Structure, electronic properties and electron energy loss spectra of transition metal nitride films

doi:10.1016/j.tsf.2012.06.086

Thin Solid Films

Volume 528, 15 January 2013, Pages 49-52

https://doi.org/10.1016/j.tsf.2012.06.086 Get rights and content

Abstract

We present a thorough and critical study of the electronic properties of the mononitrides of the group IV–V–VI metals (TiN, ZrN, HfN, NbN, TaN, MoN, and WN) grown by Pulsed Laser Deposition (PLD). The microstructure and density of the films have been studied by X-Ray Diffraction (XRD) and Reflectivity (XRR), while their optical properties were investigated by spectral reflectivity at vertical incidence and in-situ reflection electron energy loss spectroscopy (R-EELS). We report the R-EELS spectra for all the binary TMN and we identify their features (metal-d plasmon and N-p + metal-d loss) based on previous ab-initio band structure calculations. The spectral positions of p + d loss peak are rationally grouped according to the electron configuration (i.e. of the respective quantum numbers) of the constituent metal. The assigned and reported R-EELS spectra can be used as a reference database for the colloquial in-situ surface analysis performed in most laboratories.

Highlights

► Identification of the effect of ionization potential to the structure of PLD nitride films. ► Report of low energy electron loss spectra of NbN, MoN, HfN, TaN, WN. ► Correlation of the Np+Med loss peak with the metal’s valence electron configuration.

Introduction

The unique combination of properties of nitrides of the group IVb–VIb transition metals includes significant electron conductivity, refractory character, high hardness and chemical inertness. These properties make them suitable for applications in electronics, such as diffusion barriers of Al and Cu [1], [2], [3], ohmic contacts for III-nitride and phosphide optoelectronics [4], [5], [6], [7], components in micro-electro-mechanical systems (MEMS) [8], [9], in addition to the protective and decorative coatings' industry, which has been the main field of their applications [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. The optical and electrical properties of the transition metal nitrides (TMN) have been considered either as a piece of essential information for electronic applications or as a diagnostic tool for identifying in-situ their density, crystal and chemical quality [4], [21], [22], [23], [24], [25]. The optical method of choice of most authors is spectroscopic ellipsometry (SE) while there is only a handful of works dealing with the valence electron energy loss spectra (EELS) of TiN, ZrN, VN and CrN [26], [27], [28], [29]. Therefore, a thorough study of the EELS spectra of TMN and their association with the corresponding optical spectra and the existing band structure calculations [25], [30], [31], [32] are still lacking in the literature.

In this paper, we study thoroughly the structure, density, and the electronic properties of binary, single phase ZrN, HfN, NbN, TaN, MoN, and WN films of rocksalt structure grown by Pulsed Laser Deposition (PLD); TiN films grown with similar PLD conditions are also studied for comparison purposes. We report their EELS spectra and we identify their features based on existing band structure ab initio calculations. We correlate the spectral value of the p + d loss peak in EELS spectra with the plasma energy determined by optical analysis and the electronic structure of the constituent metals. We show that the values of the p + d loss peak in EELS spectra are rationally grouped according to the electron configuration (i.e. of the respective quantum numbers) of the constituent metal.

Section snippets

Experimental

The 200–300 nm thick films were grown on commercial Czochralski-grown n-type Si{001} (1–10 Ωcm) by reactive Pulsed Laser Deposition (PLD) [25], [33]. The substrates were cleaned in ultrasonic baths of tetra-chloro-ethylene, acetone and methanol, rinsed by deionized water and dried by pure N₂ gas shower prior to deposition in order to remove the possible organic contaminants on the surface. The PLD experiments were performed in a high-vacuum chamber (base pressure P_b ~ 5 × 10^− 6 Pa) at room temperature

Results and discussion

AES spectra revealed that the growth conditions implemented in this study for PLD resulted to stoichiometric MeN ([N]/[Me] = 0.98 ± 0.02), where Me = Zr, Nb, Mo, Hf, Ta, W; no traces of other elements have been detected by AES, except of minor O and C surface contamination. The crystal structure has been studied by XRD and supported by the density values determined by XRR and listed in Table 1. Fig. 1 shows the XRD patterns from the nitrides of the row-5 (having 4d valence electrons) and row-6

Conclusions

A wide variety of binary transition metal nitride films of rocksalt structure have been grown by PLD. Their texture and crystallite sizes are associated with the energetic conditions of growth (governed by the ionization potential of the metal target) and the mass of the depositing metal species. They are all good electron conductors whose density of conduction electrons has been evaluated through E_pu determination via optical modeling. Their valence EELS spectra exhibit a characteristic loss

Acknowledgement

This research project has been co-financed by the European Union (European Regional Development Fund- ERDF) and Greek national funds through the Operational Program “THESSALY- MAINLAND GREECE AND EPIRUS-2007-2013” of the National Strategic Reference Framework (NSRF 2007-2013).

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Citation Excerpt :
Going beyond the three aforementioned nitrides, of significant technological importance are all the nitrides of the transition metals of the group IVb-Vb-VIb (4–6 IUPAC) of the periodic table of elements, as shown in the reduced periodic table of elements in Fig. 1 (data taken from [184–187]); these nitrides can form cubic rocksalt-type crystals (B1-structure, Fm3m symmetry) and constitute a category of very important technological materials due to their exceptional mechanical properties, high melting point, refractory character and chemical stability over hostile environments. Thus, TMN are widely studied and used for a variety of applications, such as decorative coatings, protective and anti-corrosive coatings, in cutting tools and machining equipment; therefore, the works dealing with comparisons on their growth and properties are numerous and well-established [184,188–200]. They also exhibit electronic conductivity due to the partially filled valence d orbitals that are not completely hybridized with the N-2p electrons, as we will show below.
The nitrides of most of the group IVb-Vb-VIb transition metals (TiN, ZrN, HfN, VN, NbN, TaN, MoN, WN) constitute the unique category of conductive ceramics. Having substantial electronic conductivity, exceptionally high melting points and covering a wide range of work function values, they were considered for a variety of electronic applications, which include diffusion barriers in metallizations of integrated circuits, Ohmic contacts on compound semiconductors, and thin film resistors, since early eighties. Among them, TiN and ZrN are recently emerging as significant candidates for plasmonic applications. So the possible plasmonic activity of the rest of transition metal nitrides (TMN) emerges as an important open question. In this work, we exhaustively review the experimental and computational (mostly ab initio) works in the literature dealing with the optical properties and electronic structure of TMN spanning over three decades of time and employing all the available growth techniques. We critically evaluate the optical properties of all TMN and we model their predicted plasmonic response. Hence, we provide a solid understanding of the intrinsic (e.g. the valence electron configuration of the constituent metal) and extrinsic (e.g. point defects and microstructure) factors that dictate the plasmonic performance. Based on the reported optical spectra, we evaluate the quality factors for surface plasmon polariton and localized surface plasmon for various TMN and critically compare them to each other. We demonstrate that, indeed TiN and ZrN along with HfN are the most well-performing plasmonic materials in the visible range, while VN and NbN may be viable alternatives for plasmonic devices in the blue, violet and near UV ranges, albeit in expense of increased electronic loss. Furthermore, we consider the alloyed ternary TMN and by critical evaluation and comparison of the reported experimental and computational works, we identify the emerging optimal tunable plasmonic conductors among the immense number of alloying combinations.

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Structure, electronic properties and electron energy loss spectra of transition metal nitride films

Abstract

Highlights

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgement

J. Phys. Chem. Solids

Surf. Coat. Technol.

Surf. Coat. Technol.

Surf. Coat. Technol.

Surf. Coat. Technol.

J. Electron Spectrosc. Relat. Phenom.

Appl. Surf. Sci.

Surf. Sci.

Comp. Mater. Sci.

Surf. Coat. Technol.

Surf. Coat. Technol.

Annu. Rev. Mater. Sci.

J. Vac. Sci. Technol.

Appl. Phys. Lett.

Appl. Phys. Lett.

J. Appl. Phys.

J. Appl. Phys.

Appl. Phys. Lett.

J. Phys. D: Appl. Phys.