ISSN 00201685, Inorganic Materials, 2014, Vol. 50, No. 1, pp. 40–45. © Pleiades Publishing, Ltd., 2014. Original Russian Text © M.N. Solovan, V.V. Brus, E.V. Maistruk, P.D. Maryanchuk, 2014, published in Neorganicheskie Materialy, 2014, Vol. 50, No. 1, pp. 46–51.
Electrical and Optical Properties of TiN Thin Films M. N. Solovana, V. V. Brusa, b, E. V. Maistruka, and P. D. Maryanchuka a
Fed’kovich National University, ul. Kotsyubinskogo 2, Chernivtsi, 58012 Ukraine HelmholtzZentrum Berlin für Materialen und Energie, Kekuléstraße 5, 12489 Berlin, Germany email: solovan[email protected]
Received February 26, 2013
Abstract—TiN thin films have been grown by reactive magnetron sputtering. It has been shown that an Ohmic contact to TiN thinfilm can be made from indium. The TiN thin films have been shown to be ntype semiconductors with a carrier concentration of 2.88 × 1022 cm–3 and resistivity of ρ = 0.4 Ω cm at room tem perature. The activation energy for conduction in the TiN films at temperatures in the range 295 K < T < 420 K is 0.15 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 Eg = 3.4 eV. DOI: 10.1134/S0020168514010178
EXPERIMENTAL TiN thin films were grown on precleaned glass and glassceramic substrates in a LeyboldHeraeus L560 multipurpose vacuum system by dc reactive magne tron sputtering of a pure (99.99%) titanium target in an argon + nitrogen atmosphere. The titanium target, in the form of a disk 100 mm in diameter and 5 mm in thickness, was placed on the stage of a watercooled magnetron 7 cm below sub strates. The glass and glassceramic substrates were situ ated above the magnetron, and the stage was rotated during the sputterdeposition process to ensure trans verse homogeneity of the films. Prior to the deposition process, the vacuum chamber was pumped down to a residual pressure of 5 × 10–3 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 shortterm ion etching (bombard ment with argon ions). During the deposition process, the argon partial pressure in the vacuum chamber was 0.35 Pa and the nitrogen partial pressure was 0.7 Pa. The magnetron power was determined to be ~120 W. The deposition process was run for 15 min at a substrate temperature of ~570 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
INTRODUCTION Recent years have seen an intensive search for and investigation of various materials potentially attractive for application in highefficiency photoelectric devices. Titanium nitride (TiN) is a promising wideband gap material. It possesses an advantageous combina tion of physicochemical parameters: low resistivity, rather high transmission in the visible range, high reflectance in the infrared spectral region, high hard ness, high wear resistance, good chemical inertness, and good corrosion resistance [1–3]. Titanium nitride is used in optical filters, thinfilm resistors, and protective and decorative coatings [4, 5]. Owing to its physical properties, TiN is an attrac tive 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 consider able interest. Results of studies of some properties of thin TiN films were presented in many reports [1–10], but, to the best of our knowledge, no detailed studies of elec trical contacts to TiN thin semiconductor films or optical or electrical properties of transparent or con ductive TiN thin films have been reported in the liter ature. Such studies would be very helpful for further optimization of heterojunctionbased devices for elec tronics and solar power conversion because the effi ciency of such devices is significantly influenced by the optical and electrical characteristics of heterojunc tion components [11–13]. In this paper, we report the optical and electrical properties of TiN thin films produced by reactive mag netron sputtering. 40
ELECTRICAL AND OPTICAL PROPERTIES OF TiN THIN FILMS
0.6 Voltage, V
Current × 102, A
–10 –15 –0.4
15 Voltage, V
–10 –0.3 Fig. 1. 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 glassceramic substrates (with no peeling even when an external mechanical load was applied). To determine electrical parameters of thin films, highquality Ohmic contacts should be made to TiN thin films. To date, no detailed studies of the electrical prop erties 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 mea sured the current–voltage (I–V) characteristics of the contacts by a threeprobe 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 char acteristics) were only met by the indium contact, whose I–V curve is displayed in Fig. 1. The current– voltage characteristics of the titanium and chromium contacts are presented as plots in Fig 1 (insets a and b). INORGANIC MATERIALS
Electrical contacts for temperaturedependent electrical resistance (R) measurements were made on the opposite sides of the films through indium deposi tion at a substrate temperature of ⯝ 400 K. R was mea sured as a function of T in the temperature range T = 295–420 K. Since irreversible processes, for example, oxidation, during temperaturedependent 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 77–340 K. The effect of galvano and thermomagnetic side (parasitic) effects on mea surement results was eliminated by averaging mea surement results obtained at different current and magnetic field directions. The current through the sample was 10 μA, and the magnetic field was Н = 5 kOe.
SOLOVAN et al.
The net uncertainty in electrical conductivity was 2% 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 SF2000 spectrophotometer. Data were collected in the spectral region 200– 1000 nm at 1nm intervals. The thickness of the TiN films was measured by a standard procedure using an MII4 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 per mittivity of the medium, the Coulomb interaction between this excess electron and the nucleus is weak ened to a significant degree. Even small thermal exci tation 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 1 cm3 of TiN contains 2.6 × 1022 Ti atoms, the electron concentra tion should also be ~1022 cm–3, which is confirmed by experimental Hall effect and conductivity data (Fig. 2, insets a and b), n = 2.88 × 1022 см–3 at room tempera ture, in good agreement with previous results . Giv ing 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, 10]. Since titanium nitride has both metallic and cova lent (Ti–N) bonds, it can exhibit both metallic and semiconductor conductivity, in good agreement with data in the literature [3, 8–10]. The temperature dependence of the Hall coeffi cient RH = 1/(en) for the TiN thin films (Fig. 2, inset b) demonstrates that the conductivity of the films exhib its semiconducting behavior (RH decreases with increasing Т). The roomtemperature Hall coeffi cient, RH = –2.1 × 10–4 см3/C, agrees well with data in the literature [2, 9]. Hall coefficient and thermoelectric power mea surements for the TiN thin films suggest that the trans port process involves electrons; that is, the TiN films are ntype. Figure 2 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 ТіO2 films . The activation energy evaluated from the slope of the linear portion of the experimental R(T) curves for the TiN films is 0.15 eV and possibly corre sponds to the depth of a working energy level produced by the titanium’s electron that is not involved in cova lent chemical bonding. The sheet resistance Rs of a thin film (whose thick ness 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 lengthtowidth ratio L/W : Rs = RW/L, where R is the measured resistance. The calculated roomtemperature Rs value is 40 kΩ/䊐. Since the film thickness is d = 100 nm, we obtain ρ = 0.4 Ω cm. Optical properties. The optical properties of thin films (their refractive index n(λ), absorption coeffi cient α(λ), and extinction coefficient k(λ)) can be assessed by independent reflectance and transmit tance measurements. If the condition n2 Ⰷ k2 is fulfilled (and there is no interference), the transmittance of a sample of appro priate thickness d can be represented by the formula  T =
(1 − R ) 2 [1 + (λα 4πn)2]
(1) . e αd − R 2e −αd Since n2 Ⰷ k2, that is, αλ/4πn < 1, the absorption coefficient at transmittances in the range from (1 – R)/(1 + R) to 10% can be found as
(1 − R) 2 e −αd 1 − R 2e −2αd
⎡(1 − R ) 2 ⎤ (1 − R ) 4 α = 1 ln ⎢ + + R2⎥ . 2 d ⎢⎣ 2T 4T ⎥⎦
Relation (3) is valid when interference effects at the film–substrate interface can be neglected; that is, when there is no welldefined interference pattern in the transmission spectrum. Figure 3 presents the transmission, reflection, and absorption spectra of a TiN thin film. It can be seen in Fig. 3 that the absorption coeffi cient increases sharply near the fundamental absorp tion edge. In addition, the absorption coefficient increases at wavelengths λ > 500 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
ELECTRICAL AND OPTICAL PROPERTIES OF TiN THIN FILMS
10.6 σ, S/cm
0 120 240 Temperature, K
360 –RH × 104, cm3/C
9.8 Ea = 0.15 eV
3.0 103/T, K–1
240 T, K
Fig. 2. Arrhenius plot of resistance for TiN films. Insets: temperaturedependent (a) electrical conductivity and (b) Hall coeffi cient of the films.
(n + 1) from which we obtain 
(5) n =1+ R . 1− R As seen in Fig. 4, the refractive index n (λ) calcu lated by Eq. (5) for the TiN films increases with increasing wavelength in the range λ > 500 nm, which is caused by the increase in reflectance in the infrared spectral region. The sharp rise in refractive index at wavelengths λ < 500 nm is due to the increase in reflec tance 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)/4π . The extinc tion coefficient also rises sharply near the fundamental absorption edge of the films under investigation (Fig. 4, inset). At the same time, in the transmission window of the films (λ > 500 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 INORGANIC MATERIALS
40 2 20
2 1 0
600 800 Wavelength, nm
Fig. 3. (1) Transmission, (2) absorption, and (3) reflection spectra of TiN thin films.
α × 10–4, cm–1
( n − 1) 2
(6) α ( hν) = A ( hν − E g ) , where the coefficient А depends on the carrier effec tive mass. This α(hν) behavior suggests that the mate rial of the TiN thin films grown by dc reactive magne tron sputtering is a direct gap semiconductor. By extrapolating the linear portion of the plot of (αhν)2 = f(hν) against hν to zero absorption coefficient (Fig. 5),
R, T, %
refractive index, n(λ), for the thin films under investi gation using the relation
SOLOVAN et al. n 5
0 3 300
600 Wavelength, nm
600 800 Wavelength, nm
Fig. 4. Spectral dependence of the refractive index for TiN thin films. Inset: spectral dependence of the extinction coefficient.
(αhν)2 × 10–10, eV2/cm2
1 3.4 eV 0 1.5
Photon energy, eV Fig. 5. Plot of (αhν)2 against photon energy for TiN thin films.
the band gap of the thin TiN films was determined to be Eg = 3.4 eV. CONCLUSIONS TiN thin films have been grown by reactive magne tron 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 mea sured as a function of temperature, R = f(T). The acti vation energy for conduction in the TiN films at tem peratures in the range 295 K < Т < 420 K has been determined to be 0.15 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 roomtemperature resistivity of the TiN thin films is ρ = 0.4 Ω cm. Studies of transport processes in the TiN thin films at temperatures in the range 77 K < Т < 330 K have shown that the TiN films are ntype semiconductors with a roomtemperature carrier concentration of 2.88 × 1022 cm–3. We have measured the transmission and reflection spectra of the TiN thin films. The main optical con stants 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 Eg = 3.4 eV. REFERENCES 1. Gagnon, G., Currie, J.F., Beique, C., et al., Character ization of reactively evaporated tin layers for diffusion barrier applications, J. Appl. Phys., 1994, vol. 75, no. 3, p. 1565. 2. Andrievskia, R.A., Dashevskyb, Z.M., and Kalinni kova, G.V., Conductivity and the Hall coefficient of nanostructured titanium nitride films, Tech. Phys. Lett., 2004, vol. 30, no. 11, p. 930. 3. Kiran, M.S.R.N., Krishna, M.G., and Padmana bhan, K.A., Growth, surface morphology, optical INORGANIC MATERIALS
ELECTRICAL AND OPTICAL PROPERTIES OF TiN THIN FILMS
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Translated by O. Tsarev