Thermodynamic properties and electrical conductivity of Ta 3 N 5 and TaON

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SWISHER AND M. H. READ

TaON and Ta3N5 p o w d e r s and thin f i l m s w e r e p r e p a r e d f r o m Ta205 p o w d e r s and t a n t a l u m thin f i l m s by e q u i l i b r a t i n g s p e c i m e n s with a m m o n i a - b e a r i n g gas m i x t u r e s at l l 0 0 ~ TaON was found to be n o n s t o i c h i o m e t r i c and to have s e m i c o n d u c t i n g p r o p e r t i e s . The f r e e e n e r g y of f o r m a t i o n of TaO, .o~2qo.g5, which i s the c o m p o s i t i o n in e q u i l i b r i u m with Ta2Os, is - 9 4 . 1 • 5 kcal p e r m o l e . The r a n g e of e l e c t r i c a l conductivity o b s e r v e d at r o o m t e m p e r a t u r e was 10 -3 to 5 ( o h m - c m ) - ' . Ta3N5 has a f r e e e n e r g y of f o r m a t i o n of - 6 5 • 10 kcal p e r m o l e , and i t s e l e c t r i c a l conductivity is 1.3 • 10 -4 ( o h m - c m ) -1.

S E V E R A L n i t r i d e s of t a n t a l u m have been p r e p a r e d and c h a r a c t e r i z e d by X - r a y diffraction t e c h n i q u e s . 1 The n i t r i d e s , TaNo.05, Ta2N, TaNo.8-o.9, and TaN, f o r m when t a n t a l u m r e a c t s with n i t r o g e n u n d e r v a r i o u s cond i t i o n s . All a r e s e m i m e t a l l i c , both in a p p e a r a n c e and in e l e c t r i c a l p r o p e r t i e s . TazN thin f i l m s , which can be p r e p a r e d by s p u t t e r i n g t a n t a l u m in a s m a l l p a r t i a l p r e s s u r e of n i t r o g e n , 2 a r e u s e d c o m m e r c i a l l y a s resistors. A h i g h e r n i t r i d e , Ta3Ns, f o r m s when Ta205 r e a c t s with d r y a m m o n i a g a s at t e m p e r a t u r e s in the v i c i n i t y of 700 ~ to 900~ TasN5 is r e d and has a t e t r a g o n a l crystal structure. 3 No d i r e c t d e t e r m i n a t i o n s of the f r e e e n e r g i e s of f o r m a t i o n of any of the t a n t a l u m n i t r i d e s at e l e v a t e d t e m p e r a t u r e s have b e e n r e p o r t e d . The f r e e e n e r g y of f o r m a t i o n of TaN at e l e v a t e d t e m p e r a t u r e s h a s been e s t i m a t e d f r o m its heat of f o r m a t i o n at 25~ In g e n e r a l , both t h e r m o d y n a m i c p r o p e r t i e s and phase e q u i l i b r i a in the Ta-N s y s t e m a r e not w e l l - k n o w n . The only o x y n i t r i d e of t a n t a l u m which has b e e n p r e p a r e d and c h a r a c t e r i z e d with r e s p e c t to c r y s t a l s t r u c t u r e i s TaON.~ It was p r e p a r e d by r e a c t i n g Ta205 powd e r with a m m o n i a , u s i n g cyclic t r e a t m e n t s with wet and dry NHa gas at 700 ~ to 900~ TaON was r e p o r t e d to b e g r e e n in color and to have a m o n o c l i n i c c r y s t a l s t r u c t u r e . In the t e r n a r y T a - O - N phase d i a g r a m , s t o i c h i o m e t r i c TaON l i e s on the p s e u d o b i n a r y s e c t i o n b e tween Ta205 and Ta~N5. TaON also has been o b s e r v e d to f o r m as an i n t e r m e d i a t e product in the oxidation of Ta3N5 to Ta205 in a i r . Both TaON and TaaN5 a r e u n s t a