- Email: [email protected]
Thermally stimulated luminescence excitation spectroscopy (TSLES): a versatile technique to study electron transfer processes in solids
Journal of Luminescence 94–95 (2001) 465–469
Thermally stimulated luminescence excitation spectroscopy (TSLES): a versatile technique to study electron transfer processes in solids J. Fleniken, J. Wang, J. Grimm, M.J. Weber1, U. Happek* Department of Physics and Astronomy, The University of Georgia, Athens, GA 30602-2451, USA
Abstract We present a technique to locate the ground state of impurity ions relative to the host conduction band, utilizing thermally stimulated luminescence (TSL). The technique makes use of the concept that TSL probes the occupation of electron traps, and the fact that these traps are filled via promotion of impurity electrons into the conduction band. Using tunable radiation we determine the threshold for trap filling, which is given by the ionization threshold of the impurity. This technique is a form of excitation spectroscopy, sensitive to electron (or hole) transport processes in solids. Advantages of TSL excitation spectroscopy over complementary techniques, such as photoconductivity, include impurity specific signals, applicability to both bulk and powder samples, and high sensitivity. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Thermally stimulated luminescence; Photoionization; Excitation spectroscopy
1. Introduction The optical properties of insulators doped with rare earth ions or transition elements are dominated by the intra-ion transition of the impurity, and these transitions have been the prominent focus of spectroscopic studies in the past [1]. However, for a complete picture of the optical properties of a doped solid, we have to include transitions in the UV spectral region involving both the impurity electronic states and the extended electronic states of the host lattice, i.e. *Corresponding author. Tel.: +1-706-542-2859; fax: +1706-542-2492. E-mail address: [email protected] (U. Happek). 1 LNBL, 1 Cyclotron Road, Berkeley, CA 94720 USA.
electron transfer processes [2,3]. Electron transfer processes play an important role in phosphors and scintillators, photovoltaic and photorefractive materials, spectral (chemical) holeburning, and can influence the efficiency of solid lasers via excited state absorption. In these and other examples the location of the impurity groundstate relative to the host valence and conduction band is an important factor. Photoconductivity is a technique that allows one to determine the position of impurity energy levels with respect to the host bands. McClure and co-workers have promoted this technique for the study of doped insulators [4], and our group has applied this method to study a number of systems. As a complementary technique, we investigated a method that uses thermally stimulated
0022-2313/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 0 1 ) 0 0 3 3 9 - 8
466
J. Fleniken et al. / Journal of Luminescence 94–95 (2001) 465–469
luminescence (TSL) to locate the impurity energy levels relative to the host bands. A typical application of TSL is the study of shallow traps, an excellent overview of this subject has been given by McKeever [5]. In our application of TSL we are not interested in the location of the traps, but simply use traps as an indicator for impurity electron promotion into the conduction band.
2. Thermally stimulated luminescence excitation spectroscopy (TSLES) TSLES is a form of excitation spectroscopy that probes the promotion of impurity electrons into the conduction band or holes into the valence band. Using tunable visible and UV radiation we determine the threshold for trap filling, which coincides with the ionization threshold of a specific impurity. The concept of this technique is shown in Fig. 1. The sample is illuminated at low temperature at a fixed energy hnE for a given period of time. If the energy of the incident light is sufficiently energetic to bridge the gap EPI between the impurity ground state and the conduction band CB (solid arrows, Fig. 1a), a small fraction of the delocalized electrons will be trapped. If the incident photon energy is too small to bridge the gap (dashed arrow, Fig. 1a), no electron transfer process occurs and the traps remain empty. After illumination the sample temperature is rapidly raised. If the traps had been filled during illumination, trapped electrons are released due to thermal activation and recombine with the ionized impurities, with subsequent emission of impurity specific radiation at hnL ; i.e. TSL is observed (Fig. 1b). The cycle is then repeated, illuminating the sample at a different wavelength. Plotting the integrated TSL signal as a function of excitation wavelength then gives the ionization threshold of the impurity ion. Likewise, the threshold ECT for charge transfer transitions can be determined. Advantages of TSLES include: Impurity and impurity site specific signal. A major advantage of this technique is the impurity specific signal, that is, the TSL signal is characteristic for an impurity or site.
Fig. 1. Principle of TSLES (top). Example of the excitation wavelength dependence of the TSL signal (bottom).
Electron and hole specific TSL signal. The TSL signal as a function of temperature depends on the trap depth. Since hole traps differ from electron traps, TSLES gives the opportunity to distinguish between photoionization and charge transfer processes. Combination of TSLES and photoconductivity measurements then allows one to distinguish between electron and hole conductivity. This application is important for doped insulators, where attempts to conduct Hall measurements are generally futile.
467
J. Fleniken et al. / Journal of Luminescence 94–95 (2001) 465–469
3. Experimental setup
4. Experimental results As a first example, Fig. 2 shows the TSLES spectrum of a single crystal of Y2SiO5 : Ce3+. The ionization threshold is evident. The signal rises sharply around 330 nm, which we interpret as the
6
10
5
10
4
10
3
10
2
2.0x10
4
1.5x10
4
1.0x10
4
5.0x10
3
Y2SiO5:Ce
260
280
300
320
340
3+
360
380
excitation wavelength (nm)
normalized intensity
The experimental apparatus consists of a temperature variable nitrogen cryostat, equipped with a high power heater to rapidly ramp the sample temperature from the base temperature of 77 K to up to 600 K. Since TSLES requires a cooling, exposure, and ramping cycle at each excitation wavelength, the cryostat was designed to allow rapid cooling of the sample. The shortest cycle time is about 30 min, but must often be extended to an hour if longer exposure and/or heating to very high temperatures are required. The sample is mounted onto the coldfinger of the cryostat and is illuminated with a 100 W Xe lamp or a 30 W Deuterium lamp, filtered by a 0.125 m monochromator. After the exposure at low temperatures, the lamp is turned off and a temperature controller ramps the temperature of the cold finger. During the heating cycle, the TSL is detected by a photomultiplier tube with a narrow band filter to select the detection wavelength, which is determined by a radiative transition of the specific impurity investigated. The photomultiplier is connected to a photon-counting system that records the TSL signal as a function of sample temperature. As an alternative, the entire TSL spectrum can be recorded by a CCD camera, mounted onto a monochromator. Fig. 1 (bottom) gives an example for the dependence of the TSL signal on the excitation wavelength. The peaks are due to electron traps with different depth. For TSLES the integrated signal is plotted as a function of the excitation wavelength.
10
normalized intensity
Experiments on single crystals, ceramic samples, and microcrystalline powder. TSLES can be easily applied to ceramic samples and microcrystalline or nanocrystalline particles.
Y2SiO5:Ce
3+
0.0 260
280
300
320
340
360
380
excitation wavelength (nm)
Fig. 2. TSLES spectrum of Y2SiO5 : Ce3+ single crystal in logarithmic (top) and linear (bottom) scale.
ionization threshold of Ce3+ in this material. This result is in excellent agreement with our photoconductivity experiments, which gives an ionization threshold around 325 nm [6]. The next example shows the applicability of TSLES for microcrystalline materials. Again, we compare our results with those obtained via photoconductivity. Fig. 3 (top) shows the photoconductive response of CaS : Eu2+ powder at different temperatures. The low temperature spectrum (T ¼ 80 K) displays two thresholds, one around 650 nm and a second around 510 nm [2]. We have interpreted the low energy threshold as tunneling of electrons between Eu2+ and Eu3+ ions, while the high energy threshold was attributed to the ionization of Eu2+ ions [7]. Fig. 3 (bottom) shows the corresponding TSLES signal
468
J. Fleniken et al. / Journal of Luminescence 94–95 (2001) 465–469 100
T=80k photoexcitation signal
80
60
40
20
0 250
300
350
400
wavelength [nm]
normalized TSL (photon counts)
80000
60000
40000
? 20000
0
260
280
300
320
340
360
380
400
wavelength (nm)
Fig. 4. Photoexcitation (top) and TSLES (bottom) of Li–Mg– silicate glass doped with Ce3+.
Fig. 3. Photoconductive (top) and TSLES (bottom) spectra of CaS : Eu2+ microcrystalline powder.
when the sample was irradiated at T ¼ 80 K. This time we find only one threshold, at 500 nm, confirming both the ionization threshold obtained using photoconductivity and our interpretation that the low energy threshold of the photoconductivity threshold was due to tunneling, since this process should not result in TSLES signal. Finally, we present very recent results on a Ce3+ doped Li–Mg–silicate glass. We observe a strong rise of the TSLES signal at 280 nm (Fig. 4, bottom), which can be interpreted as the ionization threshold. It has to be pointed out that his interpretation is preliminary. However, the photoexcitation spectrum of the glass (Fig. 4, top,
detection wavelength 440 nm) shows no structure around 280 nm due to band-to-band absorption, giving support to our tentative assignment of the ionization threshold for Ce3+ in this material.
5. Conclusion The presented examples demonstrate that TSLES is an excellent technique to obtain the ionization threshold of impurity ions in insulating materials. The method can be used for single crystals, powder, glasses, and thin films. A complete spectrum can be obtained within 1 day, and systematic study of the position of impurity energy levels with respect to the host valence and conduction bands are under way.
J. Fleniken et al. / Journal of Luminescence 94–95 (2001) 465–469
Acknowledgements The research was supported by a grant from the General Electric Corporation and an NSF grant (DMR 9986693). References [1] B. Henderson, G.F. Imbusch, Optical spectroscopy of Inorganic Solids, Clarendon Press, Oxford, 1989.
469
[2] U. Happek, S.A. Basun, J. Choi, J.K. Krebs, M. Raukas, J. Alloys, Compounds 303 (2000) 198. [3] M. Raukas, S.A. Basun, W. van Schaik, W.M. Yen, U. Happek, Appl. Phys. Lett. 69 (1996) 3300. [4] W.C. Wong, D.S. McClure, S.A. Basun, M.R. Kotka, Phys. Rev. B 51 (1995) 5682. [5] S.W.S. McKeever, Thermoluminescence of Solids, Cambridge University Press, Cambridge, New York, 1985. [6] J. Choi, Thesis, The University of Georgia, Athens, GA., 2001. [7] S.A. Basun, et al., Phys. Rev. B 56 (1997) 1030.
Thermally stimulated luminescence excitation spectroscopy (TSLES): a versatile technique to study electron transfer processes in solids J. Fleniken, J. Wang, J. Grimm, M.J. Weber1, U. Happek* Department of Physics and Astronomy, The University of Georgia, Athens, GA 30602-2451, USA
Abstract We present a technique to locate the ground state of impurity ions relative to the host conduction band, utilizing thermally stimulated luminescence (TSL). The technique makes use of the concept that TSL probes the occupation of electron traps, and the fact that these traps are filled via promotion of impurity electrons into the conduction band. Using tunable radiation we determine the threshold for trap filling, which is given by the ionization threshold of the impurity. This technique is a form of excitation spectroscopy, sensitive to electron (or hole) transport processes in solids. Advantages of TSL excitation spectroscopy over complementary techniques, such as photoconductivity, include impurity specific signals, applicability to both bulk and powder samples, and high sensitivity. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Thermally stimulated luminescence; Photoionization; Excitation spectroscopy
1. Introduction The optical properties of insulators doped with rare earth ions or transition elements are dominated by the intra-ion transition of the impurity, and these transitions have been the prominent focus of spectroscopic studies in the past [1]. However, for a complete picture of the optical properties of a doped solid, we have to include transitions in the UV spectral region involving both the impurity electronic states and the extended electronic states of the host lattice, i.e. *Corresponding author. Tel.: +1-706-542-2859; fax: +1706-542-2492. E-mail address: [email protected] (U. Happek). 1 LNBL, 1 Cyclotron Road, Berkeley, CA 94720 USA.
electron transfer processes [2,3]. Electron transfer processes play an important role in phosphors and scintillators, photovoltaic and photorefractive materials, spectral (chemical) holeburning, and can influence the efficiency of solid lasers via excited state absorption. In these and other examples the location of the impurity groundstate relative to the host valence and conduction band is an important factor. Photoconductivity is a technique that allows one to determine the position of impurity energy levels with respect to the host bands. McClure and co-workers have promoted this technique for the study of doped insulators [4], and our group has applied this method to study a number of systems. As a complementary technique, we investigated a method that uses thermally stimulated
0022-2313/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 0 1 ) 0 0 3 3 9 - 8
466
J. Fleniken et al. / Journal of Luminescence 94–95 (2001) 465–469
luminescence (TSL) to locate the impurity energy levels relative to the host bands. A typical application of TSL is the study of shallow traps, an excellent overview of this subject has been given by McKeever [5]. In our application of TSL we are not interested in the location of the traps, but simply use traps as an indicator for impurity electron promotion into the conduction band.
2. Thermally stimulated luminescence excitation spectroscopy (TSLES) TSLES is a form of excitation spectroscopy that probes the promotion of impurity electrons into the conduction band or holes into the valence band. Using tunable visible and UV radiation we determine the threshold for trap filling, which coincides with the ionization threshold of a specific impurity. The concept of this technique is shown in Fig. 1. The sample is illuminated at low temperature at a fixed energy hnE for a given period of time. If the energy of the incident light is sufficiently energetic to bridge the gap EPI between the impurity ground state and the conduction band CB (solid arrows, Fig. 1a), a small fraction of the delocalized electrons will be trapped. If the incident photon energy is too small to bridge the gap (dashed arrow, Fig. 1a), no electron transfer process occurs and the traps remain empty. After illumination the sample temperature is rapidly raised. If the traps had been filled during illumination, trapped electrons are released due to thermal activation and recombine with the ionized impurities, with subsequent emission of impurity specific radiation at hnL ; i.e. TSL is observed (Fig. 1b). The cycle is then repeated, illuminating the sample at a different wavelength. Plotting the integrated TSL signal as a function of excitation wavelength then gives the ionization threshold of the impurity ion. Likewise, the threshold ECT for charge transfer transitions can be determined. Advantages of TSLES include: Impurity and impurity site specific signal. A major advantage of this technique is the impurity specific signal, that is, the TSL signal is characteristic for an impurity or site.
Fig. 1. Principle of TSLES (top). Example of the excitation wavelength dependence of the TSL signal (bottom).
Electron and hole specific TSL signal. The TSL signal as a function of temperature depends on the trap depth. Since hole traps differ from electron traps, TSLES gives the opportunity to distinguish between photoionization and charge transfer processes. Combination of TSLES and photoconductivity measurements then allows one to distinguish between electron and hole conductivity. This application is important for doped insulators, where attempts to conduct Hall measurements are generally futile.
467
J. Fleniken et al. / Journal of Luminescence 94–95 (2001) 465–469
3. Experimental setup
4. Experimental results As a first example, Fig. 2 shows the TSLES spectrum of a single crystal of Y2SiO5 : Ce3+. The ionization threshold is evident. The signal rises sharply around 330 nm, which we interpret as the
6
10
5
10
4
10
3
10
2
2.0x10
4
1.5x10
4
1.0x10
4
5.0x10
3
Y2SiO5:Ce
260
280
300
320
340
3+
360
380
excitation wavelength (nm)
normalized intensity
The experimental apparatus consists of a temperature variable nitrogen cryostat, equipped with a high power heater to rapidly ramp the sample temperature from the base temperature of 77 K to up to 600 K. Since TSLES requires a cooling, exposure, and ramping cycle at each excitation wavelength, the cryostat was designed to allow rapid cooling of the sample. The shortest cycle time is about 30 min, but must often be extended to an hour if longer exposure and/or heating to very high temperatures are required. The sample is mounted onto the coldfinger of the cryostat and is illuminated with a 100 W Xe lamp or a 30 W Deuterium lamp, filtered by a 0.125 m monochromator. After the exposure at low temperatures, the lamp is turned off and a temperature controller ramps the temperature of the cold finger. During the heating cycle, the TSL is detected by a photomultiplier tube with a narrow band filter to select the detection wavelength, which is determined by a radiative transition of the specific impurity investigated. The photomultiplier is connected to a photon-counting system that records the TSL signal as a function of sample temperature. As an alternative, the entire TSL spectrum can be recorded by a CCD camera, mounted onto a monochromator. Fig. 1 (bottom) gives an example for the dependence of the TSL signal on the excitation wavelength. The peaks are due to electron traps with different depth. For TSLES the integrated signal is plotted as a function of the excitation wavelength.
10
normalized intensity
Experiments on single crystals, ceramic samples, and microcrystalline powder. TSLES can be easily applied to ceramic samples and microcrystalline or nanocrystalline particles.
Y2SiO5:Ce
3+
0.0 260
280
300
320
340
360
380
excitation wavelength (nm)
Fig. 2. TSLES spectrum of Y2SiO5 : Ce3+ single crystal in logarithmic (top) and linear (bottom) scale.
ionization threshold of Ce3+ in this material. This result is in excellent agreement with our photoconductivity experiments, which gives an ionization threshold around 325 nm [6]. The next example shows the applicability of TSLES for microcrystalline materials. Again, we compare our results with those obtained via photoconductivity. Fig. 3 (top) shows the photoconductive response of CaS : Eu2+ powder at different temperatures. The low temperature spectrum (T ¼ 80 K) displays two thresholds, one around 650 nm and a second around 510 nm [2]. We have interpreted the low energy threshold as tunneling of electrons between Eu2+ and Eu3+ ions, while the high energy threshold was attributed to the ionization of Eu2+ ions [7]. Fig. 3 (bottom) shows the corresponding TSLES signal
468
J. Fleniken et al. / Journal of Luminescence 94–95 (2001) 465–469 100
T=80k photoexcitation signal
80
60
40
20
0 250
300
350
400
wavelength [nm]
normalized TSL (photon counts)
80000
60000
40000
? 20000
0
260
280
300
320
340
360
380
400
wavelength (nm)
Fig. 4. Photoexcitation (top) and TSLES (bottom) of Li–Mg– silicate glass doped with Ce3+.
Fig. 3. Photoconductive (top) and TSLES (bottom) spectra of CaS : Eu2+ microcrystalline powder.
when the sample was irradiated at T ¼ 80 K. This time we find only one threshold, at 500 nm, confirming both the ionization threshold obtained using photoconductivity and our interpretation that the low energy threshold of the photoconductivity threshold was due to tunneling, since this process should not result in TSLES signal. Finally, we present very recent results on a Ce3+ doped Li–Mg–silicate glass. We observe a strong rise of the TSLES signal at 280 nm (Fig. 4, bottom), which can be interpreted as the ionization threshold. It has to be pointed out that his interpretation is preliminary. However, the photoexcitation spectrum of the glass (Fig. 4, top,
detection wavelength 440 nm) shows no structure around 280 nm due to band-to-band absorption, giving support to our tentative assignment of the ionization threshold for Ce3+ in this material.
5. Conclusion The presented examples demonstrate that TSLES is an excellent technique to obtain the ionization threshold of impurity ions in insulating materials. The method can be used for single crystals, powder, glasses, and thin films. A complete spectrum can be obtained within 1 day, and systematic study of the position of impurity energy levels with respect to the host valence and conduction bands are under way.
J. Fleniken et al. / Journal of Luminescence 94–95 (2001) 465–469
Acknowledgements The research was supported by a grant from the General Electric Corporation and an NSF grant (DMR 9986693). References [1] B. Henderson, G.F. Imbusch, Optical spectroscopy of Inorganic Solids, Clarendon Press, Oxford, 1989.
469
[2] U. Happek, S.A. Basun, J. Choi, J.K. Krebs, M. Raukas, J. Alloys, Compounds 303 (2000) 198. [3] M. Raukas, S.A. Basun, W. van Schaik, W.M. Yen, U. Happek, Appl. Phys. Lett. 69 (1996) 3300. [4] W.C. Wong, D.S. McClure, S.A. Basun, M.R. Kotka, Phys. Rev. B 51 (1995) 5682. [5] S.W.S. McKeever, Thermoluminescence of Solids, Cambridge University Press, Cambridge, New York, 1985. [6] J. Choi, Thesis, The University of Georgia, Athens, GA., 2001. [7] S.A. Basun, et al., Phys. Rev. B 56 (1997) 1030.