Electrical and Optical Properties of TiN Thin Films

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1 ISSN -68, Inorganic Materials,, Vol., No., pp.. Pleiades Publishing, Ltd.,. Original Russian Text M.N. Solovan, V.V. Brus, E.V. Maistruk, P.D. Maryanchuk,, published in Neorganicheskie Materialy,, Vol., No., pp. 6. Electrical and Optical Properties of TiN Thin Films M. N. Solovan a, V. V. Brus a, b, E. V. Maistruk a, and P. D. Maryanchuk a a Fed kovich National University, ul. Kotsyubinskogo, Chernivtsi, 8 Ukraine b Helmholtz-Zentrum Berlin für Materialen und Energie, Kekuléstraße, 89 Berlin, Germany solovan-86@mail.ru Received February 6, Abstract TiN thin films have been grown by reactive magnetron sputtering. It has been shown that an Ohmic contact to TiN thin-film can be made from indium. The TiN thin films have been shown to be n-type semiconductors with a carrier concentration of.88 cm and resistivity of ρ =. Ω cm at room temperature. The activation energy for conduction in the TiN films at temperatures in the range 9 K < T < K is. ev. The optical properties of the TiN thin films have been investigated. The material of the TiN thin films has been shown to be a direct gap semiconductor with a band gap E g =. ev. DOI:./S6878 INTRODUCTION Recent years have seen an intensive search for and investigation of various materials potentially attractive for application in high-efficiency photoelectric devices. Titanium nitride (TiN) is a promising wide-bandgap material. It possesses an advantageous combination of physicochemical parameters: low resistivity, rather high transmission in the visible range, high reflectance in the infrared spectral region, high hardness, high wear resistance, good chemical inertness, and good corrosion resistance [ ]. Titanium nitride is used in optical filters, thin-film resistors, and protective and decorative coatings [, ]. Owing to its physical properties, TiN is an attractive material for application in various photoelectric devices [6, 7], so the study of the optical and electrical properties of thin titanium nitride films is of considerable interest. Results of studies of some properties of thin TiN films were presented in many reports [ ], but, to the best of our knowledge, no detailed studies of electrical contacts to TiN thin semiconductor films or optical or electrical properties of transparent or conductive TiN thin films have been reported in the literature. Such studies would be very helpful for further optimization of heterojunction-based devices for electronics and solar power conversion because the efficiency of such devices is significantly influenced by the optical and electrical characteristics of heterojunction components [ ]. In this paper, we report the optical and electrical properties of TiN thin films produced by reactive magnetron sputtering. EXPERIMENTAL TiN thin films were grown on precleaned glass and glass-ceramic substrates in a Leybold-Heraeus L6 multipurpose vacuum system by dc reactive magnetron sputtering of a pure (99.99%) titanium target in an argon + nitrogen atmosphere. The titanium target, in the form of a disk mm in diameter and mm in thickness, was placed on the stage of a water-cooled magnetron 7 cm below substrates. The glass and glass-ceramic substrates were situated above the magnetron, and the stage was rotated during the sputter-deposition process to ensure transverse homogeneity of the films. Prior to the deposition process, the vacuum chamber was pumped down to a residual pressure of Pa. An appropriate mixture of argon and nitrogen gases was prepared directly during the deposition process using two independent gas sources. Unintentional impurities (organic contamination and native oxide) on the target and substrate surfaces were removed by short-term ion etching (bombardment with argon ions). During the deposition process, the argon partial pressure in the vacuum chamber was. Pa and the nitrogen partial pressure was.7 Pa. The magnetron power was determined to be ~ W. The deposition process was run for min at a substrate temperature of ~7 K. The substrate temperature was monitored with a system of thermocouples situated in the vacuum chamber and was set by a controller on a control board. After the deposition process, the presence of TiN films on the substrates was evidenced by a change in the color of the substrate surface. The films adhered

2 ELECTRICAL AND OPTICAL PROPERTIES OF TiN THIN FILMS Current, A. (а)..6 Voltage, V Current, mа.... Voltage, V. (b) Current, A.. Voltage, V Fig.. Current voltage curve of an indium contact to a TiN thin film. Insets: I V curves of (a) a titanium and (b) a chromium contact.. well to the glass and glass-ceramic substrates (with no peeling even when an external mechanical load was applied). To determine electrical parameters of thin films, high-quality Ohmic contacts should be made to TiN thin films. To date, no detailed studies of the electrical properties of Ohmic contacts to TiN thin films have been reported, so we deposited three different metals (indium, chromium, and titanium) onto TiN thin films in order to find out which of them would ensure Ohmic contacts to titanium nitride. Next, we measured the current voltage (I V) characteristics of the contacts by a three-probe technique. I V measurements for the metallic contacts to the TiN thin films showed that criteria for Ohmic behavior of contacts (low resistivity, as well as linear behavior and symmetric shape of forward and reverse I V characteristics) were only met by the indium contact, whose I V curve is displayed in Fig.. The current voltage characteristics of the titanium and chromium contacts are presented as plots in Fig (insets a and b). Electrical contacts for temperature-dependent electrical resistance (R) measurements were made on the opposite sides of the films through indium deposition at a substrate temperature of K. R was measured as a function of T in the temperature range T = 9 K. Since irreversible processes, for example, oxidation, during temperature-dependent resistance measurements may change parameters of the film, the measurements were performed during both heating and cooling. Samples for Hall effect and electrical conductivity measurements had four Hall contacts and two Ohmic current contacts, which were made by indium thermal deposition through a mask. Transport coefficients were measured at dc in a static magnetic field in the temperature range 77 K. The effect of galvanoand thermomagnetic side (parasitic) effects on measurement results was eliminated by averaging measurement results obtained at different current and magnetic field directions. The current through the sample was μa, and the magnetic field was Н = koe. INORGANIC MATERIALS Vol. No.

3 SOLOVAN et al. The net uncertainty in electrical conductivity was % and that in the Hall coefficient was 6%. The uncertainty in thermoelectric power was within 6%. The transmission and reflection spectra of the thin films were taken on an SF- spectrophotometer. Data were collected in the spectral region nm at -nm intervals. The thickness of the TiN films was measured by a standard procedure using an MII- interferometer. RESULTS AND DISCUSSION Electrical properties. When TiN is formed, each nitrogen atom, whose outer shell contains five valence electrons, gives them away to form chemical bonds with its nearest neighbors. Each titanium atom, whose outer shell contains four valence electrons, gives away three electrons to form covalent bonds with nitrogen (Ti N), and the fourth valence electron proves to be redundant; that is, it is not involved in covalent bond formation [8, 9]. Because of the high dielectric permittivity of the medium, the Coulomb interaction between this excess electron and the nucleus is weakened to a significant degree. Even small thermal excitation is sufficient to detach the excess electron from the Ti atom. Accordingly, these electrons produce shallow donor levels in the band gap of TiN, and a low activation energy is needed to promote them to the conduction band. For this reason, TiN contains high concentration of conduction electrons, proportional to the concentration of Ti atoms. Since cm of TiN contains.6 Ti atoms, the electron concentration should also be ~ cm, which is confirmed by experimental Hall effect and conductivity data (Fig., insets a and b), n =.88 см at room temperature, in good agreement with previous results []. Giving away an electron, a titanium atom converts into a positively charged particle (ion), which resides on a lattice site, and, together with the other titanium ions and electrons, creates metallic bonding in TiN [9, ]. Since titanium nitride has both metallic and covalent (Ti N) bonds, it can exhibit both metallic and semiconductor conductivity, in good agreement with data in the literature [, 8 ]. The temperature dependence of the Hall coefficient R H = /(en) for the TiN thin films (Fig., inset b) demonstrates that the conductivity of the films exhibits semiconducting behavior (R H decreases with increasing Т). The room-temperature Hall coefficient, R H =. см /C, agrees well with data in the literature [, 9]. Hall coefficient and thermoelectric power measurements for the TiN thin films suggest that the transport process involves electrons; that is, the TiN films are n-type. Figure shows the temperature dependence of resistance for the TiN thin films. It follows from these data that the electrical conductivity of the thin films under investigation exhibits semiconducting behavior. The measurements were made during both heating and cooling. It can be seen that heating produces no changes in the films, which indicates that the TiN thin films have high thermal stability, in contrast to ТіO films []. The activation energy evaluated from the slope of the linear portion of the experimental R(T) curves for the TiN films is. ev and possibly corresponds to the depth of a working energy level produced by the titanium s electron that is not involved in covalent chemical bonding. The sheet resistance R s of a thin film (whose thickness is much smaller than a typical contact separation) is the resistance of a square sheet of the film, specified in unit of ohms per square. The sheet resistance of a sample in the form of a rectangle depends not on its linear dimensions but on its length-to-width ratio L/W : R s = RW/L, where R is the measured resistance. The calculated room-temperature R s value is kω/. Since the film thickness is d = nm, we obtain ρ =. Ω cm. Optical properties. The optical properties of thin films (their refractive index n(λ), absorption coefficient α(λ), and extinction coefficient k(λ)) can be assessed by independent reflectance and transmittance measurements. If the condition n k is fulfilled (and there is no interference), the transmittance of a sample of appropriate thickness d can be represented by the formula [] ( R) [ + ( λα πn) ] T = () αd αd. e R e Since n k, that is, αλ/πn <, the absorption coefficient at transmittances in the range from ( R)/( + R) to % can be found as Note that ( R) e T = Re αd αd () ( R) ( R) α= ln + + R. () d T T Relation () is valid when interference effects at the film substrate interface can be neglected; that is, when there is no well-defined interference pattern in the transmission spectrum. Figure presents the transmission, reflection, and absorption spectra of a TiN thin film. It can be seen in Fig. that the absorption coefficient increases sharply near the fundamental absorption edge. In addition, the absorption coefficient increases at wavelengths λ > nm, which is due to light absorption by charge carriers. From the measured reflectance of the TiN thin films, one can find the spectral dependence of the. INORGANIC MATERIALS Vol. No.

4 ELECTRICAL AND OPTICAL PROPERTIES OF TiN THIN FILMS.6 (а) Heating Cooling. σ, S/cm lnr [Ω] Temperature, K E a =. ev 6 R H, cm /C (b) 9.6 T, K /T, K.. Fig.. Arrhenius plot of resistance for TiN films. Insets: temperature-dependent (a) electrical conductivity and (b) Hall coefficient of the films. refractive index, n(λ), for the thin films under investigation using the relation ( n ) R =, ( n + ) from which we obtain [] () n = + R. () R As seen in Fig., the refractive index n (λ) calculated by Eq. () for the TiN films increases with increasing wavelength in the range λ > nm, which is caused by the increase in reflectance in the infrared spectral region. The sharp rise in refractive index at wavelengths λ < nm is due to the increase in reflectance near the fundamental absorption edge of the thin titanium nitride films. The extinction coefficient can readily be found using the relation k(λ) = λα(λ)/π []. The extinction coefficient also rises sharply near the fundamental absorption edge of the films under investigation (Fig., inset). At the same time, in the transmission window of the films (λ > nm), we observe a slight increase in extinction coefficient, which is caused by the increase in absorption coefficient. The absorption coefficient of the TiN thin films in the intrinsic absorption region was found to be well represented by the relation ( ) ( ), α hν = A hν Eg (6) where the coefficient А depends on the carrier effective mass. This α(hν) behavior suggests that the material of the TiN thin films grown by dc reactive magnetron sputtering is a direct gap semiconductor. By extrapolating the linear portion of the plot of (αhν) = f(hν) against hν to zero absorption coefficient (Fig. ), R, T, % Wavelength, nm Fig.. () Transmission, () absorption, and () reflection spectra of TiN thin films. α, cm INORGANIC MATERIALS Vol. No.

5 SOLOVAN et al. n k.. 6 Wavelength, nm 9 6 Wavelength, nm 8 Fig.. Spectral dependence of the refractive index for TiN thin films. Inset: spectral dependence of the extinction coefficient. (αhν), ev /cm..... Photon energy, ev. ev Fig.. Plot of (αhν) against photon energy for TiN thin films.. the band gap of the thin TiN films was determined to be E g =. ev. CONCLUSIONS TiN thin films have been grown by reactive magnetron sputtering. It has been shown that an Ohmic contact to a TiN thin film can be made by thermal evaporation of indium. The resistance of the films studied has been measured as a function of temperature, R = f(t). The activation energy for conduction in the TiN films at temperatures in the range 9 K < Т < K has been determined to be. ev, which seem to correspond to the depth of a working energy level produced by the titanium s electrons that are not involved in covalent chemical bonding. The room-temperature resistivity of the TiN thin films is ρ =. Ω cm. Studies of transport processes in the TiN thin films at temperatures in the range 77 K < Т < K have shown that the TiN films are n-type semiconductors with a room-temperature carrier concentration of.88 cm. We have measured the transmission and reflection spectra of the TiN thin films. The main optical constants of the TiN films have been determined and the material of the films has been shown to be a direct gap semiconductor with a band gap E g =. ev. REFERENCES. Gagnon, G., Currie, J.F., Beique, C., et al., Characterization of reactively evaporated tin layers for diffusion barrier applications, J. Appl. Phys., 99, vol. 7, no., p. 6.. Andrievskia, R.A., Dashevskyb, Z.M., and Kalinnikova, G.V., Conductivity and the Hall coefficient of nanostructured titanium nitride films, Tech. Phys. Lett.,, vol., no., p. 9.. Kiran, M.S.R.N., Krishna, M.G., and Padmanabhan, K.A., Growth, surface morphology, optical INORGANIC MATERIALS Vol. No.

6 ELECTRICAL AND OPTICAL PROPERTIES OF TiN THIN FILMS properties and electrical resistivity of -TiN x (. < x.) films, Appl. Surf. Sci., 8, vol., p. 9.. Gaoling Zhao, Tianbo Zhang, Tao Zhang, et al., Electrical and optical properties of titanium nitride coatings prepared by atmospheric pressure chemical vapor deposition, J. Non-Cryst. Solids, 8, vol., p. 7.. Li-Jian Meng and Santos, M.P., Characterization of titanium nitride films prepared by d.c. reactive magnetron sputtering at different nitrogen pressures, Surf. Coat. Technol., 997, vol. 9, p Dimitriadis, C.A., Lee, J.I., Patsalas, P., et al., Characteristics of TiNx/n-Si Schottky diodes deposited by reactive magnetron sputtering, J. Appl. Phys., 999, vol. 8, no. 8, p Kadelec, S., Musil, J., and Vyskocil, J., Growth and properties of hard coatings prepared by physical vapor deposition methods, Surf. Coat. Technol., 99, vols., p Jeyachandran, Y.L., Narayandass, Sa.K., Mangalaraj, D., et al., Properties of titanium nitride films prepared by direct current magnetron sputtering, Mater. Sci. Eng., A, 7, vols. 6, p.. 9. Samsonov, G.V., Nitridy (Nitrides), Moscow: Naukova Dumka, Samsonov, G.V and Vinitskii, I.M, Tugoplavkie soedineniya (Refractory Compounds), Moscow: Metallurgiya, Solovan, M.N., Brus, V.V., and Maryanchuk, P.D., Electrical and photoelectric properties of anisotype n-tin/p-si heterojunctions, Semiconductors,, vol. 7, no. 9, p. 7.. Brus, V.V., Ilashchuk, M.I., Kovalyuk, Z.D., et al., Electrical and photoelectrical properties of photosensitive heterojunctions n-tio /p-cdte, Semicond. Sci. Technol.,, vol. 6, paper 6.. Brus, V.V., Open-circuit analysis of thin film heterojunction solar cells, Sol. Energy,, vol. 86, p. 6.. Solovan, M.N., Maryanchuk, P.D., Brus, V.V., et al., Electrical and optical properties of TiO and TiO :Fe thin films, Inorg. Mater.,, vol. 8, no., p. 6.. Ukhanov, Yu.I., Opticheskie svoistva poluprovodnikov (Optical Properties of Semiconductors), Moscow: Nauka, 977. Translated by O. Tsarev INORGANIC MATERIALS Vol. No.

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