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Optical Materials 30 (2008) 763–766

The diamond films and single diamond micro-crystals studiedby micro-Raman spectroscopy

K. Fabisiak a,*, W. Masierak a, E. Staryga b, M. Kozanecki c

a Department of Physics, Kazimierz Wielki University, Weyssenhoffa Sq. 11, 85-072 Bydgoszcz, Polandb Institute of Physics, Technical University of Łodz, Wołczanska 219, 90-005 Łodz, Poland

c Faculty of Chemistry, Technical University of Łodz, _Zeromskiego 116, 90-924 Łodz, Poland

Available online 2 April 2007

Abstract

Diamond films and single diamond micro-crystals were deposited on a Si substrate with a hot filament chemical vapor deposition (HFCVD) method from methyl alcohol–hydrogen gas mixture. The Raman spectra of diamond micro-crystals have been measured as a func-tion of the crystals size and their orientation. The diamond Raman line was found to become broader and weaker with decreasing crystalsize. The observed result can be explained by crystal size effect but phonon confinement or strain effect can not be excluded. It was alsoobserved that the Raman spectrum depends on crystal orientation. In general the full width at half maximum of the diamond line isbroader for (111) than (100) diamond plane. For thick diamond film the in-depth profile analysis was performed using confocal Ramanmicroscopy.� 2007 Elsevier B.V. All rights reserved.

PACS: 78.30.�j

1. Introduction

The research activity in the field of diamond depositionfrom gas phase using chemical vapor deposition (CVD) hasincreased significantly in the last 20 years. The break-through in CVD diamond technology was made in 1982by Matsumoto et al. [1]. They used hot filament operatingat temperature around 2100 �C to directly activate hydro-gen and hydrocarbon necessary to diamond growth. Sincethen, different CVD methods such as for example directcurrent CVD (DC CVD) [2], radio-frequency CVD (RFCVD) [3] and microwave plasma CVD (MP CVD) [4] weredeveloped and modified. Diamond is characterized byunique combination of exceptional physical properties, itis an electrical insulator but conducts heat even five timesmore effectively than copper, and can withstand very highelectrical field [5]. When doped shows very high electronand hole mobility [6,7].

0925-3467/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.optmat.2007.02.031

* Corresponding author.E-mail address: kfab@ukw.edu.pl (K. Fabisiak).

The chief motivation of this research effort is the hope ofproducing, at low cost, a high quality material with apotential application especially in microelectronics as amicrochip substrate, high efficiency electron emitters,photo-detectors and transistors and heat sinks.

In present work for evaluation of diamond quality theRaman spectroscopy was used.

2. Experimental

The measurements were carried out on the diamondfilms synthesized on (100) single crystal silicon wafer bythe HF CVD. The details of the HF CVD reactor isdescribed elsewhere [8].

The diamond film morphology has been studied byusing Scanning Electron Microscope (SEM) LEO type1430VP operating at the voltage of 25 kV.

The Raman spectra were recorded at room temperaturein back scattering geometry using Jobin–Yvon T64000Raman spectrometer equipped with confocal Olympusmicroscope. The 514.5 nm line from argon laser was used

25

30

m]

764 K. Fabisiak et al. / Optical Materials 30 (2008) 763–766

for excitation. The spectrometer resolution was set at4 cm�1. Data were taken with a step size of 0.2 cm�1 overthe (1100–1600) cm�1range to determine exact peak posi-tion and the FWHM.

0 5 10 15 20 25

0

5

10

15

20

Cry

stal

siz

e [1

0-6

Deposition time [h]

Fig. 2. Crystal size vs. deposition time

1250 1300 1350 1400

5000

10000

15000

20000

25000

Inte

nsi

ty [

a.u

.]

Raman shift [cm-1]

a

b

c

d

e

f

Fig. 3. Raman spectrum taken from (111) (spectra: a, b, c) and (100)(spectra: c, d, e) crystal planes and different crystal sizes of 7 lm, 15 lmand 25 lm.

3. Results and discussion

Raman spectroscopy allows for ready identification ofdifferent carbon allotropes present in the sample. Naturaldiamond exhibits a single sharp line at 1332.5 cm�1 havingFull Width at Half Maximum (FWHM) of about 2 cm�1

[9].Nucleation is the first step in diamond growth process.

It is quite easy to grow diamond homoepitaxially on a sin-gle diamond crystal using different CVD methods [10,11].In the case of other non-diamond substrates, if no substratepretreatment was applied, the initial diamond nucleationprocess is very slow and nucleation density very low whatis illustrated in Fig. 1a. The estimated nucleation densityis about 103 cm�3. In the first experiment performed withinpresent work, diamonds were grown on the substrate with-out pretreatment, in order to have very low nucleation den-sity which allows for observation of the evolution of crystalsize as function of deposition time. At the beginning of dia-mond growth process, the micro-crystals have differentshapes, did not show any preferential orientation in respectto the substrate and their size increases proportionally tothe deposition time (see Fig. 1 and 2).

For the bigger micro-crystals, it was possible to take themicro-Raman spectra from (100) and (111) crystal planesand the results are shown in Fig. 3. The Raman spectrumtaken from (111) planes is twice broader in comparisonto that taken from (100) planes for comparable crystal

Fig. 1. Evolution of micro-crystal size as function of deposition time: (a) after 1 h, (b) after 5.5 h, (c) after 15 h and (d) after 25 h.

Fig. 4. Subsequent steps of the growth of continuous diamond film: (a) after 1 h, (b) after 6 h, (c) after 10 h and (d) after 20 h.

K. Fabisiak et al. / Optical Materials 30 (2008) 763–766 765

sizes which indicate that the (111) crystal planes are moredefective. The dependence of FWHM on crystal size can beexplained by the crystal size effect [12] but phonon confine-ment effect cannot be excluded [13].

In the second experiment, we grew continuous diamondon the silicon substrate polished by diamond paste andsubsequent steps of diamond layer growth are shown inFig. 4. The estimated nucleation density is about twoorders of magnitude higher than in the previous case.The growth of continuous polycrystalline diamond filmconsists of few steps: nucleation of individual crystalliteson the surface defects, subsequent three-dimensionalgrowth of individual crystallites, faceting and coalescence

1100 1200 1300 1400 15000

400

800

1200

1600

2000

2400

2800

3200

3600

Inte

nsi

ty [

a.u

.]

Raman shift [cm-1]

a

b

d

e

e

Fig. 5. An example of Raman spectra taken at different distances from thesubstrate: (a) 0.5 lm, (b) 1.5 lm, (c) 5 lm, (d) 10 lm and (e) 18 lm.

with neighboring crystallites and finally growth of continu-ous layer in the perpendicular direction to the substrate.

For diamond layer shown in Fig. 4d, confocal Ramanmicroscopy was used for in-depth profile analysis. Fig. 5illustrates only a few examples of Raman spectra takenat different distances from the substrate. The Raman peakpositions and their FWHM are presented in Fig. 6. As it isseen, close to substrate diamond layer is under strong com-pressive strain because the Raman peak is shifted by+3.3 cm�1 in respect to that observed for diamondmono-crystal. With increasing distance from the substrate,this compressive stress decreases and is almost fully relaxedat distance equal to about 10 lm. At distances greater than

-2 0 2 4 6 8 10 12 14 16 18

1332.6

1332.8

1333.0

1333.2

1333.4

1333.6

1333.8

-2 0 2 4 6 8 10 12 14 16 18

4.4

4.6

4.8

5.0

5.2

5.4

5.6

5.8

6.0

Raman peak position

FW

HM

[cm

-1]

Ram

an p

eak

po

siti

on

[cm

-1]

Distance from the substrate [10-6 m]

FWHM

Fig. 6. Evolution of diamond Raman peak parameters (Raman peakposition and FWHM) as function of distance from the substrate.

766 K. Fabisiak et al. / Optical Materials 30 (2008) 763–766

10 lm, the Raman peaks are centered at position charac-teristic for diamond mono-crystal. Similar behavior showsFWHM (see Fig. 6). The Raman peak is much broader fordiamond layer close to the substrate in comparison to thoserecorded at distances greater then 10 lm from the sub-strate. Broadening of the Raman peaks can be caused bycompressive stress or by decrease of diamond layer quality[8]. Higher stress in the parts close to the interface betweenthe substrate and diamond layer can have thermal origindue to difference between thermal expansion coefficientsof diamond and substrate [14].

4. Conclusions

Diamond single micro-crystal and continuous diamondfilm have been grown successfully on silicon substrate ina HF CVD apparatus. From SEM analysis, we see that sizeof diamond crystals increases linearly with deposition time.The diamond Raman peak taken from the (111) crystalplanes is almost twice broader in comparison to that takenfrom (100) crystal plane. The FWHM depends on the crys-tal size. In-depth profile analysis shows that diamond layerclose to the interface diamond-substrate is on compressivestress and the stress distribution along the film thickness isnot homogenously distributed. The stress is completelyrelaxed at distances bigger than 10 lm and the diamondquality improves as indicates the value of FWHM of dia-mond Raman peak.

Acknowledgement

The work was financed by the Polish State Committeefor Scientific Research-Project No. 3 T11B 050 26.

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