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edfas.org ELECTRONIC DEV ICE FA I LURE ANALYSIS | VOLUME 24 NO . 1 22 Fourier components higher than seventh order could not be acquired because of limitations inherent to the experi- mental setup. For this reason, the reconstructedwaveform was not suitably accurate to allow an analysis of those components undergoing rapid decay that can provide data concerning trap states at relatively shallow levels. The present study evaluated a modification of local DLTS analysis based on tr-SNDM. This modification allowed all the Fourier components of the capacitance transients to be obtained within the bandwidth of the experimental system. Thus, the transient waveformcould be analyzed in greater detail. During examination of a SiO 2 /4H-SiC(n-type) sample, the application of a nega- tive voltage pulse to the semiconductor layer was found to increase Δ C ts as electrons accumulated at the interface and the depletion layer width decreased. Subsequent to these effects, accumulated electrons were captured by interface states below the Fermi level. Electrons trapped in those states above the Fermi level were released after stopping the voltage pulse. The time constants for these electrons were determined by the energy depth of each state, producing a slow increase inΔ C ts . This change inΔ C ts provided data concerning the trap states at the SiO 2 /SiC interface. Assessing the transient component decay time andamplitudeallowed investigations of theenergydepths and densities of interface states, respectively. The Δ C ts values as a function of time resulting from the application of a negative pulse having an amplitude of 10 V (+2 to 8 V) and a width of 10 µs are plotted in Fig. 4a. This waveform represents the standard DLTS response obtained from an n-typeMOS capacitor. After cessation of the voltage pulse, an intense transient component exhibiting exponential decay together with an initial ampli- tude of about 6 aF appears. This phenomenon is ascribed to electrons released from interface states lying above the Fermi level. The decay component provides information concerning the density of interface states, D it , and as well the energy depth, E it . On this basis, the D it energy distribution couldbe determined by applying a standard DLTS analysis to the transient waveform. The transient waveforms acquired at all points comprising the scan matrix could be reconstructed to allow visualization of changes in the nanoscale capacitance over time. Figure 4b presents such Δ C ts images acquired from a SiO 2 / Fig. 4 (a) Δ C ts as a function of time when applying a voltage pulse with a width of 10 µs to a SiO 2 /SiC sample. (b) Reconstructed Δ C ts images obtained when applying a voltage pulse with a width of 10 µs at t = -10 µs. Note that each image is shown with different color scales to emphasize inhomogeneous contrast. From left to right the images correspond to t = -20, -4, and -0.4 µs, respectively, as also indicated in (a). (c) D it map generated by analyzing a series of capacitance transients. These D it values are averages corresponding to energy depths over the range of 0.31 – 0.38 eV. (a) (b) (c) Fig. 3 Schematic diagram showing the apparatus and basic principles of tr-SNDM. [13] Adapted with permission from Fig. 1a, Appl. Phys. Lett., 111, 163103, 2017. Copyright 2017, American Institute of Physics. SIMULTANEOUS LOCAL CAPACITANCE-VOLTAGE PROFILING (continued from page 19)