Improved surface morphology of sulfur-dopedhomoepitaxial diamond films by plasma CVD

method with SF6 Grading–Doping profile

Sachiko Fujii*, Shigeru Hino, Takeshi KobayashiDepartment of Physical Science, Graduate School of Engineering Science, Osaka University,

1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan

Abstract

Diamond films were homoepitaxially grown on type-Ib (1 0 0) diamond substrates by the microwave plasma CVD method at

800 8C with and without doping of S and compared with each other. Source gas was a mixture of H2 þ CH4 and a dopant gas of

SF6, the doping amount ranging up to S/C ratio of 24,000 ppm. The FE-SEM image taken from grown films clearly revealed a

morphological degradation when SF6 was doped uniformly during growth, and the more the doping amount, the more

degradation developed. The SIMS observation exhibited a marked pile-up of S atoms at the interface between the grown layer

and substrate, which was responsible for the morphological degradation. By introducing the Grading–Doping (starting from

non-doping), a very smooth surface has been successfully obtained.

# 2003 Elsevier Science B.V. All rights reserved.

Keywords: Diamond film; Sulfur-doping; Epitaxial growth; SF6; Surface morphology; CVD

1. Introduction

From viewpoints of diamond electronic application,

n-type diamond film growth has been attracting much

attention from researchers all over the world because

n-type diamond film is indispensable to achieve and

establish the p–n junction, bipolar transistor, highly

efficient electron cold-emitter, etc. In other words,

advantageous diamond application cannot be hatched

until n-type diamond films are in our hands. Contrast-

ing with the easy doping of acceptor boron, however,

there still has been a long way for the donor doping in

diamond films except for a few successful experimen-

tal reports.

So far, nitrogen is known as n-type donor, but the

donor level is so deep that N-doped diamond film is

semi-insulator [1]. Lately, phosphor and sulfur have

been employed as donor impurities of the chemical

vapor deposition (CVD) of diamond films and actually

realized the n-type conduction, to some extent, even at

the room temperature [2,3]. In a similar conceptual

way, sulfur ion-implantation to the diamond film has

been carried out by Katoh et al. and some researchers.

The implanted surface surely offered n-type conduc-

tion as well as the blackish color, but it appeared that

they originated from formation of graphitic sub-sur-

face layer introduced by the ion-implantation [4,5].

To date, in most CVD cases, they used hydride

dopant gas sources of the n-type diamond film: PH3 for

Applied Surface Science 216 (2003) 596–602

* Corresponding author. Tel.: þ81-6-6850-6313;

fax: þ81-6-6850-6341.

E-mail address: sachiko@sup.ee.es.osaka-u.ac.jp (S. Fujii).

0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0169-4332(03)00456-2

P-doping and H2S for S-doping. In contrast with those

previous works, the present work is characterized by

the new dopant gas SF6, which is the halogenated

compound commonly used as the etching gas of

silicon and silicon dioxide, and S-ion source for

implantation. In this case, SF6 is expected to supply

not only sulfur as a donor impurity in the diamond

semiconductor but also the partnering fluorine which

can serve as the morphology improver. The latter role

may stand at the well-known fact that incorporated

halogen species makes the diamond crystallinity bet-

ter when films are grown by the CVD with halogen

additive [6]. Actually, however, we faced an unex-

pected problem in our starting experiment. Namely,

diamond films grown in SF6 additive had suffered

from a fatal morphological degradation, though they

were CVD grown on diamond bulk substrates. Close

observation of the film surface exhibited the faint

appearance of grain-growth having a tile-shape, being

a drawback arising from addition of SF6 in the dia-

mond CVD. We need, towards the electronic applica-

tion, high quality diamond thin films free from

secondary nuclear formation and lattice defects, any-

way. According to the secondary ion mass analysis

(SIMS), anomalous accumulation of S at the interface

between the film and substrate was thought to cause

the relevant morphological degradation. To overcome

the problem and realize high quality n-type diamond

film, we proposed a new growth method with SF6

Grading–Doping profile.

2. Experimental

Diamond films were grown on high pressure and

high temperature synthetic (HPHT) Ib (1 0 0) dia-

mond substrates for the homoepitaxy and on scratched

(1 0 0) Si substrates for measurement of their growth

rate, facet-shape and so on. The growth apparatus was

a familiar microwave-plasma-assisted CVD (MP–

CVD) system equipping a tubular quartz chamber

[7]. The main source was a mixture of (H2 and

CH4) with the doping gas of SF6. The total pressure

was kept at 30 Torr, regardless the doping amount.

Film growth was done at the substrate temperature of

800 8C for 4 h. Other experimental conditions were

summarized in Table 1. We changed the additive

amount of SF6 in the concentration ratio (S/C) range

up to 48,000 ppm, while CH4 concentration of 1.0%

remained almost unchanged in any cases.

To minimize the crystal degradation due to SF6

addition during CVD, we proposed the Grading–Dop-

ing method wherein the deposition started from non-

doping for 5 min followed by SF6 addition (3000 ppm

stepwise increase in S/C ratio every 3 min up to the

destination level).

Grown diamond films were characterized by the

field-emission secondary electron microscope (FE-

SEM), reflection high electron energy diffraction

(RHEED), Raman scattering and secondary ion mass

analysis. In addition, electrical measurement was also

done.

3. Results and discussion

First, we can clearly compare surface morphologies

of diamond homoepitaxial films with and without

doping S in Fig. 1(a) and (b), where the former and

latter correspond to non-doped and uniformly-doped

(S/C ¼ 24,000 ppm) films, respectively. When S was

uniformly doped, the surface flatness was missing and

we can find a faint appearance of grain-growth having

a tile-shape discontinuity, instead. Observed degrada-

tion in morphology of the homoepitaxial diamond film

due to SF6 addition was indeed a serious problem one

could hardly expect beforehand. Although data are not

shown here, the degradation was exaggerated as

increasing SF6 addition.

As a suitable way to solve the morphological

degradation problem, we introduced the grading feed

of SF6 dopant gas during CVD, and particularly at the

very initial stage, no feed of SF6. Hereafter we tenta-

tively call it as ‘‘SF6 Grading–Doping’’. An example

Table 1

Growth conditions of S-doped diamond crystals

Source gas CH4/H2/SF6

CH4 concentration 1.0%

SF6 concentration S/C ratio 0–48000 ppm

Total gas flow 100 sccm

Total pressure 30 Torr

Microwave 2.45 GHz, 600 W

Substrate temperature around 800 8CSubstrate HPHT (1 0 0) diamond, Si

S. Fujii et al. / Applied Surface Science 216 (2003) 596–602 597

Fig. 1. FE-SEM images of (1 0 0) homoepitaxial diamond thin films. (a) Non-doped film; (b) SF6 uniform doping CVD film (S/C ratio was

24,000 ppm); and (c) SF6 Grading–Doping CVD film (S/C ratio was 24,000 ppm).

598 S. Fujii et al. / Applied Surface Science 216 (2003) 596–602

of the Grading–Doping program is given in Fig. 2. In

this program, non-doped diamond growth starts at the

onset of CVD for interval Dt, then SF6 feed increases

by DV, and afterward this cycle is repeated until the

amount of SF6 reaches the destination. Resultant

substantial improvement of the diamond morphology

was obtained as explained in the following.

By virtue of the Grading–Doping, discontinuity sign

completely disappeared though SF6 of 24,000 ppm was

fed during the growth, as shown in Fig. 1(c). Even when

we compare it with non-doped film (Fig. 1(a)), there is

no particular difference between them.

Crystallinity of S-doped diamond film was con-

firmed by the RHEED observation. Fig. 3(a) and (b)

show RHEED patterns taken under beam incidence in

the direction [1 0 0] of S-doped diamond thin films by

uniform doping and Grading–Doping methods,

respectively. In the diffraction patterns of both, spots

falling on the zeroth Laue zone and Kikuchi diffrac-

tion lines are clearly seen. Moreover, even spots of first

Laue zone were also observed when we manipulated

the electron beam angle. According to the these

results, films by uniform doping and Grading–Doping

method did not have a significant difference in their

crystallinity, although the former revealed clearly the

morphological degradation as indicated in Fig. 1(b). It

is thought that the boundary effect seldom prevails

widely as to the uniform doping film. Namely, it could

be restricted in at most the microscopic range even if it

works.

Fig. 1. (Continued )

Fig. 2. An example of SF6 (S/C ¼ 24,000 ppm) doping program

for the Grading–Doping CVD.

S. Fujii et al. / Applied Surface Science 216 (2003) 596–602 599

Fig. 3. RHEED patterns of (1 0 0) S-doped homoepitaxial diamond thin films. (a) Uniform doping CVD film and (b) Grading–Doping CVD

film. S/C ratio was 24,000 ppm.

600 S. Fujii et al. / Applied Surface Science 216 (2003) 596–602

In Fig. 4, we discuss again the S depth profiles

obtained from SIMS analyses for the uniform doping

and Grading–Doping diamond films. The former

was prepared by the MP–CVD for 4 h with uni-

form SF6 (S/C ¼ 24,000 ppm) addition. On the other

hand, the latter experienced Grading–Doping up to

24,000 ppm for a starting half hour and then it grew

successively under the uniform doping of 24,000 ppm

for 3.5 h. In this figure, it is not clear where the

substrate–film interface of the Grading–Doping sam-

ple locates because the perfect homoepitaxy pro-

gressed for the starting 3 min. In this period, at

least 10 nm thick non-doped film grew. After this, S

of 24,000 ppm was fed in the chamber, which in turn,

might induce the steep rise of S profile in the SIMS

curve (solid circles) at around 0.45 mm deep from

the top surface. As long as the curve (solid circles)

in Fig. 4 is concerned, the peak S content (incor-

porated S at the initial growth stage) is suppressed at

or less than a half of the uniform doping film (sym-

bolized by open circles). Furthermore, the profile is

followed by a slope descending so much gradually.

Contrasting with this, uniform doping film (symbo-

lized by open circles) exhibits the S profile peaking

sharply at the interface and explosively diminishing

toward the top surface. These differences are thought

to account for the observed improvement of the

epitaxial film morphology by the Grading–Doping

shown in Fig. 1(c).

It is of interest to see, in Fig. 4, that both curves fall

on the similar value around 1:0 � 1017 cm�3 in the

region from about 0.1 mm deep layer to the outer most

surface, though S profiles involve marked difference at

and in the vicinity of the interface depending on the

doping method. This fact suggests that the Grading–

Doping method, which overcame the morphological

problem, humps about no particular drawback in

manipulation of the doping amount. In other words,

one can feed SF6 dopant gas as much as one requests

when uses the Grading–Doping method. It is because

the Grading–Doping method has relaxed, to a large

extent, the limitation of SF6 doping amount without

suffering any special losses in the diamond film

growth. As to the yield of S doping in diamond film

from SF6 additive during MP–CVD growth, SIMS

data are plotted in Fig. 5, where the ordinate gives S

doping amount near the film surface region. In this

figure, the doping yield is increasing superlinearly

against the SF6 content. The reason for this non-

linearlity is not understandable at least at present. If

one bears in mind that certain amount of SF6 is

currently consumed in vain, e.g. etching of the quartz

tube, it is acceptable that the SF6 loss-fraction (loss-

amount against the total feed) becomes increasing for

less feed and vice versa. Anyway, the observed super-

linearity of S doping yield is of great favor when one

aims the heavily doping of S. Toward the heavily

Fig. 4. A comparison of S depth profile in the epitaxial diamond

films (S/C ¼ 24,000 ppm). Open and solid circles correspond to the

S concentrations of SF6 uniform doping and Grading–Doping,

respectively. Actually, Ar milling yield of the film grown by the

Grading–Doping CVD was by about 50% higher than that of the

uniform doping film. So, in this figure, the depth value of the

former film was calibrated on the basis of growth rate data.

Fig. 5. S doping amount in diamond films grown by the Grading–

Doping CVD as a function of SF6 additive amount. Data were

taken from SIMS analysis.

S. Fujii et al. / Applied Surface Science 216 (2003) 596–602 601

doping of S in diamond film, increases in the micro-

wave power and in the substrate temperature are

helpful in enhanced dissociation of SF6 molecule [8].

In our diamond films, there exists the residual deep

defects with a concentration as high as about

5 � 1017 cm�3 by the report of Otsuka et al. [9].

Taking this fact into account, we should dope S into

diamond films with the concentration in excess of

1 � 1018 cm�3, which is a similar value described by

Mort et al. [10] and Maki et al. [11] who dealt with B

doping in diamond films and evaluated the electrical

properties. Unfortunately, however, doping amount of

S was comparable to that of the deep defect in the

present work. Because of this, the Hall effect mea-

surements performed to Grading–Doping films some-

times exhibited n-type conduction and sometimes did

not. An example of the Hall effect, measurement data

(n-type conduction) is given in Fig. 6.

4. Conclusions

We proposed a new S doping method named ‘‘Grad-

ing–Doping’’ for use to SF6 addition in (H2 þ CH4)

plasma CVD process for diamond synthesis. So far,

even homoepitaxy of diamond films by using SF6

doping gas suffered from the morphological degrada-

tion, which was a serious obstacle to practically uti-

lizing n-type diamond films. SIMS analysis revealed

the anomalous pile-up of S atom at and near the

interface between the substrate and grown film, which

is responsible to the morphological degradation. On

the basis of this finding, Grading–Doping method was

proposed to solve the problem. By using the Grading–

Doping, diamond films with a lot of S doping offered

smooth surface and improved crystallinity. At the

moment, S doping amount reached around ð2�3Þ�1017 cm�3, being very close to the residual defect con-

centration in our standard films. Therefore, the grown

films are in the critical stage (in the sense of electronic

application) where sometimes Hall effect measurement

exhibits n-type conductivity and sometimes does not.

By adjusting the microwave power, substrate tempera-

ture, etc. to the appropriate ones, the SF6 Grading–

Doping CVD will promise the establishment of S

doped n-type diamond films and related multilayers.

Acknowledgements

The authors would like to thank Prof. H. Kawarada,

Dr. M. Tachiki and Mr. K. Nakazawa of Waseda Uni-

versity for discussion and collaborating experiment.

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