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Ferroelectric-like dielectric anomaly in RF-sputtered amorphous LiNbO3 films
Mat. R e s . B u l l . , Vol. 20, p p . 315-320, 1985. P r i n t e d in t h e USA. 0025-5408/85 $3.00 + .00 C o p y r i g h t ( c ) 1985 P e r g a m o n P r e s s L t d .
FERROELECTRIC-LIKE DIELECTRIC ANORALY IN RF-SPUTTERED ARORPHOUS ,1#
LiNbO 3 FILR5
K.BoR. Vatma, K.So Harshavardhan~ K.30 Rso and CoN.R, Rao + R a t e r l o l s Roses:oh L a b o r e t o r y I n d i a n I r m t / t o t o o f Sctence Bangalo:e-560012, I n d l s .
(Received January
8, 1985; C o m m u n i c a t e d b y C . N . R .
Rao)
ABSTRACT Amorphous f i l m s o f LINbO 3 prepared by RF s p u t t e r i n g e x h i b i t r e r r o e l e c t ~ l o - l l k s d £ e l e c t r £ o anomaly J u s t b a l e r the glees t ~ a n s t t l o n t e a p e r a t u r e . [1oct--on m i c r o s = e p i c s t u d i e s 8ho~ the presence o f eaalZ : ¢ y s t a l l l n e c l u s t e r s I n the amorphous fllm. The d l e l e c ~ t c anomaly h o o v e r , seems t o o r i g i n a t e from the t i s s u e nater£81 t e t h e r than from the c l u s t e r s .
I NTROOUCTION
Several p r o p e r t i e s o f eol£ds are supposed to c : l t ~ c a l l y deperx~ on the e x i s t e n c e o f l o n g - r a n g e o~dert f e r r o e l e c t ¢ l o beheviour o f l a t e r i 8 1 8 be£n9 one ouch. I t tee t h e r e f o r e outp¢£eing vben glassy r i b b o n s o f LINbO 3 ~ z a reported ~ r possess f e r t o e l e o t ¢ £ ¢ prope¢ty by Glees a t 81 ( 1 ) . There are s i m i l a r r e p o r t s uhete both the RF-oputtered and the r a p i d l y quenched l a t e : i s l e have been found to e x h i b i t f e t t o o l s c t ~ l o behaviout J u s t sO f ~ e l t parent c~ystallins states, Lines (2) has presented a t g u l e n t e to suggest t h a t t~s £nc£dence or f s r = o e l e c t r l r ~ t y i 8 not SnGonststent - ~ t h t h l absence of' c:ys t ~ l l i n t t y .
* P u b l £ c e t ~ o n No° 23 from R a t e = I s l e Research Laborato¢y. +To ~hom co~espondence should be addressed.
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I n v i o v o r the p o s s i b l e a p p l i c a t i o n s o f r a = t o e l e c t t L c t h i n f i l m s i n i n t e g r a t e d o p t i c s ( 3 , 4 ) e l o c t t o o p f . t c modulators ( 5 - 7 ) harmonic gametet a t s ( 8 , 9 ) e t c . , • d e t a i l e d study o r t h e i r d i a l e c t J r i c behavlou¢ and s t r u c t u r e i s i m p o r t a n t . The fooJs of' i n v e s t i g a t i o n i n t h i s paper I s to f,ind out i f RF-eputteted f,ilms or LINbO 3 also e x h i b i t d i e l e c t r i c anomaly and to i n v e s t i g a t e the o r i g i n or the phenomenon which appears to be g e n e r a l l y 8ssoctatad u i t h amorphous s t a t e . I n t h i s cocmunicatlon, the ocouttenom o f d t e l e c t ¢ i c anomaly Ln R r - s p u t t a r e d r ~ l n s o r LLNbO3 i s repotted. The d i e l e c t r i c behaviouts o f RF-eputteced and melt-quenched rllmm have been competed. The u l t t a m £ c c o s t t u c t u r e of, the f,ilms have been i n v e s t i g a t e d ue£ng high r e s o l u t i o n t c a n e ~ s s l o n e l e c t r o n m/ctoecopy (HRT[R). A p l a u s i b l e a n a l y s i s baaed on the c l u s t e r model ks g i v e n , h t c h sho~s t h a t the d i e l e c t r i c 8nomaZy can t e e u Z t q u i t s g e n e r a l l y i n i o n i c amorphous m a t e r i a l 8 due ta the unusual manner of, sol,teeing or v i b ¢ a t t o n a i lodes £n the r e g i o n of, glass t r a n s i t i o n ( 1 0 , 1 1 ) . CXP£RIRENTAL L i t h i u m nJ.obata r e q u i t e d re= RF s p u t t e r i n g was prepared by h e a t i n g a a t a l c ~ t o a e t t i c m i x t u r e of, reagent grade LI2CO 3 end Nb205 ~n a p l a t i n u m c r u c i b l e Par about 3 hours 8 t 8 temperature of, about 1170K. The p r o d u c t tins g z a d u a l l y cooled to 7?0 K and metntcLnad s t t h i s tempe¢atuwe f,ot ~ 8 h o u r s . The f o z ~ t l o n of, c r y s t a l l i n e LINbO 3 wee confirmed by X-ray dlf,f,tact/on. The l n t a t p l a n a t spacings (d) of' the sample compared ~ e l l w i t h the ~epotted l i t e r a t u r e (12) v a l u e s . LINbO 3 was RF-sputtered on to quactz s u b e t t a t a s held a t team temperat u r e u s i n g an RRC ( R a t e r i s l s Research C o r p o r a t i o n , USA) lode1 S[R-8822 provided u l t h 8 13.56 RHz RF g e n e : e t a t capable o f producing a maximum o u t p u t po~et l e v e l o f 1.5 kW. S p u t t e r i n g was performed i n 8n argon p l e a t s a t a conmtant pcsssute o f 4x10-3 t a r t . The 8 t c u c t u t e of, the 8puttaced f i l m s ~ete exa~Lned u s i n g a h i g h c e s o l u t l o n t ¢ s n s n t s s l o n s l e c t c o n a t c ¢ o s c o p e , model 3rOL, :ER 200 CX. The opetat:J.ng v o l t a g e of, the system uas 200 kV u t t h a p t l a a t y mgntflcatton of 3-5x105. A1unLtrttum e l e c t r o d e s wets d e p o s i t e d u s i n g masks p r i c e to s p u t t a t l n g LINb03 f,llms r e q u i t e d r o t d i a l a c t c i c c o n s t a n t measurements and s p l a n a r sample c o n f i g u r a t i o n use adopf~d r o t aeaeucementa. Capacitance ~ao Imasuted as • f , u n c t l o n of, both t l m p e t a t u t e (300-700 K) and frequency (1-100 kHz) ~ t t h 8 s i g n a l s t c e n g t h o f 8 V : a s u s i n g a t h : e e t e r m i n a l General Radio Capac£tance Bridge (USR) coupled e x t e r n a l l y ~ t h GR1316 o e c L l l a t o t and GR1232A tuned a a p l l f , i e r and n u l l d e t e c t e r . The atomy ¢apacLtar~:e tnt~oduced by loads e t c . ~ae Ittrttmtzed by r e d u c i n g the l e n g t h s of' the c o n n e c t i n g leads and by p r o v i d i n g adequate e l e c t r i c a l 8 h l e l d l n 9, D l e l e c t t a , c constants of' f i l m s tmra evaluated a f t e ¢ c o r r e c t i n g f,ot the r e s i d u a l 8tcay capacitance. RESULTS AND DISCUSSION The t e m p e r a t u r e v a r i a t i o n of' the d S a l s c t t t c c o n s t a n t of, an a s - s p u t t e r e d f,llm a t 1 kHz 18 shown An r i g . 10 The saaploo ~e¢o t e a p o t s bJra-cyr..led and the d i a l e c t : i t c o n s t a n t values wore f,~Jnd to be q u i t e r e p r o d u c i b l e upto about 700 K. D i e l e c t r i c constants of the t o i l e t - q u e n c h e d LINI:O3 glass r e p o r t e d by Glass s t 81 18 818o tnclucled I n f,ig. 1 f,ot comparison. I t may be noted t h a t the c l i e l e c ~ t c boheviout of' the s p u t t e r e d f,lZm I s q u a : L t t a ~ v e l y s i m i l a r to t h a t or the z ' o l l e r quenched
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g l a s s . The temperature o f t h e d l e l o c t r l c anomaly, howovez:, i s about 50 K l o ~ r . Unfortunately, i n our oxperlment~, ~ could not ~ 8 o u r e the d l e l e c t r i c constants afte= c r y s t a l l i z a t i o n and hence the second peak soon by Glass •
ot el,
Bulk glass (Gloss el ol~)
~s
not observed,
The
magnitude o f the anomaly l t o e l f i s ooneiderably highs= I n sputtered • Sputtered film ( Present wDrk ) f i l m s than in r o l l o t - q u e f l c h o d giassoo. The fEoquoncy dependence o f the d l o l o c t ~ i ¢ Gormt~nt8 o f LLNUO3 f i l m s l e shorn i n f i g . 2. The d i e l e c t r i c constant decreases r a p i d l y upto about 15 kHz and the 5 decrease i s l e s s r a p i d s t higher frequencies. The p:oeont study e o t a b l L o h e s t h a t sputtered amorphous f i l m s o f LIN~ 3 exhibit 8 dielectric 4 anomaly s i m i l a r to the t o l l s : I 9OO 500 700 300 quenched glassy f i l m s . Lines (2) TemD¢~otur¢(K) has considered the l e t t e r as a fettoolectrio-llko anomaly and FIG. 1 has attAmptod to e x p l a i n the anomaly assuming the e x i s t e n c e of Temperature v a r i a t i o n of the d i e l e o t r i c constant o f LLNbO3 f i l m 8 t I kHz. s m a l l , d i o l o c t r / c a l l y s o f t , ordered r e g i o n s . I n the ordered r e g i o n s , the d i e l e c t r i c bohaviou= i o considered to he s i m i l a r to t h a t in a a c r o s c o p i c a l l y c r y s t a l l i n e parent m a t e r i a l s .
~ o
_J
3 5
/-,0
80 Frequency
120
(F~HZ)
FIG. 2 The f:squency dependent8 of tho d i e l e c t r i o constant o f LiNUG3 f i l m a t 300K.
An eloct~on microg~aph of on a s - s p u t t e r e d f i l m along w i t h the corresponding diff~ac~Lon p a t t e : n to shown i n r i g . 3. Whereas the diffraction pattern clearly s u g ~ o f ~ t h a t the s p u t t e r e d f i l m s ate amorphous, the high r e s o l u t i o n mlc~o~aphs : e v e a l a mosaic o f oupetf/ne c ~ y s t a l l i t e a w i t h i n average dimension o f the order o f 15 ~. Proeonoo o f o~dored regions o f s i m i l a r tango has been t a p e , ted i n other glassy mater i a l s (10)o The sputtered f i l m s ware annealed a t v a r i o u s f~mpa: a t u r o s upto NO K f o r oevor81 hours. The mkcrograph and the d i f f C a c t i o n p a t t e r n o f the ??0 K-annealed f i l m ( f i g . 4) show evidence o f c ~ y s t a l l l z s t i o n . Mlcrographs and d i f f r a c t i o n
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p a t t e t n e o f samples annealed below 7?0 K vats e s s e n t i a l l y 8 i m i l a : to those i n r i g , 3. The tdt spacLng8 o f the c r y s t a l l i z e d Films vere obtained f:om TEM and they compared y e l l K i t h l t t e r a ~Jra values o
FIG. 3 E l e c t : o n mlc¢og:aph o f a s - s p u t t e r e d LINbO 3 f i l m togethe¢ Kith the d i f f ract.ton p a t t e r n ( I n s a t ) ,
riG. 4 Elect:con miccogcaph and the d i f f r a c t i o n p a t t e r n ( i n s e t ) o f the annealed LiNbO 3 f i l m .
Evonthough the p r e s e n t HR~EM s t u d i e s confi=m the presence o f a t c ¢ o c ¢ y e t a i Z i t e :egtorm Invoked by Lines 9 the d l a l e c t ¢ l g anomaly observed I n the amorphous m a t e : t a l e amy not ensue rr~n the u l t c a m l c : o c ¢ y a t a l l i t e s f o r the f o l l o K i n g :eaeone. ( l ) The observed anomaly i s close to the gleam t t a n e t t i o n tompatatu:a and solid 8t¢uctural entit~es like u l t c u i c ¢ o c ¢ y s t e l l i b e s are u n l i k e l y to be p r e s e n t a:ound Tg. ( i t ) There i s no evidence f o r a f e r r o e l e c t : L c h y s t e r e s i s loop i n amorphous materials ~ i o h exhl b i t the anomaly. ( £ 1 i ) A s i m i l a r inc¢oasa i n d i o l o c t c i o c o n s t a n t i s incident I n o t h e r i o n i c glasses (13) i n the nelghbouthood o f Tg. A c l o s e r examlrmt l o n o f this general phenomenon i n amorphous i o n i c m a t e r i a l s i s the:afo:o in order. Rt c ¢ o c t y s t e l l i t e regions observed I n HRTER o? the amorphous f i l m s ( F i g . 3) suggest t h a t ve could uae a r . l u s t e r model f o r s p u t t e r e d f i l m s as y e l l . I n accordance vcLth t h i s model, ~e consider the i n t e r c¢yatallite r e g i o n i n the LiNbO3 ?Lime as amotphoue t i s s u e . ~e then examine the o r i g i n o f the d i e l e c t r i c anomaly i n the l i g h t or c l u s t e r model as the o,laee t c a n a i t i o n temperabJte i s approached. Hoveve¢t va may note that the RF-sputbeted LINbO3 f i l m s e x h l h i t or~y a c:ya t a l l i z a t i o n tempeta~tuta clcse to Tg and L t amsks the much veakat glass t r a n s i t i o n evmnto
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Significant features result from the anhermonlcity of vibrations of particles, p a r t i c u l a r l y the a l k a l i ions (Li + "n the present case), i n the tissue region and on the surfaces of c r y s t a l l l t e s . I t has been sho~n earlier (10) that the v i ~ a t i o n a l e n e r g i e s are related by z l E ~ ~E 1 = 1 / n where ~E n and d [ 1 are the v i ~ a t i o n a l energy d i f f e r e n c e s i n the n th end I s t v i b r a t i o n a l l e v e l s . The corresponding v i b r a t i o n a l f r e q u e n c i e s are t h e r e f o r e r e l a t e d as oJn/~Jl = 1 / n . P a r t i c l e s v i b r a t i n g i n h i g h e r quantum l e v e l s , t h e r e f o r e , are ° a o f t e r t . Since as • f u n c t i o n o f tamperst u f a p a r t i c l e s p o p u l a t e higher l e v e l e r t h e i r e x c i t a t i o n s are governed by modes uhioh l a o f t e n l as temperature i s i n c r e a s e d . Using the Lyddans-SechaTeller relation (14), " identify .ith the t a o f t l mode so t h a t a S ~ j n = ~ J l / n = COTO decreases, ~ o i n c r e a s e s . Since the p a r t i c l e s are i n d i f f e r e n t v i b r a t i o n a l s t a t e s they ~ould make a ~eighted c o n t r i b u t i o n to the measured ~o" The t o t a l number o f p a r t i c l e s I s determined by the volume o~ the t i s s u e which inc r e a s es as TD I s approached. As the temperature i n c r e a s e s , the volume o f c l u s t e r decreases and ' l i q u i d l i k e I p r o p e r t i e s emerge i n the t i s s u e r e g i o n . Aa • consequence, the vibrations become wholly l o c a l i z e d and the phonon leedst description becomes inappropriate; the modes do not contribute to ~_. Hence, the u e f f e c t i v e number o f modes i s proportional to the volume of the cluster. An approximate expression for C-o may be wtlttan as,
rn C-°
=
constant
~
Vtiesue
x
Vcluata r
n
vhere f n i s the f r a c t i o n o f p a r t i c l e s i n n t h v i b ~ a t i o r m l l e v e l , determined by an appropriate p a r t i t i o n function. He,ever, i n this model, fn always Increases with temperature upto Tg.Vclustar le zero at Tg end Vtissue i s l o u a t OK. Hence, s maximum occurs i n the v a lu e o f ~ o, p r i o r to Tg accounting for the observed anomaly i n ~o"
The point ~e wish to make i s that the d i e l e c t r i c anomaly of amorphous materiels m~y originate from the tissue and not from the mic~ocrystalline cluster regions. This i s important because the preparation of amorphous solids involve quenching from considerably high temperatures. The nucleated clusters moat l l k s l y possess highly symmetrical nonferroe l e o t r l c structures. Fertoelsctrlc d l e t o r t l o n i n these structures provide negiiglbls energy gains as compared to the de~res of mataetablllty a l r e a d y p r e s e n t . However, since t i s s u e i s e common f e a t u r e o f s l l i o n i a glasses, it ~ould suggest that dielectric anomaly should mahifest in all ionic glasses (or sputtered films). Indeed as pointed out earlier, largo inczeesa in dielectric constants have bean noted in nitrate end sulphate glasses a t temperatures close to T ( 1 3 ) . g In conclusion, we suggest that the dielectric anomalies observed in glasses and sputtered films of LiNbO3-1ike materials is associated with the tissue. The large magnitude of dlolecttlc constants in LINGO 3materials is due to the large io~Io polorisability aeeoclated ~Lth certain ionic groups like (NbO~) ~'.
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REF'ERF'NCIES I.
A.M. Gleam, R.E. Lirms, K. h s s a n and 3.W. Shtevet, AppZ. Phys. L s t t . 31, 249 (1977).
2o
•.(o
3.
R.V. Schmidt end TaP. KaELnov, Appl. Phys. L a t t . 2 5 , 458 ( l g 7 4 ) .
4,
H . Naitohm Re NunoshLta and T. N a k s y s l ,
5.
PoK. T£en, S.S. Riva and AeA. Ballman, Appl. Phys. L e t t . 25~ 563 (1974).
6.
S. FukunLshl~ N. UchLde, S. Miyszawe and 3. Nods, Appl. Phys. L a t t . ~ , 424 ( l g ? 4 ) .
7.
I . P . Keminovm 3.ft. Catruthers, E.H. Turns~ and L.W. S ~ l z , Appl. Phys. L s t t . 22, 540 (1973). K. Nunomura, A. IshLtani~ T. H s t s u ~ r a and 3. Hayaship Crystal Grovth, 45, 355 (1978).
8. 9.
LLnos, Phys. Rave 15, 388 (1977)e Appl. Opt. 16 m 2546 (197"7).
K. 3sin and G.H. Hsv£g, Opt'. Ccmmun. 36, 483 (1981).
10.
K.3. Rao and C.N.R. Rao, Mat. flee. EUII. 17, 1337 (1982)l R. Pa~t#msarathy, K.3. Has and C.N.R. flao, Chem. Sos. Ray. 1 ~ 361 (1983)! K.3. Rao, Ptoc. %ndian Acad. a c t . (Chem. Sc~ g3, 389 (1984).
11.
S. Bharat~ m R. ParthaaaratJly, Ks3. Rao and C.N.R. Raot Solid State Communir.a~Lone, 45, 457 (1983).
12.
Standard X-ray d i f r r a c t A o n po~der patterns N;SeS. monographs 25--Sec~on 6.
13.
P,S.L. Naraaimham and K.3. Rao, 8 u l l Mater. ScL. 1, 71 (1979).
14.
C, K L t t e l t IInt~oduc~on to Solid State Physics t 3ohn WL1ey, New York (1968).
FERROELECTRIC-LIKE DIELECTRIC ANORALY IN RF-SPUTTERED ARORPHOUS ,1#
LiNbO 3 FILR5
K.BoR. Vatma, K.So Harshavardhan~ K.30 Rso and CoN.R, Rao + R a t e r l o l s Roses:oh L a b o r e t o r y I n d i a n I r m t / t o t o o f Sctence Bangalo:e-560012, I n d l s .
(Received January
8, 1985; C o m m u n i c a t e d b y C . N . R .
Rao)
ABSTRACT Amorphous f i l m s o f LINbO 3 prepared by RF s p u t t e r i n g e x h i b i t r e r r o e l e c t ~ l o - l l k s d £ e l e c t r £ o anomaly J u s t b a l e r the glees t ~ a n s t t l o n t e a p e r a t u r e . [1oct--on m i c r o s = e p i c s t u d i e s 8ho~ the presence o f eaalZ : ¢ y s t a l l l n e c l u s t e r s I n the amorphous fllm. The d l e l e c ~ t c anomaly h o o v e r , seems t o o r i g i n a t e from the t i s s u e nater£81 t e t h e r than from the c l u s t e r s .
I NTROOUCTION
Several p r o p e r t i e s o f eol£ds are supposed to c : l t ~ c a l l y deperx~ on the e x i s t e n c e o f l o n g - r a n g e o~dert f e r r o e l e c t ¢ l o beheviour o f l a t e r i 8 1 8 be£n9 one ouch. I t tee t h e r e f o r e outp¢£eing vben glassy r i b b o n s o f LINbO 3 ~ z a reported ~ r possess f e r t o e l e o t ¢ £ ¢ prope¢ty by Glees a t 81 ( 1 ) . There are s i m i l a r r e p o r t s uhete both the RF-oputtered and the r a p i d l y quenched l a t e : i s l e have been found to e x h i b i t f e t t o o l s c t ~ l o behaviout J u s t sO f ~ e l t parent c~ystallins states, Lines (2) has presented a t g u l e n t e to suggest t h a t t~s £nc£dence or f s r = o e l e c t r l r ~ t y i 8 not SnGonststent - ~ t h t h l absence of' c:ys t ~ l l i n t t y .
* P u b l £ c e t ~ o n No° 23 from R a t e = I s l e Research Laborato¢y. +To ~hom co~espondence should be addressed.
315
316
K.B.R.
VARMA, et aI.
Vol. 20, No. 3
I n v i o v o r the p o s s i b l e a p p l i c a t i o n s o f r a = t o e l e c t t L c t h i n f i l m s i n i n t e g r a t e d o p t i c s ( 3 , 4 ) e l o c t t o o p f . t c modulators ( 5 - 7 ) harmonic gametet a t s ( 8 , 9 ) e t c . , • d e t a i l e d study o r t h e i r d i a l e c t J r i c behavlou¢ and s t r u c t u r e i s i m p o r t a n t . The fooJs of' i n v e s t i g a t i o n i n t h i s paper I s to f,ind out i f RF-eputteted f,ilms or LINbO 3 also e x h i b i t d i e l e c t r i c anomaly and to i n v e s t i g a t e the o r i g i n or the phenomenon which appears to be g e n e r a l l y 8ssoctatad u i t h amorphous s t a t e . I n t h i s cocmunicatlon, the ocouttenom o f d t e l e c t ¢ i c anomaly Ln R r - s p u t t a r e d r ~ l n s o r LLNbO3 i s repotted. The d i e l e c t r i c behaviouts o f RF-eputteced and melt-quenched rllmm have been competed. The u l t t a m £ c c o s t t u c t u r e of, the f,ilms have been i n v e s t i g a t e d ue£ng high r e s o l u t i o n t c a n e ~ s s l o n e l e c t r o n m/ctoecopy (HRT[R). A p l a u s i b l e a n a l y s i s baaed on the c l u s t e r model ks g i v e n , h t c h sho~s t h a t the d i e l e c t r i c 8nomaZy can t e e u Z t q u i t s g e n e r a l l y i n i o n i c amorphous m a t e r i a l 8 due ta the unusual manner of, sol,teeing or v i b ¢ a t t o n a i lodes £n the r e g i o n of, glass t r a n s i t i o n ( 1 0 , 1 1 ) . CXP£RIRENTAL L i t h i u m nJ.obata r e q u i t e d re= RF s p u t t e r i n g was prepared by h e a t i n g a a t a l c ~ t o a e t t i c m i x t u r e of, reagent grade LI2CO 3 end Nb205 ~n a p l a t i n u m c r u c i b l e Par about 3 hours 8 t 8 temperature of, about 1170K. The p r o d u c t tins g z a d u a l l y cooled to 7?0 K and metntcLnad s t t h i s tempe¢atuwe f,ot ~ 8 h o u r s . The f o z ~ t l o n of, c r y s t a l l i n e LINbO 3 wee confirmed by X-ray dlf,f,tact/on. The l n t a t p l a n a t spacings (d) of' the sample compared ~ e l l w i t h the ~epotted l i t e r a t u r e (12) v a l u e s . LINbO 3 was RF-sputtered on to quactz s u b e t t a t a s held a t team temperat u r e u s i n g an RRC ( R a t e r i s l s Research C o r p o r a t i o n , USA) lode1 S[R-8822 provided u l t h 8 13.56 RHz RF g e n e : e t a t capable o f producing a maximum o u t p u t po~et l e v e l o f 1.5 kW. S p u t t e r i n g was performed i n 8n argon p l e a t s a t a conmtant pcsssute o f 4x10-3 t a r t . The 8 t c u c t u t e of, the 8puttaced f i l m s ~ete exa~Lned u s i n g a h i g h c e s o l u t l o n t ¢ s n s n t s s l o n s l e c t c o n a t c ¢ o s c o p e , model 3rOL, :ER 200 CX. The opetat:J.ng v o l t a g e of, the system uas 200 kV u t t h a p t l a a t y mgntflcatton of 3-5x105. A1unLtrttum e l e c t r o d e s wets d e p o s i t e d u s i n g masks p r i c e to s p u t t a t l n g LINb03 f,llms r e q u i t e d r o t d i a l a c t c i c c o n s t a n t measurements and s p l a n a r sample c o n f i g u r a t i o n use adopf~d r o t aeaeucementa. Capacitance ~ao Imasuted as • f , u n c t l o n of, both t l m p e t a t u t e (300-700 K) and frequency (1-100 kHz) ~ t t h 8 s i g n a l s t c e n g t h o f 8 V : a s u s i n g a t h : e e t e r m i n a l General Radio Capac£tance Bridge (USR) coupled e x t e r n a l l y ~ t h GR1316 o e c L l l a t o t and GR1232A tuned a a p l l f , i e r and n u l l d e t e c t e r . The atomy ¢apacLtar~:e tnt~oduced by loads e t c . ~ae Ittrttmtzed by r e d u c i n g the l e n g t h s of' the c o n n e c t i n g leads and by p r o v i d i n g adequate e l e c t r i c a l 8 h l e l d l n 9, D l e l e c t t a , c constants of' f i l m s tmra evaluated a f t e ¢ c o r r e c t i n g f,ot the r e s i d u a l 8tcay capacitance. RESULTS AND DISCUSSION The t e m p e r a t u r e v a r i a t i o n of' the d S a l s c t t t c c o n s t a n t of, an a s - s p u t t e r e d f,llm a t 1 kHz 18 shown An r i g . 10 The saaploo ~e¢o t e a p o t s bJra-cyr..led and the d i a l e c t : i t c o n s t a n t values wore f,~Jnd to be q u i t e r e p r o d u c i b l e upto about 700 K. D i e l e c t r i c constants of the t o i l e t - q u e n c h e d LINI:O3 glass r e p o r t e d by Glass s t 81 18 818o tnclucled I n f,ig. 1 f,ot comparison. I t may be noted t h a t the c l i e l e c ~ t c boheviout of' the s p u t t e r e d f,lZm I s q u a : L t t a ~ v e l y s i m i l a r to t h a t or the z ' o l l e r quenched
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317
g l a s s . The temperature o f t h e d l e l o c t r l c anomaly, howovez:, i s about 50 K l o ~ r . Unfortunately, i n our oxperlment~, ~ could not ~ 8 o u r e the d l e l e c t r i c constants afte= c r y s t a l l i z a t i o n and hence the second peak soon by Glass •
ot el,
Bulk glass (Gloss el ol~)
~s
not observed,
The
magnitude o f the anomaly l t o e l f i s ooneiderably highs= I n sputtered • Sputtered film ( Present wDrk ) f i l m s than in r o l l o t - q u e f l c h o d giassoo. The fEoquoncy dependence o f the d l o l o c t ~ i ¢ Gormt~nt8 o f LLNUO3 f i l m s l e shorn i n f i g . 2. The d i e l e c t r i c constant decreases r a p i d l y upto about 15 kHz and the 5 decrease i s l e s s r a p i d s t higher frequencies. The p:oeont study e o t a b l L o h e s t h a t sputtered amorphous f i l m s o f LIN~ 3 exhibit 8 dielectric 4 anomaly s i m i l a r to the t o l l s : I 9OO 500 700 300 quenched glassy f i l m s . Lines (2) TemD¢~otur¢(K) has considered the l e t t e r as a fettoolectrio-llko anomaly and FIG. 1 has attAmptod to e x p l a i n the anomaly assuming the e x i s t e n c e of Temperature v a r i a t i o n of the d i e l e o t r i c constant o f LLNbO3 f i l m 8 t I kHz. s m a l l , d i o l o c t r / c a l l y s o f t , ordered r e g i o n s . I n the ordered r e g i o n s , the d i e l e c t r i c bohaviou= i o considered to he s i m i l a r to t h a t in a a c r o s c o p i c a l l y c r y s t a l l i n e parent m a t e r i a l s .
~ o
_J
3 5
/-,0
80 Frequency
120
(F~HZ)
FIG. 2 The f:squency dependent8 of tho d i e l e c t r i o constant o f LiNUG3 f i l m a t 300K.
An eloct~on microg~aph of on a s - s p u t t e r e d f i l m along w i t h the corresponding diff~ac~Lon p a t t e : n to shown i n r i g . 3. Whereas the diffraction pattern clearly s u g ~ o f ~ t h a t the s p u t t e r e d f i l m s ate amorphous, the high r e s o l u t i o n mlc~o~aphs : e v e a l a mosaic o f oupetf/ne c ~ y s t a l l i t e a w i t h i n average dimension o f the order o f 15 ~. Proeonoo o f o~dored regions o f s i m i l a r tango has been t a p e , ted i n other glassy mater i a l s (10)o The sputtered f i l m s ware annealed a t v a r i o u s f~mpa: a t u r o s upto NO K f o r oevor81 hours. The mkcrograph and the d i f f C a c t i o n p a t t e r n o f the ??0 K-annealed f i l m ( f i g . 4) show evidence o f c ~ y s t a l l l z s t i o n . Mlcrographs and d i f f r a c t i o n
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p a t t e t n e o f samples annealed below 7?0 K vats e s s e n t i a l l y 8 i m i l a : to those i n r i g , 3. The tdt spacLng8 o f the c r y s t a l l i z e d Films vere obtained f:om TEM and they compared y e l l K i t h l t t e r a ~Jra values o
FIG. 3 E l e c t : o n mlc¢og:aph o f a s - s p u t t e r e d LINbO 3 f i l m togethe¢ Kith the d i f f ract.ton p a t t e r n ( I n s a t ) ,
riG. 4 Elect:con miccogcaph and the d i f f r a c t i o n p a t t e r n ( i n s e t ) o f the annealed LiNbO 3 f i l m .
Evonthough the p r e s e n t HR~EM s t u d i e s confi=m the presence o f a t c ¢ o c ¢ y e t a i Z i t e :egtorm Invoked by Lines 9 the d l a l e c t ¢ l g anomaly observed I n the amorphous m a t e : t a l e amy not ensue rr~n the u l t c a m l c : o c ¢ y a t a l l i t e s f o r the f o l l o K i n g :eaeone. ( l ) The observed anomaly i s close to the gleam t t a n e t t i o n tompatatu:a and solid 8t¢uctural entit~es like u l t c u i c ¢ o c ¢ y s t e l l i b e s are u n l i k e l y to be p r e s e n t a:ound Tg. ( i t ) There i s no evidence f o r a f e r r o e l e c t : L c h y s t e r e s i s loop i n amorphous materials ~ i o h exhl b i t the anomaly. ( £ 1 i ) A s i m i l a r inc¢oasa i n d i o l o c t c i o c o n s t a n t i s incident I n o t h e r i o n i c glasses (13) i n the nelghbouthood o f Tg. A c l o s e r examlrmt l o n o f this general phenomenon i n amorphous i o n i c m a t e r i a l s i s the:afo:o in order. Rt c ¢ o c t y s t e l l i t e regions observed I n HRTER o? the amorphous f i l m s ( F i g . 3) suggest t h a t ve could uae a r . l u s t e r model f o r s p u t t e r e d f i l m s as y e l l . I n accordance vcLth t h i s model, ~e consider the i n t e r c¢yatallite r e g i o n i n the LiNbO3 ?Lime as amotphoue t i s s u e . ~e then examine the o r i g i n o f the d i e l e c t r i c anomaly i n the l i g h t or c l u s t e r model as the o,laee t c a n a i t i o n temperabJte i s approached. Hoveve¢t va may note that the RF-sputbeted LINbO3 f i l m s e x h l h i t or~y a c:ya t a l l i z a t i o n tempeta~tuta clcse to Tg and L t amsks the much veakat glass t r a n s i t i o n evmnto
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3
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Significant features result from the anhermonlcity of vibrations of particles, p a r t i c u l a r l y the a l k a l i ions (Li + "n the present case), i n the tissue region and on the surfaces of c r y s t a l l l t e s . I t has been sho~n earlier (10) that the v i ~ a t i o n a l e n e r g i e s are related by z l E ~ ~E 1 = 1 / n where ~E n and d [ 1 are the v i ~ a t i o n a l energy d i f f e r e n c e s i n the n th end I s t v i b r a t i o n a l l e v e l s . The corresponding v i b r a t i o n a l f r e q u e n c i e s are t h e r e f o r e r e l a t e d as oJn/~Jl = 1 / n . P a r t i c l e s v i b r a t i n g i n h i g h e r quantum l e v e l s , t h e r e f o r e , are ° a o f t e r t . Since as • f u n c t i o n o f tamperst u f a p a r t i c l e s p o p u l a t e higher l e v e l e r t h e i r e x c i t a t i o n s are governed by modes uhioh l a o f t e n l as temperature i s i n c r e a s e d . Using the Lyddans-SechaTeller relation (14), " identify .ith the t a o f t l mode so t h a t a S ~ j n = ~ J l / n = COTO decreases, ~ o i n c r e a s e s . Since the p a r t i c l e s are i n d i f f e r e n t v i b r a t i o n a l s t a t e s they ~ould make a ~eighted c o n t r i b u t i o n to the measured ~o" The t o t a l number o f p a r t i c l e s I s determined by the volume o~ the t i s s u e which inc r e a s es as TD I s approached. As the temperature i n c r e a s e s , the volume o f c l u s t e r decreases and ' l i q u i d l i k e I p r o p e r t i e s emerge i n the t i s s u e r e g i o n . Aa • consequence, the vibrations become wholly l o c a l i z e d and the phonon leedst description becomes inappropriate; the modes do not contribute to ~_. Hence, the u e f f e c t i v e number o f modes i s proportional to the volume of the cluster. An approximate expression for C-o may be wtlttan as,
rn C-°
=
constant
~
Vtiesue
x
Vcluata r
n
vhere f n i s the f r a c t i o n o f p a r t i c l e s i n n t h v i b ~ a t i o r m l l e v e l , determined by an appropriate p a r t i t i o n function. He,ever, i n this model, fn always Increases with temperature upto Tg.Vclustar le zero at Tg end Vtissue i s l o u a t OK. Hence, s maximum occurs i n the v a lu e o f ~ o, p r i o r to Tg accounting for the observed anomaly i n ~o"
The point ~e wish to make i s that the d i e l e c t r i c anomaly of amorphous materiels m~y originate from the tissue and not from the mic~ocrystalline cluster regions. This i s important because the preparation of amorphous solids involve quenching from considerably high temperatures. The nucleated clusters moat l l k s l y possess highly symmetrical nonferroe l e o t r l c structures. Fertoelsctrlc d l e t o r t l o n i n these structures provide negiiglbls energy gains as compared to the de~res of mataetablllty a l r e a d y p r e s e n t . However, since t i s s u e i s e common f e a t u r e o f s l l i o n i a glasses, it ~ould suggest that dielectric anomaly should mahifest in all ionic glasses (or sputtered films). Indeed as pointed out earlier, largo inczeesa in dielectric constants have bean noted in nitrate end sulphate glasses a t temperatures close to T ( 1 3 ) . g In conclusion, we suggest that the dielectric anomalies observed in glasses and sputtered films of LiNbO3-1ike materials is associated with the tissue. The large magnitude of dlolecttlc constants in LINGO 3materials is due to the large io~Io polorisability aeeoclated ~Lth certain ionic groups like (NbO~) ~'.
320
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REF'ERF'NCIES I.
A.M. Gleam, R.E. Lirms, K. h s s a n and 3.W. Shtevet, AppZ. Phys. L s t t . 31, 249 (1977).
2o
•.(o
3.
R.V. Schmidt end TaP. KaELnov, Appl. Phys. L a t t . 2 5 , 458 ( l g 7 4 ) .
4,
H . Naitohm Re NunoshLta and T. N a k s y s l ,
5.
PoK. T£en, S.S. Riva and AeA. Ballman, Appl. Phys. L e t t . 25~ 563 (1974).
6.
S. FukunLshl~ N. UchLde, S. Miyszawe and 3. Nods, Appl. Phys. L a t t . ~ , 424 ( l g ? 4 ) .
7.
I . P . Keminovm 3.ft. Catruthers, E.H. Turns~ and L.W. S ~ l z , Appl. Phys. L s t t . 22, 540 (1973). K. Nunomura, A. IshLtani~ T. H s t s u ~ r a and 3. Hayaship Crystal Grovth, 45, 355 (1978).
8. 9.
LLnos, Phys. Rave 15, 388 (1977)e Appl. Opt. 16 m 2546 (197"7).
K. 3sin and G.H. Hsv£g, Opt'. Ccmmun. 36, 483 (1981).
10.
K.3. Rao and C.N.R. Rao, Mat. flee. EUII. 17, 1337 (1982)l R. Pa~t#msarathy, K.3. Has and C.N.R. flao, Chem. Sos. Ray. 1 ~ 361 (1983)! K.3. Rao, Ptoc. %ndian Acad. a c t . (Chem. Sc~ g3, 389 (1984).
11.
S. Bharat~ m R. ParthaaaratJly, Ks3. Rao and C.N.R. Raot Solid State Communir.a~Lone, 45, 457 (1983).
12.
Standard X-ray d i f r r a c t A o n po~der patterns N;SeS. monographs 25--Sec~on 6.
13.
P,S.L. Naraaimham and K.3. Rao, 8 u l l Mater. ScL. 1, 71 (1979).
14.
C, K L t t e l t IInt~oduc~on to Solid State Physics t 3ohn WL1ey, New York (1968).