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LiF films: absorption and luminescence of colour centres
106
Thin Solid Films, 205 (1991) 106 108
LiF films: absorption and luminescence of colour centres Rosa Maria Montereali and Giuseppe Baldacchini E N E A , Area INN, Dip. Sviluppo Tecnologie di Punta, Centro di Frascati. ('.P. 65, 00044 Frascati Rome ( ltalr ~
L. C. Scavarda Do Carmo Departamento de Fisiea. Ponfificia Universidade ('atolica do Rio de daneiro. P.O. Bo.v 38071, 22453 Rio de Janeiro ( BrazilJ
(Received in revised form May 13, 1991)
Abstract LiF films I ~m thick have been produced by thermal evaporation on amorphous silica substrates. X-ray diffraction has shown the existence of a polycrystalline structure. The films were subjected at room temperature to electron beam irradiation of energy 3 keV and current 120 taA on an area of about 0.5 cm 2. This irradiation produced F, and F~* centres stable at room temperature that were observed by light absorption measurements. Although the concentration of the centres was much higher than that obtained usually in bulk crystals, luminescence was also observed.
!. Introduction Alkali halides have been studied since the beginning of this century. At present they are among the best-known materials. Studies have been conducted in pure and doped crystals, also containing colour centres [1], which have been produced by ionizing radiations (X-rays, 7 rays, electron beams, neutron beams etc.) or additive coloration. Recently, coloured alkali halide crystals were used to obtain optically active devices such as lasers [2] or Q switches [3]. The characteristic four-level scheme of the colour centres makes them part of the so-called solid state vibronic laser family, whose importance is based on the possibility of obtaining population inversions even with low power pumping [2]. The applications of colour centres are well established in the area ofcolour centre lasers, but also applications in the area of high density mass memories have been investigated making use of inhomogeneous broadening on zero-phonon lines [4, 5]. Colour centre lasers usually operate at liquid nitrogen temperature [2], but some success has been reported on room-temperature-active colour centres in LiF crystals [6]. In these crystals, F2 and F 3-- centres (two electrons bound to respectively two or three neighbouring anionic vacancies, the last one being a charged defect) were produced and analysed for use as room temperature lasers [6, 7]. The present interest in miniaturization has called for developments in the techniques of production and characterization of thin films. The alkali halides have seldom been analysed in the form of films; nevertheless, in
0040-6090:91.$3.5/)
the 1960s comprehensive work on the production and characterization of several alkali halides was carried out [8], and we developed this subject further specifically to understand LiF growth on amorphous substrates [9]. In this work we used the low penetration depth of electron beams in the kiloelectronvolt energy range and the produced LiF films to investigate some optical properties of F 2 and F,~ ~ centres. Under weak argon ion laser excitation we have found emission bands in the green red region of the spectrum. As far as we know, these results are the first obtained in colour centre alkali halide films, and possibly they open the door to applications of alkali halides in room-temperature-active optical microdevices.
2. Experimental details LiF films with thicknesses in the 1 ~tm range were produced by thermal evaporation on heated silica substrates [9]. The films grown on substrates kept at temperatures of 250 <'C or more during evaporation are polycrystalline with (100) planes parallel to the substrate surface and the refractive index is similar to that of bulk Li F crystals ( 1.388 + 0.001 ). These films were submitted at room temperature to 3 keV electron beam irradiation for about 4 rain, with low beam current (120 gA) and large beam spot (diameter, about 4 mm) in order to avoid local heating. Since the 3 keV electron penetration is of the order of0.15 lain, only the upper film layers were activated by the production of colour centres.
(
1991 ElsevierSequoia. Lausanne
R. M. Montereali et al. / LiF films: absorption and luminescence o f colour centres
The optical properties of the samples were studied at both room and liquid nitrogen temperatures. Absorption measurements were taken with a standard commercial spectrophotometer, while the emission measurements were made with a colinear pumping source and detector, with appropriate light filtering through glass filter and monochromator. The samples were excited with the lines of an argon ion laser kept at low power and the emission was measured with a photomultiplier, S-20 response.
107
and 530 nm, which correspond to the known positions of the F 2 and the F 3 + centre emission bands [7]. The small periodic oscillation superimposed on the broader emission band at 650 nm suggests optical interference phenomena on the LiF thin layer. By analysing the results a
l
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~p:45Z9
3. Results and conclusions ..p
An optical absorption measurement at room temperature is reported in Fig. 1. Two main absorption bands peaking at 250 nm (attributed to F centres) and 460 nm (attributed to F 2 and F 3 + centres) on top of a Rayleigh scattering type of background are observed. By using the Smakula formula [10], a reasonable oscillator strength f for the F 2 and F 3 + centres of about 0.5, and assuming the centre density to be constant on the 0.15 gm depth, we obtain an [F2] + I F 3 + ] density of the order of 1019 c m 3. On cooling to 80 K, optical absorption measurements reveal a modest increase in band intensity and some narrowing. Figures 2(a) and 2(b) show the emission spectrum of an LiF film excited by two argon ion laser lines at room and liquid nitrogen temperature respectively. The luminescence is composed of two bands peaking at about 660 nm
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"-" "~ t~ "~
K
Z
0.5
J~-.
(.,.)
500
500 p,V
I
d
i
600
I
F
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I
800
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200
300
400
WAVELENGTH
500
600
700
(nm)
Fig. 1. R o o m temperature optical absorption bands taken for an LiF film irradiated by a 3 keV electron beam. The band positions and the argon ion laser lines used for excitation are also shown.
I
800
|
700
I
600
I
500
Fig. 2. Emission bands from a LiF film 1 pm thick, coloured wittl a 3 keV electron beam, and excited by two lines of an argon ion laser: (a) room temperature; (b) 80 K.
R. M. Montereali et al. / LiF.films. absorption and luminescence o f co~our centres
108
of Fig. 2 and those obtained by using the other lines of the argon ion laser, not reported here, we were able to reconstruct a part of the excitation spectrum, which is shown in Fig. 3. The following results can be derived from it. Firstly, the 530 nm excitation spectrum, being more intense at 80 K than at room temperature, reflects most probably a narrowing of the F~ + absorption band with decreasing temperature that is more pronounced than in the case of the F 2 band. Secondly, the 660 nm emission is surprisingly more intense at room temperature than at 80 K, although this can be explained by a differential absorption between the two bands and some kind of saturation process at low temperature [11]. However, in general the excitation spectrum of Fig. 3 supports the identification of the two emission bands as F2 (about 660 rim) and F 3 + about 530 nm).
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nevertheless already conclude that the F 2 and F 3 ~ centres [11] are of interest for the area of room temperature film luminescence studies since they do not seem to exhibit strong luminescence quenching owing to the extremely high density of centres produced by the 3 keV electron irradiation. A more complete study of F 2 and F 3 + centres produced by 7 irradiation and 30 keV electron beam irradiation (about 7.5 lam penetration depth in LiF crystals) is under way. Preliminary results show that in both cases the emission properties are similar, and they lead to the conclusion that consequences from concentration quenching of the luminescence should be slight, if any. However, a systematic survey at various colour centre concentration on LiF films is highly desirable and will soon be undertaken.
Acknowledgments We would like to thank M. Bottomei for his skilful technical assistance. One of the authors (L.C.S.C.) would like to acknowledge personal ICTP and CAPES (Brazilian agency) grants and kind ENEA hospitality.
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References F. Luty, in W. B. Fowler (ed.), Phvsi('s O/Color (k'lll('rA, Academic Press, New York, 1968, p. 182. 2 L. F. Mollenauer, in L. F. Mollenauer and J. ('. White reds.), Tunahh, Lasers, Springer. Berlin. 1987, p. 225. 3 G. B. Artschuler, A. V. Okishev and A. P. Shkadarevich, 5,'ol-. Phy,L Tech. Phys., 32 (1) (1987) 95. 4 C. Ortiz, R. M. Macfarlane, W. Lenth and G ('. Bjorkhmd, Apl,I. Phys., 25 (Iggl) 87. 5 ('. Ortiz, (7. N. Afonso, R. M. Shelby, A. C. [ r o p e r and G. C. Bjorklund, Phys. Status Solidi B. 123 (1984) 79. 6 L.-X. Zheng and L-F. Wan, Opt. Commun., 55 (4) (1985) 277. 7 T . T . Basiev, S. B. Mirov and V. V. Osiko, IEEE ,I. Quanmtn Electron.. 24 (1988) 1052. 8 C. Weaver, Adv. Phys., 42 (1962) $3, 9 R.M. Montereali, G. Baldacchmi, S. Martelli and L. ('. Scavarda do Carmo. Thi. Solid Film.L 196 ( 1991 ) 9 I. 10 W. B. Fowler, in W. B. Fowler (ed.), Phyvic~ ~)! ('o/or ('entcrL Academic Press, New York, 1968, p. 172. 11 A.P. Voitovich, V. S. Kalinov, I. I. Kalosha, S. A. Mikhnov and S. I. Ovseychuk, Dokl. AkadNauk B.S.S.R., 30(1986) 132. 1
E 293K
©
©
Fig. 3. Excitation spectrum for F 2 (about 660 nm) and F 3 + (about 530 nm) emission in LiF films at 80 K and at room temperature.
Although the F 2 and F~ + centres have been subjected to intense studies, and also a specific photochemical process was used to produce almost pure F3 + centres, more experiments should be carried out to understand these aggregate systems in LiF crystals and films. We can
Thin Solid Films, 205 (1991) 106 108
LiF films: absorption and luminescence of colour centres Rosa Maria Montereali and Giuseppe Baldacchini E N E A , Area INN, Dip. Sviluppo Tecnologie di Punta, Centro di Frascati. ('.P. 65, 00044 Frascati Rome ( ltalr ~
L. C. Scavarda Do Carmo Departamento de Fisiea. Ponfificia Universidade ('atolica do Rio de daneiro. P.O. Bo.v 38071, 22453 Rio de Janeiro ( BrazilJ
(Received in revised form May 13, 1991)
Abstract LiF films I ~m thick have been produced by thermal evaporation on amorphous silica substrates. X-ray diffraction has shown the existence of a polycrystalline structure. The films were subjected at room temperature to electron beam irradiation of energy 3 keV and current 120 taA on an area of about 0.5 cm 2. This irradiation produced F, and F~* centres stable at room temperature that were observed by light absorption measurements. Although the concentration of the centres was much higher than that obtained usually in bulk crystals, luminescence was also observed.
!. Introduction Alkali halides have been studied since the beginning of this century. At present they are among the best-known materials. Studies have been conducted in pure and doped crystals, also containing colour centres [1], which have been produced by ionizing radiations (X-rays, 7 rays, electron beams, neutron beams etc.) or additive coloration. Recently, coloured alkali halide crystals were used to obtain optically active devices such as lasers [2] or Q switches [3]. The characteristic four-level scheme of the colour centres makes them part of the so-called solid state vibronic laser family, whose importance is based on the possibility of obtaining population inversions even with low power pumping [2]. The applications of colour centres are well established in the area ofcolour centre lasers, but also applications in the area of high density mass memories have been investigated making use of inhomogeneous broadening on zero-phonon lines [4, 5]. Colour centre lasers usually operate at liquid nitrogen temperature [2], but some success has been reported on room-temperature-active colour centres in LiF crystals [6]. In these crystals, F2 and F 3-- centres (two electrons bound to respectively two or three neighbouring anionic vacancies, the last one being a charged defect) were produced and analysed for use as room temperature lasers [6, 7]. The present interest in miniaturization has called for developments in the techniques of production and characterization of thin films. The alkali halides have seldom been analysed in the form of films; nevertheless, in
0040-6090:91.$3.5/)
the 1960s comprehensive work on the production and characterization of several alkali halides was carried out [8], and we developed this subject further specifically to understand LiF growth on amorphous substrates [9]. In this work we used the low penetration depth of electron beams in the kiloelectronvolt energy range and the produced LiF films to investigate some optical properties of F 2 and F,~ ~ centres. Under weak argon ion laser excitation we have found emission bands in the green red region of the spectrum. As far as we know, these results are the first obtained in colour centre alkali halide films, and possibly they open the door to applications of alkali halides in room-temperature-active optical microdevices.
2. Experimental details LiF films with thicknesses in the 1 ~tm range were produced by thermal evaporation on heated silica substrates [9]. The films grown on substrates kept at temperatures of 250 <'C or more during evaporation are polycrystalline with (100) planes parallel to the substrate surface and the refractive index is similar to that of bulk Li F crystals ( 1.388 + 0.001 ). These films were submitted at room temperature to 3 keV electron beam irradiation for about 4 rain, with low beam current (120 gA) and large beam spot (diameter, about 4 mm) in order to avoid local heating. Since the 3 keV electron penetration is of the order of0.15 lain, only the upper film layers were activated by the production of colour centres.
(
1991 ElsevierSequoia. Lausanne
R. M. Montereali et al. / LiF films: absorption and luminescence o f colour centres
The optical properties of the samples were studied at both room and liquid nitrogen temperatures. Absorption measurements were taken with a standard commercial spectrophotometer, while the emission measurements were made with a colinear pumping source and detector, with appropriate light filtering through glass filter and monochromator. The samples were excited with the lines of an argon ion laser kept at low power and the emission was measured with a photomultiplier, S-20 response.
107
and 530 nm, which correspond to the known positions of the F 2 and the F 3 + centre emission bands [7]. The small periodic oscillation superimposed on the broader emission band at 650 nm suggests optical interference phenomena on the LiF thin layer. By analysing the results a
l
lO0/zV
~p:45Z9
3. Results and conclusions ..p
An optical absorption measurement at room temperature is reported in Fig. 1. Two main absorption bands peaking at 250 nm (attributed to F centres) and 460 nm (attributed to F 2 and F 3 + centres) on top of a Rayleigh scattering type of background are observed. By using the Smakula formula [10], a reasonable oscillator strength f for the F 2 and F 3 + centres of about 0.5, and assuming the centre density to be constant on the 0.15 gm depth, we obtain an [F2] + I F 3 + ] density of the order of 1019 c m 3. On cooling to 80 K, optical absorption measurements reveal a modest increase in band intensity and some narrowing. Figures 2(a) and 2(b) show the emission spectrum of an LiF film excited by two argon ion laser lines at room and liquid nitrogen temperature respectively. The luminescence is composed of two bands peaking at about 660 nm
"E
"-" "~ t~ "~
K
Z
0.5
J~-.
(.,.)
500
500 p,V
I
d
i
600
I
F
0.6
700
b
0.7
A
I
800
~4sT,gj
6
O I0.
II
~.p=476.6 I
O U)
m < 0.4
0.3
200
300
400
WAVELENGTH
500
600
700
(nm)
Fig. 1. R o o m temperature optical absorption bands taken for an LiF film irradiated by a 3 keV electron beam. The band positions and the argon ion laser lines used for excitation are also shown.
I
800
|
700
I
600
I
500
Fig. 2. Emission bands from a LiF film 1 pm thick, coloured wittl a 3 keV electron beam, and excited by two lines of an argon ion laser: (a) room temperature; (b) 80 K.
R. M. Montereali et al. / LiF.films. absorption and luminescence o f co~our centres
108
of Fig. 2 and those obtained by using the other lines of the argon ion laser, not reported here, we were able to reconstruct a part of the excitation spectrum, which is shown in Fig. 3. The following results can be derived from it. Firstly, the 530 nm excitation spectrum, being more intense at 80 K than at room temperature, reflects most probably a narrowing of the F~ + absorption band with decreasing temperature that is more pronounced than in the case of the F 2 band. Secondly, the 660 nm emission is surprisingly more intense at room temperature than at 80 K, although this can be explained by a differential absorption between the two bands and some kind of saturation process at low temperature [11]. However, in general the excitation spectrum of Fig. 3 supports the identification of the two emission bands as F2 (about 660 rim) and F 3 + about 530 nm).
©
F¢(o) F2(~) © 80K
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1
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nevertheless already conclude that the F 2 and F 3 ~ centres [11] are of interest for the area of room temperature film luminescence studies since they do not seem to exhibit strong luminescence quenching owing to the extremely high density of centres produced by the 3 keV electron irradiation. A more complete study of F 2 and F 3 + centres produced by 7 irradiation and 30 keV electron beam irradiation (about 7.5 lam penetration depth in LiF crystals) is under way. Preliminary results show that in both cases the emission properties are similar, and they lead to the conclusion that consequences from concentration quenching of the luminescence should be slight, if any. However, a systematic survey at various colour centre concentration on LiF films is highly desirable and will soon be undertaken.
Acknowledgments We would like to thank M. Bottomei for his skilful technical assistance. One of the authors (L.C.S.C.) would like to acknowledge personal ICTP and CAPES (Brazilian agency) grants and kind ENEA hospitality.
E;
0
I
a
457.9 I
I
I
I
476.6 488.1 I
I
E}
514.7 I
References F. Luty, in W. B. Fowler (ed.), Phvsi('s O/Color (k'lll('rA, Academic Press, New York, 1968, p. 182. 2 L. F. Mollenauer, in L. F. Mollenauer and J. ('. White reds.), Tunahh, Lasers, Springer. Berlin. 1987, p. 225. 3 G. B. Artschuler, A. V. Okishev and A. P. Shkadarevich, 5,'ol-. Phy,L Tech. Phys., 32 (1) (1987) 95. 4 C. Ortiz, R. M. Macfarlane, W. Lenth and G ('. Bjorkhmd, Apl,I. Phys., 25 (Iggl) 87. 5 ('. Ortiz, (7. N. Afonso, R. M. Shelby, A. C. [ r o p e r and G. C. Bjorklund, Phys. Status Solidi B. 123 (1984) 79. 6 L.-X. Zheng and L-F. Wan, Opt. Commun., 55 (4) (1985) 277. 7 T . T . Basiev, S. B. Mirov and V. V. Osiko, IEEE ,I. Quanmtn Electron.. 24 (1988) 1052. 8 C. Weaver, Adv. Phys., 42 (1962) $3, 9 R.M. Montereali, G. Baldacchmi, S. Martelli and L. ('. Scavarda do Carmo. Thi. Solid Film.L 196 ( 1991 ) 9 I. 10 W. B. Fowler, in W. B. Fowler (ed.), Phyvic~ ~)! ('o/or ('entcrL Academic Press, New York, 1968, p. 172. 11 A.P. Voitovich, V. S. Kalinov, I. I. Kalosha, S. A. Mikhnov and S. I. Ovseychuk, Dokl. AkadNauk B.S.S.R., 30(1986) 132. 1
E 293K
©
©
Fig. 3. Excitation spectrum for F 2 (about 660 nm) and F 3 + (about 530 nm) emission in LiF films at 80 K and at room temperature.
Although the F 2 and F~ + centres have been subjected to intense studies, and also a specific photochemical process was used to produce almost pure F3 + centres, more experiments should be carried out to understand these aggregate systems in LiF crystals and films. We can