Double electric layer at the LiBrNaCl interface

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Double electric layer at the LiBr/NaCI T Lewowski, P Mazur and P Wieczorek, Borna 9, 50-204 Wroclaw, Poland

institute of Experimental

48lnumber

interface

Physics, University

of Wroctaw, pl. Maxa

The effect of formation of the potential barrier at the interfaces of LiBr/NaCI and NaCYLiBr was investigated using the method of ultraviolet photoelectron spectroscopy (UPS). This barrier is created by the displacement of positive and negative ions from their equilibrium positions in the boundary region. The measured barrier height is 0.4 eV and when NaCl is evaporated on LiBr it has positive sign (electrons are accelerated in this barrier). For the system LiBr/NaCl/LiBr effective barrier height is equal to zero, as the barrier at the NaCI/LiBr has an opposite sign. The results could be important for technology of protecting of semiconductor devices using insulating layers. 0 1997 Ekevier Science Ltd. All rights reserved

Introduction Vacuum evaporation of alkali halide crystals goes principally through emission of single polar molecules (for example NaCl) with permanent dipole moment of order of several Debyes. Obviously, other mechanisms, such as dissociation of molecules and emission of dimers, trimers or bigger clusters during the evaporation process, are weak and may be eliminated by proper evaporation technology. While condensing on the surface of ionic crystal, polar molecules build an epitaxial layer, if the lattice mismatch of both substances does not exceed 20%. The positive ions of adsorbed molecules are placed above the anions, and the negative ions above the cations of crystal lattice. Thus, the (100) plane acts as interface. Owing to the difference between sizes of ions in substrate and those in surface monolayer, the centres of positive and negative ions in the interfacial region can be displaced from their normal positions and situated on separated crystal planes,’ as shown schematically in Figure 1. Therefore, as the first monolayer is formed, a double electric layer may be generated at the interface of both substances. The value of the ‘dipole moment’ of such a layer taken in the direction normal to the substrate surface is proportional to the difference between the sum of ionic radii of metal and halogen. In these considerations, only the direction normal to the surface component of the dipole moment should be taken into account. If the value of the sum of ionic radii of ions B and C (see Figure l(a)) is lower than those for ions A and D, the electric field strength is directed outwards the surface of the substrate, and when it is higher, towards the surface (Figure l(b)). If both sums have equal values, the centres of metal and halogen ions at the interface are arranged on the same planes, similar to the inside of a bulk crystal, as was observed during the deposition of NaCl on a LiJ substrate.’ Investigations of interfaces between LiBr and other alkali halides are very interesting owing to the fact that this compound, when vacuum deposited on a silicon (100) surface, grows epitaxially at

deDosited

monolaver

substrate full circles - halogen ions empty circles - metal ions

a

deposited monolayer

substrate

full circles - halogen ions empty circles - metal ions Figure 1. Displacements

b

of ions at the interface of two alkali halide

crystals.

room temperature. We suppose that this salt when used as a very thin interlayer between silicon wafer and protecting films would reduce mechanical stresses between them. NaCl was chosen for our investigations because of its lattice constant value, which is very close to those for LiBr. On the other hand the difference between the sums of ionic radii (Li + Cl) - (Na + Br) is sufficiently 305

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et a/: Double electric layer at the LiEWNaCI interface

high (0.055nm) to induce the above described effect. Figure 2 shows a sketch of ion arrangement at the interface of LiBr and NaCI; clearly the electric field in this case is directed towards the surface of the substrate. Such a field can be detected using electron energy analysis of electrons transmitted through the interface in the direction normal to its surface. When electrons pass through such a double layer their kinetic energy should decrease, and the inverse effect should be observed when an electric field has an opposite direction. The change of electron energy can be measured using electron energy spectroscopy.

Experimental The production and measurements of thin film systems were realised in UHV conditions using apparatus and methods described elsewhere.3 The Ag photocathode, about 100 nm thick, deposited by vacuum evaporation on a sapphire substrate, was applied as the source of electrons traversing the studied interface. Photoelectrons were excited from the cathode by 4.8 eV photons separated from a mercury lamp spectrum. The energy of these photons is insufficient for excitation of other electrons, particularly from investigated alkali halides. In such conditions only electrons escaping from Ag photocathode and transmitted through alkali halide layers to vacuum can be detected. As zero kinetic energy, the Fermi level of silver was chosen. On the surface of photocathode a thin (g 15 nm) layer of LiBr was vacuum deposited and the energy spectrum of electrons passing through this layer was registered. Then another layer (Z 5 nm of NaCI) was evaporated and the electron energy spectrum was registered again. This procedure was repeated several times for consecutive layers of LiBr and NaCl (each 5 nm thick). All evaporations and measurements were performed at room temperature.

Results and discussion In Figure 3 we present a typical example of a set of measurements of photoelectron energy distribution curves and the changes after deposition, by turns, of thin NaCl and LiBr layers. The solid line in Figure 3(a) represents a Ag photocathode covered with 15 nm of LiBr, the dashed line represents the same photocathode with a supplementary 5nm thick layer of NaCI. A change in the spectrum shape owing to the deposition of NaCl is distinct and reveals as the shift to the left side of the low energy part of the spectrum equal to about 0.4eV. Simultaneously, an increase of

ENERGY

[ eV]

Figure 3. Changes of photoelectron energy spectra after deposition successive layers of NaCl and LiBr. For details, see text.

of

the emission intensity is observed. After deposition of next layer of LiBr (5 nm thick) the intensity of emission decreases and the spectrum shape becomes similar to those typical for Ag/LiBr photocathode, as is shown in Figure 3(b). An inverse effect is observed when the third layer (5 nm of NaCI) is added (Figure 3(c)). The intensity of emission and the spectrum width increases again. In Figure 3(d) the spectrum for photocathode {Ag+ 15 nm LiBrJ is compared to that of {Ag+ 15 nm LiBr+ 5 nm NaCl+ 5 nm LiBr}. Both spectra are very similar and the difference between them is due to the attenuation of the electron flux in a thicker layer. The same value of the electron energy shift equal to 0.4 eV was observed for the system LiBr/NaCI deposited on a Si( 100) plane.4 The broadening of the electron energy spectrum and changes in the emission intensity could be explained by the influence of a double electric layer (formed at the interface between two alkali halides in the mechanism described above) on electron motion in this region. The ‘dipole layer’ change in the kinetic energy of transmitted electrons, which are accelerated (on a LiBr-NaC1 interface) or retarded (on a NaCl-LiBr interface) by the electric field. This also influences the electron emission intensity, as the external potential barrier at the interface of alkali halide layer vacuum is more transparent for more energetic electrons. It is very interesting that a double electric layer is formed at the interface of LiBr and NaCl in spite of fact that the difference of lattice constants of those two alkali halides is very small in a comparison with other alkali halide pairs, and does not exceed 3%. The epitaxial growth in such conditions should be easy. However, the displacements of the ions creating a dipole layer occur in a very narrow region of the interface and only in the direction normal to the interface. Similar effect was observed on free surface of ionic crystals (this phenomenon called ‘surface distortion of ions’ was reported by Benson and Claxton’).

Conclusions Ag photocathode .__________________. _._..~.................... -~-------------Figure 2. Arrangement of ions at the LiBr/NaCl interface. 306

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The potential barrier at the interface of two ionic crystals is created by displacement of ions from their equilibrium positions

TLewowski

et al: Double electric layer at the LiBr/NaCI interface

in the boundary region, even when the difference of lattice constants of both crystals is small and epitaxial growth is preferred.

References

Acknowledgements

3. Lcwowski: T., Thin SolidFihns, 1995, 259, 53. 4. Saiki, K., Nakamura, Y., Nishida, N. and Gao. W., Koma, A., SurjI

1. Lewowski, T., Grygorczyk, R. and Kisiel, W., Surf. Sci., 1977, 64, 732. 2. Lewowski, T. and Mazur, P., Acta CJnic. Wratislaoiensis, 1986, 937, 177.

This work

was sponsored

by the Polish

State Committee

Scientific Research within Project No 2 P03B 138 09.

for

Sci., 1994, 301, 29. 5. Benson. G. C. and Claxton, T. A., J. Chem. Phys., 1968,48,

1356.

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