DISLOCATION ELECTRONIC STATES FROM SCHOTTKY DIODES IN SILICON

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DISLOCATION ELECTRONIC STATES FROM SCHOTTKY DIODES IN SILICON

S. Mantovani, E. Mazzega

To cite this version:

S. Mantovani, E. Mazzega. DISLOCATION ELECTRONIC STATES FROM SCHOTTKY DIODES IN SILICON. Journal de Physique Colloques, 1979, 40 (C6), pp.C6-63-C6-65.

�10.1051/jphyscol:1979613�. �jpa-00219029�

JOURNAL DE PHYSIQUE ColZoque C6, supplhnent au n06, tone 40, j u i n 1979, page C6-63

DISLOCATION ELECTRONIC STATES FROM SCHOTTKY DIODES IN SILICON

S. Mantovani and E. Mazzega

I s t i t u t o d i Fisica deZZ 'Universitd, 41100 Modena, I t a l y

R6sum6.- Nous avons mesur6 .?i l a temp6rature de l ' a z o t e l i q u i d e l a c a p a c i t e .?i haute frequence en fonc- t i o n de l a t e n s i o n i n v e r s e d'une j o n c t i o n Schottky Pd2Si-Si dont l e s i l i c i u m de type n a @ t & pr&c@- demment d6form6. Les r e s u l t a t s v e r i f i e n t l e modele de l a bande demi-plein associge a l a pr6sence de d i s l o c a t i o n s , e t montrent que l e niveau d 1 & n e r g i e pour l a d i s l o c a t i o n neutre e s t s i t u @ a 0,40 eV au- dessus de l a bande de valence.

A b s t r a c t . - By h i g h frequency measurements a t l i q u i d n i t r o g e n temperature o f t h e capacitance as a f u n c t i o n o f the a p p l i e d bias on Schottky PdzSi-Si diodes, obtained from p r e v i o u s l y deformed Si, i t has been p o s s i b l e t o c o n f i r m t h e v a l i d i t y o f t h e model o f t h e h a l f - f i l l e d band associated w i t h t h e d i s l o c a t i o n s , w i t h t h e energy l e v e l o f t h e n e u t r a l d i s l o c a t i o n a t 0.40 eV above the valence band.

1. I n t r o d u c t i o n . - Accordingly t o t h e model o f t h e h a l f - f i l l e d band /1/ (HFB) t o t h e edge t y p e d i s l o c a - t i o n i n Ge and S i i t can be associated a one- dimensional energy band, l o c a t e d i n the forbidden gap and pinned t o t h e Fermi l e v e l EF. The d i s l o c a - t i o n i s t h e r e f o r e expected t o be s i m i l a r , i n i t s e l e c t r i c a l p r o p e r t i e s , t o a deep i m p u r i t y w i t h a dense m u l t i - l e v e l spectrum, l y i n g i n the same h a l f - gap as EF. To make a scan o f these d i s l o c a t i o n cen- t r e s we performed on Schottky diodes made o f pre- v i o u s l y deformed n-Si, low temperature, h i g h f r e - quency capacitance measurements by a simple method

/ 2 , 3 / able t o d e t e c t very small concentrations and

s p a t i a l d i s t r i b u t i o n o f deep i m p u r i t y centres.

2. The method.- The method c o n s i s t s i n g i v i n g a l a r g e reverse b i a s V R t o a Schottky diode a t room temperature, i n c o o l i n g i t s l o w l y t o t h e l i q u i d n i - trogen temperature and measuring, i n a s h o r t time, t h e j u n c t i o n capacitance C as a f u n c t i o n o f t h e a p p l i e d v o l t a g e V, w h i l e the l a t t e r i s reduced from VR t o z e r o and then increased again t o VR.

The 1 arge reverse b i a s , given a t room tempera- t u r e , and t h e subsequent slow c o o l i n g enable the space charge r e g i o n ( o f w i d t h L) t o be i n a steady s t a t e a t 77 K. F u r t h e r t h e low temperature makes t h e time constant f o r the thermal emission o f the charge c a r r i e r trapped i n the deep l e v e l s very long and t h e r e f o r e enables t h e i r c o n c e n t r a t i o n t o be assumed constant d u r i n g the C - V measurement; as a consequen- ce d u r i n g t h e b i a s decreasing (V -. 0) t h e deep

l e v e l s l y i n g i n t h e upper h a l f o f t h e band-gap, are unoccupied by e l e c t r o n s , i n the space charge r e g i o n from x = O t o x = ( L - A ) . I n ( L - A) t h e quasi-Fermi l e v e l EFn i n t e r s e c t s t h e deepest l e v e l s . During t h e

successive b i a s i n g (V ^-t VR) t h e trapped charges on the deep l e v e l s are i n t h e i r e q u i l i b r i u m concentra- t i o n a t 77 K. Therefore the C-V curves obtained i n the two cases must be d i f f e r e n t due t o t h e d i f f e r e n t n e t space charge c o n c e n t r a t i o n i n the two measure- ments.

3. Experimental.- S t a r t i n g m a t e r i a l was monocrystal- l i n e n-Si, doped w i t h P, w i t h a r e s i s t i v i t y o f 500R cm, i n form o f {111} s l i c e s o f thickness o f about 200 pm. The samples were deformed by bending and f l a t t e n i n g i n a vacuum b e t t e r than t o r r a t 750°C and then cooled down a t a r a t e o f l e s s than 5OC/min. The t o t a l d i s l o c a t i o n d e n s i t i e s so i n t r o - duced were i n t h e range (0.5-5) l o 6 cm-2 ; 75 % o f which were o f t h e edge type. P d 2 S i - S i Schottky bar- r i e r s , w i t h evaporated A1 as back contact, were obtained w i t h a method already described elsewhere / 4 / . The Pd s i l i c i d e f i l m was about 500 1 t h i c k and had a c i r c u l a r area w i t h a diameter o f 6 mm. A f t e r t h e measurements t h e diodes were etched t o count t h e e t c h p i t s b y o p t i c a l microscopy :more countings were made i n t h e evaporated area and the r e s u l t s averaged.

Only those samples w i t h a homogeneity b e t t e r than 10% were r e t a i n e d . The capacitance was measured as already described elsewhere /5/ and the r a t e o f coo- l i n g t h e diodes t o t h e l i q u i d n i t r o g e n temperature d i d n o t exceed 1°C/min.

4. Results and discussion.- I n f i g u r e l a a r e shown the C-V curves obtained by decreasing and increasing, a t 77 K, t h e b i a s o f a reference sample reverse biased a t 95 V and room temperature; t h i s sample had undergone t h e same thermal processes as t h e deformed ones.

The two curves o f f i g u r e l a coincide, showing

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979613

C6-64 JOURNAL DE PHYSIQUE

t h a t t h e c o n c e n t r a t i o n o f deep l e v e l s i n t r o d u c e d d u r i n g t h e thermal cycles, i f any, i s lower than 10" ~ m - ~ .

F i g . 1 : Capacitance f o r u n i t area, C/A, versus a p p l i e d bias V, a t 77 K. ( a ) reference sample, (b) deformed sample w i t h an edge d i s l o c a t i o n d e n s i t y o f 1 . 8 ~ 1 0 cm-'.

By means o f t h e expression :

where A i s the j u n c t i o n area, E t h e s i l i c o n p e r m i t - t i v i t y , q the a b s o l u t e e l e c t r o n i c charge, and x i s given by AE/C, one o b t a i n s a n e t space charge con- c e n t r a t i o n which i s u n i f o r m and equal t o N = 9.2 x l ~ ~ ~ c m - ~ .

I n f i g u r e l b a r e shown t h e same two curves f o r t h e deformed sample w i t h an edge d i s l o c a t i o n d e n s i t y NI = 1 . 8 ~ 1 0 ~ c m - ~ ; i n t h i s case t h e two cur- ves do n o t coincide, i n d i c a t i n g t h e presence o f deep t r a p s . From t h e curve a t i n c r e a s i n g b i a s one o b t a i n s by use o f (1) a n e t space charge concentration u n i - form up t o the w i d t h o f about 200 pm ( a t which t h e diode i s f u l l y depleted) N' = 2 . 5 ~ 1 0 ~ ~ c m - ~

.

f = (N-N1)a/Nl,where a i s the mean d i s t a n c e between two dangling bonds, and t h e n e u t r a l d i s l o c a t i o n l e v e l i n S i , E,. I n f a c t from t h e HFB model one has

where EV i s t h e top o f t h e valence band, and (EF-EV) i s given by : EG-kT I~(C~T'~/N'); 1 i s t h e Debye screening length, EG t h e band-gap width, Cc = 5 . 4 ~ 10' 5~m-3k-3/2 and T t h e a b s o l u t e temperature.

From t h e experimental values o f N and N', (Eo-EV) i s found t o be 0.40 eV, i n agreement w i t h r e c e n t r e s u l t s 161. From t h e curve a t decreasing b i a s o f t h e deformed sample one obtains, as a l r e a d y i n d i - cated above, the n e t space charge c o n c e n t r a t i o n N' ( x ) , which i n t h i s case i s n o t u n i f o r m i n t h e whole deple- t i o n region; as shown i n f i g u r e 2 up t o t h e d e p l e t i o n w i d t h ( L - A) t h e concentration i s equal t o t h a t o f t h e reference sample ( 9 . 2 ~ 1 0 ' ~ c m - ~ ) and from x = ( L - A) t o x = L i t decreases monotonously w i t h i n - creasing x, t o value o f t h e deformed sample

( 2 . 5 ~ 1 0 ' ~ c m - ~ ) a t t h e boundary o f t h e d e p l e t i o n r e - gion.

F i g . 2 : Net charge c o n c e n t r a t i o n i n t h e d e p l e t i o n r e g i o n o f the deformed sample o f f i g u r e l b w i t h de- creasing b i a s .

The p r o f i l e shown i n f i g u r e 2 i s f a r from a step f u n c t i o n as i t should be i n the case o f deep monolevel centres. From t h e values o f N' ( x ) i t i s pos- s i b l e t o o b t a i n a t each x t h e corresponding values o f f and o f (EFn-Ev), assuming (EFn-EV) = 0.40 eV i n x = ( L - A), where f = 0, EFn z EF a t t h e d e p l e t i o n r e g i o n boundary L and (EFn

-

I f we suppose t h a t equation (2) i s v a l i d a l s o i n t h e space charge region, w i t h t h e replacement o f EF w i t h EFn, we can c a l c u l a t e by i t t h e values o f

S. Mantovani and E . Mazzega

(EFn-EV) f o r d i f f e r e n t v a l u e s o f f, w i t h (Eo - E V ) = 0.40 eV, N = 9 . 2 ~ 1 O ' * c m - ~ and T = 77 K. The values so o b t a i n e d can be compared w i t h t h o s e f o u n d e x p e r i - m e n t a l l y , as shown i n f i g u r e 3. The agreement b e t - ween t h e two k i n d o f r e s u l t s i s good, c o n f i r m i n g t h e v a l i d i t y of t h e HFB model, and o f t h e assumption made, i n a wide range o f f values.

References

/1/ S c h r o t e r , W. and Labush, R., Phys. S t a t u s S o l i d i 36 (1969) 539.

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/2/ B r o t h e r t o n , S.D., Sol i d - S t a t e E l e c t r o n .

19

/3/ Sah, C.T., Rosier, L.L. and Forbes, L., Appl.

Phys. L e t t . - 15(1969) 161.

/4/ C a n a l i , C . , C a t e l l a n i , F., Mantovani, S. and P r u d e n z i a t i , M., J. Phys. D.

10

/5/ Mantovani, S., d e l Pennino, U. and Mazzega, E., Phys. S t a t u s S o l i d i ( a ) - 35 (1976) 451.

/6/ Kos, H.J. and Neubert, D., Phys. S t a t u s S o l i d i ( a ) - 44 (1977) 259.

F i g . 3 : Fermi l e v e l p o s i t i o n above t h e valence band versus t h e d i s l o c a t i o n occupation f u n c t i o n f; A ex- p e r i m e n t a l values; s o l i d l i n e - c a l c u l a t e d a c c o r d i n g t o e q u a t i o n (2) w i t h (Eo - EV) = 0.40 eV.