J. Phys. Chem. 1991,95, 819-824
819
Hydrogen Evolution at a Pt-ModHled InP Photoelectrode: Improvement of Current-Voltage Characterlstics by HCI Etching Hikaru Kobayasbi,* Fumiaki Mizuno, Yoshihiro Nakato, and Hiroshi Tsubomura* Laboratory for Chemical Conversion of Solar Energy and Department of Chemistry. Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan (Received: April 30, 1990; I n Final Form: June 20, 1990)
Hydrogen photoevolution at pInP electrodes coated with platinum and palladium has been studied. Efficient and stable solar to chemical energy conversion has been achieved after etching the electrodes with concentrated HCI. The cumnt-voltage (1-V) behavior of the as-prepared electrode covered with a continuous Pt layer is poor, due probably to the presence of defect states in InP. The photocurrent density of this electrode decreases with time under illumination, presumably due to an inmase in the defect density. After etching of the electrode with concentrated HCI, the barrier height is increased to 1.0 V, and the I-V characteristics are improved remarkably, showing no degradation under illumination. SEM, XPS, and AES analyses show that the concentrated HCI solution dissolves the InP substrate in the InP/Pt interfacial region, and simultaneously part of the Pt is removed from the InP surface. A structural model of the Pt-modified and HCI-etched InP electrode is proposed, in which the InP surface is in direct contact with the solution due to the formation of minute Pt islands. By use of this model, the high barrier formation is well explained. The cell characteristics of the Pd-deposited electrodes are poor and become worse with hydrogen bubbling. The 1-Vcharacteristics of the Pt-deposited electrodes are unaffected by hydrogen or nitrogen bubbling and the reason is discussed.
Introduction Hydrogen photoevolution at bare and metal-modified p-type semiconductors such as Si, Gap, GaAs, InP, etc. has been studied extensively in relation to solar-to-chemical energy conversion.’-’5 A small amount of metal such as platinum, rhodium, and ruthenium deposited on the semiconductor electrodes was reported to improve energy conversion efficiency remarkably. Noblemetaldeposited InP electrodes were reported to give high energy conversion efficiency by Heller and co-workers.2 They proposedSd that, during hydrogen evolution, the metal work function was decreased by adsorption of hydrogen, and accordingly a high barrier was formed in the metal-deposited semiconductor. They also reported that the barrier height did not depend on the kind of metal used. Bockris et al.,’+ on the other hand, observed a linear relation between the exchange current densities at various metal electrodes and the amounts of the shift in the hydrogen evolution onset potential of the InP electrode coated with the metal from the onset potential of the bare electrode. On the basis of this observation, they suggested that the rate-determining step was a charge-transfer process at the metal/solution interfaces. A similar result was reported by us for p-GaP,I and recently the same conclusion was obtained by using a transient photocurrent technique.“ Sammells et al.lOJ1and Goodman and WesselsI3 showed that codeposition of cobalt and platinum effectively improved the hydrogen-evolution efficiency of the InP electrodes in alkaline solutions and suggested that the improvement arose from a reduction in the density of interface states near the valence (1) Nakato, Y.; Tonomura, S.; Tsubomura, H.Ber. Bunsen-Ges. fhys. Chem. 1976,80, 1289. (2) Heller, A.; Vadimsky, R. G . fhys. Reo. Lett. 1981, 46, 1153. (3) Aharon-Shalom, E.; Heller, A. J . Electrochem. Soc. 1982,129,2865. (4) Heller, A.; Aharon-Shalom, E.; Bonner, W. A,; Miller, B. J . Am. Chem. SOC.1982, 104,6942. (5) Aspnts, D. E.; Heller, A. J . fhys. Chem. 1983,87,4919. (6) Heller, A. J . Phys. Chem. 1985, 89, 2962. (7) Szklarczyk, M.; Bockris, J. O M . J. fhys. Chem. 1984. 88, 1808. (8) Szklarczyk, M.; Bockris. J. O M . J . fhys. Chem. 1984, 88, 5241. (9) Bockris, J. OM.; Kainthla, R. C. J. fhys. Chem. 1985, 89, 2963. (IO) Ang, P. G. P.; Sammclls, A. F. J. Electrochorn.Soc. 1984, 131, 1462. (11) Cook, R. L.; Dcmpsey, P. F.; Sammells, A. F. J . Electrochem. Soc.
1985, 132, 1315. (12) Yoncyama, H.;Azuma, H.;Tamura, H.J . Electroanal. Chem. Interfacial Electrochem. 1985, 186, 247. (13) Goodman, C . E.; Wessels, B. W. Appl. fhys. Letr. 1986, 49. 829. (14) Nakato, Y.; Yano, H.;Nishiura, S.;U d a , T.; Tsubomura, H.J . Electroanal. Chem. Interfacial Electrochem. 1981, 228, 91. (1 5) Cook, R. L.; MacDuff, R. C.; Sammells, A. F. J . Electrochem. Soc. 1989, 136, 1468.
band.16 Recently, we proposed a new model for which n- and ptype semiconductor electrodes show stable and efficient I-V characteristicswhen they are coated with very small and sparsely scattered metal islands.”J* In most of the studies mentioned above,”I5 metals were photoelectrodeposited on the InP surfaces. In the present study, the metal is deposited on the electrodes by using the electron beam (EB)evaporation method, by which metals are expected to form more stable layers.
Experimental Section The InP wafers ((loo), ptype, 2.6 X lo1*cm-3 Zn-doped) were washed with boiled acetone and distilled water and then etched with concentrated HCl or concentrated HF. After ohmic contact was made by applying Zn-containing In+a alloy on the back surface and heating at 300 O C in a hydrogen atmosphere, a Cu wire was attached to the rear InP surface with silver paste for current collection, and the InP sample was encapsulated with epoxy resin. Then a 20 A thick layer of platinum or palladium was deposited by the EB evaporation method. In some cases, Pt was deposited by reduction of chloroplatinic acid (H2PtC16) in a 0.5 wt % ethanol solution. In this method, the solution was dropped onto the InP surface so as to form a 130 pm thick layer and the electrode was dried by evacuating with a mechanical pump. The electrode was then heated at 300 OC for 30 min in a hydrogen atmosphere.I7J8 The current-potential (I-V) and capacitance-potential (C-V) characteristics were measured either in a 0.5 M HCI solution or in a 1 M HC104 solution by using a plain Pt counter electrode, a saturated calomel reference electrode (SCE), a potentiostat, and a potential sweeper. The solution was stirred magnetically. A solar simulator (AMI, 100 mW cm-2) was used as a light source. The C-Vmeasurements were performed with an impedance analyzer (Hewlett Packard, Model 4192A) at a frequency ranging between 0.1 and 10 kHz and with a peak-to-peak amplitude of the ac signal of 5 mV. The scanning electron micrographs (SEM) were taken at a primary electron energy of 10-15 keV and normal incidence. The scanning Auger electron spectroscopy (AES) measurements were (16) Goodman, C. E.; Wessels, B. W.; Ang. P. G . P. Appl. fhys. Lett.
19114. - - - ., 4.7. . - , 442. . .-.
(17) Ueda, K.; Nakato, Y.; Suzuki, N.; Tsubomura, H.J . Electrochem. SOC.1989, 136, 2280. (18) Nakato, Y.; Nishiura, S.;Oshika, H.;Tsubmura, H.Jpn. J . Appl. Phys., Part 2 1989, 28, 261.
0022-3654/9 1/2095-08 19$02.50/0 0 1991 American Chemical Society
Kobayashi et al.
820 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 VOLE
VOLTS vs. SCE
-m
-02 -0.1
o
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Figure 2. I-V curves measured in a 0.5 M HCI solution for the InP electrodes under illumination on which Pt was deposited by the H2PtC16 method: (a) Pt-asdeposited electrode; (b) electrode a after HCI etching.
Figure 1. I-V curves for the Pt-modified InP electrodes under illumination (except case g): (a) Pt-as-depositedelectrode at room temperature; (b) electrode a after illumination for 10 min; (c) electrode a after keeping the potential at 0.5 V in the dark for 3 min; (d) Pt-as-deposited electrode at 300 'C; (e) electrode d after HCI etching; (9electrode b
after HCI etching: (g) electrode a in the dark.
made at an incident electron energy of 3 keV and an ac modulation amplitude of 10 V. For the measurements of X-ray photoelectron spectra (XPS),I9 photoelectrons were excited by Mg K a radiation, and the binding energies were corrected by using the Ag(3dSIz) peak as an energy standard (368.2 eV). For the measurements of the XPS depth profiles, Ar+ ion bombardment was carried out at a kinetic energy of 0.5 keV and an incidence angle of 60'.
Results Figure 1 shows the I-Vcurves of the P t d e p i t e d InP electrodes fabricated by the EB method and measured in a 0.5 M HCI solution. For the electrode with a 20 A thick Pt layer, the photocurrent density and the photovoltage were low (Figure la), and the photocurrent density was reduced to 70-80% (Figure lb) by keeping the electrode potential at the onset potential of hydrogen evolution of a Pt electrode (VR), under illumination for 10 min. As seen from the very weak cathodic current in the dark (Figure Is), the contact between InP and Pt is not ohmic; a barrier of considerable height is present. The energy position of the top of the valence band of InP, which is close to the Pt Fermi level under vacuum, suggests an ohmic contact between InP and Pt. However, in the present case, the potential barrier is formed probably between the defect states and the InP bulk as discussed later. The I-V characteristics of the Pt-as-deposited electrode were improved by keeping the potential at 0.5 V vs SCE for 3 min in the dark, as shown in Figure IC. When the electrode degraded by illumination (Figure Ib) was immersed in a concentrated HCI solution for an appropriate period (typically 10 s), the cell performance was improved remarkably as shown in Figure 1f: we obtained a photocurrent density at VR of 24.7 mA cm-2, a shift of the onset potential of hydrogen photoevolution from V, (AVH) of 680 mV, and a solar-techemical energy conversion efficiency estimated from the shaded area at 9.2%. (It should be noted that the estimation of the solar-tochemical energy conversion efficiency from the shaded area gives a lower value than Heller's The I-V characteristics of the Pt-deposited and HCI-etched electrode depended on the redox potential of a redox couple added to the solution. In the presence of an Fe2+/Fe3+redox couple whose redox level was 0.5 V vs SCE, the open-circuit photovoltage
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(19) Nakato, Y.; Hiramoto, M.;Iwakabe, Y.; Tsubomura, H. J . Elecrrochem. SOC.1985, 132, 330.
Pt
I/
06!c-ENyly (mAcm-z)
Figure 3. I-Vcurves for the Pd-covered InP electrodes under illumination: (a) with nitrogen bubbling; (b) with hydrogen bubbling.
was about 150 mV, much lower than the above-mentioned shift in the hydrogen evolution onset potential, i.e. AV, = 680 mV. For all the electrodes mentioned above, Pt was deposited immediately after etching bare InP electrodes with concentrated HCI. The amount of oxide between the InP substrate and the Pt overlayer was very small since no oxide peak was observed in the XPS spectra of the bare InP surface recorded just after HCI etching. The XPS measurements showed that oxide was formed on the InP electrodes kept in humid air for a day or treated with a Brrmethanol solution. The I-Vbehavior of such oxide-coated InP electrodes on which a 20 A thick Pt layer was deposited was as poor as that of the similarly Pt-plated electrodes having no interfacial oxide (Figure la). The I-Vcharacteristics of the InP electrodes on which Pt was deposited at 300 OC (Figure Id) were worse than those for the electrodes prepared without heating (Figure la) and were not improved by HC1 etching (Figure le). Figure 2 shows the I-V curves of the InP electrodes, under illumination, on which Pt was deposited from an ethanol solution of HzPtC1,, measured in a 0.5 M HCI solution. The SEM measurements for the case of the InP electrodes showed that the Pt layer formed was discontinuous but the Pt grains were large as described later (cf. Figure 5g). The I-Vcharacteristics of this electrode were poor before HCI etching (Figure 2a). A drastic improvement was achieved by HCI etching as shown in Figure 2b. Figure 3 shows I-V curves of the 20 A thick continuously Pd-covered InP electrodes, under illumination, measured in a 1 M HCIO4 solution. Curves a and b were measured after the solution was bubbled for 30 min with N, and H2, respectively. It was clearly seen that, by H2 bubbling, the I-Vcharacteristics were degraded, with the shift, AVH, markedly reduced. For the Pt-as-deposited electrode (Figure la) and the Pt-coated and HCI-etched electrode (Figure 1f), the I-Vcharacteristics under electrode illumination were unchanged by H2 or N2 bubbling. These results are contrary to the expectation from Heller's mode1.43 Figure 4 shows the Mott-Schottky plots of the Pt-deposited electrodes fabricated by the EB evaporation method, measured in a 0.5 M HCI solution. During the measurements, the solution was vigorously bubbled with H2 gas in order to have the solution and the electrode in a condition similar to that during hydrogen evolution under electrode illumination. For the electrode before HCI etching (Figure 4a), the plot measured a t 0.1 kHz was nonlinear. It became linear as the frequency was increased. A similar nonlinear plot was observed also in the presence of a
Hydrogen Evolution a t a Pt-Modified InP Photoelectrode
- 1
of spectrum a, a strong peak and a weak peak were observed at 129.4 and 134.4 eV, which were attributed to the InP substrate and its oxide, respectively. In spectrum b, the oxide peak was observed with higher intensity than the substrate peak and was extinguished by Ar+ ion bombardment for 10 s, indicating that the oxide species was present on the Pt surface. In spectrum c, the intensity of the oxide peak was reduced, indicating the removal of the oxide species by HCI etching. For this electrode, illumination did not increase the intensity of the oxide peak, contrary to the case before HCI etching, indicating that additional oxide was not formed after HCI etching. The In(3d) peak was observed at 444.5 eV for the Pt-as-deposited electrode. After illumination, this peak shifted to 445.7 eV due to oxidation of the surface. From the energy positions of the P(2p) (134.4 eV) and In(3d) (445.7 eV) peaks, the oxide species was attributed to InP04.MJ1 After HCI etching, the In(3d) peak returned to 444.5 eV, owing to the removal of the surface oxide. The energy positions of the Pt(4f5/,) and pf(4f7/2) peaks were not changed by HCI etching, but the intensity of these peaks decreased to 10% of that before HCI etching, indicating the removal of a large amount of Pt from the surface.
(a)
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-05
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 821
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Figure 4. Mott-Schottky plots for the Pt-deposited InP electrodes measured at 0.1, 1, and 10 kHz: (a) Pt-as-deposited electrode; (b) electrode a after HCI etching. Cr2+/Cr3+redox couple in the solution which should fix the potential of the Pt overlayer more definitely than the H+/H2 level formed by the H2 bubbling. The deviation of the plots from a straight line became much larger after the Pt-as-deposited electrode was illuminated, indicating an increase in the defect density. The plots tended to be linear after HCI etching (Figure 4b), probably because of the removal of the defect states. The slopes of the plots became smaller after HCI etching, indicating an increase in the surface area as shown later. In cases where the Ptcoated and HC1-etched electrode was immersed in the Fe2+/Fe3+ redox solution, the barrier height was estimated to be 0.2 V from the intercept of the Mott-Schottky plot with the x axis. Figure 5 shows the SEM micrographs and the scanning AES maps of the Pt-deposited InP electrodes. The Pt layer formed by the EB evaporation method was almost continuous, as shown in Figure 5a, but the electrode surface became rough, showing stripes after HCI etching (Figure 5b). Similar stripes were produced by immersing bare InP electrodes in a concentrated HCI solution, as shown in Figure 5c, suggesting that the stripes were not caused by the Pt overlayer but resulted from an inhomogeneous etching of the InP substrate. This was verified more clearly by the scanning AES maps in which high-density indium (Figure 5d) and phosphorus (Figure 5e) were observed on the stripes, while platinum was detected all over the surface (Figure 50.The Pt layer produced by the H,PtCI, deposition method was discontinuous but the Pt grains were large and dense (Figure 5g), contrary to our previous work on Si.'7J8 The scanning AES measurements confirmed that the white part seen in Figure 5g consisted of Pt. Figure 6 shows the XPS spectra of the Pt-covered InP electrodes. Spectrum a is for the Pt-as-deposited electrode, spectrum b for the same electrode after illumination in 0.5 M HCI for 10 min, and spectrum c for the electrode illuminated in 0.5 M HCI for 10 min and etched with concentrated HCI. In the P(2p) region
Discussion The Pt-deposited p-InP electrodes show a very weak dark current in the reverse-bias region (Figure lg), indicating the formation of a fairly high potential barrier at the Pt/InP interface. They show a poor photovoltaic behavior, however, before etching with concentrated HCI (Figure la), and the Mott-Schottky plots measured at low frequencies were nonlinear (Figure 4a). Both can be explained by the presence of high-density defect states in InP, which act as recombination centers. Due to the high defect density, the InP Fermi level coincides with the defect level, forming a barrier. Formation of the defect states in the InP bulk for the Pt-deposited InP electrode is also supported by the production of InW4 on the Pt surface, evident from the XPS depth profile. It is likely that the InP is decomposed at the Pt/InP interface, probably by the action of Pt catalysis under electrode illumination, and that the released indium and phosphorus atoms diffuse to the Pt surface and then react with oxygen or water, forming InPO,. Outdiffusion of phosphorus through metal layers has been reported for Au or Ag d e p o ~ i t i o n . ~On ~ . ~the ~ other hand, such diffusion of indium is observed for Ti, Cr, Fe, or Co d e p o s i t i ~ nwhere , ~ ~ ~the ~ ~metals form stable compounds with phosphorus. In other words, since the heat of formation of the phosphorus compounds with these metals is thought to be high, local heating of the InP substrate occurs, and consequently the InP is decomposed. In the present case, outdiffusion of indium and phosphorus is thought to be enhanced by illumination. The defect density is high and the I-V characteristics are poor after a large amount of InP04 is produced, since decomposition of the InP substrate is necessary to form InP04. The above discussion suggests that the defects are of vacancy type. On the basis of the above discussion, the energy diagram and the surface structure of the Pt-as-deposited electrode are illustrated in Figure 7. According to the literature on MIS solar cells, the density of defect states may be reduced by interposing an oxide layer between Pt and InP. In the present case, however, the oxide layer is not formed a t the Pt/InP interface, but is formed on the Pt outer surface, leading to an increase in the defect density. Even for the case where the oxide layer was interposed between the InP sub(20) Hollinger, G.; Bergignat, E. J . Vac. Sci. Technol. 1985, A3, 2082. (21) Clark, D. T.; Fok, T.; Roberts,G. G . ;Sykes, R. W. Thin Solid Films 1980, 70, 26 1 . (22) Spicer, W. E.; Chye, P. W.; Skeath, P. R.; Su,C. Y.; Lindau 1. J . Vac. Sci. Technol. 1919, 16, 1422. (23) Spicer, W. E.; Chye, P. W.; Garner, C. M.; Lindau, 1.; Pianetta, P. Surf. Sci. 1919, 86, 763. (24) Ken Chin, K.; Cao, R.; Kendelewicz, T.; Miyano, K.; Yeh, J. J.; Lindau, I. Spicer. W. E. Phys. Reu. 1981. 836, 5914. (25) Spicer, W. E.; Kendelewicz, T.; Newman, N.; Chin, K. K.; Lindau, 1. Surf. Sci. 1986, 168, 240.
Kobayashi et al.
822 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991
a
r
.
r
b
:1 c
..
.
1 -
*I____
Figure 5. In situ SEM and scanning AES maps of the InP electrodes: (a) SEM of the 20 A thick Pt-deposited electrode prepared by the EB evaporation method; (b) SEM after HCI etching of electrode a; (c) SEM of the bare InP electrode after HCI etching; (d) scanning AES map of indium after HCI etching of clcctrodc a; (e) scanning AES map of phosphorus after HCI etching of electrode a; (f) scanning AES map of platinum after HCI etching of electrode a; (8) SEM of the Pt-deposited electrode prepared by the H,PtCI, method.
strate and the Pt layer, the I-Vcharacteristics were not improved, indicating that the oxide layer did not decrease the dark recombination current, contrary to the case of Si MIS solar This is probably because the formation of the oxide layer accompanied the production of defects in InP. The acceptor density was calculated to be (2.4-2.5) X 1OI8cm-3 from the slope of the Mott-Schottky plot of the Pt-as-deposited (26) (27) (28) (29)
Card, H.C . Solid Srare Commun. 1974, 14, 1011. Ponpon, J. P.; Siffert, P. J. Appl. Phys. 1976, 47, 3248. Pulfrey, D. L. I € € € Trans. Electron Deuices 1978, 25, 1308. Cheek, G.; Mertens, R. Sol. Cells 1983, 8, 17.
electrode measured at 10 kHz. This value was in good agreement with the doping density of 2.6 X 10I8cm-3 specified by the manufacturer, verifying the resulting slope. The flat-band potential, determined by extrapolating the straight portion of the plot measured at 10 kHz,was found to vary from sample to sample between 0.52 and 0.72 V vs SCE. From the average flat-band potential of -0.62 V vs SCE and the acceptor density of 2.6 X 10I8~ m - the ~ , corresponding barrier height was estimated to be - -V.uJ
v
.
When the Pt-coated electrode is etched with a concentrated HCI solution, the surface becomes rough (Figure Sb). From the slope of the Mott-Schottky plot, the InP surface area is estimated
Hydrogen Evolution at a Pt-Modified InP Photoelectrode
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 823 electrode is estimated to be 0.78 V vs SCE from the MottSchottky plot measured at the high frequency of 10 kHz, where the dielectric relaxation has the minimum effect, and the corresponding barrier height is estimated to be 1.O V. The determined flat-band potential was more positive by 0.1-0.2 V than that reported in the l i t e r a t ~ r e . ~ ' -The ~ formation of such a high barrier results from direct contact of the InP surface with the solution, as discussed later. The solar-to-chemical energy conversion efficiency achieved so far is 9.2%. By optimizing the fabrication of the cells, more efficient energy conversion could be attained. A reduction in the acceptor density, for example, may increase the photocurrent density owing to the increase in the minority carrier diffusion length and the depletion layer width. (Note that the acceptor density in the semiconductor used in the present study is 1 or 2 orders of magnitude higher than those of other works which reported high efficien~y.~-'J~J~) The I-V characteristics of the InP electrode Pt-deposited by the H2PtCI6method were poor before HCI etching (Figure 2a), in contrast to the case of Si electrodes."-'* There are two reasons for this: (1) The Pt islands formed on the InP surface were larger than those on the Si surface.I7J8 (2) Defect states were introduced in the InP. By HCl etching, the cell performance improved for the same reason mentioned above. In order for the I-V characteristics to be improved by HCI etching, the HCl solution must penetrate through the Pt layer. This probably occurs along the Pt grain boundaries where the HCI solution etches the defective portion of the InP away. Good I-V characteristics were obtained by HCI etching in cases where Pt was deposited at room temperature (Figure If). On the contrary, the electrodes heated at 300 OC during Pt deposition showed no improvement of the cell characteristics by HCl etching (Figure le). The Pt grains are expected to become large upon heating, and consequently the HCl solution will find it difficult to penetrate into the InP substrate. On the other hand, the I-Vcharacteristics of the electrode with Pt deposited by the H2PtC16method were improved after HCl etching even for the case of heat treatment at 300 OC, because the Pt layer, in this case, was discontinuous (Figure 5g). Heller et aLzd proposed that the efficient cell performance with Pt-, Ru-, or Rh-deposited p-InP electrodes was due to the high barrier formation caused by a decrease in the metal work function owing to hydrogen adsorption. However, the present study using Pt- and Pd-deposited p I n P electrodes has clarified that the improvement is not caused by hydrogen adsorption, on the basis of the following results: (1) By keeping the potential of the Pt-asdeposited electrode at 0.5 V vs SCE in the dark, we improved cell characteristics (Figure IC). Hydrogen did not evolve under this condition and there was a small amount of anodic current, indicating anodic dissolution of the InP electrode. Thus, the improvement is likely to be achieved by the etching effect as discussed above. (2) The I-V characteristics of the Pd-deposited electrode became poorer after bubbling the solution with H2 (Figure 3b). In this case, a large amount of hydrogen is thought to be present at the Pd/InP interface since the diffusion rate of hydrogen in Pd is very high. Therefore, this result is contrary to Heller's model but attributable to interface states intr+uced by hydrogen.3s (3) The I-V characteristics of the 20 A thick Pt-as-deposited electrode were the same under H2 and N2 bubbling. This is also contrary to Heller's model. Hydrogen a d s ~ r p t i o n ~ ~(or - ~ removal * of preadsorbed oxygenJ9by H2 bubbling) at the Pt/solution interface
-
4'n 4 % \ BMx)3s
EERW(eV)
Figure 6. XPS spectra of the Pt-depositcd InP electrode prepared by the EB evaporation method: (a) R-as-deposited electrode; (b) electrode a after illumination for 10 min; (c) electrode a after HCI etching. (a)
-2yo A
I
0 0
0
Pt
o
In;
0
e o o 0 0
Figure 7. Energy diagram at zero bias (a) and surface structure (b) of the Pt-asdeposited InP electrode. The vacancy-type defects are probably produced at the InP/Pt interface and diffuse into the InP bulk.
to become 1.25 times larger due to its roughness. A considerably thicker InP surface layer must be etched away to produce such a rough surface. Consequently, the defect states in InP are removed. Simultaneously, a large part of the Pt overlayer is removed and very minute Pt islands are formed on the etched InP surface, as discussed later in detail. Thus, the linear Mott-Schottky plots and the good I-V characteristics are obtained. Degradation of the cell characteristics by illumination does not occur after the improvement by HCI etching. In this case, therefore, illumination does not increase the amount of InP04 or the number of defect states. From this result, it is concluded that formation of the defects occurs only near the preexistent defects, and no defects are produced without them. In other words, once good cell characteristics are achieved, they are not degraded by illumination. The Mott-Schottky plots measured at different frequencies after HCI etching are almost parallel, and we attribute this parallel shift to a dielectric relaxation related to irregularities in the thin surface layer.3o KIlhne and S ~ h e f o l d on , ~ ~the other hand, observed Mott-Schottky plots whose slopes were frequency-dependent for the case of a Pt-photoelectrodepositedInP electrode. This difference may be due to the varying Pt-deposition method. The flat-band potential for the Pt-deposited and HCI-etched (30) Dutoit, E. C.; Van Meirhaeghe, R. L.; Cardon, F. Ber. Bunsen-Ges. Phys. Chem. 1975, 79, 1206. (31) Kahne, H.-M.;Schefold, J. Ber. Bunsen-Ges. Phys. Chem. 1988,92, 1430.
(32) Van Wezemael, A.-M.; Laflere, W. H.; Cardon, F. J . Elecrroanal. Chem. Interfacial Electrochem. 1978, 87, 105. (33) Feng, Q.; Cotton, T. M. J . Electrochem. Soc. 1988, 135, 591. (34) Chandra, N.; Wheeler, B. L.; Bard, A. J. J . Phys. Chem. 1985,89, 5037. (35) Yousuf, M.; Kuliyev, B.; Lalevic, 8.; Poteat, T. L. Solid-State Electron. 1982, 25, 753. (36) Christmann, K.; Ertl, G. Sur. Sci. 1976, 60,365. (37) Collins, D. M.; Spicer, W. E. S u r . Sci. 1977, 69, 114. (38) Norton, P. R.; Goodale, J. W. Solid Srare Commun. 1979, 31, 223. (39) Yamamoto, N.; Tonomura, S.; Matsuoka, T.; Tsubomura. H. Surf. Sci. 1980, 92, 400.
824 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991
Figure 8. Structural model (a) and potential diagram (b) for the Ptdeposited and HCI-etched InP electrode.
does not change the barrier height but changes the potential drop across the Helmholtz layer. On the other hand, hydrogen that reaches the Pt/InP interface may change the barrier height. In the present case of a continuously Ptdeposited electrode, however, the barrier height is determined by the high density of defect states, not by the metal work function. (4) The Pt-deposited electrode subsequently etched with concentrated HCI showed no difference in the I-V curves under H2 and N2 bubbling. This can be explained by the model proposed for the electrode surface structure shown below. Structural Model of the Pt-Deposited Electrodes. The XPS result shows that the amount of Pt (initial thickness 20 A) on the InP surface is reduced to 10%by HCl etching, and the scanning AES result indicates that Pt is present uniformly throughout the surface. From the latter result, it is inferred that if Pt is present in the form of islands, the islands are smaller than the resolution of SEM (50 nm) and scanning AES (100 nm). The possible structure of the surface is illustrated schematically in Figure 8a. The barrier height in the area of the surface where the Pt is in direct contact with the InP is thought to be lower than in the naked area, but the modulation of the barrier height by the Pt islands decays rapidly toward the interior of the semiconductor, as shown in Figure 8b, in cases where the contact width is much smaller than the depletion-layer width as discussed in detail in our previous papers.4M2 In such a case, the effective barrier height in the (40) Nakato, Y.; Tsubomura, H. J . Phorochem. 1985, 29, 257.
Kobayashi et al. presence of the Pt islands is almost the same as that of a totally naked electrode. The effective barrier height is, therefore, not influenced by the change in the metal work function, and the I-V characteristics are unchanged by H2 bubbling as mentioned above. The I-V characteristics after HCl etching of the electrode Pt-deposited by the EB evaporation method are better than those of the electrode fabricated by the H2PtC16 method. From this result, it is likely that smaller and more uniformly distributed Pt islands are formed by the former method than the latter, so that a higher barrier is formed. The above model also explains the formation of a high barrier of ca. 1.0 V observed for the Pt-deposited and HC1-etched InP electrodes. The conduction band edge of a naked part of p-InP is determined by the adsorption-desorption equilibrium of hydrogen or hydroxyl ions between the surface hydroxyl groups and the solution and is unchanged by the variation in the redox level. In cases where charge exchange occurs between the InP and the solution, the barrier height is determined by the energy difference between the valence-band edge and the redox level of a H+/H2 couple. In this case, the barrier height is estimated to be about 1 V from the flat-band potential of 0.78 V vs SCE and the redox level of a H+/H2 couple (1 M HCI) of -0.2 V vs SCE, in agreement with the experimental value of 1.0 V. Spicer et a1.,23-24*43*4 on the other hand, found that a small amount of metal or chemisorbed oxygen (coverage
