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 and
. The associated charge transfer reactions
do not
involve any intermediate states and are therefore simple NMP processes. It is
remarked here that the transitions
require the energy minima
and
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
and
are energetically not far separated. In a TDDS measurement, the
investigated devices are stressed at a high
so that the defects are forced from the
state
into the state
or
. During this step, the defects undergo the
transition
into the state
or even further into
. The other
direct pathway
into the state
or
is assumed to go over a large
barrier
. Therefore, the transition
proceeds on much larger
timescales compared to
and can be neglected. After stressing, the
recovery traces are monitored at low
or
, respectively, at which the
energy minima of the states
and
coincide and noise is produced.
However, the state
is thermodynamically preferred due to its energetically
lower position compared to the states
and
. When the defect returns
to its initial state
, the RTN signal disappears with a time constant of
. The corresponding transition could be either
or
with a time constant of
or
, respectively (cf. Fig. 7.11). The
termination of the noise signal after a time period of
is determined by the
minimum of these time constants. Consider that the NMP barriers
and
must not be too large since otherwise trapping events will occur
too fast and are therefore not detected using a conventional measurement
equipment.
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 and
. The recurrent pauses of the noise signal (see Fig. 7.11)
originate from transitions into the metastable state
, which is electrically
indistinguishable from the state
. 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
occurs on larger time scales than the return to the state
through the
transition
. The slow capture time constant
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
. The slow emission time constant
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 and
where they produce an RTN
signal. As in aRTN, they undergo a transition to the metastable state
thereby
stopping to produce a noise signal. However, this special sort of defects is
characterized by a slow emission time constant
, which is much larger than the
typical measurement time of TDDS. As a consequence, the next transition back to
the state
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.