Effect of organic precursors on diamond nucleation on silicon

g!AMOND RELATED MATERIALS ELSEVIER

Diamond and Related Materials 4 ( 1995) 720-724

Effect of organic precursors

on diamond

M. Ece, B. Oral, J. Patscheider, Swiss Federal

Luhortctories

for Materiuls

Testing

wd

Resetuch

(EMPA),

nucleation

on silicon

K.-H. Ernst

b’eherlandstrusse

129, CH-8600

Duehendorf:

S~vitzerlantl

Abstract Diamond nucleation on silicon substrates is a very slow process unless the substrates are bias treated or scratched with diamond powder prior to diamond growth under chemical vapor deposition (CVD) conditions. In this report, we present our results concerning the effect of various organic coatings on CVD diamond nucleation on Si substrates. Results show that diamond can nucleate readily on Si coated with an organic layer even without scratching and biasing. The different organics used on Si became glassy carbon in the early stages of CVD diamond deposition. The glassy carbon coatings obtained from different organics as well as the diamond films grown on the glassy carbon/Si substrates were characterized by X-ray photoelectron spectroscopy (XPS), X-ray induced Auger electron spectroscopy (XAES), scanning electron microscopy (SEM), and Raman spectroscopy. The results are discussed in terms of the formation of glassy carbon which, in turn, enhances diamond nucleation. Ku~ords:

Diamond

nucleation;

Diamond

growth;

Glassy carbon;

1. Introduction A variety of methods has been employed to obtain a reasonable diamond nucleation density on silicon and on many other substrates. Simple substrate scratching with powders of diamond has been used extensively. The mechanism by which scratching enhances nucleation is not yet understood in detail. An ongoing debate about the effect of scratching focuses on the following explanations. One is that a high number of nucleation sites are formed in the scratches [ 11. The second and more widely accepted explanation states that diamond and other carbonaceous materials left in the scratches act as nucleation enhancement agents [ 21. Diamond nucleates on these carbonaceous materials or on intermediate layers of stable carbides [3]. Recently, an alternative theory for diamond nucleation has emerged from experimental findings which show a drastic increase in the number density of diamond crystallites if graphitic carbon is predeposited onto the surfaces of non-diamond as well as non-carbide-forming substrates 14,571. Angus et al. have shown that graphite as well as aromatic (sp”) compounds can serve as sites for nucleation of diamond. Similarly, turbostratic carbon (TC) (sp’ bonded carbon structure slightly different from graphite) has been suggested as another precursor phase for diamond nucleation [ 5.61. Furthermore, Terranova et al. have investigated diamond nucleation on thick plates of glassy carbon and shown that the morphology 0925.9635/95/$09.50 8 1995 Elsevier Science S.A. All rights reserved ssDl0925-9635(94)05300-6

Characterization

and adhesive strength of diamond films varied with deposition conditions [7]. Brewer et al. also studied diamond nucleation on titanium-implanted glassy carbon substrates and found that the adhesion and uniformity of diamond films on unimplanted glassy carbon deteriorated significantly after deposition [ 81. In this study, we investigated the effect of glassy carbon layers on the nucleation and growth of diamond on silicon. The glassy carbon can be thought of as tangled ribbons of graphite (disordered graphite) containing curved sheets [9]. These curved sheets are stacked with a spacing similar to that found in graphite. The glassy carbon coatings used in this work were obtained by pyrolysis of polymers as described below.

2. Experimental

details

A hot-filament assisted CVD method was employed for the growth of diamond films. Details of the experimental set-up can be found elsewhere [6]. The experiments were carried out using hydrogen and methane and a rhenium filament was employed. Four different types of organic were used to obtain glassy carbon coatings on Si (100) substrates: I cellulose acetate. II phenolic resin, III caramel of starch and sugar mixture, and IV caramel of starch, sugar, and citric acid mixture. Details of the preparation of these organics are given below.

M. Ece et al./Diamond

and Related Materials

I. Cellulose acetate was dissolved in acetone prior to being applied on the silicon substrates and then the coating was heated in air at 300 “C for 30 min. II. Phenolic resin coating was prepared using a ratio of 12: 1 phenol (C6H,0H, 99.5% purity) and hexamine (C,H,,N,, 99.5% purity) respectively. They were mixed and then heated to 150 “C in air [lo]. Once this mixture solidified, it was dissolved in acetone. The solution was put on the silicon substrates using a spin-coater to obtain uniform coatings and then heated in air at 350 “C. III. 10 wt.% D( +)sucrose (C12H22011, 99.5% purity) was mixed with soluble starch (C,H,,O,), and then heated to 150 “C in air. Using the spin-coater the hot liquid mixture was applied to the silicon substrate to form a film. Afterwards the sample was annealed in air at 350 “C. IV. 23 wt.% D( + )- sucrose was mixed with 70 wt.% starch and 7 wt.% citric acid and then heated to 100 “C in air. This mixture was put on the silicon substrates as described above and annealed in air at 350 “C. All of these processes produced dark brown films which could no longer be dissolved in acetone. Experimental parameters used for the deposition of diamond films onto glassy carbon coated Si substrates are summarized in Table 1. Both glassy carbon films obtained by pyrolysis of organics and diamond films grown on these glassy carbon films were characterized by X-ray photoelectron spectroscopy (XPS), X-ray induced Auger electron spectroscopy (XAES), scanning electron microscopy (SEM), and Raman spectroscopy. It is known that direct pyrolysis of a phenolic resin and cellulose polymers results in the formation of glassy carbon [ 10,111. In order to prove that the sucrose based organics, III and IV, along with the phenolic resin and cellulose acetate turned into glassy carbon under diamond deposition conditions, we annealed these organics on silicon substrates in argon (20 mbar) at 800 “C for several hours, and subsequently the XPS and XAES spectra were recorded. The XPS measurements were carried out with a Perkin Elmer PHI 5400 ESCA system using Mg Kcc excitation (hv = 1253.6 eV). The electron energy analyser was operated at a pass energy of 35.36 eV, resulting in an energy resolution of 0.89 eV (FWHM Ag 3d,,,). The sampled areas were 1 mm in diameter. The photoelectrons were collected under a Table 1 The experimental

conditions

used for deposition

of the diamond

721

4 (1995) 720-724

take-off angle of 47” with respect to the surface normal. During the time of analysis the residual gas pressure in the system was (4-6) x lo-” mbar. In the XAES spectra, differentiation of the C KLL-region was accomplished using the Savitzky-Golay algorithm with a seven-point convoluting function [ 121. Raman spectra of the diamond films were recorded by a Renishaw Raman Microscope and a spot size of 1 pm was used. The system had a spectral resolution of 1 cm-‘.

3. Results and discussion The glassy carbon films formed on silicon by annealing the organics were dark brown and shiny. They were semi-metallic like graphite. Glassy carbon is often related to graphite since it contains almost 100% sp2 sites and consists of tangled ribbons of graphite [ 133. During heating of these precursor organics, a relatively large weight and volume change occurred. Therefore, cracks, blisters and distortions often appeared in the films. They may be due to different thermal expansion coefficients of the substrate and organic coatings. These adverse effects can be reduced by using sufficiently low rates of heating and cooling. Fig. l(a) and (b) shows XPS and XAES spectra of glassy carbon coatings on Si (C-F) and the diamond film grown on phenolic resin coated Si (G) respectively. XPS and XAES spectra of highly oriented pyrolitic graphite (HOPG) (A) and TC (B) are included for comparison. All four of the organic coatings after annealing showed very similar features, namely a shoulder at the high binding energy side of the C 1s (Fig. 1 (a)) which is caused by a n-n* transition of valance band electrons [ 141. For TC (B), this shoulder is also observed, and for HOPG (A), this loss structure is well resolved. As expected, this feature is not observed for diamond (G). The loss regions of the C-KLL Auger signals of the four organics show similarities as well. A detailed discussion of these loss features can be found elsewhere [ 15,161. The SEM images of the diamond films in Fig. 2 show that glassy carbon coatings promoted nucleation and growth of diamond on silicon surfaces which were not scratched with diamond powder. The number density of nuclei of each film is listed in Table 1 and they

films

: CH,

Substrate

G (“C)

T, (“C)

t (h)

H,

Cellulose acetate/Si Phenolic resin/Si (Sucrose, starch)/Si (Sucrose, starch, citric acid)/Si

1600 1875 1900 1950

970 970 900 1000

3.5 7.5 2.5 3.5

99.5 : 0.5 99.5 : 0.5 99.5 : 0.5 99.0: 1.0

P, (mbar)

N (cm-‘)

70 70 70 70

4x lo8 2 X 108 3 x 10’ 8x10’

Tr, T,, t and P, and N are the filament temperature (read by a disappearing filament type optical pyrometer), the substrate an IR radiation optical pyrometer), the deposition time, the total gas pressure, and number density of nuclei respectively

temperature

(read by

M. Ece et al./Diamond and Rrhted Muteriab 4 (IYYS) 720-724

722

C KLL \i

/.\ 50

40

30

Ea - E;‘*

20

10

[ev]

0

2‘20

240 Kmetic

260 Energy

280

300

[eV]

Fig. 1. (a) and (b) are XPS and XAES spectra of (A) HOPG, (B) TC, (C) phenolic resin/%, (D) cellulose acetate/L%, (E) sucrose and starch/$ (F) sucrose, starch, and citric acid/%, (G) diamond on phenolic resin/Si.

are comparable (in the range 3 x lo’-4 x lo8 cm-“). Uncoated and unscratched silicon substrates did not show significant nucleation (less than 1 x IO3 cm-‘). The Raman spectra of the diamond films grown on the glassy carbon coated silicon substrates are shown in Fig. 3. The 1332 cm-’ mode of diamond is clearly seen in all the films [ 171. Fig. 3(a), (b) and (d) have other prominent features, such as the broad shoulder at around which is thought to be related to highly 1490cm-‘, disordered regions of sp’ carbon [ 18-201. The fact that one can achieve nucleation and deposition of diamond on glassy carbon coated silicon substrates without diamond powder scratching (or biasing) is the most important result of this work. We believe that two competing effects take place when the glassy carbon coated silicon substrates are put in the CVD reactor. Firstly, atomic hydrogen, under high temperature deposition conditions, will etch this layer and at the same time hydrocarbons present in the chamber will start to deposit carbonaceous layers (diamond, diamondlike carbon, amorphous carbon). Secondly, the glassy

Fig. 2. SEM images of the diamond films deposited onto (a) cellulose acetate/%, (b) phenolic resin/%, (c) caramel of sucrose and starch (d) caramel of sucrose, starch and citric acid mixture. The insets in (b) and (d) are higher magification pictures of the same films.

mixture!Si.

M. Eceetal.JDiamondandRelatedMaterials4

(1995) 720-724

I’ 1200

-

723

I’

b)

I’ 1331.4,

I’

I

I-

FWHM=8.3

L

1332.2, I

FWHM=8.4

3

1000

-

800

-

600

-

400

-

Ki s : 2

i

I

t

I

I

I

I

l

2oI:

,I,

1600

1800

2000

1000

A(wave number) (cm-‘)

I

’

I

’

I

1332.6,

_ 4

’

I

I

’

1200

1400

1600

1800

2000

A(wave number) (cm-‘)

’

,

5000

7

FWHM=7.1

I

,

- d)

,

1331.6,

I

,

I

I

,-

I

I

FWHM=10.6

I

4000-

2

I

3000-

cd E ? 2000aI. z 1000-

0-

I 1000

1200

1400

1600

1800

2000

1000

A(wave number) (cm“)

I,

I

1200

1400

I

I

I 1600

I

1800

2000

A(wave number) (cm-‘)

Fig. 3. Raman spectra of the diamond films deposited onto (a) cellulose mixture/%, and (d) caramel of sucrose, starch, and citric acid mixture/Si.

acetate/$

(b) phenolic

resin/L%, (c) caramel

of sucrose

and

starch

carbon layers may also react with the underlying silicon substrate and form silicon carbide [ 211. When the glassy carbon layers are thick and continuous, some of them will remain in spite of both atomic hydrogen etching

and silicon carbide formation. Therefore, diamond partitles will have a chance to nucleate since more sites are available for diamond nucleation on graphitic carbon than on silicon [22,23].

124

M. Ew et ul.lDicumnd

and R&ted

4. Conclusions We investigated diamond nucleation and deposition on glassy carbon coated silicon substrates without diamond powder scratching. The glassy carbon coatings were obtained by pyrolysis of four different polymers. Nucleation of diamond was not homogenous on any of the samples. The coverage of the diamond films on all substrates varied from region to region. The films were not continuous owing to the non-continuous glassy carbon layers. The defects (long cracks, blisters and large holes) formed during the formation of glassy carbon as a result of a large weight and volume change had a direct effect on nucleation and the quality of the diamond films. We believe that uniformly thick and continuous glassy carbon coatings can enhance nucleation more effectively and in turn lead to good quality diamond films. Acknowledgements We would like to thank P. Alers for his help in Raman spectroscopy and Gunther Hobi for his technical assistance. Financial support of the present work by the Foundation (ProjectsSwiss National Science No. 21-30089.90) carried out under the auspices of the trinational D-A-CH cooperation of Germany, Austria and Switzerland is gratefully acknowledged. References [1]

K. Mitsuda, Y. Kajima, Sri., 22 (1987) 1557.

T. Yoshida

and K. Akashi,

J. Muter.

Muterids

4

i 1995) 720-724

and S. Hirayama. 7’hin 121 K. Tamaki, Y. Watanabe, Y. Nakamura Solid Films. 236 (1993) 115. 131 B.E. Williams and J.T. Glass. J. Muter. Rex, 4 (1989) 373. R. Gat, A.B. Anderson. S.P. 141 J.C. Angus, 2. Li. M. Sunkara, Mehandru and M. Geiss. Proc. Int. S:nzp. on Diunrontl untl Dianrontl Materials. Electrochemical Society Meet. Washington. DC. 1991, p, 125 and references cited therein. I51 B. Oral. M. Ece. T. Rogelet and Z.M. Yu, Uiunronrl R&t. sMatrr.. 2 (1993) 225. and B. Oral, Thrn Solid Films, 253 ( 1994) I 14. C61 Patscheider R. Polini, V. Sessa, M. Braglia and G. Cocito. c71 M.L. Terranova, Diumonrl Reht. Muter., I ( 1992) 969. 1x1 M..4. Brewer. I.G. Brown, P.J. Evans and A. Hoffman. ,+lpp/. Phr\. Lett.. 63 ( 12) (1992) 1631. 191 S. Prawer and C.J. Rossouw. .I. Appl. Plz~s.. 63 ( 1988) 4435. K. Kawamura and L.L. Ban. Proc,. R. SW [lOI G.M. Jenkins. Lontlon. Srr. A, 327 ( 1972) 501. Polyrnrric, Carbons c~trhor~ [Ill G.M. Jenkins and K. Kawamura. fihre, ,&m rd char, Cambridge University Press, Cambridge. 1976. and M.S.E. Golay. Anal. C/rem.. 36 (8) (1964) 1121 A. Savitrky 1627. [I31 J. Robertson, A&. Phy., 35 (1987) 317. 1141 D. Briggs, in D. Briggs and M.P. Seah (eds.). Prwtic,d Surfucr Antr/)ks. Wiley, New York. 1985 p. 364. El51 P.G. Lurte and J.M. Wilson, Surf. Sci.. h5 (1977) 476. 1161 J.C. Lascarich and S. Scaglione, Appl. Surf: %i.. 7X ( 1994) 17. 1171 S.A. Solin and J.M. Wilson, Surf. Sci., 6.5 (1977) 476. 1181 RI. Shroeder, R.J. Nemanich and J.T. Glass. Phrs. Ku. B. 41 (1990) 133x. 1191 A.M. Bonnet, Phys. Rev. B. 42 ( 1990) 6040. Y. Sate. M. Tsutsumr and N. Setaka, J. Mater. I201 S. Matsumoto, ki.. I7 (1975) 3106. 1’11 A.K. Green and V. Rehn. .I. J’~lc. Sci. 7‘rc~hnoi. A I (4) (1983) 1x77. [2] W.R.L. Lambrecht, C.H. Lee, B. &gall. J.C. Angus, Z. LI and M. Sunkara, Nature. 364 (1993) 6075. 1231 B. Oral and M. Ece, Dicrmond R&t. ,hbter., 3 ( 1994) 495.