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Applied Surface Science 120 (1997) 279-286

Amorphous SiC film formation on Si(100) using electron beam excitation Jiazhan Xu a, W.J. Choyke b, John T. Yates Jr.

a,*

a Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA b Department of Physics, University of Pittsburgh, Pittsburgh, PA 15260, USA

Received 11 October 1996; accepted29 May 1997

Abstract The effect of electron impact on methylsilane (CH3SiH3) conversion to amorphous-Sio.sCo.5:H (a-Sio.sCo.5:H) films on Si(100) has been studied by Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and low energy electron diffraction (LEED). It is found that electron impact greatly enhances CH3SiH 3 decomposition on Si(100) at both 90 K and 300 K, resulting in a-Sio.sC0.5:H thin film formation. Thermal annealing of the film causes hydrogen desorption and amorphous silicon carbide (a-SiC) formation. Upon annealing to temperatures above 1200 K, the a-SiC film became covered by a thin silicon layer as indicated by AES studies. Ordered structures are not produced by annealing the a-SiC up to 1300 K. © 1997 Elsevier Science B.V. I. Introduction Hydrogenated amorphous silicon-carbon (aSil_xCx:H) alloy films and amorphous silicon carbide are of interest from the point of view of the fundamental understanding of physical properties of disordered alloys. Technical applications include window layers for amorphous silicon solar cells and light-emitting diodes (LED's) in the visible range [1,2]. The optical and electronic properties of these materials depend on chemical composition and more specifically on the nature of the chemical bonding. Traditionally, deposition of a-Sil_xCx:H alloy films is achieved using a silane-methane mixture in a glow discharge plasma [3]. Other deposition techniques can be used, such as low pressure chemical vapor deposition (LPCVD) [4,5], electron cyclotron

* Corresponding author. Tel.: + 1-412-6248320; fax: + 1-4126246003; e-mail:[email protected].

resonance (ECR) plasma CVD [6,7], reactive sputtering [8], plasma enhanced chemical vapor deposition (PECVD) [8,9], magnetron sputtering [10], and laser ablation [11]. The composition, including the hydrogen content, the S i / C ratio, and the structure of the films, may be controlled by changing the reaction conditions. In this study, we report a new method to grow an a-SiosC0.5:H alloy film using electron impact enhancement of the dissociation of methyl silane, CH3SiH 3. The thermal annealing effect on the chemical composition and the structure of the a-Sio 5C0.5:H film was investigated by AES, XPS, TPD and LEED. It was found that electron impact greatly enhances the rate of CHaSiH 3 decomposition on Si(100) at both 90 K and 300 K. Thermal annealing of the a-Sio.sC0.5:H film resulted in hydrogen desorption and a-SiC formation. Silicon diffusion to the surface occurred at high temperatures yielding a Si-covered surface.

0169-4332/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0169-4332(97)00232-8

280

J. Xu et al. /Applied Surface Science 120 (1997) 279-286

Effect of Electron Bombardmenton a-Sio.5Co.5:H Film Deposition on Si(100) - 300 K

2. Experimental Experiments were performed in an ultrahigh vacuum (UHV) system which contains a hemispherical Leybold-Heraeus EA-10 electron analyzer, a dual M g / A I anode X-ray source, a Perkin-Elmer cylindrical mirror electron analyzer, a home-made rearview low-energy electron diffraction apparatus, a UTI-100C quadrupole mass spectrometer, a calibrated and collimated micro capillary beam doser, and an electron gun capable of irradiating a 1 cm 2 area surface with uniform electron density [12]. The measured collected current from this gun was not corrected for secondary electron emission. The UHV chamber base pressure was --- 5 × 10 -11 Torr, which was maintained by an ion pump, a titanium sublimation pump, and a turbomolecular pump. The experimental setup for film epitaxial growth is shown schematically in Fig. 1. Methylsilane (CH3SiH 3, 99.96% from Voltaix) was dosed through the beam doser to the clean Si(100) surface and was then decomposed by the 100 eV electron beam to form an a-Si0.sC0.s:H alloy film. With this setup, the a-Si0.sC0.5:H alloy film can be grown continuously at low temperature using electron bombardment to induce the film growth process. The flux of CH3SiH 3 is 5 × 1012/cm2s. The chamber pressure during film growth is in the low 10 -1° Torr range. The grown film, with and without thermal annealing, were then characterized by AES, XPS, TPD and LEED. The heating rate for thermal annealing and TPD measurements is 3 K / s .

Schematic Experimental Setup for a-Sio.5Co.5:H Film Synthesis on Si(100) Electron Gun " ~ ~ ' i

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Si(100) single crystals (1.5 mm thick, B-doped with conductivity of 10 ~ cm, obtained from Virginia Semiconductor) were mounted between two tantalum clips with four silicon wafer spacers and two tantalum springs. The crystal was resistively heated providing temperature uniformity along the sample ( + 1 5 K at 1400 K). Detailed mounting procedures and temperature measurement are described by Nishino et al. [13]. Crystal cleaning was done by outgassing at 950 K followed by heating to 1400 K in UHV. Surface purity and ordering were checked by AES and LEED, respectively.

shutter 3. Results 3.1. CH3 Sill 3 : electron impact effect at 300 K

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Fig. 1. Schematicexperimentalsetup for a-Si05Co.5:Halloy film synthesis on Si(100), stimulatedby electronimpact. The electron energy employedhere is 100 eV.

The effect of electron impact on CH3SiH 3 deposition on Si(100) has been studied by AES at 300 K, as shown in Fig. 2. The Auger ratio, C(KVV)/

J. Xu et al. /Applied Surface Science 120 (1997) 279-286

Temperature Programmed Desorption Study of CH3SiHa and a-Sio.sCo.s:H Film on Si(100) - 300 K

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Fig. 3. TPD study of CHaSiH 3 and a-Si0.sC05:H alloy film on Si(100) after adsorption or film growth at 300 K. Heating rate is 3

K/s. S i(KVV), was monitored to indicate CH 3Sill 3 deposition. Without electron impact, adsorption of CH3SiH 3 on Si(100) resulted in a maximum of 0.022 + 0.003 for the C(KVV)/Si(LVV) ratio. With electron impact (100 eV) enhancement, the C / S i Auger ratio increased as the CH3SiH 3 and electron fluence increased. The rate of the development of the C / S i ratio was increased somewhat as the flux of electrons was increased from 6.2 × 1013/cm2s to 2.5 × 1014/cmZs. With fluences of 9 × 1018 elect r o n s / c m 2 and 7.8 × 1017 C H 3 S i H 3 / c m 2, a C / S i Auger ratio of 0 . 4 4 _ 0.03 was obtained. This result clearly shows that electron impact enhances CHaSiH 3 decomposition on Si(100) at 300 K. The effect of electron impact enhancement of CHaSiH 3 decomposition was further supported by TPD studies, as shown in Fig. 3. In this case, the hydrogen thermal desorption signal from the deposited film was used to monitor the degree of deposition on Si(100). Without electron impact enhancement (Fig. 3a-d), the H E desorption signal increased as the CH3SiH 3 exposure increased and

281

then finally approached saturation. The H 2 desorpfion temperature is = 820 K which is similar to H 2 desorption from the monohydride layer on the Si(100) surface [14]. With an electron fluence of 1.2 × 10=9/cm 2 while exposing the surface to CH3SiH 3, the hydrogen desorption peak intensity increases by a factor of about 12, compared to the saturation limit in the absence of electron impact. The desorption peak is broad with an additional low temperature contribution and a high temperature shoulder suggesting that Sill 2, Sill, and CH x are present in the film [14-16]. The amount of H 2 desorption is not proportional to the electron fluence as seen by comparing Fig. 3e and f. This is due to continuous hydrogen desorption by electron impact during the film growth process. Further experiments indicate that the film thickness appears to be unlimited when electron impact enhancement is employed at 300 K.

3.2. CH3SiH~: electron impact effects at 90 K Similar electron impact enhanced CH 3Sill 3 deposition on Si(100) is observed at 90 K, as indicated in the AES studies shown in Fig. 4. With a total fluence of 8.9 x 1017 e l e c t r o n / c m 2 and 5.2 × 1016 Effect of Electron Bombardment on a-Sio.sCo.5:H Film Deposition on Si(100) - 90 K 0.5

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282

J. Xu et al. /Applied Surface Science 120 (1997) 279-286 Temperature Programmed Desorption Study of CH3SiH3 and a-Sio.sCo.5:HFilm on Si(100) - 90 K ,

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Fig. 5. TPD study of CH3SiH3 and electron beam induced grown of an a-Sio.sC0.5:Halloy film on Si(100) after adsorption or film growth at 90 K. Heating rate is 3 K/s. C H 3 S i H 3 / c m 2, the C ( K V V ) / S i ( L V V ) Auger ratio reaches 0 . 3 7 _ 0 . 0 3 . Compared with a-Si0.sC0.s:H film growth at 300 K, electron enhanced film growth at 90 K is several times more efficient as judged by comparing the initial slopes of the C / S i Auger ratio curves shown in Fig. 2 and Fig. 4. This is due to higher coverage of precursor CH 3Sill 3 molecules on Si(100) at 90 K. A temperature-programmed desorption study of CH3SiH 3 deposition on Si(100) at 90 K with and without electron enhancement is shown in Fig. 5. For CH3SiH 3 chemisorption on Si(100), the main H 2 desorption is at = 820 K attributed to monohydride Sill species (Fig. 5a-e). At high CHaSiH 3 exposures (Fig. 5d and e), a small shoulder appears at lower temperature and may be due to dihydride (Sill 2 a n d / o r CH 2) species [14-16]. Further studies show that the integrated H 2 desorption peak area for CH3SiH 3 saturation adsorption at 90 K is much higher than that for CH3SiH 3 adsorbed at 300 K. The difference is due to molecular adsorption of CHaSiH 3 at 90 K while dissociative adsorption occurs at 300 K [17]. With electron impact enhancement, the H 2 desorption signal increases several fold (Fig. 5f and g).

The thermal annealing effect has been investigated using XPS and is shown in Fig. 6. In the case of clean Si(100) (Fig. 6a), a plasmon loss peak at 17 eV due to the Si bulk plasmon excitation is observed. As the a-Si0.sC0.5:H film grows and is subsequently annealed to 1283 K (Fig. 6b-d), a new plasmon loss peak at - 22 eV develops and finally dominates in the plasmon loss feature. The plasmon loss at 22 eV indicates that a tetrahedral silicon-carbon network is formed in this growth process [18]. The disappearance of the 17 eV Si bulk plasmon loss at higher film thickness in the annealed films is due to the limitation of the electron escape depth. As discussed in Section 4.3, the film grown in this way is mostly a-SiC with small crystalline inclusions being possible. The onset of a tetrahedral S i - C network appears even at annealing temperatures lower than that needed

XPS Study of a-Sio.sC0•s:H Film on Si(100) After 1283 K Thermal Annealing , . +. . . . . . x4 xl Plasmon Loss ! Si(2p)

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Binding Energy (eV) Fig. 6. XPS study in the Si(2p) region of SiC film growth on Si(100) after a-Si0.sC0.5:H alloy film formation and annealing to 1283 K. The films were grown at 90 K with 4×10 )7 electrons/cm 2 and 2.4× 1016 CH3SiH3/cm 2 for each sequential synthesis. A1 K,~ radiation was used as the X-ray source.

283

J. Xu et al. / Applied Surface Science 120 (1997) 279-286

XPS Study of Annealing Temperature Effect on a-Sio.sCo.s:H Film on Si(100)

Effect of Annealing Temperature on Carbon and Silicon Auger Signal '

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Fig. 8. AES study of thermal annealing on an a-Si0.sC0.5:H alloy film on Si(100). The film was grown at 300 K with 1.3X 1019 electrons/cm 2 and 9.2 × 1017 CH3SiH 3 / c m 2, then annealed to the temperatures shown.

for hydrogen desorption (onset = 600 K), as shown in Fig. 7. The 'as-grown' film (Fig. 7a) gives a broad plasmon loss peak centered below 22 eV indicating that the majority of the C is not bound tetrahedrally to Si, or vice versa [18]. Upon annealing to 575 K, a plasmon loss component at ---22 eV starts to develop (Fig. 7b). As the annealing temperature is further increased (Fig. 7c-e), the 22 eV plasmon loss feature finally dominates the plasmon loss region. This result indicates that thermal annealing of the 'as-grown' film promotes the formation of a tetrahedral Si-C network [18]. A similar thermal annealing effect is also evident in the C(KVV) AES lineshape studies, as shown in Fig. 8. The 'as-grown' film is shown in Fig. 8a with a AIc(KVV)/Ic(KVV) ratio of 0.14. As the annealing temperature is increased (Fig. 8b-f), the A Ic(KVV)/Ic(KVV) ratio increased monotonically to 0.27 indicating that thermal annealing promotes tetrahedral Si-C formation [19]. In addition to this effect, silicon diffusion from the bulk to the surface occurs upon annealing to temperatures in the range of 1160 K to 1295 K as indicated by the large increase of the Si(LVV) Auger signal (compare Fig.

8e and d). This effect has been observed previously on SiC films grown on the Si(100) surface [20,21]. Since the C(KVV) Auger signal decreases only slightly, the Si oveda),er produced by thermal annealing is only a few A in thickness. The C and Si stoichiometry and its change with annealing temperature can be calculated from the relative C and Si Auger intensity. Table 1 lists the C / S i Auger ratio and the stoichiometry of C and Si after correction for the relative sensitivity factors in Auger spectroscopy (Sc/Ssi of 0.54 [22]). The C and Table 1 AES study of carbon and silicon stoichiometry of the film grown by electron impact on adsorbed CH3SiH 3. Ve = 100 eV

C / S i Auger ratio a Stoichiometry of C / S i after correctionb

300 K

1008 K

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1160 K

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0.64 1.18

0.66 1.22

aData are obtained from Fig. 8 with an error bar of +7%. bThe C / S i stoichiometry is calculated by dividing the measured C / S i Auger ratio by the relative sensitivity factor (Sc / S s i = 0.54).

284

J. Xu et al. /Applied Surface Science 120 (1997) 279-286

Si stoichiometry is around 1 : 1. The slight increase in the C to Si ratio as a function of the annealing temperature may be related to the C Auger lineshape change. Neither the film produced at 90 K or 300 K, nor the film produced on annealing the low temperature layer to 1300 K shows any evidence of crystallographic order as indicated by LEED.

4. Discussion 4.1. Adsorption of CH3SiH3 on Si(lO0) Although CH3SiH 3 has been shown to be a good candidate for /3-SIC epitaxial growth on Si(100) at elevated temperatures [23,24], no study of CH3SiH 3 adsorption on Si(100) has been performed. On the other hand, disilane (Si2H6), a similar molecule, has been studied extensively [25]. A general picture is that Si2H 6 molecularly adsorbs on Si(100) and S i ( l l l ) at temperatures below 100 K and dissociatively adsorbs on both Si surfaces at temperatures above 120 K. CH3SiH 3 is a similar to Si2H 6 with a C atom in place of a Si atom. Since the S i - C bond strength (107.9 kcal/mol) is higher than that of the Si-Si bond (78.1 kcal/mol), it is expected that CH3SiH 3 will also adsorb molecularly on Si(100) at 90 K. Adsorption of CH3SiH 3 on Si(100) at 300 K and at 90 K is different as indicated by the TPD studies (Figs. 3 and 5). The main H 2 desorption for 300 K-adsorption has a maximum desorption rate at - 8 2 0 K, while desorption peaks at = 750 K and 820 K are observed for 90 K-adsorption. The appearance of the low temperature desorption peak (dihydride species) for adsorption at 90 K is a result of higher hydrogen coverage which is indicated by the increase of the integrated H 2 desorption peak area. This observation is consistent with the fact that CH3SiH 3 dissociatively adsorbs on Si(100) at 300 K and that the dissociation products from one molecule occupy two or more dangling bond surface sites [17]. For comparison, in the case of Si2H 6 adsorption on Si(100) at 300 K, Si-Si bond breaking occurs resulting in two Sill 3 groups; the Sill 3 groups decompose further to Sill 2 and H at higher temperatures. Since the S i - C bond strength is higher than the Si-Si bond

strength and slightly higher than the S i - H bond strength, the dissociation of CH3SiH 3 on Si(100) at 300 K may involve S i - H bond scission or Si-C bond scission.

4.2. Electron beam activation of CH3SiH3 It is well known that electron impact can induce chemical bond scission in adsorbed monolayers on metal and semiconductor surfaces [26]. By using electron-induced bond dissociation, various adsorbed hydrocarbon fragments on metal surfaces have been generated. Using methane as an adsorbed precursor, electron bombardment can dissociate the C - H bond producing a methyl group adsorbed on Pt(111) [27]. Similarly, the cyclohexyl and cyclopropyl groups can be produced on metal surfaces from electron bombardment of cyclohexane and cyclopropane precursors [28,29]. The physical reason for electron induced bond dissociation is that the incident electron impacts the physisorbed molecular precursor and is either captured by the precursor to form a temporary negative ion or is involved in some other type of electronic excitation process. The excited precursor molecule, if not quenched or desorbed, then dissociates, and dissociation of the bond in the molecule produces a surface fragment which can then be strongly bound to the surface. A similar electron impact excitation mechanism can be applied to CH3SiH 3 deposition on Si(100). As shown in Figs. 2 and 4, electron impact enhances CH3SiH 3 deposition on Si(100), and the enhancement is much more efficient at 90 K than at 300 K. This indicates that the electron enhancement effect is initialized by an electron-induced bond dissociation of CH3SiH 3 molecules during their adsorption life-

Schematic of First Step of Electron Stimulated a-Sio.sCo.5:H Film Synthesis CH3SiH3

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J. Xu et al. / Applied Surface Science 120 (1997) 279-286

time on the surface, as shown schematically in Fig. 9. Electron impact causes C - H or Si-H bond dissociation to form reactive species which further react with other molecules and deposit on the surface. The nature of the reactive species is undetermined. The study of SiC formation by thermal decomposition of CH3SiH 3 at high temperatures indicates that the isomers CH3SiH and CH z =Sill 2 are the precursors for SiC formation [30]. The study of amorphous Si formation from Sill 4 by PECVD indicates that the deposition process begins with electron impact dissociation of Sill 4 to Sill 2 in the gas phase [31]. Thus, it is possible that the electron impact enhancement effect is initialized by dissociation of CH3SiH 3 to CH3SiH which may isomerize to CH 2 =Sill 2. Both CH3SiH and CH z =Sill 2 intermediates could be the main reactive species.

4.3. Thermal annealing effect on the a-Sio.5Co.s:H alloy film As shown in Fig. 7a, the 'as-grown' film exhibits a broad plasmon loss feature centered below 22 eV indicating that the chemical bonds formed in the 'as-grown' film are mainly Si-Si, Si-H, C-H, SiCH2-Si and Si-CH 3 with little Si-C tetrahedral network formation [18]. This is consistent with other studies where carbon bonds in the silicon network mainly in the form of Si-CH2-Si and Si-CH 3 linkages [9,32]. As the annealing temperature increases, the plasmon loss at --- 22 eV develops and finally dominates the plasmon loss signal (Fig. 7b-e). This observation indicates that thermal annealing of the a-Si0.sC0.5:H alloy film results in more Si-C bond formation as hydrogen is released, finally producing tetrahedral Si-C network formation. The physical reason is that the Si-C bond formation is energetically favorable since the total energy of two Si-C bonds is greater than the sum of the energies of C - C and Si-Si bonds, despite the fact that the C - C bond has a higher energy [33]. A schematic picture of tetrahedral network formation during hydrogen removal by annealing is shown in Fig. 10. Three processes could contribute to the formation of a tetrahedral Si-C network. First, as shown in Fig. 7b, the plasmon loss at = 22 eV increases in intensity upon annealing to temperatures lower than

285

Schematic Thermal Annealing Effect on the a-Sio.sCo.5:H Film I

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the H 2 desorption temperature. As is known, annealing in this temperature range results in structural relaxation and the reduction of the broken bond concentration [15,34]. Thus, structural relaxation and the reduction of broken bonds favors Si-C bond formation. The process can be used for structure stabilization and the improvement of amorphous semiconductor film characteristics. Secondly, hydrogen desorption begins to occur upon annealing to temperatures higher than 650 K, which is followed by Si-C bond formation as is evident in Fig. 7c. Thirdly, chemical bond rearrangement occurs at high temperature favoring Si-C bond formation as indicated in Fig. 7c-e. Since crystalline SiC is not obtained as indicated by our LEED studies, the deposited and annealed film is considered to be amorphous SiC with possible traces of crystalline SiC.

5. Summary We have studied the effect of electron impact on methylsilane decomposition on Si(100) and the thermal annealing effect on the a-Si0.sC0.5:H alloy film formed. The techniques, AES, XPS, TPD and LEED, were employed. We conclude: (1) CH3SiH 3 adsorbs molecularly on Si(100) at 90 K. At 300 K, CH3SiH 3 adsorbs dissociatively on Si(100) with either Si-H or Si-C bond dissociation. (2) Electron impact enormously enhances the reactivity of CH3SiH 3 on Si(100) at both 90 K and 300 K. This process results in the formation of an a-Si0.sCo.5:H alloy film apparently with unlimited film thickness.

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J. Xu et al. /Applied Surface Science 120 (1997) 279-286

(3) Heating of an a-Si0.sC0.5:H alloy film results in the desorption of H 2. Upon annealing to higher temperatures, XPS studies show the development of a plasmon loss at --- 22 eV indicating tetrahedral SiC network formation. (4) Crystalline SiC is not produced by annealing an a-Si0.sC0.5:H alloy film up to 1300 K as indicated by LEED studies. (5) Si segregation to the a-SiC surface is found during the heating processes (T > 1200 K).

Acknowledgements We acknowledge the financial support of this research by the Materials Research Center of the University of Pittsburgh.

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[8]

[9]

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