OBIC Measurements on 6H-SiC Schottky Diodes
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ABSTRACT The OBIC (Qptical Beam Induced Current) technique is a powerful method to investigate the electric field distribution of p-n junctions in SiC. In a previous work we found strong indications for the presence of a high density of negative surface charge in n-type SiC. In order to study samples of both conductivity types under similar conditions we prepared Schottky contacts on ntype and p-type 6H-SiC CVD epitaxial layers. OBIC measurements show an extension of the depletion region of several hundreds of microns from the edge of the contact on n- and p-type samples, thus interconnecting diodes on an area up to several mm 2. Our results imply that there is no fixed surface charge but a high density of both acceptor- and donorlike surface states leading to a dependence of the net surface charge on the Fermi energy, in which case the sign of the surface charge reverses from negative on ntype material to positive on p-type 6H-SiC.
INTRODUCTION Recent progress in the growth of single crystal SiC wafers as well as CVD layers has made it possible to produce devices with a blocking capability of up to 4.5 kV [ 1]. The ability of supporting high electric fields is limited by the onset of avalanche breakdown, which can occur both within the interior regions of the device and at the edges. The resulting high electric surface fields in SiC power devices are one of the major problems to be solved to further improve device performance. A careful design of the edge termination of power devices is necessary to raise the breakdown voltage to the upper bound provided by the analysis of the semi-infinite junction. Another factor that comes into play is the passivation of the surface of discrete power devices which is in fact closely related to the problem of edge termination. The passivation induced surface charges strongly influence the electric field in the device regions lying close to the surface and a proper choice of the passivant can lower the surface electric field considerably. The higher surface fields of SiC power devices make surface passivation even more critical in comparison to Si power devices. A previous study [2] on SiC p+-n mesa diodes showed that the experimental breakdown voltage exceeded the value expected from numerical studies regardless of the specific edge termination design (mesa diodes with and without multiple floating field rings). OBIC measurements revealed a substantial expansion of the depletion layer to a value ten times as large as that provided by the numerical analysis, thus decreasing the surface electric field and in consequence increasing the maximum achievable breakdown voltage. These results implied the presence of a negative surface charge with a concentration of > 3x1012 cm-2 . Surface charge variations will result in irregularly shaped depletion layers which itself will cause premature breakdown. Locally reduced surface charge concentrations may result in increased electric surface fields in this spot. On the opposite high surface charge concentrations may lead to the incorporation
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