Transition Metal (TM) Nitrides are widely used in the coating industry because they possess high hardness, chemical inertness and high mechanical strength. Chromium Nitride (CrN) is a technologically important TM nitride, however, its growth process is not well understood. In addition, CrN is a promising material for electronic or spintronic applications, because it may exhibit a band gap as well as magnetic ordering. However, reports on the electronic structure of CrN are contradictory: CrN may be a semiconductor or a metal, may exhibit ferro- or antiferromagnetic ordering, and may have a structural phase transition near room temperature. The goal of this thesis research is to provide insight into the layer growth process of CrN and to resolve the controversy regarding its electronic structure.
In order to investigate the growth process, single crystal CrN(001) layers, 10 to 160 nm thick, were grown on MgO(001) by reactive magnetron sputtering at growth temperatures Ts = 600 and 800°C. The CrN(001) surfaces exhibit <110> step edges and 1-10 nm wide atomically smooth terraces, as observed by in situ scanning tunneling microscopy (STM). Growth at 600°C leads to surface mounds which have square shapes and edges along <110> directions for t ≤ 40 nm, develop dendritic shapes for t = 80 nm, and join during continued growth to revert back to squares at t = 160 nm. In contrast, at Ts = 800°C mounds elongate and join along <100> directions with curved edges, yielding long chains of interconnected square mounds for t ≤ 40 nm, which are subsequently overgrown by rapidly growing mounds with <110> edges. The dramatic difference in mound development for Ts = 600 and 800°C is attributed to the increasing adatom diffusion length at increasing growth temperature.
The electronic structure of CrN is investigated by optical methods. For this purpose, the refractive index n and extinction coefficient k were determined by combining optical transmittance and reflectance over a large wavelength range 250 nm ≤ λ ≤ 30 μm and for samples with thickness t = 44.11,000 nm. Features in the dielectric function at 0.64, 1.5 eV and 2.9 eV indicate direct interband transitions which allow, when comparing with band structure calculations, to estimate the fundamental indirect gap to be 0.19±0.46 eV. This is consistent with the absence of free carrier contributions over the entire measurement range, indicating a carrier density ≤ 3×1019 cm -3. The combination of infrared reflectivity and Raman spectroscopy provides CrN optical vibrational frequencies at various points in the Brillouin zone, and Born effective charges of ±4.4 for Cr and N ions, confirming the low free carrier concentration. The qualitative and quantitative similarities of the dielectric function and the phonon dispersion curves of CrN with those reported for ScN, which is a semiconducting NaCl-structure nitride, are also consistent with the presence of a band gap in CrN.
The electronic transport in CrN is investigated by temperature dependent conductivity measurements for samples grown on different substrates and at various growth temperatures, which, in turn, affect the crystalline quality and the N-vacancy concentration. Low temperature transport in single crystal CrN is attributed to localized N-vacancy defect states that facilitate variable range hopping (VRH) with a crossover at 30±10 K from Efros-Shklovskii (ES) to Mott VRH. The nitrogen vacancy concentration increases both at high and low growth temperatures, Ts > 800°C and Ts < 600°C, due to N2 desorption and kinetically limited N2 dissociation, respectively, which causes a sharp increase in the localization length &zgr; and a correspondingly higher hopping conduction. In contrast, the room temperature conductivity is dominated by electrons that are thermally activated from localized states in the band tail to extended states above the mobility edge. The density of defect states in the conduction band tail decreases with increasing growth temperature, causing a decrease in the room temperature carrier density, which is estimated from Hall effect and optical reflectance measurements to be on the order of 1020 cm-3, resulting in an increase in the resistivity by an order of magnitude, from 7.3 to 71 mΩcm. Temperature dependent x-ray diffraction indicates a phase transition at 280 K to a low temperature orthorhombic phase for polycrystalline CrN. Orthorhombic low-temperature phase is metallic with a positive or negative ρ(T) depending on degree of disorder. These results demonstrate how strongly the defect density affects CrN transport properties, and explains the large discrepancy in the previously reported room temperature values.