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Study of the effective layer for luminescence and exoelectron emission in YBa2Cu3O7−δ
Journal of Luminescence 87}89 (2000) 724 }726
Study of the e!ective layer for luminescence and exoelectron emission in YBa Cu O\d L. Oster*, J. Haddad The Technological College of Beer-Sheva, School of Engineering, P.O. Box 45, Beer-Sheva 84100, Israel
Abstract The electron-induced thermostimulated exoelectron emission (TSEE), thermoluminescence (TL) and stationary cathode luminescence (CL) in high-temperature superconducting YBa Cu O ceramic have been investigated. The \d e!ective depth of TSEE and CL from this material have been determined by the theoretical and experimental methods. Experimental results showed a linear dependence of both the CL and TSEE intensities on the energy of the exciting electrons, as predicted by theory. The results also indicated the presence of an inactive layer close to the surface of the samples. The e!ective depth ¸ of the layer responsible for the CL emission was found to be greater than 300 nm and the depth of the dead surface layer about 43 nm. The e!ective depth of both the 95 and 148 K TSEE peaks was found to be greater than 730 nm. 2000 Elsevier Science B.V. All rights reserved. Keywords: Exoelectron emission; Ceramics
1. Introduction The value of studies of high-temperature superconductors (HTSC) using luminescence and thermally stimulated exoelectron emission (TSEE) methods was shown earlier in Refs. [1,2]. The present work proves the basic possibility of determining the e!ective depth of the layer from which photons and exoelectrons are emitted, using these two methods, and also estimates its magnitude for HTSC, as derived from the dependence of cathodoluminescence (CL) and TSEE intensity on the energy of exciting electrons. 2. Theory This dependence is described by the expression
I"
J
E(x,l)g(x)exp(!x/¸) dx, (1) where E(x,l) is the energy of the excited levels created in the layer (dx); g(x)"B/l(x), where B is the CL or TSEE
* Corresponding author. Fax: #972-7-6472810. E-mail address: [email protected] (L. Oster)
yield per incident electron; l is the energy required for the formation of one CL * photon or one exoelectron, ¸ is the depth of the layer from which the CL or TSEE are emitted and l is the penetration depth of exciting electrons. For the excitation energy range, 0.5)V- )6 keV, there is a common empirical formula l"K< - ,
(2)
where l is in nm; < is in eV and K is the proportional factor. For many dielectric materials K"11.2 [3]. Theoretic calculations of l, for Y}Ba}Cu}O ceramics in this energy range also give expression (2) with only a minor di!erence in coe$cient: K"13 [4]. In the case where the energy loss of exciting electrons is proportional to the penetration depth < E"j - , l
(3)
where j is the incident electron #ux (cm\ s\), E is the rate of the energy density loss (keV cm\ s\) and l is the penetration depth (cm). For TSEE and thermoluminescence (TL) processes, j in Eq. (3) is replaced by q"jt, where t is the time of
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 3 7 5 - 0
L. Oster, J. Haddad / Journal of Luminescence 87}89 (2000) 724 }726
725
exposure to electrons, and S is introduced in Eq. (1) instead of I (S } TL or TSEE peak integral). Taking into account Eq. (3) and supposing g(x)"0, when x)1L and g(x)"constant, when x'lL , we obtain from Eq. (1)
l I"jg(<- !
¸ , l
(4)
where <- is the energy of the exciting electrons;
(5)
3. Experimental method In the course of this work, TL, CL, TSEE and the dependence of the intensity of the 512 nm CL emission band and TSEE on the energy of the exciting electrons were measured in pristine superconducting YBa Cu O ceramic as well as in non-superconduct \d ing samples at 80 K (the ¹! of this material is 92 K and o"5.7 g/cm). The experimental conditions were as follows: the vacuum was 10\ Pa, the exciting electrons' energy was <- "1.8}3.8 keV, the constant current density was 6;10\A/cm. TSEE was also measured under the same conditions in the temperature range 80}400 K after the exposure time of 10 min. The critical superconductivity temperature (¹! ), emission spectra of TL and CL, X-rayograms and temperature dependence of CL were also measured.
4. Results and discussion Pristine samples exhibited a few TL and TSEE peaks in the temperature range 80}400 K (Fig. 1). The data analysis of TL, TSEE and X-ray indicate, that the TL and TSEE in the temperature region 85}400 K are due to common trap centers belonging to the impurity phases Y O (TSEE peak at 148 K), BaCO (¹ "192 K) and to the basic material, the superconducting orthor-
Fig. 1. TL (a) and TSEE (b) glow curves of superconducting YBa Cu O ceramic irradiated by #uence 2.5;10 elec \d trons cmU.
hombic phase (TSEE peaks at 95 and 363 K and TL peak at 85 K). TSEE and TL peaks at low temperature close to the critical temperature (¹ &¹! ) were not seen in the nonsuperconducting sample. The origin of the TL and TSEE peaks obtained at 85 and 95 K, respectively, can be understood in the context of the hole bipolar model of superconductivity in which a Cooper pair (e.g. localized on oxygen in the chain Cu>}O}Cu>) can trap an electron by electron excitation. The decay of the pairs at ¹! releases the electrons, resulting in their emission (TSEE) or radiative recombination with holes (TL). The origin of the di!erence in the glow-curves of TL and TSEE is due to one (or several) of the following reasons: the location of electron storage (bulk for TL and surface for TSEE); the activation energy (in the TSEE process electrons overcome electron a$nity); the manner in which the electrons get energy for their exit into vacuum or recombination, etc. [5]. The emission spectrum of the TL peak at 85 K shows a broad band centered at 360 nm. The emission spectrum of the CL shows two emission bands in the range from 300 to 450 nm (violet band) and the yellow band centered at 512 nm. The temperature dependence of the various CL emission bands were measured during cooling and heating between 80 and 400 K and showed di!erent behaviours (Fig. 2). The violet CL bands show the main peak in the range from 210 to 220 K (Fig. 2, curves 1}3), while the yellow band shows a steep decay at these temperatures (Fig. 2, curve 4). This CL behaviour repeats during heating}cooling}heating cycles in pristine as well as in nonsuperconducting samples. The maximum of the `violeta peak is shifted upon heating and cooling (Fig. 2, curve 1 and 2). The study of the dependence of this hysteresis on heating (cooling) rates, CL pulse duration (Fig. 2, curve 3), etc., shows that CL is associated with a phase transition of `order}disordera type in the oxygen sublattice, because these CL maxima are consistent with wellknown features of thermal, electric, optical and elastic properties in the temperature range 210}220 K, which
726
L. Oster, J. Haddad / Journal of Luminescence 87}89 (2000) 724 }726 Table 1 The e!ective depth of the layer for CL and TSEE in YBa Cu O \d
¸ (nm) l (nm) L
Fig. 2. Temperature dependences of the main CL bands in YBa Cu O ceramics: (1,2) j "363 nm; sample heating and Ud !* cooling, respectively, continuous irradiation; (3) j "363 nm; !* sample heating and cooling, pulse irradiation; (4) j "512 nm; !* sample heating and cooling and continuous irradiation.
CL (512 nm)
TSEE TSEE impurity superconducting peak at 148 K `peaka at 95 K [2] [2]
*300 43
*730 78
*730 23
trapped electron are equal in both layers, enables us to suppose that the small slope of the "rst line (lL "7.8 nm) is associated with a layer containing a small concentration (10}15%) of the superconducting phase. These superconducting grains apparently occur in the partially deoxygenated YBa O O -phase (x)6.5). V
5. Conclusions
Fig. 3. Dependence of 512 nm CL band intensity (a), 148 K (b) and 95 K (c) TSEE peaks integral on the energy of the incident electrons.
have been associated with this phase transition [6,7]. The intensity of the violet band is proportional to the extent of lattice instability, whereas that of the yellow band is proportional to the concentration of the low-temperature phase. CL and TSEE energy response following electron excitation is shown in Fig. 3. Experimental results show a linear dependence of both the CL intensity and TSEE sum on the <- , as predicted by expression Eq. (4a). The results also indicated the presence of an inactive layer close to the surface of the samples. This fact disproves the possibility that obtained CL and TSEE occur in the thin adsorption layer (see expression 5). The results of our calculations, based on the above-mentioned formulae and experimental date, are presented in the Table 1. The TSEE peak at 95 K has previously been attributed to a super-conducting phase transition [2]. The dependence of this peak intensity on the energy of incident electrons shows two linear regions (Fig. 3, curve c). These regions are apparently attributed to two di!erent layers. The fact, that the same material is active in both layers and that the energies required for the formation of one
1. Our results show that the TL peak at 85 K and TSEE peak at 95 K are due to the superconducting transition. 2. The CL of YBa Cu O\d ceramic is determined by phase transitions in the oxygen sublattice. 3. The obtained value of the depth of the layer from which the exoelectrons are emitted is in a good agreement with published data for the penetration depth of thermoelectrons in solids [8].
Acknowledgements Financial support from the Rashi Foundation is gratefully acknowledged.
References [1] CH.B. Lushik, I.L. Kuusman, E.KH. Feldbakh, P.KH. Liblik, T.I. Savikhina, I.A. Merilo, Fiz. Tverd. Tela. 29 (1987) 3667. [2] V.Ya. Yaskolko, L.N. Oster, K.M. Mukimov, Phys. Stat. Sol. A 123 (1991) K35. [3] V.K. Lyapidevski, Methods of Radiation Measurement, Moskow, 1987. [4] A. Miteriv, Superconductivity 4 (1991) 544. [5] L. Oster, V. Yaskolko, J. Haddad, Phys. Stat. Sol. A. 174 (1999) 431. [6] M.K. Wu, J.R. Ashbury, C.J. Toryg et al., Phys. Rev. Lett. 58 (1987) 908. [7] J.l. Chen, L.E. Wenger, C.J. McEwan, E.M. Logathetis, Phys. Rev. Lett. 58 (1987) 1972. [8] M.P. Seah, W.A. Denoh, Surf. Interface Anal. 1 (1979) 1.
Study of the e!ective layer for luminescence and exoelectron emission in YBa Cu O\d L. Oster*, J. Haddad The Technological College of Beer-Sheva, School of Engineering, P.O. Box 45, Beer-Sheva 84100, Israel
Abstract The electron-induced thermostimulated exoelectron emission (TSEE), thermoluminescence (TL) and stationary cathode luminescence (CL) in high-temperature superconducting YBa Cu O ceramic have been investigated. The \d e!ective depth of TSEE and CL from this material have been determined by the theoretical and experimental methods. Experimental results showed a linear dependence of both the CL and TSEE intensities on the energy of the exciting electrons, as predicted by theory. The results also indicated the presence of an inactive layer close to the surface of the samples. The e!ective depth ¸ of the layer responsible for the CL emission was found to be greater than 300 nm and the depth of the dead surface layer about 43 nm. The e!ective depth of both the 95 and 148 K TSEE peaks was found to be greater than 730 nm. 2000 Elsevier Science B.V. All rights reserved. Keywords: Exoelectron emission; Ceramics
1. Introduction The value of studies of high-temperature superconductors (HTSC) using luminescence and thermally stimulated exoelectron emission (TSEE) methods was shown earlier in Refs. [1,2]. The present work proves the basic possibility of determining the e!ective depth of the layer from which photons and exoelectrons are emitted, using these two methods, and also estimates its magnitude for HTSC, as derived from the dependence of cathodoluminescence (CL) and TSEE intensity on the energy of exciting electrons. 2. Theory This dependence is described by the expression
I"
J
E(x,l)g(x)exp(!x/¸) dx, (1) where E(x,l) is the energy of the excited levels created in the layer (dx); g(x)"B/l(x), where B is the CL or TSEE
* Corresponding author. Fax: #972-7-6472810. E-mail address: [email protected] (L. Oster)
yield per incident electron; l is the energy required for the formation of one CL * photon or one exoelectron, ¸ is the depth of the layer from which the CL or TSEE are emitted and l is the penetration depth of exciting electrons. For the excitation energy range, 0.5)V- )6 keV, there is a common empirical formula l"K< - ,
(2)
where l is in nm; < is in eV and K is the proportional factor. For many dielectric materials K"11.2 [3]. Theoretic calculations of l, for Y}Ba}Cu}O ceramics in this energy range also give expression (2) with only a minor di!erence in coe$cient: K"13 [4]. In the case where the energy loss of exciting electrons is proportional to the penetration depth < E"j - , l
(3)
where j is the incident electron #ux (cm\ s\), E is the rate of the energy density loss (keV cm\ s\) and l is the penetration depth (cm). For TSEE and thermoluminescence (TL) processes, j in Eq. (3) is replaced by q"jt, where t is the time of
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 3 7 5 - 0
L. Oster, J. Haddad / Journal of Luminescence 87}89 (2000) 724 }726
725
exposure to electrons, and S is introduced in Eq. (1) instead of I (S } TL or TSEE peak integral). Taking into account Eq. (3) and supposing g(x)"0, when x)1L and g(x)"constant, when x'lL , we obtain from Eq. (1)
l I"jg(<- !
¸ , l
(4)
where <- is the energy of the exciting electrons;
(5)
3. Experimental method In the course of this work, TL, CL, TSEE and the dependence of the intensity of the 512 nm CL emission band and TSEE on the energy of the exciting electrons were measured in pristine superconducting YBa Cu O ceramic as well as in non-superconduct \d ing samples at 80 K (the ¹! of this material is 92 K and o"5.7 g/cm). The experimental conditions were as follows: the vacuum was 10\ Pa, the exciting electrons' energy was <- "1.8}3.8 keV, the constant current density was 6;10\A/cm. TSEE was also measured under the same conditions in the temperature range 80}400 K after the exposure time of 10 min. The critical superconductivity temperature (¹! ), emission spectra of TL and CL, X-rayograms and temperature dependence of CL were also measured.
4. Results and discussion Pristine samples exhibited a few TL and TSEE peaks in the temperature range 80}400 K (Fig. 1). The data analysis of TL, TSEE and X-ray indicate, that the TL and TSEE in the temperature region 85}400 K are due to common trap centers belonging to the impurity phases Y O (TSEE peak at 148 K), BaCO (¹ "192 K) and to the basic material, the superconducting orthor-
Fig. 1. TL (a) and TSEE (b) glow curves of superconducting YBa Cu O ceramic irradiated by #uence 2.5;10 elec \d trons cmU.
hombic phase (TSEE peaks at 95 and 363 K and TL peak at 85 K). TSEE and TL peaks at low temperature close to the critical temperature (¹ &¹! ) were not seen in the nonsuperconducting sample. The origin of the TL and TSEE peaks obtained at 85 and 95 K, respectively, can be understood in the context of the hole bipolar model of superconductivity in which a Cooper pair (e.g. localized on oxygen in the chain Cu>}O}Cu>) can trap an electron by electron excitation. The decay of the pairs at ¹! releases the electrons, resulting in their emission (TSEE) or radiative recombination with holes (TL). The origin of the di!erence in the glow-curves of TL and TSEE is due to one (or several) of the following reasons: the location of electron storage (bulk for TL and surface for TSEE); the activation energy (in the TSEE process electrons overcome electron a$nity); the manner in which the electrons get energy for their exit into vacuum or recombination, etc. [5]. The emission spectrum of the TL peak at 85 K shows a broad band centered at 360 nm. The emission spectrum of the CL shows two emission bands in the range from 300 to 450 nm (violet band) and the yellow band centered at 512 nm. The temperature dependence of the various CL emission bands were measured during cooling and heating between 80 and 400 K and showed di!erent behaviours (Fig. 2). The violet CL bands show the main peak in the range from 210 to 220 K (Fig. 2, curves 1}3), while the yellow band shows a steep decay at these temperatures (Fig. 2, curve 4). This CL behaviour repeats during heating}cooling}heating cycles in pristine as well as in nonsuperconducting samples. The maximum of the `violeta peak is shifted upon heating and cooling (Fig. 2, curve 1 and 2). The study of the dependence of this hysteresis on heating (cooling) rates, CL pulse duration (Fig. 2, curve 3), etc., shows that CL is associated with a phase transition of `order}disordera type in the oxygen sublattice, because these CL maxima are consistent with wellknown features of thermal, electric, optical and elastic properties in the temperature range 210}220 K, which
726
L. Oster, J. Haddad / Journal of Luminescence 87}89 (2000) 724 }726 Table 1 The e!ective depth of the layer for CL and TSEE in YBa Cu O \d
¸ (nm) l (nm) L
Fig. 2. Temperature dependences of the main CL bands in YBa Cu O ceramics: (1,2) j "363 nm; sample heating and Ud !* cooling, respectively, continuous irradiation; (3) j "363 nm; !* sample heating and cooling, pulse irradiation; (4) j "512 nm; !* sample heating and cooling and continuous irradiation.
CL (512 nm)
TSEE TSEE impurity superconducting peak at 148 K `peaka at 95 K [2] [2]
*300 43
*730 78
*730 23
trapped electron are equal in both layers, enables us to suppose that the small slope of the "rst line (lL "7.8 nm) is associated with a layer containing a small concentration (10}15%) of the superconducting phase. These superconducting grains apparently occur in the partially deoxygenated YBa O O -phase (x)6.5). V
5. Conclusions
Fig. 3. Dependence of 512 nm CL band intensity (a), 148 K (b) and 95 K (c) TSEE peaks integral on the energy of the incident electrons.
have been associated with this phase transition [6,7]. The intensity of the violet band is proportional to the extent of lattice instability, whereas that of the yellow band is proportional to the concentration of the low-temperature phase. CL and TSEE energy response following electron excitation is shown in Fig. 3. Experimental results show a linear dependence of both the CL intensity and TSEE sum on the <- , as predicted by expression Eq. (4a). The results also indicated the presence of an inactive layer close to the surface of the samples. This fact disproves the possibility that obtained CL and TSEE occur in the thin adsorption layer (see expression 5). The results of our calculations, based on the above-mentioned formulae and experimental date, are presented in the Table 1. The TSEE peak at 95 K has previously been attributed to a super-conducting phase transition [2]. The dependence of this peak intensity on the energy of incident electrons shows two linear regions (Fig. 3, curve c). These regions are apparently attributed to two di!erent layers. The fact, that the same material is active in both layers and that the energies required for the formation of one
1. Our results show that the TL peak at 85 K and TSEE peak at 95 K are due to the superconducting transition. 2. The CL of YBa Cu O\d ceramic is determined by phase transitions in the oxygen sublattice. 3. The obtained value of the depth of the layer from which the exoelectrons are emitted is in a good agreement with published data for the penetration depth of thermoelectrons in solids [8].
Acknowledgements Financial support from the Rashi Foundation is gratefully acknowledged.
References [1] CH.B. Lushik, I.L. Kuusman, E.KH. Feldbakh, P.KH. Liblik, T.I. Savikhina, I.A. Merilo, Fiz. Tverd. Tela. 29 (1987) 3667. [2] V.Ya. Yaskolko, L.N. Oster, K.M. Mukimov, Phys. Stat. Sol. A 123 (1991) K35. [3] V.K. Lyapidevski, Methods of Radiation Measurement, Moskow, 1987. [4] A. Miteriv, Superconductivity 4 (1991) 544. [5] L. Oster, V. Yaskolko, J. Haddad, Phys. Stat. Sol. A. 174 (1999) 431. [6] M.K. Wu, J.R. Ashbury, C.J. Toryg et al., Phys. Rev. Lett. 58 (1987) 908. [7] J.l. Chen, L.E. Wenger, C.J. McEwan, E.M. Logathetis, Phys. Rev. Lett. 58 (1987) 1972. [8] M.P. Seah, W.A. Denoh, Surf. Interface Anal. 1 (1979) 1.