Comparative Analysis of the GaN Nonpolar Plane Morphology by Wet Treatment and Its Effect on Electrical Properties in Trench MOSFET

The morphological characteristics of the GaN nonpolar sidewalls with different crystal plane orientations were studied under various TMAH wet treatment conditions, and the effect of different morphological features on device carrier mobility was modeled and analyzed. After TMAH wet treatment, the morphology of the a-plane sidewall presents multiplied zigzag triangular prisms along the [0001] direction, which consist of two adjacent m-plane and c-plane on top. While along the [112̅0] direction, the m-plane sidewall is represented by thin, striped prisms with three m-plane and a c-plane on the side. The density and size of sidewall prisms were studied by varying the solution temperature and immersion period. The prism density decreases linearly as the solution temperature rises. With increased immersion time, both a-plane and m-plane sidewalls show smaller prism sizes. Vertical GaN trench MOSFET with nonpolar a- and m-plane sidewall channels were fabricated and characterized. By properly treated in TMAH solution, transistors with an a-plane sidewall conduction channel exhibit higher current density, from 241 to 423 A cm^-2@V_DS = 10 V, V_GS = 20 V, and higher mobility, from 2.9 to 2.0 cm² (V s)^-1, compared to those of m-plane sidewall devices. The temperature dependence on mobility is also discussed, and a modeling analysis for the difference in carrier mobility is then performed.