Electrochemical Investigation of the Gallium Nitride-Aqueous Electrolyte Interface

Electrochemical Investigation of the Gallium Nitride-Aqueous Electrolyte Interface

Shyam S. Kocha, *'a Mark W. Peterson, a Douglas J. Arent," Joan M. Redwing, b Michael A. Tischler, b and John A. Turner *'"

~Nationa/ Renewable Energy Laboratory, Photoconversion Branch, Go~den, Colorado 80401, USA bAdvanced Techno/ogy Materials, /ncorporated Danbury, Connecticut 06810, USA

ABSTRACT

GaN (Eg = ~3.4 eV) was photoelectrochemically characterized and the energetic position of its bandedges determined with respect to SHE. Electrochemical impedance spectroscopy was employed to analyze the interface, determine the space charge layer capacitance, and, subsequently obtain the flatband potential of GaN in different aqueous electrolytes. The flatband potential of GaN varied at an approximately Nernstian rate in aqueous buffer electrolytes of different pHs indicating acid-base equilibria at the interface.

Introduction The earliest (1938) synthesis of small crystals of gallium nitride

(GaN) reported in the literature is that of Juza and Hahn l'a and involved the reaction of ammonia with gallium. Later in 1960, Grim- meiss et aL 3'4 synthesized GaN to study its luminescence properties. In 1968, Maruska and Tietjen 5 generated larger area GaN grown epitaxially on sapphire and determined its ambient temperature bandgap to be 3.4 eV. This was followed by the report of blue luminescence of GaN by Pankove, e following which, many labora- tories have worked on synthesizing and characterizing GaN. GaN has been established to be an extremely promising and useful wide-direct-gap material that has practical solid-state applications as blue p-n junction LEDs, 7 UV emitting lasers, as well as piezoelectric and electro-optic modulators. 8 Although high quality crystalline GaN is readily grown today, to the best of our knowledge, i~s electrochemical characterization has not been reported in the literature.

Wide bandgap semiconductors (>2.5 eV) are of particular photoelectrochemical interest in that they allow us to carry out fundamental investigations of semiconductor electrochemistry, probing both conduction and valence band processes. Additionally, many wide bandgap semiconductors, especially oxides, show excellent chemical stability. Wide bandgap semiconductors with bandgaps >3.0 eV that do not absorb visible light can be dye-sensitized to obtain photocurrents with visible solar radiation. 942 Requirements for dye-sensitization include adsorption of the sensitizer plus favor- able redox potentials for charge injection from sensitizer to the semiconductor band. Typical semiconductors studied as dye-sensitized electrodes are SiC, ZnO, SnO2, TiO2, SrTiO3, and AgBr. Solar energy conversion devices consisting of nanocrystalline dye-sensitized TiO2 having efficiencies in excess of 6% have been reported by Gratzel and co-workers. ~3 There are also reports of the use of Pt/TiOJRuOa aqueous suspensions to catalyze the direct conversion of light to chemical energy (H2) in the presence of a suitable sensitizer and electron relay. 1+

Literature reports regarding the chemical stability of GaN claim insolubility in acids and alkalis at ambient temperature and slow dissolution in boiling bases. 5 In fact, its stability has been problem- atic in finding suitable wet chemical etches for cleaning and re- moval of surface impurities. The stability of GaN coupled with its wide bandgap make it an interesting material to study for applica- tion as a dye-sensitized electrode as well as a system useful for fundamental investigations of semiconductor electrochemistry. Therefore, we have carried out an investigation of the GaN- aqueous electrolyte interface to determine its bandedge positions, photocurrent onset potential as well as pH dependence in addition to a few solid-state properties. Our focus in the current study is the fundamental electrochemical characterization of GaN. In the pro- cess of electrochemical characterization we can establish if a material is thermodynamically capable of electrolyzing water under bandgap illumination in the absence of an external bias and also under visible illumination with dye sensitization.

Experimental The GaN layers used in our study were grown by metallorganic

vapor phase epitaxy on an (0001) sapphire substrate. A thin AIN

* Electrochemical Society Active Member.

buffer was grown at 550~ prior to the growth of the 2 ~m GaN epitaxial layer at 1100~ Silane was used as an intentional n-type dopant source. The carrier concentration of the layer, determined by room temperature Hall measurements, was 1.0 x 1019 cm -3 with an electron mobility of 190 cm2/Vs. Since the substrate was insulating, an ohmic contact was made on the front surface of the GaN using Ga-ln paste; this procedure is somewhat different from the technique of using an indium amalgam as employed by Maruska etaL ~ based on their private communication with Liebert and Nix. The electrical contact was reinforced using silver epoxy and subsequently shielded from the electrolyte with insulating epoxy.

For electrochemical studies, the samples were rinsed with ethanol, acetone, deionized water, and given a mild etch in warm 1 MKOH for 1-2 min before each experiment. The electrolytes used were indifferent buffer solutions of different pHs. A three-electrode configuration was used in the studies with platinum gauze as coun- terelectrode and either a saturated (KCI) calomel electrode (SCE) or a sodium chloride saturated calomel electrode (SSCE) as reference. The area of the GaN working electrode in all the electrochemical experiments was 0.08 cm a. Electrochemical impedance spectroscopy (EIS) was carried out using a Solartron potentiostat (1286 El) and frequency response analyzer (1260 FRA). 20 mV p-p ac sinusoidal perturbations in the frequency range of 0.01 Hz-500 kHz superimposed on a dc bias was applied to the sample at each pH. Prior to measurements at each bias, the electrode was subjected to a dc bias (at the measurement voltage) for a period of 1-2 min for stabilization. Optical spectra were measured using a 450 W Ar arc lamp and an Oreil 1/8 meter monochromator. The beam was split with bifurcated UV fiber optic so that the photoresponse could be corrected for the lamp output by simultaneously measuring the flux with a thermopile. Photocurrent-voltage curves were obtained at a wavelength of 345 nm (10 nm bandpass) and a flux of 3.49 mW/ cm 2.

All chemicals used were reagent grade. All aqueous buffer electrolytes used were purged with nitrogen gas before carrying out

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Fig. 1. Photocur ren t s p e c t r o s c o p y of ep i layer n -GaN elec- t rodes in e lect ro ly tes of pH 2. Inset s h o w s the b a n d g a p analysis .

L238 J. Electrochem. Soc., Vol. 142, No. 12, December 1995 �9 The Electrochemical Society, Inc.

) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 193.146.32.73Downloaded on 2014-05-01 to IP

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J. Electrochem. Soc., Vol. 142, No. 12, December 1995 �9 The Electrochemical Society, Inc. L239

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Fig. 2. Frequency independent Mott-Schottky plots of n-GaN in electrolytes of different pHs, with space charge layer capacitance values obtained from electrochemical impedance spectroscopy. Inset shows pH dependence of flatband potential.

photoelectrochemical experiments and had compositions as previ- ously reported. 16'17 Deionized water having a resistivity of 18 M~- cm was utilized in electrolyte preparation and sample rinsing.

Results and Discussion The optical properties of GaN were characterized with photolu-

minescence (PL), UV absorption spectroscopy, and photocurrent spectroscopy (PCS). The room temperature PL shows 2 peaks, one at 3.41 eV believed to be band to band emission, 18 and the "yellow" peak at 2.15 eV believed to be emission from the conduction band to a deep acceptor levelJ 9 Analysis of the GaN UV-absorption data for a direct gap yields a value of 3.40 eV. Figure 1 shows the PCS of GaN in pH 2 buffer along with the direct bandgap analysis. These data give a value of 3.41 eV which is in excellent agreement with the PL and UV-absorption data.

Electrochemical impedance spectroscopy (EIS) was carried out on the GaN electrodes in aqueous buffered electrolytes of different pHs. Two time constants with considerable overlap were observed in the phase and complex impedance spectra. We attribute the time constant predominant at low frequencies (<1 kHz) to a distribution of surface states (R~C~) and the high frequency time constant to the space charge layer (R~cC, o). The spectra were fitted with a nonlinear least squares fit (NLLSF) program 2~ using an equivalent electric circuit 2~ that accounts for the two time constants in addition to the series resistance (Rs) of the electrolyte. The resistance and capacitance due to the surface states are in series with each other but parallel to the space charge layer capacitance and resistance

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Fig. 3. Photocurrent-voltage curves of n-GaN in electrolytes of pH 0 and 14 vs . a sodium chloride saturated calomel reference,

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Fig. 4. Photocurrent-time profile for n-GaN in aqueous electrolytes of pH 0 and 14. Illumination is white light from an arc lamp at approximately 1600 mW/cm 2.

and provide an alternate path for charge exchange that responds only to lower frequencies.

The space charge layer capacitance that was extracted from the equivalent electrical circuit was then used to obtain Mott-Schottky plots (C -2 vs. V). Each datum point in the Mott-Schottky plot is obtained from an NLLSF to the impedance spectrum at a particular bias. The Mott-Schottky plots were linear over a range of almost 2 V as would be expected for a wide bandgap well-behaved material. The extrapolated intercept of the plot gives the flatband potential (VFB); 22 the slope of the plots is inversely proportional to the apparent carrier concentration (Nd) and was found to be ca. 1.0 x 1019 cm s, in agreement with the solid-state Hall measurements. Figure 2 shows plots for four different pHs; the inset in the figure is the variation of the flatband potential with the pH. The relationship for the pH dependence was found to be

VFB=- -750- -55"pH (mVvs. SCE)

The slope is not significantly lower than the expected Nernstian variation of 59 mV/pH. This response is linked to acid-base equilibria due to the presence of native Ga oxides interacting with H + and OH ions at the interface, changing the Helmholtz layer charge and leading to the observed pH dependence. The bandedge shifts follow the changes in the hydrogen and oxygen redox potentials. Such relationships have been observed to be followed by most oxide semiconductors as well as III-V semiconductors that have a thin layer of native oxide on the s u r f a c e . 16'21'23

From the knowledge of the flatband potential, the bandgap and the doping densityf 1 the energetic position of the bandedges can be calculated. For this sample at pH 14, we calculate that the conduction bandedge Ec = 1.26 V and valence bandedge Ev = 2.14 V vs. SHE. Note that the GaN is highly doped leading to a small value for Ec - qVFB ~ 0.05 eV.

Photocurrent-voltage measurements (Fig. 3) were carried out to determine the onset potential of the photocurrent of GaN in electrolytes of pH 0 and 14. The reaction here is the oxygen evolution reaction for a bare GaN electrode. The photocurrent-voltage curves exhibit a small degree of hysteresis and reveal onsets that are about 0.50 V more positive than the corresponding flatband potentials. This can be attributed to the slow kinetics of the oxygen evolution reaction at this noncatalyzed surface and near edge surface states that manifest themselves in most llt-V semiconductors. The quantum efficiency was calculated to be s = 5.2 • 10 3 in solutions of pH 14 at 0 V vs. SSCE.

The conduction and valence bandedges are located such that the electrolysis of water should be possible without an external bias (with an available overpotential of 1.75 V [Ev - Eo2 (pH 14)] for the oxygen evolution reaction). Figure 4 shows the short-circuit current for water electrolysis under bandgap illumination. The surface of the sample was modified with ruthenium dioxide colloids to catalyze the oxygen evolution reaction. In view of the preliminary nature of the experiments, we do not feel at this time that the difference in photocurrent between pH 0 and pH 14 is significant. The difference can be explained by variable positions of the sample in the light beam and the aging of the RuO2 colloids, More precise measurements of the behavior of GaN will be forthcoming. While water splitting with a >3.0 eV bandgap system is not new, to our knowledge this is the first nonoxide material to accomplish it. Although no significant decomposition of the electrode surface was observed



L240 J, Electrochem. Soc., Vol. 142, No. 12, December 1995 �9 The Electrochemical Society, Inc.

after these experiments, further work has to be done as regards to the stability of the electrodes for long times in various electrolytes. Work is currently in progress to characterize p-GaN, and a compre- hensive impedance analysis together with studies on dye sensitized GaN will be reported in a forthcoming paper.

Acknowledgments This work was supported by the U.S. Department of Energy,

Office of Energy Research, Office of Basic Energy Sciences, Divi- sion of Advanced Energy Projects.

Manuscript submitted Sept. 21, 1995; revised manuscript re- ceived Oct. 4, 1995.

The National Renewable Energy Laboratory assisted in meeting the publication costs of this article.

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Electrochemical Investigation of the Gallium Nitride-Aqueous Electrolyte Interface