The nanowire geometry offers a unique way to realize epitaxial GaAs-based electronics and photonics on Si despite the large lattice mismatch. In that way, the superior electronic and optoelectronic properties of the former may be integrated with the mature complementary metal oxide semiconductor (CMOS) technology of the latter. For the realization of functional devices, it is essential to control the charge carrier concentration in the nanowires by the deliberate incorporation of dopants. Si has been widely used as donor in GaAs thin films, and its use also in nanowires is highly desirable. However, when doping with Si is attempted during the vapor–liquid–solid (VLS) growth of GaAs nanowires (core doping scheme), the dopant atoms incorporate both on Ga-sites (SiGa) as donors and on As-sites (SiAs) as acceptors, making it impossible to obtain unipolar conductivity with low carrier compensation level.
We demonstrate that the amphoteric behavior of Si is adequately controlled when a shell doping scheme is utilized, i.e. when a Si-doped GaAs shell layer is grown conformally around the undoped GaAs nanowire core in the vapor-solid (VS) mode (Fig. 1). All nanowire samples were grown on Si(111) substrates by molecular beam epitaxy (MBE). Measurements of Raman scattering by local vibrational modes (LVMs) of Si revealed the preferential formation of SiGa donors in the case of the shell doping scheme (red curve in Fig. 2), in contrast to the core doping one (blue curve in Fig. 2) where SiGa donors and SiAs acceptors co-exist. Investigating the dependence of the incorporation site of Si on the growth conditions of the doped shell (that is the substrate temperature, the V/III flux ratio, and the Si beam flux), we identified a growth window that ensures the incorporation of Si exclusively as donor. Within this window, both As-rich conditions and limited Ga adatom diffusivity are established on the nanowire sidewalls, allowing us to obtain donor concentrations up to 1×1019 cm-3, with the compensation level by Si acceptors remaining below 10 %. The achievement of n-type conductivity opens the way to realizing functional GaAs-based nanowire devices, and is particularly significant when a dopant as well understood and advantageous as Si is employed.