Growth and properties of nanocrystalline diamond films

phys. stat. sol. (a) 203, No. 13, 3375–3386 (2006) / DOI 10.1002/pssa.200671406

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Original

Paper

Review Article

Growth and properties of nanocrystalline diamond films

Oliver A. Williams1 and Miloš Nesládek

*, 2

1 Institute for Materials Research (IMO), Hasselt University, Wetenschapspark 1,

3590 Diepenbeek, Belgium 2 CEA-LIST (Recherche Technologique), CEA-Saclay, 91191 Gif sur Yvette, France

Received 15 March 2006, revised 6 June 2006, accepted 9 August 2006

Published online 11 October 2006

PACS 73.50.–h, 81.05.Kw, 81.15.Gh

The aim of this paper is to summarise recent progress in the growth of small grain-sized Chemical Vapor

Deposition (CVD) diamond often called nanocrystalline diamond, i.e., diamond with grains typically

smaller than 500 nm. Nanocrystalline (NCD) and Ultrananocrystalline diamond (UNCD) films are new

materials offering interesting applications to nanobioelectronics and electrochemistry. However NCD and

UNCD thin films comprise of entirely different structures which is highlighted here in this paper. We dis-

cuss in detail the main differences in Raman spectra, optical properties and electrical transport properties.

Finally we present a simple model of the conductivity mechanism in nitrogenated UNCD (N-UNCD) and

boron doped NCD (B-NCD) films, and show the possibility of achieving the superconductive transition in

B-NCD films.

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction

Recent progress in nanotechnology has motivated research into small grain-sized diamond films known

as nano-crystalline (NCD) and ultra-nano-crystalline diamond (UNCD) [1–6]. These materials have

rather unusual properties compared to their single-crystal diamond counterpart and can be deposited

from submicron thicknesses on wide range of substrates including glass, Si and several metals. NCD and

UNCD are both intended for the emerging Nano/Micro-Electro-mechanical Systems (N/MEMS) type of

device applications, optical coatings, thermal management and finally electrochemistry and bioelectron-

ics [3, 4]. However NCD and UNCD are two entirely different forms of diamond: UNCD is grown from

Ar-rich plasma and it is a fine grained material (5–15 nm) [1], with grain sizes practically independent

of the film thickness, due to the fact that UNCD re-nucleates continuously during film growth. Conse-

quently UNCD is composed of grain boundaries exceeding 10% of the total volume fraction and leading

to a great proportion of non-diamond or disordered carbon in the films. On the contrary, NCD films are

prepared from more conventional diamond plasma chemistry, under atomic hydrogen-rich conditions,

resulting in grain sizes which scale with the film thickness, due to the columnar type of film growth [5].

The grain sizes of NCD are usually quoted as below 100 nm, but grain sizes and films thickness from

about 30 nm are nowadays possible [6]. Based on the continuous etching of sp2 carbon by atomic hydro-

gen during growth, the re-nucleation is suppressed during NCD growth, thus NCD has far lower propor-

tions of sp2 carbon than the UNCD films. This makes the NCD advantageous for applications to electro-

chemistry, where often sp2 carbon influences the electrode stability.

* Corresponding author: e-mail: [email protected]

3376 O. A. Williams and M. Nesládek: Growth and properties of nanocrystalline diamond films

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-a.com

The electrical conduction mechanism in these two types of films is entirely different. In UNCD, by the

addition of nitrogen into the gas phase it is possible to generate a high n-type conductivity [7]. However

this type of electrical transport is dominated by the grain boundary conduction with practically 2D type

of conduction, which we demonstrate in this paper by magnetotransport measurements at low tempera-

tures, and which leads to week localisation effects [8]. By means of using an admixture of 10–20% ni-

trogen (N2) into the gas phase, high electric conductivity (∼1–10 × 102 S cm–1) can be achieved [2, 7].

On the other hand, in the NCD films the conduction relies on substitutional doping of diamond grains

with impurities such as boron [9]. The bulk doping of NCD with boron can yield superconductivity [10]

if metallic doping is achieved. The main aim of this review is to compare the structural, optical and elec-

trical properties of NCD and UNCD.

2 Growth

Undoped and boron-doped nanocrystalline diamond films can be grown from a variety of conditions,

with common features of relatively low pressures and high nucleation densities. To achieve these high

nucleation densities, several approaches can be adopted. Nucleation densities up to around 1011 cm–2 can

be realized using vibration seeding, a process where the silicon wafer is placed on a polishing cloth im-

pregnated with diamond powder on a vibration table. A weight is placed on top of the silicon wafer and

the vibration table switched on for around 30 min. This process has the advantage of being very repro-

ducible but has several disadvantages. Firstly, the maximum nucleation density is about an order of mag-

nitude below the state of the art. Secondly, as the process is used repetitively, build up of damaged sili-

con/foreign particles from previously seeded substrates can lead to scratching of new wafers. Thirdly,

some agglomerates of diamond powder may be left on the surfaces of seeded wafers, resulting in higher

surface roughness. An alternative process is ultrasonic seeding in diamond suspensions of alcohol and

diamond powders, a technique which is best described as a “black art”, relying on many proprietary

protocols. In this process diamond powders, usually ultra-dispersed diamond (UDD) [11] are mixed with

an alcohol to form a pseudo-stable suspension. The concentration of this solution is crucial and pretreat-

ments of diamond powders can also influence stability and effectiveness of the suspension. The wafer is

placed into this suspension and into an ultrasonic bath or horn. Figure 1 shows examples of these two

seeding procedures.

The image on the left side shows an example of poorly optimized seeding in ultrasonic solution. The

nucleation density shown here is rather low and can be improved substantially, however, it can be seen

that the nuclei have uniform size and there is no visible damage of the silicon wafer. The image on the

Fig. 1 Early stage of diamond nucleation obtained by a MW-CVD growth using 1% CH4 at 30 Torr for

10 minutes. (a) corresponds to ultrasonic seeding and (b) seeding by an ultrasonic vibration table, dis-

cussed in the text.

phys. stat. sol. (a) 203, No. 13 (2006) 3377

www.pss-a.com © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Original

Paper

right side of Fig. 1 shows a film seeded with the vibration table method. Here the nucleation density is higher but there are large particles left on the silicon wafer and there are also fine scratches visible to the eye. Ultimately, seeding in solution can yield higher nucleation densities (over 1012 cm–2), can treat three dimensional topographies and does less damage to silicon wafers than the vibration table technique. The principal problem with seeding in solution is the aggregation of fine particles into much bigger sizes than the core particles. This can leave hazy or opaque regions on the wafer due to the presence of residual powder. UNCD and NCD can both be grown on a plethora of substrates, Si, SiO2, SiC, Al, Cu, refractory metals, low alkaline glasses such as Corning Eagle 2000 etc. Nucleation densities tend to be highest on silicon and refractory metals. Following seeding procedures, wafers were cleaned in a pure alcohol solu-tion and placed inside a microwave plasma chemical vapour deposition reactor. Growth procedures were as follows. For NCD, films were grown in a hydrogen rich plasma with a small percentage of methane. For doping, trimethylboron was also added to the gas phase. Typical microwave powers were between 1000 W and 2500 W depending on substrate size and process pressure. Temperatures between 500 °C and 800 °C were monitored by dual wavelength pyrometry, the actual value again chosen for the relevant substrate. An example of the morphology of a ∼400 nm thick NCD films is shown on the left of Fig. 2. It can be seen from this figure that the average grain size is below 100 nm, with the occasional larger grain and there are no macroscopic pin holes visible in the film. Thus the film is totally closed at a thick-ness of 100 nm, in fact we are able to grow close films as thin as 60 nm. Further optimization of seeding procedures will allow this minimum thickness of coalesced films to be reduced. An example of a cross section of a much thicker film is shown on the right side of Fig. 2. Here, the evolution of the microstruc-ture is clearly visible, the grains increasing in size with increasing distance from the silicon substrate. This example was chosen as a particularly poor film grown with high CH4 admixture. This film is clearly approaching a microcrystalline structure rather than NCD. This phenomena is a key issue when growing NCD, as they are fundamentally a form of van der Drift growth, becoming microcrystalline as the bigger grains overtake the smaller ones. Thus the term “survival of the largest”, is a good description of the overall process. This means that high nucleation densities are essential to ensure early coalescence of films. The high density of initial grains prevents the formation of large grains which can otherwise domi-nate the morphology from the beginning. Low methane levels also help ensure that the distribution of grain size remains narrow. In stark contrast, UNCD films were grown in hydrogen poor plasmas. Here, low percentages of meth-ane (around 1%) were diluted in Ar. Typical microwave power and pressure levels were 1400 W and 150 Torr respectively. The substrate temperature was monitored by a thermocouple attached to the sub-strate holder and was maintained at 800 °C. The conductivity of UNCD can also be controlled by the addition of nitrogen into the gas phase [7], reducing the Ar rate to maintain a constant total gas flow. Transmission Electron Microscopy has shown that these films have grain sizes of 3–5 nm with abrupt grain boundaries [12]. The films appear highly reflective but are absorbing in the visible spectrum. SEM images of a typical UNCD film can be seen in Fig. 3.

Fig. 2 Top and cross-sectional view scanning electron micrographs of ∼400 nm thin nanocrystalline

diamond (NCD) films grown from hydrogen rich plasma on Si substrate.



Fig. 3 Top and cross-sectional scanning electron micrograph of ∼1000 nm thin ultrananocrystalline dia-

mond (UNCD) film grown from Ar-rich plasma on Si substrate.

The left image is the plan view SEM. It can be seen that there is no direct evidence of crystallinity at

this length scale as the grains are too small. The cross section in the right of Fig. 3 shows that this mate-

rial shows no columnar structure and is thus fundamentally different to the van der Drift growth mode

seen in NCD. This is due to the high re-nucleation rate of the hydrogen poor Ar/CH4 plasma, leading to

continuous nucleation of new crystals and thus limiting the maximum grain size. One advantage of this is

that the surface roughness is independent of the film thickness, as unlike in NCD, the grains are not ex-

panding and competing. A disadvantage of this is that this material really consists of two phases, the

material in the grains and that at the grains boundaries. Of course NCD also consists of grain boundary

material but the volume fraction is considerably less, in fact minimal relative to the 10% of UNCD car-

bon at the grain boundaries. These key differences in carbon configuration can be characterised by Ra-

man spectroscopy as shown in Fig. 4.

1.E+03

1.E+04

1.E+05

1.E+06

0 500 1000 1500 2000 2500

Wavenumber (cm-1)

Ram

an

inte

nsi

ty (

A.U

.)

8% N2

10% N2

Undoped

(a)

800 1000 1200 1400 1600 18000

1000

2000

3000

4000

Ram

anin

tens

ity(a

.u)

Wavenumber (cm-1)

1041005 450 nm3041005D 180 nm

(b)

Fig. 4 (online colour at: www.pss-a.com) Comparison of Raman spectra of undoped and N-doped ultrananocrys-

talline (UNCD) thin films, prepared from the Ar-rich plasma (a) and undoped nanocrystalline diamond (NCD) (b),

prepared from the H-rich plasma. Specifically, the UNCD films exhibit broad features centered at 1370 cm–1 and

1550 cm–1. For low thickness the NCD films show a quite similar features as UNCD. For larger thickness a clear

zone centre diamond peak at 1332 cm–1 can be established. The position of the main Raman lines is discussed in the

text.

a) b)

phys. stat. sol. (a) 203, No. 13 (2006) 3379


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3 Raman spectra of NCD and UNCD films

The Raman spectra in Fig. 4 show very typical Raman spectral shapes for UNCD and NCD samples.

There are major differences in the Raman spectra of UNCD and NCD films. The Raman spectra for

variously N-doped UNCD films, detailed in Refs. [13, 14], exhibit two broad features at about 1370 cm–1

and 1550 cm–1 with no clear zone-centre peak for diamond at 1332 cm–1. These characteristic spectra can

be explained by the fact that UNCD is comprised of nanograins of sp3 carbon surrounded by grain boun-

daries of non-diamond carbon, consisting of several bonding configuration (sp, sp2) and also disordered

carbon. The grain boundary carbon occupies around 10% of the total film volume and thus little evidence

of diamond is seen in visible Raman due to the enhancement of sp2 peaks at this wavelength. The broad-

ened zone centre Raman line can be detected with UV excitation. The Raman spectra of UNCD films are

also practically independent of nitrogen content in the gas phase, suggesting that the non-sp3 components

dominate the Raman spectra. However, the Raman spectra for NCD films which are entirely different,

reflecting the fact that NCD films are similar to columnar microcrystalline diamond, just with reduced

grain sizes and thickness. To illustrate this difference Fig. 4 compares the Raman spectra for undoped

UNCD and NCD films with thicknesses of about 500 nm. In NCD films the zone-centre phonon line at

1332 cm–1 is clearly visible together with a graphitic peak at about 1500 cm–1. It should be noted that the

Raman spectral shape also depends on the NCD thickness. I.e., for very thin NCD layers in the range of

50 nm thickness, D and G peaks appear under red light excitation, related to disordered carbon present in

larger relative proportion than that of thicker films. For such thin films, peaks at 1120 cm–1 and 1450 cm–1

associated with transpolyacetylene at the grain boundaries are often detected. An interesting phenome-

non occurs upon boron (B) doping of NCD films, detailed in Fig. 5. For high B-concentrations, in the

range around 2 × 1020 cm–3 or higher, we can see the so called Fano resonance, which will be discussed

below, confirming a substitutional B-incorporation in the diamond lattice. Figure 5 shows Raman spectra

for two heavily B-doped samples containing 5 × 1019 cm–3 and 4 × 1020 cm–3 boron, as calibrated by

SIMS measurements.

Upon increasing B-concentration the Raman zone-centre phonon line at about 1332 cm–1. The position

of the central zone phonon at 1332 cm–1 is marked by a dash line in Fig. 5. Upon increasing B-concen-

tration, an asymmetric shoulder at about 1230 cm–1 appears with a broad tail towards lower wavenum-

600 800 1000 1200 1400 1600 1800 2000500

1000

1500

2000

2500

3000

3500

4000

4500

NB

= 4x1020 cm-3

Ram

anin

tens

ity(a

rb.u

nits

)

Wavenumber ( cm-1)

Fano resonance

NB= 5x1019cm-3

1332 cm-1

Fig. 5 (online colour at: www.pss-a.com) Raman spectra of variously boron-doped nanocrystalline dia-

mond films with B:C gas doping ratio of 800 and 3000 ppm (top and bottom curve in the figure). The po-

sition of the central zone phonon at 1332 cm–1 is marked by a dash line. The Fano resonance is observable

as a broad peak at about 1230 cm–1 and as a deformation of the zone-centre phonon line.



bers. Similar results have been found for heavily B-doped single crystal homoepitaxial layers with

B-concentrations close to the Metal–Insulator Transition (MIT) transition (NB > 2 × 1020 cm–3). The peak

at 1230 cm–1, typical for heavily B-doped diamond with B concentration above MIT was explained by a

Fano resonance, e.g. quantum interferences between the continuum of electronic excitations and a zone-

centre phonon [15]. An additional interesting characteristic in Fig. 4 is that sp2 related Raman resonances

in heavily B-doped films are significantly reduced and for the highest doping levels become almost in-

visible. This fact suggests that the grain boundaries of heavily B-NCD films are partially compensated

by such way that sp2 carbon in the grain boundaries is changed to a Raman inactive mode. This hypothe-

sis needs further TEM study but one possible explanation is that segregation of boron at grain boundaries

occurs which leads by subsequent interactions with grain boundary carbon atoms to a generation of Ra-

man inactive B-rich BxC1–x phases. The segregation of boron onto the grains boundaries can be easily

seen based on comparison of SIMS and Hall measurements data. In the metallic regime (there is 1:1

ration between the boron concentration and free carrier concentration above MIT) we can clearly see that

the B-concentration obtained from SIMS is a factor of two higher than the carrier concentration obtained

from Hall measurements, suggesting the existence of some additional non-active boron in the B-NCD

grain boundaries.

4 Optical properties of UNCD and B-NCD films

Higher optical transparency of thin NCD films compared to UNCD is an important advantage for several

applications requiring a combination of high conduction with full optical transparency. In general, the

intrinsic optical absorption is related to the presence of non-diamond carbon such as sp2 bonded carbon,

present mainly at grain boundaries. Figure 6 compares the optical absorption of thin UNCD and NCD

films, deposited on quartz substrate with the optical absorption of highly transparent CVD microcrystal-

line diamond [16].

Figure 6 demonstrates that the intrinsic absorption in diamond is sensitive to the grain size and thus

the concentration of non-diamond carbon located at grain boundaries. However, the character of the

broad absorption continuum, which decays towards the IR is essentially the same. It has been suggested

by Zapol et al. [17] based on Molecular Dynamic simulations, that a highly disordered subsystem at grain

boundaries leads to an increase of the subgap density of states (DOS), located near the midgap. If nitro-

gen is introduced into the gas phase, the nitrogen further promotes the formation of midgap π-states. The

essence of this picture is that the increased DOS in nanocrystalline diamond relates to sp2 bonded carbon

PDS absorption data :UNCD and NCD films on quartz

1.E+02

1.E+03

1.E+04

1.E+05

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00Photon Energy (eV)

Abs

orp.

coef

f.(1

/cm

)

NCD film on glass

N-UNCD (10% N) film on quartz

Undoped UNCD film on quartz

Fig. 6 (online colour at: www.pss-a.com) Optical absorption spectra calculated from photothermal de-

flection spectroscopy (PDS) for nanocrystalline (NCD) and ultrananocrystalline diamond (UNCD) films.

phys. stat. sol. (a) 203, No. 13 (2006) 3381


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0 500 1000 1500 2000 2500 3000

0

20

40

60

80

100T

rans

mita

nce

(%)

Wavelength (nm)

glass substrateNCD M2, 220 nmNCD M3 90 nm

NCD ON CORNING 1737 GLASS

Fig. 7 (online colour at: www.pss-a.com) Optical transmission data measured from IR to near UV range

for two undoped nanocrystalline (NCD) films, deposited on Corning #1737 glass substrates. The thickness

of the films measured was 80 to 240 nm. Transmission data of Corning glass substrate is shown for com-

parison.

in grain boundaries as described in our previous work on microcrystalline diamond [18]. In Ref. [18] we

have used a two Gaussian distribution for π–π* states to model such broad optical absorption in dia-

mond films. Figure 6 thus clearly point towards the fact that in microcrystalline diamond, nanocrystalline

diamond and ultrananocrystalline diamond the absorption spectra are related to the same type of non-

diamond carbon defects of π-type state origin. However, its concentration increases progressively from

microcrystalline diamond towards UNCD structure. In UNCD films, the absorption coefficient is close to

that expected for metallic type of behavior (104 cm–1). Compared to UNCD, NCD films have overall

lower optical absorption. This is consistent with the columnar NCD growth, which leads to the develop-

ment of large grains, identical to microcrystalline diamond. Thus the NCD films can be truly considered

as an early stage of microcrystalline diamond growth. As an estimation, the optical absorption coefficient

of about 103 cm–1 for a 100 nm thick layer leads to the absorbance of just about 1%. This is the main

reason why NCD films show very high transparency despite the fact that they have a relatively high

absorption coefficient compared to natural or single crystal CVD diamond. Figure 7 shows the optical

transmission data for several undoped NCD films of various film thicknesses.

Figure 7 thus clearly demonstrate high optical transparency if NCD films. The transmission spectra

are modulated by an interference pattern, which gives the samples a characteristic colour. Typically, the

color of a “just coalesced” layer, i.e. a layer with grain sizes equal to the films thickness with nucleation

density at about 1011 cm–2, and about 70 nm thick, is a blue. B-doping has a significant influence on the

absorption of NCD films. When boron is incorporated in the diamond lattice it leads to the blue colour

background, independent of interference effects as discussed above. The influence of boron on the elec-

trical transport properties is discussed in detail in following paragraph.

5 Doping and transport measurements

One of the aims of this chapter is to compare the transport properties of doped UNCD and NCD films. In

summary, in nitrogenated UNCD films, nitrogen does not act as a donor (nitrogen is a deep n-type do-

pant 1.7 eV from the conduction band minimum) and therefore does not significantly influence the con-

ductivity of the UNCD diamond films. Rather, the high incorporation of nitrogen atoms in UNCD leads

just to the genesis of a transport path related to a highly conductive subsystem of π-states located in the

grain boundaries. A rough estimate for the conductivity of such a subsystem, using of Mott’s metallic



transition theorem [19]: M 0 A

(8π / ) ,Eσ κε ε= (where the relative permittivity ε = 5.5, the dimensionless

constant κ which ranges for various possible arrangements of doping centres from 0.025 to 0.05) leads

to reasonable limits for σM at room temperature of about σM ≈ 10 S cm–1 for the activation energy of

conductivity EA, with the directly deduced from the slope of temperature dependence plotted in Arhenius

coordinates. The calculate value does lead is close to the typical experimentally measured values of

10–100 S cm–1. Thus we attribute the metallic conduction in nitrogenated UNCD to transport in the grain

boundary π-states, inducing a metallic conductivity. The details can be found in Ref. [8].

On the other hand, in case of boron-doped NCD films, boron is incorporated substitutionaly into the

diamond lattice. Boron incorporates rather easy due to the fact that the boron atom has a covalent radius

which is close to that of carbon, and therefore fits into substitutional sites. An isolated boron atom be-

longs to the third group of the periodic table and forms in diamond an acceptor state (p-type impurity),

having a binding energy of ~0.37 eV. This is obviously much larger acceptor activation energy as in

silicon; however B is the shallowest impurity in diamond. Bohr’s radius (aB ≈ 3.1 × 10–10 m) of boron is

comparable with the lattice constant of diamond (a ≈ 3.56 × 10–10 m) and boron therefore behaves like a

typical shallow acceptor. B-NCD films show the same properties as bulk diamond with boron substitu-

tionaly incorporated into the diamond lattice. There are several substantiations for this statement, starting

from intense blue colour of B-NCD films, observed Fano resonances and observed superconductivity,

similar to microcrystalline or epitaxial B-doped diamond [15].

Figure 8 shows for comparison, a set of magnetoresistance (MR) curves measured at magnetic fields

perpendicular to the sample and at various temperatures ranging from 1.28 K to 3.8 K for both N-UNCD

and B-NCD films.

0 1 2 3 4 5 6 7 83000

4000

5000

6000

3.96 K

2.97 K

2.32 K

1.87 K

1.59 K

R( Ω

)

B (T)

1.29 K

B-doped UNCD(sample M1 5000 ppm B:C)

(b) Fig. 8 (online colour at: www.pss-a.com) Magentoresistance data (a) measured at LHeT for 20% N-doped UNCD

films, prepared at Argonne Laboratories U.S. The inset shows the dependence of conductivity on the critical mag-

netic field Bk for which the delocalisation of wavefunction occurs according to the Eq. (1), discussed in the text,

(starting at the Bk critical magnetic field onset). (b) The magnetorestistance data for B-doped NCD sample with

B-concentration of 5 × 1019 cm–3 showing a positive magnetoresistance based on the shrinkage of the wave function

with applied magnetic field.

a)

b)

phys. stat. sol. (a) 203, No. 13 (2006) 3383


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For N-UNCD films (see Fig. 8a), the resistance increases as the temperature decreases at zero mag-

netic fields. This transport is atypical for ordered metals and it could be interpreted as a symptom of a

disordered transport. Further on, for UNCD samples the magnetoresistance data show a reduction of

electrical resistivity with applied magnetic field. When looking carefully at the magnetoresistance curves

in Fig. 8a, after passing a certain amplitude Bk, the MR dependences starts to quickly decrease. The total

decrease of resistance due to a magnetic field of 8 T reaches a value of 28% at ∼1.28 K. As far as we

know, the only effect, which can account for such a huge (giant) NMR could be the effect of weak local-

ization (WL). Recently we have tried to model such kind of NMR using the WL model, knowing to be

used for explanation of the negative magnetoresistance in of disordered metals and semiconductors [20,

21]. Our modelling [8] led to derivation of the formula relating the critical magnetic field Bk to the film

conductivity G.

( )1/ 2

k 0( / ) .B e G Gγ= - (1)

This formula, derived by Mares at al in Ref. [8] relates a critical magnetic field Bk for the transition to

delocalised conduction, induced by the magnetic field perturbation. The term G−G0 is usually called

“quantum correction,” with G0, γ being empirical constants. When using the experimental Bk values from

Fig. 8a, an excellent agreement with experimental data suggests validity of our assumption of weak lo-

calisation transport at low temperatures (see the inset of Fig. 8a, where the square root dependence of the

experimentally determined Bk is plotted as a function of measured conductivities, and fitted using the

formulae (1)). It supports the fact that transport in this subsystem has a character comparable to low-

dimensional (2D) disordered metals, which is controlled by quantum interference effects of electrons

resulting in their weak localization. Estimates for the dimensions of the localization lengths Lϕ based on

the experimentally determined values of Bk for the weakly localized orbits leads to Lϕ about 7 nm. So the

extent of phase-coherent wave functions is comparable with the grain sizes, observed by TEM in our

UNCD films (10–16 nm). This conduction, similar as in disordered metal, explains the negative Hall

sign on one side and untypical temperature dependence of conductivity on the other.

In contrast, B-NCD films show an entirely different LT transport behaviour. To elucidate this trans-

port mechanism Fig. 8b shows the magnetoresistance at liquid helium temperatures for a B-NCD film

where the boron concentration was below MIT transition (SIMS data show the B-concentration in the

range of 5 × 1019 cm–3). In contrast to UNCD, B-NCD films show a positive magnetoresistance (e.g. an

increase of the resistance with magnetic field). Such kind of transport could be explained by localization

of the wavefunction on boron atoms [10]. With increasing magnetic field, a shrinkage of the extension of

1.0 1.5 2.0 2.5 3.0 3.5 4.0

20

40

130140150160170180190

Res

istiv

ity(Ω

)

T (K)

B=0

3000 ppm B (Nc~1.7 × 10 21cm-3; 420 nm)

Tc ~ 1.7 K

Fig. 9 LHe temperature electrical resistivity meas-

urements for boron-doped nanocrystalline diamond

(B-NCD) films, exhibiting the RT carrier con-

centration NB equal to 1.7 × 1021 cm–3. The sample

shows a superconductive transition at Tc ~ 1.7 K.



wave functions leads to a progressive localization, resulting in a decrease of the conductivity with mag-

netic field, giving rise to a positive magnetoresistance [10]. This picture is consistent with the substitu-

tional incorporation of boron into the diamond lattice and reduced effect of conductance via grain

boundaries. Recently, superconductivity has been observed for heavily B-doped CVD diamond films

with substitutional B-incorporation [22–24]. Thus for B-NCD we can also expect a superconductive

transition at low temperatures. Figure 9 depicts low temperature behaviour of conductivity in a B-doped

NCD film of thickness 420 nm, with a room temperature carrier concentration for about 1.6 × 1021 cm–3

and a room temperature mobility of about 1 cm2 V–1 s–1.

At temperatures of about 1.7 K the resistivity drops below 10–4 Ω, which is the limit of our van der

Paw LT Hall setup [8]. For comparison, the epitaxial B-doped layers grown on (100) surface in Ref. [15]

have shown the SC transition at a Tc of 1.4 K for N = 1.2 × 1021 cm–3, this is close to values found in

B-NCD diamond thin films. Thus it is clear that at LT the conduction mechanism in BNCD films is sig-

nificantly different to UNCD and at the same time almost identical to conduction in heavily substitution-

aly B-doped diamond. Finally Fig. 10 shows the conductivity values for B-NCD layers for three B-con-

centrations, specifically for 600, 800 and 3000 ppm B:C ratio.

It can be established that heavily boron doped NCD films indeed show very high conductivity, simi-

larities to heavily B-doped epitaxial films, with the MIT transition observed by a strong increase in con-

ductivity at B-concentrations of about 2–3 × 1020 cm–3 [8, 10] heavily B-doped diamond single crystals

show hopping transport just below the MIT, whilst samples with B-concentration above MIT show

semimetal transport behaviour [25]. In our case, for concentration above the MIT transition, the Hall

coefficient confirms hole type of conduction with mobilities in the range of 0.5–2 cm2 V–1 s–1. These

mobilities are lower than expected for a Mott metal, suggesting that a local disorder upon heavy boron

doping might influence the transport mechanism. For concentrations below the MIT transitions the con-

ductivity plotted in Arhenius coordinates show a rather complex behaviour. Modelling the transport data

by fitting with a variable range hopping formula in 3 dimension (σ ~ T (–1/4)) or eventually 2 dimensions

(σ ~ T (–1/3)), or alternatively nearest neighbour hopping (σ ~ T (–1/2)) does not give any acceptable fit. Be-

low the level of about 1 × 1019 cm–3 films tend to show again the temperature activated behaviour and

p-type conductivity, however mobilities being significantly lower (factor 100–1000) than for B-doped

epitaxial diamond [10].

2 4 6 8 10 12 14101

102

103

600 ppm; 80 nm

3000 ppm; 90 nm

11 meV

2 meV

0.8 meV4 meV

0.6 meV2 meV

3 meV

13 meVcond

uctiv

ity(S

cm-1)

1000/T (1/K)

3000 ppm; 420 nm

800 ppm; 330 nm

Fig. 10 (online colour at: www.pss-a.com) Conductivity measurements for a set of boron-doped nano-

crystalline (B-NCD) samples for boron concentration NB ranging for doping levels of 600, 800 and

3000 ppm B:C for various thickness.

phys. stat. sol. (a) 203, No. 13 (2006) 3385


Original

Paper

6 Conclusions

The aim of this paper is to summarise the state of the art of growth and of doping of nanocrystalline

diamond (NCD) and ultrananocrystalline diamond (UNCD) thin films. Based on experimental data we

discuss in detail the NCD and UNCD Raman, optical and electrical transport properties which are impor-

tant for understanding the nanocrystalline diamond structure and properties related to several applica-

tions such as electronics and bioelectronics. NCD films are grown mainly in H-rich plasmas/gas phase

and they resemble by their columnar growth microcrystalline diamond with the result of having similar

Raman spectra, exhibiting 1332 cm–1 zone-centre phonon Raman line. NCD films can be deposited with

thicknesses ranging from about 50 nm to several hundreds of nm on various substrates such as glass, Si,

and several metals The conduction mechanism in boron-doped NCD layers is of semiconducting type,

which can be converted to metallic type of conduction when the B-acceptor concentrations reach the

Mott’s metallic conductivity limit of 2 × 1021 cm–3. At the same time the B-NCD layers are optically

transparent. UNCD films however have very different structures, due to the growth mechanism in Ar-

rich plasmas, leading to a continuous diamond re-nucleation during the growth. This leads to a higher

proportion of non-diamond carbon in UNCD films compared to NCD films. Consequently, UNCD films

have lower optical transparency than NCD films of the same film thickness. Also UNCD films do not

show semiconducting type of behaviour; and the UNCD conductivity mechanism is governed by a

highly conductive metallic system located at the grain boundaries. However, the advantage of UNCD

films is the continuous renucleation during growth which leads to small grain sizes even for thicker

UNCD films.

Acknowledgements The authors would like to thank especially to P. Hubik and J. J. Mareš from the Institute of

Physics, Academy of Sciences of the Czech Republic in Prague for LT Hall measurements and transport data model-

ling, J. Chevallier and A. Rzepka from CNRS Meudon for the SIMS and Raman measurements. Further on, thanks

are due to C. Mer and D. Tromson, for help with the B-NCD sample growth at CEA Saclay and defect spectroscopy

study and finally to P. Bergonzo for fruitful discussions.

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Growth and properties of nanocrystalline diamond films

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