cm<sup>3</sup> · s

*<sup>D</sup>*Si cm<sup>2</sup> · <sup>s</sup>

\$<sup>1</sup> cm<sup>3</sup> · <sup>s</sup>

\$<sup>2</sup> cm<sup>6</sup> · <sup>s</sup>

*<sup>D</sup>*<sup>O</sup> cm<sup>2</sup> · <sup>s</sup>

*C*Si

*C*O

\*Assumed value

that the parameters are related to SiO2 (*i.e.* d ¦

extracted from the reference [17].

and*D*<sup>O</sup>

#### 3.3. Oxidation rate at various oxygen partial pressures

To determine the oxygen pressure dependence of the SiC oxidation process, *ex-situ*z)!/1.!¥

ments from 10-3 to 4 atm have been carried out [23, 27]. However, both of these studies did

not examine the initial oxidation process in detail, partly because *in-situ*z.!(w0%)!z+/!.2¥

tions were not possible. We performed *in-situ* real-time measurements at reduced partial

,.!//1.!/z !03!!\*z C^Dz \* z D^Cz 0)\_z \* z "+1\* z 0\$!z ,.!/!\*!z +"z 0\$!z %\*%0%(z #.+30\$z .0!z !\*¥

hancement to be similar to the case at atmospheric pressure [10]. Recently, we observed the

SiC oxidation process under low oxygen partial pressures down to 0.02 atm to examine the

initial stage of oxidation in more detail [11, 30].

%#^zJdz%\*z.!/,!0%2!z/0#!\_z 0\$!z+4% !z 0\$%'\*!//z !,!\* !\*!z+"z+4% !z#.+30\$z.0!z\*z!z,¥

acteristic lengths for the deceleration of oxide growth rate in each oxidation stage. The first and the second terms represent the rapid and gentle deceleration stage, respectively. -1¥ tion (11) means that, in the thin oxide regime, the oxide growth occurs by two ways and these proceed not in series but in parallel because the growth rate is given by the sum of two terms and is determined mainly by the faster one in each stage. Obviously, the *L <sup>r</sup>* and *L <sup>g</sup>* values correspond to the gradients of the fitted line in the rapid and gentle deceleration stage, respectively. As shown in Fig. 7, the *L <sup>r</sup>*z2(1!z !.!/!/z3%0\$z !.!/%\*#z,.0%(z,.!/¥

sure, which corresponds to the more remarkable rapid deceleration. In contrast, the *L <sup>g</sup>* 2(¥ ue is almost constant regardless of the partial pressure. This suggests that the oxidation process is different between the rapid and gentle deceleration stage. It is noted that the thickness at which the deceleration rate changes from rapid one to gentle one (termed '*Xc*') is almost constant around 7 nm regardless of oxygen partial pressure and surface polarity. \*z0\$!z/!z+"z%z+4% 0%+\*\_zz.,% z !!(!.0%+\*z/0#!z\$/z(/+z!!\*z+/!.2! z&1/0z"0!.z+4% ¥ tion starts, and the thickness corresponding to *Xc* is also almost independent of the oxygen

partial pressure, though the growth rates at *Xc* depend on the oxygen partial pressure [13,

31]. Therefore, it can be stated that *Xc* is determined only by the thickness of the oxide layer

As mentioned above, the existence of a rapid deceleration stage in the oxide growth rate just after oxidation starts (*X*<10 nm) has been observed also for Si oxidation [13, 31]. However, in the investigations on Si oxidation mechanisms, the cause for the rapid deceleration has not been clarified yet, so far. For SiC oxidation, Yamamoto *et al.*z\$2!z0.%! z0+z.!,.+ 1!z0\$!z+¥ served data using the Massoud's empirical equation [13]. Here, we will discuss the reasons why two deceleration stages exist in the thickness dependence of oxide growth rate, based

*dt* <sup>=</sup>*Rr*exp( *<sup>X</sup>* / *<sup>L</sup> <sup>r</sup>*) <sup>+</sup> *Rg*exp( *<sup>X</sup>* / *<sup>L</sup> <sup>g</sup>*) (11)

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195

*Rg*) are pre-exponential constants, and *L <sup>r</sup>* and *L <sup>g</sup>* (*L <sup>r</sup>* < *L <sup>g</sup>*dz.!z\$.¥

proximated by a sum of the two exponential functions [11], as,

*dX*

for both the Si and SiC oxidation cases.

on the Si and C emission model [19].

where, *Rr*and *Rg* (*Rr*

Figure 7. Oxide growth rates as a function of oxide thickness at various oxygen partial pressures on (0001¯ ) C-face (a) and (0001) Si-face (b). The dashed lines are fitted to the experimental data using exponential functions.

Oxide thickness dependence of the oxide growth rates on 4H-SiC C-face (a) and Si-face (b) are shown in Fig. 7. As seen in the figure, we could obtain growth rate data in the extremethin oxide region down to a few nm more precisely by reducing the oxygen partial pressure less than 0.1 atm, compared with the case of measurements above 0.1 atm. For both faces, the oxide thickness dependence of growth rate less than 0.1 atm are basically similar to those of above 0.1 atm even if the partial pressure is lowered to 0.02 atm. Namely, just after 0\$!z+4% 0%+\*z /0.0/\_z 0\$!z+4% !z#.+30\$z .0!z .,% (5z !.!/!/z \* z 0z .+1\* z Jz\*)z%\*z 0\$%'¥ ness, the deceleration rate changes to gentle one (hereafter each oxidation stage is denoted as rapid and gentle deceleration stage, respectively). Because the growth rates at each of the deceleration stage well ride on a straight line in a semi-log plot (shown by dashed lines in %#^zJdz%\*z.!/,!0%2!z/0#!\_z 0\$!z+4% !z 0\$%'\*!//z !,!\* !\*!z+"z+4% !z#.+30\$z.0!z\*z!z,¥ proximated by a sum of the two exponential functions [11], as,

$$\frac{dX}{dt} = R\_r \exp(-X \left| L\_{\,\,r} \right) + R\_\chi \exp(-X \left| L\_{\,\,g} \right) \tag{11}$$

where, *Rr*and *Rg* (*Rr Rg*) are pre-exponential constants, and *L <sup>r</sup>* and *L <sup>g</sup>* (*L <sup>r</sup>* < *L <sup>g</sup>*dz.!z\$.¥ acteristic lengths for the deceleration of oxide growth rate in each oxidation stage. The first and the second terms represent the rapid and gentle deceleration stage, respectively. -1¥ tion (11) means that, in the thin oxide regime, the oxide growth occurs by two ways and these proceed not in series but in parallel because the growth rate is given by the sum of two terms and is determined mainly by the faster one in each stage. Obviously, the *L <sup>r</sup>* and *L <sup>g</sup>* values correspond to the gradients of the fitted line in the rapid and gentle deceleration stage, respectively. As shown in Fig. 7, the *L <sup>r</sup>*z2(1!z !.!/!/z3%0\$z !.!/%\*#z,.0%(z,.!/¥ sure, which corresponds to the more remarkable rapid deceleration. In contrast, the *L <sup>g</sup>* 2(¥ ue is almost constant regardless of the partial pressure. This suggests that the oxidation process is different between the rapid and gentle deceleration stage. It is noted that the thickness at which the deceleration rate changes from rapid one to gentle one (termed '*Xc*') is almost constant around 7 nm regardless of oxygen partial pressure and surface polarity. \*z0\$!z/!z+"z%z+4% 0%+\*\_zz.,% z !!(!.0%+\*z/0#!z\$/z(/+z!!\*z+/!.2! z&1/0z"0!.z+4% ¥ tion starts, and the thickness corresponding to *Xc* is also almost independent of the oxygen partial pressure, though the growth rates at *Xc* depend on the oxygen partial pressure [13, 31]. Therefore, it can be stated that *Xc* is determined only by the thickness of the oxide layer for both the Si and SiC oxidation cases.

As mentioned above, the existence of a rapid deceleration stage in the oxide growth rate just after oxidation starts (*X*<10 nm) has been observed also for Si oxidation [13, 31]. However, in the investigations on Si oxidation mechanisms, the cause for the rapid deceleration has not been clarified yet, so far. For SiC oxidation, Yamamoto *et al.*z\$2!z0.%! z0+z.!,.+ 1!z0\$!z+¥ served data using the Massoud's empirical equation [13]. Here, we will discuss the reasons why two deceleration stages exist in the thickness dependence of oxide growth rate, based on the Si and C emission model [19].

Figure 7. Oxide growth rates as a function of oxide thickness at various oxygen partial pressures on (0001¯ ) C-face (a)

Oxide thickness dependence of the oxide growth rates on 4H-SiC C-face (a) and Si-face (b) are shown in Fig. 7. As seen in the figure, we could obtain growth rate data in the extremethin oxide region down to a few nm more precisely by reducing the oxygen partial pressure less than 0.1 atm, compared with the case of measurements above 0.1 atm. For both faces, the oxide thickness dependence of growth rate less than 0.1 atm are basically similar to those of above 0.1 atm even if the partial pressure is lowered to 0.02 atm. Namely, just after 0\$!z+4% 0%+\*z /0.0/\_z 0\$!z+4% !z#.+30\$z .0!z .,% (5z !.!/!/z \* z 0z .+1\* z Jz\*)z%\*z 0\$%'¥ ness, the deceleration rate changes to gentle one (hereafter each oxidation stage is denoted as rapid and gentle deceleration stage, respectively). Because the growth rates at each of the deceleration stage well ride on a straight line in a semi-log plot (shown by dashed lines in

and (0001) Si-face (b). The dashed lines are fitted to the experimental data using exponential functions.

194 Physics and Technology of Silicon Carbide Devices

pressed as *C*<sup>O</sup>

second term in eq. (11).

be expressed that*Rr* =*k*<sup>0</sup>

starts. As the value of *k*<sup>0</sup>

¦ *C*O

¦

pressure both for Si and SiC oxidation.

<sup>I</sup> / *<sup>N</sup>*0, where *k*<sup>0</sup>

through oxide even if the oxygen pressure is as low as 0.02 atm1

<sup>I</sup> *pC*<sup>O</sup>

<sup>0</sup> by Henry's low. Therefore, *R*, the growth rate when the oxide thickness

is the interfacial reaction rate when oxidation

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197

. Therefore, the oxide

is also unlikely to depend on oxygen partial pressure, *Rr*should be

*X* approaches 0, should be proportional to *p*\_z3\$%\$z%/z%\*z#++ z #.!!)!\*0z3%0\$z 0\$!z!4,!.%¥ mental results in the gentle deceleration stage, *i.e. Rg*. Based on the Si emission model for Si oxidation (the Si and C emission model for SiC), the interface reaction rate *k* decreases with proceeding of oxidation due to the accumulation(s) of Si interstitials (Si and C interstitials). As the number of the accumulated atoms should increase with the proceeding of oxidation and thus is proportional to the quantity of oxidized molecules, *i.e.*, thickness of the oxide *X*, the variation in *k* may be approximately given as an exponential function of *X* in the form of *C*exp( *X* / *L* ), where *C* and *L*z.!z0\$!z,.!w!4,+\*!\*0%(z0!.)z\* z\$.0!.%/0%z(!\*#0\$\_z.!/,!¥ tively, related to the accumulation(s) of Si (Si and C) interstitials at the interface. From these considerations as well as the fact that *Rg* is proportional to *p*, the gentle deceleration of oxide #.+30\$z.0!z\*z!z00.%10! z0+z0\$!z1)1(0%+\*c/dz+"z%zc%z\* zdz%\*0!./0%0%(/z\*!.z0\$!z%\*¥ terface, and given approximately as*dX* / *dt Rg*exp( *X* / *L <sup>g</sup>*), which is coincident with the

If the initial growth rate *Rr* in the rapid deceleration stage is also followed by eq. (12), it can

also proportional to oxygen pressure. While, as seen in Fig. 8, *Rr*is not proportional to *p*, though it decreases with decreasing *p* in the low *p* region. Also in the case of Si oxidation, the experimental data show almost no dependency of *Rr* with respect to *p* [13]. Here, we consider the reason why *Rr* is not proportional to but almost independent of oxygen partial

It has been considered that oxide growth occurs only or mainly at the Si-oxide (SiC-oxide) interface, so far. However, according to the interfacial Si emission model [14, 17fz"+.z%z+4%¥ dation and the Si and C emission model [19] for SiC oxidation, Si atoms (Si and C atoms) emit into the oxide layer, and some of which meet with oxidant inside the oxide to form SiO2. When the thickness of the oxide is very thin, a part of the emitted Si atoms can go 0\$.+1#\$z0\$!z+4% !z(5!.z\* z.!\$z0\$!z+4% !z/1."!\_z\* z0\$!\*z.!z%\*/0\*0(5z+4% %6! \_z.!/1(0¥ ing in the formation of a SiO2 layer on the oxide surface. Therefore, there are two oxide growth processes other than the interfacial oxide growth, *i.e.*, the oxide formation due to the oxidations of Si interstitials on the oxide surface and inside the oxide, and the total growth rate is given by the sum of these three oxidation processes, as revealed in eq. (6). In the case of oxidation inside the oxide, the possibility that an emitted Si interstitial encounter oxygen inside the oxide should be proportional to the oxygen concentration in the oxide. Therefore, this oxidation process should be proportional to *p* like*Rg*, and thus, can be excluded as a candidate of the origin of*Rr*. In contrast, in the case of oxidation on the oxide surface, the amount of oxygen is thought to be enough to oxidize all the Si atoms emitted and appearing on the surface, because the number of oxygen molecules impinging onto the surface from the gaseous atmosphere is several orders larger than that of emitted Si atoms transmitted

¦

Figure 8. Oxygen partial pressure dependence of the initial growth rate, *Rr*MF>ADD=<KQE:GDK 9F<L@=?=FLD=<=;=D=J9s tion growth rate, *Rg*(filled symbols), on C-face (circles) and Si-face (triangles).

We fitted the experimental data at each partial pressure with two straight lines, as shown by the dashed lines in Fig. 7, and derived the initial growth rate of the two deceleration stages, *Rr*and*Rg*, by extrapolating the straight line to *X*=0 in the rapid and the gentle deceleration stage, respectively. It is noticed that the meanings of *Rr* and *Rg* are the same as those in eq. (11). Figure 8 shows the oxygen partial pressure dependence of *Rr* and *Rg*\_z !\*+0! z5z1\*"%(¥ (! z\* z"%((! z/5)+(/\_z.!/,!0%2!(5\_z+\*z0\$!zwzc%.(!/dz\* z%w"!/zc0.%\*#(!/d^z%\*!z0\$!z+4¥ ide growth in thin thickness region was too fast to follow spectroscopic ellipsometry measurements in the case of 1 atm on C-face, we could not obtain the values of oxide growth rate in the rapid deceleration stage accurately, and thus the value of *Rr* for C face at 1 atm was not shown in this figure. The broken line in Fig. 8 shows the line proportional to oxygen partial pressure and fitted to the *Rg* data for C-face. For the both polar faces, the data points of *Rg* ride almost on the line, suggesting that *Rg* is proportional to partial pressure, though, for Si-face, *Rg*becomes slightly smaller as seen from the linear relation approaching 1 atm. It should be noted that the rates are almost equal to each other between C- and Sifaces at low pressure region, which is strangely different from the fact that the oxide growth rates for C-face are about eight times larger than those for Si-face at atmospheric oxygen pressure. If the oxide grows chiefly at the interface, the oxide growth rate *dX/dt* is rewritten by using eq. (6) as follows:

$$N\_0 \frac{dX}{dt} \approx kC\_0^1 \tag{12}$$

The value of *k* is unlikely to depend on oxygen partial pressure because it corresponds to the rate that one SiC molecule is changed to one SiO2 molecule, which should not depend on *p*. In the thin oxide regime discussed here, the interface oxygen concentration *C*<sup>O</sup> <sup>I</sup> \*z !z !4¥ pressed as *C*<sup>O</sup> <sup>I</sup> *pC*<sup>O</sup> <sup>0</sup> by Henry's low. Therefore, *R*, the growth rate when the oxide thickness *X* approaches 0, should be proportional to *p*\_z3\$%\$z%/z%\*z#++ z #.!!)!\*0z3%0\$z 0\$!z!4,!.%¥ mental results in the gentle deceleration stage, *i.e. Rg*. Based on the Si emission model for Si oxidation (the Si and C emission model for SiC), the interface reaction rate *k* decreases with proceeding of oxidation due to the accumulation(s) of Si interstitials (Si and C interstitials). As the number of the accumulated atoms should increase with the proceeding of oxidation and thus is proportional to the quantity of oxidized molecules, *i.e.*, thickness of the oxide *X*, the variation in *k* may be approximately given as an exponential function of *X* in the form of *C*exp( *X* / *L* ), where *C* and *L*z.!z0\$!z,.!w!4,+\*!\*0%(z0!.)z\* z\$.0!.%/0%z(!\*#0\$\_z.!/,!¥ tively, related to the accumulation(s) of Si (Si and C) interstitials at the interface. From these considerations as well as the fact that *Rg* is proportional to *p*, the gentle deceleration of oxide #.+30\$z.0!z\*z!z00.%10! z0+z0\$!z1)1(0%+\*c/dz+"z%zc%z\* zdz%\*0!./0%0%(/z\*!.z0\$!z%\*¥ terface, and given approximately as*dX* / *dt Rg*exp( *X* / *L <sup>g</sup>*), which is coincident with the second term in eq. (11).

If the initial growth rate *Rr* in the rapid deceleration stage is also followed by eq. (12), it can be expressed that*Rr* =*k*<sup>0</sup> ¦ *C*O <sup>I</sup> / *<sup>N</sup>*0, where *k*<sup>0</sup> ¦ is the interfacial reaction rate when oxidation starts. As the value of *k*<sup>0</sup> ¦ is also unlikely to depend on oxygen partial pressure, *Rr*should be also proportional to oxygen pressure. While, as seen in Fig. 8, *Rr*is not proportional to *p*, though it decreases with decreasing *p* in the low *p* region. Also in the case of Si oxidation, the experimental data show almost no dependency of *Rr* with respect to *p* [13]. Here, we consider the reason why *Rr* is not proportional to but almost independent of oxygen partial pressure both for Si and SiC oxidation.

Figure 8. Oxygen partial pressure dependence of the initial growth rate, *Rr*MF>ADD=<KQE:GDK

*N*0 *dX dt kC*<sup>O</sup>

In the thin oxide regime discussed here, the interface oxygen concentration *C*<sup>O</sup>

The value of *k* is unlikely to depend on oxygen partial pressure because it corresponds to the rate that one SiC molecule is changed to one SiO2 molecule, which should not depend on *p*.

We fitted the experimental data at each partial pressure with two straight lines, as shown by the dashed lines in Fig. 7, and derived the initial growth rate of the two deceleration stages, *Rr*and*Rg*, by extrapolating the straight line to *X*=0 in the rapid and the gentle deceleration stage, respectively. It is noticed that the meanings of *Rr* and *Rg* are the same as those in eq. (11). Figure 8 shows the oxygen partial pressure dependence of *Rr* and *Rg*\_z !\*+0! z5z1\*"%(¥ (! z\* z"%((! z/5)+(/\_z.!/,!0%2!(5\_z+\*z0\$!zwzc%.(!/dz\* z%w"!/zc0.%\*#(!/d^z%\*!z0\$!z+4¥ ide growth in thin thickness region was too fast to follow spectroscopic ellipsometry measurements in the case of 1 atm on C-face, we could not obtain the values of oxide growth rate in the rapid deceleration stage accurately, and thus the value of *Rr* for C face at 1 atm was not shown in this figure. The broken line in Fig. 8 shows the line proportional to oxygen partial pressure and fitted to the *Rg* data for C-face. For the both polar faces, the data points of *Rg* ride almost on the line, suggesting that *Rg* is proportional to partial pressure, though, for Si-face, *Rg*becomes slightly smaller as seen from the linear relation approaching 1 atm. It should be noted that the rates are almost equal to each other between C- and Sifaces at low pressure region, which is strangely different from the fact that the oxide growth rates for C-face are about eight times larger than those for Si-face at atmospheric oxygen pressure. If the oxide grows chiefly at the interface, the oxide growth rate *dX/dt* is rewritten

tion growth rate, *Rg*(filled symbols), on C-face (circles) and Si-face (triangles).

196 Physics and Technology of Silicon Carbide Devices

by using eq. (6) as follows:

9F<L@=?=FLD=<=;=D=J9s

<sup>I</sup> (12)

<sup>I</sup> \*z !z !4¥

It has been considered that oxide growth occurs only or mainly at the Si-oxide (SiC-oxide) interface, so far. However, according to the interfacial Si emission model [14, 17fz"+.z%z+4%¥ dation and the Si and C emission model [19] for SiC oxidation, Si atoms (Si and C atoms) emit into the oxide layer, and some of which meet with oxidant inside the oxide to form SiO2. When the thickness of the oxide is very thin, a part of the emitted Si atoms can go 0\$.+1#\$z0\$!z+4% !z(5!.z\* z.!\$z0\$!z+4% !z/1."!\_z\* z0\$!\*z.!z%\*/0\*0(5z+4% %6! \_z.!/1(0¥ ing in the formation of a SiO2 layer on the oxide surface. Therefore, there are two oxide growth processes other than the interfacial oxide growth, *i.e.*, the oxide formation due to the oxidations of Si interstitials on the oxide surface and inside the oxide, and the total growth rate is given by the sum of these three oxidation processes, as revealed in eq. (6). In the case of oxidation inside the oxide, the possibility that an emitted Si interstitial encounter oxygen inside the oxide should be proportional to the oxygen concentration in the oxide. Therefore, this oxidation process should be proportional to *p* like*Rg*, and thus, can be excluded as a candidate of the origin of*Rr*. In contrast, in the case of oxidation on the oxide surface, the amount of oxygen is thought to be enough to oxidize all the Si atoms emitted and appearing on the surface, because the number of oxygen molecules impinging onto the surface from the gaseous atmosphere is several orders larger than that of emitted Si atoms transmitted through oxide even if the oxygen pressure is as low as 0.02 atm1 . Therefore, the oxide #.+30\$z.0!z"+.z0\$!z+4% 0%+\*z+\*z0\$!z+4% !z/1."!z/\$+1( z!z%\* !,!\* !\*0z+"z0\$!z+45#!\*z,.¥ tial pressure, which is in good agreement with the behavior of*Rr*. Besides, the possibility 0\$0z %z %\*0!./0%0%(/z #+z 0\$.+1#\$z 0\$!z +4% !z \* z .!\$z 0\$!z +4% !z /1."!z %/z +\*/% !.! z 0+z !¥ crease rapidly with increasing oxide thickness, and can be given as a form ofexp( *X* / *L <sup>r</sup>*), where *L <sup>r</sup>* (< *L <sup>g</sup>*dz%/z0\$!z!/,!z !,0\$z+"z%z0+)/z".+)z0\$!z+4% !z(5!.^z.+)z0\$!/!z+\*/% !.¥ tions, the rapid deceleration stage of oxide growth rate observed just after oxidation starts is thought to be due to oxidation of Si interstitials on the oxide surface. The value of *Xc* +¥ tained from the experiments, around 7 nm, indicates that the escape depth of Si from the oxide is estimated to be several nm at 1100°C.

Figure 9 compares the observed oxide growth rates at various oxygen partial pressures on

sion model (solid lines). It is noted that all the parameters used in the calculations other than the solubility limit of oxygen in SiO2 <sup>2</sup>z.!z0\$!z/)!z.!#. (!//z+"z,.0%(z,.!//1.!/^z\$!z"%#¥ ures indicate that the calculated curves successfully reproduce the observed growth rates though restricted in the gentle deceleration region and that the calculated curves show the rapid reduction more remarkable as the higher partial pressure, which is opposite tendency 0+z0\$0z+"z0\$!z+/!.2! z 0^z(/+z%\*z0\$!z/!z+"z%z+4% 0%+\*\_z0\$!z%\*0!."%(z%z!)%//%+\*z)+ ¥ !(zeDG\_zDJfz\*\*+0z.!,.+ 1!z0\$!z#.+30\$z.0!z%\*z0\$!z0\$%\*z+4% !z.!#%+\*z0z/1w0)+/,\$!.%z,.!/¥ sure, as pointed out by Farjas and Roura [31]. We consider that the accurate description for 0\$!z/1."!z+4% !z#.+30\$z)5z %//+(2!z 0\$!z %/#.!!)!\*0z!03!!\*z 0\$!z(1(0! z\* z+¥ /!.2! z+4% !z#.+30\$z.0!/z%\*z0\$!z/!/z+"z(+3z,.0%(z,.!//1.!/^z+z2!.%"5z0\$%/z% !\_z3!z/!,¥ rately calculated the oxide growth rate on the surface, inside the SiO2 layer, and at the SiC-SiO2 interface (denoted as*F*sur, *F*ox, and*F*int, respectively) using the Si and C emission model.

Figure 10. Simulated oxide growth rates on the surface, inside the SiO2 layer, and at the SiC-SiO2 interface (*F*sur, *F*ox,

Figure 10 shows the calculation results of*F*sur, *F*ox, and *F*int at 0.1 and 0.01 atm on Si-face. The figure indicates that the *F*sur curve for 0.01 atm is much higher than that for 0.1 atm by a factor of about 5 orders, though the *F*sur is still lower than*F*int. This result confirms that the low pressure oxidation enhances the oxide growth on the surface, as discussed above, which is probably due to the reduction in the oxygen concentration at the interface and inside the oxide. Conversely, the *F*ox and *F*int curves for 0.01 atm are lower than those for 0.1 atm by factors of 2 orders and 1 order, respectively. Since the *F*int is higher than *F*ox in general, it is

2 The corresponding solubility limit of oxygen is derived by multiplying partial pressure by that for 1 atm, *i.e.*, *C*<sup>O</sup>

<sup>0</sup> <sup>=</sup> *<sup>p</sup>* <sup>×</sup>*C*<sup>O</sup> 0

and*F*int, respectively) at 0.1 and 0.01 atm on Si-face.

(1 atm).

¯dzw"!zcdz\* zcCCCDdz%w"!zcdz3%0\$z0\$!z#.+30\$z.0!/z#%2!\*z5z0\$!z%z\* zz!)%/¥

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199

SiC (0001

As has been mentioned above, in the rapid deceleration stage, the growth rate is determined with the oxidation rate of the emitted Si interstitials on the oxide surface (termed 'surface oxide growth'), while in the gentle deceleration stage, it is determined by the oxidation rate at the SiC-oxide interface and that of the emitted Si interstitials inside the SiO2 layer (termed 'interfacial oxide growth' and 'internal oxide growth', respectively). The surface oxide growth rate depends little on partial pressure and, in contrast, the interfacial and internal oxide growth rates are proportional to partial pressure. It is therefore predicted from these pressure dependence that the rapid deceleration becomes more remarkable (*i.e.* the smaller *L <sup>r</sup>*) at lower partial pressures, which is confirmed in Fig. 7. In addition, the reason why the #.+30\$z.0!/z%\*z 0\$!z.,% z !!(!.0%+\*z/0#!z0z(+3z,.!//1.!/z.!z\*+0z/+z)1\$z %""!.!\*0z!¥ tween C- and Si-faces is that the surface oxide growth is dominant to the oxide growth in 0\$%/z/0#!\_z/+z0\$0z0\$!z+4% 0%+\*z+\*z0\$!z+4% !z/1."!z)5z,.+!! z%\* !,!\* !\*0(5z+"z0\$!z/1.¥ face polarity.

Figure 9. Oxide growth rates as a function of oxide thickness at various oxygen partial pressures on (0001¯ ) C-face (a) and (0001) Si-face (b). The solid lines denote growth rates given by the Si and C emission model [19].

<sup>1</sup> The oxygen flux impinging from a gaseous atmosphere of pressure *p* to the solid surface is 3x1022*p*[m-2s-1]. Since the areal density of Si atoms on SiO2 is about 8x1018[m-2], the flux in the case that *p*=0.02 atm corresponds to 75 [monolayer/s], which is the oxygen flux necessary for the oxide growth rate of 105 nm/h.

Figure 9 compares the observed oxide growth rates at various oxygen partial pressures on SiC (0001 ¯dzw"!zcdz\* zcCCCDdz%w"!zcdz3%0\$z0\$!z#.+30\$z.0!/z#%2!\*z5z0\$!z%z\* zz!)%/¥ sion model (solid lines). It is noted that all the parameters used in the calculations other than the solubility limit of oxygen in SiO2 <sup>2</sup>z.!z0\$!z/)!z.!#. (!//z+"z,.0%(z,.!//1.!/^z\$!z"%#¥ ures indicate that the calculated curves successfully reproduce the observed growth rates though restricted in the gentle deceleration region and that the calculated curves show the rapid reduction more remarkable as the higher partial pressure, which is opposite tendency 0+z0\$0z+"z0\$!z+/!.2! z 0^z(/+z%\*z0\$!z/!z+"z%z+4% 0%+\*\_z0\$!z%\*0!."%(z%z!)%//%+\*z)+ ¥ !(zeDG\_zDJfz\*\*+0z.!,.+ 1!z0\$!z#.+30\$z.0!z%\*z0\$!z0\$%\*z+4% !z.!#%+\*z0z/1w0)+/,\$!.%z,.!/¥ sure, as pointed out by Farjas and Roura [31]. We consider that the accurate description for 0\$!z/1."!z+4% !z#.+30\$z)5z %//+(2!z 0\$!z %/#.!!)!\*0z!03!!\*z 0\$!z(1(0! z\* z+¥ /!.2! z+4% !z#.+30\$z.0!/z%\*z0\$!z/!/z+"z(+3z,.0%(z,.!//1.!/^z+z2!.%"5z0\$%/z% !\_z3!z/!,¥ rately calculated the oxide growth rate on the surface, inside the SiO2 layer, and at the SiC-SiO2 interface (denoted as*F*sur, *F*ox, and*F*int, respectively) using the Si and C emission model.

#.+30\$z.0!z"+.z0\$!z+4% 0%+\*z+\*z0\$!z+4% !z/1."!z/\$+1( z!z%\* !,!\* !\*0z+"z0\$!z+45#!\*z,.¥ tial pressure, which is in good agreement with the behavior of*Rr*. Besides, the possibility 0\$0z %z %\*0!./0%0%(/z #+z 0\$.+1#\$z 0\$!z +4% !z \* z .!\$z 0\$!z +4% !z /1."!z %/z +\*/% !.! z 0+z !¥ crease rapidly with increasing oxide thickness, and can be given as a form ofexp( *X* / *L <sup>r</sup>*), where *L <sup>r</sup>* (< *L <sup>g</sup>*dz%/z0\$!z!/,!z !,0\$z+"z%z0+)/z".+)z0\$!z+4% !z(5!.^z.+)z0\$!/!z+\*/% !.¥ tions, the rapid deceleration stage of oxide growth rate observed just after oxidation starts is thought to be due to oxidation of Si interstitials on the oxide surface. The value of *Xc* +¥ tained from the experiments, around 7 nm, indicates that the escape depth of Si from the

As has been mentioned above, in the rapid deceleration stage, the growth rate is determined with the oxidation rate of the emitted Si interstitials on the oxide surface (termed 'surface oxide growth'), while in the gentle deceleration stage, it is determined by the oxidation rate at the SiC-oxide interface and that of the emitted Si interstitials inside the SiO2 layer (termed 'interfacial oxide growth' and 'internal oxide growth', respectively). The surface oxide growth rate depends little on partial pressure and, in contrast, the interfacial and internal oxide growth rates are proportional to partial pressure. It is therefore predicted from these pressure dependence that the rapid deceleration becomes more remarkable (*i.e.* the smaller *L <sup>r</sup>*) at lower partial pressures, which is confirmed in Fig. 7. In addition, the reason why the #.+30\$z.0!/z%\*z 0\$!z.,% z !!(!.0%+\*z/0#!z0z(+3z,.!//1.!/z.!z\*+0z/+z)1\$z %""!.!\*0z!¥ tween C- and Si-faces is that the surface oxide growth is dominant to the oxide growth in 0\$%/z/0#!\_z/+z0\$0z0\$!z+4% 0%+\*z+\*z0\$!z+4% !z/1."!z)5z,.+!! z%\* !,!\* !\*0(5z+"z0\$!z/1.¥

Figure 9. Oxide growth rates as a function of oxide thickness at various oxygen partial pressures on (0001¯ ) C-face (a)

1 The oxygen flux impinging from a gaseous atmosphere of pressure *p* to the solid surface is 3x1022*p*[m-2s-1]. Since the areal density of Si atoms on SiO2 is about 8x1018[m-2], the flux in the case that *p*=0.02 atm corresponds to 75

and (0001) Si-face (b). The solid lines denote growth rates given by the Si and C emission model [19].

[monolayer/s], which is the oxygen flux necessary for the oxide growth rate of 105 nm/h.

oxide is estimated to be several nm at 1100°C.

198 Physics and Technology of Silicon Carbide Devices

face polarity.

Figure 10. Simulated oxide growth rates on the surface, inside the SiO2 layer, and at the SiC-SiO2 interface (*F*sur, *F*ox, and*F*int, respectively) at 0.1 and 0.01 atm on Si-face.

Figure 10 shows the calculation results of*F*sur, *F*ox, and *F*int at 0.1 and 0.01 atm on Si-face. The figure indicates that the *F*sur curve for 0.01 atm is much higher than that for 0.1 atm by a factor of about 5 orders, though the *F*sur is still lower than*F*int. This result confirms that the low pressure oxidation enhances the oxide growth on the surface, as discussed above, which is probably due to the reduction in the oxygen concentration at the interface and inside the oxide. Conversely, the *F*ox and *F*int curves for 0.01 atm are lower than those for 0.1 atm by factors of 2 orders and 1 order, respectively. Since the *F*int is higher than *F*ox in general, it is

<sup>2</sup> The corresponding solubility limit of oxygen is derived by multiplying partial pressure by that for 1 atm, *i.e.*, *C*<sup>O</sup> <sup>0</sup> <sup>=</sup> *<sup>p</sup>* <sup>×</sup>*C*<sup>O</sup> 0 (1 atm).

found that the growth rate in the gentle deceleration stage to be proportional to pressure is due to the proportional increase in*F*int. Moreover, a careful look confirms the deceleration of *F*int in the very-thin oxide region (*ca.*< 5 nm) to be enhanced as elevating the pressure, which has been seen in the calculated curves in Fig. 9. Therefore, it is necessary for the Si and C !)%//%+\*z)+ !(\_z/z3!((z/z"+.z0\$!z%\*0!."%(z%z!)%//%+\*z)+ !(z0\$0z\*z,,.+,.%0!z !/.%,¥ 0%+\*z"+.z0\$!z+4% !z#.+30\$z+\*z0\$!z/1."!z%/z%\*0.+ 1! z0+z0\$!z)+ !(z/+z/z0+z!\*\$\*!z0\$!z/1.¥ face growth rate in the initial oxidation region up to over the interfacial growth rate.

Hashimoto *et al.* \$2!z"+1\* z0\$0z0\$!.!z%/zz/0.+\*#z+..!/,+\* !\*!z!03!!\*z0\$!z.!".0%2!z%\*¥ !4z+"z0\$!z%\*0!."!z(5!.z\* z%\*0!."!z/00!z !\*/%05z+0%\*! z".+)z0\$!z
z,%0+./z".%¥ cated on the same sample for ellipsometry measurements [32]. Hence, we believed that the 1)1(0%+\*/z+"z%z\* zz%\*0!./0%0%(/z\*!.u0z0\$!z%\*0!."!z.!z(+/!(5z.!(0! z0+z0\$!z"+.)¥ tion of interface states. According to the report from Afanasev *et al.* [36f\_zz#.,\$%0!w(%'!z.¥ bon layer forms near/at the interface, which causes interface states over the whole range of forbidden energy band and, in addition, an intrinsic SiO2 defect (NIT: near interface trap), presumably originating from oxygen deficiency, exists regardless of Si or SiC-polytypes and gives rise to the interface states in the vicinity of the 4H-SiC conduction band edge, which cause a channel degradation in MOSFETs. Since the oxygen deficiency can be regarded as a Si interstitial in SiO2, Si (Si and C) emission into SiO2 layer may be the origin for this SiO2 defect. According to the *ab-initio* studies performed by Knaup *et al.* [37], the origin of NITs is Si interstitials or C dimers originating from C interstitials inside SiO2. They also reported that C dimers in the SiC-side give rise to interface states at the energy range between mid gap and valence band edge. On the other hand, according to the *ab-initio* /01 %!/z ".+)z!¥ vynck *et al.* [38], C-C pair in SiC originating from C interstitials and a Si-C-O structure give rise to a broad peak of interface state density, and a Si2-C-O structure give rise to a sharp peak near the conduction band edge, which is compatible with NIT. Cochrane *et al.* have ,!."+.)! z\*z(!0.%((5z !0!0! z)#\*!0%z.!/+\*\*!zc
dz0!\$\*%-1!/z\* z"+1\* z%z2¥ cancies (or C dangling bonds) in the SiC-side near the interface [40], which is presumably 00.%10! z 0+z 0\$!z%z!)%//%+\*z 1.%\*#z+4% 0%+\*^z!.5z.!!\*0(5\_z\$!\*z\* z\*0!(% !/z\$2!z.!¥ ported that they identified the origin of interface states that degrade the SiC MOS channel mobility as 'C di-interstitials', which are formed by a combination of the two C interstitials injected into the SiC [41]. Accordingly, we tried to simulate the formation of the interface layer based on the diffusion theory and, as a result, the calculations reproduced the interface layer with the thickness of around 1 nm [39], which agrees with the values often reported [18, 34, 39f^z\*535\_z3!z3+1( z(%'!z0+z!),\$/%6!z\$!.!z0\$0z"1.0\$!.z1\* !./0\* %\*#z+"z0\$!z%\*¥ terfacial Si and C emission phenomenon during oxidation of SiC might be the key to realize

Thermal Oxidation Mechanism of Silicon Carbide

http://dx.doi.org/10.5772/50748

201

Finally, we would like to introduce a recent innovation using oxidation about elimination of point defects. Hiyoshi and Kimoto invented an epoch-making idea, in which as-grown deep-level states such as a Z1/2 center can be eliminated by oxidation of SiC substrates [43]. Taking into account the report from Storasta *et al.* [44] in which the origin of Z1/2 is perhaps C vacancy, Hiyoshi and Kimoto considered the mechanism of deep-level state elimination by oxidation as follows: The excess silicon and carbon interstitials may be generated at the oxidizing interface, and a part of the emitted silicon and carbon interstitials may diffuse into the epilayer, and consequently, recombination of the interstitials with vacancies present in the epilayers may take place [45]. Therefore, the Si and C emission surely takes place during +4% 0%+\*z\* z0\$!z/1.2!5z+"z%z\* zz%\*0!./0%0%(z)+2!)!\*0z%/z2!.5z%),+.0\*0z0+z10%(%6!z0\$!.¥ mal oxidation as a device fabrication process, *e.g.* Ref. [46f\_z!2!\*z%\*z0\$!z !2!(+,)!\*0/z%\*z%¥ polar devices of over 10 kV class. Very recently, we found that stacking-faults are formed or extended by thermal oxidation, which is perhaps induced by the interfacial strain due to

the intrinsic performances of SiC MOSFETs.

thermal oxidation [47].

### 3.4. Discussion on Si and C emission phenomenon and its relation to the interface layer

+z".\_z3!z/,!%"5z0\$!z+4% 0%+\*z)!\$\*%/)z+"z%z".+)z0\$!z2%!3,+%\*0z+"z0\$!z%z\* zz!)%/¥ /%+\*z,\$!\*+)!\*+\*^z \*z 0\$%/z /!0%+\*\_z3!z3%((z %/1//z 0\$!z /0.101.!z \* z 0\$!z "+.)0%+\*z)!\$¥ nism of the interface layer between SiC-SiO2, which has been considered as the crucial issue for the practical use of SiC-MOSFET, in the light of interfacial Si and C emission.

 \*z,.!2%+1/z3+.'\_z3!z\$2!z"+1\* z0\$0zeFE\_zFFfz0\$!z,\$+0+\*z!\*!.#5z !,!\* !\*!z+"z0\$!z+,0%¥ cal constants *n* and *k* of the interface layer derived from the complex dielectric constants between 2 and 6 eV, covering the direct interband transition energy *E*0 of 4H-SiC of 5.65 eV, is similar to that of bulk 4H-SiC, though the absolute values of *n* are about 1 larger 0\$\*z0\$+/!z+"z%zeFGf^z!z(/+z"+1\* z".+)z0\$!z.!(z0%)!z+/!.20%+\*z+"z%z+4% 0%+\*z,.+¥ ess using an *in-situ*z/,!0.+/+,%z!((%,/+)!0.5z0\$0zeFCfz0\$!z%\*0!."!z(5!.z0\$%'\*!//z%\*.!/¥ es with increasing oxide thickness and saturates around 1.5 nm at the oxide thickness of around 7 nm, and the refractive indices of the layer are also saturated at around 7 nm in oxide thickness to the similar values obtained from *ex-situ* measurements. The similarity in the energy dispersion of the optical constants of the interface layers suggests that the interface layer is not a transition layer between SiC and SiO2, such as SiOx or SiCxOy^z0\$¥ er, it is a layer having a modified structure and/or composition compared to SiC, such as a stressed or interstitials-incorporated SiC layer, locating not on the SiO2 side but the SiC side of the SiC-oxide interface.

According to the results from the real time observation [30f\_z0\$!z0\$%'\*!//z0z3\$%\$z0\$!z%\*0!.¥ face layer thickness and the refractive index become constant (*i.e.*, 7 nm) is determined not from the surface polarity or oxygen partial pressure but from the oxide thickness. The Si and C emission model describes this behavior by considering that Si and C atoms are emitted into both directions of the SiC-oxide interface accompanying oxidation at the interface, *i.e.*, %\*0+z\*+0z+\*(5z0\$!z+4% !z(5!.z10z(/+z0\$!z%z(5!.zc/!!z%#^zEd\_z\* z1)1(0%+\*z+"z%\*0!./0%¥ tial Si and/or C atoms emitted into the SiC substrate may form a layer having similar optical ,.+,!.0%!/z/z%z10z(.#!.z.!".0%2!z%\* %!/z+),.! z0+z%^z%\*!z1)1(0%+\*z+"z%\*0!.¥ stitials is linked to the growth of the oxide, it is considered that growth of the interface layer is saturated at some intrinsic oxide thickness even if the oxygen pressure is changed. Takaku *et al.* \$2!z"+1\* z".+)z0\$!z.!(z0%)!z+/!.20%+\*z0\$0z0\$!z1/!z+"z.!(0%2!(5z(+3z+4% 0%+\*z0!)¥ perature or oxygen pressure leads to no formation of the interface layer [35], which can be explained by considering that the accumulations of Si and/or C interstitials are prevented by lowering the temperature or pressure.

Hashimoto *et al.* \$2!z"+1\* z0\$0z0\$!.!z%/zz/0.+\*#z+..!/,+\* !\*!z!03!!\*z0\$!z.!".0%2!z%\*¥ !4z+"z0\$!z%\*0!."!z(5!.z\* z%\*0!."!z/00!z !\*/%05z+0%\*! z".+)z0\$!z
z,%0+./z".%¥ cated on the same sample for ellipsometry measurements [32]. Hence, we believed that the 1)1(0%+\*/z+"z%z\* zz%\*0!./0%0%(/z\*!.u0z0\$!z%\*0!."!z.!z(+/!(5z.!(0! z0+z0\$!z"+.)¥ tion of interface states. According to the report from Afanasev *et al.* [36f\_zz#.,\$%0!w(%'!z.¥ bon layer forms near/at the interface, which causes interface states over the whole range of forbidden energy band and, in addition, an intrinsic SiO2 defect (NIT: near interface trap), presumably originating from oxygen deficiency, exists regardless of Si or SiC-polytypes and gives rise to the interface states in the vicinity of the 4H-SiC conduction band edge, which cause a channel degradation in MOSFETs. Since the oxygen deficiency can be regarded as a Si interstitial in SiO2, Si (Si and C) emission into SiO2 layer may be the origin for this SiO2 defect. According to the *ab-initio* studies performed by Knaup *et al.* [37], the origin of NITs is Si interstitials or C dimers originating from C interstitials inside SiO2. They also reported that C dimers in the SiC-side give rise to interface states at the energy range between mid gap and valence band edge. On the other hand, according to the *ab-initio* /01 %!/z ".+)z!¥ vynck *et al.* [38], C-C pair in SiC originating from C interstitials and a Si-C-O structure give rise to a broad peak of interface state density, and a Si2-C-O structure give rise to a sharp peak near the conduction band edge, which is compatible with NIT. Cochrane *et al.* have ,!."+.)! z\*z(!0.%((5z !0!0! z)#\*!0%z.!/+\*\*!zc
dz0!\$\*%-1!/z\* z"+1\* z%z2¥ cancies (or C dangling bonds) in the SiC-side near the interface [40], which is presumably 00.%10! z 0+z 0\$!z%z!)%//%+\*z 1.%\*#z+4% 0%+\*^z!.5z.!!\*0(5\_z\$!\*z\* z\*0!(% !/z\$2!z.!¥ ported that they identified the origin of interface states that degrade the SiC MOS channel mobility as 'C di-interstitials', which are formed by a combination of the two C interstitials injected into the SiC [41]. Accordingly, we tried to simulate the formation of the interface layer based on the diffusion theory and, as a result, the calculations reproduced the interface layer with the thickness of around 1 nm [39], which agrees with the values often reported [18, 34, 39f^z\*535\_z3!z3+1( z(%'!z0+z!),\$/%6!z\$!.!z0\$0z"1.0\$!.z1\* !./0\* %\*#z+"z0\$!z%\*¥ terfacial Si and C emission phenomenon during oxidation of SiC might be the key to realize the intrinsic performances of SiC MOSFETs.

found that the growth rate in the gentle deceleration stage to be proportional to pressure is due to the proportional increase in*F*int. Moreover, a careful look confirms the deceleration of *F*int in the very-thin oxide region (*ca.*< 5 nm) to be enhanced as elevating the pressure, which has been seen in the calculated curves in Fig. 9. Therefore, it is necessary for the Si and C !)%//%+\*z)+ !(\_z/z3!((z/z"+.z0\$!z%\*0!."%(z%z!)%//%+\*z)+ !(z0\$0z\*z,,.+,.%0!z !/.%,¥ 0%+\*z"+.z0\$!z+4% !z#.+30\$z+\*z0\$!z/1."!z%/z%\*0.+ 1! z0+z0\$!z)+ !(z/+z/z0+z!\*\$\*!z0\$!z/1.¥

face growth rate in the initial oxidation region up to over the interfacial growth rate.

for the practical use of SiC-MOSFET, in the light of interfacial Si and C emission.

side of the SiC-oxide interface.

200 Physics and Technology of Silicon Carbide Devices

lowering the temperature or pressure.

3.4. Discussion on Si and C emission phenomenon and its relation to the interface layer

+z".\_z3!z/,!%"5z0\$!z+4% 0%+\*z)!\$\*%/)z+"z%z".+)z0\$!z2%!3,+%\*0z+"z0\$!z%z\* zz!)%/¥ /%+\*z,\$!\*+)!\*+\*^z \*z 0\$%/z /!0%+\*\_z3!z3%((z %/1//z 0\$!z /0.101.!z \* z 0\$!z "+.)0%+\*z)!\$¥ nism of the interface layer between SiC-SiO2, which has been considered as the crucial issue

 \*z,.!2%+1/z3+.'\_z3!z\$2!z"+1\* z0\$0zeFE\_zFFfz0\$!z,\$+0+\*z!\*!.#5z !,!\* !\*!z+"z0\$!z+,0%¥ cal constants *n* and *k* of the interface layer derived from the complex dielectric constants between 2 and 6 eV, covering the direct interband transition energy *E*0 of 4H-SiC of 5.65 eV, is similar to that of bulk 4H-SiC, though the absolute values of *n* are about 1 larger 0\$\*z0\$+/!z+"z%zeFGf^z!z(/+z"+1\* z".+)z0\$!z.!(z0%)!z+/!.20%+\*z+"z%z+4% 0%+\*z,.+¥ ess using an *in-situ*z/,!0.+/+,%z!((%,/+)!0.5z0\$0zeFCfz0\$!z%\*0!."!z(5!.z0\$%'\*!//z%\*.!/¥ es with increasing oxide thickness and saturates around 1.5 nm at the oxide thickness of around 7 nm, and the refractive indices of the layer are also saturated at around 7 nm in oxide thickness to the similar values obtained from *ex-situ* measurements. The similarity in the energy dispersion of the optical constants of the interface layers suggests that the interface layer is not a transition layer between SiC and SiO2, such as SiOx or SiCxOy^z0\$¥ er, it is a layer having a modified structure and/or composition compared to SiC, such as a stressed or interstitials-incorporated SiC layer, locating not on the SiO2 side but the SiC

According to the results from the real time observation [30f\_z0\$!z0\$%'\*!//z0z3\$%\$z0\$!z%\*0!.¥ face layer thickness and the refractive index become constant (*i.e.*, 7 nm) is determined not from the surface polarity or oxygen partial pressure but from the oxide thickness. The Si and C emission model describes this behavior by considering that Si and C atoms are emitted into both directions of the SiC-oxide interface accompanying oxidation at the interface, *i.e.*, %\*0+z\*+0z+\*(5z0\$!z+4% !z(5!.z10z(/+z0\$!z%z(5!.zc/!!z%#^zEd\_z\* z1)1(0%+\*z+"z%\*0!./0%¥ tial Si and/or C atoms emitted into the SiC substrate may form a layer having similar optical ,.+,!.0%!/z/z%z10z(.#!.z.!".0%2!z%\* %!/z+),.! z0+z%^z%\*!z1)1(0%+\*z+"z%\*0!.¥ stitials is linked to the growth of the oxide, it is considered that growth of the interface layer is saturated at some intrinsic oxide thickness even if the oxygen pressure is changed. Takaku *et al.* \$2!z"+1\* z".+)z0\$!z.!(z0%)!z+/!.20%+\*z0\$0z0\$!z1/!z+"z.!(0%2!(5z(+3z+4% 0%+\*z0!)¥ perature or oxygen pressure leads to no formation of the interface layer [35], which can be explained by considering that the accumulations of Si and/or C interstitials are prevented by

Finally, we would like to introduce a recent innovation using oxidation about elimination of point defects. Hiyoshi and Kimoto invented an epoch-making idea, in which as-grown deep-level states such as a Z1/2 center can be eliminated by oxidation of SiC substrates [43]. Taking into account the report from Storasta *et al.* [44] in which the origin of Z1/2 is perhaps C vacancy, Hiyoshi and Kimoto considered the mechanism of deep-level state elimination by oxidation as follows: The excess silicon and carbon interstitials may be generated at the oxidizing interface, and a part of the emitted silicon and carbon interstitials may diffuse into the epilayer, and consequently, recombination of the interstitials with vacancies present in the epilayers may take place [45]. Therefore, the Si and C emission surely takes place during +4% 0%+\*z\* z0\$!z/1.2!5z+"z%z\* zz%\*0!./0%0%(z)+2!)!\*0z%/z2!.5z%),+.0\*0z0+z10%(%6!z0\$!.¥ mal oxidation as a device fabrication process, *e.g.* Ref. [46f\_z!2!\*z%\*z0\$!z !2!(+,)!\*0/z%\*z%¥ polar devices of over 10 kV class. Very recently, we found that stacking-faults are formed or extended by thermal oxidation, which is perhaps induced by the interfacial strain due to thermal oxidation [47].
