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γ-Ray irradiation effects on 6H-SiC MOSFET
Materials Science and Engineering B61 – 62 (1999) 480 – 484
g-Ray irradiation effects on 6H-SiC MOSFET Takeshi Ohshima *, Masahito Yoshikawa, Hisayoshi Itoh, Yasushi Aoki 1, Isamu Nashiyama Japan Atomic Energy Research Institute, 1233 Watanuki, Takasaki, Gunma 370 -1292, Japan
Abstract The effects of g-ray irradiation on the performance of enhancement-type n-channel 6H-SiC metal-oxide-semiconductors field effect transistors (MOSFETs) were studied. The gate oxide of the 6H-SiC MOSFETs was fabricated using pyrogenic or dry oxidation process. Oxide-trapped charges and interface traps produced in the MOSFETs by irradiation are evaluated from changes in the subthreshold–current curve. It is found that the net numbers of radiation-induced oxide-trapped charges and interface traps depend on the oxidation process. From a comparison of radiation response between 6H-SiC and Si MOSFETs, 6H-SiC MOSFETs are demonstrated to have higher radiation resistance. © 1999 Elsevier Science S.A. All rights reserved. Keywords: 6H-SiC; MOSFET; Pyrogenic and dry oxidation processes; g-Ray irradiation; Oxide-trapped charge; Interface traps
1. Introduction Because silicon carbide, SiC, has excellent thermal and electrical properties such as a high thermal conductivity, a wide band gap, a high saturation velocity of electrons and a high breakdown field, it is expected to be applied to high-power and high-frequency electronic devices [1–4]. It has been found that SiC has strong radiation resistance [5 – 9]. Thus, its application to electronic devices used in ionizing radiation field such as space environment is also expected. For this purpose, it is very important to understand the nature of radiationinduced defects in SiC device structures and the influence of such defects on the device performance. For SiC, radiation-induced interface traps and oxidetrapped charges in metal-oxide-semiconductor (MOS) capacitors have been investigated from capacitance– voltage measurements [6,8]. Recently, the generation of such defects in g-ray irradiated SiC MOS transistors has been reported [7,9]. However, the formation mechanisms of these defects and their influence on the performance of MOS transistors have not yet been fully understood. * Corresponding author. Fax: +81-27-346-9687. E-mail address: [email protected] (T. Ohshima) 1 Present address: Sumitomo Heavy Industries, 2-1-1 Yato, Tanashi, Tokyo 188-8585, Japan.
To study the radiation effects on SiC MOS transistors, we have fabricated enhancement-type n-channel MOS field effect transistors (MOSFETs) on p-type 6H-SiC, and performed g-ray irradiation to them. In this article, the formation of interface traps and oxidetrapped charges in gate oxide of the MOSFETs due to g-ray irradiation is described, based on the shifts of the threshold voltage, Vth, and the midgap voltage, Vmid. The dependence of the generation of these defects on oxidation process, i.e. pyrogenic or dry oxidation is also examined.
2. Experiments The MOSFETs were fabricated using p-type 6H-SiC epitaxial films (4 mm thick) grown on p-type 6H-SiC substrates (Cree). The net acceptor concentration of the epitaxial films was 4.9 or 6.8×1015 cm − 3. The source and drain were formed by nitrogen ion implantation (30 keV–N + , 1.0 ×1014 cm − 2) at 1200°C and subsequent annealing at 1300°C for 20 min in Ar atmosphere. Gate oxide with a thickness of 20 nm was formed at 1100°C for 1 h by pyrogenic oxidation (H2:O2 = 1:1) or for 6 h by dry oxidation. Here, the MOSFETs with gate oxides formed by pyrogenic and dry oxidations are referred to as MOSFET(Pyro) and MOSFET(Dry), respectively. The gate length×width
0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 8 ) 0 0 5 6 0 - 1
T. Ohshima et al. / Materials Science and Engineering B61–62 (1999) 480–484
of the MOSFETs was 10×200 mm2. g-Ray irradiation was performed up to 150 kGy (SiO2) at a rate of 8.8 kGy h − 1 at room temperature. During irradiation, no electrical bias was applied to the gate, or between the source and drain. The subthreshold characteristics for the MOSFETs were measured under the dark condition at room temperature.
3. Results and discussion Fig. 1 shows drain current, ID, versus gate voltage, VG, curves in the subthreshold region, subthreshold– current curves, for the SiC MOSFET(Pyro) before and after irradiation at three different radiation doses. The voltage marked with a cross on each curve corresponds to Vth, which is determined as the value at the intersection between the VG-axis and the line extrapolated from the curve of the square root of ID versus VG in the saturation region. Here, ID is given by ID = CmCox(VG −Vth)2,
(1)
where Cm is m(W/2L). m, W, L and Cox are the channel mobility, the gate width, the gate length and the oxide capacitance, respectively. Before irradiation, Vth was obtained to be 1.49 V. Due to irradiation, it shifts toward the lower gate voltage side at doses of 8.8 and 17.4 kGy. A slight recovery of Vth is observed at 104.4 kGy. The slope of the subthreshold – current curves does not change obviously below 104.4 kGy, and becomes gradual at 104.4 kGy. These results indicate that
Fig. 1. Subthreshold –current curves for 6H-SiC metal-oxide-semiconductors field effect transistors (MOSFET) irradiated with g-rays. The gate oxide of the MOSFET was formed by pyrogenic oxidation. The potential of 12 V was applied between source and drain during the measurement. The result before irradiation is also shown for comparison. The crosses represent the threshold voltages.
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interface traps and oxide-trapped charges are generated in the MOSFET by irradiation of g-rays. In order to separate the contributions of radiation-induced interface traps and oxide-trapped charges, the drain current corresponding to the midgap condition, Imid, is obtained by the following analysis. In the subthreshold region, ID is represented by the formula [10] ID = 2 Cm(qNALB/b)(ni/NA)2 exp(bfs)(bfs) − 1/2 (2) where NA, ni, fs and LB are the acceptor concentration in the channel, the intrinsic carrier concentration, band bending at the surface and the Debye length given by LB = [os/(bqNA)]1/2, respectively. Here, b is equal to q/kT, where q and k are the electron charge and the Boltzmann constant, respectively. Since fs at the midgap condition is equal to (kT/q) ln(NA/ni), Imid is derived from the substitution of fs for (kT/q) ln(NA/ni) in Eq. (2). Cm can be obtained from the slope of the curve of the square root of ID versus VG in the saturation region. Using the obtained value of Imid and the subthreshold–current curve, Vmid can be determined. Since Imid for 6H-SiC tends to be of the order of 10 − 30 A, it is necessary to linearly extrapolate the lower position of the subthreshold–current curve down to the lower part of the curve for the determination of Vmid. According to McWhorter and Winokur [11], an entire subthreshold curve is simply translated by the generation of oxide-trapped charges because the contribution of oxide-trapped charges to the shift of threshold voltage is independent of gate bias. On the other hand, interface traps interact with carriers in semiconductor, and the density of interface traps interacting with carriers depends on gate bias. Thus, the subthreshold–current curve is stretched by the generation of interface traps. As a result, the shift of the threshold voltage upon irradiation, DVth, is described as DVox + DVit, where DVox and DVit are the voltage shifts due to the generation of oxide-trapped charges and interface traps, respectively. The shift of the midgap voltage, DVmid, upon irradiation represents the shift owing to the formation of oxide-trapped charges only (DVox = DVmid). Since the subthreshold–current curve between the midgap and the threshold voltages is stretched by the generation of interface traps, DVit can be determined from DVit = (Vth − Vmid)post −(Vth − Vmid)pre, where ‘post’ and ‘pre’ denote after and before irradiation, respectively. For MOS capacitors fabricated on p-type substrates, the information on interface traps of which electronic levels are located in the lower side from the intrinsic level is mainly obtained from capacitance–voltage curves. On the other hand, from the fact that DVit is obtained from the subthreshold– current curve between the midgap and the threshold voltages, interface traps generated in the upper side from the intrinsic level is derived for n-channel MOSFETs.
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the argument that intrinsic interface traps in the upper half of the band gap are positively charged [12]. Since the nature of intrinsic and radiation-induced interface traps has not yet been revealed, further investigations are necessary to determine whether interface traps in the upper half of the band gap act as acceptors or donors. The net number of radiation-induced oxide-trapped charges per unit area, DNox, is determined from DVox by DNox = DVox Cox/q
(3)
where Cox is equal to oox/tox, and oox and tox are the relative dielectric constant of SiO2 and the thickness of gate oxide, respectively. The absorbed dose dependence of DNox for the MOSFET(Pyro) and MOSFET(Dry) is shown in Fig. 3. The data reported for Si MOSFET [11] and SiC MOSFET [9] are also plotted in the figure for comparison. The gate oxide of the SiC MOSFET reported in Ref. [9] has been formed by pyrogenic oxidation. The value of DNox for all SiC MOSFETs increases with absorbed dose with an exponent of 2/3. This 2/3 power-law dependence was also found in Si MOSFETs [11]. The value of DNox for SiC MOSFET(Pyro) in this study is almost the same as that previously reported for SiC MOSFET. The value of DNox for SiC MOSFET(Pyro) is approximately 1.5 times as high as that for SiC MOSFET(Dry) at the same absorbed dose. The difference may be caused by hydrogen-related species such as water or hydroxide remaining in gate oxide formed by pyrogenic oxidation. Fig. 2. Dependence of DVth, DVit and DVox on absorbed dose for 6H-SiC metal-oxide-semiconductors field effect transistors (MOSFETs). (a) The gate oxide of the 6H-SiC MOSFET was formed by pyrogenic oxidation. (b) The gate oxide of the 6H-SiC MOSFET was formed by dry oxidation.
The absorbed dose dependence of DVth, DVit and DVox for the SiC MOSFET(Pyro) and MOSFET(Dry) is shown in Fig. 2(a) and (b), respectively. The value of DVox for both SiC MOSFET(Pyro) and MOSFET(Dry) shifts to the negative voltage side by irradiation. This is attributed to the fact that positive charges are generated in their gate oxides. Similar results were also reported for Si MOSFETs [11]. As for interface traps, the value of DVit for both SiC MOSFETs shifts to the positive voltage side by irradiation. This result indicates that negatively charged interface traps are produced by irradiation. Since acceptor traps falling below the Fermi level are charged negatively, this result can be explained in terms of the formation of acceptor traps in the upper half of the band gap. In contrast to this finding, a large negative flat band shift observed for unirradiated p-type 6H-SiC MOS capacitors can be understood based on
Fig. 3. Absorbed dose dependence of the net number of radiation-induced oxide-trapped charges for 6H-SiC metal-oxide-semiconductors field effect transistors (MOSFETs). Circles and squares represent the results for the 6H-SiC MOSFETs whose gate oxides were formed by pyrogenic and dry oxidations, respectively. The data reported for Si MOSFET [11] (upside-down triangles) and 6H-SiC MOSFET [9] (triangles) are also plotted for comparison.
T. Ohshima et al. / Materials Science and Engineering B61–62 (1999) 480–484
Fig. 4. Absorbed dose dependence of the net number of radiation-induced interface traps for 6H-SiC metal-oxide-semiconductors field effect transistors (MOSFETs). Circles and squares represent the results for the 6H-SiC MOSFETs whose gate oxides were formed by pyrogenic and dry oxidations, respectively. The data reported for Si MOSFET [11] (upside-down triangles) and 6H-SiC MOSFET [9] (triangles) are also plotted for comparison.
The value of DNox for Si MOSFET is approximately twice that for the SiC MOSFET(Pyro) and about four times as high as that for SiC MOSFET(Dry). Yoshikawa et al. [8] reported that interface traps at the middle region of the band gap of 6H-SiC behave just like oxide-trapped charges because they release charges with extremely long time constants (1012 – 1017s) at room temperature. Therefore, it is likely that the obtained DNox includes a part of radiation-induced interface traps at the middle region. In spite of the over-estimated value of DNox for the SiC MOSFETs, DNox in the SiC MOSFETs is 1/4 1/2 of that in the Si MOSFET. Thus, it is concluded that the net number of radiation-induced oxide-trapped charges for the SiC MOSFETs is less than that for Si MOSFETs, suggesting strongly that the SiC MOSFETs have higher radiation resistance. Fig. 4 shows the absorbed dose dependence of the net number of radiation-induced interface traps per unit area, DNit, which is determined using DNit = DVit Cox/q
(4)
Then, DNit represents an increase in the net number of interface traps (except those behave as oxide-trapped charges) between the midgap and the threshold voltages upon irradiation. The data reported for Si MOSFETs [11] and SiC MOSFETs [9] are also plotted in the figure for comparison. The value of DNit for the SiC MOSFET(Pyro) as well as the SiC MOSFET reported in Ref. [9] increases with absorbed dose with an exponent of approximately 3/2. In contrast to this result, the power-law dependence for the Si MOSFET and the SiC
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MOSFET(Dry) has an exponent of approximately 2/3. The value of DNit for the SiC MOSFET(Pyro) is less than that for the SiC MOSFET(Dry) in a dose range below 100 kGy. Above 100 kGy, the values obtained for both MOSFETs exhibit almost the same. The generation of interface traps upon irradiation is caused by the reaction of radiation-induced charges in oxide to precursors of interface traps. Thus, the difference of the dose dependence of DNit between MOSFET(Pyro) and MOSFET(Dry) is attributable to the difference of the type of precursors formed by pyrogenic oxidation with that by dry oxidation. Precursors related to hydrogen and/or water are probably generated by pyrogenic oxidation. They can affect the generation of interface traps. It may lead to the difference of the dose dependence of DNit observed. The values of DNit for the SiC MOSFET(Pyro) and the SiC MOSFET(Dry) irradiated at 10 kGy are 3 × 1010 and 5× 1011 cm − 2, respectively. On the other hand, that for the Si MOSFET is 2× 1012 cm − 2 at 10 kGy. This value is about 70 and four times as high as those for the SiC MOSFET(Pyro) and the SiC MOSFET(Dry), respectively. The result of the generation of interface traps as well as oxide-trapped charges shows that SiC MOSFETs are quite resistant against radiation in comparison with Si MOSFETs. As for the reason why SiC MOSFETs have such high radiation resistance, residual atoms such as carbon in oxide may play a role in this behavior. Further investigations are necessary to clarify this point.
4. Summary Enhancement-type n-channel MOSFETs were fabricated on 6H-SiC using pyrogenic or dry oxidation process. These devices were irradiated with g-rays up to 150 kGy (SiO2) at room temperature. Based on the changes in the subthreshold–current curve by irradiation, radiation-induced oxide-trapped charges and interface traps were evaluated. The net number of radiation-induced oxide-trapped charges for the SiC MOSFETs fabricated using pyrogenic oxidation is approximately 1.5 times as high as that for the SiC MOSFETs fabricated using dry oxidation. The net number of radiation-induced interface traps for the SiC MOSFETs using pyrogenic oxidation is less than that for the SiC MOSFETs using dry oxidation in a g-ray dose range below 100 kGy. Above 100 kGy, almost the same number of interface traps is produced for both MOSFETs. These results indicate that the generation of oxide-trapped charges and interface traps in SiC MOSFETs by irradiation depends on the oxidation process. The net numbers of radiation-induced oxide-trapped charges and interface traps in the SiC MOSFETs are very low compared with those reported in the Si MOS-
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FETs. This result demonstrates that SiC MOSFETs have strong radiation resistance and are suitable for the electronic component used in radiation fields.
[6] [7]
References [1] W.E. Nelson, F.A. Halden, A. Rosengreen, J. Appl. Phys. 37 (1966) 333. [2] D.K. Ferry, Phys. Rev. B12 (1975) 2361. [3] H. Daimon, M. Yamanaka, M. Shibahara, E. Sakuma, S. Misawa, K. Endo, S. Yoshida, Appl. Phys. Lett. 51 (1987) 2106. [4] M. Bhatnagar, B.J. Baliga, IEEE Trans. Electron Devices 40 (1993) 645. [5] H. Itoh, M. Yoshikawa, I. Nashiyama, S. Misawa, H. Okumura, S. Yoshida, In: G.L. Harris, M.G. Spencer, C.Y. Yang (Eds.),
.
[8]
[9] [10] [11] [12]
Amorphous and Crystalline Silicon Carbide III, Springer-Verlag, Berlin, 1992, p. 143. M. Yoshikawa, H. Itoh, Y. Morita, I. Nashiyama, S. Misawa, H. Okumura, S. Yoshida, J. Appl. Phys. 70 (1991) 1309. T. Ohshima, M. Yoshikawa, H. Itoh, T. Takahashi, H. Okumura, S. Yoshida, I. Nashiyama, Proc. Int. Conf. Silicon Carbide and Related Materials, Kyoto, 1995 (Inst. Phys. Conf. Ser. No.142, 1996) p. 801. M. Yoshikawa, K. Saitoh, T. Ohshima, H. Itoh, I. Nashiyama, S. Yoshida, H. Okumura, Y. Takahashi, K. Ohnishi, J. Appl. Phys. 80 (1996) 282. T. Ohshima, M. Yoshikawa, H. Itoh, Y. Aoki, I. Nashiyama, Jpn. J. Appl. Phys. 37 (1998) L1002. J.R. Brew, In: Dawon Kahhny (Ed.), Applied Solid State Science, Academic, New York, 1981, p. 31. P.J. McWhorter, P.S. Winokur, Appl. Phys. Lett. 48 (1986) 133. J.A. Cooper Jr., Phys. Stat. Sol. A 162 (1997) 305.
g-Ray irradiation effects on 6H-SiC MOSFET Takeshi Ohshima *, Masahito Yoshikawa, Hisayoshi Itoh, Yasushi Aoki 1, Isamu Nashiyama Japan Atomic Energy Research Institute, 1233 Watanuki, Takasaki, Gunma 370 -1292, Japan
Abstract The effects of g-ray irradiation on the performance of enhancement-type n-channel 6H-SiC metal-oxide-semiconductors field effect transistors (MOSFETs) were studied. The gate oxide of the 6H-SiC MOSFETs was fabricated using pyrogenic or dry oxidation process. Oxide-trapped charges and interface traps produced in the MOSFETs by irradiation are evaluated from changes in the subthreshold–current curve. It is found that the net numbers of radiation-induced oxide-trapped charges and interface traps depend on the oxidation process. From a comparison of radiation response between 6H-SiC and Si MOSFETs, 6H-SiC MOSFETs are demonstrated to have higher radiation resistance. © 1999 Elsevier Science S.A. All rights reserved. Keywords: 6H-SiC; MOSFET; Pyrogenic and dry oxidation processes; g-Ray irradiation; Oxide-trapped charge; Interface traps
1. Introduction Because silicon carbide, SiC, has excellent thermal and electrical properties such as a high thermal conductivity, a wide band gap, a high saturation velocity of electrons and a high breakdown field, it is expected to be applied to high-power and high-frequency electronic devices [1–4]. It has been found that SiC has strong radiation resistance [5 – 9]. Thus, its application to electronic devices used in ionizing radiation field such as space environment is also expected. For this purpose, it is very important to understand the nature of radiationinduced defects in SiC device structures and the influence of such defects on the device performance. For SiC, radiation-induced interface traps and oxidetrapped charges in metal-oxide-semiconductor (MOS) capacitors have been investigated from capacitance– voltage measurements [6,8]. Recently, the generation of such defects in g-ray irradiated SiC MOS transistors has been reported [7,9]. However, the formation mechanisms of these defects and their influence on the performance of MOS transistors have not yet been fully understood. * Corresponding author. Fax: +81-27-346-9687. E-mail address: [email protected] (T. Ohshima) 1 Present address: Sumitomo Heavy Industries, 2-1-1 Yato, Tanashi, Tokyo 188-8585, Japan.
To study the radiation effects on SiC MOS transistors, we have fabricated enhancement-type n-channel MOS field effect transistors (MOSFETs) on p-type 6H-SiC, and performed g-ray irradiation to them. In this article, the formation of interface traps and oxidetrapped charges in gate oxide of the MOSFETs due to g-ray irradiation is described, based on the shifts of the threshold voltage, Vth, and the midgap voltage, Vmid. The dependence of the generation of these defects on oxidation process, i.e. pyrogenic or dry oxidation is also examined.
2. Experiments The MOSFETs were fabricated using p-type 6H-SiC epitaxial films (4 mm thick) grown on p-type 6H-SiC substrates (Cree). The net acceptor concentration of the epitaxial films was 4.9 or 6.8×1015 cm − 3. The source and drain were formed by nitrogen ion implantation (30 keV–N + , 1.0 ×1014 cm − 2) at 1200°C and subsequent annealing at 1300°C for 20 min in Ar atmosphere. Gate oxide with a thickness of 20 nm was formed at 1100°C for 1 h by pyrogenic oxidation (H2:O2 = 1:1) or for 6 h by dry oxidation. Here, the MOSFETs with gate oxides formed by pyrogenic and dry oxidations are referred to as MOSFET(Pyro) and MOSFET(Dry), respectively. The gate length×width
0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 8 ) 0 0 5 6 0 - 1
T. Ohshima et al. / Materials Science and Engineering B61–62 (1999) 480–484
of the MOSFETs was 10×200 mm2. g-Ray irradiation was performed up to 150 kGy (SiO2) at a rate of 8.8 kGy h − 1 at room temperature. During irradiation, no electrical bias was applied to the gate, or between the source and drain. The subthreshold characteristics for the MOSFETs were measured under the dark condition at room temperature.
3. Results and discussion Fig. 1 shows drain current, ID, versus gate voltage, VG, curves in the subthreshold region, subthreshold– current curves, for the SiC MOSFET(Pyro) before and after irradiation at three different radiation doses. The voltage marked with a cross on each curve corresponds to Vth, which is determined as the value at the intersection between the VG-axis and the line extrapolated from the curve of the square root of ID versus VG in the saturation region. Here, ID is given by ID = CmCox(VG −Vth)2,
(1)
where Cm is m(W/2L). m, W, L and Cox are the channel mobility, the gate width, the gate length and the oxide capacitance, respectively. Before irradiation, Vth was obtained to be 1.49 V. Due to irradiation, it shifts toward the lower gate voltage side at doses of 8.8 and 17.4 kGy. A slight recovery of Vth is observed at 104.4 kGy. The slope of the subthreshold – current curves does not change obviously below 104.4 kGy, and becomes gradual at 104.4 kGy. These results indicate that
Fig. 1. Subthreshold –current curves for 6H-SiC metal-oxide-semiconductors field effect transistors (MOSFET) irradiated with g-rays. The gate oxide of the MOSFET was formed by pyrogenic oxidation. The potential of 12 V was applied between source and drain during the measurement. The result before irradiation is also shown for comparison. The crosses represent the threshold voltages.
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interface traps and oxide-trapped charges are generated in the MOSFET by irradiation of g-rays. In order to separate the contributions of radiation-induced interface traps and oxide-trapped charges, the drain current corresponding to the midgap condition, Imid, is obtained by the following analysis. In the subthreshold region, ID is represented by the formula [10] ID = 2 Cm(qNALB/b)(ni/NA)2 exp(bfs)(bfs) − 1/2 (2) where NA, ni, fs and LB are the acceptor concentration in the channel, the intrinsic carrier concentration, band bending at the surface and the Debye length given by LB = [os/(bqNA)]1/2, respectively. Here, b is equal to q/kT, where q and k are the electron charge and the Boltzmann constant, respectively. Since fs at the midgap condition is equal to (kT/q) ln(NA/ni), Imid is derived from the substitution of fs for (kT/q) ln(NA/ni) in Eq. (2). Cm can be obtained from the slope of the curve of the square root of ID versus VG in the saturation region. Using the obtained value of Imid and the subthreshold–current curve, Vmid can be determined. Since Imid for 6H-SiC tends to be of the order of 10 − 30 A, it is necessary to linearly extrapolate the lower position of the subthreshold–current curve down to the lower part of the curve for the determination of Vmid. According to McWhorter and Winokur [11], an entire subthreshold curve is simply translated by the generation of oxide-trapped charges because the contribution of oxide-trapped charges to the shift of threshold voltage is independent of gate bias. On the other hand, interface traps interact with carriers in semiconductor, and the density of interface traps interacting with carriers depends on gate bias. Thus, the subthreshold–current curve is stretched by the generation of interface traps. As a result, the shift of the threshold voltage upon irradiation, DVth, is described as DVox + DVit, where DVox and DVit are the voltage shifts due to the generation of oxide-trapped charges and interface traps, respectively. The shift of the midgap voltage, DVmid, upon irradiation represents the shift owing to the formation of oxide-trapped charges only (DVox = DVmid). Since the subthreshold–current curve between the midgap and the threshold voltages is stretched by the generation of interface traps, DVit can be determined from DVit = (Vth − Vmid)post −(Vth − Vmid)pre, where ‘post’ and ‘pre’ denote after and before irradiation, respectively. For MOS capacitors fabricated on p-type substrates, the information on interface traps of which electronic levels are located in the lower side from the intrinsic level is mainly obtained from capacitance–voltage curves. On the other hand, from the fact that DVit is obtained from the subthreshold– current curve between the midgap and the threshold voltages, interface traps generated in the upper side from the intrinsic level is derived for n-channel MOSFETs.
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the argument that intrinsic interface traps in the upper half of the band gap are positively charged [12]. Since the nature of intrinsic and radiation-induced interface traps has not yet been revealed, further investigations are necessary to determine whether interface traps in the upper half of the band gap act as acceptors or donors. The net number of radiation-induced oxide-trapped charges per unit area, DNox, is determined from DVox by DNox = DVox Cox/q
(3)
where Cox is equal to oox/tox, and oox and tox are the relative dielectric constant of SiO2 and the thickness of gate oxide, respectively. The absorbed dose dependence of DNox for the MOSFET(Pyro) and MOSFET(Dry) is shown in Fig. 3. The data reported for Si MOSFET [11] and SiC MOSFET [9] are also plotted in the figure for comparison. The gate oxide of the SiC MOSFET reported in Ref. [9] has been formed by pyrogenic oxidation. The value of DNox for all SiC MOSFETs increases with absorbed dose with an exponent of 2/3. This 2/3 power-law dependence was also found in Si MOSFETs [11]. The value of DNox for SiC MOSFET(Pyro) in this study is almost the same as that previously reported for SiC MOSFET. The value of DNox for SiC MOSFET(Pyro) is approximately 1.5 times as high as that for SiC MOSFET(Dry) at the same absorbed dose. The difference may be caused by hydrogen-related species such as water or hydroxide remaining in gate oxide formed by pyrogenic oxidation. Fig. 2. Dependence of DVth, DVit and DVox on absorbed dose for 6H-SiC metal-oxide-semiconductors field effect transistors (MOSFETs). (a) The gate oxide of the 6H-SiC MOSFET was formed by pyrogenic oxidation. (b) The gate oxide of the 6H-SiC MOSFET was formed by dry oxidation.
The absorbed dose dependence of DVth, DVit and DVox for the SiC MOSFET(Pyro) and MOSFET(Dry) is shown in Fig. 2(a) and (b), respectively. The value of DVox for both SiC MOSFET(Pyro) and MOSFET(Dry) shifts to the negative voltage side by irradiation. This is attributed to the fact that positive charges are generated in their gate oxides. Similar results were also reported for Si MOSFETs [11]. As for interface traps, the value of DVit for both SiC MOSFETs shifts to the positive voltage side by irradiation. This result indicates that negatively charged interface traps are produced by irradiation. Since acceptor traps falling below the Fermi level are charged negatively, this result can be explained in terms of the formation of acceptor traps in the upper half of the band gap. In contrast to this finding, a large negative flat band shift observed for unirradiated p-type 6H-SiC MOS capacitors can be understood based on
Fig. 3. Absorbed dose dependence of the net number of radiation-induced oxide-trapped charges for 6H-SiC metal-oxide-semiconductors field effect transistors (MOSFETs). Circles and squares represent the results for the 6H-SiC MOSFETs whose gate oxides were formed by pyrogenic and dry oxidations, respectively. The data reported for Si MOSFET [11] (upside-down triangles) and 6H-SiC MOSFET [9] (triangles) are also plotted for comparison.
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Fig. 4. Absorbed dose dependence of the net number of radiation-induced interface traps for 6H-SiC metal-oxide-semiconductors field effect transistors (MOSFETs). Circles and squares represent the results for the 6H-SiC MOSFETs whose gate oxides were formed by pyrogenic and dry oxidations, respectively. The data reported for Si MOSFET [11] (upside-down triangles) and 6H-SiC MOSFET [9] (triangles) are also plotted for comparison.
The value of DNox for Si MOSFET is approximately twice that for the SiC MOSFET(Pyro) and about four times as high as that for SiC MOSFET(Dry). Yoshikawa et al. [8] reported that interface traps at the middle region of the band gap of 6H-SiC behave just like oxide-trapped charges because they release charges with extremely long time constants (1012 – 1017s) at room temperature. Therefore, it is likely that the obtained DNox includes a part of radiation-induced interface traps at the middle region. In spite of the over-estimated value of DNox for the SiC MOSFETs, DNox in the SiC MOSFETs is 1/4 1/2 of that in the Si MOSFET. Thus, it is concluded that the net number of radiation-induced oxide-trapped charges for the SiC MOSFETs is less than that for Si MOSFETs, suggesting strongly that the SiC MOSFETs have higher radiation resistance. Fig. 4 shows the absorbed dose dependence of the net number of radiation-induced interface traps per unit area, DNit, which is determined using DNit = DVit Cox/q
(4)
Then, DNit represents an increase in the net number of interface traps (except those behave as oxide-trapped charges) between the midgap and the threshold voltages upon irradiation. The data reported for Si MOSFETs [11] and SiC MOSFETs [9] are also plotted in the figure for comparison. The value of DNit for the SiC MOSFET(Pyro) as well as the SiC MOSFET reported in Ref. [9] increases with absorbed dose with an exponent of approximately 3/2. In contrast to this result, the power-law dependence for the Si MOSFET and the SiC
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MOSFET(Dry) has an exponent of approximately 2/3. The value of DNit for the SiC MOSFET(Pyro) is less than that for the SiC MOSFET(Dry) in a dose range below 100 kGy. Above 100 kGy, the values obtained for both MOSFETs exhibit almost the same. The generation of interface traps upon irradiation is caused by the reaction of radiation-induced charges in oxide to precursors of interface traps. Thus, the difference of the dose dependence of DNit between MOSFET(Pyro) and MOSFET(Dry) is attributable to the difference of the type of precursors formed by pyrogenic oxidation with that by dry oxidation. Precursors related to hydrogen and/or water are probably generated by pyrogenic oxidation. They can affect the generation of interface traps. It may lead to the difference of the dose dependence of DNit observed. The values of DNit for the SiC MOSFET(Pyro) and the SiC MOSFET(Dry) irradiated at 10 kGy are 3 × 1010 and 5× 1011 cm − 2, respectively. On the other hand, that for the Si MOSFET is 2× 1012 cm − 2 at 10 kGy. This value is about 70 and four times as high as those for the SiC MOSFET(Pyro) and the SiC MOSFET(Dry), respectively. The result of the generation of interface traps as well as oxide-trapped charges shows that SiC MOSFETs are quite resistant against radiation in comparison with Si MOSFETs. As for the reason why SiC MOSFETs have such high radiation resistance, residual atoms such as carbon in oxide may play a role in this behavior. Further investigations are necessary to clarify this point.
4. Summary Enhancement-type n-channel MOSFETs were fabricated on 6H-SiC using pyrogenic or dry oxidation process. These devices were irradiated with g-rays up to 150 kGy (SiO2) at room temperature. Based on the changes in the subthreshold–current curve by irradiation, radiation-induced oxide-trapped charges and interface traps were evaluated. The net number of radiation-induced oxide-trapped charges for the SiC MOSFETs fabricated using pyrogenic oxidation is approximately 1.5 times as high as that for the SiC MOSFETs fabricated using dry oxidation. The net number of radiation-induced interface traps for the SiC MOSFETs using pyrogenic oxidation is less than that for the SiC MOSFETs using dry oxidation in a g-ray dose range below 100 kGy. Above 100 kGy, almost the same number of interface traps is produced for both MOSFETs. These results indicate that the generation of oxide-trapped charges and interface traps in SiC MOSFETs by irradiation depends on the oxidation process. The net numbers of radiation-induced oxide-trapped charges and interface traps in the SiC MOSFETs are very low compared with those reported in the Si MOS-
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FETs. This result demonstrates that SiC MOSFETs have strong radiation resistance and are suitable for the electronic component used in radiation fields.
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