show the important geometrical
dimensions for the metallic electrodes, the overall size of the proposed photodetector, and the
distance between two fingers. Using the plasmon hybridization theory to analyze closely
packed nanoscale assemblies, the plasmon responses of various types of aluminum-based
nanodisk oligomers and monomers have already been investigated numerically and
analytically [28,39]. It is also shown that aluminum nanodisk heptamer antennas with a thin
oxide layer (2-25 nm, depends on the size of consisting particles) can be tailored to support
strong plasmonic FR mode across the near-UV (λ~350 nm) band [39]. However, this
wavelength is not unique and the position of FR minimum can be tuned via modifications in
the geometrical, chemical and environmental parameters of the assembly. In the proposed UV
detector, we used nanodisks with the geometrical dimensions that were calculated by
Golmohammadi et al. [28], and accordingly, the radius of nanodisks is R = 70 nm with the
thickness of t = 35 nm separated with the offset gap of D7h = 12 nm. It should be noted that
while the thickness of the oxide layer around nanoparticles is varied the size of offset gap is
kept fixed to satisfy the required near-field coupling strength. To provide a detailed study and
compare the effect of aluminum heptamer arrays on the responsivity and performance of the
structure, we also demonstrate the spectral response of the structure without presence of
antennas on GaN as the non-plasmonic regime. To this end, we used empirically measured
values for a GaN-based UV detectors reported by Li et al. [6].
The frequency-dependent absorption mechanism of the proposed plasmonic UV detector
is based on the hybridized plasmon resonant modes due to the interaction of an incident beam
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© 2016 OSA
Received 17 Mar 2016; revised 21 May 2016; accepted 25 May 2016; published 10 Jun 2016
13 Jun 2016 | Vol. 24, No. 12 | DOI:10.1364/OE.24.013665 | OPTICS EXPRESS 13669
with the metallic antenna. The maximum absorption can be achieved at the spectral position
of the antisymmetric FR dip, because of the suppression of the scattering bright resonant
dipolar peak by narrow antibonding dark mode. Such a significant absorption leads to
generation of large number of hot carriers at the metal-dielectric interface, which are
transferred to semiconductor surmounting the Schottky barrier and collected by the electrodes
resulting a remarkable photocurrent and hence, high responsivity. Figure 1(c) exhibits a twodimensional (xz-view) cross-sectional schematic of the proposed UV detector displaying the
hot electron transport in the GaN layer to the adjacent electrodes. It is well-understood that in
a metal-semiconductor system, reduced electron-electron scattering in the metallic part of the
nanoantenna increases the number of hot electrons transferred to the semiconductor layer
[33,41]. This ultrafast transition of plasmonic charges leads to accumulation of more hot
electrons and sweeping them before immediate recombination. Figure 1(d) demonstrates the
schematic band diagram profile for the proposed plasmonic device, showing the carrier
formation and transition mechanisms and sweeping opposite charges to the nearby electrodes.
When a bias is applied (here 5.0 V) between the metallic contacts one forward and one
reverse biased Schottky junctions are formed. The large electric field in between, results in
sweeping of the photogenerated hot electrons to the positive electrode and thereby producing
a photocurrent. However, due to losses via back-scattering, inelastic collisions, and heat
energy conversion (internal damping), not all of the photoexcited electrons are injected to the
semiconductor [44,45]. Thus we have to consider only the hot carriers within the mean-free
path (MFP) length (lp) distance from the interface for transferring to the semiconductor over
the Schottky barrier [45–47]. In this approach, photoexcited electrons are excited from the
energy states below the Fermi level (d-band) to the higher energy levels, and once they arrive
at the interface with an energy larger than the Schottky barrier height get injected to the GaN.
Experimental results show that MFP for electrons is strongly energy-dependent and minor
perturbations in energy level results significant changes [48–50]. In our analysis we assumed
lp = 25 nm for electrons 5 eV above Fermi level energy as reported in the literature [49].
Fig. 1. a) Schematic of the plasmonic photodetector composed of aluminum nanodisk clusters
deposited on GaN-sapphire substrates. The inset are the definitions for a nanodisk and a
heptamer cluster with geometrical parameters, b) a top-view of the photodetector with the
geometrical dimensions identification, c) the cross-sectional view of the hot electron
generation and transform under the aluminum-based nanodisk clusters at the GaN-metal
interface, d) schematic band diagram for the aluminum-GaN interface, showing the carrier
formation mechanism in the device.
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© 2016 OSA
Received 17 Mar 2016; revised 21 May 2016; accepted 25 May 2016; published 10 Jun 2016
13 Jun 2016 | Vol. 24, No. 12 | DOI:10.1364/OE.24.013665 | OPTICS EXPRESS 13670
Considering depicted band diagram for the Aluminum-GaN-Ni/Au structure, the electrical
simulation results verify formation of a Schottky barrier with the height of ΦB = 0.87 eV. In
this regime, the decayed plasmons result hot electrons that are arrived to the interface with
higher energies more than ~0.87 eV are able to pass the barrier and transit to reach the biased
electrode. In the examined device, hot electron generation rate (Ghe) by aluminum
nanoantennas at the Fano dip wavelength due to photoexcitation can be calculated using [27]:
Ghe = PCabs(λ)/ħωAh, where P is the incident light power (20 μW), Cabs(λ) is the absorption
cross section as a function of resonant wavelength, and Ah is the metallic nanoantenna area
and found to be Ghe = 5 × 1017 s−1. Then, the electron concentration can be calculated as [27]:
ne = τGhe/Ah, where τ is the relaxation time, which estimated to be 0.825 × 10−6 s (see
methods). The approximate electron concentration at the aluminum-GaN interface is defined
as ne = 1.04 × 1017 cm−2. Comparing hot electron generation rate and the associated
concentration in the proposed system with gold nonamers and gratings for the same purpose
[27,33,40], we realized a significant enhancement due to inherent and remarkable absorptive
behavior of aluminum across the UV spectrum as well as continuous electron energy
distribution [6].
For conventional semiconductor layers that have broadly been utilized for photocurrent
generation in designing plasmonic devices (e.g. silicon, cadmium selenide, etc.), the carrier
lifetime is in the range of ~100 μs. Long carrier lifetime prevents immediate recombination
and facilitates generation of large photocurrent. In contrast, the carrier lifetime and
recombination process for UV-compatible GaN around is in the range of a few nanoseconds.
In this regime, extremely short transition time (in the range of a few picoseconds) is required
to overcome the immediate recombination of the carriers. Using experimentally and
theoretically obtained values for the saturation velocity (Vsat) in n-type GaN [42,43], the
transition time (ttr) can be determined by: ttr = L/Vsat, where L is the pitch (see methods). For
the saturation velocity of 105 cm/s, with the pitch of 500 nm, the transition time is calculated
as 5 × 10−12 s (5 ps) which is extremely short (ttr<<τn) compared to the carrier lifetime in GaN
which is around 6.5 ns (see methods). Therefore, large number of electrons can be collected
before they recombine resulting photocurrent with gain. Additionally, for the uncovered parts
of the photodetector, the incident photons with the energies larger than the bandgap of GaN
can also be absorbed and generate electron-hole pairs. These electron-hole pairs in the
uncovered parts will be added to the hot electron pairs of clusters and contribute to the
photocurrent [see Fig. 1(d)]. The fast relaxation and transition times constitute the base for
very fast temporal response for the proposed devices. We estimated that the rise and fall times
are in the sub-microsecond range using the standard methods [51,52].
3. Results and discussion
Figure 2(a) represents the scattering and absorption cross-sections for the aluminum
heptamer. Clearly, an antisymmetric, narrow, and tunable plasmonic FR mode is induced
around λ~325 nm, which is between two distinct shoulders correlating with the bonding and
antibonding plasmon modes at λ~250 nm and λ~385 nm, respectively. These spectral
responses are calculated by selecting the geometrical dimensions of the molecular heptamer to
tune the UV detector frequency close to the FR dip frequency. Using previously discussed
geometries for the heptamer clusters, we set the oxide (Al2O3) layer thickness to tox = 2 nm. In
our analysis, we detected a distinct absorption extreme at the FR position with a couple of
absorption shoulders in the vicinity of the bonding and antibonding modes in both of the
examined heptamers with two different aluminum types [see Fig. 2(a)]. However, the
absorption at these wavelengths is not as high as the one at the FR dip position. When the
incident UV beam is resonant with the induced absorption window, the strong near-field
coupling of light and cluster gives rise to generation of hot electron-hole pairs. Figure 2(b)
exhibits the normalized E-field map of the plasmon resonance excitation and hybridization
corresponding to the plasmonic Fano dip mode wavelength in an isolated aluminum heptamer.
In the plotted snapshot, formation of the hotspots at the offset spots between nanodisks is
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© 2016 OSA
Received 17 Mar 2016; revised 21 May 2016; accepted 25 May 2016; published 10 Jun 2016
13 Jun 2016 | Vol. 24, No. 12 | DOI:10.1364/OE.24.013665 | OPTICS EXPRESS 13671
clearly visible. The full optical responses of simple and complex aluminum antennas such as
scattering profiles, and extinction spectra are discussed with details in previous studies
[28,53], hence, here we just considered the absorption properties of the proposed heptamer
assemblies.
Fig. 2. a) Scattering and absorption cross-sectional profiles for an aluminum heptamer antenna
with the oxide size of 2 nm around nanoparticles for Knight et al. aluminum, b) E-field map of
the plasmon resonance excitation and hybridization in the antenna, and formation of energetic
hotspots between proximal nanodisks are obvious.
Since the oxidation of aluminum in the subwavelength regime also affects the dielectric
permittivity of the particles, the thickness of the oxide layer has an important role in
hybridization of plasmon resonances in closely spaced nanoparticle assemblies. This effect is
due to the variations in both chemical and geometrical features of the oxidized nanoparticle
assembly. The chemical influence of the oxide layer is discussed in numerical setup part of
the Methods section. For the geometrical dimensions, increasing the oxide layer directly
increases the overall size of the cluster, hence, we expect noticeable variations in the spectral
response of the cluster. The effect of oxide layer on the scattering efficiency profile for an
isolated nanodisk with an oxide thickness of tox~2-3 nm have been examined, theoretically
and experimentally [18]. Besides, it is shown that increasing the thickness of the oxide layer
results in a red-shift of scattering dipolar resonance peak to the longer wavelengths (from UV
to the visible band) [53]. This red-shift also contains dramatic decrements in the scattering
efficiency peak. Therefore, we have to find an acceptable trade-off between scattering and
absorption efficiencies by finding appropriate dimensions for the oxide layer around
nanodisks in the heptamer assembly. To this end, we investigated the effect of oxide coverage
on the plasmon response of the proposed heptamer. The absorption spectra for an isolated
aluminum nanodisk heptamer on a glass host is plotted in Fig. 3(a). Increasing the thickness
of the oxide layer leads to enhancements in the absorption efficiency due to formation of
narrower FR minimum resulted by the suppression of the scattering extreme by antibonding
dark resonant mode [26,39,53]. This phenomenon includes a red-shift in the position of the
peaks to the longer wavelengths. The reason originates from the strong EM field hybridization
of plasmonic resonances in large size heptamer clusters. As a result, deeper Fano minimum in
the extinction profile can be induced, including a significant enhancement in the ratio of the
absorbed power. It is well-accepted that Fano dips are very sensitive to the minor alterations
in the structural properties of nanoparticle clusters [25,26]. Noticing in the absorption profile
in Fig. 3(a), for the heptamer assembly with thicker oxide layer, the peak of the absorption is
shifted to the visible spectrum that is not desired for our UV photodetector. In addition, for
the ideal case, for an entirely aluminum cluster without oxide layer (tox~0 nm), a noticeable
extreme is appeared at the short wavelengths around λ~280 nm, close to deep-UV band. For
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© 2016 OSA
Received 17 Mar 2016; revised 21 May 2016; accepted 25 May 2016; published 10 Jun 2016
13 Jun 2016 | Vol. 24, No. 12 | DOI:10.1364/OE.24.013665 | OPTICS EXPRESS 13672
tox = 2 nm and 4 nm two absorption extremes are obtained at λ~320 nm and 345 nm,
respectively, which have almost equal amplitude. This profile also shows the absorption
spectra for the UV detector without presence of nanoparticle clusters. Noticing in the
corresponding curve, due to the absence of metallic components and plasmonic effects, we
observe only the natural absorption of incoming UV beam by GaN substrate, which is reduced
dramatically after UV band λ>400 nm.
Fig. 3. The plasmon responses for the UV photodetector, a) the absorption spectra for heptamer
clusters deposited in GaN epilayer with variant oxide thickness and without metallic
heptamers, b) E-field enhancement diagram for the UV device with and without aluminum
clusters, c) numerically plotted absorption spectra for the oxide layer thickness as a function of
incident UV beam in a heptamer nanocluster.
Figure 3(b) represents numerically obtained absorption spectra for Al2O3 thickness
variations as a function of incident beam. The absorption ratio increased significantly
including a red-shift to the visible spectrum by increasing the thickness and the entire size of
the heptamer. The enhancement of the electric field |E| at the gap spots between central
nanodisks of the heptamer cluster is shown in Fig. 3(c). Significant enhancements of the
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© 2016 OSA
Received 17 Mar 2016; revised 21 May 2016; accepted 25 May 2016; published 10 Jun 2016
13 Jun 2016 | Vol. 24, No. 12 | DOI:10.1364/OE.24.013665 | OPTICS EXPRESS 13673
electric field is observed at the offset spots between aluminum nanodisks because of
hybridization and strong confinement of the plasmonic resonant modes. Comparing two types
of antennas with different oxide thicknesses, a slight difference in the enhancement is noticed.
This effect can be described by the effect of oxide layer thickness on the Bruggeman
dielectric function of the entire composite aluminum antenna [18], yielding different real and
imaginary permittivities at different wavelengths. Hence, modifying the oxide thickness can
lead to severe changes in the spectral response of the structure, as shown in the preceding
profiles. This plot also shows that no distinct shoulder is observed in the electric field profile
at the illumination spots for the absence of metallic heptamers and therefore the absence of
the plasmonic effects. For the case without the heptamers, a thin layer of electric field appears
at the surface of the GaN (with the magnitude of 1.15 × 105 V/cm). While for the plasmonic
case, a much larger electric field is monitored below the cluster due to hybridization of
plasmons (with the magnitude of 3.95 × 106 V/cm).
Fig. 4. a,b) carrier concentration for the detector system without heptamers with bias (5.0 V),
while the UV light is in OFF and ON states, respectively c,d) carrier concentration for the
system with heptamers with bias (5.0 V), while the UV light is in OFF and ON states,
respectively, e,f) E-field enhancement map for the device with and without clusters, while the
UV light is ON and bias is 0.0 V.
Figures 4(a) and 4(b) display the electron concentration for the proposed UV
photodetector device without presence of aluminum heptamers, while the bias is applied (5.0
V) and the light source is OFF and ON, respectively [these states are indicated inside the
corresponding profile in Fig. 4]. By applying both bias and UV beam, comparing to the
absence of the beam [Fig. 4(a)], a noticeable electron concentration is obtained under the
electrodes [Fig. 4(b)], resulting a photocurrent. On the other hand, by adding metallic
nanoscale heptamers between electrodes, we observed a dramatic enhancement in the
concentration of carriers resulted by the metallic clusters, as shown in Figs. 4(c) and 4(d),
respectively. To show the effect of plasmonic clusters on carrier generation, we used
aluminum nanoparticles with the oxide coverage of tox = 2 nm. In this regime, the generated
#261332
© 2016 OSA
Received 17 Mar 2016; revised 21 May 2016; accepted 25 May 2016; published 10 Jun 2016
13 Jun 2016 | Vol. 24, No. 12 | DOI:10.1364/OE.24.013665 | OPTICS EXPRESS 13674
large carrier concentration causes Schottky barrier lowering which could contribute to the
enhancement of the photocurrent [54,55]. The above comparison between non-plasmonic and
plasmonic UV detectors can be further illustrated by plotting corresponding E-field maps for
the excited electric field at the surface of the GaN, below the antennas, as shown in Figs. 4(e)
and 4(f), where the effect of plasmonic antennas in formation of a large field at the GaNaluminum interface is obvious.
Further, we study the electrical response of the plasmonic GaN photodetector. Figure 5(a)
illustrates the current-voltage (I-V) characteristic calculated for the peak of the absorption
profile along with the one for the device without plasmonic assemblies. In the calculated
response, the voltage is changed between 0 V to 5.0 V. For a bias of 5.0V the plasmonic
detector with the heptamer arrays with tox = 2 nm and 4 nm, yield the photocurrents of 88.56
μA and 90.25 μA, respectively. For the non-plasmonic case (absence of metallic nanoparticle
clusters), the photocurrent is found as 1.72 μA under 5.0 V bias. Dramatic enhancement in the
photocurrent due to the plasmonic heptamers is clearly visible. The inset of Fig. 5(a) shows
the extracted dark current as a function of the bias voltage, which reaches to 47.95 nA, 52.5
nA and 55.25 nA for the non-plasmonic case without the heptamers, tox = 2 nm and 4 nm,
respectively, under 5.0 V bias. Figure 5(b) shows the photocurrent as a function of the
polarization angle of the incoming light. In the plotted figure, hallow and solid circles
represent the calculated photocurrents for different incident polarization modes in two types
of heptamers with different oxide thicknesses. It is observed that the response of the proposed
structure is insensitive to the variations of the polarization angle of the incident EM energy
due to the inherent symmetry of the molecular heptamer cluster. In addition, besides the
ability to support pronounced Fano dip at the UV spectrum, it should be noted that
antisymmetric structures with more complex geometries cannot provide such a high and
polarization-independent absorption spectra [56].
Figure 6(a) represents the spectral response of the proposed UV detector with and without
aluminum antenna arrays, where the thickness of the oxide layer is changed and the bias is
kept as 5.0 V. For the plasmonic regime, the responsivity peaks are at λ~325 nm and λ~330
nm, and the cutoff wavelengths here is at λ~335 nm and λ~345 nm, for tox = 2 nm and 4 nm,
respectively. The peak responsivity (Rph) corresponds to the position of FR dip of heptamers.
At the peaks of the curve for the heptamer with tox = 2 nm and 4 nm, the responsivity of the
proposed plasmonic UV photodetector exceed 20.8 A/W and 21.9 A/W, respectively. This
outcome shows the superior responsivity of the examined UV detector in comparison to
analogous nanoscale devices [6,17]. On the other hand, for the non-plasmonic case, we
observed a conventional responsivity with a distinct shoulder at the UV spectra in the range of
λ~300 nm to 350 nm, where at the highest peak this parameter is measured as approximately
0.13 A/W [see the inset diagram in Fig. 6(a)]. Using the calculated responsivity data for the
proposed plasmonic UV photodetector, we extracted the external quantum efficiency (EQE)
for the structure in two different regimes by employing the conventional equation [11]: EQE
= hcR/eλ, where h is the Planck’s constant, c is the velocity of light, e is the electron charge, R
is the responsivity of the device, and λ is the wavelength of the incoming optical power. The
calculated EQE for the UV detector in non-plasmonic case is 64.5% while EQE is 8065% and
8116% for the devices with the presence of aluminum clusters with tox = 2 nm and 4 nm,
respectively. This dramatic enhancement in the responsivity and efficiency of the device
during transition from non-plasmonic to the plasmon regime originates from the generation of
hot carriers due to strong hybridization of plasmons at resonant frequencies. As the other
important parameter, we estimated the internal quantum efficiency (IQE) of the proposed UV
photodetector, which is the number of the produced charge carriers per incident photon and
can be calculated using the computed photocurrent profile as well as the incoming photon
energy flux on the subwavelength heptamer antennas.
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© 2016 OSA
Received 17 Mar 2016; revised 21 May 2016; accepted 25 May 2016; published 10 Jun 2016
13 Jun 2016 | Vol. 24, No. 12 | DOI:10.1364/OE.24.013665 | OPTICS EXPRESS 13675
Fig. 5. Electrical response for the UV photodetector, A) numerically achieved photocurrentvoltage (I-V) curves for two different oxide thicknesses of a heptamer cluster and without
heptamers. The inset is the dark current-voltage (I-V) curves for the non-plasmonic and
plasmonic UV photodetector for two different oxide thicknesses at λ = 325 nm and 335 nm for
tox = 2 nm, and 4 nm, respectively, B) polarization-independency of the generated photocurrent
(blue-spheres) of the device for the polarization angle variations of the incident UV beam.
Fig. 6. The spectral responses for the UV detector in both non-plasmonic and plasmonic
regimes, with variant Al2O3 thicknesses of heptamer clusters, A) responsivity profile under 5.0
V applied bias. Inset is the responsivity profile for the non-plasmonic regime, B) internal
quantum efficiencies (IQEs) for different regimes of the UV detector.
The absorbed power by the structure is given by Pabs = −0.5ω p E Im ( ε eff ) , |E| is the
2
amplitude of the incident electric field, and ε eff is the effective permittivity of the
semiconductor substrate and metallic heptamer that are contributed in the absorption
mechanism. Accordingly, the number of absorbed photon is given by:
−0.5ω p E Im ( ε eff
2
N ph =
)

(1)
On the other hand, using the equation above, we define the IQE [41]:
IQE =
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© 2016 OSA
Number of hot electrons/Sec
Total absorbed photons/Sec
(2)
Received 17 Mar 2016; revised 21 May 2016; accepted 25 May 2016; published 10 Jun 2016
13 Jun 2016 | Vol. 24, No. 12 | DOI:10.1364/OE.24.013665 | OPTICS EXPRESS 13676
Figure 6(b) exhibits numerically obtained IQE for the proposed device as a function of the
incident UV light, where the peaks for tox = 2 nm and 4 nm with the values of 38% and 40%,
respectively are induced at the FR dip positions. It is also worth noting that at these short
wavelengths, generation of hot electrons by metallic nanodisk heptamers has an undeniable
impact in having such a large photocurrent as well as a significant IQE. We also calculated
corresponding IQE for the UV detector without nanodisk clusters displayed in the profile with
dotted curve as 15.6%. A comparison of the performance of all the examined regimes for the
proposed UV photodetector shows that inducing the plasmonic effect via aluminum clusters
enhances the responsivity and photocurrent of the device with the expense of having a few
nano-amperes dark current. Additionally, comparing with the reported IQE for the recent
works,6,11 the proposed plasmonic UV photodetector shows significant efficiency. The
response of the plasmonic UV photodetector is comparable with the more complex designs
that have been proposed, such as coupling of plasmons between aluminum particles and zinc
oxide (ZnO) nanoparticles, or using multilayer substrate to enhance the electron-hole
confinement to improve generated photocurrent [6,57–63]. Finally, we estimated the
corresponding gain (Γph) of the investigated photodetector using [64,65]:
Γ ph =
R ph  hc 


IQE  qλ 
(3)
where q is the elementary charge, and c is the velocity of light. The corresponding gain is
found to be Γph = 2.1 × 102 for the aluminum cluster with the oxide thickness of tox = 2 nm.
4. Conclusion
In conclusion, a method is proposed to enhance the photoresponse of a GaN UV detector
using plasmon hybridization mechanism. Using aluminum-based symmetric heptamer clusters
deposited on a GaN active layer, we developed a structure with enhanced photocurrent due to
significant increase in the generation of hot electrons under the heptamer clusters resulted by
the decay of the plasmons on the Al disks. We also investigated the effect of oxide layer
thickness variations on the characteristics of the UV photodetector. Inducing a pronounced
Fano dip in the UV region, we obtained a significant absorption of incoming light power by
suppression of the scattering maxima. Calculating the important parameters for the proposed
photodetector, we proved its superior performance and quality in comparison to analogous
devices without plasmonic structures. Possessing high responsivity, quantum efficiency,
internal gain, and significant photocurrent across the UV spectrum make this structure as a
potential platform for designing and fabricating optoelectronic UV devices for several sensing
applications.
Methods
Definition of the optical response of the proposed device. To extract the optical properties
of the proposed UV photodetector, we investigated the excitation of plasmon resonant bright
and dark modes and their interference using the finite-difference time-domain (FDTD)
method (Lumerical FDTD). In the simulations, to determine the plasmonic responses,
following parameters were employed: The spatial cell sizes were set to dx = dy = dz = 0.8 nm,
and 48 perfectly matched layers (PMLs) were the boundaries. Additionally, simulation time
step was set to the 0.01 fs according to the Courant stability. The light source was a linear
plane wave electric source with a pulse length of 2.6533 fs, offset time of 7.5231 fs, and with
the illumination power of P = 20 μW/mm2. Using the ellipsometric data for Al thin films that
were obtained in recent works [18,19], we calculated the dielectric function for the
nanostructures composed of a thin coverage of oxide layer using Bruggeman dielectric model
for modified Drude model as following:
#261332
© 2016 OSA
Received 17 Mar 2016; revised 21 May 2016; accepted 25 May 2016; published 10 Jun 2016
13 Jun 2016 | Vol. 24, No. 12 | DOI:10.1364/OE.24.013665 | OPTICS EXPRESS 13677
ε (ω ) = ε ∞ −
ω p2
ω (ω + iΓ )
(4)
where ɛ∞ (~2-3) is the high frequency response, ωp (~13.9 eV) is the bulk plasmon frequency,
and Γ (~1.2 eV) is the damping constant [19]. It should be underlined that our FDTD
simulations, the empirical settings and conditions are employed to study the features and
plasmon responses of the proposed UV detector [6].
Definition of the electrical response of the proposed device. On the other hand, to
determine the electrical properties such as responsivity and dark current characteristic (I-V)
diagrams, we used fully physics-based Lumerical DEVICE. To this end, we applied finite
element mesh (FEM) generation method to solve the electrical properties of the plasmonic
UV detector numerically. For the GaN substrate with the presence of metallic electrodes and
clusters, the work function is taken as 5.85 eV by following Kim et al. [65] with the dc
permittivity of 9.7. Additionally, the bias voltage during the simulations is set as 0.0
V<VG<5.0 V. The majority carriers lifetime (electrons) and diffusion length were gotten from
experimentally determined works for n-type GaN epilayers on a sapphire substrate with
dislocation density of 108 cm−2, where the recombination lifetime for carriers was set to τn =
6.5 ns [66–68].
Saturation velocity (Vsat) calculation. As a general rule for III-V materials, the bulk
saturation velocity in high-field mobility can be defined by modeling as a function of lattice
temperature given by [69]:
Vsat (T ) =
Vsat (T = 300K)
T 
(1 − A) + 
A
 300K 
(5)
where Vsat is the saturation velocity at the lattice temperature (T = 300 K), and A represents
the temperature coefficient, showing the strong dependency of the various materials which are
included in the mechanism (Monte Carlo simulations model).
Relaxation time estimation. For the examined aluminum antennas, using the following
method [70,71]:
τ = ttr2 + ( RL C )
2
(6)
where RL is the aluminum antenna resistance, and C is the gate-interface capacitance that can
be defined as below:

Ah ε 0 ( ε GaN + 1) 
π

C=
L +W

8 L
 4 Ln  π + W








(7)
where ɛ0 is the permittivity of the vacuum., L and W are the pitch and electrode width,
respectively. Therefore, for L = 500 nm, W = 200 nm, RL = 250 kΩ, and ɛGaN = 9.7, the
capacitance is calculated as 330 pF. Using the computed values, the relaxation time is defined
as 0.825 × 10−6 s.
Acknowledgments
This work is supported by NSF CAREER program with the Award number: 0955013, and by
Army Research Laboratory (ARL) Multiscale Multidisciplinary Modeling of Electronic
Materials (MSME) Collaborative Research Alliance (CRA) (Grant No. W911NF-12-2-0023,
Program Manager: Dr. Meredith L. Reed). Raju Sinha gratefully acknowledges the financial
support provided through presidential fellowship by the University Graduate School (UGS) at
Florida International University.
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© 2016 OSA
Received 17 Mar 2016; revised 21 May 2016; accepted 25 May 2016; published 10 Jun 2016
13 Jun 2016 | Vol. 24, No. 12 | DOI:10.1364/OE.24.013665 | OPTICS EXPRESS 13678