In section 4-2, the [ZnO/Si] ML thin films were annealed at a shorter duration by furnace annealing (FA), a slower temperature variation process, for the nc-Si QD embedded ZnO thin films. The sample fabrication and characteristics analyses were investigated and discussed below.
4-2.1 Sample Fabrication
The [ZnO/Si] ML thin films with 20 bilayers were deposited on p-type Si(100) wafers or fused quartzes by sputtering process. The sputtering powers of ZnO and Si were fixed at 75 W and 110 W, and the effective thicknesses of each ZnO and Si layers were fixed at 5 and 3 nm respectively. After deposition, the [ZnO/Si]ML thin films were annealed in N2 environment for 5 minutes by FA under different Tann from 700°C (FA-700) to 1000°C (FA-1000) with 100°C increment for the investigation on Si QDs formation with high crystallinity.
4-2.2 Nano-Crystalline Properties
Raman spectrum measurement, a reliable technique for examining the crystalline properties of nano-scaled Si materials [47, 60], is utilized to confirm the nc-Si QDs formation in ZnO thin films after FA. The curve-fitting result of Raman spectrum for sample FA-1000 is shown in Fig. 4-5(a). In addition to the transverse optical (TO) modes of Si-Si vibrations in the a-, i-, and nc-Si phases located at 480.0, 504.6, and 513.2 cm-1, which are usually observed in the nc-Si QD thin films [47], the contribution of E2(high) mode of ZnO matrix located at 436.2 cm-1 is also taken into account in this curve-fitting process [61]. The fitting curve composed of these four components shows an excellent match with the experimental data and an obvious
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nc-Si signal. Raman spectra of the [ZnO/Si] ML thin films under different Tann are shown in Fig. 4-5(b). The intensity of nc-Si significantly increases with increasing Tann while that of a-Si decreases. The parameters of the curve-fitting results in Raman spectra for nc-Si, and the calculated fc-Si are listed in Table 4-1. Sample FA-1000 shows not only the largest integrated intensity but also the narrowest FWHM for nc-Si.
The narrower FWHM means a larger Si crystal size, and the corresponding size in sample FA-1000 can be estimated to be about 3 nm in diameter [48, 60]. In addition, the largest fc-Si of 76.8 % is obtained from sample FA-1000.Hence, our results show that Tann has a great effect on the crystallization of a-Si nano-clusters. Since the crystalline property of ZnO matrix has strong influences on the optical and electrical properties of the ZnO thin films [65], the XRD pattern of sample FA-1000 is examined and shown as inset of Fig. 4-5(b). The ZnO(0002) orientation located at 34.45° is clearly observed in sample FA-1000. In other words, the high crystallinity of Si nano-clusters can form in the crystalline ZnO matrix by utilizing a [ZnO/Si] ML deposition structure with Tann of 1000°C for a short annealing duration time of 5 minutes by FA.
Fig. 4-5: (a) The curve-fitting result of Raman spectrum for sample FA-1000. (b) Raman spectra of the [ZnO/Si] ML thin films under different Tann. Inset shows the XRD pattern for sample FA-1000.
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Table 4-1: Parameters of the curve-fitting results in Raman spectra for nc-Si phase and the calculated crystalline volume fractions of Si (fc-Si) under different Tann.
Sample ID nano-structural properties. The [ZnO/Si] ML structure can be clearly observed after deposition from inset of Fig. 4-6(a) and is well maintained after annealing shown as inset of Fig. 4-6(b). The nano-scaled rough morphology different from that of the ML structures using Si-based dielectric materials as matrix [64] originates from the slight crystallization of ZnO thin-layers during deposition [66]. From Fig. 4-6(a), we can see the formations of a-Si nano-clusters with a size distribution of 3~5 nm separated by ZnO thin-layers after deposition. And a large number of nano-crystalline clusters marked in red dashed circles are found after annealing from Fig. 4-6(b). Combined with the Raman and XRD results, these nano-crystalline clusters are composed of the nc-Si QDs embedded in the crystalline ZnO matrix. Moreover, the observed size distribution of a-Si nano-clusters is well consistent with the examined height of nano-clusters in AFM image and the estimated average crystalline size of Si in Raman spectrum for sample FA-1000, and such size is proper to produce the quantum confinement effect for electro-optical devices integrating nc-Si QDs [16, 24, 67].
Therefore, we verify that the ZnO thin film with the suitable size of nc-Si QDs can be realized after FA.
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Fig. 4-6: Cross-sectional TEM images of the (a) as-deposited and (b) 1000°C-annealed (sample FA-1000) [ZnO/Si] ML thin films. Insets show the corresponding overall images.
4-2.3 Optical Properties and Sub-bandgap Formation
The optical properties of the ZnO thin film with nc-Si QDs are also investigated and discussed. Fig. 4-7 shows the PL spectra of sample FA-1000 and pure ZnO thin film with equal ZnO thickness of 105 nm under an identical annealing condition as a reference sample. The visible PL emission (PLVIS) with a wide FWHM is observed in the pure ZnO thin film. Similar results have been reported and shown this PLVIS is from the contribution of the native defects in ZnO, which can be efficiently controlled by various post-annealing methods [68, 69]. Hence, the increased PLVIS in sample FA-1000 can be mainly inferred from the higher native defect density in ZnO matrix since it has a smaller crystalline size than that in pure ZnO thin film. In addition to the increased PLVIS, the PL spectrum accompanies increased near-infrared emission (PLNIR) and an unusual peak (PLunusual) located at 685 nm (1.81 eV), which have not been reported in pure or doped ZnO materials [68, 69], for sample FA-1000. This PLunusual may be contributed from the nc-Si QDs embedded in the ZnO matrix with wide bandgap. Because of the quantum confinement effect [53], an effective bandgap of nc-Si QD larger than the bandgap of bulk crystalline-Si (1.12 eV) is formed.
Therefore, the obvious PLNIR may be primarily resulted from the interface states between nc-Si QDs and ZnO matrix. Similar results have also been observed for nc-Si
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QDs embedded in SiO2 matrix [50, 70]. For the Tann lower than 1000°C, the PLVIS and PLNIR clearly dominate the PL emission properties and no obvious PLunusual peak is observed. This indicates the native defects in ZnO matrix and the interface states between nc-Si QDs and ZnO matrix are excessively formed because of the lower crystal quality of Si QDs and ZnO matrix under lower Tann.
Fig. 4-7: PL spectra of sample FA-1000 and pure ZnO thin film after annealing at 1000°C.
To confirm the contributions of nc-Si QDs in PL spectrum and their impact on the optical properties of ZnO thin film with nc-Si QDs, the ultraviolet/visible/near- infrared (UV-VIS-NIR) spectra were examined. Figs. 4-8(a) and (b) show the light transmission, reflection, and absorption spectra of sample FA-1000 and pure ZnO thin film for comparison. For sample FA-1000, a high transmittance (T) in the long-wavelength (long-λ) range of 650 to 1000 nm and a high absorption in the short-λ range under 380 nm similar to general ZnO thin film are observed, and the T is higher than that of pure ZnO thin film owing to the reduced reflectance (R). The average R of sample FA-1000 (~9.5%) is significantly lower than that of pure ZnO thin film (~22.0%) in the measured λ range of 320~1000 nm. The sharp R peak occurred at the absorption edge of 380 nm in pure ZnO thin film originates from
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excitonic resonant emission, which is usually observed in the high-temperature annealed or epitaxial ZnO thin films [71, 72]. The T of sample FA-1000 obviously reduces with decreasing λ in the middle-λ range of 380 to 650 nm. At the same time, an increased absorbance (A) compared to pure ZnO thin film is observed in the corresponding λ range. The increase of light absorption causes the decrease of light transmission in the middle-λ range because of the presnece of nc-Si QDs. To further verify the contributions of nc-Si QDs, Fig. 4-8(c) shows the PL spectrum and Tauc plot for the indirect allowed transition since the optical transition process in nc-Si QD system is dominated by a phonon-assisted mechanism [53]. The average indirect optical bandgap (Eg,opt.) can be evaluated by linear extrapolating the interception at the energy axis (αhν = 0) from the plot of (αhν)1/2 as a function of incident photon energy (hν), where α is the optical absorption coefficient. The evaluated indirect Eg,opt for sample FA-1000 is 1.86 eV, which quite matches the PLunusual peak energy (1.81 eV).
Thus, we show that the nc-Si QDs embedded in ZnO matrix induce a sub-bandgap formation and in turn contribute the PLunusual and significant light absorption enhancement in the middle-λ range. Other than Si-based dielectric materials [8, 73], we successfully demonstrate the ability of sub-bandgap formation in ZnO material utilizing nc-Si QDs. Moreover, the Eg,opt. of ZnO matrix can be obtained from the Tauc plot for direct allowed transition shown as inset of Fig. 4-8(c). The bandgap of 3.34 eV well agrees with that of general ZnO thin films [74]. These measurement results indicate that the ZnO matrix in sample FA-1000 still reserves its essential and advantageous optical properties even though the nc-Si QDs are embedded [74]. The realization of high crystallinity of Si QDs embedded in the crystalline ZnO matrix with a meaningful sub-bandgap provides a new and potential composite material for the development of future electro-optical devices.
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Fig. 4-8: (a) Transmission, reflection, and (b) absorption spectra of sample FA-1000 and pure ZnO thin film on quartzes. (c) PL spectrum and Tauc plot for indirect allowed transition of sample FA-1000.
Inset shows the Tauc plot for direct allowed transition of sample FA-1000.
4-2.4 Summary of Section 4-2
In section 4-2, we had successfully fabricated the nc-Si QD embedded ZnO thin films under a shorter duration time of 5 minutes by FA. Our results indicate that 1000°C of Tann can result in highest crystallinity of Si QDs embedded in crystalline ZnO matrix. Though embedded with nc-Si QDs, the optical properties of ZnO thin film can be preserved in the long- and short-λ ranges. In the middle-λ range, the significantly enhanced light absorption owing to nc-Si QDs is obtained. In addition, an optical sub-bandgap of 1.86 eV closes to the unusual PL emission peak located at 1.81 eV is observed in sample FA-1000. These results represent the sub-bandgap formation in ZnO thin film by utilizing nc-Si QDs while maintaining the essential optical properties of ZnO matrix.
60 fixed at 75 W and 110 W, and the effective thicknesses of each ZnO and Si layer were fixed at 5 and 3 nm respectively. After deposition, the [ZnO/Si] ML thin films were annealed at 500, 600, 700, and 800°C for 30 minutes in N2 environment by FA.
4-3.2 Nano-Crystalline Properties
In order to investigate the crystalline properties of the Si QDs embedded in ZnO thin films annealed at a longer duration under different Tann, Raman spectra were measured and shown in Fig. 4-9. The corresponding fc-Si values obtained from fitting the curves are shown as inset of Fig. 4-9 [47]. The nc-Si phase is formed in ZnO matrix and significantly increased by increasing Tann when Tann is higher than 600°C. It indicates the higher Tann under a longer duration can largely enhance the crystalline quality of Si QDs embedded in ZnO matrix. Besides, the required Tann for the significant nc-Si formation, using a [ZnO/Si] ML structure, can be obviously decreased by increasing the annealing duration.