Thickness-dependent nonlinear absorption behaviors in polycrystalline ZnSe thin films
Introduction
The nonlinear absorption (NA) behaviors of semiconductors have been extensively studied for many years because of scientific interests and their potential applications. Among II-IV compounds ZnSe is a significant material for the development of all-optical digital processing. Beside, these films also offer multiplicity applications in solid state device technologies [1], [2]. NA can be classified into two types: (i) transmittance increases with increasing optical intensity (saturable absorption (SA)); (ii) transmittance reduces with increasing optical intensity (two photon absorption (TPA), multiphoton absorption, and reverse saturable absorption). TPA process in semiconductor plays an important role when the laser energy is larger than half of the energy band gap of the polycrystalline films (Eg > hν > Eg/2) [3]. Recently, we investigated the effect of film thickness and doping on the NA and SA properties in very thin amorphous InSe [4], undoped GaSe, Sn and Ge doped GaSe [5] and Ga0.90In0.10Se and Ga0.85In0.15Se films [6]. Thinner films exhibited SA behaviors while thicker films exhibited NA behaviors for 4 ns, 65 ps, and 44 fs pulse durations. This observed behavior was attributed to decreasing localized defect states with decreasing film thickness. NA in amorphous semiconductors can be observed at nonresonant conditions since numerous localized defect states effectively blur the energy band gap. Therefore, one photon absorption (OPA), TPA, and free carrier absorption (FCA) and their saturations contribute to the nonlinear saturation behavior of these samples.
It is well known that the line and planar defects in the polycrystalline thin films and the crystalline size effect can lead to change energy band gap [7]. In this study, we studied the effect of the line and planar defects on the nonlinear absorption behaviors of ZnSe films. It was found that the energy band gap values decrease with increasing film thickness [8]. This behavior was attributed to increasing localized defect states on grain boundary with increasing film thickness. The band tail was found near the bottom of the conduction band according to Urbach tail. In this paper, we investigated the NA and SA properties of polycrystalline ZnSe thin films with open aperture (OA) Z-scan technique at 532 nm for 65 ps pulse duration. To derive the transmission in the OA Z-scan data, a theoretical model incorporating two photon absorption and its saturation was considered. Nonlinear absorption coefficients and saturation intensity thresholds were extracted from the fitting of the experimental results.
Section snippets
Experimental
ZnSe thin films were produced using ZnSe target by means of RF magnetron sputtering system of Vaksis. RF power was employed for the deposition of ZnSe thin film layers with the power of 60 W in the rate of ~ 2.5 Å/s under 5 × 10− 3 Torr Ar atmosphere. The rate and the thickness of the deposited layers were monitored simultaneously by a quartz crystal monitor (Inficon XTM/2). The thicknesses of the deposited ZnSe thin films were also determined by using spectroscopic ellipsometer (Woollam, -M2000V) in
Results and Discussion
In general, ZnSe has cubic zinc blende structure with the contribution of hexagonal phase. The XRD pattern obtained for ZnSe thin films was shown in Fig. 1 for different thicknesses. The XRD pattern indicates that the thin films deposited on cold glass substrate have polycrystalline structure with different orientation along (100) and (111) directions for 154 nm and 448 nm film thicknesses, respectively. The XRD spectrum shows two other weak reflection peaks arising from (110) and (112) planes
Conclusion
We prepared polycrystalline ZnSe thin films with different thickness (154, 335, 448 nm) by sputter technique and studied the nonlinear and saturable absorption properties by open aperture Z-scan and ultrafast pump-probe techniques. Energy band gap values of thin films found from linear absorption spectra increased with decreasing film thickness. The observed red shift in the polycrystalline thin films can be attributed to increasing the band tailing with increasing film thickness. The 65 ps open
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