Generation and Quenching of Luminescence in n-Type Porous Silicon

In this mechanism, an extension of a previous model, adsorbed hydrogen plays an important role in both the generation of electroluminescence and the ...
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Langmuir 1999, 15, 3666-3671

Generation and Quenching of Luminescence in n-Type Porous Silicon/Solution Diodes: Role of Adsorbed Hydrogen J. J. Kelly,*,† E. S. Kooij, and D. Vanmaekelbergh Debye Institute, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands Received October 14, 1998. In Final Form: January 29, 1999 The electroluminescence generated during cathodic reduction of peroxydisulfate and hydrogen peroxide at n-type porous silicon electrodes is reconsidered. On the basis of electrochemical results, a mechanism is proposed to account for some unusual features of the reduction reactions and the corresponding light emission. In this mechanism, an extension of a previous model, adsorbed hydrogen plays an important role in both the generation of electroluminescence and the quenching of light emission.

Introduction Shortly after the first reports of strong visible photoluminescence from porous silicon,1,2 solid-state light emitting diodes were fabricated.3,4 The efficiency of such devices, i.e., the ratio of photons emitted to charge carriers passing through the diode, was extremely low (e10-5). It was soon realized that much more efficient emission could be obtained from a porous silicon/solution diode.5,6 In this case minority carriers are injected into the semiconductor from a redox species in a solution which effectively penetrates the whole porous structure. The most widely studied system is that in which the peroxydisulfate anion (S2O82-) is used to generate electroluminescence in n-type porous silicon. It is generally accepted that reduction of the oxidizing agent is a twostep reaction.5,7 The first step involves capture of a conduction band (CB) electron by the anion.

S2O82- + e-(CB) f SO42- + SO4•-

(1)

The SO4•- radical anion formed in reaction 1 is, in the nonadsorbed state, an extremely strong oxidizing agent capable of extracting an electron from, i.e., “injecting a hole” into, the valence band (VB) of most semiconductors.

SO4•- f SO42- + h+(VB)

(2)

In the case of n-type porous silicon, the hole, injected into a nanocrystallite, can recombine radiatively with a conduction band electron supplied from the silicon substrate to give visible light emission. The onset of emission is expected at the onset potential for reaction 1, i.e., when the electron concentration in the porous structure becomes appreciable. This should occur at a potential close to or more negative than the flat band potential Ufb of n-type silicon. †

Tel: 30-2532220. Fax: 30-2532403. E-mail: [email protected].

(1) Canham, L. T. Appl. Phys. Lett. 1990, 57, 1046. (2) Cullis, A. G.; Canham, L. T. Nature 1991, 353, 335. (3) Koshida, N.; Koyama, H. Appl. Phys. Lett. 1992, 60, 347. (4) Kozlowski, F.; Sauter, M.; Steiner, P.; Richter, A.; Sandmaier, H.; Lang, W. Thin Solid Films 1992, 222, 196. (5) Bressers, P. M. M. C.; Knapen, J. W. J.; Meulenkamp, E. A.; Kelly, J. J. Appl. Phys. Lett. 1992, 61, 108. (6) Canham, L. T.; Leong, W. Y.; Beale, M. I. J.; Cox, T. I.; Taylor, L. Appl. Phys. Lett. 1992, 61, 2563. (7) Memming, R. J. Electrochem. Soc. 1969, 116, 785.

The essential features of the results obtained with peroxydisulfate are shown in Figure 1.8-16 The currentpotential curve (a) is that typical of n-type porous silicon prepared from moderately doped material.5,16 Reduction of S2O82- begins between -0.5 and -0.7 V, measured with respect to the saturated calomel electrode (SCE).5,6,15,16 A current plateau is often observed due to mass transport of the oxidizing agent to the electrode surface. At more negative potentials, the current increases as a result of hydrogen evolution. Light emission (curve b) begins at the onset of hydrogen evolution.17 Various groups have shown that the spectral distribution of the emitted light changes as the potential is scanned to negative values;6,10,11-14 the emission maximum shifts considerably to shorter wavelengths. At more negative potentials the electroluminescence is quenched, as shown in curve b, Figure 1. A scan to positive potentials is required to restore the emission. Electroluminescence is also observed in hydrogen peroxide (H2O2) solution.10 A two-step mechanism similar to that of S2O82- (eqs 1 and 2) is thought to be responsible for the reduction of H2O2.7 The photoluminescence, generated by illuminating an n-type porous silicon electrode, is, like the electroluminescence, quenched at negative potentials, the longer wavelength emission disappearing first.11-14 The two forms of emission are strongly coupled. For a given wavelength the drop in photoluminescence coincides with the rise in (8) Bsiesy, A.; Muller, F.; Ligeon, M.; Gaspard, F.; He´rino, R.; Romestain, R.; Vial, J. C. Appl. Phys. Lett. 1994, 65, 3371. (9) Hory, M. A.; Bsiesy, A.; Herino, R.; Ligeon, M.; Muller, F.; Vial, J. C. Thin Solid Films 1996, 276, 130. (10) Meulenkamp, E. A.; Bressers, P. M. M. C.; Kelly, J. J. Appl. Surf. Sci. 1993, 64, 283. (11) Meulenkamp, E. A.; Peter, L. M.; Riley, D. J.; Wielgosz, R. I. J. Electroanal. Chem. 1995, 392, 97. (12) Peter, L. M.; Riley, D. J.; Wielgosz, R. I.; Snow, P. A.; Penty, R. V.; White, I. H.; Meulenkamp, E. A. Thin Solid Films 1996, 276, 123. (13) Bsiesy, A.; Vial, J. C.; Gaspard, F.; He´rino, R.; Ligeon, M.; Mihalcescu, I.; Muller, F.; Romestain, R.; J. Electrochem. Soc. 1994, 141, 3071. (14) Bsiesy, A.; Muller, F.; Ligeon, M.; Gaspard, F.; He´rino, R.; Romestain, R.; Vial, J. C. Phys. Rev. Lett. 1993, 71, 637. (15) Ogasawara, K.; Momma, T.; Osaka, T. J. Electrochem. Soc. 1995, 142, 1874. (16) Kooij, E. S.; Noordhoek, S. M.; Kelly, J. J. J. Phys. Chem. 1996, 100, 10754. (17) With the exception of the results of Herino and co-workers,13,14 the electroluminescence is found to lag behind the current due to S2O82reduction.5,6,15 Current onset in refs 13 and 14 is at a potential more negative than that reported by other groups.

10.1021/la981435r CCC: $18.00 © 1999 American Chemical Society Published on Web 04/09/1999

Luminescence in Porous Silicon/Solution Biodes

Figure 1. Curve a shows the current-potential characteristics of an n-type porous silicon electrode in a 0.1 M S2O82-, 1.0 M H2SO4 solution. Curve b shows the potential dependence of the emitted light intensity corresponding to curve a. Curve c gives the photoluminescence intensity as a function of potential for a solution without S2O82-. The emission wavelength (650 nm) is the same for curves b and c. Ufb is the flat band potential in the peroxydisulfate solution.

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Figure 2. Reaction scheme proposed by Peter et al.11,12 to explain electroluminescence and photoluminescence from ntype porous silicon/solution diodes.

instead nonradiative Auger recombination (step 7) occurs. If the potential is made more negative so that an electron enters a particle already containing an electron-hole pair (step 8), Auger recombination then causes quenching of the electroluminescence (step 7). This explains why, for a given wavelength, the appearance of electroluminescence coincides with the disappearance of photoluminescence. At sufficiently negative potentials hydrogen can be evolved cathodically (step 9). In this reaction scheme steps 3 and 5 are considered to be reversible; as the potential is made positive, the Fermi level drops and with it the degree of occupation of the particles. In an electrochemical study of redox chemistry at silicon and porous silicon electrodes we found results which led us to question the validity of reaction 216 and, as a consequence, aspects of the model described above. In this paper we reconsider the problem of light emission from forward-biased n-type porous silion/solution diodes. Results which seem to be at odds with the existing model are considered. We argue that hydrogen, evolved cathodically, plays an important role in light emission from porous silicon, and we adapt the model to account for this aspect. Like its predecessor, this new model is based largely on circumstantial evidence. The remainder of the paper is divided into three parts. In the first section we concentrate on the problems encountered with the present model. In the second section we argue a case for the role of hydrogen, while in the last part a hydrogen-based model is described and its consequences considered.

electroluminescence (see Figure 1, curves b and c). An elegant model has been suggested to explain these results.11-14 The model is based on three premises. (i) Light emission results from radiative recombination within quantized particles having a distribution of sizes. (ii) The observation of electroluminescence depends on populating these particles with electrons from the silicon substrate; this initiates reaction 1 which results in hole injection (reaction 2) followed by radiative recombination. Since smaller structures emitting shorter wavelength light have a larger band gap and a conduction band edge at higher energy,1,18 a more negative potential is required to switch on the S2O82- reduction. (iii) Quenching of both the electroluminescence and photoluminescence is attributed to Auger recombination; the energy that becomes available from electron/hole recombination is used not for emission of a photon but to excite an electron (or hole) within the band. The essential features of the model can be best represented by the reaction scheme proposed by Peter and co-workers11,12 (see Figure 2). This “scheme of squares” describes the occupation by electrons and holes of a single quantized particle. At positive potentials the Fermi level in the system is well below the conduction band edge of the particle and its conduction band is empty. Radiative recombination as a result of photoexcitation of the electronfree particle (step 1) gives rise to photoluminscence (step 2). The electron-hole pair is denoted by the circled asterisk. To observe electroluminescence the particle has to be occupied by an electron (step 3). This can be achieved by making the potential negative and thus raising the Fermi level of the system toward the conduction band edge of the particle. The potential at which an electron is present in the particle depends on the degree of quantization, i.e., on the size of the structure. Reaction of the electron with an S2O82- ion gives rise to hole injection into the valence band (step 4; this corresponds to reactions 1 and 2). Introduction of an electron (step 5) gives an electron-hole pair which on relaxing emits a photon characteristic of the dimensions of the particle (step 2). Photoexcitation of a particle already occupied by an electron (step 6) does not lead to photoluminescence;

Before discussing the unusual features of the reduction of S2O82- and H2O2 at silicon, we first consider the energetics and kinetics of charge transfer of a simple oneelectron oxidizing agent (the Ce4+ ion), which also has a strongly positive redox potential.19 It should be noted that in fluoride-free solutions of oxidizing agents such as Ce4+, S2O82-, or H2O2, silicon is expected to be covered with a thin oxide.19 Such thin oxide films have only a slight influence on the flat band potential, i.e., on the position of the band edges.16 After considering the Si/Ce4+ system, we describe the problems encountered with S2O82- and

(18) Van Buuren, T.; Tiedje, T.; Dahn, J. R.; Way, B. M. Appl. Phys. Lett. 1993, 63, 2911.

(19) Kooij, E. S.; Butter, K.; Kelly, J. J. J. Electrochem. Soc. 1998, 145, 1232.

Anomalies in the Present Model

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Figure 5. Schematic energy band diagrams showing the interface between a bulk silicon electrode and solution (a) at flatband potential and (b) under accumulation. Case c is for the interface between bulk and size-quantized silicon under accumulation.

Figure 3. Curve a is the current-potential curve for an n-type porous silicon electrode in a 0.1 M Ce4+, 1.0 M H2SO4 solution. Curve b shows the potential dependence of the electroluminescence intensity (λ ) 720 nm). Ufb is the flat band potential.

Figure 4. Schematic energy band diagrams showing the band bending at the interface between bulk silicon and solution (a and b) and between bulk and size-quantized silicon (c and d) under strong depletion (a and c) and weak depletion (b and d).

H2O2 as oxidizing agents. We end this section by comparing briefly some features of the electroluminescence from solid state and solid/solution diodes. A previous study has shown that holes can be injected directly into silicon and porous silicon from Ce4+ and IrCl62- ions in solution.19 A considerable dark current is measured with the p-type electrode, proving that reduction of the oxidizing agent indeed occurs via the valence band. A cathodic current due to Ce4+ reduction at n-type porous silicon occurs at a potential slightly positive with respect to the flat band value (Figure 3, curve a). At positive potentials the bulk silicon is in depletion, both at the interface with solution (as in Figure 4a) and with quantized structures (Figure 4c). The electron concentration at the solution interface is low; the concentration in the quantized structures is expected to be even lower due to the mismatch of the band edges.18 The probability that a hole injected by Ce4+ ions into a quantized region will encounter an electron is negligible. Injected holes cause oxidation of the semiconductor,19 and as a result, no current is observed (Figure 3, curve a). In porous silicon this oxidation reaction is accompanied by light emission (Figure 3, curve b, (20) Minks, B. P.; Oskam, G.; Vanmaekelbergh, D.; Kelly, J. J. J. Electroanal. Chem. 1989, 273, 119.

potentials more positive than approximately +0.2 V). This is similar to the anodic luminescence of porous silicon.19 At more negative potentials, as the Fermi level is raised, the electron concentration increases at both the bulk silicon/solution and bulk silicon/porous silicon interfaces (Figure 4b,d, respectively). Holes injected by Ce4+ ions at the solution interface no longer oxidize the semiconductor but instead recombine with electrons supplied from the bulk, giving rise to a cathodic current. That recombination also occurs in the porous layer is clear from the considerable light emission observed in this range (Figure 3, curve b). The decrease in emitted intensity when cathodic current begins to flow at +0.2 V is very likely due to a difference in the ratio of radiative to nonradiative recombination rates for the “anodic” and “cathodic” luminescence processes. The reduction of Ce4+ is completely diffusion controlled;10 i.e., all holes injected into the semiconductor must recombine with conduction band electrons. Despite the fact that at potentials close to the flat band value of bulk silicon the concentration of electrons in the conduction band of quantized nanocrystallites will not be very high, there are clearly sufficient electrons available to ensure radiative recombination with injected holes. Electroluminescence is maintained as the Fermi level is raised further and the electron concentration in the nanostructures increases. Emission is quenched at negative potentials when hydrogen begins to evolve (Figure 3, curve b). Reduction of S2O82- and H2O2 at porous silicon electrodes shows two distinctive features. The first is that the reactions are kinetically unfavorable. While reduction of both oxidizing agents occurs at n-type III-V electrodes at a potential of at least 0.5 V positive with respect to Ufb,7,20 the onset of reduction at silicon and porous silicon is markedly negative with respect to Ufb (Figure 1). For a bulk silicon electrode this potential range corresponds to accumulation; i.e., the electron concentration at the surface is higher than in the bulk. As the potential is made negative with respect to Ufb (Figure 5a), the Fermi level passes through the conduction band edge at the surface and the semiconductor becomes quasi-metallic. Since the space charge layer capacitance becomes very large, a change in applied potential leads to a change in the potential drop across the Helmholtz layer. The band edges are unpinned and move upward (Figure 5b). In an indifferent electrolyte solution hydrogen is evolved at n-type silicon when the overpotential becomes large, i.e., when the Fermi level is sufficiently far above the energy corresponding to the H2O/H2 equilibrium. As at many metal electrodes, a considerable overpotential is required to generate hydrogen at n-type silicon.The consequences of strong accumulation for the bulk silicon/porous silicon interface are sketched in Figure 5c. It is unlikely that the band edges become unpinned as for the case of the solution

Luminescence in Porous Silicon/Solution Biodes

interface. Instead, the band bending will continue to increase as the Fermi level is raised. The second unusual feature relates to hole injection. If reactions 1 and 2 occur at silicon, then the phenomenon of “photocurrent doubling”7,20 is expected for S2O82- and H2O2 at a p-type bulk silicon electrode under depletion conditions. A photon is required to generate an electron and a hole, which are separated by the electric field of the depletion layer. The hole is registered as photocurrent in the external circuit. The electron is driven to the solution interface where it reduces an S2O82- ion. The resulting SO4•- radical ion injects a second hole into the valence band, which also contributes to the measured photocurrent. This means that for each photon absorbed, two charge carriers should be detected in the external circuit; this corresponds to a “quantum efficiency” of two, i.e., to photocurrent doubling. Extensive measurements with p-type silicon in both H2SO4 and HF solutions containing S2O82- have shown a quantum efficiency of one in a wide range of photon flux.16 The quantum efficiency for the photocathodic reduction of hydrogen peroxide (H2O2), a reaction which like that of S2O82- shows photocurrent doubling at various semiconductors,7,21 is also one for silicon.19 Similar results are observed with both oxidizing agents at p-type porous silicon electrodes. These results suggest that, for light intensities at which photocurrent doubling should be observable, the second step of the reduction of S2O82- or H2O2 at p-type silicon occurs via the conduction band rather than the valence band of the semiconductor. The anomaly in the reduction mechanism is also illustrated in measurements involving the transistor technique.16 The n-type surface of a silicon p-n junction is made the electrode of an electrochemical cell. When the potential of the n-type layer is scanned in an S2O82-/H2SO4 solution, a current-potential curve similar to that shown in Figure 1 is found with the p-n configuration. Holes injected into the valence band of the n-type layer during the second reduction step should be detected as a short-circuit current between the n-type and p-type regions of the junction. The short-circuit current, however, is very low in the potential range in which only S2O82- is reduced; it becomes appreciable only when hydrogen evolution begins. This parallels the potential dependence of the electroluminescence of the n-type porous silicon electrode (Figure 1, curve b). Appreciable light emission is only observed at the end of the S2O82- reduction current plateau. These results show that for the plateau range the mechanism of reduction of S2O82- (and H2O2) at silicon is different from that given by reactions 1 and 2; holes are not injected. That hole injection followed by electron/hole recombination is possible in this potential range is demonstrated by the Ce4+ results of Figure 3. From Figure 1 it is clear that the electroluminescence from porous silicon/solution diodes is effectively quenched at negative potentials (i.e. as the Fermi level in the system is raised (Figure 5)). In the existing model (Figure 2) Auger recombination accounts for quenching of the emission. One might expect analogous results with solid-state devices. Much improved light emitting porous silicon diodes are currently available.22-25 However, we are not (21) Kelly, J. J.; Minks, B. P.; Verhaegh, N. A. M.; Stumper, J.; Kelly, J. J. Electrochim. Acta 1992, 37, 909. (22) Cox, T. I. In Properties of Porous Silicon; EMIS Datareviews Series No. 18; Canham, L., Ed.; INSPEC: London, 1997; Chapter 10.2. (23) Tsybeskov, L.; Duttagupta, S. P.; Hirschman, K. D.; Fauchet, P. M. Appl. Phys. Lett. 1996, 68, 2058. (24) Oguro, T.; Koyama, H.; Ozaki, T.; Koshida, N. J. Appl. Phys. 1997, 81, 1407.

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aware of any evidence for quenching of the electroluminescence, even at very high current densities. Role of Hydrogen Quenching of photoluminescence and electroluminescence from bulk n-type semiconductor electrodes in aqueous solutions has been observed at negative bias for a variety of systems.26-31 In the potential range in which the luminescence is quenched, strong hydrogen evolution generally occurs. It has been suggested that this reaction is responsible for the decrease in the intensity of the emission;30,31 adsorbed or absorbed hydrogen may act as a nonradiative surface or near-surface recombination center. That hydrogen is indeed important is clear from luminescence experiments on GaP in acetonitrile solution32 and ZnO in aqueous solution.33 In both cases the emission is stable at negative potentials. Obviously hydrogen cannot be evolved in the aprotic solvent. For ZnO the overpotential for hydrogen generation is very large; reduction of water becomes appreciable only at a potential more than 1 V negative with respect to the flat band value.34 There is also considerable evidence to support the important role of hydrogen in electron-hole recombination in illuminated p-type electrodes in aqueous solutions. Peter and co-workers35 demonstrated with photoelectrochemical measurements that absorption of hydrogen in p-type GaP gives rise to near-surface states which promote effective electron-hole recombination. They suggest a cycle involving adsorbed hydrogen atoms:

H+ + e-(CB) f Hads•

(3)

Hads• + h+(VB) f H+

(4)

De Mierry et al.36 used SIMS to follow absorption of deuterium in p-type Si during photocathodic reduction of heavy water; a considerable concentration is established in the near-surface region, to a depth that depends on the doping density. They showed that hydrogen, photocathodically incorporated, caused a marked increase in the blocking current under reverse bias conditions. This increase was attributed to electron-hole generation via hydrogen-related band gap states. A shift in the onset potential for photocurrent indicates that such states can also act as recombination centers. Using in-situ infrared spectroscopy Chazalviel and coworkers37,38 have provided evidence for the electrochemical (25) Hirschman, K. D.; Tsybeskov, L.; Duttagupta, S. P.; Fauchet, P. M. Nature 1996, 384, 338. (26) Smandek, B.; Chmiel, G.; Gerischer, H. Ber. Bunsen-Ges. Phys. Chem. 1989, 93, 1094. (27) Smandek, B.; Gerischer, H. Electrochim. Acta 1985, 30, 1101. (28) Nakato, Y.; Tsumura, A.; Tsubomura, H. Bull. Chem. Soc. Jpn. 1982, 55, 3390. (29) Nakato, Y.; Morita, K.; Tsubomura, H. J. Phys. Chem. 1986, 90, 2718. (30) Schoenmakers, G. H.; Bakkers, E. P. A. M.; Kelly, J. J. J. Electrochem. Soc. 1997, 144, 2329. (31) Chmiel, G.; Gerischer, H. J. Phys. Chem. 1990, 94, 1612. (32) McIntyre, R.; Smandek, B.; Gerischer, H. Ber. Bunsen-Ges. Phys. Chem. 1985, 89, 78. (33) Schoenmakers, G. H.; Vanmaekelbergh, D.; Kelly, J. J. J. Phys. Chem. 1996, 100, 3215. (34) de Wit, A. R.; Janssen, M. D.; Kelly, J. J. Appl. Surf. Sci. 1990, 45, 21. (35) Li, J.; Peat, R.; Peter, L. M. J. Electroanal. Chem. 1984, 165, 41. (36) De Mierry, P.; Etcheberry, A.; Rizk, R.; Etchegoin, P.; Aucouturier, M. J. Electrochem. Soc. 1994, 141, 1539. (37) Mandal, K. C.; Ozanam, F.; Chazalviel, J.-N. Appl. Phys. Lett. 1990, 57, 2788. (38) Belaı¨di, A.; Chazalviel, J.-N.; Ozanam, F.; Gorochov, O.; Chari, A.; Fotouhi, B.; Etman, M. J. Electroanal. Chem. 1998, 44, 55.

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incorporation of hydrogen into n-type Si and Ge. On prolonged cathodic polarization a thin, disordered, and highly hydrogenated surface layer is produced on both semiconductors. Allongue et al.39 have shown that deuterium is incorporated into n-type porous Si under conditions of cathodic polarization. Finally it is interesting to note that Lowe-Webb et al.40 report the rapid quenching of efficient visible photoluminescence from nanoscale silicon clusters upon exposure to atomic hydrogen. Subsequent photooxidation in air restored the light emission. Since the results described above show convincingly that ad- or absorbed hydrogen influences the photoelectrical properties of semiconductors and it is clear that hydrogen is absorbed into both porous and nonporous n-type Si, we suggest that quenching of luminescence in porous Si could be connected to a hydrogen reaction cycle of the type given by eqs 3 and 4. If hydrogen is responsible for quenching the light emission, then there must be a link between hydrogen evolution and peroxydisulfate and peroxide reduction; the decay of the photoluminescence and the rise of the electroluminescence are clearly linked. This raises the question as to why the SO4•- radical anion fails to inject a hole into p-type Si under conditions in which currentdoubling is expected (and found with other semiconductors). A somewhat similar situation is encountered with the reduction of hypohalites (ClO- and BrO-) at p-type GaAs in alkaline solution.21 Under illumination, these strong oxidizing agents give a cathodic photocurrent with a variable quantum efficiency: two at low light intensity, decreasing to one as the photon flux is raised. These results have been interpreted by a reaction scheme similar to that of eqs 1 and 2; the intermediate from the first step (in the case of the hypohalites a Cl• or Br• radical) is, however, adsorbed forming an intra-band gap surface state which can interact with both bands of the semiconductor. The results for the reduction of S2O82- or H2O2 at Si could be explained in a similar way. The SO4•- or OH• radical formed either by dissociation or electrochemical reduction could adsorb on the surface at sites denoted by A (to give an occupied site SO4•-(A)):

S2O82- + 2A f 2SO4•-(A)

(5)

Figure 6. Reaction scheme similar to that shown in Figure 2 in which adsorbed hydrogen plays an essential role.

cathodic reduction of H2O2 at p-type Si is one, a markedly higher efficiency (up to 1.6) is measured when Fe3+ ions are present in the acidic H2O2 solution. This result can be explained by assuming that Fe3+ ions are preferentially reduced

Fe3+ + e-(CB) f Fe2+

and the Fe2+ product reacts chemically with H2O2 (the Fenton reaction)

Fe2+ + H2O2 f Fe3+ + OH- + OH•

S2O82- + A + e-(CB) f SO42- + SO4•-(A)

(6)

No hole injection is observed indicating that the resulting surface state intermediate SO4•-(A) is energetically unfavorable for hole injection into the valence band so that electron capture predominates, in this case, even to low light intensity. •-

-

2-

SO4 (A) + e (CB) f SO4

+A

(7)

(9)

giving an OH• radical in solution which injects a hole before it can adsorb on the surface. While an adsorbed SO4•- or OH• intermediate could explain the absence of photocurrent doubling for p-type Si, it does not account for the change in mechanism of S2O82- reduction at n-type Si on going to more negative potentials where hydrogen is evolved. Adsorbed hydrogen atoms might block surface sites A at which the oxidizing agent or its radical intermediate adsorbs.

A + H+ + e-(CB) f H•(A)

or

(8)

(10)

At such a site, the S2O82- anion can inject a hole

S2O82- f SO42- + SO4•- + h+(VB)

(11)

A reaction between the radical anion and the adsorbed hydrogen atom

SO4•- + H•(A) f SO42- + H+ + A

(12)

regenerates the surface site.

41

Smandek and Gerischer provide evidence from electroluminescence measurements for the specific adsorption of the SO4•- radical ion on TiO2. Adsorption leads to a stabilization of the radical, shifting its energy level markedly to a higher energy in the band gap of TiO2. That with silicon an adsorbed species is important is supported by experiments involving a variation on the Fenton reaction.19 While the quantum efficiency for the photo(39) Allongue, P.; de Villeneuve, C. Henry; Pinsard, L.; Bernard, M. C. Appl. Phys. Lett. 1995, 67, 941. (40) Lowe-Webb, R. R.; Lee, H.; Ewing, J. B.; Collins, S. R.; Yang, W.; Sercel, P. C. J. Appl. Phys. 1998, 83, 2815. (41) Smandek, B.; Gerischer, H. Electrochim. Acta 1989, 34, 1411.

A Hydrogen-Based Model The ideas described above suggest a possible alternative scheme for the generation of electroluminescence from S2O82- and quenching of the light emission (Figure 6). As in the original scheme (Figure 2) photoexcitation of an unoccupied particle (step 1) gives rise to photoluminescence (step 2). At a negative potential, reduction of a proton by an electron present in a quantized particle gives a hydrogen center (step 3) which triggers the S2O82reduction and the generation of a hole in the particle (step 4); this sequence corresponds to reactions 10-12 described in the previous section. The subsequent injection of an

Luminescence in Porous Silicon/Solution Biodes

electron (step 5) produces an electron-hole pair, which is responsible for electroluminescence (step 2). While an electron-hole pair photogenerated in a hydrogen-free particle can recombine radiatively (step 2), absorption of a photon in a particle containing a hydrogen center (step 6) leads only to nonradiative recombination (step 7); this is the result of reactions 3 and 4, described in the previous section. Similarly, the combination of an electron-hole pair and a hydrogen center in the particle (step 8) leads to quenching of the electroluminescence (step 2). In this way the rise in electroluminescence is coupled to the decay of the photoluminescence (Figure 1). Step 9 accounts for hydrogen evolution in the system. It is clear that the schemes given in Figures 2 and 6 are equivalent; in Figure 6 hydrogen assumes the role of the electron that causes Auger recombination in the model of Figure 2. The scheme of Figure 6 implies that the reaction mechanism for S2O82- and H2O2 is distinctive in a number of respects. Reduction of both species occurs almost exclusively via the conduction band unless hydrogen is evolved, in which case hole generation in nanocrystallites becomes possible. Switching on the hydrogen reaction requires the conduction band of the nanocrystallite to be occupied by an electron. This step is determined by the position of the Fermi level, i.e., by the applied potential, and depends on the crystallite size in the same way as for the model shown in Figure 2. On the other hand, the Ce4+ ion injects a hole directly into the valence band of silicon and porous silicon. If no electrons are available at the surface, as at positive potentials, oxidation of the semi(42) Noguchi, H.; Kondo, T.; Uosaki, K. J. Phys. Chem. B 1997, 101, 4978. (43) Peter, L. M.; Wielgosz, R. I. Appl. Phys. Lett. 1996, 69, 806.

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conductor ensues (giving “anodic” luminescence in porous materials). Since S2O82- and H2O2 do not inject holes to a significant extent at positive potentials, only very weak or no luminescence is observed in this range.19 At around the flat band potential, Ce4+ reduction leads to a cathodic current due to electron-hole recombination. The fact that the diffusion-limited recombination current is accompanied by light emission has two important consequences: Electron supply to the nanocrystallite, which is necessary to generate luminescence, is obviously not a problem; since no light emission is observed from S2O82- in the same potential range, reduction of the oxidizing agent does not involve hole injection. It is interesting to note that Peter and Wielgosz observed electroluminescence from porous p-type silicon in 0.2 M peroxydisulfate solution under illumination with white light from the backside of the wafer.43 The light intensity used in these experiments was high giving a limiting photocurrent density of 60 mA/cm2. This means that the photocurrent very likely was due not only to S2O82reduction but that a significant amount of hydrogen must have been evolved. According to our model hydrogen is necessary to ensure hole injection from S2O82- and thus light emission. The importance of hydrogen is also clear from work of Uosaki and co-workers,42 who report a quenching at negative potentials of the photoluminescence from p-type porous silicon due to illumination with shortwavelength light (