ELSEVIER Diamond and Related Materials 7 ( 19981491 494

D| OND AND

RELATED T|R|AL

Optical properties in infra-red region of nitrogen-incorporated amorphous carbon films

Z.Y. C h e n "'*, Y .H . Yu ", J .P. Z h a o ~, X. W a n g ~, S.Q. Y a n g ", T .S. Shi ~, X . H . L iu ~, S.P. W o n g b I .H. W i l s o n b, J .B. X u b E .Z . L u o b

a hm Beam Laboratory. Shanghai h~stitute of Metallurgy, Chinese Academy of Sciences, Shanghai 2(;0050, PR China b Departme~,tt q/'Electronic Engineering. The (_Tfinese Universitj, ofHong Kong, Shatin, NT, Hong Kong

Received 18 June 1997: accepted 5 September 1997

Abstract

Nitrogen-incorporated amorphous carbon (a-C:N) films are prepared by filtered arc deposition. Optical properties of a-C:N films were characterized by infra-red reflectance spevtrometer. The experimental results show that the reflectivity of a-C:N films increases as the nitrogen content increases. Using detailed theoretical analysis and computer simulation, the complex dielectric constant e =e~ -+ it2,, refractive index ~ =n + ik and absorption coefficient ~ of a-C:N films in infra-red region were investigated. Simulation results indicate that the optical constants of a-C:N show a considerable variation with wave number and nitrogen content. The a-C:N film with a low nitrogen content possesses a low g and corresponding low ~ and :~ values. The variation of optical properties and optical constants of a..C:N fihns may be due to the development of graphite-like structure with the increasing of nitrogen content in these films. ~i,~.~ 1998 Elsevier Science S.A.

Kevwm'ds." Amorphous carbon: Optical: Infra-red: Con,puter simulation

I. introduction

Thin films of amorphous carbon (a-C) deposited fi'om a filtered cathodic vacuum arc plasma have attracted a great deal of in'terest in recent years [13] . Such a type of film shows no dete~table hydrogen and typically contains about 80 90% sp "~ bonded carbon, giving rise to remarkable properties close to that of diamond[l,2]. The usefulnes.s of this films for electronic applications has been greatly enhanced by the recent demonstration of n-type doping by nitrogen and r~hosphorous [4-8].

Significant efforts have been made in this area, but let;,; work has been carried out on the optical properties of this kind of N-doped amorphous carbon (a-C:N) film. Investigated optical properties of a-C:N films mainly focused on the optical band gap and absorption coefficient in the ultraviolet visible region. However, there is no study on the 9ptical constants (dielectric constant, refractive index, extinction coefficient and absorption coefficient) in the infra-red region. In this paper, the optical properties of a-C:N films prepared by filtered arc deposition have been investigated using an

* Corresponding author. Tel: + 86 21 625 i 3510.

(1925-9635/98/$19.00 ,:~'~ 1998 Elsevier Science B.V. All rights reserved. Pfi S0925-9635(97)00242-2

infra-red reflectance spectrometer. The optical constants in the infra-red region were also ~tudied using a com- puter simulation of IR retlection spectra.

2. Experimental

In the present study, a-C:N films ~,ere prepared by filtered arc deposition {FAD). Fhe details of this FAD system have been reported previously[3]. The films were deposited on to Si ( I i I ) substrate at a substrate bias of -200 V. The base pressure of the vacuum chamber is 3.5 x 10 3 Pa. Nitrogen is introduced into the films by admitting high-purity nitrogen gas into the deposition chamber with a mass flow controller. After the nitrogen is introduced, the pressure of the vacuum chamber is i x 10 2~! × 10 ~ Pa. According to the Rutherford back-scattering measurements, the nitrogen concen- tration in these films increases from 8 to 20 at.% with nitrogen pressures increasing. IR reflection measure- ments were carried out at room temperature by means of a Perkin-Elmer 983 double-beam spectrometer for

- | the fi'equency range 1000-4000 cm

492 , Z. }'. ('hen ('t al D immmd ami Rt'htted MateriaL~' 7 ¢/00,~') 401 494

3. Theoretical consideration

The complex refractive index of film can be obtained from dielectric function, e.g. a = n + i k = V e , and g=Et +i¢_,. The two representations are related at any

- - k 2 given photon frequency according to et n 2 and E2 = 2nk.

The equation for the amplitude reflectivity of a thin absorbing film on a substrate in air from Ref. [9] is:

]~ = I'af + I'fs (' - 2i,~

! +/'afrtse- 2i,~" ( 1 )

where the subscripts a, f, and s refer to air, film, and substrate respectively, and r is the Fresnel reflection coefficient. 6 is the change in phase of the beam on traversing the film.

2rr 6 = - - m/cos ~, ( 2 )

where 2 is the wavelength of incidence light in vacuum. n is the refi'acfive index of film, d is the thickness of film,,;, and ~p is *,he refractive angle of incidence light in films. Finally, the reflectivity is given by:

R= R. R*. (3)

A computer code is established to calculate the retlec- tivity of the film lit an arbitrary angle incidence from the thin film model described above. The experimental rellection spectrum is fitted by using a computer codc that adjusts the values of" the pan'ameten's of the thin lilm model to minimize i"..

%,

i 1

where N is the number of data points of the experimental retlection spectrum and N, is the number of parameters of the model described above.

4. Results and discussion

For a-C:N tilms deposited on Si substrates by FAD, the optical properties of a-C:N films were described by:

B (" D ct = A + (,;)

(U (t)2 (p)3 "

,~., = A' exp( - B'(,~ ~ -' ), ( 6 )

where ¢,J is lhe ~,ave number. The parameters A, B, C, D, A'. and B' were variables to be determined in the fitting algorithm.

Fig. I shows the infi'a-red rettection spectra of a-C:N films. From Fig. I, it can be seen that: ( i ) the reflectivity of a-C:N films decreases with the increase of wave numb,:r, i.e. it decreases with pl~oton energy, and (2)

45

4 0

3 5

~ 3o

25

20

15

10

~ 8 at.% o 11 at.%

~ . . . ~ _~ 12 at.%

I I I I , I , I ~ I

I000 Isoo 2000 2500 3000 3~00 4ooo

. (cm "1)

Fig. I. The infra-red reflection spectra of a-C:N lilms with different nitrogen content . The solid lines are the best-tit curves.

the reflectivity of a-C:N films increases with the nitrogen content. Film 1, with a nitrogen content 8 at.%, has a lower reflectivity. In the wave number region of 1000-4000 cm- ~, the reflectivity of film 2 with a nitrogen content of !! at.% is close to that of Si substrate, and for film 3 with a nitrogen content of 20at.%, the reflectivity is even higher than that of the Si substrate. The solid lines in Fig. I are the results of fitting the reflection spectra using the empirical model. The non- linear least-squares-adjusted curves are in good agreement with the experimental results. The good agreement between experiment and calculation indicates that the model described above is appropriate tbr use in studying the optical properties of a-C:N lilms.

Fig. 2a shows the real part of the dielectric constant c~ of a-C:N films obtained from a computer simulation. it is found that ct is approximately constant in the measurement range. However, there is an obvious depen- dence of ct on nitrogen content. Film ! possesses lower Et values than that of films with a hig, her nitrogen content. The imaginary part of the dielectric constant c2 increases with nitrogen content (Fig. 2b), reflecting the increasing absorption in film due to the development of the graphic phase with a short range order. For comparison, the dielectric constant of 9ure amorphous carbon films prepared by direct carbon arc deposition without nitrogen was also given in Fig. 2a and b. It should be noted that there is a large change in E2 for a change in nitrogen concentration from 8% to I1%, and then very little change between i!% and 20%. This indicated that graphitization in a-C:N films becomes distinctly with nitrogen content increasing fiom 8% to 1 I%, trigonal bonds predominates in the films with 1 I% nitrogen, which is responsible for the large change in ¢2. This effect becomes weak under a higher nitrogen content, 20%, resulting in films with little change in c,. This iact is demonstrated by Hall electrical resistivity

2. Y. Chert el aL/ Diamond and Rektted Matcrial.~ 7 ( i 998) 491 494 493

60

~- 12

10

(a)

8 a t . %

. . . . . . 1 1 a t . %

. . . . . . . . . . 2 0 a t . %

- ~ - Oat .%

I I , I

1000 1500 200o I , I • I ,,_1 ,., I ,

2500 3000 3500 4000

o (cm' l l

4.5

4,0

3.5

(a)

3.0

2.5

2,0

8 a t .%

. . . . . . 11 a t .%

... . . . . . . 20 a t .%

' " . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i . . . . . . . . . . . . . . . . . . .

" ' ' ' ' ' ' - - - . . . . . . . . . . . . . . . . . . . . . . . . . .

I , ...I , I , ! I I I I I

1000 1500 2000 2500 3000 3500 4000

o (cm "1)

4

(hi

8 a t . %

. . . . . . 1 1 a t . % \

X - ......... 2 0 a t . %

"N . . . . O a t . %

"..'.:.

"" :"::: '" :" :" :" ": '" :" :': :':': ."..L', :-'..2 :-.

1000 %'10 2000 2500 3000 3500 4000

u (cm "11

I:'ig, 2, Real p a r t ~ ( a and imag ina ry part ~, (b) o fd i c l cc l r i c func t ion vs. wave nu~nber Io: a - ( ' :N lilnls wilh di l lcrcnl n i t rogen t,'onlcnl ;llltl pure t |-(, lilm.~ v~ithout n i t rogen.

18

16

14

1,2

1.0

08

06

04

02

O0

8 a t .%

. . . . . . 11 at .%

.......... 20 at.% " ' . " x •

" ' . . % , . •

• -..... • . • , .

. .

• . .... q ? . .

.......... • L.?

, . . .

1000 1500 2000 2500 3000 3500 40t30

qbj o (era 1)

Ii~. Y [,~cfracti\c index n (a )and cxlinc| i~m coclliclcnl /, (h) ~,, ~ ; l \ c I l l l l l l ]) t , l ' for ;I,-(':N l i lms ~'i lh t l i l | c re l l l i i i [ Io~t ' l l L~l',lClll,

measurements. Results indicated that the resistivity of a-C:N films decreases rapidly from 2.56x 10 ~ to 3.25 x 10- -" ~cm at room temperature when the nitrogen content is increased from 8 to i i%, and then changes to 2.16 x 10 -2 ~cm at 20% ni~,rogen content.

The simulation values of refractive index n and extinc- tion coefficient k are shown in Fig. 3a and b. The tendency of n and k of films changed with nitrogen content is similar to that of £~ and % respecthely. As can be seen fi'om Fig. 3a, n is almost independent of wave number and approaches 2.6, 3.3. and 3.7, corre- sponding to a nitrogen content increasing fl-om 8 to 20 at.%. T!;e extinction coefficient is generally a decrease function of wave number.. The high nitrogen content lilms in Fig. 3b appears 1o be predominantly sp-' in character with a strong, broad absorption feature in t spanning the full infra-red range. The k of tilm 1 is close to zero at higher wave number, indicating that liim I has a ',,:,,. transparency in this region. Since the values of n and k are related to the structure of the film, such

changes show that the lilms became more graphitc4ike with the increase in nitrogen con len t .

Fig. 4 shows the absorp t ion coel t ic icn, :~ (~ .~4~k ,,. I

of a-C:N tilms as a function of wa,¢c number for a dilt"erent nitrogen content. It is J~und that the :~ of film I is significantly lower than that of lilms 2 and 3. It is known thai tile absorption as a function o f wa~e number is determineu by two factors, e.g. retlcction and transmis- sion. The summation of the thre" pans is equal to i. Combining Figs. I and 4. it can be .~;een that the transmit- tahoe of film ! is much higher than thai of film 2 and 3. It is well known that diamond and diamond-like tilms are transparent in the infra-red region. Therefore, il can be concluded that tilm I, will1 a low nitrogen content. shows a more diamond-like character. This indicalcs that iiim 1 has a higher sp 3 I:ond contenl lhan t]lnl 2 and 3. This is supported by the change in optical band gap of films, which decreases from 2.5 to 1.6 with increasing nitrogen concentration in the tilrns.

494 Z. ): C/ten et al. / Diamomt and Related Material,~ 7 (1998) 491 494

25000

20000

15O0O

E v

100OO

500O

. . . . . . - . . . . . . . . . . . .

s # . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . s p . . . . . " . . . . . . . . . . . . . . . . . . . .

8 at.% . . . . . . 1 ! at.% . . . . . . . . . . 2 0 a t . %

0 I i I I I I t I , I

Iooo I ~ 2ooo 2soo 3ooo 3soo 4ooo

u (cm'll

Fig. 4. Absorption coefficient • vs. wave number for a-C:N films with different nitrogen content.

5. Conclusion

In summary, the optical properties of a-C:N films prepared by filtered arc deposition have been investi- gated by IR reflectance spectrometer. The results indi- cate that the reflectivity of a~C:N films increases as the nitrogen content increases. By detailed theoretical analy- sis and computer simulation of infra-red reflection spectra, the dielectric constants ~, and e2 and, conse- quently, the refl'active indices, n ~nd k, and at)sorption

coefficient ~ of a-C:N films in infra-red range are obtained. The a-C:N film with a low nitrogen content exhibits low e, and e2 values and corresponding low n, k, and ~ values. The variation of optical properties and optical constants of a-C:N films may be due to the development of a graphite-like structure with the increasing nitrogen content in films.

References

[I] D.R. McKenzie, D.A. Muller, B.A. Paithorp, Phys. Rev. Lett. 67 ( 1991 ) 773.

[2] D.R. McKenzie, Y. Yin, N.A. Malk , C.A. Davis, B.A. Paithorp, G.A.J. Amaratunga, V.S. Veerasamy, Diamond Relat. Mater. 3 (1994) 353.

[3] J.P. Zhao, X. Wang, Z.Y. Chen, S.Q. Yang, T.S. Shi, X.H. Liu, J. Jang, K.C. Park, Nucl. lnsht]m. Meth. B 127- 128 (1997) 817.

[4] V.S. Veerasamy, G.A.J. A,naratunga, C.A. Davis, A.E. Timbs, W.I. Milne, D.R. McKenzie, J. Phys. Condens. Matter 5 (1993) LI69.

[5] C.A. Davis, Y. Yin, D.R. McKenzie, L.E. Hail, E. Karvtchinskaia, V. Keast, G.A.J. Amaratunga, V.S. Veerasamy, J. Non-Cryst. Solids 170 ( i 994) 46.

[6] J. Robertson, C.A. Davis. Diamond Relat. Mater. 4 (1995) 441. [7] G.A.J. Amaratt,,nga, VS. Veerasamy, C.A. Davis, W.I. Milne,

D.R. McKenzie, J. Yuan, M. Weiler, J. Non-Cryst. Solids 164165166 (1993) 1119.

[8] V.S. Veerasamy, J. Yuan, G.A.J. Amaratunga, W.I. Milne, K W.R. Gilkes, M. Weiler, L.M. Brown, Phys. Rev. B 48 (1993) 17954.

[9] O.S. Heavens, Optical Properties of Thin Solid Films, Academic Press, New York, 1955.