Hot Spots In 4H Sic P + N Diodes Studied By The Optical Beam Induced Current Technique
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The physical properties make SiC one of the most promising candidates to supplant Si for high power semiconductor electronic devices. These properties include the high breakdown electric field (about 10 times higher than that of Si), the wide bandgap energy (ranging from 2.4 eV for the 3C polytype to 3.3 eV for the 2H polytype) and the high thermal conductivity (about three times that of Si). In power device applications SiC would permit devices with higher operating voltages and temperatures with dramatically reduced switching losses while at the same time requiring less area than a Si device with the same ratings. Progress in the growth of single crystal SiC wafers as well as CVD layers has made it possible to produce P+N diodes with a blocking capability of up to 4.5 kV [1]. To live up to these high expectations raised by the outstanding fundamental physical properties the SiC technology has to be mature enough to achieve the superior device performance, reliability and cost-effectiveness demanded by commercial applications. One of the main obstacles to commercialization for SiC has been the quality of the material itself that made it possible to produce only small-area high-voltage devices that could not handle the high currents required in high-power applications. The most well known and, for large-area high-voltage devices, most harmful material defect has been the micropipe a hollow screw dislocation with Burgers vectors > 2 [2]. Research efforts all over the world focused on reducing the micropipe density which subsequently decreased from several hundreds per cm 2 to about 30 per cm 2 on currently commercially available wafers with best 157
Mat. Res. Soc. Symp. Proc. Vol. 512 0 1998 Materials Research Society
reported wafers already passing the 1 per cm2 mark [3, 4]. Recently it was pointed out that the next performance-limiting defect on the road to a commercial SiC high-power devices could be the elementary screw dislocations with densities of several thousands per cm2 observed in 4H SiC wafers by X-ray topography [4, 5]. Methods like statistical descriptions e.g. device yield produce only a very crude picture of the device performance and its limitations by describing an ensemble behaviour. Studying devices on an individual level by analysing leakage current and breakdown behaviour gives an already more detailed insight but is still limited by averaging over a large portion of the active device area. The lack of information on whether the device is limited by localised phenomena or a uniform behaviour has been achieved is a major problem when it comes to a comparison between simulations and experimental results. OBIC has proven to be a valuable tool for collecting local information on power semiconductor devices in Si and SiC [6, 7, 8]. We have recently built an OBIC set-up especially designed for characterisation of SiC devices. Due to its current sensitivity the technique can be applied to detect failures in the device that manifest themselves as peaks or "hot spots" in the photocurrent distribution
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