4.4. Aluminium oxide (Al2O3)

based structures. Additionally, due to poorer interface properties and low value of dielectric constant of SiO2 is not seems to be implement in future MOS structures on SiC substrate.

HfO2 is second most promising dielectric on SiC surface after SiO2 due to its high dielectric constant and very high breakdown voltage. A good quality and desired thickness can be easily achieved in laboratory that is why HfO2 based MOS device are seems to me future devices. Pure form of HfO2, and its silicate are the potential candidate SiO2 as a gate material in a scaled down MOS technology. A continuous research and development (R&D) on this material is considerably seems to be more advanced compared to other high-k dielectrics (2\*/j!2z^^\_zDLLJaz\*\*!.z^z
^\_zECCJd^z
+.!+2!.\_zzz\$!+\*#z!0z(z\$/z+/!.2! zz/%#¥ nificant improvement in the performance of HfO2/SiO2z/0'z#0!z %!(!0.%z+\*zGw%z/1.¥ face (Cheong K.Y., 2007). Atomic layer deposition (ALD) is the most advisable and recommended process to deposit HfO2 (Cho M., 2002). However, large variations in growth rate, dielectric constant, and fixed charge are reported for HfO2 !,+/%0! z +\*z /%(%+\*z /1¥ strate. The interface stability is one of the most important issues in the deposition process. When HfO2 is considered to be deposited on SiC surface, the accurate knowledge of thermal

Titanium dioxide (TiO2) is another gate dielectric, which is explained in this chapter. The electronic bandgap energy of this material is relatively small (3.5 eV), but dielectric constant can be varied from 40 to 110. TiO2 exist in two important phases, Anatase and Rutile, which depends on growth process. Rutile phase of TiO2 is the thermally stable phase that presents the higher dielectric constant around 80. Other form i.e. Anatase is a thermally unstable phase, which shows a lower dielectric constant. The Anatase form can be transforming to Rutile phase by annealing the deposited material at temperatures more than 600° C. A high leakage current values and higher interface density are the most drawback of this material, which is unacceptable in the fabrication of transistor structure. In order to minimize these problems it is interesting to employ a stack layer of thin SiO2 and TiO2 on SiC substrate. In this way, the interface quality can be improved and the other problems may be minimized, turning this material viable and very attractive to substitute the current dielectric material on SiC surface. Variable-energy positron annihilation spectroscopy (VEPAS) was employed to investigate the atomic scale structure of TiO2/SiO2T gate dielectric stack on 4H-SiC surface (Coleman P.G., 2007). In this study a vacancy type defects was observed. Thin film of TiO2 film can be deposited with many techniques likes chemical vapour deposition (CVD), RF sputtering, e-beam evaporation, metal-organic chemical vapour deposition (MOCVD) and so on. The dielectric constant of TiO2 was reported to be 31, which is stable in the frequency range from 100 Hz to 1 MH. The critical breakdown field is 3 MV/cm. TiO2 is seems to of be very promising material in the development of gas sensors particularly Hydrogen sensors

4.2. Hafnium dioxide (HfO2)

212 Physics and Technology of Silicon Carbide Devices

stability at elevated temperatures is a must.

4.3. Titanium dioxide (TiO2)

(Weng M-H, 2006; Shafiei M., 2008)

Aluminium oxide (Al2O3) is another gate dielectric, which has proven the demanded gate material SiC MOS structures. This material has a broad scope in semiconductor industry and the single crystal wafer of Al2O3 is commercially available. Crystalline form of Al2O3 %/z'\*+3\*z/z((! z/,,\$%.!\_z+.zw(2O3,z3\$%\$z\$/z0\$!z.\$+)+\$! .(z/5))!0.5^z\$!z,¥ ,(%0%+\*z+"z/,,\$%.!z/z,//%20%+\*z)0!.%(z "+.z%z%/z2!.5z\$. z 1!z 0+z.5/0((%\*!z)%/¥ match and polycrystalline Al2O3 may cause large leakage trough grain boundaries of material. This material belongs to the family of wide bandgap (8.8 eV) and having the ,+0!\*0%(z..%!.z+"zE^Kz!z3%0\$z%z+\* 10%+\*z\* ^z\$!z(1(0! z+\* 10%+\*z\* z+""¥ /!0z "+.z Gw%z /5/0!)z%/z +10z Dz!\_z3\$%\$z%/z /)((!.z 0\$\*z 0\$0z+"z 0\$!z)!/1.! z+\*z % system. But this value is high enough to effectively prevent carrier injection at interface. However, amorphous form of Al2O3 z/!!)/z0+z!z\*z00.0%2!z\* % 0!z/zz#0!z %!(!¥ 0.%z "+.z %z/! z /0.101.!/^z\$%/z)0!.%(z)5z!z !,+/%0! z5z)\*5z %""!.!\*0z 0!\$\*%¥ ques such as sputtering (Jin p., 2002), plasma deposition (Werbowy A., 2000), Atomic layer deposition LD (Gao, K.Y. 2005) and so on with the suitable gaseous inlet of the precursors. ALD is seems to have the largest interest for fabrication of devices. K.Y. Gao et al has demonstrated a very good result and explained very nicely (Gao, K.Y. 2005). Post deposition annealing of Al2O3 in presence of H2 environment at 500°C demonstrates z!""!0%2!z.! 10%+\*z%\*z%\*0!."!z/00!/z !\*/%05z%\*z0\$!z)% z\* #,z+"z0\$!zIw%^z+.( ¥ 3% !z\*1)!./z+"z.!/!.\$!./zc2%!z
^zECCJaz\$!z ^\_zECCCdz.!z%\*0!\*/%2!(5z3+.'%\*#z0+z!4¥ ,(+.! z 0\$! z (EFu% z %\*0!."! z ,.+,!.0%!/^ z / z 0\$! z .!/1(0 z 0\$! z "%./0 z Gw% z
 z 3%0\$ Al2O3 as a gate dielectric was successfully demonstrated (Hino S., 2007).

#### 4.5. Aluminium nitride (AlN)

Aluminium nitride (AlN) is also one of the very promising gate dielectric materials, which can be associated with SiC system. Its lower bandgap of 6 eV in comparison with Al2O3 or SiO2 )%#\$0z!z %/,,+%\*0%\*#\_z10zz(00%!z)%/)0\$z0+z%z+"z+\*(5zDM\_z()+/0z0\$!z/)!z0\$!.¥ )(z!4,\*/%+\*z1,z0+zDCCCz[z\* zz\$%#\$z %!(!0.%z+\*/0\*0z.!z)+.!z!\*+1.#%\*#^z!\*!.((5\_ (z%/z1/! z/zz1""!.z(5!.z,.%+.z0+z#.+3zz/0.101.!/z+\*z%z/1/0.0!/^z\$%/z%/z0\$!z¥ sic cause for largest number of research associated with the epitaxial growth at very high temperatures. Low temperature deposition is also possible over verity of substrate like other techniques that are of interest for low temperature deposition of passivation layers like 0+)%z(5!.z !,+/%0%+\*\_zwz/,100!.%\*#z,1(/! z(/!.z !,+/%0%+\*^z\$!.!z.!z\*+0z/+z)\*5z/01 ¥ ies were focused on electrical characterization of AlN layers on SiC surface. Some results, \$+3!2!.\_z/\$+3/z/0%/"0+.5z%\*/1(0%\*#z,.+,!.0%!/z"+.z)+\*+w.5/0((%\*!z(z3%0\$z!,0(! (!'#!z1..!\*0/z+"z0\$!z+. !.z+"zDCwLzu)<sup>2</sup> z\* zz.!' +3\*z"%!( z+"z.+1\* zGz
u)zc\*+¥ jima N., 2002). AlN/SiC interface do not shows the promising characteristics because of \$.#!z0.,,%\*#z0z%\*0!."!^z+3!2!.z+6+\*!z(!\*%\*#z\* z(z,.!w0.!0)!\*0z+"z%z/1."! /\$+3/z z 0.!)!\* +1/z%),.+2!)!\*0z +"z 0\$!z,.+,!.0%!/z +"z %!(!0.%z(5!.z \* z,.+2% !/z%\*0!.¥ face quality sufficient for the fabrication of MOS structures. Introduction of thin SiO2 as a buffer layer between SiC and AlN is an additional barrier to prevent electron injection from semiconductor to dielectric, which may further decrease leakage current. This type of stack layer of AlN/SiO2uIw%z3/z,.!/!\*0! z%\*zc-%/!.%z^\_zECCCdz\* z.!2!(/zz(+3z\$.#%\*#z!"¥ fect when 100 Å SiO2 layer was used.

(!//z )#!z0\$\*z+4% !z/,100!.%\*#z\* z/\$+1( z,.+ 1!z0\$!z,1.!/0z+4% !^z-

act method for commercial production.

Vacuum Pump

good coverage.

Figure 4. Schematic diagram of e-Beam evaporation system

Sputtering is generally used to deposit refractory materials, compound and alloys, which are difficult to evaporate by e-Beam method. Sputtering exists in the category of the Physical Vapor Deposition (PVD) process, in which metals are removed from the solid cathode. The whole process is carried out by bombarding the cathode with positive ions emitted from ..!z#/z %/\$.#!^z\$!\*z%+\*/z3%0\$z\$%#\$z'%\*!0%z!\*!.#5z.!z%\*% !\*0z+\*z0\$!z0\$+ !\_z0\$!z/1¥ sequent collision knocks loose or sputters atoms from materials. The schematic of Sputtering system is given in Figure 5. Its advantage is that it is broadly available and can produce pure oxides. Its disadvantages are that oxides are insulators so sputtered oxides tend to have plasma-induced damage. Also, PVD methods deposit in line of sight, so they do not give

10z%0z%/z\*+0z\*z!4¥

215

http://dx.doi.org/10.5772/52553

Thermionic Filament

Water-cooled crucible

e-Beam

Target

Substrate

Materials and Processing for Gate Dielectrics on Silicon Carbide (SiC) Surface
