www.elsevier.com/locate/tsf

Thin Solid Films 482

Electrochemically induced surface modifications in boron-doped diamond

films: a Raman spectroscopy study

P.C. Ricci*, A. Anedda, C.M. Carbonaro, F. Clemente, R. Corpino

Dipartimento di Fisica, INFM Universita di Cagliari, Cittadella Universitaria S.P. Monserrato-Sestu Km 0.700-09042 Monserrato (Ca), Italy

Available online 5 January 2005

Abstract

Owing to its unique properties such as chemical stability, large potential windows of water stability (up to 3 V) and mechanical resistance,

boron-doped diamond (BDD) thin films have been widely used as electrodes for electro-analysis and electrolysis.

In this work we have studied surface structural modifications in BDD thin films grown by hot filament chemical vapor deposition

(HFCVD) due to usage as anodic working electrode in H3PO4 aqueous solution (pH=1, 2) up to 1000 h. With the powerful tool of

Micro-Raman spectroscopy we show nonuniform structural variations of the diamond lattice. Modifications in the electrochemical

properties has been observed while BDD electrodes were used as anodes. Moreover, by modelling the Fano interaction between discrete

phonon states and the degenerate continuum of states, we point out an increase of the acceptor levels as a function of the electrode

working hours.

These experimental evidences indicate an anomalous behaviour of hydrogen loaded in the films during the growth process. A model

based on the hydrogen–boron interaction could explain the observed modifications in Fano coupling.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Raman scattering; Boron-doped diamond (BDD); Fano interaction

1. Introduction

Because of its peculiar properties such as wide band gap,

high breakdown voltage, high thermal conductivity, and

small dielectric constant, diamond has been recognized as

suitable material for electronic devices which can operate at

high temperature, high frequency, and high power range.

Semiconducting p-type diamond can be prepared by

chemical vapor deposition with doping boron into the

lattice, adding diborane gas to methane–hydrogen gas

mixtures [1]. Boron-doped Diamond (BDD) films are

considered as one of the most ideal electrodes for electro-

analysis and electrolysis [2,3]. Actually, conductive dia-

mond has several superior characteristics compared to other

carbon materials and metals, including very low back-

ground current density and wide potential window in

aqueous solution [4,5]. However the stability of BDD thin

films working as anode is still a mandatory task to achieve

0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.tsf.2004.11.169

* Corresponding author.

E-mail address: carlo.ricci@dsf.uica.it (P.C. Ricci).

[5,6]. In this work we analyse a commercial BDD thin films

working as anode in acid solution at different time step up

to 1000 h. The analysis has been performed by Raman

spectroscopy, which can provide useful information on

crystal structure without affecting the sample quality.

2. Experimental

Commercial Diamond films (CSEM, the Swiss Center

for Electronics and Microtechnology), 1 Am thick, were

grown by hot filament chemical vapor deposition at 850 8Cfrom a 0.5% H2/CH4 mixture at a pressure of 40 torr. The

substrate was a p-type polished silicon wafer (1 mm) with a

resistivity of about 10 mV cm�1. The nominal concen-

trations of Boron atoms in diamond lattice is 1020 cm�3.

Micro-Raman measurements have been carried out in

backscattering geometry by using the polarized 514.5 nm

line of an Argon-ion laser. Raman scattering measurements

have been performed in air at room temperature with a triple

spectrometer Jobin–Yvonne Dilor integrated system with a

(2005) 311–317

P.C. Ricci et al. / Thin Solid Films 482 (2005) 311–317312

spectral resolution of about 1 cm�1. Spectra have been

recorded in the Stokes region by a 1200 grooves/mm grating

monochromator and CCD detector system. Confocal micro-

scope Olympus B-201 has been used, with an objective of�100 with 0.90 numerical aperture. The spatial resolution

was less than 1 Am.

The electrochemical measurements have been performed

in a conventional three electrode cell with a platinum

counter-electrode and a saturated calomel electrode (SCE)

as reference. The electrode potential is given with respect to

the potential of the SCE.

The BDD films worked as anode in H3PO4 aqueous

solution (pH=1, 2), in dark condition, up to 1000 h. The

procedure was interrupt at regular step in order to measure

the Raman spectra and the voltage–current curves. The

density current was kept at 12 mA/cm2 and the relative

potential varied between 2.7 and 2.8 eV vs. SCE.

The V–I characteristic curves were taken in a 0,1 M

concentration H2SO4 acid solution in potentio-dinamic

regime with a scan rate of 20 mV s�1.

3. Results

Fig. 1 reports the Raman spectrum of the as-prepared

sample. Silicon substrate peaks are located at 520.7 and

950 cm�1. The Raman peak at 1332 cm�1 is attributed to

the contribution of undoped diamond [7]. The two broad

band at the low energy side (about 1200 cm�1) and high

energy side (about 1500 cm�1) of the diamond line have

been previously attributed to reticular distortion of the

diamond structure [7,8] and to the presence of a graphite

Fig. 1. Raman spectrum of the as-prepared BDD sample, no variation have been o

taken after (a) 0, (b) 20, (c) 40, and (d) 60 working minutes.

phase, respectively [9]. The graphite phase in commercial

samples is not unusual; the possibility to reveal a few

percentage of the graphite phase in the diamond lattice

through the Raman analysis is due to the high Raman

efficiency of the graphite band with respect to the diamond

signal (about two orders of magnitude greater) [7,9]. It is

worth to note that no relievable difference has been

observed during the first 200 working hours in the Raman

spectra.

The inset in Fig. 1 reports the characteristic voltage–

current (V–I) curve registered during the first working hour.

Although Raman spectra are unchanged during the first 200

working hours, the V–I curves change drastically during the

first hour, there is a relative high increase of the resistivity

with working time; it is reasonable to suppose that the

electrochemical reaction at the electrode surface does not

change in this short period and the variation in the current

value could be due only to changes in the electrode

properties.

After 350 working hours, the sample presents some

regions (less of 2 Am2) that look paler at optical microscope.

We detect some small area in which is totally absent the

diamond thin film, actually only the 520 and 930 cm�1 both

arising from Si substrate are detected (Fig. 2A). These zones

enlarge with the working time and in the samples after 800

working hours, they are about 10 mm2 wide. On the

boundary of these regions, two bands centered at around

1350 and 1550 cm�1 attributed to sp2 carbon phases

(graphite) [7] are present in addition to the first and second

order silicon peaks, the sharp peak observable in the spectra

can be assigned to a weak contribution of the diamond peak

at 1332 cm�1 (Fig. 2B).

bserved during the first 200 working hours. Inset: V–I characteristic curves

Fig. 2. Raman spectrum of the central part (A) and on the boundary (B) of 1 Am2 zones (inhomogeneous region) observed after 350 working hours of the BDD

thin film.

P.C. Ricci et al. / Thin Solid Films 482 (2005) 311–317 313

Outsides these zones, the samples appear homogeneous;

in Fig. 3, the Raman spectrum taken at different working

hours is reported.

Drastic changes in the spectrum take place at higher

working time; in particular:

– the appearance of a new broad band centered at about

500 cm�1

Fig. 3. Raman spectra of the BDD thin film taken after (A) 0, (B) 200, and (C) 35

taken after (A) 1, (B) 200, (C) 350, and (D) 800 working hours.

– the great enhance of the band at 1200 cm�1

– the increasing asymmetry of one phonon band at 1332

cm�1

The bands at 500 and at 1200 cm�1 were previously

attributed to amorphous diamond and to reticular dis-

tortion [1,7,8]. The reported downshift and the increasing

asymmetry of the 1332 cm�1 diamond peak were

0 working hours in the homogenous region. Inset: V–I characteristic curves

P.C. Ricci et al. / Thin Solid Films 482 (2005) 311–317314

reported by several authors and it was related to the

increasing of the so-called Fano’s interaction induced by

quantum mechanical interference between the discrete

phonons state with a degenerate electronic continuum

[10–15]. The continuum could be identified as the hole

continuum of the acceptors to valence excitations or

inter-valence transitions from the low energy level to

high energy level. Both these phenomena have been

related to the variation of Boron concentration in the

diamond structure [13,16]. Ager et al. [16] considered

that the Fano interference of the Raman phonon in

heavily B-doped diamond due to the transitions between

impurity band and valence band states as the more likely

mechanism, while in heavily boron-doped Silicon inter-

valence band, electronic Raman scattering generates the

Fano-type interference [17,18].

The inset in Fig. 3 shows the V–I characteristic curves

taken at different working time steps. Contrary to the trend

Fig. 4. Fitting curves in the 1250–1350 cm�1 range of the as-prepared samp

of the first hour, an increase of the conductivity in the

sample with working time was observed.

4. Discussion

A drastic degradation of the BDD thin films has been

observed after about 800 hours of work as anode in acid

solution. The samples show some small regions in which the

thin film has been removed and at the boundary of these

zones, only a graphitic phase have been observed.

The Raman spectra outside these zones reveal an increase

of the bands related to an amorphization of the diamond

lattice. Moreover, the variation of the one phonon line of the

diamond structure seems to be well reproduced by the Fano

interaction [15]. To Fit the Raman phonon line, it was

necessary to assume a superposition of a Fano profile and a

Lorenzian line which may indicate inhomogeneous incor-

le (A) and after 800 working hours (B). Refer to the text for details.

Table 1

Fitting parameters obtained Fano’s equation

Lorentzian N/cm-1 W/cm-1

1332 7

Fano’s interaction q G/cm�1 E/cm�1

Sample asQprepared �16 11 1330

Sample after 800 working hour �2,3 16 1324

The parameters of the Lorentzian curve have been fixed during the

deconvolution procedure.

P.C. Ricci et al. / Thin Solid Films 482 (2005) 311–317 315

poration of the boron in the films [19]. We have simulated

the curves using the equation:

y¼A0

� jqþ� x�$

C

�j2

1þ� x�$

C

�2�þ2

A1

p

�w

4 x�x0ð Þ2þw2

�ð1Þ

where the first part represent the interaction between the

discrete phonon state of the diamond at 1332 cm�1 and the

electronic continuum generated by the Boron incorporation:

q is a line-shape parameter, C is the broadening

parameter of the one-phonon line width, - is the peak

frequency in the doped crystal. It is worth to note that the

Fano’s equation becomes a Lorentzian as qYFl.

The second part in Eq. (1) is a Lorentzian curve and

represent the contribution of the intrinsic diamond crystal

x0 and w are the phonon frequency and phonon width of the

intrinsic diamond crystal, respectively. The computing

simulation process was conduct on the Raman spectra of

the sample as-prepared and after 800 working hours in the

1260–1380 cm�1 region (Fig. 4A and B, respectively). The

Fig. 5. Raman spectra of the BDD thin film taken after (A) 0 and (B) 800 worki

region.

parameter of the intrinsic crystal lattice have been fixed at

x0=1332 cm�1 and w=7 cm�1 while the relative intensity

parameters A0 and A1 and the three Fano’s parameter q, C,

and - have been set free. The results, reported in Table 1,

show a decreasing of the q absolute value, an increase of the

broadening parameter C and a downshift of the phonon

frequency - for the sample after 800 working hours respect

to the as-prepared condition. Similar results have been

obtained by several authors in sample with increasing Boron

concentration [10–14]. For example, Ushizawa et al. [14]

assigned to 1018 cm�3 and to 1020 cm�3 boron concen-

tration in samples with fitting parameters comparable to the

ones obtained in this work for the as-prepared and after 800

working hours sample, respectively.

No new Boron atoms were added to the BDD thin film

during the electrochemical process, but what was varied in

the BDD thin film is the acceptor levels concentration and

the hole as free carrier. During the HFCVD growth process,

the Hydrogen atoms can form B–H pairs which gives rise

to a passivation of Boron acceptors. According to Mehand-

ru and Anderson [20], during the anodization process, the

B–H complex can dissociate following two different

reactions:

B : HYB� þ Hþ

B : HYB� þ H0 þ hþ ð2Þ

Both processes can free the acceptor level associated to

the Boron atom. The observed increase of the Fano

resonance (Fig. 3) can be associated to the increase of

the interaction between the discrete phonon and the

electronic continuum. The dissociation process both

ng hours in the homogenous region in the high energy frequency spectrum

P.C. Ricci et al. / Thin Solid Films 482 (2005) 311–317316

increases the acceptor levels density (Fano interference due

to the interactions between the discrete phonon with

valence to impurity transitions) and increases the holes as

free carriers (Fano interference due to the interference

between the discrete phonon state and the inter-valence

band transitions).

Moreover, the conduction mechanism in BDD elec-

trode is strongly related to the concentration of the

acceptors levels and to the hole free carriers [21].

Opposite to the first working hour, during which the

resistivity increases with working time (inset of Fig. 1),

the resistivity decreases increasing the working time

(inset of Fig. 3). It has been proposed that the behavior

during the first hour arises from a passive layer,

generated on diamond surfaces by anodic pretreatment

[5]. The passive films is generated by the removal of

Hydrogen acting as acceptor in the subsurface region (H-

terminated diamond surface), leaving the trapped Hydro-

gen as H+B� dimers (O- termination surface). It is well

know that Hydrogen affects the electrical properties of

semiconductors, because of its fast diffusion and its

strong tendency to passivate electrically active sites. In

the case of p-type diamond, Hydrogen–Boron interactions

generate B–H pairs leading to a passivation of Boron

acceptors [5,22–24], reducing the hole concentration and

therefore the current intensity.

Therefore, the increased value of the current measured in

the V–I characteristic curves in along range of working time

can be related to the B–H dissociation process.

Moreover, in diamond films, the reduction of the

resistivity may also be due to the formation of the graphitic

phase. The distortion of the diamond structure has been

revealed by the formation of the band at 500 and 1200

cm�1 and can be due to the accumulation of Hydrogen at

the grain boundary of the polycrystalline film and in

interstitial sites. The increased concentration of Hydrogen

at the surface can be proved examining the Raman

spectrum of the degraded electrode in the high-frequency

region (Fig. 5). The broad and composite Raman band

between 2850 and 3000 cm�1 can be assigned to CHx

bonds [25]. It has been observed [25,26] that the CHx

bonds appear only after electrochemical treatment and,

according to Kondo et al. [5], the hydrogen-terminated

diamond surface is related to high concentration of hydro-

gen on the BDD surface.

The increased concentration of free H+ and H0, as well

their migration related to the electric field and diffusion

process generated by the fast oxidation process, can

accelerate the degradation of the diamond crystal and the

formation of the graphite phase. The process is not

uniform on the sample surface and we observe a random

degradation of the sample. This can be due to the

inhomogeneous formation of a graphite phase that under

the working electric voltage (about 2.7 V) determines fast

breaking of sp2 C–C bonds and removal of the BDD thin

film.

5. Conclusions

In the present study, the electrochemical stability of

Boron-doped Diamond thin film electrodes was studied

through Raman scattering analysis. Modifications of the

structure were observed from about 350 anode working

hours and a significant degradation of the sample was

revealed after 800 working hours. The Raman spectra show

the appearance and the enhancing of the band at 500 and

1200 cm�1 related to lattice distortion of the diamond

structure and the increasing of the Fano interaction, induced

by quantum mechanical interference between the diamond

discrete phonons state and the continuum (as acceptor to

band excitations and/or inter-valence transitions) by Boron

acceptor levels. We propose that the dissociation of the B–

H pairs during the anodising process enhances the accept-

ors concentration and the accumulation of free Hydrogen in

grain boundaries and in interstitial sites. This process can

affect the diamond crystal structure with the formation of

the graphitic phase randomly distributed in the electrode

and the progressive removal of the BDD thin film.

Acknowledgments

The authors would like to thank the Prof. Anna Maria

Polcaro for helping in the electrochemical measurements

and for the interesting discussions.

References

[1] R.J. Zhang, S.T. Lee, Y.W. Lam, Diamond Relat. Mater. 5 (1996)

1288.

[2] D. Gandini, E. Mahe, P.A. Michaud, W. Haenni, A. Perret, C.

Comminellis, J. Appl. Electrochem. 30 (2000) 1345.

[3] H.B. Suffredini, S.A.S. Machado, L.A. Avaca, J. Braz. Chem. Soc. 15

(2004) 16.

[4] R. DeClements, G. Mswain, J. Electrochem. Soc. 144 (1994) 3382.

[5] T. Kondo, K. Honda, D.A. Tryk, A. Fujishima, Electrochim. Acta 48

(2003) 2739.

[6] K. Hayashi, S. Yamanaka, H. Watanabe, T. Sekiguchi, H. Okushi, K.

Kajimura, J. Appl. Phys. 81 (1997) 744.

[7] P.G. Buckley, T.D. Moustakas, L. Ye, J. Varon, J. Appl. Phys. 66

(1989) 3595.

[8] P. Gonon, E. Gheeraert, A. Deneuville, F. Fontaine, L. Abello, G.

Lucazeau, J. Appl. Phys. 78 (1995) 7059.

[9] Y. Hishiyama, H. Irumano, Y. Kaburagi, Y. Soneda, Phys. Rev., B 63

(2001) 245406.

[10] K.W.R. Gilkes, S. Prawer, K.W. Nugent, J. Robertson, H.S. Sands, Y.

Lisfshitz, X. Shi, J. Appl. Phys. 87 (2000) 7283.

[11] K. Ushizawa, K. Watanabe, T. Ando, I. Sakaguchi, M. Nishitani-

Gamo, Y. Sato, H. Kanda, Diamond Relat. Mater. 7 (1998) 1719.

[12] Y. Wang, H. Li, Y. Xiong, S. Zhang, Z. Lin, K. Feng, Diamond Relat.

Mater. 9 (2000) 1708.

[13] F. Pruvost, A. Deneuville, Diamond Relat. Mater. 10 (2001) 531.

[14] K. Ushizawa, M.N. Gamo, F. Watanabe, I. Sakaguchi, Y. Sato, T.

Ando, J. Raman Spectrosc. 30 (1999) 957.

[15] M. Cardona, G. Guntherodt, Light scattering in solids IV 1984,

Springer, Berlin, 1978.

P.C. Ricci et al. / Thin Solid Films 482 (2005) 311–317 317

[16] J.W Ager, W. Walukiewicz, M. McCluskey, M.A. Plano, M.I.

Landstrass, Appl. Phys. Lett. 66 (1995) 616.

[17] F. Cerdeira, T.A. Fjeldly, M. Cardona, Phys. Rev., B 8 (1973)

4734.

[18] R. Gajic, D. Braun, F. Kuchar, A. Golubovic, R. Kortntner, H.

Loschner, J. Butschke, R. Springer, F. Letzkus, J. Phys. Chem., B

Condens. Mater. Surf. Interfaces Biophys. 15 (2003) 2923.

[19] R. Locher, J. Wagner, F. Fuchs, C. Wild, P. Hiesinger, P. Gonon, P.

Koidl, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 29 (1995)

211.

[20] S.P. Mehandru, A.B. Anderson, J. Mater. Res. 9 (1994) 383.

[21] B. Massarani, J.C. Bourgoin, R.M. Chrenko, Phys. Rev., B 17 (1978)

1758.

[22] K. Thonke, Semicond. Sci. Technol. 8 (2003) S20.

[23] D.A. Tryk, K. Tsunozaki, T.N. Rao, A. Fujishima, Diamond Relat.

Mater. 10 (2001) 1804.

[24] R. Zeisel, C.E. Nebel, M. Stutzmann, Diamond Relat. Mater. 9 (2000)

413.

[25] M. Bernard, A. Deneuville, E. Bustarret, Diamond Relat. Mater. 11

(2002) 662.

[26] G.H. Goeting, F. Marken, A. Gutierrez Sosa, R.G. Compton, J.S.

Foord, Diamond Relat. Mater. 9 (2000) 390.