Compositional and structural properties of deuterated

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by plasma enhanced chemical vapour deposition (PECVD) starting from silane ... since hydrogen plays a fundamental role in improving the quality of the ..... Cotton, F. A., and Wilkinson, G. W., 1972, Advanced Inorganic Chemistry 3rd edition.

PHILOSOPHICAL MAGAZINE B, 1999, VOL. 79, NO. 10, 1685± 1694

Compositional and structural properties of deuterated plasma enhanced chemical vapour deposited silicon± carbon alloys L. Calcagno{ k , F. Giorgis{ , A. Makthari} , P. Musumeci{ and F. Pirri{ { Dipartimento

di Fisica and Istituto Nazionale di Fisica per la Materia, Corso Italia 57 ± Catania, Italy { Dipartimento di Fisica and Istituto Nazionale di Fisica per la Materia, Politecnico di Torino, Via Duca degli Abruzzi ± Torino, Italy } DeÂpartement de Physique, Faculte des Sciences, Universite Moulay Ismail, BP 4010, Beni M’Hamed, MekneÁs, Morocco [Received 20 May 1998 and accepted 17 June 1999]

Abstract

Hydrogenated and deuterated amorphous silicon carbon ® lms were prepared by plasma enhanced chemical vapour deposition (PECVD) starting from silane and deuterated methane gas mixtures. The gas percentages was varied in order to produce ® lms with di€ erent carbon and silicon content. The elemental composition was determined by Rutherford backscattering and elastic recoil detection analysis and the bonding structure by infrared spectroscopy. The hydrogen plus deuterium atomic fraction, in the grown ® lms, is about 0.36, almost independent of the ® lm composition. However, the concentration of hydrogen or deuterium depends on the carbon content. In silicon-rich samples both hydrogen and deuterium atoms are contained in the ® lms; with increasing carbon content, the hydrogen concentration decreases and the deuterium concentration increases. At the highest carbon concentration (0.28) the resulting ® lms are fully deuterated with the deuterium atoms attached both to silicon and carbon. From infrared absorption analysis, information on plasma chemistry and surface or bulk reactions during ® lm growth was obtained. For the ® rst time the experimental determination of the origin of the bonded hydrogen in amorphous SiC:H ® lms, grown by methane± silane PECVD, is reported. Comparison of elemental composition and infrared spectra of ® lms grown from hydrogenated and deuterated methane shows that hydrogen exchange occurs between carbon and silicon atoms. Moreover, as the ® lms approach stoichiometry the hydrogen incorporated into the sample originates mainly from methane gas. } 1. Introduction Hydrogenated amorphous silicon carbon alloys (a-SiC:H) have been widely studied in the past few years because of their applications in p-doped window layers of a-Si:H solar cells, for thin ® lm transistors and as photoreceptor active layers in electroluminescent devices (Tawada et al. 1982, Carson 1989, Hamakawa et al. 1989, Jwo 1990, Kanicki 1991). An interesting application is the construction of light k

E-mail: [email protected] Philosophical Magazine B ISSN 0141± 8637 print/ISSN 1463± 6417 online # 1999 Taylor & Francis Ltd http://www.tandf.co.uk/JNLS/phb.htm http://www.taylorandfrancis.com/JNLS/phb.htm

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emitting diodes (LEDs), as a-SiC based LEDs have a number of advantages including low power dissipation (Hamakawa et al. 1988). a-SiC:H ® lms are generally grown by plasma enhanced chemical vapour deposition (PECVD) and the bandgap can be varied from 1.9 to 3.2 eV by controlling the carbon and the hydrogen concentration. The physics of these alloys is quite complex due to the presence of carbon: the carbon concentration can be varied from 0 (a-Si:H) to 1 (a-C:H) and consequently the optical gap can be tailored over a wide range of values. Furthermore the ® lm properties change because of the di€ erent carbon con® gurations and of the ® lm microstructure. It has been reported that in silicon-rich ® lms, the tetrahedral Si± Si bonds dominate, with randomly distributed SiC clusters. At high carbon content, the CÐ C network becomes prevalent with the presence of CHn groups and aromatic CÐÐ C clusters (Demichelis et al. 1995a,b). Many attempts have been made to improve the optoelectronic properties of aSiC:H ® lms by carefully optimizing the deposition conditions and by employing di€ erent carbon sources (CH4 , C2H2) or by adding hydrogen to the reactant gases of the PECVD reactor (Baker et al. 1990, Hicks et al. 1990). It has been reported that the ® lm composition and properties depend on the deposition conditions and on the gases used for the plasma. For the characterization of a-SiC:H alloys, knowledge of the content of all the components is important, however, that of the hydrogen is particularly important, since hydrogen plays a fundamental role in improving the quality of the alloys by decreasing both defect density and topological disorder. Moreover the presence of hydrogen in¯ uences the optical and photoluminescence properties (Baker et al. 1990). a-SiC:H alloys prepared by PECVD generally contain a large hydrogen fraction, which is in the range 20± 50 at.%. So a more pronounced e€ ect of hydrogen on aSiC:H network structure compared with hydrogenated amorphous silicon (a-Si:H) ® lms is expected. The hydrogen concentration in a-SiC:H alloys depends on the deposition parameters and the hydrogen bonds to silicon and carbon atoms in di€ erent con® gurations (Munekata et al. 1984, Robertson 1992). However, it is unclear what the origin of the incorporated hydrogen is and how hydrogen elimination occurs from the adsorbed SiHn and CHn precursor groups. In order to clarify this point a-SiC alloys were prepared using silane and deuterated methane at di€ erent gas percentages to obtain ® lms with di€ erent stoichiometry and di€ erent hydrogen and deuterium concentrations. Elastic recoil detection analysis (ERDA) was used to measure the hydrogen and deuterium concentration and Rutherford backscattering spectrometry (RBS) was used to determine the silicon and carbon content. The bonding structure was studied with infrared spectroscopy, following the band structures characteristic of Si± H, Si± D, C± H and C± D over the entire compositional range. The results have been compared with those obtained for the fully hydrogenated material. } 2. Experimental procedure Amorphous hydrogenated and deuterated silicon carbon alloy ® lms were prepared by an ultrahigh vacuum-PECVD system by the decomposition of silane and deuterated methane mixtures. The gas percentages was varied in order to obtain ® lms with di€ erent composition. The reference parameter for the composition of the gas is the ratio Y …CD4 † ˆ CD4 = …CD4 ‡ SiH4†, which in our experimental conditions was varied from 0.7 to 0.98.

Deuterated a-Si:C ® lms

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The ® lms were deposited on a double polished silicon substrate maintained at a temperature of 3808 C. The 13.56 MHz radiofrequency power density e€ ectively dissipated into the plasma was 20 mWcm 2 and the deposition pressure 60 Pa. Ion beam analysis was used to characterize the elemental ® lm composition. The determination of silicon and carbon content was obtained from RBS using 2.0 MeV He‡ as the incident beam (Chu et al. 1978). The hydrogen and deuterium concentrations were measured by ERDA using also 2.0 MeV He‡ impinging on the sample with an incidence and emergence angle of 158 with respect to the sample surface (Tesmes and Nastasi 1995). The nature of the chemical bonding was studied by infrared spectroscopy using a Fourier transformed infrared Perkin-Elmer 2000 spectrometer working in the range 400± 4000 cm 1 with a resolution of 4 cm 1 . The ® lm absorbance was deduced from the recorded infrared spectra taking into account the absorption in the silicon substrate and the in¯ uence of interference fringes. ¡

¡

¡

} 3. Results and discussion To measure the hydrogen and deuterium concentration of the hydrogenated± deuterated silicon carbon ® lms ERDA spectra were recorded by using a 2.0 MeV He beam. In ® gure 1 the spectra of two samples obtained by two di€ erent values of the ratio Y …CD4 † ˆ CD4 = …CD4 ‡ SiH4†, are reported. In these spectra the edge at channel 580 corresponds to the deuterium atoms recoiling from the sample surface, and the edge at channel 405 to the hydrogen atoms. The hydrogen signal results overlapped the deuterium spectrum. The ® lm thickness is higher than the maximum depth which can be analysed (about 300 nm) and the ® lm± substrate interface is not detectable. The spectrum height is proportional to the hydrogen or deuterium concentration, however, the proportionality factor includes many parameters which are di cult to

Figure 1. Elastic recoil spectra of two a-SixCy:HzDz alloys obtained by PECVD with two di€ erent percentages of deuterated methane Y …CD4† ˆ CD4= …CD4 ‡ SiH4†. The spectrum of a silicon carbide target implanted with a know amount of hydrogen (standard) is also shown. 0

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measure, so the hydrogen concentration has been determined using a standard sample obtained by the implantation of a know amount of hydrogen into a silicon carbide substrate. The hydrogen implantation was performed at di€ erent energies and ¯ uences so to reach a hydrogen atomic fraction of 0.33 uniformly distributed through a depth of 100 nm. The spectrum of this sample is shown in ® gure 1. The spectrum height of hydrogen and deuterium in silicon carbon alloys is almost constant with depth so the concentration can be considered uniform inside the ® lms. Comparison of the spectrum height of PECVD ® lms with the height of the standard sample gives a hydrogen atomic fraction of z ˆ 0:1 for Y …CD4 † ˆ 0:95 and z ˆ 0:22 for Y …CD4 † ˆ 0:7. To determine the deuterium atomic fraction (z ) the ratio of the hydrogen height and deuterium height is measured. In fact, the spectrum height is proportional to the atomic concentration (Tesmes and Nastasi 1995) and for deuterium and hydrogen the ratio of the concentration is given by: 0

C D= C H

ˆ

z 0 =z

ˆ

SiCH

H D¼ H‰" ŠD

SiCH

=H H¼ D‰" ŠH

… 1†

;

where H H and H D are the spectrum height of hydrogen and deuterium atoms respectively, ¼ H and ¼ D are the recoil cross-section of hydrogen and deuterium respectively, ‰" ŠSiCH and ‰" ŠSiCH are the energy loss factors of deuterium and hydrogen D H respectively into the compound and z= z is the ratio of atomic fractions, which is equal to the ratio of atomic concentrations. For the recoil cross-sections and the energy loss factors we have used the values reported by Tesmes and Nastasi (1995). The experimental measurements give for the spectra reported in ® gure 1 z ˆ 0:28 for Y …CD4† ˆ 0:95 and z ˆ 0:12 for Y …CD4† ˆ 0: 7. In order to determine the silicon and carbon concentration, RBS analysis was performed and the spectra of two samples corresponding to Y …CD4† ˆ 0: 95 and Y …CD4† ˆ 0:98 are reported in ® gure 2. In these spectra the edge at channel 354 relates to the scattering from the surface silicon atoms and the edge at channel 312 to the silicon atoms of the silicon substrate. The region 354± 312 is due to silicon atoms contained into the silicon carbide ® lms and the box from 160 to 120, overlapping the silicon substrate spectrum, corresponds to the carbon atoms in the silicon± carbon ® lms. The spectrum height is proportional to the concentration of the corresponding atoms: as the backscattering yield in the region 354± 312, related to silicon in the ® lms, is almost constant, then the ® lm composition as a function of depth is uniform. The stoichiometry of these ® lms can be obtained from the spectrum heights of the edges corresponding to the scattering from the surface atoms. The atomic fractions of silicon (x ) and carbon (y ) are given by 0

0

0

C Si = C C

ˆ

x=y

ˆ

SiCH

H Si ¼ C‰" ŠSi

SiCH

=H C¼ Si ‰" ŠC

;

… 2†

where C Si and C C are the atomic concentration of silicon and carbon respectively in the compound, H C and H Si are the carbon and silicon heights respectively, ¼ C and ¼ Si are the backscattering cross-sections of carbon and silicon atoms respectively, ‰" ŠSiCH and ‰" ŠSiCH are the backscattering energy loss factors for silicon or carbon C Si in the compound (Chu et al. 1978), and x =y is the ratio of atomic fractions of silicon and carbon. For the spectra reported in ® gure 2 we obtained x = y ˆ 3:1 (dashed line) and x =y ˆ 1:1 (solid line). The experimental determination of absolute hydrogen atomic fraction (z) and deuterium atomic fraction (z ) and of the ratio x =y , allows us to calculate the 0

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Deuterated a-Si:C ® lms

Figure 2. Rutherford backscattering spectra of two a-Six Cy :Hz Dz alloys obtained by PECVD with di€ erent percentages of silane and deuterated methane. 0

absolute atomic fraction of each element by using the relation x ‡ y ‡ z ‡ z ˆ 1. The ® lm composition depends on the plasma composition, and in ® gure 3(a) the hydrogen plus deuterium atomic fraction and in 3(b) the carbon fraction as functions of deuterated methane percentage in the plasma Y …CD4 † ˆ CD4 = CD4 ‡ SiH4 are reported. The data are comparable with those of fully hydrogenated a-SiC:H ® lms grown in the same deposition system using silane± methane gas mixtures (Demichelis et al. 1995a,b). The hydrogen plus deuterium content in the ® lm is almost independent of the methane content in the plasma. Only a small increase from 0.35 up to values of 0.38 can be evidenced, at least in this Y (CD4) range. The carbon content, reported as the atomic percentage ratio C=…C ‡ Si† of the carbon plus silicon (® gure 3(b)), reaches a value of 0.1 only for Y …CD4† ˆ 0:7 and to obtain a carbon fraction C= …C ‡ Si† ˆ 0:5 (stoichiometric composition) we must use a value of Y … CD4† > 0:98: The results reported in ® gure 3 agree with those of fully hydrogenated silicon carbon alloys. This behaviour indicates that all the processes occurring in the plasma during deposition are independent on the presence of deuterated gas. In previous studies (Wieder et al. 1979, Rava et al. 1996) it was veri® ed that for methane± silane gas mixtures at radiofrequency power densities below 30 mW cm 2, the deposition occurs in the so-called `low power regime’ (Rava et al. 1995). In these conditions only silane radicals (SiH3 , SiH2) and H are created in the plasma. There is no primary dissociation of methane. The incorporation of carbon into the alloy is due to the reaction between silicon species and CH4 molecules: 0

¡

SiHn ‡ CH4 ! SiHn ¡ CH3 ‡ H2 : To obtain detailed information on ® lm composition and plasma reactions, the measured atomic fractions of silicon, deuterium and hydrogen as functions of carbon content are reported in ® gure 4. The silicon atomic fraction (x ) decreases from 0.6 to 0.32 when the carbon fraction (y ) increases from 0.05 to 0.28. In the same range the

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Figure 3.

L. Calcagno et al.

(a) Hydrogen plus deuterium content and (b) carbon content as a function of CD4 percentage in the plasma.

hydrogen fraction (z) decreases, becoming almost negligible, while the deuterium fraction (z ) reaches the value of 0.34. We can see that at low carbon content both atoms, hydrogen and deuterium, are contained in the ® lms; for y > 0: 2, the ® lms are fully deuterated. In order to understand how hydrogen and deuterium are bonded to carbon and silicon atoms, infrared spectroscopy has been applied. A typical spectrum of aSiC:HD ® lm consists of four regions of interest corresponding to wavenumbers 0

Figure 4. Atomic fractions of silicon, deuterium and hydrogen versus carbon atomic fraction for ® lms obtained by PECVD.

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Deuterated a-Si:C ® lms

between 450± 1200 cm 1 , 1200± 1800 cm 1 , 1900± 2500 cm 1 and 2750± 3100 cm 1 (Bullot and Schmidt 1987, Mastelaro et al. 1996). The region 450± 1200 cm 1 contains the sum of three contributions: (i) 600± 650 cm 1, attributed to Si± H wagging mode: (ii) 650± 900 cm 1 , attributed to Si± C stretching mode; and (iii) 950± 1100 cm 1, CHn wagging or rocking vibrations. In the region 1200± 1800 cm 1 the stretching vibration of a single Si± D bond (1550 cm 1 ) is located, as the 2090 cm 1 Si-H bond stretching band is shifted upon deuteration down to 1550 cm 1. The ratio between the two frequencies (1.35) is close to the square root of the ratio of the reduced masses of the Si± H and Si± D bonds (1.39). In the range 1900± 2500 cm 1 both the Si± H (2090 cm 1 ) and C± D (2150 cm 1 ) vibrations occur, the latter is due to the shift of C± H stretching modes after deuteration. In the region 2750± 3100 cm 1 the C± H stretching modes are present. Because of the fact that in the region 450± 1200 cm 1 some bands overlap, to obtain the concentration of deuterated and hydrogenated bonds we have focused attention on the last three regions. In ® gure 5 the infrared spectra in these regions of interest are reported for samples with di€ erent compositions. The spectra show that with changing the ® lm composition the areas of the peaks change, indicating a modi® cation of the bonding con® guration. In order to follow the behaviour of the di€ erent bonding as a function of the carbon content in the ® lms, we have calculated the integrated intensities of the peaks as ¡

¡

¡

¡

¡

¡

¡

¡

¡

¡

¡

¡

¡

¡

¡

¡

¡

I …! b †



ˆ

¬ …! † d!; !

… 3†

where ! b is the central frequency of the vibrational mode, ¬ …! † is the absorption coe cient and the integration is over the absorption band of interest. The Si± H and C± D bonds are very close and give almost a single feature, so the individual contributions were obtained by ® tting the band 1900± 2500 cm 1 with two gaussians centred at 2150 cm 1 and 2090 cm 1 . We note that in the ® tting procedure only the positions of the gaussians were ® xed. ¡

¡

¡

Figure 5. Infrared spectra of a-Six Cy:HzDz ® lms prepared by PECVD for di€ erent values of carbon concentration. 0

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From the integrated absorption the oscillator density is given by NX¡ Y

ˆ

… 4†

A X ¡ Y I …! b †;

where A X Y is a proportionality factor. For the hydrogenated CHn and SiHn vibrational modes several determinations of A CH and A SiH have been reported (Freeman and Paul 1978, Gat et al. 1992, El Khakani et al. 1993). For the Si± H stretching mode the most accepted value is A SiH ˆ 1:4 1020 cm 2 , from which the density of hydrogen bonded to silicon can be calculated. For the Si± D bonds we have used a proportionality factor A SiD ˆ … 1: 92†1=2 A SiH ˆ 1:9 1020 cm 2 , where the numerical factor (1.92) 1= 2 is derived from theory (Brodsky et al. 1977). The determination of C± H (or C± D) bond concentration is more complicated, as it has been reported that the use of a single A CH (or A CD) constant, independent of carbon content and ® lm structure, is unreliable (Bullot and Schmidt 1987). So we have determined the C± H (or C± D) bond concentration through the di€ erence between the total hydrogen (or deuterium) concentration, obtained from ERDA measurements, and the Si± H (or Si± D) bond concentration, obtained by infrared absorption, supposing that the amount of free hydrogen (or deuterium) is negligible. In ® gure 6 the calculated Si± H, Si± D, C± H and C± D bond concentrations as functions of carbon atomic fraction are reported. The C± H bond density is almost constant with the carbon concentration and is only a few per cent of the total bonds. The Si± H bond density decreases with carbon content and becomes very low when the ® lms approach the stoichiometric composition. As regards the deuterated bonds we observe a linear increase of C± D bonds with increasing the carbon concentration, while the Si± D bond density increases at low carbon content and then saturates. The results of ® gure 6 show that in silicon-rich samples up to a carbon fraction y ˆ 0:15 both hydrogenated and deuterated bonds are formed. At higher carbon fractions hydrogenated bonds are only a few per cent and deuterated bonds dominate. In addition the density of deuterium bonded to carbon is always higher than the density of deuterium bonded to silicon. ¡

¡

¡

Figure 6. Si± H, Si± D, C± H and C± D bond densities versus carbon atomic fraction.

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Deuterated a-Si:C ® lms

The results of ® gures 4 and 6 can give some indication of the reactions occurring at the surface or bulk during a-SiC:H ® lm growth. Taking into account that the ® lm formation in the low power regime is due to the adsorption at the hydrogenated± deuterated surface of SiHx (x ˆ 2, 3) and SiHy ± CD3 (y ˆ 1, 2) groups (Solomon et al. 1988, DellaSala et al. 1985, Gallagher et al. 1989), we expect that the creation of Si± D and C± H bonds occurs through reactions at the surface or bulk during or after the incorporation of the precursor groups. The possible reactions, which could justify the trends of ® gure 6, are: Si- Si…wb† ‡ C- D ! Si- C ‡ Si- D

… 5†

C- C ‡ Si- H ! C- H ‡ Si- C

… 7†

Si- C…wb† ‡ C- D ! C- C ‡ Si- D

… 6†

C…db† ‡ Si- H ! C- H ‡ Si…db†

… 8†

Si- C…wb† ‡ Si- H ! Si- Si ‡ C- H

… 9†

where wb stands for weak bond and db for dangling bond, if the following bond energies are considered (Cotton and Wilkinson 1972): Si± Si, 226 kJ mol 1; Si± C, 301 kJ mol 1 ; Si± H, 323 kJ mol 1 ; C± C, 356 kJ mol 1 ; and C± H 416 kJ mol 1 . It is possible to observe from ® gure 6 that the total concentration of Si± H and Si± D bonds is almost constant around 1.3± 1.4 1022 cm 3 for C content y in the range 0.05± 0.28. This result is in agreement with previous reports where it was found that in a-SiC:H ® lms the SiH concentration is constant up to C= …C ‡ Si† ˆ 0:8 (Demicheliset al. 1995a), corresponding to a hydrogenation of the silicon network of about 15± 20 at.%. Also from ® gure 6 it is possible to evaluate that carbon atoms have an average hydrogenation/deuteration CHp Dq with p ‡ q ˆ 3 for y ˆ 0:05 down to p ‡ q ˆ 1: 1 for y ˆ 0: 28. Since carbon is introduced into the network as CD3 groups, processes of de-deuteration or D ! H exchange have to take place. We observe that C± H bond concentration is almost independent of C content in the ® lms, as it is justi® ed by reactions (7)± (9) where the limiting factor is the Si± H bond concentration, incorporated from the silane species. On the other hand, the Si± D concentration versus carbon content in the ® lm seems to increase with carbon atomic fraction. This is in agreement with reactions (5) and (6) where the limiting factor is the C± D bond concentration, deriving from deuterated carbon precursors. We are able to deduce that, during the growth of a-SiC:H ® lms from silane± methane gas mixtures in the low power regime, a process of hydrogen elimination takes place in the growing ® lm to reach the ® nal atomic composition in equilibrium with the substrate temperature. While this process operates, a di€ usion mechanism of H atoms takes place causing a H-exchange between Si± H and C± H bonds. ¡

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} 4. Conclusions Hydrogenated± deuterated silicon carbon alloy ® lms have been deposited by PECVD from silane and deuterated methane gas mixtures. The ® lm composition has been investigated by ion beam analysis and the structural properties by infrared absorption spectra, focusing the attention on Si± H, Si± D, C± H and C± D bands. The ® lm composition depends on the percentage of deuterated methane contained in the gas mixture and the silicon and carbon concentration are similar to those obtained for fully hydrogenated material. The concentrations of hydrogen and deuterium

1694

Deuterated a-Si:C ® lms

and their related bond densities depend on the carbon content: up to a carbon atomic fraction of 0.15 both atoms are contained into the ® lms, hydrogen is mainly bonded to silicon and deuterium to carbon. At high carbon concentration the hydrogen content and the Si± H bond density decrease, while the deuterium content increases. All these results when compared with the data obtained for fully hydrogenated material suggest that during the growth process hydrogen atoms originate mainly from methane and only a small fraction from silane gas. References Baker, S. H., Spear, W. E., and Gibson, R. A. G., 1990, Phil. Mag. B, 62 , 213. Brodsky, M. H., Cardona, M., and Cuomo, J. J., 1977, Phys. Rev. B, 16, 3556. Bullot, J., and Schmidt, M. P., 1987, Phys. Stat. sol. (b), 143 , 348. Carson, D. E., 1989, IEEE Trans. Electron devices ED, 36 , 2775. Chu, W. K., Mayer, J. W., and Nicolet, M. A., 1978, Backscattering Spectrometry (New York: Academic Press).

Cotton, F. A., and Wilkinson, G. W., 1972, Advanced Inorganic Chemistry 3rd edition (Chichester, UK: Wiley).

DellaSala, D., Fiorini, P., Frova, A., Gregori, A., Skumanic, A., and Amer, N. M., 1985, J. non-crystalline Solids, 77/78 , 853.

Demichelis, F., Giorgis, F., Pirri, C. F., and Tresso, F., 1995a, Phil. Mag. A, 72 , 913. Demichelis, F., Giorgis, E., Pirri, C. F, Tresso, E., Della Mea, G., Rigato, V., and Zandolin, S., 1995b, Diamond Relat. Mater., 4, 357. El Khakani, M. A., Chaker, M., Jean, A., Boily, S. Peípin, H. Kieffer, J. C., and Guj rathi, S. C., 1993, J. appl. Phys., 74, 2834. Freeman, E. C., and Paul, W., 1978, Phys. Rev. B, 18, 4288. Gallagher, A., Doyle, J., and Daughty, D., 1989, Amorphous Silicon Technology, Material Research Society Symposium Proceedings, Vol. 149, edited by A. Madan, E. A. Schi€ , M. J. Thompson, K. Tanaka and P. G. LeComber (Pittsburgh, Pennsylvania, Material Research Society), p. 23.

Gat, E., El Khakani M. A. Chaker, M., Jean, A. Boily, S., Peípin, H., Kieffer, J. C., Durand, J., Cros, B., Rousseaux, F., and Gujrathi, S. C., 1992, J. Mater. Res., 7, 2478.

Hamakawa, Y., Kruangam, D., Deguchi, M., Hattori, Y., Toyama, T., and Okamoto, T., 1988, Appl. Surf. Sci., 33/34 , 1142. Hamakawa, Y., Kruangam, D., Toyama, T., Yoshimi, M., Paasche, S., and Okamoto, H., 1989, Optoelectronic Devices Technol. , 4, 281. Hicks, S. E., Fitzgerald, A. G., and Baker, S. H., 1990, Phil. Mag. B, 62, 193. Jwo, S. C., 1990, Jpn. J. appl. Phys., 29 , L746. Kanicki, J., 1991, Amorphous and Microcrystalline Semiconductor Devices. Optoelectronic Devices, Vol. 1 (Boston, MA: Artech House), chap. 6.

Mastelaro, V., Flank, A. M., Fantini, M. C. A., Bittencourt, D. R. S., Carrero, M. N. P., and Pereyra, I., 1996, J. appl. Phys., 79 , 1324. Munekata, H., Murasato, S., and Kunimoto, H., 1984, Appl. Phys. L ett., 23 , 810. Rava, P., Crovini, G., Demichelis, F., Giorgis, F., Galloni, R., Rizzoli, R., and Summonte, C., 1995, J. Phys. Colloque, C5, 1125. Rava, P., Crovini, G., Demichelis, F., Giorgis, F., and Pirri, C. F., 1996, J. appl. Phys., 80 , 4116.

Robertson, J., 1992, Phil. Mag. B, 66 , 615. Solomon, I., Schmidth, M. P., Senemand, C., and Drisskhodjia, M., 1988, Phys. Rev. B, 38 , 13 263.

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