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Langmuir 1996, 12, 1056-1060
Surface Arsenic Enrichment of n-GaAs Photoanodes in Concentrated Acidic Chloride Solutions Mahmoud M. Khader† Chemistry Department, Faculty of Science, UAE University, Al-Ain, P.O. Box 17551, United Arab Emirates Received November 10, 1994. In Final Form: September 11, 1995X The photoelectrochemical properties of n-GaAs are studied in acidified chloride electrolytes of different compositions, with the objective of preventing its photocorrosion. After irradiation of a n-GaAs electrode, under fixed anodic potential, current-voltage curves (J-V) experience two modifications: a negative shift in the onset potential of the photocurrent, Vp, and a dark cathodic peak at ∼-0.38 V (saturated calomel electrode, SCE). During irradiation at +0.4 V (SCE) in an acidified chloride solution, the illumination current increased with time rather nonlinearly. In an electrolyte composed of 2.5 M H2SO4 and 6.0 M LiCl at +0.4 V (SCE) the illumination current reached a saturation value of 18 mA cm-2 after ∼40 min of irradiation. The relationship between the illumination current density and the square root of the irradiation time is linear; however, it has a changing slope at certain time. Therefore, according to the predictions of the diffusivity equation solution, the electrode reactions are limited by diffusion-controlled processes. The origin of these processes is due to the preferential dissolution of surface gallium atoms and the formation of gallium vacancies. As a result of this dissolution, the surface of GaAs is enriched with arsenic. The negative shift in Vp is attributed to the junction between the arsenic-rich layer and the GaAs surface. Furthermore, the cathodic peak at ∼-0.38 V (SCE) is assigned to either the adsorption of chloride ions on this arsenic layer or the reduction of arsenic chloride on the surface. Regarding the stability of the photogenerated elemental arsenic in contact with the electrolytes, the values of the standard redox potentials as well as the shape of the illumination current-time curves (J-t) suggest that arsenic is oxidized to As2O3 in electrolytes of relatively low acid concentration. On the other hand, in electrolytes having H2SO4 concentrations as high as 2.5 M in the presence of high chloride concentrations, arsenic chloride, arsenic oxychloride, as well as elemental arsenic are expected to exist on GaAs surface. The diffusion coefficient of gallium ions or vacancies, as calculated from the value of the slope of the curves of illumination current versus the square root of irradiation time in a 2.5 M H2SO4 + 6.0 M LiCl mixture, is zero after 40 min of irradiation. Therefore it is expected that the photodissolution of gallium ions has completely stopped.
I. Introduction The photocorrosion of n-GaAs electrodes in aqueous electrolytes represents the major difficulty in using such an active solar energy material in wet solar cells.1-3 Numerous publications have dealt with various aspects of GaAs corrosion; some work focused on different methods for preventing corrosion,4-11 while other work dealt with kinetics and mechanisms.12-15 Moreover, etching of GaAs was also dealt with.16-21 †
On leave from Cairo University, Giza, Egypt. Abstract published in Advance ACS Abstracts, December 15, 1995. X
(1) Freese, K. W., Jr.; Madou, M. J.; Morrison, S. R. J. Phys. Chem. 1980, 84, 3172; J. Electrochem. Soc. 1981, 128, 1527. (2) Nakato, Y.; Tsumura, A.; Tsubomura, H. J. Electrochem. Soc. 1981, 128, 1300; 1980, 127, 1502. (3) Memming, R.; Schwandt, G. Electrochim. Acta 1968, 131, 1299. (4) Khader, M. M.; Hannout, M. M.; El-Dessouki, M. S. Int. J. Hydrogen Energy 1991, 16 (12), 797. (5) Vanmaekelberg, D.; Gomes, W. P. J. Phys. Chem. 1990, 94, 157. (6) Allongue, P.; Souteyrand, E. J. Electroanal. Chem. 1989, 269, 361. (7) Allongue, P.; Cachet, H. Electrochim. Acta 1988, 33, 79; J. Electrochem. Soc. 1984, 131, 2861. (8) Nakato, Y.; Tsubomura, H. Ber. Bunsen-Ges. Phys. Chem. 1987, 91, 405. (9) Lincot, D.; Vedel, J. J. Phys. Chem. 1988, 92, 4103. (10) Gerischer, H.; Lubke, M. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 123. (11) Reichman, B.; Fan, R. F.; Bards, A. J. J. Electrochem. Soc. 1980, 127, 333. (12) Allongue, P.; Blonkowski, S. J. Electroanal. Chem. 1991, 300, 261. (13) Lorenz, W.; Wolf, B. Electrochim. Acta 1983, 28, 191. (14) Mandler, D.; Bard, A. J. J. Electrochem. Soc. 1990, 137, 2468. (15) Menezes, S.; Miller, B. J. Electrochem. Soc. 1983, 130, 517. (16) Chun, H.; Ra, K. H. J. Electrochem. Soc. 1993, 140, 2568. (17) Mannhein, E.; Alkire, R. C.; Sani, R. L. J. Electrochem. Soc. 1994, 141, 546.
0743-7463/96/2412-1056$12.00/0
The products of GaAs anodic photocorrosion depend strongly on the pH of electrolyte. In neutral and less acidic electrolytes, insoluble oxides are formed on the GaAs surfaces and tend to deteriorate its photoactivity.22,23 In alkaline solutions, these oxides are soluble and thus unoxidized GaAs surfaces are always preserved.4,7 Furthermore, in concentrated acidic electrolytes, the work on passivation by Morrison et al.24,25 evidences that anodic oxidation of n-GaAs leads to the formation of unoxidized elemental arsenic in the oxide and in the GaAs/oxide interfacial region. In addition, Allongue et al.26 postulated that an arsenic-rich surface layer is formed due to the preferential anodic dissolution of surface gallium atoms at pH ) 0. This is in agreement with predictions by Li and Peter.27 Regarding the different approaches which have dealt with the inhibition of GaAs corrosion, the method which depends on the competition between the photocorrosion of the semiconductor and the oxidation of a redox couple (18) Fink, T.; Osgood, R. M., Jr. J. Electrochem. Soc. 1993, 140, 2572. (19) Ruberto, M. N.; Zhang, X.; Scarmozzino, R.; Willner, A. E.; Podlesnik, D. V.; Osgood, R. M., Jr. J. Electrochem. Soc. 1991, 138, 1174. (20) Van de Ven, J.; Nabben, H. J. P. J. Electrochem. Soc. 1991, 138, 144. (21) Hollan, L.; Tranchart, J. C.; Memming, R. J. Electrochem. Soc. 1979, 126, 855. (22) Schneemeyer, L. F.; Miller, B. J. Electrochem. Soc. 1982, 129, 1977. (23) Menezes, S.; Miller, B. J. Electrochem. Soc. 1983, 130, 517. (24) Freese, K. W., Jr.; Madou, M. J.; Morrison, S. R. J. Electrochem. Soc. 1981, 128, 1939. (25) Freese, K. W., Jr.; Morrison, S. R. J. Electrochem. Soc. 1979, 128, 1235. (26) Allongue, P.; Blonkowski, S. J. Electroanal. Chem. 1991, 317, 77. (27) Li, J.; Peter, L. M. J. Electroanal. Chem. 1986, 199, 1.
© 1996 American Chemical Society
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in solution has been particularly investigated.7,12-14,28,29 However, complete stability of GaAs with these species depends greatly on their concentrations. While dilute solutions of highly reducing species, e.g., selenide, can stabilize GaAs completely,12-14 concentrated electrolytes of less reducing capacities, e.g., I- and Cl-, are necessary to achieve the same goal.7,28,29 The present work investigates the issue of the preferential dissolution of gallium. This is achieved by analyzing the J-V curves as well as the illumination current-time, J-t, relationships under galvanostatic conditions. In addition, the mechanism of GaAs photocorrosion is investigated, in acidic electrolytes of different compositions, with the objective of reaching complete corrosion prevention. II. Materials and Methods Samples of Si-doped n-GaAs (6 × 1017 cm-3) produced from MCP Wafer Technology Limited were used. Electrodes were prepared by mounting the GaAs wafer on a copper base by using silver epoxy. The copper base and the surface of the crystal, except the face which was exposed to the electrolyte, were coated with silicone rubber. The surface area in contact with the electrolyte was 25 mm2, and the wafer thickness was 0.66 mm. Electrolytes were sulfuric acid and lithium chloride solutions. Electrochemical measurements were made in a three-electrode cell with n-GaAs serving as the working electrode, a Pt wire as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. All measurements were performed on a EG & G Princeton Applied Research PotentiostatGalvanostat (Model 273) with a corrosion analysis software program. The electrodes were etched by rinsing in a mixture of equal volumes of H2SO4 + H2O2 and H2O until a mirror-like surface was obtained. They were illuminated by white light filtered from IR and UV. The intensity of the light, as measured by a calibrated luxmeter (LX-101), was 20 mW cm-2. Coating of GaAs with arsenic was performed electrochemically by dipping the GaAs electrodes into 1 × 10-3 M solution of AsCl3 in methanol. The electrodes were biased at -0.4 V (SCE), and the quantity of electricity passed through the GaAs electrode was estimated. The corresponding amount of arsenic deposited was calculated. The number of monolayers of the deposited element was calculated by assuming that the number of arsenic atoms in one monolayer on GaAs is 1 × 1015 atom cm-2. Throughout this paper the electrode potential was measured with respect to a SCE.
Figure 1. Current-potential relations of n-GaAs in 6 M Cl+ 2.5 M H2SO4 (A) in the dark and (B) under white light illumination. Curves a and a′ are for fresh electrode, and b and b′ and c and c′ are after prebiasing the n-GaAs electrode at +0.4 V for 15 min and 120 min, respectively.
III. Results III.1. Current-Voltage Curves. Figure 1 shows the dark and the illumination current-voltage curves, J-V, for an n-GaAs electrode. Curves a, b, and c of Figure 1A are from data in the dark, and curves a′, b′, and c′ of Figure 1B represent data under illumination. Curves a and a′ are recorded for a fresh GaAs electrode, b and b′ for prebiased GaAs electrodes at +0.4 V for 15 min, and c, and c′ for GaAs prebiased for 120 min at +0.4 V. All these curves were collected from an electrolyte composed of 2.5 M H2SO4 and 6.0 M LiCl. Curves a and a′ are similar to the J-V curves reported in the literature.18,20,21 On the other hand, the J-V curves b′ and c′ show negative shifts of their illumination current onset potentials, Vp. This behavior is necessarily related to the pretreatment which the GaAs electrode experienced before measuring the J-V curves. Under a biasing potential of +0.4 V, only gallium atoms undergo anodic dissolution from the GaAs surface.26,27 This leads to preferential enrichment of arsenic (28) Khader, M. M. Presented at The First UAE University Conference on Material Science, United Arab Emirates; 12-14 December 1993; Al-Ain, Br. Corros. J. 1995, 30, 221. (29) Kubiac, C. P.; Schneemeyer, L. F.; Wrighton, M. S. J. Am. Chem. Soc. 1980, 102, 6900.
Figure 2. Dark current-voltage curves in 6 M Cl- + 2.5 M H2SO4 (A) n-GaAs coated with 5 monolayers of arsenic (s) and (B) n-GaAs prebiased at +0.4 V for 120 min (‚‚‚).
at the topmost atomic layers on the GaAs surface. Furthermore, the dark J-V curve measured after 120 min polarization, curve c, shows a cathodic peak at ∼-0.38 V. To probe the nature of this peak, the dark J-V curve was measured for an electrode of GaAs coated with a film of five atom layers of arsenic. This J-V curve along with that of a naked, prebiased GaAs electrode are shown in Figure 2, curves A and B, respectively. The anodic peak marked I in Figure 2, curve A, corresponds to the anodic dissolution of arsenic from the pure arsenic film. On the other hand the cathodic peak II in the same curve is assigned either to the reduction of the anodic product produced under peak I or to the adsorption of a reducing species on arsenic sites. This cathodic peak II is experienced by the GaAs electrode after contact with an acidified chloride solution for a period longer than 15 min (Figure 1); it has not been observed in the presence of sulfuric acid alone, and moreover, it has not been seen with other acidified halides,
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Figure 3. The relation between the illumination current density of n-GaAs biased at +0.4 V in 2.5 M H2SO4 and (a) 2 M Cl- (- -), (b) 4 M Cl- (s), and (c) 6 M Cl- (- ‚ -).
e.g., iodide ions.28 Therefore this cathodic peak could be either assigned to the adsorption of chloride ions on the arsenic-rich surface layer or to the reduction of an oxidized species on the GaAs surface, e.g., arsenic chloride, according to
AsCl3 + 3e- f As + 3Cl-
Figure 4. The illumination current density versus the square root of illumination time relations. The data are taken from Figure 3. ((b) 2 M Cl-, (O) 4 M Cl-, and (×) 6 M Cl-).
(1)
It is noticed that the anodic peak I does not appear in the case of the prepolarized GaAs electrode. Instead, this electrode exhibits an anodic current over a broad potential range. This indicates that the arsenic on the prepolarized GaAs electrode is oxidized via a mechanism different from that for pure arsenic. However, it is clear that the cathodic peak at ∼-0.38 V is similar in both cases, and then, similarly, this peak is assigned to either the adsorption of Cl- from the solution on an arsenic surface layer or to a reduction process according to reaction 1. III.2. Illumination Current-Time Transients (Jt). Figure 3 illustrates the relationships between the illumination current density, measured at +0.4 V, versus the time of irradiation by white light in 2.5 M H2SO4 solution containing various concentrations of LiCl (2, 4, and 6 M). In all cases the illumination currents have increased with the time of illumination at a rate which is clearly dependent on the Cl- concentration. Furthermore, in 6 M LiCl solution, the illumination current has reached a plateau after ∼40 min of irradiation, Figure 3, currve c. When this experiment was conducted for 24 h the steady state illumination current did not show any sign of deterioration. Figure 4 shows the relationship between the illumination currents and the square root of the irradiation time for the above three electrolytes. This relationship is linear with a changing slope at a specific stage. The J-t curves in an electrolyte of 6 M LiCl and 2.5 M H2SO4 under different polarization potentials are shown in Figure 5. For comparison the J-t curve measured at +0.4V, shown in Figure 3, is also included in Figure 5. It is clear from this figure that the illumination currents have increased with the increase in the applied anodic potential. For electrolytes of constant chloride and variable H2SO4 concentrations (0.1, 1.0, and 2.5 M), the J-t relations are shown in Figure 6. With 0.1 M H2SO4, the illumination current has increased gradually and reached a maximum after ∼20 min of irradiation before it decreased with time nonlinearly (curve a). In 1.0 M H2SO4 the maximum is
Figure 5. The relation between the illumination current and the time of irradiating n-GaAs in 6 M Cl- + 2.5 M H2SO4 at (a) -0.2 V (- -), (b) +0.2 V (s), and (c) +0.4 V (- ‚ -)
Figure 6. The illumination current density as a function of illumination time of n-GaAs electrodes at +0.4 V in 6 M Cland (a) 0.1 M H2SO4 (- ‚ -), (b) 1.0 M H2SO4 (- -), and (c) 2.5 M H2SO4 (s).
reached at ∼40 min (curve b). As shown in curve c, the 6 M Cl- + 2.5 M H2SO4 electrolyte has an increased illumination current reaching a plateau at ∼40 min. IV. Discussion The anodic corrosion of n-GaAs in aqueous electrolytes results in either the dissolution of the electrode material (eq 2)20,26 or the oxidation of its surface, (eq 3).24,25
n-GaAs Photoanode in Acidic Chloride Solutions
GaAs + 3h+ f Ga3+(aq) + As(surf)
Langmuir, Vol. 12, No. 4, 1996 1059
(2)
Table 1. Effect of Chloride Ion Concentration on the Diffusion Coefficient of Gallium Vacanciesa,b D, cm2 s-1
GaAs + 3H2O + 6h+ f 1
/2Ga2O3(surf) + 1/2As2O3(surf) + 6H+ (3)
The carriers involved in both cases are the valence band holes. If reaction 3 was dominant, one would expect a decrease in the photocurrent, under constant applied potential, with the increase of irradiation time due to oxide growth.16,17 The results represented in Figures 3 and 5 show an opposite effect, i.e., an increase in the illumination current with time. Consequently, reaction 3 is excluded in analyzing the results of Figures 3 and 5. It is particularly noteworthy that the onset as well as the saturation illumination currents have shifted toward more negative potentials after anodic polarization (curves b′ and c′ in Figure 1B). This negative shift in the onset potential, Vp, is attributed to a corresponding negative shift in the flat-band potential, Vfb, of the semiconductor. As reaction 2 proceeds, gallium atoms are dissolved in the electrolyte, and an arsenic rich layer is formed at the outer surface of the GaAs. It is thus suggested that the junction between the arsenic-rich surface layer and the semiconductor substrate produces photovoltages analogous to those found at a semiconductor/metal interface. These are known to produce a negative shift in the flat-band potential.11,19,26-28 The formation of an arsenic-rich surface layer is proved by comparing the dark cathodic peak at ∼-0.38 V for the prebiased electrode with the corresponding peak of an arsenic-coated GaAs, Figure 2, curves A and B, respectively. As suggested previously, this comparison predicts that the cathodic peak at ∼-0.38 V is either due to the adsorption of Cl-, from the electrolyte, on arsenic atoms or to a reduction process, e.g., reaction 1. The arsenic layer is formed on the GaAs surface due to the preferential photodissolution of gallium. Although one expects that the produced elemental arsenic at +0.4 V is to be oxidized at higher anodic potential during the forward scan, this oxidation may not proceed to completion, i.e., elemental as well as oxidized arsenic can both exist in the surface region of GaAs.24,25 This unoxidized arsenic can be responsible for the adsorption of Cl- and the appearance of the cathodic peak at ∼-0.38 V, Figure 1A, curve c. The selective dissolution of Ga out of the GaAs sample, according to eq 2, leads to the preferential enrichment of arsenic at the outer surface. As gallium atoms dissolve out of the surface, they leave behind vacancies in the GaAs lattice. The process is controlled by the rate of diffusion of Ga atoms, or vacancies, in the direction normal to the sample surface. Within this framework the outward diffusion of Ga atoms is equivalent to the inward diffusion of vacancies.30,31 The diffusivity equation under this condition is given by the one-dimensional equation of Fick’s second law as
chloride ion concentration, M
first diffusion process
second diffusion process
2 4 6
4.0 × 10-6 3.8 × 10-6 4.0 × 10-6
1.9 × 10-5 6.3 × 10-6 1.4 × 10-6
a The diffusion coefficients are calculated by using eq 7 from the slopes of Figure 4. b The electrode was biased at +0.4 V in 2.5 M H2SO4.
increasing photocurrents, are all related. Therefore, the solution of the diffusivity eq (4), which gives the concentration profile of Ga vacancies within the GaAs crystal, can be expressed in terms of photocurrent as32
Jt
xDt )
J∞
L
[
n)∞
1
xπ
+
nL
]
(-1)ni erfc ∑ xDt n)1
(5)
where Jt is the illumination current, under constant applied potential, at time t and J∞ is the maximum illumination current. J∞ corresponds to the maximum enrichment of the GaAs surface with arsenic. The magnitude of J∞ from Figure 3 is 18 mA cm-2. L is the thickness of the GaAs crystal. At relatively short time, low values of D and/or thick crystals (i.e., large L), such that
nL nL f ∞; i erfc f0 xDt xDt the above solution converges to
Jt ) J∞
x
Dt πL2
(6)
Equation 6 predicts a straight line relationship between Jt and R(t). Figure 4 illustrates these relations. At fixed applied anodic potential, constant acid concentration, and variable chloride ion concentration, the Jt vs R(t) relations of Figure 4 predict two diffusion-controlled processes. The diffusion coefficient of both processes is related to the slope, S, of the straight lines in Figure 4 by
D)
[ ]
πL2 2 S J∞2
(7)
where C(x,t) is the concentration of gallium vacancies within the GaAs lattice at a time t and a distance x normal to the surface, and D is the interdiffusivity. The previous discussion shows that three processes, Ga vacancy formation, surface arsenic enrichment, and
The values of the diffusion coefficient for the two processes in different electrolytes are listed in Table 1. This table shows that the diffusion coefficient of the first process is about the same and independent of the chloride ion concentration. On the other hand, the diffusion coefficient of the second process decreases with an increase in the chloride ion concentration. To explain the nature of these two diffusion processes, the stability of different species on the GaAs surface, in contact with the electrolyte, is discussed. Since the redox potential, E°, of Ga/Ga3+ is -0.67 V in 2.5 M H2SO4 solution,33 i.e., at any potential positive of -0.67 V, Ga is readily soluble in the electrolyte; consequently Ga vacancies are formed. Therefore, the first diffusion process is assigned to the diffusion of these vacancies from the surface to the bulk of GaAs. According to reaction 2, the
(30) Khader, M. M.; El-Anadouli, B. E.; El-Nagar, E.; Ateya, B. G. J. Solid State Chem. 1991, 93, 283. (31) Khader, M. M.; Keiri, F. M.-N.; El-Anadouli, B. E.; Ateya, B. G. J. Phys. Chem. 1993, 97, 6074.
(32) Crank, J. In The Mathematics of Diffusion, 2nd ed.; Clarendon Press: Oxford, 1975; p 47. (33) Pourbiax, M. Atlas of electrochemical equilibria in aqueous solution; Pergamon Press: New York, 1966; pp 428, 493.
∂C(x,t) ∂2C(x,t) )D ∂t ∂2t
(4)
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rate of Ga dissolution is increased with an increase in the number of holes. It should be mentioned that when a blank experiment was carried out in the dark at +0.4 V, the measured anodic current did not change during a period of 2 h. This result indicates that in the dark, where there are no holes reaching the GaAs surface, the electrode is stable, and thus the current did not change with time. Conversely, in the light the number of photogenerated holes increases and, therefore, reaction 2 is favored. As a result, the illumination current has increased. Furthermore, the assignment of reaction 2 to the first diffusion process is supported by the experimental results of Figure 5 which shows the relation between the photogenerated current density and time of illumination at different applied potentials. These results indicate that the rate of increasing the illumination current has increased with the increase in the magnitude of the anodic potential. Such an increase in the illumination current is a result of the increased efficiency of the process of electron/hole separation. The anodic potential forces the photogenerated holes to reach the GaAs surface and thus increases the illumination current. The stability of elemental arsenic on the GaAs surface depends on the pH of the electrolyte as well as the value of the applied potential. At pH > -0.34 and a potential positive of -0.45 V, arsenic is oxidized to the stable As2O3 according to33
2As(surf) + 3H2O f As2O3(surf) + 6H+ + 6e-
(8)
The formation of As2O3 on the GaAs surface is known to deteriorate its photoactivity.22,23 Indeed, in electrolytes of 0.1 and 1.0 M H2SO4, the photoactivity of GaAs has reached a maximum after ∼30 and ∼40 min, respectively, and then decreased with further illumination as shown in Figure 6. The initial increase of the illumination current, as in curves a and b of Figure 6, is attributed to the formation of an arsenic-rich surface layer. Moreover, the subsequent decrease in the illumination current, curves a and b Figure 6, is due to the formation of an As2O3 layer on the GaAs electrode according to eq 8. Regarding the oxidation of arsenic in electrolytes of high acid concentration, the Pourbaix diagram of the arsenic system33 shows that at pH e -0.6 and a potential positive of -0.47 V, AsO+ is formed due to the oxidation of arsenic by H2O according to
As + H2O f AsO+ + 2H+ + 3e-
(9)
The species AsO+ can be stabilized via the reaction with Cl- to give, e.g., AsOCl. The surface species AsOCl and/or AsCl3 are supposed to be transparent to visible light and thus will not deteriorate the photocurrent. Furthermore, at higher concentrations of Cl-, it is suggested that the surface compounds AsOCl and/or AsCl3 resist oxidation by H2O due to the extremely high activity of the chloride ion at such concentrations.29 The high activity of Cl- is accompanied by a reduction in the activity of H2O.29 Therefore, the oxidation of arsenic and/or AsCl3 by H2O is minimized at high concentrations of Cl-. This means
that at high Cl- and acid concentrations, the surface of GaAs can have stable species like arsenic, AsCl3, and AsOCl. In addition, Cl- can also be adsorbed on arsenic, as previously suggested. The second diffusion process is also assigned to the diffusion of Ga vacancies, however, after enriching the GaAs surface with arsenic and chlorinated arsenic. The decrease in the value of the diffusion coefficient, for the second diffusion process, with the increase in the chloride ion concentration is a subsequent result of formation of a stable surface layer of arsenic and chlorinated arsenic on GaAs. This layer protects the GaAs surface from further attack by the electrolyte and thus reduces the rate of formation of Ga vacancies. After ∼40 min illumination in an electrolyte of chloride ion concentration as high as 6 M and 2.5 M H2SO4, the photogenerated current density at +0.4 V has reached a maximum of 18 mA cm-2 and remained constant during an experimental run of 14 h. This is the most important experimental finding in the present work. This experiment signifies two desirable results, namely, increasing the photocurrent density from ∼4 to 18 mA cm-2 and stabilizing the GaAs electrode against further attack by the electrolyte, presumably, due to the formation of an arsenic and chlorinated arsenic layer on the electrode surface. The slope of the illumination current vs time relationship is zero in the steady state region (Figure 3 curve c). Therefore, according to eq 7, the diffusion coefficient of gallium ions for this region is zero. Accordingly, it is predicted that the photocorrosion of a GaAs electrode is completely stopped in an electrolyte composed of 2.5 M H2SO4 and 6.0 M Cl-. The oxidation process which consumes the photogenerated holes, after reaching the steady state condition, remains as yet untreated. It is possible that the change in the surface properties due to the preferential dissolution of gallium could produce surface states. These states could make the redox potential of species like Cl-/Cl2 and/or OH-/O2 compatible with the position of the valence band edge of GaAs and thus allow the oxidation of such species. V. Conclusions Irradiation of n-GaAs in acidified chloride electrolytes, under anodic polarization, enriches the surface with arsenic. The arsenic-rich surface layer enhanced the photoactivity by shifting the onset potential of the photocurrent, Vp, toward more negative values. This shift is a result of the junction between the arsenic-rich layer and the GaAs surface. The Ga vacancies, which are formed due to the preferential dissolution of surface Ga ions, are diffused toward the bulk of the GaAs crystal. Initially, the diffusion coefficient for this process is independent of the chloride ion concentration, but it then decreases with the increase in the chloride ion concentration, presumably due to the formation of an arsenic and/or chlorinated arsenic-rich layer. In an electrolyte of chloride ion concentration 6.0 M and sulfuric acid concentration of 2.5 M, the diffusion coefficient of gallium ions is zero. This predicts the complete prevention of the corrosion process. LA940895R
