Different nanostructures identified in boron nitride thin films grown on Si (100) by rf magnetron sputtering

Diamond & Related Materials 18 (2009) 6–12

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Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / d i a m o n d

Different nanostructures identified in boron nitride thin films grown on Si (100) by rf magnetron sputtering N. Frangis ⁎, I. Tsiaoussis, Y. Panayiotatos, S. Logothetidis Solid State Physics Section, Department of Physics, Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece

a r t i c l e

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Article history: Received 11 June 2007 Received in revised form 4 July 2008 Accepted 9 July 2008 Available online 13 July 2008 Keywords: Boron nitride (BN) Nanostructures High-resolution electron microscopy

a b s t r a c t Boron nitride (BN) thin films grown on Si (100) substrates by radio frequency magnetron sputtering, with varying growth parameters, are studied by high-resolution transmission electron microscopy (HRTEM). The HRTEM study reveals the presence of several interesting nanostructures in the BN films. Nanotube-like configurations, nanoarches and nanohorns are observed, as well as well-orientated cubic BN nanocrystals. It is found that several configurations of the turbostratic BN planes (properly orientated or curved) can act, in the same film, as nucleation sites for the growth of the cubic phase. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Boron nitride (BN), in a similar way with carbon, can form several crystalline phases, with sp2 or sp3 types of bonds. The two main phases are the room temperature hexagonal (h-BN) and the highpressure and high-temperature cubic (c-BN) phase. The hexagonal BN (h-BN) has a graphite-like structure with sp2 type of bonds and an AA′AA′…. stacking sequence of the hexagonal graphitic layers [1]. Other phases with sp2 type of bonds are the rhombohedral (r-BN) phase, with an ABCABC… stacking sequence and the turbostratic phase (t-BN). The latter is the special phase of BN, which very often appears in the BN films. In this phase the two dimensional order on the hexagonal basal plane is largely retained, but these planes are almost stacked randomly along the c-axis and their interplanar spacing does not remain absolutely constant [2]. The crystal structure of cubic BN (c-BN) was determined by Bundy and Wentorf [3]. It is a zinc-blende (sphalerite) type of structure, with a lattice constant a = 0.361 nm and sp3 type of bonds. Another phase with sp3 type of bonds is the hexagonal wurtzitic (w-BN) polymorph, which has an ABAB…. stacking sequence, instead of the cubic ABCABC…. layer configuration. Cubic BN thin films have potentially significant technological applications, due to their excellent physical, chemical and mechanical properties such as high hardness and thermal conductivity, chemical inertness, wide band gap, and their ability to be both p- and n- type doped [4–7]. Moreover BN thin films are candidates for optical applications [2], due to their optical gap and good transmittance from ultraviolet to visible and near infrared light. Amorphous BN (a-BN) ⁎ Corresponding author. Tel.: +30 2310998177; fax: +30 2310998019. E-mail address: [email protected] (N. Frangis). 0925-9635/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2008.07.008

films, rich in sp3 bonds, are considered to be superior for optical applications due to their structural homogeneity [8,9]. BN films with a high percentage of the cubic phase can now be deposited by a variety of techniques [1,10–17], including the techniques involving ion beam irradiation (usually by Ar ions) during the deposition [18,19]. The synthesis of homogeneous c-BN films on Si substrates, using the radio frequency (rf) magnetron sputtering (MS) technique [20], has encountered many difficulties. In addition to that, successful nucleation of c-BN occurs only after a critical thickness, threshold energy and flux of ions are exceeded and only for a rather narrow “window” of the values of the deposition parameters [1]. On the contrary, thick and stable c-BN films, with improved crystalline quality, can be grown heteroepitaxially on diamond, due to the compatibility of c-BN and diamond [21]. Until now it is widely accepted (e.g. ref. [18]) that the BN films, grown on Si (100) substrates by Physical Vapor Deposition methods, consist of four individual layers. First an interfacial amorphous layer is observed, consisting of substrate and deposited atoms, followed by a turbostratic BN (t-BN) layer highly oriented, with the hexagonal layers having a preferred orientation normal to the substrate. Then a third layer, rich in c-BN nanocrystals, follows (in the optimum case this layer is a pure c-BN layer). On top of these layers a thin sp2 bonded surface layer exists. This top sp2 layer is absent in films grown by Chemical Vapor Deposition [22]. Several orientations of c-BN, in relation to the Si substrate, have been reported [23–29]. The most frequently observed is an [111] inplane texture, where one family of the {111} cubic planes is perpendicular to the substrate and roughly parallel to the hexagonal planes of the second layer. (A crystallographic axis [hkl] is characterized as in- or out-of-plane if it is parallel or perpendicular to the substrate-thin film interface, respectively).

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A number of authors [23,30,31] suggest that this texture of the t-BN and c-BN is a result of the minimisation of the thermodynamic Gibbs free energy, considering a biaxial compressive stress field and only elastic strain energy. However, others [32–34] consider the above consideration inadequate, proposing that plastic deformation has to be taken into account and that the observed texture arises probably from the growth mechanism. Lifshitz et al [35] propose that this specific orientation relationship is a result of preferential displacement of atoms, because the bond energy on the graphite planes is much larger than the bond energy perpendicular to them. Furthermore it has been reported that the formation of the rhombohedral BN (r-BN) phase is an important factor for the subsequent growth of c-BN [19]. However it is unclear if r-BN acts as a structural precursor (i.e. under compression the rhombohedral layers are transformed to cubic structure), or forms a preferred nucleation site for the growth of c-BN. In the latter case h-BN is also considered as a nucleation site. In both cases the (0002) hexagonal planes are parallel to one of the {111} crystallographic cubic planes. A review of these considerations is given in ref [1]. Li et al [25] presented experimental evidence that apart from these mechanisms, c-BN can grow on curled t-BN planes, i.e. the dangling bonds of the curved hexagonal planes favour the formation of c-BN. In this case the c-BN planes have no specific orientation relationship with the t-BN layers. In the present work the BN thin films were deposited on Si (100) substrates employing radio frequency (rf) magnetron sputtering under various deposition parameters (Table 1). Several interesting nanostructures are identified in the films and are studied by electron microscopy. 1.1. Experimental The BN films were deposited by rf magnetron sputtering using a 6″ hot pressed h-BN target (99.999% purity) in a high vacuum chamber (Pb b 1×10− 7 mbar). The Si (100) substrates were chemically and dry etched. The substrates were coated using a sputtering target power between 200 W and 400 W and a working pressure in the range of 1.4×10− 3 to 8.0×10− 3 mbar. The films were deposited at room temperature (RT), the only exception being the sample A, which was deposited at 300 °C. The energy and the flux of the ions were varied by applying an external bias voltage (Vb ) to the substrate in the range from +15 V to −150 V. The deposition parameters are summarised in Table 1. Cross-sectional and plane view specimens suitable for electron microscopy observations were prepared by mechanical polishing followed by ion thinning with 6 keV Ar ions. During the last thinning step, the energy was reduced to 3 keV and the angle of incidence was about 5°. The high-resolution transmission electron microscopy (HRTEM) study was performed in a JEOL 2011 microscope, which operates at 200 kV and has a

Substrate bias Voltage Vs (V)

Substrate Working Temperature Pressure o ( C) (mbar)

A

400

Floating (+15)

300

B

350

−70

25

1.5×10− 3

110

C1 C2 C3

200 350 200

−60 −70 −150

25 25 25

8.0×10− 3 8.0×10− 3 8.0×10− 3

160 130 160

Film Film thickness structure (nm)

1.4×10− 3 160 (Ar: 1.2×10− 3, −3 N2: 0.2×10 )

point resolution of 0.194 nm. For conventional low magnification observations a JEOL 120CX microscope, operating at 100 kV, was also used. 2. Electron microscopic study The electron microscopic characterization results are summarised in Table 1. In the following, emphasis is on the description of some original nanostructural features of the films.

Table 1 Deposition parameters of BN thin films, their thickness and structure Sample Target power P(W)

Fig. 1. A HRTEM image from a cross-section specimen from sample of type A, showing the structure of the film close the BN/Si interface. Note that above the amorphous layer, an extra layer exists, with two types of overlapping t-BN layers, i.e. running parallel and perpendicular to the interface. Above these two layers the t-BN layers are roughly perpendicular to the interface. The electron diffraction pattern shown as inset was taken from a selected area containing part of the thin film and the substrate (electron beam parallel to [011̄]Si). The Si reflections and the t-BN “reflections” are indicated.

t-BN, c-BN, h-BN, r-BN nanotubes, nanoarches, nanohorns t-BN, c-BN nanocrystals in triangular shape t-BN, a-BN t-BN, a-BN a-BN, t-BN

2.1. Low pressure-high temperature film (sample type A) The electron microscopic study of cross-section specimens from films of type A reveals the structural features of the film. In the electron diffraction patterns (e.g. the pattern shown as inset in Fig. 1), apart of the Si reflections, the t-BN broad diffraction “arcs” are present. It is very well known that the t-BN phase produces a broad and diffuse diffraction pattern, distinct from that of the other phases [3]. Using the Si reflections as an internal standard, the d spacings in the real space, corresponding to the 0002 “reflections” of the t-BN phase were found in the range of 0.35–0.36 nm, in very good agreement with the values reported in the literature (0.35–0.38 nm). The same is also valid for the 10 and 11 rings. The fact that the 0002 “reflections” from t-BN form small arcs and not a complete circle

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indicates that the hexagonal planes are highly and nearly perpendicular oriented to the BN/Si interface. The HRTEM study reveals that next to the interface an amorphous zone (of thickness about 3.5 nm) exists (Fig. 1). It is remarkable that in some areas, above the amorphous zone, an extra layer (of thickness 5 nm), is revealed (Fig. 1). It has a configuration, which implies the presence of two types of overlapping t-BN layers, i.e. running parallel and perpendicular to the interface. Such a layer, with two overlapping orientations, has not been reported so far. Yang et al [27] reported that the t-BN layers are randomly oriented just above the amorphous layer and then they are oriented perpendicular to the substrate. In the present case two distinct orientations are observed. This exceptional and occasional observation of BN hexagonal layers parallel to the interface and only for a limited thickness, is consistent with the theoretical treatments presented in the introduction, which suggest that the growth of hexagonal BN planes parallel to the interface is not favoured energetically. The next layer is a t-BN layer, with the hexagonal planes running normal to the substrate, i.e. it confirms the conclusion from the electron diffraction study. In some cases the layers appear not to be simply perpendicular to the substrate, but they tend to have their c-axis parallel to the [011]Si (Fig. 1). Their “period” is roughly 0.35 nm. c-BN nanocrystals are found in the t-BN layer of the film, in several distances from the BN/Si interface. They have a round shape and a diameter in the range of 3-8 nm. In most of the cases the {111} planes of the cubic phase are not parallel to the hexagonal t-BN planes. Some of the nanocrystals are well-orientated and the configuration of the projected structure is observed. An example is shown in Fig. 2, where the c-BN nanocrystal has an orientation such that the electron beam is parallel to the [011 ̄]BN axis. The same orientation holds also for the Si substrate (corresponding electron diffraction pattern that of Fig. 1). Hence, the c-BN nanocrystal and the Si substrate share a common axis. This relation occasionally occurs for this sample type, but as we shall see in the following it is the rule for the sample type B. As can be seen in the image of Fig. 2, twinning occurs in the nanocrystal. The density and the size of the c-BN nanocrystals increase closer to the surface, but they do not form a complete layer of c-BN. Observations in plane view specimens (thinned only from the substrate) reveal the presence of crystals with size larger than 10 nm (exceptionally up to 35 nm). The c-BN nanocrystals do not present a clear preferential orientation. However, some crystals seem to have an [110] out of plane preferential orientation, which means that they have two {111} planes perpendicular to the substrate. As described in [1],

Fig. 3. a) A nanotube-like configuration observed in films of type A. b) The curved t-BN layers form arcs, which are parts of concentric circles.

this orientation is a special case of the [111] in-plane orientation. Occasionally the hexagonal, the rhombohedral and the wurtzitic phases were detected. A t-BN layer terminates the BN films. We estimate that about 20% of the film is in the c-BN phase. 2.2. Nanotube-like configurations and other nanostructures in film of type A

Fig. 2. A c-BN nanocrystal formed in a t-BN matrix and well-orientated in relation to the substrate: electron beam//[011̄]Si //[011̄]BN (corresponding electron diffraction pattern that of Fig. 1). Note the presence of twin planes.

The h-BN and r-BN structures and the semi-ordered t-BN phase are constructed by the same building units: the graphite-like hexagonal planes. The graphite-like hexagonal planes are also the building units of BN nanotubes. The existence of BN nanotubes has already been reported in references [36–40]. The local formation of the h- or r-BN structures in a t-BN matrix has been reported in most of the BN films and it was also

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observed in the studied films of type A. In a similar way one would plausibly expect the local formation of BN nanotubes (and other related nanostructures or nanoparticles, like fullerenes, onions etc) in a t-BN matrix. The possible existence of nanotubes in BN films was predicted by Li et al [25], without providing any experimental results for this. The investigation for the existence of these nanostructures in a film is constrained referring to nano-objects that probably overlap with other materials features. In the studied BN film of type A, nanotube-like configurations were identified. The example shown in Fig. 3a, is taken from a cross-section specimen. The observed configuration could be a nanotube with its axis parallel to the growth direction, i.e. to the [100]Si. Another image, from the same specimen, indicating nanotubes is shown in Fig. 3b, where the bent t-BN planes form arcs, which are parts of concentric circles. They could be the ends of nanotubes with their axes parallel to the Si surface. Alternatively they could be nanoonions [41] or part of nanoarches. Evidence for the existence of nanotubes is also provided from plane view specimens observations. In Fig. 4 the t-BN planes are bent and in two cases they form a circle, which could be the end of a single wall nanotube. Especially in area 1, the circle is formed in the core of a species of coaxial nanoarches, having their axis parallel to [100]Si, i.e. normal to the BN/Si interface. All the above observations can be considered at least as indications for the existence of BN nanotubes (probably deformed) in the film of type A. An alternative interpretation of the circles in Fig. 4 is that they are the projections of BN fullerenes. The diameter of the “nanoparticle” 2 is about 1.45 nm, i.e. very close to the size of the BN fullerene reported by Bengu and Marks [42]. In Fig. 5a (image taken from a cross-section specimen) two families of hexagonal planes are intersected and form a configuration similar to that known for nanohorns (compare for example with images presented in refs. [43,44]). Moreover in the area between the two families of lines, some weak dots are observed which locally form a slightly deformed hexagonal configuration, i.e. the whole configuration is similar to that of nanotubes. The hexagonal configuration is better seen in the processed Fourier filtered image of Fig. 5b. In fact, nanohorns are considered as nanotubes (distorted or not), terminated

Fig. 5. Images from a cross-section specimen from film of type A: a) A configuration of the tBN layers which indicates the formation of nanohorns. b) Processed image after Fourier filtering of the area in the white frame in panel a. A slightly deformed hexagonal configuration is locally observed. c) An isolated nano-arch (half nanotube), indicated by the arrow.

in a way to resemble to horns [45,46]. So, nanotubes exist in the present film, at least as part of nanohorns. Isolated nanoarches (Fig. 5c) are in some cases also present. It is known that the nanoarches are considered to be half-nanotubes [47,48]. All the above observations clearly demonstrate that several nanostructures are formed in the film of type A, embedded in the t-BN environment. The observed nanostructures can be interpreted in a few ways. However, it can be concluded that discussed nanostructures may exist in the BN film of type A in forms of: nanotubes, nanoarches (half nanotubes), nano-onions, nanohorns, fullerenes. 2.3. Low pressure- low temperature film (sample type B) Fig. 4. A plane view image from film of type A. In area 1 nanoarches are formed, with their axis normal to the film plane. In the core of the nanoarches a circle is formed, indicating the formation of a single wall BN nanotube. In area 2 another circle is observed. The circles can be considered as the upper part of single wall nanotubes or fullerenes.

This type of film exhibits some special features. In the electron diffraction patterns (e.g. Fig. 6a) apart of the Si reflections, the t-BN broad diffraction “arcs” are present, having a larger angular width compared with that of sample of type A, i.e. a smaller percentage of t-BN layers are

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Fig. 6. a) An electron diffraction pattern from a selected area containing part of the thin film of type B and the substrate (electron beam parallel to [011̄]Si). The Si reflections, the t-BN “reflections” and the c-BN 111 ring are indicated. b) A bright field image from the film of type B revealing the presence of triangular c-BN nanocrystals. c) A HRTEM image of a c-BN nanocrystal in the film of type B. The presence of twin planes and stacking faults, with several orientations is obvious.

perpendicularly oriented to the substrate. Moreover a ring, consisting of a rather small number of spots, is present in the pattern. Using the Si reflections as an internal standard, the radius of the ring is found to correspond to the 111 of c-BN, revealing the presence of c-BN nanocrystals with several orientations. In bright film images of the film, several triangular precipitates are observed, which are surprisingly rather well ordered pointing always towards to the substrate (Fig. 6b). All the observed triangles are approximately isosceles, with the angle of the apex pointing to the substrate at angles from 90-100° and with the side opposite to this apex almost parallel to the BN/Si interface. Their minimum distance from the substrate is about 6 nm and the maximum size of the triangles' sides is about 30 nm. HRTEM images of these precipitates elucidate their structure, confirming that they are cubic BN (c-BN) nanocrystals. An example is given in Fig. 6c. The image was taken with the electron beam parallel to the [011̄]Si (electron diffraction pattern of Fig. 6a). It is revealed from the HRTEM images that the nanocrystals contain usually more than one grain (orientation variants) of c-BN. All the grains obey approximately the relation: [011̄]Si//[011̄]BN (texture formation). The same orientation relationship was observed in [24]. The nanocrystals are usually highly defected, presenting their own internal interesting structure. An example was presented in details elsewhere [49]. It is also remarkable that in the c-BN nanocrystals, several orientations of the twin planes are observed (Fig. 6c).

The nanocrystals are well orientated concerning their shape as well as their crystallographic orientation. It is plausible to characterise the constructed nanostructure as a self-organized nanosystem. We estimate that about 35% of the film is in the c-BN phase. 2.4. Orientation relationship between the t-BN and the {111} c-BN planes Several orientations of cubic {111} planes, in relation with the t-BN layers, are observed. In some cases (Fig. 7a) it is clear that the hexagonal planes of the t-BN phase seem to continue as cubic {111} planes in the c-BN nanocrystal, being almost perpendicular to the substrate (maximum deviation of about 10o). Also, it can be seen that four cubic planes match with three hexagonal layers. This relationship indicates the formation of the r-BN (or the h-BN) phase, before the growth of the cubic BN. For some time this formation was considered by several authors as a necessity for the growth of c-BN [1]. Alignment of the t-BN planes with the cubic planes was also observed in other cases (see for example Fig. 6c), with the planes deviating from the growth direction by a larger angle (up to 23°). In the image of Fig. 7b, the t-BN planes indicated by curved arrows 1 deviate from the growth direction and they seem to be transformed to the one family of the {111} c-BN planes. The t-BN planes indicated by curved arrows 2 deviate and they seem to be transformed to the second family of the {111} c-BN planes. The whole configuration indicates that the c-BN is nucleated at the end of a nanoarche (or half-nanotube), as

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suggested by Collazo et al [47,48]. This transformation was clearly demonstrated first by Nistor et al [50,51]. Formation of c-BN from curled t-BN planes was also reported by Li et al [25], who claim that the dangling or deformed bonds of the bent graphitic type layers act as nucleation sites for the c-BN growth. In some cases the {111} c-BN planes deviate from the t-BN basal planes and are in a more or less random relationship. Fig. 7c reveals a very interesting case. The orientation of the t-BN planes varies gradually as we see from the left side of the figure to the right (the layer indicated by arrow 1 is almost parallel to the growth direction while the layer indicated by arrow 2 deviates by an angle of about 34o), but all the t-BN planes lead to the growth of the same {111} planes in the c-BN nanocrystal. All the above observations lead to the conclusion that growth of c-BN can start on several nucleation sites. The r-BN (or h-BN) can act as a precursor, but it is not a necessity. Cubic BN can grow also on curled hexagonal planes or nanoarches. The {111} cubic planes are in some cases aligned to the t-BN layers, but in other cases they are not. Li et al [25] have reported the existence of the different nucleation mechanisms in films, grown under different growth parameters. Studying the B-type films shows that different phase transitions may exist in a single film. 2.5. High pressure–low temperature films (samples type C) Samples prepared at room temperature and with the most intense Ar beam (samples C1 to C3) present a 3 nm thick amorphous layer next to the Si surface. The rest of the film consists of a mixture of amorphous and turbostratic BN phases. No other phases were revealed. For the sample A3, the amorphous BN seems to be the predominant phase. Panayiotatos et al [8] concluded from spectroscopic ellipsometry measurements that the films contain a high percentage (up to 30%) of sp3 bonds. Note that the film C2 was grown with the same parameters as the film B, except for the Ar pressure, which was much higher (Table 1). The structural features of the two films are apparently different. The films of type C are more homogeneous than the others and due to their homogeneity they have been proposed for optical applications [8,9]. 3. Conclusions The electron microscopic study reveals that thin BN films with special nanostructural features can be grown by reactive and nonreactive rf magnetron sputtering. The main conclusions can be summarized as follows:

Fig. 7. Three different relationships between the t-BN and the c-BN {111} planes: a) The two families of planes are aligned and they are approximately parallel to the normal to the substrate (deviation of about 10°). Four cubic planes match with three hexagonal layers, indicating the formation of the r-BN (or the h-BN) phase, before the growth of c-BN. b) The two families of planes are not aligned. The t-BN planes curve and the nucleation of c-BN on the top of a nano-arch is indicated by arrows 1 and 2. c) The orientation of the t-BN planes varies (the t-BN planes indicated by arrows 1 and 2 form an angle of about 34°), but they lead to the growth of the same {111} planes in the c-BN nanocrystal.

1) The film of type A contains several interesting nanostructures like nanotubes, nanoarches (half-nanotubes), onions, nanohorns, fullerenes, as well as c-BN nanocrystals with rather small size (b10 nm). 2) Cubic BN can be formed even during growth at room temperature (film of type B), provided that the other growth parameters are properly selected (see also ref. [52]). The important role of the ion bombardment for the growth of c-BN is confirmed (already reported earlier in ref. [1]). The local heating and the increase of the stress, due to the bombardment by Ar ions, are probably responsible for the formation of the cubic phase, which is known to be the high-temperature and high-pressure phase of BN. It seems however that above a certain working pressure the crystallisation is prevented and films containing amorphous and turbostratic BN are grown (films type C). 3) Several types of nucleation sites (such as properly oriented or curved t-BN planes) for the growth of the cubic phase are found to exist in the same film. The previous formation of other BN phases is not a necessity. 4) The B-type film contains c-BN nanocrystals of a larger size than those of film A (a few tens of nms), which seem to be selforganised, since they are well-orientated concerning their

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