7.5 Explanation for Noise in TDDS Measurements

So far it has been shown that the eNMP model accounts for all features seen in the time constant plots for the ‘normal’ as well as the ‘anomalous’ defects. Beyond that, the model can also give an explanation for tRTN observed in TDDS (see Section 1.3.4). The generated noise stems from defects switching forth and back between states 2  and 1′ . The associated charge transfer reactions T2↔1 ′ do not involve any intermediate states and are therefore simple NMP processes. It is remarked here that the transitions T2↔1 ′ require the energy minima 2  and 1′ in the configuration coordinate diagram to be on approximately the same level at the relaxation voltage. This is only the case for a group of defects whose energy minima 1  and 1′ are energetically not far separated. In a TDDS measurement, the investigated devices are stressed at a high VG  so that the defects are forced from the state 1  into the state 2  or 1′ . During this step, the defects undergo the transition T1→2′→2  into the state 2  or even further into 1′ . The other direct pathway T1→1′ into the state 1′ or 2  is assumed to go over a large barrier ε11′ . Therefore, the transition T1→1 ′ proceeds on much larger timescales compared to T1→2′→2  and can be neglected. After stressing, the recovery traces are monitored at low VG  or Fox  , respectively, at which the energy minima of the states 2  and 1′ coincide and noise is produced. However, the state 1  is thermodynamically preferred due to its energetically lower position compared to the states 2  and 1′ . When the defect returns to its initial state 1  , the RTN signal disappears with a time constant of τsem  . The corresponding transition could be either T2→2′→1  or T1′→1  with a time constant of  ′
τe2m  or   ′
τ1e,min  , respectively (cf. Fig. 7.11). The termination of the noise signal after a time period of τsem  is determined by the minimum of these time constants. Consider that the NMP barriers ε21′ and ε1′2  must not be too large since otherwise trapping events will occur too fast and are therefore not detected using a conventional measurement equipment.


Figure 7.11: Top Left: The hole occupancy during tRTN. At t = 0  the stress voltage has been removed and the defect is in its positive state 2  . After a time τsem  the defect ceases to produce noise. Bottom Left: Configuration coordinate diagram for a tRTN defect. The thick arrow indicates the fast switches between the states 2  and 1′ related to the occurrence of noise. The possibilities to escape from these states are shown by the thin arrows. Top Right: Hole occupancy during aRTN. Bottom Right: Configuration coordinate diagram for an aRTN defect. Since this defect is a hole trap, the red solid and the blue dashed line correspond to the positive and neutral charge state, respectively. The double-sided thick arrow is associated with aRTN while the thin one represents the transitions into and out of the metastable state 2′ .

Interestingly, there also exists a sort of defects which repeatedly produce noise for a some time (see Section 1.3.4). This kind of noise has been referred to as aRTN and will be discussed for hole traps in the following. Just as in the case of tRTN, the noise signal is generated by charge transfer reactions between the states 2  and 1′ . The recurrent pauses of the noise signal (see Fig. 7.11) originate from transitions into the metastable state 2′ , which is electrically indistinguishable from the state 2  . These interruptions correspond to the time during which the defect dwells in this state and no charge transfer reaction can take place. Thereby it has been presumed that the NMP transition T2′→1  occurs on larger time scales than the return to the state 2  through the transition T2′→2  . The slow capture time constant τscap  in Fig. 7.11 defines the mean time interval during which noise is observed. Its value is given by the inverse of the transition rate 1∕r22′ . The slow emission time constant τsem = 1∕r2′2  corresponds to the mean time interval until the next noise period starts.

One should keep in mind that defects showing an aRTN behavior can also be responsible for tRTN seen in TDDS measurements. During TDDS stress, this sort of defects are forced into one of the states 2  and 1′ where they produce an RTN signal. As in aRTN, they undergo a transition to the metastable state 2′ thereby stopping to produce a noise signal. However, this special sort of defects is characterized by a slow emission time constant τsem  , which is much larger than the typical measurement time of TDDS. As a consequence, the next transition back to the state 2  and the subsequent noise period are shifted out of the experimental time window of TDDS and will not be recorded during the measurement run. According to this explanation, tRTN can also be explained as a stimulated variant of aRTN.

In summary, the eNMP can account for the features from the time constant plots and is consistent with the observation of tRTN as well as aRTN. This fact is presented here since it is regarded as an additional support for the validity of this model.