Download Photoelectrochemical hydrogen generation with linear gradient Al

Photoelectrochemical hydrogen generation with
linear gradient Al composition dodecagon faceted
AlGaN/n-GaN electrode
W. C. Lai,1* M. H. Ma,1 B. K. Lin,1 B. H. Hsieh,1 Y. R. Wu,2 and J. K. Sheu1
1
Institute of Electro-Optical Science and Engineering, National Cheng Kung University, Tainan City 70101, Taiwan
Institute of Photonics and Optoelectronics and Department of Electrical Engineering, National Taiwan University,
10617, Taipei, Taiwan
*
[email protected]
2
Abstract: We demonstrated photoelectrochemical cells (PECs) with
dodecagon faceted AlGaN/n-GaN heterostructure electrode for H2
generation, where the AlGaN/n-GaN heterostructure has a linear gradient Al
composition (LGAC). The separation efficiency of the photo-generated
electron–hole pairs in the electrode performs a key function in the H2
generation efficiency of PEC cells. The linear gradient Al composition,
AlGaN, could create more internal field and light absorption because of the
linear graded band gap. Therefore, the zero-bias photocurrent density of PEC
cells with dodecagon facet LGAC AlGaN/n-GaN heterostructure electrode is
around 5.9 times larger than that of dodecagon faceted n-GaN electrode.
©2014 Optical Society of America
OCIS codes: (230.0230)
Electro-optical devices.
Optical
devices;
(230.0250)
Optoelectronics;
(230.2090)
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1. Introduction
Solar hydrogen (H2) generation via water splitting poses an advantageous challenge for
developing clean eco-friendly energy systems [1]. The photoelectrochemical method of
generating H2 is one of the most desirable renewable routes to address this challenge [2].
Fujishima et al. have reported water splitting into H2 and O2 with TiO2 electrode under
ultraviolet (UV) light illumination [3]. Since then, several metal-oxide semiconductors have
been widely investigated [4]. Besides metal-oxide semiconductors, various semiconductors
have been demonstrated as water splitting photoelectrochemical cells (PECs) photoelectrode or
photocatalyst, such as GaN [5–7], MoS2/Si [8], GaP [9], and CdS [10]. Semiconductors for use
in PECs H2 generation should possess a conduction band potential that is less than that of the
cathode reduction half-reaction, and its valence band-edge potential must be higher than that of
the anode oxidation half-reaction [11]. GaN-based materials have suitable band gap that fits the
criteria for photoelectrochemical H2 generation [12,13]. In addition, III-nitride materials are
#223682 - $15.00 USD Received 23 Sep 2014; revised 30 Oct 2014; accepted 30 Oct 2014; published 13 Nov 2014
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potentially resistant to aqueous solutions [14]. Tuning the band gap of GaN-based material
from 3.4 eV to 0.7 eV by adding indium content would result in the PECs fitting the solar
spectrum for enhancing light absorption. Moreover, Ono et al. have also reported that n-GaN
electrode PECs split water without extra bias [15]. However, the photocurrent density is not
sufficiently high to produce adequate hydrogen gas within a short time. Increasing the area of
GaN surface would be one method to enhance the photocurrent of PECs for water splitting.
Waki et al. have reported on GaN area increase by introducing a select area GaN growth
technique [16]. The group created faceted GaN strips on a patterned n-GaN template to increase
the area of GaN and, consequently, the photocurrent of PECs. However, Liu et al. have reported
that PECs with a naturally textured rough n-GaN electrode did not exhibit photo-current
enhancement compared with a smooth surface n-GaN electrode under illumination [17]. This
result should be attributed to the highly defective low-temperature growth of textured n-GaN
electrode. Moreover, Tu et al. have reported that the photocurrent of PECs with p-GaN
electrode could be enhanced by introducing Au nanoparticles on the p-GaN electrode, because
the Au nano-particles could bend down the energy band of p-GaN larger than p-GaN in direct
contact with the solution [18]. Therefore, the band bending of the GaN electrode would also
affect the photocurrent of the PECs. In this study, we aimed to demonstrate a faceted
AlGaN/n-GaN heterostructure with linear gradient Al composition (LGAC) AlGaN layer as the
electrode of PECs. The optoelectrical characteristics of the fabricated PEC cell with flat n-GaN,
faceted n-GaN, and facet LGAC AlGaN/n-GaN heterostructure electrode will be discussed.
2. Experiments
All samples were grown on a (0001) sapphire substrate by vertical MOCVD. In n-GaN and
heterostructure AlGaN/n-GaN epitaxy, trimethylgallium (TMGa), trimethylaluminum (TMAl),
and ammonia are the source materials for Ga, Al, and N, respectively. Silane is an n-type
doping source of the III-nitride material. A 25 nm thick, low-temperature GaN nucleation layer
was first deposited on the substrate at 520 °C after a standard H2 etch-back process at 1060 °C.
A 1.5 μm thick, high-temperature undoped GaN (u-GaN) was deposited at 1050 °C after the
NH3 / H2 etch-back process. A 2 μm thick, Si-doped GaN layer (n-GaN) with doping
concentration of 1 × 1019cm−3 was then grown on the u-GaN layer. The samples were then
removed from reactor and then a 250 nm thick SiO2 deposited on n-GaN. Standard
photolithography was then used to define a circular disk pattern of 3 μm diameter and 3 μm
spacing between two neighboring disks. An inductively coupled (ICP) plasma etcher was used
to etch the exposed SiO2 regions to form a SiO2-pillar mask. Subsequently, a faceted n-GaN
layer was grown on the n-GaN template with the SiO2 pillar mask where the faceted n-GaN has
thickness and doping concentration of 5 μm and 1 × 1019cm−3, respectively. A 100 nm thick
undoped AlGaN layer with and without gradient Al composition was grown on the faceted
n-GaN layer to form the faceted AlGaN/n-GaN heterostructure. We varied the TMAl flow rate
linearly from 40 sccm to 5 sccm to create AlGaN layers with a linear gradient Al composition
from 20% at the AlGaN/GaN interface to 2.5% on the surface. And we keep the TMAl flow rate
of 40 sccm to create constant 20% Al composition of AlGaN layer. We performed scanning
electron microscopy (SEM) to observe the surface morphology of samples. Transmission
electron microscope (TEM) was performed to index dodecagonal planes of faceted n-GaN
structure.
For ohmic contact of the faceted AlGaN/n-GaN heterostructure, a portion of the AlGaN
layer was removed via ICP dry etching to expose the n-GaN layer. A bilayer metal Cr (50 nm)
/ Au (80 nm) was then deposited on the n-GaN and exposed n-GaN to form an ohmic contact.
The PECs with flat n-GaN electrode was named as PECs I for comparison. The PECs with
faceted n-GaN, constant Al composition (CAC) AlGaN/n-GaN, and LGAC AlGaN/n-GaN
electrode were named as PECs II, III, and VI, respectively. A potentiostat
(Autolab-PGSTAT128N) was employed to supply the external bias. The current density was
measured to evaluate the electrical properties of the PECs. A 300 W Xe lamp was utilized as
#223682 - $15.00 USD Received 23 Sep 2014; revised 30 Oct 2014; accepted 30 Oct 2014; published 13 Nov 2014
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15 December 2014 | Vol. 22, No. S7 | DOI:10.1364/OE.22.0A1853 | OPTICS EXPRESS A1855
light source, and 1 mol/L NaCl was used as electrolyte at room temperature. The light
illumination power at position of PECs is 2.2 W/cm2. An Ag/AgCl reference electrode and a
platinum (Pt) wire counter electrode were used to measure the potential. The bias voltage was
applied to the working electrode with respect to the Ag/AgCl reference electrode. Hydrogen
gas was generated at the Pt wire counter electrode.
3. Results and discussions
Figure 1 shows surface morphology SEM images of faceted n-GaN, CAC AlGaN/n-GaN, and
LGAC AlGaN/GaN electrode. The n-GaN layer with facets presented epitaxy lateral
overgrowth (ELO) on top of the SiO2 pillars at standard growth conditions, as shown in Fig. 1.
Therefore, faceted dodecagon V-shaped pits were created on top of the SiO2 pillars. As the
growth time elapsed, the bottom of the dodecagon V-shaped pits decreased because of the ELO
process. However, the growth rate of dodecagon V-shaped pit facet surfaces is considerably
less than that of the (0001) surface. Therefore, the facet surfaces of dodecagon V-shaped pits
would eventually coalesce with the facet surfaces of the neighboring V-shaped pits, and the
(0001) surface disappeared. Finally, we could obtain a dodecagon faceted n-GaN structure
layer having the average size of the bottom and top dodecagon V-shaped pits of 1.02 and 6.2
μm, respectively. Moreover, the dodecagon faceted n-GaN structure layer was consisted with
rough and smooth facets as indicated in Fig. 1(a). We have done the TEM on the sample of
faceted GaN structure to index the faceted GaN planes. Two TEM samples were prepared by
Focus Ion Beam (FIB) to cut along line 1 and line 2 from the dodecagon faceted n-GaN
structure as indicated by the Fig. 1(a). TEM analysis was then performed in a FEI Tecnai F20
TEM operated at 200 kV. TEM bright field (BF) images and their corresponding selected area
diffraction patterns (SADPs) were recorded. The rotation angle between images and SADPs
was calculated to be 90 clockwise (SADP fixed) from the over-focused shadow image which
contains the image inside the transmitted beam. The rough surface of the dodecagon faceted
n-GaN structure is identified to be {1 1 −2 2} by a pair of TEM BF cross-section image cut
along the line 1 presented in Fig. 1(a) and its corresponding SADP, as indicated in Fig. 1 (d).
The smooth surface of the dodecagon faceted n-GaN structure is identified to be {-1 1 0 1} by a
pair of TEM BF cross-section image cut along the line 2 presented in Fig. 1(a) and its
corresponding SADP, as indicated in Fig. 1(e). Both {1 1 −2 2} and {-1 1 0 1} should belong to
the semi-polar surface. The {1 1 −2 2} facets have a rougher surface than the {-1 1 0 1}. To
date, the details for the formation of rough {1 1 −2 2} surfaces are not yet understood and need
to be investigated. The following AlGaN layer growth for the CAC AlGaN/GaN and LGAC
AlGaN/GaN heterostructures should not change the dodecagon facet shape of the faceted
n-GaN layer.
#223682 - $15.00 USD Received 23 Sep 2014; revised 30 Oct 2014; accepted 30 Oct 2014; published 13 Nov 2014
(C) 2014 OSA
15 December 2014 | Vol. 22, No. S7 | DOI:10.1364/OE.22.0A1853 | OPTICS EXPRESS A1856
Fig. 1. Surface morphology SEM images of (a) faceted n-GaN, (b) CAC AlGaN/n-GaN, and (c)
LGAC AlGaN/GaN electrode (d) The TEM cross-section image cut on the rough surface of the
dodecagon faceted n-GaN structure and its corresponding SADP with z = [1-100]. (e) The TEM
cross-section image cut on the smooth surface of the dodecagon faceted n-GaN structure and its
corresponding SADP with z = [11–20].
Figure 2 demonstrates the dark and illuminated current densities of PECs I, II, III, and VI as
a function of external bias voltage (Vext). All PECs show similar dark current density (Jd) – Vext
characteristics and possess cathodic currents associated with proton reduction onset potential of
around –0.8 V. Under illumination and at a photocurrent density of 2 mA/cm2, the voltage of
PECs II (–0.035 V) is larger than that of PECs I (–0.27 V). In addition, the zero-bias voltage
photocurrent density of PEC II (2.34 mA/cm2) is less than that of PECs I (6.06 mA/cm2).
However, the photocurrent of PECs II is larger than PECs I when the applied Vext is larger than
0.6 V. The PECs with dodecagon faceted n-GaN electrode have the least and the largest zero
bias photocurrent density and voltage at 2 mA/cm2, respectively. Lin et al. have reported that
the polarization direction of GaN would affect the energy band bending of the GaN and
electrolytes junction [19]. Ga-polar n-GaN with polarization pointing to GaN would have
larger band bending than the N-polar n-GaN with polarization pointing out off the GaN surface.
This trend imparts Ga-polar n-GaN with more effective carrier separation and less carrier
recombination than the N-polar n-GaN. Moreover, Jung et al. have reported that the surface
Fermi level of semipolar (11-22) n-GaN/metal contact pinned around 0.82 eV below the
conduction band energy [20]. The reduced Schottky barrier height of the semipolar
n-GaN/metal contact would suppress the band bending of n-GaN. The dodecagon faceted
n-GaN electrode consisted of semipolar surface would have less polarization than the smooth
surface n-GaN electrode. In addition, the dodecagon faceted n-GaN electrode/electrolyte
interface would be similar to the semipolar n-GaN/metal contact, which reduces the band
bending of the dodecagon faceted n-GaN electrode and the electrolyte junction because of the
polarization reduction and surface Fermi level pinning effect. The carrier recombination
process then increases. Therefore, PECs with dodecagon faceted n-GaN electrode have lower
zero bias photocurrent density than PECs with smooth surface n-GaN electrode. Fuji revealed
the PECs properties on (11-20) nonpolar and (11-22) semipolar n-GaN layer [21], where PECs
with (11-20) nonpolar and (11-22) semipolar n-GaN layer electrodes have lower zero bias
photocurrent density than that with (0001) polar n-GaN electrode.
#223682 - $15.00 USD Received 23 Sep 2014; revised 30 Oct 2014; accepted 30 Oct 2014; published 13 Nov 2014
(C) 2014 OSA
15 December 2014 | Vol. 22, No. S7 | DOI:10.1364/OE.22.0A1853 | OPTICS EXPRESS A1857
Fig. 2. (a) Dark and (b) illuminated current densities of PECs I, II, III, and IV as a function of
Potential vs. Ag/AgCl (Vext).
However, the photocurrent density (Jp)-Vext characteristics of PECs with dodecagon faceted
n-GaN electrode could be improved by adding an AlGaN layer. PECs III presents a voltage at 2
mA/cm2 and zero bias photocurrent of –0.32 V and 8.62 mA/cm2, respectively. PECs III shows
more negative voltage at 2 mA/cm2 than of PECs II and I. Furthermore, the zero-bias
photocurrent density of PECs III is around 3.7 and 1.5 times larger than those of PECs II and I,
respectively. In addition, the photocurrent density of PECs III is larger than those of PECs I and
II for an applied bias ranging from –0.6 V to 2.0 V. Fujii et al. [22] have reported that the energy
differences between the conduction band-edge energy and the energy of the H2 evolving
half-reaction increases with Al composition. The electrons in the conduction band of
AlxGa1−xN are expected to have higher energy for H2 gas production than those in GaN. Thus,
easier H2 gas evolution at the counter electrode is expected when AlxGa1−xN is used than when
GaN is used with the same number of electrons. Moreover, the group also revealed that the
onset voltage of PECs with AlGaN electrode would have negative voltage with increasing Al
composition of AlGaN. Although the larger band gap of AlGaN is directly linked to lower
photo absorption, the internal field of AlGaN/GaN heterostructure from the piezoelectric
polarization effect would enhance the generated electron–hole pair separation and suppress the
carrier recombination process in the AlGaN layer. Therefore, the PECs with dodecagon faceted
AlGaN/n-GaN electrode (PECs III) has considerably better zero-bias photocurrent density and
more negative voltage at 2 mA/cm2 than the PECs with flat n-GaN and dodecagon facet n-GaN
electrodes.
The Jp-Vext characteristic of PECs with dodecagon facets AlGaN/n-GaN heterostructure
electrode could be further improved by band engineering of the AlGaN layer with linear
gradient Al composition. The PECs IV has the largest zero-bias photocurrent density of 13.77
mA/cm2 and has almost the same voltage of −0.36 V at 2 mA/cm2 as PECs III. The zero-bias
photocurrent density of PECs IV is around 1.6, 5.9, and 2.3 times larger than those of PECs III,
II, and I, respectively. Therefore, the PECs IV should have larger amount gas generation than
the PECs I at zero-bias as indicated in Fig. 3. Figure 4 (a) shows the numerical simulated
schematic band diagrams of electrolyte and dodecagon faceted n-GaN, dodecagon faceted
CAC AlGaN/n-GaN heterostructure, and dodecagon faceted LGAC AlGaN/n-GaN
heterostructure electrode interface at zero-bias. And we have taken the derivation of conduction
band (Ec) and valance band (Ev) to find the electric field profiles at conduction band and
valence band of all PECs samples as shown in Fig. 4(b). The details of the simulation software
have been reported by the Wu et al. and Li et al. [23,24]. The Fermi level is zero in the whole
junction at zero bias. The band-offset ratio ( ΔE c / ΔEg ) is 63%. The basic parameters
especially the setting of polarization value can be found in report of Ambacher et al. [25].
#223682 - $15.00 USD Received 23 Sep 2014; revised 30 Oct 2014; accepted 30 Oct 2014; published 13 Nov 2014
(C) 2014 OSA
15 December 2014 | Vol. 22, No. S7 | DOI:10.1364/OE.22.0A1853 | OPTICS EXPRESS A1858
According to Ref. 25, since GaN is the substrate, the polarization of different x% Al
composition is the function of x%. For the linear gradual Al composition AlGaN layer case, we
divided 100nm thick AlGaN layer into 33 sections where each section’s Al composition
gradually changes from 20% to 5% in the growth direction. As shown in the insect of Fig. 3,
each section of Ec and Ev of LGAC AlGaN has small band offsets ΔE c and ΔE v due to the
composition changes. The polarization charge difference induced at each interface can be
obtained by taking the divergence of P(x) along the growth direction, i.e. ∇P(x) = ρ pol . In our
setting, the equivalent negative polarization charge density is induced as Simon et al. [26] and
Zhang et al. [27–30] suggested. And the polarization charge at the semipolar plane is changed
as well due the induced of shear strain and projection angle [31,32]. Two {-1 1 0 1}, {1 1 −2 2}
planes have close polarization charge because of similar tilt angle of 60 degree as shown in Fig.
1 (d) and (e). The flat n-GaN electrode/electrolyte interface shows a rapid band bending upward
with depletion thickness of 27 nm and the largest electric field. The dodecagon faceted n-GaN
electrode/electrolyte interface has less upward band bending and electric field than that of flat
n-GaN electrode/electrolyte interface because of polarization reduction and surface Fermi level
pinning effect. The dodecagon faceted CAC AlGaN/n-GaN heterostructure
electrode/electrolyte interface demonstrated the whole 100 nm thick CAC AlGaN layer having
linear band bending up and constant Ec and Ev electric field because of the assistance of the
internal spontaneous and piezoelectric polarization field. Therefore, PECs with dodecagon
faceted CAC AlGaN/GaN electrode having much thicker thickness of the constant Ec and Ev
electric field region than that with n-GaN and dodecagon faceted n-GaN electrode would
improve the light absorption and separation efficiency of the photo-generated electron–hole
pair in the AlGaN layer, which would consequently improve the zero-bias current density and
have more negative voltage at 2 mA/cm2 of PECs III.
Fig. 3. Gas generation picture of the PECs I and IV under illumination. Left side of picture is
PECs IV and right side of picture is PECs I. Both PECs are generating H2 gas at the Pt cathode
(see Media 1).
Unlike the linear upward bending band of CAC AlGaN/GaN heterostructure electrode of
PECs, the LGAC AlGaN/GaN hetero-structure electrode PECs have a curve-bending band as
shown in Fig. 3 (a). The LGAC AlGaN/GaN heterostructure electrode shows more rapid band
bending up at the interface of the AlGaN/GaN than CAC AlGaN/GaN heterostructure
electrode. In addition, the band bending of LGAC AlGaN/GaN heterostructure electrode slows
down and flattens near the electrode surface. Therefore, the Ec and Ev electric field is not
constant in the whole LGAC AlGaN layer but gradually reduces from AlGaN/GaN interface
toward the surface. Since small band offsets ΔE c and ΔE v due to the composition changes in
each section of LGAC AlGaN layer, there were Ec and Ev electric field pikes at each section
junction after taking the derivation of Ec and Ev. However, the pikes electric field directions at
#223682 - $15.00 USD Received 23 Sep 2014; revised 30 Oct 2014; accepted 30 Oct 2014; published 13 Nov 2014
(C) 2014 OSA
15 December 2014 | Vol. 22, No. S7 | DOI:10.1364/OE.22.0A1853 | OPTICS EXPRESS A1859
conduction band and valence band are different. The overall electric field of LGAC
AlGaN/GaN heterostructure is larger than that of CAC AlGaN/GaN heterostructure in the
region between 1000 nm and 1045 nm. For the conduction band, the overall Ec electric field is
smaller including those Ec electric field pikes due to the band offset. However, because the
electron mobility is much higher than holes, the diffusion length is much longer to avoid
recombination. The advantage part is in the valence band, the Ev electric field pikes direction
(induced by the valence band offset) is the same as the depletion field direction. As mentioned,
the hole mobility is much lower than electron. Therefore, the hole needs a larger electric field to
be pulled away from the AlGaN/GaN interface especially near the AlGaN/GaN interface where
a large two dimensional electron gas is there. In the LGAC case, the hole is much easier to
move away due to the larger electric field near AlGaN/GaN interface and also those additional
Ev electric field pikes. Therefore, the PECs with LGAC AlGaN/GaN hetero-structure electrode
should have better photo-generated electron–hole pair separation efficiency than that with CAC
AlGaN/GaN heterostructure electrode. Moreover, the LGAC AlGaN layer might have better
light absorption than the AlGaN layer because of the extra light absorption wavelength from
334 nm to 362 nm. Therefore, PECs IV has the largest zero-bias photocurrent density and has
almost the same voltage at 2 mA/cm2as PECs III.
Fig. 4. Schematic (a) band diagrams, (b) Ec and Ev electric field of electrolyte and dodecagon
faceted n-GaN, dodecagon faceted CAC AlGaN/n-GaN heterostructure, and dodecagon faceted
LGAC AlGaN/n-GaN heterostructure electrodes interface.
Figure 5 shows the surface morphology SEM images of PECs I to IV after Jp-Vext
characteristics measurement. High-density tiny pits were formed on the surface of PEC cells
with flat n-GaN electrode (PECs I) after photoelectrochemical etching. In addition, the
photoelectrochemical-etched facets n-GaN electrode also show high density tiny pits on all
facets of {-1 1 0 1}, {1 1 −2 2}, and (0001) surface. The {-1 1 0 1} surfaces of the etched
dodecagon faceted n-GaN electrode present similar surface morphology to etched flat n-GaN
electrode. However, the {1 1 −2 2} surfaces of etched dodecagon faceted n-GaN electrode
seem to present a rougher surface than {-1 1 0 1}. Rougher photoelectrochemical etched {1 1
−2 2} facets should be attributed to the rough surface of the as grown {1 1 −2 2} facets.
Furthermore, adding AlGaN layer on the dodecagon faceted n-GaN enlarged the
photoelectrochemical etching pits size of the facet surface. The enlarged pits size of dodecagon
faceted AlGaN/n-GaN heterostructure indicates high photoelectrochemical reaction rate on the
facet surface. This finding also implies that PECs III has a larger photo current density with
applied bias than PECs II and I. Moreover, the dodecagon faceted AlGaN/n-GaN
heterostructure electrode also shows rougher PEC etched surfaces of {1 1 −2 2} than that of {-1
1 0 1}. Finally, the dodecagon faceted LGAC AlGaN/n-GaN hetero-structure electrode PECs
have the roughest etched facet surfaces. In addition, the boundary of the etched {-1 1 0 1} and
{1 1 −2 2} faceted LGAC AlGaN/n-GaN hetero-structure electrode was barely recognized. The
#223682 - $15.00 USD Received 23 Sep 2014; revised 30 Oct 2014; accepted 30 Oct 2014; published 13 Nov 2014
(C) 2014 OSA
15 December 2014 | Vol. 22, No. S7 | DOI:10.1364/OE.22.0A1853 | OPTICS EXPRESS A1860
roughest dodecagon facet surface of the LGAC AlGaN/n-GaN heterostructure electrode
indicates that PECs IV underwent the most severe photoelectrochemical reaction. Therefore,
PECs IV should have the largest photocurrent density with applied bias.
Fig. 5. Surface morphology SEM images of (a) PECs I, (b) PECs II, (c) PECs III, and (d) PECs
IV after the Jp-Vext characteristics measurement.
4. Conclusions
In summary, we demonstrated PECs with dodecagon faceted n-GaN, CAC AlGaN/GaN
hetero-structure and LGAC AlGaN/n-GaN heterostructure electrode for H2 generation. The
separation efficiency of the photo-generated electron–hole pairs in the electrode performs a key
function for H2 generation efficiency of PECs. Dodecagon faceted n-GaN electrode has less
zero-bias photocurrent density than flat n-GaN electrode because of the reduced polarization
field and surface Fermi level pinning effect to reduce photo-generated electron–hole pair
separation efficiency. Adding an AlGaN layer on the dodecagon faceted n-GaN electrode
would help improve the zero-bias photocurrent density by 3.7 times because of the assistance of
internal spontaneous and piezoelectric polarization fields. Furthermore, the LGAC AlGaN
could create more internal field and light absorption because of the linear graded band gap.
Therefore, zero-bias photocurrent density of PECs with dodecagon faceted LGAC
AlGaN/n-GaN heterostructure electrode is around 5.9 times larger than that with dodecagon
faceted n-GaN electrode.
Acknowledgments
The authors are grateful to the Ministry of Science and Technology of Taiwan for their
financial
support
under
Contract
Nos.
NSC101-2221-E-006-066-MY3
and
102-3113-P-009-007-CC2. This research was also made possible through the Advanced
Optoelectronic Technology Center, National Cheng Kung University as a project of the
Ministry of Education of Taiwan and through the financial support of the Bureau of Energy,
Ministry of Economic Affairs of Taiwan, under Contract No. 102-E0603. We would like to
especially thank Dr. J. S. Bow for performing the TEM observation on indexing the planes of
faceted n-GaN structure.
#223682 - $15.00 USD Received 23 Sep 2014; revised 30 Oct 2014; accepted 30 Oct 2014; published 13 Nov 2014
(C) 2014 OSA
15 December 2014 | Vol. 22, No. S7 | DOI:10.1364/OE.22.0A1853 | OPTICS EXPRESS A1861