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C: Plasmonics, Optical Materials, and Hard Matter
Structural and Optical Properties of Ti-Doped InTe Thin Films Yafei Yuan, Chunmin Liu, Jing Su, Ling Cheng, Jie Fang, Xintong Zhang, Yan Sun, Yanqing Wu, Hao Zhang, and Jing Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10862 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018
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The Journal of Physical Chemistry
Structural and Optical Properties of Ti-doped InTe Thin Films Yafei Yuan,† Chunmin Liu,† Jing Su,† Ling Cheng,† Jie Fang, † Xintong Zhang,† Yan Sun,‡ Yanqing Wu,# Hao Zhang,†,§ and Jing Li*,†,§ †
Department of Optical Science and Engineering, Fudan University, Shanghai 200433, China Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China # Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China § Shanghai Ultra-Precision Optical Manufacturing Engineering Center, Shanghai 200433, China ‡
ABSTRACT: Thin films of Ti-doped InTe chalcogenide were prepared by magnetron co-sputtering method. The optical and structural properties were investigated by various tools after the thermal treatment. All annealed films were highly polycrystalline with (211) as preferred orientation. Raman spectra results confirm that the structure of InTe films can be adjusted by Ti dopant. The transmittance is high in near infrared region. The refractive index (n) and extinction coefficient (k) of the films were measured by Spectroscopic Ellipsometer. The optical band gaps increase from 1.56 eV to 1.85 eV with increasing concentration of Ti. The third-order nonlinear optical properties of thin films were investigated by applying open aperture Z-scan method with femtosecond laser excitation at 800 nm wavelength. The results show reverse saturation absorptions which show the potential value of the films in optical limiting applications. The nonlinear absorption coefficient was calculated using a model developed by Sheik-Behae. The femtosecond pump-probe technique was employed to measure the transient reflectivity of the samples. The results indicate that the lifetime of the carrier can be adjusted by Ti dopant due to carrier trapping effect.
1. INTRODUCTION Chalcogenide glass has attracted considerable interest due to its interesting electrical and optical properties 1-4, i.e. relatively wide transparency window 5 , low-energy phonons 6, photo-induced phenomena 7 and high linear and nonlinear refractive indices 8. All of these properties make chalcogenide glass a promising candidate material in the next generation of photonic chip platform for ultrafast all-optical signal processing 9-13. However, the poor thermal stability limits chalcogenide in real applications. It has been reported that Ti dopant can improve the thermal stability of chalcogenide 14-16. Meanwhile when used as a common electrode, it might diffuse into functional layer. Therefore Ti dopant chalcogenide is widely used and investigated 14-20.
Indium telluride (InTe), a binary chalcogenide, is a well-known direct band gap semiconductor 21, regarded as a kind of ideal material for radiation detector, switching and photovoltaics 22-25. Generally, Ti-doped method is used to stabilize InTe in experiments. However, there is still lack of investigations on the three-order nonlinearity and effects of Ti-doped InTe up till now. In this paper, Ti-doped InTe thin films were fabricated by the magnetron co-sputtering method with different Ti concentrations. We focus on the incorporation of titanium as a tunable tool to affect the properties of the InTe thin films. Furthermore, the structural and physical properties of the Ti-doped InTe films have also been investigated.
2. EXPERIMENTAL METHODS
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Thin films of Ti-doped InTe films were deposited on Si (100) and fused quartz substrates by the magnetron co-sputtering technique at room temperature. The size of targets InTe (purity: 99.99%) and Ti (purity: 99.99%) is about 100 mm in diameter and 5 mm in thickness. The substrates were cleaned in an ultrasonic cleaner, rinsed in analytically pure acetone, alcohol and then dried using a nitrogen gun. The Background vacuum and working pressure are approximately 710-6 and 2.810-3 mbar, respectively. The sputtering power of the InTe target was set at 100 W in RF mode. And the sputtering power of Ti target was changed from 0 W to 25 W by a step of 5 W in DC mode. The temperature was maintained at 300 K for the 15-mindepostion. And then the films were annealed for 45 min in flowing nitrogen at 450 . The atomic ratios of the film samples were analyzed by electron dispersive spectrometry (EDS) equipped in a Tescan VEGA 3SBH scanning electron microscope (SEM). The crystallinity of the Ti-doped InTe films were identified by X-ray diffractometer (Bruker D8 ADVANCE) with Cu-Kα (λ 1.54056 Å) radiation. The diffraction angle was set from 20° to 60° at 0.02° interval in each step. A Raman micro-spectroscopy (Nanofinder 30) was employed to measure the spectral characteristics. The optical transmission spectra were obtained by using a dual beam UV–VIS–NIR (Lambda 1050) spectrophotometer with a wavelength range from 400 nm to 2500 nm. The V-VASE typed Spectroscopic Ellipsometer was employed to measure the optical parameters of the films. In the measurements of third-order nonlinearities, a Ti:sapphire laser (Spectra Physics, Spitfire Ace) with pulse duration of 100 fs at the repetition frequency of 1 kHz was used as the excitation source, and the operating wavelength was set at 800 nm. In the section of open-aperture (OA) Z-scan process, the power of incident laser density I0 was set to be as low as 60 GW/cm2, so that the thermal effect and damage to the sample can be avoided. To explore the dynamics time response of the films, the femtosecond pump-probe technique was employed to measure the transient reflectivity of the samples.
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The composition results of the Ti-doped InTe films measured by EDS are listed in Table 1. The concentration of Ti dopant increases with the sputtering power increasing of Ti target. The structural analysis of the annealed InTe films by the X-ray diffraction is shown in Figure 1. The sharp Bragg peaks bring out the high crystallinity of the samples. The observed diffraction peaks (200), (211), (420) and (322) were identified as tetragonal InTe structure 3, 21. The preferential orientation of the films in (211) plane is obviously shown. The main peaks corresponding to the (211) orientation of the films lie respectively at 25.36°, 24.99°, 24.91°and 24.87° with Ti dopant concentrations increasing. The (211) diffraction peaks shifted to the low angle direction slightly with the increase of Ti dopant concentrations which is attributed to the expansive lattice and larger cell volume after Ti doped 20. The (200) orientation appears in the pure InTe film and disappears in the Ti doped films which means the structure is tuned by the Ti dopant.
Figure 1. XRD pattern of the Ti-doped InTe films.
Table 1 Composition of Ti-InTe films tested by EDS. Samples
Ti Power
1
3. RESULTS AND DISCUSSION
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0
Composition
Abs. error
In50.07Te49.93
0.97%
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2
15
Ti3.66In49.54Te46.80
1.05%
3
20
Ti11.01In45.54Te43.45
1.12%
4
25
Ti16.65In40.20Te43.15
1.08%
To identify the structure change caused by Ti dopant, the Raman spectra of the films were measured and shown in Figure 2. The energy range of vibrations is nearly the same for all the samples and extends to around 300 cm-1. This indicates the similarity in mass and bond forces among these three elements. The main Raman peak of pure InTe film is consistent with previous work 3, 21. The Raman peaks at 101 cm-1 and 123 cm-1 are related to the In-Te phase 4. The Raman spectra of Te chains display main bands at 120 cm-1, 140 cm-1, 176 cm-1 and 260 cm-1. Therefore the strong peak at 123 cm-1 should come from a mixed mode formed by both In-Te and Te-Te vibrations 26. The Te rings and long Te chains would be broken with increasing of Ti dopant concentration which leads to the lack of the peaks at 193 and 260 cm-1. The existence of Ti-Te bonds will greatly enrich with the Ti dopant concentrations increasing, which increases the complexity of the random bonding model analysis. Theoretical calculation results reveal that Ti prefers to adopt distorted octahedral geometries in chalcogenide 27, 28 . Hence, we reasonably assume that TiTe2 is the configuration for Ti-Te compounds in the doped films. The Raman active vibration modes of the TiTe2 are A1g and Eg. The frequencies of the A1g and Eg vibrational mode are at around 118 and 140 cm-1, respectively 29. Thus, the bonding between Ti and Te will weaken the peak at 101 and 123 cm-1 and enhance the peak at 118 cm-1. Besides, the peak at 140 cm-1 of the doped films may be a mixed mode for both Ti-Te and Te-Te vibrations. It should be noted that the participation of Ti in the forming lattice should lead to the structure modification. The analysis is coincident with XRD result.
Figure 2. Raman spectra of the Ti-doped InTe films The optical transmission spectra of the films measured in the wavelength range from 300 nm to 2500 nm are shown in Figure 3. The sharp fall of transmittance spectra at the lower wavelength confirms that the deposited films are uniform and crystalline. The transmittance for the films in visible light region is lower than that in the near-infrared ranges. The transmittance of Ti-doped InTe thin films is lower than that of undoped InTe film in near-infrared region from 1200 nm to 2500 nm. The visible light transmittances of the sample films increase with the wavelength increasing. The maximum optical transmittance of the doped films is enhanced with the Ti dopant concentrations increasing at the wavelength of 800 nm.
Figure 3. Transmittance spectra for Ti-doped InTe films
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The wavelength dependence of refractive index (n) and extinction coefficient ( κ ) of the films were acquired by a spectroscopic ellipsometer and shown in Figure 4, which clearly reveals that the refractive index and extinction coefficient have an inverse relation. Moreover, a notable discrepancy at around 800 nm wavelength between n and κ spectra confirms that optical transition in InTe energy levels will appear here. It is obvious that the refractive index increases with increasing Ti dopant concentrations in visible light region in Figure 4(a). It is shown in Figure 4(b) that the extinction coefficient curve shifts towards the shorter wavelengths. The variation of the optical constants of InTe thin film with the doping of Ti element should be associated with the fact that Ti improves crystallinity of InTe, which is already proved in XRD as well as in Raman spectra analysis. The real part of the dielectric constant shown in Figure 4(c) is deduced from following relation 30, n κ
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The optical band gap broadening is primarily attributed to the Burstein-Moss effect.
(1)
The has the same trend as the refractive index. This observation is caused by the lower value of the extinction coefficient compared with that of the refractive index. The absorption coefficients ( ) can be calculated from the extinction coefficient by the following expression 31, α 4κ⁄
(2)
As the InTe films were doped with the Ti element, the energy level will be broadened due to the formation of new defect bands caused by Ti impurities. The optical band gap ( ) of the annealed Ti-doped InTe films can be calculated by using Tauc relation 32, h h
Figure 4. The refractive index, extinction coefficient and curves of Ti-doped InTe films
(3)
where h is the photo energy, is a constant, and n is equal to 1/2 for direct band gap materials. The optical band gap is obtained by extrapolating the linear portion of the plot to the energy axis. The plot of variation of (h )2 versus h is shown in Figure 5. It is clear that the optical band gap increases from 1.56 eV to 1.85 eV with increasing concentration of Ti dopant. This is consistent with previous research work 33, 34.
Because of the filling of the electronic states near the bottom of the conduction bands, the Fermi level was lifted into the conduction band when the thin films were doped with Ti, which leads to the energy band broadening. The increase of bandgap due to the Burstein-Moss effect can be written as ∆"# ,34 ∆"#
$%& &⁄' ∗ ()*
(4)
∗ Where n is the carrier concentration and ,-. is the effective mass. The equation shows that the increase of carrier concentration results in an increase in photonic bandgap, and photonic bandgap depends positively with n2/3. In this work, the carrier concentration increases with increasing Ti-doped concentration, which is confirmed by the following discussion based on femtosecond pump-probe measurements.
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The Journal of Physical Chemistry
films, as shown in Figure 6, are obtained by the incident intensity (I0) of about 60 GW/cm2. It can be seen that the curve consists of a normalized transmittance valley, which indicates a strong reverse saturation absorption (RSA) in this condition. It is clear that undoped InTe film exhibits stronger RSA than the Ti-doped films which should be associated with the structure changes. However, the RSA subsequently enhanced with increasing Ti doped concentration which should be related with free carrier absorption (FCA).
Figure 5. Allowed optical band gaps of the Ti-doped InTe films
The raw data are fitted by using typical OA Z-scan theory proposed by Sheik-Behae. The nonlinear absorption coefficient /011 , defined as α 2 3 /011 4, can be calculated by the following equation 11. 567 ∑I (J2
9:;? @
