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The nature of anomalous temperature dependence of photoluminescence in glasses
Journal of Non-Crystalline Solids 90 (1987) North-tlolland. Amsterdam
THE NATURE OF PHOTOLUMINESCENCE
453 .456 453
ANONALOUS TEMPERATURE IN GLASSES
V.K.MALlNOVSKY,
V.N.NOVIXOV,
DEPENDENCE
OF
A.P.SOKOLOV
Institute of Automation and Electrometry of Siberian Branch of the USSR AC. Sci., 630090, Novosibirsk, 90, USSR The anomalouos temperature dependence of photoluminescence (PL) in glasses is related to local increase in temperature around the PL center. The temperature increases due to tiduoed by disorder localization of vibration energy which arises together with the Stokes shift. In the frames of this model some experimental rewults on PL are explained. 1. INTRODUCTION It ie well known, that PL properties are similar in glassy and crystalline chalcogenides of the same composition'. However the temperature dependence-of PL in glasses is quite different from simple activation law typical of crystal8 and is described by inverse Arrhenius dependence' : IpL N exp ( - T/T,), I PL =
conwt,
TZT,
T -CT,
where T, = T, = 20 + 60 K and depends on composition and thermal history of the sample. This dependence has been observed alwo in In spite of silica glasses 2 and in other disordered wolidw3. a number of models wuggested for interpretation of (1) the origin of parameters T, and T, is not clear. 2. MODEL In order to explaine the dependence (1) and wome other properties of PL in glasses we use Local Heating Model (LHJd). Previously this model has been used by uw for explaination of the photoinduced structure transformations in chalcogenide glasses4-6 . First attempt to relate the temperature dependence of PL in glasses to LHM has been done in 78. We assume that PL centers are similar in glassy and crywtalline modifications of the wame compounds'. Large Stokew shift ( Eg/2) of PL epectra shows, that a great part of energy around 002%3093/87/$03.500 ElsevierSciencePublishersB.V. (North-Holland PhysicsPublishing Division)
the PL center is converted to phonons. Localization of these phonons due to structure disordering leads to local heating of some microregion around the PL center (in crystals this heating is absent as phonons leave region around the PL center for the time of about one period of atomic vibration). As a result, activation of non-radiative recombination of relaxed photoelectrons in glasses is realized efficiently at higher local temperature than in crystals. This local temperature can sufficiently exceed mean temperature of the sample. Based on this idea we shall receive firstly expression (1). It is known that the thermal quenching of PL in glasses is The determined by the non-radiative channel of recombination'. quantum efficiency of PL is connected to the rates of radiative and non-radiative recombination 'r 'r So, the temperature
dependence lpL
In crystals
= 'pm
N w;’
at temperatures w&.
(2)
+ 'nr of PL is
determined
by
Wm
(T):
(T) above Bebye temperature
CT) = voexp
(-E/T)
(4)
where E is PL activation energy, VoI 10'3 s-1 - atomic vibration frequency. In accordance with our model PL intensity in glass is just the same as in a crystal, but with different effective temperature: (5) IPL N exp (-E/(T+T*)) Here T* is contribution to the effective temperature of microregion around the PL center, that is related to the non-equilibrium phonons accompanying the appearance of Stokes shift. Taking into account the value of Stokes shift Eg/2, the LHM predicts that T* 2 T /2 4-6, where T is glass temperature. At low T, T < T* we imm:diately get fro: (5) (expanding the exponent in powers of T/T*): W&
(T)
= v, exp(-E/T*)
exp (T/T,)
(6)
where T, = T*2/E
(7)
3. DISCUSSION Typical values of parameters in (5)-(7) are the following: E = 200-300 mev, T* = Eg/2N = some hundreds of degrees (N - the number of atoms in a local heated microregion, N -100). So, T* = 091 E, T,=O,l T* = 20 + 60 K. It is difficult to make a more accurate estimation of T* because of non-equilibrium and nonplanckian spectrum of vibrations leading to local heating5. In fact PL data for the first time give experimental estimation of local temperature. For example in case of As2S3 T, = 20 K ', E = 230 mev ' , T* = 230 K=Tg/2. According to (6) at TLT* activaI& (T) have demonstrated tion dependence with the same activation energy as in corresponding crystal. This situation has been observed experimentally in g-As2Se3 '. Let us note, that T* and consequently T, depend on fictive temperature of the sample If. The point is increase in disorder with increasing Tf. This leads to more efficient localization of the high-frequency phonons and so to increase in T* and T,. This explaines some scattering in T, values in different works. From the-other hand, changing of single parsmeter Tf leads to changing of two measuring parameters - T, and IpL. This allows us using (31, (61, (7) to find E. Thus, in ' the experimental curves IpL (T) for g-As2S3 at two different values of Tfare shown. From these low-temperature data we find for E a value 260 mev, which is in a good agreement with PL activation energy in corresponding crystals. Recent picosecond measurements 10 of PL intensity in g-As 23S have shown that 1, t5 300 pa, (8) IPL N exp (- t/r, and t, depends =.a -1
=
on temperature -1 =I
+
t2'
(0)
as exp (T/T,),
-1 where 4 ' IO8 s-1 is radiative T;' (I$ ==4 is non-radiative T = 0. Our model gives for the latter T;' (0) = v. exp (- E/T*). Supposing we have T-1 (o)=4.108s-' which is in 2 experiment. In the frames of the model results are explained. l
108
s-1
(9)
recombination rate, recombination rate at expression (6): vo= 1013 s-l, E/T* = 10 a good agreement with the some other experimental
4. C0NCLUS10N It is shown that the difference in temperature dependence of PL in glasses and corresponding crystals at low T may be due to localization of high-frequency phonons in glasses, which leads to local heating of microregion around PL center. On the basis of this model some experimental results are explained. REFERENCES 1)
R.A.
Street,
Adv.
Phys.
25 (1976)
2) C.M. Gee and M. Kaether, 3) I. Hirabayashi, Jap. 52 (1983)
Phys.
K. Morigaki
397.
Rev. Lett.
42 (1979)
and S. Yamanaka,
V.K.
Malinovsky,
V.G.
Zdanov,
5)
V.K. Malinovsky, print.
V.N.
Novikov,
J. Non-Cryst.
A.P.
Sokolov,
V.G.
Malinovskg,
51
7) I.
(1984)
J. Phys.
Sot.
671.
4)
6) V.K.
1765.
J. Non-Cryst.
Sol.
Zdanov,
51
Sol. Sol.
(1982)
(1986)
St.
31.
in
Comm.
647.
Kosa Somogyi,
M. KOOE, Preprint
KFKI N 126,
Budapest,
(1984).
8) M. KOOB, I.
Kosa Somogyi,
J. Non-Cryst.
Sol.
77 & 78 (1985)
1145.
9) M.A. Kastner, 10)
T.E.
Orlowsky,
J. Non-Cryst. and B.A.
Sol.
Weinstein,
77
eC 78
Phil.
(1985)
1173.
Mag. B52 (885)
1.
THE NATURE OF PHOTOLUMINESCENCE
453 .456 453
ANONALOUS TEMPERATURE IN GLASSES
V.K.MALlNOVSKY,
V.N.NOVIXOV,
DEPENDENCE
OF
A.P.SOKOLOV
Institute of Automation and Electrometry of Siberian Branch of the USSR AC. Sci., 630090, Novosibirsk, 90, USSR The anomalouos temperature dependence of photoluminescence (PL) in glasses is related to local increase in temperature around the PL center. The temperature increases due to tiduoed by disorder localization of vibration energy which arises together with the Stokes shift. In the frames of this model some experimental rewults on PL are explained. 1. INTRODUCTION It ie well known, that PL properties are similar in glassy and crystalline chalcogenides of the same composition'. However the temperature dependence-of PL in glasses is quite different from simple activation law typical of crystal8 and is described by inverse Arrhenius dependence' : IpL N exp ( - T/T,), I PL =
conwt,
TZT,
T -CT,
where T, = T, = 20 + 60 K and depends on composition and thermal history of the sample. This dependence has been observed alwo in In spite of silica glasses 2 and in other disordered wolidw3. a number of models wuggested for interpretation of (1) the origin of parameters T, and T, is not clear. 2. MODEL In order to explaine the dependence (1) and wome other properties of PL in glasses we use Local Heating Model (LHJd). Previously this model has been used by uw for explaination of the photoinduced structure transformations in chalcogenide glasses4-6 . First attempt to relate the temperature dependence of PL in glasses to LHM has been done in 78. We assume that PL centers are similar in glassy and crywtalline modifications of the wame compounds'. Large Stokew shift ( Eg/2) of PL epectra shows, that a great part of energy around 002%3093/87/$03.500 ElsevierSciencePublishersB.V. (North-Holland PhysicsPublishing Division)
the PL center is converted to phonons. Localization of these phonons due to structure disordering leads to local heating of some microregion around the PL center (in crystals this heating is absent as phonons leave region around the PL center for the time of about one period of atomic vibration). As a result, activation of non-radiative recombination of relaxed photoelectrons in glasses is realized efficiently at higher local temperature than in crystals. This local temperature can sufficiently exceed mean temperature of the sample. Based on this idea we shall receive firstly expression (1). It is known that the thermal quenching of PL in glasses is The determined by the non-radiative channel of recombination'. quantum efficiency of PL is connected to the rates of radiative and non-radiative recombination 'r 'r So, the temperature
dependence lpL
In crystals
= 'pm
N w;’
at temperatures w&.
(2)
+ 'nr of PL is
determined
by
Wm
(T):
(T) above Bebye temperature
CT) = voexp
(-E/T)
(4)
where E is PL activation energy, VoI 10'3 s-1 - atomic vibration frequency. In accordance with our model PL intensity in glass is just the same as in a crystal, but with different effective temperature: (5) IPL N exp (-E/(T+T*)) Here T* is contribution to the effective temperature of microregion around the PL center, that is related to the non-equilibrium phonons accompanying the appearance of Stokes shift. Taking into account the value of Stokes shift Eg/2, the LHM predicts that T* 2 T /2 4-6, where T is glass temperature. At low T, T < T* we imm:diately get fro: (5) (expanding the exponent in powers of T/T*): W&
(T)
= v, exp(-E/T*)
exp (T/T,)
(6)
where T, = T*2/E
(7)
3. DISCUSSION Typical values of parameters in (5)-(7) are the following: E = 200-300 mev, T* = Eg/2N = some hundreds of degrees (N - the number of atoms in a local heated microregion, N -100). So, T* = 091 E, T,=O,l T* = 20 + 60 K. It is difficult to make a more accurate estimation of T* because of non-equilibrium and nonplanckian spectrum of vibrations leading to local heating5. In fact PL data for the first time give experimental estimation of local temperature. For example in case of As2S3 T, = 20 K ', E = 230 mev ' , T* = 230 K=Tg/2. According to (6) at TLT* activaI& (T) have demonstrated tion dependence with the same activation energy as in corresponding crystal. This situation has been observed experimentally in g-As2Se3 '. Let us note, that T* and consequently T, depend on fictive temperature of the sample If. The point is increase in disorder with increasing Tf. This leads to more efficient localization of the high-frequency phonons and so to increase in T* and T,. This explaines some scattering in T, values in different works. From the-other hand, changing of single parsmeter Tf leads to changing of two measuring parameters - T, and IpL. This allows us using (31, (61, (7) to find E. Thus, in ' the experimental curves IpL (T) for g-As2S3 at two different values of Tfare shown. From these low-temperature data we find for E a value 260 mev, which is in a good agreement with PL activation energy in corresponding crystals. Recent picosecond measurements 10 of PL intensity in g-As 23S have shown that 1, t5 300 pa, (8) IPL N exp (- t/r, and t, depends =.a -1
=
on temperature -1 =I
+
t2'
(0)
as exp (T/T,),
-1 where 4 ' IO8 s-1 is radiative T;' (I$ ==4 is non-radiative T = 0. Our model gives for the latter T;' (0) = v. exp (- E/T*). Supposing we have T-1 (o)=4.108s-' which is in 2 experiment. In the frames of the model results are explained. l
108
s-1
(9)
recombination rate, recombination rate at expression (6): vo= 1013 s-l, E/T* = 10 a good agreement with the some other experimental
4. C0NCLUS10N It is shown that the difference in temperature dependence of PL in glasses and corresponding crystals at low T may be due to localization of high-frequency phonons in glasses, which leads to local heating of microregion around PL center. On the basis of this model some experimental results are explained. REFERENCES 1)
R.A.
Street,
Adv.
Phys.
25 (1976)
2) C.M. Gee and M. Kaether, 3) I. Hirabayashi, Jap. 52 (1983)
Phys.
K. Morigaki
397.
Rev. Lett.
42 (1979)
and S. Yamanaka,
V.K.
Malinovsky,
V.G.
Zdanov,
5)
V.K. Malinovsky, print.
V.N.
Novikov,
J. Non-Cryst.
A.P.
Sokolov,
V.G.
Malinovskg,
51
7) I.
(1984)
J. Phys.
Sot.
671.
4)
6) V.K.
1765.
J. Non-Cryst.
Sol.
Zdanov,
51
Sol. Sol.
(1982)
(1986)
St.
31.
in
Comm.
647.
Kosa Somogyi,
M. KOOE, Preprint
KFKI N 126,
Budapest,
(1984).
8) M. KOOB, I.
Kosa Somogyi,
J. Non-Cryst.
Sol.
77 & 78 (1985)
1145.
9) M.A. Kastner, 10)
T.E.
Orlowsky,
J. Non-Cryst. and B.A.
Sol.
Weinstein,
77
eC 78
Phil.
(1985)
1173.
Mag. B52 (885)
1.