Transition metal (TM) nitrides are well known for their remarkable physical properties including high hardness and mechanical strength, chemical inertness, and electrical resistivities that vary from metallic to semiconducting. As a result, they are widely studied and have become technologically important for applications such as hard wear resistant coatings on cutting tools, as diffusion barriers in microelectronic devices, and as corrosion and abrasion resistant layers on optical components. Transition metals from the left side of the periodic table, including Sc, Ti, V, Cr, Y, Zr, Nb, Hf, and Ta, form nitrides with a B1-NaCl structure [1, 2]. The excellent mechanical properties of these materials are due to strong covalent-ionic bonds between the TM and N ions resulting from the fully occupied N 2p bands [2, 3].
The chemical bonding in NaCl structure transition metal nitrides occurs along perpendicular (100) directions. This strong bonding directionality results in dramatic changes in surface atom mobilities as a function of crystalline facet orientation, leading to a range of highly anisotropic effects including strong preferred texture development in polycrystalline thin films, anisotropic microstructures, and orientation-dependent properties including hardness, wear strength, adhesion, chemical inertness, and diffusion.
Recent advances in the growth of single crystalline layers of NaCl structure transition metal nitrides have shown that these highly anisotropic effects cause another remarkable feature: Epitaxial layers exhibit, due to atomic shadowing from kinetically roughened periodic surface mound structures, arrays of self-organized rectangular 1-nm-wide nanopipes. These nanopipes are open "holes" that extend through the entire layer thickness along the  growth direction. Their high degree of self-organization, due to kinetic faceting, leads to extremely high aspect ratios on the order of 1000. This, in turn, offers tremendous potential for technological applications in a wide range of fields including nanoelectronics, single molecule chemistry, sensors, and integrated low-dimensionality devices.
This chapter summarizes current understanding for the growth of such nanopipes, the underlying physical properties of their formation, and methods to control their shape, size, and separation. The first section reviews recent findings describing the atomistic processes that govern the growth of transition metal nitrides, including the effects of anisotropic surface diffusion, atomic N flux, adatom potential energies, and ion irradiation. This is followed by a discussion on the key processes leading to nanopipe formation, that is, kinetic roughening, surface morphological evolution, and atomic shadowing. The last section describes the actual growth on nanopipes, strongly utilizing the concepts described in the preceding sections, and provides insight into nanopipe manipulation.
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