Conducting transition metal nitride thin films with

Conducting transition metal nitride thin films with tailored cell sizes: The case of δ-TixTa1−xN L. E. Koutsokeras, G. Abadias, Ch. E. Lekka, G. M. Matenoglou, D. F. Anagnostopoulos et al. Citation: Appl. Phys. Lett. 93, 011904 (2008); doi: 10.1063/1.2955838 View online: http://dx.doi.org/10.1063/1.2955838 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v93/i1 Published by the AIP Publishing LLC.

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APPLIED PHYSICS LETTERS 93, 011904 共2008兲

Conducting transition metal nitride thin films with tailored cell sizes: The case of ␦-TixTa1−xN L. E. Koutsokeras,1,2 G. Abadias,2 Ch. E. Lekka,1 G. M. Matenoglou,1 D. F. Anagnostopoulos,1 G. A. Evangelakis,3 and P. Patsalas1,a兲 1

Department of Materials Science and Engineering, University of Ioannina, GR-45110 Ioannina, Greece Laboratoire PHYMAT, Université de Poitiers-CNRS, UMR 6630, SP2MI, Téléport 2, Bd Marie et Pierre Curie, 86962 Chasseneuil-Futuroscope, France 3 Department of Physics, University of Ioannina, GR-45110 Ioannina, Greece 2

共Received 21 April 2008; accepted 6 June 2008; published online 7 July 2008兲 We present results on the stability and tailoring of the cell size of conducting ␦-TixTa1−xN obtained by film growth and ab initio calculations. Despite the limited solubility of Ta in Ti, we show that TiN and TaN are soluble due to the hybrization of the d and sp electrons of the metal and N, respectively, that stabilizes the ternary system to the rocksalt structure. The stress-free cell sizes follow the Vegard’s rule; nevertheless, process-dependent stresses expand the cell size of the as-grown films. The electronic properties of ␦-TixTa1−xN films 共␳ = 180 ⍀ cm兲 are similar to those of TiN and TaN. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2955838兴 TiN and TaN films have been extensively studied for applications in electronics and optoelectronics.1–7 It comes out that in some cases their growth on lattice-mismatched substrates results in the formation of misfit dislocations at interfaces that may alter device’s integrity.5 Conducting ␦-TixTa1−xN with tailored cell size can be very promising candidates replacing TiN and TaN, in order to achieve low mismatch and epitaxial growth on various active semiconductor layers. Nevertheless, it is not clear whether this ternary system can be formed, due to the limited solubility of Ta in Ti. In addition, TiN and TaN have different crystal structures in the bulk: ␦-TiN 共rocksalt兲 and ␧-TaN 共hcp兲, although ␦-TaN has been also grown8. In this work, we investigate the stability of ternary ␦-TixTa1−xN using various growth techniques and detailed ab initio calculations and we evaluate their electronic properties demonstrating their potential use in electronic applications. ␦-TixTa1−xN films, 200– 300 nm thick, were grown on Si兵001其 by reactive pulsed laser deposition9 共PLD兲 and by dual ion beam sputtering 共DIBS兲.10,11 The sample composition x was changing by using mixed Ti–Ta targets 共99.95% purity兲 of varying fractions; x was determined by Auger electron spectroscopy 共AES兲 and by energy dispersive x rays 共EDX兲. The crystal structure was studied by ␪-2␪ x-ray diffraction 共XRD兲. The optical properties were studied using a rotating polarizer spectroscopic ellipsometer 共SE兲. The total ground energy, the cell size, and the electron density of states of ␦-TixTa1−xN have been calculated using the linear augmented plane wave method within the density functional theory 共DFT兲.12 The exchange-correlation functional was treated within the generalized gradient approximation13 共GGA兲 or the local spin density approximation 共LSDA兲.14 The growth conditions implemented in this study for PLD and DIBS were used previously to grow stoichiometric ␦-TiN 共Refs. 9–11兲 and pure ␦-TaN 共关N兴 / 关Ta兴 = 0.98⫾ 0.02兲 only for PLD. AES spectra revealed that all

films consist exclusively of Ti, Ta, and N; minor surface impurities of O and C have been traced mainly on DIBS films 共ex situ measurements兲. EDX did not detect any O or C in the bulk of the films. Figure 1 shows AES spectra from a PLD- and a DIBS-grown film exhibiting Ta 共170– 180 eV兲, Ti and N 关360– 420 eV 共Ref. 9兲兴 patterns; the DIBS sample also exhibits the ArLMM pattern 共220 eV兲, resulting from the entrapment of backscattered 共BS兲 Ar reflected from the target surface. The energetic BS species move toward the film surface and are implanted in the film due to their high kinetic energy 共1.2 keV兲. A close correlation is found between the fraction of the BS Ar, calculated using SRIM 共sampling of 107 ions per x, impinging at an angle of 45° on a composite TixTa1−x target兲,15 and the variation of the Ar content in the film 共Fig. 2, inset兲. After vacuum annealing at 850 ° C for 3 h the stress of the films is almost completely relieved10 and the ArLMM pattern reduces down to the detection limit of AES, indicating a strong outdiffusion of Ar. The XRD patterns exhibited exclusively the characteristic 共111兲 and 共200兲 peaks of the rocksalt structure10,16,17 over the full range of x 共0 ⬍ x ⬍ 1兲. This finding contrasts the data of Ref. 18 concerning the limited solubility of Ta in TiN. Instead, it is in agreement with the stabilization of ␦-TaxZr1−xN,19 which is a structurally similar system. The cell sizes 共␣111兲 were calculated from the 共111兲 interplanar spacings16 and are displayed in Fig. 2. The ␣111 versus x of

a兲

FIG. 1. 共Color online兲 AES spectra from two ␦-TixTa1−xN films grown by PLD and DIBS.

Author to whom to correspondence should be addressed. Electronic mail: [email protected].

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FIG. 2. 共Color online兲 The cell size of ␦-TixTa1−xN films, of TaN and TiN 共Ref. 17兲, along with results from GGA and LSDA calculations. Inset: the relative intensity of the ArLMM pattern over the TiLMV and TaMNN AES patterns, which is proportional to the 关Ar兴 content in the films, and the fraction of 1.2 keV BS Ar calculated by SRIM.

the two sets of films follow two distinct straight lines 共Fig. 2, solid symbols兲, which do not coincide with Vegard’s rule 共defined by the corresponding TiN and TaN bulk values17兲. In all cases ␣111 is expanded compared to the expected values from Vegard’s rule; this is attributed to in-plane compressive stresses.10 Values of ␣111 for DIBS films are more expanded compared to those for PLD films due to excessive stress attributed to the contribution of energetic BS neutrals. The slope of the DIBS line is different from that of the PLD line and Vegard’s rule, resulting in a wider expansion in the Ta-rich samples. No pure ␦-TaN films could be grown by DIBS, most likely due to this excessive stress.

Appl. Phys. Lett. 93, 011904 共2008兲

In the case of annealed, stress-relieved films, all the cell sizes follow Vegard’s rule 共Fig. 2, open symbols兲, thus providing the ability of tailoring them to the desired lattice cell that matches the conducting epilayers for metallizations or diffusion barriers onto various sublayers. This is also supported by GGA and LSDA calculations that yield the corresponding cell size in the range of the stress-free ␣111. It is worth noting that the calculated cell size as well as the slope difference compared with the experimental data is attributed to the periodic crystalline bulk system used in the calculations. In addition, in the cases of pure TiN and TaN, these results are in excellent agreement with previous calculations.19–21 The wide solubility range between TiN and TaN, in contrast to metallic Ti and Ta, can be understood considering the chemical bonding between the metal and N atoms. Our ab initio calculations show that the metallic TaN besides the purely ionic bonding between the Ta 共5d36s2兲 and N 共2s22p3兲, presents also covalent-like bonding due to the strongly hybridized energy states around −7 eV of the Ta5d and N2p. This mixed bonding competition is clearly manifested in the ␦-TixTa1−xN, Fig. 3. The TaN features of the N strong s-character, along with the Ta dt2g pronounced lobes that point toward the nearest metal atoms denote ␴-bonding and characterize the Ti25Ta75N共010兲 plane, Figs. 3共a兲 and 3共b兲. The corresponding TiN plane in the case of Ti75Ta25N共010兲 exhibits more metallic than ionic character 共Ti: 3d24s2兲, Fig. 3共d兲. In the Ti25Ta75N共100兲 plane, N loses its perfect electron saturation regaining charge via ␲-bonding formation with the Tidt2g orbitals in the 共110兲 plane, Fig. 3共a兲. As the Ti inclusions increase 关Figs. 3共c兲 and 3共d兲兴, the Tidt2g-Tadt2g hybridization is more pronounced reaching in the case of Ti75Ta25N共100兲 the picture of the pure TaN plane of Ti25Ta75N共010兲, Fig. 3共a兲. Finally, the small hybridization of Tadt2g orbitals with N atoms in the 关001兴 direction of

FIG. 3. 共Color online兲 Total valence charge density for the 共010兲, 共100兲, and 共110兲 planes. 关共a兲 and 共b兲兴 Ti25Ta75N, 共c兲 Ti50Ta50N and 共d兲 Ti25Ta75N. Red 共blue兲 indicates positive 共negative兲 values.


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range of 0.424– 0.445 nm can be grown based on the ␦-TixTa1−xN 共0 ⬍ x ⬍ 1兲 system. DFT calculations have identified a mixed ionic-covalent bonding between metal and N atoms that stabilizes the rocksalt structure. The stress-free cell size follows Vegard’s rule; deviations from Vegard’s rule for the as-grown films are due to stress, which expand the cell size and are associated with the energetic growth process. The films are typical conductors of varying density of electrons. The resistivity values are comparable to those of polycrystalline TiN and TaN films, making these films excellent candidates for substitution of TiN and TaN in electronic applications, where a better lattice match is required. P.P. acknowledges the University of Poitiers for granting him a visiting professorship during which part of this work has been performed. The authors are grateful to Dr. Ph. Guerin for the DIBS growth. The Central Facilities of the University of Ioannina and Professors C. Kosmidis and P. Pomonis are acknowledged for providing the laser and EDX equipment. FIG. 4. 共Color online兲 Dielectric function spectra of representative ␦-TixTa1−xN films.

Ti25Ta75N共110兲 plane, Fig. 3共a兲, is enhanced in the Ti50Ta50N共110兲 and Ti75Ta25N共110兲 planes, resulting in positive charge of the corresponding N atoms. Thus, the bonding nature of ␦-TixTa1−xN is characterized by a mixed covalent-ionic character that coexists with the metallic features. The charge transfer between the cations 共Ti,Ta兲 and the anion 共N兲 increases when the mean metal electronegativity increases, resulting in stronger ionic character for TaN compared to TiN along with stronger covalent bond features. The potential applications of the ␦-TixTa1−xN epilayers require their electronic properties to be at least similar to those of TiN and TaN films. The optical properties of ␦-TixTa1−xN films have been studied by SE 共Fig. 4兲 showing that the films are typical conductors exhibiting a strong Drude behavior resembling TiN,6,7 even for small x 共Ta-rich兲. The spectra were fitted by a Drude term and two Lorentz oscillators that correspond to the intraband absorption and the interband transitions, respectively.7 The differences between the various spectra come mainly from their plasma energy, increasing from 7.8 to 9.4 eV upon reduction of x, which is associated with the variation of conduction electron density. Secondary differences include the spectral position of interband transitions manifested above 3 eV; however, such a study is beyond the scope of this communication. The film resistivity can be determined from the Drude term,7 yielding values in the 210– 290 ␮⍀ cm range for polycrystalline ␦-TixTa1−xN films, which are higher than that of ␦-TiN 共Ref. 7兲 but similar to that of pure ␦-TaN.8,22 Further reduction of the resistivity 共down to 180 ␮⍀ cm兲 was achieved by annealing. In conclusion, stable, conducting transition metal nitrides with rocksalt structure and varying cell size over the

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