Nanometer-Scale Investigation of Schottky Contacts and Conduction Band Structure on 4H-, 6H- and 15R-SiC Using Ballistic Electron Emission Microscopy

Abstract

We have performed ballistic-electron emission microscopy (BEEM) of Pd and Pt Schottky contacts on 6Hand 4H-SiC and Pd Schottky contacts on 15R-SiC. Measured Schottky barrier heights (SBH’s) appear spatially uniform up to the fitting error due to noise (~0.03 and ~0.1 eV for 6Hand 4H-SiC, respectively). In 4H-SiC, we observed an additional conduction band minimum (CBM) ~0.14 eV above the lowest CBM, which is in good agreement with our band theoretical calculation. Preliminary results on Pd/15R-SiC indicate a higher CBM ~0.5 eV above the lowest CBM, and possibly another higher CBM ~0.3 eV above the lowest CBM. Also, in Pd/15RSiC, large variations in BEEM spectra at different locations were observed, suggesting an inhomogeneous interface. Additionally, we sometimes observed enhancement in ballistic transmittance over regions intentionally stressed by hot electron injection. We report ballistic-electron emission microscopy (BEEM) measurements of Pd and Pt Schottky contacts on 6Hand 4H-SiC, and Pd contacts on 15R-SiC. The BEEM technique is an extension of scanning tunneling microscopy (STM) and can be used to probe local electronic properties of buried metal/semiconductor interfaces with nanometer-scale spatial resolution and high energy resolution [1,2]. Figure 1 shows the schematic experimental setup with corresponding energy-level diagrams for BEEM. The STM tip is used to inject hot electrons into a thin metal film, with the hot-electron peak energy controlled by the applied tip bias VT . Provided the metal film is thin enough compared with the electron mean free path, a small fraction of these electrons can cross the metal film elastically and can enter the semiconductor conduction band if injected with sufficient energy. Hence, the local Schottky barrier height (SBH) [3] qVB can be directly determined by measuring BEEM Ic-VT curves (also called BEEM spectra) and evaluating the threshold voltage Vth for Fig. 1: (a) Physical setup of the BEEM experiment on SiC and (b) the corresponding energy level diagram (only the lowest CBM is shown). By measuring the dependence of Ic (flux of ballistic electrons from the STM tip into the SiC) on tunnel voltage VT (at constant tunnel current IT) we can probe the local Schottky barrier height and SiC band structure. A