Electroless plated nickel contacts to hydrogenated amorphous silicon

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Thin Solid Films 252 (1994) 78 Sl

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Electroless plated nickel contacts to hydrogenated amorphous silicon T. Balaji Suresh, S. Satish Kumar, S. Karmalkar, Enakshi Bhattacharya I:'h'~ trwal l','n~hwering I)~7u~rtment. liT..~&ah'a.v 600 036. India

Rccci,,cd 23 I'cbruar.', 1994, acccpled 7 June 1994

Abstract We report for the tirst timc investigations on electroless plated nickel phosphorus alloy (Ni-P) contacts to undopcd amorphous silicon (a-Si:H). 1 • l" characteristics of eleclroless Ni P were compared with those of Nickel deposited by thermal evaporation. 11 was l\-mnd that as-deposited Ni P makes a rcctit}'ing contacl to undoped a-Si:H. The effects of plasma annealing on the contacts were studied. Ni P contacts on Im~ pressure chemical vapour deposited a-Si are also reported here. K c v w o r d ~ : Amorphous materials; Contacts, Nickel: Schottky barrier

I. Introduction Electroless nickel plating is a well known technique for making ohmic contacts to both n-type and p-type crystalline silicon (c-Si) [1]. The technique has the advantages o f simplicity, low cost and the requirement of low annealing temperatures. Electroless N i - P plating involves autocatalytic reduction o f metal salts such as nickel chloride to the metal at the silicon surface with the liberation o f hydrogen. The composition o f the plating bath used in the present experiment is essentially the same as that o f the Brenner solution [2]. that is N i C L . 6 H . O 30 g 1 k NH4C 50 g I '. N a H ~ P O s - H p O 1 0 g l i, (_'~H~Na30..HzO 8 4 g l i The reactions involved arc divided into anodic and cathodic, forming m a n y local cells at the interface between substrate and solution. Thc different reactions occuring can be summed up as follows [3]. (1) diffusion o f reactants to the interface. (2) oxidation o f H , P O , (anodic) H,PO~ - HzO--,H.~PO

+2c

-2H

(3) migration o f the electrons in the substrate. (4) reduction o f N i : and H - (cathodic) Ni:' 2H

l- 2c 4-2c

For well-prepared substratcs vigorous reaction with evolution o f hydrogcn begins as soon as thc sample is introduced into the bath. The composition o f the deposit on well-prcparcd samples depends primarily on thc pH and termpcraturc o f the bath. The percentage o f phosphorus in thc nickcl deposit is reported to bc inversely proportional to the pH and directly proportional to the operating temperature of the bath. The phosphorus conlent varies from 3% to 15% when the operating temperaturc of the bath is kept at about 95 C [4, 5]. Elcctron X-ray diffraction patterns showed that the as-deposits arc supersaturated solutions o f p h o s p h o r u s dissolved in tinely crytalline nickcl [6]. As-deposited N i - P makes a Schottky barrier contact to n-type c-Si. Application of electroless Ni .-P as Schottky barrier metal on c-Si for solar cclls has bccn reported [6, 7]. Sintcring above 580 C is observed to make the contact ohmic. Often, to make ohmic contact to u n d o p e d a-Si: H a thin n " layer is grown before depositing the metal. Phosphinc is mixed with silanc to grow the n layer. Phosphinc is a highly toxic gas and its use is hazardous, t left ~vc explore the possibility of making ohmic contacts to a-Si:H by electroless or autocatalytic plating o f nickel.

--*Ni --,[I.

(5) diffusion o f reaction products from the interface into the solution. 004(I-6090..94.s7.o0 ¢ 1994 l!lsc,,icrScience S A All right:, rc,,cr',cd S S D I 0040-6090(94) 06229 I!

2. Experimental procedure U n d o p e d a-Si:H was deposited by plasma-enhanced chemical v a p o u r deposition ( P E C V I ) ) onto heavily

79

T. Balaji Suresh et al. / Thin Solid Films, 252 (1994) 78-81

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doped n-type c-Si wafer which serves as a back ohmic contact. The deposition conditions were as follows: substrate temperature 280 ~C, silane flow rate 15 sccm, r.f. power 10 W, chamber pressure 0.6 mbar. The thickness of the film was 1.56 ~tm and the activation energy was found to be 0.7 eV. The cleaned sample was chemically etched for 2 min in a solution containing HNO3, H F (48%) H20 in the ratio 100:1:100. This etch is extremely important for proper substrate preparation and ensures a thick and adherent coating. The plating solution was heated to 90 °C and adjusted to pH 8 - 1 0 by adding a m m o n i u m hydroxide until the colour of the solution changed from green to blue owing to complex formation. The sample was then dipped for l min in 48% HF, washed with water and transferred quickly to the plating bath. The reaction began immediately and was continued for 3 min. A thick metallic golden yellow coloured deposition on a-Si:H and a dull yellow deposition on the bottom n* c-Si were obtained. The N i - P was patterned with 1 m m diameter dots by photolithography. N i - P on bottom n ÷ c-Si forms the back ohmic contact as shown in Fig. l(a). Nickel was also thermally evaporated through masks onto similar substrates for comparison of characteristics with N i - P (Fig. 1(b)). The evaporation was done at a chamber pressure of 1 0 - 6 m b a r and substrate temperature of 150 ~C.

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3. Results and discussion

Fig. 3. 1- V characteristics of samples after hydrogen plasma annealing at 280 'C for 100 rain.

Fig. 2 compares the I - V characteristics of the two samples considered above. Nickel is known to form a stable Schottky barrier with undoped a-Si:H [8]. From Fig. 2 it can be clearly seen that as-deposited N i - P makes a Schottky barrier to undoped amorphous silicon and the I - V characteristics are similar to that of a nickel Schottky barrier. The ideality faction n for nickel Schottky was found to be 1.35. The ideality factor of the N i - P Schottky varied from 1.4 to 1.7. Hence, for Schottky diodes on undoped a-Si:H, electroless plated N i - P could be a viable alternative to evaporated nickel. N i - P deposited onto n-type c-Si and sintered above 500 "C is known to make a good ohmic contact owing to phosporus diffusion [9]. After annealing, a heavily doped donor layer is formed at the surface of c-Si owing to the migration of Si atoms into the N i - P alloy

and phosphorus diffusion. Similar experiments to drive the phosphorus from the N i - P alloy into the undoped a-Si:H were tried. Annealing the samples in vacuum at 270 °C for 1 h produced no change in the I - V characteristic of sample l(a). The substrate temperature was not sufficient for phosphorus diffusion. The samples could not be subjected to temperatures above 280"C (the deposition temperature) as this would cause hydrogen effusion. Previously we had grown a-Si:H on N i - P plated c-Si and found the contact to be ohmic. To examine the role of plasma, the samples were subjected to hydrogen plasma at a substrate temperature of 280 C , r.f. power 10 W and chamber pressure of 0.8 mbar. Fig. 3 compares the 1 - V characteristics of as-deposited N i - P and

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h y d r o g e n p l a s m a treated Ni P contacts. It is seen that N i - P on a-Si:H remains a rectit}'ing contact. It is possible that the h y d r o g e n p l a s m a did not have any effect because the N i - P d e p o s i t i o n was quite thick and the h y d r o g e n could not reach the interface. Hence, a n o t h e r s a m p l e was p r e p a r e d by plating a thin layer o f Ni P on a-Si:H. The thickness o f the Ni P layer was not m e a s u r e d but the plating time was a b o u t 30 s. This s a m p l e was subjected to h y d r o g e n p l a s m a at 280 C for a d u r a t i o n o f 100rain and then replated to give a thicker c o a t i n g ( f o r 2 3 min) on the previous one. N i P was p a t t e r n e d with I m m dots. The I-. V c h a r a c teristics tire p l o t t e d in Fig. 4, showing that h y d r o g e n p l a s m a treated thin Ni P m a k e s o h m i c c o n t a c t to a - S i : l t with nearly equal currents in both directions. A s h o k and c o w o r k e r s have r e p o r t e d the creation o f d o n o r - l i k e defects at the m e t a l - c-St interface due to ion b o m b a r d m e n t , causing a decrease in the barrier height in A l / n - l y p e c-Si and the creation o f a barrier in A1/p-type c-St [10, 11]. A t this stage we d o not k n o w the exact m e c h a n i s m leading to the o h m i c i t y o f the c o n t a c t .... w h e t h e r it is e n h a n c e d P diffusion in the presence o f the p l a s m a or the f o r m a t i o n o f d o n o r - l i k e states at the interface. We also studied Ni P c o n t a c t s to L P C V I ) a-Si. U n d o p e d a-St was d e p o s i t e d o n t o n-type c-St at 580 (', SiHa flow rate 40 s t a n d a r d cm ~ m i n ' c h a m b e r pressure 0.33 mbar. The thickness o f the film was 5700 ,~. C o n ductivity m e a s u r e m e n t s were taken on Cr c o p l a n a r c o n t a c t s on tilm grown on q u a r t z substrate. F r o m r o o m t e m p e r a t u r e to a b o u t 100 C the c o n d u c t i o n m e c h a n ism was d u e to h o p p i n g . A b o v e that the c o n d u c t i v i t y was a c t i v a t e d with an a c t i v a t i o n energy o f 0 . 5 9 c V . N i - P was plated on these samples using the same bath as m e n t i o n e d a b o v e and p a t t e r n e d with I m m

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0.60 Voltage (VI

0.80

100

Fig 5. I lcharacleristics of Ni P tm IP('VD a-St

dots. The I I," characteristics are given in Fig. 5 and show that N i - - P m a k e s a g o o d o h m i c c o n t a c t to L P C V D a-St. We see that there is s o m e difference between the I l characteristics in Figs. 4 and 5. F o r both samples. a b o v e a certain a p p l i e d bias, the current is lower when the b o t t o m c o n t a c t is injecting, showing the limitation o f the c-Si/a-Si contact. This difference is m o r e lk~r p l a s m a e n h a n c e d ( P E ) ( ' V I ) a-Si:H sample, being magnilied at higher voltages. W e a t t r i b u t e this to the difference in the h e t e r o j u n c t i o n tit the back between n-type c-St with P E C V D a-Si:H with tin optical b a n d g a p o f 1.7 eV a n d I , P C V D a-St with an optical b a n d g a p o f 1.5 eV. W h e n the Ni P c o n t a c t is injecting a super linear log I log V characteristic is o b s e r v e d tit higher voltages, as seen in n i---n or n i metal structures o f a-Si:H [12. 13].

4. Conclusion W e have d e m o n s t r a t e d that electroless p l a t e d Ni P is a simple way to form S c h o t t k y barriers on a-Si:H. A thin layer o f N i - P on t m d o p e d a-Si:H when subjected to H2 o r Sill4 p l a s m a at 280 C m a k e s an o h m i c contact. Ni P on L P C V D a-Si is shown to m a k e a g o o d o h m i c contact. Hence. in devices such as thin tilm transistors ( T F T s ) N i - P can be an alternative to the n ' layer for source and d r a i n contacts.

Acknowledgement We thank Professor. K.N. Bhat tbr his c o n t i n u o u s e n c o u r a g e m e n t and constructive discussions.

T. Balaji Suresh et al. / Thin Solid Films, 252 (1994) 78 gl

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[8] D. E. Heller, M. Gunes, F. Rubinelli, R. M. Dawson, S. Nag, S. J. Fonash and C. R. Wronski. Proc. M R S Spring Meet., San Francisco CA, 1992, Materials Research Society, Pittsburgh, PA, 1992. [9] B. K. Singh and R. N. Mitra, J. Electrochem. Soc., 127(1980) 2578. [10] S. Ashok and K. Srikanth, in J. J. Pouch and A. S. Alterovitz (eds.) Plasma Processing oJ Materials, Materials Science Forum, TransTech, Aedermannsdorf, 1993. [11] S. J. Fonash, S. Ashok and R. Singh, Appl. Phys. Lett., 39 (1981) 423. [12] E. Bhattacharya, S. Guha. K. V. Krishna and D. R. Bapat, J. Appl. Phys., 53 (1982) 6285. [13] R. L. Welisfield, J. Appl. Phys., 54 (1983) 6401,