Defects studies of semiconductor materials
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T H E p r e s e n c e of defects in bulk and e p i t a x i a l s e m i conductor m a t e r i a l s can be r e a d i l y d e t e r m i n e d by s e v e r a l t e c h n i q u e s . U n f o r t u n a t e l y , due to l i m i t a t i o n s of each t e c h n i q u e , it has b e e n found that the n a t u r e and cause of defects is r a r e l y r e s o l v e d by one tool alone. This work shows that a b e t t e r e v a l u a t i o n and u n d e r s t a n d i n g of defects can be a c h i e v e d by c o m b i n i n g the i n f o r m a t i o n obtained from etch pit count, X - r a y diff r a c t i o n for m e a s u r e m e n t of p r e c i s e cell p a r a m e t e r , X - r a y topography, s c a n n i n g e l e c t r o n m i c r o s c o p y (SEM), and e l e c t r o n m i c r o p r o b e a n a l y s i s (EMA).
A modified C a m b r i d g e m i c r o p r o b e is used for studying m a t e r i a l s or d e v i c e s in depth (1 to 6 p) by s e v e r a l m o d e s : s a m p l e c u r r e n t i m a g e , conductive mode o p e r a t i o n , and c a t h o d o l u m i n e s c e n c e . In addition the probe is equipped with a light e l e m e n t d e t e c t o r ~~ and a 400 c h a n n e l a n a l y z e r to p e r m i t the d e t e r m i n a t i o n of i m p u r i t i e s in the p p m range within an a n a l y t i c a l spot of 1 to 2 p in d i a m .
I) TECHNIQUES
The following t h r e e e x a m p l e s i l l u s t r a t e the n e c e s s i t y of u s i n g a c o m b i n a t i o n of s e v e r a l t e c h n i q u e s for the e v a l u a t i o n and u n d e r s t a n d i n g of defects. Homogeneity Studies of GaAs W a f e r s . F o u r GaAs ingots with e q u i v a l e n t doping (Sn, 3 • 10 ~6 p e r cc GaAs) but p r e p a r e d u n d e r d i f f e r e n t conditions were selected. F r o m each ingot, at a p p r o x i m a t e l y the c e n t e r , a slice was cut and m e c h a n i c a l l y polished. The slice was then c h e m i c a l l y p o l i s h e d in a (5:1:1) m i x t u r e of H2SO4-HzOH202 a g a i n s t a Teflon block in a r o t a t i n g b e a k e r . The d i s l o c a t i o n d e n s i t y of each wafer, a s d e t e r m i n e d by the s t a n d a r d etch pit count p r o c e d u r e 1 is: 1.1 • 103, 1.0 • 104, 1.0 • l0 s, and 5.0 • 105 d i s l o c a t i o n s p e r sq cm. Since the d i s l o c a t i o n etch y i e l d s a n u m b e r which is g r e a t l y i n f l u e n c e d by the p r o c e d u r e itself, and s i n c e it does not indicate the n a t u r e of defects, X - r a y topographs were obtained as a m e a n s to f u r t h e r e v a l u ate and c o m p a r e the s a m p l e s . F i g s . 1 to 4 a r e (444) r e f l e c t i o n topographs taken with MoK~ r a d i a t i o n , 10 h r e x p o s u r e on Ilford L - 4 nuclear plates. A v i s u a l i n s p e c t i o n and a m i c r o s c o p i c e x a m i n a t i o n , 50X, of the topographs show l a r g e d i f f e r e n c e s between w a f e r s of v a r i o u s d i s l o c a t i o n d e n s i t y . The r e l a t i v e a m o u n t of c r y s t a l defects p e r unit a r e
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