**1. Part 1: GaAs growth on Si(001)**

The growth of polar zinc-blende GaAs on a non-polar Si(001) substrate can lead to planar defects named antiphase boundary (APB). The APB planes are made of III-III or/and V-V bonds that can propagate in the layer through the {110}, {111} or higher index planes (**Figure 1**) [2, 3].

The elastic strain field due to APB changes atomic distances and hence electronic states, acts as a carrier diffusion and/or non-radiative recombination centers. APBs nucleate at the edges of the single-layer steps present at nominal (001) silicon surfaces. Until now, the APBs formation was mainly inhibited by using (i) off-axis Si(001) substrates tilted by 4–6° towards [110] direction [4, 5] or (ii) Si(211) substrates [6] where Si double-layer steps could be formed easily. However, these wafers are not compatible with industrial Si CMOS standards which uses nominal Si (001) wafers, i.e. with a miscut angle equal or lower than 0.5°. The best option to prevent the APBs nucleation is to promote double layer step formation on nominal Si(001) substrate as it is the case for off-axis wafers.

**Figure 1.**

*Ball-and-stick model of III-V-on-Si with {110} and {111}-APBs. The single-layer step edges initiate the formation of the APBs while the surface with double-layer steps allows a single-domain III-V cristal. From reference [1].*
