Scanning tunneling microscopy studies of the GaAs(001) surface and the nucleation of ZnSe on GaAs(001)

A critical issue in the epitaxial growth of ZnSe based blue laser diodes is the formation of stacking faults in the ZnSe which originate at the interface with the GaAs substrate. These stacking faults are known to cause rapid optical degradation of the lasers. In this paper we describe scanning tunneling microscopy (STM) studies of the initial stages of the growth of ZnSe by molecular beam epitaxy (MBE) on the GaAs(001) surface, which are aimed at understanding how the growth mode can be influenced and stacking fault densities reduced. It has yet been established whether or not a GaAs buffer layer will be required to bring the stacking fault density down to an acceptable level. We looked at the morphology of an oxide desorbed substrate and found that it is much rougher than an MBE grown GaAs buffer layer. The growth mode of ZnSe on GaAs depends on the chemistry of the GaAsZnSe interface. If Se is first reacted with the GaAs surface, a stable and highly ordered Se-terminated surface with a (2 × 1) reconstruction is formed. Our STM images show that ZnSe does not readily nucleate on this surface, and grows by the formation of small three-dimensional islands. The resulting stacking fault density is in excess of 108 cm−2. The stacking fault density is reduced by at least two orders of magnitude if the reaction between Se and GaAs is inhibited by growing ZnSe at 300°C directly on the GaAs(001)-(2 × 4) As-rich surface. Although this results in a more two-dimensional growth mode, the first few monolayers are observed to be very disordered. The origin of this disorder has yet to be determined Ways must be found to reduce further the stacking fault density to below 104 cm−2 in order to achieve commercially viable blue lasers in ZnSe.