  Scanning tunneling microscopy experiments show that the unstable growth
morphology observed during molecular beam homoepitaxy on slightly vicinal
Si(001) surfaces consists of straight step bunches. The instability occurs
under step- flow growth conditions and vanishes both during low-temperature
island growth and at high temperatures. An instability with the same
characteristics is observed in a 2D Kinetic Monte Carlo model of growth with
incorporated Si(001)- like diffusion anisotropy. This provides strong evidence
that the diffusion anisotropy destabilizes growth on Si(001) and similar
surfaces towards step bunching. This new instability mechanism is operational
without any additional step edge barriers.
