Studies of Al$_x$Ga$_{1-x}$N/AlN, nanosized columnar heterostructures grown on silicon substrates with various surface modifications

The growth of nanosized columnar Al$_x$Ga$_{1-x}$N/AlN heterostructures on the surface of silicon substrates of three types, namely, on a standard atomically smooth $c$-Si substrate, a Si substrate with a transition layer of porous silicon por-Si/$c$-Si, and a hybrid a substrate containing a layer of silicon carbide grown by the method of coordinated substitution of atoms on the surface of porous silicon SiC/por-Si/$c$-Si. The complex structural-spectroscopic analysis carried out showed that the epitaxial growth of the AlN nucleation layer on all types of substrates under N-enriched conditions leads to the formation of Al$_x$Ga$_{1-x}$N/AlN heterostructures with a Ga-polar surface. It was found that a layer of an ordered Al$_x$Ga$_{1-x}$N solid solution was formed only on the SiC/por-Si/c-Si hybrid substrate. On $c$-Si and por-Si/$c$-Si substrates, the Al$_x$Ga$_{1-x}$N layer is in the state of a disordered solid solution with an excess content of gallium atoms. It has been demonstrated that Al$_x$Ga$_{1-x}$N nanosized columns formed on a SiC/por-Si/$c$-Si substrate are tilted relative to the $c$-axis, which is associated with the peculiarities of the formation of the SiC layer by the method of coordinated substitution of atoms on a porous Si substrate, which leads to the formation of inclined (111) SiC facets. at the interface between the (111) Si surface and pores in Si. Optical studies of the grown samples showed that the optical band-to-band transition for the Al$_x$Ga$_{1-x}$N solid solution with $E_g$ = 3.99 eV was observed only when studying the heterostructure grown on the SiC/por-Si/$c$-Si substrate. The results obtained in this work demonstrate the promise of using SiC/por-Si/$c$-Si substrates for the integration of silicon technology and the technology of synthesizing nanosized Al$_x$Ga$_{1-x}$N columnar heterostructures by molecular beam epitaxy with nitrogen plasma activation.