ELECTRON SPIN-LATTICE RELAXATION : A TOOL FOR THE INVESTIGATION OF PARAMAGNETIC CENTERS IN AMORPHOUS SEMI-CONDUCTORS

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ELECTRON SPIN-LATTICE RELAXATION : A TOOL FOR THE INVESTIGATION OF PARAMAGNETIC CENTERS IN AMORPHOUS SEMI-CONDUCTORS

A. Deville, B. Gaillard, C. Blanchard, J. Livage

To cite this version:

A. Deville, B. Gaillard, C. Blanchard, J. Livage. ELECTRON SPIN-LATTICE RELAX- ATION : A TOOL FOR THE INVESTIGATION OF PARAMAGNETIC CENTERS IN AMOR- PHOUS SEMI-CONDUCTORS. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-1001-C4-1004.

�10.1051/jphyscol:19814218�. �jpa-00220847�

page

ELECTRON SPIN-LATTICE RELAXATION

:

A TOOL FOR THE INVESTIGATION OF

PARAMAGNETIC CENTERS I N AMORPHOUS SEMI-CONDUCTORS A . Deville, B . Gaillard, C . Blanchard and J. ~ i v a ~ e *

Ddpartement d 'Electronique, E. R. A . 375, Universitl de Provence, Centre Saint-Jlr6me, 13397 Marseille Cedes 13, France

*

Spectrochimie du SoZide, L.A. 302, Universitl Pierre e t Marie i k r i e , 75000 Paris, France

A b s t r a c t

-

The s p i n - l a t t i c e (5.L. ) r e l a x a t i o n p r o p e r t i e s o f pure and W-doped V205 a r e examined. The S.L. r a t e s f o r pure c r y s t a l l i n e and amorphous oxi.de have been measured from 1.5 t o 50 and 100 K r e s p e c t i v e l y . The knowledge o f t h e S.L. r a t e s i s used t o c l a r i f y t h e spectrum o f W-doped V20S.

Semi-conductors o f t e n possess paramagnetic defects which can be detected by E l e c t r o n Spin Resonance (E.S.R.). I n an E.S.R. experiment, t h e energy absorbed by t h e spins from t h e radiofrequency f i e l d i s given up t o a thermostat ( l a t t i c e ) . I t i s p o s s i b l e t o g e t i n f o r m a t i o n s on t h e d e f e c t o r on t h e l a t t i c e by d i r e c t l y measuring t h e e l e c t r o n s p i n - l a t t i c e r e l a x a t i o n t i m e T1. Eloreover, when s e v e r a l centers a r e p r e s e n t , i t i s sometimes p o s s i b l e t o separate each c o n t r i b u t i o n i n t h e E.S.R. spectrum through t h e i r s p i n - l a t t i c e properties,We h e r e a f t e r p r e s e n t r e s u l t s obtained w i t h pure o r W doped V23,. Non s t o i c h i o n e t r j c !I2D5 i s 3 mixed valence semiconductor. I t s e l e c t r i c a l p r o p e r t i e s a r i s e from t h e hopping o f unpaired e l e c t r o n s betweenv4+.and V'.+

A s t r o n g e l e c t r o n phonon c o u p l i n g i s observed i n such oxides and t h e charge c a r r i e r s are a c t u a l l y small polarons. The semi -conducting p r o p e r t i e s t h e r e f o r e s t r o n g l y depend on t h e i n t e r a c t i o n s o f t h e unpaired e l e c t r o n s w i t h t h e l a t t i c e . fioreoverinthe amorphous oxide, t h e s t r u c t u r a l desorder leads t o Anderson l o c a l i z a t i o n Any i n f o r m a t i o n about e l e c t r o n - l a t t i c e o r e l e c t r o n s p i n - l a t t i c e i n t e r a c t i o n w i l l t h e r e f o r e be v e r y u s e f u l i n order t o g e t a b e t t e r understanding o f t h e semi-conduc- t i n g p r o p e r t i e s o f t r a n s i t i o n - m e t a l oxides.

I

-

Pure V205

A

-

O r i g i n o f t h e paramagnetic centers v5'(3d0) has no permanent magnetic moment. V205 heated i n t h e a i r above i t s m e l t i n g p o i n t (650' C ) loses oxygen.

Therefore t h e c r y s t a l l i n e oxide obtained by slow c o o l i n g from t h e m e l t and t h e amorphous oxide prepared by s p l a t c o o l i n g 111 a r e non s t o i c h i o ~ t r i c . T h i s leads t o t h e c r e a t i o n o f paramagnetic d e f e c t s associated w i t h oxygen vacancies : i n c r y s t a l l i n e V2O5, the E.S.R. spectrum c o n s i s t s o f 15 l i n e s o r i g i n a t i n g from an a d d i t i o n a l e l e c t r o n (S = 1/2) d e l o c a l i z e d over two 5 ' ~(I = 7/2), the e l e c t r o n s p i n being coupled t o both n u c l e a r moments. On t h e o t h e r hand, i n amorphous V2O5, t h e E.S.R. spectrum c o n s i s t s o f 8 l i n e s ; t h e a d d i t i o n a l e l e c t r o n i s l o c a l i z e d on a s i n g l e vanadium (v4':3d1), t h e e l e c t r o n s p i n being coupled t o t h e n u c l e a r moment o f t h e vanadium i o n .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814218

C4- 1002 JOURNAL DE PHYSIQUE

B

-

I n f l u e n c e o f conduction : V?05 i s a n-type semi-conductor, b u t i n the i n v e s t i g a t e d temperature range, t h e frequency o f hopping between centers i s much lower than t h e s p i n - l a t t i c e r e l a x a t i o n frequency. We may t h e r e f o r e i g n o r e conduc- t i o n i n o u r experiments and the e l e c t r o n must be considered bound t o a given para- magnetic center. The s t r o n g electron-phonon coupling, and t h e Anderson l o c a l i z a t i o n present i n t h e amorphous s t a t e , l e a d t o a low m o b i l i t y f o r t h e hopping e l e c t r o n s i n V205 and t h i s supports o u r p o i n t o f view. A t 100 K b o t h m a t e r i a l s have h i g h r e s i s t i v i t y indeed : p i s equal t o 2.107n.cm i n c r y s t a l l i n e V2O5, and t o 5.1011~.cm i n amorphous V2O5.

C

-

Experimental r e s u l t s . - We d i r e c t l y measured t h e s p i n - l a t t i c e r e l a x a t i o n time T1 by the s a t u r a t i n g p u l s e method : f o r a given sample, r e s u l t s a r e t h e same f o r any h y p e r f i n e s t r u c t u r e l i n e . I n t h e amorphous as w e l l as i n t h e c r y s t a l l i n e m a t e r i a l , t h e recovery s i g n a l c o n s i s t s o f two exponentials, whose time constants are i n a constant r a t i o o f some u n i t s . The l o n g e s t time-constant f o r t h e c r y s t a l - l i n e o x i d e ( f i g . 1) obeys t h e f o l l o w i n g expression i n t h e 1.4-50 K temperature range :

T,-I = 12.5 T

+

3.1 l o 3 / s h -f- l5 (TIS, TK) (1 )

F i g . l : Temperature v a r i a t i o n o f T,-'(T) f o r c r y s t a l l i n e V205. a p u l s e satu-

r a t i o n , o p r o g r e s s i v e s a t u r a t i o n . The s o l i d curve corresponds t o eq. ( l ) .

Fig.2 : Temperature v a r i a t i o n o f

-

T,-'(T) f o r amorphous V205. a p u l s e s a t u r a t i o n .

-

: Experimental v a r i a - t i o n .

----

: T h e o r e t i c a l v a r i a t i o n corresponding t o eq.(2).

-

¹

D

- Discussion.- In c r y s t a l l i n e V205 , t h e usual d i r e c t process (T1 T) i s observed when T

< 4

K. The l i n e a r temperature-dependence observed when

T >

20

K

indicates t h a t when T

>

5 K, the dominant process characterized by an energy

Er =

15.5 K , i s also a one phonon process. We i n t e r p r e t t h i s dependence with the help of Murphy model 121

:

the spin i s coupled t o the phonons via a system with two energy l e v e l s

QA

,

QB

separated by

E r ;

one must then consider the s t a t e s

I f ^{1/2 ,} The e f f i c i e n t relaxation t r a n s i t i o n s a r e

:

lf1/2,QA>

<-->

l j 1 / 2 , @ 2 .

The energy of t h e e f f i c i e n t phonons f o r t h i s process i s about Er. The origin of the two energy l e v e l s % ,

@B

i s s t i l l uncertain

;

t h e delocalization of the elec- tron over t h e two nuclei suggests, by analogy with H ~ ' , t h e existence of a bonding and an antibonding s t a t e , but the corresponding energy difference seems too impor- t a n t compared t o 15 K.

A

more l i k e l y explanation i s t o consider t h a t t h e electron i s submitted t o a symmetrical double well potential

131, the tunnel s p l i t t i n g

being much l e s s than t h e quantum of vibrational energy. When

kT < f l R

we have t o consider only the vibrational ground s t a t e , s p l i t by tunnel-effect. This agrees with recent simulation of optical spectra f o r vanadium dimers

141.

In amorphous

-

¹

V205 , T1 obeys a quadratic law f o r T

<

10 K

;

a t higher temperatures, we observe a tendency towards a l i n e a r temperature v a r i a t i o n , which indicates a one-phonon process. We discard the p o s s i b i l i t y of a phonon-bottleneck (low concentration

:

1 0 - ~ centers/v5'

;

high temperature). We can i n t e r p r e t t h i s behaviour supposing t h a t the spins a r e coupled t o the phonons via Two-Level-Systems (T.L.S.). The existence of T.L.S. seems t o be q u i t e general i n amorphous systems, although t h e i r nature i s s t i l l unknown. The T.L.S. phonons coupling i s known t o be s t r o n g ; the spin-T.L.S. coupling i s l e s s known

;

our r e s u l t s suggest i t i s e f f i c i e n t i n

V2O5;

i t consists of a g o r

A

modulation

;

according t o a model proposed by S.R. Kurtz .and H.J. Stapleton (51 , we consider t h a t a spin f l i p needs one phonon (c) and one T.L.S. ( % f l u ) . We determine the T ~ - ' ( T ) law supposing t h a t the T.L.S. have a constant density of s t a t e s between 0 and Emax

;

choosing

Emax =

56

K,

we get expression ( 2 ) .

I1 - W-doped

V2O5.

Hexavalent ions such a s w6' can be substituted f o r t h e vanadium ions.

A

charge compensation occurs and v4+ ions a r e associated t o the w6+ impurity.

A t low temperature one observes an E.S.R. spectrum consisting of

8

l i n e s and

corresponding t o an unpaired electron which i n t e r a c t s with the moment of a

51V

nucleus ( f i g . 3a). Pulse experiments show t h a t t h i s defect i s f a r l e s s coupled t o

the l a t t i c e than t h a t observed i n pure V205 ( i n t r i n s i c defect)

:

a t 6 K ,

C4- 1004 JOURNAL DE PHYSIQUE

we e x p e r i m e n t a l l y f i n d T, = 15 ms ( W d e f e c t ) and TI = 2 ms ( i n t r i n s i c d e f e c t ) 161.

Fig. 3 : E.S.R. spectrum o f c r y s t a l -

v

^{: E.S.R.} spectrum o f c r y s t a l l i n e l i n e powder o f W-doped V205. pow e r o f pure V,05. H, = 7.3 10-GT.

a) H, = 7.3 IO-~T, T - 6 K .

b)

H,

= 7.3 10-6T ; T = 6 K.

It may then be p o s s i b l e t o determine whether i n t r i n s i c d e f e c t s are s t i l l present i n W-doped V,O,usinga r - f f i e l d a b l e t o s a t u r a t e t h e W d e f e c t w i t h o u t s a t u r a t i n g the i n t r i n s i c one ; th e spectra recorded a t 6 K f o r t h e HI values i n d i c a t e d i n f i g . 5 show t h a t besides t h e W d e f e c t , t h e r e e x i s t s a second c e n t e r ( f i g . 3b) which has t h e same E.S.R. spectrum as t h e i n t r i n s i c d e f e c t ( f i g . 4 ) . Both d e f e c t s have t h e same concentration. I n t r i n s i c d e f e c t s may t h e r e f o r e c o n t r i b u t e t o t h e e l e c t r i c a l p r o p e r t i e s o f W-doped V205.

References

111 !l. Flichaud, P. Pineau, J . Livage, R. Collongues. Rev. Phys. Appl.,

12

(1977), 715.

121 J . Hurphy. Phys. Rev.,

145

(1966), 241.

13) R.J. Rollefson. Phys. Rev. B,

5

(1972, 3235.

14) F. Babonneau, C. Sanchez ( P r i v a t e communication).

( 5 ) S.R. K u r t z , H.J. Stapleton. Phys. Rev. B,

22

(1980), 2195.

161 B. G a i l l a r d , C. Blanchard, A . D e v i l l e , J. Livage. Phys. S t a t . Sol.,

104,

(1981), 627.