Cylindrical monocrystals, with a diameter of 27mm and a length of 15 to 30mm, were prepared. No typical conversion multi-twin patterns were observed anywhere within the body of the crystal. Similar results were obtained for 3 successive growth runs. There was a metamorphosis of the seed crystal from a monocrystalline state, with numerous parallel twins at room temperature, to a polycrystalline state without twins. This was attributed to a 3C-2H solid-solid transition which occurred during seed heating and involved the maximum possible 4-fold wurtzite orientation. It was suggested that the polycrystallinity of the seed crystal effectively reduced twinning in the adjacent as-grown monocrystalline section. This was in good agreement with published results where twin-free growth had also been obtained in the polycrystalline first-to-freeze region. It had also been shown that the twin density was inversely proportional to the number of grain boundaries.

P.Rudolph, K.Umetsu, H.J.Koh, T.Fukuda: Journal of Crystal Growth, 1994, 143[3-4], 359-61