Luminescence studies of semiconductor electrodes

Luminescence studies of semiconductor electrodes

Electrochimica Acta 45 (1999) 561 – 574 www.elsevier.nl/locate/electacta Luminescence studies of semiconductor electrodes J.J. Kelly a,*, E.S. Kooij ...

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Electrochimica Acta 45 (1999) 561 – 574 www.elsevier.nl/locate/electacta

Luminescence studies of semiconductor electrodes J.J. Kelly a,*, E.S. Kooij a, E.A. Meulenkamp b a

Debye Institute, Uni6ersity of Utrecht, PO Box 80000, 3508 TA Utrecht, The Netherlands Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA Eindho6en, The Netherlands

b

Received 15 January 1999; received in revised form 5 February 1999

Abstract In this paper we review our recent results of in-situ luminescence studies of semiconductor electrodes. Three classes of materials are considered: single crystal compound semiconductors, porous silicon and semiconducting oxides doped with luminescent ions. We show how photoluminescence (PL) and electroluminescence (EL) measurements can give information about the optical and optoelectrical properties of the solid and about mechanisms of surface reactions. The relevance of time- and frequency-dependent measurements, which have been largely neglected, is stressed. © 1999 Elsevier Science Ltd. All rights reserved. Keywords: Luminescent ions; Semiconductor electrodes; Porous silicon

1. Introduction In considering sustained light emission from semiconductor electrodes, we must distinguish two forms [1]. In photoluminescence (PL) the semiconductor is excited with a light source which creates an electron in the conduction band and a hole in the valence band. Radiative recombination of electron and hole either directly (band–band) or indirectly (via a bandgap state) gives light emission. In electroluminescence (EL), current flow through the semiconductor/solution interface as a result of an electron transfer reaction causes light emission. The ‘fate’ of electron–hole pairs generated by light in a semiconductor electrode is generally determined by the electric field within the space charge layer of the solid. The situation is shown schematically in Fig. 1 for an n-type bulk semiconductor. At positive potentials corresponding to depletion, the electron–hole pair is separated by the electric field. The hole oxidizes either the semiconductor or a species in solution, while the * Corresponding author.

electron is registered as a current in the external circuit. It is clear that, in this case, photocurrent (PC) is observed but not PL. As the potential is made negative approaching the flat band value, electron – hole recombination occurs, reducing the PC. If recombination is radiative, then bandgap or sub-bandgap emission will be observed. There are various ways in which an electrochemical reaction can cause light emission from a semiconductor. The most straightforward is shown schematically in Fig. 2, again for an n-type electrode. An oxidizing agent in solution extracts an electron from, i.e. ‘injects’ a hole into the valence band. At positive potentials corresponding to depletion conditions, the majority carrier concentration at the surface is low. The minority carriers are held at the interface where they cause oxidation of the semiconductor or a species in solution. Clearly, neither current nor luminescence is observed in this case. At negative potential the band bending is reduced and the surface concentration of electrons increases. Recombination of injected holes with electrons supplied via the external circuit gives a cathodic current. If recombination is radiative, then photons with

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Fig. 1. Schematic representation of the potential dependence of the photocurrent (PC) and the photoluminescence intensity (PL) of an n-type semiconductor in an indifferent electrolyte solution. Energy band diagrams are shown for the illuminated semiconductor under flat band and depletion conditions.

an energy equal to the bandgap are emitted. Alternatively, radiative recombination can occur via an energy level in the gap, either from the conduction band to the level or from the level to the valence band. If the rate of reduction of the oxidizing agent becomes diffusion controlled (i.e. independent of potential, see Fig. 2), as is often the case, then one expects a saturation of the emitted intensity. For a p-type semiconductor, electrons can be injected into the conduction band from an electron donor in solution. Radiative recombination of electrons with holes supplied via the external circuit again converts current into light. As will become clear, Figs. 1 and 2 are naive in the sense that virtually all reported data show a more complicated potential dependence. This is due to surface recombination of electrons and holes, a process which, almost without exception, is non-radiative. Surface recombination modulates the PL and EL intensities shown in Figs. 1 and 2. It therefore constitutes a third route, besides PC and bulk recombination, for the removal of minority carriers.

Fig. 2. Schematic representation of the potential dependence of the cathodic current and the EL intensity of an n-type semiconductor in a solution containing an oxidizing agent which injects holes into the valence band of the solid. Schematic energy band diagrams are shown for flat band and depletion conditions.

This paper is mainly concerned with the type of EL depicted in Fig. 2. Other less common mechanisms for EL generation are known. When the luminescent center is at the surface, one can observe what we have termed electron transfer luminescence (ETL) [2]. An electron is transferred from the conduction band of an n-type semiconductor to the excited state of the adsorbed species. A hole is injected directly into the ground state from an oxidizing species in solution. Relaxation of the electron from the excited to the ground state results in light emission. Another type of EL can only be observed at high electric fields in the solid. It is related to processes such as field-enhanced tunneling, impact excitation and impact ionization of luminescent centers [3]. This EL mechanism is generally not observed in the potential range of electrochemical systems (low electric field case). In this paper we review some in-situ luminescence studies recently performed in our group at the Debye Institute. After a brief discussion of equipment and methods (Section 2), results from a range of electrochemical systems will be described. These illustrate how PL and EL yield information on three topics: solid state properties, electron transfer reactions and surface recombination. The systems considered include single crystal electrodes (Section 3), nanoporous systems (Section 4) and wide bandgap semiconductors doped with luminescent ions (Section 5). The emphasis in these sections is on the potential dependence of the emission. In Section 6 time- and frequency-dependent experiments are considered. Where appropriate, work from other groups will be referred to. This paper is, however, not intended as a comprehensive review of the subject.

2. Equipment and methods Five components can be distinguished in the set-up for a typical EL experiment: (i) standard electrochemical equipment to control the potential; (ii) an electrochemical cell designed to collect the emitted light efficiently; (iii) the electrode under investigation; (iv) a suitable electroactive species in solution; (v) an optical set-up for detection of the emitted light. Here, some features of the latter two aspects are briefly discussed. A simple detector such as a Si photodiode suffices to measure the luminescence intensity. The emission spectrum can be determined by combination with a monochromator. Variations of this basic set-up include the use of a photomultiplier tube (PMT) or a chargecoupled device (CCD) as detector, providing higher sensitivity and speed, and multi-channel analysis, respectively. A detection system can be home built, but is generally available commercially. A spectrofluorimeter offers the additional possibility of recording photo-

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luminescence excitation and emission spectra. This type of measurement can be very useful if not essential for a proper understanding of electroluminescence. Some examples are given below. Time- or frequency-resolved measurements down to approximately 10 ms step size are feasible without too much effort, using the equipment described. This time resolution is of the same order of magnitude as the RC time constant of many electrochemical systems. Thus, one of the attractive features of luminescence is that it can be measured easily, at low intensity, and with relatively inexpensive equipment that is compatible with the usual electrochemical cells and set-ups, and with electrochemical time scales. A disadvantage of the luminescence technique is the difficulty of measuring the absolute intensity and, hence, the EL or PL efficiency. Several routes are available. The set-up can be calibrated using PL systems or chemiluminescent materials with known quantum efficiency. An actinometer can also be used. With a well-designed set-up the percentage of the light emitted that falls on the detector can be estimated. A calibrated source and detector (i.e. traceable to a standard) then yield the absolute emitted light intensity and the PL efficiency (QEPL). The quantum efficiency for EL (QEEL) is usually defined as the number of photons emitted per second divided by the number of charge carriers injected. Values of QEEL in the most efficient systems are generally of the order of 10 − 2. In the visible spectral range such EL is typically visible to the eye under daylight conditions. However, values as low as 10 − 8 have also been reported. An elegant way to measure such low light levels is the integration of a Si photodetector in the electrode mounting [4]. Subbandgap emitted light passes through the electrode and is collected by the detector at the back. Most EL experiments have been carried out using n-type electrodes and aqueous solutions containing oxidizing agents (hole injectors) such as Ce4 + , Fe(CN)36 − , H2O2 and S2O28 − . This is due equally to the technological interest in etchants based on these agents, and to the limited availability of wide bandgap p-type semiconductors and of strong reducing agents. However, the latter restriction can be circumvented by in-situ generation. Bulk electrolysis is possible. However, a more elegant method, which we have used successfully a number of times [5–7], involves a rotating ring-disc electrode (RRDE), comprising a disc at which the electroactive species is generated, and a ring of the semiconductor of interest. Since the transit time from disc to ring is small (typically ]10 ms [8]), rather unstable species can be studied. The local generation also makes it possible to use strongly colored reducing agents, which would normally absorb the emitted light. Some examples are discussed below. A possible variation on the RRDE would be the use of a thin layer flow cell with an auxiliary electrode upstream.

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From an analysis point of view, there are two difficulties which deserve some attention because of their relevance to (semi)quantitative interpretation of EL and PL measurements. First, the uncompensated resistance or iR drop in solution can be rather large since EL and PL are often measured under forward bias. Thus, the true potential dependence can be obscured. iR compensation techniques can be used to get around this problem [9 – 11]. Second, comparison of data and determination of the true peak shape require that the wavelength-dependent spectral sensitivity of the detection set-up (monochromator throughput, detector response, windows, filters, etc.) be determined. This can be done using photoluminescent or chemiluminescent reference samples with known emission properties. Note also that the spectra should actually be presented on an energy scale for quantitative analysis, and the conversion from a wavelength scale requires multiplication by l 2 [12]. These corrections are particularly important when analyzing broad spectra centered at different wavelengths.

3. Single crystal electrodes

3.1. Photoluminescence Many groups have used in-situ PL measurements to get information about the physical properties of single crystal semiconductors and their interface with solution [13 – 19]. In the simplest case in which electron – hole recombination at the surface can be disregarded, the potential dependence of the emitted intensity is described by the Ga¨rtner model [15,18]. This model was developed to account for the PC of reverse-biased Schottky diodes. A minority carrier such as a hole in an n-type semiconductor, that is generated in the depletion layer (of thickness W) or within a diffusion length (Lp) of the edge of this layer, is effectively separated from the majority carrier and is transferred across the interface. Such carriers contribute to the PC but are lost to luminescence; there is, in effect, a ‘dead layer’ within which no emission is generated. Electron – hole pairs formed deeper within the semiconductor recombine and can give rise to luminescence. The criterion for observing emission is that the penetration depth of the exciting light 1/a(l) (where a(l) is the absorption coefficient at wavelength l) is larger than W+Lp. The PL intensity IPL is given by [18] IPL =

kf(0) e − a(l)W 1+a(l)Lp

(1)

where f(0) is the incident photon flux and k is the ratio of the rate of radiative recombination to the total recombination rate. In principle, it should be possible to obtain values for Lp, W and a(l) from the potential

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dependence of IPL. However, since it is difficult to measure the absolute PL quantum yield (IPL/f(0)) and the ratio k, it is customary to relate the PL intensity to the maximum intensity Imax which is observed at flat band potential (W=0; Imax =kf(0)/(1+a(l)Lp)) IPL/Imax =exp[ −a(l)W]

(2)

We have used in-situ PL measurements to characterize epitaxial layers of n-type GaN, a material which has recently become important for optoelectronic applications [20]. Two emission bands are observed, one with a maximum at 3.4 eV corresponding to band–band recombination, the other at 2.2 eV (‘yellow’ luminescence) due to sub-bandgap recombination. In Fig. 3 we show a plot of IPL for the latter as a function of potential. The typical form shown schematically in Fig. 1 can be recognized. From impedance measurements the depletion layer width was determined as a function of potential. Using these results we could show the validity of Eq. (2) (see inset, Fig. 3) and determine the absorption coefficient a(l). By normalizing IPL as in Eq. (2) one loses information on the minority carrier diffusion length (Eq. (1)). However, Lp can be determined by taking the ratio of IPL values measured at two different wavelengths, if the absorption coefficients at the wavelengths are known [20]. It is clear from Eq. (2) that if a(l) is known accurately, PL measurements can be used to follow changes in the depletion layer width and band bending. Ellis and co-workers [15] have used such measurements for GaAs. Free electrons and holes, generated within the dead layer, are very rapidly separated by the strong electric field of the depletion layer (on a ps scale). We have shown [20] that sub-bandgap emission resulting from exciton absorption in GaN is much less sensitive to the electric field than that from free carriers because of the

relatively large exciton binding energy in GaN (  30 meV). With the suppression of the contribution to PL from free carriers (the dead layer effect) it is possible to get detailed information about excitons in the semiconductor at room temperature from luminescence excitation measurements. Such information is less accessible from optical absorption or PC measurements, where band – band absorption obscures the exciton contribution. The Ga¨rtner model has been extended to account for PL for the case in which surface recombination is important [18,19,21]. This approach can give information on surface recombination rates. Generally, electron – hole recombination at the surface is non-radiative and does not result in luminescence. In many cases the PL intensity of n-type electrodes does not attain a constant value as shown in Fig. 1, but is quenched as the potential is scanned to negative values [17 – 19]. A considerable hysteresis is often observed in the reverse scan. In aqueous solution hydrogen is evolved in this potential range. Hydrogen may be incorporated into the semiconductor to a considerable depth. In addition, the semiconductor can be electrochemically reduced. Such chemical changes may induce surface or near-surface states which can greatly enhance non-radiative surface recombination. Results we have obtained with ZnO [22] and GaN [20] are exceptional in this respect, showing stable PL to markedly negative potentials (for example, see Fig. 3). The result with ZnO may be attributed to the absence of hydrogen evolution; ZnO shows a very large hydrogen overpotential [23]. The PL stability in the GaN case may be due to the unusual chemical stability of the nitride. Obviously, analogous effects may be expected with p-type electrodes where surface oxidation will affect the surface recombination rate. Rappich and co-workers have performed very interesting PL measurements on Si during anodic polarization in buffered HF solution [24]. Under such conditions the current can oscillate due to temporal changes in the nature of the oxide on the semiconductor. Such oscillations in anodic current are accompanied by oscillations in emission. These results were related to changes in the rate of non-radiative recombination at the silicon/silicon oxide interface.

3.2. Electroluminescence

Fig. 3. The intensity of the PL measured at 2.2 eV (the ‘yellow’ emission) from an n-type GaN electrode as a function of potential. The excitation energy was 4 eV. In the inset the results are plotted according to Eq. (2) (see text).

There are many examples of light emission due to hole injection into n-type semiconductors from simple and complex oxidizing agents in solution. Reduction of a one-electron agent such as Fe(CN)36 − in aqueous solution generates luminescence in GaAs [25,26], GaP [27] and ZnSe [28]. Ce4 + reduction also causes emission from GaAs [25]. Organic radical cations can be used to produce EL in aprotic solvents [29]. The valence band edge of a number of n-type semiconductors is at low

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Observation of EL means that the chlorine radical intermediate can inject a hole into the valence band (VB)

[30], where two charge carriers are detected as PC for each photon absorbed by the electrode. The photon produces an electron – hole pair. The electron reduces the oxidizing agent (Eq. (3a) or (3b)) giving an intermediate which generates a second hole (Eq. (4)). A mechanism similar to that given by Eqs. (3) and (4) also holds for the other two-electron systems. The n-GaAs/H2O2 case is particularly interesting. We have found spontaneous current oscillations during H2O2 reduction and these are accompanied by oscillations in the EL intensity [33]. It was shown that such oscillations occur when a negative impedance is caused by an anomalous dependence of the band bending on the electrode potential [35]. This effect was attributed to a hydride layer which is formed electrochemically during reduction. Examples of EL from single crystal p-type electrodes are rare. To inject electrons one needs a strong reducing agent with donor levels above the conduction band edge of the semiconductor. Such agents are generally not stable in aqueous solution. As far as we are aware, there are only two examples where oxidation of a simple reducing agent in aqueous solution gives light emission. We have observed EL from p-type InP with Cr(CN)46 − at high pH [5] and the methylviologen radical cation MV+’ at low pH [36]. The MV+’ radical was produced at a gold disk and the EL measured at a p-type InP ring in an RRDE set-up (see Section 2). In both cases band – band emission was involved. The position of the conduction band edge, which depends on, for example, the solution pH, is critical in such experiments. GaAs whose conduction band is at slightly higher energy does not show EL with these reducing agents. It is interesting to note that Uosaki and Kita report EL from p-type GaAs near the flat band potential after pulsing the potential to a strongly negative value [37]. Emission was attributed to electron injection from adsorbed hydrogen radicals formed during strong cathodic polarization. In the EL experiments of p-type InP with MV+’ at low pH described above two peaks were found in the intensity – potential curves [36]. The peak at more negative potential was due to the redox system. The second peak was also observed in indifferent electrolyte solution [38] and coincided with anodic dissolution of the semiconductor (see Fig. 5). From these experiments it could be concluded that electrons are injected into the conduction band from intermediates of the dissolution reaction. The results can be represented schematically by the localization of a valence band hole in a surface bond (In:P)

Cl’ “Cl − +h+ (VB)

In:P+ h+ (VB) “ In+’P

Fig. 4. The potential dependence of the electroluminescence intensity IEL and of the current density j for a rotating CdS electrode in a 0.01 M HOCl, 0.5 M K2SO4 solution of pH 7.5. The scan rate was 50 mV s − 1.

energy and the band may not be accessible to these simple ‘hole donors’. Two-electron systems, which at p-type semiconductors show PC doubling [30], invariably give EL at n-type electrodes. Examples include S2O28 − [14,31,32], H2O2 [33], the halogens [4] and the hypohalites [4,34]. We illustrate this case with results we obtained on hypochlorite reduction at n-type CdS [4]. Two plateaus are observed in the cathodic current–potential curve (Fig. 4). At more positive potentials, only HOCl is reduced, while in the second plateau both HOCl and its conjugate base (OCl−) react; the ratio of the limiting current in the two plateaus depends on pH, as expected. A strong luminescence with an efficiency of about 0.5% is observed during reduction. The emission involves both band–band and sub-bandgap recombination [4]. These results are consistent with a two-step reduction mechanism. Since the energy corresponding to the redox potential of the HOCl/Cl− couple is more than 1 eV above the valence band edge of CdS, it is highly unlikely that the first step involves hole injection. Instead, an electron from the conduction band (CB) is required. HOCl +e − (CB)“ OH − +Cl’ −

OCl +H2O+e



(3a)

(CB)“2OH +Cl’ −

(3b)

(4)

That two bands of the semiconductor can be involved in the reduction reaction is clear from experiments with the p-type form of other semiconductors

(5)

The intermediate, an electron deficient bond, can have an energy level in the bandgap. The intermediate can be oxidized by the trapping of a second hole (Eq.

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Fig. 5. The potential dependence of the electroluminescence intensity IEL and of the current density j for a p-type InP electrode, dissolving anodically in a 1 M HCl solution. The inset shows the EL spectrum.

(6)) or by electron excitation to the conduction band (Eq. (7)) In+’P+h+ (VB)“ In+ In+’P“In+

+

+

P

(6)

P+ e − (CB)

(7)

The latter reaction involving minority carrier generation gives rise to band –band emission (see inset Fig. 4). This emission is quenched when anodic dissolution is suppressed by a competing reaction, in which the oxidation of Fe2 + ions repairs surface bonds In+’P+Fe2 + “In:P+Fe3 +

(8)

The general mechanism indicated by Eqs. (5)–(7) is consistent with PC doubling results obtained for the anodic dissolution of n-type InP [39]. Theuwis et al. found that EL from p-type InP, anodically dissolving in H2SO4 solution, is considerably enhanced when H2O2 is added to solution [40]. They propose that the oxidizing agent induces chemical changes in the decomposition intermediate, thereby facilitating electron injection into the conduction band. As in the case of PL, recombination of electrons and holes at the surface invariably decreases the efficiency of EL. An exception to the ‘rule’ of non-radiative

surface recombination was reported by Smandek and Gerischer [41] who attributed sub-bandgap emission in the TiO2/peroxydisulfate system to a surface process. ETL described above [2] is, of course, a special case involving radiative surface recombination. From the results described above it is clear that EL can be used as a diagnostic tool for investigating minority carrier participation in surface reactions. The method is, however, not quantitative. As for PL, it is difficult to determine absolute efficiencies and the ratio of radiative to non-radiative recombination. In addition, the potential dependence of the emitted intensity is generally not as simple as shown in Fig. 2. For example, in the hypochlorite case of Fig. 4 two peaks are observed with a marked hysteresis in the return scan (the latter is not shown in the figure). For n-type GaAs in alkaline Fe(CN)36 − solutions a peak is also observed in the scan from positive to negative potentials [25]. After the peak, the EL intensity drops to a very low value before increasing again markedly at more negative potentials. Such effects, which will be considered in Section 5, again indicate the importance of chemical changes at the surface during potentiodynamic measurements. Exceptional results have been reported by Mc Intyre et al. [29], who found stable EL in the negative potential range with n-GaP in aprotic solvents. Obviously, in this case hydrogen evolution does not occur. The measured EL intensity – potential curve resembled the ‘ideal’ curve shown in Fig. 2. Despite the complications in aqueous solution EL can, in combination with other techniques, give important information on the mechanism of charge transfer reactions at semiconductor interfaces. 4. Porous silicon Although porous silicon, obtained by anodic etching in HF solution, had been known since the fifties, it was not until 1990 that Canham [42] demonstrated the very surprising luminescent properties of the material. Bulk silicon, which has an indirect bandgap, shows only very weak PL in the near IR. Porous silicon, on the other hand, gives a broad and strong emission band in the visible spectral range. This discovery triggered an extensive research effort aimed mainly at producing an all-silicon light emitting device. Various explanations for the visible emission have been proposed [43]. The most widely accepted is that in which the light is generated in nanometer-sized crystallites within the porous silicon layer. Due to the local confinement of the charge carriers within these small silicon particles the effective band gap is widened (Fig. 6) while the ratio of radiative to non-radiative recombination is enhanced considerably. The large size distribution of the crystallites within the porous silicon structure accounts for the broad PL spectrum: particles of different sizes emit at different wavelengths.

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Fig. 6. Schematic energy band diagrams for heterojunctions consisiting of bulk silicon/solution and bulk silicon/porous silicon nanoparticle/solution. Due to size quantization, the bandgap of the nanoparticle is larger than that of the bulk semiconductor (two particle sizes are shown). Because of the larger bandgap, there is a mismatch of the valence band (VB) and conduction band (CB) edges which is more pronounced for the smaller crystallite.

Porous silicon shows a wealth of luminescent phenomena. Here we shall consider briefly some aspects of PL and EL from electrodes in solution. The potential dependence of the PL from porous silicon in indifferent electrolyte solutions is markedly different from that observed from single crystal electrodes [10,11,44,45]. As described in the previous section, illumination of n-type electrodes under anodic polarization generally leads to PC as the photogenerated electron–hole pairs are effectively separated within the depletion layer; PL is not observed. On the other hand, PL is observed from n-type porous silicon electrodes in a broad potential range (Fig. 7). The light emitting silicon particles are too small to sustain a depletion layer. Charge carriers, generated within these crystallites, are not separated and PC is consequently not observed. Instead electrons and holes recombine. At potentials more negative than the flat band value, the PL is quenched. The quenching has been attributed to Auger recombination [10,11,44,45]. Radiative recombination of the electron–hole pair within a crystallite is suppressed by the presence of an electron supplied from

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the substrate. The potential at which the emission decreases depends on the wavelength; emission at lower energy from larger crystallites is quenched at more positive potentials than the higher energy emission from the smallest particles. This has been attributed to the fact that the barrier for the transfer of an electron from the bulk to smaller, more confined particles is larger than that to less confined regions (Fig. 6). There are many ways to generate light emission from porous silicon electrodes by means of electron exchange reactions with species in solution. Various possibilities are summarized in Tables 1 and 2. The first case (I A and II A) represents conventional EL as a result of minority carrier injection from solution. For example, we have shown that, as at InP, the MV+’ radical cation formed in the reduction of divalent methylviologen MV2 + + e − “ MV+’

(9)

is able to inject electrons into the conduction band of crystalline and porous silicon [6,7]. An RRDE configuration, with a gold disc and a p-type porous silicon ring, was used to generate radical cations. At the positively polarized ring electrode electrons, injected by the MV+’ species, recombine with holes, giving rise to weak emission (I A). Considerably more attention has been devoted to EL from n-type porous silicon in solutions containing strong oxidizing agents. It has been shown that Ce4 + , 2− IrCl26 − , MnO− and NO− ions in 4 , H2O2, S2O8 3 aqueous solutions inject holes into the valence band of n-type porous silicon thereby giving rise to light emission when the electrode is cathodically polarized (case II A) [46,47]. EL has also been observed from porous silicon as a result of hole injection in non-aqueous solutions containing thiantrene [48]. The most efficient emission has been found with peroxydisulfate [49,50]. It is generally accepted that reduction of the oxidizing agent is, like OCl− described above, a two-step reaction. The first step involves capture of a conduction band electron − S2O28 − + eCB “ SO24 − + SO4− ’

(10)

The radical anion is an extremely strong oxidizing agent capable of injecting a hole into the valence band of most semiconductors SO4− ’ “SO24 − + h+ VB

Fig. 7. Schematic representation of the potential dependence of the PL and EL intensity of an n-type porous silicon electrode. PL is measured in an indifferent electrolyte solution. For the EL measurement an oxidizing agent in solution injects holes into the valence band of the semiconductor.

(11)

A hole injected into luminescent regions within the porous structure may recombine radiatively with electrons under forward bias. Recent results [51,52] have shown that the reaction mechanism is more complex than that indicated by Eqs. (10) and (11). An interesting aspect of the EL is its voltage tunability. Upon scanning the potential to more negative values the emission maximum shifts to shorter wavelengths until finally the EL is quenched [10,11,44,45]. EL can be

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Table 1 Possible mechanisms for EL from p-type porous silicon (see text)

Electrolyte Holes from: Electrons from: Luminescence Current

IA

IB

IC

ID

electron donor substrate solution conv. electrolum anodic

hole donor solution illum. substrate

indifferent substrate Si oxidation anodic lum. anodic

hole donor solution Si oxidation ‘chernilum.’ electroless

photocathodic

expected from a particle if it is populated by an electron supplied from the bulk (Eq. (10)). This requires that the Fermi energy of the bulk silicon be raised to a level close to the conduction band edge of the particle. This will occur for larger particles at less negative potential since the mismatch of the conduction band edges is small in this case (see Fig. 6). As a result, long wavelength emission will be turned on first. Gradually as the Fermi level is raised further, i.e. as the potential is made more negative, the smaller particles will participate and the emission maximum will shift to shorter wavelength. Quenching of EL at negative potentials has also been attributed to Auger recombination [10,11,44,45]. This can explain why the rise in EL on going to negative potentials is coupled to the quenching of PL (Fig. 7) [10,11]. To give EL, an electron is required in a particle in order to create a hole via S2O28 − reduction. On the other hand, photoexcitation of a particle already occupied by an electron leads to Auger recombination, i.e. the PL is quenched. At more negative potentials, the supply of an electron to a particle in which an electron–hole pair is present, leads to Auger quenching of the EL. Case I B is an interesting variation on the luminescence described above. When the electrode is illuminated with light that is only absorbed in the substrate, photogenerated minority carriers may be transferred to the porous structure under reverse bias. Injection of a majority carrier into the crystallites from solution leads to visible emission. This type of luminescence was illustrated for p-type porous silicon by illuminating the substrate from the back side [53]. Under cathodic polarization the electrons accumulate in the porous struc-

ture and injection of a hole from the peroxydisulfate containing solution (reactions (10) and (11)) leads to visible emission. A similar blue-shift is observed as in the case of conventional EL from n-type material. As far as we are aware, the equivalent case (II B) has not been reported for n-type porous silicon in solutions containing a reducing agent. Experiments by Green et al. [54] suggest that formic acid could perhaps show such an effect. The strong visible emission observed during anodic oxidation of p-type porous silicon electrodes in indifferent electrolyte solution (case I C) was actually the first type of EL reported [55,56]. Initially it was not clear which species was responsible for the electron injection which is necessary to explain luminescence. Since neither anodic oxidation nor light emission were observed with n-type silicon in the dark [57] it seemed likely that a valence band process with holes was involved. We confirmed this by experiments in which IR light was used to excite selectively the silicon substrate [57]. Under reverse bias both PC and visible light emission (case II C) were found. Electrons and holes are separated by the electric field at the bulk silicon/porous silicon interface (Fig. 8). The electrons are registered as a PC in the external circuit. The holes are injected into the porous layer where, as in p-type porous silicon, they cause oxidation of the semiconductor. There is strong evidence to suggest that electrons can be thermally excited to the conduction band from surface state intermediates of the oxidation reaction (electron-deficient back bonds). As in the case of anodic luminescence from p-type InP described above, radiative recombination of electrons and holes accounts for the EL. Light emission from porous silicon is much stronger than from InP.

Table 2 Possible mechanisms for EL from n-type porous silicon (see text)

Electrolyte Holes from: Electrons from: Luminescence Current

II A

II B

II C

II D

hole donor solution substrate conv. electrolum cathodic

electron donor ilium. Substrate solution

indifferent ilium. substrate Si oxidation anodic lum. photoanodic

hole donor solution Si oxidation ‘chemilum.’ electroless

photoanodic

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Fig. 8. Energy level diagram of an n-type porous silicon electrode in contact with an indifferent electrolyte. The origin of IR induced oxidation and visible light emission is schematically represented.

The spectrum of the anodic emission from both pand n-type porous silicon depends on the degree of oxidation [55–58]. This is shown in Fig. 9 for IR illuminated n-type material. The emission intensity first increases while the maximum clearly shifts towards shorter wavelengths. This blue-shift has been attributed to size-selective hole injection [57,58]. As mentioned above, the nanoporous layer consists of particles with a large distribution of sizes. The barrier for hole transfer is smallest for the largest, bulk-like regions which have an effective band gap close to that of bulk silicon; these regions will be oxidized first, giving longer wavelength emission. As oxidation proceeds, current flow to such regions is hindered and holes are injected into smaller, more confined crystallites; this results in a shift of the emission to shorter wavelengths. Eventually, the porous

569

electrode is oxidized to such an extent that current can no longer flow and consequently the luminescence is quenched. Billat has also studied PL in combination with the EL during anodic oxidation [58]. The PL intensity is not dependent on charge carrier injection and thus provides direct information on the efficiency of radiative and non-radiative recombination. When the EL intensity rises in the initial stages of oxidation, the PL also increases. Apparently, the formation of oxide decreases the rate of non-radiative recombination, either by passivating the surface of the emitting crystallites or by blocking the ‘leakage’ of charge carriers to nonconfined regions within the porous structure. As the EL decreases towards the end of the oxidation process, the PL intensity of low doped material remains unchanged while highly doped porous silicon exhibits a decline of the radiative decay rate. The origin of this doping related effect remains unclear. Finally, luminescence can be generated by an ‘electroless’ mechanism (I D and II D). Immersion of either p-type or n-type porous silicon in a solution containing 2− − strong oxidizing agents like NO− 3 , S2O8 , MnO4 , Ce4 + and IrCl26 − under open-circuit conditions gives rise to visible emission [46]; this has been termed ‘chemiluminescence’. Recently, we suggested a mechanism similar to that proposed for the luminescence accompanying anodic oxidation [47]. The oxidizing agent injects holes into the valence band causing oxidation of the porous silicon electrode and the corresponding luminescence. The emission is in the same range of the spectrum as that of anodic emission, while a similar blue-shift and the transient nature of the luminescence further support the idea of a common mechanism.

5. Doped wide bandgap semiconductors

Fig. 9. Time-dependence of the emission spectrum, observed during anodic oxidation of a stationary n-type porous silicon electrode in 1 M KCl solution polarized at 1.5 V, under illumination with IR light. The time at which the spectra were recorded after the light was switched on is indicated by a–f and varied from 2 s (curve a) to 50 s (curve f). The strong increase above 850 nm is due to stray light from the IR diode.

Although many materials are ‘doped’ or ‘activated’ in the sense that they show sub-bandgap PL/EL, in this section the benefits of doping, intended to understand better the nature of EL, are discussed. An in-depth study of PL and EL of oxide films (Ta2O5 and Al2O3) clearly demonstrated the usefulness of dopants. These systems were first developed by a Finnish group [59,60] as an analytical tool for the determination of H2O2 and heavy metal ions. The doped oxide films were prepared by anodization in the presence of the metal ions in solution. The work on Ta2O5:Tb3 + in our group shows two major advantages of doping [2,61]. Firstly, the incorporation of Tb3 + in Ta2O5 led to a 100-fold increase in light intensity compared to the intrinsic tantalate emission. This also enabled PL experiments. Analysis of the potential dependence and the effect of H2O2 concentration showed that Tb3 + EL was similar to the intrinsic EL and, hence, could be used as a probe of the elec-

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570

EL=hole injection rate × recombination rate × radiative quantum efficiency PL=recombination rate × radiative quantum efficiency (excited in Ta2O5) PL=radiative quantum efficiency (excited in 4f8 level)

Fig. 10. Schematic picture of the radiative (—) and non-radiative (- - -) transitions relevant to the PL and EL of the Ta2O5:Tb3 + system. The Tb3 + 4f8 energy levels are shown on the left. The three transitions labeled ‘insensitive’ show intensities which, to a good approximation, do not depend on the surroundings of the ion. The intensities of the three other 4f8 transitions shown on the right do depend on the surroundings. PL (— ) and EL (- - -) formation of electron–hole pairs in Ta2O5 is also shown.

trode processes at the Ta2O5/electrolyte interface. The dopant acts as a recombination center: Tb3 + +h+ “Tb4 + 4+

Tb

+e “(Tb −

3+

(12a) )*“Tb

3+

Additionally, evidence for EL of surface-adsorbed ions was obtained from these measurements. The decay time of the Tb3 + 5D4 emission depends strongly on the rate of the non-radiative 5D4 – 7F0 transition. In the present case, this was mainly determined by the number of H2O or D2O molecules attached. The decay time was investigated in pulsed EL measurements. An example is shown in Fig. 11. It is clearly seen that the decay of the EL at t \8 ms is much slower in D2O than in H2O. A quantitative analysis showed that, on average, 2.5 9 0.5 water molecules were attached to the Tb3 + ion. This is a strong indication for EL of surface-adsorbed ions. The relative peak intensities of the 5D4 – 7FJ transitions depend on the degree of distortion from centrosymmetry of the environment. The ratio R of the 5D4 – 7F6/ 5 D4 – 7F5 peak intensities is a good measure: the higher it is, the higher is the degree of distortion. R was larger for EL than for PL, in accordance with a higher contribution from adsorbed ions. Additionally, REL decreased in the potential range in which EL (and PL) quenching occurred. Quenching was shown to be due to the introduction of additional recombination centers (proton intercalation) from the oxide/solution interface. Again, this pointed to EL of surface-adsorbed ions. A few other EL studies of doped electrodes have been reported. Bard and co-workers used Mn-doped

(12b)

Secondly, the luminescence of rare earth (RE) ions such as Tb3 + is determined by the local environment. A schematic picture of all the non-radiative (dashed lines) and radiative (full lines) transitions involved is given in Fig. 10. On the left hand side of the Tb3 + energy level scheme, three transitions are indicated (one in excitation, two in emission) which are relatively insensitive to the environment. These serve as internal references. On the right hand side, three emission processes are shown, the rates of which depend strongly on the parameter indicated (water, symmetry). Electron– hole formation in the Ta2O5 matrix is shown on the right side. Eq. (12b) is included as electron transfer to the upper 4f levels. The Ta2O5:Tb3 + system is rather complicated and not all aspects can be summarized here. However, it clearly represents a unique case since the combination of the special luminescence properties of the dopant RE ions and use of EL and PL made it possible to separate all potential-dependent contributions to EL and PL. The intensities are given by:

Fig. 11. Time-resolved EL from a Ta2O5:Tb3 + anodic film. A solution of 10 − 3 M H2O2, 1.0 M NaClO4 (pH 3) in either H2O or D2O was used. At t =0 the potential is pulsed cathodically to a value at which H2O2 is reduced; the Tb3 + emission intensity increases. At 8 ms the potential is returned to a value at which the reduction reaction does not occur; the emission decays. Note the logarithmic intensity scale.

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ZnS [62,63]. The main effects of doping were a high light output and long rise time and decay time of the luminescence. Kouyate and co-workers studied the cathodic and anodic (high-field) EL of RE3 + -doped ZnO [64]. Under cathodic bias, only ZnO emission was observed. Hence, the RE ions did not act as recombination centers. The anodic EL was characteristic of the RE ions. The spectra thus illustrate the different excitation mechanisms for these types of EL. Athanassov et al. [65] recently reported on a system which resembles the surface Tb3 + case described above. Instead of an inorganic luminescent center, they used a ruthenium bipyridyl complex adsorbed on nanoporous TiO2 to generate EL under forward bias in the presence of peroxydisulfate. This system is actually very similar to ETL (described in Section 1) and studied for Al/Al2O3 electrodes [66].

6. Time- and frequency-resolved experiments Most of the work on PL and EL reported in the literature has been carried out under (quasi) steadystate conditions. Relatively little attention has been paid to time-resolved measurements. An exception is PL pulsed on a picosecond time scale, used to investigate surface recombination [67–70]. Frequency-resolved experiments are rare. This is surprising since such measurements, and in particular impedance-based methods, have gained wide popularity in semiconductor electrochemistry in recent years. In this section, some examples of modulated EL/PL experiments known to us are discussed. They illustrate the kind of information that can be obtained and indicate that this fertile field remains largely unexplored. An early paper by Beckmann and Memming [71] on the PL of n-GaP describes the use of a small sinusoidal potential modulation during a potential scan and lockin detection of the PL response. This method was necessary to investigate accurately a small ‘discontinuity’ in the (PL–U) curve in the onset region of the PC. d(PL)/dU is effectively determined. The ‘discontinuity’ was attributed to charge carrier transfer across the interface, resulting in a lower recombination rate. Thus, investigation of the d(PL)/dU function at several frequencies and phase-sensitive detection should yield the time constant for this reaction. Analogous potentialmodulated, frequency-resolved experiments on EL have, as far as we are aware, not been performed. However, processes such as filling of surface states, changes in the surface chemistry, and charge carrier injection should be accessible to study in this way. In this sense, such measurements may be complementary to (photo)electrochemical impedance spectroscopy (PEIS/EIS), and optoelectrical impedance spectroscopy (IMPS), and provide an additional means to study the semiconductor/electrolyte interface.

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Several papers have described time-resolved EL measured upon application of a potential step. The intensity is governed by the time-dependent (i) charge injection rate, (ii) the surface recombination rate, and by the kinetics of (iii) charge carrier recombination, and (iv) light emission decay. Clearly, the interpretation is complicated. The nature of the luminescent centers was identified by their decay times for the case of metal ion-doped oxides [66]. One of these systems was already mentioned in Section 5. Bard and co-workers have investigated the kinetics of charge recombination in (Mn-doped) ZnS electrodes [62,63]. The slow rise and the slow decay of the EL were attributed to trapping and de-trapping of charge carriers, respectively. Manivannan et al. [72,73] used fast measurements (sub-ms time resolution) in their studies of the S2O28 − induced EL of n-SiC. The analysis of the ‘dead time’ and the rise time pointed to the important roles of the RC time constant of the semiconductor/electrolyte interface and ’ the hole transfer rate from the intermediate SO− 4 radical. Time-dependent studies of changes in the surface recombination velocity have been described in several papers [74 – 78]. An interesting variation on this technique was developed by Decker and co-workers [79]. It exemplifies a general approach to investigate changes in the surface recombination velocity which are due to changes in the surface composition. The EL of n-GaAs in the presence of Fe(CN)36 − was studied. The unusual potential dependence of the emission observed during a potential scan was briefly outlined in Section 3. This was attributed to a change in the surface termination from hydroxide to hydride coverage on going from positive to negative potentials. The potential dependence was now studied using a train of potential pulses with an incremental increase in amplitude. This is schematically shown in Fig. 12. The pulse frequency was varied and the average EL intensity was measured. The results are also shown in Fig. 12. It is clear that the intensity maximum at −1.2 V disappears when potential pulses shorter than about 50 ms are applied. Hence, the time constant for the transition from an oxidized to a reduced surface was estimated to be 0.1 ms. Unfortunately, this approach has not been applied to many other systems [80]. It would also be interesting to combine such studies with other techniques such as EIS or PL to obtain a more complete picture. It is clear that the possibilities of pulsed EL and PL experiments have not been fully exploited. So far, only potential-modulated experiments have been described. Other system parameters can also be varied. Some preliminary work [81] we have performed on modulation of the charge carrier injection rate illustrates both the complexity of many EL systems, and the usefulness of this modulation approach. The systems studied were the diffusion-limited reduction of Fe(CN)36 − at n-GaP and n-InP in 1.0 M KOH. The

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Fig. 12. Plots of ac EL intensity vs. potential pulse height for n-type GaAs in a 0.05 M Fe(CN)36 − , 0.5 M NaOH solution as a function of the cycling frequency. The shape of the square wave pulse is shown in the inset (results from Decker et al. [79]).

usual dc measurements (potential step, cyclic voltammetry) indicated that the EL intensity did not show the expected dependence on the hole injection current, especially at relatively positive potential. At U\Uoc the system showed slow etching. Thus, it was suspected that a change in the hole injection current jh also induced a change in the surface recombination rate such that IEL was no longer proportional to jh. Small amplitude modulation of jh was used to verify this assumption. Two techniques were used. For n-GaP the rotation rate of the RDE was ac modulated (1.0 Hz; B10% relative amplitude) and (dIEL/IEL)/(djh/jh) was determined. The other experiments used a Au disc/InP ring RRDE in a Fe(CN6)4 − solution. The disc potential was set at the half-wave potential of the FeII to FeIII oxidation and modulated at 0.5–5 Hz with B 10% relative current amplitude. This yielded an ac hole injection current at the InP ring and (dIEL/IEL)/(djh/jh) was again determined. Both experiments showed that (dIEL/IEL)/(djh/jh) was close to 1.0. Hence, IEL was proportional to jh, in contrast to the results of the dc experiments.

Although much work has been done on EL and PL, relatively little attention has been paid to (quantitative) time- and frequency-resolved measurements. This can be ascribed to three factors, which have been addressed in this review. (i) The transient response can be due to a number of effects and the analysis is complicated. Unambiguous interpretation in terms of a single parameter has generally not proved possible. (ii) Due to changes in the surface recombination velocity the intensity at a particular potential is, in most cases, not sufficiently stable to allow reproducible frequency-resolved measurements. Thus, one of the criteria of a true impedance approach cannot be met. (iii) There is no general quantitative model, such as the dead-layer model for PL, which describes the dependence of the EL intensity on potential, hole injection rate and surface recombination rate. Nevertheless, the examples discussed above show convincingly that valuable insight into complex electrochemical systems can be obtained by time- and frequency-resolved experiments. We feel, therefore, that there is considerable scope for such EL and PL measurements. The importance of simultaneous PL measurements under EL conditions has been illustrated by the work on several systems, including porous Si and doped oxide films. It is hoped that the recognition of the value of this combination of luminescence techniques will lead to an increasing number of studies in this field. A detailed comparison of PL and EL also provides a route to a more quantitative EL model. The inherent simplicity of EL and PL methods makes them very attractive to study the semiconductor solid state properties, the semiconductor/electrolyte interface, and the electrochemical reactions.

Acknowledgements We are pleased to acknowledge the contribution of former graduate students Andre´ de Wit, Gerko Oskam, Ben Erne´, Marc Koper, Peter Bressers, Geert Schoenmakers and Jao van de Lagemaat to the work described in this paper.

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