Design, Processing and Characterization of Silicon Carbide Diodes

Abstract: Electronic power devices made of silicon carbide promisesuperior performance over today's silicon devices due toinherent material properties. As a result of the material'swide band gap of 3.2eV, high thermal conductivity, itsmechanical and chemical stability and a high critical electricfield, 4H-silicon carbide devices have the potential to be usedat elevated temperatures and in harsh environments. Shortercarrier lifetimes and a reduction in the necessary width of thelow-doped drift zone in silicon carbide devices compared totheir silicon counterparts result in faster switching speedsand lower switching losses and thus in much more efficientpower devices.High-voltage 4H-silicon carbide diodes have been fabricatedin a newly developed processing sequence, using standardsilicon process equipment. Epitaxial layers grown by chemicalvapor deposition (CVD) on commercial 4H-silicon carbidesubstrates were used as starting material for both mesa-etchedepitaxial and implanted p+n-n+ planar diodes, Schottky diodesand merged pn-Schottky (MPS) diodes, together with additionaltest structures. The device metallization was optimized to givea low contact resistivity on implanted and epitaxial layers anda sufficiently high Schottky barrier with a singlemetallization scheme. Different high-field termination designshave been tested and breakdown voltages of up to 4 kV onimplanted, field-ring terminated diodes were achieved,corresponding to 80% of the critical electric field. A 5kVepitaxial diode design with a forward voltage drop of 3.5V at acurrent density of 100Acm-2 equipped with an implanted junctiontermination extension (JTE) was also fabricated.A new measurement setup was designed and built with thecapability of measuring current-voltage and capacitance-voltagecharacteristics of semiconductor devices at reverse biases upto 10kV. Together with these electrical measurements, theresults of other characterization techniques were used toidentify performance limiting defects in the fabricated siliconcarbide diodes. Increased forward voltage drop of bipolardevices during on-state operation was studied and it was shownthat the stacking faults causing forward degradation arevisible in scanning electron microscopy. With the help ofsynchrotron white-beam X-ray diffraction topographs (SWBXT),electron beam induced current (EBIC) and electroluminescencemeasurements of silicon carbide diodes, the role of screwdislocations as a dominant source of device failure in the formof localized microplasma breakdown was identified. Screwdislocations with and without open core have been found tocause a 20-80% reduction in the critical electric field of4H-silicon carbide diodes, both for low-voltage (150V) andhigh-voltage (~5kV) designs. While micropipes have almost beeneliminated from commercial silicon carbide material,closed-core screw dislocations are still abundant withdensities in the order of 10000cm-2 in state-of-the-art siliconcarbide epitaxial layers.