  Intrinsic catalytic process by capillary condensation of Ga-atoms into
nanotrenches, formed among impinging islands during the wurzite-GaN thin film
deposition, is shown to be an effective path to growing GaN nanorods without
metal catalysts. The nano-capillary brings within it a huge imbalance in
equilibrium partial pressure of Ga relative to the growth ambient. GaN nanorods
thus always grow out of a holding nanotrench and conform to the boundaries of
surrounding islands. The nanorods are epitaxially orientated with <0001>GaN //
<111>Si and <2110>GaN // <110>Si similar to the matrix. Concaved geometry is
essential and is a condition that limits the axial dimension of the nanorods
protruding above the base (matrix) material region. Revelation of the growth
mechanism in the current context suggests that fabrication of nano quantum
structures with controlled patterns is enabling for any attainable dimensions
