ABSTRACT
The importance of heteroepitaxial growth of GaAs on Si substrate has long been recognized and it has been a topic of intense research over the years. However, there yet remain several technical issues in the area of epitaxial growth [1], and most critically, the reduction of threading dislocation density. A dislocation density of ~ 104 cm'2 is generally taken as a desirable goal to realize the potential advantages in most applications. For example, the energy conversion efficiency of GaAs solar cells is known to decrease inversely with dislocation density [2]. Although prior efforts using a two-step growth technique [3], thermal cycle anneal (TCA) treatment [4-6], and use of strained layer superlattice (SLS) [7,8] layers have all led to a significant improvement, the dislocation densities at distances of a few pm from GaAs/Si heterointerface were still in the 105 ~ 106 cm'2
range. Tachikawa and Mori [9] showed that most of the residual dislocations are in fact due to thermal stress that arises as the substrate is cooled from the growth temperature. For this reason, low temperature growth techniques have recently received much attention. One such method combines a SLS and migration enhanced epitaxy (MEE) achieved at 300°C [10], while another uses a surfactant-assisted epitaxy [11] with atomic hydrogen-assisted molecular beam epitaxy (H-MBE) performed at a growth temperature of ~ 350°C [12,13]. Both growth techniques have been shown to successfully reduce the threading dislocation densities to the 104 cm'2 range.
