F+-desorption mechanism from a CaF2(111) surface by low-energy electron irradiation

Surface Science Letters 253 (1991) L407-L410 North-Holland

IA07

Surface Science Letters

F+-desorption mechanism from a CaF2(lll ) surface by low-energy electron irradiation Kouji Miura, Kazuhiko Sugiura and Hiroshi Sugiura Department of Physics, Aichi University of Education, Igaya-eho, Kariya, Aichi 448, Japan Received 23 February 1991; accepted for publication 23 April 1991

Electron stimulated desorption (ESD) of F ÷ ions from the (111) surface of an epitaxially grown CaF2 film is observed in the incident electron energy region above 28 eV. The threshold energy of ion desorption corresponds to the binding energy of the Ca 3p energy level in the CaF2 crystal. ESD of F + ions arises from the interatomic Auger decay of a Ca 3p core-hole. The energy produced by the core-hole decay turns out to be used for the formation energy of a new level near the Fermi level and the kinetic energy of the desorbed ion. The kinetic energy distribution of the desorbed ions can be expressed as a convolution of the Ca 3p and F 2p spectral functions deduced from photoemission data.

Defects such as color centers and halogen deficiencies are known to exist in alkaline earth fluorides. Recently, alkaline earth fluorides (CaF 2, BaF2, SrF2) were also noted as the most promising candidates for achieving the long-term goal of building three-dimensional integrated circuits. Therefore, especially from a technological point of view, the influence on electronical characteristics of defects formed in the crystal is considered to be of great importance. However, the detailed mechanism of defect formation has not been well understood. There have been several studies on halogen deficiencies formed in CaF 2 crystals by electron and photon irradiations [1-4]. It was shown by Karlsson et al. [1] that the new structure induced by ultraviolet irradiation is created near 0.5 eV below the Fermi level and forms a metallic band. Also, this structure was observed by the low energy electron loss spectroscopy (EELS) experiments of Saiki et al. [2] and Miura et al. [3,4]. The new state created near the Fermi level by the electron irradiation forms a metallic band due to ordered surface F-centers. However, the high-energy electron diffraction ( R H E E D ) pattern from the C a F 2 ( l l l ) surface with a metallic band shows a very sharp 1 × 1 order structure, which indicates

that there is no disorder of the atomic structure of the surface by electron irradiation [4]. In addition, ion scattering spectroscopy (ISS) studies exhibit that the ordered surface F-centers are formed at the third layer without an accompanying change of atomic structures of the first and the second layer of the surface [4]. The conservation of the atomic structure of the surface is considered to be due to the formation of surface F-centers resulting in charge neutralization of the surface. The Knotek and Feibelman ( K - F ) mechanism [5,6] is well known for desorption processes from materials such as ionic crystals. Since in materials such as CaF2, charge transfer from the metal atom to the halide atom arises, its valence band is known to consist of orbitals of fluorine atoms. Accordingly, if the desorption of surface fluorine atoms arises from the K - F mechanism, the threshold energy for desorption m a y turn out to be the binding energy of the Ca 3p or F 2s core levels. At present, there is little experimental observation of the threshold energy. In this Letter, using quadrupole mass spectroscopy (QMS), we shall report in detail on the desorption mechanism of fluorine, resulting in the determination of its sign (F °, F - , F+), the

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K. Miura et a L / F '+desorption mechanism from CaI'T(I l 1) studied bY ESD

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threshold energy and the kinetic energy distribution of the desorbed gas, respectively. All experiments for sample preparation and electron stimulated desorption (ESD) were carried out in situ in an ultrahigh vacuum chamber (ANELVA VA-I) operating at a base pressure below 1 × 10 9 Torr. Thick CaF 2 films epitaxially grown were made on a clean S i ( l l l ) surface by molecular beam epitaxy (MBE) of CaF 2. It was confirmed by ion scattering spectroscopy (ISS) that the outermost layer of the surface is formed by F atoms. The specimen was mounted on a rotatable sample holder. The spacing between an electron gun and a specimen was about 2 cm. The incident angle of electrons was about 30 ° from the surface normal. Desorbed ions were detected at right angles to the surface by QMS (ULVAC MSQ-150A), which was operating without filament emission for measuring only the + 1 charged mass. Also, the desorbed ion kinetic energy distribution was measured by supplying a retarding voltage to the grids situated in front of the specimen. The specimen was heated to about 150 ° C by a halogen lamp to avoid electronic charging during measurements, but the data obtained were almost identical to those obtained without heating. Shown in fig. 1 is the total ion yield from the C a F 2 ( l l l ) surface, induced by an incident electron energy of 40 eV. Peaks due to H 2 and F + ions were observed. With the increase of incident electron energy,

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both peaks due to H 2 and F + ions drastically increase. Also, the F + ion yield from the CAF2(111) surface was measured as a function of incident electron energy and is shown in fig. 2. The initial increase of the yield is at about 28 eV and the yield prominently enhances as the incident energy increases. The spectral features for a heavily irradiated surface were almost the same as for the above spectrum. It is known from photoemission spectroscopy [1,7] that the C a 3 p core-level and F 2p valence band for CaF 2 are respectively above 28 and 32 eV. While it is known that there exists the case [8] that both the anion and cation desorb at the same time, in this experiment only fluorine as anion desorbs as a F + ion. In fig. 3, we note that the threshold energy of the F + ion corresponds to the binding energy of the C a 3 p corelevel. Shown in fig. 3 is the electron stimulated desorption ion kinetic distribution (ESDIKED). The kinetic energy distribution of desorbed F ÷ ions was measured by the retarding method. The energy integrated spectra obtained are shown in fig. 3. These spectra do not significantly change as the incident energy increases. Shown in fig. 4 is the kinetic energy distribution in case of an incident electron energy of 40 eV, which was obtained by smoothing the obtained energy integrated spectra and differentiating numerically. The spectrum is distributed in a range from 0 to 10 eV and has a m a x i m u m at about 4 eV. The E S D I K E D did not vary with increasing incident electron energy up to 200 eV. Also, the E S D I K E D from a heavily irradiated surface did almost not vary. We propose the core-hole Auger decay model originating from a Ca 3p core-hole, as illustrated in fig. 5. First, 3p holes are created in the Ca atoms. Since there are no valence electrons on the Ca's in CaF 2, the dominant channel for 3p hole decay comes from valence band electrons formed by the F 2p level. The maximal energy obtained by the Auger process is the energy difference ( E c , 3pEF2p) , where Eca3p and E F 2 p a r e the energy levels of C a 3 p and F2p, respectively. This energy excites a fluorine atom as: F--+F*+e

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previous papers [1-4], the new level near the Fermi level was shown to be created by low-energy irradiation. Therefore, the electrons produced by the Auger process are trapped by this new level (defect level; see fig. 5) above the valence band and form a metallic band near the Fermi level, due to the ordered F-centers produced by F ÷ desorption. Then, fluorine is electronically +1 charged and likely to desorb by the Coulomb

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Fig. 5. The core-hole Auger decay model originated from the Ca3p core-hole. The defect level forms a metallic band.

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repulsive force. If the surface lattice is not d a m a g e d by electron i r r a d i a t i o n [9], the energy o b t a i n e d b3 the C o u l o m b repulsion is c o n v e r t e d into the kinetic energy of the F + ion. This a s s u m p t i o n is reasonable in view of our previous R H E E D and ISS e x p e r i m e n t a l results [4]. A c c o r d i n g l y , in this des o r p t i o n process, we can r e p r e s e n t the energy conservation relation, as follows EF-Zp -- Eca3p = EDE F -- EF2 p + E K q- A E ,

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and E 0 = EDE v -- EF2 p + E K q- A E ,

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where EDE v, E K a n d A E are the new energy levels p r o d u c e d b y F + ion d e s o r p t i o n , the kinetic energy of the F + ion a n d the energy difference between EDE F a n d the v a c u u m energy level, respectively. Therefore, the E S D I K E D , F ( E o ) of the F + ions can be expressed as a c o n v o l u t i o n of Ca 3p and F 2p spectral functions as follows, F(Eo) = ff(E

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where f ( E ' ) a n d g ( E ' ) are respectively the C a 3p a n d F 2p spectral functions d e d u c e d f r o m the p h o toemission d a t a of C a F 2 o b t a i n e d b y K a r l s s o n et al. [1]. The c a l c u l a t e d E S D I K E D using eq. (4) is shown in fig. 6, where A E is 2 eV. T h e c a l c u l a t e d E S D I K E D is in excellent a g r e e m e n t with o u r experiment, with respect to the c o m p l e t e f e a t u r e of the p e a k p o s i t i o n a n d the total energy range. If F - c e n t e r s are formed, the influence o n the energy state of ions close to F - c e n t e r s is p r e d i c t e d , b u t this effect is negligible since the o b t a i n e d result is given from the surface with the lower electron irradiation. Even if the surface is heavily i r r a d i a ted b y low-energy electrons, the a t o m i c structures of the first a n d second layers of the surface d o a l m o s t not vary, except for the c r e a t i o n of the u n d e r l y i n g o r d e r e d F - c e n t e r s [4]. Thus, the E S D I K E D of F + ions from the heavily electron i r r a d i a t e d surface is a l m o s t identical to that f r o m the surface i r r a d i a t e d with lower electron i r r a d i a tion. W e conclude: (1) F l u o r i n e a t o m s d e s o r b as F + ions due to low-energy electron i r r a d i a t i o n . (2) C a 3 p holes are c r e a t e d b y electron i r r a d i a t i o n . After interatomic Auger decay originating from C a 3p hole occurs, a F * state is p r o d u c e d . (3) F *

2 4 6 8 Kinetic Energy(eV)

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Fig. 6. Photoemission data of CaF2 obtained by Karlsson et al. [1], where E F shows the Fermi level. The calculated ESDIKED is deduced from the above data. forms a n e w state n e a r the F e r m i level a n d then d e s o r b s as a F + ion. (4) T h e E S D I K E D of F + ions can b e e x p r e s s e d as a c o n v o l u t i o n of C a 3p a n d F 2p spectral functions, respectively.

References [1] U.O. Karlsson, F.J. Himpsel, J.F. Morar, F.R. McFeely, D. Rieger and J.A. Yarmoff, Phys. Rev. Lett. 57 (1986) 1247. [2] K. Saiki, Y. Sato, K. Ando and A. Koma, Surf. Sci. 192 (1987) 1. [3] K. Miura, R. Souda, T. Aizawa, C. Oshima and Y. Ishizawa, Solid State Commun. 72 (1989) 605. [4] K. Miura, K. Sugiura, R. Souda, T. Aizawa, C. Oshima and Y. Ishizawa, Jpn. J. Appl. Phys. 30 (1991) 809. [5] M.L. Knotek and P.J. Feibelman, Phys. Rev. Lett. 40 (1978) 964. [6] M.L. Knotek and P.J. Feibelman, Surf. Sci. 90 (1979) 78. [7] M.A. Olmstead, R.I.G. Uhrberg, R.D. Bringans and R:Z. Bachrach, Phys. Rev. 35 (1987) 7526. [8] T. Yasue, T. Gotoh, A. Ichimiya, Y. Kawaguchi, M~ Kotani, S. Ohtani, Y. Shigeta, S. Takagi, Y. Tazawa and G. Tominaga, Jpn. J. Appl. Phys. 25 (1986) L363. [9] R.E. Walkup and Ph. Avouris, Phys. Rev. Lett. 56 (1986) 524.