Defect-induced band gap states and the contact charging effect in wide band gap insulators

Surface Science 408 (1998) 237–251

Defect-induced band gap states and the contact charging effect in wide band gap insulators U. Malaske, C. Tegenkamp, M. Henzler, H. Pfnu¨r * Institut fu¨r Festko¨rperphysik, Universita¨t Hannover, Appelstraße 2, D-30167 Hannover, Germany Received 31 October 1997; accepted for publication 17 March 1998

Abstract Color centers, created by electron bombardment, have been studied as model defects on the (100) surfaces of the wide band gap insulators NaCl and KCl with electron energy loss spectroscopy ( EELS ) and UV photoelectron spectroscopy ( UPS ). Both salts were grown as thin epitaxial films on the same Ge(100) surface so that they could be directly compared in situ. At substrate temperatures below 200 K, characteristic losses of F and M centers could be identified on both materials. Close to room temperature, however, high electron exposures resulted in additional losses in the band gap due to surface and bulk plasmons of Na and K clusters. Only the defects turn out to be chemically reactive. Color centers dissociate water, but leave salicylic acid (SA) intact as a molecule, which, however, is more strongly bound compared with the undistorted surfaces. Both the color centers and the adsorbed molecules provide unoccupied electronic states in the band gap to which thermal electronic excitation is possible. We propose that these states are essential for charge exchange during contact charging and discuss this phenomenon in context with our experiments. © 1998 Published by Elsevier Science B.V. All rights reserved. Keywords: Alkali halides; Electron loss spectroscopy; Insulating surfaces; Organic acids; Photoelectron spectroscopy; Water

1. Introduction Alkali halides are prototype wide band gap insulators, the bulk properties of which have been studied extensively in the past [1,2]. However, investigations of insulating surfaces were started only recently [3], since only a small number of the usual surface sensitive techniques can be used on bulk insulating material. This problem can mostly be circumvented by the use of thin insulating films epitaxially grown on conducting substrates, which show only little charging effects [4,5]. Since the electronic equivalence of these layers with bulk * Corresponding author. Fax: (+49) 511 7624877; e-mail: [email protected] 0039-6028/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 98 ) 0 02 4 7 -7

crystals has been proven for several systems [6,7], this technique allows us to study properties of these surfaces with common experimental methods like UPS and EELS under well-defined UHV conditions. Especially the epitaxy of NaCl on Ge(100) [8] and of KCl on NaCl(100) [9] has been improved in recent years and, where a direct comparison was possible, the results on single crystals and on single crystalline thin films have been shown to be the same [6,7]. A common and interesting property of most insulating surfaces is that stoichiometric and atomically perfect surfaces are highly inert [3]. However, they become chemically reactive once defects (isolated vacancies, steps, etc.) are formed. In fact, surfaces are never free of defects.

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Therefore, it is of considerable relevance to study the properties of various defects on such surfaces. In this paper we extend previous investigations [10,11] of single anion vacancies on the (100) surfaces of NaCl and KCl, which are commonly known as color centers. We use them here as model defects, which are shown to widely determine the chemical properties of these surfaces. For example, water is dissociated at color centers [12], but only physisorbed as a molecule on the perfect surface [10]. In fact, the properties of defects and their adsorption behavior turn out to be not only chemically relevant, they also seem to play an important role in the long-known phenomenon of contact charging [13,14]. Although the principal mechanism of charge exchange between different materials is not debatable, neither the location nor the origin of charges nor the sign of net charge transfer is understood in terms of an atomic picture. This is even more surprising, since the process of contact charging is widely used technically to separate electrostatically materials like NaCl and KCl, but also various polymers from each other [15]. In most cases, conditioners, i.e. substances in small quantities which change selectively the surface properties, have to be added to achieve high selectivity [16,17]. Salicylic acid (SA) together with water vapor in particular was shown to be efficient for separating NaCl and KCl electrostatically, to some extent also water vapor alone, but only at elevated temperatures [16 ]. We therefore concentrate on these two adsorbates. This study, however, although using the same substances as in the technical process mentioned, is rather aimed at a basic understanding of the role of defects than at a detailed clarification of the mechanisms in the technical process, since color centers are very unlikely to be produced there. Nevertheless, we think that defects are essential in the conditioning process. We take this process as a specific example to illustrate the importance of defects, which are able to modify drastically the properties of an insulating surface both electronically and chemically. In the case of adsorption on defect-free alkali halide surfaces, the kinetic and electronic properties of water on KCl(100) and NaCl(100) have

already been extensively described [6,10,18–20] so that we can concentrate on the electronic properties of salicylic acid in the first section of our results. Thick molecular layers, i.e. layers without electronic contributions from the substrate, are compared with monolayers and submonolayers. We then turn to the generation of color centers, our model defects. As a side-step we show that these are able to ‘‘collapse’’ to form alkali clusters, if the concentration of defects and the mobility for the cations are high enough, and investigate changes in the electronic structure of water and SA exposed to color centers, so that we are able to draw an energy scheme including occupied and unoccupied states of both substrates and adsorbates (Section 3.4). Finally, these results are discussed in the context of contact charging.

2. Experiment The experiments have been carried out in a UHV chamber operated at a base pressure of 1×10−8 Pa. It was equipped with an optical LEED to control the morphology of the Ge sample and the alkali halide films. The electronic structure was studied using HeI and HeII radiation for UPS, and a twin anode with Mg and Al targets for XPS. All UPS data were taken in normal emission, i.e. we detected only excitations along the CDX direction of the fcc Brillouin zone (BZ). The UV photoelectrons and the backscattered electrons from EELS have been detected by an angle resolved hemispherical analyzer (LH GEA 21). The Ge sample was mounted on a transferable sample holder. This holder also included a thermocouple (Ni/Ni–Cr) connected close to the sample surface to determine the temperature, and a filament for heating located behind the surface. The (100)-oriented, p-doped Ge sample was polished by diamond pastes down to 0.25 mm grain size in several steps and cleaned by sputtering and annealing cycles (Ar+ sputtering at 300 K for 60 min at 800 eV and a crystal current of 1.5 mA, annealing up to 1100 K ) until optical LEED revealed a sharp (4×2) surface reconstruction at temperatures below 200 K. NaCl films of 2–3 ML thickness were prepared

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Fig. 1. He(II )–UP spectra in normal emission (top) and EEL spectrum (bottom; E =80 eV, 30° off-normal, specular direc0 tion) of a 3 nm thick salicylic acid film at 180 K. Lines represent results from fits (see text).

by evaporation from an Al O tube filled with 2 3 NaCl and heated by a tungsten wire at substrate temperatures of 170 K. By subsequent annealing to 630 K, well-ordered NaCl layers were obtained [6,21]. Similarly, KCl layers were evaporated onto the NaCl(100) films. As pointed out earlier [22], KCl on NaCl forms a regular array of stacking faults for up to three monolayers of KCl due to the large lattice misfit of 10%, whereas thicker layers grow with their own lattice constant, but the grain boundaries are no longer ordered. By evaporation of more than three monolayers of KCl at 200 K followed by slight annealing up to 300 K, we obtained a minimum in the FWHM of the LEED (00) spot of about 8% of the diameter of the BZ. At this point, it is important to note that this formation of small grains neither produces new electronic states nor does it change the width of the band gap [23], as seen by EELS.

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The water adsorption experiments have been done by isobaric adsorption at 5×10−7 Pa background pressure. Purity and partial pressure of H O were controlled by a quadrupole mass 2 spectrometer. The sample was exposed to SA at 180 K, using an Al O tube closed at one end, which was 2 3 mounted on a transferable holder. After purification of the SA by pumping with a turbomolecular pump for 6 h in the preparation chamber, the SA holder was transferred to a position in front of the sample. Typical exposure times to grow multilayers at 180 K substrate temperature were 30–60 s. Monolayer coverages were produced by annealing to 220 K. F centers were generated by electron bombardment from the EELS electron gun with typical electron energies of 100–200 eV and current densities around 100 nA/mm2. The electron gun was both used to record EELS spectra under quasistationary conditions and, at the higher current densities just mentioned, to perturb the layers. Thus the influence of current density and of temperature could be tested almost in real time.

3. Results 3.1. Salicylic acid on undistorted surfaces Fig. 1 shows UPS and EELS measurements for a condensed SA multilayer at T=180 K. The thickness was estimated to be more than 3 nm by the damping of the Na 1s substrate emission in XPS. Therefore, NaCl or KCl substrate excitations are not visible any more for UPS and EELS. The fact that no indication of radiation damage was found with these spectroscopies does not exclude electron-induced dissociation and desorption, but indicates that either electron stimulated desorption dominates strongly or that possible reaction products desorb before detectable amounts are accumulated in these layers. Out of the series of SA-induced photoemission peaks seen in the top panel of Fig. 1 we concentrate on those with kinetic energies above 24 eV kinetic energy ( labeled U1 to U4 in Fig. 1, upper panel ), which are most prominent and most specific to

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this molecule with a single aromatic ring. These are also more sensitive to different side groups than those with higher binding energies [24]. The assignment of peaks U1 to U4 can be attempted assuming that different groups of molecular orbitals have characteristic emissions which are clearly separable from others (composite molecule method [25]). The determination of unknown transitions in a molecule by composition of molecular fragments depends on the possibility of cutting the molecule at positions where the molecular interaction between different atomic groups is weak. This seems to be possible for organic acids. In our case the SA molecule (C H O ) is described as a 7 6 3 composition of formic acid (HCOOH ) and phenole (C H OH ). We can also try an assignment 6 5 by comparison with the isoelectronic antranilic acid, which has already been investigated by Meeks et al. [25]. Using this method, the emissions below 27 eV kinetic energy in the UP spectra represent excitations from the C–C and C–H orbitals of the aromatic system [24]. By comparison with the spectra of formic acid, phenole and antranilic acid, we conclude that U2 must contain both the p 0 emission, which is due to the phenolic hydroxy group, and in the right tail an additional contribution from the carboxylic oxygen (n). Due to symmetry reduction of the benzene ring by the COOH– and OH– groups of the SA molecule, the characteristic p excitation of the benzene ring splits into a

p and a p peak at 30 eV and 31 eV kinetic energy, 2 3 respectively, which are both contained in the U1 peak. For a characterization of modifications in the UP spectra in the monolayer and on surfaces with high concentrations of color centers described below, we fitted the spectrum shown in Fig. 1 by a set of Gaussians. The resulting peak positions and half-widths are given in Table 1. In the EEL spectrum in Fig. 1 four main loss peaks are shown ( labeled E1 to E4). By comparison with UV absorption measurements [26 ], the loss at 5.1 eV can be identified as a transition from the occupied p state to the unoccupied p* state of the aromatic system and the 4.0 eV loss as the corresponding CNO excitation in the COOH group. The other two losses E3 and E4 are also characteristic for organic acids, but without a definite assignment. In order to locate precisely the peak positions we fitted also this spectrum by Gaussians, i.e. we assumed again that the width is dominated by instrumental resolution. The result is given in Table 1. The increase of temperature to 220 K resulted in desorption of the multilayers of SA. Only the more stable monolayer remained on the surface. The resulting UP spectra are shown in the upper part of Fig. 2. Fitting these spectra also by Gaussians, a direct comparison of these spectra with those of the condensed multilayer was made. It turned out that the monolayer spectra could be

Table 1 Peak positions and full-widths at half-maximum (FWHM ) as obtained by He(II )–UPS and EELS of condensed salicylic acid on undistorted NaCl(100) and KCl(100) as shown in Fig. 1. Binding energies (BE) are given with respect to the valence band maximum (C ) of NaCl(100) and KCl(100), respectively. The binding energies for the physisorbed species are equal to the condensed species. 15 Results obtained by UV absorption spectroscopy ( UV-abs.) are given in the last line for comparison with the EELS data Condensed SA species – UPS and EELS results

Reference

UPS

U4

U3

U2 p ,n 0

U1 p ,p 2 3

BE (eV ) FWHM (eV )

−6.4 2.7

−4.6 1.3

−3.0 2.1

−0.9 1.7

EELS

E4

E3 p–p*

E2 CNO

E1

Loss (eV ) UV-abs.

7.0 —

6.05 6.11

5.1 5.24

4.0 4.10

this work

this work [26 ]

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Fig. 2. He(II )–UP spectra of the physisorbed monolayer (top) and of the chemisorbed submonolayer coverage (bottom) for salicylic acid on NaCl(100) ( left) and KCl(100) (right). The strong peak in the bottom spectra centered at 1.7 eV is due to the valence bands. Dashed and thin lines represent fits in order to separate the emission of the clean surfaces from the adsorbate induced features (see text).

perfectly fitted by keeping the separation between all peaks of the multilayer fixed, allowing only for a rigid shift of the whole spectrum so that it could be adjusted relative to the substrate valence band emission. Intensities and half-widths of adsorbateinduced peaks were also allowed to vary in the fits. Spectra of the clean surface were added to the fits as a whole, and their relative weight was adjusted. Fits of both the adsorbate- and substrateinduced parts are shown as thin and dashed lines, respectively, in Fig. 2 together with their sum. From the identical peak separations for multilayers and monolayers on both substrates, we conclude that SA remains molecular also in the monolayer. Relative intensities, however, change for all peaks U1–U4. Since our measurements only select a small angular segment, this may be caused by structural order, which changes the angular distribution of emitted electrons, and/or a change

of the cross-section for photoionisation due to electronic interaction with the substrate. Further annealing to 300 K reveals a third species, which is characterized by its different bonding strength in comparison to the physisorbed and condensed layers. This minority species is stable up to 450 K. XPS measurements of NaCl show that its coverage is close to the detection limit of 0.01 ML. The concentration of this chemisorbed species is considerably higher on KCl than on NaCl, as is obvious from the comparison of the bottom spectra of Fig. 2. This is caused by the higher intrinsic defect density of KCl compared to NaCl as found previously with LEED [8,9]. Again, fits of the spectra using the same procedures as just described show that all characteristic emissions of molecular SA are still present. A tendency for energetic shifts to higher binding energies with respect to the valence bands was obtained in the

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fits, which is expected due to the formation of stronger bonds. A quantitative analysis is only possible with higher concentrations of this species. As we will show below, color centers allow us to increase its concentration. 3.2. Generation of color centers and alkali clusters by electron bombardment Point defects, which are observed as loss peaks in the band gap region of both alkali halide spectra, can easily be produced by bombarding the surfaces with electrons. The energetic threshold for this process has not been investigated in detail, but it seems to be close to 30 eV [10]. As we will show below, they are chemically reactive. In earlier studies of electron bombardment on NaCl(100) layers, basically two defect-induced electronic losses in the band gap at 1.9 eV and 3.1 eV [7,10] have been found, which were interpreted as excitations of the bulk ( F ) and the surface F centers B (F ), respectively. The proposed mechanism was S that chlorine on the surface is excited and desorbed by an ESD process, while it is excited to interstitial sites in the bulk. Surprisingly, the ‘‘bulk’’ loss at 1.9 eV did not show up at temperatures lower than 130 K [11,27], so that this assignment seemed to be unsatisfactory. On the other hand, measurements by Roy et al. [28] indicate that it is possible to make Na clusters by electron bombardment using 500 eV electrons. It turns out that these results can easily be reconciled taking into account the mobility of alkali ions close to and above room temperature. Our experiments carried out at room temperature show clearly that extensive electron bombardment results in the combined formation of color centers, mainly on the surface, and alkali clusters. Only the loss peak assigned to the clusters grows as a function of time after prolonged bombardment. For the EELS results shown in Fig. 3, the NaCl(100) surface was bombarded at room temperature (RT ) with electrons of 195 eV energy and a current density of 140 nA/mm2. The EEL spectra were taken in time intervals of several minutes. After 2 min of bombardment (second curve) the typical losses at 2 eV and 3.4 eV developed. By further bombardment the peak at 2 eV

Fig. 3. EEL spectra of NaCl(100) at 300 K taken with an electron intensity of 140 nA/mm2 and an impact energy of 195 eV. BP, MSP, and SP denote excitations of bulk and surface plasmons from clusters of metallic sodium (see text). For better visibility, the spectra are shifted with respect to each other.

saturates. We assign this peak to F centers, in S agreement with Roy et al. [28]. The loss peak at 3.4 eV, however, grows with increasing electron exposure, and new peaks at 4.5 eV and 5.7 eV appear. Following the interpretation and results by Roy et al. [28], the peaks marked as BP and SP in Fig. 3 correspond to bulk and surface plasmon excitations in Na clusters at characteristic loss energies around 5.7 and 3.5 eV, respectively. Their energetic positions and their relative intensities change, which is natural as the cluster size is expected to change with increasing amounts of Na present on the surface. The energetic shift of the surface plasmon of Na clusters may also be partially due to a change in shape of these clusters. According to classical electrodynamics [29,30], the surface plasmon energy for spherical particles should be Bvsphere =Bv /E3, where Bv denotes SP BP BP the energy of the bulk plasmon. For deviations from this form, the surface plasmon energy increases up to the limit of a thin film

U. Malaske et al. / Surface Science 408 (1998) 237–251

(Bvfilm =Bv /E2). The shift of the surface plasSP BP mon energy might be due to a transition of the cluster form from spherical to a more disk-like form. For much higher intensities of irradiation (1011 electrons/lattice site), other authors [31] observe plasmon frequencies on NaCl single crystals still similar to ours on the alkali halide films. The shoulder which appears at 4.6 eV at high bulk plasmon intensities (denoted MSP in Fig. 3) is most likely due to a higher order loss of the surface plasmon (multipole excitation) that should have a loss energy of 0.8Bv , with v the bulk plasmon BP BP frequency [32]. The energy loss spectra of the KCl(100) surface in Fig. 4 can be interpreted in an analogous way. The major differences are that the coverage of F S centers (peak at 1.6 eV ) at the beginning of the electron bombardment is already higher than for NaCl (as mentioned above), and that the loss peaks are energetically closer to each other, so

Fig. 4. EEL spectra of KCl(100) at T=260 K taken with high electron intensity (40 nA/mm2) and an electron energy of 120 eV. BP, MSP, and SP denote excitations of bulk and surface plasmons from clusters of metallic potassium (see text). Spectra are shifted as in Fig. 3.

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that a possible shift of the surface plasmon (SP) peak cannot be resolved. The small loss located at 5.1 eV seems to be due to traces of dissociatively adsorbed water (see below), and is therefore not related to excitations in the metal clusters. In Table 2 the results of the alkali clusters of NaCl and KCl are summarized, together with those of Roy et al. [28] and Kunz [33], who made measurements of alkali films evaporated on graphite. As is obvious from Table 2, the excitation energies of our clusters vary between the values of clusters and the bare alkali films. This corroborates our interpretation of shape changes. At temperatures below 200 K, the reduced mobility of the alkali atoms strongly prevents the generation of metallic clusters and colloids, as is obvious from Fig. 5. As seen from this figure, the spectra are dominated by losses with maxima at 2.0 eV (NaCl ) and 1.6 eV ( KCl ), which have already been assigned to the characteristic losses of F centers, whereas the losses just assigned to S alkali metal clusters are at best present as traces in the tails of these peaks on the high energy loss side. It is worth noting that the loss peaks measured on both surfaces are considerably wider than the instrumental resolution, which indicates that there might be a mixture of different configurations of color centers. Indeed, a high resolution EELS investigation of NaCl(100) [27] done at a temperature of 100 K under similar treatment by electron bombardment was able to resolve four loss peaks in the energy range 1–2.7 eV, which were interpreted as losses due to generation of triples (R at S 1.2 eV ), pairs (M at 1.5 eV ), and two energetically S non-degenerate losses of single color centers [F S (1s–2p ) at 2.0 eV and, with low intensity, F x,y S (1s–2p ) at 2.7 eV ]. In agreement with the results z of Fig. 5, the dominant loss peak was found to be at 2.0 eV. Because of the lower resolution of our EELS instrument, we were not able to resolve these characteristic losses. Qualitatively, the same behavior was found for KCl, where the F S (1s–2p ) transition is located at 1.6 eV. For this x,y system no high resolution experiments are available yet, but also for KCl the existence of a loss due to the M center around 1 eV has to be S postulated to describe the measured peak in a fit satisfyingly. The origin of the long tails at energies

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Table 2 Excitation energies of bulk (BP) and surface plasmons (SP) for metallic sodium and potassium at T=300 K Excitation energies for plasmons (eV ) Sodium

Cluster Thick films

Reference Potassium

BP

SP

BP

SP

5.7 5.5 5.72

3.4–3.75 3.4 3.85

3.6 3.6 3.72

2.3 2.3 2.63

this work [28] [33]

following we test their chemical reactivity by adsorbing water and SA. 3.3. Adsorption of water and salicylic acid at defects

Fig. 5. EEL spectra of NaCl(100) (top) and KCl (bottom) with high coverage of F and M centers at temperatures below S S T=200 K.

between 3 and 4.5 eV for NaCl and between 2 and 3 eV for KCl cannot be clearly identified, but, as mentioned, there might be a small probability for production of very small alkali clusters at the measuring temperatures around 180 K used in this experiment, in contrast to the HREELS experiment done at 100 K. While details of kinetics and assignment of these losses will be presented in ref. [27], the essential result of the experiments just described is that we are able to produce a layer that contains a high concentration of F centers as the main defects by electron bombardment at energies below 200 eV and at sufficiently low temperatures. In the

From previous investigations [10] we know that water exposed to a NaCl surface, which was bombarded with electrons of similar energy as used here and at a surface temperature of 130 K, adsorbs dissociatively at the color centers, filling them with OH−. We carried out similar measurements at room temperature. In order to avoid the decay of color centers by metal cluster formation, we bombarded the NaCl(100) surfaces with 120 eV electrons at a current density of 70 nA/mm2 and at a surface temperature of 300 K in a background pressure of 5×10−7 Pa of H O. The same type of 2 measurements was also carried out for KCl(100). As a comparison of the UP spectra for both surfaces shows (see Fig. 6), water dissociates at color centers produced by the electron beam on KCl(100) as well. In both spectra the emissions from 3s and 1p orbitals of OH were identified by a comparison with the bare alkali halide surfaces and a deconvolution carried out by fitting the data with Gaussians. The binding energies are given with respect to the valence band maximum C of each 15 salt [34,35]. As demonstrated in Fig. 6, all features in both spectra can be perfectly interpreted if peaks induced by the He( II ) satellite with 48.2 eV photon energy are taken into account too. However, the small emission at −9 eV binding energy in the OH−/KCl spectra, which looks like split off the 3s emission, cannot be interpreted so easily. The majority of species formed on both surfaces are

U. Malaske et al. / Surface Science 408 (1998) 237–251

Fig. 6. UPS–He(II ) spectra of NaCl(100) (top) and KCl (bottom) covered with OH− ions at previously generated color centers. Shadowed peaks are induced by OH−, as obtained from fits (thin lines) using the spectra of the clean surfaces. VB X ∞ denote the edges of the substrate valence bands. He(II ) 4,5 sat. are peaks induced by the 48 eV satellite. Excitations due to He(I ) are visible at the far left end.

clearly different from NaOH and KOH, respectively, as seen from the energetic differences between 3s and 1p peaks, DE . DE for s−p s−p NaCl(100) is 4.8 eV, significantly greater than DE =4.2 eV for NaOH [36 ]. This difference s−p deviates even more for OH− on KCl(100): our measured value is 5.4 eV, compared to 4.0 eV for KOH [37]. This indicates that, instead of formation of alkali hydroxide on the surface, isolated ionically bound OH− ions are formed at the F center, which are surrounded by alkali ions. This conclusion agrees with the results from calculations [38] of the diffusion barrier for OH− ions on the NaCl(100) surface (E =3.5 eV ). diff Interestingly, the emission feature in the KCl–OH spectrum at −9 eV binding energy (see bottom of Fig. 6) is not observed in the NaCl–OH spectrum. Its appearance is clearly separated from the K 3p emission induced by He(II ) satellite radiation. An assignment to a possible 3s emission of potassium hydroxide is also not plausible, because we do not observe the corresponding p emission, which should appear in the intensity minimum between the KCl valence band and the

245

OH− 3s peak (DE =4.0 eV ). An explanation s−p for the occurrence of this peak is possible, if we take into account the higher cross-section for color center generation on KCl compared to NaCl (see Fig. 5). This property of KCl also enhances the probability of MS center formation during electron bombardment on the alkali halide surfaces. These MS centers have to be filled by two OH− ions, which see an environment closer to a stoichiometric hydroxide, but still significantly different from it. Therefore, it seems to be a plausible explanation that we observe a split 3s state due to strong lateral interaction of two neighboring OH− ions, whereas the corresponding p state is still contained in the strong valence band emission. We have also carried out EELS experiments at these OH− covered surfaces. Instead of the typical color centers and plasmon losses we found a new peak at 2 eV above the exciton loss for both NaCl( l00) and KCl(100), as shown in Fig. 7 in the middle spectrum. This loss is due to the electronic excitation in OH− from the occupied 1p to the unoccupied 4s state. This indicates that we have saturated all generated color centers with adsorbed OH− on both surfaces in our experiments. The influence of color centers on SA is less dramatic than on water. Binding of SA is much stronger at color centers than on the perfect surfaces, but the SA molecule stays intact as far as we can judge from UPS and EELS. The corresponding EELS spectra are shown in the top panel of Fig. 7. For the data shown, we bombarded the surfaces with electrons of 120 eV at a substrate temperature of T=190 K until the characteristic loss peak of F centers was saturated, exposed S them to 50 Langmuir of SA, and annealed them subsequently to 300 K. In contrast to the undistorted surfaces, these layers turned out to be stable at this temperature. Compared to the undamaged surfaces, where we found only traces of SA left at 300 K (see Section 3.1), the coverage of SA is at least one order of magnitude higher on the surfaces bombarded with electrons, as judged from the intensities measured in EELS and in UPS. From these findings it is obvious that the SA species which is still stable at and above 300 K must be

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Fig. 7. EEL spectra for NaCl(100) ( left) and KCl(100) (right) with different treatments. Top: adsorbed SA at F centers; middle: S dissociative water adsorption at F centers; bottom: SA adsorbed at OH− centers. T=300 K. S

adsorbed at defects, especially at the color centers created by our treatment. The UP spectra (not shown) look almost identical to the monolayer spectra of physisorbed SA. In particular, all characteristic peaks seen there can again clearly be identified. In comparison with the SA physisorbed at lower temperatures we only observed a slight energetic shift of 0.3 eV to higher binding energies of the p-ring emissions with respect to the valence bands. Therefore, it seems that the molecule remains intact. From the EEL spectra (see Figs. 1 and 7) of SA the same conclusion is drawn. All peaks characteristic of the SA molecule seen in Fig. 1 also appear in Fig. 7. Only a slight shift of the three peaks E1–E3 to higher loss energies by 0.1 eV was found for SA at color centers on both salts, which we further on call chemisorbed, compared with the condensed layer. The peak positions and half-widths for SA chemisorbed at color centers are collected in Table 3 for

Table 3 Energy losses DE and the FWHM of the SA adsorbed at F S centers. (T=300 K ) SA at color centers – EELS results

DE FWHM

( E4)

( E3)

p–p* ( E2)

CNO (E1)

7.0

6.10 0.9

5.2 0.8

4.1 0.6

both salts. The broadening of the peaks in the chemisorbed SA by 30–40% compared to the physisorbed species can be explained by the enhanced electronic interaction of the SA with the band structure of the substrates. Even if the color centers are filled with OH−, they can still act as point defects at the surface, because the OH filled color centers are electronically not fully equivalent to the perfect surfaces.

U. Malaske et al. / Surface Science 408 (1998) 237–251

However, the OH− centers are expected to be less reactive than the bare F centers, because the S charge is mostly concentrated at the hydroxide ion. In agreement with this expectation, water, for example, was found not to dissociate any more at the OH centers, but is more strongly bound there than the physisorbed species [11]. Similar behavior is found for SA. Even with OH filled color centers, about the same amount of SA as on the surface with bare color centers was found to be stable at and even above room temperature. The positions of the loss peaks E1–E3 in Fig. 7 (bottom) are identical to those found for the adsorption on the bare color centers, whereas the half-widths, as obtained from fitting the EELS curves, are now only 10–15% wider than for the physisorbed species. This indicates that also in this case SA is directly coupled to the point defects filled with OH. As is seen from the reduced halfwidths of the EELS peaks, the chemical interaction between SA and the defects has become weaker, which may partly be compensated by the much larger dipolar interaction due to the strong dipole of the OH− ion. Interestingly, we cannot identify any more the loss peak due to OH after adsorption of SA in the EEL spectrum. This finding does not allow us to answer the question of whether the SA molecule as a whole is attached to OH filled color centers or the acidic group of SA replaces the OH in the color center. The fact that we observe slight changes between SA adsorbed at bare color centers and at OH filled centers favors the first possibility, i.e. we think that the molecule is still intact. For the case of SA adsorption at bare color centers we therefore suggest a bonding of the carboxylic oxygen with the anion vacancy after detachment of its hydrogen atom, analogous to the dissociative water adsorption. In the case of the adsorption at the OH− ion, the SA molecule could be bonded mainly by van der Waals forces to the surface dipole. This weaker bonding is reflected also by the lower desorption temperature for SA/OH (400 K ) in contrast to about 500 K for the OH− ions. 3.4. Energy diagram All obtained data are summarized in the energy diagram shown in Fig. 8. The valence and conduc-

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Fig. 8. Energy diagram deduced from our data including defect and adsorbate states. Filled symbols mark occupied and unfilled symbols unoccupied states. Shaded vertical bars indicate the lowest possible electronic excitations in the presence of OH and SA. E1 and E2 are the lowest transitions within the SA molecule.

tion band edges of Ge(100), bare NaCl(100), and of bare KCl(100) are shown with respect to the vacuum level, as obtained from our UP spectra. The edges of conduction and valence bands for the alkali halides are indicated by the dashed lines. Solid lines mark the high symmetry points of the valence band, where the maxima of the density of states are located [34,39,40]. One obvious question to be answered is the size of possible band bending in both NaCl and KCl films due to photon-induced charging. Because we evaporated KCl on NaCl, this problem is more severe for KCl than for NaCl. Thick layers (H >8 ML) show for KCl a NaCl+KCl shift of up to 2 eV between valence band level and vacuum level [6,7], which is reduced by increasing the temperature from 100 K to room temperature. This shows that also the films can be electrically charged up to the breakdown fields, which we estimate from these findings to be around 107 V/cm. On the other hand, since for the results collected in Fig. 8 we worked with 3 ML KCl evaporated on one double layer of NaCl, we get a maximum shift of the energetic positions of less than 0.5 eV for KCl, and of 0.1 to 0.2 eV for

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NaCl. These small shifts do not influence our main conclusions drawn below. The energy diagram in Fig. 8 shows further the occupied and unoccupied states of the OH− centers (filled and open circles, respectively). The boxes represent the occupied and unoccupied states for SA. For the sake of clarity we plot only the two transitions with the lowest energies of excitation, E1 and E2. The locations of the unoccupied states directly follow from the UP spectra of the occupied molecular states, and the characteristic energies in EELS together with the assignments made there. Therefore, we will not repeat them here. For the discussion about contact charging conducted later, however, we note already here that in the presence of OH, the gap between the highest occupied and the lowest unoccupied state is reduced to 5.1 eV for NaCl–OH and to 4.9 eV for KCl–OH, whereas the threshold energy remains constant. With adsorbed SA, this gap is even further reduced to 1.2 eV on both systems.

4. Discussion Our experiments give direct evidence for the common trend, that the surfaces of wide band gap insulators become chemically reactive only by introduction of defects. There are several general properties of adsorption at defects on these surfaces, which we discuss first. It seems that we can distinguish between a true chemical activity and an enhanced dipolar (or multipolar) interaction caused by the defects. The dissociation of water at the color centers produced by the electron bombardment is a clear example of a chemical reaction induced by the presence of point defects. In the case of molecular adsorption of SA, on the other hand, there are only minor signs of a ‘‘classical’’ chemical interaction, i.e. electron redistribution and shifts of peaks relative to each other in the electron spectroscopies used in our experiments. Instead, we see only small global shifts and changes of half-widths, indicating a modification of coupling strength to the substrate relative to the molecule physisorbed on the perfect surface. The physical reason for this finding could be simply an enhanced dipole–dipole interaction between the electron in the color center and the

functional group(s) of the SA molecule. This mechanism of enhanced dipolar interaction would still be active after filling the color centers with OH, since the OH− ion clearly has a much higher dipole moment than the Cl− of the perfect surface. This would also explain why the adsorption strength does not change very much after filling the color centers with OH. At this point it remains an open question whether the increase of coupling to the substrate compared to the adsorption on the perfect surface would be sufficient to explain the increase of half-widths, especially in EELS, but the experiments show the correct trend. Some additional contribution of electronic interaction between the surfaces and the SA molecules in the monolayer obviously remains still, since in EELS we see a change in half-width depending on the location to which the molecule is bound (perfect surface, F center or OH filled F center). If the dipolar mechanism were important in general for molecular adsorption on insulator surfaces and their defects, it should not be limited to adsorption at color centers, but would be effective at all kinds of defects which enhance dipolar interaction. This can happen also at steps, kinks, dislocations and grain boundaries. As mentioned at the beginning in the experimental section, KCl layers grown on the NaCl surface contain grain boundaries and dislocations. These, however, do not show up either in UPS or in EELS as splitting states. Instead, we found even on the KCl layers a nearly perfect band gap. Nevertheless, they could still enhance a van der Waals type of interaction. The concentration of these defects, however, should still be of the order of 1% on KCl [9]. From an agreement of the amounts of strongly bound SA and the density of color centers produced on this surface, it is clear that on KCl the latter type of defect does not change appreciably the amount of strongly bound SA. However, the precision of our measurements is not high enough to exclude their effectiveness in binding SA. The fact that other defects may also play a role is obvious from surfaces annealed to 550 K, at which all color centers are annihilated and the surplus of alkali atoms is desorbed. These surfaces are still able to bind an amount of SA at room temperature that is significantly enhanced over previously undamaged surfaces, but again no characteristic

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losses in the band gap are visible in EELS from these surfaces. The result that in SA electronic transitions to energetic levels seem to be possible that are located below the Fermi level of the Ge substrate, but appear as unoccupied in EELS, is puzzling at first (see Fig. 8). It is obvious that we cannot explain this finding by band bending either in the substrate or in the insulating layers. For the insulating layers we already estimated above that band bending would only account for an energetic shift of less than 0.5 eV. Band bending in the substrate can be excluded since we see only the electrons emitted from the topmost layers of the substrate, and we have calibrated our spectra with these emissions. Please note, however, that the measured losses are characteristic for the neutral SA molecule, whereas the transition of an electron from the Ge substrate can only take place to the affinity level of a negatively charged SA ion. Estimates from the location of this level in similar organic acids [41] show that this level must be at least 8 eV above the occupied states of the neutral molecule so that it cannot be occupied without additional activation, in agreement with our findings. We turn now to the problem of contact charging, which according to our opinion is intimately related to the existence of defects at the surfaces of insulating materials, and even only possible in their presence for wide band gap material, as we will show. We first discuss (hypothetically) perfect (100) surfaces of NaCl and KCl. For electrons to become mobile, which is necessary for any kind of charge exchange, they have to be excited to previously unoccupied electronic states. Since there are no surface states available on these surfaces, the lowest possible excitation is to the excitonic states, which are located 7.4 and 7.7 eV above the valence band edge, respectively. The excitation probability in thermal equilibrium close to room temperature (300–400 K ) is therefore practically zero, so that any charge exchange between different crystals in contact with each other is impossible. Hence, it is obvious that the effect of contact charging cannot take place on the perfect stoichiometric (100) surfaces of either KCl or NaCl. As a consequence, the Fermi levels in this situation can be pinned by an arbitrarily small amount of impurities with

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occupied states in the band gaps, but these would not allow any systematic behavior. This situation is strongly altered by introducing a high concentration of either defect- or adsorbateinduced empty states in the bad gap. The defect states are more or less chemically reactive, as also shown by our experiments, so that they will be quickly filled by adsorbates in an atmospheric environment, and the adsorbate-induced electronic states will only be relevant. This situation is well illustrated by our experiments for the systems investigated here, although they were carried out at much lower pressure. In order to make close connection with our experiments, we concentrate on the situation close to room temperature, where, as mentioned, the concentration of adsorbate molecules is determined by the defect concentration. Similar temperatures are also used in the technological process mentioned. As seen from our experiments (see Fig. 8), adsorbate-induced empty states are able to strongly lower the activation barrier necessary to make electrons mobile. Of course, they are still confined to the surface states of the adsorbate molecules, but can now hop from one empty state to the next. In the experimental systems investigated here, the average concentration of molecules responsible for the unoccupied states at room temperature is only of the order of 10%, so that band formation is not effective, and the transport of electrons is due to hopping. However, a thermal equilibrium distribution at the surface according to Fermi statistics can now be much better defined than in the previous situation, since it involves a finite probability of occupation of states that are empty at T=0. Thus local charge exchange is possible now in the presence of the adsorbate molecules. This process does not require the formation of isolated ions, but only the formation of dipoles oriented parallel to the surface, which have a much lower energy of formation compared to the isolated ions. If adsorbate covered surfaces are now brought into contact, the hopping process will be extended to molecules adsorbed on both surfaces. Since we are now able to exchange electronic charge and to establish thermal equilibrium, Fermi levels at both surfaces can be established, if true thermal equilib-

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rium can be reached. The process, although confined to the interface, would then be completely analogous to the charge exchange in metals, i.e. the Fermi levels EA and EB are originally at F F different energetic positions before making contact with regard to the common vacuum level E . vac This is schematically illustrated in the left and middle part of Fig. 9. In contact, there will be a charge transfer from system A to system B until the combined surfaces are in thermodynamic equilibrium (EA =EB ) and the Fermi levels are at the F F same energetic positions (see Fig. 9c). Due to the net charge transfer from one side to the other, a contact voltage appears outside which is given by U =(WA−WB)/e. However, the charge exchange C in our case can only occur by hopping, which is a slow process compared with charge exchange between metals (see Fig. 9), and which is confined to the thin adsorbate layers. If the materials are now separated mechanically, there is a high probability that the charge exchange back to the original situation of the separated materials is incomplete. If thermal equilibrium is not reached completely, the direction of net charge exchange is still determined by this model. We point out here that this proposed scenario needs as essential ingredients the formation of defects at the insulating surfaces, which are able to form a bond with adsorbed molecules that is strong enough to guarantee the presence of the molecules at the relevant temperatures on the surface. The properties of the defects and their relative concentration together with the electronic properties of the adsorbed molecules determine the size and sign of the contact charging effect. Contrary to the assumptions made in the litera-

ture [42,43] that cation vacancies are responsible for the charging effect, we show that anion vacancies (but also other defects still to be identified) are effective in binding surface molecules with sufficient strength, which provide the empty surface states to allow charge transfer between different materials. According to our model just outlined, and according to our determination of the energetic levels (see Fig. 8), KCl should be negatively charged with respect to NaCl. The reasons are that before contact the local Fermi level is energetically lower on KCl than on NaCl with regard to the vacuum level E . Secondly, the higher crossvac section for vacancy formation enhances the charge transfer due to the higher density of states produced by the higher concentration of defects on KCl, i.e. the corresponding concentration of strongly adsorbed SA at these defects. This sign of contact charging was indeed found technologically [16 ]. The latter property, of course, is specific to color centers, but is obviously the same as for those defects produced technically during grinding the raw salts. However, the interaction strengths with other kinds of defects might be very different on the surfaces investigated here, so that even a reversal of sign in charging is possible. A sign opposite to that indicated by our scheme shown in Fig. 8 was actually found in the technical process with pure water as conditioner at elevated temperatures between 590 and 750 K [44]. This finding shows that the investigation of just one kind of defect – color centers – is not sufficient to explain all details of contact charging, but does not necessarily contradict the proposed model. Instead, we conclude that the specific interactions of other sorts of defects are worth investigation since they may be different from those found with the color centers.

5. Conclusions

Fig. 9. Model of the contact charging for insulators. (a) Bare defect-free NaCl and KCl, for which Fermi levels are not well defined. (b) The boxes in the band gap denote occupied and unoccupied adsorbate induced surface states. (c) Insulators in contact with a contact voltage U . C

The adsorption properties for water and salicylic acid on the (100) surfaces of NaCl and KCl with and without the presence of a concentration of color centers – used as model defects – of approximately 10% of a monolayer were studied by UPS and EELS. Our experiments directly illustrate the

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dominant role of defects for the chemical properties of insulating surfaces with wide band gaps, at least for those insulators without surfaces states. Adsorbed molecules provide additional occupied and unoccupied electronic states at the surfaces, as shown for the two molecules investigated here. Defects can induce chemical reactions, e.g. the dissociation of water, as shown. The unoccupied states of these molecules in particular have the important property of lowering the electronic activation energy necessary to make electronic transport along the surfaces possible and to allow charge transfer at the interface of insulating materials once brought into contact. We propose this property to be essential for the phenomenon of contact charging. The mobility of electronic charge due to thermal excitation to these low lying electronic states makes Fermi level equilibration possible at the interface, while during mechanical separation of the insulators the hopping mechanism for charge exchange is slow enough so that the reversal of charge exchange may remain incomplete. Within this model, the sign of contact charging depends mainly on three parameters: the relative concentration of the adsorbed molecules on both surfaces, the location of the vacuum level on the clean surface relative to the valence bands, and the interaction strength of the adsorbed molecules with defects, which is able to shift the electronic levels of the adsorbate.

Acknowledgements Support by the Deutsche Forschungsgemeinschaft and by the Kali und Salz AG is gratefully acknowledged.

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