**6.1. NO annealing**

In 1997, the group of Prof. Dimitrijev, at Griffitth University in Australia, demonstrated that high temperature (1100 ◦C) nitric oxide (NO) annealing reduces the *Dit* at SiO2/6H-SiC interfaces [76]. In 2000, Chung *et al.* published results on the effects of NO at the SiO2/4H-SiC interface revealing that, in addition to removing deep states, it is also very efficient at reducing the density of slow states (by a factor of ≈ 10), and yields an order of magnitude increase in the channel mobility from about 5 to 50 cm2/V.s along the Si-face [40, 41]. This breakthrough discovery, which originated from the joint effort between Auburn University and Vanderbilt University, led to the adoption of the NO process by the scientific and industrial communities as it enables the fabrication of high-quality oxide-based SiC power devices, facilitating their commercial release (Fig. 1).

The benefits of NO annealing have been directly correlated with the incorporation of nitrogen, which is confined to the SiO2/SiC interface, as detected by various techniques such as secondary ion mass spectroscopy (SIMS) [82, 106], nuclear reaction analysis (NRA) [81], electron energy loss spectroscopy (EELS) [33], and medium energy ion scattering (MEIS) [47, 137]. To study the impact of nitrogen, the amount incorporated can be tailored by the NO annealing time as illustrated in Fig. 5(a). The N density is then extracted by integrating SIMS interface peaks resulting from 1175 ◦C NO exposure of a dry thermal oxide for up to 2 hours. The nitrogen content is found to saturate around 6 × <sup>10</sup><sup>14</sup> cm−<sup>2</sup> or about a half monolayer coverage of the SiC surface. The nitridation kinetics result from a balance between N incorporation and removal. Indeed, at this temperature, NO decomposes partially into N2 and O2. While 1175 ◦C is required to enable NO diffusion to the interface and subsequent nitridation, the presence of oxygen limits its effect as interfacial nitrogen is unstable against the slow re-oxidation occurring in parallel [37]. Moreover, additional defect formation can also result from the presence of the excess oxygen.

8 Physics and Technology of Silicon Carbide Devices

**4. Argon anneal**

**5. Hydrogen passivation**

density throughout the semiconductor band gap.

devices, facilitating their commercial release (Fig. 1).

schemes.

**6. Nitridation**

**6.1. NO annealing**

the U-shaped D*it* distribution. It rises sharply towards the SiC band edges because of the Si-related defects not dominant at silicon interfaces. Therefore, the efficiency of passivation techniques are expected to be very different at interfaces formed on the two semiconductors. In the following Sections, we will discuss how to reduce *Dit* and its relationship with mobility. Although there is extensive literature dedicated to various orientations and polytypes, this overview is dedicated to devices fabricated on the (0001) Si-face of 4H-SiC.

Oxidation conditions and post-oxidation annealing (POA) can affect the trap density at the SiO2/4H-SiC interface. Both Ar anneal performed at growth temperature and re-oxidation at lower temperatures (e.g. 900 ◦C) have indeed proven to slightly reduce the amount of deep states [40, 129]. This can be explained by the removal of excess carbon without additional oxide formation, as corresponding atomic configurations yield defects populating interface states toward the middle of the gap. Since it does not reduce the density of levels close to the conduction band edge of 4H-SiC, Ar POA alone is not sufficient to enable efficient SiC devices. Nevertheless, it is typically used after thermal oxidation and before other annealing

Wet oxidation of 4H-SiC also yields a small reduction of interface states with energies away from the semiconductor band edges when compared to SiO2 formation in dry oxygen [2, 26, 55, 90, 130]. It correlates well with the effects of H2 POA. While at silicon interfaces hydrogen annealing yields a *Dit* from about 10<sup>11</sup> to 1010 cm−2*eV*−<sup>1</sup> in the middle of the gap and a mobility close to half the one of the bulk [20, 21], its impact at SiC interfaces is much less efficient, highlighting the differences between the two semiconductors [27, 58, 96]. Molecular hydrogen can indeed passivate Si- or C- dangling bonds, and insert long Si-Si bonds [28]. But it does not significantly affect split carbon interstitials and slow near interface states which populate the majority of the *Dit* at the 4H-SiC band edges. Like Ar annealing, wet oxidation and/or H2 POA can be used together with other annealing techniques to optimize the trap

In 1997, the group of Prof. Dimitrijev, at Griffitth University in Australia, demonstrated that high temperature (1100 ◦C) nitric oxide (NO) annealing reduces the *Dit* at SiO2/6H-SiC interfaces [76]. In 2000, Chung *et al.* published results on the effects of NO at the SiO2/4H-SiC interface revealing that, in addition to removing deep states, it is also very efficient at reducing the density of slow states (by a factor of ≈ 10), and yields an order of magnitude increase in the channel mobility from about 5 to 50 cm2/V.s along the Si-face [40, 41]. This breakthrough discovery, which originated from the joint effort between Auburn University and Vanderbilt University, led to the adoption of the NO process by the scientific and industrial communities as it enables the fabrication of high-quality oxide-based SiC power Progressive reduction of the *Dit* across the 4H-SiC band gap corresponding to the tailored introduction of nitrogen has been measured in metal-oxide-semiconductor capacitors (MOSCAPs), as shown in Fig. 5(b). The density of states shows a strong correlation to the nitrogen content and is reduced by up to an order of magnitude close to the conduction band edge [82, 106, 108]. The sensitivity of the inversion mobility of electrons to the *Dit* reduction was studied in lateral field-effect transistors containing different amounts of nitrogen at the SiO2/4H-SiC interface. From the results depicted in Figs. 6(a) & 10, the peak field-effect mobility is found to be inversely proportional to the density of charged states, which reveals a Coulomb-scattering-limited transport. It is important to note that this is true even in devices with the lowest *Dit* so that further defect passivation is projected to increase the mobility from 50 to more than 100 cm2/V.s, which cannot be achieved by NO POA alone as nitrogen density becomes saturated. These conclusions are in agreement with separate mobility studies using Hall effect measurements on nitrided samples [13, 114, 125]. Such experiments also reveal that at higher fields, mobility becomes limited by surface-roughness scattering. Although NO POA has been shown to yield smoother interfaces [51], it is not clear what else can be done to further reduce that particular component.

**Figure 5.** (a) SIMS Nitrogen profiles showing progressive accumulation at the SiO2/SiC interface with increasing NO annealing time. Adapted from Ref. [106]. (b) Density of interface traps across the 4H-SiC band gap. Longer NO anneals yield lower D*it*. Reproduced with permission from Ref. [108].©2011 IEEE

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e- h+ M O S

10 eV

nitrogen must be at play [106]. In fact, similar conclusions have been drawn from N-induced negative bias temperature instability (NBTI) on silicon which has been attributed to Si-N bonds [24, 25], which also suggests that the defects must originate from the SiO2 conversion

Tailoring Oxide/Silicon Carbide Interfaces: NO Annealing and Beyond

From the impact of nitrogen incorporation on *Dit*, electron traps, and hole traps, summarized in Fig. 6(b), it is possible to paint a picture of what happens at the atomic level. First, it is important to note that although the term "passivation" is often used to describe the effects of nitrogen, the large density introduced by the NO process more likely yields a complete reconfiguration of the interface from SiO/SiC to SiON/SiC. But for simplicity, let us look at the effect of nitrogen on single defects. As mentioned previously, the majority of traps at the thermally grown interface are considered to be single and split carbon interstitials, as well as Si-Si bonds. Unlike hydrogen, nitrogen can reduce the energy of most of these atomic configurations by substituting for threefold-coordinated carbons or by inserting short suboxide bonds [90, 130]. While it can suppress acceptor levels close to the 4H-SiC conduction band edge, the donor nature of nitrogen and its 5 valence electrons can then yield states close to, or even within, the semiconductor valence band [104]. If the resulting atomic configurations are located on the oxide side of the interface, they can therefore act as stable hole traps. Let us take the example of a short Si-Si suboxide bond expected to have an unoccupied state close to the 4H-SiC conduction band. When inserted by NO, it yields a Si-NO-Si bridge and moves the trap level close the valence band. Theory suggests that the nitrogen lone electron pair leads to a partially occupied state that is a favorable hole trap, since giving away an electron makes the atomic configuration reduce its energy by a few eV. As mentioned earlier, nitrogen might not discriminate between defects and stoichiometric oxide sites as it indeed converts SiO to SiON. Therefore, the same configuration could result from N insertion in bridging Si-O-Si, which can create trap levels where there was none

In summary, the presence of nitrogen at the oxide/semiconductor interface is beneficial as it reduces *Dit*, increases mobility, and suppresses electron-induced interface state generation. But on the other hand, re-oxidation during NO POA limits the amount of N that can be inserted, and nitrogen generates a quantity of new traps by bonding in the near-interface

e-

3.5 eV

M O S

**(b)**

**Figure 7.** Effective trapped charge as a function of (a) injected electron density and (b) emitted hole density by internal photoemission. NO POA suppresses electron-induced interface state generation but increases the amount of hole traps in the

into a nitride.

before.

**(a)**

oxide. Adapted from Refs. [103, 104]

**Figure 6.** (a) Field-effect mobility extracted from lateral n-channel MOSFETs on 4H-SiC fabricated using different N doses. Reproduced with permission from Ref. [107]. ©2010 TTP (b) Dependence of integrated interface trap density (N*it*), electron-generated levels, and positive oxide traps, on N content. Reproduced with permission from Ref. [107]. ©2010 TTP

The benefits of nitrogen incorporation also extend to the reliability of SiC devices when it comes to electron injection in the gate oxide, which is inherent to n-channel transistor operation, as the dielectric is exposed to leakage currents and charge tunneling from the inversion layer towards the biased gate contact [31, 32, 52, 73, 132]. While the resulting degradation can take a long time to develop under normal operation, accelerated techniques can be used to study device response to excess carriers penetrating the gate structure. To best simulate actual bias conditions, electrons were injected at low oxide fields (< 2 MV/cm) using a mercury lamp promoting carriers from the negatively-biased gate metal to the conduction band of 4H-SiC in MOSCAPs fabricated using dry oxidation followed by various NO annealing times [103, 106]. As shown in Fig. 7(a), the density of trapped negative charge, extracted from the flatband voltage shift of capacitance-voltage (CV) curves, can be plotted as a function of the integrated gate current, i.e. the injected electron density. If no nitrogen is present at the oxide/semiconductor interface, device characteristics continuously drift towards positive voltages. From the observation of hysteresis behavior of CV curves, this has been correlated to electron-induced acceptor state generation at the interface [5]. However, the presence of even the smallest amount of nitrogen can suppress the degradation, exposing the secondary component of the negative charge, the bulk electron traps in SiO2 [103], Fig. 6(b).

When it comes to positive charge trapping however, the presence of nitrogen proves to be detrimental to device stability [73, 74]. This can be of concern even for n-channel transistors to which a negative gate bias can be applied to ensure that it is OFF in its idle state. Several methods have been used to accelerate hole exposure of the gate oxide such as x-ray irradiation [52], Fowler-Nordheim tunneling, and internal photoemission [104]. Figs. 6(b) & 7(b) show the positive trapped charge as a function of the injected hole carrier density using such a technique with samples containing different amount of nitrogen. There is a clear correlation between nitrogen content and oxide trap density. Because the nitrogen is contained at the interface and the charge is stable against bias reversal, it is attributed to near-interface traps in the SiON layer. ESR experiments have ruled out oxygen vacancies as the main positive trap in the oxide, another indication that atomic configurations involving nitrogen must be at play [106]. In fact, similar conclusions have been drawn from N-induced negative bias temperature instability (NBTI) on silicon which has been attributed to Si-N bonds [24, 25], which also suggests that the defects must originate from the SiO2 conversion into a nitride.

10 Physics and Technology of Silicon Carbide Devices

0 2 4 6 8 10 12 Gate bias (V)

**(a) (b)**

 2 hr NO 1 hr NO

 15 min NO 7.5 min NO

As-Oxidized

1011

0 1 2 3 4 5 6 Nitrogen density (1014cm-2 )

NO time (min) 7.5 15 30 60 120

1012

 Nit e- traps h+ traps

Trap density (cm-2 )

**Figure 6.** (a) Field-effect mobility extracted from lateral n-channel MOSFETs on 4H-SiC fabricated using different N doses. Reproduced with permission from Ref. [107]. ©2010 TTP (b) Dependence of integrated interface trap density (N*it*), electron-generated levels, and positive oxide traps, on N content. Reproduced with permission from Ref. [107]. ©2010

The benefits of nitrogen incorporation also extend to the reliability of SiC devices when it comes to electron injection in the gate oxide, which is inherent to n-channel transistor operation, as the dielectric is exposed to leakage currents and charge tunneling from the inversion layer towards the biased gate contact [31, 32, 52, 73, 132]. While the resulting degradation can take a long time to develop under normal operation, accelerated techniques can be used to study device response to excess carriers penetrating the gate structure. To best simulate actual bias conditions, electrons were injected at low oxide fields (< 2 MV/cm) using a mercury lamp promoting carriers from the negatively-biased gate metal to the conduction band of 4H-SiC in MOSCAPs fabricated using dry oxidation followed by various NO annealing times [103, 106]. As shown in Fig. 7(a), the density of trapped negative charge, extracted from the flatband voltage shift of capacitance-voltage (CV) curves, can be plotted as a function of the integrated gate current, i.e. the injected electron density. If no nitrogen is present at the oxide/semiconductor interface, device characteristics continuously drift towards positive voltages. From the observation of hysteresis behavior of CV curves, this has been correlated to electron-induced acceptor state generation at the interface [5]. However, the presence of even the smallest amount of nitrogen can suppress the degradation, exposing the secondary component of the negative charge, the bulk electron traps in SiO2

When it comes to positive charge trapping however, the presence of nitrogen proves to be detrimental to device stability [73, 74]. This can be of concern even for n-channel transistors to which a negative gate bias can be applied to ensure that it is OFF in its idle state. Several methods have been used to accelerate hole exposure of the gate oxide such as x-ray irradiation [52], Fowler-Nordheim tunneling, and internal photoemission [104]. Figs. 6(b) & 7(b) show the positive trapped charge as a function of the injected hole carrier density using such a technique with samples containing different amount of nitrogen. There is a clear correlation between nitrogen content and oxide trap density. Because the nitrogen is contained at the interface and the charge is stable against bias reversal, it is attributed to near-interface traps in the SiON layer. ESR experiments have ruled out oxygen vacancies as the main positive trap in the oxide, another indication that atomic configurations involving

50

40

30

FE mobility (cm2 / V.s)

TTP

20

10

0

[103], Fig. 6(b).

From the impact of nitrogen incorporation on *Dit*, electron traps, and hole traps, summarized in Fig. 6(b), it is possible to paint a picture of what happens at the atomic level. First, it is important to note that although the term "passivation" is often used to describe the effects of nitrogen, the large density introduced by the NO process more likely yields a complete reconfiguration of the interface from SiO/SiC to SiON/SiC. But for simplicity, let us look at the effect of nitrogen on single defects. As mentioned previously, the majority of traps at the thermally grown interface are considered to be single and split carbon interstitials, as well as Si-Si bonds. Unlike hydrogen, nitrogen can reduce the energy of most of these atomic configurations by substituting for threefold-coordinated carbons or by inserting short suboxide bonds [90, 130]. While it can suppress acceptor levels close to the 4H-SiC conduction band edge, the donor nature of nitrogen and its 5 valence electrons can then yield states close to, or even within, the semiconductor valence band [104]. If the resulting atomic configurations are located on the oxide side of the interface, they can therefore act as stable hole traps. Let us take the example of a short Si-Si suboxide bond expected to have an unoccupied state close to the 4H-SiC conduction band. When inserted by NO, it yields a Si-NO-Si bridge and moves the trap level close the valence band. Theory suggests that the nitrogen lone electron pair leads to a partially occupied state that is a favorable hole trap, since giving away an electron makes the atomic configuration reduce its energy by a few eV. As mentioned earlier, nitrogen might not discriminate between defects and stoichiometric oxide sites as it indeed converts SiO to SiON. Therefore, the same configuration could result from N insertion in bridging Si-O-Si, which can create trap levels where there was none before.

In summary, the presence of nitrogen at the oxide/semiconductor interface is beneficial as it reduces *Dit*, increases mobility, and suppresses electron-induced interface state generation. But on the other hand, re-oxidation during NO POA limits the amount of N that can be inserted, and nitrogen generates a quantity of new traps by bonding in the near-interface

**Figure 7.** Effective trapped charge as a function of (a) injected electron density and (b) emitted hole density by internal photoemission. NO POA suppresses electron-induced interface state generation but increases the amount of hole traps in the oxide. Adapted from Refs. [103, 104]

region of the oxide. Since we have shown that more nitrogen would increase the mobility even further, other nitridation methods could maximize its density while confining it to the interface boundary.

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Another elegant way to introduce nitrogen is the exposure of thermal oxides to nitrogen radicals [116, 134, 137]. It can be achieved using a remote plasma generating highly reactive N<sup>+</sup> ions. SIMS measurements have shown that, like NO-POA, this results in nitrogen accumulation strictly at the interface between the oxide and the semiconductor. One advantage being that it can potentially occur without re-oxidation, allowing for N maximization. Studies on devices fabricated on the Si-face of 4H-SiC again show that the D*it* is reduced proportionally to the amount of incorporated nitrogen, in line with results from NO POA. In fact, similar and even better performance in terms of peak field-effect mobility has been demonstrated using that technique. However, prolonged plasma exposure can also reduce the integrity of the gate oxide, implying that this promising nitridation method still

Tailoring Oxide/Silicon Carbide Interfaces: NO Annealing and Beyond

In 2009, about a decade after the introduction of NO annealing, Okamoto *et al.*, from the Nara Institute of Science and Technology in Japan, proposed another post-oxidation annealing technique that significantly reduces *Dit* at SiO2/4H-SiC interfaces. As mentioned in the previous section, implantation of nitrogen in SiC prior to oxidation has proved to be a beneficial nitridation technique. Hence, Prof. Yano and his group cleverly extended this logic to a screening method for various potential passivating species [87]. This is how phosphorus caught their attention as oxidation of P-implanted SiC also showed a lower density of electrically active defects than as-oxidized un-implanted interfaces. Following this discovery, they implemented a more gentle way to introduce P at the interface in order to avoid ion-induced damages and undesirable doping of the substrate, by flowing gas through

When performed at 1000 ◦C on SiO2, grown on the Si-face of 4H-SiC, POCl3 POA leads to <sup>a</sup> *Dit* below 10<sup>11</sup> cm<sup>−</sup>2eV<sup>−</sup><sup>1</sup> close to E*c*, or several times lower than following NO POA [88]. This is reflected in the efficiency of lateral nFETs as the peak value of the field-effect mobility almost doubles compared to NO POA to about 90 cm2/V.s. This has been correlated with the presence of phosphorus at the interface. Another proposed method to reach similar mobility values is exposing a thermal oxide to P2O5 extracted from a solid phosphosilicate glass (PSG) diffusion source [115]. Device properties following POCl3 or PSG POA are reported in Figs.

Note that from SIMS analysis, it is found that both POCl3 and PSG POA convert the dielectric into a phosphosilicate by yielding phosphorus throughout the gate. This compromises the reliability of the devices. Recently, forming a thin P-containing interfacial oxide, using POCl3 and O2, followed by dielectric deposition, was shown to reduce trapping by narrowing the

The benefits of phosphorus at SiO2/SiC interfaces represent a milestone for silicon carbide research; not only because of mobility improvements, but also because it shines light on the nature of passivation at the atomic level. Indeed, both N and P are among the group V elements of the periodic table, possessing similar chemistry due to their 5 valence electrons. For example, it has fueled the discussion of the role of sub-surface SiC doping in improving device characteristics [36]. But while the physics of N and P binding at interfaces is still being debated, we are one step closer to a more comprehensive understanding of post-oxidation

a POCl3 bubbler during a high temperature post-oxidation anneal.

requires optimization.

**7. Phosphorus**

8(a), 8(b) & 10.

phosphorus profile [11].

annealing mechanisms.
