February_EDFA_Digital

edfas.org ELECTRONIC DEV ICE FA I LURE ANALYSIS | VOLUME 24 NO . 1 24 generated from the increasing and decreasing regions of the pulse, respectively. The local d C /d V - V characteristics could be acquired at the same time based on band-pass filtering of capacitance signals. The resulting amplitudes were correlated with d C /d V and were also modulated depending on the voltage swept by the slower triangular pulse. The envelopes of the filtered responses therefore provide simultaneous d C /d V - V characteristics. The tr-SNDMtechniquedescribedabovewas employed to simultaneously perform local CV, d C /d V - V and local DLTS analyses of n-type 4H-SiC(0001) wafers that had been thermally oxidized. Note that these wafers were not subjected to post oxidation annealing (POA) and had a dopant concentration of 10 16 cm -3 alongwith a 10 nmthick thermal oxide layer. The CV, d C /d V - V and local DLTS data were acquired by subjecting the specimens to 160 voltage pulseswith respect to the grounded tip. The signal at each point was averaged to obtain a higher S/N ratio. The local DLTS data were generated with a negative rectangular pulse (0 to -5 V) having adurationof 5μs that promoted the accumulation of electrons at the sample interface, and D it imageswere producedbased on the capacitance respons- es to these pulses, employing a Fourier DLTS process. [20] The D it was determined to have an energy depth situated 0.35 eV from the conduction band edge. The rectangular pulse was applied and, 30 μs later, a triangular pulse with a 100 μs period and 1.5 cycle was initiated to perform the local CV and d C /d V - V analyses. This process employed an amplitude of 40 V pp to allow the complete analysis of both depletion and accumulation regions, while a sinusoidal voltage at a frequency of 1 MHz was superimposed on the triangular pulse to acquire d C /d V - V data at the same time. The amplitude of the sinusoidal voltage was set at 4 V pp . A cut-off frequency of 200 kHz was employed in conjunc- tion with the low-pass filter used to process the local CV data, while the d C /d V - V signal was filtered topass frequen- cies from 0.8 to 1.2 MHz. Typical results obtained from a SiO 2 /SiC specimen without POA are shown in Fig. 6. The expected n-type local CV data are evident in Fig. 6a and indicate that highly positive and negative voltages caused depletion and accumulation at the interface, respectively. The d C /d V - V characteristics for the same sample are shown in Fig. 6b and exhibit significant hysteresis between the backward and forward signals. This hysteresis is also apparent in the local CV characteristics and suggests a large value of D it . Prior work observed a pronounced exponential decay in the case of the local DLTS response, as is also evident in Fig. 6c. [13] The hysteresis in the local CV data as well as the voltage shift and the spatial inhomogeneity of the data can be examined by defining the parameter V peak as equal to the voltage at which the d C /d V signal is minimized. Subsequently, Δ V peak is defined as the difference between Fig. 6 Typical local (a) CV, (b) d C /d V - V and (c) DLTS characteristics of a SiO 2 /SiC interface without POA. Fig. 7 Reconstructed (a) V peak (backward), (b) V peak (forward), (c) Δ V peak , and (d) D it images acquired from a SiO 2 /SiC interface without POA. (a) (b) (c) (a) (b) (c) (d)