- Email: [email protected]
Energy transfer processes and photoluminescence properties of homogeneously- and delta-doped ZnS : Mn
Journal of Crystal Growth 184/185 (1998) 1123-1127
ELSEVIER
Energy transfer processes and photoluminescence of homogeneously- and delta-doped ZnS
:
properties Mn
W. Park, T.C. Jones, S. SchGn, W. Tong, M. Chaichimansour, B.K. Wagner, C.J. Summers* Phosphor Technology Center of Excellence, Manufacturing Research Center, Georgia Institute ofTechnology, Atlanta GA 30332.0560,
USA
Abstract An investigation is reported of the energy transfer processes in ZnS : Mn by photoluminescence spectroscopy. At temperatures > 50 K, the luminescence decay of homogeneously doped ZnS : Mn was strongly non-exponential due to non-radiative energy transfer processes. The concentration dependence of the effective lifetime was found to change with temperature. Analysis of the temperature dependence showed that the energy transfer between Mn ions was active only for Mn concentrations > 2%, and that the energy transfer between Mn ions was mediated by an electric dipole-dipole interaction. The delta-doped ZnS : Mn showed a faster decay due to the enhanced overlap between 3d and s-p host states caused by lattice strain. From the temperature dependence, the two-dimensional confinement of energy transfer was observed for large spacings between doping planes. When the doping planes were brought close together, the delta-doped samples behaved similarly to the homogeneously doped ZnS : Mn, indicating that the energy transfer was no longer two-dimensionally confined. 0 1998 Published by Elsevier Science B.V. All rights reserved. PACS:
78.20. - e; 78.55.Et
Keywords:
ZnS : Mn; Delta-doping;
Photoluminescence;
ZnS : Mn exhibits concentration quenching at Mn concentrations > 2% due to the non-radiative transfer processes between Mn ions and defects [l]. This concentration quenching is especially detrimental in display applications because it imposes a limit on the brightness achievable with energy
*Corresponding author. Tel.: + 1 404 894 1260; fax: + 1 404 894 1258; e-mail: [email protected].
Energy transfer
this material. Unfortunately, there is little known about the energy transfer process and its effect on the luminescence properties of ZnS : Mn. In this work, the results of a comprehensive spectroscopic study indicated that the energy transfer between Mn ions was mediated by dipole-dipole interaction. Also, a new doping technique, delta-doping, was carried out in order to suppress the energy transfer processes, and the luminescence properties were investigated.
0022-0248/9X/$19.00 CI 1998 Published by Elsevier Science B.V. All rights reserved. PII SOO22-0248(97)00590-3
1124
W. Park et al. /Journal
of Crystal Growth 1841185 (1998) 1123-1127
Homogeneously doped ZnS : Mn thin films with Mn concentrations between 0.4% and 2.5% and delta-doped samples with 14% 2D Mn concentrations and plane spacings between 90 and 300 A were investigated. The details of the growth conditions were published elsewhere [2]. The ZnS : Mn samples were excited with the 480 nm output of a Spectra-Physics MOP0 laser in order to achieve resonant excitation of Mn ions, and, thus, to avoid any influence of host-Mn energy transfer. The typical excitation intensity was 0.5 mJ per pulse and the pulse width was about 8 ns. The detection apparatus has been described elsewhere [3]. At 10 K, the homogeneously doped ZnS : Mn samples exhibited two-exponential decays. The slow component exhibited a time constant of 1.6 ms and was attributed to a single Mn ion in a cubic site. The fast decaying component with a time constant of 120 ps was assigned to exchange-coupled Mn pairs. At temperatures above 50 K, the pair emission disappeared due to the thermal fluctuation of electronic spins which quenched the exchange interaction between Mn ions. This was consistent with the previous observation that the exchange interaction between Mn ions was found to be thermalized at temperatures above 50 K [4]. At these temperatures, the decay pattern also became non-exponential due to the energy transfer processes which provided non-radiative recombination paths for the Mn ions. For such non-exponential decays, it is not straightforward to define the lifetime of the decay. Since our decay curves were not fitted well with the stretched exponential law proposed by Benoit et al. [S], a more general definition of lifetime shown in Eq. (1) was used in this study,
L(t) dt.
(1)
Fig. 1 shows the effective lifetime measured for various Mn concentrations at various temperatures. The lifetime exhibited a power law dependence on the Mn concentration. At 300 K, the lifetime was proportional to C,,“,‘. From electroluminescence decay measurements, Benoit et al.
1
.OOE-4 4
1.OOE-5
1 0.10
Fig. 1. The effective lifetime as a function measured at various temperatures.
ld.00 of Mn concentration
have found a C,,” dependence, using the stretched exponential law [S]. Considering the difference in the two definitions for the lifetime, both results were in reasonable agreement. In an attempt to explain the results of Benoit et al. [S], De Visschere and Neyts [6] have suggested the existence of two kinds of defects whose concentrations depend on the Mn concentration [6]. However, any supporting evidence for such an assumption has yet to be found. Furthermore, the exponent of the power law dependence was found to change with temperature and the concentration dependence became C,,‘.’ at 100 K, as shown in Fig. 1. Thus, any attempt to identify the microscopic interaction responsible for the energy transfer from the concentration dependence was unreliable. It seemed very unlikely that the interaction mechanism would change with temperature. Fig. 2 shows the temperature dependence of the luminescence lifetime for various Mn concentrations. Two concentration regimes with contrasting temperature dependence were found. For Mn concentrations < l%, the lifetime had a very weak dependence on temperature. In contrast, a strong temperature dependence was observed for Mn concentrations larger than 2%. The energy transfer rate for ions with strong phonon coupling has an
W. Park et al. 1 Journal of Crystar Growth 1841185 (1998) 1123-1127
1
0.5% Mn t-
. -
fer process became active for Mn concentrations between 1% and 2% suggested that the Mn-Mn transfer rate became equal to the intrinsic radiative decay rate when the average Mn-Mn distance was between 13 and 16 A. Using the r-dependence of the Mn-Mn transfer rate for various multipole interactions, we can then calculate the transfer rate rhln for a nearest neighbor Mn pair:
t
4 2
1 OOE-4
P
d
1
ZET(r = r,) = 0 E
0.004
0 006
“1 _=-
1
ZMn
TR
(3)
>
where d is the nearest neighbor distance, 1/rR is the radiative transition rate, and s has values of 6, 8, . for dipole-dipole, dipole-quadrupole, and highef order interactions, respectively. Setting r, = 15 A, d = 3.8 A, and rR = 1.6 ms, one finds
O.OOB
Reciprocal Temperature (l/K,) Fig. 2. The temperature homogeneously doped tions.
1125
dependence of the effective lifetime for ZnS : Mn with various Mn concentra-
TM” = 4 x 1O-7 s for dipole-dipole TM” = 3 x lo-*
interaction,
s for dipole-quadrupole
interaction, exponential
temperature
dependence
as follows
c71:
ZMn= 2 x 1o-9 s for quadrupoleequadrupole interaction.
(2) Using Eq. (2), an activation energy of 3.5 f 0.2 meV was found for the low concentration region, and for concentrations >2%, the temperature dependence exhibited an activation energy of 16.0 f 1.0 meV. The observation of different temperature dependence clearly indicated that different interactions were involved in the two concentration regimes. The most plausible interpretation was that Mn-Mn transfer was not active at concentrations lower than 1% and only became active at concentrations larger than 2%. Since the luminescence decay was non-exponential at all concentrations, a Mn-defect transfer process was believed to be active at all concentrations. Thus, the weak temperature dependence observed for the low concentration regime was attributed to the temperature dependence of the Mn-defect energy transfer. The average Mn-Mn distance for Mn concentrations of 1% and 2% are approximately 16 and 13 A, respectively. The fact that the MnMn trans-
(4)
An independent estimate for rMn was also possible using Dexter’s formula [8],
Here, n is the index of refraction, r is the separation of the pair, tR is the intrinsic radiative decay time of Mn, Qa is the area under the absorption band, and gn and gA are the normalized emission and absorption lineshape functions, respectively. For a dipole-dipole interaction, rM,, was calculated to be approximately 2 x 10m8 s. Unfortunately, it was not possible to estimate rMvlnusing Dexter’s formula for other types of multipole interaction due to the lack of experimental data. However, the ratio of the energy transfer rates mediated by dipole-dipole and dipole-quadrupole interactions is known to be roughly 10m2. Therefore, tMn for dipole-quadrupole and quadrupole-quadrupole interactions are expected to be 2 x lO-‘j and 2 x 1O-4 s, respectively. By comparing these estimates from Dexter’s equation with the results shown in Eq. (4) one finds
1126
W. Park et al. 1 Journal
of CrystalGrowth
that the only reasonable agreement is for dipoledipole interactions. Therefore, it was concluded that the Mn-Mn transfer processes in ZnS were mediated by electric dipole-dipole interaction. Delta-doping was proposed to confine the energy transfer processes in separate two-dimensional planes. It has been observed that delta-doped ZnS : Mn showed much brighter luminescence than the homogeneously doped samples having comparable number of Mn ions, indicating an enhanced luminescence efficiency in the delta-doped samples [4,9]. The delta-doped samples investigated in this work had identical two-dimensional concentrations of Mn, 9.3 x lOi cmm2, but different spacings between the doping planes of 300,130 and 90 A. All of the samples showed non-exponential decays, indicating the existence of energy transfer processes. At 10 K, the effective lifetimes were found to be 69, 68 and 61 us for 300,130 and 90 A spacings, respectively. These small lifetimes were attributed to the increased overlap between s-p host state wave functions with the localized d electron wave functions due to the strain induced by the planar incorporation of Mn. The lifetime was reduced the most to 5 us at 300 K for the sample with 90 A spacing, whereas other samples with larger spacing exhibited a lifetime of 11 us. The temperature dependence of the effective lifetime for the delta-doped samples it shown in Fig. 3. The samples with 130 and 300 A spacings had the same temperature dependence with DEA = 12.0 x 0.9 meV while the sample with 90 A spacing exhibited a slightly stronger temperature dependence with EA = 15.8 x 1.3 meV. The 12 meV activation energy observed from the samples with large spacings between doping planes was believed to be characteristic of the energy transfer process when confined in two dimensions. This assignment was supported by the observation of a 16 meV activation energy for the sample with a spacing of 90 A. This activation energy was the same as that observed from the homogeneously doped samples, indicating that delta-doped samples with small plane spacings approached the limit of homogeneous doping. This was expected because the excitation energy could then naturally hop from one plane to another as the doping planes were brought closer together, and thus the energy transfer processes were no longer
184/185 (1998) 1123-1127
EA=12.0*0.9meV (spacing = 130, 300 A) .OOE+
. E,, = 15.8 f 1.3 meV Z (spa&g = 90 A) - the same as homogeneous doping
/
1.OOE-6-i 0.000
0.010
0.020 0.030 0.040 Reciprocal Temperature (l/K)
0.050
Fig. 3. The temperature dependence of effective lifetime delta-doped ZnS : Mn with various doping plane spacings.
for
two-dimensionally confined. For 2D Mn concentration of 9.3 x 10’ 3 cme2, this was observed when the spacing was reduced to 90 A. In conclusion, the nature of the energy transfer processes responsible for concentration quenching in ZnS : Mn was identified and an advanced doping technique to increase the luminescence efficiency was proposed. At temperatures > 50 K, the decay of Mn luminescence was strongly non-exponential due to the non-radiative energy transfer processes. The concentration dependence of the effective lifetime was found to change with temperature. Two regimes of concentration with distinct temperature dependence were found. For Mn concentrations lower than l%, the effective lifetime was nearly temperature independent. For concentrations larger than 2%, the effective lifetime showed strong temperature dependence with an activation energy of 16 meV. This observation led to a conclusion that the energy transfer rate between Mn ions became equal to the radiative transition rate at a Mn concentration between 1% and 2%. Comparing with the estimates from Dexter’s theory, the energy transfer between Mn ions was found to be mediated by electric dipole-dipole interaction. Delta-doped ZnS : Mn showed fast luminescence decay with lifetimes ranging from 61 to 69 us at 10 K. The fast decay was attributed to the
W. Park et al. 1 Journal of Crystal Growth 184/185 (1998) 1123-1127
enhanced overlap between 3d and s-p host state wave functions due to the lattice strain caused by the planar incorporation of Mn. When the doping planes were far apart, the delta-doped ZnS : Mn exhibited a weaker temperature dependence with an activation energy of 12 meV. This temperature dependence was the characteristic of two-dimensionally confined energy transfer processes. This conclusion was supported by the observation that the delta-doped ZnS : Mn with a small plane spacing (90 A) showed temperature dependence identical to the homogeneously doped ZnS: Mn, indicating that the energy transfer between doping planes became active.
1121
References Cl1 M. Katiyar, A.H. Kitai, J. Lumin. 46 (1990) 227. W. Park, B.K. Wagner, C.J. PI S. Schon, M. Chaichimansour, Summers,
J. Crystal
Growth,
to be published.
c31 T.K. Tran, W. Park, W. Tong, M.M. Kyi, B.K. Wagner, C.J. Summers,
J. Appl. Phys. 81 (1997) 2803.
f41 W. Park, T.K. Tran, W. Tong, M.M. Kyi, S. Schon, B.K. Wagner, C.J. Summers, Proc. Mater. Res. Sot. Symp. 424 (1997) 465. c51 J. Benoit, P. Benalloul, A. Geoffroy, C. Barthou, Phys. Stat. Sol. A 105 (1988) 637. C61P. De Visschere, K. Neyts, J. Lumin. 52 (1992) 313. f71 T.F. Soules, C.B. Duke, Phys. Rev. B 3 (1971) 262. PI D.L. Dexter, J. Chem. Phys. 21 (1953) 836. 191 W. Tong, T.K. Tran, W. Park, S. Schon, B.K. Wagner, C.J. Summers, J. SID 4 (1996) 325.
ELSEVIER
Energy transfer processes and photoluminescence of homogeneously- and delta-doped ZnS
:
properties Mn
W. Park, T.C. Jones, S. SchGn, W. Tong, M. Chaichimansour, B.K. Wagner, C.J. Summers* Phosphor Technology Center of Excellence, Manufacturing Research Center, Georgia Institute ofTechnology, Atlanta GA 30332.0560,
USA
Abstract An investigation is reported of the energy transfer processes in ZnS : Mn by photoluminescence spectroscopy. At temperatures > 50 K, the luminescence decay of homogeneously doped ZnS : Mn was strongly non-exponential due to non-radiative energy transfer processes. The concentration dependence of the effective lifetime was found to change with temperature. Analysis of the temperature dependence showed that the energy transfer between Mn ions was active only for Mn concentrations > 2%, and that the energy transfer between Mn ions was mediated by an electric dipole-dipole interaction. The delta-doped ZnS : Mn showed a faster decay due to the enhanced overlap between 3d and s-p host states caused by lattice strain. From the temperature dependence, the two-dimensional confinement of energy transfer was observed for large spacings between doping planes. When the doping planes were brought close together, the delta-doped samples behaved similarly to the homogeneously doped ZnS : Mn, indicating that the energy transfer was no longer two-dimensionally confined. 0 1998 Published by Elsevier Science B.V. All rights reserved. PACS:
78.20. - e; 78.55.Et
Keywords:
ZnS : Mn; Delta-doping;
Photoluminescence;
ZnS : Mn exhibits concentration quenching at Mn concentrations > 2% due to the non-radiative transfer processes between Mn ions and defects [l]. This concentration quenching is especially detrimental in display applications because it imposes a limit on the brightness achievable with energy
*Corresponding author. Tel.: + 1 404 894 1260; fax: + 1 404 894 1258; e-mail: [email protected].
Energy transfer
this material. Unfortunately, there is little known about the energy transfer process and its effect on the luminescence properties of ZnS : Mn. In this work, the results of a comprehensive spectroscopic study indicated that the energy transfer between Mn ions was mediated by dipole-dipole interaction. Also, a new doping technique, delta-doping, was carried out in order to suppress the energy transfer processes, and the luminescence properties were investigated.
0022-0248/9X/$19.00 CI 1998 Published by Elsevier Science B.V. All rights reserved. PII SOO22-0248(97)00590-3
1124
W. Park et al. /Journal
of Crystal Growth 1841185 (1998) 1123-1127
Homogeneously doped ZnS : Mn thin films with Mn concentrations between 0.4% and 2.5% and delta-doped samples with 14% 2D Mn concentrations and plane spacings between 90 and 300 A were investigated. The details of the growth conditions were published elsewhere [2]. The ZnS : Mn samples were excited with the 480 nm output of a Spectra-Physics MOP0 laser in order to achieve resonant excitation of Mn ions, and, thus, to avoid any influence of host-Mn energy transfer. The typical excitation intensity was 0.5 mJ per pulse and the pulse width was about 8 ns. The detection apparatus has been described elsewhere [3]. At 10 K, the homogeneously doped ZnS : Mn samples exhibited two-exponential decays. The slow component exhibited a time constant of 1.6 ms and was attributed to a single Mn ion in a cubic site. The fast decaying component with a time constant of 120 ps was assigned to exchange-coupled Mn pairs. At temperatures above 50 K, the pair emission disappeared due to the thermal fluctuation of electronic spins which quenched the exchange interaction between Mn ions. This was consistent with the previous observation that the exchange interaction between Mn ions was found to be thermalized at temperatures above 50 K [4]. At these temperatures, the decay pattern also became non-exponential due to the energy transfer processes which provided non-radiative recombination paths for the Mn ions. For such non-exponential decays, it is not straightforward to define the lifetime of the decay. Since our decay curves were not fitted well with the stretched exponential law proposed by Benoit et al. [S], a more general definition of lifetime shown in Eq. (1) was used in this study,
L(t) dt.
(1)
Fig. 1 shows the effective lifetime measured for various Mn concentrations at various temperatures. The lifetime exhibited a power law dependence on the Mn concentration. At 300 K, the lifetime was proportional to C,,“,‘. From electroluminescence decay measurements, Benoit et al.
1
.OOE-4 4
1.OOE-5
1 0.10
Fig. 1. The effective lifetime as a function measured at various temperatures.
ld.00 of Mn concentration
have found a C,,” dependence, using the stretched exponential law [S]. Considering the difference in the two definitions for the lifetime, both results were in reasonable agreement. In an attempt to explain the results of Benoit et al. [S], De Visschere and Neyts [6] have suggested the existence of two kinds of defects whose concentrations depend on the Mn concentration [6]. However, any supporting evidence for such an assumption has yet to be found. Furthermore, the exponent of the power law dependence was found to change with temperature and the concentration dependence became C,,‘.’ at 100 K, as shown in Fig. 1. Thus, any attempt to identify the microscopic interaction responsible for the energy transfer from the concentration dependence was unreliable. It seemed very unlikely that the interaction mechanism would change with temperature. Fig. 2 shows the temperature dependence of the luminescence lifetime for various Mn concentrations. Two concentration regimes with contrasting temperature dependence were found. For Mn concentrations < l%, the lifetime had a very weak dependence on temperature. In contrast, a strong temperature dependence was observed for Mn concentrations larger than 2%. The energy transfer rate for ions with strong phonon coupling has an
W. Park et al. 1 Journal of Crystar Growth 1841185 (1998) 1123-1127
1
0.5% Mn t-
. -
fer process became active for Mn concentrations between 1% and 2% suggested that the Mn-Mn transfer rate became equal to the intrinsic radiative decay rate when the average Mn-Mn distance was between 13 and 16 A. Using the r-dependence of the Mn-Mn transfer rate for various multipole interactions, we can then calculate the transfer rate rhln for a nearest neighbor Mn pair:
t
4 2
1 OOE-4
P
d
1
ZET(r = r,) = 0 E
0.004
0 006
“1 _=-
1
ZMn
TR
(3)
>
where d is the nearest neighbor distance, 1/rR is the radiative transition rate, and s has values of 6, 8, . for dipole-dipole, dipole-quadrupole, and highef order interactions, respectively. Setting r, = 15 A, d = 3.8 A, and rR = 1.6 ms, one finds
O.OOB
Reciprocal Temperature (l/K,) Fig. 2. The temperature homogeneously doped tions.
1125
dependence of the effective lifetime for ZnS : Mn with various Mn concentra-
TM” = 4 x 1O-7 s for dipole-dipole TM” = 3 x lo-*
interaction,
s for dipole-quadrupole
interaction, exponential
temperature
dependence
as follows
c71:
ZMn= 2 x 1o-9 s for quadrupoleequadrupole interaction.
(2) Using Eq. (2), an activation energy of 3.5 f 0.2 meV was found for the low concentration region, and for concentrations >2%, the temperature dependence exhibited an activation energy of 16.0 f 1.0 meV. The observation of different temperature dependence clearly indicated that different interactions were involved in the two concentration regimes. The most plausible interpretation was that Mn-Mn transfer was not active at concentrations lower than 1% and only became active at concentrations larger than 2%. Since the luminescence decay was non-exponential at all concentrations, a Mn-defect transfer process was believed to be active at all concentrations. Thus, the weak temperature dependence observed for the low concentration regime was attributed to the temperature dependence of the Mn-defect energy transfer. The average Mn-Mn distance for Mn concentrations of 1% and 2% are approximately 16 and 13 A, respectively. The fact that the MnMn trans-
(4)
An independent estimate for rMn was also possible using Dexter’s formula [8],
Here, n is the index of refraction, r is the separation of the pair, tR is the intrinsic radiative decay time of Mn, Qa is the area under the absorption band, and gn and gA are the normalized emission and absorption lineshape functions, respectively. For a dipole-dipole interaction, rM,, was calculated to be approximately 2 x 10m8 s. Unfortunately, it was not possible to estimate rMvlnusing Dexter’s formula for other types of multipole interaction due to the lack of experimental data. However, the ratio of the energy transfer rates mediated by dipole-dipole and dipole-quadrupole interactions is known to be roughly 10m2. Therefore, tMn for dipole-quadrupole and quadrupole-quadrupole interactions are expected to be 2 x lO-‘j and 2 x 1O-4 s, respectively. By comparing these estimates from Dexter’s equation with the results shown in Eq. (4) one finds
1126
W. Park et al. 1 Journal
of CrystalGrowth
that the only reasonable agreement is for dipoledipole interactions. Therefore, it was concluded that the Mn-Mn transfer processes in ZnS were mediated by electric dipole-dipole interaction. Delta-doping was proposed to confine the energy transfer processes in separate two-dimensional planes. It has been observed that delta-doped ZnS : Mn showed much brighter luminescence than the homogeneously doped samples having comparable number of Mn ions, indicating an enhanced luminescence efficiency in the delta-doped samples [4,9]. The delta-doped samples investigated in this work had identical two-dimensional concentrations of Mn, 9.3 x lOi cmm2, but different spacings between the doping planes of 300,130 and 90 A. All of the samples showed non-exponential decays, indicating the existence of energy transfer processes. At 10 K, the effective lifetimes were found to be 69, 68 and 61 us for 300,130 and 90 A spacings, respectively. These small lifetimes were attributed to the increased overlap between s-p host state wave functions with the localized d electron wave functions due to the strain induced by the planar incorporation of Mn. The lifetime was reduced the most to 5 us at 300 K for the sample with 90 A spacing, whereas other samples with larger spacing exhibited a lifetime of 11 us. The temperature dependence of the effective lifetime for the delta-doped samples it shown in Fig. 3. The samples with 130 and 300 A spacings had the same temperature dependence with DEA = 12.0 x 0.9 meV while the sample with 90 A spacing exhibited a slightly stronger temperature dependence with EA = 15.8 x 1.3 meV. The 12 meV activation energy observed from the samples with large spacings between doping planes was believed to be characteristic of the energy transfer process when confined in two dimensions. This assignment was supported by the observation of a 16 meV activation energy for the sample with a spacing of 90 A. This activation energy was the same as that observed from the homogeneously doped samples, indicating that delta-doped samples with small plane spacings approached the limit of homogeneous doping. This was expected because the excitation energy could then naturally hop from one plane to another as the doping planes were brought closer together, and thus the energy transfer processes were no longer
184/185 (1998) 1123-1127
EA=12.0*0.9meV (spacing = 130, 300 A) .OOE+
. E,, = 15.8 f 1.3 meV Z (spa&g = 90 A) - the same as homogeneous doping
/
1.OOE-6-i 0.000
0.010
0.020 0.030 0.040 Reciprocal Temperature (l/K)
0.050
Fig. 3. The temperature dependence of effective lifetime delta-doped ZnS : Mn with various doping plane spacings.
for
two-dimensionally confined. For 2D Mn concentration of 9.3 x 10’ 3 cme2, this was observed when the spacing was reduced to 90 A. In conclusion, the nature of the energy transfer processes responsible for concentration quenching in ZnS : Mn was identified and an advanced doping technique to increase the luminescence efficiency was proposed. At temperatures > 50 K, the decay of Mn luminescence was strongly non-exponential due to the non-radiative energy transfer processes. The concentration dependence of the effective lifetime was found to change with temperature. Two regimes of concentration with distinct temperature dependence were found. For Mn concentrations lower than l%, the effective lifetime was nearly temperature independent. For concentrations larger than 2%, the effective lifetime showed strong temperature dependence with an activation energy of 16 meV. This observation led to a conclusion that the energy transfer rate between Mn ions became equal to the radiative transition rate at a Mn concentration between 1% and 2%. Comparing with the estimates from Dexter’s theory, the energy transfer between Mn ions was found to be mediated by electric dipole-dipole interaction. Delta-doped ZnS : Mn showed fast luminescence decay with lifetimes ranging from 61 to 69 us at 10 K. The fast decay was attributed to the
W. Park et al. 1 Journal of Crystal Growth 184/185 (1998) 1123-1127
enhanced overlap between 3d and s-p host state wave functions due to the lattice strain caused by the planar incorporation of Mn. When the doping planes were far apart, the delta-doped ZnS : Mn exhibited a weaker temperature dependence with an activation energy of 12 meV. This temperature dependence was the characteristic of two-dimensionally confined energy transfer processes. This conclusion was supported by the observation that the delta-doped ZnS : Mn with a small plane spacing (90 A) showed temperature dependence identical to the homogeneously doped ZnS: Mn, indicating that the energy transfer between doping planes became active.
1121
References Cl1 M. Katiyar, A.H. Kitai, J. Lumin. 46 (1990) 227. W. Park, B.K. Wagner, C.J. PI S. Schon, M. Chaichimansour, Summers,
J. Crystal
Growth,
to be published.
c31 T.K. Tran, W. Park, W. Tong, M.M. Kyi, B.K. Wagner, C.J. Summers,
J. Appl. Phys. 81 (1997) 2803.
f41 W. Park, T.K. Tran, W. Tong, M.M. Kyi, S. Schon, B.K. Wagner, C.J. Summers, Proc. Mater. Res. Sot. Symp. 424 (1997) 465. c51 J. Benoit, P. Benalloul, A. Geoffroy, C. Barthou, Phys. Stat. Sol. A 105 (1988) 637. C61P. De Visschere, K. Neyts, J. Lumin. 52 (1992) 313. f71 T.F. Soules, C.B. Duke, Phys. Rev. B 3 (1971) 262. PI D.L. Dexter, J. Chem. Phys. 21 (1953) 836. 191 W. Tong, T.K. Tran, W. Park, S. Schon, B.K. Wagner, C.J. Summers, J. SID 4 (1996) 325.