7. Charge management in SiO2 on 4H-SiC

#### 7.1. Origin and basic theory of oxide charges in SiO2

In the prospects of technological issues on Silicon carbide based MOS system, the almost similar consideration has been adopted to investigate the charge management as silicon based MOS system. In this section, the oxide charges associated with Ni/SiO2uGw%z /5/¥ tems have been examined with varying oxide thickness. There are general four types of charges associated with the SiO2-Si system as shown in figure 13. They are fixed oxide charge, mobile oxide charge, oxide trappedcharge and interface trapped charge (Afanas'ev, V, V., 1996 and Schroder, D. K., 2006). The basic origin of all oxide charges and experimental methods in order to calculate these charge are presented here one by one.

Figure 13. Allocation of different oxide charges associated with SiC MOS system

### 7.2. Fixed oxide charges (Qfix)

( / ) exp *B i q qE*

 ' ( )

Where, J is the current density; T, the absolute temperature; q, the electronic charge; £B, the

In the prospects of technological issues on Silicon carbide based MOS system, the almost similar consideration has been adopted to investigate the charge management as silicon based MOS system. In this section, the oxide charges associated with Ni/SiO2uGw%z /5/¥ tems have been examined with varying oxide thickness. There are general four types of charges associated with the SiO2-Si system as shown in figure 13. They are fixed oxide charge, mobile oxide charge, oxide trappedcharge and interface trapped charge (Afanas'ev, V, V., 1996 and Schroder, D. K., 2006). The basic origin of all oxide charges and experimental

methods in order to calculate these charge are presented here one by one.

Figure 13. Allocation of different oxide charges associated with SiC MOS system

 

(9)

, dielectric

*J E kT* 

,+0!\*0%(z..%!.z0z)!0(z\* z0\$!z%\*/1(0+.z%\*0!."!az\_z!(!0.%z"%!( z%\*z%\*/1(0+.az<sup>i</sup>

constant; and k, the Boltzmann constant.

226 Physics and Technology of Silicon Carbide Devices

7. Charge management in SiO2 on 4H-SiC

7.1. Origin and basic theory of oxide charges in SiO2

These are positive or negative charges located near the SiO2/4H-SiC (less than 25 Å from the interface) interface due to primarily structural defects in the oxide layer. The origin of fixed \$.#!z !\*/%05z%/z.!(0! z0+z0\$!z+4% 0%+\*z,.+!//\_z+4% 0%+\*z)%!\*0z\* z0!),!.01.!\_z++(¥ ing condition of the furnace and also on polytypes of Silicon carbide. Qfix highly depends on the final oxidation temperature. For higher oxidation temperature, a lower Qfix3%((z !z +¥ served. However, if it is not permissible to oxidize the wafer at high temperatures, it is also possible to reduce the Qfixby annealing the oxidized wafer in a nitrogen or argon ambient after oxidation.

The fixed charge can be determined by comparing the flatband voltage shift of an experimental xz1.2!z3%0\$zz0\$!+.!0%(z1.2!\_z,.+2% ! z5z0\$!z+4% !z0\$%'\*!//z\* z3+.'z"1\*0%+\*z %""!.!\*¥ ces of metal and 4H-SiC. Fixed charge related to the flatband voltage is given by:

$$\mathcal{Q}\_{f\text{fix}} = (\mathcal{q}\_{ms} - V\_{F\mathcal{B}})\mathcal{C}\_{out} \tag{10}$$

Where, £MS is the difference of work function between metal and semiconductor, which must be known in order to determine the value of Qfix.

#### 7.3. Oxide trapped charge (Qoxt)

This oxide charge may be positive or negative due to the hole and electron trapped in the bulk of the oxide. These trapping may results from ionizing radiation, avalanche injection, Fowler-Nordheim tunneling or other similar processes. Unlike the fixed charge, this chare \*z(/+z!z.! 1! z5z\*\*!(%\*#z0.!0)!\*0^z4% !z\$.#!/z\*z!z0.,,! z%\*z0\$!z+4% !z 1.¥ ing device operation, even if not introduced during device fabrication. During the device operation electrons and/or holes can be injected from the substrate or from the gate material. \*!.#!0%z . %0%+\*z (/+z ,.+ 1!/z !(!0.+\*w\$+(!z ,%./z%\*z 0\$!z +4% !z \* z /+)!z +"z 0\$!/!z !(!¥ 0.+\*/z\* u+.z\$+(!/z.!z/1/!-1!\*0(5z0.,,! z%\*z0\$!z+4% !^z\$!z+4% !z0.,,! z\$.#!z%/z1/1(¥ ly not located at the oxide/4H-SiC, but is distributed through the oxide. The distribution of Qoxtmust be known for proper interpretation of C–V curves. Oxide trapped charge can be determined by:

$$\mathcal{Q}\_{\rm out} = -V\_{FB}\mathcal{L}\_{\rm out} \tag{11}$$

Where, the symbols have their usual meaning.

#### 7.4. Mobile oxide charge (Qmob)

The origin of this oxide charge is due to the presence of ionic impurities such as Na+ , Li+ , K+ and possible H+ in the oxide films. These ionic impurities may be resulted from the ambient, which, was used for thermal oxidation. Negative ions and heavy metals ions may also contribute to this charge. Sodium ion is the dominant contaminant. The other ionic impurities like potassium may !z%\*0.+ 1! z 1.%\*#z\$!)%(w)!\$\*%(z,+(%/\$%\*#^z+.z)+%(!z\$.#!z(1(0%+\*z0\$!z)!/¥ urement temperature must be sufficient high so the charge to be mobile. Typically, the devices are heated to 2000 C to 3000 C. A gate bias, to produce an oxide field of around 106 V/cm is applied "+.zz/1""%%!\*0(5z(+\*#z0%)!z%\*z+. !.z0+z .%"0z\$.#!z".+)z%\*0!."!^z\$!z)+%(!z\$.#!z\*z!z !¥ termined from the flatband voltage shift, according to the equation:

$$\mathcal{Q}\_{mab} = -\Delta V\_{FB} \mathcal{C}\_{out} \tag{12}$$

**SiO2** 

q <sup>B</sup>

**EC** 

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

229

**Ei EF** 

**EV** 

**X** 

**SiC** 

**q(x)** 

q S

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

**EC** 

**Vacuum Energy** 

**EV** 

1. The energy difference between the metal work-functionand the semiconductor workfunction is zero. Under this condition, the Fermi levels of the metal and semiconductor are aligned at equilibrium. This is equivalent with no charge flowing when they are put

2. The only charges that can exist in the structure under any biasing conditions are those %\*z0\$!z/!)%+\* 10+.z\* z0\$+/!z3%0\$z0\$!z!-1(z10z3%0\$z+,,+/%0!z/%#\*z+\*z0\$!z)!0(z/1.¥

3. The semiconductor Fermi level is constant from the SiC-bulk toward the interface. It is

4. There are no traps at the metal/SiO2 or SiO2/4H-SiC interface. The SiO2 is free of defects (structural defects, impurities, vacancies, etc.). The only allowed charge in the structure

5. The resistivity of the insulator (oxide) is infinity so that there are no carrier transports

 \*z.!(\_z0\$!z+4% !/z+"z\*5z
z,%0+.z"!01.!/zz\*1)!.z+"z\$.#!/z"+.z!4),(!z"%4! z+4¥ ide charge, mobile charge, oxide trap charge and interface trap level density. There is also a non-zero difference between the gate metal and semiconductor work function. The electric

q M q <sup>B</sup> M

**Metal** 

Figure 14. Flatband energy band diagram of an ideal MOSiC structure

determine by the shallow doping (usually nitrogen N).

exists in the semiconductor and, with opposite sign in the metal.

An ideal MOS diode is defined as follows:

face adjacent to the insulator.

under Bias conduction.

8.2. The real MOSiC capacitor

in contact.

**E** 
