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Anodic photodissolution of GaP single crystal electrodes in aqueous media
Solar Energy Materials 3 (1980) 347-355 © North-Holland Publishing Company
ANODIC PHOTODISSOLUTION OF GaP SINGLE CRYSTAL ELECTRODES IN AQUEOUS MEDIA Arnaud ETCHEBERRY, Jean Lou SCULFORT • Laboratoire d'Electrochimie Interfaciale du C N R S , 1 Place Aristide Briand, F-92190-Meudon-Bellevue, France
and Alain MARBEUF Laboratoire de Physique des Solides du C N R S , 1 Place Aristide Briand, F- 9 2190- M eudon- Be llevue , France
Received 20 March 1980
In this work, we show the influence of etching on the process of anodic dissolution of a GaP single crystal in an alkaline medium. The photodissolution of the electrode was studied from electrochemical measurements and the process was explained in terms of the influence of surface defects and particularly of dislocations.
1. Introduction Though GaP dissolution has been studied a great deal for some years [1-4], it seemed of interest to investigate the process occurring at the electrode surface during anodic polarization. The purpose of this investigation was the correlation between the surface metallographic state and the stability interface of the semiconductorsolution.
2. Experimental Anodic polarization has been investigated on single crystals (GaP n-type Te doped, N D- N A= 2 x 101 a cm-3)with (111) B surface orientation (phosphorus at the surface). All the samples were cut from the same ingot. Ohmic contacts were achieved on the back side of the samples, by depositing an indium sphere at 400°C under hydrogen and fusing a gold wire to it. The measurements were made under large anodic polarization in the dark and under illumination with monochromatic light of variable wavelength and intensity. All potentials were referred to the calomel reference electrode in a separated compartment but are presented here with respect to the normal hydrogen electrode (NHE) Before putting the samples in contact with 1 M KOH electrolyte, the following 347
348
.t. Etcheherry et al. . Anodic photodissolution
operations were performed successively: mechanical polishing, washing in boiling trichlorethylene solution, etching in concentrated acid medium ( H C I + H N O 3 + H 2 0 in volumic proportions 2; 1; 1.5) for one minute at a temperature around 40°C, ultrasonic rinsing in "millipore" water (p > 18 Mf~) and finally drying under argon flow. The main compartment of the cell contained a large gold counter electrode. The surface was analyzed by systematic observations before and after electrochemical treatment using a metallographic microscope fitted with differential inter-. ference "Nomarski" contrast (Mag 1000 x ) and also a scanning electronic micr~scope fitted with an X-ray analyser. Colorimetric titrations of the solution were also carried out.
3. Results and discussion
All GaP-electrolyte junctions show blocking behaviour over a wide range of potential. We mainly investigated the anodic polarization range corresponding to a depletion layer at the surface. In the dark the anodic currents start, from the first voltammogram at + 1.8 V/NHE. This current increases with the number of polarization cycles and an anodic peak appears (fig. 1). The potential Vp of this peak shifts with time to higher potentials. If we compare the microscopic views of the surface (400 × ) before and after polarization, we observe the presence of pits after a long time (5 to 6 h). The damage to the surface (fig. 2) is very similar to that obtained by a mild chemical development of the dislocations. Thus we conclude that the observed defects are related to the emergence of dislocations. The existence of anodic current can be explained by the dissolution of the crystal ; if i < 4 mA cm- 2 [4] the dissolution equation in an alkaline medium is : GaP + 4 O H - + 3 h +--+GaO~ + P + 2 H 2 0 .
(1)
Our observations show that the dissolution process is initiated at the defects. Anodic dissolution requires holes at the surface. In n-type GaP, holes are minority ' Ixl06A c m-2 .'-10
.,. . . . . . . . . S..p4 SmV s-1 ~
-10
~
V / NHE
Fig. 1. Current-voltagecurvesrecordedwithGaP electrodesin the dark : (1) after ½h undercyclicpolarization from - 1 V to + 3 V/NHE; (2) after 1 h ; 13)after 2 h; (4) after 4 h ; (5) after 6 h.
A. Etcheberry et al. / Anodic photodissolution
349
Fig. 2. Surface aspect of an electrode after five hours in the dark under anodic polarization ( V + 3 V/NHE).
carriers and their concentration is small in the bulk. However, under the combined effects of a large anodic polarization and a very negative fiat band potential (liEn = -- 1.9 V/NEH) [5] the energy band bending is very pronounced and the space charge la4'er contains a hole accumulation region. In this case, a uniform dissolution should be obtained on the semiconducting electrode. Thus to explain the localized aspect of the attacks which show pits corresponding to crystals imperfections, it is necessary to involve another process giving a longer hole availability and particular interface properties in the defect neighbourhood. By analogy with the work of Hollan et al. [6, 7], we assume the existence of localized breakdowns at some points on the interface. Two mechanisms may be considered: a predominant avalanche process and a possible tunnelling contribution because of the high doping N D --N A ~ 1018 cm-3. The observed slow progression of defect development along the surface can be explained by a polarization set just below the avalanche potential. Thus the observed attack depth is small because it corresponds to the pre-breakdown region. This fact is confirmed by colorimetric measurements which show no presence of dissolved Ga above the sensitivity threshold of l 0 - 6 M Ga. However the density of pits observed on the surface is very high (fig. 2). If we illuminate the GaP electrode with monochromatic light of photon energy above Ecap(2 < 540 nm), a specific attack appears on the surface. The current (fig. 3) due to the crystal dissolution appears at very negative potentials, very close to the flat band potential if the light intensity is high. In this case, the creation of electron-hole pairs can lead to a hole concentration, sufficient even under negative polarization, to obtain dissolution following eq. (1). Specific pits appear with time (fig. 4); these pits are equilateral triangles with their sides parallel to the three characteristic directions < 110>. The area of the pits increases with illumination time but their density seems related to the number of dislocations revealed in the dark. With increasing light intensity, the concentration of holes available at the surface increases and the photodissolution reaction also extends in depth (from the surface) in proportion to the increased light intensity.
A. Etcheherry et al. : A nodic photodissolution
350
smv~
0
VINHE ! 0
I * I
I *2
0
1 *3
Fig. 3. Curve i = f l V) : ( 1 ) in the dark ; (2) under m o n o c h r o m a t i c light, 2 = 450 nm.
Fig. 4. Surface aspect alter two h o u r s illumination with 2 = 4 5 0 nm.
In fig. 5, we compare the pits developed on two parts of the same partially illuminated electrode. The density of pits is nearly independent of the light intensity but the attack is more pronounced where the electrode is strongly illuminated. In the latter case the surface is very badly damaged and the colorimetric titration reveals a certain quantity of gallium in the solution ; the concentrations measured in the electrolyte seem to confirm the electrochemical reaction given in eq. (1). Additional observations have been made with a scanning electron microscope: fig. 6 shows the existence of shallow flat-bottomed pits. Moreover fig. 7 shows that the granulated surface obtained after attack always has the composition GaP as the same pattern is observed on a flesh surface. The pits do not arise from any point on the surface but only from the defects. As an example dissolution pits appear when a scratch exists on the surface (fig. 8); this attack is quickly enhanced at the surface and also at depth by illumination.
A. Etcheberry et al. / Anodic photodissolution
351
Fig. 5. Surface aspect after partial illumination with 2 =450 nm: (a) total surface area; (b) part of the surface corresponding to the cross in (a) and observed with a larger magnification.
This study has demonstrated some important points a b o u t the utilization o f semiconductor electrodes in energy conversion: it is necessary to use semiconducting surfaces free of defects because their presence leads to crystal decomposition when the electrode is put in the electrolyte; it is necessary to modify the interface description in the neighbourhood of defects; finally, the appearance and development of the pits is enhanced as the surface is more damaged. Very clean surfaces obtained after a suitable etching have to be used.
52
A. Et cheberrv e t al. / ,4 n odic pho todisso lu t ion
Fig. 6. Surface aspect observed with a scanning electron microscope : (a) observation of pits with a weak magnification ; (b) pits around the cross observed with a larger magnification.
A. Etcheberry et al. / Anodic photodissolution
Fig. 7. Composition analysis of the surface detected by X-ray analysis : (a) detection of P. The surface region is the same as that of fig. 6b.
353
Ga ; tb) detectior
354
A. Etcheberry et al. /Anodic photodissolution
Fig. 8. Attack observed on a scratched surface.
4. Conclusion We have reported on the influence of surface defects, especially of dislocations, in the process of the dissolution of the semiconductor electrodes. In future work we intend to show the influence of the electrode preparation, such as etching or crystalline orientation, on dissolution and also to make a comparison with epitaxial layers of n-type doped ternary alloys Gaxlnl _ xP (1 > x > 0, 9). In this case, the substitution of In for Ga brings about a sharp increase in the density of dislocations which should permit a more definite correlation between the defects and the instability of the semiconductor-electrolyte interface.
Acknowledgements We are grateful to the "Unit of electro-optics" of the University of Newcastle upon Tyne (UK) and principally Dr. E. Bault for the measurements achieved with the scanning electron microscope during the stay of J. L. Sculfort at the School of Chemistry of this University.
References [1] R. L. Meek and N. E. Shumaker, J. Electrochem. Chem. 119 (1972) 1148. [2] M. J. Madou, F. Cardon and W. P. Gomes, Ber. Buns. Ges. Phys. Chem. 82 (1978) 819. [3] R. Memming and G. Schwandt, Electrochimica Acta 13 (1968) 1299. R. Memming and K. H. Beckmann, J. Electrochem. Soc. 116 (1969) 363. [4] A. Uragaki, H. Yadanaka and M. Inona, J. Electrochem. Soc. 123 (1976} 680.
A. Etcheberry et al. / Anodic photodissolution
355
[5] J. L. Sculfort, A. M. Baticle and G. Gautron, C. R. Acad. Sci. S6rie C (Paris) 287 (1978) 317. [6] J. C. Tranchart, A. Farrage and L. Hollan, Proc. Colloq. Mat~riaux et technologie pour la micro 61ectronique, Montpellier (1978). [7] J. C. Tranchart, L. Hollan and R. Memming, J. Electrochem. Soc. 125 (1978) 1185.
ANODIC PHOTODISSOLUTION OF GaP SINGLE CRYSTAL ELECTRODES IN AQUEOUS MEDIA Arnaud ETCHEBERRY, Jean Lou SCULFORT • Laboratoire d'Electrochimie Interfaciale du C N R S , 1 Place Aristide Briand, F-92190-Meudon-Bellevue, France
and Alain MARBEUF Laboratoire de Physique des Solides du C N R S , 1 Place Aristide Briand, F- 9 2190- M eudon- Be llevue , France
Received 20 March 1980
In this work, we show the influence of etching on the process of anodic dissolution of a GaP single crystal in an alkaline medium. The photodissolution of the electrode was studied from electrochemical measurements and the process was explained in terms of the influence of surface defects and particularly of dislocations.
1. Introduction Though GaP dissolution has been studied a great deal for some years [1-4], it seemed of interest to investigate the process occurring at the electrode surface during anodic polarization. The purpose of this investigation was the correlation between the surface metallographic state and the stability interface of the semiconductorsolution.
2. Experimental Anodic polarization has been investigated on single crystals (GaP n-type Te doped, N D- N A= 2 x 101 a cm-3)with (111) B surface orientation (phosphorus at the surface). All the samples were cut from the same ingot. Ohmic contacts were achieved on the back side of the samples, by depositing an indium sphere at 400°C under hydrogen and fusing a gold wire to it. The measurements were made under large anodic polarization in the dark and under illumination with monochromatic light of variable wavelength and intensity. All potentials were referred to the calomel reference electrode in a separated compartment but are presented here with respect to the normal hydrogen electrode (NHE) Before putting the samples in contact with 1 M KOH electrolyte, the following 347
348
.t. Etcheherry et al. . Anodic photodissolution
operations were performed successively: mechanical polishing, washing in boiling trichlorethylene solution, etching in concentrated acid medium ( H C I + H N O 3 + H 2 0 in volumic proportions 2; 1; 1.5) for one minute at a temperature around 40°C, ultrasonic rinsing in "millipore" water (p > 18 Mf~) and finally drying under argon flow. The main compartment of the cell contained a large gold counter electrode. The surface was analyzed by systematic observations before and after electrochemical treatment using a metallographic microscope fitted with differential inter-. ference "Nomarski" contrast (Mag 1000 x ) and also a scanning electronic micr~scope fitted with an X-ray analyser. Colorimetric titrations of the solution were also carried out.
3. Results and discussion
All GaP-electrolyte junctions show blocking behaviour over a wide range of potential. We mainly investigated the anodic polarization range corresponding to a depletion layer at the surface. In the dark the anodic currents start, from the first voltammogram at + 1.8 V/NHE. This current increases with the number of polarization cycles and an anodic peak appears (fig. 1). The potential Vp of this peak shifts with time to higher potentials. If we compare the microscopic views of the surface (400 × ) before and after polarization, we observe the presence of pits after a long time (5 to 6 h). The damage to the surface (fig. 2) is very similar to that obtained by a mild chemical development of the dislocations. Thus we conclude that the observed defects are related to the emergence of dislocations. The existence of anodic current can be explained by the dissolution of the crystal ; if i < 4 mA cm- 2 [4] the dissolution equation in an alkaline medium is : GaP + 4 O H - + 3 h +--+GaO~ + P + 2 H 2 0 .
(1)
Our observations show that the dissolution process is initiated at the defects. Anodic dissolution requires holes at the surface. In n-type GaP, holes are minority ' Ixl06A c m-2 .'-10
.,. . . . . . . . . S..p4 SmV s-1 ~
-10
~
V / NHE
Fig. 1. Current-voltagecurvesrecordedwithGaP electrodesin the dark : (1) after ½h undercyclicpolarization from - 1 V to + 3 V/NHE; (2) after 1 h ; 13)after 2 h; (4) after 4 h ; (5) after 6 h.
A. Etcheberry et al. / Anodic photodissolution
349
Fig. 2. Surface aspect of an electrode after five hours in the dark under anodic polarization ( V + 3 V/NHE).
carriers and their concentration is small in the bulk. However, under the combined effects of a large anodic polarization and a very negative fiat band potential (liEn = -- 1.9 V/NEH) [5] the energy band bending is very pronounced and the space charge la4'er contains a hole accumulation region. In this case, a uniform dissolution should be obtained on the semiconducting electrode. Thus to explain the localized aspect of the attacks which show pits corresponding to crystals imperfections, it is necessary to involve another process giving a longer hole availability and particular interface properties in the defect neighbourhood. By analogy with the work of Hollan et al. [6, 7], we assume the existence of localized breakdowns at some points on the interface. Two mechanisms may be considered: a predominant avalanche process and a possible tunnelling contribution because of the high doping N D --N A ~ 1018 cm-3. The observed slow progression of defect development along the surface can be explained by a polarization set just below the avalanche potential. Thus the observed attack depth is small because it corresponds to the pre-breakdown region. This fact is confirmed by colorimetric measurements which show no presence of dissolved Ga above the sensitivity threshold of l 0 - 6 M Ga. However the density of pits observed on the surface is very high (fig. 2). If we illuminate the GaP electrode with monochromatic light of photon energy above Ecap(2 < 540 nm), a specific attack appears on the surface. The current (fig. 3) due to the crystal dissolution appears at very negative potentials, very close to the flat band potential if the light intensity is high. In this case, the creation of electron-hole pairs can lead to a hole concentration, sufficient even under negative polarization, to obtain dissolution following eq. (1). Specific pits appear with time (fig. 4); these pits are equilateral triangles with their sides parallel to the three characteristic directions < 110>. The area of the pits increases with illumination time but their density seems related to the number of dislocations revealed in the dark. With increasing light intensity, the concentration of holes available at the surface increases and the photodissolution reaction also extends in depth (from the surface) in proportion to the increased light intensity.
A. Etcheherry et al. : A nodic photodissolution
350
smv~
0
VINHE ! 0
I * I
I *2
0
1 *3
Fig. 3. Curve i = f l V) : ( 1 ) in the dark ; (2) under m o n o c h r o m a t i c light, 2 = 450 nm.
Fig. 4. Surface aspect alter two h o u r s illumination with 2 = 4 5 0 nm.
In fig. 5, we compare the pits developed on two parts of the same partially illuminated electrode. The density of pits is nearly independent of the light intensity but the attack is more pronounced where the electrode is strongly illuminated. In the latter case the surface is very badly damaged and the colorimetric titration reveals a certain quantity of gallium in the solution ; the concentrations measured in the electrolyte seem to confirm the electrochemical reaction given in eq. (1). Additional observations have been made with a scanning electron microscope: fig. 6 shows the existence of shallow flat-bottomed pits. Moreover fig. 7 shows that the granulated surface obtained after attack always has the composition GaP as the same pattern is observed on a flesh surface. The pits do not arise from any point on the surface but only from the defects. As an example dissolution pits appear when a scratch exists on the surface (fig. 8); this attack is quickly enhanced at the surface and also at depth by illumination.
A. Etcheberry et al. / Anodic photodissolution
351
Fig. 5. Surface aspect after partial illumination with 2 =450 nm: (a) total surface area; (b) part of the surface corresponding to the cross in (a) and observed with a larger magnification.
This study has demonstrated some important points a b o u t the utilization o f semiconductor electrodes in energy conversion: it is necessary to use semiconducting surfaces free of defects because their presence leads to crystal decomposition when the electrode is put in the electrolyte; it is necessary to modify the interface description in the neighbourhood of defects; finally, the appearance and development of the pits is enhanced as the surface is more damaged. Very clean surfaces obtained after a suitable etching have to be used.
52
A. Et cheberrv e t al. / ,4 n odic pho todisso lu t ion
Fig. 6. Surface aspect observed with a scanning electron microscope : (a) observation of pits with a weak magnification ; (b) pits around the cross observed with a larger magnification.
A. Etcheberry et al. / Anodic photodissolution
Fig. 7. Composition analysis of the surface detected by X-ray analysis : (a) detection of P. The surface region is the same as that of fig. 6b.
353
Ga ; tb) detectior
354
A. Etcheberry et al. /Anodic photodissolution
Fig. 8. Attack observed on a scratched surface.
4. Conclusion We have reported on the influence of surface defects, especially of dislocations, in the process of the dissolution of the semiconductor electrodes. In future work we intend to show the influence of the electrode preparation, such as etching or crystalline orientation, on dissolution and also to make a comparison with epitaxial layers of n-type doped ternary alloys Gaxlnl _ xP (1 > x > 0, 9). In this case, the substitution of In for Ga brings about a sharp increase in the density of dislocations which should permit a more definite correlation between the defects and the instability of the semiconductor-electrolyte interface.
Acknowledgements We are grateful to the "Unit of electro-optics" of the University of Newcastle upon Tyne (UK) and principally Dr. E. Bault for the measurements achieved with the scanning electron microscope during the stay of J. L. Sculfort at the School of Chemistry of this University.
References [1] R. L. Meek and N. E. Shumaker, J. Electrochem. Chem. 119 (1972) 1148. [2] M. J. Madou, F. Cardon and W. P. Gomes, Ber. Buns. Ges. Phys. Chem. 82 (1978) 819. [3] R. Memming and G. Schwandt, Electrochimica Acta 13 (1968) 1299. R. Memming and K. H. Beckmann, J. Electrochem. Soc. 116 (1969) 363. [4] A. Uragaki, H. Yadanaka and M. Inona, J. Electrochem. Soc. 123 (1976} 680.
A. Etcheberry et al. / Anodic photodissolution
355
[5] J. L. Sculfort, A. M. Baticle and G. Gautron, C. R. Acad. Sci. S6rie C (Paris) 287 (1978) 317. [6] J. C. Tranchart, A. Farrage and L. Hollan, Proc. Colloq. Mat~riaux et technologie pour la micro 61ectronique, Montpellier (1978). [7] J. C. Tranchart, L. Hollan and R. Memming, J. Electrochem. Soc. 125 (1978) 1185.