"In the III-nitride semiconductor device and the epitaxial substrate according to the foregoing aspects, the value of the second angle ALPHA2 is in the range of not less than -0.30 degrees and not more than +0.30 degrees. In the epitaxial semiconductor region provided on the underlying semipolar surface, the above configuration suppresses the pit creation and the pit expansion that are caused by slight variation in the inclination direction of the c-axis with respect to the foregoing crystal axis. Hence, the semipolar epitaxial film of the epitaxial semiconductor region is provided with an excellent surface morphology.
"Still another aspect of the present invention is a method for fabricating a III-nitride semiconductor device. This method comprises: (a) preparing a substrate comprising a III-nitride semiconductor and having a primary surface which along a first reference plane perpendicular to a reference axis inclined at a predetermined angle ALPHA with respect to a c-axis of the III-nitride semiconductor; and (b) growing an epitaxial semiconductor region, which comprises plural GaN-based semiconductor layers, on the primary surface of the substrate. The epitaxial semiconductor region comprises the GaN-based semiconductor layers. The reference axis is inclined at a first angle ALPHA1 in a range of not less than 10 degrees and less than 80 degrees with respect to the c-axis of the III-nitride semiconductor toward a first crystal axis; the first crystal axis is either one of m-axis and a-axis of the III-nitride semiconductor. The reference axis is inclined at a second angle ALPHA2 in a range of not less than -0.30 degrees and not more than +0.30 degrees with respect to the c-axis of the III-nitride semiconductor toward a second crystal axis, and the second crystal axis is the other of the m-axis and a-axis. The predetermined angle, the first angle, and the second angle have a relation of ALPHA=(ALPHA1.sup.2+ALPHA2.sup.2).sup.1/2. A morphology of an outermost surface of the epitaxial semiconductor region includes a plurality of pits, and a pit density of the pits is not more than 5.times.10.sup.4 cm.sup.-2.
"According to this aspect, the value of the second angle ALPHA2 is in the range of not less than -0.30 degrees and not more than +0.30 degrees. In the growth of the epitaxial semiconductor region on the underlying semipolar plane, this suppresses the pit creation and the pit expansion that are caused by slight variation in the inclination direction of the c-axis with respect to the aforementioned crystal axis. Hence, an excellent surface morphology is provided to the semipolar epitaxial film in fabrication of the III-nitride semiconductor device.
"In the invention according to the foregoing aspects, at an opening of each of the pits, a first width of the opening in a first direction of a line of intersection of the pit and a second reference plane, which is defined by the c-axis of the III-nitride semiconductor and the first crystal axis, is smaller than a second width of the opening in a second direction perpendicular to the first direction. According to this invention, the shape of the pits in the semipolar surface is different from that of pits in a c-plane. The pits in the semipolar surface of the epitaxial film have an asymmetric shape, which has a horizontally long or vertically long shape with a large aspect ratio. Hence, the pits in the semipolar surface occupy a large area in the epitaxial film surface, and thus the reduction in the pit density is effective in enlargement of an excellent morphology surface.
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