Study of Surface Defects in 4H-SiC Schottky Diodes Using a Scanning Kelvin Probe

In the present paper we attempt to study and explain the increased leakage currents in Schottky diodes with an integrated p-n-structure. By a scanning Kelvin probe method (vibrating capacitor) were obtained the local variations of surface contact potential difference (CPD) for the chips with large and small leakage currents. It is shown that samples with higher leakage currents have smaller surface potential barrier. The SEM investigations revealed that a critical role in increasing leakage currents play the dislocations penetrating from the substrate into the epitaxial layer.

Manipulation of Surface Charge on GaN

We have characterized the surface charge on a variety of GaN samples using two surface potential techniques, conventional Kelvin probe and scanning Kelvin probe microscope (SKPM). Kelvin probe was primarily used to measure the change in surface potential under UV illumination, otherwise known as the surface photovoltage (SPV). Due to band bending near the semiconductor surface of about 1 eV in dark conditions, the SPV signal for n-type GaN typically reaches 0.5 to 0.6 eV upon switching on UV light. This value can slowly decrease by up to 0.3 eV during UV illumination in air ambient for 2-3 hours. We report that samples with many hours of ambient UV exposure do not show this slow decrease during SPV measurements, consistent with the UV-induced growth of a thicker surface oxide that limits charge transfer. In addition to prolonged UV exposure, the surface contact potential was also manipulated by local charge injection. In this procedure, the surface is charged using a metallized atomic force microscope tip which is scanned in contact with the sample. Subsequent SKPM measurements indicate an increase or decrease in the surface contact potential for the charged region, depending on the applied voltage polarity. Measurements of the discharge behavior in dark for these regions show a logarithmic time behavior, similar to the decay behavior during our observations of SPV transients after switching off the light. As expected, illumination of the surface increases the discharge rate and restores the charged area to its original state.