improvement of energetic efficiency for homoepitaxial diamond growth in a h 2 /ch 4 pulsed discharge

p s sapplications and materials science

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Improvement of energetic efficiency for homoepitaxial diamond

growth in a H2/CH

4pulsed discharge

O. Brinza, J. Achard, F. Silva, X. Duten, A. Michau, K. Hassouni, and A. Gicquel

LIMHP, CNRS, Université Paris 13, France

Received 22 February 2007, revised 27 June 2007, accepted 29 July 2007

Published online 4 September 2007

PACS 52.65.–y, 52.77.Dq, 81.05.Uw, 81.15.Gh

The use of pulsed discharges for diamond deposition has been demonstrated to ensure a better control of

heat transfer from the plasma to the walls in microwave plasma reactors. It also favours the production of

CH3

species while keeping constant or higher the H-atom density. Higher growth rates can then be ob-

tained. In this paper is reported an increase of the growth rate by 25% while decreasing the input micro-

wave mean power by 15%. These results are discussed in terms of atomic hydrogen and methyl radicals

densities calculated with a unstationary 1-D axial plasma model. The results show in particular that the

gas temperature, which directly controls atomic hydrogen production, needs a ton

of around 8 ms to reach

the steady state, and that 50% of atomic hydrogen is lost by recombination after a toff

of 2 ms.

phys. stat. sol. (a) 204, No. 9, 2847–2853 (2007) / DOI 10.1002/pssa.200776305

phys. stat. sol. (a) 204, No. 9, 2847–2853 (2007) / DOI 10.1002/pssa.200776305

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Original

Paper

Improvement of energetic efficiency for homoepitaxial diamond

growth in a H2/CH4 pulsed discharge

O. Brinza, J. Achard*, F. Silva, X. Duten, A. Michau, K. Hassouni, and A. Gicquel

LIMHP, CNRS, Université Paris 13, France

Received 22 February 2007, revised 27 June 2007, accepted 29 July 2007

Published online 4 September 2007

PACS 52.65.–y, 52.77.Dq, 81.05.Uw, 81.15.Gh

The use of pulsed discharges for diamond deposition has been demonstrated to ensure a better control of

heat transfer from the plasma to the walls in microwave plasma reactors. It also favours the production of

CH3 species while keeping constant or higher the H-atom density. Higher growth rates can then be ob-

tained. In this paper is reported an increase of the growth rate by 25% while decreasing the input micro-

wave mean power by 15%. These results are discussed in terms of atomic hydrogen and methyl radicals

densities calculated with a unstationary 1-D axial plasma model. The results show in particular that the

gas temperature, which directly controls atomic hydrogen production, needs a ton

of around 8 ms to reach

the steady state, and that 50% of atomic hydrogen is lost by recombination after a toff

of 2 ms.

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction

Improvements achieved during the last ten years in the area of diamond growth allow to obtain

monocrystalline diamond films of very high purity and with thicknesses of several hundred of microns

[1–5]. However, in spite of these improvements, the cost for producing high purity diamond remains

high, because of the high electrical energy needed for dissociating molecular hydrogen. Efforts for reduc-

ing the energy needed to produce the active species must be still undertaken.

The first way to reach this goal could be to substitute, even partially, molecular hydrogen by mole-

cules consuming less energy for their dissociation. As established by Bachman et al. through the well

known C–H–O diagram [6], diamond can grow under different gas systems. However, as shown in the

literature the thermal properties of the diamond films obtained through different gas mixtures strongly

vary, and the best performance in 1991 was reported to be obtained with hydrogen-rich H2/CH4 mixtures

[7]. This is one of the reason why the large majority of the research in the field has been devoted to this

gas system. Nevertheless, considering the current state of knowledge, it could be eventually interesting to

reconsider alternative gas mixtures.

The second way to improve the creation of active species may consist in increasing the working pres-

sure keeping constant the input power. In these conditions, the Microwave Power Plasma Density

(MWPD) strongly increases since the volume of the plasma decreases, but simultaneously the area avail-

able for diamond growth decreases preventing the growth of several samples during the same run. As a

consequence, the production cost does not decrease.

The third way could be to use pulsed plasmas. As a matter of fact, we have shown recently, that

pulsed discharges allow to push forward the limits of performance of a given reactor by injecting higher

microwave power, keeping similar the mean consumed power and the thermal management. Higher

* Corresponding author: e-mail: [email protected], Phone: +33 149 403 426, Fax: +33 149 403 414

2848 O. Brinza et al.: Improvement of energetic efficiency for homoepitaxial diamond

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-a.com

growth rates have been achieved while keeping the same the diamond quality [8]. Furthermore, since the

plasma area available for growth was kept constant, several diamond crystals have been grown simulta-

neously showing an issue to reduce the production costs [9].

In this paper, is reported an analysis of the phenomena occurring in the plasma phase and at the

plasma/surface interface of a pulsed discharge while keeping constant the peak microwave power and

varying the mean power. Also is reported homoepitaxial diamond growth using different frequencies and

duty cycles showing similar, or even higher growth rates while coupling to the plasma lower microwave

mean power (MWMP). The analysis of the diamond surface morphology and defect concentration is

presented. These results are discussed in terms of atomic hydrogen and methyl radicals densities deter-

mined from simulations based on a unstationary 1-D axial plasma model.

2 Experimental details

The different monocrystalline diamond samples have been grown in a metallic home-made microwave

plasma reactor starting from (100) Ib HPHT substrates. The substrates have been pre-treated before

growth by a H2/O2 plasma operating at a high microwave power during one hour as already reported

[10]. The growth conditions in terms of substrate temperature, methane concentration in the gas phase,

pressure and microwave power were 850 °C, 4%, 200 mbar and 3 kW respectively for all the samples.

The only parameters that have been modified from a sample to another one were ton and toff, where ton

denotes the time during which the plasma is switched on, and toff the time during which the plasma is

switched off. As a constant input power was applied, the microwave mean power (MWMP) injected in

the gas discharge was varied. All the growth conditions are summarized in Table 1. The different ranges

of ton and toff used for this study have been chosen thanks to the studies performed previously in LIMHP,

based on pulsed plasma modelling [11–13] and spectroscopic analysis.

The characterization of the diamond films have been carried out by Differential Interference Contrast

Microscopy (DICM) and the diamond purity and quality were assessed by high resolution Raman Spec-

troscopy and Photoluminescence performed at liquid nitrogen temperature. The growth rate has been

estimated by measuring the thickness of the sample before and after growth with an accuracy of ±2 µm.

3 Results and discussion

3.1 Diamond growth

Figure 1 shows the evolution of the growth rate as a function of ton and toff. The dotted line represents a

guide for the eyes corresponding to the growth rate obtained for the sample 1 i.e. in continuous mode

(CW). Most of the results show a decrease in the growth rate as MWMP decreases, i. e. as ton decreases

Table 1 Growth conditions of the samples considered in the paper.

samples ton

(ms)

toff

(ms)

MWMP

(kW)

deposition

duration (h)

thickness

(µm)

growth rate

(µm/h)

1 ∞ 0 3 6 34 5.6

2 6 27 4.5

3 8 2 2.4

6 22 3.6

4 6 31 5.2

5 11 2 2.5

3 14 4.7

6 15 1 2.8 7 37 5.3

7 6 45 7.5

8 15 2 2.65

8 51 6.4

9 15 3 2.5 8 30 3.8

phys. stat. sol. (a) 204, No. 9 (2007) 2849

www.pss-a.com © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Original

Paper

3

4

5

6

7

8

9

(15;3)

(15;1)(11;2)

(15;2)

(8;2)

(CW)

Gro

wth

rate

(µm

.h-1)

Experiments

and/or toff varies. However, it clearly appears that an increase in the growth rate is obtained while inject-

ing a lower MWMP for some very specific conditions, which is the case for a ton of 15 ms and a toff of

2 ms, noted (15;2) below.

The resulting morphology, observed by DICM (not presented here) on the different samples, is very

similar for all the samples. None unepitaxial crystallite is observed and the final roughness is close to

that obtained in CW mode. The diamond quality and purity characterized by Raman and photolumines-

cence spectroscopy is similar whatever the growth conditions. Actually, even if the plasma is stopped

several millisecond during the process, the high microwave power density used in this study leads, as we

will see later, to high atomic hydrogen density (several 1016 cm–3) in the plasma phase ensuring the high

quality of the material. A typical spectrum is given in Fig. 2 showing that no nitrogen nor silicon related

centres are present.

It is then possible to increase the growth rate by 25% while injecting a MWMP 15% lower keeping

similar characteristics of the grown layer.

3.2 Plasma analysis

Plasma simulations, based on a unstationary 1-D axial model developed previously in LIMHP have been

performed [11, 12, 14]. Briefly, the model involves coupled equations in order to describe the main phe-

nomena occurring in the plasma and at the plasma-surface interface. Thus, continuity, transport (molecu-

lar diffusion and forced convection) and energy equations are solved simultaneously. The kinetics model

used to describe the chemistry in H2/CH4 discharges involves three reaction groups. The first one corr-

500 550 600 650 700 750 800

ton

=15ms

toff

=2ms

R

PL

Inte

nsi

ty(a

.u.)

Wavelength (nm)

Fig. 1 Growth rate of the different samples for differ-

ent ton

and toff

. The dot line represents a guide for the

eyes corresponding to the growth rate obtained in con-

tinuous mode. For specific conditions, an increase of

the growth rate is obtained while injecting a lower

MWMP.

Fig. 2 Photoluminescence spectrum of the sample

grown using a ton

of 15 ms and a toff

of 2 ms. The exci-

tation wavelength was 514 nm.



esponds to the chemistry of pure hydrogen discharge and involves electron impact reactions leading to

H2 dissociation, H2 and H ionization and H atom excitation. It also involves ionization through the

quenching of H-atom excited states, ion conversion reactions and thermal dissociation of H2. The second

reaction group describes the thermal hydrocracking of CH4/H2 mixture. The third group describes the

chemistry of hydrocarbon ions which includes electron impact ionization reactions of hydrocarbon mole-

cules, charge transfer reactions between H 3

+ and hydrocarbon molecules, charge transfer reactions be-

tween hydrocarbon, molecules and hydrocarbon ions and dissociative recombination of hydrocarbon

ions. The rate constants for these reactions have been already reported in our previous papers. Those

involving electron collision reactions have been obtained by solving the Boltzmann equation for the

electrons.

The model allows calculations of the species densities all along the axial direction and as a function of

the time. The calculation is stopped when the steady state is reached i.e. when the densities of the differ-

ent species are similar from a pulse to another. We concentrated our attention on the main growth species

for diamond under the conditions studied here which are H atoms and CH3 radicals. As a matter of fact, a

linear variation of the experimental (100) planes growth rates as a function of those calculated from the

equation proposed by Goodwin is obtained, with CH3 radical and H atom densities provided by the

model [15–17]. On the opposite, the variations of C2H2 and atomic C densities (C2H2 or C atoms might

be growth precursors under specific conditions) cannot explain the evolution of the growth rate for our

experimental conditions.

Under these plasma conditions, we have shown that the production of CH3 is mainly governed by the

set of chemical reactions CHx + H ⇔ CH

x–1 + H2, with x ranging from 1 to 4. High CH3 radical density is

obtained under conditions of high atomic hydrogen density i.e. at high gas temperature [18, 19]. How-

ever, a too high gas temperature leads to the conversion of CH3 into other carbon species, such as CH2

[14, 20, 21]. The optimal net balance for CH3 radical density is obtained for a gas temperature range of

1500 K to 2000 K [22, 23], range which is existing at less than 1 mm from the substrate thanks to the

temperature gradient between the plasma core and the substrate maintained at 1100 K. Thus, in continu-

ous mode, the hot plasma core insures the production of H atoms which diffuse until the surface while

the colder plasma/surface interface insures that of CH3 radicals (see Fig. 3).

In pulsed plasmas, H atom and CH3 radical production and loss depend not only on the axial position

(due to the axial gas temperature profile) but also on the time since the gas temperature varies as a func-

tion of the time. In particular, while CH3 is totally dissociated in the plasma bulk in continuous mode, in

pulsed regime, it is present at the ignition and the shut down of the discharge, as shown in Fig. 4.

Fig. 3 (online colour at: www.pss-a.com) Calculation performed using a 1-D axial plasma model (the substrate is

located at 0 abscissa) for a continuous microwave power of 3 kW, 4% of CH4 in the H

2/CH

4 gas discharge and a

substrate temperature of 1100 K. (a) Gas temperature; (b) atomic hydrogen (squares, left scale) and methyl radical

(circles, right scale) concentrations.

phys. stat. sol. (a) 204, No. 9 (2007) 2851


Original

Paper

0.000 0.004 0.008 0.012 0.016 0.0201200

1400

1600

1800

2000

2200

2400

TG

toff

ton

CH

3 density(cm

-3)

t (s)

Gas

tem

pera

ture

(K)

5.0x1013

1.0x1014

1.5x1014

2.0x1014

2.5x1014

CH3

density

Fig. 4 (online colour at: www.pss-a.com) Temporal evolution of [H] and [CH3] calculated in the plasma

bulk for ton

= 15 ms and toff

= 3 ms. CH3 species production is favoured at the ignition and at the shut down

of the plasma when the gas temperature is in the range 1500–2000 K.

At the plasma/surface interface, typical calculated variations of [H] and [CH3] are plotted in Figure 5

during a pulse for a ton of 15 ms and a toff varying between 1 ms and 3 ms. A periodic shut down of the

plasma is seen to favour the CH3 production at the beginning of the pulse as well as during toff, particu-

larly when high microwave power is used to efficiently dissociate molecular hydrogen.

The shut down of the plasma during few ms allows to temporarily decrease the gas temperature and

maintain it longer in the range 1500–2000 K than in continuous mode. So, at the beginning of the pulse

and during toff, an increase of CH3 radical production is obtained. As soon as the gas temperature de-

creases, the H production obviously decreases. The compromise leading to optimal local conditions for

diamond growth is then a function of the characteristic times of the different phenomena involved in-

cluding reactions and transport. Concerning toff, the best compromise found up to date is around 2 ms in

order to limit the lost of atomic hydrogen to 50%, during the plasma off.

A thorough analysis of the temporal evolution of [H] and [CH3] during ton can also highlight the ex-

perimental results, in particular those concerning the variation of the growth rate for ton of 8 ms and

15 ms respectively, at constant toff equal to 2 ms. The observation of the temporal evolution plot of [H]

and [CH3] for ton varying from 8 to 15 ms (see Fig. 6) indicates that [H] needs around 4 ms to reach 80%

of the steady state value. As a consequence, for ton = 8 ms, only 50% of the on-state presents a high hy-

drogen density while for ton = 15 ms, 75% of the on state allows to obtain high hydrogen density.

0.000 0.004 0.008 0.012 0.016 0.0201015

1016

1017

H-a

tom

icde

nsity

(cm

-3)

t (s)

(15;1)(15;2)(15;3)CW

0.000 0.004 0.008 0.012 0.016 0.020

CH

3de

nsity

(cm

-3)

t (s)

(15;1)(15;2)(15;3)CW

2.5x1014

1.0x1014

1.5x1014

2.0x1014

Fig. 5 (online colour at: www.pss-a.com) Temporal evolution of [H] (left) and [CH3] (right) calculated at a dis-

tance of 900 µm from the substrate during the pulse for a ton

of 15 ms and a toff

varying beetwen 1 ms and 3 ms.



0.000 0.004 0.008 0.012 0.016 0.0205.0x1015

1.0x1016

1.5x1016

2.0x1016

2.5x1016

3.0x1016

3.5x1016

4.0x1016

H-a

tom

icde

nsity

(cm

-3)

t (s)

(8;2)(11;2)(15;2)CW

1.0x1014

1.5x1014

2.0x1014

2.5x1014

CH

3de

nsity

(cm

-3)

(8;2)(11;2)(15;2)CW

0.000 0.004 0.008 0.012 0.016 0.020

t (s)

Fig. 6 Temporal evolution of [H] (left) and [CH3] (right) calculated at a distance of 900 µm from the substrate

during the pulse for different ton

and a toff

of 2 ms.

Thus, the higher growth rate obtained for (15;2) than for (8;2), can be interpreted by the higher net

production of both CH3 radicals and H atoms for (15;2), than for (8;2).

Moreover, it is worth noting that the time during which the sample is exposed to the plasma varies as a

function of the duty cycle: for ton = 8 ms and toff = 2 ms, the exposure time of the sample to the plasma

has been estimated to be only 80% while this time reaches 88% for ton = 15 ms and toff = 2 ms. This may

play a role on the observed gap between the growth rates obtained for these two experimental conditions.

4 Conclusion

In this paper, growth of single crystal diamond has been performed using pulsed microwave plasmas. By

a judicious choice of ton and toff, it has been shown that an increase in the growth rate can be obtained

even while coupling less microwave mean power to the plasma. This can be an issue for reducing sig-

nificantly the production cost of single CVD-diamond crystals. In particular, an increase of the growth

rate by 25% has been obtained while the microwave power was reduced by 15%. Thanks to a unstation-

ary 1D-axial plasma model, the increase of the growth rate has been mainly attributed to an improvement

of CH3 radicals net production by modulating gas temperatures during the process, which reduces the

CH3 radicals conversion into other carbon species occurring for temperature higher than 2000 K. A com-

promise between CH3 radicals and H atoms production and loss, which are strong functions of space and

time, was found for ton = 15 ms and toff = 2 ms.

Acknowledgements This work was supported in part by European Project RTN DRIVE of the 6th PCRD, No.

MRTN-CT-2004-512224.

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improvement of energetic efficiency for homoepitaxial diamond growth in a h 2 /ch 4 pulsed discharge

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