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Energy transfer in ZnS:Cu:In phosphors
J. Phys. Chem. Solids
Pergamon
ENERGY
Press 1959. Vol. 9. pp. 149-152.
TRANSFER
Printed in Great Britain.
IN ZnS:Cu:In
PHOSPHORS
N. T. MELAMED Westinghouse
Research Laboratories,
Pittsburgh
35, Pennsylvania
(Received 25 July 1958)
Abstract-Transport of energy between luminescence centers has been observed in ZnS:Cu:In phosphors at room temperature and at 77°K. The results support a photoconductive transfer mechanism. Energy transfer at low temperatures can be interpreted as resulting from a process in which a given wavelength is capable of putting an electron into an impurity level from the valence band, as well as exciting it from the same impurity level to the conduction band.
1. INTRODUCTION IN a recent
in 2nS:Cu:Cl phosphors, we observed that, from 300” to 4”K, irradiation into the excitation band of the blue emitting centers resulted in an emission from the green luminescence centers. We inferred that excitation energy was transported from blue centers to green centers, and came to the opinion that the most likely mechanism of transfer was photoconductive. An important feature in a photoconductive transfer process was the assumption that optically induced transitions can take place between an impurity level and either band. It was thought possible to test this assumption, as well as several other possible transfer mechanisms which could not be entirely eliminated, by observing the transport of energy in phosphors similar in composition to ZnS:Cu:Cl, but in which the chlorine is replaced by a different coactivator. Indium was chosen because glow-curve data indicate that, of the known coactivators, it forms the deepest level and therefore differs most in this respect from the chlorine it replaces. paper(l)
dealing
2. EXPERIMENTAL
with energy
transfer
of ZnS :Cu:In. The blue emission band, attributed to zinc vacancies, lies at about 4600 A. The green copper band is at 5200 8, and the orange
)O,
:“,
I
,
Electron
Volts
I 2.5 , I
I
2-O
1 1 ,
1 .,
.9 -
4 -
Emission Spectra -298OK --1160K ...... ...__ 770K
RESULTS
The phosphors were prepared from luminescence-grade zinc sulfide and were fired 1 hr at 950” in HsS. The impurities were added as copper acetate and indium sulfide. Emission and excitation spectra were obtained in a manner described for ZnS:Cu:Cl.(l) Fig. 1 shows the emission spectrum of a sample
K
6
h
40000
h
1 I
4500
~~~Ifi~~L’~~~~’
““‘I
5000 5500 6000 Wovelength in &
7000
FIG. 1. Emission spectrum of ZnS:C~(l0-~):1n at 298”, 116”, and 77°K. Because of their large half-widths, the individual blue, green, and red emission bands are not clearly distinguishable. 149
N.
150
T.
MELAMED
emission band of indium has its peak at 5850 A. The positions of the emission bands agree with the data obtained by KR&ER and DIKHOFF.@) The presence of a blue emission in ZnS:Cu:In suggests that indium, like chlorine, can give rise to zinc vacancies at low copper concentrations, apart from its role as charge compensator for copper. (2)
absorption of radiation at blue centers with a subsequent transfer of energy to green centers.(l) In ZnS:Cu:In, the excitation bands are most numerous, due in part to the greater number of emission bands. The orange emission of indium is excited in three bands: at 4250,3850, and 3450 b. Excitotlon
Excitollon 40 I
45
Energy
in Electron 3.5 I Excitoiion
Volts 30 I Spectra
-
Energy
in Electron
Volts
+/;I
IOBlue Emission -- - Green Emission ......“--. Red Emission
-
T =77*K
0
(
2800
I
,
3000
,
,
v
,
I
3200 3400 3600 Excitation Wovelength,
4000
44po
2900
3000
4ooO 3200 3400 3600 Excitation Wovelength, %
4400
FIG. 2. Excitation spectra for the blue, green, and red emissions of ZnS:Cu(lO-3:In at 298” and 77°K.
FIG. 3. Excitation spectra for the blue and green emis-
Fig. 2 shows the excitation spectra for the blue, green, and orange emissions of ZnS:Cu (0.0001 per cent) :In at room temperature and at 77°K. For purposes of comparison the excitation spectra of ZnS:Cu(O*OOOl per cent):Cl are shown in Fig. 3. Considering first the excitation bands of ZnS:Cu:Cl, we find an excitation band for the blue emission at 3450 a, which we believe corresponds to the absorption band for blue centers. We find two excitation bands for the green emission, at 3850 and 3450 8. The 3850 A excitation band corresponds to the absorption band of the green centers; the 3450 a excitation band arises from the
The 4250 i% band we attribute to the absorption band of indium. The two bands at shorter wavelengths we believe result from a transfer of energy from green and from blue centers to orange centers. The blue and green emissions of ZnS:Cu:In show excitation bands similar to those found in ZnS :Cu :Cl, with, however, several additional excitation bands which are not present in ZnS :Cu:Cl. For the blue emission, these additional bands lie at 3850 and 4250 A. The appearance of two new excitation bands for the blue emission in ZnS:Cu:In has significance towards establishing the mecharism of energy transfer in ZnS. These bands, lying at 3850 and
sions of Z~S:CU(~O-~):CI
at 298”, 77”, and 4°K.
ENERGY
TRANSFER
IN
4250 A, coincide with what we believe are the excitation peaks for direct absorption in copper and indium. They indicate that in the 2nS:Cu:In phosphor a transfer of energy takes place from the copper and indium centers to the zinc vacancies. In Fig. 4 is shown an energy-level diagram for ZnS:Cu:In, which we deduce from our optical measurements. Conduction
-
36e.V. 3450%)
Bond
T I
t (b)
I
(‘.
2.7e.V. (460061 32e.V (38SOii (c
I
Zn Vac-
I
-: (d) Valence
Band
FIG. 4. Energy-level diagram for the ZnS:Cu:In phosphor deduced from measurements of excitation and emission spectra. The excitation spectra for transitions (b) and (d) are assumed to extend from the near-infrared through the visible to the near-ultraviolet region. 3. DISCUSSION
There is evidence in various phosphors that energy can be transported from one luminescence center to another by three mechar isms: by resonance, by cascade, or by a photoconductivity process. The presence of the 3850 ar.d 4250 A excitation bands for the blue emission in ZnS :Cu :In strongly supports a photoconductive transfer mechar ism. Both resonance transfer and cascade trarsfer require that the emission band of the absorl i g center (or sensitizer) overlaps the absorption band of the emittir g cer,ter.(s) Since the emission bards of both copper ar_d indium lie too far to the lotg-waveler gth side of the 3450 A blue excitation band for overlap to occur, it is not likely
ZnS:Cu:In
PHOSPHORS
151
that either a resonance or a cascade process can play an important role in the transfer from red or green centers to blue. To account for a photoconductive transfer which results from excitation with energy of less than the band gap, two processes have been proposed.(l) In the first process, an electron is excited from the valence band into an unoccupied donor, leaving a free hole (for example, transition Q, in Fig. 4). In addition, an electron is excited into the conduction band from an occupied acceptor level (c in Fig. 4). Energy transfer results from the recombination of the electron and the free hole at a third center. Such a process requires an overlap in the excitation spectra of a particular set of donor and acceptor impurity levels; it predicts an eventual depletion of one or both of these levels, with a consequent diminution of transfer. Because we have not been able to observe any evidence of exhaustion, we do not think this process, taken by itself, is the major cause of transfer. As an alternative process we proposed one which is more general, and of which the first process may be regarded as a special case. In this, the more general process, optical transitions can take place between an impurity level and either band. In particular, the same radiation which can fill vacant indium levels from the valence band, can also excite electrons from occupied indium levels to the conduction bard (transitions a and b, Fig. 4) or, by analogy, radiation which excites electrons from occupied copper levels to the conduction band can also partially fill vacant copper centers from the valence bar d (transitiors c ar-d d). Such a process leads to a photoconductive transfer which requires rLo overlap in the excitation spectra of donors and acceptors, predicts no depletion of impurity centers, ar.d can occur to a significant extent at low temperatures, We believe the more general photoconductive process to be the explanation for the energy transfer in ZnS. It is consistert with our observation that no reduction in trarsfer occurs with time. It is further supported by evidence which indicates that electron traps can be emptied by nearultraviolet radiation. The emptyirg of traps by short-waveler gth radiation had been previously proposed by ANTONOV-ROMANOVSKIIand SHUKEN to explain the increased absorption at 4500 A which results from excitation in ZnS:Cu:Co,(4) by
152
N.
T.
MELAMED
SMITH and TURKEVICH to account for the emptying of traps in copper activated ZnS by visible and near-ultraviolet radiation(s) and by HOOGENSTRAATENand KLASENS to explain the intensitydependence of the light sum of ZnS:Cu:Co under 3650 A excitation.(s) Transitions such as (6) and (d) also can be used to explain the increase in absorption throughout the entire visible range that has been observed in a number of other sulfide phosphors during excitation.(T) The proposed photoconductive mechanism further suggests that transfer can occur from any one center to any other. That this indeed appears to be the case is seen in the excitation spectrum for the orange emission. This consists of three excitation bands: the long-wavelength band, resulting from direct excitation of orange centers, and two at shorter wavelengths which correspond respectively to the excitation bands for the green centers and for the blue centers, and result from a transfer of energy from these to the indium centers. The green emission in ZnS :Cu :In is also seen to include an excitation band at 4250 A which is not present in ZnS :Cu :Cl and which suggests that a transfer of energy takes place from indium to copper. The presence of these kinds of multiple-transfer processes favors a photoconductive mechanism. In discussing the transfer of energy in ZnS:Cu:Cl,(l) several possible complications were described. We considered the presence within the blue excitation band, of either an exciton band or of an excited state for the copper emission. If excited states or excitons exist, it is not likely they would contribute to the long-wavelength excitation bands in ZnS:Cu:In. A further complication was the possibility that dissimilar centers associate. If this were the case, one might assume that in ZnS:Cu:In all three centers are mutually associated. While such a situation could result, for ex-
ample, from segregation at grain boundaries or at stacking faults, there appears to be no direct evidence at present that association of this kind takes place. 4. CONCLUSIONS The results obtained on ZnS:Cu:In indicate that a photoconductive energy transfer takes place in ZnS, which can account for the transfer observed at low temperatures and for low impurity concentrations. The detailed process appears to be one in which particular wavelengths are capable of both filling and emptying impurity levels. We believe such processes occur quite generally in sulfide phosphors. They will be most in evidence at low temperatures, where the available thermal energy is insufficient to supply the additional energy needed to empty traps, and at low concentrations, where competing processes and added complications are least likely to occur. Acknowledgements-The author wishes to thank Dr. E. N. ADAMSfor his critical comments and Miss SALLY WARNER for assistance in obtaining the measurements. He is grateful to Mr. A. WACHTEL of the Westinghouse Research Laboratory, Blootield, New Jersey, for supplying the specimens.
REFERENCES 1. MELAMEDN. T., J. Phys. Chem. Solids 7, 146 (1958). 2. KRUGER F. A. and DIKHOFF J. A. M., Physica 16, 297 (1950). 3. DEXTER D. L., J. Chem. Phys. 21, 836 (1953). 4. ANTONOV-ROMANOVSKII V. V. and SHUKEN I. P., Dokl. Akad. Nauk. SSSR 71, 445 (1950). 5. SMITH A. W. and TURKEVICHJ., Phys. Rev. 87, 306 (1952). 6. HOOGENSTRAATEN W. and KLASENSH. A., J. Electrothem. Sot. 100, 366 (1953). 7. Final Rep. Polytech. Inst. Brooklyn, Contract: NObsr 39045 (June 1949).
Pergamon
ENERGY
Press 1959. Vol. 9. pp. 149-152.
TRANSFER
Printed in Great Britain.
IN ZnS:Cu:In
PHOSPHORS
N. T. MELAMED Westinghouse
Research Laboratories,
Pittsburgh
35, Pennsylvania
(Received 25 July 1958)
Abstract-Transport of energy between luminescence centers has been observed in ZnS:Cu:In phosphors at room temperature and at 77°K. The results support a photoconductive transfer mechanism. Energy transfer at low temperatures can be interpreted as resulting from a process in which a given wavelength is capable of putting an electron into an impurity level from the valence band, as well as exciting it from the same impurity level to the conduction band.
1. INTRODUCTION IN a recent
in 2nS:Cu:Cl phosphors, we observed that, from 300” to 4”K, irradiation into the excitation band of the blue emitting centers resulted in an emission from the green luminescence centers. We inferred that excitation energy was transported from blue centers to green centers, and came to the opinion that the most likely mechanism of transfer was photoconductive. An important feature in a photoconductive transfer process was the assumption that optically induced transitions can take place between an impurity level and either band. It was thought possible to test this assumption, as well as several other possible transfer mechanisms which could not be entirely eliminated, by observing the transport of energy in phosphors similar in composition to ZnS:Cu:Cl, but in which the chlorine is replaced by a different coactivator. Indium was chosen because glow-curve data indicate that, of the known coactivators, it forms the deepest level and therefore differs most in this respect from the chlorine it replaces. paper(l)
dealing
2. EXPERIMENTAL
with energy
transfer
of ZnS :Cu:In. The blue emission band, attributed to zinc vacancies, lies at about 4600 A. The green copper band is at 5200 8, and the orange
)O,
:“,
I
,
Electron
Volts
I 2.5 , I
I
2-O
1 1 ,
1 .,
.9 -
4 -
Emission Spectra -298OK --1160K ...... ...__ 770K
RESULTS
The phosphors were prepared from luminescence-grade zinc sulfide and were fired 1 hr at 950” in HsS. The impurities were added as copper acetate and indium sulfide. Emission and excitation spectra were obtained in a manner described for ZnS:Cu:Cl.(l) Fig. 1 shows the emission spectrum of a sample
K
6
h
40000
h
1 I
4500
~~~Ifi~~L’~~~~’
““‘I
5000 5500 6000 Wovelength in &
7000
FIG. 1. Emission spectrum of ZnS:C~(l0-~):1n at 298”, 116”, and 77°K. Because of their large half-widths, the individual blue, green, and red emission bands are not clearly distinguishable. 149
N.
150
T.
MELAMED
emission band of indium has its peak at 5850 A. The positions of the emission bands agree with the data obtained by KR&ER and DIKHOFF.@) The presence of a blue emission in ZnS:Cu:In suggests that indium, like chlorine, can give rise to zinc vacancies at low copper concentrations, apart from its role as charge compensator for copper. (2)
absorption of radiation at blue centers with a subsequent transfer of energy to green centers.(l) In ZnS:Cu:In, the excitation bands are most numerous, due in part to the greater number of emission bands. The orange emission of indium is excited in three bands: at 4250,3850, and 3450 b. Excitotlon
Excitollon 40 I
45
Energy
in Electron 3.5 I Excitoiion
Volts 30 I Spectra
-
Energy
in Electron
Volts
+/;I
IOBlue Emission -- - Green Emission ......“--. Red Emission
-
T =77*K
0
(
2800
I
,
3000
,
,
v
,
I
3200 3400 3600 Excitation Wovelength,
4000
44po
2900
3000
4ooO 3200 3400 3600 Excitation Wovelength, %
4400
FIG. 2. Excitation spectra for the blue, green, and red emissions of ZnS:Cu(lO-3:In at 298” and 77°K.
FIG. 3. Excitation spectra for the blue and green emis-
Fig. 2 shows the excitation spectra for the blue, green, and orange emissions of ZnS:Cu (0.0001 per cent) :In at room temperature and at 77°K. For purposes of comparison the excitation spectra of ZnS:Cu(O*OOOl per cent):Cl are shown in Fig. 3. Considering first the excitation bands of ZnS:Cu:Cl, we find an excitation band for the blue emission at 3450 a, which we believe corresponds to the absorption band for blue centers. We find two excitation bands for the green emission, at 3850 and 3450 8. The 3850 A excitation band corresponds to the absorption band of the green centers; the 3450 a excitation band arises from the
The 4250 i% band we attribute to the absorption band of indium. The two bands at shorter wavelengths we believe result from a transfer of energy from green and from blue centers to orange centers. The blue and green emissions of ZnS:Cu:In show excitation bands similar to those found in ZnS :Cu :Cl, with, however, several additional excitation bands which are not present in ZnS :Cu:Cl. For the blue emission, these additional bands lie at 3850 and 4250 A. The appearance of two new excitation bands for the blue emission in ZnS:Cu:In has significance towards establishing the mecharism of energy transfer in ZnS. These bands, lying at 3850 and
sions of Z~S:CU(~O-~):CI
at 298”, 77”, and 4°K.
ENERGY
TRANSFER
IN
4250 A, coincide with what we believe are the excitation peaks for direct absorption in copper and indium. They indicate that in the 2nS:Cu:In phosphor a transfer of energy takes place from the copper and indium centers to the zinc vacancies. In Fig. 4 is shown an energy-level diagram for ZnS:Cu:In, which we deduce from our optical measurements. Conduction
-
36e.V. 3450%)
Bond
T I
t (b)
I
(‘.
2.7e.V. (460061 32e.V (38SOii (c
I
Zn Vac-
I
-: (d) Valence
Band
FIG. 4. Energy-level diagram for the ZnS:Cu:In phosphor deduced from measurements of excitation and emission spectra. The excitation spectra for transitions (b) and (d) are assumed to extend from the near-infrared through the visible to the near-ultraviolet region. 3. DISCUSSION
There is evidence in various phosphors that energy can be transported from one luminescence center to another by three mechar isms: by resonance, by cascade, or by a photoconductivity process. The presence of the 3850 ar.d 4250 A excitation bands for the blue emission in ZnS :Cu :In strongly supports a photoconductive transfer mechar ism. Both resonance transfer and cascade trarsfer require that the emission band of the absorl i g center (or sensitizer) overlaps the absorption band of the emittir g cer,ter.(s) Since the emission bards of both copper ar_d indium lie too far to the lotg-waveler gth side of the 3450 A blue excitation band for overlap to occur, it is not likely
ZnS:Cu:In
PHOSPHORS
151
that either a resonance or a cascade process can play an important role in the transfer from red or green centers to blue. To account for a photoconductive transfer which results from excitation with energy of less than the band gap, two processes have been proposed.(l) In the first process, an electron is excited from the valence band into an unoccupied donor, leaving a free hole (for example, transition Q, in Fig. 4). In addition, an electron is excited into the conduction band from an occupied acceptor level (c in Fig. 4). Energy transfer results from the recombination of the electron and the free hole at a third center. Such a process requires an overlap in the excitation spectra of a particular set of donor and acceptor impurity levels; it predicts an eventual depletion of one or both of these levels, with a consequent diminution of transfer. Because we have not been able to observe any evidence of exhaustion, we do not think this process, taken by itself, is the major cause of transfer. As an alternative process we proposed one which is more general, and of which the first process may be regarded as a special case. In this, the more general process, optical transitions can take place between an impurity level and either band. In particular, the same radiation which can fill vacant indium levels from the valence band, can also excite electrons from occupied indium levels to the conduction bard (transitions a and b, Fig. 4) or, by analogy, radiation which excites electrons from occupied copper levels to the conduction band can also partially fill vacant copper centers from the valence bar d (transitiors c ar-d d). Such a process leads to a photoconductive transfer which requires rLo overlap in the excitation spectra of donors and acceptors, predicts no depletion of impurity centers, ar.d can occur to a significant extent at low temperatures, We believe the more general photoconductive process to be the explanation for the energy transfer in ZnS. It is consistert with our observation that no reduction in trarsfer occurs with time. It is further supported by evidence which indicates that electron traps can be emptied by nearultraviolet radiation. The emptyirg of traps by short-waveler gth radiation had been previously proposed by ANTONOV-ROMANOVSKIIand SHUKEN to explain the increased absorption at 4500 A which results from excitation in ZnS:Cu:Co,(4) by
152
N.
T.
MELAMED
SMITH and TURKEVICH to account for the emptying of traps in copper activated ZnS by visible and near-ultraviolet radiation(s) and by HOOGENSTRAATENand KLASENS to explain the intensitydependence of the light sum of ZnS:Cu:Co under 3650 A excitation.(s) Transitions such as (6) and (d) also can be used to explain the increase in absorption throughout the entire visible range that has been observed in a number of other sulfide phosphors during excitation.(T) The proposed photoconductive mechanism further suggests that transfer can occur from any one center to any other. That this indeed appears to be the case is seen in the excitation spectrum for the orange emission. This consists of three excitation bands: the long-wavelength band, resulting from direct excitation of orange centers, and two at shorter wavelengths which correspond respectively to the excitation bands for the green centers and for the blue centers, and result from a transfer of energy from these to the indium centers. The green emission in ZnS :Cu :In is also seen to include an excitation band at 4250 A which is not present in ZnS :Cu :Cl and which suggests that a transfer of energy takes place from indium to copper. The presence of these kinds of multiple-transfer processes favors a photoconductive mechanism. In discussing the transfer of energy in ZnS:Cu:Cl,(l) several possible complications were described. We considered the presence within the blue excitation band, of either an exciton band or of an excited state for the copper emission. If excited states or excitons exist, it is not likely they would contribute to the long-wavelength excitation bands in ZnS:Cu:In. A further complication was the possibility that dissimilar centers associate. If this were the case, one might assume that in ZnS:Cu:In all three centers are mutually associated. While such a situation could result, for ex-
ample, from segregation at grain boundaries or at stacking faults, there appears to be no direct evidence at present that association of this kind takes place. 4. CONCLUSIONS The results obtained on ZnS:Cu:In indicate that a photoconductive energy transfer takes place in ZnS, which can account for the transfer observed at low temperatures and for low impurity concentrations. The detailed process appears to be one in which particular wavelengths are capable of both filling and emptying impurity levels. We believe such processes occur quite generally in sulfide phosphors. They will be most in evidence at low temperatures, where the available thermal energy is insufficient to supply the additional energy needed to empty traps, and at low concentrations, where competing processes and added complications are least likely to occur. Acknowledgements-The author wishes to thank Dr. E. N. ADAMSfor his critical comments and Miss SALLY WARNER for assistance in obtaining the measurements. He is grateful to Mr. A. WACHTEL of the Westinghouse Research Laboratory, Blootield, New Jersey, for supplying the specimens.
REFERENCES 1. MELAMEDN. T., J. Phys. Chem. Solids 7, 146 (1958). 2. KRUGER F. A. and DIKHOFF J. A. M., Physica 16, 297 (1950). 3. DEXTER D. L., J. Chem. Phys. 21, 836 (1953). 4. ANTONOV-ROMANOVSKII V. V. and SHUKEN I. P., Dokl. Akad. Nauk. SSSR 71, 445 (1950). 5. SMITH A. W. and TURKEVICHJ., Phys. Rev. 87, 306 (1952). 6. HOOGENSTRAATEN W. and KLASENSH. A., J. Electrothem. Sot. 100, 366 (1953). 7. Final Rep. Polytech. Inst. Brooklyn, Contract: NObsr 39045 (June 1949).