Electrical Characterization of Ion Implantation into GaAs Topography and Depth Profiles

Recen t advances in the character iza t ion of ionimplan ted GaAs samples have inc luded whole wafer mapp ing (topography) and dep th profi l ing techniques . We rev iew several me thods for m a p p i n g electr ical parameters , inc lud ing the darkspot res is tance (DSR), and the m i c r o w a v e pho toconduc t ance techniques . In addit ion, we sugges t a new photo-Hal l techn ique which wou ld al low mobi l i ty (~) and carr ier -concentra t ion (n) m a p p i n g as well as that of res is t ivi ty (p). Finally, we r e v i e w m e t h o d s for ob t a in ing p, Ix, and n dep th profiles, w i th pa r t i cu la r e m p h a s i s on the app l i ca t ion of the magne to r e s is tance t echn iques in actual field-effect t ransis tor s tructures. C h a r a c t e r i z a t i o n is an i m p o r t a n t e l e m e n t in t h e process of i m p r o v i n g s e m i c o n d u c t o r mate r ia l s and devices . N o w h e r e is this s t a t e m e n t m o r e t rue than in t he p roduct i on o f a G a A s M E S F E T (meta l s e m i c o n d u c t o r f ie ld-eL f ee t t r ans i s t o r ) by d i r e c t ion i m p l a n t a t i o n in to a semi-i n su l a t i ng (SI) subs t ra te , because the cha rac te r i za t ion m u s t e n c o m p a s s t h e b e g i n n i n g subs t r a t e , t he a c t i v e layer , and the f inal dev ice . B e c a u s e o f t he m u l t i d e v i e e G a A s d ig i t a l and ana log c i r cu i t s n o w b e i n g d e v e l o p e d , and also for s i m p l e e c o n o m i c r easons , i t is b e c o m i n g n e c e s s a r y to use large area wafers , typ ica l ly of 3 in. diam. S u c h wafe r s a re o f ten l a t e ra l ly i n h o m o g e n e o u s and so shou ld be cha rac t e r i zed over the who le surface, wh ich is p r a c t i c a l on ly w i t h a u t o m a t e d t e c h n i q u e s . In r e c e n t years , t h e p r o p e r t i e s e x p l o i t e d by n o n d e s t r u c t i v e topog r a p h i c t e c h n i q u e s i n c l u d e : p h o t o a s s i s t e d r e s i s t a n c e (1-5), m i c r o w a v e p h o t o e o n d u c t a n c e (6, 7), p h o t o l u m i n e s c ence (8, 9), c a t h o d o l u m i n e s c e n c e (10), p h o t o a b s o r p t i o n (11, 12), and xray sca t t e r ing (13-15). F u r t h e r m o r e , a comm e r c i a l a p p a r a t u s is a v a i l a b l e for t o p o g r a p h i c d e v i c e m e a s u r e m e n t s , s u c h as t r a n s c o n d u c t a n c e and p i n c h o f f vo l t age . Thus , it is n o w p o s s i b l e to e f f i c i en t ly m e a s u r e and d i sp lay large v o l u m e s of mate r ia l and d e v i c e data on t h e s a m e w a f e r (or a d j a c e n t wafers) , and t h e n c h e c k for co r re l a t ions b e t w e e n the var ious sets of data. B e s i d e s t o p o g r a p h i c i n f o r m a t i o n , d e p t h d e p e n d e n t d a t a are a lso e s s e n t i a l for a c o m p l e t e u n d e r s t a n d i n g of ion i m p l a n t e d samples . A w e l l k n o w n m e t h o d for determ i n a t i o n of the car r ie r c o n c e n t r a t i o n (n) profi le is cap a c i t a n c e vs. vo l t age (C-V) (16). Mobi l i ty (~)-profile analysis , h o w e v e r , has d e v e l o p e d on ly in t he las t f ew years (17-20). A t h e o r e t i c a l ana lys i s of t he c o m b i n e d ~ and n p rof i l es a l lows the d e t e r m i n a t i o n of d o n o r (ND) and aee e p t o r (NA) prof i les , w h i c h l eads to a m o r e d e t a i l e d und e r s t a n d i n g of i m p l a n t a c t i v a t i o n p r o c e s s e s . In th is paper , we e m p h a s i z e t he m e t h o d s for o b t a i n i n g s u c h prof i l es f r o m ac tua l F E T s t r u c t u r e s by the use of t he m a g n e t o r e s i s t a n c e p h e n o m e n o n . M e t h o d s b a s e d on o t h e r p h e n o m e n a , such as C-V and Hal l e f fec t , are disc u s s e d in less de ta i l s ince t h e y are o lde r and h a v e b e e n r e v i e w e d e x t e n s i v e l y e l s e w h e r e (17). A n o t h e r sub jec t of h igh r e l e v a n c e for dev i ce pe r fo rma n c e is pa ra s i t i c r e s i s t a n c e (Rp). The ga t ed T L M p a t t e r n *Electrochemical Society Active Member. (20, 21) (a se r ies of F E T ' s w i th d i f f e r e n t ga t e l eng ths ) is use fu l for m e a s u r i n g Rp in the F E T dev ices w h i c h m a k e up the pa t t e rn , w h i l e t he s t a n d a r d T L M p a t t e r n (22) (no gates) is ideal for i nves t iga t ing con tac t r e s i s t ance (R~.). I f on ly a s ing le F E T is be ing s tudied, t h e n R,, can be determ i n e d by o t h e r t e c h n i q u e s (23, 24). C o n t a c t r e s i s t a n c e s tud ies are espec ia l ly r e l evan t for ion i m p l a n t a t i o n charac te r i za t ion because of the w ide ly used se l f -a l igned ga te (SAG) m e t h o d of source and dra in con tac t fo rmat ion . A final p o i n t to be e m p h a s i z e d is t h e i m p o r t a n c e of c h a r a c t e r i z i n g all p h a s e s of t h e GaAs ion i m p l a n t a t i o n p rocess , f r o m t h e b e g i n n i n g s u b s t r a t e to t he f inal devices . In th is paper , we will cons ide r r e p r e s e n t a t i v e techn i q u e s f rom each s tage, i.e., subs t r a t e , a c t i ve layer , and v a r i o u s d e v i c e s tages . I t is f u r t h e r i m p o r t a n t to e m p l o y all t he c h a r a c t e r i z a t i o n t e c h n i q u e s ava i lab le , b e c a u s e it is easy to d raw e r roneous conc lu s ions f rom the resu l t s of a s ingle e x p e r i m e n t . Final ly , it is usefu l to h a v e o the r res e a r c h e r s r e p e a t t he e x p e r i m e n t s , i f poss ib le . S u c h r o u n d r o b i n i n t e r a c t i o n s are v e r y u s e f u l for p r o d u c i n g accu ra t e and r e p r o d u c i b l e results .