Progressive reduction of the interface state density

SiO 2 GROWTH KINETICS ON SiC

6.3 Results and Discussion

6.3.3 Progressive reduction of the interface state density

Figure 6.4: Density of interface states (a) and positive charge trapping (b) in the control samples and in the one annealed in NO for 2 hours. The temperature and the slow re- oxidation have no significant effect on the properties, only the nitrogen incorporated during the NO anneal does.

Figure 6.5: Density of interface states of the control sample (2 hours Ar at 1175 ^◦C) and of the ones annealed for 7.5, 15, 30, and 120 minutes in NO. D⁰_it and D_it^∗ refer to the Dit

prior to NO annealing and at N saturation respectively.

Because the impact on the Dit of the temperature and of the slow re-oxidation have been shown to be negligible compared to the observed impact of NO anneals, this reduction is attributed to the passivation and/or the removal of defects by nitrogen binding at the interface. Theory suggests that nitrogen can indeed substitute for three-fold coordinated atoms or bind at defect sites, which reduces theD_it in the upper part of the 4H-SiC band- gap,¹³ as described in Chapter I (Section 1.3.3). These defects could be of different nature and therefore have different energy levels, explaining the efficiency of nitrogen over a wide energy range. For example, single carbon interstitials at the interface or silicon suboxide bonds in the oxide (of various lengths) are expected to have levels close to the conduction band edge. Also, carbon clusters at the interface (8 atoms or less) are thought have levels located deeper in the gap. All these defects can theoretically be passivated by nitrogen.

This is supported by experiments, such as X-ray photoelectron spectroscopy measurements (XPS), which indicate that NO anneals lead to the reduction of excess carbon, and to the formation of C-N and Si-N bonds while complex oxide/carbon states are removed.^25,26

Figure 6.6: D_itreduction (a), and normalized D_it reduction (b), after nitrogen saturation at the SiO₂/SiC interface.

The obtained D_it values can be fit by an exponential function which has the form

D_it[E_c−E] = D₀e^−B(Ec−E) (6.7)

whereEis the energy of the level,Ec is the energy at the conduction band edge,D₀is a pre- exponential constant, andB an exponential argument. It is observed that, although deeper levels seem to reach saturation early, the value of B does not change much as a function of nitrogen content. Indeed, the fits lead to an average B of 7.2 eV⁻¹ with a standard deviation of 0.9 eV⁻¹. This indicates that the shape of theD_itprofile remains similar during nitridation and that the amplitudeD₀ is progressively reduced. The difference betweenD_it⁰ and theD_it at N saturation (D_it^∗) is shown in Figure6.6(a). It reveals that N removes more levels towards the conduction band edge. This is only because their density is higher prior to NO annealing. Indeed, the reduction of theD_it normalized byD⁰_it, Figure6.6(b), indicates that the efficiency of nitrogen is comparable throughout the monitored energy window, as it removes about 70% of all levels. This suggests that the corresponding defects are of similar nature. However, the shape of the curve indicates that levels within 0.3 eV of the conduction band could be associated to a distinct category of atomic configurations. Actually, it has been observed that theD_itof unpassivated samples rises very sharply in that region,²⁷ and reaches a value of several times 10¹³ cm⁻² eV⁻¹, which cannot be predicted by Eq.(6.7).

These shallow states are thought to originate mostly from Si-Si suboxide bonds and to be very sensitive to nitrogen incorporation.²⁸ Another important parameter to extract, is the total density of levels removed after NO annealing between 0.2 and 0.6 eV. This is done by integrating the curve in Figure 6.6(a). It yields a value of approximately 7.6×10¹¹ cm⁻² which is only about 0.1% of the N area density at saturation. Even when considering the shallow states and the levels in the rest of the band-gap, it appears that there are two orders of magnitude more nitrogen incorporated than there are defects to passivate. This surprisingly large number will be discussed below.

Figure 6.7: (a) to (e); evolution of the Dit at different energy levels as a function of the area density of nitrogen incorporated by NO annealing. The amount of nitrogen required to achieve saturation seems to be less for levels deeper in the gap, it varies from5×10¹⁴ cm² at Ec−E= 0.2 eV to3×10¹⁴ cm² atEc−E = 0.6 eV. The dashed lines are fits by Eq.(6.12). (f) Normalized passivation cross-sections extracted form the fits.

The evolution of the D_it at specific levels is shown as a function of the nitrogen area density in Figure 6.7(a-e). This allows to study the kinetics of the passivation at different energies. A passivation model has previously been derived by McDonald et al.²³It assumed the reduction in size of carbon clusters which progressively moved the defect levels deeper in the band-gap. This does not seem to apply to the data presented here as there is no evidence of a step formation inDit vsη(the nitrogen area density) for deeper levels, which motivated the model. On the contrary, in this work, the density of deeper levels is found to saturate faster. It should be noted that the processing of the samples was different in the two experiments. McDonald et al. used a wet oxidation to grow the oxides, as well as a re-oxidation process; they report a higherD_it⁰ andD^∗_itwhich is likely related to the presence of other dominant defects.

Here, it is proposed that to first order N removes levels within the studied energy window without inducing any. Therefore, the passivation rate is expected to be proportional to a cross-section s_j (in cm²) and to the density of available binding sitesD_j (in cm⁻²) of type j. It should also be inversely proportional to the sum ofs_jD_j over all competing locations.

This leads to the differential equation dDj[η]

dη = − s_j(D_j[η]−D_j^∗)

is_i(D_i[η]−D_i^∗) (6.8) where D_j^∗ is the remaining density when η reaches η^∗, the N area density at saturation. It can be solved for all sites by defining an average cross-section

s_av =

is_i(D_i−D_i^∗)

i(D_i−D_i^∗) (6.9)

noting that

i

(D_i−D^∗_i) = η^∗−η (6.10)

Using Eqs. (6.9) and (6.10), Eq.(6.8) becomes

dD_j

dη = −sj(Dj −D^∗_j)

sav(η^∗−η) (6.11)

which can be solved exactly with the boundary condition Dj[0] =D_j⁰;

D_j[η] = (D⁰_j −D_j^∗)

η^∗−η η^∗

s_j/s_av

+D_j^∗ (6.12)

A particular solution of Eq.(6.8) arises when s_j =s_av, as Eq.(6.12) then yields a linear relationship between D_j and η. From the definition of the average cross-section, Eq.(6.9), sj =savcould occur either coincidentally, or if all cross-sections are equal, or if the nitrogen occupancy of a dominating binding site (D_j−D^∗_j η^∗) is studied.

When Dj corresponds to an atomic configuration that is electrically active, it can be interpreted as the density of levels at a given energy E_j, s_j becoming a passivation cross- section. Indeed, Eq.(6.12) is found to successfully model the evolution of theD_it at the ex- tracted levels considered in Figure6.7(a-e). The obtained normalized cross-sections (s_j/s_av) are shown in Figure 6.7(f). Their increasing values for levels deeper in the band-gap cor- respond to the earlier saturation of the D_it at these energies. It indicates that although the density of corresponding defects is small prior to nitridation when compared to the rest of the band-gap, they could be binding sites favored by N. Moreover, a distinction can be made between the levels within 0.3 eV of the conduction band edge and the ones deeper in the band-gap. Indeed, ones between 0.2 and 0.3 eV have similar passivation cross-sections.

This again suggests that they relate to a separate category of defects.

Note that if more than one nitrogen atom is required to remove a level, say αN, the D_it at E_j would correspond to D_j/α. Could this explain why there are apparently two orders of magnitude more N incorporated than there are defects to passivate? It is hard to conceive, because it would not only imply that there is a progressive redistribution of the energy levels, which is not observed here between 0.2 and 0.6 eV, but also that apparently

about 100 N atoms are needed to remove a single defect level. Instead, it is possible that the binding of nitrogen is not driven by defect passivation but by the formation of a SiON layer at the interface (η^∗ 1 monolayer). Nitrogen can indeed be incorporated in SiO₂, regardless of the interface state density. For example, it can bind in the bulk of the oxide following plasma or NH₃ nitridation.^17,18,19 In the case of NO-annealing, the fact that nitrogen resides at the interface is related with the cracking of NO molecules in that region and with the re-oxidation process, as explained earlier. Moreover, similar characteristics of the nitrogen uptake are observed at the SiO₂/Si interface which is very different from the SiO₂/SiC one.¹⁶

In fact, nitridation is known to generate D_it in the case of Si and to increase NBTI, indicating that N binding can have drawbacks.^9,10,11,12 This correlates with the theoretical calculations which predict that N can disturb a perfect oxide by inserting within a Si-O- Si bridge which yields an oxygen protrusion and a new defect level close to the valence band edge of 4H-SiC.^13,3,2 Therefore, the majority of the nitrogen incorporated at the SiO₂/SiC is probably not associated with defect passivation and could be detrimental to the interface. Actually, this excess nitrogen is thought to enhance hole trapping in the NO-annealed samples as discussed in a following section.