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Abstract
In2Te5 films were deposited by the magnetron sputtering method. The structural properties of both the as-deposited and annealed films have been studied respectively by X-ray diffractometer and Raman spectrometer. As a result, the formation of the Te-Te bond was observed in Raman spectra of the film sample annealed at 400 ºC. A spectroscopic ellipsometer was employed to measure the optical constants of the films. The optical band gap of the films varies with the annealing temperatures, which is attributed to the Burstein-Moss effect. In the meantime, the nonlinear optical characterization of the films was investigated by the open-aperture Z-scan technique, and it shows an apparent reverse saturation absorption. The effect of the nonlinear absorption coefficient should be related with the unsaturated defects and free carrier absorption.
© 2017 Optical Society of America
1. Introduction
Chalcogenide has fascinated a lot of researchers for its promising electrical and optical properties [1–5]. Due to the relatively wide transparency window, low-energy phonons, photo-induced phenomena and high indices of linear and nonlinear refraction [6], it is meaningful to study chalcogenide in advancing the next generation photonic chip platform for ultrafast optical signal processing [7–11]. Indium telluride, a binary chalcogenide, is a well-known direct band gap semiconductor with defect structure [12]. It has always been regarded as a kind of ideal material for radiation detector, switching and photovoltaics [13–16]. However, few studies have been reported on the In2Te5 film. On the other hand, it is well known that thermal treatment influences greatly on the physical properties of thin films [17–19]. Also, the In2Te5 film is a kind of thermal sensitive phase change materials. Therefore, it is necessary to investigate the physical properties of In2Te5 thin films.
In this work, the In2Te5 thin films were prepared by the magnetron sputtering method and annealed in nitrogen at different temperatures. We focus on the thermal treatment as a tunable tool to affect the properties of In2Te5 films. Furthermore, the structures, physical and optical properties of the films have been discussed.
2. Experimental
Thin films of In2Te5 were deposited on both Si (100) and fused quartz substrates by the magnetron sputtering technique at room temperature. The substrates were cleaned using an ultrasonic cleaner according to the standard cleaning process. The purity of target In2Te5 is about 99.999%. The chamber was evacuated down to approximately 7 × 10-6 mbar. And the working pressure was approximately 2.8 × 10-3 mbar. The sputtering power of the In2Te5 target was controlled at 70 W in radio-frequency mode. The thickness of the sample films is about 100 nm. Then, the film samples prepared under the same conditions were annealed under nitrogen atmosphere at 150 , 250 , 300 , 350 and 400 respectively.
The fabricated In2Te5 films were detected by X-ray diffractometer (XRD) (Bruker D8 Advance) with Cu-Kα (λ = 1.54056 Å) radiation. The diffraction angles were set from 10°to 60°at 0.02 ° interval each step. The Raman spectra of the films were recorded by Raman micro-spectroscopy (Nanofinder 30). The optical transmission spectra were obtained using a dual light path UV–VIS–NIR (Lambda 1050) spectrophotometer in the wavelength range from 400 nm to 2500 nm. The Spectroscopic Ellipsometer (V-VASE) was employed to obtain the films optical parameters. In the measurements of third-order nonlinearities, a Ti: sapphire laser (Spectra Physics, Spitfire Ace) with pulse duration of 100 fs at the repetition rate of 1kHz 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 was set to be low, which is equivalent to the laser density I0 of 60 GW/cm2, so that thermal effect and damage to the sample could be avoided.
3. Results and discussion
The structural features of all the films were analyzed according to the XRD pattern. Figure 1 shows the XRD patterns of both as-deposited and annealed In2Te5 film samples. The absence of structural peaks for as-deposited film confirms the amorphous state of In2Te5 as well as the sample at the annealing temperature of 150 . It indicates different polycrystalline characteristics of remaining samples that the emergence of sharp Bragg diffraction peaks at higher annealing temperature such as 250 , 300 350 and 400 . The crystalline state of In2Te5 samples annealing under low temperatures is monoclinic. Obviously, the preferential orientation of samples was changed into (404) and (321) peaks at high annealing temperature of 400 from that of in (008) and (310) peaks at low annealing temperatures including 300 and 350 . It indicates that the thermal treatment acts as a tunable role affecting two crystalline phases. The (021) orientation at the annealing temperature of 400 corresponds to the Te crystalline peak in hexagonal phase, which is consistent with other literature reports [17]. In this case, part of In2Te5 changed into In2Te3 in cubic phase due to the separation of Te.
As shown in the Fig. 2, the Raman spectra of the films selected can identify that the phase transition of the samples is caused by the annealing effect. The energy range of vibrations extends to 300 cm-1 or so and is almost the same for all samples. This indicates the similarity in mass and bond forces among the two elements of In and Te. The main Raman peaks of pure In-Te films are consistent with other works [12, 20]. The peaks at 111 cm-1, 130 cm-1 and 149 cm-1 are related to the In-Te phase. The Raman spectra of Te chains display main band at 120 cm-1, 140 cm-1, 173 cm-1 and 260 cm-1 [21], therefore the bulge around 170 cm-1 is attributed to Te-Te vibrations in the film sample annealed at 400 °C. The peaks at both 130 cm-1 and 149 cm-1 shift slightly with the increasing of the annealing temperatures duo to the Te-Te formation. The disappearance of the peak at 111 cm-1 in the film annealed at 400 °C indicates the second phase transition occurrence. The results of Raman spectrum analysis are in agreement with those measured by XRD.
As a function of wavelength, the optical transmission spectra of the as-deposited and annealed In2Te5 films is shown in Fig. 3. The spectrum range is from 300 nm to 2500 nm. Obviously, the transmittance is less than about 5% at the short wavelength region of 300-600 nm due to photon absorption. It is observed that the transmittance of the films in visible light region is lower than that in the near-infrared light region. The transmission spectrum of the as-deposited film is similar to that of the film sample annealed at 150 °C. It reflects the absence of phase transition in the annealed sample, which was coincident with the results of Raman and XRD measurements. All of the absorption edges of the films shift toward longer wavelength (red-shift) during the process of the first phase transition, which indicates the narrowing of band gap. However, the absorption edge of the sample annealed at 400 °C shifts toward shorter wavelength (blue-shift) during the process of the second phase transition, which indicates the widening of band gap. For the film annealed at 150 °C, the largest transmittance is obtained over the whole spectrum range, also it is not less than about 50% in the near-infrared region. Therefore, it is possible for the chalcogenide glasses to be applied in infrared fiber optics and other infrared systems [19]. Moreover, it is clear that thermal treatment can be as an effective mean for tunable of transmittance.
Both of refractive index (n) and extinction coefficient (k) curves for the as-deposited and annealed In2Te5 films is given in Fig. 4. As a pair of important optical constants, the refractive index and extinction coefficient curves (n and k spectra) can reflect the properties of the film samples. A stronger absorption region in k spectra confirm that optical transition energy levels in In2Te5 film will appear at the region around 400~500 nm. With the wavelength of the photon increases in the visible light region, the value variation tendency between n and k is opposite. The reason why the films at higher temperature (250 °C, 300 °C and 350 °C) have higher values of n during the first phase transition in visible region is that the unsaturated defects gradually disappear during annealing process. While the value of n decreases obviously at annealing temperature of 400 °C in the second phase transition, the values of n are 2.89-4.21 in the weak absorption region. In the wavelength region around 400~500 nm, it is the stronger absorption region of the samples. This should be the result of the resonance effect between photons and electrons, which causes coupling of electrons in the oscillating electromagnetic filed [22]. In near-infrared region, the values of k change gently. The study of refractive index n and extinction coefficient k is one of extremely important parameters as it is the prepared material for application in integrated optics, photo-sensor, photo-electronic, phase transition material, memory devices etc. [23].
The absorption coefficient α plays as an important parameter in the constants calculation of semiconductor films. It can be calculated from the extinction coefficient k by the following formula [24].
After the annealing process, the energy levels of the films seem to be changed which should be related to the combination of the new energy states or the decrease of the defects.The optical band gap () of the films can be expressed by using Tauc equation [25],
where is the photon energy, is a constant, 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 ()2 versus is shown in Fig. 5. The obtained values of are listed in Table 1. It is clear that the optical band gap is about 1.91 eV in the as-deposited film. The decreases from about 1.89 eV to 1.68 eV corresponding to the annealing temperature (at 250 , 300 and 350 ) increasing in the first crystalline phase transition. It is observed that the band gap widen to 1.99 eV in the second crystalline phase (at 400 ). The widening of the optical band gap was most primarily attributed to the Burstein-Moss effect shift [26]. These similar optical properties were observed in the previous works as well [27].
Table 1. The and for the as-deposited and annealed In2Te5 films
For a better understanding on the optical properties of the as-deposited and annealed films, it is necessary to investigate the dielectric constant. The components of the complex dielectric constants, () and () are related to the optical constants. The real part () represents the refraction of the radiation while the imaginary part represents the absorption of energy due to dipole dislocation [28]. Figure 6 shows the plots of and as a function of photon energy. It is obvious that the has the same behavior as the refractive index n because the values of the extinction coefficient k is smaller than that of the refractive index n of the samples. With the increasing of annealing temperature, there is a notable change in of the sample annealed at 400 , it should be related to the second phase transition.
The open-aperture (OA) Z-scan technique was employed to study the nonlinear optical properties of the In2Te5 films with femtosecond laser pulses at the wavelength of 800 nm. No absorption band appears at the measuring wavelength, which implies that the third-order optical nonlinear properties originate from pure electronic distortion with ultrafast response time. In order to ensure that no permanent photo-induced change on each sample has occurred during laser irradiation, we have repeated the experiment by illuminating the sample on the same position. The OA Z-scan results of the films were tested under the incident intensity (I0) of about 60 GW/cm2 which is shown in Fig. 7. It can be seen that the curve consists of a normalized transmittance valley, which indicates a reverse saturation absorption (RSA) in the In2Te5 films. Obviously, the amorphous films (as deposited and 150 °C annealed) exhibit stronger RSA than the films in the polycrystalline phase (annealed at 250 °C, 300 °C, 350 °C and 400 °C). It should be related with the decrease of both the unsaturated defects and free carrier absorption [29].
The raw data was fitted by using typical OA Z-scan theory proposed by Sheik-Behae. The nonlinear absorption coefficient , defined as , can be calculated by the follow equation [10],
where is the normalized transmittance of the OA Z-scan result, is the incident laser intensity in the focal plane, is the longitudinal displacement of the samples, is the Rayleigh length, is the effective thickness of samples which is given by the follow expression [9].Here is the linear absorption coefficient at 800 nm wavelength, and L is the physical thickness of the thin films which is obtained by a step profiler. The obtained value of nonlinear absorption coefficient is shown in Table 1. The absorption coefficient of the as-deposited film is apparently larger than that of the annealed films. The absorption coefficient and reverse saturation absorption can be tunable by the annealing process in crystalline phases of the samples, which is important in nonlinear optical application [7, 9, 10].4. Conclusions
The In2Te5 chalcogenide films were prepared by our magnetron sputtering system and annealed in nitrogen at different temperatures. The XRD analysis shows that the annealing process induces amorphous-crystalline transformation. The Raman spectra results are coincident with XRD analysis and reveal the Te-Te formation occurrence in the film annealed at 400 °C. The Spectroscopic Ellipsometer analysis indicates that the optical constants can be effective tunable by the thermal treatment. The variation of the optical band gap of the In2Te5 films is attributed to the Burstein-Moss effect. Nonlinear optical characterization of the films studied by the OA Z-scan technique, it shows apparently reverse saturation absorption. This property has potential applications in nonlinear devices. All the results reveal that the thermal treatment with the In2Te5 films is an effective mean to modulate the structure, physical and optical properties of them.
Funding
Natural Science Foundation of Shanghai (17ZR1402200,13ZR1402600); National Natural Science Foundation of China (60578047,61427815).
Acknowledgments
The authors would like to express their sincere thanks for the financial support by the funding under Grant Nos. 17ZR1402200, 13ZR1402600, 60578047 and 61427815. The authors thank Prof. L. Y. Chen for effective backup.
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