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Title: Raman scattering from epitaxial HfN layers grown on MgO(001)

Abstract

Stoichiometric single-crystal HfN layers grown on MgO(001) are analyzed by Raman spectroscopy. Second-order Raman scattering predominates, but first-order modes in the acoustic and optical ranges are also visible. The latter indicates that the O{sub h} symmetry of NaCl-structure HfN is broken. The large mass difference between Hf and N leads to a correspondingly large separation, 250 cm{sup -1}, between the first-order acoustic and optical bands. Within this gap, four Raman lines are clearly observed. The first three are the second-order transverse acoustic mode (240 cm{sup -1}), the sum of the first-order transverse and longitudinal acoustic modes (280 cm{sup -1}), and the second-order longitudinal acoustic mode (325 cm{sup -1}). The fourth line at 380 cm{sup -1} is identified as the difference between the first-order optical and acoustic modes. The observed first-order Raman scattering, as well as the width of the gap between the first-order acoustic and optical modes, is in good agreement with previously calculated HfN phonon density of states.

Authors:
; ; ;  [1];  [2]
  1. Department of Materials Science and the Frederick Seitz Materials Research Laboratory, University of Illinois, 104 South Goodwin Avenue, Urbana, Illinois 61801 and Institut Universitaire de Technologie, Universite de Haute Alsace, 68093 Mulhouse Cedex (France)
  2. (United States)
Publication Date:
OSTI Identifier:
20787881
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Applied Physics; Journal Volume: 99; Journal Issue: 4; Other Information: DOI: 10.1063/1.2173037; (c) 2006 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; EPITAXY; HAFNIUM NITRIDES; LAYERS; MAGNESIUM OXIDES; MASS DIFFERENCE; MONOCRYSTALS; OPTICAL MODES; PHONONS; RAMAN EFFECT; RAMAN SPECTRA; RAMAN SPECTROSCOPY; SODIUM CHLORIDES; STOICHIOMETRY; SYMMETRY

Citation Formats

Stoehr, M., Seo, H.-S., Petrov, I., Greene, J.E., and Department of Materials Science and the Frederick Seitz Materials Research Laboratory, University of Illinois, 104 South Goodwin Avenue, Urbana, Illinois 61801. Raman scattering from epitaxial HfN layers grown on MgO(001). United States: N. p., 2006. Web. doi:10.1063/1.2173037.
Stoehr, M., Seo, H.-S., Petrov, I., Greene, J.E., & Department of Materials Science and the Frederick Seitz Materials Research Laboratory, University of Illinois, 104 South Goodwin Avenue, Urbana, Illinois 61801. Raman scattering from epitaxial HfN layers grown on MgO(001). United States. doi:10.1063/1.2173037.
Stoehr, M., Seo, H.-S., Petrov, I., Greene, J.E., and Department of Materials Science and the Frederick Seitz Materials Research Laboratory, University of Illinois, 104 South Goodwin Avenue, Urbana, Illinois 61801. Wed . "Raman scattering from epitaxial HfN layers grown on MgO(001)". United States. doi:10.1063/1.2173037.
@article{osti_20787881,
title = {Raman scattering from epitaxial HfN layers grown on MgO(001)},
author = {Stoehr, M. and Seo, H.-S. and Petrov, I. and Greene, J.E. and Department of Materials Science and the Frederick Seitz Materials Research Laboratory, University of Illinois, 104 South Goodwin Avenue, Urbana, Illinois 61801},
abstractNote = {Stoichiometric single-crystal HfN layers grown on MgO(001) are analyzed by Raman spectroscopy. Second-order Raman scattering predominates, but first-order modes in the acoustic and optical ranges are also visible. The latter indicates that the O{sub h} symmetry of NaCl-structure HfN is broken. The large mass difference between Hf and N leads to a correspondingly large separation, 250 cm{sup -1}, between the first-order acoustic and optical bands. Within this gap, four Raman lines are clearly observed. The first three are the second-order transverse acoustic mode (240 cm{sup -1}), the sum of the first-order transverse and longitudinal acoustic modes (280 cm{sup -1}), and the second-order longitudinal acoustic mode (325 cm{sup -1}). The fourth line at 380 cm{sup -1} is identified as the difference between the first-order optical and acoustic modes. The observed first-order Raman scattering, as well as the width of the gap between the first-order acoustic and optical modes, is in good agreement with previously calculated HfN phonon density of states.},
doi = {10.1063/1.2173037},
journal = {Journal of Applied Physics},
number = 4,
volume = 99,
place = {United States},
year = {Wed Feb 15 00:00:00 EST 2006},
month = {Wed Feb 15 00:00:00 EST 2006}
}
  • While many transition metal (TM) nitrides - including TiN, ZrN, and TaN - have been widely studied and are currently used as hard wear-resistant coatings, diffusion barriers, and optical coatings, little is known about a related TM nitride, HfN. Here, we report the results of a systematic investigation of the growth and physical properties of HfN{sub x} layers, with 0.80{<=}x{<=}1.50, deposited on MgO(001) by ultrahigh vacuum reactive magnetron sputtering at 650 deg. C in mixed N{sub 2}/Ar discharges. HfN{sub x} layers with 0.80{<=}x{<=}1.20 crystallize in the B1-NaCl structure with a cube-on-cube epitaxial relationship to the MgO(001) substrate, while films withmore » 1.24{<=}x{<=}1.50 contain a N-rich second phase. The relaxed bulk lattice parameter of HfN{sub x}(001) decreases only slightly with increasing N/Hf ratio, ranging from 0.4543 nm with x=0.80 to 0.4517 nm with x=1.20. The room-temperature resistivity {rho} of stoichiometric HfN(001) is 14.2 {mu}{omega} cm and {rho}(x) increases with both increasing and decreasing x to 140 {mu}{omega} cm with x=0.80 and 26.4 {mu}{omega} cm with x=1.20. The hardness H and elastic modulus E of HfN(001) are 25.2 and 450 GPa, respectively. H(x) initially increases for both over- and understoichiometric layers due to defect-induced hardening, while E(x) remains essentially constant. Single-phase HfN{sub x}(001) is metallic with a positive temperature coefficient of resistivity (TCR) between 50 and 300 K and a temperature-independent carrier density. It is also superconducting with the highest critical temperature, 9.18 K, obtained for layers with x=1.00. In the two phase regime, {rho} ranges from 59.8 {mu}{omega} cm with x=1.24 to 2710 {mu}{omega} cm with x=1.50. TCR becomes positive with x{>=}1.38, no superconducting transition is observed, and both H and E decrease.« less
  • Epitaxial Ag(001) layers were deposited on MgO(001) in order to study electron surface scattering. X-ray reflection indicates 3D layer nucleation with a high rms surface roughness of 1.0 nm for a layer thickness d = 3.5 nm. X-ray diffraction shows that {l_brace}111{r_brace} twins form at d < 11 nm, followed by 2nd generation twinning for 11 nm < d < 120 nm. Increasing the growth temperature from 25 to 150 deg. C suppresses 2nd generation twinning and reduces the twin density by 2 orders of magnitude. In situ deposition of epitaxial 2.5-nm-thick TiN(001) underlayers prior to Ag deposition results inmore » twin-free single-crystal Ag(001) with 10 x smoother surfaces for d = 3.5 nm. This is attributed to a better wetting on the higher energy TiN(001) than MgO(001) surface, resulting in the absence of 3D nuclei with exposed {l_brace}111{r_brace} facets, which facilitate twin nucleation. The twinned Ag/MgO layers have a higher resistivity {rho} than the single crystal Ag/TiN layers at both 298 and 77 K, due to electron scattering at grain and twin boundaries. The {rho} for single-crystal Ag layers increases with decreasing d, which is well explained with known surface scattering models and provides specularity parameters for the Ag-vacuum and the Ag-air interfaces of p = 0.8 {+-} 0.1 and 0.4 {+-} 0.1, respectively. A comparison with corresponding epitaxial Cu(001) layers shows that {rho}{sub Ag} < {rho}{sub Cu} for d > 50 nm, consistent with known bulk values. However, {rho}{sub Ag} > {rho}{sub Cu} for d < 40 nm. This is attributed to the larger electron mean free path for electron-phonon scattering and a correspondingly higher resistivity contribution from surface scattering in Ag than Cu. In contrast, air exposure causes {rho}{sub Ag} < {rho}{sub Cu} for all d, due to diffuse scattering at the oxidized Cu surface and the correspondingly higher Cu resistivity.« less
  • We report on the mechanism of magnetization reversal in epitaxial Co/Fe bi-layers grown by molecular beam epitaxy on MgO(001) substrates. For Co films thicker than 5 nm, the crystal structure is hexagonal. The Fe layer follows an epitaxial relation relative to the MgO substrate of (001)[100]Fe//(001)[110]MgO. When deposited on a cubic Fe layer, the Co layer follows a bi-crystal epitaxial relation of (11{bar 2}0)[0001]Co//(001) 100 Fe as previously reported [Popova et al., Appl. Phys. Lett. 81, 1035 (2002); Wang et al., J. Appl. Phys. 101, 09D103 (2007)]. The magnetization reversal in-plane follows a cubic fourfold symmetry, which coincides with thatmore » of the underlying bcc Fe layer. In this study, we find that the area of each Co crystal domain spans 200-1500 nm{sup 2} and that these two domains are approximately evenly distributed. The micromagnetic reversal mechanism is a combination of coherent rotational processes and domain wall displacement. These magnetic domains are sized tens of {micro}m and separated by predominately 90{sup o} or occasionally 180{sup o} domain walls along the Fe<110> and Fe<100> directions, respectively. The cubic anisotropy of the bi-crystalline Co layer is explained by exchange-coupling between hcp grains with perpendicular crystallographic orientation, each having in-plane uniaxial magnetic anisotropy along its respective [0001] direction.« less
  • We report on the mechanism of magnetization reversal in epitaxial Co/Fe bi-layers grown by molecular beam epitaxy on MgO(001) substrates. For Co films thicker than 5 nm, the crystal structure is hexagonal. The Fe layer follows an epitaxial relation relative to the MgO substrate of (001)[100]Fe//(001)[110]MgO. When deposited on a cubic Fe layer, the Co layer follows a bi-crystal epitaxial relation of (1120)[0001]Co//(001)<100>Fe as previously reported [Popova et al., Appl. Phys. Lett. 81, 1035 (2002); Wang et al., J. Appl. Phys. 101, 09D103 (2007)]. The magnetization reversal in-plane follows a cubic fourfold symmetry, which coincides with that of the underlyingmore » bcc Fe layer. In this study, we find that the area of each Co crystal domain spans 200-1500 nm{sup 2} and that these two domains are approximately evenly distributed. The micromagnetic reversal mechanism is a combination of coherent rotational processes and domain wall displacement. These magnetic domains are sized tens of {mu}m and separated by predominately 90 deg. or occasionally 180 deg. domain walls along the Fe<110> and Fe<100> directions, respectively. The cubic anisotropy of the bi-crystalline Co layer is explained by exchange-coupling between hcp grains with perpendicular crystallographic orientation, each having in-plane uniaxial magnetic anisotropy along its respective [0001] direction.« less
  • Epitaxial Fe/MgO layers have been grown on In{sub x}Ga{sub 1-x}As substrates to examine the epitaxial relationship and the morphological variation with respect to indium content, x and the growth temperature of MgO interlayer. The in-plane epitaxial relationship of Fe[010]//MgO[110]//In{sub x}Ga{sub 1-x}As[110] is found in the structures of all x values for 4 nm thick MgO layers grown at room temperature. Epitaxial MgO interlayers grow in two-dimensional layer regardless of x while the morphology of subsequent Fe changes from two-dimensional layer to three-dimensional islands with the increase of x. Furthermore, the average size of Fe islands becomes smaller at higher xmore » value due to enhanced underlying strain. The elevated growth temperature of MgO has led to partial strain relaxation, resulting in the suppression of three-dimensional Fe island formation.« less