BEEM Reveals Electrical Properties Of Dislocations And Nanopipes (Research Review)
It has been shown that SiC power devices fabricated on micropipe-free SiC wafers exhibit a dramatic improvement in electrical properties compared with devices fabricated on wafers containing micropipe defects; see P G Neudeck and J A Powell in Electron Device Letters 1994, 15, p63. Micropipe-related defects cause pre-avalanche breakdown in SiC power devices by facilitating electrical discharge through the hollow core of nanopipes under high electric field operation. This mechanism contributes to large leakage currents. A team led by C V Reddy and V Narayanamurti at Harvard University have employed ballistic electron-emission microscopy (BEEM) to image dislocations and nanopipes in SiC that have so far proved difficult to detect with either SEM and AFM techniques (Journal of Applied Physics 2001, 89, p5797). Topographic AFM images taken from 300 300 nm areas on two different SiC samples reveal different kinds of defects (see ). Image (a) shows a defect with an associated deep trench that is commonly observed all over the SiC matrix. A nanopipe-related defect is shown in the image labeled (b), and the area is enlarged in the inset. BEEM spectroscopy performed on the defect and the adjacent area revealed that large electric currents are concentrated around the nanopipe/dislocation sites. The authors note that the nanopipe may not actually have a hollow core to facilitate the electrical discharge. However, they conclude that since the defect is highly conducting, it is responsible for the large leakage currents observed in SiC power devices.