Nitride semiconductor structure, nitride semiconductor device, and method for fabricating the device

A nitride semiconductor structure comprising:

• a single-crystalline Group III nitride semiconductor portion containing a Group III element and having a predetermined crystallographic plane, where the Group III element is an element selected from the group 13 elements of the periodic table;
• and
• a single-crystalline Group II-IV nitride semiconductor portion provided on the predetermined crystallographic plane of the Group III nitride semiconductor portion, the Group II-IV nitride semiconductor portion containing a Group II element and a Group IV element, the Group II-IV nitride semiconductor portion forming a heterojunction with the Group III nitride semiconductor portion, where the Group II element is an element selected from either the group 2 elements of the periodic table or the group 12 elements of the periodic table and the Group IV element is an element selected from the group 14 elements of the periodic table,
• the predetermined crystallographic plane is a crystallographic plane other than a (0001) plane.