Small area deposition of Ge on Ge or GaAs substrates via the disproportionation of GeI 2
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d e p o s i t i oH n of s e m i c o n d uE c t o r s onto s e l e c t e d a r e a s T of a s u b s t r a t e h as been of i n t e r e s t f o r a n u m b e r of y e a r s a s an a l t e r n a t i v e to diffusion i s o l a t i o n m e t h o d s in i n t e grated circuit technology for applications requiring m i n i m a l p a r a s i t i c c a p a c i t a n c e . A m o n g the d i f f i c u l t i e s e n c o u n t e r e d have been s u r f a c e q u a l i t y and s p u r i o u s growth of c r y s t a l l i t e s on o x i d e - m a s k e d a r e a s . In addition, a r e s t r i c t i o n to the p o s s i b l e d e v i c e a p p l i c a t i o n s f o r d e p o s i t i o n s into s p e c i f i e d a r e a s h a s been the t e m p e r a t u r e of the v a r i o u s p r o c e s s e s ( i . e . , s i l i c o n at l l 0 0 ~ 1 and GaAs at 750~ In the p r e s e n t w o r k the GeI2 d i s p r o p o r t i o n a t i o n r e a c tion was u s e d to grow e p i t a x i a l g e r m a n i u m at 350~ s e l e c t i v e l y in s m a l l e x p o s e d a r e a s on g e r m a n i u m and GaAs s u b s t r a t e s . The r e s u l t s of the d e p o s i t i o n e x p e r i m e n t s w e r e e s s e n t i a l l y the s a m e f o r both s u b s t r a t e m a t e r i a l s and w e r e i n d e p e n d e n t of doping l e v e l . Since the t e m p e r a t u r e of the iodide p r o c e s s is low, c a . 350~ the m e t h o d h a s the c a p a b i l i t y of f o r m i n g e p i t a x i a l j u n c t i o n s without d i s r u p t i n g p r e v i o u s l y f o r m e d j u n c t i o n s . F o r e x a m p l e , an e p i t a x i a l e m i t t e r can be g r o w n o v e r a p r e v i o u s l y diffused b a s e - c o l l e c t o r j u n c t io n . The p r o c e s s a l s o e n a b l e s the f a b r i c a t i o n of i s o l a t e d i s l a n d s of g e r m a n i u m on an o x i d e - m a s k e d s e m i - i n s u l a t i n g GaAs s u b strate.
shown in F i g . 1. It c o n s i s t s of a d e t a c h a b l e p y r e x bulb (2.5 by 9.0 in.) which c o n t a i n s a p p r o x i m a t e l y 500 g of iodine ( G a l l a r d - S c h l e s s i n g e r - u l t r a high p u r i t y 99.9999 pct) m i x e d with 4 m m p y r e x b e a d s to p r e v e n t packing of the i o d i n e. F l o w through the iodine i s c o n t r o l l e d by two T e f l o n - P y r e x v a l v e s . The e n t i r e a s s e m b l y w a s contained in an e n c l o s u r e upon which h e a t i n g t a p e s w e r e wound. The output of the iodine c h a m b e r w as fed d i r e c t l y into a h e a t e d p l a t i n u m bed (450~ and the r e s u l t i n g HI w a s diluted to the d e s i r e d v a p o r p r e s s u r e at a point b e f o r e e n t e r i n g the g e r m a n i u m bed. The ad v an t ag e of this type of g e n e r a t o r l i e s in e a s e of r e p l e n i s h i n g the iodine supply and the e l i m i n a t i o n of the bulky oil bath used to m a i n t a i n iodine t e m p e r a t u r e s in p r e v i o u s g e n e r a t o r s . Oxygen and w a t e r i m p u r i t i e s in the c a r r
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