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Luminescence excitation of colour centers in beryllium oxide
148
Nuclear Instruments and Methods in Physics Research A261 (1987) 148-149 North-Holland, Amsterdam
L U M I N E S C E N C E EXCITATION OF C O L O U R C E N T E R S IN BERYLLIUM OXIDE S.V. G O R B U N O V ,
V.Y. I V A N O V , A.V. K R U Z H A L O V ,
V.A. P U S T O V A R O V ,
a n d B.V. S H U L G I N
Ural Polytechnical Institute, Sverdlovsk, 620002, USSR
Synchrotron radiation is used in the measurement of the spectral luminescence characteristics of F-centers in BeO. The electron-hole and the exciton mechanism of energy transfer to F-centers is discussed. The photon multiplication effect (E > 25 eV) is connected with the creation of secondary carriers by hot photoelectrons. The anomalous absorption of polarized light by F-centers in BeO is due to the splitting of the 1p-excitation level of the F-centers in BeO under the influence of a C3,, crystal symmetry field.
In this paper we concentrate on the spectral luminescence characteristics of F-centers in BeO. An absorption band with a 6.6 eV maximum appears in BeO as a result of BeO annealing in Be vapours or BeO irradiation by fast particles. At the same time an intense luminescence band with 3.4 eV maximum is observed in such samples. The absorption (6.6 eV) and luminescence (3.4 eV) bands were identified as F-bands earlier [1]. Luminescence excitation, reflection and absorption spectra of additively coloured BeO crystals have been measured using synchrotron radiation from the storage ring VEPP-2M (100-670 MeV) of the Nuclear Physics Institute of the Siberian Division of the Academy of Sciences of U S S R in Novosibirsk. The separated beam of synchrotron radiation, with the electric vector almost completely polarized in the horizontal plane of the storage ring, was monochromatized using a Wadsworth's vacuum monochromator equipped with a gold concave grating with a groove spacing of 600 l i n e s / m m and a radius of curvature of 2 m. F-luminescence was registered with a photoelectron multiplier FEU-71 with a violet filter (FS-1). All the measured data have been continuously monitored by the minicomputer Electronika 60M and further processed by the computer Electronika 100-25 [2]. BeO single crystals ( E I I C and E ± C) with an F-center concentration up to 1017 cm ~ were prepared by additive colouring in Be vapour. 3.4 eV luminescence is effectively excited in the crystal transparency region (5-10 eV) and the fundamental absorption region ( E > 10.4 eV) (fig. 1). The excitation band at 6.6 eV coincides with the F-absorption band. The shoulder 8.4 eV in the excitation spectrum also manifests itself in the absorption spectrum of additively coloured BeO crystals. This peculiarity is due to electron transitions to higher energy excited levels of the F-center. The maximum 9.5 eV in the luminescence excitation spectrum of F-centers in BeO may result 0168-9002/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
from the creation of excitons near F-centers as in alkali-halides (B-band). For E > 10 eV, luminescence excitation of F-centers is due to exciton and electron-hole processes. The maximum at 10.14 eV on the long-wave edge of the exciton absorption arises from the absorption coefficient increase in this region. The observed minimum in the luminescence excitation of the F-center at 10.4 eV corresponds to the maximum of the reflection spectrum which is due to exciton creation in /'-point of the Brillouin zone. The intensity decrease of luminescence at E > 11 eV arises from the increase of the number of nonradiative recombinations in the defect surface layer of single crystals. In the energy region E > 25 eV in the luminescence excitation spectrum of F-centers, an increase of the luminescence quantum yield is observed which results from the photon multiplication effect. Since for BeO E v < Eg < E~, secondary electron-hole pairs are mainly produced by hot photoelectrons. We have studied the dichroism of the F-centers
30
L~
0.f-~
|l
I
t0
i
I
)
1~
2~
25-
E,, eX/
Fig. 1. The luminescence excitation spectrum of the F-center at 3.4 eV (a) and the reflection spectrum (b) of BeO single crystal at the temperature T = 300 K for additively coloured sample.
S. V. Gorbunov et al. / Luminescence exeitation of colour centers in BeO
//!1
~a ~- 0 , 6
// 02
Fig. 2. The optical absorption spectra of additively coloured BeO crystals in polarized synchrotron radiation at the temperature T = 300 K: E IIC (a) and E l C (b).
optical absorption in BeO by means of polarized synchrotron radiation. The absorption band of the F-center (6.5-6.6 eV) in BeO is due to optical transitions from 1s-ground states to excited lp-singlet states. A characteristic feature of an F-center is the absence of photoconversion F ~ F + which make it possible to suppose the splitting of lp-excitation level of the F-center in BeO under the influence of C3v symmetry crystal field. A n analysis based on group theory showed that if such
149
a splitting takes place optical transitions 1A1 - - + 1A 1 and 1A1 --+ E 1 for F-centers are possible with the help of light absorption polarized parallel and perpendicular to the C-axis of the crystal. The data on the studied absorption spectra of additively coloured BeO crystals are presented in fig. 2. One can see that the F-band in BeO is the superposition of two absorption bands with maxima 6.3 eV for E H C and 6.55 eV for E _1_C. The halfwidth of one of these bands is the same and is equal to 0.82 + 0.02 eV. So the F-center optical absorption of BeO is due to the ~r-transitions 1Ax-IA 1 (EIIC, h m = 6.3 eV) and the o-transitions 1A1-E 1 ( E _ I _ C , h m = 6 . 5 5 eV). The observed splitting of the F-center lp-singlet state in Beo is about 0.25 eV and it demonstrates the essential difference between its optical properties and those of similar centers in other crystal oxides.
Acknowledgement The authors are indebted to the personnel of VEPP2M for their permission to work with synchrotron radiation beams and V.A. Maslov for providing us with BeO crystals.
References [1] A.V. Kruzhalov, S.V. Gorbunov, B.V. Shulgin and V.A. Maslov, Pisma Zh. Techn. Fiz. 10 (1984) 1503. [2] E.S. Gluskin and V.A. Kochubei, Kosmicheskie issledovania 18 (1980) 476.
III. VUV/SOFT X-RAY SPECTROSCOPY
Nuclear Instruments and Methods in Physics Research A261 (1987) 148-149 North-Holland, Amsterdam
L U M I N E S C E N C E EXCITATION OF C O L O U R C E N T E R S IN BERYLLIUM OXIDE S.V. G O R B U N O V ,
V.Y. I V A N O V , A.V. K R U Z H A L O V ,
V.A. P U S T O V A R O V ,
a n d B.V. S H U L G I N
Ural Polytechnical Institute, Sverdlovsk, 620002, USSR
Synchrotron radiation is used in the measurement of the spectral luminescence characteristics of F-centers in BeO. The electron-hole and the exciton mechanism of energy transfer to F-centers is discussed. The photon multiplication effect (E > 25 eV) is connected with the creation of secondary carriers by hot photoelectrons. The anomalous absorption of polarized light by F-centers in BeO is due to the splitting of the 1p-excitation level of the F-centers in BeO under the influence of a C3,, crystal symmetry field.
In this paper we concentrate on the spectral luminescence characteristics of F-centers in BeO. An absorption band with a 6.6 eV maximum appears in BeO as a result of BeO annealing in Be vapours or BeO irradiation by fast particles. At the same time an intense luminescence band with 3.4 eV maximum is observed in such samples. The absorption (6.6 eV) and luminescence (3.4 eV) bands were identified as F-bands earlier [1]. Luminescence excitation, reflection and absorption spectra of additively coloured BeO crystals have been measured using synchrotron radiation from the storage ring VEPP-2M (100-670 MeV) of the Nuclear Physics Institute of the Siberian Division of the Academy of Sciences of U S S R in Novosibirsk. The separated beam of synchrotron radiation, with the electric vector almost completely polarized in the horizontal plane of the storage ring, was monochromatized using a Wadsworth's vacuum monochromator equipped with a gold concave grating with a groove spacing of 600 l i n e s / m m and a radius of curvature of 2 m. F-luminescence was registered with a photoelectron multiplier FEU-71 with a violet filter (FS-1). All the measured data have been continuously monitored by the minicomputer Electronika 60M and further processed by the computer Electronika 100-25 [2]. BeO single crystals ( E I I C and E ± C) with an F-center concentration up to 1017 cm ~ were prepared by additive colouring in Be vapour. 3.4 eV luminescence is effectively excited in the crystal transparency region (5-10 eV) and the fundamental absorption region ( E > 10.4 eV) (fig. 1). The excitation band at 6.6 eV coincides with the F-absorption band. The shoulder 8.4 eV in the excitation spectrum also manifests itself in the absorption spectrum of additively coloured BeO crystals. This peculiarity is due to electron transitions to higher energy excited levels of the F-center. The maximum 9.5 eV in the luminescence excitation spectrum of F-centers in BeO may result 0168-9002/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
from the creation of excitons near F-centers as in alkali-halides (B-band). For E > 10 eV, luminescence excitation of F-centers is due to exciton and electron-hole processes. The maximum at 10.14 eV on the long-wave edge of the exciton absorption arises from the absorption coefficient increase in this region. The observed minimum in the luminescence excitation of the F-center at 10.4 eV corresponds to the maximum of the reflection spectrum which is due to exciton creation in /'-point of the Brillouin zone. The intensity decrease of luminescence at E > 11 eV arises from the increase of the number of nonradiative recombinations in the defect surface layer of single crystals. In the energy region E > 25 eV in the luminescence excitation spectrum of F-centers, an increase of the luminescence quantum yield is observed which results from the photon multiplication effect. Since for BeO E v < Eg < E~, secondary electron-hole pairs are mainly produced by hot photoelectrons. We have studied the dichroism of the F-centers
30
L~
0.f-~
|l
I
t0
i
I
)
1~
2~
25-
E,, eX/
Fig. 1. The luminescence excitation spectrum of the F-center at 3.4 eV (a) and the reflection spectrum (b) of BeO single crystal at the temperature T = 300 K for additively coloured sample.
S. V. Gorbunov et al. / Luminescence exeitation of colour centers in BeO
//!1
~a ~- 0 , 6
// 02
Fig. 2. The optical absorption spectra of additively coloured BeO crystals in polarized synchrotron radiation at the temperature T = 300 K: E IIC (a) and E l C (b).
optical absorption in BeO by means of polarized synchrotron radiation. The absorption band of the F-center (6.5-6.6 eV) in BeO is due to optical transitions from 1s-ground states to excited lp-singlet states. A characteristic feature of an F-center is the absence of photoconversion F ~ F + which make it possible to suppose the splitting of lp-excitation level of the F-center in BeO under the influence of C3v symmetry crystal field. A n analysis based on group theory showed that if such
149
a splitting takes place optical transitions 1A1 - - + 1A 1 and 1A1 --+ E 1 for F-centers are possible with the help of light absorption polarized parallel and perpendicular to the C-axis of the crystal. The data on the studied absorption spectra of additively coloured BeO crystals are presented in fig. 2. One can see that the F-band in BeO is the superposition of two absorption bands with maxima 6.3 eV for E H C and 6.55 eV for E _1_C. The halfwidth of one of these bands is the same and is equal to 0.82 + 0.02 eV. So the F-center optical absorption of BeO is due to the ~r-transitions 1Ax-IA 1 (EIIC, h m = 6.3 eV) and the o-transitions 1A1-E 1 ( E _ I _ C , h m = 6 . 5 5 eV). The observed splitting of the F-center lp-singlet state in Beo is about 0.25 eV and it demonstrates the essential difference between its optical properties and those of similar centers in other crystal oxides.
Acknowledgement The authors are indebted to the personnel of VEPP2M for their permission to work with synchrotron radiation beams and V.A. Maslov for providing us with BeO crystals.
References [1] A.V. Kruzhalov, S.V. Gorbunov, B.V. Shulgin and V.A. Maslov, Pisma Zh. Techn. Fiz. 10 (1984) 1503. [2] E.S. Gluskin and V.A. Kochubei, Kosmicheskie issledovania 18 (1980) 476.
III. VUV/SOFT X-RAY SPECTROSCOPY