Orientation dependence of X-ray reflection from BNhex in the energy region of B and N K-edges

Physica B 208&209 (1995) 417 418

ELSEVIER

Orientation dependence of X-ray reflection from BNhex in the energy region of B and N K-edges E. Filatova a, A.A. Pavlychev a, C. Blessing b, J. Friedrich b'* aInstitute of Physics, St.-Petersburg University, Russian Federation bII. lnstitut J~r Experimentalphysik der Universitiit Hamburg, Luruper Chaussee 149, D-22761 Hamburg, Germany

Abstract

The reflection spectra fine structure from two surfaces of the hexagonal BNh,~ cut parallely and perpendicularly to the c-axis of the crystal in the energy region 40-850 eV, i.e. including the BK and NK absorption edges, have been obtained for different angles of incidence using s-polarized synchrotron radiation. The BK and NK absorption spectra fine structure for both crystal orientations were calculated from the reflection spectra at 86° using the Kramers-Kronig dispersion relations and were combined in a common scale by means of XPS-data. A strong orientation dependence of both reflection and absorption spectra was found. The comparison of the BK and N t absorption spectra leads to the following conclusion: the NK-spectra demonstrate features similar to the B~:-spectra, that means both B and N 2p states participate in the formation of N2pn*, B2pn*, N2p~* and B2p~* resonance states in BNhex.

The anisotropy of chemical bonding via sp 2 hybridization between B and N atoms within and perpendicular to the layers of hexagonal BNhex crystals, i.e. perpendicular, resp. parallel, to the optical axis c, leads to strong anisotropy in the interaction of BNh,x with polarized radiation. Absorption must originate from transitions from B ls (N Is) to a states in the case of E l c and from B ls (N Is) to n states in the case of ELIc, where E is the electric field vector. By turning the crystal and using s-polarized radiation one can thus change the contributions of different transitions to the BK (N~) absorption spectra. The reflection spectra from two surfaces cut parallely and perpendicularly to the c-axis of the crystal in the energy region 40-850 eV, were obtained for different angles of incidence. Fig. 1 shows the reflection fine structure near the BK and NK absorption edges. The BK reflection spectra of BNll and BN± are similar in the * Corresponding author.

number of features and their energy dependence. At the same time, the intensity of peak a is considerably higher in the case of the BNIL orientation than in the other orientation. Quite different behaviour occurs for the intensity of peak b (higher intensity in the case of BNx). Analogous orientation dependence of Bt reflection spectra were obtained in Ref. [1] using the unpolarized radiation of an X-ray tube. Fig. 2 shows the BK and NK absorption spectra for both crystal orientations calculated from the reflection spectra using the Kramers-Kronig dispersion relations and combined in a common scale by means of XPS-data [2]. Although application of the Kramers-Kronig relations in this case does not guarantee accuracy of the absolute values of the optical constants, it describes the shape of the spectra quite correctly. There is a good agreement in the number of features of the spectra and their energy positions with results for polycrystalline BNhex obtained by the photoyield method [3]. The comparison of the BK and NK absorption spectra (Fig. 2)

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E. Filatova et al./ Physica B 208&209 (1995) 417-418

0.0014

b

NK 86 ° ,

0.0010 '

C t

;

%

//

"

0.0006

'\~-'~,

/

.

: ,s

~.~_ 0.0002

400

420

440 Enersy leVI

460

480

0.0080

ja

R

B K 82 °

r;

0.0015

,I i I

0.0010

i'

0.0005 0.0000

180

200 Energy [eV]

220

240

Fig. 1. Reflection spectra of BNhex at the B and N K-edges. Closed lines: BNN,dotted lines: BNs.

shows that the N~-spectra demonstrate features similar to the Bt-spectra, that means both boron and nitrogen p states participate in the formation of rt- and a-like molecular orbitals of BNhox. In order to explain the spectral behaviour of the X-ray reflection and the absorption coefficients near the K-edges in BNho, crystals the quasiatomic approach [-4] is applied. The calculations of the oscillator strengths spectral distribution (OSSD) for the transitions from N and B Is-levels are carried out using the analytical expressions obtained in Ref. [5] and connecting the OSSD with the local electron-optical characteristics of matter. According to the calculations: the main spectral features near the K-edges are originated from the splitting of the intense Is-12p-resonances at nitrogen and boron atoms by anisotrope surrounding potentials into two 2pa~- and e'(2p)-components, respectively; the a~ components are low-lying; the resonance effects inside surroundings play an important role in formation of the OSSD for transitions from NK level to Ee' states; the N l s -1 e'(2p)-resonance is more delocalized in comparison with the Bls 1 e'(2p) one. The comparison of the experimental and the theoretical data allows to assign the a and a' spectral features to the B2pa~ and N2pa~ resonances, respectively, and the b and b' features with the superposition of the e'(B2p) and e'(N2p) states. Orientation dependencies support this conclusion.

References -

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I-1] E.O. Filatova and T.A.Blagoveschenskaya, J. X-Ray Sci.Technol. 4 (1993). [2] K. Hamrim et al., Phys. Scripta 1 (1970) 277. I-3] V.A.Formichev and M.A.Rumsh, J. Phys. Chem. Solids 29 (1968) 1015. [4] A.A.Pavlychev and A. Barry, Phys. Seripta 41 (1990) 157. [5] U. Fano and J.W.Cooper, Rev. Mod. Phys. 40 (1968) 441.

b

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Energy [eV] Fig. 2. Absorption spectra of BNhc,. 1: BKI, 2: BKI, 3: N K I , 4: NK,I.