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Semiconductor electrode modifications: influence on the state distribution at the interface
Elrcrrochimlca Acta. “0,. Printed I” oreat FJrrtmn.
34, NO. 12. pp. 1717-1722.
1989.
00134686,*9 Pcq.m””
S3m+O.M Press pk.
SEMICONDUCTOR ELECTRODE MODIFICATIONS: INFLUENCE ON THE STATE DISTRIBUTION AT THE INTERFACE P. ALLONGUE
and E. SOUTEYRAND
LP15 du CNRS, Physique des Liquides et Electrochimie, Laboratoire de I’Universiti P. & M. Curie, Tour 22, 4 place Jussieu, 75252 Paris Cedex 05, France (Received
1 October 1988; in revisedform 18 January 1989)
Abstract-Surface modifications including a surface preparation (etching, photoetching _) followed by electrodeposition of a discontinuous metal film are presented. Photocapacitance spectra and surface observations reveal that such a procedure is equivalent to modify both the energy and the spatial distribution of the interface states in the case of Pt and Ru.
INTRODUCTION Surface modifications of semiconductor electrodes are currently investigated by a number of research groups in view of improving or limiting some electrochemical reactions occurring at the interface. Metal deposition remains an attractive technique for modifying the surface although it has been extensively studied to date. In that field the main goals are either to achieve the complete stabilization of photoanodes or to catalyse hydrogen evolution at photocathodes with optically transparent metal films to avoid light absorption[ l-51. In number of publications the presence of some surface states in the bandgap of the semiconductor is invoked to interpret the results even though no direct characterization of these energy levels is performed. Moreover, the morphology of the deposited films is generally not considered although it plays a key role for such modifications[Sj. In the present paper, we report experimental informations upon the last two above mentioned points in the case of n-GaAs electrodes modified by metal electrodeposition: (i) the surface state distribution is determined by photocapacitance measurements[6-81; (ii) the surface topography, before and after deposition, is observed by transmission electron microscopy (TEM). In the present work, the metal is electrodeposited onto the semiconductor under potentiostatic conditions[5]. In the case of Pt and Ru, the photocapacitance results show that both metals induce surface states in the bandgap of GaAs and also passivate states which are intrinsic to the GaAsselectrolyte interface[7, 81. The surface observations reveal, for a given metal, that the distribution of the islets (which constitute the film) depends strongly on the surface preparation prior to deposition (the electrochemical parameters for deposition being kept constant[5]); in some cases it is also observed that for a given surface preparation, the metal grain distribution depends on the deposited metal. Considering together both types of results (photocapacitance and surface observations) suggests that our surface modifications (including a surface preparation followed by a metal electrodeposition)
induce a modification of both the energy and the spatial distributions of the interface states on GaAs. Possible applications will be briefly mentioned.
EXPERIMENTAL Semiconductor electrodes are made of n-GaAs single crystals, (100) oriented and Si doped (lOI cmm3). The surface is coated either with Pt or Ru by electrodeposition, under potentiostatic conditions, from (NH4)2 PtCl, and RuCl, solutions[6]. Before each experiment the surface is polished with a 1 nm diamond past and softly etched with the usual mixture (H,O: H,O, : HzSO,) (vol. 25: 2: 2) at room temperature. The deposition is performed onto (i) as prepared, (ii) photoetched or (iii) chemically corroded surfaces. The photoetching consists in corroding the surface under illumination in 1 M KOH (I,,= 0.6 mAcm’) for 10 min. The chemical corrosion is performed by injection of holes from a lo-‘M ferri/ferrocyanide (pH 14) solution. In both cases the alkaline pH prevents the oxide formation[9]; the photoetching and the chemical corrosion remove layers of about 340 and 160 nm respectively after the soft etching. The state distribution at the interface is studied by the photocapacitance technique as far as the energy distribution is concerned. The technique consists in measuring the variations of the space charge capacitance of the semiconductor as a function of the incident sub-gap wavelength (hv < energy bandgap); refs [f&8] give more details about the theory and the experimental set-up. For a n-type semiconductor, one has to know that each positive discontinuity in a photocapacitance spectrum AC(hv) is associated with a transition between a surface (or bulk) state and the conduction band; by contrast, a transition between the valence band and a state is accounted for by a negative step in the spectrum. The spectrum analysis is presented in the literatureC6, 71 as well as the method to separate bulk states from surface states. Transmission electron microscopy (TEM) is used for observing the surface topography and/or the film
1717
1718
P. ALLONGUE
and
morphology (particle size, density, spatial distribution). Samples are prepared by means of the extractive replica technique. The smallest discernible metal particle is about 1 nm diameter.
RESULTS Figures 1 and 2 present the photocapacitance speotra for both a Ru coated and a bare n-GaAs surface in contact with a 1 M KOH solution. In both cases, the applied potential V(us see) is adjusted to promote the same band bending V,=O.95 V in the semiconductor (V is determined by plotting the Mott-Schottky diagram in each case). Therefore, every difference be-
I
I
I
0.8
1.0
I
I Ec -1.30
.I0
:I0
0.6 I--L--
ENERGY
1.2
1.4
(eV)
Fig. 1. Photocapacitance spectrum for a bare n-GaAs electrode in contact with 1 M KOH. The surface is only chemically etched (ie “as prepared” in the text). The applied potential is V= -0.9 V/see (flat band potential - 1.85 V/see) so that the band bending is 0.95 V. The capacitance in darkness is 14 nF. The spectrum is normalized with respect to incident photon flux +,,[6]
1
I
I
I
E. SOUTEYRAND
tween the spectra can be attributed to surface effects[7, 81. The comparison between the photocapacitance spectra of Figs 1 and 2 reveals that (i) there are several transitions which are common to both spectra and (ii) Ru induces a small additional peak around 1.25 eV (Fig. 2). The four common transitions occur at 0.85, 0.98, 1.10 and 1.30 eV; the main negative peak centered at 0.85 eV has been attributed elsewhere to the well-known bulk state EL2 in GaAs; this deep level is located at 0.85 eV below the conduction band[6]. The last three transitions are related to the interactions between the GaAs surface and the solvent; the corresponding surface states act as corrosion intermediates and are located at 0.98, 1.10 and 1.30 eV below the conduction band edge [7, 81 because the associated discontinuities are positive steps (this also explains the indexations E, -0.98 eV )_The peak at 1.25 eV; which only exists in the presence of Ruthenium (Fig. 2) can be regarded as the combination of a positive step at 1.17 eV and a negative one at 1.25 eV; in other words, this peak accounts for the presence of two Ruthenium-induced states located at EC1.17 eV and E,+ 1.25 eV respectively. Figure 3 visualizes the energy distribution of the interface states derived from Figs 1 and 2: E, = EC-O.98 eV and E, = E, - 1.10 eV are the corrosion states; the states E& = EC--O.2 eV and E,,= E, +0.26 eV are the Ruinduced states; for the sake of simplicity, the band tail (E, - 1.30 eV) and the bulk state EL2 are not figured. The last difference between Figs 1 and 2 concerns the amplitude 6, of the transition observed at l.lOeV. Note that a1 remains unchanged. The smaller amplitude 6, after Ruthenium electrodeposition indicates that the density of the state E, has been lowered. In other words, Ru-adatoms passivate the state E, but not the state E; To summarize, Figs 1 and 2 show that Ru creates two new states (ERU and E&j and passivates the state E, only. A quite similar result is found with Pt: Pt induces also two states close to the band edges (at EC -0.16 eV and Ev+0.21 eV) but it passivates the state E, in contrast to the Ru effect[7, 81. Before going further it is worth recalling the conclusions derived from previous photocapacitance exper-
I
CONDUCTION
BAN
J-
+
--E”R”
G:
IO.1 ev
E,=1.43 eV
a
;;
a
21;
0.6
0.0
1.0
1.2
ENERGY
(eV)
1.4
Fig. 2. Photocapacitance spectrum for a Ru-coated n-GaAs electrode in contact with 1 M KOH: applied potential -0.88 V/see (flat band potential - 1.83 V/XC). The capacitance in darkness is 14 nF. The spectrum is normalized as in Fig. 1.
VALENCE a
BAND
]
CcmROSlON STATES
by
Fig. 3. Energy distribution of the surface states for a GaAs surface coated with Ru islands. E, (E,) is the energy at the bottom of the conduction (top of the valence) band, the states E, =I?, -0.98 eV and E, =E, - 1.10 eV are the corrosion states (see text); the states E&=E, -0.2 eV and E,, = E, +0.26 eV are the Ru-induced states. Note that Ru also passivated the state E, (see text).
Semiconductor
electrode
iments[7, S]: (i) under photocorrosion conditions in 1 M KOH photoholes are transferred to solution through both the states E, and E,; (ii) under chemical corrosion conditions in a ferri/ferrocyanide (pH 14) solution, holes are injected preferentially in the state E,. These results suggest that the surface preparations used in this work (photoetching, chemical corrosion) are likely to affect the spatial distribution and probably the density of both states E, and E,. Keeping in mind the above remarks and the conclusions derived from Figs 1 and 2 we have studied the evolution of Pt and Ru electrodeposits as a function of the surface preparation (Figs 5 and 6) to examine whether there exists correlations between the surface
Fig. 4. TEM observations
modifications
1719
pretreatment and the morphology of the deposited film. In fact, the following observations show that the changes in the morphology concern the distribution of the metallic islets (about 1 nm diameter) which compose the film. At first, Fig. 4 shows that the topography of the surface actually depends on its preparation. When it is “as prepared” the surface is practically smooth (not shown); on the contrary, a photoetched surface (Fig. 4a) exhibits a marked topography with “waves” parallel to the
of a (GaAs) surface, (100) orientation, as a function photoetching; (b) chemical corrosion.
of the surface
preparation:
(a)
1720
P. ALLONGUE and E. SOUTEYRAND
Fig. 5. TEM observations of a Ru-film, coated on n-GaAs, as a function of the surface preparation: prior to deposition the surface is “as prepared” (a) and chemically corroded (b). The conditiom for electrodeposition are the same in both cases.
regularly distributed valleys over the whole surface (although almost the same thickness of material has been removed). Figures 5 and 6 show micrographs of electrodeposited Pt and Ru films onto variously prepared surfaces. Note that the parameters for electroplating are kept constant for a given metal; Pt is deposited at -0.7 V/see for 2 s; the respective parameters are - 1.0 V/see and 90 s for Ru[S]. Figure 5 depicts the influence of the surface preparation for a given metal (eg Ru). If the surface is as prepared the film consists in large cells (200 -400 nm wide) regularly spaced in which the microcrystallites of Ru (l-2 nm
diameter) have uniformly nucleated and grown (Fig. 5a). Onto a chemically corroded surface, the film morphology is quite different (Fig. 5b): except for regularly distributed cells (30 nm wide), free of metal, the metal grains of Ru have uniformly nucleated. It is worth noticing that for each of these two surface preparations of Fig. 5 the distribution of the Pt islands is similar to that of the Ru islands[8]. At variance, Fig. 6 shows that onto a photoetched surface Pt and Ru islets present completely different distributions: Pt crystallizes along parallel lines, that recall the topography in Fig. 4a, whereas Ru nucleates almost uni-
Semiconductor
electrode modifications
1721
Fig. 6. TEM observations of Pt and Ru films electrodeposited onto a photoetched surface: (a) Pt; (b) Ru
formly over the surface. Finally one can remark that independently of the surface preparation the films are always made of very small particles (l--2 nm) while their organization changes; this is probably due to similar growth modes of the Pt and Ru microcrystallites.
DISCUSSION With respect to the above results it must be emphasized that the expectations derived from photocapacitance measurements are quite consistent with the surface observations. As a matter of fact, the surface
topography, ie the spatial distribution of the surface defects, depends on the preparation procedure (Fig. 4). In the same way, a given metal deposits quite differently when the surface preparation changes (see Figs 5 and 6). Finally for some surface preparations, the distribution of the crystallites depends on the metal (see Fig. 6). Figure 6 and the photocapacitance results suggest that there are two types of surface defects D, and D,, probably related to the states E, and E,, on which Pt and Ru would nucleate respectively. Figure 7 sketches the surface topography of a photoetched surface (Fig. 4); it is assumed that the defect D, is located at the bottom of the waves while D, is regularly distribu-
1722
P.ALLONGUE
and E.SOUTEYRANU
CONCLUSION D,(E,-0.98eV)
SIDE
VIEW
Fig. 7. Schematic representation of a photoetched n-GaAs surface, (100) orientation, according to Fig. 4a. The symbols (0) and (+) represent the localization of the surface defects D, and D, on which Pt and Ru nucleate respectively (see text).
ted over the wave-sides. Notice that this assumption is not in contradiction with crystallographic considerations because the defects of highest and lowest energy (Dl and D2 respectively) are effectively related to the higher (E,) and the lower (E,) surface state with respect to the top of the valence band (see Fig. 3). Considering such an assumption it is now obvious that Pt must nucleate along parallel lines and Ru uniformly, in agreement with Fig. 6, because they respectively passivate the state E,(D,) and E2(D2). The fact that the distance between two alignments of Pt crystaliites is close to the width of the waves of Fig. 4a makes the assumption more convincing and the spacial identification of D, and D, very likely in this peculiar case. Contrary to the above favourable case, the Pt and Ru grains which are electrodeposited onto a “as prepared” surface (Fig. 5a) present the same distribution. This suggests that the soft etching is not able to spatially separate the surface defects D, and D,. All happens as if D, and D, were paired-off. The last remark applies also to chemically corroded surfaces (Fig. 5b). However the distribution of the pairs (Dl, D2) must be quite different from that observed with “as prepared” surface when comparing Figs 5a and b. The above observations tend to prove that there is a close correlation between the occurrence of a marked surface topography and the spatial discrimination of the defects D, and D, (see Figs. 5 and 6). This is not unrealistic and does not question our assumption about D, and D,. So far we have shown (i) that the surface states E, and E, are related to surface defects which have been identified in some cases (Fig. 7), (ii) that these defects act as nucleation sites for Pt and Ru respectively, (iii) that it is possible to change their spatial distribution by varying the surface preparation and (iv) that each metal induces surface states and passivates others (Fig. 3).
AND
PERSPECTIVES
As a main conclusion we have shown, from photocapacitance-measurements and surface observations, that a surface modification, including a surface pretreatment and a metal deposition, should be regarded as a modification of both the energy and the spatial distribution of the interface states. This can be explained as follows: (i) the surface preparation determines the density and the spatial distribution of the nucleation sites on which the film growth initiates; (ii) the subsequent nucleation of electrodeposited adatoms shifts the energy of some sites (nucleated sites). The last stage is well accounted for by the occurrence of the metal-induced states. The other interesting point of this work is that we have been able to associate the nucleation sites with the surface states pre-existing to the deposit. The results which we have presented should find applications in the optimization of the metal grain distribution for a given (photo)electrochemical purpose with a given semiconductor/electrolyte junction. Indeed it has been shown elsewhere that there exists a critical morphology of the film beyond which any improvement of its properties is obtained only at the expense of the light absorptionC5, lo]. Considering the energy localization of the metal-induced states is therefore not enough for explaining the improved stability of n-GaAs photoanodes[5] or the enhanced hydrogen evolution at p-GaAs photocathodes[S]; one must also consider their density (cm-‘) and their spatial distribution, ie the density and the distribution of the associated metal grains according to the present work. As a last remark, one may imagine double deposits because they are expected to passivate both types of defects. Preliminary results effectively show that Pt and Ru can be deposited successively onto the same surface and that they do not deposit on each other. Acknowledgement-This work has been supported CNRS-PIRSEM GRECO 130061.
by the
REFERENCES 1.
2. 3.
4. 5.
6. 7. 8. 9. 10.
Y. Nakato, K. Abe and H. Tsubomura,
Ber. Bw7senges. phys. Chem. 80, 1002 (1976). S. Menezes, A. Heller and B. Miller, J. electrochem. Sm. 128, 1939 (1981). B. J. Tufts, J. L. Abrahams, P. G. Santangelo. G. N. Ruba, L. Casagrande and N. S. Lewis, Nature 326, 861 (1987). A. Heller, E. Aharon-Shalom, W. A. Banner and B. Miller, .I. Am. &em. Sot. 104, 6942 (1982). P. Allongue, E. Souteyrand. L. Allemand and H. Cachet, Proceedings of the Symposium on Photoelectrochemistry and Electrosynthesis on Semiconductor Materials, 172”d Electrochemical Society Meeting, Honolulu (1987). P. Allongue and H. Cachet, EPY. Bunsznges. phys. Chem. 92, 566 (1988). P. Allongue, Ber. Bunsenges. phys. Chem. 92, 895 (1988). P. Allongue, Thesis, Paris (1988). D. E. Aspnes and A. A. Stunda, Appl. Phys. Lat. 46, 1701 (1985). P. Allongue and 8. Souteyrand, 39’h ISE Meeting, Glasgow (1988).
34, NO. 12. pp. 1717-1722.
1989.
00134686,*9 Pcq.m””
S3m+O.M Press pk.
SEMICONDUCTOR ELECTRODE MODIFICATIONS: INFLUENCE ON THE STATE DISTRIBUTION AT THE INTERFACE P. ALLONGUE
and E. SOUTEYRAND
LP15 du CNRS, Physique des Liquides et Electrochimie, Laboratoire de I’Universiti P. & M. Curie, Tour 22, 4 place Jussieu, 75252 Paris Cedex 05, France (Received
1 October 1988; in revisedform 18 January 1989)
Abstract-Surface modifications including a surface preparation (etching, photoetching _) followed by electrodeposition of a discontinuous metal film are presented. Photocapacitance spectra and surface observations reveal that such a procedure is equivalent to modify both the energy and the spatial distribution of the interface states in the case of Pt and Ru.
INTRODUCTION Surface modifications of semiconductor electrodes are currently investigated by a number of research groups in view of improving or limiting some electrochemical reactions occurring at the interface. Metal deposition remains an attractive technique for modifying the surface although it has been extensively studied to date. In that field the main goals are either to achieve the complete stabilization of photoanodes or to catalyse hydrogen evolution at photocathodes with optically transparent metal films to avoid light absorption[ l-51. In number of publications the presence of some surface states in the bandgap of the semiconductor is invoked to interpret the results even though no direct characterization of these energy levels is performed. Moreover, the morphology of the deposited films is generally not considered although it plays a key role for such modifications[Sj. In the present paper, we report experimental informations upon the last two above mentioned points in the case of n-GaAs electrodes modified by metal electrodeposition: (i) the surface state distribution is determined by photocapacitance measurements[6-81; (ii) the surface topography, before and after deposition, is observed by transmission electron microscopy (TEM). In the present work, the metal is electrodeposited onto the semiconductor under potentiostatic conditions[5]. In the case of Pt and Ru, the photocapacitance results show that both metals induce surface states in the bandgap of GaAs and also passivate states which are intrinsic to the GaAsselectrolyte interface[7, 81. The surface observations reveal, for a given metal, that the distribution of the islets (which constitute the film) depends strongly on the surface preparation prior to deposition (the electrochemical parameters for deposition being kept constant[5]); in some cases it is also observed that for a given surface preparation, the metal grain distribution depends on the deposited metal. Considering together both types of results (photocapacitance and surface observations) suggests that our surface modifications (including a surface preparation followed by a metal electrodeposition)
induce a modification of both the energy and the spatial distributions of the interface states on GaAs. Possible applications will be briefly mentioned.
EXPERIMENTAL Semiconductor electrodes are made of n-GaAs single crystals, (100) oriented and Si doped (lOI cmm3). The surface is coated either with Pt or Ru by electrodeposition, under potentiostatic conditions, from (NH4)2 PtCl, and RuCl, solutions[6]. Before each experiment the surface is polished with a 1 nm diamond past and softly etched with the usual mixture (H,O: H,O, : HzSO,) (vol. 25: 2: 2) at room temperature. The deposition is performed onto (i) as prepared, (ii) photoetched or (iii) chemically corroded surfaces. The photoetching consists in corroding the surface under illumination in 1 M KOH (I,,= 0.6 mAcm’) for 10 min. The chemical corrosion is performed by injection of holes from a lo-‘M ferri/ferrocyanide (pH 14) solution. In both cases the alkaline pH prevents the oxide formation[9]; the photoetching and the chemical corrosion remove layers of about 340 and 160 nm respectively after the soft etching. The state distribution at the interface is studied by the photocapacitance technique as far as the energy distribution is concerned. The technique consists in measuring the variations of the space charge capacitance of the semiconductor as a function of the incident sub-gap wavelength (hv < energy bandgap); refs [f&8] give more details about the theory and the experimental set-up. For a n-type semiconductor, one has to know that each positive discontinuity in a photocapacitance spectrum AC(hv) is associated with a transition between a surface (or bulk) state and the conduction band; by contrast, a transition between the valence band and a state is accounted for by a negative step in the spectrum. The spectrum analysis is presented in the literatureC6, 71 as well as the method to separate bulk states from surface states. Transmission electron microscopy (TEM) is used for observing the surface topography and/or the film
1717
1718
P. ALLONGUE
and
morphology (particle size, density, spatial distribution). Samples are prepared by means of the extractive replica technique. The smallest discernible metal particle is about 1 nm diameter.
RESULTS Figures 1 and 2 present the photocapacitance speotra for both a Ru coated and a bare n-GaAs surface in contact with a 1 M KOH solution. In both cases, the applied potential V(us see) is adjusted to promote the same band bending V,=O.95 V in the semiconductor (V is determined by plotting the Mott-Schottky diagram in each case). Therefore, every difference be-
I
I
I
0.8
1.0
I
I Ec -1.30
.I0
:I0
0.6 I--L--
ENERGY
1.2
1.4
(eV)
Fig. 1. Photocapacitance spectrum for a bare n-GaAs electrode in contact with 1 M KOH. The surface is only chemically etched (ie “as prepared” in the text). The applied potential is V= -0.9 V/see (flat band potential - 1.85 V/see) so that the band bending is 0.95 V. The capacitance in darkness is 14 nF. The spectrum is normalized with respect to incident photon flux +,,[6]
1
I
I
I
E. SOUTEYRAND
tween the spectra can be attributed to surface effects[7, 81. The comparison between the photocapacitance spectra of Figs 1 and 2 reveals that (i) there are several transitions which are common to both spectra and (ii) Ru induces a small additional peak around 1.25 eV (Fig. 2). The four common transitions occur at 0.85, 0.98, 1.10 and 1.30 eV; the main negative peak centered at 0.85 eV has been attributed elsewhere to the well-known bulk state EL2 in GaAs; this deep level is located at 0.85 eV below the conduction band[6]. The last three transitions are related to the interactions between the GaAs surface and the solvent; the corresponding surface states act as corrosion intermediates and are located at 0.98, 1.10 and 1.30 eV below the conduction band edge [7, 81 because the associated discontinuities are positive steps (this also explains the indexations E, -0.98 eV )_The peak at 1.25 eV; which only exists in the presence of Ruthenium (Fig. 2) can be regarded as the combination of a positive step at 1.17 eV and a negative one at 1.25 eV; in other words, this peak accounts for the presence of two Ruthenium-induced states located at EC1.17 eV and E,+ 1.25 eV respectively. Figure 3 visualizes the energy distribution of the interface states derived from Figs 1 and 2: E, = EC-O.98 eV and E, = E, - 1.10 eV are the corrosion states; the states E& = EC--O.2 eV and E,,= E, +0.26 eV are the Ruinduced states; for the sake of simplicity, the band tail (E, - 1.30 eV) and the bulk state EL2 are not figured. The last difference between Figs 1 and 2 concerns the amplitude 6, of the transition observed at l.lOeV. Note that a1 remains unchanged. The smaller amplitude 6, after Ruthenium electrodeposition indicates that the density of the state E, has been lowered. In other words, Ru-adatoms passivate the state E, but not the state E; To summarize, Figs 1 and 2 show that Ru creates two new states (ERU and E&j and passivates the state E, only. A quite similar result is found with Pt: Pt induces also two states close to the band edges (at EC -0.16 eV and Ev+0.21 eV) but it passivates the state E, in contrast to the Ru effect[7, 81. Before going further it is worth recalling the conclusions derived from previous photocapacitance exper-
I
CONDUCTION
BAN
J-
+
--E”R”
G:
IO.1 ev
E,=1.43 eV
a
;;
a
21;
0.6
0.0
1.0
1.2
ENERGY
(eV)
1.4
Fig. 2. Photocapacitance spectrum for a Ru-coated n-GaAs electrode in contact with 1 M KOH: applied potential -0.88 V/see (flat band potential - 1.83 V/XC). The capacitance in darkness is 14 nF. The spectrum is normalized as in Fig. 1.
VALENCE a
BAND
]
CcmROSlON STATES
by
Fig. 3. Energy distribution of the surface states for a GaAs surface coated with Ru islands. E, (E,) is the energy at the bottom of the conduction (top of the valence) band, the states E, =I?, -0.98 eV and E, =E, - 1.10 eV are the corrosion states (see text); the states E&=E, -0.2 eV and E,, = E, +0.26 eV are the Ru-induced states. Note that Ru also passivated the state E, (see text).
Semiconductor
electrode
iments[7, S]: (i) under photocorrosion conditions in 1 M KOH photoholes are transferred to solution through both the states E, and E,; (ii) under chemical corrosion conditions in a ferri/ferrocyanide (pH 14) solution, holes are injected preferentially in the state E,. These results suggest that the surface preparations used in this work (photoetching, chemical corrosion) are likely to affect the spatial distribution and probably the density of both states E, and E,. Keeping in mind the above remarks and the conclusions derived from Figs 1 and 2 we have studied the evolution of Pt and Ru electrodeposits as a function of the surface preparation (Figs 5 and 6) to examine whether there exists correlations between the surface
Fig. 4. TEM observations
modifications
1719
pretreatment and the morphology of the deposited film. In fact, the following observations show that the changes in the morphology concern the distribution of the metallic islets (about 1 nm diameter) which compose the film. At first, Fig. 4 shows that the topography of the surface actually depends on its preparation. When it is “as prepared” the surface is practically smooth (not shown); on the contrary, a photoetched surface (Fig. 4a) exhibits a marked topography with “waves” parallel to the
of a (GaAs) surface, (100) orientation, as a function photoetching; (b) chemical corrosion.
of the surface
preparation:
(a)
1720
P. ALLONGUE and E. SOUTEYRAND
Fig. 5. TEM observations of a Ru-film, coated on n-GaAs, as a function of the surface preparation: prior to deposition the surface is “as prepared” (a) and chemically corroded (b). The conditiom for electrodeposition are the same in both cases.
regularly distributed valleys over the whole surface (although almost the same thickness of material has been removed). Figures 5 and 6 show micrographs of electrodeposited Pt and Ru films onto variously prepared surfaces. Note that the parameters for electroplating are kept constant for a given metal; Pt is deposited at -0.7 V/see for 2 s; the respective parameters are - 1.0 V/see and 90 s for Ru[S]. Figure 5 depicts the influence of the surface preparation for a given metal (eg Ru). If the surface is as prepared the film consists in large cells (200 -400 nm wide) regularly spaced in which the microcrystallites of Ru (l-2 nm
diameter) have uniformly nucleated and grown (Fig. 5a). Onto a chemically corroded surface, the film morphology is quite different (Fig. 5b): except for regularly distributed cells (30 nm wide), free of metal, the metal grains of Ru have uniformly nucleated. It is worth noticing that for each of these two surface preparations of Fig. 5 the distribution of the Pt islands is similar to that of the Ru islands[8]. At variance, Fig. 6 shows that onto a photoetched surface Pt and Ru islets present completely different distributions: Pt crystallizes along parallel lines, that recall the topography in Fig. 4a, whereas Ru nucleates almost uni-
Semiconductor
electrode modifications
1721
Fig. 6. TEM observations of Pt and Ru films electrodeposited onto a photoetched surface: (a) Pt; (b) Ru
formly over the surface. Finally one can remark that independently of the surface preparation the films are always made of very small particles (l--2 nm) while their organization changes; this is probably due to similar growth modes of the Pt and Ru microcrystallites.
DISCUSSION With respect to the above results it must be emphasized that the expectations derived from photocapacitance measurements are quite consistent with the surface observations. As a matter of fact, the surface
topography, ie the spatial distribution of the surface defects, depends on the preparation procedure (Fig. 4). In the same way, a given metal deposits quite differently when the surface preparation changes (see Figs 5 and 6). Finally for some surface preparations, the distribution of the crystallites depends on the metal (see Fig. 6). Figure 6 and the photocapacitance results suggest that there are two types of surface defects D, and D,, probably related to the states E, and E,, on which Pt and Ru would nucleate respectively. Figure 7 sketches the surface topography of a photoetched surface (Fig. 4); it is assumed that the defect D, is located at the bottom of the waves while D, is regularly distribu-
1722
P.ALLONGUE
and E.SOUTEYRANU
CONCLUSION D,(E,-0.98eV)
SIDE
VIEW
Fig. 7. Schematic representation of a photoetched n-GaAs surface, (100) orientation, according to Fig. 4a. The symbols (0) and (+) represent the localization of the surface defects D, and D, on which Pt and Ru nucleate respectively (see text).
ted over the wave-sides. Notice that this assumption is not in contradiction with crystallographic considerations because the defects of highest and lowest energy (Dl and D2 respectively) are effectively related to the higher (E,) and the lower (E,) surface state with respect to the top of the valence band (see Fig. 3). Considering such an assumption it is now obvious that Pt must nucleate along parallel lines and Ru uniformly, in agreement with Fig. 6, because they respectively passivate the state E,(D,) and E2(D2). The fact that the distance between two alignments of Pt crystaliites is close to the width of the waves of Fig. 4a makes the assumption more convincing and the spacial identification of D, and D, very likely in this peculiar case. Contrary to the above favourable case, the Pt and Ru grains which are electrodeposited onto a “as prepared” surface (Fig. 5a) present the same distribution. This suggests that the soft etching is not able to spatially separate the surface defects D, and D,. All happens as if D, and D, were paired-off. The last remark applies also to chemically corroded surfaces (Fig. 5b). However the distribution of the pairs (Dl, D2) must be quite different from that observed with “as prepared” surface when comparing Figs 5a and b. The above observations tend to prove that there is a close correlation between the occurrence of a marked surface topography and the spatial discrimination of the defects D, and D, (see Figs. 5 and 6). This is not unrealistic and does not question our assumption about D, and D,. So far we have shown (i) that the surface states E, and E, are related to surface defects which have been identified in some cases (Fig. 7), (ii) that these defects act as nucleation sites for Pt and Ru respectively, (iii) that it is possible to change their spatial distribution by varying the surface preparation and (iv) that each metal induces surface states and passivates others (Fig. 3).
AND
PERSPECTIVES
As a main conclusion we have shown, from photocapacitance-measurements and surface observations, that a surface modification, including a surface pretreatment and a metal deposition, should be regarded as a modification of both the energy and the spatial distribution of the interface states. This can be explained as follows: (i) the surface preparation determines the density and the spatial distribution of the nucleation sites on which the film growth initiates; (ii) the subsequent nucleation of electrodeposited adatoms shifts the energy of some sites (nucleated sites). The last stage is well accounted for by the occurrence of the metal-induced states. The other interesting point of this work is that we have been able to associate the nucleation sites with the surface states pre-existing to the deposit. The results which we have presented should find applications in the optimization of the metal grain distribution for a given (photo)electrochemical purpose with a given semiconductor/electrolyte junction. Indeed it has been shown elsewhere that there exists a critical morphology of the film beyond which any improvement of its properties is obtained only at the expense of the light absorptionC5, lo]. Considering the energy localization of the metal-induced states is therefore not enough for explaining the improved stability of n-GaAs photoanodes[5] or the enhanced hydrogen evolution at p-GaAs photocathodes[S]; one must also consider their density (cm-‘) and their spatial distribution, ie the density and the distribution of the associated metal grains according to the present work. As a last remark, one may imagine double deposits because they are expected to passivate both types of defects. Preliminary results effectively show that Pt and Ru can be deposited successively onto the same surface and that they do not deposit on each other. Acknowledgement-This work has been supported CNRS-PIRSEM GRECO 130061.
by the
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