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INTRODUCTION. Titanium nitride (TiN) thin films are finding wide practical application in various semiconductor devices. [1–3] due to their favorable ...
ISSN 0030400X, Optics and Spectroscopy, 2014, Vol. 117, No. 5, pp. 753–755. © Pleiades Publishing, Ltd., 2014. Original Russian Text © M.N. Solovan, V.V. Brus, P.D. Maryanchuk, I.M. Fodchuk, V.M. Lorents, A.M. Sletov, M.M. Sletov, M. Gluba, 2014, published in Optika i Spektroskopiya, 2014, Vol. 117, No. 5, pp. 775–778.

CONDENSEDMATTER SPECTROSCOPY

Structural and Photoluminescent Properties of TiN Thin Films M. N. Solovana, V. V. Brusa, P. D. Maryanchuka, I. M. Fodchuka, V. M. Lorentsa, A. M. Sletova, M. M. Sletova, and M. Glubab a

b

Chernivtsi National University, Chernivtsi, 58012 Ukraine Helmholtz Zentrum Berlin für Materialien und Energie, Berlin, D12489 Germany email: [email protected] Received March 27, 2014

Abstract—Structural and photoluminescent properties of TiN thin films deposited by dc reactive magnetron sputtering are studied. It is found that TiN thin films are polycrystalline with a grain size of ~15 nm and have a NaCltype cubic crystal structure with a lattice constant of 0.42 nm. The TiN films under study exhibit pho toluminescence in the spectral range hν ≈ 2.1–3.4 eV at 300 K. DOI: 10.1134/S0030400X14110198

INTRODUCTION Titanium nitride (TiN) thin films are finding wide practical application in various semiconductor devices [1–3] due to their favorable combination of physico chemical characteristics (low specific resistance, suffi ciently high transmission coefficients in the visible spectral region and reflection coefficients in the UV region, high hardness and wear resistance, chemical inertness, and corrosion resistance [4–6]. In recent years, a large number of publications have been devoted to investigations of the properties of tita nium nitride [1–6], but there have been no detailed studies of the structural and photoluminescent prop erties of TiN semiconductor thin films deposited by reactive magnetron sputtering. These studies are very important for practical applications—in particular, for further development of devices based on het erotransitions for electronics and solarpower engi neering—because their efficiency strongly depends on the characteristics of the semiconductor heterostruc tures [7–10]. Previously [11–13], we studied heterotransitions obtained by deposition of thinfilm TiN on silicon and cadmium telluride substrates and found relatively moderate photoelectric parameters. In the present work, we study the structural and photoluminescent properties of TiN thin films depos ited by reactive magnetron sputtering. These investi gations are of practical importance for increasing the photoelectric conversion efficiency of such hetero structures. EXPERIMENTAL The TiN thin films were deposited onto preliminar ily purified glass substrates (typical size 5 × 5 × 1 mm) in a multipurpose LeyboldHeraeus L560 vacuum sys

tem by reactive magnetron spattering of a pure (99.9999%) titanium target in the atmosphere of an argon–nitrogen mixture at a constant voltage. The partial pressures in the vacuum chamber dur ing the deposition were ~0.35 Pa for argon and ~0.7 Pa for nitrogen. The magnetron power used was ~120 W. The deposition duration was ~15 min with a substrate temperature of ~570 K. The TiN film deposition tech nique is described in more detail in [14]. The thickness of TiN films was measured using an MII4 interferom eter. The Xray diffractograms for TiN films were obtained at 295 K on a Thermo Scientific ARL X’TRA Xray diffractometer using an Xray tube with a copper anode (λ ≈ 0.154 nm) at the Lashkaryov Institute of Semiconductor Physics, National Academy of Sci ences of Ukraine. This diffractometer operated in the vertical θ–θ Bragg–Brentano geometry. The Ramanscattering spectra were recorded on a LabRAM microscope at a laser wavelength of 632.82 nm. The photoluminescence spectra were measured on a multipurpose measuring complex, which included an MDR23 diffraction monochromator and a stan dard synchronous detection system [15]. The photolu minescence was excited by an LGN21 nitrogen laser with λ ≈ 0.337 µm. The spectra were plotted taking into account the complex sensitivity. The spectra are given as the numbers of photons in unit energy range Nω versus photon energy ћω. RESULTS AND DISCUSSION The Ramanscattering spectrum of a TiN thin film 100 nm thick is shown in Fig. 1. This spectrum shows peaks at 218, 320, and 547 cm–1, and their broadening indicates that the film under study is textured. The

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Intensity, arb. units 30

Intensity

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0

200

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600 800 Raman shift, cm–1

Fig. 1. Ramanscattering spectra of TiN thin films. See explanations in the text.

peaks at 218 and 320 cm–1 correspond to the acoustic phonon modes of TiN, which agrees well with 207 and 310 cm–1 previously obtained for stoichiometric TiN [16], while the peak at 547 cm–1 belongs to the optical phonon modes of TiN and also agrees with the 550 cm–1 previously measured for TiN [16]. It is known that the firstorder scattering for TiN with a NaCltype cubic lattice is forbidden [17]. Therefore, the occurrence of the firstorder Raman scattering points to the existence of a system of point defects, which exist even in stoichiometric TiN sam ples [18, 19]. The firstorder peaks of acoustic modes are related to the vibrations of heavy Ti ions (usually at 150–300 cm–1), while the firstorder peaks in the opti cal region appear as a result of vibrations of lighter N ions (as a rule, at 400–650 cm–1). Analysis of the Xray diffractograms given in Fig. 2 shows that the intensity maxima observed at angles θ = 37.05° and 43.04° correspond to reflections from the (111) and (200) planes. The existence of these peaks agrees with the data obtained in [20] for TiN films with a NaCltype cubic lattice. The calculated lattice con stants in this case are а = 0.42 ± 0.01 nm, which almost coincides with the lattice constant а = 0.424 nm for bulk stoichiometric TiN samples [21, 22]. However, to determine the lattice constant more precisely, it is nec essary to take into account internal stresses (deforma tions), and well as the size and disorientation of indi vidual crystallites. Using the wellknown Debye–Sherrer equation, we can determine the TiN grain size by the formula [20] (1) D = 0.94λ / β cos θ, where D is the grain size, β is the full width at half max imum of the diffraction peak, λ is the Xrayradiation wavelength, and θ is the diffraction angle. It is established that film grain size D is ~15 nm. This grain size testifies to a developed structure and is

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TiN (200)

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Fig. 2. Diffractograms of TiN thin films.

favorable for smoothing of the surface morphology, which can positively affect the film microhardness [23] and considerably improve the corrosion resis tance of the material [24]. Microdeformation ε and dislocation density δ in thin films are calculated from the relations [25] ε = β cos θ/4 ,

(2)

δ = 15(ε / a)D .

(3)

Note that we measure dislocation density δ as the dislocation length per unit volume of the crystal. The microdeformation and dislocation density are ε ≈ 2.4 × 10–3 and δ ≈ 5.8 × 1011 cm–2. Figure 3 presents the photoluminescence spectrum of TiN thin films at 300 K, which covers a large range of photon energies, ћω ≈ 2.1–3.4 eV. This spectrum may be formed by radiative transitions with participa tion of localized energy states in the forbidden gap. This is evidenced by the relation ћω < Eg. between the radiation energy and the forbidden gap width. Our investigation of the optical transmission by the well known method of [26] allowed us to determine the for bidden gap width to be 3.4 eV. The form of the depen dence of the absorption coefficient on the photon energy points to the occurrence of direct interband optical transitions, which determine the optical prop erties of TiN. The luminescence spectrum has a max imum in the region of ћω = 2.84 eV, which can be explained by recombination processes via simple donor states. These processes are caused by point defects of the crystal lattice. Among these defects, it is the nitrogen vacancies that form donor states. Their energy depth is estimated to be Ed = Eg – ћωm = 3.4 – 2.84 = 0.56 eV. Note that these states can capture excess electrons of titanium, which ensures a long radiative lifetime of nonequilibrium carriers. Because of this, the experimentally observed photolumines cence intensity at 300 K is low, which does not allow OPTICS AND SPECTROSCOPY

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STRUCTURAL AND PHOTOLUMINESCENT PROPERTIES OF TiN THIN FILMS Nω, arb. units 4

3

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0 2.0

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3.5 បω, eV

Fig. 3. Photoluminescence spectra of TiN thin films at 300 K.

one to perform complete detailed analysis of the recombination radiation nature. Since the layers were deposited by reactive magnetron sputtering, the possi bility of the presence of oxygen atoms as a background impurity is also not excluded [27]. CONCLUSIONS The structural and photoluminescent properties of a series of TiN thin films deposited by reactive magne tron sputtering have been studied. The Raman scatter ing spectra and Xray diffractograms of titanium nitride films showed intensity peaks typical for bulk stoichiometric polycrystalline TiN samples with a NaCltype cubical lattice. Based on the analysis of the Xray diffractograms for the TiN films studied, we determined lattice con stant а = 0.42 nm, grain size D ~ 15 nm, microdefor mation ε ≈ 2.4 × 10–3, and dislocation density δ ≈ 5.8 × 1011 cm–2. These films exhibit a lowintensity photolumines cence at 300 K in the rather wide spectral range បω ≈ 2.1–3.4 eV with a maximum at បω ≈ 2.84 eV. REFERENCES 1. X. Lu, G. Wang, T. Zhai, M. Yu, S. Xie, Y. Ling, C. Liang, Y. Tong, and Y. Li, Nano Lett. 12, 5376 (2012). 2. V. M. Vinokur, T. I. Baturina, M. V. Fistul, A. Yu. Mi ronov, M. R. Baklanov, and C. Strunk, Nature 452, 613 (2008). 3. M. Tao, D. Udeshi, S. Agarwal, E. Maldonado, and W. P. Kirk, Sol. St. Electron. 48, 335 (2004). 4. G. Gagnon, J. F. Currie, C. Beique, J. L. Brebner, L. Gujrathi, and S. G. Onllet, J. Appl. Phys. 75, 1565 (1994). OPTICS AND SPECTROSCOPY

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Translated by M. Basieva