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Double space charge injection in solids
Volume 4, number 3
PHYSICS LETTERS
References 1) W. S. Porter, B. Roth and J. L. Johnson, Phys. Rev. 111 (1958) 1579.
DOUBLE
SPACE
CHARGE
1April 1963
2) J.E. Perry and S. J. Bame J r . , Phys. Rev. 99 (1955) 1368. 3) F. Ajzenberg-Selove and T. Lauritsen, Nuclear Phys. 11 (1959) 1.
INJECTION
IN SOLIDS
G. A. COX and R. H. TREDGOLD Department of Physics, University College of North Wales, Bangor Received 18 March 1963
The possibility of injecting electrons and holes simultaneously into an insulator was f i r s t suggested by Smith 1, 2). Using gallium e l e c t r o d e s on CdS he obtained e l e c t r o l u m i n e s c e n c e at a wavelength c o r responding to the absorption edge of CdS. He argued that this behaviour could only be explained by double injection. In this experiment there r e m a i n s a c e r tain element of uncertainty beacause the s a m e material was used for both anode and cathode. It is possible that an e l e c t r o c h e m i c a l change at one or both of the elctrodes may have changed the relative effective work functions of the m a t e r i a l s . Otherwise simultaneous electron and hole injection would be impossible. Subsequently, s e v e r a l approximate theoretical t r e a t m e n t s of the subject have been given "-"J. Negative r e s i s t a n c e c h a r a c t e r i s t i c s of p - i - n s t r u c t u r e s have also been attributed to double injection 7). In the work d e s c r i b e d in this letter we have attempted to obtain a completely unambiguous demonstration of double injection using BaTiO 3 single c r y s t a l s at a t e m p e r a t u r e substantially above the Curie point. E a r l i e r work on BaTiO 3 c r y s t a l s 8, 9) has elucidated the following relevant f a c t s about the conductivity of this substance in this t e m p e r a ture range. 1. At low field strength it behaves as an i m p u r i ty semiconductor. 2. At fields > 1 kV cm-1 most of the c u r r e n t is c a r r i e d by space c h a r g e injected f r o m the cathode. This c u r r e n t is limited by the diffusion field at the cathode, not by volume effects 10). The nature of the cathode m a t e r i a l affects the current. 3. Gold and s i l v e r anodes ' f o r m e d ' for s e v e r a l days at fields ~ 5 kV c m - 1 lead to an enhancement of the c u r r e n t by about one o r d e r of magnitude. T r a c e r experiments 9) using Au 198 indicate that the anode m a t e r i a l does not t r a v e l m o r e than about 100 A into the BaTiO 3 and thus the f o r m i n g p r o c e s s must be a s s o c i a t e d with the electrode insulator
interface in a manner analogous to that postulated by Smith 1,2). It thus appears that the forming probably leads to hole injection. In the experiment described here we have used o r d i n a r y thermal injection of electrons at the cathode and have used optical injection of holes at the anode. Carefully selected c l e a r BaTiO 3 c r y s t a l plates about 0.1 m m thick were employed. They were p r e p a r e d in this l a b o r a t o r y by the Remeika 11) method and the e l e c t r o d e s were evaporated on to the s u r f a c e s under a vacuum of 1 0 - ° t o r r after the conventional cleaning techniques had been employed. In an initial experiment thin gold e l e c t r o d e s were used having a t r a n s m i s s i o n coefficient virtually independent of wavelength in the region of interest. Using light in a n a r r o w band centered around 3725 A (which is on the short wavelength side of the absorption edge) the following results were obtained. Illumination through the cathode ted to not detectable i n c r e a s e of the c u r r e n t above the ' d a r k ' value even at 180°C. Illumination through the anode produced a c u r r e n t about 30 times l a r g e r than the dark c u r r e n t for the particular light intensity used. The c u r r e n t was strongly dependent on t e m p e r a t u r e as may be seen f r o m the figure. As the light is on the short wavelength side of the absorption edge it will be absorbed in a distance short as c o m p a r e d with the c r y s t a l thickness. It is thus evident that the light c r e a t e s holes at the anode. It will obviously c r e a t e electrons at the cathode when that electrode is illuminated but, for the injecting contact postulated, there will already be a high density of electrons in the conduction band at the cathode and the c u r r e n t is limited by the diffusion field 10), not by the lack of available c a r r i e r s at the electrode. Why is the c u r r e n t strongly t e m p e r a t u r e dependent? This r e s u l t can only be explained by supposing that the electron and hole c u r r e n t s interact 199
Volume 4, number 3
PHYSICS
.--<
1 A p r i l 1963
ALUMINtUM CATHODE , GOLD ANODE.
6, ~
00
LETTERS
G
O
L
D
CATHODE
AND A N O D E .
O -- 5, --X--)C-- NICKEL C A T H O D E , G O L D . . ~ A N O D E , X C ~ Y S T A L T H I C K N E S S O'O10 CM. cD CL
<(4, }-Z ELI 3 n" n-
14 S~C.
O
U O2 O T ~L
o
~ O
4
8
[2
APPLIED
16
20
2.4
POTENTIAL
28
32
36
40
(VOLTS).
Fig. 1. The influence of the cathode m a t e r i a l on the photocurrent on illuminating the anode in barium titanate single c r y s t a l s for two different t e m p e r a t u r e s . Work functions of c a thode m a t e r i a l s (preffered values): nickel: 5.01 eV; gold: 4.82 eV; aluminium 4.08 eV. b y v i r t u e of t h e i r a s s o c i a t e d c h a r g e s . T h e d a r k ( e l e c t r o n ) c u r r e n t i s known to b e s t r o n g l y t e m p e r a t u r e d e p e n d e n t 8) a n d w i l l t h u s i n f l u e n c e t h e h o l e current. To t e s t t h i s h y p o t h e s i s we c a r r i e d out the f o l l o w i n g e x p e r i m e n t . A c r y s t a l h a d a l a r g e thin g o l d a n o d e e v a p o r a t e d on o n e s i d e . On t h e o t h e r s i d e two s m a l l s i m i l a r c a t h o d e s w e r e e v a p o r a t e d , one b e i n g of g o l d a n d t h e o t h e r of a l u m i n i u m . P h o t o c o n d u c t i v i t y w a s now o b s e r v e d u n d e r o t h e r w i s e i d e n t i c a l c o n d i t i o n s ()~ = 3725 ,~) f i r s t with t h e g o l d c a t h o d e a n d t h e n w i t h t h e a l u m i n i u m c a t h o d e . In b o t h c a s e s i t w a s t h e anode w h i c h w a s i l l u m i n a t e d . The dark current was subtracted from the total curr e n t and the r e s u l t p l o t t e d a g a i n s t f i e l d . A s w i l l b e s e e n f r o m the f i g u r e t h e r e i s a s u b s t a n t i a l d i f f e r e n c e b e t w e e n t h e r e s u l t s f o r the two d i f f e r e n t cathodes. It i s e v i d e n t t h a t t h e l i g h t c a n n o t i n f l u e n c e the c a t h o d e d i r e c t l y . T h e s e r e s u l t s c a n t h u s o n l y be e x p l a i n e d on the b a s i s of d o u b l e i n j e c t i o n . T h e e x p e r i m e n t w a s r e p e a t e d u s i n g two o t h e r s i m *****
200
i l a r c r y s t a l s and the r e s u l t s a g r e e d to w i t h i n a few percent. Similar results were also obtained with s h o r t e r w a v e l e n g t h s . On one c r y s t a l r e s u l t s w e r e o b t a i n e d u s i n g a n i c k e l c a t h o d e in a d d i t i o n to g o l d and a l u m i n i u m c a t h o d e s . I t i s h o p e d to r e p o r t on the p h o t o c o n d u c t i v i t y of B a T i O 3 and S r T i O 3 m o r e f u l l y e l s e w h e r e .
References 1) R.W. Smith, Phys. Rev. 100 (1955) 760. 2) R.W. Smith, Phys. Rev. 105 (1957) 900. 3) R. H. P a r m e n t e r and W.Ruppel, J Appl. Phys. 30 (1959) 1548. 4) M.A. Lampert, R.C.A.Rev. 20 (1959) 682. 5) M.A.Lampert and A.Rose, Phys. Rev. 121 (1961) 26. 6) M.A. Lampert, Phys. Rev. 125 (1962) 126. 7) N.Holonyaket al., Phys. Rev. Letters 8 (1962) 426. 8) A. Branwood and R.H.Tredgold Proe. Phys. Soe. 76 (1960) 93. 9) A. Branwood et a l . , Proe. Phys. Soc. 79 (1962) 1161. 10) G. G. Roberts and R. H. Tredgold, Physics Letters 2 (1962) 6. 11) J . P . R e m e i k a , J. Am. Chem. Soc. 76 (1954) 940.
PHYSICS LETTERS
References 1) W. S. Porter, B. Roth and J. L. Johnson, Phys. Rev. 111 (1958) 1579.
DOUBLE
SPACE
CHARGE
1April 1963
2) J.E. Perry and S. J. Bame J r . , Phys. Rev. 99 (1955) 1368. 3) F. Ajzenberg-Selove and T. Lauritsen, Nuclear Phys. 11 (1959) 1.
INJECTION
IN SOLIDS
G. A. COX and R. H. TREDGOLD Department of Physics, University College of North Wales, Bangor Received 18 March 1963
The possibility of injecting electrons and holes simultaneously into an insulator was f i r s t suggested by Smith 1, 2). Using gallium e l e c t r o d e s on CdS he obtained e l e c t r o l u m i n e s c e n c e at a wavelength c o r responding to the absorption edge of CdS. He argued that this behaviour could only be explained by double injection. In this experiment there r e m a i n s a c e r tain element of uncertainty beacause the s a m e material was used for both anode and cathode. It is possible that an e l e c t r o c h e m i c a l change at one or both of the elctrodes may have changed the relative effective work functions of the m a t e r i a l s . Otherwise simultaneous electron and hole injection would be impossible. Subsequently, s e v e r a l approximate theoretical t r e a t m e n t s of the subject have been given "-"J. Negative r e s i s t a n c e c h a r a c t e r i s t i c s of p - i - n s t r u c t u r e s have also been attributed to double injection 7). In the work d e s c r i b e d in this letter we have attempted to obtain a completely unambiguous demonstration of double injection using BaTiO 3 single c r y s t a l s at a t e m p e r a t u r e substantially above the Curie point. E a r l i e r work on BaTiO 3 c r y s t a l s 8, 9) has elucidated the following relevant f a c t s about the conductivity of this substance in this t e m p e r a ture range. 1. At low field strength it behaves as an i m p u r i ty semiconductor. 2. At fields > 1 kV cm-1 most of the c u r r e n t is c a r r i e d by space c h a r g e injected f r o m the cathode. This c u r r e n t is limited by the diffusion field at the cathode, not by volume effects 10). The nature of the cathode m a t e r i a l affects the current. 3. Gold and s i l v e r anodes ' f o r m e d ' for s e v e r a l days at fields ~ 5 kV c m - 1 lead to an enhancement of the c u r r e n t by about one o r d e r of magnitude. T r a c e r experiments 9) using Au 198 indicate that the anode m a t e r i a l does not t r a v e l m o r e than about 100 A into the BaTiO 3 and thus the f o r m i n g p r o c e s s must be a s s o c i a t e d with the electrode insulator
interface in a manner analogous to that postulated by Smith 1,2). It thus appears that the forming probably leads to hole injection. In the experiment described here we have used o r d i n a r y thermal injection of electrons at the cathode and have used optical injection of holes at the anode. Carefully selected c l e a r BaTiO 3 c r y s t a l plates about 0.1 m m thick were employed. They were p r e p a r e d in this l a b o r a t o r y by the Remeika 11) method and the e l e c t r o d e s were evaporated on to the s u r f a c e s under a vacuum of 1 0 - ° t o r r after the conventional cleaning techniques had been employed. In an initial experiment thin gold e l e c t r o d e s were used having a t r a n s m i s s i o n coefficient virtually independent of wavelength in the region of interest. Using light in a n a r r o w band centered around 3725 A (which is on the short wavelength side of the absorption edge) the following results were obtained. Illumination through the cathode ted to not detectable i n c r e a s e of the c u r r e n t above the ' d a r k ' value even at 180°C. Illumination through the anode produced a c u r r e n t about 30 times l a r g e r than the dark c u r r e n t for the particular light intensity used. The c u r r e n t was strongly dependent on t e m p e r a t u r e as may be seen f r o m the figure. As the light is on the short wavelength side of the absorption edge it will be absorbed in a distance short as c o m p a r e d with the c r y s t a l thickness. It is thus evident that the light c r e a t e s holes at the anode. It will obviously c r e a t e electrons at the cathode when that electrode is illuminated but, for the injecting contact postulated, there will already be a high density of electrons in the conduction band at the cathode and the c u r r e n t is limited by the diffusion field 10), not by the lack of available c a r r i e r s at the electrode. Why is the c u r r e n t strongly t e m p e r a t u r e dependent? This r e s u l t can only be explained by supposing that the electron and hole c u r r e n t s interact 199
Volume 4, number 3
PHYSICS
.--<
1 A p r i l 1963
ALUMINtUM CATHODE , GOLD ANODE.
6, ~
00
LETTERS
G
O
L
D
CATHODE
AND A N O D E .
O -- 5, --X--)C-- NICKEL C A T H O D E , G O L D . . ~ A N O D E , X C ~ Y S T A L T H I C K N E S S O'O10 CM. cD CL
<(4, }-Z ELI 3 n" n-
14 S~C.
O
U O2 O T ~L
o
~ O
4
8
[2
APPLIED
16
20
2.4
POTENTIAL
28
32
36
40
(VOLTS).
Fig. 1. The influence of the cathode m a t e r i a l on the photocurrent on illuminating the anode in barium titanate single c r y s t a l s for two different t e m p e r a t u r e s . Work functions of c a thode m a t e r i a l s (preffered values): nickel: 5.01 eV; gold: 4.82 eV; aluminium 4.08 eV. b y v i r t u e of t h e i r a s s o c i a t e d c h a r g e s . T h e d a r k ( e l e c t r o n ) c u r r e n t i s known to b e s t r o n g l y t e m p e r a t u r e d e p e n d e n t 8) a n d w i l l t h u s i n f l u e n c e t h e h o l e current. To t e s t t h i s h y p o t h e s i s we c a r r i e d out the f o l l o w i n g e x p e r i m e n t . A c r y s t a l h a d a l a r g e thin g o l d a n o d e e v a p o r a t e d on o n e s i d e . On t h e o t h e r s i d e two s m a l l s i m i l a r c a t h o d e s w e r e e v a p o r a t e d , one b e i n g of g o l d a n d t h e o t h e r of a l u m i n i u m . P h o t o c o n d u c t i v i t y w a s now o b s e r v e d u n d e r o t h e r w i s e i d e n t i c a l c o n d i t i o n s ()~ = 3725 ,~) f i r s t with t h e g o l d c a t h o d e a n d t h e n w i t h t h e a l u m i n i u m c a t h o d e . In b o t h c a s e s i t w a s t h e anode w h i c h w a s i l l u m i n a t e d . The dark current was subtracted from the total curr e n t and the r e s u l t p l o t t e d a g a i n s t f i e l d . A s w i l l b e s e e n f r o m the f i g u r e t h e r e i s a s u b s t a n t i a l d i f f e r e n c e b e t w e e n t h e r e s u l t s f o r the two d i f f e r e n t cathodes. It i s e v i d e n t t h a t t h e l i g h t c a n n o t i n f l u e n c e the c a t h o d e d i r e c t l y . T h e s e r e s u l t s c a n t h u s o n l y be e x p l a i n e d on the b a s i s of d o u b l e i n j e c t i o n . T h e e x p e r i m e n t w a s r e p e a t e d u s i n g two o t h e r s i m *****
200
i l a r c r y s t a l s and the r e s u l t s a g r e e d to w i t h i n a few percent. Similar results were also obtained with s h o r t e r w a v e l e n g t h s . On one c r y s t a l r e s u l t s w e r e o b t a i n e d u s i n g a n i c k e l c a t h o d e in a d d i t i o n to g o l d and a l u m i n i u m c a t h o d e s . I t i s h o p e d to r e p o r t on the p h o t o c o n d u c t i v i t y of B a T i O 3 and S r T i O 3 m o r e f u l l y e l s e w h e r e .
References 1) R.W. Smith, Phys. Rev. 100 (1955) 760. 2) R.W. Smith, Phys. Rev. 105 (1957) 900. 3) R. H. P a r m e n t e r and W.Ruppel, J Appl. Phys. 30 (1959) 1548. 4) M.A. Lampert, R.C.A.Rev. 20 (1959) 682. 5) M.A.Lampert and A.Rose, Phys. Rev. 121 (1961) 26. 6) M.A. Lampert, Phys. Rev. 125 (1962) 126. 7) N.Holonyaket al., Phys. Rev. Letters 8 (1962) 426. 8) A. Branwood and R.H.Tredgold Proe. Phys. Soe. 76 (1960) 93. 9) A. Branwood et a l . , Proe. Phys. Soc. 79 (1962) 1161. 10) G. G. Roberts and R. H. Tredgold, Physics Letters 2 (1962) 6. 11) J . P . R e m e i k a , J. Am. Chem. Soc. 76 (1954) 940.