Impurities and trapping states in calcium tungstate

Volume 24A, number 5

PHYSICS LETTERS

IMPURITIES

AND T R A P P I N G

27 February 1967

S T A T E S IN C A L C I U M T U N G S T A T E

M. SAYER and A. SOUDER

Department of Physics, Queen's University, Kingston, Ontario, Canada Received 23 January 1967

The influence of impurities on thermoluminescence and thermally stimulated current in calcium tungstate is shownto be large. No definite correlation is observed between the two phenomenaand it is suggested that thay arise from electron and hole traps respectively.

Cook [1] and B r a u n l i c h et al. [2] have r e p o r t e d i n v e s t i g a t i o n s of photoconductivity, t h e r m o l u m i n e s c e n c e and t h e r m a l l y s t i m u l a t e d c u r r e n t in c a l cium tungstate, which d i f f e r in the s p e c t r a l r e sponse and the t e m p e r a t u r e s of the t h e r m o l u m i n e s c e n c e m a x i m a o b s e r v e d . The influence of i m p u r i t i e s on t h es e r e s u l t s may have been l a r g e , p a r t i c u l a r l y s i n c e s p e c t r o s c o p i c a n a l y s i s of c o m m e r c i a l l y g ro wn c r y s t a l s in our l a b o r a t o r y i n dic a t e d that i m p u r i t y c o n c e n t r a t i o n s in the r a n g e 0.001 to 0.02 at.% a r e c o m m o n . R e s u l t s of t h e r m o l u m i c e s c e n c e (TL) and t h e r m a l l y s t i m u l a t e d c u r r e n t (TSC) m e a s u r e m e n t s on s e v e r a l n o m i n al l y p u r e single c r y s t a l s a r e shown in table 1. T h r e e b as i c t h e r m o l u m i n e s c e n c e p e a k s w e r e o b s e r v e d with s o m e e v i d e n c e of s u b s i d i a r y p e a k s . The heating r a t e was 8°K/rain and the c r y s t a l s w e r e e x c i t e d at 85°K e i t h e r by 7 r a y s f r o m an 22Na s o u r c e or by u l t r a v i o l e t e x c i t a t i o n . The t e m p e r a t u r e s and r e l a t i v e a m p l i t u d e s of the peaks differed between crystals over a range g r e a t e r than e x p e r i m e n t a l e r r o r . R e p r e s e n t a t i v e a c t i v a tion e n e r g i e s c a l c u l a t e d f r o m the i n i t i a l r i s e of the peaks a r e shown, although t r e a t m e n t in t e r m s of a s i n g l e a c t i v a t i o n e n e r g y was an o v e r s i m p l i fic a t i on . T h e r m a l l y s t i m u l a t e d c u r r e n t was obs e r v e d as a single peak at 220°K and the c o r r e l a -

THERMALLY

'HERMOLUMINESGENGE NTENSITY ARBITRARY U N I T S ) - -

STIMULATED C/JRRENT

(AMPERES ItlO-li) . . . . . . [';

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Table 1 Results for unactivated crystals Peak temperatures ~- 2°K

120

160

200

240

280

320

360

Temperature OK

Crystal TSC

34 41 51

154 165 154

200 218 239

300 300 291/325

220 220 220

q5 (eV)± 0.03 eV

0.32

0.44

0.62

0.5±0.2

246

i° 2O

0

TL

6O

Fig. 1. Thermoluminescence and thermally stimulated current for single crystals of calcium tungstate doped with 1 at.% AgAG, 0.4 at.% In and 0.4 at.% Na respectively. Heating rate 8OK/min. 2537 .~ ultraviolet excitation.

Volume24-A, number 5

PHYSICS LETTERS

tion be t ween c u r r e n t and l u m i n e s c e n c e m a x i m a r e p o r t e d by B r a u n l i c h [2] s e e m e d to be a s p e c i a l c a s e r a t h e r than a g e n e r a l r e l a t i o n s h i p . The u l t r a - v i o l e t s p e c t r a l r e s p o n s e was r e s t r i c t e d to w a v e l e n g t h s < 3000 ~ . C r y s t a l s a c t i v a t e d with 0.4 at.% Na, 0.4 at.% In and 1.0 at.% AgA1 w e r e e x a m i n e d to a s c e r t a i n w h e t h e r the above v a r i a t i o n s o r i g i n a t e d in i m p u r i t i e s . The r e s u l t s a r e shown in fig. 1. The i n t e n sity of t h e r m o l u m i n e s c e n c e a f t e r 2537 /~ u. v. e x c i t a t i o n was l a r g e r (100x) than that f r o m the u n a c t i v a t e d c r y s t a l s and was independent of any p r e v i o u s h i s t o r y of r a d i a t i o n d a m a g e [3]. A l a r g e t h e r m a l l y s t i m u l a t e d c u r r e n t peak o c c u r r e d at about 220°K s i m i l a r to that o b s e r v e d f o r the una c t i v a t e d c r y s t a l s . The r e s u l t s f o r the sodium doped c r y s t a l s duplicated the work of Cook [1] a l m o s t e x a c t l y . The w a v e l e n g t h r e s p o n s e f o r photoconductivity extended to 4800 A and that f o r t r a p p i n g e f f e c t s to 4000/~. An i n c r e a s e in the d a r k c u r r e n t with t e m p e r a t u r e o b s e r v e d by Cook and a t t r i b u t e d by him to a s e m i - c o n d u c t i n g t r a n s i t i o n a c r o s s a band gap of 2.1 eV was o b s e r v e d with the s a m e a c t i v a t i o n e n e r g y . The c o n s i d e r a b l e influence of the i m p u r i t i e s on the trapping b e h a v i o u r s u p p o r t s the contention that the l e v e l s o r i g i n a t e at i m p u r i t y c e n t r e s e x t e r n a l to the WO~- l u m i n e s c e n c e c e n t r e [4]. Howe v e r , the lack of g e n e r a l c o r r e l a t i o n b e tw e e n t h e r m o l u m i n e s c e n c e and t h e r m a l l y s t i m u l a t e d c u r r e n t s u g g e s t s that a s i n g l e p r o c e s s does not g i v e r i s e to both p h e n o m e n a . A s s u m i n g that in the wide band gap m a t e r i a l f r e e c a r r i e r s o r i g i nate in t r a p p i n g s t a t e s , the s e p a r a t e b e h a v i o u r

27 February 1967

could a r i s e if t h e r m o l u m i n e s c e n c e a r i s e s f r o m e l e c t r o n c a p t u r e f r o m l u m i n e s c e n c e c e n t r e s and t h e r m a l l y s t i m u l a t e d c u r r e n t o r i g i n a t e s in hole t r a p s c h a r a c t e r i s t i c of the l a t t i c e . R e c e n t m e a s u r e m e n t s [5] which show that both e l e c t r o n and hole m o b i l i t i e s a r e low and that the mobile c a r r i e r s during photoconduction a r e h o l e s , support the p r e s e n t i d e a s . Application of the peak shape methods of H a l p e r i n and B r a n e r [6] in o r d e r to e s t i m a t e the type of k i n e t i c s involved in t h e r m o l u m i n e s c e n c e indicated k i n e t i c s of f i r s t o r d e r . Such k i n e t i c s a r i s e in a conduction band model if the p r o b a b i l i t y f o r cap t u r e of the r e l e a s e d c a r r i e r is high. In the c a s e of s o d i u m doping, it is unlikely that the c r y s t a l acts as an i n t r i n s i c s e m i co n d u ct o r of band gap 2,1 eV as s u g g e s t e d by Cook but that an i m p u r i t y influenced p r o c e s s with an a c t i v a t i o n e n e r g y of 1.05 eV is involved. This work was c a r r i e d out under a g r an t f r o m the National R e s e a r c h Council of Canada. Thanks a r e due to Mr. W. R. Hardy f o r a n u m b e r of p r e liminary measurements.

References 1. J.R,Cook, Proc. Phys.Soc. B68 (1955) 148. 2. P.Braunlich, K.Reiber and A.Scharmann, Z. Physik 183 (1965) 431. 3. D. Hahn and K. Lertes, Z. Physik 169 (1962) 331. 4. M.I.Tombak and A. M. Lurvich, Zh. Pricklad Spectrosk (USSR) 6 (1966) 564. 5. K.Reiber and A.Scharmann, Z. Physik 191 (1966) 480. 6. A.Halperin and A.A.Braner, Phys.Rev. 117 (1960) 408.

SUPERHEATING AND SUPERCOOLING IN T Y P E SUPERCONDUCTORS: TIN $ F. W . S M I T H and M. C A R D O N A

I

*

Physics Department, Brown University, Providence, Rhode Island Received 19 January 1967 The superheating and supercooling fields of unplated and copper-plated tin spheres are reported. The data suggest an increase in Hc3/Hc2 with decreasing temperature. We have m e a s u r e d the s u p e r h e a t i n g and s u p e r cooling f i e l d s [1] of unplated and c o p p e r - p l a t e d tin s p h e r e s f o r t e m p e r a t u r e s b e tw e e n 1.1OK and Tc. The s p h e r e s w e r e p r e p a r e d by u l t r a s o n i c d i s p e r s i o n of m o l t e n tin (99.999%) in oil [2]. They w e r e

c o p p e r - p l a t e d by i m m e r s i o n in a s a t u r a t e d s o l u tion of CuSO 4. The s p h e r e s w e r e p l a c e d inside a Supported by the Advanced Research Project Agency. * Alfred P.Sloan Foundation Fellow. 247