4-1.1 Sample Fabrication
In section 4-1, the [ZnO/Si] ML thin films with 24 bilayers were deposited on p-type Si(100) wafers or fused quartzes at room-temperature by radio-frequency (RF) magnetron sputtering method. The sputtering power of Si (PSi) is varied from 25 (S25) to 110 W (S110) while that of ZnO was fixed at 75 W. The effective thicknesses of each ZnO and Si thin-layers were fixed at 5 and 3 nm, respectively. After deposition, the [ZnO/Si] ML thin films were annealed by a rapid thermal annealing (RTA) process at 1000°C for 50 seconds in N2 environment.
4-1.2 Nano-Crystalline Properties
To confirm the nano-crystalline Si (nc-Si) QDs formation, Raman spectra measurements, a well-known and credible technique for examining the nc-Si properties [47, 60], were performed. Fig. 4-1(a) shows the Raman spectra of the annealed [ZnO/Si] ML thin films under different PSi and its inset shows the
48
curve-fitting result of Raman spectrum for sample S110. The fitting curve, which is decomposed into four components with peaks located at 436.1, 480.0, 508.3, and 519.7 cm-1, shows an excellent match with the measured data. The peaks at 480.0, 508.3, and 519.7 cm-1 are usually observed in the nc-Si QD, and they are contributed from the transverse-optical (TO) modes of Si-Si vibrations in the amorphous (a-Si), intermediate (i-Si), and nc-Si phases of Si, where the i-Si phase is caused by grain boundaries or smaller crystallites [47]. The full-width at half-maximum (FWHM) of nc-Si phase is 7.4 cm-1, corresponding to nc-Si size about 4 nm [60]. The peak at 436.1 cm-1 comes from the E2(high) mode of ZnO. The lower peak position than 439 cm-1 of bulk ZnO is attributed to the presence of intrinsic defects in the ZnO nano-clusters [61]. The peak near 520 cm-1 is not observed in sample S25 and significantly increases from sample S75 to S110. Besides, sample S110 shows not only the largest Si crystalline intensity but also a great Si crystal volume fraction (fc) of 88%, where fc-Si is estimated from the sum of the integrated intensities of nc-Si and i-Si phases divided by the total sum of the integrated intensities of nc-Si, i-Si, and a-Si phases ((Ii-Si+Inc-Si)/(Ia-Si+Ii-Si+Inc-Si)×100%) [62].
Fig. 4-1: (a) Raman spectra and (b) XRD patterns of the annealed [ZnO/Si] ML thin films under different PSi. Inset of (a) shows the curve-fitting result of Raman spectrum for sample S110.
49
The crystalline property of ZnO matrix has strong influences on the optical and electrical properties of ZnO thin films [34]. Fig. 4-1(b) shows the X-ray Diffraction (XRD) patterns of the annealed [ZnO/Si] ML thin films under different PSi for the examination of the c-axis (0002) preferred orientation of ZnO matrix. The [ZnO/Si]
ML thin films exhibit a narrower FWHM and higher intensity when increasing PSi. This means a better crystallization of ZnO matrix can be obtained with a higher PSi. Hence, the results in Raman spectra and XRD patterns indicate that a high enough PSi is necessary for the formation of nc-Si embedded in ZnO matrix and the increased PSi
can improve the crystalline properties of both nc-Si and ZnO matrix.
4-1.3 Formation Mechanism
In order to understand the formation mechanism, we analyze the atomic force microscope (AFM) images of the ZnO single-layer with a 5 nm thickness and the [ZnO/Si] single-bilayer under different PSi after deposition, as shown in Fig. 4-2.
Significant variations on the surface morphologies are observed. The AFM image of sample S25 shows a smaller root-mean-square (RMS) surface roughness than that of the ZnO single-layer, and the deposited Si layer can be seen as a thin layer-like.
However, the AFM images of samples S75, S90, and S110 show larger RMS surface roughnesses than that of ZnO single-layer and clear formation of a-Si nano-clusters.
Moreover, the RMS surface roughness increases with increasing PSi and the density of nano-clusters in samples S90 and S110 can be estimated to be 3.1×1010 and 1.9×1010 cm-2. The similar results are also obtained in the [ZnO/Si] double-bilayers. Since an a-Si nano-film needs a higher crystallization temperature of 1100°C than that for a-Si nano-clusters [63], the nc-Si is hard to efficiently form in sample S25 during annealing. The more obvious formation of a-Si nano-clusters with increasing PSi indicates that a higher PSi can remarkably assist the sputtered Si atoms gaining more
50
kinetic energy to self-aggregate as a-Si nano-clusters during deposition. Therefore, nc-Si QDs can be formed more easily during annealing. This observation is in good agreement with the Raman results. The peak intensity of the nc-Si is greatly enhanced with increasing PSi. Furthermore, because each ZnO thin-layer is separated by the a-Si thin-layer in sample S25, the crystallization of ZnO matrix is impeded during annealing. Hence, a lower quality of ZnO crystallization is obtained in sample S25.
Fig. 4-2: AFM images of (a) the ZnO single-layer with a 5 nm thickness and the [ZnO/Si]
single-bilayer thin films under (b) 25, (c) 75, (d) 90, and (e) 110 W of PSi after deposition.
The as-deposited and after-annealing cross-sectional transmission electron microscope (TEM) images of sample S110 are shown in Fig. 4-3. In Figs. 4-3(a) and (b), we can observe the obvious ML structure with a slightly rough morphology and the formation of a-Si nano-clusters with a size distribution of 3~5 nm separated by ZnO thin-layers after deposition. The slightly rough morphology is different from the ML structures using Si-based dielectric materials as matrix [64]. This result is reasonable since ZnO is easy to crystallize during deposition [34]. We can adjust the morphology of each ZnO thin-layer by tuning the ZnO sputtering power or the
51
working pressure during deposition. The observed size distribution of a-Si nano-clusters is highly consistent with the examined height of a-Si nano-clusters in AFM image and the estimated size of nc-Si about 4 nm in Raman spectrum for sample S110, and such size is suitable for various electro-optical devices using the quantum-confinement effect. From Fig. 4-3(c), the ML structure can still be clearly seen after annealing and a high-density of nano-crystalline clusters with a size distribution of 2~6 nm can be observed from the zoom-in HRTEM image shown in Fig. 4-3(d). Combined with the Raman and XRD results, these nano-crystalline clusters are the nc-Si QDs embedded in crystalline ZnO matrix. Therefore, we can conclude that a high PSi can assist the formation of self-aggregated a-Si nano-clusters on ZnO layers during deposition, and such result is advantageous to form the nc-Si QDs embedded in crystalline ZnO matrix during annealing, as illustrated in Fig. 4-4.
Thus, we demonstrate that the good crystallization of nc-Si QDs and ZnO matrix can be simultaneously achieved with a high enough PSi for the nc-Si QDs embedded ZnO thin films.
Fig. 4-3: The overall and zoom-in cross-sectional TEM images of the [ZnO/Si] ML thin film. (a) and (b) are as-deposited, and (c) and (d) are after annealing for sample S110.
52
Fig. 4-4: Illustration of the formation of nc-Si QDs embedded in the crystalline ZnO matrix with a high enough PSi by using a [ZnO/Si] ML structure.
4-1.4 Summary of Section 4-1
In section 4-1, we had successfully fabricated the nc-Si QDs embedded in ZnO thin films and demonstrated their formation by using a [ZnO/Si] ML structure and annealing process. The sample with PSi of 110 W shows a large fc-Si of 88% and a highly uniform size about 4 nm of nc-Si QDs. Our results indicate that an obvious self-aggregation of the sputtered Si atoms as nano-clusters with a high enough PSi
during deposition is essential and helpful for the nc-Si QDs formation and the better crystallization of ZnO matrix during annealing. Therefore, we demonstrate the feasibility of fabricating nc-Si QDs embedded in crystalline ZnO thin films.
53