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Abstract
Herein, an Al:InSe thin film is designed and synthesized by the direct current-radio frequency sputtering method. The obtained Al:InSe films possess smooth surface morphology with Al:InSe particles dispersion on sapphire substrates. Via aluminum (Al) doping, two-photon absorption and the doping energy level together induced free-carrier absorption, established on the conduction band. Al:InSe thin film displays a broad-band nonlinear absorptive band arising from 532 nm to 1064 nm, and exhibits effective third-order (βeff) and fifth-order nonlinear absorption coefficients (γeff) and relaxation time (τ) under femtosecond pulsed laser. This study provides a potential method for preparing Al:InSe thin film to investigate their nonlinear optical properties.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Selenide semiconductors have emerged as ideal materials for a wide range of applications in optical communication systems, nonlinear optical devices, photoelectric conversion, and photocatalysis [1–3]. Among the various selenide semiconductors materials, Indium selenide (InSe) semiconductors with high nonlinearity and ultrafast response time, high thermal conductivity and good thermal stability attract more and more attention from researchers [4,5]. Nowadays, InSe has been widely applied in multiple optical devices, such as optical sensors, optoelectronic amplifiers and fiber couplers [6–8]. To modulate and enhance its physical properties, InSe has been doped with group I, III and IV elements [9–13]. Among the various doped elements, aluminum (Al) can be act as an excellent dopant owing to its excess electrons relative to InSe, which can boost the absorption on ground state. Doping into InSe, Al will be effectively changed the band gap energy of InSe, which make the nonlinear signal move to broad-band.
The in-depth study of the nonlinear optical absorption in photo-excited semiconductor materials is essential to enhancing the efficiency of photodetector and photoelectric converter devices [14–18]. The femtosecond Z-scan technique and transient absorption spectroscopy technique have been attested to be the effective methods to investigate the mechanism of nonlinear absorption, electron-phonon scattering and carrier mobility [19,20]. Pervious literatures have reported that the variation of free carrier density in InSe plays a significant role in carrier dynamics, resulting in two-photon absorption (TPA) process [21,22]. Moreover, in consideration of the special properties for Al doped InSe, further research need to concentrate on the potential micro mechanisms. Generally, more mechanisms will compete in the optical nonlinearities of Al:InSe thin film, and make the interpretation much more complicated [23–25]. Therefore, it is essential to separate different nonlinear processes and determine their corresponding response time in Al:InSe thin film for its potential applications.
In this work, the morphology, structure and nonlinear optical properties of Al:InSe thin films were investigated. The Al:InSe thin films were prepared by direct current-radio frequency sputtering method. The scanning electronic microscopy (SEM) indicate that the Al:InSe films possess smooth surface morphology with Al:InSe particles dispersing on sapphire substrates, the mean size of Al:InSe nano particle are 20–30 nm and films thickness is 140 nm. The ultrafast nonlinear optical properties of Al:InSe thin films were analyzed via femtosecond Z-scan technique and transient absorption spectroscopy technique. The nonlinear absorption process of pure InSe and Al:InSe thin film are both TPA. However, Al:InSe thin film also possesses free-carrier absorption (FCA), which is induced by two-photon absorption and doping energy level. Thus the Al:InSe thin film displays a broad-band nonlinear absorptive band arising from 532 nm to 1064 nm. The effective third-order nonlinear absorption coefficients (βeff) and effective fifth-order nonlinear absorption coefficients (γeff) of Al:InSe thin film were increased with the increasing of incident beam energy (from 50 nJ to 500 nJ). In addition, the relaxation time values of Al:InSe thin film under 350 nm excitation (3.54 eV) were extracted to be τ1=1.85 ± 0.35 ps, τ2=12.5 ± 0.95 ps and τ3=2.7 ± 0.18 ns, respectively.
2. Preparation and characterization of the pure InSe and Al:InSe thin film
The pure InSe and Al:InSe thin films were prepared by direct current-radio frequency sputtering method, Al:InSe nano particle can well deposited on sapphire substrates and obtained the Al:InSe thin films [26,27]. The radio frequency power, reactant pressure, work temperature and sputtering time of the deposition process were 50 W, 3.0 Pa, 150 °C and 1 h, respectively. The surface morphology of Al:InSe thin film was examined by scanning electron microscopy (SEM), as shown in Fig. 1(a). The Al:InSe thin film tend to present uniform and smooth structures with mean size of 20–30 nm for Al:InSe nano particle. In addition, cross-section SEM of the Al:InSe thin film indicate the uniform and continuous distribution with the thickness is 140 nm (Fig. 1b). The quantitative elemental analysis of the Al:InSe thin film was measured by energy dispersive spectroscopy (EDS) in Fig. 1(c). It is clearly shown that the concentration of incorporated Al in the Al:InSe thin film was 7.29%, and according to the corresponding elemental mapping images (Fig. 1d), Al, In, and Se elements are found to be uniformly dispersed throughout the whole Al:InSe thin film.
Fig. 1. (a) SEM micrograph of the Al:InSe thin film. (b) Cross-section SEM micrograph of the Al:InSe thin film. (c) EDS image of the Al:InSe thin film. (d) Elemental mapping images of the Al:InSe thin film.
The linear absorption spectrum of the pure InSe and Al:InSe thin film were measured by UV-Vis-NIR spectrophotometer in Fig. 2(a). The results of linear absorption spectrum indicate that the optical absorption edge of Al:InSe thin film has a blue shift compared with that of the pure InSe. The reason for this phenomenon is that the absorption threshold moves towards the direction of high energy accompanying by the carrier concentration increases with Al doping, leading to Moss-Burstein drift [28,29]. The absorption data adheres to the following relation [30]
where α is the absorption coefficient, A is a constant, hν is the photon energy, Eg is the optical band gap and n is an index (n = 1, 2, 3. . .). The values of optical band gap (Eg) for pure InSe and Al:InSe thin film were estimated as 1.89 eV and 2.44 eV, respectively. In addition, the X-ray diffraction (XRD) spectra of the pure InSe and Al:InSe thin film is shown in Fig. 2(b). The diffraction peak at 2θ of 21.290 is indexed to (004) plane of InSe (PDF#34-1431). It can be clearly seen that (004) orientation becoming more manifest as Al doping. Meanwhile, the XRD spectra of the pure InSe and Al:InSe thin film were broad and featureless, manifesting the amorphous nature of the films. Furthermore, Al doping does not influence the crystal structure of the pure InSe.Fig. 2. (a) Linear absorption spectrum of the pure InSe and Al:InSe thin film. The inset shows the graphs of absorbance vs. photon energy. (b) XRD spectra of the pure InSe and Al:InSe thin film.
3. Nonlinear optical properties of the pure InSe and Al:InSe thin film
The magnitude of nonlinear absorption coefficient for pure InSe and Al:InSe thin film was measured by performing the femtosecond open aperture Z-scan. The Z-scan measurements were performed using a Nd:YAG tunable laser with a pulse width of 190 femtosecond at 532 nm, 800 nm and 1064 nm. The femtosecond open aperture Z-scan signals from the pure InSe under 800 nm excitation display reverse saturation absorption as shown in Fig. 3(a), yet no signals were observed when the excitation at 532 nm and 1064 nm in Fig. 3(b). In consideration of the linear absorption spectrum results for pure InSe in Fig. 2(b), when the energy of incident photon (hν) and optical band gap (Eg) meet to the requirement (hν<Eg<2 hν), TPA occurred [31]. The absorption change induced by TPA could be expressed as [32]
where α is the total absorption coefficient, α0 is the linear absorption coefficient, β is the nonlinear absorption coefficient and I is the energy of incident beam. The nonlinear absorption coefficient based on reverse saturation absorption (fitting result) is shown in Fig. 3(a). It was found that when the excitation is at 800 nm, although the energy of incident beam increased from 100 nJ to 800 nJ, β was evaluated as the same value (=2.1×10−7 cm/W).Fig. 3. (a) The femtosecond open aperture Z-scan signals of pure InSe measured at 800 nm. (b) The femtosecond open aperture Z-scan signals of pure InSe measured at 532 nm and 1064 nm.
As is well-known that doping is an excellent method to modulate the band gap structure of semiconductor, which is capable of overcoming the potential barriers between neutrally charged ground state and metastable charged state. On this occasion, doping energy level can make contribution to non-radiative recombination, carrier trapping, or persistent photoconductivity [33,34]. The following study focus on the nonlinear optical properties of Al:InSe thin film. Reverse saturation absorption behavior at three different excitation (532 nm, 800 nm and 1064 nm) under different energy of incident beam were measured as shown in Fig. 4. The illustration of the optical band gap for Al:InSe thin film, one photon absorption (OPA), TPA and FCA were shown in Fig. 5.
Fig. 4. The femtosecond open aperture Z-scan signals of Al:InSe thin film measured at 532 nm (a), 800 nm (b) and 1064 nm (c).
Fig. 5. Illustration of optical band gap for Al:InSe thin film, one photon absorption (OPA), two-photon absorption (TPA), free-carrier absorption (FCA) and carrier recombination processes.
The excitation wavelengths are far away from resonant wavelengths, TPA is believed to dominate the reverse saturation absorption in Al:InSe thin film. Moreover, it has been found that the effective third-order nonlinear absorption coefficients (βeff) and effective fifth-order nonlinear absorption coefficients (γeff) of Al:InSe thin film were increased with the increasing of incident beam energy (from 50 nJ to 500 nJ) under different excitation (532 nm, 800 nm and 1064 nm). The data of Z-scan experiments at 532 nm to 1064 nm under 190 femtosecond are selected as an example that is typical of two-photon absorption and doping energy level together induced FCA. Generally, two-photon absorption and doping energy level together induced FCA can be considered as a fifth-order nonlinear absorption. The total nonlinear absorption coefficient can be expressed as [35]
where α is the total absorption coefficient, α0 is the linear absorption coefficient, βeff is the effective third-order nonlinear absorption coefficient, γeff is the effective fifth-order nonlinear absorption coefficient and I is the energy of incident beam. It should be noted that the combination of two-photon absorption and doping energy level together induced FCA result. As for open aperture Z-scan, the normalized transmittance could be expressed as [36]
Table 1. Parameters of femtosecond open aperture Z-scan experiments at different excitation and incident beam energy.
In order to separate the different nonlinear processes and determine the corresponding response time in Al:InSe thin film, the transient absorption spectra of Al:InSe thin film were performed (Fig. 6). The excitation pulse at 350 nm (3.54 eV) emitted from an optical parametric amplifier pumped by a Yb:KGW femtosecond laser (1.20 eV, 190 fs, and 6 kHz) was chopped with a frequency of 137 Hz and focused loosely on the sample films. The probe pulses from the white-light supercontinuum (450 nm–1100 nm) were generated by focusing 1030 nm laser pulses onto a YAG crystal. The typical pump fluence was 75 µJ/cm2. The alteration of absorption intensity at specific wavelength obtained by transient absorption spectrum experiment is expressed by the alteration of optical density (ΔOD). The calculation method could be expressed as [37,38]
where λp is the wavelength of probe beams, t is the delay time and Ton and Toff is the transmission of probe beams with and without pump beams, respectively.Fig. 6. (a)Transient absorption spectra of Al:InSe thin film under 3.54 eV photon excitations. Transient absorption curves of Al:InSe thin film on short timescale (b) and long timescale (c) under 3.54 eV photon excitations.
Fig. 6(a) shows the transient absorption spectra of Al:InSe thin film excited at various delay times. The pump photon energy (3.54 eV) was utilized to overcome the optical band gap in Al:InSe thin film (2.44 eV). At 0.3 ps delay time, we observe a Drude like free carrier response, simultaneously a reverse saturation absorption signal (ΔOD>0) almost straightway centered at 680 nm after excitation. As delay time increases, the signal peak of transient absorption spectra does not shift obviously, indicating the reverse saturation absorption of the excited state in the whole delay time comes from the same excited state energy level. Furthermore, the spectral shapes at 300 ps and 1800 ps time delays are pretty close, manifesting the carrier lifetime is quite long. During the cooling of the hot carrier, the energy was released in the form of phonons, and these phonons are positioned at conduction band to raise the temperature of the host lattice [39–41]. As the delay time increases, hot spots can be generated within the focal volume by inter-band transition via TPA. These hot spots with significant phonon concentration could eventually enhance the nonlinear absorption and keep a broad-band transient absorption spectra persisting up to a few nanoseconds.
Fig. 6(b) and (c) present the transient absorption curves of the Al:InSe thin film on short and long timescales extracted under 3.54 eV photon excitations. The results show that the pump fluence does not affect the decay dynamics, which indicates that the measurements were carried out under low perturbation regime. The transient absorption curves of the Al:InSe thin film on total delay time via global analysis were fitted, and the triple exponential function could be expressed as [37]
4. Summary
In summary, the morphology, structure and nonlinear optical properties of Al:InSe thin film are investigated. Al doping provides more carriers, resulting in two-photon absorption and doping energy level together induced FCA, which were detected by femtosecond open aperture Z-scan technique and transient absorption spectroscopy technique in Al:InSe thin film. The Al:InSe thin film displays a broad-band nonlinear absorptive band arising from 532 nm to 1064 nm and large nonlinear absorption coefficients. Furthermore, it is worth mentioning that the transient absorption decay processes have three parts which are ascribed to carrier-phonon interaction, carrier recombination and bound carrier relaxation, respectively. This study proved the practicability of modulating the ultrafast nonlinear optical properties for Al:InSe thin film by utilizing the Al doping.
Funding
National Natural Science Foundation of China (NSFC) (11504072, 11704273); Natural Science Foundation of Jiangsu Province (BK20170375).
Acknowledgment
The authors would like to thank Xingzhi Wu for fruitful discussions and valuable input in the analysis of near-infrared optical nonlinearity enhancement. The authors also acknowledge the technical support from Prof. Yinglin Song.
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