Metal/semiconductor phase transition in chromium nitride(001) grown by rf-plasma-assisted molecular-beam epitaxy

Abstract

Structural and electronic properties of stoichiometric single-phase CrN(001) thin films grown on MgO(001) substrates by radio-frequency N plasma-assisted molecular-beam epitaxy, are investigated. In situ room-temperature scanning tunneling microscopy clearly shows the 1 ϫ 1 atomic periodicity of the crystal structure as well as long-range topographic distortions which are characteristic of a semiconductor surface. This semiconductor behavior is consistent with ex situ resistivity measurements over the range 285 K and higher, whereas below 260 K, metallic behavior is observed. The resistivity-derived band gap for the high-temperature region, 71 meV, is consistent with the tunneling spectroscopy results. The observed electronic (semiconductor/metal) transition temperature coincides with the temperature of the known coincident magnetic (para-antiferro) and structural (cubic-orthorhombic) phase transitions.For many years, CrN has received a lot of attention due to its high hardness and corrosion resistance. 1,2 In addition, CrN is also interesting due to its magnetic, optical, and electronic properties. It is known that CrN is paramagnetic (PM) with a B1 NaCl crystal structure at room temperature; at T Néel , reported in the range of 273-283 K, the material undergoes a phase transition to antiferromagnetic (aFM) with an orthorhombic P nma crystal structure. [3][4][5] found theoretically that the magnetic stress is the driving force for the lattice distortions, and thus linked the magnetic and structural transitions. So far, however, the electronic properties have been less understood.Filippetti et al. 5 reported, based on theoretical calculations, that CrN is a metal in its PM state but a weak metal in the aFM state. This agrees well with an earlier experimental work, based on CrN polycrystalline powders, which found metallic behavior with, however, an abrupt increase of the resistivity with increasing temperature at ϳ286 K. 7 On the other hand, Herle et al., 8 based on synthesized CrN powders, reported that CrN is a semiconductor with a band gap of 90 meV as measured by resistivity. Recently, Gall et al. 9 grew crystalline thin films of CrN by sputtering and reported that CrN behaves as a semiconductor (Mott-type insulator) with an optical gap of 0.7 eV. Although the electronic behavior at room temperature has become more clear, the value of the band gap as well as its detailed nature, remain to be determined; moreover, the discrepant results reported for low temperatures, remain to be resolved.In this letter, we present results concerning the unique structural and electronic properties of CrN(001). We show that high-quality epitaxial layers are grown by molecularbeam epitaxy (MBE), having crystalline (001) orientation and atomically smooth surface [as measured by scanning tunneling microscopy (STM)] and semiconducting behavior at 300 K. Bulk ex situ measurements confirm the semiconducting behavior at 300 K, but furthermore a transition to the metallic state is observed at ϳ285 K. This transition coincides with the ...

Microstructure and thermoelectric properties of CrN and CrN/Cr2N thin films

“…As for semiconducting properties, previous reports have announced different band gaps for CrN. Constantin et al [12] prepared 1:1 stoichiometric CrN thin films deposited by molecular beam epitaxy, and reported a resistivity-derived 71 meV band gap at room temperature, by using the ln(ρ/ρ 0 ) = (E g /2k B )(1/T) equation which relates the resistivity of a semiconductor to ambient temperature and the energy band gap where ρ is the electrical resistivity, ρ 0 is the resistivity pre-factor, E g is the semiconductor band gap, k B is the Boltzmann constant and T is temperature. Zhang et al [13] measured the optical properties of CrN and concluded that the band gap is 0.19 eV with an error bar of 0.46 eV, demonstrating difficulties in this regard.…”

A comparative study of transport properties in polycrystalline and epitaxial chromium nitride films

“…3,4 Considerable experimental and theoretical work have demonstrated that CrN undergoes a magnetic and structural phase transition from a paramagnetic NaCl structure at room temperature to a low-temperature antiferromagnetic orthorhombic P nma phase at N eel temperature of 273-286 K. [5][6][7] A variety of electrical transport properties in CrN have been observed, such as, a semiconducting behavior with dq/dT < 0, [8][9][10][11] a metallic behavior with dq/dT > 0, [12][13][14] and continuous and discontinuous q(T) curves at 260-280 K. [8][9][10][11][12][13]15 These differences in electrical transport properties have been attributed to the sensitivity of the transport properties to N stoichiometry. 12,15 In addition, the discontinuity in q(T) curves has usually been observed in CrN powders or polycrystalline CrN films, rather than in epitaxial films, suggesting that the epitaxial constraints could affect the transition.…”

Epitaxial suppression of the metal-insulator transition in CrN

“…3,4 Electronic transport studies report controversial results for CrN, including (i) values for the resistivity ρ at room temperature range over more than two orders of magnitudes, from 1.7×10 -3 to 3.5×10 -1 Ωcm, 3,[5][6][7][8][9][10] even when only considering the most reliable data for single crystal CrN layers; (ii) the temperature dependence of ρ shows metallic behavior with dρ/dT > 0 in some studies, 6,7,11 but an increase in ρ with decreasing temperature in other reports, 3,5,9,12 which has been attributed to the presence of a band gap 5 or carrier localization due to crystalline defects 13 or N-vacancies; 14 (iii) some studies report a discontinuity in ρ(T) at 260-280 K, 3,6,7,14 which is associated with a magnetic and structural phase transition from a paramagnetic NaCl structure at room temperature to a low-temperature antiferromagnetic orthorhombic P nma phase 11,15 with a 0.56-0.59% higher density, 11 and a 25% lower bulk modulus, 16 while other reports show no evidence for a phase transition in the ρ(T)-curves. 5,7,9 Electronic structure calculations suggest that magnetic stress relief couples magnetic ordering with the structural phase transition, 17 and that CrN exhibits a band gap if the Hubbard Coulomb interaction term is sufficiently large.…”