© Copyright 1996 American Chemical Society
JUNE 12, 1996 VOLUME 12, NUMBER 12
Letters Dynamics of the Hydrogen Oxidation and Silicon Dissolution Reactions in the Formation of Porous Silicon D. M. Soares,* O. Teschke, and M. C. dos Santos UNICAMP/IFGW/DFA, 13081-970, Campinas, SP, Brasil Received August 17, 1995. In Final Form: March 14, 1996X Porous silicon layers were grown in hydrofluoric acid solutions under constant anodic currents periodically interrupted during 100 ms every second. By monitoring the time dependent electrode potential, dynamic characteristics of the porous silicon formation were determined. Two reactions occur during the process: at the bottom of the pores the anodic silicon dissolution reaction proceeds and on the walls of the pores the hydrogen oxidation reaction takes place.
Porous silicon (PS) has attracted much attention since Canham1 reported luminescence observed under UV exposure of anodized crystalline silicon. The development of silicon-based optoelectronic devices became a possibility.2 The strong visible photoluminescence was mainly explained in terms of quantum effects occurring in nanostructures formed on the surface of the material.2,3 In order to produce PS with unique properties, an understanding of the mechanism of its formation is important. Basic models for PS formation were given by Beale,4 Go¨selle,3 and Smith.5 All of them provide possible explanations for PS formation but possess seemingly divergent pore generation mechanisms. Important processes are still poorly understood2,3 such as the reason for the dissolution resistance of the pore walls, the pore propagation mechanism, the dissolution surface chemistries, and the pore formation mechanism itself. In a recent paper we proposed a distribution of passivated * To whom correspondence may be addressed: e-mail,
[email protected]. X Abstract published in Advance ACS Abstracts, May 15, 1996. (1) Canham, L. T. Appl. Phys. Lett. 1990, 57, 1046. (2) Smith, R. L.; Collins, S. D. J. Appl. Phys. 1992, 71, R1. (3) Lehmann, V.; Go¨sele, U. Appl. Phys. Lett. 1991, 58, 856. (4) Beale, M. I. J.; Benjamin, I. D.; Uren, M. J.; Chew, N. G.; Cullis, A. G. Appl. Phys. Lett. 1985, 46, 86. (5) Smith, R. L.; Chuang, S. F.; Collins, S. D. J. Electron. Mater. 1988, 17, 533.
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H-covered sites and active F-covered sites at the silicon surface as responsible for the PS formation in HF solutions.6 In this work the pore formation mechanism is investigated by observing the current versus voltage curves and the time dependent electrode potential during anodization. The electrochemical reactions and their dynamics are described. The electrochemical cell (60 cm3), made of Teflon, consisted of a 50:50 (by volume) solution of HF (aqueous 48%) and ethanol (95%), both Merck products. The counter electrode was a platinum grid. The reference electrode was a saturated calomel electrode (SCE). Single crystal, polished silicon 〈100〉 oriented, 4.9 and 0.009 Ω cm p- and n-types wafers were used. They were fixed in a PTFE support. A Ga-In ohmic contact was prepared at the rear side of the silicon sample. The Si electrodes were held vertically in the electrochemical cell. The Si frontal side was in contact with the etching solution. The exposed area was 0.38 cm2. During the electrochemical etching process, the samples were illuminated by the radiation from a 60 W tungsten lamp. Before each test, the Si samples were degreased with methanol and washed in double distilled water. A potentiostat (PAR-273A) was used for the measurements. PS layers were grown on polished silicon surfaces according to the following procedure: The samples were (6) Teschke, O.; Santos, M. C.; Kleinke, M. U.; Soares, D. M.; Galva˜o, D. S. J. Appl. Phys. 1995, 78, 590.
© 1996 American Chemical Society
2876 Langmuir, Vol. 12, No. 12, 1996
Letters
illuminated and a cathodic current of 3 mA was applied during 5 min to eliminate oxides. In the sequence they were anodized with a current of 3 mA for 60 min, in order to form PS layers. The current was periodically interrupted for 100 ms every second, and the discharging and charging potentials were monitored. In order to understand the PS layer formation, it is necessary to deal with the mechanism of the anodic surface dissolution. Although the detailed mechanism is still not clear, it probably occurs as follows: At the onset of the anodic current the silicon surface is all H-covered.7,8 A field built up across the space charge layer, moves holes toward the surface at kinks, defects, or tensioned regions, in a manner similar to that proposed by the diffusionlimited theory.5 This induces a nucleophilic attack on the Si-H bonds by F- (or HF2-, ref 2) ions, forming SiF2 groups at these sites and ions H+ in the solution. The Si-Si back bonds of the SiF2 groups are stretched due to the fluorine electronegativity allowing the insertion of F- (or HF2-) ions. The reaction detaches the SiF2 group from the surface into the solution and forms two new Si-F bonds which react again as
[SiN]SiF2 + 2h+ + 2F-sol. w [SiN-1]SiF2 + (SiF2)Sol. (1) This changes the surface geometry and the electric field distribution so that the next hole transference will preferentially occur at this location thus enlarging the “pore”. Equation 1 describes the silicon dissolution reaction (SDR). The intermediary product (SiF2)Sol. reacts rapidly with HF, forming H2 and SiF410,11
(SiF2)Sol. + 2(HF)Sol. w (H2)Sol. + (SiF4)Sol.
(2)
In the dark, the electrolyte/semiconductor junction behaves as a diode. In the cathodic scan the electrolyte/ silicon junction is forward biased and there is H2 evolution. In the anodic scan the electrolyte/silicon junction is a reverse-biased diode and the inverse anodic current (IR) is practically zero.2,12,13 Figure 1a shows the iV curves of an n-type, 4.9 Ω cm 〈100〉 silicon sample after the cathodic polarization, Si curve, and after the PS layer formation, PS curve. By illumination of the samples, holes are created at the surface, and the SDR takes place. The rest potential (zero current potential, VR) occurs at the potential value of VR ) -400 mV. A value of VR ) -260 mV was found by ref 12 for a defect-free surface. Our measurement was probably affected by surface defects introduced by the hydrogen evolution reaction (HER). We will fix our attention on potential values in the region close to VR. The process of porous silicon formation shifts VR to more cathodic potentials, increases the anodic reverse current (IR), and decreases the hydrogen evolution rate (smaller slope of the iV curve at potentials more cathodic than VR) as shown by the PS curve in Figure 1a. The electrochemical equilibrium potential, VR, corresponds to the H2 oxidation reaction, which occurs at the Si-H surface bonds covering the electrode surface, as follows6 (7) Teschke, O.; Galembeck, F.; Goncalves, M. C.; Davanzo, C. U. Appl. Phys. Lett. 1994, 64, 26. (8) Trucks, G. W.; Raghavachari, K.; Higashi, G. S.; Chabal, Y. J. Phys. Rev. Lett. 1990, 65, 504. (9) Judge, J. S. J. Electrochem. Soc. 1971, 118, 1772. (10) Memming, R.; Schwandt, G. Surf. Sci. 1966, 4, 109. (11) Theunissen, M. J. J. J. Electrochem. Soc. 1972, 119, 351. (12) Chazalviel, J. N. Surf. Sci. 1979, 88, 204. (13) Gerischer, H. Electrochim. Acta 1990, 35, 1677.
Figure 1. (a) Current vs electrode potential curves for an illuminated n-type, 4.9 Ω cm 〈100〉 silicon sample, 0.38 cm2 electrode area. The Si curve was obtained after 3 min of cathodic polarization at 3 mA. After the first anodic scan it can be observed that VR shifts gradually to more cathodic values. The PS curve was plotted after 60 min of anodic polarizaton at a current of 3 mA. The zero current potential (VR) value is -400 mV for silicon and -720 mV for porous silicon. (b) The same for a p-type, 4.9 Ω cm 〈100〉 silicon sample. For silicon VR ) -260 mV vs SCE and for porous silicon VR ) -530 mV vs SCE.
(Si-H)Crys. + (H2)Sol. / (Si-H)Crys. + 2e-Crys. + 2H+ (3) Since the flat band potential value, VFB, of an n-type, 4.9 Ω cm, is about VFB ) -450 mV2,12,14 and for PS, VR ) -720 mV, one may claim that the H2 oxidation reaction injects electrons into the conduction band and H+ into the solution resulting in an anodic current of ≈150 µA at V ) -400 mV, Figure 1. The quasi-apolar Si-H bonds enhance the chemical stability of the Si surface against attacks by polar molecules like H2O, HF, etc. For more anodic potentials than VR a second electrochemical wave is started, which corresponds to the SDR, eq 1 and eq 2, where H2 is the reaction subproduct. Similar results were obtained for a p-type sample, see Figure 1b. In order to determine the dynamics of this process, we show in Figure 2a-c the transient electrode potential for a periodically interrupted anodic current shown in Figure 2d. During the current interruption period, the electrode potential values are close to VI ) -700 mV, which corresponds to the H+/H2 redox potential at the Si-H (14) Memming, R.; Schwandt, G. Surf. Sci. 1966, 5, 97.
Letters
Figure 2. Time dependent electrode potential curves for illuminated silicon samples during the onset of a periodically interrupted anodic current (IA). (a) 4.9 Ω cm, n-type silicon sample after 7, 27, and 62 min of polarization at IA ) 3 mA. (b) 0.009 Ω cm, n-type silicon. For 7, 18, 60, and 100 min of polarization at IA ) 3 mA. (c) The same sample as in (b), after 100 min of anodic polarization at IA ) 3 mA. A sudden increase of the current by a factor of 2 decreases the plateau duration time (tT) by half and doubles the potential difference value VI - VP. (d) Diagram of the periodically interrupted current at the current onset.
covered surface, according to eq 3. At the onset of the current a voltage plateau slowly started to develop preventing the electrode potential from increasing to its value VA where the SDR would also occur; see Figure 2. Only samples with a fully grown PS layer showed this behavior. The initial plateau potential value, VP, is more cathodic for highly doped samples, curves b and c of Figure 2, than for lowly doped ones, Figure 2a. If the current is instantaneously doubled as shown in curves Figure b and c of 2, the potential difference VI - VP is also doubled.
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Consequently we assume that the potential drop from VI to VP at the current onset is due to an ohmic contribution. The oxidation of H2 initially fixes the electrode potential at the value VP, which takes place at the bottom and at the walls of the pores, accounting for all the anodic current. As the H2 concentration decreases to zero, it allows the local electrode potential to increase anodically and the SDR to take place preferentially at the bottom of the pores rather than at the surface and pore walls. Equation 2 shows that the silicon dissolution produces H2 in a ratio 1:1. Part of this H2 is fed back to the pore walls preventing their dissolution by fluoride ions. The cathodic shift of the PS curve in Figure 1 and the increase of the reverse anodic current also concur with this hypothesis. A sudden increase of the anodic current by a factor of 2 decreases the plateau duration to a half, see Figure 2b,c. No significant electrode potential changes were observed at the onset of the current with the exception of the ohmic drop contribution. Consequently the electron injection rate depends only on the concentration of H2 at the electrode surface, characterizing a space charge controlled process.13 The voltage plateau duration period tT versus time plot (not shown), shows that after about 30 min tT starts to saturate departing from the linearly increasing curve. This behavior shows that the H2 oxidation reaction, which fixes the tT value, is restricted to the region near the bottom and not to the whole of the pore surface since this same behavior is observed for various depths of pores. The small capacitances of the active F-covered regions of the pores are in parallel with a much larger capacitance of the H-covered surface. Therefore the measured capacitance values of the electrode will be determined by the H-covered surface which allows a Mott-Schottky behavior as observed in the literature.12,14,15,16 In conclusion, by periodically interrupting the anodic current for a short time interval, the dynamics of the PS formation was observed. The H-covered sites allow the exchange of electrons with the redox pair H+/H2. This process cathodically shifts VR and increases the anodic reverse current IR measured at the original rest potential (-400 mV vs SCE). It is also shown that the H2 oxidation reaction protects the pore walls by injecting electrons and that the dissolution reaction takes place at the pore bottoms. On the basis of these conclusions we propose a chemical dissolution process and a mechanism for PS formation. Acknowledgment. The authors thank L. O. Bonugli and J. R. Castro for technical assistance. This work was supported by CNPq 520060/93-8 and FAPESP 93\0961-5 and 93\2412-9. LA950692I (15) Meissner, D.; Memming, R. Electrochim. Acta 1992, 37, 799. (16) Chazalviel, J. N. Electrochim. Acta 1992, 37, 865.
