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Comparison of luminescence- and ESR-investigations in ZnS: Fe
Journal of Luminescence 20 (1979) 403—408 © North-Holland Publishing Company
COMPARISON OF LUMINESCENCE- AND ESR-INVESTIGATIONS IN ZnS:Fe H. NELKOWSKI, 0. PFUTZENREUTER and W. SCHRITTENLACHER Institut für Festkörperphysik der Technischen Universitit Berlin, Germany Received 23 February 1979
It is possible to change the ionization state of iron in ZnS by light excitation in the visible and uv and by changing sample temperature. This can be confirmed by ESRmeasurements. To clarify the nature of a new emission band (Xmax = 980 nm) in ZnS:Fe a comparison between luminescence- and ESR-measurements has been performed. These 3~-ionsas the emitting centers for the 980 nm results can be interpreted by assuming Fe luminescence. Furthermore a sensitization of this emission by the self-activated luminescence is concluded.
1. Introduction ZnS: Fe has been studied for a long time in different respects. It is well known that iron is a killer of the visible luminescence in ZnS [1—4].On the other hand by incorporation of iron new luminescence bands arise. One emission in the far IR (Xmax = 3.3 jim) could be attributed to an internal transition in Fe2~-centers[5T 5D)— 2( as a ~E(5 D)] [5,6]. Another band in the visible (Amax = 660 nm) was explained donor—acceptor transition [7]. Recently we found a further iron luminescence band in ZnS (Xmax = 980 nm) [8]. This emission band was examined in the ZnS~Se 1~ solid solutions. The band maxima turned out to be independent of composition. This fact together with half-width and shape of this emission band favours an internal transition in iron centers. It is known from ESR-measurements [7,9] that the ionization state of iron in ZnS can be influenced by radiation and temperature. Comparison of the ESR- and luminescence measurements in the same crystals should provide criteria whether it is possible to attribute the 980 nm emission to iron centers and to determine the corresponding ionization state. Furthermore, some conclusions can be drawn about the kinetics of these transitions. 2. Phosphors and equipment The single crystals under investigation had pure cubic structure and were doped with 100 ppm Fe. They contained also manganese as an accidental impurity. 403
404
II. Nelkow~kjetai /Lumjnescence and ESR in ZnS:Fe
UV-irradiation was performed with the 365 nm line of a mercury lamp (200 W) and for the irradiation in the visible spectral region a xenon lamp in combination with interference filters or grid monochromator was used. The luminescence was measured by a three-prism-spectrograph in connection with cooled multipliers with S-8 cathode for the blue-red- and with Si-cathode for the red-infrared spectral region. The ESR-measurements were made with a Varian V-4502 spectrometer equipped with a Varian V-4531 multipurpose cavity.
3. Experimental results and discussion 3.1. Growth and decay ofluminescence- and ofESR-signal during and after uv-irradiation The unexcited crystals showed only the ESR-signal of Mn2~.After irradiation with uv at 77 K we observed the two well known additional signals of the A- and the Fe3~-centers* [9]. The rise and decay of the intensities of these two ESRsignals at 77 K during and after uv-excitation is shown in fig. 1. The two signals increase slowly and reach a stationary value after a few minutes. Switching off the excitation, there is a pronounced difference in the behaviour of the two signals. In some minutes the Ak-signal decreases for about 50% while the Fe3~-signalincreases for about 10%. The decrease of the A ‘-signal is possibly due to the recombination of conduction electrons or electrons from shallow traps with Ak-centers and has also been observed by Räuber, Schneider and Matossi [9]. The Fe3~-signalremained nearly constant in their measurement. Jazczyn-Kopec and Lambert [7] measured a slight decrease of the Fe3~-signalwhile in CdS: Fe and CdSe: Fe Hoshina et al. [10,11] observed a behaviour similar to our results in ZnS:Fe. They explained the increase of the Fe3~-signalby hole capture of Fe2~centers. The increase in Fe3~-concentrationmay be compensated more or less by recombination of Fe3~-centerswith electrons of appropriate donors. This recombination process was proposed as the mechanism for the red Fe-luminescence [7]. The rise of the infrared iron luminescence (Amax = 980 nm) is very similar to that of Fe3~-ESR-signal.Excited by uv-light the luminescence increases slowly (curve 1 in fig. 2) and reaches a stationary value after a few minutes. After switching off the excitation the luminescence decreases rapidly to about 15% of the initial value and shows a long after glow. When irradiated again with uv-light (curve 2—4 in fig. 2) the growth depends on the time elapsed after the preceding excitation. A possible explanation of this behaviour is the assumption of an energy transfer process from A-centers to Fe3~-centers.UV-irradiation simultaneously creates Fe3~*
The transition of an electron from a shallow donor to an excited A-center (D—A pab) is accepted to be responsible for the blue self-activated luminescence (SAL) 14.
H. Nelkowski et al. / Luminescence and ESR in ZnS:Fe UV on
405
UV off
r
~ Time [mm] 3~-(1) and A~-ESRsignal (2) during and after uv-irradiaFig. 1. Increase and decrease of the Fe tion of a previously unexcited ZnS:Fe crystal at 80K.
centers and A~-centers (fig. 1). The recombination of electrons in the A~-centers gives rise to the SAL but the most part of the energy is transferred to Fe3~-centers. During the uv-excitation the 980 nm luminescence depends on the existence of both, Fe3~-and Ak-centers. Therefore the luminescence increases slowly, if the crystal was previously unexcited. (Curve 1 in fig. 2.) After switching off the excitation the A~signal decreases slowly and the Fe3~-signalincreases even, as just discussed (fig. 1). Switching on the uv-light again, the Fe3~-centersare still present and the 980 nm emission increases very fast to a value dependent on the actual A~-centerconcentration and then slowly to the stationary value. The fast decrease of the 980 nm emission after switching off the uv-light is due to the sudden decrease of the number of generated electrons and holes and therefore of recombination of electrons in A~centers. The long afterglow corresponds to the remaining rate as can be seen from
I
IJVon
UVoff
Lum
~1’2
14’ Time [mm]
Fig. 2. Increase and decrease of the 980 nm emission of ZnS:Fe at 77 K during and after uvirradiation. A previously unexcited crystal (1) after waiting for 2 s (2), 30 s (3) and 45 mm in the dark (4).
406
H. Nelkowski et al.
/ Luminescence and ESR in ZnS.Fe
the decrease of the At-signal (fig. 1). The time characteristics of the 980 nm emission are very similar to the results obtained for the SAL in undoped ZnS [12]. This close correlation suggests the assumption on an energy transfer process. 3.2. Dependence of the ESR-signals and luminescence intensity on excitation in the visible spectral region By uv-irradiation ZnS: Fe is transferred in an excited state where the incorporated iron partly changes its ionization state to Fe3~(and traps are filled). At 77 K this excited state is stable in the dark, Irradiating the excited crystal in the visible spectral region causes a decay of the Fe3tESR~signalintensity as was found by Räuber et al. [9]. They observed a quenching maximum at a wavelength of 580 nm. This was connected with an increase in the A4’-signal. They explained this effect by assuming an excitation of valence band electrons in Fe3tcenters and a subsequent hole capture by the A-centers. Our ESR-measurement shows also the decrease of the Fe3~-signal during 578 nm irradiation but there was a weak decrease of the A’~-signal,too. In our luminescence measurement we observe at 77 K a fast increase of the 980 nm emission with a slow decay when irradiating the excited crystal with light of 578 nm wavelength. The behaviour of the SAL is reversed, it increases monotonously and remains nearly constant up to the longest measured time of 45 mm. The time dependence of these two bands is shown in fig. 3. Irradiating the uv-excited crystal at 77 K with 578 nm light, empties the traps, as is seen in the decrease of luminescence- and conductivity-glowpeaks (curve lb, 2b and 3b in fig. 4). The recombination of electrons from traps in A4’-centers causes
I
100 Lum [nu]
Y
Time [mm]
Fig. 3. Normalized emission intensity of the SA- and 980 nm luminescence of a previously uvirradiated ZnS:Fe crystal at 77 K during 578 nm excitation.
H, Nelkowskj et al.
la
A
r~00
~ ~
/ Luminescence and ESR in ZnS:Fe
2a
3
~(
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~-J I
1!~-~~L1R ~[‘ a ~
u,
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,~
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ta
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*~
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~. 0/
a
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I /1b~
407
I
100
\~ 0
\ \
f 200
I
5
•
-~
\
300
600 Temperature
[x]
Fig. 4. Luminescence and conductivity glow curves of ZnS:Fe. The crystal was for 5 mm excited by 365 nm radiation at 77 K and then left for 10 mm in the dark (a-curves) or bradiated by 578 nm light for 10 mm (b-curves); 1: 980 nm emission, 2: red Fe emission, 3: conductivity.
3~-centers sensitizing the SAL. The part of recombination energy transferred Fe 980 rim luminescence depends on the number of thesetocenters. This number decreases by capture of electrons from the conduction band or the valence band by 578 nm irradiation. Therefore after a fast increase the 980 nm luminescence decreases and SAL increases (fig. 3). These results are a further strong support for the hypothesis of the proposed energy transfer process. —
—
3.3. Temperature characteristics Fig. 5 shows the temperature dependence of the luminescence intensity (during stationary 365 nm excitation) of the blue SA- the red Fe- and the 980 nm band in ZnS : Fe. The 980 nm emission decreases strongly with increasing temperature and has an intermediate maximum at 90 K. This maximum coincides with the only glow peak of this band (fig. 4). In contrast to the 980 nm emission the red band increases with temperature and remains nearly constant up to 450 K. The SAL increases with temperature for T> 100 K, has a maximum at 200 K and falls off again. Jaszczyn-Kopec and Lambert L7] measured a descend in Fe3’~-concentrationto about 30% between 100 and 200 K. This diminution causes a decrease of the 980 nm emission and a simultaneous increase of the SAL-emission analogous to the discussion in section 3.2. Above 150 K the red Fe emission (Fe3~+ e = Fe2” + hv) and therefore the Fe3tconcentration remains nearly constant up to 500 K while the 980 nm emission descends very strongly. To explain this contradiction we assume
/1. Nel/cowski at al, / Luminescence and ESR in ZnS:Fe
408
I Lum [au]
b
1:
100
200
300
400 Temperature
[K]
Fig. 5. Temperature dependence of the normalized emission intensity of ZnS:Fe under stationary uv-excitation; (a) blue SA emission, (b) red Fe emission, (c) 980 nm emission.
strong increase of the radiationless transition rate in Fe~~centers at about 100 K. A confirmation to this is given by the quite similar temperature dependence in the radiationless transition rate of the IR-photoluminescence of ZnSe :Fe3~[13]. a
Acknowledgement We thank the Deutsche Forschungsgemeinschaft for providing the ESR-spectrometer.
References Ii]
N. Riehl and H. Ortmann, Z. Phys. Chem. A188 (1941) 109. [2] R.H. Bube, S. Larach and RE. Schrader, Phys. Rev. 92 (1953) 1135. [31 G. Gergeley, J. Phys. Radium 17 (1956) 679. [4] N. Arpiarian, J. Phys. Radium 17 (1956) 674. [51 GA. Slack and B.M. O’Meara, Phys. Rev. 163 (1967) 335. 161 F.S. Ham and G.A. Slack, Phys. Rev. B4 (1971) 777. [71 P. Jaszczyn-Kopec and B. Lambert, J. Luminescence 10 (1975) 243. [81 H. Nellcowski, 0. Pfbtzenreuter and W. Schrittenlacher, J. Luminescence 17 (1978) 419. [9] A. R’auber, J. Schneider and F. Matossi, Z. Naturforsch. 17a (1962) 654. [10] K. Morigaki and T. Hoshina, J. Phys. Soc. Japan 21(1966)842. [11] T. Hoshina, J. Phys. Soc. Japan 22 (1967) 1049. [121 0. PfOtzenreuter, Thesis, TU Berlin (1976). [13] H. Nelkowski, 0. Pfbtzenreuter and W. Schrittenlacher, to be published. [14] 5. Oda and K. Kukimoto, J. Luminescence 18/19 (1979) 829.
COMPARISON OF LUMINESCENCE- AND ESR-INVESTIGATIONS IN ZnS:Fe H. NELKOWSKI, 0. PFUTZENREUTER and W. SCHRITTENLACHER Institut für Festkörperphysik der Technischen Universitit Berlin, Germany Received 23 February 1979
It is possible to change the ionization state of iron in ZnS by light excitation in the visible and uv and by changing sample temperature. This can be confirmed by ESRmeasurements. To clarify the nature of a new emission band (Xmax = 980 nm) in ZnS:Fe a comparison between luminescence- and ESR-measurements has been performed. These 3~-ionsas the emitting centers for the 980 nm results can be interpreted by assuming Fe luminescence. Furthermore a sensitization of this emission by the self-activated luminescence is concluded.
1. Introduction ZnS: Fe has been studied for a long time in different respects. It is well known that iron is a killer of the visible luminescence in ZnS [1—4].On the other hand by incorporation of iron new luminescence bands arise. One emission in the far IR (Xmax = 3.3 jim) could be attributed to an internal transition in Fe2~-centers[5T 5D)— 2( as a ~E(5 D)] [5,6]. Another band in the visible (Amax = 660 nm) was explained donor—acceptor transition [7]. Recently we found a further iron luminescence band in ZnS (Xmax = 980 nm) [8]. This emission band was examined in the ZnS~Se 1~ solid solutions. The band maxima turned out to be independent of composition. This fact together with half-width and shape of this emission band favours an internal transition in iron centers. It is known from ESR-measurements [7,9] that the ionization state of iron in ZnS can be influenced by radiation and temperature. Comparison of the ESR- and luminescence measurements in the same crystals should provide criteria whether it is possible to attribute the 980 nm emission to iron centers and to determine the corresponding ionization state. Furthermore, some conclusions can be drawn about the kinetics of these transitions. 2. Phosphors and equipment The single crystals under investigation had pure cubic structure and were doped with 100 ppm Fe. They contained also manganese as an accidental impurity. 403
404
II. Nelkow~kjetai /Lumjnescence and ESR in ZnS:Fe
UV-irradiation was performed with the 365 nm line of a mercury lamp (200 W) and for the irradiation in the visible spectral region a xenon lamp in combination with interference filters or grid monochromator was used. The luminescence was measured by a three-prism-spectrograph in connection with cooled multipliers with S-8 cathode for the blue-red- and with Si-cathode for the red-infrared spectral region. The ESR-measurements were made with a Varian V-4502 spectrometer equipped with a Varian V-4531 multipurpose cavity.
3. Experimental results and discussion 3.1. Growth and decay ofluminescence- and ofESR-signal during and after uv-irradiation The unexcited crystals showed only the ESR-signal of Mn2~.After irradiation with uv at 77 K we observed the two well known additional signals of the A- and the Fe3~-centers* [9]. The rise and decay of the intensities of these two ESRsignals at 77 K during and after uv-excitation is shown in fig. 1. The two signals increase slowly and reach a stationary value after a few minutes. Switching off the excitation, there is a pronounced difference in the behaviour of the two signals. In some minutes the Ak-signal decreases for about 50% while the Fe3~-signalincreases for about 10%. The decrease of the A ‘-signal is possibly due to the recombination of conduction electrons or electrons from shallow traps with Ak-centers and has also been observed by Räuber, Schneider and Matossi [9]. The Fe3~-signalremained nearly constant in their measurement. Jazczyn-Kopec and Lambert [7] measured a slight decrease of the Fe3~-signalwhile in CdS: Fe and CdSe: Fe Hoshina et al. [10,11] observed a behaviour similar to our results in ZnS:Fe. They explained the increase of the Fe3~-signalby hole capture of Fe2~centers. The increase in Fe3~-concentrationmay be compensated more or less by recombination of Fe3~-centerswith electrons of appropriate donors. This recombination process was proposed as the mechanism for the red Fe-luminescence [7]. The rise of the infrared iron luminescence (Amax = 980 nm) is very similar to that of Fe3~-ESR-signal.Excited by uv-light the luminescence increases slowly (curve 1 in fig. 2) and reaches a stationary value after a few minutes. After switching off the excitation the luminescence decreases rapidly to about 15% of the initial value and shows a long after glow. When irradiated again with uv-light (curve 2—4 in fig. 2) the growth depends on the time elapsed after the preceding excitation. A possible explanation of this behaviour is the assumption of an energy transfer process from A-centers to Fe3~-centers.UV-irradiation simultaneously creates Fe3~*
The transition of an electron from a shallow donor to an excited A-center (D—A pab) is accepted to be responsible for the blue self-activated luminescence (SAL) 14.
H. Nelkowski et al. / Luminescence and ESR in ZnS:Fe UV on
405
UV off
r
~ Time [mm] 3~-(1) and A~-ESRsignal (2) during and after uv-irradiaFig. 1. Increase and decrease of the Fe tion of a previously unexcited ZnS:Fe crystal at 80K.
centers and A~-centers (fig. 1). The recombination of electrons in the A~-centers gives rise to the SAL but the most part of the energy is transferred to Fe3~-centers. During the uv-excitation the 980 nm luminescence depends on the existence of both, Fe3~-and Ak-centers. Therefore the luminescence increases slowly, if the crystal was previously unexcited. (Curve 1 in fig. 2.) After switching off the excitation the A~signal decreases slowly and the Fe3~-signalincreases even, as just discussed (fig. 1). Switching on the uv-light again, the Fe3~-centersare still present and the 980 nm emission increases very fast to a value dependent on the actual A~-centerconcentration and then slowly to the stationary value. The fast decrease of the 980 nm emission after switching off the uv-light is due to the sudden decrease of the number of generated electrons and holes and therefore of recombination of electrons in A~centers. The long afterglow corresponds to the remaining rate as can be seen from
I
IJVon
UVoff
Lum
~1’2
14’ Time [mm]
Fig. 2. Increase and decrease of the 980 nm emission of ZnS:Fe at 77 K during and after uvirradiation. A previously unexcited crystal (1) after waiting for 2 s (2), 30 s (3) and 45 mm in the dark (4).
406
H. Nelkowski et al.
/ Luminescence and ESR in ZnS.Fe
the decrease of the At-signal (fig. 1). The time characteristics of the 980 nm emission are very similar to the results obtained for the SAL in undoped ZnS [12]. This close correlation suggests the assumption on an energy transfer process. 3.2. Dependence of the ESR-signals and luminescence intensity on excitation in the visible spectral region By uv-irradiation ZnS: Fe is transferred in an excited state where the incorporated iron partly changes its ionization state to Fe3~(and traps are filled). At 77 K this excited state is stable in the dark, Irradiating the excited crystal in the visible spectral region causes a decay of the Fe3tESR~signalintensity as was found by Räuber et al. [9]. They observed a quenching maximum at a wavelength of 580 nm. This was connected with an increase in the A4’-signal. They explained this effect by assuming an excitation of valence band electrons in Fe3tcenters and a subsequent hole capture by the A-centers. Our ESR-measurement shows also the decrease of the Fe3~-signal during 578 nm irradiation but there was a weak decrease of the A’~-signal,too. In our luminescence measurement we observe at 77 K a fast increase of the 980 nm emission with a slow decay when irradiating the excited crystal with light of 578 nm wavelength. The behaviour of the SAL is reversed, it increases monotonously and remains nearly constant up to the longest measured time of 45 mm. The time dependence of these two bands is shown in fig. 3. Irradiating the uv-excited crystal at 77 K with 578 nm light, empties the traps, as is seen in the decrease of luminescence- and conductivity-glowpeaks (curve lb, 2b and 3b in fig. 4). The recombination of electrons from traps in A4’-centers causes
I
100 Lum [nu]
Y
Time [mm]
Fig. 3. Normalized emission intensity of the SA- and 980 nm luminescence of a previously uvirradiated ZnS:Fe crystal at 77 K during 578 nm excitation.
H, Nelkowskj et al.
la
A
r~00
~ ~
/ Luminescence and ESR in ZnS:Fe
2a
3
~(
/
~-J I
1!~-~~L1R ~[‘ a ~
u,
j
,~
C
ta
t1
L~5~ tC
>w —u
H
*~
i
—
~. 0/
a
\
p
+
~
1 e
I \
•4I,
3b
0,~’
\
IS
I /1b~
407
I
100
\~ 0
\ \
f 200
I
5
•
-~
\
300
600 Temperature
[x]
Fig. 4. Luminescence and conductivity glow curves of ZnS:Fe. The crystal was for 5 mm excited by 365 nm radiation at 77 K and then left for 10 mm in the dark (a-curves) or bradiated by 578 nm light for 10 mm (b-curves); 1: 980 nm emission, 2: red Fe emission, 3: conductivity.
3~-centers sensitizing the SAL. The part of recombination energy transferred Fe 980 rim luminescence depends on the number of thesetocenters. This number decreases by capture of electrons from the conduction band or the valence band by 578 nm irradiation. Therefore after a fast increase the 980 nm luminescence decreases and SAL increases (fig. 3). These results are a further strong support for the hypothesis of the proposed energy transfer process. —
—
3.3. Temperature characteristics Fig. 5 shows the temperature dependence of the luminescence intensity (during stationary 365 nm excitation) of the blue SA- the red Fe- and the 980 nm band in ZnS : Fe. The 980 nm emission decreases strongly with increasing temperature and has an intermediate maximum at 90 K. This maximum coincides with the only glow peak of this band (fig. 4). In contrast to the 980 nm emission the red band increases with temperature and remains nearly constant up to 450 K. The SAL increases with temperature for T> 100 K, has a maximum at 200 K and falls off again. Jaszczyn-Kopec and Lambert L7] measured a descend in Fe3’~-concentrationto about 30% between 100 and 200 K. This diminution causes a decrease of the 980 nm emission and a simultaneous increase of the SAL-emission analogous to the discussion in section 3.2. Above 150 K the red Fe emission (Fe3~+ e = Fe2” + hv) and therefore the Fe3tconcentration remains nearly constant up to 500 K while the 980 nm emission descends very strongly. To explain this contradiction we assume
/1. Nel/cowski at al, / Luminescence and ESR in ZnS:Fe
408
I Lum [au]
b
1:
100
200
300
400 Temperature
[K]
Fig. 5. Temperature dependence of the normalized emission intensity of ZnS:Fe under stationary uv-excitation; (a) blue SA emission, (b) red Fe emission, (c) 980 nm emission.
strong increase of the radiationless transition rate in Fe~~centers at about 100 K. A confirmation to this is given by the quite similar temperature dependence in the radiationless transition rate of the IR-photoluminescence of ZnSe :Fe3~[13]. a
Acknowledgement We thank the Deutsche Forschungsgemeinschaft for providing the ESR-spectrometer.
References Ii]
N. Riehl and H. Ortmann, Z. Phys. Chem. A188 (1941) 109. [2] R.H. Bube, S. Larach and RE. Schrader, Phys. Rev. 92 (1953) 1135. [31 G. Gergeley, J. Phys. Radium 17 (1956) 679. [4] N. Arpiarian, J. Phys. Radium 17 (1956) 674. [51 GA. Slack and B.M. O’Meara, Phys. Rev. 163 (1967) 335. 161 F.S. Ham and G.A. Slack, Phys. Rev. B4 (1971) 777. [71 P. Jaszczyn-Kopec and B. Lambert, J. Luminescence 10 (1975) 243. [81 H. Nellcowski, 0. Pfbtzenreuter and W. Schrittenlacher, J. Luminescence 17 (1978) 419. [9] A. R’auber, J. Schneider and F. Matossi, Z. Naturforsch. 17a (1962) 654. [10] K. Morigaki and T. Hoshina, J. Phys. Soc. Japan 21(1966)842. [11] T. Hoshina, J. Phys. Soc. Japan 22 (1967) 1049. [121 0. PfOtzenreuter, Thesis, TU Berlin (1976). [13] H. Nelkowski, 0. Pfbtzenreuter and W. Schrittenlacher, to be published. [14] 5. Oda and K. Kukimoto, J. Luminescence 18/19 (1979) 829.