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Thermally stimulated conductivity in sulphur single crystals excited by X-rays
Volume 34A, number 2
PHYSICS LETTERS References
s t r a h l u n g p r o c e s s [5]. This t h r e s h o l d may i n c r e a s e with i n c r e a s i n g optical frequency a c c o r ding to the theory which s e e m s to be c o n s i s t e n t with the e x p e r i m e n t a l r e s u l t s . It should be also noted that the dc i m p u l s e breakdown field of KC1 (0.4MV/cm) i s quite n e a r to the optical breakdown field when the i n i tial e l e c t r o n supply is a s s u r e d .
THERMALLY SINGLE
STIMULATED CRYSTALS
8 February 1971
[1] H. Dupont, A. Donzel and J. Ernest, Appl. Phys. Letters 11 (1967) 271. [2] D. Olness, J. Appl. Phys. 39 (1968) 6. [3] Y. Yasojima, M. Takeda and Y. Inuishi, Japan. J. Appl. Phys. 7 (1968) 552. [4] Y.Yasojima and Y. Inuishi, Digest of Technical Paper (6th 1970 IQEC) 6-9. [5] A. Wasserman, Appl. Phys. Letters 10 (1967) 132.
CONDUCTIVITY EXCITED
IN SULPHUR
BY X - R A Y S * .
A. FERREIRA C~)ME Z ** Departarnento de Ffsica, gscola de Engenharia de Sao Carlos (Est. S . P . ), Brazil. and F. S ~ N C H E Z S I N E N C I O Escuela Superior de Ffsica y Maternt~ticas, I . P . N . , M~xieo, 14 D . F . , Mexico and Departamento de Ffsica, Escola de Engenharia de Sao Carlos (Est. S . P . ), Brazil. Received 3 December 1970
Thermally stimulated conductivity (TSC) data for sulphur single crystals (pure and doped with selenium) excited by X-rays, indicates that (a) trap filling is a bulk process, (b) traps are net created by radiation with the doses used and, (c) the observed traps are not related to selenium impurities.
It was r e p o r t e d in a p r e v i o u s paper [1] that t r a p filling, d u r i n g t h e r m a l l y s t i m u l a t e d conductivity (TSC) in s u l p h u r s i n g l e c r y s t a l s , p r e v i o u s ly photoexcited with uv r a d i a t i o n , was l i m i t e d to a v e r y n a r r o w r e g i o n of the c r y s t a l close to the i l l u m i n a t e d electrode. In this work we have used X - r a y s i n s t e a d of uv r a d i a t i o n . The filling of t r a p s in this case was in the bulk. Our r e s u l t s show no r a d i a t i o n damage in the c r y s t a l s with the doses used. Solution grown c r y s t a l s , p u r e and doped with s e l e n i u m , w e r e studied by the TSC method. Both type of c r y s t a l s have s i m i l a r TSC s p e c t r a . X - r a y s u s e d were f r o m a molybdenum t a r g e t tube operated at 45 kV and 25 mA. P u r e and Sedoped c r y s t a l s w e r e f r o m CS 2 solution. L a r g e single c r y s t a l s were obtained and cut into plates 0.5 m m thick or l e s s and c r o s s - s e c t i o n of about * Work supported by National Research Council of Brazil, Foundation for Research of the State of Sao Paulo, Bank of National Development (BNDE) and partial support of ONR. ** Permanent Address: Instituto Tecnol6gico de Aeronautics (ITA), S. Josb dos Campos, S.P., Brazil. 130
r
x 1 0 -~2
-g D U
3,~0 3~,0 260
2~0
260 r~o ~,b ~40 80
Temperature ('K) Fig. i. Thermally stimulated conductivity (TSC) of pure sulphur single crystal excited by X-rays.
0.25 cm2. T h e s e were polished with fine a b r a s ive and etched lightly with a dilute solution of CS 2 in alchohol. E l e c t r o d e s were of vacuum d e posited gold, cathode s p u t t e r e d p l a t i n u m or alcohol-dag painted in a guard r i n g configuration. F o r the TSC m e a s u r e m e n t the c r y s t a l s were heated in a n i t r o g e n a t m o s p h e r e at a r a t e of 0.3OK/sec.
Volume 34A, number 2
PHYSICS LETTERS
.j
10.0 " x l ( ~ s
.o 7.5 E
_00
~ 5.(3
g
u 2.~
o
~b
15o
,~o
266
39o
x,O~
rads
Fig. 2. Growth curve of the total charge measured by the TSC method. A c h a r a c t e r i s t i c TSC s p e c t r u m i s shown in fig. 1. The d o s e a b s o r b e d by the c r y s t a l in t h i s c a s e was 105 r a d s . The m a i n d i f f e r e n c e s f r o m the uv r e s u l t s a r e , (a) the f o r m of t h e TSC s p e c t r u m i s independent of the p o l a r i t y of the a p p l i e d e l e c t r i c f i e l d d u r ing i r r a d i a t i o n (EL) o r d u r i n g the TSC m e a s u r e m e n t (F~I), (b) the a r e a s under the TSC p e a k s a r e p r o p o r t i o n a l to the s a m p l e v o l u m e s , (c) e x p e r i m e n t s with d i f f e r e n t k i n d s of e l e c t r o d e s show that the TSC s p e c t r u m i s independent of the e l e c t r o d e u s e d . T h e s e r e s u l t s i n d i c a t e bulk f i l ling of t r a p s . The t r a p s c r e a t e d by r a d i a t i o n a r e not s i g n i f i cant in n u m b e r by d o s e s up to 1Mrad, c o m p a r e d with the n u m b e r found in p u r e c r y s t a l s . T h i s i s
8 February 1971
shown in fig. 9, by the growth c u r v e of the t o t a l c h a r g e a s c a l c u l a t e d f r o m the a r e a s of the TSC p e a k s . The c h a r g e s a t u r a t e s with a p p r o x i m a t e l y 105 r a d s . I n c r e a s i n g the d o s e by a f a c t o r 3 does not a p p r e c i a b l y change the t o t a l c h a r g e . T h i s i s u n d e r s t o o d a s the f a i l u r e of the r a d i a t i o n to c r e a t e new t r a p s . P e a k s o t h e r than those i d e n t i f i c a t e d with uv excitation a r e not o b s e r v e d . The t o t a l d e n s i t y of t r a p s found was 1014 c m - 3 in a g r e e m e n t with our p r e v i o u s r e s u l t s [1], u s i n g the p e n e t r a t i o n v o l u m e f o r uv light. The s h a p e and magnitude of the TSC r e s u l t s f o r S e - d o p e d suphur c r y s t a l s was s i m i l a r to t h o s e for p u r e c r y s t a l s independent of the amount of s e l e n i u m in the c r y s t a l s . D i s p l a c e m e n t of the a b s o r p t i o n edge to l o n g e r k, up to 300 ~ , i n d i c a t e d the p r e s e n c e of Se i m p u r i t i e s . It a p p e a r s then that the o b s e r v e d t r a p s a r e not r e l a t e d to s e l e n i u m , one of the m o s t c o m m o n i m p u r i t i e s in sulphur. The a u t h o r s a r e indebted to D r . R. W i l l i a m s f o r e n c o u r a g e m e n t and v a l u a b l e a d v i c e . A. F. G. e x p r e s s e s his a p p r e c i a t i o n to ITA for i t s s u p p o r t . F . S . S . e x p r e s s e s his g r a t i t u d e to the I . P . N . , and Latin A m e r i c a n C e n t e r of P h y s i c s for their support.
RefeT"ences [1] F. S~ncbez Slnenclo, S. Mascaxenhas and B. S. H. Royce, :Phys. Letters 26A (1967) 70. $$~$s#
131
PHYSICS LETTERS References
s t r a h l u n g p r o c e s s [5]. This t h r e s h o l d may i n c r e a s e with i n c r e a s i n g optical frequency a c c o r ding to the theory which s e e m s to be c o n s i s t e n t with the e x p e r i m e n t a l r e s u l t s . It should be also noted that the dc i m p u l s e breakdown field of KC1 (0.4MV/cm) i s quite n e a r to the optical breakdown field when the i n i tial e l e c t r o n supply is a s s u r e d .
THERMALLY SINGLE
STIMULATED CRYSTALS
8 February 1971
[1] H. Dupont, A. Donzel and J. Ernest, Appl. Phys. Letters 11 (1967) 271. [2] D. Olness, J. Appl. Phys. 39 (1968) 6. [3] Y. Yasojima, M. Takeda and Y. Inuishi, Japan. J. Appl. Phys. 7 (1968) 552. [4] Y.Yasojima and Y. Inuishi, Digest of Technical Paper (6th 1970 IQEC) 6-9. [5] A. Wasserman, Appl. Phys. Letters 10 (1967) 132.
CONDUCTIVITY EXCITED
IN SULPHUR
BY X - R A Y S * .
A. FERREIRA C~)ME Z ** Departarnento de Ffsica, gscola de Engenharia de Sao Carlos (Est. S . P . ), Brazil. and F. S ~ N C H E Z S I N E N C I O Escuela Superior de Ffsica y Maternt~ticas, I . P . N . , M~xieo, 14 D . F . , Mexico and Departamento de Ffsica, Escola de Engenharia de Sao Carlos (Est. S . P . ), Brazil. Received 3 December 1970
Thermally stimulated conductivity (TSC) data for sulphur single crystals (pure and doped with selenium) excited by X-rays, indicates that (a) trap filling is a bulk process, (b) traps are net created by radiation with the doses used and, (c) the observed traps are not related to selenium impurities.
It was r e p o r t e d in a p r e v i o u s paper [1] that t r a p filling, d u r i n g t h e r m a l l y s t i m u l a t e d conductivity (TSC) in s u l p h u r s i n g l e c r y s t a l s , p r e v i o u s ly photoexcited with uv r a d i a t i o n , was l i m i t e d to a v e r y n a r r o w r e g i o n of the c r y s t a l close to the i l l u m i n a t e d electrode. In this work we have used X - r a y s i n s t e a d of uv r a d i a t i o n . The filling of t r a p s in this case was in the bulk. Our r e s u l t s show no r a d i a t i o n damage in the c r y s t a l s with the doses used. Solution grown c r y s t a l s , p u r e and doped with s e l e n i u m , w e r e studied by the TSC method. Both type of c r y s t a l s have s i m i l a r TSC s p e c t r a . X - r a y s u s e d were f r o m a molybdenum t a r g e t tube operated at 45 kV and 25 mA. P u r e and Sedoped c r y s t a l s w e r e f r o m CS 2 solution. L a r g e single c r y s t a l s were obtained and cut into plates 0.5 m m thick or l e s s and c r o s s - s e c t i o n of about * Work supported by National Research Council of Brazil, Foundation for Research of the State of Sao Paulo, Bank of National Development (BNDE) and partial support of ONR. ** Permanent Address: Instituto Tecnol6gico de Aeronautics (ITA), S. Josb dos Campos, S.P., Brazil. 130
r
x 1 0 -~2
-g D U
3,~0 3~,0 260
2~0
260 r~o ~,b ~40 80
Temperature ('K) Fig. i. Thermally stimulated conductivity (TSC) of pure sulphur single crystal excited by X-rays.
0.25 cm2. T h e s e were polished with fine a b r a s ive and etched lightly with a dilute solution of CS 2 in alchohol. E l e c t r o d e s were of vacuum d e posited gold, cathode s p u t t e r e d p l a t i n u m or alcohol-dag painted in a guard r i n g configuration. F o r the TSC m e a s u r e m e n t the c r y s t a l s were heated in a n i t r o g e n a t m o s p h e r e at a r a t e of 0.3OK/sec.
Volume 34A, number 2
PHYSICS LETTERS
.j
10.0 " x l ( ~ s
.o 7.5 E
_00
~ 5.(3
g
u 2.~
o
~b
15o
,~o
266
39o
x,O~
rads
Fig. 2. Growth curve of the total charge measured by the TSC method. A c h a r a c t e r i s t i c TSC s p e c t r u m i s shown in fig. 1. The d o s e a b s o r b e d by the c r y s t a l in t h i s c a s e was 105 r a d s . The m a i n d i f f e r e n c e s f r o m the uv r e s u l t s a r e , (a) the f o r m of t h e TSC s p e c t r u m i s independent of the p o l a r i t y of the a p p l i e d e l e c t r i c f i e l d d u r ing i r r a d i a t i o n (EL) o r d u r i n g the TSC m e a s u r e m e n t (F~I), (b) the a r e a s under the TSC p e a k s a r e p r o p o r t i o n a l to the s a m p l e v o l u m e s , (c) e x p e r i m e n t s with d i f f e r e n t k i n d s of e l e c t r o d e s show that the TSC s p e c t r u m i s independent of the e l e c t r o d e u s e d . T h e s e r e s u l t s i n d i c a t e bulk f i l ling of t r a p s . The t r a p s c r e a t e d by r a d i a t i o n a r e not s i g n i f i cant in n u m b e r by d o s e s up to 1Mrad, c o m p a r e d with the n u m b e r found in p u r e c r y s t a l s . T h i s i s
8 February 1971
shown in fig. 9, by the growth c u r v e of the t o t a l c h a r g e a s c a l c u l a t e d f r o m the a r e a s of the TSC p e a k s . The c h a r g e s a t u r a t e s with a p p r o x i m a t e l y 105 r a d s . I n c r e a s i n g the d o s e by a f a c t o r 3 does not a p p r e c i a b l y change the t o t a l c h a r g e . T h i s i s u n d e r s t o o d a s the f a i l u r e of the r a d i a t i o n to c r e a t e new t r a p s . P e a k s o t h e r than those i d e n t i f i c a t e d with uv excitation a r e not o b s e r v e d . The t o t a l d e n s i t y of t r a p s found was 1014 c m - 3 in a g r e e m e n t with our p r e v i o u s r e s u l t s [1], u s i n g the p e n e t r a t i o n v o l u m e f o r uv light. The s h a p e and magnitude of the TSC r e s u l t s f o r S e - d o p e d suphur c r y s t a l s was s i m i l a r to t h o s e for p u r e c r y s t a l s independent of the amount of s e l e n i u m in the c r y s t a l s . D i s p l a c e m e n t of the a b s o r p t i o n edge to l o n g e r k, up to 300 ~ , i n d i c a t e d the p r e s e n c e of Se i m p u r i t i e s . It a p p e a r s then that the o b s e r v e d t r a p s a r e not r e l a t e d to s e l e n i u m , one of the m o s t c o m m o n i m p u r i t i e s in sulphur. The a u t h o r s a r e indebted to D r . R. W i l l i a m s f o r e n c o u r a g e m e n t and v a l u a b l e a d v i c e . A. F. G. e x p r e s s e s his a p p r e c i a t i o n to ITA for i t s s u p p o r t . F . S . S . e x p r e s s e s his g r a t i t u d e to the I . P . N . , and Latin A m e r i c a n C e n t e r of P h y s i c s for their support.
RefeT"ences [1] F. S~ncbez Slnenclo, S. Mascaxenhas and B. S. H. Royce, :Phys. Letters 26A (1967) 70. $$~$s#
131