2. Dopant incorporation during growth of SiC thin films

#### 2.1. In situ doping

+/0z/01 %!/z+\*z%z0\$%\*z"%()/\_z!/,!%((5z%\*z0\$!%.z)+.,\$+1/z"+.)\_z%/z\*+0z"+1/! z+\*z%\*0.%\*¥ sic films. In general, the electrical properties of wide band gap semiconductor materials as the SiC are controlled by introducing dopants into the bulk material (Oliveira, 2002). Hence, determining the best material doping concentration is one important issue to be considered during a device development. SiC-based thin films with variable electrical conductivity, ".+)z/!)%+\* 10+.z0+z%\*/1(0+.\_z\$2!z!!\*z,.+ 1! z5z &1/0%\*#z+\*(5z0\$!z +,\*0z+\*!\*¥ tration. This allows the use of these films in a variety of devices such as solar cells, different diode types, TFTs, MEMS sensors, among others (Vetter, 2006; Oliveira, 2004).

The main chemical elements used as SiC dopants are in group III (aluminum, Al, and boron, B) and group V (nitrogen, N, and phosphorus, P) of the periodic table. Therefore, n-type doping of SiC is commonly achieved by the use of nitrogen or phosphorus whereas p-type is by aluminium or boron. The in situ doping process is done by adding doping gas, such as nitrogen (N2) or ammonia (NH3) as nitrogen precursors, diborane (B2H6) as boron precursor, phosphine (PH3) as phosphorus precursor and trimethylaluminum (TMA) as aluminium precursor, during SiC epitaxial growth (Miyajima, 2006d^z\$%/z,.+!//z%/zz\*+\*w/!(!0%2!z +,¥ %\*#z+"z%z!,%04%(z(5!./z#.+3\*z+\*z %""!.!\*0z/1/0.0!z05,!/zc!^#^z/%(%+\*\_z/%(%+\*w+\*w%\*/1(¥ tor (SOI) and quartz) and it has allowed the preparation of SiC films with high electrical conductivity and low defect density. In situ doping exhibits advantages when compared to post-deposition doping methods as ion implantation and thermal diffusion, such as the easy incorporation of dopants in CVD or PVD processes and the reduction of processing steps. It %/z\*+0!3+.0\$5z0\$0z0\$!z%\*+.,+.0%+\*z+"z +,\*0/z!/% !/z""!0%\*#z0\$!z+\* 10%2%05z+"z0\$!z)¥ terial also influences other properties namely Young's modulus, hardness, optical gap, transmittance, morphology, etc. (Murooka, 1996; Sundaram, 2004).

In situ doping of amorphous and crystalline SiC films is well-documented in the literature. Historically, the first papers on the electrical properties of a-SiC films were published during 0\$!zJCj/^z \*zDLJJ\_z\* !./+\*z\* z,!.z%\*2!/0%#0! z0\$!z!(!0.%(z,.+,!.0%!/z+"z,(/)z!\*¥ hanced chemical vapor deposition (PECVD) hydrogenated amorphous SiC (a-SiC:H) films. In the 80's, several papers on doping of a-SiC and a-SiC:H films and their potential applications were published (Tawada, 1982; Beyer 1985; Pereira 1985). Since then, numerous studies have demonstrated the great potential of the SiC-based thin films for electronic device applications.

#### *2.1.1. Nitrogen incorporation*

In general, the use of amorphous SiC films has been preferred due to relatively their low growth temperature, which guarantees a larger compatibility with silicon-based technology

Nowadays, SiC-based thin films, such as SiCN, SiCO, SiCNO, SiCB, SiCBN and SiCP, have been extensively used in electronic and MEMS devices either as a semiconductor or as an insulator, depending on the film composition. These films have been shown promising for applications in diodes, thin-film transistors (TFTs) and MEMS devices (Yih, 1994; Patil, 2003;

The goal of this chapter is to discuss the role of in situ incorporation of nitrogen, oxygen, aluminum, boron, phosphorus and argon on the properties of SiC films. Special attention is given to the low temperature SiC growth processes. An overview on the applications of SiC- /! z0\$%\*z"%()/z%\*z!(!0.+\*%z\* z

z !2%!/z%/z,.!/!\*0! z\* z %/1//! ^z1.z.!!\*0z.!¥

+/0z/01 %!/z+\*z%z0\$%\*z"%()/\_z!/,!%((5z%\*z0\$!%.z)+.,\$+1/z"+.)\_z%/z\*+0z"+1/! z+\*z%\*0.%\*¥ sic films. In general, the electrical properties of wide band gap semiconductor materials as the SiC are controlled by introducing dopants into the bulk material (Oliveira, 2002). Hence, determining the best material doping concentration is one important issue to be considered during a device development. SiC-based thin films with variable electrical conductivity, ".+)z/!)%+\* 10+.z0+z%\*/1(0+.\_z\$2!z!!\*z,.+ 1! z5z &1/0%\*#z+\*(5z0\$!z +,\*0z+\*!\*¥ tration. This allows the use of these films in a variety of devices such as solar cells, different

The main chemical elements used as SiC dopants are in group III (aluminum, Al, and boron, B) and group V (nitrogen, N, and phosphorus, P) of the periodic table. Therefore, n-type doping of SiC is commonly achieved by the use of nitrogen or phosphorus whereas p-type is by aluminium or boron. The in situ doping process is done by adding doping gas, such as nitrogen (N2) or ammonia (NH3) as nitrogen precursors, diborane (B2H6) as boron precursor, phosphine (PH3) as phosphorus precursor and trimethylaluminum (TMA) as aluminium precursor, during SiC epitaxial growth (Miyajima, 2006d^z\$%/z,.+!//z%/zz\*+\*w/!(!0%2!z +,¥ %\*#z+"z%z!,%04%(z(5!./z#.+3\*z+\*z %""!.!\*0z/1/0.0!z05,!/zc!^#^z/%(%+\*\_z/%(%+\*w+\*w%\*/1(¥ tor (SOI) and quartz) and it has allowed the preparation of SiC films with high electrical conductivity and low defect density. In situ doping exhibits advantages when compared to post-deposition doping methods as ion implantation and thermal diffusion, such as the easy incorporation of dopants in CVD or PVD processes and the reduction of processing steps. It %/z\*+0!3+.0\$5z0\$0z0\$!z%\*+.,+.0%+\*z+"z +,\*0/z!/% !/z""!0%\*#z0\$!z+\* 10%2%05z+"z0\$!z)¥ terial also influences other properties namely Young's modulus, hardness, optical gap,

searches on heterojunction diodes and MEMS sensors are emphasized.

2. Dopant incorporation during growth of SiC thin films

diode types, TFTs, MEMS sensors, among others (Vetter, 2006; Oliveira, 2004).

transmittance, morphology, etc. (Murooka, 1996; Sundaram, 2004).

(Hatalis, 1987).

Hwang, 1995; Fraga, 2011c).

314 Physics and Technology of Silicon Carbide Devices

2.1. In situ doping

)+\*#z 0\$!z %""!.!\*0z%w/! z "%()/\_z 0\$!z/%(%+\*z.+\*%0.% !z c%dz\$/z!!\*z 0\$!z)+/0z%\*¥ 2!/0%#0! z 1!z0+z%0/z!/5z/5\*0\$!/%/^z!\*!.((5\_z%z"%()/z.!z,.+ 1! z5z%\*0.+ 1%\*#z\*%¥ trogen gas (N2) during SiC film growth by CVD and PVD processes. The use of N2 as doping gas is advantageous due to non-toxicity, low cost and high efficiency (Fraga, 2008d^z\$!z+\*¥ trol of N2 gas flow during deposition process has been shown as a convenient and effective 35z0+z\$\*#!z0\$!z!(!0.%(z,.+,!.0%!/z+"z%z"%()/z%\*z+. !.z0+z+0%\*z"%()/z3%0\$z !/%.! z!(!¥ trical conductivities for each application type. According to Alizadeh and Sundaram, for N2/Ar ratios from 0.2 to 0.4, the N2 gas acts like a dopant in a-SiC films prepared onto glass substrate by radiofrequency (RF) magnetron sputtering of a SiC target in N2/Ar atmosphere, reducing their electrical resistivities from the range of 109 .cm to 104 .cm. However, for N2/Ar ratios between 0.6 and 0.8 the film resistivity reach values in the range of 1010 ^)\_ indicating the formation of insulator SiCN films. Other important correlations between N2/Ar ratio and the properties of a-SiCN films were shown in their previous work: the bandgap and the percentage of optical transmission of these films increase with the N2/Ar ratio increases. The electrical conductivity of sputtered a-SiCN films was also studied by Wu !0z (^z3\$!.!\_z%\*z 0\$%/z3+.'\_z 0\$!z w%z "%()/z3!.!z !,+/%0! z +\*0+z -1.06\_z #(//z \* z %z /1¥ strates at room temperature by RF reactive sputtering of a SiC target in Ar/N2/H2/CH4 0¥ mosphere. They observed that the dark conductivity decreases with increases in N2 flow rate. Besides sputtering, other processes are being used to grown a-SiCN films. Gomez et al. used the electron cyclotron resonance (ECR) PECVD to prepare a-SiCN films using nitrogen, )!0\$\*!\_z\* z.#+\*z %(10! z/%(\*!z/z,.!1./+.z#/!/^z))+0+z!0z(^z%\*2!/0%#0! z0\$!z+.¥ .!(0%+\*z!03!!\*z\*%0.+#!\*z%+\*z!\*!.#5z\* z0\$!z"+.)! z\$!)%(z+\* /z%\*zw%z"%()/z !,+/¥ ited on (100) Si substrates by nitrogen ion-assisted pulsed-laser ablation of a SiC target.

 \*z/%01z\*%0.+#!\*z +,%\*#z+"z.5/0((%\*!z%z"%()/z\$2!z(/+z!!\*z+))+\*(5z.!,+.0! ^z%&!/1\*¥ dara et al. investigated the nitrogen doping of polycrystalline 3C-SiC films grown on (100) Si substrates by LPCVD at various growth temperatures 650–850ºC using 1,3-disilabutane and NH3 as precursors. They concluded that the electrical resistivity of the polycrystalline lms is further controlled by adjusting the NH3 ow rate in the reactor and that nitrogen-doped 3C-SiC lms exhibit lower resistivities (around 0.02 .cm) than the undoped (around 10 ^)dz+0%\*! z0zKCCB^z\$!z!""!0/z+"z<sup>2</sup> ow rate and growth temperature on the electrical properties of nitrogen-doped 3C-SiC thin lms, grown on Si3N4/p-Si (111) substrates by LPCVD at temperature 1100-1250ºC using organosilane-precursor trimethylsilane ((CH3)3%d\_z3!.!z %/1//! z5z\$!\*#z!0z(^z 0z3/z+/!.2! z0\$0z%\* !,!\* !\*0z+"z0\$!z0!),!.¥ ature process the film resistivity decreases continuously with N2 "(+3z%\*.!/!/^z%1z!0z(^z.!¥ ported the synthesis of nitrogen-doped polycrystalline 3C-SiC thin films by LPCVD at 800ºC with optimized properties for MEMS applications: resistivity of 0.026 .cm, residual stress of 254 MPa and strain of 4.5 × 10PG. The deposition conditions to produce films with these characteristics were established through studies on the effects of the precursor gases flow rate, NH3 and diclorosilane, on the material properties such resistivity, residual stress, strain, crystallinity and surface morphology. It is necessary to underline that for crystalline SiC films prepared by CVD processes, the site-competition epitaxy model has been shown as an efficient method to control in situ doping. This model is based on the variation of the Si/C ratio within the CVD reactor in order to control the dopant incorporation. In the case of the nitrogen doping in SiC, its incorporation is directly related to Si/C ratio (Larkin, 1997).

Al-doped SiC are of p-type conductivity. The film resistivity, measured at room temperature by a standard four-point probe system, was of 0.31 ^)^z\$z!0z(^z %/1//! z0\$!z,\$+0+(1¥ minescence properties of Al-doped SiC films deposited on Si substrates by RF magnetron /,100!.%\*#z+"zz/%\*#(!z.5/0((%\*!z%z0.#!0z+\*0%\*%\*#z/!2!.(z,%!!/z+"z(1)%\*1)z+\*z%0/z/1.¥ face. The influence of substrate temperature (300–390°C) on the aluminum doping of µc-SiC:H prepared by hot-wire CVD using TMA as gas dopant was discussed by Chen et al.. They concluded that a process temperature higher than 350°C is needed to obtain effective aluminum doping of µc-SiC:H thin films. An important observation on in situ aluminum +,%\*#z%\*z%z%/z0\$0z+. %\*#z0+z0\$!z/%0!w+),!0%0%+\*z!,%045z)+ !(\_z0\$!z(z +,\*0z%\*+.¥ poration has been found to be inversely related to the Si/C ratio within the CVD reactor. This behavior is opposite to that observed in nitrogen incorporation in SiC (Larkin, 1997).

Applications of SiC-Based Thin Films in Electronic and MEMS Devices

http://dx.doi.org/10.5772/50998

 \*z/%01z+.+\*z +,%\*#z%/z+0\$!.z,.+!//z1/! z0+z+0%\*z%z"%()/z3%0\$z,w05,!z+\* 10%2%05^z-

+\*w +,! z /,100!.! z w%`z "%()/\_z ,.!,.! z +\*0+z +.\*%\*#z #(//z \* z w%z /1/0.0!/z )%\*¥ tained at temperatures of 125–250°C by magnetron sputtering of silicon in Ar+H2+B(CH3)3 atmospheres, were reported by Uthanna et al. The highest values of dark conductivity and doping efficiency were achieved at a carbon content x= 0.04. It was also found that the film 3%0\$z4zRzC^DG\_z !,+/%0! z0zDJH[\_z\$/z!(!0.%(z,.+,!.0%!/z/z.!-1%.! z"+.z/+(.z!((z,,(%¥ tions. Yoon et al. studied the effects of the diborane (B2H6dz(!2!(/z+\*z0\$!z !,+/%0%+\*z.0!\_z+,0%¥ cal band gap and conductivity of boron-doped SiC:H films prepared using ECR PECVD technique from a mixture of methane, silane, hydrogen and diborane gases. It was observed that at a low microwave power of 150 W the band gap of the SiC:H film decreases as the diborane flow increases, whereas the films deposited at a high microwave power of 800 W remains relatively unaffected throughout the entire range of diborane levels investigated. Highly conductive boron-doped nanocrystalline SiC ()/z3%0\$z z(+3z +\*!\*0.0%+\*z +"z \$5¥ .+#!\*w %(10%+\*z c,w\*w%`dz #.+3\*z 5z z)!.1.5w/!\*/%0%6! z,\$+0+wz)!0\$+ z1/%\*#z /%¥ lane, hydrogen, diborane and ethylene as a carbon source, were reported by Myong et al. These films were tested as window material for amorphous silicon solar cells and a good conversion efficiency of 10.4%, without use any back reflectors, was obtained. It has also been noticed the improvement of the Young's modulus of polycrystalline SiC film grown by LPCVD through the introduction of B2H6 in the precursor gas mixture (Murooka, 1996). This study concluded that the Young's modulus of SiC films increases with the addition of B2H6, and a maximum value of 600 GPa, which was 25% higher than in the case without B2H6, was

Although boron-doped SiC films exhibit suitable properties for different applications, these films are still little used because, like the PH3, the B2H6 dopant gas is toxic. An alternative method for the formation of SiCB films has been shown in the literature: the deposition of SiCB films using sputtering target containing boron in its composition. This method has also allowed the growth of quaternary compound SiCBN by introducing N2 gas into deposition process. Optical properties of amorphous SiCBN thin films obtained, by co-sputtering from SiC and BN targets using N2/Ar gas mixtures, were studied by Vijayakumar et al. It was

+.¥

317

*2.1.4. Boron incorporation*

reached at a source gas ratio B/Si=0.02.

#### *2.1.2. Phosphorus incorporation*

In situ phosphorus doping, although little used, is another way used to obtain n-type SiC. Ruddell et al. employed a thermal CVD reactor for the deposition of phosphorus-doped SiC layers on Si substrates using silane/propane/phosphine gas chemistry over the temperature range 720-970ºC. SiC films with a phosphorus concentration of 5×1020 cm-3 and resistivity of C^Iz^)z3!.!z+0%\*! \_z3\$%\$z%\* %0!/zz#++ z!""%%!\*5z+"z,\$+/,\$+.1/z +,%\*#^z\$!z+..!¥ (0%+\*/z!03!!\*z 0\$!z)%.+32!z,+3!.z c".+)zDHCz 0+zLCCzdz\* z 0\$!z,.+,!.0%!/z+"z,\$+/,\$+¥ rus-doped SiC:H films, prepared by ECR-CVD from a mixture of methane/silane/ hydrogen/ ,\$+/,\$%\*!\_z3!.!z%\*2!/0%#0! z5z++\*z!0z(^z\$!z,.+,!.0%!/z+"z\*w05,!z\*\*+.5/0((%\*!z\$5 .+¥ genated cubic silicon carbide (nc-3C-SiC:H) prepared by hot-wire chemical vapor deposition cd\_z1/%\*#z,\$+/,\$%\*!z\* z\$!4)!0\$5( %/%(6\*!zc
dz/z +,\*0/\_z0z(+3z0!),!.¥ 01.!/z.+1\* zFCCBz3!.!z/01 %! z5z
%5&%)z!0z(^z \*z/%01z,\$+/,\$+.1/z +,%\*#z 1.%\*#z/,10¥ tering process was reported by Pereira et al. that performed the substitutional doping of RF sputtered amorphous SiC by adding controlled amounts of phosphine (PH3dz0+z0\$!z.#+\*z0¥ mosphere at a constant substrate temperature of 200 °C. They observed that the conductivity of the SiC film increases about one order of magnitude when doped with phosphorus and in the presence of 0.5 m Torr of hydrogen. In recent work, Loubet et al. reported an epitaxy process based on a cyclical deposition-etch (CDE) technique to obtain ultra-low resistivity in /%01z,\$+/,\$+.1/w +,! z/%(%+\*z.+\*zc%dz(5!./z"+.z.%/! z/+1.!u .%\*z,,(%0%+\*/^z!¥ spite studies have demonstrating the efficiency of in situ phosphorus doping during SiC film growth, this process is not much used due to high toxicity and flammability of PH3 gas. When used, in general, PH3 is highly diluted (< 1%) in hydrogen because the risks in diluted form are less critical.

#### *2.1.3. Aluminum incorporation*

Considerable efforts have also been made to prepare p-type SiC films. In situ aluminum doping of SiC films is one of the processes used for this purpose. Wang et al. presented a +,%\*#z)!0\$+ z"+.z#.+30\$z+"z(w +,! z/%\*#(!w.5/0((%\*!zFw%z!,%(5!./z+\*0+zcDCCdz%z/1¥ /0.0!/z 5z 0+)%w(5!.z!,%045z 0z DCCC[z1/%\*#z /%(\*!\_z !05(!\*!z \* z
z #/!/z%\*z z +\*¥ ventional LPCVD reactor. The hot-probe and Hall effect measurements confirmed that the Al-doped SiC are of p-type conductivity. The film resistivity, measured at room temperature by a standard four-point probe system, was of 0.31 ^)^z\$z!0z(^z %/1//! z0\$!z,\$+0+(1¥ minescence properties of Al-doped SiC films deposited on Si substrates by RF magnetron /,100!.%\*#z+"zz/%\*#(!z.5/0((%\*!z%z0.#!0z+\*0%\*%\*#z/!2!.(z,%!!/z+"z(1)%\*1)z+\*z%0/z/1.¥ face. The influence of substrate temperature (300–390°C) on the aluminum doping of µc-SiC:H prepared by hot-wire CVD using TMA as gas dopant was discussed by Chen et al.. They concluded that a process temperature higher than 350°C is needed to obtain effective aluminum doping of µc-SiC:H thin films. An important observation on in situ aluminum +,%\*#z%\*z%z%/z0\$0z+. %\*#z0+z0\$!z/%0!w+),!0%0%+\*z!,%045z)+ !(\_z0\$!z(z +,\*0z%\*+.¥ poration has been found to be inversely related to the Si/C ratio within the CVD reactor. This behavior is opposite to that observed in nitrogen incorporation in SiC (Larkin, 1997).

#### *2.1.4. Boron incorporation*

ature process the film resistivity decreases continuously with N2 "(+3z%\*.!/!/^z%1z!0z(^z.!¥ ported the synthesis of nitrogen-doped polycrystalline 3C-SiC thin films by LPCVD at 800ºC with optimized properties for MEMS applications: resistivity of 0.026 .cm, residual stress of 254 MPa and strain of 4.5 × 10PG. The deposition conditions to produce films with these characteristics were established through studies on the effects of the precursor gases flow rate, NH3 and diclorosilane, on the material properties such resistivity, residual stress, strain, crystallinity and surface morphology. It is necessary to underline that for crystalline SiC films prepared by CVD processes, the site-competition epitaxy model has been shown as an efficient method to control in situ doping. This model is based on the variation of the Si/C ratio within the CVD reactor in order to control the dopant incorporation. In the case of the nitrogen doping in SiC, its incorporation is directly related to Si/C ratio (Larkin, 1997).

In situ phosphorus doping, although little used, is another way used to obtain n-type SiC. Ruddell et al. employed a thermal CVD reactor for the deposition of phosphorus-doped SiC layers on Si substrates using silane/propane/phosphine gas chemistry over the temperature range 720-970ºC. SiC films with a phosphorus concentration of 5×1020 cm-3 and resistivity of C^Iz^)z3!.!z+0%\*! \_z3\$%\$z%\* %0!/zz#++ z!""%%!\*5z+"z,\$+/,\$+.1/z +,%\*#^z\$!z+..!¥ (0%+\*/z!03!!\*z 0\$!z)%.+32!z,+3!.z c".+)zDHCz 0+zLCCzdz\* z 0\$!z,.+,!.0%!/z+"z,\$+/,\$+¥ rus-doped SiC:H films, prepared by ECR-CVD from a mixture of methane/silane/ hydrogen/ ,\$+/,\$%\*!\_z3!.!z%\*2!/0%#0! z5z++\*z!0z(^z\$!z,.+,!.0%!/z+"z\*w05,!z\*\*+.5/0((%\*!z\$5 .+¥ genated cubic silicon carbide (nc-3C-SiC:H) prepared by hot-wire chemical vapor deposition cd\_z1/%\*#z,\$+/,\$%\*!z\* z\$!4)!0\$5( %/%(6\*!zc
dz/z +,\*0/\_z0z(+3z0!),!.¥ 01.!/z.+1\* zFCCBz3!.!z/01 %! z5z
%5&%)z!0z(^z \*z/%01z,\$+/,\$+.1/z +,%\*#z 1.%\*#z/,10¥ tering process was reported by Pereira et al. that performed the substitutional doping of RF sputtered amorphous SiC by adding controlled amounts of phosphine (PH3dz0+z0\$!z.#+\*z0¥ mosphere at a constant substrate temperature of 200 °C. They observed that the conductivity of the SiC film increases about one order of magnitude when doped with phosphorus and in the presence of 0.5 m Torr of hydrogen. In recent work, Loubet et al. reported an epitaxy process based on a cyclical deposition-etch (CDE) technique to obtain ultra-low resistivity in /%01z,\$+/,\$+.1/w +,! z/%(%+\*z.+\*zc%dz(5!./z"+.z.%/! z/+1.!u .%\*z,,(%0%+\*/^z!¥ spite studies have demonstrating the efficiency of in situ phosphorus doping during SiC film growth, this process is not much used due to high toxicity and flammability of PH3 gas. When used, in general, PH3 is highly diluted (< 1%) in hydrogen because the risks in diluted

Considerable efforts have also been made to prepare p-type SiC films. In situ aluminum doping of SiC films is one of the processes used for this purpose. Wang et al. presented a +,%\*#z)!0\$+ z"+.z#.+30\$z+"z(w +,! z/%\*#(!w.5/0((%\*!zFw%z!,%(5!./z+\*0+zcDCCdz%z/1¥ /0.0!/z 5z 0+)%w(5!.z!,%045z 0z DCCC[z1/%\*#z /%(\*!\_z !05(!\*!z \* z
z #/!/z%\*z z +\*¥ ventional LPCVD reactor. The hot-probe and Hall effect measurements confirmed that the

*2.1.2. Phosphorus incorporation*

316 Physics and Technology of Silicon Carbide Devices

form are less critical.

*2.1.3. Aluminum incorporation*

 \*z/%01z+.+\*z +,%\*#z%/z+0\$!.z,.+!//z1/! z0+z+0%\*z%z"%()/z3%0\$z,w05,!z+\* 10%2%05^z-+.¥ +\*w +,! z /,100!.! z w%`z "%()/\_z ,.!,.! z +\*0+z +.\*%\*#z #(//z \* z w%z /1/0.0!/z )%\*¥ tained at temperatures of 125–250°C by magnetron sputtering of silicon in Ar+H2+B(CH3)3 atmospheres, were reported by Uthanna et al. The highest values of dark conductivity and doping efficiency were achieved at a carbon content x= 0.04. It was also found that the film 3%0\$z4zRzC^DG\_z !,+/%0! z0zDJH[\_z\$/z!(!0.%(z,.+,!.0%!/z/z.!-1%.! z"+.z/+(.z!((z,,(%¥ tions. Yoon et al. studied the effects of the diborane (B2H6dz(!2!(/z+\*z0\$!z !,+/%0%+\*z.0!\_z+,0%¥ cal band gap and conductivity of boron-doped SiC:H films prepared using ECR PECVD technique from a mixture of methane, silane, hydrogen and diborane gases. It was observed that at a low microwave power of 150 W the band gap of the SiC:H film decreases as the diborane flow increases, whereas the films deposited at a high microwave power of 800 W remains relatively unaffected throughout the entire range of diborane levels investigated. Highly conductive boron-doped nanocrystalline SiC ()/z3%0\$z z(+3z +\*!\*0.0%+\*z +"z \$5¥ .+#!\*w %(10%+\*z c,w\*w%`dz #.+3\*z 5z z)!.1.5w/!\*/%0%6! z,\$+0+wz)!0\$+ z1/%\*#z /%¥ lane, hydrogen, diborane and ethylene as a carbon source, were reported by Myong et al. These films were tested as window material for amorphous silicon solar cells and a good conversion efficiency of 10.4%, without use any back reflectors, was obtained. It has also been noticed the improvement of the Young's modulus of polycrystalline SiC film grown by LPCVD through the introduction of B2H6 in the precursor gas mixture (Murooka, 1996). This study concluded that the Young's modulus of SiC films increases with the addition of B2H6, and a maximum value of 600 GPa, which was 25% higher than in the case without B2H6, was reached at a source gas ratio B/Si=0.02.

Although boron-doped SiC films exhibit suitable properties for different applications, these films are still little used because, like the PH3, the B2H6 dopant gas is toxic. An alternative method for the formation of SiCB films has been shown in the literature: the deposition of SiCB films using sputtering target containing boron in its composition. This method has also allowed the growth of quaternary compound SiCBN by introducing N2 gas into deposition process. Optical properties of amorphous SiCBN thin films obtained, by co-sputtering from SiC and BN targets using N2/Ar gas mixtures, were studied by Vijayakumar et al. It was found that the transmittance of the SiCBN films increases with nitrogen incorporation increases. Petrman et al. prepared SiBCN films by reactive magnetron sputtering of a Sig(B)25 target in No/Ar gas mixtures. They showed the dependence of electrical resistivity and optical gap of SiBCN films on the N2 content used in the No/Ar gas mixture.

#### 2.2. Unintentional doping

The incorporation of dopants during SiC growth can also be unintentional, 1.e., when elements presented during the deposition process are incorporated into film due to unwanted chemical reactions. Oxygen and hydrogen have been shown as the most common unintentional dopants of SiC films.

The contamination sources by hydrogen are well clear. It is known that the presence of hydrogen in the plasma and its consequent incorporation into the SiC film, in CVD or PVD deposition processes, is due to use of hydrogenated precursor gases as source of carbon (e.g. CHy, C2H2, C2H2, C2Hg, C3Hg and C,H10) and as source of silicon (e.g. SiH4, and Si2H2). The hydrogen incorporation is significant, when the SiC films are produced at low temperature processes as PECVD and sputtering. This has motivated several studies on how the hydrogen incorporation aftects the properties of SiC films and on the applications of a-SiC:H films in devices. Studies on PECVD and sputtered a-SiC:H films have shown that the hydrogen incorporation induces the formation of voids (Beyer, 1985) and increases the compressive stress (Kim, 1995). On the other hand, increasing hydrogen content in a-SiC:H films increases the optical gap (Shimada, 1979), improves the photoconductivity and increases the electrical resistivity. The optical and electrical changes due to hydrogen incorporation have stimulated the research on growth of a-SiC:H films by using the hydrogen dilution method, i.e, adding hydrogen gas (H₂) to the deposition process. This method is attractive because the optical gap of the a-SiC:H films can be varied by changing the H2 flow rate while the other deposition parameters are kept constant. In recent years, a-SiC:H films have been successfully employed as electronic surface passivation of c-51 in photovoltaic applications exhibiting performance comparable to thermal SiO2 and a-SiN, that are the most used materials. As the photoconductivity of a-SiC:H films is high, another attractive application of these films is replacing Si:H top layer of pin solar cells (Vetter, 2006).

Regarding the unintentional contamination by oxygen, the sources have been more discussed in the literature. Many studies have showed that the oxygen contamination sources during the deposition of SiC films are mainly the residual gas in reactant chambers, possible air leak in the deposition system and adsorbed gas molecules on the reactor inner walls (Medeiros, 2011). Moreover, post-deposition film surface contamination by oxygen, that can occurs when the SiC film is exposed to atmospheric air, has also been described by difterent authors (Fraga, 2008). The oxygen incorporation in SiC films and the consequent formation of silicon oxycarbide (SiCO) can be interesting for different applications such as thin film anodes for lithium ion batteries (Shen, 2011) and as doping contacts for solar cell applications (Martins, 1996). The excellent physical and chemical properties identified in SiCO films have motivated studies on the growth and characterization of these films intentionally deposited through oxygen gas addition during their growth process. Amorphous SiCO films have been produced mainly by sputtering of a SiC target in Os/Ar atmosphere or by PECVD using C,H,/Si,H,/O2 gas mixture which result at hydrogenated films (a-SiCO:H). It has been reported that these films exhibit mechanical and electrical properties strongly dependent of the oxygen-to-carbon ratio in their chemical composition. A potential application of SiCO and SiCO:H films is to replace SiO2 in microelectronic devices because it possesses advantages over other lower dielectric materials since it is formed as an interface phase between SiC and SiO2. Apart from applications in surface passivation, dielectric insulation and copper diffusion barrier, SiCO films have also been used in photodetectors and for oxygen detection in high temperature and corrosive environments. The literature also shows the synthesis of silicon oxycarbonitride (SiCNO) films by unintentional oxygen incorporation during low temperature reactive magnetron co-sputtering of silicon and graphite targets in mixed Ar/N2 atmosphere (Medeiros, 2011). As others SiC-based films, SiCNO films exhibit high thermal stability, tunable bandgap characteristics and high gauge factor values (Cross, 2010; Terauds, 2010). A comparison among the physical, electrical, and reliability characteristics of SiC, SiCN, SiCO, SiCNO and SiN thin films was performed by Chen et al. It was observed that SiCNO films are the most appropriate to be used as copper diffusion barrier because exhibit more reliable electro-migration and stress-migration besides to present physical and electrical performance comparable to those of SiN films.

Although less studied, the argon is another atom that can be incorporated during the growth of SiC films. For this, bias assisted CVD or PVD technique with high potentials applied to the substrate holder allows tuning the argon ion (Ar') flux on the substrate surface and, consequently, the concentration of incorporated argon into film. Some studies on chemical composition of SiC films using RBS and XPS analysis have indicated argon content up to 8 at.% in film composition. A.K. Costa and co-workers investigated the influence of the substrate negative bias increases (from 0 to -100 V) on the properties of sputtered SiC films. They observed that the substrate bias leads to substantial argon incorporation into the SiC film, which results in the increase of the hardness. A negative characteristic was also observed. The microscopic examination of the film surface showed a large number of detects and pinholes as the argon content increases together with a reduction in the deposition rate due to the sputtering of the film surface by argon 10ns. In a recent work, Medeiros et al. showed that increasing the argon incorporated in Six C , films, deposited onto (100) Si by dc magnetron cosputtering technique under different negative substrate bias, promotes an increase of the elastic modulus and a reduction of the electrical resistivity. In addition, it is also noticed that co-sputtered Si.C., films grown at high negative substrate bias (between -100 and -300 V) are free of oxygen contamination.
