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.