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Diamond & Related Materials

journal homepage: www.elsevier.com/locate/diamond

Mechanical properties of high-crystalline diamond films grown via laser

MPCVD

Meijun Yanga

, Sunan Baia

, Qingfang Xua

, Jun Lib

, Toshihiro Shimadac

, Qizhong Lid

Takashi Gotoa

, Rong Tua

, Song Zhanga,⁎

a State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, People's

Republic of China

b National Key Laboratory for Shock Wave and Detonation Physics, Institute of Fluid Physics, P.O. Box 919-102, Mianyang 621900, People's Republic of China

c Hokkaido University, Fac Engn, Div Appl Chem, Kita Ku, Kita 13, Nishi 8, Sapporo, Hokkaido 0608628, Japan

d Hubei Key Laboratory Advanced Technology of Automobile Parts, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, People's Republic of China

ARTICLE INFO

Keywords:

Diamond films

Laser microwave plasma chemical vapor

deposition (laser MPCVD)

Crystalline

Mechanical properties

ABSTRACT

High-crystalline diamond films were prepared by laser (wavelength: 532 nm) microwave plasma chemical vapor

deposition (Laser MPCVD). The effects of laser power density (E) on the microstructure and properties of dia- mond films were investigated. The laser enhanced the precursor reaction process which reduces the non-dia- mond components such as graphite and amorphous carbon. At E = 40 W/cm2

, more methyl radicals and atomic

hydrogen were generated with coupling of laser and plasma, which promoted crystal growth until the grain size

reaches the maximum value of 0.72 μm. The phase purity of film reached the highest as the full width at half

maximum (FWHM) of Raman peak reached the minimum of FWHMmin = 4.2 cm−1. And hardness (H) and

Young's modulus (M) values reached the maximum of Hmax = 91 GPa and Mmax = 721 GPa, respectively. The

discovery of the advantages of laser in diamond growth is of great significance for improving the synthesis of

many technically crucial materials.

1. Introduction

Chemical vapor deposition (CVD) is the preferred method for de- positing high quality diamond films [1,2]. However, there are several

problems with this method, such as low depositing efficiency, poor

mechanical properties and amorphous carbon impurities [3–5]. We

have previously used intermediate frequency induction heated micro- wave plasma chemical vapor deposition (IH-MPCVD) to prepare dia- mond with good mechanical properties [6]. Furthermore, the high- crystalline diamond with better mechanical behavior is still an im- portant issue [7,8].

There is crucial to study the laser growth process of CVD diamond as

the lowest cost method of preparing diamond [9]. By illuminating one

or more laser beams into a reactive gas mixture [11,12], it is possible to

control or even divert the direction of product formation. Studies have

prepared diamond films and diamond-like films by incorporating a laser

into the CVD [12–14], called as laser chemical deposition (LCVD). Sun

[15] deposited 3C-SiC (111) epitaxial films with high crystallinity on Si

(110) substrate via LCVD. Table 1 shows the previous reports for LCVD

diamond films. T.V. Kononenko [16] explored the nano-ablation of a

diamond surface exposed to femtosecond laser pulses, diamond gra- phitized by laser etching, which reduces crystallinity. Zhang [17] de- posited rapid nucleation of diamond films by pulsed laser chemical

vapor at a low substrate temperature. Fan [18,19] used combustion

CVD (CCVD) to improve diamond quality by adjusting laser wave- length, confirming the suppression effect of UV laser irradiation on

nondiamond carbon formation. Constantin [20] used different wave- lengths of UV laser irradiation on the diamond-forming combustion

flame, suppressed secondary nucleation and leads to the direct photo

dissociation of hydrocarbon precursors. The achievements of other la- sers CVD have been summarized in our paper, and they all illustrate

that laser is useful for the fabrication of diamond film in the CVD

procedure. MPCVD is the popular method to obtain diamond film these

days, but growth rate and quality of the film is still deficient. So we

introduced laser into MPCVD for the first time in order to coupling the

thermal and light effects of laser and the electromagnetic field effect of

the microwave to deposit the high quality diamond film.

Crystallite size, film thickness and phase purity are the factors de- termining important properties of nanocrystalline diamond films [21].

With the purpose of obtaining diamond films in both superior

https://doi.org/10.1016/j.diamond.2020.108094

Received 18 July 2020; Received in revised form 1 September 2020; Accepted 11 September 2020

⁎ Corresponding author.

E-mail address: kobe@whut.edu.cn (S. Zhang).

Diamond & Related Materials 109 (2020) 108094

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crystallinity and mechanical behavior [22], we prepared diamond films

by laser-microwave plasma chemical vapor deposition (LMPCVD).

Combined the intermediate frequency induction heated MPCVD system

and LCVD system, a continuous laser with super-Gaussian distribution

is introduced on MPCVD to form a LMPCVD device. The influence of

laser power density (

E), CH

4/H

2 ratio and microwave power on the

growth rate (

G), full width at half maximum (FWHM) value, hardness

(

H) and Young's modulus (

M) values of diamond films were in- vestigated.

2. Experimental

Fig. 1 shows the experimental schematic of the home-made laser- microwave plasma CVD. The LMPCVD system consisted of a laser

(HK1064-500FC, ZK-LASER, 500 W), a microwave generator network

(MPS-15D, NISSIN, Tokyo, Japan, 2.45 GHz, 1.5 kW), a quartz tube

chamber, an intermediate frequency induction heating device (S08-

6093, Daiichi Kiden, Tokyo, Japan, 6 kHz, 12.5 kW), and a vacuum

system. Single-crystalline silicon (100) wafers with dimensions of

10.0 × 15.0 × 0.5 mm were used as substrates. In order to remove the

impurities adsorbed on the surface of the Si substrate and create micro- defects on the substrate surface to increase nucleation, the Si substrates

scratched via ultrasonic bath in acetone with diamond powders

(10–40 μm) for 1 h [23]. The scratched substrates were taken ultrasonic

cleaning in deionized water, and then dried in a N

2 flow.

During the experiment, the substrates were preheated at 923 K in

30 min after quartz chamber was evacuated to 10−1 Pa. The hydrogen

2) flow rate was set to 400 sccm, the methane concentration (

c) was

1.0%, the methane (CH

4) flow rate was 4 sccm, the deposition pressure

was 3 kPa, and the deposition time was 2 h (Fig. 2(c)), the microwave

power was 750 W, and the laser power density (

E) was 0–100 W/cm

(Fig. 2(a)). The substrate temperatures (

Tsub) were controlled in the

range of 923–1123 K by the change of laser power density (

E) and

microwave power. To compare the effect of laser, we had deposited

MPCVD diamond films in the range of 923–1123 K without laser in our

previous work [

6]. The process conditions for depositing diamond films

at different laser power density (

E) conditions are shown in Table 2 and

Fig. 2(b).

Phase identification of the films was obtained by Raman spectra

(LabRAM HR Evolution; Horiba, Paris, France) with the excitation of a

laser 532 nm in wavelength. Crystalline phases were examined by X-ray

diffraction with CuKa radiation (XRD; Ultima III, Rigaku, Tokyo,

Japan, at 40 kV and 40 mA). A field-emission scanning electron mi- croscope (SEM; Quanta-250, FEI, Houston, TX, at 20 kV) was used to

observe the film thickness and microstructure. The surficial roughness

was analyzed by atomic force microscopy (AFM; Multimode 8-HR,

Bruker, Santa Barbara, USA). The hardness and young's modulus were

analyzed by an MTS Nano-indenter (Agilent Technologies G200,

California, USA). The measurements were performed with loads ran- ging from 120 to 600 mN at a fixed penetration depth of 100 nm.

Young's modulus hardness of the films were determined from the load

versus displacement curves in complete load/unload cycles.

3. Results and discussion

Fig. 3(a) shows the XRD patterns of diamond films prepared at

different laser power density, and the diffraction peaks at 2θ = 43.9° is

corresponds to diamond (111) plane. The drum peak indicates that the

crystallinity of the film is poor at

E = 100 W/cm

. The full width at half

maximum (FWHM) of 43.9° XRD peaks of diamond films deposited by

LMPCVD are shown in Fig. 3(b), the LMPCVD diamond films with the

best crystallinity are between 40 W/cm

2 and 60 W/cm

The SEM surface and cross-sectional morphology of diamond films

prepared by LMPCVD at different laser power densities (

E = 0–100 W/

) are shown in Fig. 4. In our previous work, we used parameter α to

Table 1

Comparison of different CVD methods with laser for diamond films.

describe the growth rate ratio of the 〈100〉 and 〈111〉 crystal directions

References Year Laser (nm) Method Precursor and carrier gas Tsub (K) FHWM of Raman (cm−1) Note

Zhang [17] 2001 532 HFCVD CH4 + Ar + H2 923 N/A Rapid nucleation of diamond films by pulsed laser chemical vapor deposition at a low temperature.

Fan [18] 2012 9219, 1035, 10,719 CCVD C2H4 + O2 + NH3 1043–1053 N/A Adjust the laser frequency to match the vibration of the precursor to synthesize Nitrogen-doped diamond.

Fan [14] 2014 10,532 CCVD C2H2 + C2H4 + O2 1043–1053 N/A Resonant vibrational excitation of ethylene molecules by IR-laser to deposit combustion CVD diamond

Fan [19] 2018 248 CCVD C2H2 + C2H4 + O2 1043–1053 5.7 Suppress nondiamond carbon by UV laser photolysis of hydrocarbons.

Constantin [20] 2018 193, 248 CCVD CH4 + Ar + H2 2450 N/A UV laser irradiation on the diamond-forming combustion flame to suppress secondary nucleation.

This study 2020 532 LMPCVD CH4 + Ar + H2 973–1123 4.2 Increased the crystallinity of diamond films by LMPCVD to improve mechanical behavior.

M. Yang, et al. Diamond & Related Materials 109 (2020) 108094 2

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and reflect the evolution of the structure and morphology of diamond

[6]. When α is 1, 〈111〉 direction is the fastest growing direction and

the growth morphology is cubic; when α is 3, 〈100〉 direction is the

fastest growing direction and the growth morphology is octahedron. As

the increasing of E, the morphology presents the trend of transition

from cubic to octahedron. At E = 0–60 W/cm2

, the pyramidal micro- metric diamond crystals can be observed at the terminal surface, and

the triangular facet corresponds to the {111} plane. When E increases

from 40 W/cm2 to 80 W/cm2

, both triangular and square facets cor- respond to {111} and {100} lattice planes are observed [6]. The SEM

image presents a field with both faceted diamond and nano-grained

diamond at E = 80 W/cm2

. When E > 60 W/cm2

, the high-power

laser destroys the coupling effect with the plasma. The {111} surface is

firstly ablated by laser etching, then the laser will continue to etch the

{100} surface. At E = 100 W/cm2

, the diamond grew in the form of

nanoparticles. The cross-sectional view reveals the film thickness

reached the maximum of 0.76 μm at E = 40 W/cm2

Fig. 5 shows the average grain size (La) and growth rate measured

by SEM. Compared with the values obtained without laser irradiation,

more methyl radicals and atomic hydrogen were generated with cou- pling of laser and plasma, which promoted crystal growth [24]. During

the deposition process, the enriched atomic hydrogen can enhance the

quality of the diamond film [25]. The La increases from 0.50 μm to

0.72 μm accompanied by an increasing of E until the laser power

Fig. 1. The apparatus of laser-microwave plasma chemical vapor deposition.

Fig. 2. (a) The relationship between the substrate temperatures (Tsub) and the laser power density (E), (b) The relationship between the laser power (PL), laser current

(CL) and the laser power density (E), (c) Diagram for the growth of diamond films by LMPCVD.

Table 2

Deposition conditions.

Process parameters Value

Laser power density (E, W/cm2

) 0; 20; 40; 60; 80; 100

Laser spot diameter (mm) 5, 8, 11, 14, 18

Laser focus length (mm) 120

Microwave power (W) 750

Flow of CH4 (sccm) 4

Flow of H2 (sccm) 400

Deposition pressure (kPa) 3

Deposition time (h) 2

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density exceed 40 W/cm2

, then La decreases from 0.72 μm to 0.08 μm

with E increases to 100 W/cm2

. The growth rate (RG) increases as E

increases from 0 W/cm2 to 40 W/cm2 and reaches a maximum of

0.38 μm/h at E = 40 W/cm2 (Fig. 4). High power laser will etch the

{111} face, leading to amorphization of the material and reducing film

quality [26]. Continue to increase the power density, the {100} plane

will also be etched, resulting in the amorphization of the material in- tensified, generating nano-diamonds and preventing grain growth. The

RG declines to 0.28 μm/h with further increasing of E to 100 W/cm2

Fig. 6(a) shows the Raman spectrum of different carbon materials

(diamond, cluster diamond, graphite, amorphous carbon, etc.), in

which the diamond material has a sharp peak only at a Raman shift of

1332 cm−1; the Raman peak of cluster diamond is also displayed only

at 1332 cm−1, but the peak is wide and not sharp; the amorphous

carbon has a bulge-like peak at a Raman shift of about 1580 cm−1. In

Fig. 6(b), the laser power densities correspond to six curves. There are

obvious differences among the six curves at Raman shifts of 1332 cm−1

and ~1580 cm−1: Curve of E = 20 W/cm2 has a sharp peak at a Raman

shift of 1332 cm−1, which is a characteristic Raman peak of the dia- mond phase, then the existence of the diamond phase in the film is

confirmed. Furthermore, there is only a small bulge peak at

~1580 cm−1. Curve of E = 40 W/cm2 has a sharp peak only when the

Raman shift is 1332 cm−1, and there is no peak at ~1580 cm−1, this is

consistent with the Raman spectrum of the diamond in Fig. 6(a). Which

Fig. 3. (a) XRD patterns of diamond films deposited at various E, (b) FWHM of 43.9° XRD peaks of diamond films.

Fig. 4. Surficial and cross-sectional SEM image of diamond films.

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indicates that this sample has a high phase purity and it does not

substantially contain non-diamond impurities such as graphite and

amorphous carbon. As E increases, both curves of E = 60 W/cm2 and

80 W/cm2 have sharp diamond Raman peaks at 1332 cm−1, and drum

peaks with amorphous carbon at ~1580 cm−1 are more obvious. When

E reaches 100 W/cm2

, Curve of E = 100 W/cm2 still has a bulge peak at

around 1580 cm−1, and at 1332 cm−1 is a bulge-like spike, which is

basically consistent with the Raman spectrum of “cluster diamond” in

Fig. 6(a) [27], indicating that the sample E = 100 W/cm2 is a cluster of

diamonds. This is consistent with the SEM morphology in Fig. 4. The

high-power laser (E > 60 W/cm2

) reduces the coupling effect of laser

and plasma, superabundant photons leads to the generation of excess

methyl [28]. Thus, the excess graphite is formed with diamond phase

and prohibited the film growth, as shown in Fig. 5.

Fig. 6(c) shows the E and the FWHM of 1332 cm−1 Raman peaks of

diamond films. The FWHM of the Raman peak is negatively related to

the phase purity of the diamond films [29]. The FWHM of 1332 cm−1

Raman peaks of diamond films in Fig. 6(c) have a similar function curve

with the FWHM of 43.9° XRD peaks of diamond films deposited by

LMPCVD in Fig. 3(b). The FWHM of CVD synthetic diamond is in the

range of 5–25 cm−1, and the FWHM of natural diamond is in the range

of 2–3 cm−1 [29]. The FWHM of the Raman peak decreases from

30 cm−1 to 4.2 cm−1 with E increases from 0 W/cm2 to 40 W/cm2

, the

crystallite size and phase purity of diamond improves within E in- creasing from 20 W/cm2 to 60 W/cm2

. The combined effect of laser and

plasma on the precursor is called coupling. The coupling of laser and

plasma breaks the HeH bonds and CeH bonds, and promotes the de- composition of the precursor, increases the concentration of atomic

hydrogen during the synthesis. Then the content of sp3 increased and

the FWHM of diamond films reduced [30]. When the laser power

density exceeds 60 W/cm2

, superabundant photons break the CeC

bonds of diamond and induced the rich of [CxHy] and other methyl

radicals, which results in the increase of sp2 hybridized carbon and

diamond crystal distortion, and leads to the reduction of phase purity

[21]. When E increases from 40 W/cm2 to 100 W/cm2

, the FWHM value

of the Raman peak at 1332 cm−1 increases from 4.2 cm−1 to

49.9 cm−1. The FWHM value of the diamond film with E = 100 W/cm2

is even higher than that of film prepared by the MPCVD method.

Fig. 7 is the transmission electron microscope images of a cross

section of a film at E = 40 W/cm2

. In Fig. 7(a), the cross-sectional TEM

image of diamond film at E = 40 W/cm2 shows the typical CVD dia- mond columnar grains, and the width of columnar grain is 0.32 μm

[31]. The competitive growth of diamond in CVD results in the surface

grain size of preferentially grown grains being larger than columnar

[32]. The SAED pattern is a polycrystalline diffraction ring, and each

diffraction ring corresponds to a (111) crystal plane, a (220) crystal

plane, and a (311) crystal plane of the sample, respectively. The high- resolution TEM image of the top of the film as Fig. 7(c), clearly shows

continuous diamond {111} planes with interplane value of 0.206 nm,

also confirming the high crystalline of the film.

Fig. 8(a) shows the hardness (H) and Young's modulus values (M) of

the diamond film deposited by LMPCVD as a function of grain size. The

mechanical behavior of films appears to be mostly determined by the

grain size, grain shape, orientation and hydrogen incorporated in the

grain boundaries [33]. From the curve trend graph, Young's modulus

and hardness increase with increasing grain size. When the grain size

gradually increases from La = 0.08 μm to 0.72 μm, the Young's modulus

of the diamond film gradually increases from M = 376 GPa to 721 GPa,

and the hardness value increases from H = 43 GPa to 91 GPa. The

minimum grain size is La = 0.08 μm, Mmin = 376 GPa, and

Hmin = 47 GPa. The H and M values of the diamond film deposited by

LMPCVD at La = 0.08 μm are even lower than that of the diamond film

deposited by MPCVD at La = 0.5 μm. The hardness value H and the

Young's modulus value M of the diamond film prepared by the MPCVD

method are 70 GPa and 612 GPa (La = 0.5 μm), respectively. When the

grain size reaches the maximum value of 0.72 μm, the maximum

hardness is 91 GPa and the maximum Young's modulus is 721 GPa. This

trend is consistent with the trend of FWHM versus laser power density

in Fig. 6(c). And both at La = 0.72 μm (E = 40 W/cm2

), the numerical

value reaches the optimal value.

The hardness and Young's modulus of diamond films prepared by

LMPCVD are in a relatively excellent region as shown in Table 3 and

Fig. 5. Grain size and growth rate.

Fig. 6. (a) Raman spectra of different carbon materials [27], (b) Raman spectra of diamond films deposited at various E, (c) FWHM of 1332 cm−1 Raman peaks of

diamond films.

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Fig. 7. TEM of diamond film sample at E = 40 W/cm2

, (a) microscopic morphology, (b) selected area electron diffraction pattern, (c) high resolution TEM.

Fig. 8. Hardness (H) and Young's modulus (M) of diamond films deposited at various E (a), diamond and diamond-like carbon films (b).

M. Yang, et al. Diamond & Related Materials 109 (2020) 108094