GaAs heterostructures

GaAs heterostructures

Superlattices and Microstructures, Vol. 4, No. 2, 1988 P-TYPE OHMIC CONTACTS 1 97 TO A I G a A s / G a A s HETEROSTRUCTURES J.-H. R e e m t s m a...

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Superlattices and Microstructures, Vol. 4, No. 2, 1988

P-TYPE OHMIC CONTACTS

1 97

TO A I G a A s / G a A s

HETEROSTRUCTURES

J.-H. R e e m t s m a , K. H e i m e U n i v e r s i t i t - G e s a m t h o c h s c h u l e - Duisburg. D-4100 D u i s b u r g i, FRG W . S c h l a p p , G.Weimann, F o r s c h u n g s i n s t i t u t d e r D e u t s c h e n Bundespost, D-6100 Darmstadt, FRG

(Received

17 August,

1987)

Ohmic contacts to p-AiGaAs/GaAs heterostructures have l a r g e r r e s i s t i v i t i e s c o m p a r e d to n - t y p e s t r u c t u r e s b e c a u s e h o l e s are h e a v i e r t h a n e l e c t r o n s and the h e t e r o i n t e r f a c e b a r r i e r for h i g h m o b i l i t y s t r u c t u r e s is thicker. This w o r k s h o w s t h a t d i f f u s i o n of Zn d u r i n g an a l l o y p r o c e s s of a Au/Zn/Au m e t a l l i z a t i o n e s t a b l i s h e s high a c c e p t o r c o n c e n t r a t i o n s t h r o u g h o u t the h e t e r o s t r u c t u r e r e s u l t i n g in a thin m e t a l - s e m i c o n d u c t o r b a r r i e r and p r o b a b l y a d e g r a d a t i o n of the heterointerface barrier by disordering, while the heterostructure outside the contact region remains unchanged. Contact resistivities down to 3.5~mm at 25K c o u l d t h u s be achieved.

i. I n t r o d u c t i o n Heterostructure -Field-Effect-Transistors w i t h a t w o - d i m e n s i o n a l h o l e gas (2DHG) as c h a n n e l (p-HFETs) are v e r y i n t e r e s t i n g d e v i c e s for an a p p l i c a t i o n in c o m p l e m e n t a r y c i r c u i t s I-5. T h e beh a v i o u r of the d e v i c e s can be i m p r o v e d by c o o l i n g to low t e m p e r a t u r e s b e c a u s e the m o b i l i t y in the 2DHG i n c r e a s e s 6. The m a i n p r o b l e m is the h i g h r e s i s t a n c e of o h m i c contacts, e s p e c i a l l y at l o w e r temperatures. 2. E x p e r i m e n t s

and R e s u l t s

O h m i c c o n t a c t s are m e t a l - s e m i c o n d u c t o r c o n t a c t s in w h i c h h i g h d o p a n t c o n c e n t r a t i o n s b e l o w the c o n t a c t s lead to low c o n t a c t r e s i s t a n c e s due to f i e l d - e m i s sion (FE) a n d / o r t h e r m i o n i c f i e l d - e m i s sion (TFE). For a g i v e n b a r r i e r the contact resistance increases exponenti a l l y w i t h the c a r r i e r mass. Thus in GaAs c o n t a c t r e s i s t a n c e to p - t y p e m a t e r i a l is h i q h e r t h a n to n - t y p e m a t e r i al b e c a u s e m h / m e=6.7. In h e t e r o s t r u c tures an additional barrier occurs (fig.l). This b a r r i e r is b r o a d e n e d by the undoped spacer necessary for a c h i e v i n g h i g h m o b i l i t i e s (typical 40nm for p - t y p e and 5nm for n - t y p e h e t e r o structures). Therefore in c o n t r a s t to n - t y p e h e t e r o s t r u c t u r e s it is not suff i c i e n t to c o n t a c t just the s e m i c o n d u c -

0749-6036/88/0201 97 + 03 $02.00/0

t o r surface, the h e t e r o i n t e r f a c e barrier m u s t also b e reduced. A first step can be the i n c r e a s e of d o p a n t conc e n t r a t i o n b e l o w the m e t a l c o n t a c t d o w n into the 2DHG w h i c h d e c r e a s e s the b a r r i e r widths. A s e c o n d step can be the impurity induced disordering which leads to the r e d u c t i o n of the b a r r i e r h e i g h t or e v e n the e l i m i n a t i o n of the barrier. The d i f f u s i o n of Zn at t e m p e r a t u r e s b e t w e e n 500 and 6 0 0 ° C i n t r o d u c e s d i s o r d e r i n g in A I G a A s / G a A s h e t e r o s t r u c t u r e s 7. A l s o Zn d i f f u s e s r a p i d l y comp a r e d to Be u s e d d u r i n g the M B E growth. T h e r e f o r e w e c h o s e the d i f f u s i o n of Zn for m a k i n g l o w - r e s i s t i v i t y o h m i c contacts. The c o n t a c t s were fabricated on Bedoped GaAs/AIGaAs heterostructures g r o w n by M B E on s e m i - i n s u l a t i n g GaAs substrates. The A1 c o n t e n t was 40%. The l a y e r s e q u e n c e was as follows : l~m GaAs buffer, u n d o p e d 40nm A l x ~ a l_x A s, spacer, u n d o p e d 40nm AlxGal_xAS , nominally doped to 4 . 5 , i 0 1 8 c m -3 20nm GaAs, caplayer, u n d o p e d The f o l l o w i n g m e t h o d s w e r e tested: i) S e l e c t i v e d ~ f f u s i o n from spin on Znd o p e d films ~ 2) A l l o y i n g of A u / Z n / A u (20/50/20nm) contacts under different conditions a) u n c a p p e d metal, a l l o y e d at 4 5 0 0 C b) S i 3 N 4 - c a p p e d metal, all. at 5 5 0 ° C c) same as b), but all. at 4 5 0 ° C

© 1 988 Academic Press Limited

198

Superlattices and Microstructures, Vol. 4, No. 2, 1988 p-AIGaAs

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T h e c o n t a c t r e s i s t a n c e was d e t e r m i n e d b y the t r a n s m i s s i o n line m e t h o d 9 u s i n g c o n t a c t s of 5, 15, 30 a n d 60#m spacing. Due to the l a r g e r a t i o of s h e e t to cont a c t r e s i s t a n c e the a c c u r a c y of d e t e r m i n i n g the s p e c i f i c c o n t a c t r e s i s t a n c e is limited. P r i o r to t h e d e p o s i t i o n of t h e m e t a l l i z a t i o n and the spin on films t h e s a m p l e s w e r e c l e a n e d in c o n v e n t i o n al ways. 200nm Si3N 4 was r.f.sputter deposited. Result: i) For a c h i e v i n g s u f f i c i e n t l y h i g h diffusion depths within reasonable time a diffusion temperature of 675°C was chosen. A t this t e m p e r a t u r e the 2DHG is destroyed. 2) This p r o c e s s c o m b i n e s a l l o y i n g a n d d i f f u s i o n from the a l l o y e d region. T h e h i g h a m o u n t of Zn a v a i l a b l e is f a v o r able for higher acceptor concentrations. a) If the m e t a l is u n c a p p e d Zn is lost due to its h i g h v a p o r pressure. Fig. 2 shows the I-V-characteristics of t w o c o n t a c t s w i t h 5#m s p a c i n g at d i f f e r e n t t e m p e r a t u r e s . The t e m p e r a t u r e b e h a v i o u r b e t w e e n 40K and 70K c a n n o t be e x p l a i n e d b y the m e t a l - s e m i c o n d u c t o r - b a r r i e r but b y the h e t e r o i n t e r f a c e barrier, i n d i c a t i n g t h a t the d i f f u s i o n d e p t h w a s not s u f f i c i e n t . To c o n f i r m this w e p r e p a r e d c o n t a c t s to b u l k GaAs and t h i c k l a y e r s of A I G a A s in a s i m i l a r way; t h e y s h o w e d o h m i c b e h a v i o u r b e t w e e n 14K and 300K. b) Fig. 3 shows a I - V - c h a r a c t e r i s t i c of t w o c o n t a c t s w i t h 5~m spacing. C o n t a c t resistances were negative at all temperatures because lateral diffusion led to reduced but unknown contact spacings. The sheet resistance decreases as the mobility increases with increasing temperatures indicating that the 2DHG is contacted. (The saturation-like behavior at higher v o l t a g e s is due to the f i e l d - d e p e n d e n t

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h o l e mobility. This e f f e c t is k n o w n for the t w o - d i m e n s i o n a l e l e c t r o n gas I0. c) The c o n t a c t s s h o w e d o h m i c b e h a v i o r (similiar to fig.3) at all t e m p e r a tures, indicating that the alloying front has r e a c h e d the 2DHG. The c o n t a c t resistance of this sample decreased from 8~mm at 300K to 3.5~mm at 25K. S a m p l e 2b) m i g h t have b e t t e r values. H i r a n o et al. 1 p u b l i s h e d v a l u e s of l~mm w i t h o u t f u r t h e r d e t a i l s of the m e a s u r e ment. C o m p a r i s o n of s a m p l e 2a) and 2c) leads to the c o n c l u s i o n that Zn loss from u n p a s s i v a t e d c o n t a c t s p l a y s an imp o r t a n t role, even at t e m p e r a t u r e s as

Superlattices and Microstructures, Vol. 4, No. 2, 1988 low as 450 °C. The d i f f u s i o n depth was i n c r e a s e d by the p a s s i v a t i n g film because of the c o n c e n t r a t i o n dependent d i f f u s i o n c o e f f i c i e n t of Zn II.

199 for their support in technological work. This w o r k was in part s u p p o r t e d by D e u t s c h e F o r s c h u n g s g e m e i n s c h a f t . References

3. C o n c l u s i o n s Low r e s i s t i v i t y ohmic contacts to pAIGaAs heterostructures require high a c c e p t o r c o n c e n t r a t i o n s b e l o w the contact throughout the heterostructure down into the 2DHG (buffer layer), because both the m e t a l - s e m i c o n d u c t o r barrier has to be thin (hole mass high) and the h e t e r o i n t e r f a c e b a r r i e r has to be thin and low. The d i f f u s i o n of Zn is an a p p r o p r i a t e means for a c h i e v i n g this goal. The d i f f u s i o n induced d i s o r d e r i n g seems to reduce or even e l i m i n a t e the heterointerface barrier. Thus contact resistivities of 3.5~mm at 25K were reached. In addition the contacts are mechanically stable against repeated thermal cycling (300K ~ 14K and vice versa) and can easily be bonded after adding a post alloy-processing Aulayer. They are t h e r e f o r e appropriate for p-HFET application.

Acknowledgement - M.B6hm, R.Tilders, and U . D o e r k are g r a t e f u l l y a c k n o w l e d g e d

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