  We present a study of the polarization properties of emission lines from
single InGaN/GaN quantum dots (QDs). The QDs, formed by spinodal decomposition
within ultra-thin InGaN quantum wells, are investigated using single-QD
cathodoluminescence (CL). The emission lines exhibit a systematic linear
polarization in the orthogonal crystal directions [1 1 -2 0] and [-1 1 0 0]--a
symmetry that is non-native to hexagonal crystals.
  Eight-band k.p calculations reveal a mechanism that can explain the observed
polarizations: The character of the hole(s) in an excitonic complex determines
the polarization direction of the respective emission if the QD is slightly
elongated. Transitions involving A-band holes are polarized parallel to the
elongation; transitions involving B-type holes are polarized in the orthogonal
direction. The energetic separation of both hole states is smaller than 10 meV.
The mechanism leading to the linear polarizations is not restricted to InGaN
QDs, but should occur in other wurtzite-nitride QDs and in materials with
similar valence band structure.
