The influence of electrodeposited gold on the properties of III–V semiconductor electrodes—Part 1. Results of current—potential measurements on p-GaAs

Erecrnx66ncn Acm, Val. 38, Printed in Great Britain .

Nu. 213,

pP 29 1-300, 1993

0x13-4686/93 $6.00+am C 1992. Perpmon Press Ltd.

THE INFLUENCE OF ELECTRODEPOSITED GOLD ON THE PROPERTIES OF III-V SEMICONDUCTOR ELECTRODES-PART 1 . RESULTS OF CURRENT-POTENTIAL MEASUREMENTS ON p-GaAs G . OsKAr6,• D. VANMAEKELBERGH and J. J. KELLY Debye Research Institute, University of Utrecht, P.O. Box 80000, 3508 TA Utrecht, The Netherlands (Received 18 May 1992)

Abstract-The influence of electrodeposition of gold on the electrochemical properties of p-type GaAs has been studied From current-potential and impedance measurements performed in solutions containing various redox-couples it was concluded that gold-related surface states play a central role in the interaction between semiconductor, metal and electrolyte solution- A kinetic model is presented which comprises three processes : (i) excitation of valence band electrons to the surface states, (ii) recombination of these electrons with holes in the valence band and (iii) transfer of electrons from the surface states to an electron acceptor in solution . Key words : electrodeposition, GaAs, semiconductor/metal electrodes, Schottky diode, Ohmic contact .

INTRODUCTION

on both gold-plated and bare p-GaAs electrodes in electrolyte solutions with various redox couples are presented . According to the Marcus-Gerischer theory, the empty levels corresponding to the oxidized states of a redox-couple are distributed around an energy, positive with respect to the standard redox energy . The filled levels, corresponding to the reduced species, are located at energies negative with respect to the standard redox energy[18] . In Fig. 1, the standard energies of the redox-couples used are shown with respect to the position of the band-edges of GaAs in acidic and alkaline solution . As an oxidizing agent can only be reduced at the semiconductor surface if its levels have an overlap with filled levels in the semiconductor, the results of the (i, V) measurements can be predicted on the basis of this

In the last two decades, much work has been devoted to the study of the metallization of semiconductors since metal contacts, both Ohmic and Schottky, are essential for a variety of device applications[1-3] . Apart from sputtering or evaporation, semiconductor/metal contacts can be formed by (electro)chemical deposition from an electrolyte solution[4, 5] . The advantages of electrodeposition are obvious : it is a simple and inexpensive process operating at room temperature which enables a good control of the structure and thickness of the metal or alloy layer . Only a few studies have been devoted to the characterization of electrochemically formed Schottky diodes ; the electrical properties were found to be similar to those of the diodes made by evaporation or sputtering[4, 6-8] . Electrodeposition of gold has been used for Schottky diode formation on n-type GaAs[4] while gold alloys can give Ohmic contacts on p-type GaAs[9, 10] . Because of possible application in photoelectrolytic solar cells, much more attention has been paid to the photocathodic reduction of protons or water at p-type semiconductor electrodes on which (noble) metal islands were deposited[11-16] . The presence of metal islands often leads to a considerable enhancement of the rate of photoreduction of water . In the interpretation of this phenomenon the electrocatalytic properties of the deposited metal are generally taken into account[12, 14] . We investigated the electrochemical properties of GaAs electrodes on which gold islands or porous gold layers are present . This system was chosen because the electrochemical deposition of gold on nand p-type GaAs is well-defined[10, 17] . In this article, the results of current-potential measurements

-E/eV vs SCE pH

14

-2 .0

CB ` \ .' ,

--1 .6 pH - 2.5

cn

-H¢0/Ha (pH-14) T

vB

•

-0 .5

.-0 .4

-Au(CN),/Au

-H•1 H t (pH-2 .5)

.0 .0 -Fe(CN)a~4

vn

.0 .4

0.8

GaAs

electrolyte solution

Fig. 1 . Energy level scheme showing the standard energies of various redox couples with respect to the band-edges of GaAs electrodes at pH = 14 and pH = 2.5.

* Author to whom correspondence should be addressed . 291



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scheme . From Fig. 1 it is clear that Fe(CN)', - in alkaline solution can be reduced by injecting holes into the valence band (VB)[19] . In acidic solution, however, hole injection is not possible and, as there is also no overlap with the conduction band (CB), reduction does not occur . For the reduction of H,O and Au(CN)z, electrons from the CB are needed[10] . In this and following papers, the influence of a gold layer or gold islands on the reduction of these oxidizing agents at GaAs in the dark will be discussed in detail. In Part 1, the results obtained with p-GaAs under steady-state conditions are interpreted on the basis of a kinetic model which considers electron transfer at both the GaAs/Au and the Au/electrolyte interfaces. This model is supported by results of impedance measurements which will be reported in Part 2 (this issue, p . 301). The results obtained at n-GaAs and InP electrodes will be considered separately .

EXPERIMENTAL Current-potential and impedance measurements were performed on both p-type and n-type GaAs/Au dry junctions and on p-GaAs/electrolyte interfaces . The GaAs crystals [(100) orientation] were obtained from MCP Electronic Materials (UK) . The n-type material was doped with Si [(2 .02.2) x 10 17 cm -3] and the p-type with Zn [(1 .03.0) x 10 17 cm -3] . Wafers were mechanochemically polished by the supplier. Before use, the surface was rinsed successively in acetone, ethanol and water . Before each measurement, the electrode was dipped in 8 .0 M HCI for 3 min to remove oxides before being etched in a 3/1/1 mixture of H,SO,(98%), H,O and H,O,(30%) for 20s . Afterwards, the electrode was again dipped in 8 .0 M HCI for 30s . The electrochemical measurements were performed in a conventional cell with a GaAs (rotating) disk with a geometric area of 0.125cm 2 as working electrode, a platinum sheet (20cm2 ) as counter electrode and a saturated calomel electrode (see) as reference . The potential of thick gold layers deposited on GaAs was measured with respect to see . For this purpose, a contact was made to the gold layer with silver paste which was subsequently insulated from the solution with Apiezon . For measurements in solution, potentials are given with respect to see . For dry measurements, the potential of the semiconductor was measured with respect to that of the gold layer . The impedance measurements were performed with a Solartron HF Frequency Response Analyzer (ST 1255) and a Solartron Electrochemical Interface (ECI 1286) . The measured impedance was corrected for the resistance due to the electrolyte solution . A GaAs/Au surface was studied in air with Scanning Tunneling Microscopy (STM) . The potential difference between the Pt/Ir tip and the sample was -2 V and the tunnel current was about 0 .15 nA . All chemicals were of p .a . quality . Before each measurement oxygen was purged from solution by bubbling through high purity N, . The measurements were carried out at room temperature.

RESULTS

Deposition of gold on GaAs Gold was deposited from Au(CN)2 in an alkaline solution (pH = 14) with 1 .0 M KCN added to stabilize the complex . At n-GaAs, a large density of nuclei (10 70-10" cm-2) was formed by applying a potential step from the open-circuit value to a value close to the flat-band potential (Vs) for about 30 ms . The nuclei were slowly grown by stepping to a potential just negative of the open-circuit value where the cathodic current is small (CO .!mAcm-'). After deposition, the amount of deposited gold could be determined by subsequent stripping under illumination at potentials anodic with respect to the opencircuit value . At p-GaAs the same strategy was followed. In this case, gold was deposited under illumination and dissolved in the dark . In deposits corresponding to about 5 to 25 monolayers of gold, the gold was present on the surface in the form of (isolated) islands[10] . From the STM picture of a relatively thin gold layer (-75 monolayer) on an n-GaAs electrode (Fig . 2), it can be seen that the layer consists of spheres with radii of 10-15 nm and is obviously porous . The structure strongly resembles that of an evaporated gold layer on GaAs measured with STM by Phaner et al. : the surface structure is much more regular than that of electroless-deposited gold films[20]. Even thick layers proved to be porous : the flat-band potential of an n-GaAs electrode provided with about 500 monolayers gold was the same as that of a bare electrode and showed the same pH dependence . Furthermore, it was possible to oxidize p-GaAs plated with a thick gold layer in the same potential range as a bare p-GaAs electrode .

Measurements at GaAs/Au dry junctions Current-potential and impedance measurements were performed on p- and n-type GaAs with a relatively thick gold layer (ca . 150nm) . The n-GaAs/Au junction showed good rectifying properties . The capacitance was found to be independent of the frequency in the range 100Hz100kHz. The Mott-Schottky plot (C;2, V) was linear between -0.5 V and 2 V. The free carrier concentration derived from the slope (2 x 10 17 cm -3 ) was in good agreement with the suppliers specifications. The barrier height 4' determined from the Mott-Schottky plot was 1 .leV. In Fig. 3, the forward current-potential curve is shown . The results agree with the thermionic emission model and log(i) vs . V is linear over 5 orders of magnitude, as shown in the insert in Fig . 3 . The barrier height calculated from the current density at the opencircuit potential (obtained by extrapolation) was 0 .85-0 .9 eV . This value is not corrected for the image force lowering which is about 45meV in this case. The ideality factor ranged from 1 .03 to 1 .2[21] . Reineke and Memming reported a barrier height of 1 .15-1 .2eV for electrochemically formed n-GaAs/Au diodes[6] . For n-GaAs/Au diodes prepared by evaporation, lower values of 0e (0 .8-1 .0eV) have been mainly reported[22, 23] although some authors found equally high barriers[24, 25] .

Gold and Ill-V semiconductor electrodes-I

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Fig. 2. STM image (uncorrected data) of an n-GaAs electrode provided with a thin gold layer (about 75 monolayers) measured in air . The potential difference between the sample and the Pt/Ir trip was -2 V and the tunnel current was ca . 0.15 nA. The barrier height of p-GaAs/Au junction was found to be low. The current under reverse bias was, however, not very reproducible : it varied from _ 2 contact to contact between 0.01-1 .2 A em at V = -0.2V. The current generally increased with increasing reverse potential . The impedance and current-potential measurements suggest a barrier height ranging from 0 .3 to 0.5eV ; a value of about 0.3eV was reported in literature[6, 10] . The barrier V/V -0.3

-0 .2

-0 .1

0.0

-0 .3

Fig. 3. Forward current-potential curve of an n-GaAs/Au dry junction. In the insert a semi-logarithmic plot is shown . The barrier height, not corrected for image force lowering, was 0.86eV and the ideality factor was 1.1 .

height of junctions prepared by evaporation reported to be 0.4-0.6 eV [22, 23] .

is

(i, V)-characteristics at gold electrodes In Fig . 4, current-potential (i, V)-curves for a bulk gold electrode in aqueous solutions containing (A) 1 .0 M NaCIO, (pH = 2 .5) and (B) 0 .01 M K,Fe(CN) 6 + 0.25 M NaCIO, (pH = 2.5) are shown. At potentials more negative than -0 .7 V hydrogen is evolved (Fig. 4A). The Nernst potential for the H'/H2 couple is -0 .4V so an overpotential is required for H' reduction (see Fig . 1) . A current limited by the diffusion of H' is observed between -1 V and -1 .4 V . Negative of - 1 .4 V, the cathodic current increases as a result of the H 2O reduction. In the solution with Fe(CN)6 - (Fig. 4B), an extra diffusion limited plateau is found which can be ascribed to the reduction of Fe(CN)6 - to Fe(CN)6 - . The onset-potential is determined by the redox potential of 0.3 V (see Fig. 1) . The measured current at potentials more negative than -0 .7V is the sum of the two diffusion limited reduction currents . (i, V)-characteristics at gold-plated p-GaAs electrodes In Fig . 5, the (i, V)-curves for p-GaAs electrodes in a 0.01 M H 2 SO, solution (pH = 2.5) are shown . The cathodic current at a bare electrode is very small (curve a) . In contrast, a relatively large cathodic



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er al.

V/V vs SCE

V/V vs SCE -2 .0

-1 .5

-1 .0

-0 .5

0 .0

0 .5 1 .0

(A) (a)

(a)

(b (b) -3 .0

Fig. 6 . Current-potential curves of (a) a bare and (b) a gold-plated (-'25 monolayer) p-GaAs electrode in a 0 .01 M Fe(CN)6 - + 0 .01 M H,SO, (pH - 2.5) solution . Scan and rotation rates are 20 mV s - ' and 1000 rpm, respectively .

V/V vs SCE -2 .0

-1 .0

-0 .5

0 .0

0.5 0

(B) (a)

-5 E

aE -10

(b)

-15

Fig. 4 . Current-potential characteristics of a gold electrode in (A) a 0.01 M H2SO4 + t M NaCIO, solution and (B) a 0.01M K 3 Fe(CN), + 0.01 M H2SO4 + 0.25 M NaCIO, solution. Two rotation rates are shown : (a) 500 rpm and (b) 2000 rpm and the scan rate was 50 mV s - ' in both cases.

a significant cathodic current due to Fe(CN),3 reduction and corresponding to about 50% of the diffusion limited value is observed at potentials more negative than 0 .2V. In Fig. 7, the (i, V)-curves of an electrode with a thick gold layer in a 0 .01 M KS Fe(CN) s + 0 .25 M (pH = 2.5) NaCIO, are depicted . On the basis of electrode rotation rate dependence, it can be concluded that the current due to reduction of Fe(CN)s - and H 4 can become diffusion limited. A striking feature of this (i, V)-curve is

V/V vs SCE -2 .0

current corresponding to hydrogen evolution is observed at potentials more negative than -0 .6V with p-GaAs provided with a thick gold layer (curve c). At a p-GaAs electrode with gold islands (equivalent to 10 monolayers), a smaller cathodic current is observed (curve b). In Fig. 6, the (1, V)-curves for a bare (a) and a gold-plated electrode (b) in a 0 .01 M K 3 Fe(CN) s + 0.01 M H,SO 4 solution are presented. As in the previous case, no cathodic current flows at the bare electrode. With gold islands (25 monolayer) present,

-1 .5

-1 .0

-0.5

0 .0

0 .5 5

(A) 0

(a)

ao (b) -15

V/V vs SCE V/V vs SCE -2 .0 -2 .0

-1 .5

-1 .0

-0 .5

-1 .5

-1 .0

-0.5

0 .0

0 .0

1v U y

-2 .0 -0 .2

Fig. 5 . Current-potential characteristics of bare and goldplated p-GaAs electrodes in a 0 .01 M H,SO, (pH = 2 .5) solution : (a) bare electrode, (b) p-GaAs with gold islands (-10 monolayer) and (c) p-GaAs with a thick gold layer . The scan rate was 20mVs - ' and the rotation rate was 1000 rpm .

Fig. 7 . Measurements on a p-GaAs electrode with a thick gold layer (-'150nm) in a solution of 0 .01 M K,Fe(CN), + 0.01 M H2SO4 + 0.25 M NaCIO 4 . (A) Current-potential characteristics. The scan rate was 50mVs" and the rotation rates were : (a) 500rpm and (b) 2000rpm. (B) The potential of the gold layer vs . the potential applied to the disk electrode (both vs . see) for the two rotation rates.



Gold and Ill-V semiconductor electrodes-I

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V/V vs SCE -2 .0

-1 .5

-1 .0

-0.5

0 .0 1 .0

(b)

-0 .5

-1 .0

Fig. S. Current-potential curves of a bare (a) and goldplated (b) p-GaAs electrode in a 0 .01 M KAu(CN),/l .0 M KCN/1 .0M KOH solution . Curve (b) was measured at a stationary electrode with a low density of gold islands during impedance measurements . The effective scan rate is 2mV s - ' . the strong resemblance to that of a bulk gold electrode (see Fig . 4B) . At potentials more positive than 0.2V, an anodic current due to the oxidation of GaAs is observed . Similar experiments were also performed with bare and gold-plated p-GaAs electrodes in alkaline solution (pH = 14) . The results of the measurements in 1 .OM KOH resembled those obtained in acidic solution (see Fig. 5). At bare electrodes no current is observed, while the cathodic current increases with increasing thickness of the gold layer . At thick gold layers, the (i, V)-curve again strongly resembles that measured at bulk gold . In Fig. 8, the (1, V)-curves of p-GaAs electrodes in a solution of 0.01 M KAu(CN), + 1 M KCN (pH = 14) are shown . At a bare electrode the cathodic current is very small ; in this solution, gold cannot be deposited in the dark (curve a)[10] . In contrast, if gold islands are present a current due to the reduction of Au(CN)z is observed at potentials more negative than -1 .OV (curve b). At a GaAs electrode with a sufficiently thick gold layer, a diffusion limited current is observed. An anodic peak due to the dissolution of gold can be seen at potentials positive with respect to the open-circuit potential. The amount of deposited gold could be determined from the charge under the anodic peak, as indicated above. Impedance measurements at p-GaAs electrodes The Mott-Schottky plots of a bare p-GaAs electrode and a p-GaAs electrode provided with gold

V/V vs SCE Fig. 9. Mott-Schottky plots of bare and gold-plated p-GaAs electrodes : curves (0) and (0) represent a bare electrode and an electrode with gold-islands, respectively, in a 1 .0 K KOH solution . Curve (0) was measured at an electrode with gold-islands in a 0.01 M KAu(CN), + 1 .0 M KCN + LOM KOH solution . The measuring frequency was 10 kHz .

islands measured in alkaline solution are shown in Fig. 9. The results are summarized in Table 1 . For the sake of clarity only one frequency is shown. The plots were linear for frequencies between 250 Hz and 100kHz and converged at the flat-band potential (Vj At bare electrodes, impedance measurements in three different electrolyte solutions (A) : 1 .0 M KOH, (B) 1 .0 M KCN + 1 .0 M KOH and (C) : 0 .01 M KAu(CN), + 1 .0 M KCN + 1 .0 M KOH lead to the same results (open circles) . The Mott-Schottky plots are linear over the entire potential range and the V, b is -0.5v . At p-GaAs electrodes provided with gold-islands (-25 monolayer) the situation is different . In solution (A), the (C;2, V) plot (open squares) undergoes a parallel shift of ca. 0.3V towards more negative potentials with respect to that of the bare electrode . This indicates that the band-edges at the surface are shifted to higher energy when gold islands are present. Hence, the potential drop over the Helmholtz-layer is changed. Furthermore, a "knee' is found in the Mott-Schottky plot at about -1 .3 V due to the presence of gold ; its position depends on the measuring frequency. In solution (C), containing Au(CN)a , the (C,- ', V)-plot (filled squares) is not displaced with respect to that of the bare electrode . However, in this case, a knee is also found at about -1 .1 V . The increase of C, z at potentials more positive than -0 .85V can be ascribed to the dissolution of the gold islands (see Fig. 8) . Similar results are obtained in acidic solution with p-GaAs electrodes . In Table 1, the results of impedance measurements at both bare and gold-plated

Table 1 . The flat-band potentials of both p-GaAs electrodes provided with gold islands and bare electrodes in various electrolyte solutions Solution

V, b(bare) (V vs. see)

0 .01 M H,SO, 0.01 M H,SO, + 0.01 M K,Fe(CN)6 I M KOH I M KOH + 1 M KCN I M KOH + I M KCN + 0.01 M KAu(CN),

0.25 0.25 -0.50 -0.50 -0.50

V, b(gold-plated)

(V vs . sce)

-0.05 0 .25 -0.80 -0 .80 -0 .5



296

G . OSKAM

p-GaAs electrodes in 0.01 M H 2 SO, and in 0 .01 M K,Fe(CN)6 + 0.01 M H,SO, are shown . In H 2S0„ the Vrb measured with the electrode provided with gold islands is displaced by about 0.3 V in negative direction with respect to that of the bare electrode . In the solution containing Fe(CN)6 - , the same V,b is found for the gold-plated as for the bare electrode . In both solutions, a knee in the Mott-Schottky plot is observed at a potential about 0.5 V more negative than V,,, . The knee, which results from an additional capacitance parallel to that of the depletion layer, is discussed in detail in Part 2 . Mott-Schottky behaviour is not found for p-GaAs electrodes provided with a 150nm thick gold layer . The measured capacitance is large in the whole potential range. From this it can be concluded that the applied potential does not lead to a corresponding change of the band-bending within GaAs . In Fig. 7B, the potential of the gold layer with respect to see (VA.) is plotted vs. the potential V which is applied to the p-GaAs electrode provided with a -150nm thick gold layer in a solution of 0 .01 M K,Fe(CN) 6 + 0.25 M NaClO, (pH = 2 .5). It can be concluded that VA „ is proportional to V in the whole potential range and the slope of the line is 1 . That means that an applied potential falls completely over the Helmholtz-layer at the Au/electrolyte solution interface and, consequently, the band-bending in the semiconductor under the gold does not change .

DISCUSSION Before considering the interpretation of the (1, V) and (C -2 , V) characteristics of gold-plated GaAs electrodes in various electrolyte solutions, we shall first discuss briefly the electrical properties of the GaAs/Au dry junctions . GaAs/Au dry junctions

From the (Cp 2 , V) and (i, V) results it follows that gold deposited electrochemically on n-GaAs forms a Schottky diode. The barrier height determined from the (Cp 2 , V) plots is 1 .1 eV. From the slope of the Mott-Schottky plot the correct value of the donor concentration N o is found. This is possible despite incomplete coverage of the surface by gold as the mean distance between the gold-contacted areas is smaller than the width of the depletion layer . The value of the barrier height of n-GaAs/Au diodes depends on the measurement method used : 0,(i, V) is ca . 0.2eV smaller than r3(Cp 2 , V). The reasons for this effect have been much discussed in the literature[l, 3, 26] . In a recent publication, Werner and Gdttler argue that the discrepancy can be caused by a modulation of the barrier height at the interface[26] . From impedance measurements in the depletion region a mean value of 0e is found. The forward current, however, essentially flows where the barrier height is lowest . A lower value of Qa is, therefore, obtained from (i, V) measurements . Experimental support for this model was found with CdTe/Au junctions where a modulation of the barrier height of up to 0.3eV was observed with ballistic electron emission microscopy (BEEM)[27] .

et atL

The p-GaAs/Au junctions have, as the (1, V) results indicate, a much smaller barrier height . The reverse current, which is determined by the barrier height, was found to vary considerably from contact to contact . Consequently, we assume that the barrier height also varies ; the experimental results indicate values between 0 .3-0 .5 eV . As the barrier height of GaAs/metal contacts is only weakly dependent on the work function of the metal, it is generally assumed that the barrier height is determined by pinning of the Fermi level at surface states introduced by the metal[1] . As a result, a potential difference is formed over a dipole layer between GaAs and the metal . The capacitance of this dipole layer is generally much larger than that of the depletion layer. Consequently, an applied potential generally falls completely over the depletion layer . In view of the assumption that the Fermi level is pinned, our results indicate that gold deposition gives rise to surface states, located at about 0 .30.5 eV above the valence band edge . A model for reduction at gold-plated GaAs electrodes The (i, V) results show that Fe(CN)6 - is not reduced at bare p-GaAs electrodes in a H 2 S0, solution (pH = 2.5) in the dark . This agrees with the fact that the empty levels of the Fe(CN)6 - "4- couple show no overlap with the valence band (see Fig . 1). In contrast, reduction of Fe(CN)6 - occurs at p-GaAs electrodes provided with gold islands, even though the band-edges at the surface are at the same energy as those of the bare electrodes . Direct hole injection into the valence band can, therefore, be excluded . To account for these observations, we propose a model which includes thermal excitation of electrons from the valence band into gold-related surface states . From the (i, V) characteristics measured at p-GaAs/ Au dry junctions (thick gold layer) it follows that thermal excitation is effective and can lead to high currents via the surface states. In the model we assume that the gold-related surface states, located at 0 .3-0.5eV above the valence band edge, play a key role in three processes (see Fig. 10) : (i) thermal excitation of electrons from the valence band to empty surface states S° : S°

S - +h +

(ii) trapping of free holes in filled surface states S Bp

s - + h+ -- . S° (iii) transfer of an electron from the deposited gold to an oxidizing species in solution . It will be assumed that the electrons in the gold are in equilibrium with the electrons in the gold-related surface states . If an electron is transferred from gold to the solution, the equilibrium between the surface states and gold is restored immediately . Only the oxidizing species is considered to be present in solution . Hence, the overall reduction reaction can be written as : S - + Ox+ • S° + Red.

Gold and

111-V

semiconductor electrodes-1

From Equations (2) and (3) it follows that reduction at gold-coated GaAs electrodes depends on the magnitude of fp p, relative to s„ N + f k,(ox)c a . If f o p,~>sN+fkAox)c4, no cathodic current flows. The surface states are empty as the trapping of holes [process (ii)] is much faster than the excitation process (i) . In the case of gold-coated p-GaAs electrodes, holes are the majority carriers and the condition Pp p,aa„N+fk,(ox)co, prevails at sufficiently small band-bending (close to Vim). If f p p.
EIeV vs EVB

CB

297

-1 .2

-0.8

i) -0.4 i)

i,=-esfk,(ox)c,, .

VB

-o Fig. 10. Schematic representation of the model involving gold-related surface states . The three processes via these states (i), (ii) and (iii) represent the excitation of electrons from the valence band to the surface states (SS), recombination of electrons in the surface states with holes in the valence band and transfer of electrons from the surface states to an electron acceptor in the solution, respectively .

i, _ -ess„N . It is assumed that the kinetics of electron transfer between the deposited gold and the electroactive species are the same as at a bulk gold electrode . The rate constant kAox) depends on the oxidizing agent and on the potential drop over the Helmholtz layer between the gold and the electrolyte solution . As the reduction of the oxidizing species occurs at the surface of the deposited gold with electrons which are thermally excited into surface states, a geometric factor f is introduced which is the ratio of the total contact areas between gold and solution and between GaAs and gold (f 3 1) . Under steady-state conditions one has : ds t = 0 = a. Ns' - [p.p. +fkr(ox)c ..]s .

(1)

In Equation (1), so and s - refer to the density of empty and occupied surface states, respectively . The total density of gold-related surface states is denoted by s, the concentration of free holes at the semiconductor surface by p„ the density of states is the top of the valence band by N and the concentration of the oxidizing species in solution at the surface by c,, . As long as the current density is much smaller than the diffusion limited value, c„ is equal to the bulk concentration . From Equation (1), the fraction of occupied surface states is found to be : qe. N (2) . + Yy p. +fk,(ox)c,, The current density i, corresponding to the reduction of the oxidizing agent is :

S_

s

e, N

i, _ - efkr(ox)ca.s _ -efs

k,(ox)c4,a„N E. N + So p, + fk,(ox)c„

(3)

(4)

As thermal excitation of electrons to the gold-related surface states is not rate-determining, the reduction current density is limited by the charge transfer between the gold and the oxidizing agent . From Equation (2) it follows that the surface states are completely occupied . The negative charge in the surface states leads to an additional potential drop over the Helmholtz layer . When fk,(ox)c„ s, N Equation (3) reduces to : (5)

The cathodic current density is now limited by thermal excitation of electrons from the valence band to the surface states and not by the kinetics at the gold/solution interface. The surface states are empty ; no additional potential drop over the Helmholtz layer occurs. In this model we assume that electron transfer from the semiconductor via the gold to an oxidizing agent in the solution takes place through surface states. Another possible mechanism for charge transfer is classical thermionic emission . For the explanation of the results presented in this article, these two possibilities lead to the same conclusions . In Part 2, however, it is convincingly shown that charge transfer does, in fact, occur via the surface states and, for that reason, this mechanism is used here . In the following section, the experimental results obtained with p-GaAs electrodes provided with gold will be interpreted on the basis of the model . Furthermore, this model will be compared with other models presented in literature. Gold-plated p-GaAs electrodes From the results obtained in acidic and alkaline solution with p-GaAs electrodes provided with gold islands it is clear that the flat-band potential is largely determined by the pH, ie by H* and OH adsorption, as is found for bare GaAs . It can be concluded that a significant part of the electrode is bare . The interface charge between GaAs and gold is not a function of the pH but does depend on the presence of a strong oxidizing agent, as is discussed in more detail below . From impedance measurements it was concluded that the presence of gold islands on a p-GaAs electrode in a 0.01 M H 2 SO4 solution leads to an additional potential drop over the Helmholtz layer at potentials negative with respect to -0 .6 V. As a con-



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sequence, an overlap between the surface states and the H* levels is created and the rate constant for hydrogen evolution is increased. In terms of the model, this potential drop originates from goldrelated surface states occupied by electrons, which means that the condition E. N > fk,(ox)c,, is fulfilled . In this case, s - /s 1 and the expression for the current density reduces to Equation (4) . The current is controlled by the transfer of electrons across the gold/electrolyte interface only . As in the potential range negative with respect to -0 .6V MottSchottky behaviour is found, the potential drop over the Helmholtz layer and, thus, the rate constant for hydrogen evolution do not change as a function of the applied potential . This agrees with the observation that the cathodic current at these gold-coated p-GaAs electrodes is only slightly potential dependent (see Fig . 5, curve b). It was found that the band-edges at the surface of a p-GaAs electrode with gold islands in a 0 .01 M K,Fe(CN) 6 + 0.01 M H 2 SO 4 solution are located at the same energy as those of bare electrodes during the reduction of Fe(CN)6 - . Hence, no charge is present in the gold-related surface states . That means that for the reduction of Fe(CN) 3- the boundary conditionfk,(ox)c o , ~> s„N holds . The rate-constant k,(ox) consists of a potential independent component k°(ox) and a part which is exponentially dependent on the overpotential. For hydrogen evolution at gold a considerable overpotential is needed to observe a small current (see Fig . 4A). It is obvious that k°(ox) is very small . Furthermore, the overlap of the filled surface states with the H` levels is slight even after the shift of the band-edges . The reduction current of Fe(CN)6 - at gold is observed close to the Nernst potential . That means that k°(ox) for this reaction is much larger than for hydrogen evolution . From Fig. I it can be concluded that the overlap between the surface states and the empty states of the Fe(CN)b - r4couple is good . As a consequence, for the reduction of Fe(CN)6 - a rate constant k r(ox) can be expected which is orders of magnitude larger than for hydrogen evolution . When Fe(CN)6 - is present Equation (5) is valid, which means that the cathodic current density is now limited by thermal excitation and is, therefore, potential independent . In Fig. 6, a potential independent current is indeed found in the range more negative than 0 V . This current is smaller than the diffusion limited value. When the potential approaches the flat-band value (V,, = 0 .25 V) the cathodic current decreases to zero ; fi,p, is no longer negligible with respect to s„ N and on approaching the redox-potential of 0 .3 V the cathodic current decreases . From the experimental results presented in Figs 8 and 9 it can be concluded that the phenomena observed for p-GaAs electrodes with gold islands in a I M KOH and a 0 .01 M KAu(CN) 2 + 1 M KCN + 1 M KOH solution are similar to those observed in a 0.01M H 2SO 4 and a 0 .01M K 3 Fe(CN)6 + 0 .01 M H 2 SO 4 solution, respectively . In KOH, a small potential independent current due to the evolution of hydrogen is observed and the band-edges are displaced by about 0 .3 V in negative

direction with respect to the bare electrode . When KAu(CN) 2 is added, the cathodic current increases and the band-edges shift back to their original position . It can be concluded that the rate constant for reduction of Au(CN)z is much larger than that for hydrogen evolution for the same reasons as described above for the Fe(CN)6 - case . Consequently, Au(CN)2 can effectively empty the surface states preventing a shift of the band-edges . As a result, at a gold-coated p-GaAs electrode, gold can be deposited in the dark due to thermal excitation of electrons from the valence band to surface states and subsequent transfer to Au(CN),- in solution . This explains why at p-GaAs electrodes gold deposition proceeds in the dark once gold islands are formed photocathodically[l0] . It can be concluded that the results obtained in alkaline and acidic solutions can be interpreted in terms of the model in the same way . In principle, the same conditions should be fulfilled for p-GaAs electrodes with a thick gold-layer as for p-GaAs with gold islands . That in 0.01 M H 2 SO4 the condition s 4 N > fk,(ox)ca , holds is supported by a number of observations . From the experimental results it was concluded that the (i, V} curve of a p-GaAs electrode with a thick gold layer closely resembles that of a gold electrode in a H 2 SO 4 solution (pH = 2 .5) . Furthermore, in the measured potential range the applied potential falls completely over the Helmholtz-layer at the gold/electrolyte interface. From this we must conclude that the concentration of surface states increases considerably compared to the case of gold islands . The current increases to the diffusion limited value which is due to both the increase of the concentration of surface states and to the increase of the geometric factor f [see Equation (4)] . Furthermore, the rate constant k,(ox) for hydrogen evolution can be different for thin and thick gold layers ; its value is a function of the potential drop over the Helmholtz layer, which is, for thick layers, proportional to the applied potential . These results are in agreement with the predictions of our model if the above mentioned condition holds . This conclusion is supported by the capacitance measurements at p-GaAs electrodes with thick gold layers. In contrast to bare electrodes, the (Cp 2, V)-plots do not agree with the Mott-Schottky relation which indicates that the position of the band-edges at the surface varies with the applied potential. Hence, at a p-GaAs electrode with a thick gold layer the capacitance of the Helmholtz-layer is smaller than that of the semiconductor due to a high concentration of interface states between GaAs and gold . Our model cannot, however, explain why the concentration of surface states in the presence of a thick gold layer is much larger (by a factor of at least 10) than when gold islands are present . In the solution containing Fe(CN)6 the condition fk,(ox)c o , - c„ N was found to hold for p-GaAs electrodes provided with gold islands . Surprisingly, the (i, V) and (VA ,,, V) characteristics obtained at electrodes with thick gold layers in this solution are not in accordance with that condition . In Figs 7A and 7B it can be seen that the applied potential falls completely over the Helmholtz-layer and, as a consequence, the (i, V)-curve strongly resembles that of a bulk gold electrode (see Fig . 4). It must be concluded

Gold and Ill-V semiconductor electrodes-I that, in this case, the condition a, N ~> fk r(ox)c„ is fulfilled . As in the foregoing case, the (i, V)-characteristics are determined by the kinetics at the gold/ electrolyte solution interface . On increasing the K,Fe(CN) e concentration to 0 .1M, however, a potential range was found where thermal excitation was the rate determining step . A possible explanation for the different results obtained at thick and thin gold layers might be that the energetic distribution of the surface states is a function of the layer thickness with, for thicker layers, more states having an energy of -0.3eV above the valence band edge. The thermal excitation step is an exponential function of the energy difference between the surface states and the valence band so e. N would increase strongly if more shallow surface states are created . A surprising result is the observation that anodic oxidation of p-GaAs electrodes provided with a thick gold layer is possible (Figs 5 and 7A) . From Fig. 7B it can be concluded that at positive potentials the band-bending in GaAs under the gold is still independent of the applied potential . Consequently, there is no reason for an anodic current to flow . It must be assumed, therefore, that in this potential range the band-bending at gold-coated parts of the surface is different from that at the bare parts. The band-bending at the bare parts must become smaller when a more positive potential is applied thus allowing the oxidation reaction to occur. A detailed theory considering spatially inhomogeneous bandbending was introduced by Nakato et al E28] . Relation to other work

The model presented here can be compared to that proposed for hydrogen evolution at metal coated InP electrodes[12, 14] . It was found that noble metals islands catalyse the evolution of hydrogen at InP[11, 12, 14,15] . As a measure for the catalytic effect, the change in the flat-band potential observed at illuminated p-type electrodes was plotted against the limiting photocurrent density . These plots resemble the Tafel-plots for the corresponding metal electrodes[12, 29, 30] . From these plots it follows that hydrogen evolution is most effectively catalysed at Pt-islands . This could also be concluded from photocurrent-potential characteristics[12]. KGhne and Scbefold[14] developed a model in which electrons photogenerated in p-InP are supplied to the metal islands via interface states . The hydrogen evolution is determined by the kinetics of electron transfer at the metal/solution interface . The interface states between Pt and InP are located 1 .1 eV above the valence band edge . In contrast to the case of gold-plated GaAs, thermal excitation of electrons from the valence band to these states cannot occur and electrons created by light absorption are needed to evolve hydrogen . In the model proposed by KGhne and Schefold the supply of photogenerated carriers (electrons in the conduction band) does not determine the electrode kinetics . This corresponds in our model to the case for which e,,N> wp,+fkAox)c„ holds ; the supply of electrons to the metal by thermal excitation is not rate determining ; the cathodic current is controlled by the kinetics of electron transfer at the metal/

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electrolyte interface [see Equation (7)] . In our work, the most obvious example of kinetic control at the metal/electrolyte interface is found at GaAs electrodes provided with a thick gold layer in H 2 SO, and KOH solutions . The (i, V)-characteristics corresponding to hydrogen evolution at p-type electrodes are the same as at gold electrodes . This can be explained on the basis of plots of the potential of the gold layer as a function of the applied potential : the applied potential falls completely over the Helmholtz-layer at the gold/electrolyte solution interface . In combination with the corresponding current-potential plot it can be concluded that the (i, V)-curves are totally determined by the gold/ electrolyte solution interface . Our model, however, also allows for a cathodic current limited by the (thermal) supply of electrons. Experimental results corresponding to that case have been obtained with p-type GaAs electrodes provided with gold islands. An additional parallel capacitance due to the gold-related surface states was observed, which supports the important role of surface states at the semiconductor/metal interface . These results are discussed in Part 2. CONCLUSIONS The current-potential characteristics of goldcoated GaAs electrodes have been investigated in various electrolyte solutions . It was found that these electrodes exhibit unusual electrochemical properties. These properties were interpreted on the basis of a model in which electrons from the valence band are supplied to the metal (islands) by thermal excitation via interface states. It is suggested that these states are the same as those which determine the barrier height at dry junctions . Furthermore, it was concluded that our model is compatible with that of KGhne and Schefold which describes the catalysed hydrogen evolution at semiconductor (photo) elec. trodes Acknowledgements-The authors would like to thank Philips Research Laboratories for the use of STM facilities

and E. P . Boonekamp for his help with the experiments . This work was supported by the Netherlands Foundation for Chemical Research (SON), with financial aid from the Netherlands Organization for Scientific Research (NWO) .

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