Laser induced crystallization of hydrogenated amorphous silicon-carbon alloys

Transcription

1 JOURNAL OF APPLIED PHYSICS VOLUME 96, NUMBER 7 1 OCTOBER 2004 Laser induced crystallization of hydrogenated amorphous silicon-carbon alloys C. Summonte, R. Rizzoli, M. Servidori, S. Milita, S. Nicoletti, and M. Bianconi CNR-IMM Section of Bologna, Via Gobetti 101, I Bologna, Italy A. Desalvo and D. Iencinella DICASM, University of Bologna, Viale Risorgimento 2, I Bologna, Italy (Received 13 April 2004; accepted 14 June 2004) Laser induced crystallization of hydrogenated amorphous silicon carbon alloy a-si 1 x C x :H films has been investigated by means of synchrotron x-ray diffraction. The a-si 1 x C x :H films were deposited on (100) silicon wafers by very high frequency plasma enhanced chemical vapor deposition at 100 MHz in hydrogen diluted silane-methane gas mixtures. The substrate was kept at 250 C or 350 C and the stoichiometry was changed from x = 0.20 to The structural characterization of the as-grown films has been carried out by Rutherford backscattering (hydrogen concentration) and infrared spectroscopy (film ordering). The films were irradiated by a KrF excimer laser 248 nm with varying energy density and number of pulses. After irradiation, the formation of SiC crystallites has been revealed by synchrotron x-ray diffraction. Besides SiC nanocrystals, the formation of crystalline Si and graphite is observed for under- x 0.50 and over-stoichiometric x 0.50 samples, respectively. The essential role played by hydrogen concentration and hydrogen bonding configuration in determining the melting threshold and the consequent SiC grain formation is highlighted American Institute of Physics. [DOI: / ] I. INTRODUCTION Crystalline silicon carbide is a semiconductor which possesses excellent mechanical, chemical, thermal, and electrical properties, and is presently subject of large scientific interest, both on fundamental aspects, and in view of a variety of applications. 1 Optical applications of nanocrystalline films are also regarded with interest. 2 However, such a semiconductor can only be fabricated at high temperature, with consequent limitations, related both to the substrate, and to the device fabrication technology. It is therefore of great interest to attempt to crystallize a-si 1 x C x :H deposited at low temperature, as full crystallization of amorphous layers would open the way to specific applications in electronic devices, with further potentiality coming from the large area attainable with plasma deposition. The use of laser crystallization, besides not involving heating of the substrates, has the additional advantage of allowing surface patterning. 3 However, although laser crystallization of amorphous silicon is a rather mature technique, both from the point of view of the physical understanding of the process, 4,5 and of the applications in electronic and optoelectronic devices, 6,7 the nature of the same process if applied to a-sic:h is still under investigation. 3,8 10 Prediction of the regrowth process after melting is made difficult by the lack of knowledge of the thermal properties of the molten phase, 9 both as a function of the C to Si relative stoichiometry, of hydrogen concentration, and of Si, C, and H bonding configuration. A possible reference could be the phase diagram of unhydrogenated crystalline SiC, that, however, still shows controversial aspects, an exhaustive review being reported in. 11 In the usually reported phase diagram, which was obtained by Scace and Slack 12 under a pressure of 35 atm, SiC decomposes before melting and graphite coexists with a silicon rich liquid phase. Urban and Falk 10 pointed out that no liquid phase occurs at ambient pressure in the equilibrium Si-C phase diagram and SiC decomposes in graphite and a vapor phase (see, e.g., Ref. 13). However, Kleykamp and Schumacher found a liquid phase region for SiC between 2830 and about 3000 C at 1 bar. 11 Reference 8 and 10, basing on reflectivity measurements, reported evidence of melting upon laser irradiation, such a liquid phase, according to Ref. 10, having a metastable character. Baeri et al. 8 suggest that laser heating in the time scale of the nanosecond involves diffusion lengths of the order of the atomic distances. Melting therefore occurs before solid phase transformation or decomposition. In this paper we report the results of excimer laser irradiation on a set of plasma deposited a-si 1 x C x :H samples with different stoichiometry, hydrogen concentration, and bonding configuration. The influence of such initial structural parameters on subsequent crystallization is reported. Finally, we note that the mixture of the hydrogenated amorphous silicon carbon alloy with microcrystalline silicon is often referred to as microcrystalline silicon carbide, although such an expression can be misleading in some cases. II. EXPERIMENT A. Sample preparation and characterization The samples were deposited by very high frequency plasma enhanced chemical vapor deposition (VHF PECVD) at 100 MHz, 0.3 hpa, using CH 4 and SiH 4 as gas precursors. We fabricated four series of samples with varying CH 4 % =CH 4 / CH 4 +SiH 4, at two substrate temperatures (250 C /2004/96(7)/3998/8/$ American Institute of Physics

2 J. Appl. Phys., Vol. 96, No. 7, 1 October 2004 Summonte et al and 350 C) and two hydrogen dilution fractions H% =H 2 / H 2 +CH 4 +SiH 4 =48% and 90%, both on c-si and Corning glass. Shortly, the four series are indicated as , , , and For all samples reported in this work, the thickness ranges from 0.15 m to 1.2 m (about 0.5 m in most cases). Fourier transform infrared spectroscopy (IR) on asdeposited samples was performed using double beam Lambda 9 Perkin Elmer spectrometer. The absorption was computed after baseline subtraction and subsequent normalization by the sample thickness. Details on the optical characterization of the samples are reported elsewhere. 19 The stoichiometry of the as-deposited samples was determined by 3500 kev He Rutherford backscattering (RBS) analysis. Absolute spectra were collected with an uncertainty of 2%. 20 The areal density of H was indirectly determined 21 from the spectrum yield reduction. The sensitivity in the stoichiometry is 1% for Si and C and 5% for H. The accuracy in the ratio C/Si is of the order of 1% 2%. The accuracy of the H areal density is related to the accuracy of the H stopping power 22 in the matrix and it is of difficult evaluation (in lack of specific measurements it can be as large as 20%). The samples were irradiated under vacuum by using a KrF excimer laser, =248 nm, pulse duration 23 ns. The size of the irradiated spot was about 4 4 mm 2. The sample holder was kept at room temperature. The laser energy density was changed by adjusting the beam focus on the sample. The actual energy density on the sample surface, based on a direct measurement accurate within ±5%, was determined by taking into account the beam size at the sample surface, and the measured sample reflectance at 248 nm, as well as energy loss due to reflectance of the focalization lenses and the vacuum chamber quartz window. The spectral reflectance at 248 nm in the investigated stoichiometric range varies between 18% and 28%. Single or multiple irradiation were performed. FIG. 1. Geometry used at ELETTRA for grazing incidence diffraction from the samples. ranging up to 75 in each pattern, a systematic study of a large number of samples has been possible. The intensity was radially integrated overall the detector area and reported as a function of the 2 angle. In the figures, the ordinates of the diffraction patterns were translated for clarity. III. RESULTS AND DISCUSSION A complete characterization (optical gap, refractive index spectra, infrared spectra, composition) of the entire set of a-sic:h samples deposited by VHF PECVD as a function of deposition parameters is reported elsewhere. 19 Among all results, in this paper we only mention those related to the interpretation of diffraction measurements. Figure 2 shows typical diffraction patterns obtained from a sample before and after laser irradiation. The two relevant B. X-ray diffraction X-ray diffraction experiments were performed on asdeposited and laser-irradiated alloy films at the XRD1 beamline of ELETTRA synchrotron facility (Trieste, Italy). A fixed beam-exit, two-crystal silicon monochromator was used to select the energy of kev from the white emission spectrum of the source. The beam cross section at the sample was limited by slits to 0.1 mm both in horizontal and vertical directions. The high x-ray flux available at the beamline allows one to detect intense signal scattered from the films, by keeping the incidence angle of the monochromatic x-ray beam on the sample equal to 1 (Fig. 1). At such an angle, the beam footprint on the surface was mm 2, and hence almost fully included in the film surface irradiated by the laser. The beam scattered from the sample was collected by a MarCCD 2D detector of 165 mm radius, placed normal to the incident beam direction at about 40 mm from the sample. Moreover, in such a configuration, thanks to the very short exposure time 1 s and the wide scattering angle 2 FIG. 2. Images of scattering patterns taken on a sample before (a) and after (b) laser irradiation.

3 4000 J. Appl. Phys., Vol. 96, No. 7, 1 October 2004 Summonte et al. TABLE I. Thickness, C atomic fraction x, H concentration, number of laser pulses N, and laser energy density E/A for different samples. Sample Thickness m x H N pulses E/A mj/cm 2 k k k k k k k k k FIG. 3. Scattering angle 2 vs Si-C bond concentration for as-deposited samples with different H 2 dilutions (48% and 90%) in the gas mixture and different substrate temperatures (250 C and 350 C). The line is a guide to the eye. features are (i) the outer halo [Fig. 2(a)] observed for the as-deposited amorphous films and (ii) the appearance of Debye rings after laser irradiation [Fig. 2(b)], indicating the formation of crystal phases during laser processing. An inner halo, due to the intensity diffused by the beam-stopper, and the diffraction spots from the silicon substrate, are also visible in the figure. All as-deposited films result to be amorphous from x-ray diffraction point of view, as indicated by the single broad scattering halo observed in all cases [see, e.g., Fig. 2(a)]. This halo originates from an interference phenomenon based on the short range order in the films, essentially due to a given average nearest neighbour atomic distance. As the Si-Si bond length is larger than the C-C bond length, the 2 position of the halo is expected to increase with increasing carbon atomic fraction, x, in the film, or, more specifically, with Si-C bond concentration. Figure 3 shows the correlation between the two quantities. The Si-C bond concentration was calculated from the intensity of the IR absorption band at 780 cm 1 attributed to Si-C stretching vibration via the proportionality factor cm Table I reports the film thicknesses, the carbon fraction and the hydrogen concentration in the film (x and H, respectively), the number of laser shots and the single shot laser energy density, for some samples analyzed by x-ray diffraction. Among all measured samples, those reported in Table I were chosen to evidence the effects of thickness and sample composition on the crystallization of the film. The crystalline phases involved are reported in Table II along with their Miller indices, scattering angles, and relative intensities of the reflections in the angular ranges in which the Debye rings of the films were observed. After laser irradiation, depending on laser energy density and on sample thickness and stoichiometry, the diffraction spectra show peaks characteristic of the crystalline phases. In the following we assume that crystallization is associated with melting. The assumption is based on Refs. 8 and 10, that report evidence of melting associated with crystallization, while no structural modification is observed if the melting threshold is not reached. In Fig. 4 we report the single shot effective irradiation energy E/A 1 R as a function of x for all samples. A is the irradiated area and R is the sample reflectivity measured at 248 nm. Open circles refer to samples remained amorphous after laser treatment, while solid symbols refer to samples that show diffraction peaks, indicating that E E M in that case. A sharp discontinuity is seen in the figure: all understoichiometric to almost stoichiometric samples x 0.4 crystallize, at least partially, upon single shot irradiation at the laser energy density used in this work. For x 0.5, the melting threshold is no longer reached for energy densities up to 190 mj/cm 2. Although hydrogenated amorphous silicon carbon is not expected to exhibit the same thermal behavior of the crystalline unhydrogenated counterpart, the discontinuity at x 0.45 reflects the two different equilibrium melting points in the Si-C phase diagram, 1413 C and 2830 C for x 0.5 and x 0.5, respectively. 11 The melting threshold depends on several parameters of the irradiated layer. It depends (i) on the thermal parameters of the irradiated layer 8 and on its thickness, decreasing for increasing thickness, 9 (ii) on substrate thermal conductivity, 8,24 (iii) and on laser wavelength and pulse duration. 9 In the following, the results of x-ray diffraction measurements are reported, grouped in such a way to isolate the effect of a single structural or irradiation parameter. A. General considerations The amorphous to crystal transition is not complete in almost all irradiated samples, irrespective of laser energy density and pulse number. The contribution of the amorphous phase comes from regions that did not reach the melting threshold, which, keeping in mind that the x-ray beam probes the whole sample thickness, are presumably localized in deep regions of the sample. Such occurrence hinders the determination of the actual crystallized fraction.

4 J. Appl. Phys., Vol. 96, No. 7, 1 October 2004 Summonte et al TABLE II. Miller indices hkl, diffraction angles 2 and peak intensities for crystalline graphite, Si and SiC polytypes. The PDF identification numbers are indicated between brackets. Phase (PDF id) hkl 2 deg at kev Relative intensity Graphite ( ) Silicon ( ) H SiC ( ) H SiC ( ) H SiC ( ) C SiC ( ) While the formation of crystalline Si is clearly identified when x does not reach the stoichiometry value x=0.36, distinguishing between the different SiC polytypes is not possible. In fact, the coincidence of the 2 angular position of the more intense diffraction peaks due to the basal planes of the different polytypes (Table II), and the peak broadening due to the small grain size and the instrumental resolution, prevent the univocal identification of the SiC polytypes. The crystalline domain size, roughly estimated by deconvolving the observed peaks with the instrumental broadening function obtained from Si powder with diffraction domains as large as several micrometers, turns out to be about 10 nm for the largest Si and SiC particles. However, the presence of larger microtwinned grains cannot be excluded. 25 Figure 5 shows the diffraction patterns of sample k23, as deposited and after irradiation under two different laser energy densities. In both cases, the amorphous to crystal transition is not complete, as indicated by the residual halo. After the pulse at 140 mj/cm 2, a halo shift to higher scattering angles, and a very beginning of crystallization, evident from the shoulder at about 34, are observed. After the pulse at 187 mj/cm 2, we observe Si and SiC crystallization and an increase in the crystallite size (indicated by peak narrowing), associated with a slightly higher angular peak position with respect to the case of lower irradiation energy density. A shift to higher angular position larger than in the previous case is also observed for the residual halo. The shift of Debye rings to higher scattering angles can be explained by taking into account the mass density in- B. Effect of laser energy density FIG. 4. Single shot irradiation energy density vs C atomic fraction. Solid symbols indicate crystallized samples; open symbols indicate samples that did not melt. FIG. 5. Scattering spectra of sample k23 before laser irradiations, and after one pulse irradiation under two different energy densities.

5 4002 J. Appl. Phys., Vol. 96, No. 7, 1 October 2004 Summonte et al. FIG. 7. Scattering spectra of sample k49 before laser irradiations, and after one and three pulses irradiation, at two different energy densities. FIG. 6. Scattering spectra of sample k20 before laser irradiations, and after one and two pulse irradiation at the same energy density. crease as a consequence of the amorphous-to-crystal transition. Crystallization and increase in crystallite size involve the development of more compact structures with reduced unit cell dimensions. The halo shift is attributed to hydrogen evolution due to sample heating, with consequent density increase within the unmelted sample volume fraction. Lower hydrogen concentration can be hypothesized also within the nanocrystals. C. Effect of different number of laser pulses Figure 6 shows that one laser pulse at 131 mj/cm 2 is not sufficient to melt the film. Only a small shift of the halo to higher scattering angles, whose origin is discussed above, is observed after the first pulse. The second pulse produces film melting and subsequent formation of SiC domains. For this sample, as well as for other samples that will be illustrated below, the presence of the small signal at 2 =41.35, due to (200) cubic SiC, and the absence of the peak at 38.12, related to the hexagonal pyramidal planes, suggest the prevalence of the 3C SiC polytype. Residual Si crystallites also form in this sample, because of the understoichiometric composition x=0.43. Hydrogen evolution during heating induced by the first pulse 26 is believed to be responsible for the lower melting threshold observed for the second pulse. In Ref. 26 substantial hydrogen evolution was indeed observed for silicon after one laser pulse irradiation. In the same reference, a lower melting threshold was found for nonhydrogenated, compared to hydrogenated, samples. Complete evolution of hydrogen in a-sic:h films was observed to take place for furnace annealing at temperatures above 650 C. 27 The lowering of the melting threshold upon dehydrogenation is also confirmed by observing Fig. 7, where the diffraction curves are reported for sample k49, as deposited, after one pulse at 183 mj/cm 2, and after three pulses at lower energy density 137 mj/cm 2. Note the higher scattering angle of the halo, in agreement with the larger carbon fraction x=0.56. After one pulse at 183 mj/cm 2, a small angular shift of the halo towards the SiC peak position is observed, pointing out a very beginning of atomic ordering as SiC alloys. Conversely, after three pulses at lower energy density, SiC nanocrystalline particles form. Residual carbon forms graphite nanoparticles. No silicon crystallites are observed in this overstoichiometric sample. D. Effect of thickness Samples k34 and k35 (0.44 and 0.11 m, respectively) differ only by deposition time, all other deposition and irradiation parameters being identical. The composition of sample k35 was not directly measured, however, it is assumed to be the same as for k34 x=0.42, H =29%. The hypothesis is supported by very similar optical properties, and the same value of the halo scattering angle, although the halo is less intense for the thinner sample. Figure 8 shows, for the thinner sample, an evident SiC signal with at least some cubic component (see discussion above), and a very limited signal for crystallized residual silicon. Although a quantitative evaluation of the crystalline silicon fraction was not attempted, this result is in agreement with x rather close to stoichiometry. However, the thicker sample shows a much more marked Si crystallization, while the intensity of the crystalline SiC signal is lower than for k35. This result suggests that the thinner sample k35 almost completely melted and crystallized, as also supported by the vanishing of the halo after irradiation. On the contrary, a melting and crystallization profile is to be figured out for the thicker k34 sample. While the surface region completely crystallized, deeper regions did undergo a more limited heating, with consequent formation of Si crystallites embedded within an unmelted SiC region, still originating a halo. Although the phase diagram for Si-C 11,13 is not quantitatively valid for amorphous hydrogenated alloys under rapid annealing, a region exists for x 0.5 where solid stoichiometric SiC coexists with a Si rich liquid phase, which gives rise to the formation of Si nanocrystals.

6 J. Appl. Phys., Vol. 96, No. 7, 1 October 2004 Summonte et al FIG. 8. Scattering spectra of the samples k35 (a) and k34 (b) before and after laser irradiation with two pulses at similar energy density. The thickness of k35 and k34 is 0.11 m and 0.44 m, respectively. E. Different carbon fraction, same H concentration Figure 9 shows the diffraction patterns for k20 and k22 samples before and after irradiation with two pulses at similar energy density. Basing on the peak intensities, we observe that the amount of crystallized SiC after two laser pulses is greater for sample k20, in agreement with its larger value of x (Table I), also responsible for the higher scattering angle of the halo. Both samples are understoichiometric, and show crystallization of the excess Si. FIG. 10. Scattering spectra of the samples k20 (a) and k34 (b) before and after laser irradiation. For k20 and k34, the hydrogen concentrations are 36% and 29%, respectively. F. Effect of hydrogen concentration Figure 10 shows the diffraction spectra for samples k20 and k34. The two samples have similar x, as confirmed by the same 2 position of the scattering haloes, and the same thickness. Hydrogen content of k34 is lower: 29%, compared to 36%. Accordingly, as reported in Fig. 11, the infrared FIG. 9. Scattering spectra of the samples k20 (a) and k22 (b) before and after laser irradiation. For k20 and k22, the values of x are 0.43 and 0.24, respectively. FIG. 11. Infrared spectra after thickness normalisation and baseline subtraction, taken on samples k34 and k20 before irradiation.

7 4004 J. Appl. Phys., Vol. 96, No. 7, 1 October 2004 Summonte et al. FIG. 12. Infrared spectra after thickness normalisation and baseline subtraction, taken on samples k22 and k24 before irradiation. spectra of k20 shows a larger C-H bond concentration, and a lower Si-C bond concentration. Details on the interpretation of the IR spectra are reported in Ref. 19. The lower hydrogen content is associated with a stronger incorporation of carbon in crosslinked configuration in the amorphous silicon network. 28 After laser irradiation, both Si and SiC phases crystallize. However, the signal-to-background ratio is higher for sample k34, indicating that the initial better crosslinked structure makes the subsequent crystallization energetically easier. This result can be interpreted in terms of an increase of the melting threshold due to hydrogen. G. Effect of microstructure Samples k24 and k22 share the same carbon fraction (x =0.25 and 0.24, respectively) and similar H (37% and 39%, respectively). Although the hydrogen content in these two samples is almost the same, the infrared spectrum of k22 (see Fig. 12) shows a lower content of SiCH 3 groups and a larger concentration of SiC bonds, indicating again the presence of a more crosslinked structure. Upon irradiation (two pulses at 127 and 116 mj/cm 2, respectively), sample k22 shows higher intensity of the Si and SiC peaks over the background (Fig. 13). Correspondingly, the diffraction peaks are narrower. This indicates not only the formation of a large amount of crystallites, but also an increase in the crystallite size. The result underlines that the crystallization is favored by a more cross-linked network. The thickness and the laser irradiation energy density for the two samples are somewhat different. k24 is the thicker sample, which was also the one irradiated at higher energy. For both reasons, a greater temperature is expected to be reached during the laser processing. The larger crystallized volume of k22 therefore indicates that a better cross-linked structure has a major role on the formation of crystallites. FIG. 13. Scattering spectra of the samples k24 (a) and k22 (b) before and after laser irradiation. The different structural arrangement of hydrogen in the two samples can also be deduced by evaluating silicon and carbon hydrogenation. In Table III, we reported the silicon and carbon concentrations measured by RBS combined with thickness measurements, and C-H and Si-H bond densities measured by integrating the Si-H stretching vibration cm 1 and C-H n stretching cm 1 measured by IR. The proportionality constants were cm 2 (Ref. 29) and cm 2 (Ref. 27), respectively. The ratio of the two pairs of numbers, which indicates carbon and silicon hydrogenation, is also reported. From the table, it is seen that, although the silicon hydrogenation is similar for the two samples, the carbon hydrogenation is almost twice for sample k24, in agreement with the presence of Si-CH 3 groups also observed by IR. It is likely that the stronger C-H bond results in a less efficient hydrogen abstraction for this sample after the first laser pulse, thus resulting in a more efficient lowering of the melting threshold in the case of sample k22. IV. SUMMARY AND CONCLUSIONS (i) We resume here the obtained results. Upon irradiation below the melting threshold, we observe an increase in the sample density, which is at- TABLE III. Carbon and silicon concentration; C-H and Si-H bond density; carbon and silicon hydrogenation for samples k22 and k24. C Si C-H Si-H cm 3 cm 3 cm 3 cm C-H /C Si-H /Si k k

8 J. Appl. Phys., Vol. 96, No. 7, 1 October 2004 Summonte et al (ii) (iii) (iv) tributed to hydrogen abstraction due to heating. Following the considerations given for Fig. 13, hydrogen abstraction is observed to occur more efficiently for a better cross-linked amorphous network, possibly because a lower number of C-H bonds is involved. For understoichiometric films, an evidence of an increase in the melting threshold with x was not observed. Instead, we observe a discontinuity in the melting threshold for x The melting threshold decreases for lower hydrogen concentration. Upon melting, we observe the formation of SiC crystallites, and Si or graphite crystallites, depending on sample stoichiometry. If the film is thermally thick, i.e., does not undergo complete melting, deeper regions, which experienced a lower temperature, show a larger concentration of Si crystallites compared to the surface regions. This result, taken along with the discontinuity of the melting threshold for x 0.45, indicates the formation of a silicon rich liquid phase, in agreement with the phase diagram valid for an unhydrogenated crystalline material under equilibrium. In rough words, we can figure out that melting begins in silicon rich regions. In conclusion, we have performed laser crystallization of a-si 1 x C x :H alloys with composition ranging from x=0.24 to We have observed the formation of SiC crystallites, which is accompanied by either Si or graphite crystallites, depending on composition. Coherent domains up to 10 nm are detected, although the presence of larger microtwinned grains cannot be excluded. If the sample does not undergo complete melting (insufficient energy supply), preferential formation of Si crystallite occurs, while the SiC solid skeleton remains amorphous. The major role of hydrogen has been shown. 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