GaZnO 2 layer InO 2 layer O (2) Ga, Zn InO (1) 6 Fig. 1. Crystal structure of InGaZnO 4 . Structural Study on a Layered Thermoelectric Material: InGaZnO 4 Thermoelectric energy conversion is attracting m u c h i n t e r e s t a s a p o s s i b l e a p p l i c a t i o n f o r “environmentally friendly” electric-power generators a n d h i g h l y r e l i a b l e , a c c u r a t e t e m p e r a t u r e - c o n t r o l l a b l e r e f r i g e r a t o r s u s e d a s e l e c t r o n i c d e v i c e s . T h e p e r f o r m a n c e o f a t h e r m o e l e c t r i c device is defined by its material properties through th e fi gu re of me ri t, Z T = S 2 T / ρ( κ e +κ l ) , wh er e S is Seebeck coefficient, T is operating temperature, ρ i s re si st i vi ty , a n d κ e a n d κ l a re ca rr i e r a n d l a tt i ce t h e r m a l c o n d u c t i v i t i e s , r e s p e c t i v e l y . A l t h o u g h a thermoelectric material, such as Bi 2 Te 3 , which has a Z T o f a b o u t 1 , i s u s e d i n p a r t i c u l a r f i e l d s , i t s performance is still insufficient for wider commercial use. Thus, the development of new materials with l a r g e Z T i s t h e o n e o f t h e m a i n i s s u e s i n thermoelectric research. Electronically and structurally two-dimensional ( 2 - D ) m a t e r i a l s c a n b e g o o d t h e r m o e l e c t r i c s , be ca us e S in cr ea se s in a 2- D el ec tr on ic st at e [1 ] a n d κ l i s r e d u c e d i n t h e 2 - D c r y s t a l s t r u c t u r e . A Y F e 2 O 4 - t y p e l a y e r e d c o m p o u n d , I n G a Z n O 4 , i s a c a n d i d a t e f o r 2 - D m a t e r i a l . I t h a s a 2 - D c r y s t a l structure, as shown in Fig. 1 , with InO 2 layers and d o u b l e G a Z n O 2 l a y e r s . M o r e o v e r , c a l c u l a t i o n o f the electronic structure by means of a cluster model p r e d i c t e d t h a t o n l y t h e I n O 2 l a y e r s p r o v i d e 2 - D electrical conduction, if the z coordinates of Ga and Zn sites are significantly different [2]. However, no expe rime ntal resu lts on the loca l stru ctur e arou nd Ga and Zn sites have been reported, though such a structural feature plays a key role in understanding t h e e l e c t r i c a l s t r u c t u r e o f I n G a Z n O 4 . W e h a v e , th us , stu di ed th e cr yst al str uc tu re of In GaZ nO 4 to the 2-D electronic and structural properties by using synchrotron radiation X-rays. A powder sample of InGaZnO 4 was prepared by solid-state reaction. X-ray powder diffraction data w e r e c o l l e c t e d w i t h a D e b y e - S c h e r r e r - t y p e d i f f r a c t o m e t e r a t b e a m l i n e B L 0 2 B 2 . T h e c r y s t a l structure was refined by the Rietveld method using the program RIETAN. Figure 2 shows the X-ray diffraction pattern and its Rietveld refinement profile. The crystal structure o f I n G a Z n O 4 i s r h o m b o h e d r a l s p a c e g r o u p R 3 m , and Ga and Zn atoms are distributed randomly at 6c crys tall ogra phic posi tion s betw een InO 2 laye rs. discuss 75 However, since the valence state and ionic radii of Ga and Zn are different, the positions of these atoms are also d i f f e r e n t . T h e s t r u c t u r e r e f i n e m e n t b a s e d o n a m o d e l w i t h d i f f e r e n t z c o o r d i n a t e s f o r G a a n d Z n g i v e s a f a i r l y g o o d f i t t o t h e o b s e r v e d d i f f r a c t i o n pattern. The results of Rietveld refinement show that the z coordinates of the Ga a n d Z n s i t e s d i f f e r b y 3 t o 4 % . T h e d i f f e r e n c e i n l o c a l s t r u c t u r e s a r o u n d Ga Zn O (2) O (1) O (2) O (1) References [1] L. D. Hicks and M. S. Dresselhaus, Phys. Rev. B 47 (1993) 12727. [2] M. Orita et al. , Phys. Rev. B 61 (2000) 1811. [3] H. Imai, Y. Shimakawa, H. Kimura and Y. Kubo, in preparation. Fig. 2. High-resolution X-ray diffraction pattern of InGaZnO 4 and its Rietveld profile. 10 20 30 40 10000 5000 0 InGaZnO 4 R 3m R wp = 0.96%, R p = 0.62% 2 θ θ (degree) Intensity Fi g. 3. Sc he ma ti c of re fi ne d lo ca l structures around Ga and Zn atoms. t h e G a a n d Z n a t o m s i s s h o w n i n F i g . 3 . T h e random displacement of Ga and Zn atoms causes small overlapping of the 4 s orbital of Ga and/or Zn a s p r e d i c t e d b y t h e c l u s t e r - m o d e l c a l c u l a t i o n , l e a d i n g t o t h e i n s u l a t i n g n a t u r e o f t h e G a Z n O 2 layer. The In-In atomic distance in the InO 2 layer, o n t h e o t h e r h a n d , i s s m a l l e r t h a n t h a t i n t h e S n - dop ed In 2 O 3 ( ITO : a tra nsp are nt con duc tin g oxi de wit h hig h car rie r mob ili ty) . Thi s sug ges ts tha t the direct overlapping of In 5 s orbitals causes the high electrical conductivity in the layer. These structural f e a t u r e s r e v e a l e d b y o u r a n a l y s i s a r e c o n s i s t e n t w i t h t h e 2 - D m o d e l i n t h e c l u s t e r c a l c u l a t i o n . However, a preliminary result of the band-structure ca lc ul at io n in di ca te s ne ar ly 3- D el ec tr ic al co nd uc ti on due to the orbital overlap between In 5 s and Ga 4 s , in spite of the difference in the z coordinates of Ga a n d Z n [ 3 ] . A p r e c i s e e l e c t r o n i c - b a n d - s t r u c t u r e cal cul ati on usi ng the ref ine d stru ctu ral par ame ter s is now in progress. Hideto Imai, Yuichi Shimakawa and Yoshimi Kubo Fundamental Research Lab., NEC Corporation E-mail: h-imai @ ce.jp.nec.com 76
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