February_EDFA_Digital

edfas.org 23 ELECTRONIC DEV ICE FA I LURE ANALYSIS | VOLUME 24 NO . 1 SiC specimen both before and after applying a voltage pulse. Note that these data were generated using the same experimental parameters and voltage pulse shape detailed earlier in the text. Prior to the voltage pulse, at t = -20 µs, a lack of significant contrast is evident, although the subsequent image corresponding to t = -4 µs displays a lateral Δ C ts distribution that is inhomogeneous over a range of a few hundred nanometers. The application of a suitably high forward voltage to the specimen generated a peak capacitance that was very close to the capacitance of the oxide itself, regardless of the local D it . Therefore, the lack of homogeneous contrast observed at this timing can likely be attributed to changes in the initial capacitance, C ts . After the application of the voltage pulse, the contrast is seen to be different at t = 0.4 µs. Thus, the inhomoge- neity appears to be primarily a result of variations in the capacitance transient amplitude. These variations are, in turn, ameasure of the interface state distribution. A D it map summarizing the lateral dis- tribution of this variable is provided in Fig. 4c. Using this technique, the D it energy distribution couldbe determined for every individual pixel on the scan matrix, and the average D it values acquired over energy depths from 0.31 to 0.38 eV are plotted. The D it map shows maximum and minimum values of approximately 2.3×10 13 and 1.9×10 13 cm -2 eV -1 , respectively, which confirms the nonuniform nature of the SiO 2 /4H-SiC interface. SIMULTANEOUS LOCAL CAPACITANCE- VOLTAGE ANALYSIS, VOLTAGE DERIVATIVE PROFILING, AND DEEP LEVEL TRANSIENT SPECTROSCOPY USING TR-SNDM [18] The tr-SNDM technique allows directmonitoring of the sensor (SNDM probe) output over time via a high-speed digitizer and permits the capacitance response to be demodulated to give a desired applied voltage at each data point based on offline post- processing. The group therefore extended this technique to allow local CV, [19] d C /d V - V and local DLTS [9,13] analyses to be performed simultaneously. Figure 5 presents diagrams showing the procedure, which involved multiplexing in both the time and frequency domains. Local DLTS was performed by applying an initial rectangular voltage pulse, after which a triangular voltage pulse was employed together with a high-frequency sinusoidal voltage that permitted local CV and d C /d V - V data to be acquired at each point of measurement. Numerical demodulation was carried out offline employing a Hilbert transform in conjunction with suitable numerical zero-phase filtering. In this manner, capacitance responses were monitored that could be used to estimate the local D it values and also to obtain local CV and d C /d V - V data. Both the low ac voltage and the triangular pulse generated capacitance responses, and low- and band-pass filters were used to separate these responses in the frequency domain. Following the elimination of high frequency components by low- pass filtering, local CV characteristicswere acquired using a process previously suggested in the litera- ture. [19] Because the capacitance response obtained over time was equivalent to voltage-dependent data based on the linear correlation between time and voltage for a specific triangular pulse, both forward and backward local CV data could be easily Fig. 5 Plots for one cycle of an applied voltage pulse used for simul- taneous local DLTS, CV and d C /d V - V measurements by tr-SNDM, and corresponding demodulated capacitance responses.