A discussion of the mechanisms involved in the observed anomalous
Vth shifts is now presented. While these have been reported for TFTs employing high-κ gate dielectrics (such as Ta
2O
5 [
62,
63,
64], ZrO
2 [
9] and HfO
2 [
65]), other reports often show normal shift directions. High-κ dielectrics are known to be prone to have high defect density (probably playing a role in the anomalous shift) and it should be expected that different processing methodologies can result in distinct material qualities, leading to different device behaviors. The mechanisms that are normally used to explain anomalous
Vth shifts [
34] are: charge (de)trapping from the gate dielectric [
62,
65] ionic migration within the dielectric [
63] (understandable as a slow polarization of the dielectric material [
64]) and defect creation [
66]. Regarding defect creation, it can lead to an increase in the carrier concentration, resulting in the decrease of the
Vth, and it was shown before for poor quality semiconductors [
66]. In this case, this can be disregarded as no change in the transfer curves’ SS was observed, demonstrating the stability of the semiconductor (even considering the low thermal budget employed here). Furthermore, it is expected that the involved mechanism is dielectric related as this anomalous shift is not observed in our IGZO TFTs when employing other dielectrics. Regarding the charge trapping mechanisms, a positive net charge change is required for a negative
Vth shift. Hole trapping at the dielectric/semiconductor interface can be disregarded as the voltage polarity is opposite to what would be required, and wide band gap n-type oxides require optical excitation for holes to be generated in the first place. Then, electron detrapping from the dielectric to the gate must be considered (e.g., from negatively charged oxygen interstitials:
IO– or
IO2– [
65]). However, detrapping at this interface cannot change the electron concentration at the semiconductor, and thus cannot explain the anomalous shift if charge migration is not assumed [
34,
64]. To investigate the mechanism, the time dependency of the threshold voltage shift during the gate bias stress was studied and a power-law dependence
was found, as shown in
Figure 8a for the T
39S
61 and T
75S
25 compositions. While exponential dependencies are seen when charge (de)trapping is the relevant process, power dependencies are often related to reaction–diffusion mechanisms. For the compositions presented in
Figure 8a, the exponent
n changed from ≈0.5 to ≈0.6 after approximately 1 min. For the intermediate T
xS
100 − x compositions, the exponent changed from ≈0.33 to ≈0.5 after approximately 1–5 min (
Figure S4a,b). Similar dependencies were noted for the multilayered stacks (
Figure S4c), further suggesting that the mechanism is not dominated by either of the interfaces. Power law time dependency of
is known for the diffusion of H species in MOSFETs during negative bias temperature stress, where exponents of 1/3 and 1/2 are associated to trap-generation and diffusion of the charged species H
+ and H
2+, respectively [
67]. Aleksandrov et al. also associated
n = 1 to first-order chemical-reaction kinetics and
n closer to 0.5 to diffusion and drift kinetics of H
+ ions [
68]. While further investigation is needed to understand the involved species in our devices, it is interesting to notice that Ta
2O
5 dielectrics are known for their ionic conductivity, with H
+ and V
O migration in its bulk being known [
69] and partly responsible for their application as resistive switching layers [
70]. Regarding the
Vth recovery (for
VGS = 0 V in
Figure 6a), an exponential dependency with time is apparent: while recovering quickly initially, it seems that, at least in the presented time frame,
Vth is slowly tending to values significantly larger than the initial ones. Charge migration is often ruled out when fast recoveries are seen (as without driving force the species cannot quickly return to their initial positions), but the relatively small recovery seen across all compositions seems to imply that migration has to be considered. For further evaluation, the full recovery for the dielectric composition presenting the higher
Vth shift (T
39S
61) was studied and is shown in
Figure 8b. The recovery took place in a much larger time frame (>1 month) than that of the stress process (1 h) and followed an exponential behavior with a time constant
s which suggests a different recovery process than that seen up to 1 h immediately after the stress. As stated before, applying a negative
VGS resulted in a much faster recovery of
Vth (
Figure S3a), of just a few minutes. This is in agreement with negative bias stress measurements made in as-fabricated devices (
Figure S3b), where a fast increase of
Vth is observed for the first minutes of stress, with the reverse ∆
Vth direction in relation to the applied gate bias. Afterwards,
Vth is fairly stable along the stress time, given the absence of photoinduced holes (measurements under dark conditions) to sustain the commonly observed negative
Vth shifts under negative bias illumination stress (NBIS) [
71].
These results point to the anomalous shift being related to the migration of charge within the dielectric, but the nature of the migrating species or type of defects involved is not understood yet.
While Ta
2O
5 is known to have a considerable defect density, tending to form suboxides, the
Vth instability was found to be more pronounced for SiO
2-richer T
xS
100 − x compositions, implying these are more defective. Oxygen displacement in the network due to Si’s higher oxygen affinity than Ta is a possible mechanism. In fact, SiO
2/high-κ dielectrics interfaces are known to form defects, such as dipoles (V
O-I
O pairs) caused by oxygen displacement (due to deferring oxygen areal densities in these materials [
72,
73], or oxygen vacancies [
74,
75]. Note, that the oxygen vacancy formation energy in Ta
2O
5 is low compared to other high-k dielectrics [
76]. Detrapping from either these defects to the gate, their migration inside the dielectric layer, activated by gate bias, or both, can result in the observed anomalous
Vth shifts. Another possible mechanism is proton migration though the bulk of Ta
2O
5 [
77,
78,
79], but in this case it would be expected that the anomalous
Vth shift would have to increase with increased Ta
2O
5 content, contrary to our findings. Further investigation should be conducted to determine the nature of the charged species involved in the anomalous
Vth shift.