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ESR of Ni doped amorphous chalcogenide semiconductors
~
Solid State Communications, Vol.31, pp.967-970. Pergamon Press Ltd. 1979. Printed in Great Britain.
ESR OF Ni DOPED AMORPHOUS CHALCOGENIDE SEMICONDUCTORS
M. Ktuneda and T. S h l m l z u Department of Electronics,
F a c u l t y o f T e c h n o l o g y , Kanazawa U n i v e r s i t y ,
Kanazawa 920, J a p a n
(Received 18 June 1979 by T. Toyozawa)
ESR signals with g - 2.08 and 2.13 due to Ni were observed for evaporated (Ge0.32Se0.32Te0.32As0.4)100. XNIX and (Gel/3Se2/3)100-KN~K films respectively. The large increase of the electrical conductivity by the addition of Ni is discussed in connection with the ESR signal. Bulk glasses prepared by melt-quenchlng are also investigated for comparison.
i. Introduction Amorphous semiconductors have been a great concern in recent years in connection with developing cheap solar cells. Multl-component amorphous chalcogenlde semiconductors have an advantage of wide variety of controllability for physlcal properties such as optical energy gap and electrical conductivity by continuously changing the composition. Although doping of amorphous chalcogenlde semiconductors had been considered to be difficult, Flasck et al. found that transition metal element NI has a large effect on the electrical conductivity of amorphous chalcogenlde films with a composition of Ge 0 32Se 0 32Te 0 32As0 4 (Ge-Se-Te-As) prepared by°co-sputtering. 1 m e reason of the large increase of the electrical conductivity is suggested by Ovshlnsky et al, 2,3 but the mechanism has not yet been clear. Frltzsche et al. discussed on the difference in the effect of charged additives in chalcogenldes between samples prepared by melt-quenchlng and by co-sputterlng or co-evaporatlon.4, 5 In meltquenched bulk samples, the additives and the intrinsic defects are allowed to equilibrate at around the glass transition temperature, but they are free from the equilibration in fllm samples prepared by co-sputterlng or co-evaporation. As a result, charged additives have a possibility of drastically increasing the electrlcal conductivity of film samples. The authors previously reported on ESR of Mn impurity in chalcogenide glasses prepared by melt-quenchlng and revealed its mechanism for changing the physical properties of the host materials. 6 In this note we report on ESR of NI doped amorphous semiconductors, Ge-Se-Te-As: Ni and GeSe2: Ni.
rating melt-quenched bulk glasses Ge-Se-Te-As or GeSe 2 and NI from separate sources. Thin polyester films (20 ~m thick) were used as a substrate in order to get large sample volume for good signal to noise ratio of ESR measurements. The deposition rate was about 15 A/sec and the thickness of the samples were 1 % 2 ~m. The composition of the fllm prepared from Ge0.32Se0.32Teo.32As0. 4 was determined by fluorescent X-ray analysis to be close to Ge0.30Se0.30Te0.30As0.10. The amount of Ni was also determined by fluorescent X-ray analysis. The polyester substrates with the evaporated chalcogenlde films were rolled up, bound by a thread and put into an ESR sample tube. ESR measurements were performed with a JEOL PE3X spectrometer operating at X-band between room temperature and llquld nitrogen temperature. The center density of the ESR signal was obtained by comparing absorption areas with a JEOL standard sample and by assemlng the spin S - 1/2. The absorption area of a sllghtly asymmetric ESR signal was calculated by choosing a set of Gaussian parameters to fit the lower field part and the another set of Gauss4an parameters to fit the higher fleld part. 3. Results Typical ESR signals for Ge-Se-Te-As: NI films are shown in Fig. i. A slightly asymmetric ESR signal with a peak-to-peak llnewldth of about 190 G is observed at g - 2.08 at liquid nitrogen temperature as shown in Fig. 1 (a). The center density of this ESR signal is shown by open circles in Fig. 2 as a function of Ni content. Films without adding Ni exhibit a large asymmetric ESR signal at room temperature as shown in Fig. 1 (e). The peak-to-peak llnewldth of thls signal is about 500 G. The g-value defined as an intersect of the derivative absorption curve with the base llne is 2.19. The ESR
2. Experimental Sample films were prepared by co-evapo967
Vol. 31, No. 12
ESR OF Ni DOPED AMORPHOUS CHALCOGENZDE SEMICONDUCTORS
968
center density, which was estimated by numerically inte~ratlng the derivative curve, is about 8 x I02u cm -3 at room temperature. ESR spectra at room temperature for Ge-Se-Te-As: NI films with various amounts of Ni are shown in Fig. 1 (b) % (d). The ESR spectra can be decomposed into two signals: one is a broad signal of type (e) and the other a narrow signal of type (a) in Fig. i. The former is subtracted from the total spectrum and remaining spectrum is shown by a dotted curve in Flg. i. The ESR canter density corresponding to the dotted curve is shown by closed circles in Fig. 2. The center densities of the former are of the order of 1019 cm 3which is much less than that of (e) in Fig. 1. The broad ESR signal for Ge-Se-Te-As films without Ni in Fig. 1 (e) broadens and its center density decreases at liquid nitrogen temperature by more than an order of magnitude. Correspondingly, the ESR signals in Fig. 1 (b), (e) and (d) become a narrow single curve of the type (a) at llquid nitrogen temperature. The temperature dependence of the narrow ESR signal is 'shown for Ge-Se-Te-As films with 2.5 at. Z NI in Fig. 3. The narrow signal does not have a large temperature dependence.
X.ffiO
(e)
•
(d)
I
s'%~/
ot RT
(c) ot RT (b)
~2.5
(a)
X: 3.1
at77K 10
% x Fig. 1. Typical ESR signals for Ge-Se-Te-As films with various amounts of Ni at room temperature (RT) and liquid nitrogen temperature (77K). The atomic percent of Ni is designated by x. Note that the scale of the ordinate is different for different signals. Dotted curves in (b), (c) and (d) show the remaining spectra of the corresponding solid curves subtracted by a pertinent fraction of (e).
.-
102°
2.151 U
2.1°1"
o o
.-
-
.
o
s
e
10"
!
0 @
5 1000/T
!
10 ( K-' )
e
8
{ o 77K~
o 10*a
I
~e RT / m
0.1
X
I
I
I
10 1.0 (at.% of Ni)
Fig. 2. The center density for the narrow ESR signal as a function of NI content in Ge-Se-Te-As: Ni films at room temperature (RT) and liquid nitrogen temperature (77K).
Fig. 3. Temperature dependence of the narrow ESR signal in Ge-Se-Te-As films with 2.5 at.Z Ni. Annealing decreases the center density of the narrow signal as shown for Ge-Se-Te-As films with 1.4 at.g Ni in Fig. 4. A preliminary ESR measurements were also performed for Ni doped amorphous chalcogenide films with less complicated composition, GeSe2: Ni. A narrow ESR signal with a slightly
969
ESR OF Ni DOPED AMORPHOUS CHALCOCENIDE SEMICONDUCTORS
Vol. 31, No. 12
Table i. Comparisons between bulk and film samples of Ge-Se-Te-As: NI for the ESR center d e n s i t y Ns a t l i q u l d n i t r o g e n t e m p e r a t u r e and the electrical c o n d u c t i v i t y o a t room t e m p e r a t u r e . The c o n d u c t i v i t y d a t a i n f i l m s a m p l e s a r e t a k e n f r o m r e f e r e n c e s 1 and 7. N
(cm-3)
o (fl-lcm-I)
77 K
a t RT
S
at
~, 0.5 00
\ 0
i
0
I
100
,
I v
200
,
300
annealing temperature('C) Fig. 4. Change of the narrow ESR signal in Ge-Se-Te-As films with 1.4 at.% Ni by isochronal annealing for 30 mln measured at liquid nitrogen temperature. asymmetric shape was observed at liquid nitrogen temperature for films with 0.4, 1.0 and 3.0 at. % Ni. The llnewidth and the g-value of this ESR signal are 150 % 160 G and 2.13, respectively, The center density tends to saturate with the increase of Ni content and decreases above 1 at.%. The center density for films with 1.0 at.% Ni is 3 x 1019 c~n-3. A broad asymmetric ESR signal with a llnewidth of about 600 G and g - 2.07 was observed in GeSe 2 films without Ni at room temperature. But the signal is considerably weaker than that for Ge-Se-Te-As films without Ni. Since soma polyester films used as a substrate exhibited a weak broad background signal, it is difficult to distinguish the proper signal in GeSe 2 films from that in the polyester substrate. Bulk samples of Ge-Se-Te-As: Ni with O, 0.3, 1.0 and 3.0 at.% Ni were prepared by melt-quenching. The sample with 3.0 at.% Ni, however, is crystallized. The results of ESR and electrical conductivity measurements are shown in Table 1 together with those for film samples. The bulk samples with NI e x h i b i t a narrow ESR signal (£Hr~ - 230 G and g - 2.09 at liquid nitrogen temperature) similar to that for the film samples, but the ESR center density is smaller than that for the film samples. The increase of the electrical conductivity by the addition of Ni is also very small in contrast to the case of the film samples. 4. Discussion All of the Ni doped amorphous chalcogenide semiconductors, Ge-Se-Te-As: NI films, GeSe2: NI films and bulk Ge-Se-Te-As: Ni glasses, exhibit a characteristic ESR signal with a linewldth of 150 ~ 230 G and a g-value of 2.09 ~ 2.13. Although the ESR center density saturates with the increase o f NI content at different amounts of Ni for different chalcogenlde systems, it increases with the increase of NI content in the r e g i o n of a small amount of Ni as shown for Ge-Se-Te-As: Ni films in Fig. 2. Therefore, this
bulk without NI bulk with 0.3 at. % Ni bulk with 1.0 at.% NI
not detected
lO - 1 0
8.0 x 1017
10-9 % i0 -I0
8.0 × 1017
10 -8
film without Ni film with 0.4 at.% Ni
not d e t e c t e d
10 -9
5.0 x 1018
10 -4
ESR signal is considered to originate from Ni. ESR measurements of Ni in ]]I-V and H-VI compounds have been reported, in which Ni 3÷ ions (3d 7) in T~ symmetry sites have a spin S - 3/2 and exhibit ESR signals with g-values ranging from 2.09 ~ 2.20. 8 In our case of Ni in chalcogenide systems, the g-value of the ESR signal is rather close to the above values, so it is possible that Ni (3d 7) replaclng fourfold coordinated Ge atoms is responsible for the origin of the narrow ESR slgnal. Although the ESR center density was deduced by assuming the spin S = 1/2, a factor 1/5 should be multiplied if the spin S - 3/2 is taken. Annealing decreases the ESR center density of the narrow ESR signal as shown in Fig. 4. By annealing, soma fractions of Ni atoms exhibiting the ESR signal may change their electron configuration to ones which exhibit no ESR signals. Ge-Se-Te-As films exhibit a large ESR signal although they are not intentionally doped with magnetic impurities. The center density of this broad ESR signal is so large that it could not be considered to originate directly from impurities. It is also unlikely chat this signal originate from the polyester substrate because of the large center density. Chalcogenlde glasses is usually believed not to exhibit ESR signals because dangling bonds are either doubly occupied (D') or unoccupied (D÷) by electrons due to negative effective electron correlation energy. 3 But even in bulk chalcogenlde glasses, ESR signals which can be ascribed to "quenched-ln" defect centers have been observed depending on preparation condltlons. I0,11 It is not impossible that the broad ESR signal is due to defect centers in chalcogenide films, but details are not clear at present. The center density of the ESR signal from Ni is smaller b y about an order of magnitude than the amount of the doped Ni in film samples, and it is smaller by about two orders of magnitude in bulk samples. Therefore, only a small fraction of the added Ni in both film and bulk samples contributes to the ESR signal and the majority of the doped Ni does not contribute to ESR. As for the effect on electrical conductivity, the addition of Ni brings about far more prominent effect in film samples than in bulk
970
ESR OF Ni DOPED AMORPHOUS CHALCOGENIDE SEMICONDUCTORS
s a m p l e s f o r a n e q u a l amount o f added Ni. A l t h o u g h Ni can n o t b e added by m e l t - q u e n c h l n 8 beyond a c e r t a i n amount b e c a u s e c r y s t a l l i z a t i o n o c c u r s by i n c r e a s i n g t h e emount o f N i , t h e l a r g e increase of the electrical c o n d u c t i v i t y by a d d i n g Ni i n f i l m s a m p l e s i s n o t a t t r i b u t e d t o a f a c t t h a t m e t a l i m p u r i t i e s c a n b e added a b u n d a n t l y without crystallization. It is noticeable that t h e c e n t e r d e n s i t y o f t h e ESR s i g n a l due t o Ni i s a l s o l a r g e r f o r f i l m s a m p l e s t h a n f o r b u l k s a m p l e s . The f a c t s u g g e s t s t h a t Ni e x h i b i t i n g t h e ESR s i g n a l h a s s o m e t h i n g t o do w i t h t h e increase of the electrlcal c o n d u c t i v i t y . The following consideration, however, appears to be contrary to this suggestion. It has been reported that the electrica! conductivity of film samples doped with Ni increase by annealing. ! On the contrary, the center density of the ESR signal from Ni decreases by anneallng as shown in Fig. 4. It appears, therefore, that Ni contributing to the increase of the electrical conductivity is different from the paramagnetic Ni center contributing to ESR. Foregoing results suggest that a large fraction of added Ni contributes to neither the
Vol. 31, No. 12
ESR nor the increase of the electrlcal conductivity for bulk samples in comparison with the case of film samples. These Ni atoms may be present in the form of Ni-chalcogen clusters. The remaining small fraction of Ni can contribute to the ESR and the increase of the electrical conductivity. In conclusion, Ni additives in amorphous chalcogenlde semiconductors have been detected microscopically by ESR for the first time, and the results show that the incorporation scheme of Ni atoms in film samples is not the same as that in bulk samples.
Acknowledsements - The authors wish to thank Drs. Y. NakaJima and Y. Mizobuchi of Research Laboratory of Fuji Photo Film Co., Ltd. for providing the film samples. They also thank Dr. M. Suhars for the use of the ESR spectrometer at Faculty of Science of Kanazawa University. This work is partly supported by the Grant-ln-Aid for the Scientific Research from the Ministry of Education in Japan.
REFERENCES
1. FLASCK, R., IZU, M., SAPRU, K . , ANDERSON, T . , OVSHINSKY, S. R. and FRITZSCHE, H., P r o c . 7 t h I n t . Conf. on Amorphous and L i q u i d S e m i c o n d u c t o r s , E d i n b u r g h 1977, (CICL, U n i v e r s i t y o f E d i n b r u g h , 1977) p. 524. 2. OVSHINSKY, S. R., P r o c . 7 t h I n t . Conf. on Amorphous and L i q u i d S e m i c o n d u c t o r s , E d i n b u r g h , 1977, (CICL, U n i v e r s i t y o f E d i n b u r g h , 1977) p. 519. 3. OVSHINSKY, S. R. and ADLER, D., Contemp. Phys. 1 9 , 109 ( 1 9 7 8 ) . 4. FRITZSCHE, H . , P r o c . 7 t h I n t . Conf. on Amorphous and L i q u i d S e m i c o n d u c t o r s , E d i n b u r g h , 1977, (CICL, U n i v e r s i t y o f E d i n b u r g h , 1977) p. 3. 5. FRITZSCHE, H. and KASTNER, M., P h i l . Mag. B 37, 285 ( 1 9 7 8 ) . 6. KUI~DA, M., JINNO, Y . , SUZUKI, M. and SHIMIZU, T . , J p n . J . Appl. Phys. 15, 201 ( 1 9 7 6 ) . 7. KAWAJIRI, K . , TABEI, M., HIGASHI, A., MIZOBUCHI, Y. and NAKAJIMA, Y . , p r e s e n t e d a t t h e 2 6 t h S p r i n g M e e t i n g o f t h e J a p a n S o c i e t y o f A p p l i e d P h y s i c s and o f t h e R e l a t e d S o c i e t i e s , Tokyo, March, 1979. 8. KAUFMANN, U. and SCHNEIDER, J . , S o l i d S t a t e Co~mun. 25, 1113 ( 1 9 7 8 ) . 9. STREET, R. A. and MOTT, N. F . , P h y s . Rev. L e t t . 35, 1293 ( 1 9 7 5 ) . 10. KUMEDA, M., KOBAYASRI, N., MARUYAMA, E. and SHIMIZU, T . , p h y s . s t a t . s o l . (b) 73, K19 ( 1 9 7 6 ) . ii. BISHOP, S. G., STROM, U. and TAYLOR, P. C., The Structur e of Non-Crystalline Materials, Ed. GASKELL, P. H., (Taylor and Francis Ltd., London, 1977) p. 109.
Solid State Communications, Vol.31, pp.967-970. Pergamon Press Ltd. 1979. Printed in Great Britain.
ESR OF Ni DOPED AMORPHOUS CHALCOGENIDE SEMICONDUCTORS
M. Ktuneda and T. S h l m l z u Department of Electronics,
F a c u l t y o f T e c h n o l o g y , Kanazawa U n i v e r s i t y ,
Kanazawa 920, J a p a n
(Received 18 June 1979 by T. Toyozawa)
ESR signals with g - 2.08 and 2.13 due to Ni were observed for evaporated (Ge0.32Se0.32Te0.32As0.4)100. XNIX and (Gel/3Se2/3)100-KN~K films respectively. The large increase of the electrical conductivity by the addition of Ni is discussed in connection with the ESR signal. Bulk glasses prepared by melt-quenchlng are also investigated for comparison.
i. Introduction Amorphous semiconductors have been a great concern in recent years in connection with developing cheap solar cells. Multl-component amorphous chalcogenlde semiconductors have an advantage of wide variety of controllability for physlcal properties such as optical energy gap and electrical conductivity by continuously changing the composition. Although doping of amorphous chalcogenlde semiconductors had been considered to be difficult, Flasck et al. found that transition metal element NI has a large effect on the electrical conductivity of amorphous chalcogenlde films with a composition of Ge 0 32Se 0 32Te 0 32As0 4 (Ge-Se-Te-As) prepared by°co-sputtering. 1 m e reason of the large increase of the electrical conductivity is suggested by Ovshlnsky et al, 2,3 but the mechanism has not yet been clear. Frltzsche et al. discussed on the difference in the effect of charged additives in chalcogenldes between samples prepared by melt-quenchlng and by co-sputterlng or co-evaporatlon.4, 5 In meltquenched bulk samples, the additives and the intrinsic defects are allowed to equilibrate at around the glass transition temperature, but they are free from the equilibration in fllm samples prepared by co-sputterlng or co-evaporation. As a result, charged additives have a possibility of drastically increasing the electrlcal conductivity of film samples. The authors previously reported on ESR of Mn impurity in chalcogenide glasses prepared by melt-quenchlng and revealed its mechanism for changing the physical properties of the host materials. 6 In this note we report on ESR of NI doped amorphous semiconductors, Ge-Se-Te-As: Ni and GeSe2: Ni.
rating melt-quenched bulk glasses Ge-Se-Te-As or GeSe 2 and NI from separate sources. Thin polyester films (20 ~m thick) were used as a substrate in order to get large sample volume for good signal to noise ratio of ESR measurements. The deposition rate was about 15 A/sec and the thickness of the samples were 1 % 2 ~m. The composition of the fllm prepared from Ge0.32Se0.32Teo.32As0. 4 was determined by fluorescent X-ray analysis to be close to Ge0.30Se0.30Te0.30As0.10. The amount of Ni was also determined by fluorescent X-ray analysis. The polyester substrates with the evaporated chalcogenlde films were rolled up, bound by a thread and put into an ESR sample tube. ESR measurements were performed with a JEOL PE3X spectrometer operating at X-band between room temperature and llquld nitrogen temperature. The center density of the ESR signal was obtained by comparing absorption areas with a JEOL standard sample and by assemlng the spin S - 1/2. The absorption area of a sllghtly asymmetric ESR signal was calculated by choosing a set of Gaussian parameters to fit the lower field part and the another set of Gauss4an parameters to fit the higher fleld part. 3. Results Typical ESR signals for Ge-Se-Te-As: NI films are shown in Fig. i. A slightly asymmetric ESR signal with a peak-to-peak llnewldth of about 190 G is observed at g - 2.08 at liquid nitrogen temperature as shown in Fig. 1 (a). The center density of this ESR signal is shown by open circles in Fig. 2 as a function of Ni content. Films without adding Ni exhibit a large asymmetric ESR signal at room temperature as shown in Fig. 1 (e). The peak-to-peak llnewldth of thls signal is about 500 G. The g-value defined as an intersect of the derivative absorption curve with the base llne is 2.19. The ESR
2. Experimental Sample films were prepared by co-evapo967
Vol. 31, No. 12
ESR OF Ni DOPED AMORPHOUS CHALCOGENZDE SEMICONDUCTORS
968
center density, which was estimated by numerically inte~ratlng the derivative curve, is about 8 x I02u cm -3 at room temperature. ESR spectra at room temperature for Ge-Se-Te-As: NI films with various amounts of Ni are shown in Fig. 1 (b) % (d). The ESR spectra can be decomposed into two signals: one is a broad signal of type (e) and the other a narrow signal of type (a) in Fig. i. The former is subtracted from the total spectrum and remaining spectrum is shown by a dotted curve in Flg. i. The ESR canter density corresponding to the dotted curve is shown by closed circles in Fig. 2. The center densities of the former are of the order of 1019 cm 3which is much less than that of (e) in Fig. 1. The broad ESR signal for Ge-Se-Te-As films without Ni in Fig. 1 (e) broadens and its center density decreases at liquid nitrogen temperature by more than an order of magnitude. Correspondingly, the ESR signals in Fig. 1 (b), (e) and (d) become a narrow single curve of the type (a) at llquid nitrogen temperature. The temperature dependence of the narrow ESR signal is 'shown for Ge-Se-Te-As films with 2.5 at. Z NI in Fig. 3. The narrow signal does not have a large temperature dependence.
X.ffiO
(e)
•
(d)
I
s'%~/
ot RT
(c) ot RT (b)
~2.5
(a)
X: 3.1
at77K 10
% x Fig. 1. Typical ESR signals for Ge-Se-Te-As films with various amounts of Ni at room temperature (RT) and liquid nitrogen temperature (77K). The atomic percent of Ni is designated by x. Note that the scale of the ordinate is different for different signals. Dotted curves in (b), (c) and (d) show the remaining spectra of the corresponding solid curves subtracted by a pertinent fraction of (e).
.-
102°
2.151 U
2.1°1"
o o
.-
-
.
o
s
e
10"
!
0 @
5 1000/T
!
10 ( K-' )
e
8
{ o 77K~
o 10*a
I
~e RT / m
0.1
X
I
I
I
10 1.0 (at.% of Ni)
Fig. 2. The center density for the narrow ESR signal as a function of NI content in Ge-Se-Te-As: Ni films at room temperature (RT) and liquid nitrogen temperature (77K).
Fig. 3. Temperature dependence of the narrow ESR signal in Ge-Se-Te-As films with 2.5 at.Z Ni. Annealing decreases the center density of the narrow signal as shown for Ge-Se-Te-As films with 1.4 at.g Ni in Fig. 4. A preliminary ESR measurements were also performed for Ni doped amorphous chalcogenide films with less complicated composition, GeSe2: Ni. A narrow ESR signal with a slightly
969
ESR OF Ni DOPED AMORPHOUS CHALCOCENIDE SEMICONDUCTORS
Vol. 31, No. 12
Table i. Comparisons between bulk and film samples of Ge-Se-Te-As: NI for the ESR center d e n s i t y Ns a t l i q u l d n i t r o g e n t e m p e r a t u r e and the electrical c o n d u c t i v i t y o a t room t e m p e r a t u r e . The c o n d u c t i v i t y d a t a i n f i l m s a m p l e s a r e t a k e n f r o m r e f e r e n c e s 1 and 7. N
(cm-3)
o (fl-lcm-I)
77 K
a t RT
S
at
~, 0.5 00
\ 0
i
0
I
100
,
I v
200
,
300
annealing temperature('C) Fig. 4. Change of the narrow ESR signal in Ge-Se-Te-As films with 1.4 at.% Ni by isochronal annealing for 30 mln measured at liquid nitrogen temperature. asymmetric shape was observed at liquid nitrogen temperature for films with 0.4, 1.0 and 3.0 at. % Ni. The llnewidth and the g-value of this ESR signal are 150 % 160 G and 2.13, respectively, The center density tends to saturate with the increase of Ni content and decreases above 1 at.%. The center density for films with 1.0 at.% Ni is 3 x 1019 c~n-3. A broad asymmetric ESR signal with a llnewidth of about 600 G and g - 2.07 was observed in GeSe 2 films without Ni at room temperature. But the signal is considerably weaker than that for Ge-Se-Te-As films without Ni. Since soma polyester films used as a substrate exhibited a weak broad background signal, it is difficult to distinguish the proper signal in GeSe 2 films from that in the polyester substrate. Bulk samples of Ge-Se-Te-As: Ni with O, 0.3, 1.0 and 3.0 at.% Ni were prepared by melt-quenching. The sample with 3.0 at.% Ni, however, is crystallized. The results of ESR and electrical conductivity measurements are shown in Table 1 together with those for film samples. The bulk samples with NI e x h i b i t a narrow ESR signal (£Hr~ - 230 G and g - 2.09 at liquid nitrogen temperature) similar to that for the film samples, but the ESR center density is smaller than that for the film samples. The increase of the electrical conductivity by the addition of Ni is also very small in contrast to the case of the film samples. 4. Discussion All of the Ni doped amorphous chalcogenide semiconductors, Ge-Se-Te-As: NI films, GeSe2: NI films and bulk Ge-Se-Te-As: Ni glasses, exhibit a characteristic ESR signal with a linewldth of 150 ~ 230 G and a g-value of 2.09 ~ 2.13. Although the ESR center density saturates with the increase o f NI content at different amounts of Ni for different chalcogenlde systems, it increases with the increase of NI content in the r e g i o n of a small amount of Ni as shown for Ge-Se-Te-As: Ni films in Fig. 2. Therefore, this
bulk without NI bulk with 0.3 at. % Ni bulk with 1.0 at.% NI
not detected
lO - 1 0
8.0 x 1017
10-9 % i0 -I0
8.0 × 1017
10 -8
film without Ni film with 0.4 at.% Ni
not d e t e c t e d
10 -9
5.0 x 1018
10 -4
ESR signal is considered to originate from Ni. ESR measurements of Ni in ]]I-V and H-VI compounds have been reported, in which Ni 3÷ ions (3d 7) in T~ symmetry sites have a spin S - 3/2 and exhibit ESR signals with g-values ranging from 2.09 ~ 2.20. 8 In our case of Ni in chalcogenide systems, the g-value of the ESR signal is rather close to the above values, so it is possible that Ni (3d 7) replaclng fourfold coordinated Ge atoms is responsible for the origin of the narrow ESR slgnal. Although the ESR center density was deduced by assuming the spin S = 1/2, a factor 1/5 should be multiplied if the spin S - 3/2 is taken. Annealing decreases the ESR center density of the narrow ESR signal as shown in Fig. 4. By annealing, soma fractions of Ni atoms exhibiting the ESR signal may change their electron configuration to ones which exhibit no ESR signals. Ge-Se-Te-As films exhibit a large ESR signal although they are not intentionally doped with magnetic impurities. The center density of this broad ESR signal is so large that it could not be considered to originate directly from impurities. It is also unlikely chat this signal originate from the polyester substrate because of the large center density. Chalcogenlde glasses is usually believed not to exhibit ESR signals because dangling bonds are either doubly occupied (D') or unoccupied (D÷) by electrons due to negative effective electron correlation energy. 3 But even in bulk chalcogenlde glasses, ESR signals which can be ascribed to "quenched-ln" defect centers have been observed depending on preparation condltlons. I0,11 It is not impossible that the broad ESR signal is due to defect centers in chalcogenide films, but details are not clear at present. The center density of the ESR signal from Ni is smaller b y about an order of magnitude than the amount of the doped Ni in film samples, and it is smaller by about two orders of magnitude in bulk samples. Therefore, only a small fraction of the added Ni in both film and bulk samples contributes to the ESR signal and the majority of the doped Ni does not contribute to ESR. As for the effect on electrical conductivity, the addition of Ni brings about far more prominent effect in film samples than in bulk
970
ESR OF Ni DOPED AMORPHOUS CHALCOGENIDE SEMICONDUCTORS
s a m p l e s f o r a n e q u a l amount o f added Ni. A l t h o u g h Ni can n o t b e added by m e l t - q u e n c h l n 8 beyond a c e r t a i n amount b e c a u s e c r y s t a l l i z a t i o n o c c u r s by i n c r e a s i n g t h e emount o f N i , t h e l a r g e increase of the electrical c o n d u c t i v i t y by a d d i n g Ni i n f i l m s a m p l e s i s n o t a t t r i b u t e d t o a f a c t t h a t m e t a l i m p u r i t i e s c a n b e added a b u n d a n t l y without crystallization. It is noticeable that t h e c e n t e r d e n s i t y o f t h e ESR s i g n a l due t o Ni i s a l s o l a r g e r f o r f i l m s a m p l e s t h a n f o r b u l k s a m p l e s . The f a c t s u g g e s t s t h a t Ni e x h i b i t i n g t h e ESR s i g n a l h a s s o m e t h i n g t o do w i t h t h e increase of the electrlcal c o n d u c t i v i t y . The following consideration, however, appears to be contrary to this suggestion. It has been reported that the electrica! conductivity of film samples doped with Ni increase by annealing. ! On the contrary, the center density of the ESR signal from Ni decreases by anneallng as shown in Fig. 4. It appears, therefore, that Ni contributing to the increase of the electrical conductivity is different from the paramagnetic Ni center contributing to ESR. Foregoing results suggest that a large fraction of added Ni contributes to neither the
Vol. 31, No. 12
ESR nor the increase of the electrlcal conductivity for bulk samples in comparison with the case of film samples. These Ni atoms may be present in the form of Ni-chalcogen clusters. The remaining small fraction of Ni can contribute to the ESR and the increase of the electrical conductivity. In conclusion, Ni additives in amorphous chalcogenlde semiconductors have been detected microscopically by ESR for the first time, and the results show that the incorporation scheme of Ni atoms in film samples is not the same as that in bulk samples.
Acknowledsements - The authors wish to thank Drs. Y. NakaJima and Y. Mizobuchi of Research Laboratory of Fuji Photo Film Co., Ltd. for providing the film samples. They also thank Dr. M. Suhars for the use of the ESR spectrometer at Faculty of Science of Kanazawa University. This work is partly supported by the Grant-ln-Aid for the Scientific Research from the Ministry of Education in Japan.
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