The structure of acetylene flames for diamond synthesis

Diarnoml and Related Materials, 1 (1991) 19 24

19

The structure of acetylene flames for diamond synthesis Y. Matsui, H. Yabe and T. Sugimoto Central Research Laboratory, Mitsuhishi Electric Corporation, Tsukaguchi Honmachi 8-1-1, Amagasaki, ttyogo 661 (Japant

Y. Hirose Department o! Electrical Engineering and Electronics, Nippon Institute ol Technoh~gy, M(vashirocho, Minami Saitama, Saitama 345 (Japan)

Abstract An acetylene flame was photographed with an SIT camera through optical filters, and the signals were Abel transformed to obtain two-dimensional emission-intensity profiles. It was found that the intensive emissions of CH* and C* are localized at the feather boundary, while the OH* emission is localized at the intermediate zone. The structure of the acetylene flame was made clear: it consists of a carbon-radical-rich feather and an oxygen-radical-rich intermediate zone. The carbon-radical concentrations are approximately in equilibrium near the burner exit while they decrease almost linearly via interdiffusion and reactions with the oxygen-radicals in the intermediate zone. Numerical simulations including detailed gaseous and surface reactions reproduced well previous preliminary calculations showing that CH4 is rapidly produced in the boundary layer near the substrate followed by an increase xn CH~. Th~s result satisfactorily explains the measured dependence of growth rate on the substrate temperature and C2H2/0 2 ratio.

I. Introduction Hirose and Kondoh first reported diamond growth using acetylene oxygen (C2H2-O21 flames at atmospheric pressure with a linear growth rate of 100 2 0 0 # m h ~ [1]. The acetylene flame is the easiest and lowest-cost technique for diamond synthesis, and highquality optically transparent crystals have been deposited onto an inclined silicon substrate with a flow-rate ratio R =- C2H2/O2 = 1.0 1.05 [2]. To give an insight into the growth mechanism of the acetylene feather, we carried out gas analyses [3] with laser-induced fluorescence (C2 and OH) and mass spectrometry (C2H 2, H, and CO), as well as numerical simulations including both gaseous and surface reactions [4]. The gas analyses revealed that CO and H 2 are the main gaseous species, and the concentrations of C2H 2 and other carbon-containing radicals in the feather are approximately in equilibrium near the burner exit and decrease almost linearly towards the feather tip. It was also shown that a large peak in the growth-rate profile along the feather axis corresponds to the radical overshoot in the reaction zone of the premixed primary flame (inner cone). However, the surface of deposited diamond crystals, whose morphologies are a complex function of the substrate temperature T,, materials, and incoming radical fluxes, consists of various facets of diamond and graphitic components. The growth kinetics may be different for each surface, i.e. the diamond growth rate for a ( 1001 surface is reported to be about 1.7 0.9 times that of a (1111 surface [5]. However, a simplified sur-

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face reaction model (i.e. overall sticking probability) has been used previously [4], which found that the stable species (e.g. C H 4 and C2H 4) are rapidly produced in the low-temperature boundary layer near the substrate, followed by methyl radical formation due to the fast partial equilibrium of the rection CH4 + H ~ CH 3 + H 2. Moreover, it was also shown that the dependence of growth rate on T, and R is satisfactorily explained by the CH3 precursor model. In the present paper, we will give an overview of our previous studies. In order to clarify the flame structure, we will explain additional measurements of light emissions from the flame in the next section.

2. Imaging technique The acetylene torch and the gas supply system are the same as those used in previous studies [3, 4]. The flame shape is schematically shown in Fig. 1. The inner cone, 1 2 mm high, is stabilized on a concentric slit of width 0.2 ram, and the feather (or mantle) is formed for R = 1.0-1.2, surrounded by a blue secondary flame. The feather length was very sensitive to the value of R. When a water-cooled substrate was inserted into the feather region, diamond crystals were obtained on the substrate in the shape of the feather. The etching reaction was so fast in the intermediate and outer zones that diamond crystals of several micrometres disappeared within a few minutes. Optical emissions from acetylene flames have been investigated by many authors: typical emission spectra

c 1991--Elsevier Science Publishers B.V.

Y. Matsui et al. / The structure of acetylene flames ]'or diamond synthesis

20

been found that the strongest emissions are concentrated in the inner cone (h = 2 mm), while strong emissions of CH* and C* are localized at the feather boundary with OH* at the secondary flame. Figure 6 reproduces the previous LIF measurements of C, and OH radicals in the ground state for several heights h above the burner head. The uniform C~ profile is obtained near the burner, but the boundaries are diffused at higher positions. A trace of the overshoot at the inner cone is retained on the shoulder of the profile at h = 3 mm. OH was observed in the intermediate and outer zones, and the maxima are almost independent of height. The flame structure is schematically shown in Fig. 7: the feather boundary is characterized by bright non-luminous C* and CH*, which are activated by the chemiluminescent reactions between carbon and oxygen radicals (O and OH), e.g.

~- secondary flame outer zone /!/ -/~--- intermediate

/1/ /

zone

I J~

feather

/J//

,oon.e,

/ I I t / i n n e r cone ~(reaction zone) '~A hk<&~ slit

~

°=-"6mm

C2H2 + 02

Fig. 1. Acetylene torch and flame structure.

are shown in Fig. 2. The chemiluminescent emissions of OH*, CH* and C* are common to usual hydrocarbon flames, but CN* implies that the atmosphere diffuses into the flame through the secondary flame. This phenomenon is consistent with previous mass spectrometry measurements [3] which detected a large amount of N, in the intermediate zone. The present experimental apparatus is shown in Fig. 3. The acetylene flame was photographed with an SIT camera (400 × 484 pixels) equipped with an image intensifier (I.I.), using the respective interference filters for OH*, CH* and C*. The axisymmetric flame is favorable to image processing, and the signals were Abel transformed to obtain their two-dimensional intensity distributions. Typical results for R = 1.1 are shown in Fig. 4 for two different color gains: the left-hand side of each figure is the original image and the right-hand side shows the relative intensity. Figure 5 shows plots of the relative intensity profiles at three typical heights. It has

C2 + OH ~ CH* + CO C~H + O H ~ C *

+H20

However, OH* is in the highest temperature region and may be formed according to the reaction: O+H+M~OH*+M where M denotes the third body. The intense OH* emission from the inner cone may be due to the well known reaction: CH + 02 ~ OH* + CO, 3. Numerical simulation

A diamond surface is considered to be stabilized by hydrogen atoms, and the graphitization becomes signifi-

C2

C2

7OH

CH I

CH

C2

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/ .(

CN

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CH

OH \

250

300

350

400 Wave

'\

450 Iength

500 (nm)

Fig. 2. Typical emission spectrum from an acetylene flame.

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600

650

Y. Malsui el al. /' The .vtruclure o/acetylene flames lor diamoml svnthesix Frame

* + H ---~H(ad)

Memory SIT with

Tube I.I

Processor

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21

(s-l)

H(ad) + H--,, + H~

Is-2)

H(ad) -~ H -~ Hd ad)

(s-3)

H2(ad) + H ~ H ( a d ) + H,

(s-41

denotes the dangling bonds, and H(ad) and H2(ad) are H-absorbed and H2-physisorbed sites on the growing diamond surface respectively. The activation energy was calculated to be E , - 1 7 . 4 kcal tool ~ for Is-2) [6] and may be smaller for the other reactions (zero in the present easel. A CH 3 radical is considered to be a diamond precursor and chemisorbed on the dangling bond prior to its incorporation (E 5 - 5 kcal tool ' [4]) into the diamond crystals C(dia}. Other carbon radicals (represented by C) are also chemisorbed on the dangling bond to form graphitic components C(gra), but are quickly etched by hydrogen and oxygen atoms and desorbed as CH 4 and CO respectively. where •

Interference

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2...... (A)

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C* 5165 CH* 4315 OH* 3080

30 20 100

65 40 25

* + CH~-~C(dia) + H(ad) + H~ * + (" - , C(gra)

Fig 3. Experimental apparatus for image processing.

(s-6)

C(gra) + 4H ~ • + ('H a C(gra) + O - ~ . cant when the surface density of the dangling bonds increases with increasing substrate temperature. Hydrogen coverage H(ad) may be determined by the balance between the addition of a hydrogen atom to the dangling bond (s-l) and the subtraction of hydrogen atoms Is-2). Hydrogen atom recombination on the surface is also considered by reactions (s-3) and Is-4) with a sticking probability of 0.01 [4],

OH*

CH*

(s-5)

(s-7)

+CO

(s-g)

C(gra) + OH ~. H( ad~ + CO (s-9) The calculation procedure and the gaseous reactions are the same as described previously [4], and the surface reaction rate Y, for the ith species is expressed as } ' , - I/4c, n, [.v],4 e x p ( - F / R T s ) where r, and n, are the thermal velocity and the nearsurface concentration of the ith species respectively,

C2"

ii ii

Fig. 4. Typical emission profiles for R = 1.1 with two different gains: original images (upper) and intensity distribunons (lower).

Y. Matsui et al. / The structure o f acetylene flames for diamond synthesis

22

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reactions were determined by comparisons with the measured [4] and calculated CH~ concentrations on the substrate and the growth rate respectively, for one typical condition of R = 1.1 and T s = 1250K. The rates for other reactions were assumed to be fast (A = 1.0 and E = 0).

10 b3 [.L ..J

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28 feather

5 Radial

Distance

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I0 (mm

outer zone

/2,

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Fig. 6. LIF measurements fo C 2 and OH radical concentration profiles [4]. and [x] is the fractional coverage o f the surface site. The constants A, E and R are the pre-exponential factor, activation energy and gas constant respectively, and represent the reaction rate o f the endothermic reactions (s-2) and (s-5). The values o f A for these

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lated Fourier transform IR spectroscopy in hotfilament chemical vapor deposition because the k~C molar fraction in the diamond film correlated with that of CH~ in the fuel gas and not with that of C,H.. However, Goodwin [10] carried out a two-dimensional simulation of diamond growth, supporting the QHe-precursor model, while rejecting the CH~-model on the basis of the result that production in the boundary layer was insufficient to account for the high growth rate obtained for an atmospheric pressure arc jet [I 1]. However, an anomalously large growth rate should be expected for acetylene flames from the C~H~ concentrations and his sticking probability of 10 ~. Therefore. other candidates or effects (e.v,. electron bombardment) may be needed to explain the growth rate l\~r intensive arc jets. The Rideal-type surface reactions have been considered throughout the present calculation, but the Langmuir-type reaction between C - - H bonds (s-2') may play a role m hydrogen subtraction from the surface, because thermal desorption is usually referred to as the onset of the graphitization: H(ad) + Hlad) ~ * + * + H,

I0

0

-

~ 0,5 ~ Distance

23

The structure o/acetylene/lames /or diamond svnthe,~i,s

1.0

(s-2')

Even in such a case. however, the calculated results are not significantly changed, because the activation energy for Is-T) is in the same range [12] as that used in the present calculation.

from Substrate (mm)

Fig. 8 Typical example o f calculated concentration profiles in the boundary la~er for R = I.I and T, = 1250 K.

Figure 8 shows the concentration profiles of the typical species for R = 1.1 and a boundary layer thickness of 0.1 ram. reproducing well the previous preliminary calculations. The dependence of growth rate on 7-, and R is also reproduced, agreeing well with the measurements. The detailed reaction scheme and the good agreement between the measured and the calculated CH 4 concentrations at the substrate are explained in ref. 4.

4. Discussion

By' means of simulations including both detailed gaseous and surface reactions, it is verified that the CH,-precursor model is favorable for explaining the measured dependence of growth-rate on T, and R. Moreover, the present model was able [4] to explain the temperature dependence of the growth rate with a microwave plasma torch found by Mitsuda et al. [7]. Recently, Pederson et al. [8] calculated surface adsorption heights and relevant potential curves on a diamond (111) surface, and showed that CH 3 is more important than acetylene-like adsorbates in the first stage of epitaxial growth. Chu et al. [9] supported the CH3-precursor model from gas analyses by matrix-iso-

5. Conclusions

The structure of the acetylene flame for diamond synthesis has been made clear by previous gas analyses and the present image processing; it consists of a carbonradical-rich feather and an oxygen-radical-rich intermediate zone. The carbon radical concentrations show an overshoot in the reaction zone of the primary flame, and are approximately in equilibrium near the burner exit, decreasing almost linearly via interdiffusion and reactions with oxygen radicals in the intermediate zone. Numerical simulations were carried out including both surface reactions on the growing diamond crystals and gaseous reactions in the low-temperature boundary layer near the substrate. It was found CH4 is rapidly produced in the layer, followed by an increase in CH 3, which is enough to explain the diamond growth rate. The CH~-precursor model was found to be fi~vorable t\~r explaining the measured dependence of the growth rate on the substrate temperature and C~H2/Ox ratio.

References I Y, Hirose and N. Kondoh, Extemted Ah.s'tracts 3&h Spr#~g

Meet. o/ the Japanese Societ.v t!/' ,4pplied Physic,~ and Related Societies, Tokyo, Alarch, 198& Japan Societ~ of Ap-

plied Physics, Tokyo, 1988, p. 343. 2 Y. Hirose, S. Amanuma, N. Odaka and K. Komaki,

24

Y. Matsui et al. / The structure of aceo,lene flames for diamond synthesis Extended Abstracts Spring Meet. of Electrochemical SocieO,, Los Angeles, CA, May, 1989, Electrochemical Society,

tended Abstracts, Technology Update on Diamond Films, San Diego, CA, 1989, Materials Research Society, Pitts-

Pennington, NJ, 1989, p. 114. 3 Y. Matsui, A. Yukki, M. Sahara and Y. Hirose, Jpn. J. Appl. Phys., 28(1989) 1718. 4 Y. Matsui, H. Yabe and Y. Hirose, Jpn. J. Appl. Phys., 29 (1990) 1552. 5 B. V. Spitsyn, L. L. Bouilov and B. V. Berjaguin, J. Cryst. Growth, 52 (1981) 219. 6 D. Huang, M. Frenklach and M. Moroncelli, J. Phys. Chem., 92 (1988) 6379. 7 Y. Mitsuda, T. Yoshida and K. Akashi, Rev. Sci. lnstrum., 60 (1989) 249. 8 M. R. Pederson, K. A. Jackson and W. E. Pickett, Ex

burgh, PA, 1989, p. I57. 9 C. J. Chu, P. H. Hauge, M. P. D'Evely and J. L. Margrave,

Extended Abstracts Diamond Technology Initiative Syrup., Crystal CiO', VA, 1989, W7. 10 D. G. Goodwin, Extended Abstracts, Technology Update on Diamonds Fihns, San Diego, CA, 1989, Materials Research Society, Pittsburgh, PA, 1989, p. 153. 11 S. Matsumoto, Extended Abstracts, Diamonds and Diamond-like Materials Synthesis, Materials Research Society, Pittsburgh, PA, 1988, p. 119. 12 M. Balooch and D. R. Olander, J. Chem. Phys., 63 (1975) 4772.