ISSN 10637834, Physics of the Solid State, 2013, Vol. 55, No. 11, pp. 2234–2238. © Pleiades Publishing, Ltd., 2013. Original Russian Text © M.N. Solovan, V.V. Brus, P.D. Maryanchuk, T.T. Kovalyuk, J. Rappich, M. Gluba, 2013, published in Fizika Tverdogo Tela, 2013, Vol. 55, No. 11, pp. 2123–2127.
SEMICONDUCTORS
Kinetic Properties of TiN Thin Films Prepared by Reactive Magnetron Sputtering M. N. Solovana, *, V. V. Brusa, b, P. D. Maryanchuka, T. T. Kovalyuka, J. Rappichb, and M. Glubab a
Yuriy Fedkovych Chernivtsi National University, ul. Kotsubinskogo 2, Chernivtsi, 58012 Ukraine * email: solovan
[email protected] b HelmholtzZentrum Berlin für Materialien und Energie, Kekuléstr. 5, Berlin, 12489 Germany Received April 23, 2013
Abstract—Results of investigations of kinetic properties of TiN thin films prepared by dc reactive magnetron sputtering are presented. It is established that the TiN thin films are polycrystalline and possess semiconduc tor ntype conduction. The carrier concentration is ~1022 cm–3, while electron scattering occurs at ionized titanium atoms. DOI: 10.1134/S1063783413110255
1. INTRODUCTION Titanium nitride (TiN) is a promising broadband material, which has a successful set of physicochemi cal parameters, such as low resistivity, rather high transmittance in the visible spectral range, high reflec tance in the infrared spectral range, high hardness, high wear resistance, good chemical inertness, and resistance to corrosion [13]. The TiN thin films are often used for microelec tronic devices [4, 5], solar cells [6], and as protective and decorative coatings [7]. In addition, due to the biocompatibility, TiN is successfully used as a surface layer and electrical contact in orthopedic prostheses, cardiac valves, and other biomedical devices [8, 9]. Various methods of TiN film deposition are used [1–3, 10–12]. The most convenient method is the reactive magnetron sputtering. The feature of reactive sputter ing is that this method makes it possible to control some production parameters, such as the pressure of working gases during sputtering, the magnetron power, mixing, and the substrate temperature, and to obtain the highquality films with necessary properties. There are very few publications on studying the kinetic properties of titanium nitride [2, 13] and results of detailed temperature investigations of the kinetic properties of the TiN thin semiconductor films obtained by reactive magnetron sputtering. These investigations are important for the development of heterojunctionbased devices for electronics and solar power engineering since the operational efficiency of these devices is substantially affected by the electrical characteristics of semiconductor components of het erostructures [14–16].
In this study, we investigate the kinetic properties of TiN thin films prepared by the reactive magnetron sputtering. 2. EXPERIMENT The TiN thin films were deposited onto the prelim inarily cleaned glass and ceramic glass substrates 10 × 5 × 1 mm in size using a LeyboldHeraeus L560 uni versal vacuum installation with the help of the magne tron reactive sputtering of the pure titanium target in a mixture of argon and nitrogen with a constant voltage. Glass and ceramic glass substrates were arranged over the magnetron with the subsequent rotation of the table to provide the film uniformity over the thickness. Before the onset of the deposition process, the vacuum chamber was evacuated to a residual pressure of 5 × 10–3 Pa. The gas mixture of argon and nitrogen in the nec essary proportion proceeded from two independent sources during the deposition. To remove the uncontrollable surface contamina tion of the target and substrates, shortterm etching by bombarding argon ions was used. During the deposition, partial pressures in the vac uum chamber were ~0.35 Pa for argon and ~0.7 Pa for nitrogen. The magnetron power was ~120 W. Deposi tion was performed for ~15 min at a substrate temper ature of ~570 K. The kinetic conductivity coefficients were investi gated in a temperature range of 77–340 K. The sam ples for the Hall effect and electrical conductivity measurements had four Hall contacts and two current ohmic contacts, which were formed using a mask by the thermal deposition of indium at a substrate tem perature of ~400 K [14]. Kinetic coefficients were
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Intensity, arb. units 40
30
Optical phonon mode 547 cm−1 Acoustic phonon modes 320 cm−1 −1 218 cm
20 TiN 10
500 nm
Glass
0
(b)
200
400 600 Raman shift, cm−1
800
Fig. 2. Raman scattering spectrum of the TiN thin film.
3. RESULTS AND DISCUSSION
L = 100 nm
TiN
Glass
500 nm
Fig. 1. Microphotographs of the TiN thin films. (a) Surface and (b) crosssection.
measured at the dc current in a constant magnetic field. The influence of side (“parasitic”) galvanomag netic and thermomagnetic effects on the measurement results was excluded by means of averaging the results of measurements at various directions of the current and the magnetic field. The current flowing through the sample was ~10 μA and magnetic field was H = 5 kOe. The total error in the determination of the electri cal conductivity was ~2%, that of the Hall coefficient was ~6%, and when measuring the thermopower, it was no larger than 6%. The thickness of the TiN films was measured using a MII4 interferometer according to the standard pro cedure. The surface of the films and their cleavage was per formed using a Hitachi S4100 electron scanning microscope. The Raman spectra were measured using a LabRAM Raman microscope at a wavelength of laser radiation of 632.82 nm. PHYSICS OF THE SOLID STATE
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Microphotographs of the surface and the transverse section of the TiN thin films deposited by the reactive magnetron sputtering are shown in Fig. 1. A micro photograph of the surface (Fig. 1a) shows a rather high uniformity of the films under study (punctures are absent). It follows from a microphotograph of the transverse section (Fig. 1b) that the film thickness is ~100 nm. This value agrees well with the film thickness of ~100 nm, which was found using a MII4 interfer ometer. The stepped growth of the TiN thin films (Fig. 1b) (the stepped growth mode agrees well with the data [17, 18]) is caused by the fact that the sub strate temperature during the film deposition is ~570 K. This is much lower than the melting point of TiN (3200 K). The Raman spectrum of a thin TiN film is pre sented in Fig. 2. Peaks at 218, 320, and 547 cm–1 are observed in this spectrum. The presence of spread peaks indicates that the TiN thin film is polycrystal line. The peaks at 218 and 320 cm–1 correspond well to the acoustic phonon modes of TiN, which agrees well with the previously found values of 207 and 310 cm–1 for stoichiometric TiN [19], while the peak at 547 cm–1 belongs to optical phonon modes of TiN and also agrees well with the previously measured value of 550 cm–1 for TiN [19]. It is known that the firstorder scattering at 547 cm–1 is referred to the optical phonon modes of TiN and also agrees well with the previously measured value of 550 cm–1 for TiN [19]. It is known that the firstorder scattering for TiN with the NaCl type cubic lattice is forbidden [20]. Therefore, the presence of the firstorder Raman scattering indicates the presence of point defects, which are present even in the TiN stoichiometric samples [21, 22]. The first order peaks of acoustic modes are associated with vibrations of heavy Ti ions (usually 150–300 cm–1), 2013
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−RH × 104, cm3/C
T, K 60 0
120
180
240
300
360
4 −1.5 3 −3.0 2 −4.5 60
120
180 240 Raman shift, cm−1
300
360
α, μV/K
Fig. 3. Temperature dependence of the Hall coefficient for the TiN thin films.
Fig. 4. Temperature dependence of the thermopower for the TiN thin films.
while the firstorder peaks in the optical range appear as a result of oscillations of lighter N ions (as a rule, of 400–650 cm–1). The temperature dependence of the Hall coeffi cient RH = 1/(en) for the TiN thin films (Fig. 3) indi cates that the films are of the semiconductor type of conduction (RH decreases as T increases), while its roomtemperature value RH = –2.1 × 10–4 cm3/C agrees well with the published data [2]. Using the measurements of the Hall coefficient and the thermopower for the TiN thin films, it is estab lished that the electrons participate in the transfer phenomena, i.e., TiN films have the ntype conduc tion. The thermopower for thin TiN films increases modulo as the temperature increases, which is associ ated with an increase in the electron concentration with increasing T (Fig. 4). Measured values of electrical conductivity are pre sented in inset to Fig. 5. It is seen that thin films under study possess the semiconductor type of conduction, while the low electrical conductivity compared with that presented in [2, 13] is associated with the presence of oxygen in thin films, which passivates the nitrogen vacancies and thereby decreases the material conduc tivity. The presence of oxygen is caused by the insuffi ciently high vacuum in the production process. How ever, the electrical conductivity of these oxygencon taining TiN films is much higher than in the case of TiO2 [23–25]. Figure 5 shows the dependence of electrical con ductivity σ of the TiN thin films on the inverse temper ature in a semilogarithmic scale. Two straightlinear segments can be distinguished in it, which indicates the exponential dependence of electrical conductivity. The activation energy, which was determined from the straightlinear segment of dependence lnσ =
f(103/T) for the TiN film, is 0.01 eV in the temperature range 77 K < T < 210 K and, possibly, corresponds to the burial depth of the working level, which is formed by the excess (not participating in the covalent bond) electron of titanium, the nature of the appearance of which is explained further. As the temperature increases (210 K < T < 330 K), the activation energy increases and reaches 0.15 eV. An increase in the acti vation energy is associated with the fact that the elec trons start to occupy higher levels in the conduction band (above the Fermi level) as T increases, while the Fermi level increases higher in the conduction band as lnσ [Ω−1 cm−1] 1.6 ΔEa = 0.15 eV
0.8
σ, Ω−1 cm−1 4 2
0
120
0
240 T, K
360
ΔEa = 0.01 eV
−0.8
3
6 103/T, K−1
9
12
Fig. 5. Dependence of the electrical conductivity of the TiN thin films on the inverse temperature in a semiloga rithmic scale. The inset shows the temperature depen dence of the electrical conductivity.
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KINETIC PROPERTIES OF TiN THIN FILMS
charged center should pass. Therefore, the free path length is inversely proportional to the ion concentra tion. As the ion concentration increases, the mobility maximum lowers and shifts towards higher tempera tures. Thus, in the case of appearance of carrier scat tering by ionized atoms, the mobility increases as the temperature increases, which is observed in our case (Fig. 6), while low mobilities are caused by a higher ion concentration.
μ × 104, cm2/V s n, 1022 cm−3
3
9
6
2
1
100
200 300 T, K
400
3 60
120
180
240 T, K
300
360
Fig. 6. Temperature dependence of the mobility of the TiN thin films. The inset shows the temperature dependence of the electron concentration.
the temperature increases. Let us explain this phe nomenon in more detail. During the formation of the TiN compound, nitro gen, which has five valence electrons on the outer shell, gives them for the formation of chemical bonds with nearest neighbors. The titanium atom, which has four valence electrons on the outer shell, gives three electrons for the formation of covalent bonds with nitrogen (Ti–N), while the fourth electron is excess, i.e., it does not participate in the formation of covalent bonds. Due to high permittivity of the medium, the Coulomb interaction of this excess electron with the nucleus is considerably weakened. A weak thermal excitation is sufficient in order to detach the excess electron from the Ti atom. These electrons form shal low donor levels in the TiN band gap, and the low acti vation energy is necessary to transfer them into the conduction band. Therefore, a high concentration of conductivity electrons proportional to the number of Ti atoms occurs in TiN. Since ~2.6 × 1022 Ti atoms are arranged in 1 cm3 of TiN, the electron concentration also should be ~1022 cm–3, which agrees in the experi ment (insets in Fig. 6) and agrees well with [2, 26]. The titanium atom, when having lost the electron, is transformed into the positively charged particle (ion), which is immobile arrange in the crystal lattice site. The interaction of these ions with free electrons leads to that the metallic bond occurs in TiN in addi tion to the covalent bond. Each ionized atom induces the Coulomb field around it. The charge carriers, when entering the action region of this field, are sub jected to the Coulomb interaction, due to which, their initial trajectory is distorted. The ion concentration also substantially affects scattering. The larger the number of ions is, the smaller the distance between them is, and the closer free charge carriers to the PHYSICS OF THE SOLID STATE
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4. CONCLUSIONS The investigation of the surface morphology of thin films showed that the TiN film 100 nm thick is high quality. The Raman scattering spectra investigated contain peaks that are characteristic of titanium nitride and indicate that the titanium nitride film is polycrystal line. Using the investigations of the kinetic phenomena in the TiN thin films in a temperature range of 77 K < T < 330 K, it is established that the TiN films possess the semiconductor ntype conduction and the carrier concentration is ~1022 cm–3, while the electron scat tering occurs at the ionized titanium atoms. The activation energy, which was determined by the straightlinear segment of dependence lnσ = f(103/T) in a temperature range of 77 K < T < 210 K for the TiN film, is 0.01 eV. As the temperature increases (210 K < T < 330 K), it increases and reaches 0.15 eV. REFERENCES 1. G. Gagnon, J. F. Currie, C. Beique, J. L. Brebner, S. G. Gujrathi, and L. Onllet, J. Appl. Phys. 75, 1565 (1994). 2. R. A. Andrievski, Z. M. Dashevsky, and G. V. Kalinni kov, Tech. Phys. Lett. 30 (11), 930 (2004). 3. M. S. R. N. Kiran, M. Ghanashyam Krishna, and K. A. Padmanabhan, Appl. Surf. Sci. 255, 1934 (2008). 4. M. Tao, D. Udeshi, S. Agarwal, E. Maldonado, and W. P. Kirk, Solid State Electron. 48, 335 (2004). 5. M. N. Solovan, V. V. Brus, and P. D. Maryanchuk, Semiconductors 47 (9), 1174 (2013). 6. G. B. Smith, A. BenDavid, and P. D. Swift, Renew able Energy 22, 79 (2001). 7. F. Vaz, P. Cerqueira, L. Rebouta, S. M. C. Nascimento, E. Alves, Ph. Goudeau, J. P. Riviere, K. Pischow, and J. de Rijk, Thin Solid Films 447–448, 449 (2004). 8. K. H. Chung, G. T. Liu, J. G. Duh, and J. H. Wang, Surf. Coat. Technol. 188–189, 745 (2004). 9. W. Franks, I. Schenker, P. Schmutz, and A. Hierle mann, IEEE Trans. Biomed. Eng. 52, 1295 (2005). 10. Y. L. Jeyachandran, Sa. K. Narayandass, D. Man galaraj, Sami Areva, and J. A. Mielczarski, Mater. Sci. Eng., A 445–446, 223 (2007). 11. L. EscobarAlarcon, E. Camps, M. A. Castro, S. Muhl, and J. A. MejiaHernandez, Appl. Phys. A: Mater. Sci. Process. 81, 1221 (2005). 2013
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Translated by N. Korovin
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