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Although TiN thin metallic film has attractive prospects in plasma applications, its optical properties still remain less well explored. Here we have fabricated TiN thin metallic film with an average of about 50 nm thickness by pulsed laser deposition on p-GaN substrates at 650°C. The morphology and crystal structure are characterized by atomic force microscopy and X-ray diffraction. The results exhibit a smooth surface and an obviously preferred (111) orientation. A combination of cathodoluminescence spectroscopy and a cross-sectional scanning electron microscopy is developed to study the absorption behavior of TiN thin metallic film systematically. The absorption coefficients of TiN thin metallic film obtained here are in good agreement with the values obtained by the spectroscopic ellipsometry.
© 2017 Optical Society of America
1. Introduction
Recently plasmonics has been of great use in many technological applications such as information technology, biological molecule detection, solar cells and data storage [1]. Though some devices have been experimentally realized and exhibit interesting properties, the conventional plasmonic materials still rely on the use of noble metals such as gold and silver [2–8]. Since their large real parts of metal permittivities and high losses at optical frequencies hinder most applications in real-world devices [9], new materials need to be developed. With the discovery of plasmonic ceramic materials, there are many alternative materials other than conventional metallic compounds which exhibit metallic properties and also provide advantages in device performance [9–11]. Titanium nitride (TiN), one of those materials, exhibits optical properties similar to gold with the added advantage of being very mechanically and thermally durable. As the carrier concentration is smaller, the magnitude of the real permittivity of TiN is much smaller than that of noble metals in the visible range [12–16]. Additionally, TiN belongs to ceramic materials that are non-stoichiometric and hence their properties combine metal-like and covalent characteristics. Unlike the case of noble metals, the optical properties of TiN may be tuned simply by changing the processing conditions. Another major advantage of titanium nitride is that they offer easy fabrication and integration with standard silicon manufacturing processes. All of the above factors make TiN promising alternative plasmonic materials in the visible and near-IR regions [17–20].
In our work, we have prepared TiN thin metallic film with about 50 nm by pulsed laser deposition on p-GaN substrate at 650°C. The morphology and crystal structure are characterized by Atomic Force Microscopy and X-ray Diffraction. By gradually varying the acceleration voltage of cathodoluminescence spectroscopy at room temperature, the absorption behavior of TiN thin metallic film is investigated systematically via the Beer-Lambert law [21]. Finally, we compare the obtained absorption coefficients with those obtained from the spectroscopic ellipsometry and give a discussion.
2. Experimental details
The TiN thin metallic film was deposited on p-GaN by pulsed laser deposition (PLD) method. The substrate was Mg-doped c-GaN 2-2.5 μm on Al2O3 with about 430 μm thickness. To clean the surface of substrates, p-GaN was rinsed with deionised water and then degreased by ethanol and acetone with an ultrasonic cleaner, respectively. After that, the substrates were dried by blowing with high pressure nitrogen and loaded into the vacuum chamber immediately. The target we used is 4 mm thick stoichiometric TiN target (purity of 99.9%) with 50.8 mm in diameter. An excimer KrF laser (λ = 248 nm, τ = 25 ns) was focused onto the rotated TiN target at 45° angle of incidence. The distance between the target and substrates was 80 mm. First, the substrates were heated to 600°C for an hour for purification and then rose to 650°C for the next step. Before the deposition, the chamber was pumped down to a base pressure of 5 × 10- 6 Pa and then N2 was injected with the flow rates of 30 sccm controlled by gas flow meters. The substrate was treated with N2 radio frequency plasma for 15 mins in order to remove the oxide layer on the p-GaN surface. During the deposition, we kept the pressure of the chamber at 3 Pa. The laser energy was 250mJ and the pulse frequency was 2Hz. There are two factors which determine the surface roughness of the thin film deposited by pulsed laser deposition system: the rate of the deposition determined by the laser frequency; the atom mobility on the surface of the p-GaN substrate. After deposition for 1 hour, the sample was in situ annealed for 1 hour. The thin film was golden and then split into several pieces for characterization.
3. Results and discussion
Figure 1 shows the X-ray diffraction (XRD) θ/2θ pattern of the TiN thin film deposited on c-plane p-GaN substrate by pulsed laser deposition. It was characterized by using Cu Kα radiation with a scan step of 0.02° (XRD, Bruker D8 Discover diffractometer). There are two strong peaks appeared in the Fig. 1. One peak at 36.347°is corresponding to the (111) reflection from TiN, the other peak at 34.880° is related to the (0002) reflection from p-GaN substrate. There is no other diffraction peak occurred here. It indicates that the TiN thin film preferred (111) orientation, which is the close packed plane paralleled to c-axis. According to the diffraction angle, the lattice parameter and the N/Ti ratio of the TiN thin film is about 0.427 nm and bigger than 1.00, respectively. Thus the TiN thin film we obtained here is weakly metallic film [22].
Fig. 1 XRD θ/2θ patterns of TiN thin film deposited on c-plane p-GaN substrates by pulsed laser deposition at 650°C.
In order to classify the surface morphology of the TiN thin film, atomic force microscopy (AFM, Veeco Dimension 3100) measurement was carried out. The 2D and 3D 1 × 1 μm2 AFM micrographs of the TiN thin film are shown in Fig. 2. The results show that the surface of the TiN thin film is quite smooth and dense. The root-mean-square (rms) surface roughness of the sample is only 0.259 nm.
Fig. 2 2D and 3D 1 × 1 μm2 AFM micrographs of the TiN thin film on p-GaN substrate by pulsed laser deposition at 650°C.
Then we measured the fracture surface of the TiN thin film by scanning electron microscopy (SEM, Quanta 400 FEG, FEI). Figure 3 shows the cross-sectional scanning electron microscopy (SEM) images of the TiN thin film. We can obviously see that the film is smooth with a distinct interface. Combined with the XRD and AFM result, high quality TiN thin film is obtained here by pulsed laser deposition. The average thickness is about 49.7 nm and the estimated growth rate of the TiN thin film by pulsed laser deposition is about 0.0138nm/s.
Fig. 3 The cross-sectional SEM images of the TiN thin film on p-GaN substrate by pulsed laser deposition at 650°C.
Figure 4 shows the CL spectrum of the p-GaN substrate and TiN thin film on p-GaN substrate based on the scanning electron microscope instruments (MonoCL3 + , Gatan), respectively. Compared with conventional surface science techniques, cathodoluminescence (CL) spectroscopy has several advantages. It can permit nanoscale depth resolution by varying incident beam energies. Especially, electron beams excitation can be tuned from surface to bulk sensitivity. There is also no doubt about exciting electron-hole pairs in wide band gap semiconductors such as III-N materials.
Fig. 4 (a) The CL spectrum of p-GaN substrate; (b) The CL intensity of p-GaN increases with the increasing accelerating voltage; (c) The CL spectrum of TiN thin film on p-GaN substrate; (d) The CL intensity of TiN thin film on p-GaN increases with the increasing accelerating voltage.
In Fig. 4(a), we can see that the intensity of the CL spectrum of the p-GaN substrate increases with the increasing accelerating voltage. Figure 4(b) shows the relationship between the peak value of the intensity and the accelerating voltage. When the accelerating voltage ranges from 8kV to 20kV, the intensity of CL from p-GaN substrate approximately linearly increases. Since the CL intensity sharply increases when the accelerating voltage was 24kV, it may reach saturation.
Figure 4(c) and 4(d) show the CL spectrum of TiN thin film on p-GaN substrate. Since the CL intensity from p-GaN substrate sharply increases when the accelerating voltage was 20kV, we only choose the results of the range of linearity from 10kV to 16kV for discussion. During this process, the TiN thin metallic film plays a role as an optical attenuator. The electron beam passes through the TiN thin metallic film and excites the p-GaN substrate luminescence. Before the CL spectrum of p-GaN substrate is to be collected, the luminescence also passes through the TiN thin metallic film. Thus the attenuation of the CL intensity from p-GaN substrate is mainly caused by the absorption of the TiN thin metallic film.
According to the thickness and the attenuation of the CL intensity, we can estimate the absorption coefficients of the samples (Fig. 5) by the Beer-Lambert law:
where I0 is the CL intensity of the p-GaN substrate, I is the CL intensity of TiN thin metallic film on p-GaN substrate, L is the thickness of the TiN thin metallic film and α is the absorption coefficient of the TiN thin metallic film. From the former relation, α can be denoted as this Eq. (2):The average value and the absorption coefficients of the TiN thin metallic film are given in Table 1. As the accelerating voltages increase, the absorption coefficient we calculate becomes gradually larger. It may be ascribed to the influence of electron scattering on the TiN thin metallic film with increasing accelerating voltages. The average value of the absorption coefficients we calculate is about 2.93 × 105 cm−1.
Table 1. The CL intensity of p-GaN, TiN thin film coated p-GaN and the absorption coefficients of the TiN thin film.
In order to test the absorption coefficient we obtained, the films were characterized from 300 to 1100 nm in 5 nm steps by a variable angle spectroscopic ellipsometer (V-VASE, J.A. Woollam Co.). As well known, spectroscopic ellipsometry is a widely used tool to measure the optical properties of the index of refraction (n) and the extinction coefficient (k) [23–25] of materials, owing to its non-destructive, non-contact, and high-precision characteristics. For non-absorbing films, the imaginary index is small so we only have to extract two parameters, real index and thickness, which we can do uniquely. Since TiN thin metallic films belong to absorptive metal thin films, the measurement of the thickness and the refractive index becomes correlated. In this case we have to fit the real index, imaginary index, and thickness with just Δ and Ψ. Here we utilize Drude-Lorentz models to fit the optical constants of TiN thin metallic films [28–30]. The model includes two parts: the Drude part represents the optical response from arising free carriers and the Lorentz part represents the interband losses. Suppose there are three layers in our sample: GaN film on Al2O3 substrate, the deposited TiN layer and a surface oxide layer when the sample is exposed to atmosphere.
Figures 6(a) and 6(b) show that the optical constants like Δ and Ψ are well fitted via these models. Since the mean square error (MSE) is only 0.72, the index of refraction (n) and the extinction coefficient (k) of the TiN thin metallic film showed in Fig. 7 are of credibility. When the wavelength is 430 nm, the extinction coefficient (k) is 1.0742. The absorption coefficient (α) is then calculated by Eq. (3):
λ is the wavelength of light in centimeters. The absorption coefficient we obtained here is about 3.066 × 105 cm−1. The result agrees well with the value obtained via the former method.Fig. 6 The fitting picture of the TiN thin film on p-GaN substrate by pulsed laser deposition at 650°C. (a) Δ, (b) Ψ.
4. Conclusion
High quality TiN thin metal films are deposited on p-GaN Substrates by pulsed laser deposition. The X-ray Diffraction and Atomic Force Microscopy measurements show that the films are of high quality with smooth surface and a (111) -preferring orientation. By gradually varying the acceleration voltage of the cathodoluminescence (CL) spectroscopy, the different CL intensities of TiN thin metallic film on p-GaN substrate are obtained compared with that of the p-GaN substrate. Combined with the SEM measurements, the absorption coefficient of the TiN thin metallic film is estimated about 2.93 × 105 cm−1 based on the Beer-Lambert law. The result obtained here agrees well with the value obtained by the spectroscopic ellipsometry.
Funding
National Natural Science Foundation of China (No. 61674163) and (No. 61474133).
Acknowledgments
The authors are grateful for the technical support of Nano-X from Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences (SINANO).
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