Defect Dominant Junction Characteristics of 4H-SiC p + /n Diodes
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and reverse current-voltage-temperature (I-V-T), capacitance-voltage-temperature (C-V-T), deep level transient spectroscopy (DLTS), electroluminescence (EL), and reverse breakdown measurements were performed to correlate junction characteristics with defects in the material. EXPERIMENTAL The SiC diodes used in this study were mesa isolated structures, off-axis MOCVD grown on the (0001) surface of n-conductivity, 4H polytype wafers. Asymmetric abrupt pt/n junctions were obtained by doping Al and N in p- and n-type regions to -1x1019 and 5x1015 cm"3, respectively. Al and Ni standard ohmic contact metalliztions were utilized on the p-and n-type surfaces, respectively. Temperature dependent I-V measurements were performed in a closed cycle He refrigerated cryostat, which includes a heated stage enabling measurements to temperatures in excess of 750 K during a single scan. Fully automated computer controlled instrumentation and data acquisition capabilities allow the performance of a wide range of detailed electrical characterizations including C-V and several variations of DLTS. RESULTS AND DISCUSSION Forward biased current conduction characteristics of the 4H-SiC diodes were first compared to the classical Sah-Noyce-Schockley 5 (SNS) recombination-generation theory. Typical forward biased I-V data taken on the 4H-SiC diodes are illustrated in Fig. 1 for both a well-behaved and a leaky diode with a significant shunting characteristic. Solid lines in the 6 figure are numerical fits to the classical SNS equation , I
= JdYexp(q VA,1kT)+
1rec
exp(q V
where I is the total forward bias dark current and V is the applied junction voltage. The first and second terms account for diffusion and recombination mechanisms, and the third term is included to account for current 6 conduction due to tunneling6, with B an empirical fitting parameter. As can be seen from the figure, the 4H-SiC diodes exhibit bias characteristics, classical forward including the dominance of recombination over ideal diffusion conduction in the wellbehaved devices as expected for large bandgap (EG) materials. An average value for lrec/Idiff obtained from several ideal diodes was in the range of 1025. Typical values of the diode ideality factors are illustrated next to their respective conduction regions in the figure. The A2 factor corresponding to recombination in the space charge region (SCR) is indicative of recombination via traps near the center of the bandgap.
(1)
1, jo2kT)+ exp(BV), 10-1 10-2 10-
3
10-4 10-1 C
w
1
Q
10-6 10 -7
108
1010
'behaved"
Lwel
0.0 0.5
1.0 1.5 2.0 2.5 Voltage (V)
3.0 3.5
Figure 1. Forward I-V characteristics for near ideal (o) and strongly shunted ( ) diodes. 58
Although it is difficult to obtain the diffusion ideality factor, A1 , due to the dominance of
10-1
recombination currents at lower voltages and
102
due to series resistance effects at larger forward biases, it was observed to be consistently greater than the ideal Schockley value of one. This dominant recombination processes along with the lar
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