  The influence of Sb content, substrate type and cap layers on the quantum
anomalous Hall effect observed in V-doped (Bi,Sb)$_2$Te$_3$ magnetic
topological insulators is investigated. Thin layers showing excellent
quantization are reproducibly deposited by molecular beam epitaxy at growth
conditions effecting a compromise between controlled layer properties and high
crystalline quality. The Sb content can be reliably determined from the
in-plane lattice constant measured by X-ray diffraction, even in thin layers.
This is the main layer parameter to be optimized in order to approach charge
neutrality. Within a narrow range at about 80% Sb content, the Hall resistivity
shows a maximum of about 10 k$\Omega$ at 4 K and quantizes at mK temperatures.
Under these conditions, thin layers grown on Si(111) or InP(111) and with or
without a Te cap exhibit quantization. The quantization persists independently
of the interfaces between cap, layer and substrate, the limited crystalline
quality, and the degradation of the layer proving the robustness of the quantum
anomalous Hall effect.
