A sapphire substrate 1 is etched so that each trench has a width of 10
m
and a depth of 10 m were formed at 10 m of intervals in a stripe
pattern. Next, an AlN buffer layer 2 having a thickness of approximately
40 nm is formed mainly on the upper surface and the bottom surface of the trenches
of the substrate 1. Then a GaN layer 3 is formed through vertical
and lateral epitaxial growth. At this time, lateral epitaxial growth of the buffer
layer 21, which was mainly formed on the upper surface of the trenches,
filled the trenches and thus establishing a flat top surface. The portions of the
GaN layer 3 formed above the top surfaces of the mesas having a depth of
10 m exhibited significant suppression of threading dislocation in contrast
to the portions formed above the bottoms of the trenches.