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Diamond and Related Materials 12 (2003) 201207

0925-9635/03/$ - see front matter 2003 Elsevier Science B.V. All rights reserved.PII: S 0925- 9635 0 3.0 0 0 2 3 - 2

Effect of the RF power and deposition temperature on the electrical andvibrational properties of carbon nitride films

G. Lazar , M. Clin , S. Charvet , M. Therasse , C. Godet , K. Zellama *a,1 a a a b a,

Laboratoire de Physique de la Matiere Condensee, Faculte des Sciences, 33 rue Saint-Leu, 80039 Amiens Cedex, Francea `

Laboratoire de Physique des Interfaces et des Couches Minces UMR 7647, Ecole Polytechnique, 91128 Palaiseau Cedex, Franceb

Abstract

We present a detailed investigation of the effect of the radio frequency (RF) power and substrate temperature on the electricaland vibrational properties of amorphous carbon nitride films prepared by radio frequency magnetron sputtering using pure N gas2and a graphite target. A combination of electrical conductivity, visible transmission, infrared and Raman spectroscopy measurementsis applied to fully characterise the films. The optical gap decreases with increasing temperature or RF power to a minimum valuealmost equal to zero. An important increase in the room electrical conductivity by approximately ten orders of magnitude(5=10 10 V cm ) is also observed in the same range of RF power and temperature. The dependence on temperature ofy9 2 y1 y1

the conductivity is better explained in terms of bandtail hopping mechanism rather than a thermally activated process. The resultsas a whole are discussed in comparison with the earlier ones in the field. 2003 Elsevier Science B.V. All rights reserved.

PACS: 78.30.Ly; 63.50.qx; 61.43.Dq

Keywords: Amorphous carbon; Infrared; Raman spectroscopy; Electrical conductivity

1. Introduction

The study of the amorphous carbonnitrogen alloys(a-CN ) has recently received considerable attention

x

since it has been shown that they are promising candi-dates for a number of mechanical and electronic appli-cations, for example as protective coatings for magneticdisks w1x or as electronic materials in cold cathodedisplays w2,3x. The properties of the amorphous carbonnitride films depend on the bonding configurations ofnitrogen. It has been, indeed, reported that structural,

electrical, optical and mechanical properties of the filmscan be modified by nitrogen incorporation w29x. It isfound, for example, that the nitrogen incorporationincreases the electrical conductivity, accompanied by adecrease in the associated activation energy as well asin the band gap energy w5,1012x. The effects ofnitrogen involved in the amorphous network differ when

*Corresponding author. Tel.: q33-3-22-82-75-97; fax: q33-3-22-82-78-91.

E-mail address: [email protected] (K. Zellama).Permanent address: Bacau University, Calea Marasesti 157, 55001

Bacau, Romania.

nitrogen atoms enter the sp clusters, by modifying the2

p-bond states distribution or the sp network w13x.3

However, the way how nitrogen will influence thechemical organisation of the sp clusters as well as the2

sp -hybridised C atoms is not fully understood.3

This work aims at obtaining more information aboutthe effect of nitrogen on the microstructure and thereforeon the resulting electrical and vibrational properties ofamorphous carbon nitride films (a-CN ) deposited by

x

radio frequency (RF) magnetron sputtering (RFMS) atdifferent RF power and substrate temperature. Electrical

conductivity measurements, correlated with infrared andRaman spectroscopies are used to characterise the filmsin their as-deposited state. The visible transmission isalso used to estimate the optical gap of the films.

2. Experimental

The amorphous carbon nitride films, of approximately1 mm thick, were prepared by RFMS using pure nitrogenas sputtering gas and a 7.5 cm diameter graphite target.Prior deposition, the total base pressure was lower than10 Torr and was maintained at 15 mTorr for all they7

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202 G. Lazar et al. / Diamond and Related Materials 12 (2003) 201207

Fig. 1. (a) Typical IR absorption spectra (i.e. a) obtained for samplesdeposited at RF power of 50 W and substrate temperature of 150(solid line) and 300 8C (dashed curve). When the temperature increas-es, the intensity of the bands located at 30003800 and at 20002500cm decreases; (b) Comparison between IR absorption spectray1

obtained for a sample deposited at RF power of 50 W and substratetemperature of 150 8C (solid line) and a sample deposited at RFpower of 300 W and substrate temperature of 300 8C (dashed curve).The figure shows the effect of the deposition power on the bandslocated at 30003800 and at 20002500 cm .y1

films during growth. The targetsample holder distancewas 7 cm. The samples were co-deposited onto quartzand undoped crystalline silicon substrates for each spe-cific characterisation technique.

Three different substrate temperatures equal to 150,300 and 450 8C were used. For each temperature, three

types of films were deposited at RF power equal to 50,150 and 300 W, corresponding to a self-bias voltage ofy470, y870 and y1250 V, respectively. These condi-tions led to growth rates increasing in the range of 0.41.5 A s .y1

The electrical conductivity measurements were con-ducted in the coplanar configuration on samples depos-ited on quartz substrates, at room temperature as wellas for temperature ranging from 250 to 450 K. Theanalysis of the different C and N bondings was obtainedfrom IR transmission measurements using Fourier trans-form infrared (FTIR) spectrometer Bruker Vector 33, in

the range of 4004000 cm , with a resolution of 4

y1

cm and averaging over 500 scans. To deduce the IRy1

absorption coefficient a spectra, a special care wasdevoted to the determination of the baseline (i.e. as0), taking into account the interference fringes in themeasured transmission spectra. In order to follow thechanges in the film microstructure, the IR experimentswere completed with Raman scattering ones in the 8001800 cm wavenumbers range and in the back scatter-y1

ing mode, using a Jobin Yvon T6400 spectrometer with514.5 nm Argon laser excitation line. Complementaryoptical transmission measurements in the UV VISNIR, were also performed to get information about the

optical gap E and the Tauc gap E . In Ref. w14x it04 Taucis indicated that direct optical measurement of thereflectance and transmittance using spectrophotometersshows a sharp drop in sensitivity at low absorbance, butin our case the samples present a great absorption overthe whole investigated spectrum (4001200 nm) andthe absorption coefficient was calculated assuming aconstant reflectance. On the other hand, for highlyabsorbing films, other indirect techniques, such as pho-toacoustic or photothermal deflection spectroscopy, arenot appropriate to estimate the absorption coefficient.

3. Results and discussion

We present in Fig. 1a typical IR absorption spectra(i.e. a) obtained in the 5004000 cm range fory1

samples deposited with RF power of 50 W at substratetemperature of 150 and 300 8C. This figure shows fourmain absorption regions located, respectively: (i) below900 cm , a low intensity peak usually assigned to they1

presence of sp C sites in aromatic configurations, (ii)2

between 1000 and 1800 cm , (iii) between 2000 andy1

2500 cm assigned to the vibrations of CN bonds iny1

configurations such as isocyanate (C_N_O), isonitrileand nitrile and (iv) a large band between 3000 and

3800 cm which can be related to the OH andyory1

NH vibrational modes w4,1518x. The hydrogen incor-poration in the films must be only due to contamination,as the samples were deposited in pure nitrogen plasma.The origin of the 10001800 cm absorption peak isy1

still controversial. It is the almost generally acceptedhypothesis that carbonnitrogen bonding breaks thesymmetry in sp domains and causes the C_C sym-2

metric bonds to become IR active w4,13,15,19x. Basedon the work of Victoria et al. w17x, in which the resultsof isotopic substitution of N with N are presented,14 15

some authors w4,13,15,17x suppose that in the band1000 1800 cm there is contribution from neither CNy1

or NH bonds. However, the absence of the isotopic shiftin this region can be explained by the results of thenumerical simulation presented in the same work w17x.According to these results, in the aromatic systems andfor clusters containing sp N the isotopic frequency shift3

is below 10 cm and consequently very difficult to bey1

observed. On the other hand, visual examination ofcarbon nitride spectra shown in Refs. w2025x indicatedthe presence of a minimum of four individual bands,but without clear assignations. Moreover, it has beenrecently reported, for sputtered a-CN films, using FTIR,

x

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203G. Lazar et al. / Diamond and Related Materials 12 (2003) 201207

Fig. 2. Variation of the total integrated intensity of the 10001800cm absorption peak as a function of the deposition temperature fory1

different RF powers. The solid lines are guides for the eye.

XPS, XANES and NMR measurements, that the 10001800 cm absorption band must involve contributionsy1

from CN complexes in sp domains w26x. Finally, it is2

to be noticed that no clear vibrational band can bedetected in the range 2600 3000 cm , suggesting they1

quasi-absence of CH complexes in carbon nitride filmsx

w4,1518x.We have also observed that by increasing the depo-

sition temperature or the RF power, the intensity of thebands located at 30003800 and at 20002500 cmy1

decreases significantly and they almost vanish for RFpower and temperature values higher than 150 W and300 8C, respectively. These results are clearly evidencedby Fig. 1a which shows the effect of temperature andby Fig. 1b which indicates the effect of the depositionpower on these bands. This result suggests the signifi-cant decrease and the elimination of, on the one handthe OH and NH bonds and, on the other hand, the

complexes like C^

N and C_

N_

O. Moreover, for thelowest power and temperature (50 W and 150 8C,respectively; solid line in Fig. 1), the total integratedintensity of the 2000 2500 cm band is approximatelyy1

two orders of magnitude lower than that of the bandlocated in the range 10001800 cm , which, in con-y1

trast, increases with increasing power and temperature.We will, therefore, essentially focus on this band whichwill provide us a better analysis of the changes, withthe RF power and the deposition temperature, of thedifferent CC and CN bondings.

To avoid any controversy about the origin of the1000 1800 cm absorption peak, the total integratedy1

intensity of the absorption peak was used to characterisethe films. It is, however, to be noticed that the attemptto use only two Gaussians (corresponding to Raman Gand D bands) to fit this absorption peak was notpossible. The intensity of this peak can result from twofactors. The first factor is the incorporated nitrogenpercentage and the location of the nitrogen atoms in thestructure. A larger nitrogen quantity replacing carbonatoms in the aromatic rings leads to a greater perturba-tion in the electronic structure and the absorption peakintensity must increase. On the other hand, even in greatquantity, the incorporation of nitrogen in large clusters

containing sp N has no great influence on the absorption3

spectrum w17x. The second factor is the structural dis-order and the size of graphitic domains, by directinfluence on the Raman G and D bands whichbecame IR active when the nitrogen breaks the sym-metry of the aromatic rings.

The total integrated intensity of the 10001800cm absorption peak is presented in Fig. 2 as a functiony1

of the deposition temperature for the three different RFpowers 50, 150 and 300 W. Ion bombardment, enhancedby the use of a greater RF power, is a key factor forthe structure and composition of the films; it tends toreduce nitrogen incorporation and introduces optically

active centres; it probably increases the film densityalong with the disorder in the atomic network, by theformation of bonds that promote crosslinking w13x. Forreactive magnetron sputtering deposited films the nitro-gen content tends to diminish with rising depositiontemperature w13,27x. In these conditions, the initialdecrease of the 10001800 cm absorption peak wheny1

the deposition RF power increases from 50 to 150 Wcould be due to the decrease in the nitrogen proportion

in the films. When the RF power is increased to 300 W,the effect of the ion bombardment of the films must bethe main factor leading to an absorption increase. Theincrease in the intensity of this IR absorption band ismore drastic when the temperature increases from 300to 450 8C for the same 300 W RF power. A possibleexplanation of the great variation for the peak intensitycan be the existence of a transition deposition tempera-ture to sp -rich material observed for amorphous carbon2

films w28,29x.To check this hypothesis, let us discuss now the data

obtained from the Raman experiments. We have fitted

the most intense feature, 9001800 cm , of the Ramany1

spectra with three Gaussian lines, the D peak (1375cm ), the G peak(1550 cm ) and a CN band (1150y1 y1

cm ) w30x with a small contribution (below 8%). Ay1

simple two Gaussian lines fit was not suitable for ourfilms. Another widely used alternative to a Gaussian fit,a BreitWignerFano line for the G peak and a Lor-entzian line for D peak w14x also failed. From thedecomposition of the Raman spectra, we can determinethe positions of the D and G peaks, respectively locatedapproximately 1375 and 1550 cm and their corre-y1

sponding integrated intensity, I and I , respectively. WeD Gpresent in Fig. 3a and b the changes in the G peak

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Fig. 3. Variation of the G peak position (a) and the I yI ratio (b),D Gdeduced from the Raman spectra as a function of the deposition tem-perature for different RF powers. The solid lines are guides for theeye.

Fig. 4. Variation of the optical gap E as a function of the depositionTauctemperature for different RF powers as indicated. The solid lines areguides for the eye.

Fig. 5. Variation of the room temperature conductivity s as a func-RTtion of the deposition temperature for different RF powers as indicat-ed. The lines are guides for the eye.

position and I yI ratio, respectively as a function ofD Gdeposition temperature, for three different RF powers(50, 150 and 300 W). This figure clearly shows a shiftof the G peak position towards higher frequency, as wellan increase in the I yI ratio with increasing depositionD G

temperature or RF power. These results are consistentwith an increase in the degree of ordering of the C sp2

clusters w5,12x.The variation of the I yI ratio seems to contradictD G

the hypothesis of the existence of a transition tempera-ture to sp -rich material in the temperature range from2

300 to 450 8C for 300 W RF power, as in this range,Fig. 3b only shows a moderate increase in the I yID Gratio, compared to the drastic increase in the totalintegrated intensity of the 10001800 cm band report-y1

ed in Fig. 2. We can also suppose that the depositionconditions (high temperature, high impact energy for

nitrogen species) favour the replacement of the carbonatoms from the aromatic rings with nitrogen atoms. Thesp regions could be organised in aromatic domains2

which must contain N bonded atoms, as it has beenrecently reported for a-CN sputtered films w26x. Another

x

possible explanation is the transition in the same intervalto a fullerene-like structure w31x.

Concerning the optical gaps E and E , they04 Taucbehave similarly and decrease with increasing the dep-osition temperature or RF power. The values of ETaucrange from a maximum value of 0.87 eV to a minimumvalue almost equal to 0 eV, for samples deposited at(150 8C, 50 W) and at (450 8C, 300 W), respectively.

An example of the results obtained for E is presentedTaucin Fig. 4 as a function of temperature for different RFpowers as indicated.

Let us now turn to discuss the results obtained fromthe electrical conductivity measurements. We present inFig. 5 the variation of the conductivity measured atroom temperature s as a function of the depositionRTtemperature for different RF powers as indicated. Thisfigure clearly shows that s increases significantlyRTwhen the RF power or the deposition temperatureincreases. The values ofs range from a minimum ofRT5=10 to a maximum of 10 V cm for samplesy9 2 y1 y1

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Fig. 6. The Arrhenius plots of ln s(T) as a function of the inverse ofT for the samples deposited at different RF power and substrate tem-perature as indicated.

Fig. 7. Example of plot of ln s(T) as a function of T , obtainedy1

y4

for samples, deposited at 150 W and 150 8C, assuming a bandtailhopping transport mechanism. The solid line is a guide for the eye.

Fig. 8. Variation of the pre-factor s as a function of for all the1y4T00 0studied films deposited at different RF powers as indicated, assumingthe bandtails hopping transport mechanism.

deposited, respectively at (150 8C, 50 W) and at (4508C, 300 W).

We also investigated for the same samples the varia-tion of the electrical conductivity as a function oftemperature s(T) in the range of 250450 K. We triedfirst to interpret the results obtained for s(T) in termsof a thermally activated process, for which s(T) can bewritten as: s(T)ss exp(yE ykT), where s and E0 s 0 sare, respectively the pre-factor and the apparent activa-

tion energy associated with s. The Arrhenius plots ofln s(T) as a function of the inverse of T, presented inFig. 6, clearly shows a non-linear variation.

We therefore attempted to interpret the results interms of bandtail hopping transport for which thedependence on temperature of s can be expressed by:s(T)ss exp(y(T yT) ). A hopping transport model1y400 0has been previously developed w32x for an arbitraryDOS distribution and it has been shown that manytransport data for amorphous semiconductors are consis-tent with an exponential tail state distribution rather thanwith an energy-independent DOS distribution. For band-

tail hopping, the model predicts a linear relationshipbetween the pre-factor ln s and the slope . We1y4T00 0present in Fig. 7 an example of variation of ln s(T) asa function of T obtained for samples deposited aty1y4

150 8C and 150 W. This figure shows a good linearrelationship between ln s(T) and T in the wholey1y4

temperature range. We also obtain the same linearvariation for all the studied films. Fig. 8 shows a strongpositive correlation between the pre-factor s and the00slope , in good agreement with previous results w32x.1y4T0

The conductivity data considered as a whole suggestthat the electrical transport process is strongly dependenton the localised pp* states distribution which appar-

ently gets less localised with increasing deposition tem-perature and RF power.

Another important piece of information is gainedfrom the comparison of the electrical conductivity andoptical gap results with those obtained from the Ramanexperiments, presented in Fig. 9.

Starting with the electrical conductivity data, wepresent in Fig. 9 the variation of s as a function ofRTthe I yI ratio. The figure shows a good correlationD Gbetween the increase in s and that of the I yI ratio.RT D GFig. 9 also shows two different regimes for s . ForRT

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Fig. 9. Variation of the room temperature conductivity s (solidRTsymbols) and of the optical gap E (open symbols) as a functionTaucof the I yI ratio obtained for all the films. The solid line is a guideD Gfor the eye.

I yI ratio lower than 1.9, s is in the rangeD G RT5=10 10 V cm , and when this ratio is highery9 y2 y1 y1

than 2.5, s varies in the range 210 V cm . It2 y1 y1RTis also to be noticed that these particularly high valuesof s are obtained for films deposited either at highRTRF power (300 W) and temperatures equal to 300 and450 8C or at 150 W and high temperature of 450 8C.

The two regimes observed for s are also obtainedRTfor the temperature dependence of the conductivitys(T). Indeed, s(T) exhibits, as already emphasised,almost a metallic behaviour for the films deposited athigh RF power and temperature (300 W and 300 and450 8C), while for lower power and temperature, thevariation ofs with T is similar to that usually observedfor semiconductor materials.

These results are well corroborated by those obtainedfor the optical gap E . Indeed, the variation of ETauc Taucas a function of the I yI ratio, presented in Fig. 9,D Gclearly shows that for I yI ratio lower than 1.9, ED G Taucvaries in the range 0.90.4 eV, while it drops down to

0.10 eV for I yI higher than 2.5. The increase of I yD G DI ratio, explained by an increase in the ordering of theGC sp graphitic clusters, is consistent with the decrease2

of the optical gap, which corresponds to a larger sp2

content and a smaller distance between the localised pp* states, as it has been previously reported w33x.

The changes observed in s and s(T) might be dueRTto a combination of two effects, the increase in theamount of the CN bonds and the increase in the amountof the sp C graphitic clusters.2

We may therefore, make the assumption that for lowdeposition temperature (-300 8C) and low RF power(-150 W), the conduction occurs through a hopping

process in localised bandtail states, which are modifiedby the incorporation of nitrogen, as it has been previ-ously reported w32x. For high temperature ()300 8C)and high RF power ()150 W), the Urbach tail energyand the nitrogen lone pair band increase w34,35x and theconduction switches from a semiconductor to a semi-

metallic behaviour due to a strong overlap of the pp* electronic states near the Fermi level. Further XPSexperiments are now underway to have a better estimateof the amount of N incorporated in the films.

4. Conclusion

We have shown in the present study that increasingthe RF power andyor the deposition temperature of a-CN films in a pure nitrogen gas plasma, brings impor-

x

tant modifications in structure and nitrogenincorporation. As a result, a significant increase in the

electrical conductivity as well as a significant decreasein the optical gap are observed. The electrical conduc-tivity results are better explained in terms of bandtailhopping transport mechanism rather than a thermallyactivated process. We also showed that for high RFpower and deposition temperature, the conduction pro-cess switches from semiconductor to semi-metallic-likebehaviour. Complementary investigations are needed toestimate the absolute concentration of N incorporated inthe films.

Acknowledgments

This work was partially supported by the Ministere`

de la Recherche.

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