Growth behavior of boron-doped diamond in microwave plasma-assisted chemical vapor deposition using trimethylboron as the dopant source

~:ELATED D

TER|AL$

ELSEVIER

Diamond and Related Materials 7 (1998) 88.-95

Growth behavior of boron-doped diamond in microwave plasma-assisted chemical vapor deposition using trimethylboron as the dopant source H i d e a k i M a e d a ~, K y o O h t s u b o a M a s a n o r i K a m e t a ", T a k e y a s u S a i t o ~, K a t s u k i K u s a k a b e a S h i g e h a r u M o r o o k a "'*, T a n e m a s a A s a n o t, Department o.f MateriaL~" Physics ami Chemistry, Graduate School o.f Engineering, Kyushu University, Fukuoka 812-81, Japan b Center for Microelectronic S)'stems, K)'llshu h~stitute of Teclmoh~gy, Iizuka 820, Japatt Received 16 April 1097; accepted 28 Au~,us~ 1997

Abstract

Cubo-octahedral diamond cr;:stals were tbrmed by microwave plasma-assisted chemical vapor deposition of methane and hydrogen on a Si(l(l(J) wafer. Trimethylboron was then added to the gas phase as the boron source, and diamond was homoepitaxially det,osited on the I!001 and Ii I ! ] of lhe seed crystals, The growth rate. which was determined from geometrical changes in the crystals, was affected by the type of diamond faces, as well as tlae boron to carbon (B/C) ratio in the gas phase. The rate decreased with increasing the B.'C ratio at concentrations below 400 ppm, and was independent of the B/C ratio at

concentrations in excess of 500 ppm. Furtl',ermore, boron-doped diamond fih'ns were formed on single-crystalline (100) and ( I I ! ) diamond substrates by varying the B/C ratio in the gas phase, and depth profiles of boron and hydrogen were determined by secondary iota mass spectroscopy. The boron content in the (100) and (l ! l) diamond increased with increasing B/C ratio in the gas phase and was 9 x l0 t-' cm 3 and Ix ilJt'JClrl"~. respectively, when the B/C ratio in the gas phase was 2000 ppm. ,t~ 1998 Elsevi,:r Science S.A. K,'vw,,'d~," Boron doping; Dianaond lilms: Growlh: Homocpitaxy: Polycrystalline: SIMS

i. Int,'Jduction Diamond possesses unique properties with respect to band gap {5.48eV), breakdown field tl 20×10 -~ Vm-~}, electron mobility {0.22 m ~ V - t s-t), hole mobility {0.16 m2V - t s- t} and thermal conductivity (2 k W m - ~ K - ~ ) . These unique properties rwake diamond a very promising candidate for use in electronic devices which tkmction under extreme conditions, or. Ibr high power and high frequency devices [I,2]. In order to achieve diamond-based semiconductors, a deposition technique of p- and n-type single crystal films is required. Although low resistive n-type diamond films and large area single crystalline diamond films have net yet been synthesized, p-type polycrystalline and homoepitaxiai diamond films have been successfully tbrmed [3 .... 10]. Ttte most prevailing gas source for boron doping is diborane (B:H~,h which is both extremely poisonous and explosive. Fujimori et al. [3] syn~,hesized polycrystab * Corresponding author, Tel: + ~1-92-642-3551: Fax: + 81-92-651-5606; e-mail: smorotcf(umbox.nc.kyushu-u . . . . ac .iP 11925-9635 98519.110 ~3:1998 Elsevier Science S.A. All rights reserved. • Pll S0925-96351 97)00192- I

line boron-doped diamond tilms oll silicon as well as borort-dopcd homoepitaxial diamond tiims on natural diamond using diborane in conjunction with microwa~,e plasma*assisted chemical vapor deposition (MPCVD). Mort et al. [4] also reported the synthesis of p-type polycrystalline diamond fihns on a silicon wafer from a mixture of CH4, H,, 02 and B,H,, using hot filament chemical vapor deposition (HFCVD) and obtained a doped film containing 10ts--10S°cm-3 of boron, but they failed to mention the relationship between the boron concentration in the gas phase and that in the diamond film. Okano et al. [5] used a boron alkoxide. a safer reagent than diborane, as the dopant in HFCVD. and successfully formed the p-type semiconducting .!iamond film. Using trimethylborate as the dopant, Koidi and co-workers [6,7] also in~,estigated the eft'cot of boron doping on the epitaxy of diamond. They found that boron was incorporated preferential[./ in the { l l l l growth sector and that the B/C ratio in the film was one order of magnitude less than that in the gas phase. However, alkoxide contains oxygen whose effects on the formation of epitaxial diamond is not completely uo.derstood.

H. Mm,da et al. / Diamond and Rehtted Materials 7 ~ 199A'1 88- 95

Recently,

Cifre et al. [8]

used

trimethyll~9ron

[B(CH3) 3, hereafter referred to as TMB] and formed

polycrystalline diamond films using the MPCVD method. Polo et al. [9] also synthesized polycrystalline diamond films using TM,3 by the HFCVD method. TMB has a boiling point of - 20 C and is not reported to be highly toxic. They investigated growth rates, boron concentration in the film, crystallinity, electrical properties and morphologies of diamond crystal faces formed with TMB. However, more detailed studies, focusing on the differences in the diamond faces, needs to be systematically investigated. For the synthesis of highly oriented diamond films on substrates other than diamond crystal, the control the crystallographic structure of diamond is essential. As reported by Spitsyn et al. [11], the crystal habit of chemical vapor deposition (CVD) diamond varies from octahedral to cubic via cubo-octahedral forms, depending on the gas phase composition, deposition temperature and other factors. Clausing et al. [12] described such morphological changes by introducing a relative growth rate I/I 1001/'VII I1 ] [referred to as R, hereafter], x~here V11001 and I/~ i i 1 } are the growth rates on the dimnond I I00} and {111}, respectively. Based on the relative growth rate, Wild et al. [i 3,14] formed a highly oriented diamond thin film (HOD) textured on a (160) silicon wafer. The relative growth rate was determined experimentally assuming that it remained constant during the entire period of nucleation and growth. However, the conditions during the growth stage are often different fi'om those of the nucleation stage. In such case,;, the rela:ive growth rate cannot be calculated fi'om the idiomorphic shape and can be determined by measuring the change in crystal size over a given time span during which the reaction conditions remain unchanged. Maeda ct ~11.[15] developed a reliable method to determine the growth rates of the diatnond II00} and I!l II by obse;ving the change in the shape of identical crystals and synthesized an HOD lilm on a carburized Si1100). in the present study, the growth rates on diamond I I00} and I I Ill by MPCVD from a mixture of methane, hydrogen and TMB were determined as functions of substrate temperature and TMB concentration. Boron concentrations in homoepitaxially grown diamond films were quantitatively deterrnined, and the relationship between boron content in the solid phase and the boron to carbon cok:,:entration ratio (B,/C ratio) in the gas phase was investigated. The effect of ,~¢tded boron on the surface morphology of the diamond |iinlS was also examined.

2. ExpcrimcnIalprocedures Diamond growth was c:~,rricd out by MPCVD u,~;ing CH4 and TMB diluted in Fla. Details of the MPCVD

89

apparatus used have been described in a previous report [16]. The conditions used for the diamond growth are summarized in Table 1. The pressure was maintained at 5.3 kPa throughout the present experiment, and the concentration o i ' C H 4 in H 2 was 1.0%. A mirror-polished p-type Si(100) wafer, pretreated as reported previously [17], was used as the substrate. The substrate temperature was measured by an optical pyrometer assuming an emissivity of tmity, and maintained at 600-900 ~C by adjusting the microwave power in the range of 200-800 W. Initially, regular cubo-octahedral diamond crystals of ca 1.5 ~tm in size were observed on the Si(100) substrate among diamond crystals formed at a population density of 10s cm-2. Five diamond cryqals which contained a (111) face parallel to the Si(100} were chosen as substrates for the growth stage. The change in size of the identical particle before and after the growth was measured by a field-emission scanning electron microscope (FE-SEM, Hitachi S-900) with no conductive coating on the specimen. Boron-doped diamond was also deposited homoepitaxially on polished (I00) and (!11) diamond single crystals synthesized by the high pressure and high temperalure method. The aeposition temperature was kept at 750 C , corresponding to 420-430 W of the input m~crowave power. All other conditions were the ,~tme as those shown in Table i. Depth profiles of boron and hydrogen in the diamond films were Jetermired by secondary ion mass spectroscopy tSlidS, Atomica SIMS-4000M) using ox~g,c, ions. t h e depth of the ,etched area was nlcasurcd with a surfz,ce prolilomctcr.

3. Results and discussion 3. i. Dt, termimttion o.]growth raft'

A regular cubo-octahedral seed crystal nlah'ttains its original morphology during tke growtll o n b when tbc relative growth rate, R, is V3:2 (% 0.87 ~ [ 12 ]. ~,t other R values, the growing crystal undergoes continuous changes in morphology [13,14]. Fig. i schem;~tic;diy illustrates the growth of a rcgtflar cubo-~ ctahccar;~l crystal lbrmed on a (10()) substratc. The .side view indi:~ltcs the geometric relationship on the vertical pkme A A*.

"l;iblc I MPccVD condiiions tiscd H,.:

I~O c m '~ rain '

CI-I~ T M B Cfi~:

I c m -~r a i n ' 0 20!)0 p p m

Substn'atc temperature: Input microwave po~er: Pressure:

~{I;) q(10 (' 200 ~lJ() W 5. • k P~

H. Maeda et ai. / Dianlond and Rekited Materials 7 (1998) 88-95

90

A

/ .,,~--: ~. I I

"',_

"--,I_ql

"1"'I ..... f ..... i'-~---; !

""

Ii

I

i

i~..

I. . . . . . . . . i. . . . .

:'1 /

-I~,..~ : ,

8(m) ,, ,

(Ill)

!

,'

T(

R ~ v~J/2

which passes through the center of the crystal. The initial crystal, indicated by the dashed line, grows as described by tile solid line. The thickness deposited on the (100) and ( i l l ) faces during this period. T(100) and 7"( I i I ). is obtained from tile cha,lge in crystal size on the plane view. 6(100) and 6(ll I J. The relative growth rate must be calculated differently for R >_V~-3/2 and g <_~3/2. For R>_ V~/2 ( 1)

(2)

For R ~ ~ / 2 T(lll) .... T( 100)= ~

6(1111 tan(½02) I

6(111) tan(½02)-6(100) sin 0,

', ,,

6ml) -~,

"

.

,

~

L

R a v'J/2

Fig, I. G e o m e t r i c a l r e l a t i o n s h i p o f g r o w i n g crystals for (a)

6(111 ) tan(½0,.,)+ fi(100) sin 01

o

............... ii!iiiii{i!i@!iliiiiiiiiiiiiiii!iiiliiiiiii!ii ......Siii,iiraii .......iiii!iiiiiiiiiiiii!ii!iiiii!ii!i!iii!!i!ii!ii!iii!iii!iiiiii!i ,..................~ .....................~

I

ii

\,

.ii!ii ~!iii:ii~!!!iii!i!ii!i!}!i!ii!ii!iiiii?% !!i!i ~;~i~i;:'~ :~iiiiliii i iiiiii!iiiiiiiiiiiiiliiliii!iiiiiiiiiiiiiii

T( 100)= ~

!

i

\~ ,

T( l l l)---6( l I i ) tan(~fl2)

/.L

~r- A* ','9

i

~--~, \

}

i

I '-4"

., ...i." j

':i

~,,too)

Ii

! J.,,,r--"~" ..--

A-[ ....... ,. . . . i1"^~W~ ?,. i I , ij,,c-~ i',,

:."il .... f

(31 (4)

where 0t = c o s - t ( - I/V~) and 02 - c o s - 1 1 - ! / 3 ) and are indicated in Fig. I. Details of these measurements have been published previously [15]. As long as a cubo-octahedral crystal exposes the top surface parallel to the substrate, the growth rates of the (100~ and (l 1l) can be obtained

R>_V~3/2 a n d ( b ) R<_V~3/2.

independently by SEM observation from the vertical direction. Theoretically, the method developed is valid for a growth rate between zero and infinity, since it is calculated directly from the change in size of cubooclahedral crystals. Fig. 2 shows the homoepitaxial {I00} and {1111 growth rates of diamond crystals. Growth rates were different for the { 100} and {111 } faces. The apparent activation energy for these faces were 28-40 kJ mol- i and 80-100kJ m o l - l , respectively. Data on the activation energy for the growth rate of boron-doped diamond faces appear not to have been reported to date. The activation energy for the growth rate of nondoped MPCVD from a mixture of methane and hydrogen was ca 40 kJ mol- 1 [18,19], which is equivalent to the value for the nondoped (100) growth in the present study. As indicated in Fig. 3, the growth rates initially decreased with increasing B/C ratio in the gas phase and were independent of the B/C ratio at ratios >500ppm. Gonon et al. [20] synthesized B-doped polycrystalline diamond using diborane by MPCVD from a 0.5% methane diluted in hydrogen at 730 °C and found that the dopant concentrations between 10 to 2000 ppm in the gas phase decreased the growth rate from 150 to 80 nm h-l. This tendency was contrast to that of the other dopant such as phosphorus or nitrogen, which were reported to promote the diamond growth [21-23]. Additional studi s will be required to clarify this point.

H. Maeda et aL / Diamond and Related Materials 7 (1998) 88-95 Tempetature (*C)

Tempetature (°C)

900

800

700

600

!

!

!

!

10 3

91

9OO

10 3

•

e

8OO -

7OO

!

6OO

!

!

(a) {1o0}

(b) { l l l }

,5=

10 2 0.8

' 0.9

,

I

,

1.0

I

1. i

,

10 2

1.2

1000/T(K-]) 0.1

I

I

!

0.9

i.0

I. 1

!.2

1000/T (K l ) Fig. 2. Effects of B/C ratio and substrate temperature on (a) {100} and (b) { I 11 } growth rate. B/C ratio: O , 0 ppm; O, 200 ppm; ~ , 400 ppm; m 1000 ppm: A, 2000 ppm.

Fig. 4 shows that the R value rapidly decreases with increasing temperature in the range of 600-750 °C. At higher temperatures, however, it decreases moderately with increasing temperature and the {11 t } growth becomes predominant. The broken lines in Fig. 4 show the range where the idiomorphic shape method is applicable [12]. The method developed in the present study is usable even when the R value exceeds V~. As shown in Fig. 4, the R value is not altered by the B/C ratio, when the deposition temperature exceeds 750 ~C. This trcnd is convenient for the synthesis of boron-doped diamor~d because face-selective growth is controllable by varying the substrate temperature, irrespective of the B/C ratio. Fig. 5 shows a comparison of surface morphologies 600

,

,

,

•

of the polycrystalline diamond thin films formed at 750 °C, where the R ~s close to V~/2, as indicated in Fig. 4. Fig. 5a reveals the morphology of nondoped diamond films, and Fig. 5b and c reveal the morphology after the further deposition of a 1 I.tm thick film on the nondoped polycrystalline diamond. Although the B/C ratio in the gas phase was increased from 0 to 1000 ppm, the grain size of the diamond films remained unchanged. This was not inconsistent to the result shown in Fig. 4, and suggested that the seconda,, nucleation or twin formation led to the change of film structure and morphology did not occur on the polycrystalline diamond substrate. However, Locher et al. [6] reported that the structure and morphology of the boron-doped polycrystalline film were dependent on the texture of

,

I

I

I

~r'

I 'r

'l

o

500 N

400

E

.~

o

300

>

3

>

2

---~-..... a {J~F

200

o,

I{X)

l

0

.

500

!1

1000

o

I

1500

,

I

2000

2500

0 50(

,

I

600

I

I

700

i

I

800

~

.....

•

900

•

- -

000

B/C in the gas phase (ppm) Temperature (°C) Fig. 3. Effect of B/C ratio on growth rate at 800 C . [g, (100) growth of homoepitaxial film; D, { 1001 growth of crystals: A, ( i ! ! ) growth of homoepitaxial film; ,~, { i I ! } growth of crystals.

Fig. 4. Effect of temperature on relative growth rate. B/C ratio: O. 0 ppm: @, 200 ppm; [], 400 ppm; [I, 1000 ppm; A, 2000 ppm.

92

H. Maeda et al. / Dutmond ami Rektted Materials 7 (1998) 88-95

(a) Non-dO

~I|)

I

!.5 ,ttm

I

I-

1.5 ~tm

!

I

1.5 .tun

I

Fig. 5. Boron concentration dependence on surface morphologies of polycrystalline diamond. Deposition temperature=750 :C. (a) Nondoped (before deposition); (b) nondoped (after deposition): and (c) boron-doped (B/C = 1000ppm). Growth on the (100) diamond proceeds by layer and layer mechanism, and that on the (111 ) occurs by a multi-nucleation mechanism [24-26]. The boron doping appeared to decrease the nucleation rate and result in a smoother (l 1 l) surface. Wang et al. [27] also reported that the average surface roughness of each crystal decreased from 28 ~, for a B/C ratio of 4 ppm to 18 for a B/C ratio of 400 ppm. This is consistent with the findings herein. From the qualitative aspects, the effect of TMB on the growth behavior of diamond was almost similar to that of diborane. However, recent works indicate that

the polycrystalline diamond used as the substrate. They found that the boron doping led to a deterioration of the (100) textured film, whereas the structure and morphology of the ( 110)/(111 ) textured film remained unchanged by the boron doping. This result cannot directly compare to our result, because Koidl et al. used trimethylborate [B(OCH3)3] as the doping source. The effect of the boron-oxygen interaction on the structure and morphology of growing film has not been clarified. Fig. 6 shows the effect of boron doping on the surface roughness of the seed crystal. The roughness of the ( I l l ) face was especially improved by the doping. ta~ Xtm-tl~ped

....

......

I

(b)

I

120 nm

"t~~ r t l , ! - t ! t ~' p;-e t ,

' "L"b't"~

=

4(10 I)pm) .....

I

I

120 nm

Fig. 6. Surface morphologies of diamond ( I ! l Depositiontemperature= 750 C. Ca) Nondoped and (b) boron-doped ( B/C =400 ppmL

H. Maeda et al. / Diamond and Related Materials 7 (1998) 88-95

electrical properties of diamond are considerably different between diamond films doped with TMB and with diborane [28]. Therefore, the role of TMB for the formation of p-type diamond should be characterized thorough the quantitative comr~arison with that of diborane in order to elucidate the effectiveness of TMB as the boron dopant for CVD diamond.

1020 l

,..,..1

,

.

, . , . r l

(100) 1018

0

Fig. 7a and b show the depth profiles of boron and hydrogen in diamond formed homoepitaxially on the (100) and ( 111 ) diamond substrates, respectively. Yasu et al. [29] reported that surface morphology of a homoepitaxial (100) diamond, doped with diborane, was smooth when the methane concentration was 1%. Thus, we chose a methane concentration of 1% and a substrate temperature of 750 °C. The B/C ratio in the gas phase was increased at hourly intervals from 0 to 2000 ppm. Accordingly, the vertical distribution of boron content in the diamond also varied in a stepwise fashion. Fig. 8 indicates the effect of B/C ratio in the gas phase on the boron content of the diamond. Quantitative analyses for boron were conducted by SIMS analysis

.

e~ @

1017

10 16

|

i

,

, , ,..||

.

,

,

.i.,

b

!00

1000

i0000

B/C in the gas phase (ppm) Fig. 8. Relationship between boron concentration in diamond and B/C ratio in the gas phase. Deposition temperature = 750 C .

t

,

"

10 4

i

I0 19

10 3 ,... rj~

oE

......... -"

¢¢J

H

I018

10 2 ~

"~ 1017 1016

,

10 19

'~

,

.

(111)

3.2. Depth profiles of boron and hydrogen

1020~

93

1o' B/C (ppm)

B

2000i 1000 i 600 i 400 :: ; ~; ; t ; 0.5 1.0

200

1 ',, 1.5

0

~ '

,=,

10 0 ~

Substrate I 2.0

2.5

Depth (tam)

10 21

I

'

I~

~

I

10 5

(b) ( l i d

E @

8

10 20

10 4

10 19

10 3

10 18

10 2 "-"

IOI7

10 I

I0 16

8

B/C (ppm) 2000

1000

B 600 i 400

i

I;

0.5

1.0

:: 200

i

:i

;

1.5

0

~

I0 o

Substrate

I

I

i

2.0

2.5

3.0

3.5

Depth (..m)

Fig. 7. Depth profiles of boron in homoepitaxial (a) (100) and (b) { ! i I ) diamond films. Diamond was deposited at 750 C for 1 h for each B/C ratio.

94

H. Maeda et al. / Diamond and Related Materials 7 (1998) 88-95

using standard "substrates, which were prepared by implantation of tlB onto (100) and (111) diamond substrates synthesized by a high pressure and temperature process. The dose was in the range of 5 x 1013 and 2 x 1015cm -a, and 11B was implanted at 200 keV. The concentration in the solid phase was determined by the reactive sensitivity factor calculated from standard samples. When the B/C ratio in the gas phase was 200-2(100 ppm,. the boron content in diamond (100) and diamond (111) was proportional to the 1.5 and 2.0 power of the B/C ratio, respectively. Gonon et al. [20] evaluated the incorporation efficiency of boron in polycrystalline diamond films using diborane as the boron source, and reported that the boron concentration was proportional to the 2.0 power of the B/C ratio in the gas phase in the range of 10 and 10 000 ppm. When the B/C ratio was 2000 ppm, the polycrystalline film contained boron at 3 x 1019 cm -3. Fujimori et al. [3] determined the boron concentration in polycrystalline and homoepitaxial (100) diamond films doped with boron. Their data indicated that, when the B/C ratio was 2000 ppm, both diamond films contained boron at concentrations at 1020cm -a. Miyata et al. [30] synthesized a homoepitaxial (100) diamond film from methane and diborane diluted in hydrogen. The boron concentration in the film was 3 x 1020cm-3 at a B/C in the gas phase of 4000 ppm. These values are, when compared at the same B/C ratio, one order of magnitude higher than the values which were obtained from a mixture of methane, TMB and hydrogen in the present study. The growth rates of doped diamond layers were calculated from the thickness of the film, as determined by SIMS. The data are in agreement with those obtained by the seed crystal method, as indicated in Fig. 3. The hydrogen content in the homoepitaxial (100) diamond formed in the present study was at the same level as in the diamond substrate, synthesized by the high temperature and high pressure method, but that in the (111) lace was higher than the content in the substrate. Both boron and hydrogen were enriched in the diamond ( 111 ). This trend is commonly observed for a variety of other impurities. A larger number of nuclei were formed on the (!11) face during the growth stage than on the (100) face, suggesting that the probability for inclusion of impurities is higher on the former face.

with increasing amount of TMB at a B/C ratio below 400 ppm and were nearly independent of the B/C ratio in excess of 500 ppm. Boron content in the diamond increased with increasing B/C ratio. Boron content increased with increasing B/C ratio in the gas phase and boron concentration in the diamond (100) and (111) were 9 x 1017 and 1 x 1019 cm -3, respectively, when the B/C ratio in the gas phase was 2000 ppm. Boron is incorporated in (111 ) diamond more readily than in (100) diamond.

Acknowledgement This study was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan, and the Research Project for Fundamental Engineering of CVD organized by Professor Hiroshi Komiyama, The University of Tokyo. Silicon substrates were supplied by Sumitomo Sitix Corporation. The advice of Dr Toshihiro Ando of NIRIM is gratefully acknowledged.

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[21 K.E. Spear, J.P. Dismukes, Synthetic Diamond: Emerging CVD Science and Technology. John Wiley, New York, 1993.

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[10] A. Masood, M. Aslam, M,A. Tamor, T.J. Potter, Appl. Phys. Lett. 61 (1992) 1832.

[111 B.V. Spitsyn, L.L. Bouilov, B.V. Derjaguin, J. Cryst. Growth 52 (1981) 219.

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4. Conclusions Cubo-octahedral diamond crystals were formed on a Si(100) surface by MPCVD from a mixture of CH4 and H2. The substrate was then subjected to homoepitaxial growth in the presence of a gaseous mixture of CH4, H2 and TMB, and growth rates on the {100} and {111} faces were obtained from the changes in the geometries of seed crystals. Growth rates were decreased

[131 C. Wild, P. Koidl, W. MilUer-Sebert, H. Walcher, R. Kohl, N. Herres, R. Lecher, R. Samlenski, R. Brenn, Diamond Relat. Mater. 2 (199~,) 158. [14] C. Wild, R. Kohi, N. Herres, W. MtiUer-Sebert, P. Koidi, Diamond Relat. Mater. 3 (1994) 373. [15] H. Maeda, K. Ohtsubo, M. Irie, N. Ohya, K. Kusakabe, S. Morooka, J. Mater. Res, 10 (1995) 3115. [161 H. Maeda, S. Masuda, K. Kusakabe, S. Morooka, J. Cryst. Growth 121 (1992) 507. [171 H. Maeda, M. lrie, T. Hino, K. Kusakabe, S. Morooka, J. Mater. Res. 10 (1995) 158.

H. Maeda et al. / Diamondand Related Materials 7 (1998) 88-95

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