C.-S. Shin

Vacancy hardening in single-crystal TiNx(001) layers

We investigate the effect of N vacancies on the mechanical properties of epitaxial δ-TiNx(001) layers with x=0.67–1.0. The relaxed lattice parameter increases linearly with x in good agreement with ab initio density functional calculations, indicating that deviations from stoichiometry are entirely due to anion vacancies. Hardness values increase continuously, while the elastic modulus decreases with increasing N-vacancy concentration. We attribute the observed vacancy hardening to a reduced dislocation mobility arising from an increase in the rate-limiting activation energy for cation migration.

Growth, surface morphology, and electrical resistivity of fully strained substoichiometric epitaxial TiNx (0.67⩽x<1.0) layers on MgO(001)

We have grown single-crystal NaCl-structure δ-TiNx layers with x ranging from 0.67 to 1.00 on MgO(001) at 700 °C by ultra-high-vacuum reactive magnetron sputtering of Ti in mixed Ar/N2 discharges in order to investigate microstructural evolution and the physical properties of TiNx as a function of the N vacancy concentration. High-resolution x-ray diffraction and transmission electron microscopy results show that all layers grow with a cube-on-cube epitaxial relationship to the substrate, (001)TiN∥(001)MgO and [100]TiN∥[100]MgO. The relaxed lattice parameter ao(x) decreases linearly from 4.240 A with x=1.00 to 4.226 A with x=0.67. Stoichiometric TiN(001) layers are fully relaxed at the growth temperature while layers with 0.67⩽x⩽0.92 are fully coherent with their substrates. Surface morphologies vary dramatically with x. TiNx(001) layers with x=0.67–0.82 have very flat surfaces arising from large cation surface diffusion lengths approaching values corresponding to step flow. However, the surfaces of the T...

Phase composition and microstructure of polycrystalline and epitaxial TaNx layers grown on oxidized Si(001) and MgO(001) by reactive magnetron sputter deposition

TaNx is presently used in a variety of hard coating, wear-resistant, and diffusion barrier applications. However, the Ta–N system is inherently complex with more than 11 reported equilibrium and metastable phases and there has been little systematic study of the synthesis of these materials. Here, we report the results of an investigation of the ultrahigh vacuum reactive magnetron sputtering of Ta as a function of the N2 fraction fN2 in mixed Ar/N2 discharges together with the phase composition and microstructure of TaNx layers grown on MgO(001) and oxidized Si(001). Unlike the Ti–N system, for which TiN is the terminal phase, the abundance of N-rich phases in the Ta–N system results in the film deposition rate R and the N/Ta ratio of as-deposited layers varying continuously with fN2, even for values >fN2*, where fN2* is the N2 fraction corresponding to the maximum rate of N2 uptake by deposited Ta. Phase composition results are summarized in a phase map plotted as a function of film growth temperature Ts (100–800°C) and fN2. In pure Ar, the films are tetragonal β-Ta at Ts 400°C, and a mixture of the two phases at intermediate temperatures. α-Ta layers grown on MgO(001) at Ts>500°C are epitaxial with a strain-driven 45° in-plane rotation with respect to the substrate: (001)α-Ta∣∣(001)MgO with [110]α-Ta∣∣[100]MgO. A series of lower nitrides — TaN0.1, Ta4N, and Ta2N — are formed over narrow ranges of fN2 between 0 and 0.10. This is followed by a wide single-phase field at Ts ≤ 650°C corresponding to the growth of metastable B1 NaCl-structure δ-TaNx with x ranging from 0.94 (fN2=0.10) to 1.37 (fN2=0.275). The thermodynamically-stable hexagonal e-TaN phase is formed at higher growth temperatures. δ-TaNx layers grown on MgO(001) at Ts=550–650°C are epitaxial, exhibiting a cube-on-cube relationship: (001)δ-TaN∣∣(001)MgO with [100]δ-TaN∣∣[100]MgO. The relaxed lattice constant of δ-TaNx(001) layers decreases linearly from 0.4350 nm with x=0.94 to 0.4324 nm with x=1.37. Finally, layers grown in pure N2 are a two-phase mixture of δ-TaNx and body-centered tetragonal TaNx.

Growth of single-crystal CrN on MgO(001) : Effects of low-energy ion-irradiation on surface morphological evolution and physical properties

CrN layers, 0.5 ╡m thick, were grown on MgO(001) at Ts=570-775░C by ultrahigh vacuum magnetically unbalanced magnetron sputter deposition in pure N2 discharges at 20 mTorr. Layers grown at Ts=700░C ...

Development of preferred orientation in polycrystalline NaCl-structure δ-TaN layers grown by reactive magnetron sputtering: Role of low-energy ion surface interactions

We have investigated the effects of low-energy ion irradiation on texture evolution during growth of B1–NaCl-structure polycrystalline δ-TaN layers on SiO2 by ultra-high-vacuum reactive magnetron sputter deposition at 350 °C in mixed Ar+15%N2 discharges. In parallel sets of experiments, the ion-to-metal flux ratio Ji/JTa was varied from 1.3 to 10.7 while maintaining the incident ion energy Ei constant at 20 eV, and Ei was varied from 8.5 to 30 eV with Ji/JTa=10.7. All TaN layers, irrespective of Ji/JTa or Ei, were overstoichiometric with N/Ta=1.13±0.03. Layers grown with Ei=30 eV are multiphase consisting of hexagonal e-TaN and δ-TaN, while those grown with Ei⩽20 eV are single-phase δ-TaN. With Ei=8.5 eV, the δ-TaN layers have a mixed 111/002/022/113 texture, even for film thicknesses t up to 500 nm. In contrast, δ-TaN layers deposited with Ei=20 eV initially exhibit competitive texture evolution until a single texture dominates at t≳200 nm. The preferred orientation of 500-nm-thick Ei=20 eV layers can be...

Band gap in epitaxial NaCl-structure CrN(001) layers

B1–NaCl-structure CrN(001) layers were grown on MgO(001) at 600 °C by ultrahigh vacuum reactive magnetron sputter deposition in pure N2 discharges. X-ray diffraction analyses establish the epitaxial relationship as cube-on-cube, (001)CrN∥(001)MgO with [100]CrN∥[100]MgO, while temperature-dependent measurements show that the previously reported phase transition to the orthorhombic Pnma structure is, due to epitaxial constraints, absent in our layers. The resistivity increases with decreasing temperature, from 0.028 Ω cm at 400 K to 271 Ω cm at 20 K, indicating semiconducting behavior with hopping conduction. Optical absorption is low (α<2×104 cm−1) for photon energies below 0.7 eV and increases steeply at higher energies. In situ ultraviolet photoelectron spectra indicate that the density of states vanishes at the Fermi level. The overall results provide evidence for CrN exhibiting a Mott–Hubbard type band gap.

Epitaxial NaCl structure d-TaNx(001): Electronic transport properties, elastic modulus, and hardness versus N/Ta ratio

While metastable Bl-NaCl-structure d-TaNx. is presently used in a variety of hard coating, wear-resistant, and diffusion barrier applications, it is a complex material exhibiting a wide single-phase field, x?0.94-1.37, and little is known about its fundamental properties. Here, we report physical properties of epitaxial d-TaNx. layers grown as a function of x on MgO(00) by ultrahigh vacuum reactive magnetron sputter deposition. The room-temperature resistivity (? = 225 µO cm), hardness (H = 30.9 GPa), and elastic modulus (E = 455 GPa) of d-TaNx(001) are independent of x over the range 0.94-1.22. However, changes in the electronic structure associated with increasing x>1.22 lead to an increase in ? with a decrease in H and E. All d-TaNx(001) layers exhibit negative temperature coefficients of resistivity between 20 and 400 K due to weak carrier localization. d-TaNx is superconducting with the highest critical temperature, 8.45 K, obtained for layers with the lowest N/Ta ratio, x=0.94. Based upon the above results, combined with the fact that the relaxed lattice constant a0 shows only a very weak dependence on x, we propose that the wide phase field in d-TaNx is due primarily to antisite substitutions of Ta on N (and N on Ta) sites, rather than to cation and anion vacancies. To first order, antisite substitutions in TaNx are isoelectronic and hence have little effect on charge carrier density. At sufficiently high N/Ta ratios, however, simple electron-counting arguments are no longer valid since large deviations from stoichiometry alter the character of the band structure itself.

Growth and physical properties of epitaxial metastable cubic TaN (001)

We report the growth of epitaxial metastable B1 NaCl structure TaN(001) layers. The films were grown on MgO(001) at 600 °C by ultrahigh vacuum reactive magnetron sputter deposition in mixed Ar/N2 discharges maintained at 20 mTorr (2.67 Pa). X-ray diffraction and transmission electron microscopy results establish the epitaxial relationship as cube-on-cube, (001)TaN∥(001)MgO with [100]TaN∥[100]MgO, while Rutherford backscattering spectroscopy shows that the layers are overstoichiometric with N/Ta=1.22±0.02. The room-temperature resistivity is 225 μΩ cm with a small negative temperature dependence between 20 and 400 K. The hardness and elastic modulus, as determined by nanoindentation measurements, are 30.8±0.9 and 457±16 GPa, respectively.

Interfacial reactions in epitaxial Al/TiN(111) model diffusion barriers: Formation of an impervious self-limited wurtzite-structure AIN(0001) blocking layer

Single-crystal TiN(111) layers, 45 nm thick, were grown on MgO(111) by ultrahigh vacuum reactive magnetron sputter deposition in pure N2 discharges at Ts=700 °C. Epitaxial Al(111) overlayers, 160 nm thick, were then deposited at Ts=100 °C in Ar without breaking vacuum. Interfacial reactions and changes in bilayer microstructure due to annealing at 620 and 650 °C were investigated using x-ray diffraction and transmission electron microscopy (TEM). The interfacial regions of samples annealed at 620 °C consist of continuous ≃7-nm-thick epitaxial wurtzite-structure AlN(0001) layers containing a high density of stacking faults, with ≃22 nm thick tetragonal Al3Ti(112) overlayers. Surprisingly, samples annealed at the higher temperature are more stable against Al3Ti formation. TEM analyses of bilayers annealed at 650 °C (10 °C below the Al melting point!) reveal only the self-limited growth of an ≃3-nm-thick interfacial layer of perfect smooth epitaxial wurtzite-structure AlN(0001) which serves as an extremely e...

Growth and physical properties of epitaxial CeN layers on MgO(001)

While NaCl-structure transition-metal nitrides have been widely studied over the past two decades, little is known about the corresponding NaCl-structure rare-earth nitrides. Polycrystalline CeN, for example, has been reported by different groups to be both a wide band-gap semiconductor and a metal. To address this controversy, we have grown epitaxial CeN layers on MgO(001) and measured their physical properties. The films were grown at 700 °C by ultrahigh vacuum reactive magnetron sputter deposition in mixed Ar/N2 discharges maintained at 4 mTorr (0.53 Pa). X-ray diffraction and transmission electron microscopy results establish the film/substrate epitaxial relationship as cube-on-cube, (001)CeN‖(001)MgO with [100]CeN‖[100]MgO, while Rutherford backscattering spectroscopy shows that the layers are stoichiometric with N/Ce=0.99±0.02. CeN is metallic with a positive temperature coefficient of resistivity and a temperature-independent carrier concentration, as determined by Hall effect measurements, of 2.8±...