Influence of Different Surface Morphologies on the Performance of High-Voltage, Low-Resistance Diamond Schottky Diodes
Vertical diamond Schottky diodes with blocking voltages VBD > 2.4 kV and ON-resistances RON <; 400 mOcm2 were fabricated on homoepitaxially grown diamond layers with different surface morphologies. The morphology (smooth as-grown, hillock-rich, polished) influences the Schottky barrier, the carrier transport properties, and ultimately the device performance. The smooth as-grown sample exhibited a low reverse current density JRev <; 10-4 A/cm2 for reverse voltages up to 2.2 kV. The hillock-rich sample blocked similar voltages with a slight increase in the reverse current density (JRev <; 10-3 A/cm2). The calculated 1-D breakdown field, however, was reduced by 30%, indicating a field enhancement induced by the inhomogeneous surface. The polished sample demonstrated a similar breakdown voltage and reverse current density as the smooth as-grown sample, suggestingthat a polished surface can be suitable for device fabrication. However, statistical analysis of several diodes of each sample showed the importance of the substrate quality: a high density of defects both reduces the feasible device area and increases the reverse current density. In forward direction, the hillock-rich sample exhibited a secondary Schottky barrier, which could be fit with a modified thermionic emission (TEM) model employing the Lambert W-function. Both polished and smooth samples showed nearly ideal TEM with ideality factors 1.08 and 1.03, respectively. Compared with the literature, all three diodes exhibit an improved Baliga figure of merit for diamond Schottky diodes with VBD > 2 kV.
Nebel, Christoph E.