4. SiC thin film based electronic devices: diodes and TFTs

#### 4.1. Diodes

#### 4.1.1. Heterojunction diodes

A SiC/Si heterojunction diode is formed by the growth of a SiC thin film with opposite doping impurities to the Si substrate used. Thus, it forms a p-n heterojunction with principle similar to a homojunction. However, the energy band for a heterojunction diode is much more complicated than that of a p-n homojunction because uses two semiconductors of different band gaps (Li, 2006).

Figure 1. Schematic illustration of cross-section of heterojunction diode structures reported in literature: (a) and (b) Yih et al., (c) Chung and Ahn, (d) Oliveira et al.

The SiC/Si heterojunction diodes with high breakdown voltage and performance dependent on the quality of the SiC film used have been reported in the literature. Yih et al. developed SiC/Si heterojunction diodes using two different rapid thermal chemical vapor deposition (RTCVD) processes: one through the formation of crystalline ß-SiC by propane carbonization of the Si substrate in regions unprotected by SiO2 layer forming planar diodes, as shown in Figure 1 (a), and another by growing polycrystalline β-SiC through the decomposition of methylsilane (CH3SiH3) at 1300ºC forming mesa diodes, Figure 1 (b). Both diodes used Ni on the SiC film as ohmic contact and Al on Si as backside contact. These SiC/Si heterojunction %+ !/\_z "+.z +0\$z !2%!z +\*"%#1.0%+\*/\_z !4\$%%0z #++ z .!0%"5%\*#z ,.+,!.0%!/^z !2!./!z .!'¥ +3\*z2+(0#!/z+"zHCzz\* zDHCzz3!.!z+0%\*! z"+.z0\$!z,(\*.z\* z)!/z\$!0!.+&1\*0%+\*z %¥ odes, respectively. These results demonstrated the potential use of SiC/Si heterojunction for the fabrication of bipolar transistor.

We have also studied heterojunctions formed at low temperatures using RF magnetron sputtering of a SiC target under different N2/Ar gas flow ratio (from 0.1 to 0.3) to prepare a-SiCxNy film with different compositions on p-type (100) Si substrates (Fraga, 2011b). After deposition, the films were submitted to a thermal annealing at 1000ºC for 30 min. The n-type conductivity of the sputtered a-SiCxNy thin films was verified by hot probe technique. The electrical contacts were fabricated through deposition of Al dots on the a-SiCxNy surface and subsequently a layer of Al was sputtered on the back side of Si substrate (see Fig. 2 (a)). The a-SiCxNyz "%()z 0\$%'\*!//!/z .!z !03!!\*z FECz \* z FHCz \*)\_z3\$!.!/z 0\$!z (z(5!./z \$2!z 0\$%'¥ \*!//!/z.+1\* zEEHz\*)^z\$!z)+0%20%+\*z+"z0\$%/z/01 5z3/z!2(10!z0\$!z%\*"(1!\*!z+"z"%()z+)¥ position on I-V characteristics of a-SiCxNy/Si heterojunction diodes. The I-V characteristics

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

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

325

The analyses were performed in a voltage range from -10 to +10 V at room temperature. As can be observed in Figure 3, the N2u.z"(+3z.0%+z%\*.!/!/z".+)zC^Dz0+zC^Ez.! 1! z0\$!z!(!¥ trical current in three orders of magnitude (from mA to µA). On the other hand, the current

was not affected significantly increasing the N2/Ar flow ratio from 0.2 to 0.3.

Figure 3. I-V characteristics of a-SiCxNy/Si heterojunction diodes at room temperature.

overlap with the curve obtained at 135ºC.

Regarding the temperature effects, the I-V characteristics at different temperatures of the a-SiCxNy/Si heterojunction diode, fabricated with the a-SiCxNy deposited at N2/Ar of 0.1, is /\$+3\*z%\*z%#1.!zG^z 0z3/z+/!.2! z 0\$0z 0\$!z!(!0.%(z1..!\*0z%\*.!/!/z3\$!\*z 0\$!z 0!),!.¥ ture is increased from 35 to 135 ºC. However, at temperature of 160 ºC there is an almost

were measured using an Agilent B1500A semiconductor.

A 3C-SiC/Si heterojunction diode fabricated by a more simple process was reported by Chung and Ahn. This diode was fabricated by the deposition of poly 3C-SiC thin films on ptype substrates using Ar, H2 and HMDS gases in an APCVD system at 1100ºC. The ohmic contacts were prepared by the deposition of Al circle electrodes on poly 3C–SiC surface and an Au layer on the Si substrate side. Figure 1 (c) shows schematic diagram of the 3C-SiC/Si heterojunction diode formed. They concluded that p–n junction diode fabricated by poly 3C–SiC film has similar characteristics to single 3C–SiC p–n junction diode.

(0\$+1#\$z/01 %!/z\$2!z/\$+3\*z 0\$0z 0\$!z !,+/%0%+\*z+"z%z "%()/z+\*z%z/1/0.0!/z0z(+3z 0!)¥ peratures processes results in heterojunctions with low breakdown voltage and high reverse leakage current, Oliveira et al. developed PECVD SiC/Si heterojunction diodes (Figure 1 (d)) with satisfactory electrical performance exhibiting good rectifying properties. An interesting conclusion of this work is that the post-deposition thermal annealing improves the electrical ,.+,!.0%!/z+"z 0\$!zz%z "%()/^z 0z3/z+/!.2! z 0\$0z 0\$!z\$!0!.+&1\*0%+\*z "+.)! z5z \*¥ nealed PECVD SiC film, at 550ºC for 120 min, has a leakage current approximately one order of magnitude smaller than that formed by as-deposited films.

Figure 2. SiCxNy/A@=L=JGBMF;LAGF<AG<= 9/=IM=F;=G>>9:JA;9LAGF9F< :K;@=E9LA;<A9?J9EG>=D=;LJA;9D;@9J9;s terization.

We have also studied heterojunctions formed at low temperatures using RF magnetron sputtering of a SiC target under different N2/Ar gas flow ratio (from 0.1 to 0.3) to prepare a-SiCxNy film with different compositions on p-type (100) Si substrates (Fraga, 2011b). After deposition, the films were submitted to a thermal annealing at 1000ºC for 30 min. The n-type conductivity of the sputtered a-SiCxNy thin films was verified by hot probe technique. The electrical contacts were fabricated through deposition of Al dots on the a-SiCxNy surface and subsequently a layer of Al was sputtered on the back side of Si substrate (see Fig. 2 (a)). The a-SiCxNyz "%()z 0\$%'\*!//!/z .!z !03!!\*z FECz \* z FHCz \*)\_z3\$!.!/z 0\$!z (z(5!./z \$2!z 0\$%'¥ \*!//!/z.+1\* zEEHz\*)^z\$!z)+0%20%+\*z+"z0\$%/z/01 5z3/z!2(10!z0\$!z%\*"(1!\*!z+"z"%()z+)¥ position on I-V characteristics of a-SiCxNy/Si heterojunction diodes. The I-V characteristics were measured using an Agilent B1500A semiconductor.

methylsilane (CH3SiH3) at 1300ºC forming mesa diodes, Figure 1 (b). Both diodes used Ni on the SiC film as ohmic contact and Al on Si as backside contact. These SiC/Si heterojunction %+ !/\_z "+.z +0\$z !2%!z +\*"%#1.0%+\*/\_z !4\$%%0z #++ z .!0%"5%\*#z ,.+,!.0%!/^z !2!./!z .!'¥ +3\*z2+(0#!/z+"zHCzz\* zDHCzz3!.!z+0%\*! z"+.z0\$!z,(\*.z\* z)!/z\$!0!.+&1\*0%+\*z %¥ odes, respectively. These results demonstrated the potential use of SiC/Si heterojunction for

A 3C-SiC/Si heterojunction diode fabricated by a more simple process was reported by Chung and Ahn. This diode was fabricated by the deposition of poly 3C-SiC thin films on ptype substrates using Ar, H2 and HMDS gases in an APCVD system at 1100ºC. The ohmic contacts were prepared by the deposition of Al circle electrodes on poly 3C–SiC surface and an Au layer on the Si substrate side. Figure 1 (c) shows schematic diagram of the 3C-SiC/Si heterojunction diode formed. They concluded that p–n junction diode fabricated by poly

(0\$+1#\$z/01 %!/z\$2!z/\$+3\*z 0\$0z 0\$!z !,+/%0%+\*z+"z%z "%()/z+\*z%z/1/0.0!/z0z(+3z 0!)¥ peratures processes results in heterojunctions with low breakdown voltage and high reverse leakage current, Oliveira et al. developed PECVD SiC/Si heterojunction diodes (Figure 1 (d)) with satisfactory electrical performance exhibiting good rectifying properties. An interesting conclusion of this work is that the post-deposition thermal annealing improves the electrical ,.+,!.0%!/z+"z 0\$!zz%z "%()/^z 0z3/z+/!.2! z 0\$0z 0\$!z\$!0!.+&1\*0%+\*z "+.)! z5z \*¥ nealed PECVD SiC film, at 550ºC for 120 min, has a leakage current approximately one order

Figure 2. SiCxNy/A@=L=JGBMF;LAGF<AG<= 9/=IM=F;=G>>9:JA;9LAGF9F< :K;@=E9LA;<A9?J9EG>=D=;LJA;9D;@9J9;s

3C–SiC film has similar characteristics to single 3C–SiC p–n junction diode.

of magnitude smaller than that formed by as-deposited films.

the fabrication of bipolar transistor.

324 Physics and Technology of Silicon Carbide Devices

terization.

The analyses were performed in a voltage range from -10 to +10 V at room temperature. As can be observed in Figure 3, the N2u.z"(+3z.0%+z%\*.!/!/z".+)zC^Dz0+zC^Ez.! 1! z0\$!z!(!¥ trical current in three orders of magnitude (from mA to µA). On the other hand, the current was not affected significantly increasing the N2/Ar flow ratio from 0.2 to 0.3.

Figure 3. I-V characteristics of a-SiCxNy/Si heterojunction diodes at room temperature.

Regarding the temperature effects, the I-V characteristics at different temperatures of the a-SiCxNy/Si heterojunction diode, fabricated with the a-SiCxNy deposited at N2/Ar of 0.1, is /\$+3\*z%\*z%#1.!zG^z 0z3/z+/!.2! z 0\$0z 0\$!z!(!0.%(z1..!\*0z%\*.!/!/z3\$!\*z 0\$!z 0!),!.¥ ture is increased from 35 to 135 ºC. However, at temperature of 160 ºC there is an almost overlap with the curve obtained at 135ºC.

Figure 4. I-V characteristics of a-SiC.N./Si heterojunction diode (N2/ Ar flow ratio= 0.1) at different temperatures.

#### 4.1.2. Other diode types

SiC thin film electronic devices also include Schottky diode and light emitting diode (LED). Komiyama et al. fabricated Schottky diodes through the heteroepitaxial growth of 3C-SiC on a (001) Si substrate by introducing low-temperature growth (700–900ºC) of 3C-SiC using methylsilane single source, as an intermediate buffer layer, prior to the subsequent 3C-SiC active layer growth at a higher temperature (1150 ℃) using SiHz and C JH2 as precursors. The diode achieved a breakdown voltage of 190 V. A correlation between the film thickness and the leakage current of the Schottky diode was observed: on reverse bias the leakage current decreases when the 3C-SiC film thickness is increased. Figure 5 (a) shows the Schottky diode with Au,Al/poly 3C-SiC/SiO%Si substrate structure developed by Chung and Ahn. This diode exhibited a breakdown voltage of over 140 V together with a high leakage current. The authors suggested that the problem of the high leakage current is associated to random grooves, due to existence of anti-phase boundaries (APB) in the poly 3C-SiC film which demonstrates the dependence between the diode performance and the film characteristics.

Wahab et al. reported a Schottky diode formed by ß-SiC thin films grown on (100) Si substrates, using reactive magnetron sputtering of a Si target in CH4/Ar mixed plasma, with Au electrical contacts. Good electrical properties were observed such as ideality factor of 1.27 and leakage current density of 4 µA/cm².

A hot wire deposited a-SiC:H based thin film light emitting p=i=n diode was fabricated by Patil et al. The diode structure is formed by glass/TCO (SnO:F)/p-a-SiC:H/f-SiC:H/f-SiC:H/n-a-SiC:H/Al as illustrated in Figure 5 (b). The p-type a-SiC:H film was grown using SiH,/ C2Hz/B2Hz gas mixture, the n-type using SiH:/CHz and the intrinsic using SiH:/C2H2.

The deposition conditions of each film were optimized to obtain the p-, i- and the n-layers with desired electrical and optical properties. The layers exhibit the following bandgaps: 2.0 eV for p-type a-SiC:H, 2.06 for n-type a-SiC:H and 3.4 eV for intrinsic a-SiC:H.

Figure 5. Schematic illustration of cross-section of device structures: (a) Schottky diode and (b) light emitting diode.

The diode was characterized and emits light in the visible region with low intensity. The authors attributed the low emission efficiency to the fact of the device be made in a single chamber and the same filament has been used to deposit all the a-SiC:H layers, which can has caused the contamination across the p-i interface.

#### 4.2. Thin-film transistors (TFTs)

For many years the SiC thin film transistors prepared at low temperature have attracted special attention. In 1994, Hwang et al. developed two models of SiC submicron MOSFETs with vertical channel. The first model, as illustrated in Figure 6 (a), uses a sputtered SiC thin film, grown on a Si substrate at 600°C and annealed at 1300°C for 5 h under Ar atmosphere, as channel layer. It was observed that this structure can attain higher current, but the I-V characteristics can not be saturated because the channel depth is too large to be depleted. The second model fabricated (Figure 6 (b)) is formed by SiC thin film deposited by RF sputtering at 600°C on the sidewall of SiO2 insulator. With this model, a complete saturation was achieved at drain voltage of 8 V for a 400 nm channel length. Furthermore, a drain breakdown voltage more than 16 V was achieved due to the wide bandgap of the SiC film used (2.2 eV). Both models were characterized under 600 K and the I-V curves do not show turn-off indicating that the TFT's fabricated can operate in this temperature range.

In 2006, Garcia et al. reported the first PECVD amorphous silicon carbide TFTs. The a-Sij. xC.H films were deposited on glass substrates by PECVD at 300℃ using SiH.JCH./Hz gas mixture. Subsequently, n-type a-51:H layer was deposited using SiH //Hy/PH3 gas mixture and a photolithography was performed. Then, a PECVD SiO2 layer, for the gate dielectric, was deposited using SiH //N2O and a second photolithography was performed. The metal contacts were formed through the deposition of an Al layer tollowed by photolithography. Finally, an annealing in H2 atmosphere at 350℃ was performed for 30 min. The TFT structure obtained is illustrated in Figure 6 (c). The TFT was tested at different temperatures. The drain current increased two orders of magnitude as temperature increased from 30℃ to 200℃. This work compared this a-Si- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . KrF excimer laser annealing of the a-Si2,C.;H films. The polycrystalline TFT exhibited output current at least an order of magnitude higher, when operated at room temperature, with respect to its amorphous being Vcs = 10 V for both.

Figure 6. Schematic illustration of cross-section of TFT structures reported in literature: (a) and (b) Hwang et al., (c) Garcia et al.
