Optical properties and sub-bandgap formation of nanocrystalline Si quantum dots embedded ZnO thin film

In this study, we fabricate ZnO thin films with nano-crystalline Si (nc-Si) quantum dots (QDs) using a ZnO/Si multilayer deposition structure and a post-annealing process, and the formation of high crystallinity of Si QDs embedded in the crystalline ZnO matrix is demonstrated. For optical properties, the essential features of ZnO material, high transmission in long-wavelength and high absorption in shortwavelength ranges, are preserved. We observe significantly enhanced light absorption and an unusual photoluminescence emission peak contributed from the nc-Si QDs in the middle-wavelength range. In addition, we confirm the formation of optical sub-bandgap and the obtained value is quite close to the unusual PL emission peak. We show that meaningful subbandgap can form in ZnO thin film by embedding nc-Si QDs while maintaining the advantageous properties of ZnO matrix. 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Introduction
Nano-crystalline silicon (nc-Si) quantum dots (QDs) embedded in Si-based dielectric materials like SiO 2 and Si 3 N 4 had been extensively studied and integrated with various electro-optical devices, e.g.light emitting devices (LEDs) [1], transistors [2], photovoltaic (PV) cells [3], and so on.They exhibit the feasibility and show great potential for devices integrating nc-Si QDs.In this work, we embed nc-Si QDs into ZnO thin films because ZnO has many desirable features to function as nc-Si QDs' matrix material, such as wide and direct bandgap, high transparency, and highly tunable electrical properties [4].So far, only few materials can simultaneously possess these properties.Hence, ZnO can serve as the nc-Si QDs' matrix material for bandgap engineering, efficiently avoid the visible and near-infrared light absorption, and decrease carrier's transportation loss from matrix to enhance the performance of electro-optical devices with nc-Si QDs.ZnO itself is also widely applied to a variety of applications like LEDs [5], PV cells [6], and transistors [7] and shows superior results.Undoubtedly, there are many advantages to embed nc-Si QDs in ZnO thin films.Therefore, it is important to demonstrate its fabrication feasibility and understand its optical properties.In this study, we fabricate the ZnO thin films with nc-Si QDs using a ZnO/Si multilayer (ML) structure by sputtering and a post-annealing process.The nano-structural, crystalline, and optical properties of ZnO thin films with nc-Si QDs are investigated and discussed.

Experiment
The ZnO/Si ML thin films with 20-bilayers were deposited on Si(100) wafers or fused quartzes at room-temperature by radio-frequency (RF) magnetron sputtering method.The sputtering powers of ZnO and Si are fixed at 75 W and 110 W, and the effective thicknesses of each ZnO and Si layers are fixed at 5 and 3 nm respectively.After deposition, the ZnO/Si ML thin films were annealed in N 2 environment for 5 minutes by furnace under different annealing temperatures from 700°C (FA-700) to 1000°C (FA-1000) with 100°C increment for the investigation on Si QDs formation with high crystallinity.For the post-annealing process, when the set temperature is arrived, the as-deposited samples were quickly put into, stayed for 5 minutes, and quickly taken out the furnace.The surface morphologies were analyzed by a Digital Instrument D3100 atomic force microscope (AFM).The high-resolution transmission electron microscope (HRTEM) images were observed by a JEOL JEM-2010F transmission electron microscope.The X-ray diffraction (XRD) patterns were examined by a Bede-D1 Xray diffractometer with Cu K α radiation.The Raman and photoluminescence (PL) spectra were measured using a 488 nm diode-pumped solid-state laser (Horiba LabRam HR).The transmission and reflection spectra were obtained by a UV-VIS-NIR spectrophotometer (Hitachi U-4100).

Results and discussions
In order to know the growth morphology of the sputtered Si atoms during deposition, Fig. 1 shows the tapping mode AFM images before and after depositing the Si layer with effective thickness of 3 nm on ZnO thin-layer.The clear formation of amorphous Si (a-Si) nanoclusters with maximum height of 5 nm and the much larger root-mean-square (RMS) surface roughness than that of ZnO thin-layer are clearly observed in the ZnO/Si single-bilayer.The density of nano-clusters is estimated to be ~2.4× 10 10 cm −2 .Similar results are also obtained from the ZnO/Si double-bilayers.It indicates that the sputtered Si atoms on ZnO thin-layer can self-aggregate to form a-Si nano-clusters, which will make the nc-Si QDs formation easier during annealing.Raman spectrum measurement, a reliable technique for examining the crystalline properties of nano-scaled Si materials [8,9], is utilized to confirm the nc-Si QDs formation in ZnO thin films after annealing.The curve-fitting result of Raman spectrum for sample FA-1000 is shown in Fig. 2(a).In addition to the transverse optical (TO) modes of Si-Si vibrations in the a-Si, intermediate Si (i-Si), 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 [8], the contribution of E 2 (high) mode of ZnO matrix located at 436.2 cm −1 is also taken into account in this curvefitting process [10].The signal of i-Si phase comes from the grain boundaries or smaller crystallites [8].The fitting curve composed of these four components shows an excellent match with the experimental data and an obvious nc-Si signal.Raman spectra of the ZnO/Si ML thin films under different annealing temperatures are shown in Fig. 2(b).The intensity of nc-Si significantly increases with increasing annealing temperature while that of a-Si decreases.The parameters of the curve-fitting results in Raman spectra for nc-Si and the calculated crystalline volume fraction of Si (f c ), where f c is estimated from (I i-Si + I nc-Si )/(I a-Si + I i-Si + I nc-Si ) × 100% [8], are listed in Table 1.Sample FA-1000 shows not only the largest integrated intensity but also the narrowest full-width at half-maximum (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 [9,11].In addition, the highest f c of 76.8% is obtained from sample FA-1000.Hence, our results show that annealing temperature 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 [12], the XRD pattern of sample FA-1000 is examined and shown as inset of Fig. 2(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 annealing temperature of 1000°C.Figures 3(a) and 3(b) shows the cross-sectional HRTEM images of the as-deposited and 1000°C-annealed ZnO/Si ML thin films for further understanding of the nano-structural properties.The ZnO/Si ML structure can be clearly observed after deposition from inset of Fig. 3(a) and is well maintained after annealing shown as inset of Fig. 3(b).The nano-scaled rough morphology different from that of the ML structures using Si-based dielectric materials as matrix [13] originates from the slight crystallization of ZnO thin-layers during deposition [14].From Fig. 3(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 nanocrystalline clusters marked in red dashed circles are found after annealing from Fig. 3(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 electrooptical devices integrating nc-Si QDs [1][2][3].Therefore, we verify that the ZnO thin film with the suitable size of nc-Si QDs can be realized after furnace annealing.
The optical properties of the ZnO thin film with nc-Si QDs are also investigated and discussed.Figure 4 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 (PL VIS ) with a wide FWHM is observed in the pure ZnO thin film.Similar results have been reported and shown this PL VIS is from the contribution of the native defects in ZnO, which can be efficiently controlled by various post-annealing methods [15,16].Hence, the increased PL VIS 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 PL VIS , the PL spectrum accompanies increased near-infrared emission (PL NIR ) and an unusual peak (PL unusual ) located at 685 nm (1.81 eV), which have not been reported in pure or doped ZnO materials [15,16], for sample FA-1000.This PL unusual may be contributed from the nc-Si QDs embedded in the ZnO matrix with wide bandgap.Because of the quantum confinement effect [17], an effective bandgap of nc-Si QD larger than the bandgap of bulk crystalline-Si (1.12 eV) is formed.Therefore, the obvious PL NIR may be primarily resulted from the interface states between nc-Si QDs and ZnO matrix.Similar results have also been observed for nc-Si QDs embedded in SiO 2 matrix [18,19].For annealing temperatures lower than 1000°C, the PL VIS and PL NIR clearly dominate the PL emission properties and no obvious PL unusual peak is observed.This indicates the native defects in ZnO matrix and the interface states between Si QDs and ZnO matrix are excessively formed because of the lower crystal quality of Si QDs and ZnO matrix under lower annealing temperatures.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 UV-VIS-NIR spectra are examined.Figures 5(a) and 5(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 range of 650 to 1000 nm and a high absorption in the shortwavelength 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) resulted from the nano-scaled rough surface of sample FA-1000.The average R of sample FA-1000 (~9.5%) is significantly lower than that of pure ZnO thin film (~22.0%) in the measured wavelength range of 320~1000 nm.The sharp R peak occurred at the absorption edge of 380 nm in pure ZnO thin film originates from excitonic resonant emission, which is usually observed in the high-temperature annealed or epitaxial ZnO thin films [20,21].The T of sample FA-1000 obviously reduces with decreasing wavelength in the middle-wavelength 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 wavelength range.The increase of light absorption causes the decrease of light transmission in the middle-wavelength range because of the presnece of nc-Si QDs.To further verify the contributions of nc-Si QDs, Fig. 5(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 [17].The average indirect optical bandgap (E g,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 E g,opt for sample FA-1000 is 1.86 eV, which quite matches the PL unusual 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 PL unusual and significant light absorption enhancement in the middle-wavelength range.Other than Sibased dielectric materials [22,23], we successfully demonstrate the ability of sub-bandgap formation in ZnO material utilizing nc-Si QDs.Moreover, the E g,opt. of ZnO matrix can be obtained from the Tauc plot for direct allowed transition shown as inset of Fig. 5(c).The bandgap of 3.34 eV well agrees with that of general ZnO thin films [24].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 [24].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.

Conclusion
In summary, we had successfully fabricated the ZnO thin film with nc-Si QDs using a ZnO/Si ML structure with a post-annealing process.Our results indicate that 1000°C annealing temperature 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-wavelength ranges.In the middle-wavelength 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.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.We believe this novel composite material integrating ZnO thin film with nc-Si QDs has great potential for the future electrooptical device applications.

Fig. 2 .
Fig. 2. (a) The curve-fitting result of Raman spectrum for sample FA-1000.(b) Raman spectra of the ZnO/Si ML thin films under different annealing temperatures.Inset shows the XRD pattern for sample FA-1000.Table 1. Parameters of the Curve-Fitting Results in Raman Spectra for Nc-Si Phase and the Calculated Crystalline Volume Fraction of Si under Different Annealing Temperatures.

Fig. 5 .
Fig. 5. (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.