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The influence of electrodeposited gold on the properties of III–V semiconductor electrodes—part 3. Results on n-GaAs provided with thick gold l
Ekctroc~ Acta, Vol. 38, No. 8, pp. 111%1121.1993 Rimed itt Great Britin.
0013~4686/93s6.00 + a00 Q 1993.Pcqamcm Rcr Ltd.
THE INFLUENCE OF ELECTRODEPOSITED GOLD ON THE PROPERTIES OF III-V SEMICONDUCTOR ELECTRODES-PART 3. RESULTS ON n-GaAs PROVIDED WITH THICK GOLD LAYERS G. OSKAM,* D. VANMAEKELBERGH and J. J. KELLY Debye Research Institute, University of Utrecht, P.O. Box 80.000,3508 TA Utrecht, The Netherlands (Received 7 September 1992; in revisedform 18 November 1992)
Abstract-T’he influence of electrodeposited gold on the current-potential
and impedance characteristics of n-GaAs electrodes was studied in various electrolyte solutions. The potential of the gold layer on n-GaAs was measured as a function of the bias applied to the electrode. It is concluded that the same processes occur as previously proposed for gold-plated p-GaAs electrodes. A model involving goldrelated surface states which interact with both the valence band and the electrolyte solution was used to explain the results for p-GaAs. For n-GaAs, an additional process is found to play an important role: recombination of thermally produced holes with conduction band electrons. Key words: electrodeposition, Schottky diode, semiconductor/metal
1. INTRODUCTION In two previous papers we proposed a kinetic model to account for the electrochemical properties of pGaAs electrodes provided with electrodeposited gold islands or layers[l, 21. The model includes three processes: (i) thermal excitation of electrons from the valence band to empty surface states; (ii) trapping of free holes in filled surface states; and (iii) transfer of electrons from filled surface states to an oxidizing agent in solution. In the present work, the electrochemical characteristics of n-GaAs electrodes provided with gold layers were studied in various electrolyte solutions. Furthermore, the potential of the deposited gold layer was measured as a function of the bias applied to the electrode. It is concluded that the three processes which occur at gold-plated p-GaAs electrodes[l] also take place at n-GaAs. At gold-plated n-GaAs, however, recombination is shown to play a key role in determining the experimental results.
2. EXPERIMENTAL The n-GaAs crystals [(lOO) orientation], doped with Si [(2.0-2.2) 10” cm-9, were obtained from MCP Electronic Materials Ltd (U.K.). 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 8.0M HCl 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 HCl for 30 s. +Author to whom correspondence should be addressed.
electrode, GaAs, electrochemistry.
The electrochemical measurements were performed in a conventional cell with a GaAs rotating disk (geometric area 0.125 cm2) as working electrode, a platinum sheet as counter electrode and a saturated calomel electrode (see) as reference. For rotating ring disk measurements a platinum ring was used. All potentials are given with respect to see. In order to measure the potential of gold layers deposited on n-GaAs, a contact was made to the gold layer with silver paste which was subsequently insulated from the solution with Apiezon. The impedance measurements were performed with a Solartron HF Frequency Response Analyzer (SI 1255) and a Solartron Electrochemical Interface (EC1 1286). The measured impedance was corrected for the resistance due to the electrolyte solution. The gold layers (cu. 150nm) were deposited from a 0.01 M KAu(CN),/l M KCN solution at pH 14. The potential was first pulsed from the open-circuit value to - 1.8 V for about 20ms to create a high density of nuclei. These were grown at cu. - 1.45 V for about 1000 s. An n-GaAs/Au surface was studied in air with Scanning Tunneling Microscopy (STM). It was found that the gold layer consists of spheres with radii of around 20 nm and was extremely porous[ 11. 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 and in the dark. 3. RESULTS 3.1. (i, V) cliaracteristics The (i, V)-curves of bare (curves a and b) and gold-plated (curves c and d) n-GaAs electrodes in a l.OM NaClO, + 0.01 M H2S04 (pH 2.5) solution are presented in Fig. 1A. At gold-plated electrodes, hydrogen is evolved at potentials 0.4 V more positive
1115
1116
G. Ox.4 V/V vs SCE
(4 -2.0 ,
-1.5 1
‘/‘/-
-1.0
0.0 1,
-0.5 I,
-2.0
0.5 I,
5
L
1.0 ,o
’
-2.0
-10
V/V vs SCE -1.5
-1.0
-0.5
0.0
V/V vs SCE
(4
L
@I
et al.
-1.5
-1.0
1.0
-2.0 O3
’ -2.0
0.0
0.5
L
1.0
’
-15
V/V vs SCE
(B) 0.5
-0.5
-1.5
-1.0
-0.5
0.0
0.5
1.0
~
OS
4
-2.0
Fig. 1. Results obtained with bare (a and b) and goldplated (c and d) n-GaAs electrodes (N 1SOnmAu) in a solution of 0.01 M H,SO, + 1 M NaClO,. (A) Currentpotential characteristics. The scan rate was 50 mV s-l and the rotation rates were (a and c): SOOrpmand (b and d): 2ooOrpm. (B) The potential of the gold layer vs. the potential applied to the disk electrode (both vs. see) for the two rotation rates.
Fig. 2. Results obtained with bare (a) and gold-plated (b and c) n-&As electrodes (- 15OnmAu) in a solution of 0.01 M K,Fe(CN), + 0.01 M H,SO, + 0.25 M NaClO,. (A) Current-potential characteristics. The scan rate was 50mV s-l and the rotation rates were (a and b): 5OOrpm and (c): 2000rpm. (B) The potential of the gold layer vs. the potential applied to the disk electrode (both vs. see) for the two rotation rates.
than at bare electrodes. From the two rotation rates shown in Fig. lA, it can be seen that the reduction of H+ is diffusion limited. At potentials negative with respect to the H+/H1 plateau, H,O is reduced. The (i, Qcurves of gold-plated n-GaAs closely resemble those measured at a bulk gold electrode but are shifted cu. 0.2 V to more negative potentials (see Part
both Fe(CN)iand Fe(CN)zshowed that Fe(CN)z- cannot be oxidized at bare and goldplated electrodes. The onset-potential of a gold-plated n-GaAs electrode for hydrogen evolution in a l.OM KOH solution is shifted 0.25 V in the positive direction with respect to that of a bare electrode. The (i, Y)-characteristics of both a bare (a) and a gold-plated (b) n-GaAs electrode in a solution of 0.01 M K,Fe(CN), + l.OM KOH are presented in Fig. 3. At this pH, Fe&N):- can inject holes into the valence band[3, 43. At sufficiently small bandbending, these holes can recombine with electrons from the conduction band and a cathodic recombination current is observed. At a bare electrode (curve a), the onset of the cathodic current is at - l.OV and diffusion limitation is reached at - 1.3 V. At an electrode with a 150nm thick gold layer, the cathodic current starts at -0.6 V and becomes diffusion limited at - 1.0 V.
WI).
In Fig. 2A, the (i, V)-characteristics of bare (curve a) and gold-plated (curves b and c) n-GaAs electrodes in a 0.01 M K,Fe(CN), + 0.25 M NaClO, (pH 2.5) solution are shown. Ring-disk measurements showed that at bare electrodes the reduction of Fe(W);can take place with conduction band electrons at potentials negative with respect to the flat-band value. At an electrode provided with gold a cathodic current is observed between 0 and - l.OV. At an electrode with a 150nm thick gold layer, the reduction of the Fe(CN)istarts at a potential about 0.8V positive with respect to the flat-band value and becomes diffusion limited at potentials more negative than -0.7V (curves b and c). Rotating ring-disk measurements at gold-plated electrodes showed that Fe(CN)g- is not reduced at potentials more positive than OV and, as a consequence, the electrode is not etched. This result agrees with the observations that the gold-plated electrodes are stable in these acidic solutions. Current-potential measurements performed in solutions containing
3.2. Impedance measurements In Fig. 4, the Mott-Schottky plots of bare and gold plated n-GaAs electrodes in both 0.01 M H,SO, and 0.01 M H,SO, + 0.01 M K,Fe(CN), are depicted. The flat-band potential, I&, of a bare electrode is the same (- 1.1 V) in both solutions. The V,, of gold-plated electrodes in the HISO, solution is
Electrodeposited gold and III-V semiconductor electrodes-Part
-1.5
-1.0
-0.5
1117
solution. The flat-band potential of a gold-plated n-GaAs electrode in 0.01 M K,Fe(CN), + l.OM KOH is roughly the same as that of a bare electrode. During the impedance measurements, however, GaAs is etched under the gold, which is then dislodged from the surface.
V/V vs SCE -2.0
3
0.0
0.0 c!
E
2 -2.0
E .>
-4.0
Fig. 3. Current-potential curves of an n-GaAs electrode (a) bare and (b) with a cu. 150 nm thick gold layer in a 0.01 M K,Fe(CN), + LOM KOH solution. The scan rate was ZOmVs-’ and the rotation rate was 5OOrpm.
the same as that of a bare electrode. In K,Fe(CN), solution the Vfb undergoes a shift of about 0.3V in the positive direction with respect to the value for the three above cases. This indicates that in the K,Fe(CN), solution the band-edges at the goldplated surface are lowered in energy. Impedance measurements were also performed in alkaline solution. Some results are shown in Table 1. The flat-band potential of n-GaAs, both bare and provided with a gold layer, in 1.0 M KOH is - 1.8 V. For a bare electrode, the Vfb in 0.01 M K,Fe(CN), + l.OM KOH is shifted by about 0.2V in the positive direction with respect to the value in the KOH 10.0
“E 0 p! a 0 ‘0 2
7.5
z
2.5
,
I
5.0
0.0 -1.5
-1.0
-0.5
0.0
0.5
1.0
V/V vs SCE Fig. 4. Mott-Schottky plots of bare and gold-plated n-GaAs electrodes. Curves (0) and (0) represent a bare electrode and an electrode with a thick gold layer, respectively, in a 0.01 M H,SO, (pH 2.5) solution. Curves (0) and (B) were measured in a 0.01 M Fe&N):-/O.Ol H,SO, @H 2.5) solution: (0) represents a bare electrode and (m) an electrode with a thick gold layer. The measuring frequency was 10 kHz.
3.3. Measurements of the potential of the gold layer In Figs 1B and 2B, the potential of the gold layer, V,,, is plotted against the applied potential, V, for an n-GaAs electrode provided with a 150nm thick gold layer in 0.01 M H,SO, + 1 M NaClO, and in 0.01 M K,Fe(CN), + 0.25 M NaClO, (pH 2.Q respectively. At potentials more positive than about OV, the potential of the gold layer is independent of the bias. Hence, a change of the applied potential results only in a change of the band-bending inside the GaAs and, in this region, C;’ depends linearly on the applied potential (see Fig. 4). When Y is more negative than about OV, V’, changes linearly with V with a slope of 1. In this range, the 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. Around -0.9V, a slight arrest is observed in the plot. In that region, the bandbending is decreased slightly. The (V,, , P’)-plot shows a hysteresis at around OV in the H,SO, solution which is not observed in the solution containing K,Fe-(CN),. At positive potentials, the constant value of V’, is different for the two solutions: V,, N 0 to + 0.1 V in H,SO, while V,, = +0.3 V in the solution containing Fe(CN)z-. The value of V,, was measured as a function of the redox potential by changing the concentration ratio of Fe&N):- (c,,) to Fe(CN)z- (c,,,). In Fig. 5, the (V,, , Qplot is shown for three different solutions. It was found that at applied potentials more positive than about OV, the potential of the gold layer V u is the same as the redox potential of the Fe(CN$-‘4-- couple. This is visualized in the insert of Fig. 5, where V,, is plotted vs. the redox potential which was measured with a bulk gold electrode. The potential of the inflexion point in the plot also depends on the redox potential: when c,,jc,_, increases the inflexion point shifts to more negative potentials. Impedance measurements in the solutions corresponding to curve (a) and (c) in Fig. 5 showed that the flat-band potential is displaced by about 0.15V in positive direction when c,&., increases (see Table 1). It can be concluded that the inflexion points in all three solutions are located at approximately the same band-bending of about 0.65 eV and
Table 1. The flat-band potentials of bare and gold-plated n-GaAs electrodes in various solutions
5,
@a@
Solution
(V vs. see)
0.01 M H,SO, 0.01 M H,SO, + 0.01 M K,Fe(CN), 0.01 M H,SO, + 0.1 M K,Fe(CN), + 0.001 M K,Fe(CN),
- 1.10 - 1.10 -1.1
y; M&SO, + 0.1 M K,Fe(CN), + 0.001 M K,Fe(CN), 1 M KOH + 0.01 M K,Fe(CN),
--1.1 1.80 -1.60
V,, (gold-plated) (V vs. see) - 1.10 -0.76 -0.70 -1.80 -0.86 C(I. -1.6
G. OSKAM et 01.
1118 V/V vs SCE 0
-0.50
0.00
0.50
1
0
0.50 0.25 0.00
!i
-0.25
>
F
2 > -0.50
-0.75
Fig. 5. The potential of the gold layer vs. the potential applied to the working electrode for an n-GaAs electrode with 1SOnm gold in three electrolyte solutions with 0.01M H,SO,: (a) 0.1 M K,Fe(CN), + 0.001M K,Fe(CN),; (b) 0.01MeK,Fe(cN), + 6.01M K;F+zN), + 0.25M ~Zid,t (cl 0.001M K,FdCNL + 0.1 M K,Fe(CNL . The scan rate Was 5OmVs -’ aid t& rotation rate &as %00 rpm. In the insert, the potential of the gold layer is plotted vs. the rest potential measured at a bulk gold electrode. _
that the cathodic branches of the plot, when corrected for the shift of the flat-band potential, almost coincide.
4. DISCUSSION
same way as the recombination rate on the electron concentration at the surface. It is, therefore, dithcult to distinguish between these processes. Impedance results obtained at gold-plated n-GaAs in acidic K,Fe(CN), solution show that holes can be present at the surface and that thermal excitation of VB electrons in the SS indeed occurs (see the next paragraph). As the recombination rate strongly influences the experimental results at gold-plated n-GaAs electrodes, the recombination process is first considered at bare n-GaAs in alkaline K,Fe(CN), solution. At pH 14, the distribution function of Fe(CN)i- levels shows a good overlap with the valence band (VB) and reduction of the oxidizing agent takes place via hole injection into the VB[3, 43. Depending on the band-bending, the holes can either be involved in the anodic dissolution of GaAs or recombine with electrons from the conduction band (CB), which leads to a cathodic current. At n-GaAs, the recombination takes place on a donor type centre[5], which implies a large rate constant for electron capture from the CB. Consequently, it is possible to observe a cathodic current at a relatively large band-bending. A typical value for the band-bending at which recombination sets in at n-GaAs is -0.6eV[5]. This value agrees with the (i, V) results in Fig. 3: the cathodic current starts at - l.OV while the flat-band potential is - 1.6 V.
General
Gold-plated n-GaAs in K,Fe(CN),/K,Fe(CN),
In Part 1, a model was proposed to account for the (f, v), (V,, , V) and (C;‘, V) results of p-GaAs electrodes provided with gold islands or layers in various electrolyte solutions. At p-GaAs/Au dry junctions, it was found that the Fermi-level is pinned on surface states (SS) at 0.3-0.5 eV above the valence band (VB) edge. To explain the electrochemical results, a model was presented in which these goldrelated surface states play a key role. The model involves thermal excitation of VB electrons into the SS. These electrons can subsequently be transferred to an electron acceptor in the electrolyte solution. The holes produced in the VB are in equilibrium with the holes in the bulk of the semiconductor. This model could explain the reduction of Fe(CN)z- at gold-coated p-GaAs electrodes at pH 2.5; this reaction is not found at bare electrodes[l]. In Part 1, it was also concluded that the barrier heights at p-type and n-type GaAs/Au dry junctions are determined by pinning of the Fermi-level on the same SS[l]. It can, therefore, be expected that at gold-coated n-GaAs electrodes, electrons from the VB can also be excited to the SS and, in the presence of an oxidizing agent, be transferred to the solution. In contrast to p-GaAs, the electric field in the depletion layer is such that the holes remain at the surface. As a consequence, a cathodic current must originate from recombination of conduction band (CB) electrons with the thermally produced holes. A cathodic current can be generated in two other ways, eg electrons from the CB can be trapped directly in the SS or electrons can be emitted to the metal and subsequently equilibrate with the SS. At large bandbending, the rates of these processes depend in the
It was found that Fe(CN)z- at pH 2.5 cannot be reduced by hole injection into the VB at bare n-GaAs. The flat-band potential of bare electrodes in acidic K,Fe(CN), solution is the same as in 0.01 M H,SO, . In the (i, V), (VA,, V) and (C; ‘, V) results of electrodes with a 150nm thick gold layer in electrolyte solutions containing 0.01 M K,Fe(CN), (pH 2.5), two regions can be clearly distinguished. At potentials more positive than about OV, the current is very small, the potential of the gold layer is constant and the Mott-Schottky relationship is valid. In the potential range negative with respect to about OV, a cathodic current is observed while the potential of the gold layer depends linearly on the applied potential with a slope of 1 and the Mott-Schottky relationship is no longer valid. In the potential region more positive than OV, impedance measurements show that the bandbending is larger than about 0.7eV (see Fig. 4); no current is observed in this range. From Fig. 5 it can be concluded that the potential of the gold layer is equal to the redox potential of the electrolyte solution and, as a consequence, the current via the gold layer must be very small. From the impedance measurements it follows that the band-edges at the surface of gold-plated n-GaAs electrodes in a 0.01 M K,Fe(CN), solution at pH 2.5 are shifted to a position ca. 0.3eV lower in energy than those of bare electrodes. When a gold layer is present, electrons can be thermally excited to the gold-related surface states (SS), leaving behind holes in the VB. The electrons can subsequently be transferred to the solution in order to achieve equilibrium between the gold and the electrolyte solution. Due to the high band-
Electrodeposited gold and III-V semiconductor electrodes-Part
the holes are trapped at the surface and, as a result, the band-edges shift to lower energy. In Fig.6A, the situation is shown for a gold-plated n-GaAs electrode before equilibrium is reached between the gold and the solution (in this case the band-edges are located at the same energy as for a bare electrode) and in Fig. 6B the situation in which the gold layer has equilibrated with the solution is depicted. Note that the semiconductor is not in equilibrium with the SS. The potential of the gold layer was found to be equal to the redox potential, V&, in the solution. From Fig. 6, it can be understood why on changing Kedor the displacement of the band-edges with respect to those of the bare electrode changes; when P&,,. is close to OV, the shift of the band-edges is much smaller than when VrcdoX is about +0.3 V. This agrees with the results of impedance measurements as shown in Table 1. bending,
V/Vvs
(A)
SCE
CB &--
_
_
_
VB
(B)
V/V vs SCE
EF---
-
-
0
Fig. 6. Energy band diagram of a gold-plated n-GaAs electrode in a 0.01 M K,Fe(CN). solution (OH 2.5) at the inflexion point in the” (Vi., ‘k$plot at -0.1 V. iA) The system before equilibrium between the gold layer and the solution is reached. (B) The equilibrated system; note that the n-GaAs is not in equilibrium with the gold layer.
3
1119
The observation that the band-edges at goldplated n-GaAs electrodes shift to lower energy with respect to bare electrodes in acidic K,Fe(CN), solution, shows that holes are present at the surface. It can, therefore, be concluded that thermal excitation of VB electrons into the SS occurs. It has, so far, been assumed that the space charge layer of a gold-coated electrode is spatially homogeneous. However, when a metal is deposited on semiconductor electrodes, the band structure becomes complicated as the band-bending at the metal-coated and bare parts of the surface may be different. A model which considers this possibility was presented by Nakato et aI.[6, 71. The spatial inhomogeneity of the band-bending is, however, difficult to measure. In a previous paper, we discussed the influence of gold deposits on the electrochemical properties of pGaAs. At p-GaAs electrodes with a thick gold layer, a potential dependent anodic current due to the oxidation of GaAs was observed at potentials more positive than 0.2 V. From the (VA,, V)-curve it was concluded that, in this potential range, the band-bending under the gold was unaltered. It was, therefore, concluded that the band-bending at the bare parts is different from that at the gold-coated parts. No conclusive argument for spatial inhomogeneity of the band-bending is obtained from the impedance measurements at gold-plated n-GaAs electrodes. As the Mott-Schottky plots of goidplated n-GaAs electrodes in acidic K,Fe(CN), solution are linear, it must be assumed that either the spatial inhomogeneity of the band-bending is small in this potential range or the area of one of the interfaces (n-GaAs/electrolyte solution or n-GaAs/Au) is much larger. The . .band-bending at the potential of the inflexion pomt m the (VA,, V)-plot can be obtained from the Mott-Schottky plot and is (generally) found to be about 0.65 eV. At that potential, VA, is still +0.3 V, which means that the Fermi-level in the gold and in the electrolyte solution is located 0.4eV lower in energy than that of the semiconductor (see Fig. 6B). The SS are still in equilibrium with the redoxcouple; this situation can be understood from kinetic considerations. The SS can be filled and emptied via two paths: via the semiconductor and via the redox-couple in the electrolyte solution. Whether the SS are in equilibrium with the semiconductor and/or the solution depends on the rates of these processes. At positive potential the occupancy of the SS is determined by the redox-couple. When the potential becomes more negative, the SS should be filled to equilibrate with the semiconductor. It was argued before that the thermal excitation of VB electrons is fast but that the SS can only equilibrate with the semiconductor if the holes produced can be removed from the surface by recombination; it can be concluded that in the acidic K,Fe(CN), solution, the recombination process is too slow as long as the band-bending is larger than 0.65 eV. When the band-bending is larger than 0.7eV, the holes produced in the VB will be trapped in filled SS and the current is zero. When the band-bending decreases, some holes may recombine with CB electrons and a small cathodic current is observed. On
1120
G. OSKAMet al.
decreasing the band-bending further, the recombination can become very fast. The rate of filling of surface states by thermal excitation of VB electrons can become faster than the rate of emptying of the states by the oxidizing agent. This occurs in the potential range negative with respect to OV, where a cathodic current is found, VA, depends linearly on the applied potential and the Mott-Schottky relationship is no longer valid. In this range, the occupancy of the SS is determined by the semiconductor. That means that when a more negative potential is applied, a potential difference between the gold and the electrolyte solution develops. Due to this overpotential a cathodic current can flow. As all the applied potential falls over the Hehnholtzlayer of the Au/electrolyte solution interface, the band-bending under the gold is constant at a value of about 0.65eV (see Fig. 6B). As previously noted, recombination generally starts at a band-bending of about 0.6 eV. In contrast to bare electrodes, however, the band-bending at the GaAsfAu interface stays the same (cu. 0.65eV) when the potential is made more negative and it is, therefore, difficult to understand how the current can increase to the high values observed. A likely explanation involves spatial inhomogeneity of the band-bending as suggested by Nakato and coworkersC6, 71. It is possible that, when the potential is made more negative, the bandbending at the bare surface decreases. As a consequence, the concentration of electrons at the surface is higher at the bare parts. Due to the lateral concentration gradient, diffusion of electrons from the bare to the gold-plated parts and of holes from the goldplated to the bare parts of the electrode can occur, leading to a recombination current. It can be concluded that, in the potential range negative with respect to OV, the (i, q-curve is determined by a complicated interplay of various processes and, in contrast to gold-plated p-GaAs electrodes, it is difficult to describe the results in terms of a simple quantitative model. The three processes used to explain the results on gold-plated p-type electrodes (thermal excitation of VB electrons into the SS, trapping of free holes in the SS and transfer of SS electrons to an oxidizing agent in solution) play an important role. However, the recombination kinetics of the holes largely determine the results. The electrochemical results can only be explained if it is assumed that CB electrons needed for recombination are provided via parts of the surface where the band-bending is smaller than at the n-GaAs/Au interface. Gold-plated n-GaAs in 0.01 M H,SO,
The (i, V) and (C;‘, V)-curves obtained at goldplated and bare n-GaAs electrodes in 0.01 M H2S0, + 1 M NaClO, (pH 2.5) are surprisingly similar. The flat-band potential of both electrodes is the same; the only difference in the (i, V)-curve is that hydrogen evolution at the gold-plated electrode starts at a potential 0.4V more positive. For the results obtained at the gold-plated electrode, two regions can again be distinguished: at potentials positive with respect to about OV, the current is very small, V’, is constant and the Mott-Schottky plot is
linear. At potentials more negative than OV, the current is still very low until V = -0.75V, but the potential of the gold layer depends linearly on the applied potential while the Mott-Schottky relation is no longer valid. In the potential range more positive than OV, it was found that the gold layer in the solutions containing a strong oxidizing agent equilibrates with the solution and that VA, is equal to V,,. In this case, the redox potential of the H+/H,-couple is -0.4V. It can be concluded from Fig. IB that at positive potentials the gold is not in equilibrium with the H+/H,-couple as VA, N 0 to +O.l V. As no welldefined redox-couple is present, equilibration of the gold with the semiconductor might be faster than with the solution. As a consequence, the gold probably assumes a mixed-potential, determined by processes within the semiconductor and by the presence of traces of 0, in the solution. The V, is the same as that of a bare electrode so, obviously, the influence of the potential of the gold layer on the V, is small in this case. In the potential range positive with respect to OV, the band-bending changes according to the Mott-Schottky relation while the potential of the gold layer does not change. The band-bending is too large to allow holes, which are produced when electrons from the VB are excited into the SS, to recombine with CB electrons. The current is, therefore, very small. At about 0 V, the (VA,, I/)-plot changes and V’, becomes linearly dependent on the applied potential with a slope of about 1. This indicates that electrons excited from the VB are trapped in SS and the holes recombine with CB electrons. As a consequence, the surface is charged as a steady-state current cannot flow when VA, is still about 0.4V more positive than the redox potential of the H+/H,-couple. It is surprising that this process can take place while the band-bending at OV is still extremely large: about 1.1 eV (Fig. 7). Apparently, even at this large bandbending holes can recombine with CB electrons. However, the current needed to charge the surface is V/V
-I
I
Iv_ovI
vs SCE
-1.2
-
/
CB EF_
1 _
/Iss _
1
_
/ “B
/
VH./H*
I
n I
I
-
-I -I -0.8
-0.4
-I
I”
co.4
1 -
+0.8
-I
-I +‘.2
Fig. 7. Energy band diagram of a gold-plated n-GaAs electrode in a 0.01 M H,SO, solution (pH 2.5) at the inflexion point in the (V,, , Qplot at cu. 0 V.
Electrodeposited gold and III-V semiconductor electrodspart
small. The hysteresis observed at around OV in the (V,, , V)-plot may be due to a small rate for the charging of the surface at this large band-bending compared to the scan rate. Both bare and goldplated electrodes have the same V,, so a large spatial inhomogeneity of the band-bending is not very likely. In the 0.01 M K,Fe(CN), + 1 M KOH solution, it was found that the cathodic current due to recombination of injected holes at gold-plated electrodes can occur at very large band-bending. In this case, however, the measurements of the flat-band potential were not reliable. It is interesting to note that the spatial inhomogeneity of the band-bending may be very large: vrrcdox is +0.3 V while the position of the VB edge at the surface of bare GaAs is -0.4 v. When the potential is made more negative, the band-bending (under the gold) stays the same and the total band structure shifts to higher energy as the SS are charged negatively. At V = -0.75 V, the potential of the gold layer is -0.6V and, at that potential, the reduction of H+ starts, as at bulk gold electrodes[l]. As can be seen in Fig. lB, an arrest in the (V,,, V)-plot is observed at the potential of current-onset. Apparently, at first the potential of the gold layer is the limiting parameter for current but as V,, becomes sufficiently negative, the supply of electrons from the semiconductor becomes the limiting step. This is not surprising as the band-bending under the gold is still about 1 eV and the removal of holes from the surface by recombination with CB electrons cannot be very fast. In the arrest, the bandbending decreases to about 0.8 eV. During the potential sweep towards more negative potential, the band-bending at the bare parts of the surface might decrease considerably. Such a spatial inhomogeneity of the band-bending could help to increase the recombination current in the same way as described
3
1121
in the previous section. Furthermore, if the bandbending at the bare parts becomes small enough, hydrogen can be evolved with CB electrons. It can be concluded that the experimental results obtained at gold-plated n-GaAs electrodes in various electrolyte solutions can be described in terms of the three processes proposed before for gold-plated p-GaAs electrodes[l], (i) excitation of VB electrons to gold-related surface states, (ii) trapping of holes in filled surface states and (iii) transfer of electrons from filled surface states to the electrolyte solution, extended with a fourth process, (iv) the recombination of CB electrons with thermally formed VB holes, which plays a decisive role in determining the results. Acknowledgements-This work was supported by the Netherlands Foundation for Chemical Research (SON), with financial aid from the Netherlands Organization for Scientific Research (NWO).
REFERENCES 1. G. Oskam, D. Vanmaekelbergh and J. J. Kelly, Electrochim. Acta 38,291 (1993). 2. G. Oskam, D. Vanmaekelbergh and J. J. Kelly, Electrochim. Acta 38, _301(1993). 3. F. Decker, B. Pettinger and H. Gerischer, J. electrochem. Sot. 130, 1335 (1983). 4. P. H. L. Notten, J. E. A. M. van den Meerakker and J. J. Kelly, Etching of III-V Semiconductors: An Electrochemical Approach. Elsevier, Oxford (199 1). 5. D. Vanmaekelbergh, R. P. ter Heide and W. Kruijt, Ber. Bunsenges. phys. Gem. 93, 1103 (1989). 6. Y. Nakato, H. Yano, S. Nishiura, T. Ueda and H. Tsubomura, J. electroanal. Chem. 228; 97 (1987). 7. Y. Nakato, K. Ueda, H. Yano and H. Tsuboumura, J. phys. Chem. 92,2316 (1988).
0013~4686/93s6.00 + a00 Q 1993.Pcqamcm Rcr Ltd.
THE INFLUENCE OF ELECTRODEPOSITED GOLD ON THE PROPERTIES OF III-V SEMICONDUCTOR ELECTRODES-PART 3. RESULTS ON n-GaAs PROVIDED WITH THICK GOLD LAYERS G. OSKAM,* D. VANMAEKELBERGH and J. J. KELLY Debye Research Institute, University of Utrecht, P.O. Box 80.000,3508 TA Utrecht, The Netherlands (Received 7 September 1992; in revisedform 18 November 1992)
Abstract-T’he influence of electrodeposited gold on the current-potential
and impedance characteristics of n-GaAs electrodes was studied in various electrolyte solutions. The potential of the gold layer on n-GaAs was measured as a function of the bias applied to the electrode. It is concluded that the same processes occur as previously proposed for gold-plated p-GaAs electrodes. A model involving goldrelated surface states which interact with both the valence band and the electrolyte solution was used to explain the results for p-GaAs. For n-GaAs, an additional process is found to play an important role: recombination of thermally produced holes with conduction band electrons. Key words: electrodeposition, Schottky diode, semiconductor/metal
1. INTRODUCTION In two previous papers we proposed a kinetic model to account for the electrochemical properties of pGaAs electrodes provided with electrodeposited gold islands or layers[l, 21. The model includes three processes: (i) thermal excitation of electrons from the valence band to empty surface states; (ii) trapping of free holes in filled surface states; and (iii) transfer of electrons from filled surface states to an oxidizing agent in solution. In the present work, the electrochemical characteristics of n-GaAs electrodes provided with gold layers were studied in various electrolyte solutions. Furthermore, the potential of the deposited gold layer was measured as a function of the bias applied to the electrode. It is concluded that the three processes which occur at gold-plated p-GaAs electrodes[l] also take place at n-GaAs. At gold-plated n-GaAs, however, recombination is shown to play a key role in determining the experimental results.
2. EXPERIMENTAL The n-GaAs crystals [(lOO) orientation], doped with Si [(2.0-2.2) 10” cm-9, were obtained from MCP Electronic Materials Ltd (U.K.). 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 8.0M HCl 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 HCl for 30 s. +Author to whom correspondence should be addressed.
electrode, GaAs, electrochemistry.
The electrochemical measurements were performed in a conventional cell with a GaAs rotating disk (geometric area 0.125 cm2) as working electrode, a platinum sheet as counter electrode and a saturated calomel electrode (see) as reference. For rotating ring disk measurements a platinum ring was used. All potentials are given with respect to see. In order to measure the potential of gold layers deposited on n-GaAs, a contact was made to the gold layer with silver paste which was subsequently insulated from the solution with Apiezon. The impedance measurements were performed with a Solartron HF Frequency Response Analyzer (SI 1255) and a Solartron Electrochemical Interface (EC1 1286). The measured impedance was corrected for the resistance due to the electrolyte solution. The gold layers (cu. 150nm) were deposited from a 0.01 M KAu(CN),/l M KCN solution at pH 14. The potential was first pulsed from the open-circuit value to - 1.8 V for about 20ms to create a high density of nuclei. These were grown at cu. - 1.45 V for about 1000 s. An n-GaAs/Au surface was studied in air with Scanning Tunneling Microscopy (STM). It was found that the gold layer consists of spheres with radii of around 20 nm and was extremely porous[ 11. 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 and in the dark. 3. RESULTS 3.1. (i, V) cliaracteristics The (i, V)-curves of bare (curves a and b) and gold-plated (curves c and d) n-GaAs electrodes in a l.OM NaClO, + 0.01 M H2S04 (pH 2.5) solution are presented in Fig. 1A. At gold-plated electrodes, hydrogen is evolved at potentials 0.4 V more positive
1115
1116
G. Ox.4 V/V vs SCE
(4 -2.0 ,
-1.5 1
‘/‘/-
-1.0
0.0 1,
-0.5 I,
-2.0
0.5 I,
5
L
1.0 ,o
’
-2.0
-10
V/V vs SCE -1.5
-1.0
-0.5
0.0
V/V vs SCE
(4
L
@I
et al.
-1.5
-1.0
1.0
-2.0 O3
’ -2.0
0.0
0.5
L
1.0
’
-15
V/V vs SCE
(B) 0.5
-0.5
-1.5
-1.0
-0.5
0.0
0.5
1.0
~
OS
4
-2.0
Fig. 1. Results obtained with bare (a and b) and goldplated (c and d) n-GaAs electrodes (N 1SOnmAu) in a solution of 0.01 M H,SO, + 1 M NaClO,. (A) Currentpotential characteristics. The scan rate was 50 mV s-l and the rotation rates were (a and c): SOOrpmand (b and d): 2ooOrpm. (B) The potential of the gold layer vs. the potential applied to the disk electrode (both vs. see) for the two rotation rates.
Fig. 2. Results obtained with bare (a) and gold-plated (b and c) n-&As electrodes (- 15OnmAu) in a solution of 0.01 M K,Fe(CN), + 0.01 M H,SO, + 0.25 M NaClO,. (A) Current-potential characteristics. The scan rate was 50mV s-l and the rotation rates were (a and b): 5OOrpm and (c): 2000rpm. (B) The potential of the gold layer vs. the potential applied to the disk electrode (both vs. see) for the two rotation rates.
than at bare electrodes. From the two rotation rates shown in Fig. lA, it can be seen that the reduction of H+ is diffusion limited. At potentials negative with respect to the H+/H1 plateau, H,O is reduced. The (i, Qcurves of gold-plated n-GaAs closely resemble those measured at a bulk gold electrode but are shifted cu. 0.2 V to more negative potentials (see Part
both Fe(CN)iand Fe(CN)zshowed that Fe(CN)z- cannot be oxidized at bare and goldplated electrodes. The onset-potential of a gold-plated n-GaAs electrode for hydrogen evolution in a l.OM KOH solution is shifted 0.25 V in the positive direction with respect to that of a bare electrode. The (i, Y)-characteristics of both a bare (a) and a gold-plated (b) n-GaAs electrode in a solution of 0.01 M K,Fe(CN), + l.OM KOH are presented in Fig. 3. At this pH, Fe&N):- can inject holes into the valence band[3, 43. At sufficiently small bandbending, these holes can recombine with electrons from the conduction band and a cathodic recombination current is observed. At a bare electrode (curve a), the onset of the cathodic current is at - l.OV and diffusion limitation is reached at - 1.3 V. At an electrode with a 150nm thick gold layer, the cathodic current starts at -0.6 V and becomes diffusion limited at - 1.0 V.
WI).
In Fig. 2A, the (i, V)-characteristics of bare (curve a) and gold-plated (curves b and c) n-GaAs electrodes in a 0.01 M K,Fe(CN), + 0.25 M NaClO, (pH 2.5) solution are shown. Ring-disk measurements showed that at bare electrodes the reduction of Fe(W);can take place with conduction band electrons at potentials negative with respect to the flat-band value. At an electrode provided with gold a cathodic current is observed between 0 and - l.OV. At an electrode with a 150nm thick gold layer, the reduction of the Fe(CN)istarts at a potential about 0.8V positive with respect to the flat-band value and becomes diffusion limited at potentials more negative than -0.7V (curves b and c). Rotating ring-disk measurements at gold-plated electrodes showed that Fe(CN)g- is not reduced at potentials more positive than OV and, as a consequence, the electrode is not etched. This result agrees with the observations that the gold-plated electrodes are stable in these acidic solutions. Current-potential measurements performed in solutions containing
3.2. Impedance measurements In Fig. 4, the Mott-Schottky plots of bare and gold plated n-GaAs electrodes in both 0.01 M H,SO, and 0.01 M H,SO, + 0.01 M K,Fe(CN), are depicted. The flat-band potential, I&, of a bare electrode is the same (- 1.1 V) in both solutions. The V,, of gold-plated electrodes in the HISO, solution is
Electrodeposited gold and III-V semiconductor electrodes-Part
-1.5
-1.0
-0.5
1117
solution. The flat-band potential of a gold-plated n-GaAs electrode in 0.01 M K,Fe(CN), + l.OM KOH is roughly the same as that of a bare electrode. During the impedance measurements, however, GaAs is etched under the gold, which is then dislodged from the surface.
V/V vs SCE -2.0
3
0.0
0.0 c!
E
2 -2.0
E .>
-4.0
Fig. 3. Current-potential curves of an n-GaAs electrode (a) bare and (b) with a cu. 150 nm thick gold layer in a 0.01 M K,Fe(CN), + LOM KOH solution. The scan rate was ZOmVs-’ and the rotation rate was 5OOrpm.
the same as that of a bare electrode. In K,Fe(CN), solution the Vfb undergoes a shift of about 0.3V in the positive direction with respect to the value for the three above cases. This indicates that in the K,Fe(CN), solution the band-edges at the goldplated surface are lowered in energy. Impedance measurements were also performed in alkaline solution. Some results are shown in Table 1. The flat-band potential of n-GaAs, both bare and provided with a gold layer, in 1.0 M KOH is - 1.8 V. For a bare electrode, the Vfb in 0.01 M K,Fe(CN), + l.OM KOH is shifted by about 0.2V in the positive direction with respect to the value in the KOH 10.0
“E 0 p! a 0 ‘0 2
7.5
z
2.5
,
I
5.0
0.0 -1.5
-1.0
-0.5
0.0
0.5
1.0
V/V vs SCE Fig. 4. Mott-Schottky plots of bare and gold-plated n-GaAs electrodes. Curves (0) and (0) represent a bare electrode and an electrode with a thick gold layer, respectively, in a 0.01 M H,SO, (pH 2.5) solution. Curves (0) and (B) were measured in a 0.01 M Fe&N):-/O.Ol H,SO, @H 2.5) solution: (0) represents a bare electrode and (m) an electrode with a thick gold layer. The measuring frequency was 10 kHz.
3.3. Measurements of the potential of the gold layer In Figs 1B and 2B, the potential of the gold layer, V,,, is plotted against the applied potential, V, for an n-GaAs electrode provided with a 150nm thick gold layer in 0.01 M H,SO, + 1 M NaClO, and in 0.01 M K,Fe(CN), + 0.25 M NaClO, (pH 2.Q respectively. At potentials more positive than about OV, the potential of the gold layer is independent of the bias. Hence, a change of the applied potential results only in a change of the band-bending inside the GaAs and, in this region, C;’ depends linearly on the applied potential (see Fig. 4). When Y is more negative than about OV, V’, changes linearly with V with a slope of 1. In this range, the 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. Around -0.9V, a slight arrest is observed in the plot. In that region, the bandbending is decreased slightly. The (V,, , P’)-plot shows a hysteresis at around OV in the H,SO, solution which is not observed in the solution containing K,Fe-(CN),. At positive potentials, the constant value of V’, is different for the two solutions: V,, N 0 to + 0.1 V in H,SO, while V,, = +0.3 V in the solution containing Fe(CN)z-. The value of V,, was measured as a function of the redox potential by changing the concentration ratio of Fe&N):- (c,,) to Fe(CN)z- (c,,,). In Fig. 5, the (V,, , Qplot is shown for three different solutions. It was found that at applied potentials more positive than about OV, the potential of the gold layer V u is the same as the redox potential of the Fe(CN$-‘4-- couple. This is visualized in the insert of Fig. 5, where V,, is plotted vs. the redox potential which was measured with a bulk gold electrode. The potential of the inflexion point in the plot also depends on the redox potential: when c,,jc,_, increases the inflexion point shifts to more negative potentials. Impedance measurements in the solutions corresponding to curve (a) and (c) in Fig. 5 showed that the flat-band potential is displaced by about 0.15V in positive direction when c,&., increases (see Table 1). It can be concluded that the inflexion points in all three solutions are located at approximately the same band-bending of about 0.65 eV and
Table 1. The flat-band potentials of bare and gold-plated n-GaAs electrodes in various solutions
5,
@a@
Solution
(V vs. see)
0.01 M H,SO, 0.01 M H,SO, + 0.01 M K,Fe(CN), 0.01 M H,SO, + 0.1 M K,Fe(CN), + 0.001 M K,Fe(CN),
- 1.10 - 1.10 -1.1
y; M&SO, + 0.1 M K,Fe(CN), + 0.001 M K,Fe(CN), 1 M KOH + 0.01 M K,Fe(CN),
--1.1 1.80 -1.60
V,, (gold-plated) (V vs. see) - 1.10 -0.76 -0.70 -1.80 -0.86 C(I. -1.6
G. OSKAM et 01.
1118 V/V vs SCE 0
-0.50
0.00
0.50
1
0
0.50 0.25 0.00
!i
-0.25
>
F
2 > -0.50
-0.75
Fig. 5. The potential of the gold layer vs. the potential applied to the working electrode for an n-GaAs electrode with 1SOnm gold in three electrolyte solutions with 0.01M H,SO,: (a) 0.1 M K,Fe(CN), + 0.001M K,Fe(CN),; (b) 0.01MeK,Fe(cN), + 6.01M K;F+zN), + 0.25M ~Zid,t (cl 0.001M K,FdCNL + 0.1 M K,Fe(CNL . The scan rate Was 5OmVs -’ aid t& rotation rate &as %00 rpm. In the insert, the potential of the gold layer is plotted vs. the rest potential measured at a bulk gold electrode. _
that the cathodic branches of the plot, when corrected for the shift of the flat-band potential, almost coincide.
4. DISCUSSION
same way as the recombination rate on the electron concentration at the surface. It is, therefore, dithcult to distinguish between these processes. Impedance results obtained at gold-plated n-GaAs in acidic K,Fe(CN), solution show that holes can be present at the surface and that thermal excitation of VB electrons in the SS indeed occurs (see the next paragraph). As the recombination rate strongly influences the experimental results at gold-plated n-GaAs electrodes, the recombination process is first considered at bare n-GaAs in alkaline K,Fe(CN), solution. At pH 14, the distribution function of Fe(CN)i- levels shows a good overlap with the valence band (VB) and reduction of the oxidizing agent takes place via hole injection into the VB[3, 43. Depending on the band-bending, the holes can either be involved in the anodic dissolution of GaAs or recombine with electrons from the conduction band (CB), which leads to a cathodic current. At n-GaAs, the recombination takes place on a donor type centre[5], which implies a large rate constant for electron capture from the CB. Consequently, it is possible to observe a cathodic current at a relatively large band-bending. A typical value for the band-bending at which recombination sets in at n-GaAs is -0.6eV[5]. This value agrees with the (i, V) results in Fig. 3: the cathodic current starts at - l.OV while the flat-band potential is - 1.6 V.
General
Gold-plated n-GaAs in K,Fe(CN),/K,Fe(CN),
In Part 1, a model was proposed to account for the (f, v), (V,, , V) and (C;‘, V) results of p-GaAs electrodes provided with gold islands or layers in various electrolyte solutions. At p-GaAs/Au dry junctions, it was found that the Fermi-level is pinned on surface states (SS) at 0.3-0.5 eV above the valence band (VB) edge. To explain the electrochemical results, a model was presented in which these goldrelated surface states play a key role. The model involves thermal excitation of VB electrons into the SS. These electrons can subsequently be transferred to an electron acceptor in the electrolyte solution. The holes produced in the VB are in equilibrium with the holes in the bulk of the semiconductor. This model could explain the reduction of Fe(CN)z- at gold-coated p-GaAs electrodes at pH 2.5; this reaction is not found at bare electrodes[l]. In Part 1, it was also concluded that the barrier heights at p-type and n-type GaAs/Au dry junctions are determined by pinning of the Fermi-level on the same SS[l]. It can, therefore, be expected that at gold-coated n-GaAs electrodes, electrons from the VB can also be excited to the SS and, in the presence of an oxidizing agent, be transferred to the solution. In contrast to p-GaAs, the electric field in the depletion layer is such that the holes remain at the surface. As a consequence, a cathodic current must originate from recombination of conduction band (CB) electrons with the thermally produced holes. A cathodic current can be generated in two other ways, eg electrons from the CB can be trapped directly in the SS or electrons can be emitted to the metal and subsequently equilibrate with the SS. At large bandbending, the rates of these processes depend in the
It was found that Fe(CN)z- at pH 2.5 cannot be reduced by hole injection into the VB at bare n-GaAs. The flat-band potential of bare electrodes in acidic K,Fe(CN), solution is the same as in 0.01 M H,SO, . In the (i, V), (VA,, V) and (C; ‘, V) results of electrodes with a 150nm thick gold layer in electrolyte solutions containing 0.01 M K,Fe(CN), (pH 2.5), two regions can be clearly distinguished. At potentials more positive than about OV, the current is very small, the potential of the gold layer is constant and the Mott-Schottky relationship is valid. In the potential range negative with respect to about OV, a cathodic current is observed while the potential of the gold layer depends linearly on the applied potential with a slope of 1 and the Mott-Schottky relationship is no longer valid. In the potential region more positive than OV, impedance measurements show that the bandbending is larger than about 0.7eV (see Fig. 4); no current is observed in this range. From Fig. 5 it can be concluded that the potential of the gold layer is equal to the redox potential of the electrolyte solution and, as a consequence, the current via the gold layer must be very small. From the impedance measurements it follows that the band-edges at the surface of gold-plated n-GaAs electrodes in a 0.01 M K,Fe(CN), solution at pH 2.5 are shifted to a position ca. 0.3eV lower in energy than those of bare electrodes. When a gold layer is present, electrons can be thermally excited to the gold-related surface states (SS), leaving behind holes in the VB. The electrons can subsequently be transferred to the solution in order to achieve equilibrium between the gold and the electrolyte solution. Due to the high band-
Electrodeposited gold and III-V semiconductor electrodes-Part
the holes are trapped at the surface and, as a result, the band-edges shift to lower energy. In Fig.6A, the situation is shown for a gold-plated n-GaAs electrode before equilibrium is reached between the gold and the solution (in this case the band-edges are located at the same energy as for a bare electrode) and in Fig. 6B the situation in which the gold layer has equilibrated with the solution is depicted. Note that the semiconductor is not in equilibrium with the SS. The potential of the gold layer was found to be equal to the redox potential, V&, in the solution. From Fig. 6, it can be understood why on changing Kedor the displacement of the band-edges with respect to those of the bare electrode changes; when P&,,. is close to OV, the shift of the band-edges is much smaller than when VrcdoX is about +0.3 V. This agrees with the results of impedance measurements as shown in Table 1. bending,
V/Vvs
(A)
SCE
CB &--
_
_
_
VB
(B)
V/V vs SCE
EF---
-
-
0
Fig. 6. Energy band diagram of a gold-plated n-GaAs electrode in a 0.01 M K,Fe(CN). solution (OH 2.5) at the inflexion point in the” (Vi., ‘k$plot at -0.1 V. iA) The system before equilibrium between the gold layer and the solution is reached. (B) The equilibrated system; note that the n-GaAs is not in equilibrium with the gold layer.
3
1119
The observation that the band-edges at goldplated n-GaAs electrodes shift to lower energy with respect to bare electrodes in acidic K,Fe(CN), solution, shows that holes are present at the surface. It can, therefore, be concluded that thermal excitation of VB electrons into the SS occurs. It has, so far, been assumed that the space charge layer of a gold-coated electrode is spatially homogeneous. However, when a metal is deposited on semiconductor electrodes, the band structure becomes complicated as the band-bending at the metal-coated and bare parts of the surface may be different. A model which considers this possibility was presented by Nakato et aI.[6, 71. The spatial inhomogeneity of the band-bending is, however, difficult to measure. In a previous paper, we discussed the influence of gold deposits on the electrochemical properties of pGaAs. At p-GaAs electrodes with a thick gold layer, a potential dependent anodic current due to the oxidation of GaAs was observed at potentials more positive than 0.2 V. From the (VA,, V)-curve it was concluded that, in this potential range, the band-bending under the gold was unaltered. It was, therefore, concluded that the band-bending at the bare parts is different from that at the gold-coated parts. No conclusive argument for spatial inhomogeneity of the band-bending is obtained from the impedance measurements at gold-plated n-GaAs electrodes. As the Mott-Schottky plots of goidplated n-GaAs electrodes in acidic K,Fe(CN), solution are linear, it must be assumed that either the spatial inhomogeneity of the band-bending is small in this potential range or the area of one of the interfaces (n-GaAs/electrolyte solution or n-GaAs/Au) is much larger. The . .band-bending at the potential of the inflexion pomt m the (VA,, V)-plot can be obtained from the Mott-Schottky plot and is (generally) found to be about 0.65 eV. At that potential, VA, is still +0.3 V, which means that the Fermi-level in the gold and in the electrolyte solution is located 0.4eV lower in energy than that of the semiconductor (see Fig. 6B). The SS are still in equilibrium with the redoxcouple; this situation can be understood from kinetic considerations. The SS can be filled and emptied via two paths: via the semiconductor and via the redox-couple in the electrolyte solution. Whether the SS are in equilibrium with the semiconductor and/or the solution depends on the rates of these processes. At positive potential the occupancy of the SS is determined by the redox-couple. When the potential becomes more negative, the SS should be filled to equilibrate with the semiconductor. It was argued before that the thermal excitation of VB electrons is fast but that the SS can only equilibrate with the semiconductor if the holes produced can be removed from the surface by recombination; it can be concluded that in the acidic K,Fe(CN), solution, the recombination process is too slow as long as the band-bending is larger than 0.65 eV. When the band-bending is larger than 0.7eV, the holes produced in the VB will be trapped in filled SS and the current is zero. When the band-bending decreases, some holes may recombine with CB electrons and a small cathodic current is observed. On
1120
G. OSKAMet al.
decreasing the band-bending further, the recombination can become very fast. The rate of filling of surface states by thermal excitation of VB electrons can become faster than the rate of emptying of the states by the oxidizing agent. This occurs in the potential range negative with respect to OV, where a cathodic current is found, VA, depends linearly on the applied potential and the Mott-Schottky relationship is no longer valid. In this range, the occupancy of the SS is determined by the semiconductor. That means that when a more negative potential is applied, a potential difference between the gold and the electrolyte solution develops. Due to this overpotential a cathodic current can flow. As all the applied potential falls over the Hehnholtzlayer of the Au/electrolyte solution interface, the band-bending under the gold is constant at a value of about 0.65eV (see Fig. 6B). As previously noted, recombination generally starts at a band-bending of about 0.6 eV. In contrast to bare electrodes, however, the band-bending at the GaAsfAu interface stays the same (cu. 0.65eV) when the potential is made more negative and it is, therefore, difficult to understand how the current can increase to the high values observed. A likely explanation involves spatial inhomogeneity of the band-bending as suggested by Nakato and coworkersC6, 71. It is possible that, when the potential is made more negative, the bandbending at the bare surface decreases. As a consequence, the concentration of electrons at the surface is higher at the bare parts. Due to the lateral concentration gradient, diffusion of electrons from the bare to the gold-plated parts and of holes from the goldplated to the bare parts of the electrode can occur, leading to a recombination current. It can be concluded that, in the potential range negative with respect to OV, the (i, q-curve is determined by a complicated interplay of various processes and, in contrast to gold-plated p-GaAs electrodes, it is difficult to describe the results in terms of a simple quantitative model. The three processes used to explain the results on gold-plated p-type electrodes (thermal excitation of VB electrons into the SS, trapping of free holes in the SS and transfer of SS electrons to an oxidizing agent in solution) play an important role. However, the recombination kinetics of the holes largely determine the results. The electrochemical results can only be explained if it is assumed that CB electrons needed for recombination are provided via parts of the surface where the band-bending is smaller than at the n-GaAs/Au interface. Gold-plated n-GaAs in 0.01 M H,SO,
The (i, V) and (C;‘, V)-curves obtained at goldplated and bare n-GaAs electrodes in 0.01 M H2S0, + 1 M NaClO, (pH 2.5) are surprisingly similar. The flat-band potential of both electrodes is the same; the only difference in the (i, V)-curve is that hydrogen evolution at the gold-plated electrode starts at a potential 0.4V more positive. For the results obtained at the gold-plated electrode, two regions can again be distinguished: at potentials positive with respect to about OV, the current is very small, V’, is constant and the Mott-Schottky plot is
linear. At potentials more negative than OV, the current is still very low until V = -0.75V, but the potential of the gold layer depends linearly on the applied potential while the Mott-Schottky relation is no longer valid. In the potential range more positive than OV, it was found that the gold layer in the solutions containing a strong oxidizing agent equilibrates with the solution and that VA, is equal to V,,. In this case, the redox potential of the H+/H,-couple is -0.4V. It can be concluded from Fig. IB that at positive potentials the gold is not in equilibrium with the H+/H,-couple as VA, N 0 to +O.l V. As no welldefined redox-couple is present, equilibration of the gold with the semiconductor might be faster than with the solution. As a consequence, the gold probably assumes a mixed-potential, determined by processes within the semiconductor and by the presence of traces of 0, in the solution. The V, is the same as that of a bare electrode so, obviously, the influence of the potential of the gold layer on the V, is small in this case. In the potential range positive with respect to OV, the band-bending changes according to the Mott-Schottky relation while the potential of the gold layer does not change. The band-bending is too large to allow holes, which are produced when electrons from the VB are excited into the SS, to recombine with CB electrons. The current is, therefore, very small. At about 0 V, the (VA,, I/)-plot changes and V’, becomes linearly dependent on the applied potential with a slope of about 1. This indicates that electrons excited from the VB are trapped in SS and the holes recombine with CB electrons. As a consequence, the surface is charged as a steady-state current cannot flow when VA, is still about 0.4V more positive than the redox potential of the H+/H,-couple. It is surprising that this process can take place while the band-bending at OV is still extremely large: about 1.1 eV (Fig. 7). Apparently, even at this large bandbending holes can recombine with CB electrons. However, the current needed to charge the surface is V/V
-I
I
Iv_ovI
vs SCE
-1.2
-
/
CB EF_
1 _
/Iss _
1
_
/ “B
/
VH./H*
I
n I
I
-
-I -I -0.8
-0.4
-I
I”
co.4
1 -
+0.8
-I
-I +‘.2
Fig. 7. Energy band diagram of a gold-plated n-GaAs electrode in a 0.01 M H,SO, solution (pH 2.5) at the inflexion point in the (V,, , Qplot at cu. 0 V.
Electrodeposited gold and III-V semiconductor electrodspart
small. The hysteresis observed at around OV in the (V,, , V)-plot may be due to a small rate for the charging of the surface at this large band-bending compared to the scan rate. Both bare and goldplated electrodes have the same V,, so a large spatial inhomogeneity of the band-bending is not very likely. In the 0.01 M K,Fe(CN), + 1 M KOH solution, it was found that the cathodic current due to recombination of injected holes at gold-plated electrodes can occur at very large band-bending. In this case, however, the measurements of the flat-band potential were not reliable. It is interesting to note that the spatial inhomogeneity of the band-bending may be very large: vrrcdox is +0.3 V while the position of the VB edge at the surface of bare GaAs is -0.4 v. When the potential is made more negative, the band-bending (under the gold) stays the same and the total band structure shifts to higher energy as the SS are charged negatively. At V = -0.75 V, the potential of the gold layer is -0.6V and, at that potential, the reduction of H+ starts, as at bulk gold electrodes[l]. As can be seen in Fig. lB, an arrest in the (V,,, V)-plot is observed at the potential of current-onset. Apparently, at first the potential of the gold layer is the limiting parameter for current but as V,, becomes sufficiently negative, the supply of electrons from the semiconductor becomes the limiting step. This is not surprising as the band-bending under the gold is still about 1 eV and the removal of holes from the surface by recombination with CB electrons cannot be very fast. In the arrest, the bandbending decreases to about 0.8 eV. During the potential sweep towards more negative potential, the band-bending at the bare parts of the surface might decrease considerably. Such a spatial inhomogeneity of the band-bending could help to increase the recombination current in the same way as described
3
1121
in the previous section. Furthermore, if the bandbending at the bare parts becomes small enough, hydrogen can be evolved with CB electrons. It can be concluded that the experimental results obtained at gold-plated n-GaAs electrodes in various electrolyte solutions can be described in terms of the three processes proposed before for gold-plated p-GaAs electrodes[l], (i) excitation of VB electrons to gold-related surface states, (ii) trapping of holes in filled surface states and (iii) transfer of electrons from filled surface states to the electrolyte solution, extended with a fourth process, (iv) the recombination of CB electrons with thermally formed VB holes, which plays a decisive role in determining the results. Acknowledgements-This work was supported by the Netherlands Foundation for Chemical Research (SON), with financial aid from the Netherlands Organization for Scientific Research (NWO).
REFERENCES 1. G. Oskam, D. Vanmaekelbergh and J. J. Kelly, Electrochim. Acta 38,291 (1993). 2. G. Oskam, D. Vanmaekelbergh and J. J. Kelly, Electrochim. Acta 38, _301(1993). 3. F. Decker, B. Pettinger and H. Gerischer, J. electrochem. Sot. 130, 1335 (1983). 4. P. H. L. Notten, J. E. A. M. van den Meerakker and J. J. Kelly, Etching of III-V Semiconductors: An Electrochemical Approach. Elsevier, Oxford (199 1). 5. D. Vanmaekelbergh, R. P. ter Heide and W. Kruijt, Ber. Bunsenges. phys. Gem. 93, 1103 (1989). 6. Y. Nakato, H. Yano, S. Nishiura, T. Ueda and H. Tsubomura, J. electroanal. Chem. 228; 97 (1987). 7. Y. Nakato, K. Ueda, H. Yano and H. Tsuboumura, J. phys. Chem. 92,2316 (1988).