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Recombination luminescence process in NaCl
~
Solid S t a t e Communications, Vol.36, pp.1001-1005. Pergamon P r e s s L t d . 1980. P r i n t e d i n G r e a t B r i t a i n .
R E C O M B I N A T I O N LUMINESCE~ICE PROCESS F.J.LSpez,
IN
NaCI
F.Jaque and F . A ~ u l l ~ - L ~ p e z
SectiOn de Optica y Estructura de Solidos, Instituto de ffsica de1 Estado Solido and Departamento de Optica y Estructura de la ~lateria, Facttltad de Cienclas Universidad Aut~noma de Madrid, Cantoblanco, M a d r i d 34 ( S p a i n ) and M.Aguilar Uepartamento
de Optica
y Escructura Cantoblanco,
de la Materia, Universidad M a d r i d 34 ( S p a i n )
(Rece£ued by S. A m e ~ n c h x
-
Autonoma Madrid
Octobe~ S, 1980}
The r e c o m b i n a t i o n luminescence i n NaC1 crystals pure end doped with Cu + a n d Mn ÷÷ has been studied. A unique mechanisme that implies the recombination VK-eleccron is proposed Co explain the thermoluminescence as well as the radioluminesccnce and the after-glow following an i r r a d i a t i o n .
I INTRODUCTION The recombination luminescence in pore and doped alkali halide crystals has been the object o f an i n t e n s i v e work in recent years. Three kinds of techniques are, ~enerally, used for these studies: thermoluminescence (TL) ( L - 5 ) , radiatlon-induced lumlnescence (RL) (h.7) and after-glow(AG) subsequent Co irradiation (8-10). Some a u t h o r s ( S , 1 1 ) have established a relation between the AG p r o c e s s and the TL glow peak which is due t o the thermal diffusion o f t h e VK c e n t e r s . The light emittin~ process consists in both cases in the tunnelinF of the electron trapped in the impurity (which has changed its valence state during the irradiation) to a neighbouring VK center. On t h e other hand, Aguilar et al. (12) have recently stressed the relationship between RL and TL mechanisms, as inferred from the close similarity between the wavelength spectra obtained at the same temperature within a very wide range for various pure and doped alkali halide,. This last work is concerned wlth alkali halide crystals doped with divalent impurities, but the correlation observed between AG and TL has been studied in a l k a l i halide crystals doped with monovalent imp.pities. It must be pointed out that this kind of correlation can only be established for TL g l o w p e a k s f o r which the light emitting mechanism has been unambiguosly identified. This is the c a s e o f t h e TL g l o w p e a k p r o d u c e d by t h e
thermal diffusion of VK c e n t e r s whose maximum t a k e s p l a c e a b o u t 208 a n d 170 K i n KC1 a n d NaC1 r e s p e c t i v e l y . The aim o f t h e p r e s e n t work has been to ascertain w h e t h e r an unique electron-hole recombination mechanism can explain the main spectral features of the luminescence obtained i n TL (glow peak associated t o VK c e n t e r s ) , AC e n d RL e x p e r l m e n C s . Pure as well as Mn a n d Cu d o p e d NaCI s a m p l e s h a v e b e e n u s e d .
2 EXPERIMENTAL Samples of Nat1 pure and doped with Mn o r Cu w e r e g r o w t h i n our laboratory from suprapure Merck materials under an inert atmosphere. Doped samples were quenched previously to X-irradlatlon by h e a t l n B t h e m a t 3 5 0 " C f o r 30 m i n u t e s end dropping them onto a copper block at room temperature. X-irradiatlon was achieved with a Kristalloflex 2H S i e m e n s X - r a y m a c h i n e a t 50 kV, 30 mA t h r o u g h a 1 . 5 mm a l u m i n i u m f i l t e r . For emission detection an EMI 6 2 5 6 5 photomultiplier tube was c h o s e n due to the low dark current end noise. The s p e c t r a l range was comprised b e t w e e n 190 a n d 650 rim. The photomultlpller output current was measured with a CARY-401 v i b r a t i n g reed electrometer. A Bausch & Lomb h i g h intensity monochromacor was used to analyze the emission spectra. 3 RESULTS for end was
I001
F i g . 1 shows t h e AC s p e c t r a at 80 K NaC1 p u r e as w e l l as d o p e d w i t h Mn Cu ( 5 5 a n d 3 ppm respectively). It ascertained that at this temperature
V o l . 36, No. 12
RZCOMBINATION LUMINESCENCE PROCESS IN NaCl
1002 |
I
x 10
t~
I
'
1
'
I
I
1
S m
c
~4 m
o
--3 z
w ~z
2 iI
p==
-r 0
i /"~
I
',00
~ I
,
I00
500
400 WAVELENGTH
Fig. I: E m i s s i o n s p e c t r a 80 K after 10 minutes Cu-doped NaCI (dashed (continuous curve).
600
(nm)
of the after-glow ( A C ) m e a s u r e d at X-irradiation of pure and 3 ppm c u r v e ) and 55 ppm Hn-doped NaCI
no emission from some close glow peak was occurring. }lain results are the following: i) Pure an,I Cu d o p e d NaCI show the two intrinsic emission bands of the self-trapped e x c i t e s (STE) at 2 3 5 (O) and 360 (w) no. It s h o u l d be n o t e d that the l a t t e r band c a n n o t be a s s o c i a t e d to t h e t y p i c a l n a r r o w (0.23 eV) e m i s s i o n of Cu + p e a k i n ~ at 355 nm. ii) For NaCl:~:n, the emission spectr.m consists of the 2 3 5 nm i n t r i n sic hand and another one at 4 0 0 am, which has been previously attributed to an intrinsic self-trapped excitnn (STE) emission perturbed (6) or made allowed (7) by the divalent cation impurity (Mn). iii) For b o t h NaCI:Cu and NaCl:Mn the characteristic impurity emissions have n o t b e e n d e t e c t e d . F i ~ . 2 s h o w s the e m i s s i o n s p e c t r a of the TI. glow peak associated tn VK diff.sinn (~170 K) for NaCI:Hn. More important feat.res are: i ) For NnCI:~n three bands at 235, 4f}0 and 5RO nn a p p e a r . The former one is very weak snd was not d e t e c t e d in a p r e v i o t , s work. The 5RD nm b a n d iS the typical e m i s s i o n of Mn ++. ii) F o r N a C I : C u , the 360 nm broad intrinsic hand i s o b s e r v e d to~ether with the narrow 355 nm e m i s s i o n hand correspondin~ t o C . + . T h e 2 3 5 nm b a n d h a s n o t been detected, prnhahly due t o t h e much lower levels o f lieht for :;aCI:Cu in r e l a t i o n to ;:aC!:Mn.
I n s u m m a r y , t h e TL p r o c e s s e s associated to VK-e recombination show the typical impurity emission as well as the intrinsic (perturbed or not) lumlnescence.
Fig. 3 shows the emission spectra during X-ray irradiation at 80 K f o r N a C l : M n (55 ppm) and NaCI samples doped w i t h 3 a n d I 0 0 ppm o f C u . T h e RL s p e c t r a consist o f t h e STE b a n d s as well an the characteristic impurity emissions. For Hn doped samples the 360 nm emission band is predominant a t 80 K whereas t h e 4 0 0 nm e m i s s i o n band appears as a shoulder. On increasing temperature, the 3 6 0 nm e m i s s i o n band disappears at ~ foe K and t h e band centered a t 4 0 0 nm b e c o m e s apparent. The thermal evolution of this band is similar to that reported previously for NaCl:Ca (7). On t h e o t h e r band, the intensity of Mn ++ e m i s s i o n r e m a i n s c o n s t a n t up to the temperature where the VK center becomes mobile and then the i m p u r i t y emission strongly inereases. For the Cu doped samples, two concentrations have been used in order to clearly appreciate the presence of both the Cu + a n d ~ intrinsic emissions due to the very strong overlapping (peak positions almost coinciding). 4 DISCUSSION All processes iron-hole
TL, RL and AC l i g h t emitting appear to involve an eleerecombination. F o r t h e TL g l o w
V o l . 36, I~o. 12
1003
RECOMBINATION LUMINESCENCE PROCESS IN NaCl
200
&00
600 NQC| :Mn** 1 10
m
=c 50
>.
~
0
NoCI:Cu"
u.I z
~3 0 ,.J
l
,
200
, ~
s/
t
%
300 ~00 WAVELENGTH ( n m )
500
Fi E . 2: Emission spectra o f t h e Glow p e a k (TL) n e a r 170 E o f N a C l : H n ( 5 5 ppm) a n d N a C 1 : C u (3 ppm) s a m p l e s X - i r r a d i a t e d by 3 hours after quenchinR. The d e c o m p o s i t i o n of she spectrum for N a C l : C u into t w o b a n d s at 355 ( n a r r o w ) and 360 (wide) nm is also shorn.
peak correspondlnR to the onset of VK mobility, it has been proposed that the light arises from tunneller of electrons trapped at cation impurities or F' centers (for pure crystals)(13) to VK centers arrivinR into close vicinity. A STE i s t h e n f o r m e d i n an e x c i t e d level. whose de-excitation yields the observed intrinsic luminescence. A completely similar tunnelins process should be operatinE between close pairs of the same electron and hole centers after irradiation ( A t ) . On t h e other h a n d . EL processes can be more complex since they may a d d i t i o n a l l y involve trappinR of a free electron st STE l e v e l , formation end de-excitation of non-relaxed excitons, and possibly some partially ionic processes as F-ll recombination (althouRh the efficiency of this process, has been sugsested to be very small (14)). The detailed process, which has been proposed to operate during the TL R l o w
peak, c a n be impurity, H++)
H÷ + VK
written (for as ~ n l l o v s :
~ (H + + ) *
a
+
tl ++ + h V ( i m p )
divalent
(V K + e)*
h~(STE)
T h u s , when the VK center traps the electron from the impurity, an e x c i t e d STE i s formed and the impurity returns to its initial valence state but i n an excited state. T h e t w o STE emissions obtained From process (b) are the typical o and emissions i n p u r e a n d Cu d o p e d c r y s t a l s . On t h e c o n t r a r y , for NaCI:Hn it has been proposed that the q-emission is stronRly perturbed by t h e i m p u r i t y (~) or corresponds to a different transition made allowed (7) by t h e p e r t u r b i n g field oF the impurity. T h i s i s t h e 400 nm e m i s -
lOOt,
Vol. 36, No. 12
RECOMBIIqATION LUMINESCENCE PROCESS IN NaCl
200
400 I
6OO
I
'
I
I
NaCl:Mn +* I X-3
I x 30
--4 m P
~2 .w
),
~-0 z
3 ppm
Io_o__/,
z m
T3
I
NaCl: Cu ÷
X
O .J
\
\/,,//
,
0
200
3O0 WAVELENGTH
400
(nm}
Fi~. 3: Rmlssion spectra of the radiation-lnduced l u m i n e s c e n c e (RL) m e a s u r e d a t 80 K o f NaCI:Hn ( 5 5 ppm) and N s C I : C u (3 and 100 ppm) sanples.
sion. Although one could also think of a direct tunneller transition ss suggested by DeZbecq e t e l . (ll), It is difficult t o u n d e r s t a n d why t h e band i s e s s e n t i a l ly independent of the divalent cation impurity. The d e - e x c i t a t i o n process (a) leaves the impurity in an excited state and therefore ylelds the characteristic impurity emission for both NaCI:Cu and NaCl:Mn. In prlnciple, the same spectral features should be present in the AG emission. However, the impurity emission has not been observed in the experiments p~rformed st 80 K. To a c c o u n t for this different behevlonr o n e may modify the shove mechanism end assume that in the tunnellng process the impurity remains in its ground state. The impurity lumlnescence should then arise from s resonant transfer between t h e STE and the nearby impurity. If this transfer is thermally activated, it w o u l d be p o n s i -
ble for the transfer t o be o p e r a t i v e in e x p e r i m e n t s p e r f o r m e d a t 170 K ( T L ) end not in those carried o u t a t 80 K ( A G ) . A reasonable possibillty i s t h a t i n o u r A~ experiments one i8 Zooklng at more distant STE-impurlty pairs then in our TL e x p e r i m e n t s w h e r e t h e VK c e n t e r has been able Co arrive Into the clone proximity of the impurity. On t h e o c h e r hand, the closest STE-Impurlty pairs produced during irradiation must have been exhausted in • very short time before t h e AG s p e c t r a l measurements have been inlcisted (few seconds). For the remaining more distant psir8 the life-time of t h e STE level ia shorter than the transfer time end the occurfence of transfer is, therefore, prevented. This proposal is consistent vlth the spectral d a t e f o r RL s t 80 K, w h e r e t h e impurity emission is, indeed, observed. In this case. close enough $TR-Impurlty pairs are responsible for the observed
i
Vol,
36, No. 12
RECOMBINATION LUMINESCENCE PROCESS IN NaCI
Cu and Mn e m i s s i o n s . Also, the proposal is coherent with the insensitivity of the impurity emissions with temperat.re up t o ~ 160 g where VK centers become m o b i l e . M o r e o v e r it can e x p l a i n the o b s e r v e d f a c t that the ratio between'the intensities of Mn and 400 nm emissions is h i g h e r i n TL with relation t o RL e x p e r i m e n t s at the same temperattsre. Therefore, an unique m e c h a n i s m can now account for the main spectral features of TL, AG and RL experiments. However, time-resolved spectroscopy s h o u l d be a p p l i e d t o c h e c k t h e v a r i a t i o n
1005
in the s p e c t r u m of the recombination luminescence according to impurity-V K separation to confirm the proposed scheme. It may b e , finally, n o t e d that f o r some doped alkali halides (KCI:Ag, KCI:TI, etc.) (II) the AG emission includes a band apparently not corres p o n d i n g t o STE t r a n s i t i o n s , but attributed to a donor-acceptor transition f r o m t h e impurity t o t h e g r o u n d s t a t e o f V g. The work p r e s e n t e d h e r e is now b e i n ~ extended to other c a t i o n impurities in NaCl, to see in w h a t cases this alternative p o s s i b i l i t y can operate.
5 REFERENCES
l.-A.E.Purdy and R.B.Murrey, Solid State Commun. 16, 1293 (1975) 2 . - V . O s m i n i n and S . Z a z u b o v i c h , Phys. stat. sol.(b) 7 1 , 435 ( 1 9 7 5 ) 3.-gh. Egemberdiev, A.Elango and S.Zazubovich, Phys. stat. sol.(b) 97, 449 ( 1 9 8 0 ) 4.-J.M.Herreros and F.Jaque, J. L u m i n e s c e n c e 18/19, 231 (1979) 5.-F.J.Lopez, M.Aguilar, F . J a q u e and F.Agull~-L~pez, Solid State Commun. 3 4 , 869 ( 1 9 8 0 ) 6.-M.Ikeya Phys. stat. sol.(b) 69, 275
(1975)
7.-M.Aguilar, F.Agull~-L6pez,
Phys.
F.Jaque star. sol.(b)
and 84,
595
(1977)
8.-I.Jaek and H.Kink, Phys. stat. s o l . 33, 905 ( I 9 6 9 ) 9.-C.J.Delbecq, D.L.Dexter and P.H.Yuster, P h y s . R e v . 17, 4765 ( 1 9 7 8 ) 10.-T.Tashiro, S.Takeuchi, H.Saidoh and N . I t o h , Phys. stat. sol.(b) 9 2 , 611 (1979) ll.-C.J,Delbecq, ¥.Toyozawa and P.H.Yuster, Phys. Rev. B, 9,4497 (1974) I2.-M.Aguilar, F.J.L~pez and F . J a q u e , Solid State Commun. 28,699 (1978) 13.-F.J. L~pez, M. A R u i l a r and F. A g u l l ~ - L ~ p e z . To he p u b l i s h e d . 1 4 o - F . A g u l l ~ - L ~ p e z . To be p u b l i s h e d .
Solid S t a t e Communications, Vol.36, pp.1001-1005. Pergamon P r e s s L t d . 1980. P r i n t e d i n G r e a t B r i t a i n .
R E C O M B I N A T I O N LUMINESCE~ICE PROCESS F.J.LSpez,
IN
NaCI
F.Jaque and F . A ~ u l l ~ - L ~ p e z
SectiOn de Optica y Estructura de Solidos, Instituto de ffsica de1 Estado Solido and Departamento de Optica y Estructura de la ~lateria, Facttltad de Cienclas Universidad Aut~noma de Madrid, Cantoblanco, M a d r i d 34 ( S p a i n ) and M.Aguilar Uepartamento
de Optica
y Escructura Cantoblanco,
de la Materia, Universidad M a d r i d 34 ( S p a i n )
(Rece£ued by S. A m e ~ n c h x
-
Autonoma Madrid
Octobe~ S, 1980}
The r e c o m b i n a t i o n luminescence i n NaC1 crystals pure end doped with Cu + a n d Mn ÷÷ has been studied. A unique mechanisme that implies the recombination VK-eleccron is proposed Co explain the thermoluminescence as well as the radioluminesccnce and the after-glow following an i r r a d i a t i o n .
I INTRODUCTION The recombination luminescence in pore and doped alkali halide crystals has been the object o f an i n t e n s i v e work in recent years. Three kinds of techniques are, ~enerally, used for these studies: thermoluminescence (TL) ( L - 5 ) , radiatlon-induced lumlnescence (RL) (h.7) and after-glow(AG) subsequent Co irradiation (8-10). Some a u t h o r s ( S , 1 1 ) have established a relation between the AG p r o c e s s and the TL glow peak which is due t o the thermal diffusion o f t h e VK c e n t e r s . The light emittin~ process consists in both cases in the tunnelinF of the electron trapped in the impurity (which has changed its valence state during the irradiation) to a neighbouring VK center. On t h e other hand, Aguilar et al. (12) have recently stressed the relationship between RL and TL mechanisms, as inferred from the close similarity between the wavelength spectra obtained at the same temperature within a very wide range for various pure and doped alkali halide,. This last work is concerned wlth alkali halide crystals doped with divalent impurities, but the correlation observed between AG and TL has been studied in a l k a l i halide crystals doped with monovalent imp.pities. It must be pointed out that this kind of correlation can only be established for TL g l o w p e a k s f o r which the light emitting mechanism has been unambiguosly identified. This is the c a s e o f t h e TL g l o w p e a k p r o d u c e d by t h e
thermal diffusion of VK c e n t e r s whose maximum t a k e s p l a c e a b o u t 208 a n d 170 K i n KC1 a n d NaC1 r e s p e c t i v e l y . The aim o f t h e p r e s e n t work has been to ascertain w h e t h e r an unique electron-hole recombination mechanism can explain the main spectral features of the luminescence obtained i n TL (glow peak associated t o VK c e n t e r s ) , AC e n d RL e x p e r l m e n C s . Pure as well as Mn a n d Cu d o p e d NaCI s a m p l e s h a v e b e e n u s e d .
2 EXPERIMENTAL Samples of Nat1 pure and doped with Mn o r Cu w e r e g r o w t h i n our laboratory from suprapure Merck materials under an inert atmosphere. Doped samples were quenched previously to X-irradlatlon by h e a t l n B t h e m a t 3 5 0 " C f o r 30 m i n u t e s end dropping them onto a copper block at room temperature. X-irradiatlon was achieved with a Kristalloflex 2H S i e m e n s X - r a y m a c h i n e a t 50 kV, 30 mA t h r o u g h a 1 . 5 mm a l u m i n i u m f i l t e r . For emission detection an EMI 6 2 5 6 5 photomultiplier tube was c h o s e n due to the low dark current end noise. The s p e c t r a l range was comprised b e t w e e n 190 a n d 650 rim. The photomultlpller output current was measured with a CARY-401 v i b r a t i n g reed electrometer. A Bausch & Lomb h i g h intensity monochromacor was used to analyze the emission spectra. 3 RESULTS for end was
I001
F i g . 1 shows t h e AC s p e c t r a at 80 K NaC1 p u r e as w e l l as d o p e d w i t h Mn Cu ( 5 5 a n d 3 ppm respectively). It ascertained that at this temperature
V o l . 36, No. 12
RZCOMBINATION LUMINESCENCE PROCESS IN NaCl
1002 |
I
x 10
t~
I
'
1
'
I
I
1
S m
c
~4 m
o
--3 z
w ~z
2 iI
p==
-r 0
i /"~
I
',00
~ I
,
I00
500
400 WAVELENGTH
Fig. I: E m i s s i o n s p e c t r a 80 K after 10 minutes Cu-doped NaCI (dashed (continuous curve).
600
(nm)
of the after-glow ( A C ) m e a s u r e d at X-irradiation of pure and 3 ppm c u r v e ) and 55 ppm Hn-doped NaCI
no emission from some close glow peak was occurring. }lain results are the following: i) Pure an,I Cu d o p e d NaCI show the two intrinsic emission bands of the self-trapped e x c i t e s (STE) at 2 3 5 (O) and 360 (w) no. It s h o u l d be n o t e d that the l a t t e r band c a n n o t be a s s o c i a t e d to t h e t y p i c a l n a r r o w (0.23 eV) e m i s s i o n of Cu + p e a k i n ~ at 355 nm. ii) For NaCl:~:n, the emission spectr.m consists of the 2 3 5 nm i n t r i n sic hand and another one at 4 0 0 am, which has been previously attributed to an intrinsic self-trapped excitnn (STE) emission perturbed (6) or made allowed (7) by the divalent cation impurity (Mn). iii) For b o t h NaCI:Cu and NaCl:Mn the characteristic impurity emissions have n o t b e e n d e t e c t e d . F i ~ . 2 s h o w s the e m i s s i o n s p e c t r a of the TI. glow peak associated tn VK diff.sinn (~170 K) for NaCI:Hn. More important feat.res are: i ) For NnCI:~n three bands at 235, 4f}0 and 5RO nn a p p e a r . The former one is very weak snd was not d e t e c t e d in a p r e v i o t , s work. The 5RD nm b a n d iS the typical e m i s s i o n of Mn ++. ii) F o r N a C I : C u , the 360 nm broad intrinsic hand i s o b s e r v e d to~ether with the narrow 355 nm e m i s s i o n hand correspondin~ t o C . + . T h e 2 3 5 nm b a n d h a s n o t been detected, prnhahly due t o t h e much lower levels o f lieht for :;aCI:Cu in r e l a t i o n to ;:aC!:Mn.
I n s u m m a r y , t h e TL p r o c e s s e s associated to VK-e recombination show the typical impurity emission as well as the intrinsic (perturbed or not) lumlnescence.
Fig. 3 shows the emission spectra during X-ray irradiation at 80 K f o r N a C l : M n (55 ppm) and NaCI samples doped w i t h 3 a n d I 0 0 ppm o f C u . T h e RL s p e c t r a consist o f t h e STE b a n d s as well an the characteristic impurity emissions. For Hn doped samples the 360 nm emission band is predominant a t 80 K whereas t h e 4 0 0 nm e m i s s i o n band appears as a shoulder. On increasing temperature, the 3 6 0 nm e m i s s i o n band disappears at ~ foe K and t h e band centered a t 4 0 0 nm b e c o m e s apparent. The thermal evolution of this band is similar to that reported previously for NaCl:Ca (7). On t h e o t h e r band, the intensity of Mn ++ e m i s s i o n r e m a i n s c o n s t a n t up to the temperature where the VK center becomes mobile and then the i m p u r i t y emission strongly inereases. For the Cu doped samples, two concentrations have been used in order to clearly appreciate the presence of both the Cu + a n d ~ intrinsic emissions due to the very strong overlapping (peak positions almost coinciding). 4 DISCUSSION All processes iron-hole
TL, RL and AC l i g h t emitting appear to involve an eleerecombination. F o r t h e TL g l o w
V o l . 36, I~o. 12
1003
RECOMBINATION LUMINESCENCE PROCESS IN NaCl
200
&00
600 NQC| :Mn** 1 10
m
=c 50
>.
~
0
NoCI:Cu"
u.I z
~3 0 ,.J
l
,
200
, ~
s/
t
%
300 ~00 WAVELENGTH ( n m )
500
Fi E . 2: Emission spectra o f t h e Glow p e a k (TL) n e a r 170 E o f N a C l : H n ( 5 5 ppm) a n d N a C 1 : C u (3 ppm) s a m p l e s X - i r r a d i a t e d by 3 hours after quenchinR. The d e c o m p o s i t i o n of she spectrum for N a C l : C u into t w o b a n d s at 355 ( n a r r o w ) and 360 (wide) nm is also shorn.
peak correspondlnR to the onset of VK mobility, it has been proposed that the light arises from tunneller of electrons trapped at cation impurities or F' centers (for pure crystals)(13) to VK centers arrivinR into close vicinity. A STE i s t h e n f o r m e d i n an e x c i t e d level. whose de-excitation yields the observed intrinsic luminescence. A completely similar tunnelins process should be operatinE between close pairs of the same electron and hole centers after irradiation ( A t ) . On t h e other h a n d . EL processes can be more complex since they may a d d i t i o n a l l y involve trappinR of a free electron st STE l e v e l , formation end de-excitation of non-relaxed excitons, and possibly some partially ionic processes as F-ll recombination (althouRh the efficiency of this process, has been sugsested to be very small (14)). The detailed process, which has been proposed to operate during the TL R l o w
peak, c a n be impurity, H++)
H÷ + VK
written (for as ~ n l l o v s :
~ (H + + ) *
a
+
tl ++ + h V ( i m p )
divalent
(V K + e)*
h~(STE)
T h u s , when the VK center traps the electron from the impurity, an e x c i t e d STE i s formed and the impurity returns to its initial valence state but i n an excited state. T h e t w o STE emissions obtained From process (b) are the typical o and emissions i n p u r e a n d Cu d o p e d c r y s t a l s . On t h e c o n t r a r y , for NaCI:Hn it has been proposed that the q-emission is stronRly perturbed by t h e i m p u r i t y (~) or corresponds to a different transition made allowed (7) by t h e p e r t u r b i n g field oF the impurity. T h i s i s t h e 400 nm e m i s -
lOOt,
Vol. 36, No. 12
RECOMBIIqATION LUMINESCENCE PROCESS IN NaCl
200
400 I
6OO
I
'
I
I
NaCl:Mn +* I X-3
I x 30
--4 m P
~2 .w
),
~-0 z
3 ppm
Io_o__/,
z m
T3
I
NaCl: Cu ÷
X
O .J
\
\/,,//
,
0
200
3O0 WAVELENGTH
400
(nm}
Fi~. 3: Rmlssion spectra of the radiation-lnduced l u m i n e s c e n c e (RL) m e a s u r e d a t 80 K o f NaCI:Hn ( 5 5 ppm) and N s C I : C u (3 and 100 ppm) sanples.
sion. Although one could also think of a direct tunneller transition ss suggested by DeZbecq e t e l . (ll), It is difficult t o u n d e r s t a n d why t h e band i s e s s e n t i a l ly independent of the divalent cation impurity. The d e - e x c i t a t i o n process (a) leaves the impurity in an excited state and therefore ylelds the characteristic impurity emission for both NaCI:Cu and NaCl:Mn. In prlnciple, the same spectral features should be present in the AG emission. However, the impurity emission has not been observed in the experiments p~rformed st 80 K. To a c c o u n t for this different behevlonr o n e may modify the shove mechanism end assume that in the tunnellng process the impurity remains in its ground state. The impurity lumlnescence should then arise from s resonant transfer between t h e STE and the nearby impurity. If this transfer is thermally activated, it w o u l d be p o n s i -
ble for the transfer t o be o p e r a t i v e in e x p e r i m e n t s p e r f o r m e d a t 170 K ( T L ) end not in those carried o u t a t 80 K ( A G ) . A reasonable possibillty i s t h a t i n o u r A~ experiments one i8 Zooklng at more distant STE-impurlty pairs then in our TL e x p e r i m e n t s w h e r e t h e VK c e n t e r has been able Co arrive Into the clone proximity of the impurity. On t h e o c h e r hand, the closest STE-Impurlty pairs produced during irradiation must have been exhausted in • very short time before t h e AG s p e c t r a l measurements have been inlcisted (few seconds). For the remaining more distant psir8 the life-time of t h e STE level ia shorter than the transfer time end the occurfence of transfer is, therefore, prevented. This proposal is consistent vlth the spectral d a t e f o r RL s t 80 K, w h e r e t h e impurity emission is, indeed, observed. In this case. close enough $TR-Impurlty pairs are responsible for the observed
i
Vol,
36, No. 12
RECOMBINATION LUMINESCENCE PROCESS IN NaCI
Cu and Mn e m i s s i o n s . Also, the proposal is coherent with the insensitivity of the impurity emissions with temperat.re up t o ~ 160 g where VK centers become m o b i l e . M o r e o v e r it can e x p l a i n the o b s e r v e d f a c t that the ratio between'the intensities of Mn and 400 nm emissions is h i g h e r i n TL with relation t o RL e x p e r i m e n t s at the same temperattsre. Therefore, an unique m e c h a n i s m can now account for the main spectral features of TL, AG and RL experiments. However, time-resolved spectroscopy s h o u l d be a p p l i e d t o c h e c k t h e v a r i a t i o n
1005
in the s p e c t r u m of the recombination luminescence according to impurity-V K separation to confirm the proposed scheme. It may b e , finally, n o t e d that f o r some doped alkali halides (KCI:Ag, KCI:TI, etc.) (II) the AG emission includes a band apparently not corres p o n d i n g t o STE t r a n s i t i o n s , but attributed to a donor-acceptor transition f r o m t h e impurity t o t h e g r o u n d s t a t e o f V g. The work p r e s e n t e d h e r e is now b e i n ~ extended to other c a t i o n impurities in NaCl, to see in w h a t cases this alternative p o s s i b i l i t y can operate.
5 REFERENCES
l.-A.E.Purdy and R.B.Murrey, Solid State Commun. 16, 1293 (1975) 2 . - V . O s m i n i n and S . Z a z u b o v i c h , Phys. stat. sol.(b) 7 1 , 435 ( 1 9 7 5 ) 3.-gh. Egemberdiev, A.Elango and S.Zazubovich, Phys. stat. sol.(b) 97, 449 ( 1 9 8 0 ) 4.-J.M.Herreros and F.Jaque, J. L u m i n e s c e n c e 18/19, 231 (1979) 5.-F.J.Lopez, M.Aguilar, F . J a q u e and F.Agull~-L~pez, Solid State Commun. 3 4 , 869 ( 1 9 8 0 ) 6.-M.Ikeya Phys. stat. sol.(b) 69, 275
(1975)
7.-M.Aguilar, F.Agull~-L6pez,
Phys.
F.Jaque star. sol.(b)
and 84,
595
(1977)
8.-I.Jaek and H.Kink, Phys. stat. s o l . 33, 905 ( I 9 6 9 ) 9.-C.J.Delbecq, D.L.Dexter and P.H.Yuster, P h y s . R e v . 17, 4765 ( 1 9 7 8 ) 10.-T.Tashiro, S.Takeuchi, H.Saidoh and N . I t o h , Phys. stat. sol.(b) 9 2 , 611 (1979) ll.-C.J,Delbecq, ¥.Toyozawa and P.H.Yuster, Phys. Rev. B, 9,4497 (1974) I2.-M.Aguilar, F.J.L~pez and F . J a q u e , Solid State Commun. 28,699 (1978) 13.-F.J. L~pez, M. A R u i l a r and F. A g u l l ~ - L ~ p e z . To he p u b l i s h e d . 1 4 o - F . A g u l l ~ - L ~ p e z . To be p u b l i s h e d .