Photoelectrochemical activity on Ga-polar and
N-polar GaN surfaces for energy conversion
Yan-Gu Lin,1,2,3 Yu-Kuei Hsu,4 Antonio M. Basilio,1,5,6 Yit-Tsong Chen,1,5 Kuei-Hsien
Chen,1,2,7 and Li-Chyong Chen2,*
1
Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan
Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan
3
Chemical Sciences and Engineering Division, Argonne National Laboratory, IL 60439, USA
4
Department of Opto-Electronic Engineering, National Dong Hwa University, Hualien, 97401, Taiwan
5
Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan
6
Department of Chemistry, Ateneo de Manila University Quezon City, 1108, Philippines
7
[email protected]
*
[email protected]
2
Abstract: Hydrogen generation through direct photoelectrolysis of water
was studied using photoelectrochemical cells made of different facets of
free-standing polar GaN system. To build the fundamental understanding at
the differences of surface photochemistry afforded by the GaN {0001} and
{000 − 1} polar surfaces, we correlated the relationship between the surface
structure and photoelectrochemical performance on the different polar
facets. The photoelectrochemical measurements clearly revealed that the
Ga-polar surface had a more negative onset potential relative to the N-polar
surface due to the much negative flat-band potential. At more positive
applied voltages, however, the N-polar surface yielded much higher
photocurrent with conversion efficiency of 0.61% compared to that of
0.55% by using the Ga-polar surface. The reason could be attributed to the
variation in the band structure of the different polar facets via MottSchottky analyses. Based on this work, understanding the facet effect on
photoelectrochemical activity can provide a blueprint for the design of
materials in solar hydrogen applications.
©2013 Optical Society of America
OCIS codes: (260.5130) Photochemistry; (350.6050) Solar energy; (160.6000) Semiconductor
materials.
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1. Introduction
In the course of developing new strategies to convert and store the energy from sunlight to
chemical fuels, solar water splitting has recently attracted a lot of attention [1,2]. Over the
past 4 decades, the development of photocatalysis has primarily focused upon large band gap
metal oxides involving ions with filled or empty d-shell bonding configurations [3–5] and
oxynitrides [6]. Recently, the use of group III nitride semiconductors for water splitting has
attracted considerable attention [7,8]. Due to the more negative potential of the nitrogen 2p
orbital, compared to that of the oxygen 2p orbital, metal nitrides often possess a narrow band
gap and can potentially encompass nearly the entire solar spectrum. Moreover, the inherent
chemical stability of nitrides also favors their use in the harsh photocatalysis reaction
environment [9]. Particularly, GaN demonstrates considerable resistance to corrosion in many
aqueous solutions [10] and its band edge potentials are situated in positions that allow for
zero-bias hydrogen generation [11]. Indeed, recent ab initio molecular dynamic simulations
further showed that the overall water oxidation reaction at GaN surfaces can be energetically
driven by photogenerated holes [12].
Considering that photocatalytic reactions take place on the surfaces of semiconductors, the
exposed crystal facets play a critical role in determining the photocatalytic reactivity and
efficiency. For example, Hsu et al. reported the polar facets of ZnO yielded three times
increase in conversion efficiency than nonpolar-facets because of the high surface energy,
spontaneous polarization and negative flat-band potential of a polar-orientated surface [13].
Similarly, the polar GaN surfaces have also been proposed as being sufficiently accessible for
photoelectrochemistry compared to the semipolar and nonpolar facets [14]. Although the
polarity-dependent photocatalytic activities in the GaN case has been intensively studied, the
differences of surface photochemistry between Ga-polar and N-polar GaN is still not
understood. For the wurtzite structure of GaN, there are two typical faces of the polar surface,
which give rise to a microscopic spontaneous polarization as shown in Fig. 1(a) [15]. The
presence of the spontaneous polarization may induce the adsorption of positive or negative
ions from electrolyte at polar surface, causing a partial compensating of polarization charges
and altering the charge-transfer behavior. Owing to their peculiar nature, in this study,
#190238 - $15.00 USD Received 8 May 2013; revised 14 Sep 2013; accepted 10 Oct 2013; published 12 Nov 2013
(C) 2014 OSA
13 January 2014 | Vol. 22, No. S1 | DOI:10.1364/OE.22.000A21 | OPTICS EXPRESS A22
comparative studies to correlate the surface structure and photoelectrochemical performance
on Ga-polar {0001} and N-polar {000 − 1} surface are systematically investigated.
2. Experimental
A free-standing GaN film was obtained commercially from Everlight Electronics Co., Ltd.
(Taipei, Taiwan). The thickness of both GaN films is approximately 100 µm. Our Ga-face
and N-face free-standing GaN films are all commercial bulk products, which was fabricated
through hydride vapor phase epitaxy (HVPE) growth method. The HVPE system used in this
study was the horizontal home-built quartz reactor with the rotating quartz susceptor. GaCl
was supplied vertically, just over the surface of the susceptor. The growth temperature of
1030-1050°C, temperature of GaCl synthesis of 870°C, HCl flow in the range of 18-24
mls/min diluted in 500 mls/min of N2, NH3 flow of 1200 mls/min, and 3000 mls/min of N2 as
a carrier gas were applied for runs of 8 to 15h. The growth rates observed for this geometry
and the set of conditions varied from 100 to 200 μm /h. The different polarities of the two
surfaces of the bulk sample were distinguished through the well-known reaction of the Npolar surface in KOH solutions [10]. X-ray diffraction measurements confirm the polar face,
showing the (002) and (004) peaks of the wurtzite structure (Fig. 1b).
Hall measurements indicate that free-standing GaN has a carrier density of about 2.4 x
1018 cm−3, whether measured from the Ga-polar or N-polar faces. The mobility was measured
as 231 cm2V−1s−1 at the Ga-polar surface and 209 cm2V−1s−1 at the N-polar surface. Two 0.5 x
0.7 cm2 samples (Ga-polar and N-polar) were cut from the free standing GaN film for our
study. In order to obtain the efficient current collection during the solar hydrogen gas
production process, two strips of 0.1x1 cm of Ti(100 nm)/Au(20nm) were deposited at the
edge of the samples. The electrodes were prepared by connecting Cu wires to the metal
contacts with Ag glue. Afterwards, the metal area was covered with epoxy. The
photoelectrochemical measurement was carried out in 1 M HCl under a 150-Watt Xe lamp
light source at an intensity of 100 mW/cm2. A platinum counter electrode and a Ag/AgCl
reference electrode under a Solartron 1470 E multichannel system were used for the
measurements. The impedance spectra were measured from frequency range of 10 to 20 kHz
in 1 M HCl solution with an applied potential ranging from 0.2 to 1.4 V.
#190238 - $15.00 USD Received 8 May 2013; revised 14 Sep 2013; accepted 10 Oct 2013; published 12 Nov 2013
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13 January 2014 | Vol. 22, No. S1 | DOI:10.1364/OE.22.000A21 | OPTICS EXPRESS A23
Fig. 1. (a) Scheme of the wurtzite crystallographic cell of GaN for Ga-polar and N-polar cdirection with the corresponding polarization field. (b) X-Ray Diffraction pattern of GaN for
Ga-polar and N-polar.
3. Results and discussion
Figure 2(a) shows the photocurrent density versus potential curves of Ga-polar and N-polar of
GaN photoelectrodes. Compared to the N-polar electrode, the Ga-polar electrode has a more
negative onset potential, which is highly desirable for n-type semiconductors since the
hydrogen gas generation is observed even at a potential that is much lower than the standard
oxidation potential of water [16]. Previous studies [17,18] show that under the experimental
conditions, the anodic current is dominated by the oxidation of the Cl- in the working
electrode, while the corresponding hydrogen gas production occurred at the cathodic platinum
electrode. It is also shown that at higher applied voltages, about 0.4 V vs. Ag/AgCl, the
photo-anodic current measured from the N-polar surface overtakes the photocurrent of the
Ga-polar surface.
In order to assess the hydrogen gas generation efficiency, the photocurrents were
measured in 1 M HCl using a two-electrode system, with a platinum sheet as the counter
electrode as shown in the insert of Fig. 2(b). The photocurrent from the Ga-polar surface is
higher than the N-polar surface at lower potential versus the counter electrode, VCE, but
reverses at higher applied bias, similar to the profile we obtained from the three-electrode
configuration. The saturated photocurrent is related to the maximum efficiency of chargeseparation resulted from strong internal electric field via applied bias. At this time, the
photocurrent cannot increase even more bias is further applied. This is why the decreasing
efficiencies at high operating bias voltage. The surface morphology did not significantly
change through SEM analysis. The projected efficiency values may be computed according to
the formula:

ηeff =  j p

(E
o
rev
− VCE ) 
 *100%
Io

(1)
#190238 - $15.00 USD Received 8 May 2013; revised 14 Sep 2013; accepted 10 Oct 2013; published 12 Nov 2013
(C) 2014 OSA
13 January 2014 | Vol. 22, No. S1 | DOI:10.1364/OE.22.000A21 | OPTICS EXPRESS A24
o
where ηeff is the percent efficiency, j p is the current density, Erev
is the voltage for the
electrode voltage difference of the hydrogen generation from the oxidation reaction, and I o is
o
= 1.4 V for the standard Cl2/Clthe intensity of light illuminating the sample. We used Erev
reduction potential since we used HCl solution [17]. Apparently, the N-polar surface produces
much higher conversion efficiency of 0.61% compared to that of 0.55% by using the Ga-polar
surface.
Fig. 2. (a) Photoelectrochemical response of the two polar surfaces in 1 M HCl solution in
three-electrode system. (b) Photoconversion efficiency of the Photoelectrochemical cells for
two polar surfaces as a function of applied potential in two-electrode system. Inset:
Photoelectrochemical response of the two polar surfaces in 1 M HCl solution in two-electrode
system.
In order to understand the observed phenomena, Mott-Schottky plot is applied to analyze
the Ga- and N-polar systems. By using an equivalent circuit according to circuit B in
reference 11, Mott-Schottky plot of capacitance versus bias voltage as shown in Fig. 3(a) can
be obtained. The capacity for a semiconducting material to demonstrate photoactivity depends
upon the band edge potentials and the resulting band bending profile of the material in
solution. The Mott-Schottky equation relates the capacitance of the semiconductor to the
carrier concentration (Nd) and the other constants such as the fundamental charge constant
(e0), dielectric constant( ε ), vacuum permittivity( ε o ), applied potential (Vapp), and the
flatband potential( V fb ):
#190238 - $15.00 USD Received 8 May 2013; revised 14 Sep 2013; accepted 10 Oct 2013; published 12 Nov 2013
(C) 2014 OSA
13 January 2014 | Vol. 22, No. S1 | DOI:10.1364/OE.22.000A21 | OPTICS EXPRESS A25

1 
2
kT 
=
(2)
 (Vapp − V fb ) − 
2
C
eo 
 eoεε o N d  
The AC impedance measurements in the dark allow the estimation of the semiconductor
capacitance in solution. Plotting 1/C2 versus Vapp allows the estimation of the flat band
potential as the x-intercept. The Mott-Schottky results indicate that the flat band potential of
the Ga-polar surface is −0.8 V vs. Ag/AgCl while that of the N-polar is −0.4 V. From the flatband potentials, the band diagrams of the two polar surfaces can be constructed (Fig. 3(b)).
The corresponding band edges are consistent with the literature report that the Ga-polar
surface is about 0.3 ± 0.1 V more negative than the N-polar surface [19].
The difference in band-edge position greatly affects the onset potential of the two
surfaces. At applied bias equal to the flat-band edge, no photocurrent results because of the
absence of internal electric field that allows a more effective separation of the photogenerated
charges. At applied biases more negative than the flat-band edge, the band bends up, electron
accumulation occurs at the surface and substantial cathodic current is observed even in the
absence of light. At applied biases more positive than the flat-band potential, the band bends
downward, and in the presence of light, oxidative current can be observed. Hence it can be
observed that the onset potential of Ga-polar surface is more negative than that of the N-polar.
The 0.4 V-difference in the conduction band-edge potentials between the Ga-polar and the Npolar well agrees with the difference in their onset potentials. It should be noted, furthermore,
that the onset potentials do not correspond to the exact positions of the band edges. For the
Ga-polar electrode, the flat band is at −0.8V with the onset potential at −0.5 V. This is due to
the overpotential of the system, which may be caused by the presence of surface states that is
initially filled-up by the photogenerated charges. Certainly, more surface states could induce
stronger surface band pinning effect. However, presently we cannot determine the amounts of
surface states in both samples. The investigation on this issue is still in progress.
The relative photocurrents in photoelectrochemical behavior may be explained by the
relative positions of the valence-band edges of the two surfaces and the barrier heights
resulting from the different applied potentials. First, the barrier heights, ϕGa and ϕ N ,
dominate the rate of charge recombination (R). Calarco estimates that the recombination rate,
is approximated by the exponential term [20],
 φ 
R ∝ exp  −

 kT 
(3)
where k is Boltzmann constant and T is the temperature. At negative applied potentials, the
Ga-polar surface with larger band bending would have less charge recombination compared
to the N-polar material. Second, the potential difference between the valence band-edge and
the oxidizing species in the solution may govern the oxidation capacity of the holes generated
from the photoexcitation process. If the valence band edge potential is more negative than the
redox potential of the oxidizing species, the oxidation process cannot happen. This implies
that the more positive valence band edge has a better oxidation capacity due to large potential
difference between the valence band-edge and the oxidizing species in the solution.
We therefore propose that these two factors play an important role in the
photoelectrochemical behavior during the potential scan. At lower applied bias, the Ga-polar
surface significantly experiences less recombination losses compared to the N-polar surface,
and thus the photocurrent density of Ga-polar surface is much higher. While the applied
potential is increased to more positive, the effect of valence band-edge becomes the
dominating factor, and thus the N-polar surface would consequently yield much higher
photocurrent density. This model assumes that other sensitive factors, such as carrier
concentration and structural qualities, are quite similar for the two polar surfaces.
#190238 - $15.00 USD Received 8 May 2013; revised 14 Sep 2013; accepted 10 Oct 2013; published 12 Nov 2013
(C) 2014 OSA
13 January 2014 | Vol. 22, No. S1 | DOI:10.1364/OE.22.000A21 | OPTICS EXPRESS A26
Fig. 3. Mott-Schottky plots of the (a) Ga-polar and N-polar surfaces, extrapolated to their
corresponding flat band potentials. (b) The resulting band diagram for the Ga-polar and Npolar free standing thin film.
4. Conclusions
The photoelectrochemical behavior of the two different polar surfaces of n-GaN system was
investigated. The Ga-polar surface was demonstrated to show a more negative onset potential
compared to N-polar surface. At more positive applied voltages, however, the N-polar surface
yielded much higher photocurrent with conversion efficiency of 0.61% compared to that of
0.55% by using the Ga-polar surface. A model was proposed to explain these observations
through the variation of band structure at different polar facets. Based on this work,
understanding the facet effect on photoelectrochemical activity can provide a blueprint for the
design of materials in solar hydrogen applications. Moreover, as regards the decoration of
sensitizer on GaN surface for improving visible light absorption, the facet effect can further
play a significant role on the photo-conversion efficiency.
Acknowledgments
This work was supported by the National Natural Science Council, Ministry of Education,
Taiwan, and AOARD under AFSOR, US. We gratefully thank NSC, IAMS, and NTU for
financial support for this project.
#190238 - $15.00 USD Received 8 May 2013; revised 14 Sep 2013; accepted 10 Oct 2013; published 12 Nov 2013
(C) 2014 OSA
13 January 2014 | Vol. 22, No. S1 | DOI:10.1364/OE.22.000A21 | OPTICS EXPRESS A27