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Local oxide growth on the n-GaAs surface studied by small area XPS
Surface Science 433–435 (1999) 131–135 www.elsevier.nl/locate/susc
Local oxide growth on the n-GaAs surface studied by small area XPS I. Ge´rard *, C. Debiemme-Chouvy, J. Vigneron, F. Bellenger, S. Kostelitz, A. Etcheberry IREM, Universite´ de Versailles-St-Quentin, 45 Av. des Etats Unis, Versailles 78 035 cedex, France
Abstract The photoanodic dissolution of n-GaAs was investigated in a buffered solution at an intermediate pH (pH 9). At this pH, the GaAs dissolution rate was limited by the growth of an anodic oxide film. A laser illumination leads to a local oxide growth only in the illuminated part on to GaAs. Transient photocurrent and capacitance measurements clearly show dissolution kinetics in two steps: the first step is about a few tens of seconds and is correlated with gallium oxide dissolution, and the second step is greater than a few minutes, indicating the origin of at least two surface films onto GaAs. A parallel approach using in-situ electrochemistry and ex-situ surface analysis ( XPS ) was performed to characterise the growth of an anodic film on to GaAs. A gallium enrichment was detected on this oxide film. The arsenic deficiency in this oxide results from a higher arsenic oxide solubility. The limitation of the GaAs dissolution is due to the dissolution of gallium oxide at pH 9. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Electrochemical method; Gallium arsenide; Gallium oxide; Growth; Oxidation; Photoelectron spectroscopy ( X-ray photoelectron); Semiconducting interfaces
1. Introduction Although extensively studied, the electrical and chemical behaviour of the III–V oxides are not entirely understood. The literature [1] states that the dispersions of oxide composition result essentially from the competition between two or more elements of III–V compounds, which is also observed for the growth as for the ageing process. Generally, the characteristics of oxide layers depend on the ambient conditions. Since there are many parameters such as the presence of water (wet atmosphere, thin water layer, …), it is difficult to understand accurately these kinds of phenome* Corresponding author. Fax: +33-1-39-25-43-81. E-mail address: [email protected] (I. Ge´rard)
non. To study the thin oxide layers (10 nm range) in interactions with water, a parallel method can be performed using in-situ electrochemistry studies and ex-situ surface analyses ( XPS). Localised anodic layers that had grown under illumination on the surface of n-GaAs samples were studied in this paper. First of all, analyses of transient photocurrent and capacitance responses have been used in order to characterise in situ the growth of a thin anodic oxide film and its chemical stability. Secondly, using a small X-ray spot configuration, XPS analyses have been performed to characterise two parts of the SC. One of them was the illuminated part of the sample where the oxide had grown, and the other was maintained in the dark. In a previous paper [2], gaussian or flat-bottom etching profiles were observed under a laser-beam
0039-6028/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 99 ) 0 00 7 1 -0
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I. Ge´rard et al. / Surface Science 433–435 (1999) 131–135
illumination. These two profiles involved two kinds of electrical responses. For a gaussian profile, a constant photocurrent was detected, whereas for a flat-bottom profile, a transient photocurrent was observed with a time-dependent response. The flatbottom profile results from an interfacial thin and continuous oxide layer. This film becomes the limiting factor of the anodic photodissolution process for an intermediate range of pH (4–12). In order to study this thin oxide film, our experiments were performed at pH 9. From our previous work, the photodissolution of n-GaAs could be limited by the dissolution of gallium oxide, which has relatively poor dissolution kinetics.
2. Experimental The [100] n-GaAs samples, with a carrier density about 1018 cm−3, were obtained from MCP Ltd Inc. Electrodes were etched mechanochemically at room temperature with a Br /methanol 2 solution and rinsed with pure methanol. Just before the experiments, the samples were dipped in a 2 M H SO solution. 2 4 The electrode was placed in a classical threeelectrode cell. All potentials were referred to the saturated mercury sulphate electrode (mse). The electrolyte solutions were prepared using buffered concentrate Titrisol at pH 9. The electrode polarisation were performed using a EG&G potentiostat Parc 176 and Parc Model 175 generator. A Gould Model (DSO) Digital Memory Oscilloscope was used to capture the transient photocurrent. All experiments were performed at a fixed potential V=–0.3 V . mse A 5-mW HeNe laser Uniphase (Ø=2 mm) was used to illuminate the sample. A calibrated polarising filter was also used to choose the incident power light. The laser power light was measured at P =1 mW by a Liconix laser power meter model i 45 PM. The X-ray photoelectron spectroscopy ( XPS) measurements were performed on a VG ESCALAB 220i-XL spectrometer. The X-ray source was monochromatic AlKa radiation. The crater resulting from the photodissolution and the part of GaAs that was kept in the dark were
analysed using an XPS line scan. This line scan was recorded using a 150 mm X-ray spot size with a step size of 250 mm. For XPS analyses, the exit of the sample from the solution was performed under a solution drop, dried under a nitrogen flux and stored in an argon atmosphere.
3. Results and discussion At pH 9, the polarisation of the electrode at – 0.3 V means that the interface was in a deplemse tion regime. As soon as the laser beam illuminated a part of the surface, a local time-dependent photocurrent was generated ( Fig. 1a). The initial current intensity is proportional to the laser power [2–4]. Then, the photocurrent decreased until a steadystate photocurrent was reached. This photocurrent value gave the etching rate of the local dissolution which induced a flat bottom cylindrical groove ( Fig. 1b). These characteristics of the time-dependent current [5,6 ] and flat-bottom profile [2] were
Fig. 1. (a) Transient photocurrent response of n-GaAs at pH 9 and V=−0.3 V ; (b) etching profile obtained with (a) condimse tions with an He–Ne laser beam.
I. Ge´rard et al. / Surface Science 433–435 (1999) 131–135
interpreted as a result of the formation of an anodic oxide film on n-GaAs. The steady-state photocurrent was associated with a dynamic phenomenon whose rates of oxide growth and oxide dissolution were identical and entirely controlled by the pH of the solution [2]. In our previous work, the steady-state photocurrent variation against pH also gave a characteristic ‘‘U’’ shape. This behaviour was in good agreement with Pourbaix’s thermodynamic data for the dissolution of arsenic and gallium oxides [7]. According to our pH conditions, the photodissolution of n-GaAs should be entirely controlled by the gallium oxide dissolution. In such a situation, a homogeneous coverage of the illuminated surface was able to smooth over the gaussian distribution of the laser energy. This phenomenon induced the formation of a flat crater, which was experimentally observed. Thus, kinetic investigations of the growth or the dissolution of this oxide film are crucial. Evidence of the presence of the oxide layer must be demonstrated by electrochemical measurements and by local XPS analyses. Since the photocurrent decay was associated with oxide growth, the recovery of the initial response was interpreted as a total dissolution of the surface film. Thus, the study of the recovery time of the initial clean surface could give information about the stability and dissolution kinetic of the oxide that had grown. Successive illuminations and dark periods were used to determine the recovery time of the photocurrent and to determine the recovery time of the capacitance interface. Laser pulses were recorded during 5 min in order to reach the steady-state photocurrent. Concerning the capacitance measurements after an illuminated pulse, the period in the dark to obtain the initial measurement of the capacitance was associated with the recovery time. For transient photocurrent measurements, the recovery time was obtained when the same instantaneous photocurrent value was reached for two successive light pulses. The instantaneous photocurrent value of the initial light pulse was used as a reference value for a clean GaAs surface. It is important to note that the recovery time depends also on the oxide thickness, which is related to the polarisation [3,4]. In any case, the superficial film thickness is generally
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thin for this range of potential. The results obtained with these two techniques gave the same recovery times. Two steps in the dissolution process were distinguished. The first step occurred in a few tens of seconds and corresponded to at least 95% of the initial recovery signal. A shorter recovery time was obtained ($few seconds scale) in stirred conditions. This point allowed us to confirm that the diffusion of species to the bulk of the solution controls the photodissolution GaAs. Therefore, diffusion of species is the limiting factor of the dissolution mechanism. Concerning the second step of the dissolution, one order of magnitude was observed in comparison with the first step. The signal was entirely recovered after a few minutes and depended very little on the stirring conditions. These two different steps suggested the presence of at least two kinds of films on the GaAs surface. From this electrochemical approach, the dissolution of the oxide film should be rather efficient, even at pH=9, but with a critical time in the 10-s range. The draining of oxidising species in the bulk of the solution was a relevant parameter on the recovery time and on the stability of the oxide layer. The exit and drying of the sample from the solution for XPS analyses depend on this parameter. Since the oxide film thickness, resulting from the extracting conditions, could not be controlled, we firstly chose to work on the dissolution of samples covered with their native oxide (thickness$3 nm). After various dipping times at pH 9, studies of the Ga , As , Ga and As 3d 3d 2p 2p levels were performed by XPS. An efficient dissolution process was established as well for gallium oxide as for arsenic oxide ( Fig. 2). A critical immersion time of less than 1 min was established. A reproducible chemical steady state was obtained with almost no arsenic or gallium oxide. Since a specific component on the As level centred at 3d 42.5 eV was often observed, an elementary arsenic film was present under the native oxide. This residual phase was barely soluble by simple immersion at pH 9. This latter point could be correlated with the very long dissolution process step observed in electrochemical investigations. Thus, the instability of the native oxides at pH 9 observed by XPS was in good agreement (same critical time
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Fig. 2. Relative variation of As and Ga phases on a native oxide immersed in pH 9 solution.
range) with the instability of the anodic grown films studied by electrochemistry. From this result, we have established a sample extracting procedure that maintains an oxide/solution contact between 10 and 20 s. Thus, anodic oxides with the same or greater thickness than that of the native oxides could be detected by XPS analyses. Indeed, in most cases, the anodic oxide is easily detected but only in the illuminated part. Fig. 3 illustrates, for the Ga level, the 3d transition between the illuminated and dark part of the electrode. Inside the crater, a main contribution centred at 23 eV could be associated with a Ga oxide phase. Outside the crater, only a component at 20.2 eV appeared specific to the GaAs substrate. The same kind of profiles could be obtained for the As level. This anodic oxide was 3d enriched in Ga (75% Ga oxide and 25% As oxide) [8]. A small charge effect (DE$+1.5 eV ) was generally observed (see Fig. 3) for the contribution due to the Ga oxides. Concerning the thinness of the anodic oxide layer or after slight Ar+ abrasion, the region near the GaAs interface was richer in As. In this case, in the As level, the component 3d associated with As0 was higher than that mentioned above for the native oxide.
Fig. 3. Ga levels registered in a line from the crater obtained 3d under photocurrent to a point outside the crater. Step size 250 mm, X-ray spot size 150 mm. Levels 1–3 inside the crater, level 4 at the ring, and levels 5–7 outside the crater (see Fig. 1b).
I. Ge´rard et al. / Surface Science 433–435 (1999) 131–135
4. Conclusion In this paper we have shown that parallel electrochemical investigations and surface analyses are powerful tools with which to study oxide layers in interactions with aqueous media. Kinetic studies of anodic dissolution and native oxides dissolution by XPS were in good agreement with the regeneration studies (associated with the oxide dissolution) by electrochemistry. These kinetic investigations showed that even at pH 9, the dissolution kinetic was not so slow as it was usually admitted. All of these results showed also that, irrespective of the pH, contact between GaAs and aqueous phase could strongly modify the chemical composition of the surface. A rich Ga oxide phase growing anodically was also clearly detected by XPS. This point emphasizes the fact that the limiting factor of the anodic dissolution of n-GaAs is due to the gallium oxide solubility. The advantage of using a local XPS measurement is that an internal refer-
135
ence can be used to qualify the treated part of the sample.
References [1] C.N. Willsen, Physics and Chemistry of III–V Compound Semiconductor Interfaces, Plenum Press, New York, 1985. [2] I. Ge´rard, J. Vigneron, L. Baudouin, C. Mathieu, C. Debiemme-Chouvy, A. Etcheberry, in: Proc. Symp. Photoelectrochemistry 97-20, The Electrochemical Society, Inc., NJ, USA, 1997, p. 182. [3] P.H.L. Notten, J.E.A.M. Van den Meeraker, J.J. Kelly, Etching of III–V Semiconductors: an Electrochemical Approach, Elsevier Science, Amsterdam, 1991. [4] J.J. Kelly, P.H.L. Notten, J. Electrochem. Soc. 130 (1983) 2452. [5] F. Decker, Electrochim. Acta 30 (1985) 301. [6 ] T. Gruszecki, S. Eriksson, B. Holmstrsˇm, J. Electroanalyt. Chem. 274 (1989) 117. [7] W. Pourbaix, Atlas des e´quilibres e´lectrochimiques, Gauthier-Villard, Paris, 1971. [8] C.C. Chang, B. Swartz, P. Murarka, J. Electrochem. Soc.: Solid-State Sci. Technol. 124 (6) (1977) 922.
Local oxide growth on the n-GaAs surface studied by small area XPS I. Ge´rard *, C. Debiemme-Chouvy, J. Vigneron, F. Bellenger, S. Kostelitz, A. Etcheberry IREM, Universite´ de Versailles-St-Quentin, 45 Av. des Etats Unis, Versailles 78 035 cedex, France
Abstract The photoanodic dissolution of n-GaAs was investigated in a buffered solution at an intermediate pH (pH 9). At this pH, the GaAs dissolution rate was limited by the growth of an anodic oxide film. A laser illumination leads to a local oxide growth only in the illuminated part on to GaAs. Transient photocurrent and capacitance measurements clearly show dissolution kinetics in two steps: the first step is about a few tens of seconds and is correlated with gallium oxide dissolution, and the second step is greater than a few minutes, indicating the origin of at least two surface films onto GaAs. A parallel approach using in-situ electrochemistry and ex-situ surface analysis ( XPS ) was performed to characterise the growth of an anodic film on to GaAs. A gallium enrichment was detected on this oxide film. The arsenic deficiency in this oxide results from a higher arsenic oxide solubility. The limitation of the GaAs dissolution is due to the dissolution of gallium oxide at pH 9. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Electrochemical method; Gallium arsenide; Gallium oxide; Growth; Oxidation; Photoelectron spectroscopy ( X-ray photoelectron); Semiconducting interfaces
1. Introduction Although extensively studied, the electrical and chemical behaviour of the III–V oxides are not entirely understood. The literature [1] states that the dispersions of oxide composition result essentially from the competition between two or more elements of III–V compounds, which is also observed for the growth as for the ageing process. Generally, the characteristics of oxide layers depend on the ambient conditions. Since there are many parameters such as the presence of water (wet atmosphere, thin water layer, …), it is difficult to understand accurately these kinds of phenome* Corresponding author. Fax: +33-1-39-25-43-81. E-mail address: [email protected] (I. Ge´rard)
non. To study the thin oxide layers (10 nm range) in interactions with water, a parallel method can be performed using in-situ electrochemistry studies and ex-situ surface analyses ( XPS). Localised anodic layers that had grown under illumination on the surface of n-GaAs samples were studied in this paper. First of all, analyses of transient photocurrent and capacitance responses have been used in order to characterise in situ the growth of a thin anodic oxide film and its chemical stability. Secondly, using a small X-ray spot configuration, XPS analyses have been performed to characterise two parts of the SC. One of them was the illuminated part of the sample where the oxide had grown, and the other was maintained in the dark. In a previous paper [2], gaussian or flat-bottom etching profiles were observed under a laser-beam
0039-6028/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 99 ) 0 00 7 1 -0
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I. Ge´rard et al. / Surface Science 433–435 (1999) 131–135
illumination. These two profiles involved two kinds of electrical responses. For a gaussian profile, a constant photocurrent was detected, whereas for a flat-bottom profile, a transient photocurrent was observed with a time-dependent response. The flatbottom profile results from an interfacial thin and continuous oxide layer. This film becomes the limiting factor of the anodic photodissolution process for an intermediate range of pH (4–12). In order to study this thin oxide film, our experiments were performed at pH 9. From our previous work, the photodissolution of n-GaAs could be limited by the dissolution of gallium oxide, which has relatively poor dissolution kinetics.
2. Experimental The [100] n-GaAs samples, with a carrier density about 1018 cm−3, were obtained from MCP Ltd Inc. Electrodes were etched mechanochemically at room temperature with a Br /methanol 2 solution and rinsed with pure methanol. Just before the experiments, the samples were dipped in a 2 M H SO solution. 2 4 The electrode was placed in a classical threeelectrode cell. All potentials were referred to the saturated mercury sulphate electrode (mse). The electrolyte solutions were prepared using buffered concentrate Titrisol at pH 9. The electrode polarisation were performed using a EG&G potentiostat Parc 176 and Parc Model 175 generator. A Gould Model (DSO) Digital Memory Oscilloscope was used to capture the transient photocurrent. All experiments were performed at a fixed potential V=–0.3 V . mse A 5-mW HeNe laser Uniphase (Ø=2 mm) was used to illuminate the sample. A calibrated polarising filter was also used to choose the incident power light. The laser power light was measured at P =1 mW by a Liconix laser power meter model i 45 PM. The X-ray photoelectron spectroscopy ( XPS) measurements were performed on a VG ESCALAB 220i-XL spectrometer. The X-ray source was monochromatic AlKa radiation. The crater resulting from the photodissolution and the part of GaAs that was kept in the dark were
analysed using an XPS line scan. This line scan was recorded using a 150 mm X-ray spot size with a step size of 250 mm. For XPS analyses, the exit of the sample from the solution was performed under a solution drop, dried under a nitrogen flux and stored in an argon atmosphere.
3. Results and discussion At pH 9, the polarisation of the electrode at – 0.3 V means that the interface was in a deplemse tion regime. As soon as the laser beam illuminated a part of the surface, a local time-dependent photocurrent was generated ( Fig. 1a). The initial current intensity is proportional to the laser power [2–4]. Then, the photocurrent decreased until a steadystate photocurrent was reached. This photocurrent value gave the etching rate of the local dissolution which induced a flat bottom cylindrical groove ( Fig. 1b). These characteristics of the time-dependent current [5,6 ] and flat-bottom profile [2] were
Fig. 1. (a) Transient photocurrent response of n-GaAs at pH 9 and V=−0.3 V ; (b) etching profile obtained with (a) condimse tions with an He–Ne laser beam.
I. Ge´rard et al. / Surface Science 433–435 (1999) 131–135
interpreted as a result of the formation of an anodic oxide film on n-GaAs. The steady-state photocurrent was associated with a dynamic phenomenon whose rates of oxide growth and oxide dissolution were identical and entirely controlled by the pH of the solution [2]. In our previous work, the steady-state photocurrent variation against pH also gave a characteristic ‘‘U’’ shape. This behaviour was in good agreement with Pourbaix’s thermodynamic data for the dissolution of arsenic and gallium oxides [7]. According to our pH conditions, the photodissolution of n-GaAs should be entirely controlled by the gallium oxide dissolution. In such a situation, a homogeneous coverage of the illuminated surface was able to smooth over the gaussian distribution of the laser energy. This phenomenon induced the formation of a flat crater, which was experimentally observed. Thus, kinetic investigations of the growth or the dissolution of this oxide film are crucial. Evidence of the presence of the oxide layer must be demonstrated by electrochemical measurements and by local XPS analyses. Since the photocurrent decay was associated with oxide growth, the recovery of the initial response was interpreted as a total dissolution of the surface film. Thus, the study of the recovery time of the initial clean surface could give information about the stability and dissolution kinetic of the oxide that had grown. Successive illuminations and dark periods were used to determine the recovery time of the photocurrent and to determine the recovery time of the capacitance interface. Laser pulses were recorded during 5 min in order to reach the steady-state photocurrent. Concerning the capacitance measurements after an illuminated pulse, the period in the dark to obtain the initial measurement of the capacitance was associated with the recovery time. For transient photocurrent measurements, the recovery time was obtained when the same instantaneous photocurrent value was reached for two successive light pulses. The instantaneous photocurrent value of the initial light pulse was used as a reference value for a clean GaAs surface. It is important to note that the recovery time depends also on the oxide thickness, which is related to the polarisation [3,4]. In any case, the superficial film thickness is generally
133
thin for this range of potential. The results obtained with these two techniques gave the same recovery times. Two steps in the dissolution process were distinguished. The first step occurred in a few tens of seconds and corresponded to at least 95% of the initial recovery signal. A shorter recovery time was obtained ($few seconds scale) in stirred conditions. This point allowed us to confirm that the diffusion of species to the bulk of the solution controls the photodissolution GaAs. Therefore, diffusion of species is the limiting factor of the dissolution mechanism. Concerning the second step of the dissolution, one order of magnitude was observed in comparison with the first step. The signal was entirely recovered after a few minutes and depended very little on the stirring conditions. These two different steps suggested the presence of at least two kinds of films on the GaAs surface. From this electrochemical approach, the dissolution of the oxide film should be rather efficient, even at pH=9, but with a critical time in the 10-s range. The draining of oxidising species in the bulk of the solution was a relevant parameter on the recovery time and on the stability of the oxide layer. The exit and drying of the sample from the solution for XPS analyses depend on this parameter. Since the oxide film thickness, resulting from the extracting conditions, could not be controlled, we firstly chose to work on the dissolution of samples covered with their native oxide (thickness$3 nm). After various dipping times at pH 9, studies of the Ga , As , Ga and As 3d 3d 2p 2p levels were performed by XPS. An efficient dissolution process was established as well for gallium oxide as for arsenic oxide ( Fig. 2). A critical immersion time of less than 1 min was established. A reproducible chemical steady state was obtained with almost no arsenic or gallium oxide. Since a specific component on the As level centred at 3d 42.5 eV was often observed, an elementary arsenic film was present under the native oxide. This residual phase was barely soluble by simple immersion at pH 9. This latter point could be correlated with the very long dissolution process step observed in electrochemical investigations. Thus, the instability of the native oxides at pH 9 observed by XPS was in good agreement (same critical time
134
I. Ge´rard et al. / Surface Science 433–435 (1999) 131–135
Fig. 2. Relative variation of As and Ga phases on a native oxide immersed in pH 9 solution.
range) with the instability of the anodic grown films studied by electrochemistry. From this result, we have established a sample extracting procedure that maintains an oxide/solution contact between 10 and 20 s. Thus, anodic oxides with the same or greater thickness than that of the native oxides could be detected by XPS analyses. Indeed, in most cases, the anodic oxide is easily detected but only in the illuminated part. Fig. 3 illustrates, for the Ga level, the 3d transition between the illuminated and dark part of the electrode. Inside the crater, a main contribution centred at 23 eV could be associated with a Ga oxide phase. Outside the crater, only a component at 20.2 eV appeared specific to the GaAs substrate. The same kind of profiles could be obtained for the As level. This anodic oxide was 3d enriched in Ga (75% Ga oxide and 25% As oxide) [8]. A small charge effect (DE$+1.5 eV ) was generally observed (see Fig. 3) for the contribution due to the Ga oxides. Concerning the thinness of the anodic oxide layer or after slight Ar+ abrasion, the region near the GaAs interface was richer in As. In this case, in the As level, the component 3d associated with As0 was higher than that mentioned above for the native oxide.
Fig. 3. Ga levels registered in a line from the crater obtained 3d under photocurrent to a point outside the crater. Step size 250 mm, X-ray spot size 150 mm. Levels 1–3 inside the crater, level 4 at the ring, and levels 5–7 outside the crater (see Fig. 1b).
I. Ge´rard et al. / Surface Science 433–435 (1999) 131–135
4. Conclusion In this paper we have shown that parallel electrochemical investigations and surface analyses are powerful tools with which to study oxide layers in interactions with aqueous media. Kinetic studies of anodic dissolution and native oxides dissolution by XPS were in good agreement with the regeneration studies (associated with the oxide dissolution) by electrochemistry. These kinetic investigations showed that even at pH 9, the dissolution kinetic was not so slow as it was usually admitted. All of these results showed also that, irrespective of the pH, contact between GaAs and aqueous phase could strongly modify the chemical composition of the surface. A rich Ga oxide phase growing anodically was also clearly detected by XPS. This point emphasizes the fact that the limiting factor of the anodic dissolution of n-GaAs is due to the gallium oxide solubility. The advantage of using a local XPS measurement is that an internal refer-
135
ence can be used to qualify the treated part of the sample.
References [1] C.N. Willsen, Physics and Chemistry of III–V Compound Semiconductor Interfaces, Plenum Press, New York, 1985. [2] I. Ge´rard, J. Vigneron, L. Baudouin, C. Mathieu, C. Debiemme-Chouvy, A. Etcheberry, in: Proc. Symp. Photoelectrochemistry 97-20, The Electrochemical Society, Inc., NJ, USA, 1997, p. 182. [3] P.H.L. Notten, J.E.A.M. Van den Meeraker, J.J. Kelly, Etching of III–V Semiconductors: an Electrochemical Approach, Elsevier Science, Amsterdam, 1991. [4] J.J. Kelly, P.H.L. Notten, J. Electrochem. Soc. 130 (1983) 2452. [5] F. Decker, Electrochim. Acta 30 (1985) 301. [6 ] T. Gruszecki, S. Eriksson, B. Holmstrsˇm, J. Electroanalyt. Chem. 274 (1989) 117. [7] W. Pourbaix, Atlas des e´quilibres e´lectrochimiques, Gauthier-Villard, Paris, 1971. [8] C.C. Chang, B. Swartz, P. Murarka, J. Electrochem. Soc.: Solid-State Sci. Technol. 124 (6) (1977) 922.