  Crystals grow by gathering and bonding of atoms to form an ordered structure.
Typically, the growth is equally probable in all crystalline directions, but
sometimes, it is not, as is the case of nanowire growth. Nanowire growth is
explained, in most cases, by the presence of liquid metal droplets that mediate
between an incoming flux of atoms and a substrate or an existing crystal
nucleus, while defining the lateral dimension. Here, we report and explain a
previously unknown mode of non-isotropic crystal growth observed in two
wurtzite semiconductors, InN and ZnO. Being of polar structure, wurtzite
crystals possess a built-in internal electric field. Thermally-excitied charges
screen the built-in electric field during growth in a non-uniform, yet
symmetric, manner, causing the formation of symmetric domains of inverted
polarity. These domains limit the lateral expansion of the crystal, inducing a
fiber growth mode. The mechanism described here elucidates previously
unexplained phenomena in the growth of group III-nitrides on sapphire,
emphasizing the need to consider the effects of built-in electric fields in the
growth of polar semiconductors.
