H3PO4 interface

J. Electroanal. Chem., 151 (1983) 79-87 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

79

P H O T O E F F E C T S ON T H E C u / H 3 P O 4 INTERFACE PART III. I N T E R P R E T A T I O N OF P H O T O C U R R E N T S AT T H E INTERFACE

B E R N A R D P O I N T U , M I R E I L L E BRAIZAZ, PIERRE P O N C E T and J A C Q U E S R O U S S E A U

Laboratoire de Physwo-Chimw, U.E.R. de Physique, Umversit~ Lyon 1, 69622 Vdleurbanne Cedex (France) (Received 30th October 1981; in revised form 6th December 1982)

ABSTRACT We study the alterations, under illumination of the interface model in the band scheme described in the previous paper (Part II). This leads us to a description of the interface allowing the observed photoeffects to be explained.

INTRODUCTION

In the preceding paper [la] we have proposed a model, within the energy band scheme, for the interface C u / H 3 P O 4. This model leads to an account of the interfacial processes in darkness; here we shall show that we can thus also interpret the existence of photoeffects. THE BAND SCHEME WITH ILLUMINATION

The photocurrents observed are of positive sign and therefore reveal a photoeffect that cannot be localized, according to Williams' theory [ 1b], at the (CuO, p ) / solution interface. We must search for its origin in the deeper oxide layers as suggested also by its dependence towards the crystalline orientation of the metal.

The CuO, p/solution interface Taking into account Williams' theory, illumination can promote the passage of a positive ion in solution only on a n-type semiconductor. In other words, the oxidization can be made easier by illumination only for a n-type semiconductor in contact with the solution. And yet, in the above mentioned model, it is the (CuO, p) which dissolves in solution, hence it is scarcely probable that an important photoeffect may occur at the (CuO, p ) / solution interface. In fact, as a result of illumination the electrons are no longer in equilibrium with the holes in the space-charge layer: the free energy of the electrons becomes nEF and that of the holes becomes pE F. 0022-0728/83/$03.00

© 1983 Elsevier Sequoia S.A.

80

These new free energy values satisfy [2] the relations nEF

--

E F = kTlog(n*/no)

E z - pEv = k T l o g ( P * / P o )

where nEF and p E r designate respectively the quasi-Fermi levels for the electrons and for the holes, n * ( x ) and p * ( x ) designate the concentrations of carriers under

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Cu20, n

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IDIFFUSION BULK LAYER

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Fig. 1. Interface model with anodic polarization and illumination: (a) evolution of different layers of the interface a n d of limiting concentrations u n d e r illumination ( ) f r o m darkness positions ( . . . . . . ); (b) interface model u n d e r illumination in a b a n d diagram.

81 illumination, and n o and P0, the corresponding equilibrium concentrations. The highest free-energy variation occurs for the minority carriers (the electrons) whose equilibrium concentration n o is the lowest. It is nEF which draws more apart from E F, whereas pE F changes little in the surface dose to the illuminated region. The movement of photogenerated electron-hole pairs is controlled by diffusion which separates the two types of carriers, as indicated in Fig. 1. The electrons move towards the interior of the oxide in the very high gradient of the quasi-Fermi energy nEF; the negative charge density in the vicinity of the reorganization zone B increases. The holes move towards the semiconductor/solution interface where they produce a slight accumulation of positive charges. This makes the passage of the oxygens from the solution to the oxide easier where they behave as additional acceptor centres that diffuse towards the interior. In this region where the copper ions are not numerous, their passage into the solution is accelerated. As we have already seen, P*/Po is small. This appears clearly in the very small relative variation of the number of copper ions passing into the solution under the effect of illumination. The concentration gradient (c z - c l ) / r 2 1 increases slightly as well as the current i2~ which is proportional to it. We are acquainted with the fact that an important photoeffect may occur at the semiconductor/solution interface when these two conditions are satisfied simultaneously. (1) The concentration at the equilibrium of the minority carriers must be very low. This is the case here if we take into account the E v variation which is very far from Eo close to the surface. (2) The external field (here the gradient of the quasi-Fermi energy nEF) must follow such a direction that the photogenerated minority carriers are directed towards the surface where they can react with the solution. However, this is not the case here, because the minority carriers (the electrons) are drawn away from the surface by diffusion. Moreover, the reacting species are the majority carriers (passage of the copper ions into the solution) whose relative concentration variation is low under illumination. Therefore if a photoeffect exists at the (CuO, p)/solution interface, it must be very low in comparison with that intervening in another region. The Cue0 , n / Cu interface Let us note first that as the total thickness of the oxide film is very small, it is very likely that the light will reach the m e t a l / o x i d e interface. The Mott-Schottky model [3,4] is not directly applicable because no stoichiometric oxide layer exists on the metal and the observed photocurrent in such a model for a n-type oxide should be negative. Lastly, the interface model presented here excludes any rectification effect at the contact with the metal, because there is no exhaustion in majority carriers in this region. We shall nevertheless analyse the effect of illumination at the m e t a l / o x i d e interface.

82 The absorption of light leads to the creation of electron-hole pairs and a new distribution of charges within the space-charge layer; we may then again introduce the quasi-Fermi levels nEF and pE F in this region where the light is absorbed. The electron-hole pairs thus created as separated by the gradient of the quasi-Fermi energy. As opposed to the case of the (CuO, p ) / solution interface, where the photogenerated electrons were directed towards the interior of the semiconductor, here they are created in the immediate vicinity of the metal and their recombination probability is very low. Hence, this may result in the increase of the anode current under illumination which actually corresponds to the experimental observation. Let us note, however, that the relative variation n * / n o of the number of majority carriers produced by illumination is rather low. It cannot alone explain the high photocurrent which is observed. Nevertheless, this negative charge which is developed in the vicinity of the metal makes the passage of copper into the oxide easier in the form of additional donor centers. The photogenerated holes are directed towards the interior of the oxide under the action of a very strong concentration gradient of pE z. The concentration in holes in (CuzO, p) then increases considerably. On the whole, the concentration gradient (C4 --¢3)/843 increases somewhat, as well as the current i43 proportional to it. The internal region (CueO, p) / B / (CuO, n)

The oxides (CuO, n) and (Cu:O, p) found inside the anode film have a rather low conductivity. More specifically, the density of minority carriers is only slightly lower than that of the majority carriers and no important photocurrent whatsoever can result from light absorption by this region. However, as we have just seen, as a result of the very strong gradients of quasi-Fermi levels for minority carriers in the (Cu20, n) and (CuO, p) oxides, there is an accumulation of positive charges in (Cu20, p) and negative charges in (CuO, n). This corresponds to an expansion of these oxides as indicated in Fig. 1 and consequently to a decrease of 843 and 821 to the values 8~3 and 8~1. The positive space charge of (Cu20, p) increases and the Fermi level of this oxide is lowered. This negative space charge of (CuO, n) increases and the Fermi level of this oxide rises. 4¢ From these charge accumulations there results, moreover, an electrical field E32 >> E32 and hence a migration current i~2 >> i32. The diffusion currents i43 , i:l and i~0 must undergo the same increase as the migration current i32, thus they take the values i~3, i~ and i~0. Within the increased field E~2 matter transport is itself more important and the copper ions are accelerated; hence the copper concentrations at the limits of the various oxide layers vary and the current increase must appear an increase of the concentration gradients (¢4 -- C3)/843' ((?2 -- ¢1)/821 and (c I - Co)/Slo to the values (c~ - c~)/8~3, (c'2 - c'1)/8~1 and (c'l - C'o)/8'lo. (1) The thickness 810 of the diffusion layer in the liquid phase is not affected by illumination and the increase of ilo can appear only by the presence of a concentration c'1 > c I.

83 (2) The thickness 8~3 and 8~1 must be lower than 843 and 82~ as we have seen. Moreover the acceleration of copper-ion transport and the increase of c I must appear in the concentrations under illumination c~ > c a and c~ > c 2. These inequalities may finally induce a concentration c~ > c 3.

Global analysis of photoeffects Thus, we can see that the primary effect of the illumination arises at the m e t a l / o x i d e and oxide/solution interfaces to appear as a secondary effect in the central region of the oxide: (1) The exhaustion of minority carriers in the extreme regions of the oxide film constitutes a situation favourable to the production of important photoeffects [5,6]. It is the primary effect which consists in the displacement of the minority in the high gradient of their own carriers quasi-Fermi energy. Hence, photogenerated the electron-hole pairs are separated and their recombination probability is low. (2) These shifts lead to an increase of space charges of opposed signs in the oxides (Cu 20, p) and (CuO, n), hence to an increased field and an increase of the migration current density i32 in this intermediate region: this is the secondary effect. Hence, a modification of the conditions at the limits controlling the diffusional processes arises. Because of the illumination, there is a reorganization of the set of oxide layers in order to satisfy the equality: -p

__

.!

__

.t

/43 -- /32 -- /21 ~

,!

/10 ~

.t

/Dm

Because of this reorganization process, the response to photo-excitation is not instantaneous. This appears clearly on the curves indicating the photocurrent variation as a function of time [7]. INTERPRETATION OF OBSERVED PHOTOCURRENTS

Influence of the acidic concentration and the rotation speed co of the anode If the concentration cA in acid decreases, so does the viscosity v of the solution and hence the thickness 810 of the diffusion layer in the liquid phase also decreases. The concentration gradient (c I - C o ) / S l o increases as well as the current il0. Similarly, if the rotation speed to increases, 8~0 decreases and il0 increases. These two parameters, cA and co, act directly on the diffusion current in the liquid phase; the concentration gradients in the oxides (CuO, p) and (Cu20, n) must have such values that the currents i21 and i43 are equal to i~0, which corresponds to values of the copper concentrations cl, c2, c 3 and c 4 and thicknesses of these oxides depending on cA and to. The same dependence is established for the field E32 in the intermediate zone of the oxide layer. When the density of the current ill m increases as a result of variations of cA or to, the copper transport within the oxide is accelerated. If we illuminate, the photogenerated electrons and holes are submitted to higher gradients

84 in (CuO, p) and C u 2 0 , n), and this leads to an increase of the space charges of (CuO, n) and (Cu20, p), hence of the field 232 and consequently of ill m. More specifically, we can see that as the illumination increases the speed of ionization of copper in the oxide, it makes matter transport easier and increases the diffusion currents in the solid phase. The concentration c~ increases, which reduces the limitation imposed by this concentration on the diffusion in solution. This appears in the fact that Levich's criterion is satisfied better by the limiting current under illumination than by the limiting current in darkness. Matter transport in the solid phase being slower if we illuminate the interface, the limiting role of the process in the oxide phase will itself be less dominant.

Influence of the temperature 0 We have seen [7] that the appearance of the photoeffect occurs for potentials VRA that are higher as the temperature increases. On the other hand, on the characteristic i(VRA ) corresponding to these different temperatures [8], we observe that the slope of the linear ascending part increases with 0, and the potential VRA at which the curvature of the characteristic appears also increases with 0. However, this potential, at which the characteristic diverges from the linear part, corresponds to the appearance of a notable photoeffect; this corresponds [7] (for the potentials VRA < Vl) to potentials for appearance of the photoeffect that increase with temperature. This also enables us to eliminate the hypothesis of a photochemical processes for potentials below V1. The photocurrent obtained on the polishing plateau increases with the temperature just like ihm" This evolution enhances the conductivity variations of the semiconductor film with temperature according to the law of Meyer-Neldel [7,9]. There ensues an evolution of activation energy E 1 with the anode potential VRA. AS we have seen during presentation of the results, the values for E 1 are well below the values for the gaps of Cu20 and CuO and may be due to transitions which involve energy levels situated within the forbidden band of an oxide. These levels in defects are linked to the structure of the material, and the abrupt variation of E~ when we pass from frontier 1 to frontier 2 of the oscillations is due to the difference of structure of the oxide film between the two extremities of the instability region. If we connect this observation with R H E E D analysis of the electrode surfaces, we find a confirmation for the existence of oxides of different nature on either side of the oscillation zone: Cu20 for VRA ~ V1 and (Cu20 + CuO) for VRA >I V2. Finally, the evolution of E 1 for the values of lIRA >7 V2 suggests that the structure acquired by the oxide film after the oscillations clearly evolves when the electrode potential increases to reach a stationary configuration with respect to potential.

Photocurrents and anode potential The study of the evolution of the photocurrent as a function of the anode potential [7] enables us to distinguish different regions according to increasing potentials:

85 (1) In the vicinity of zero current, the photocurrent presents a large increase with the anode potential. This persists until a maximum roughly corresponding to the appearance of the anode current for VRA = 80 inV. This important photocurrent is probably attributable to the oxide film existing on the electrode and formed before the measurements are begun. For the electrode is actually studied in "normal conditions", i.e. it has been subjected immediately before the measurements to an electrolytic polishing of 30 rain on the polishing plateau. This corresponds to the formation of an anode oxide film. Once this previous polishing is complete, the circuit is open and then the electrode potential is increased; as long as no noticeable current is observed, the film is not destroyed and we obtain an increasing photocurrent with increase in the potential. This view is reinforced by the absence of noticeable photocurrents on an electrode which has not been submitted to a previous polishing, and is thus not covered with the oxide film formed by anodic prepolarization. (2) After having reached a maximal value, the photocurrent decreases strongly with the potential. This decreases corresponds to the appearance of the anode current and must be related to the progressive destruction of the oxide film. This phenomenon continues until a value of lIRA ~ 140 mV which corresponds to the minimal decomposition potential of the electrode. The photocurrent is then minimal and remains quite stable until a value of VRA ~ 400 mV is reached. (3) For VRA > 400 mV, the photocurrent increases again. This corresponds to the appearance of an oxide film on the electrode. When the curvature of the characteristic appears, for VRA -- 550 mV, the oxide film must have reached such a thickness and a structure that its semiconductor properties are revealed and the photocurrent increases strongly with VRA until a high value corresponding to the potential V1 of frontier A of the oscillations. The film thickness must then be maximal. (4) For the potential V2 of frontier 2 of the oscillations, the photocurrent is generally higher than that at frontier 1. If we increase VRA beyond V2 we observe a decrease of photocurrent up to potential Vm,n. This decrease of the photoeffect must reflect an evolution of the structure of the oxide towards a stable form. This progressive organization of the semiconductor, from an amorphous state into an organized state, is linked to a decrease of its conductivity, which leads to a reduction of the photoeffect amplitude. As for the film thickness, it must be constant [10] for V R A ~ V2 .

(5) For VRA > Vm,,, on the current density plateau, the photocurrent increases very slightly with the potential. In practice, the structure and thickness of the film do not evolve and this situation corresponds to the model previously described [la]. From these different observations we must emphasize the important points that allow a better characterization of the nature of the interface: (1) The behaviour of the anode on either side of the oscillation zone is very different: the photocurrent Ai increases strongly with VRA for VRA < Vj and Ai decreases with VRA for VRA > V2. If we put this fact with the observations of the anode in R H E E D [7] and the description of the interface model, the evolution of Ai confirms that a C u 2 0 film is formed in the last third of the increasing part of the

86 characteristic, the thickness of the film increases up to the potential V1 where the oscillations appear. For the potentials VRA > V2, the CuO film exists and we obtain the structure for the oxide film corresponding to the interface model already presented [ 1a].

Influence of the crystalline orientation of the anode The rate of dissolution of an anode and the corrosion of copper are anisotropic. This appears in the dependence of ihm on the crystalline face of copper. The differences obtained for the photocurrents with the three faces come, therefore, from the presence of the oxide whose crystalline nature is thus confirmed. We have seen [7] that the classification of the crystalline faces of copper, relative to the photocurrents obtained on the polishing plateaux with 1 s light pulses, was (100) > (111) > (110) and that this classification corresponded to that of the copper oxidization speeds [11,12] in the oxygen current at 100°C, i.e. to the thicknesses of the oxide layers. However, in the interface model that we have described, the amplitude of the photoeffect per time unit is determined in particular by the speed of creation under illumination of the electron-hole pairs in the vicinity of the C u / ( C u 2 0 , n) interface. When the illumination continues the electrons, in greater and greater numbers, are transferred from the valence band of the oxide to the conduction band. Therefore, we tend towards an exhaustion situation in "ionizable" atoms. However, in fact there is a continual formation of the oxide and a permanent renewing of the species allowing the creation of electron-hole pairs; hence the number of free photogenerated carriers per second (finally the photocurrent per time unit) does increase with the oxidization rate of the metal. The value of the amplitude of the observed photocurrent reflects, therefore, a balance between the oxidization rate of copper and that of the creation of electron-hole pairs with light energy brought in (Cu20, n) in the vicinity of the metal. The localization of an important photoeffect in this region is compatible with the proposed model: in fact ( E F - E v ) is very large in the vicinity of the metal and the concentration in minority carriers is therefore very low; this constitutes, as is well known, a condition for obtaining important photoeffects. In a recent paper [13] we have shown how this interface model enabled us to interpret equally the classification of the crystalline faces as regards saturation photocurrent obtained under long duration illumination. CONCLUSION The study of photoeffects enabled us to confirm the existence of distinct interfacial compounds at the two boundaries of the oscillation region and to propose a model for the interface in the band model of semiconductors. This model is complementary to that we proposed [8] to interpret the oscillations. Although we presented [7] a few experimental results concerning the photopoten-

87

tials in open circuit, we have not included the analysis of these phenomena in the presentation of the interface model. In fact, the complementary experimental studies are in hand and we will present a specific study of this phenomenon later. REFERENCES la lb 2 3 4 5 6 7 8 9 10 11 12 13

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