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Excitation transfer in CaF2:Eu2+, Sm2+ multilayered structures grown by MBE on Si(111)
JOURNAL
OF
LUMINESCENCE ELSEVIER
Journal
of Luminescence
76&77
( 1998) 41 I-41 5
Excitation transfer in CaF2:Eu2 +, Sm2 ’ multilayered structures grown by MBE on Si(1 1 1) G.T. Warren”.
S.A. Holmstrom”,
N.L. Yakovle@,
W.M. Yen”, W.M. Dennis”‘”
Abstract
Multilayer CaFl epitaxial structures, selectively doped with ELI’+ and Sm’+ were grown on Si( 1 I I) by molecularbeam epitaxy. We investigate excitation transfer both within a single codoped layer and between two separated layers doped with Eu’+ and Sm’+. respectively. We use the tripled output of a Q-switched YAG : Nd”+ laser to selectively excite the lowest 4f + 5d transition of ELI’ _. The Strokes shifted ELI’+ emission overlaps the absorption of Sm’+ at 420 nm. The time-resolved emission from Sm’ + at 706 nm is then monitored to provide an indication of excitation transfer to this ion. The buildup on Sm’ + emission is correlated to the quenching of the time-resolved Eu2’ emission. In addition. all measurements in the codoped structures are compared with similar measurements in singly doped materials. (‘ 1998 Elsevier Science B.V. All rights reserved. Ke~~woru’s: Excitation
transfer:
Epitaxial
films; CaFz
I. Introduction In this work we investigate excitation transfer [l] in a series of structures grown by molecularbeam epitaxy (MBE). This technique allows the control of materials properties such as the crystalline morphology, dopant concentration/distribution, and crystalline quality. In particular we are able to engineer structures in which the donor and acceptor ions are confined to planes, and the thickness and separation between these planes can be precisely controlled. This control enables us to systematically investigate the behavior of the
excitation transfer as a function of the donor acceptor ion distance and geometry. Experimentally, excitation transfer is determined by a combination of steady-state and time-resolved laser spectroscopy. Experimental results can be compared with existing (bulk) theory of excitation transfer and also with models that explicitly take into account the reduced geometry of these structures. We believe that the study of energy transfer in these structures may have implications in the efficiency of thin-film structures such as those implemented in electroluminescent devices.
2. Sample preparation *Corresponding author. billfqhal.phgsast.uga.edu.
Fax:
+
I 706 542 2492: e-mail:
0022-23 1?,98/$19.00 c 1998 Elsevier Science B.V. All rights reserved PI/ soo22-23 13(97)00277-5
The structures used in this study were grown on Si( 111) by MBE at 750 C using CaF?. CaFz:Eu2+
(a) CaF,
Pnm)
Sm:Eu:CaF* CaF,
(Snm) (1Onm)
Si(ll1)
3. Experimental
(4 CaF2
(5nm)
Eu:CaF,
(snm)
CaF2
(5nm)
Sm:CaF, CaF,
WW (1Onm)
Si(l11)
Fig. I. Multilayeredthin-film structures: (a) a single codoped layer; (b) two doped layers separated by a buffer layer.
and CaF,:Sm” sources. The doped sources had a nominal concentration of 0.1%. In order to reduce the effects of interactions with the Si substrate, all structures were grown with an initial 10 nm layer of undoped CaF, at the silicon interface (see Fig. 1). To enhance the signal-to-noise ratio in the spectroscopic measurements, multilayer structures were grown. The repeating units for our two initial structures are detailed below: (a) A single doped layer codoped with both Eu’+ and Sm2 +: CaF2:Sm2f,E~2+(CaF2. (b) Two doped
Each of the individual layers within these repeating units was 5 nm thick. Additional repeating units were grown until the total thickness of the structure was 220 nm. The total film thickness was monitored using the interference of a reflected helium-neon laser beam during deposition. Reflection high-energy electron diffraction (RHEED) was used during the substrate cleaning process to monitor contamination. RHEED measurements also indicated that the films were single crystal with flat surfaces [2].
layers separated
CaF2:Sm2+lCaF21CaFz:Euz+~CaF2.
by a buffer layer:
considerations
All spectroscopic measurements were performed at cryogenic temperature since the Sm2+ luminescence is quenched at room temperature [3]. The films were cooled by mounting on the cold finger of a two-stage closed-cycle refrigerator which was cooled to a temperature of 9 K. The tripled output of a Q-switched YAG:Nd3+ laser (i, = 355 nm, pulse width = 7 ns, pulse energy = 100 uj) was used to selectively excite the lowest 4f -+ 5d transition of Eu2+[4]. We note that the Stokes shifted ELI’+ emission overlaps the absorption of Sm’+ at 420 nm [S]. The time-resolved emission from both the Eu2+ ion at 420 nm and the Sm’+ ion at 706 nm was monitored to provide an indication of excitation transfer between these ions. The fluorescence was spectrally filtered using a Spex 0.34 m monochromator, detected using an Hamamatsu R5600U-01 photomultiplier tube and single photon counting. The signal was averaged using an EC&G Turbo MCS multichannel scaler. Transient measurements in the above multiply doped structures were compared with single doped materials with comparable dopant concentrations.
4. Results and discussion The time-resolved emission due to the Eu’+ ion for three MBE-grown tructures is shown in Fig. 2.The structure with a buffer layer between the donor and acceptor ions exhibit a single exponential
-
Single
...------. Buffer ---
Eu2’
Layer
Layer
Codoped
Layer
1 o-3 0.0
0.5
1 .o
1 .5
2.0 Time
Fip. 2. ELI’
’ fluorescent
transients
for the MBE-grown
decay with a lifetime of 0.5 us which is in good agreement with the singly doped material. In comparison, the structure with the codoped layer shows significant quenching of the Eu2+ emission. The time-resolved SmZC emission (not shown) in this structure exhibits a finite rise time which is not observed in the singly doped Sm2+ sample. We compare our experimental data to a simple model which describes energy transfer in a layered system. Our model makes the following simplifying assumptions: (i) donor-donor transfer is negligible; (ii) donor-acceptor transfer occurs by a dipole-dipole process, i.e. the transfer rate is x l/r”; (iii) the intrinsic donor-acceptor transfer rate is the same for all donors and acceptor pairs;
2.5
3.0
3.5
4.0
(ps) structures
described
in the text
(iv) no back transfer is present: (v) both donors and acceptors are randomly substituted into a simple cubic lattice. Although this structure differs from the real crystal structure and orientation, it offers an improvement over continuum models in that donors and acceptors are separated by a finite distance. The radiative decay rate used in the model was determined from the singly doped Eu”+ structure. The intrinsic donor--acceptor transfer rate was chosen to be in agreement with the experimental data. The thickness of the doped films was chosen to be 5 nm for comparison with our experimental data. In order to eliminate finite size effects. the transverse dimensions of the film was
414
---
Buffer Layer Codopsd Layer
Time (ps) Fig. 3. Fluorescent
transients
predicted
increased until no change in the transients could be detected. The results of our model are shown in Fig. 3.The model successfully predicts a non-exponential decay for the Eu’ + fluorescence in the codoped sample, while predicting a single exponential for the sample with a 5 nm buffer layer. We point out that the quenching of the fluorescence in the codoped layer was less than that predicted by our model for a bulk material.
by the simple model described
in the text.
epitaxy. We have compared the temporal evolution of both Eu’+ and Smzf emission in these structures with that observed in singly doped materials. Our results are consistent with excitation transfer from the Eu’+ to the Sm2+ ions. We have compared our results to a simple model which takes into account the layered geometry of our structures.
Acknowledgements 5. Conclusions We have grown multilayer structures of CaF, doped with Eu’ ’ and Sm2 + using molecular-beam
The authors wish to thank R.S. Meltzer for useful discussions. This work was supported by DARPA under the auspices of The Phosphor Technology Center of Excellence.
References [I]
W.hl.
Yen,
Springer. [?I
N.L
1-71S.A. Payne. 11x1. P.M.
Selrer.
Law
Spectroscopy
of Solids.
[4]
Yaho\.le\.
G.T.
Lawson.
Chase.
S.A
.I. Opt.
Payne.
Phy\.
Sot Kc\
Am
I3 3 (19X6) I3 47
(1941)
IIOO?.
Berlin. 1986.
LV.‘LI.Dcnnik
L.K.
L.L.
Warren.
.1. Luniin.
R.S.
Meltzer.
72 73 (1997) .57X.
W.M.
Yen.
[5]
J.K. 1404.
I.awwn.
S..4. Payne.
d, Opt.
SCIC. Am
13 X
I 1991 I
OF
LUMINESCENCE ELSEVIER
Journal
of Luminescence
76&77
( 1998) 41 I-41 5
Excitation transfer in CaF2:Eu2 +, Sm2 ’ multilayered structures grown by MBE on Si(1 1 1) G.T. Warren”.
S.A. Holmstrom”,
N.L. Yakovle@,
W.M. Yen”, W.M. Dennis”‘”
Abstract
Multilayer CaFl epitaxial structures, selectively doped with ELI’+ and Sm’+ were grown on Si( 1 I I) by molecularbeam epitaxy. We investigate excitation transfer both within a single codoped layer and between two separated layers doped with Eu’+ and Sm’+. respectively. We use the tripled output of a Q-switched YAG : Nd”+ laser to selectively excite the lowest 4f + 5d transition of ELI’ _. The Strokes shifted ELI’+ emission overlaps the absorption of Sm’+ at 420 nm. The time-resolved emission from Sm’ + at 706 nm is then monitored to provide an indication of excitation transfer to this ion. The buildup on Sm’ + emission is correlated to the quenching of the time-resolved Eu2’ emission. In addition. all measurements in the codoped structures are compared with similar measurements in singly doped materials. (‘ 1998 Elsevier Science B.V. All rights reserved. Ke~~woru’s: Excitation
transfer:
Epitaxial
films; CaFz
I. Introduction In this work we investigate excitation transfer [l] in a series of structures grown by molecularbeam epitaxy (MBE). This technique allows the control of materials properties such as the crystalline morphology, dopant concentration/distribution, and crystalline quality. In particular we are able to engineer structures in which the donor and acceptor ions are confined to planes, and the thickness and separation between these planes can be precisely controlled. This control enables us to systematically investigate the behavior of the
excitation transfer as a function of the donor acceptor ion distance and geometry. Experimentally, excitation transfer is determined by a combination of steady-state and time-resolved laser spectroscopy. Experimental results can be compared with existing (bulk) theory of excitation transfer and also with models that explicitly take into account the reduced geometry of these structures. We believe that the study of energy transfer in these structures may have implications in the efficiency of thin-film structures such as those implemented in electroluminescent devices.
2. Sample preparation *Corresponding author. billfqhal.phgsast.uga.edu.
Fax:
+
I 706 542 2492: e-mail:
0022-23 1?,98/$19.00 c 1998 Elsevier Science B.V. All rights reserved PI/ soo22-23 13(97)00277-5
The structures used in this study were grown on Si( 111) by MBE at 750 C using CaF?. CaFz:Eu2+
(a) CaF,
Pnm)
Sm:Eu:CaF* CaF,
(Snm) (1Onm)
Si(ll1)
3. Experimental
(4 CaF2
(5nm)
Eu:CaF,
(snm)
CaF2
(5nm)
Sm:CaF, CaF,
WW (1Onm)
Si(l11)
Fig. I. Multilayeredthin-film structures: (a) a single codoped layer; (b) two doped layers separated by a buffer layer.
and CaF,:Sm” sources. The doped sources had a nominal concentration of 0.1%. In order to reduce the effects of interactions with the Si substrate, all structures were grown with an initial 10 nm layer of undoped CaF, at the silicon interface (see Fig. 1). To enhance the signal-to-noise ratio in the spectroscopic measurements, multilayer structures were grown. The repeating units for our two initial structures are detailed below: (a) A single doped layer codoped with both Eu’+ and Sm2 +: CaF2:Sm2f,E~2+(CaF2. (b) Two doped
Each of the individual layers within these repeating units was 5 nm thick. Additional repeating units were grown until the total thickness of the structure was 220 nm. The total film thickness was monitored using the interference of a reflected helium-neon laser beam during deposition. Reflection high-energy electron diffraction (RHEED) was used during the substrate cleaning process to monitor contamination. RHEED measurements also indicated that the films were single crystal with flat surfaces [2].
layers separated
CaF2:Sm2+lCaF21CaFz:Euz+~CaF2.
by a buffer layer:
considerations
All spectroscopic measurements were performed at cryogenic temperature since the Sm2+ luminescence is quenched at room temperature [3]. The films were cooled by mounting on the cold finger of a two-stage closed-cycle refrigerator which was cooled to a temperature of 9 K. The tripled output of a Q-switched YAG:Nd3+ laser (i, = 355 nm, pulse width = 7 ns, pulse energy = 100 uj) was used to selectively excite the lowest 4f -+ 5d transition of Eu2+[4]. We note that the Stokes shifted ELI’+ emission overlaps the absorption of Sm’+ at 420 nm [S]. The time-resolved emission from both the Eu2+ ion at 420 nm and the Sm’+ ion at 706 nm was monitored to provide an indication of excitation transfer between these ions. The fluorescence was spectrally filtered using a Spex 0.34 m monochromator, detected using an Hamamatsu R5600U-01 photomultiplier tube and single photon counting. The signal was averaged using an EC&G Turbo MCS multichannel scaler. Transient measurements in the above multiply doped structures were compared with single doped materials with comparable dopant concentrations.
4. Results and discussion The time-resolved emission due to the Eu’+ ion for three MBE-grown tructures is shown in Fig. 2.The structure with a buffer layer between the donor and acceptor ions exhibit a single exponential
-
Single
...------. Buffer ---
Eu2’
Layer
Layer
Codoped
Layer
1 o-3 0.0
0.5
1 .o
1 .5
2.0 Time
Fip. 2. ELI’
’ fluorescent
transients
for the MBE-grown
decay with a lifetime of 0.5 us which is in good agreement with the singly doped material. In comparison, the structure with the codoped layer shows significant quenching of the Eu2+ emission. The time-resolved SmZC emission (not shown) in this structure exhibits a finite rise time which is not observed in the singly doped Sm2+ sample. We compare our experimental data to a simple model which describes energy transfer in a layered system. Our model makes the following simplifying assumptions: (i) donor-donor transfer is negligible; (ii) donor-acceptor transfer occurs by a dipole-dipole process, i.e. the transfer rate is x l/r”; (iii) the intrinsic donor-acceptor transfer rate is the same for all donors and acceptor pairs;
2.5
3.0
3.5
4.0
(ps) structures
described
in the text
(iv) no back transfer is present: (v) both donors and acceptors are randomly substituted into a simple cubic lattice. Although this structure differs from the real crystal structure and orientation, it offers an improvement over continuum models in that donors and acceptors are separated by a finite distance. The radiative decay rate used in the model was determined from the singly doped Eu”+ structure. The intrinsic donor--acceptor transfer rate was chosen to be in agreement with the experimental data. The thickness of the doped films was chosen to be 5 nm for comparison with our experimental data. In order to eliminate finite size effects. the transverse dimensions of the film was
414
---
Buffer Layer Codopsd Layer
Time (ps) Fig. 3. Fluorescent
transients
predicted
increased until no change in the transients could be detected. The results of our model are shown in Fig. 3.The model successfully predicts a non-exponential decay for the Eu’ + fluorescence in the codoped sample, while predicting a single exponential for the sample with a 5 nm buffer layer. We point out that the quenching of the fluorescence in the codoped layer was less than that predicted by our model for a bulk material.
by the simple model described
in the text.
epitaxy. We have compared the temporal evolution of both Eu’+ and Smzf emission in these structures with that observed in singly doped materials. Our results are consistent with excitation transfer from the Eu’+ to the Sm2+ ions. We have compared our results to a simple model which takes into account the layered geometry of our structures.
Acknowledgements 5. Conclusions We have grown multilayer structures of CaF, doped with Eu’ ’ and Sm2 + using molecular-beam
The authors wish to thank R.S. Meltzer for useful discussions. This work was supported by DARPA under the auspices of The Phosphor Technology Center of Excellence.
References [I]
W.hl.
Yen,
Springer. [?I
N.L
1-71S.A. Payne. 11x1. P.M.
Selrer.
Law
Spectroscopy
of Solids.
[4]
Yaho\.le\.
G.T.
Lawson.
Chase.
S.A
.I. Opt.
Payne.
Phy\.
Sot Kc\
Am
I3 3 (19X6) I3 47
(1941)
IIOO?.
Berlin. 1986.
LV.‘LI.Dcnnik
L.K.
L.L.
Warren.
.1. Luniin.
R.S.
Meltzer.
72 73 (1997) .57X.
W.M.
Yen.
[5]
J.K. 1404.
I.awwn.
S..4. Payne.
d, Opt.
SCIC. Am
13 X
I 1991 I