The Larger Grain and ( 111 )-Orientation Planes of Poly-Ge Thin Film Grown on SiO 2 Substrate by Al-Induced Crystallization

Al-induced crystallization yields the larger grain and (111)-orientation planes of poly-Ge thin film grown on SiO2 substrate, the (111)-orientation planes of poly-Ge thin film grown on SiO2 substrate are very important for the superior performance electronics and solar cells. We discussed the 50 nm thickness poly-Ge thin film grown on SiO2 substrate by Al-induced crystallization focusing on the lower annealing temperature and the diffusion control interlayer between Ge and Al thin film. The (111)-orientation planes ratio of poly-Ge thin film achieve as high as 90% by merging the lower annealing temperature (325 ̊C) and the GeOx diffusion control interlayer. Moreover, we find the lack of defects on poly-Ge thin film surface and the larger average grains size of poly-Ge thin film over 12 μm were demonstrated by electron backscatter diffraction measurement. Our results turn on the feasibility of fabricating electronic and optical device with poly-Ge thin film grown on SiO2 substrate.


Introduction
The high quality poly-Ge thin film has many applications, for example, thin film transistors and highly conversion efficiency solar cells [1].A larger grain and (111)-orientation planes ratio of poly-Ge thin film is specifically desirable because it is very suitable for acting as the epitaxial template for III-V group semiconductors materials [2].In particular, the (111)-orientation planes of poly-Ge thin film can provide the highest carrier mobility and treated as the epitaxial template, which require lower growing temperature (<400˚C for integrated circuits, <550˚C for glass substrates) [3].Hence, fabricating such poly-Ge thin film on SiO 2 substrate may develop novel devices with advanced function.But Gebased flexible devices desire lower temperature growth technique of high quality poly-Ge thin film grown on SiO 2 substrate.Al-induced crystallization is one of the metal induced solid phase crystallization techniques for a-Si thin film grown on SiO 2 substrate, in order to form the larger grain poly-Si thin film at lower growing temperature (420˚C -550˚C) through the layer exchange between Al and Si thin film [4].Recently, the preferentially (111)-orientation planes poly-Ge thin film with the larger grain can be achieved through the layer exchange between Ge and Al thin film during Al-induced crystallization [5].Moreover, K. Toko

Experimental Details
During our first experiment, all Al thin films were deposited on SiO 2 substrate at first, Al thin films were exposed to air for 1 min, 5 min and 30 min (t air ) to grow native AlO x thin film as the diffusion control interlayer subsequently.Because the different exposure times corresponding to the different thicknesses of AlO x thin film.Afterwards, all a-Ge thin films were grown on these AlO x thin films.
The thickness of Al and a-Ge thin film was measured to be 50 nm, this thickness size is advantageous for the optimize (111)-orientation planes [5].All thin films Journal of Materials Science and Chemical Engineering were grown by using a RF magnetron sputtering method at room temperature.
Finally, these thin films were annealed in N 2 at the annealing temperature (T a ) 325˚C, 350˚C, 375˚C for 10 -400 h.Because the annealing temperature should impact the (111)-orientation planes ratio of the ploy-Ge thin film.These thin films preparation procedure is schematically shown in Figure 1.During Al-induced crystallization, the surface morphology of crystallized poly-Ge thin film was estimated by Normarki optical microscopy, Ge and Al elemental composition in crystallized ploy-Ge thin film were evaluated by energy dispersive X-ray (EDX) analysis, the crystal state of crystallized poly-Ge thin film was appraised by X-ray diffraction (XRD) measurement, the (111)-orientation planes of crystallized ploy-Ge thin film was observed by electron backscattered diffraction (EBSD) measurement.Before EBSD measurement, these Al thin films and AlO x diffusion control interlayers on poly-Ge thin film were removed by HF solutions (~1.5%).
In our second experiments, four kinds of thin film were prepared and summarized in Table 1.Thin film A was grown as follows: 50 nm thickness Al thin film was grown on SiO 2 substrate, and bared to air for 5 min to get the native AlO x diffusion control interlayer, followed by a 50 nm thickness a-Ge thin film preparation.Thin film B has the same stacked layer structure as thin film A expect that it additionally has 1 nm thickness a-Ge insertion interlayer below Al thin film or on SiO 2 substrate.Thin film C has the native GeO x diffusion control interlayer in substitution for the AlO x diffusion control interlayer in thin film A.
Here, the GeO x diffusion control interlayer was prepared by 1 nm thickness a-Ge thin film grown on Al thin film which is exposed to air for 24 h.Thin film D has both of a-Ge insertion interlayer and GeO x diffusion control interlayer.Al and Ge thin film were prepared at room temperature using a RF magnetron sputtering method.Finally, these thin films were annealed at 180˚C -350˚C in N 2 for 0.   (111)-orientation planesof poly-Ge thin film was roughly evaluated by X-ray diffraction (XRD) rocking curve measurement.In addition, the detailed crystal (111)-orientation planes and the grain size were measured using electron backscattered diffraction (EBSD) measurement.

Results and Discussions
The changes of a-Ge thin film with annealing timeare revealed in Figure 3 under t air = 5 min and T a = 350˚C during Al-induced crystallization process.Figure 3(a)-(c) show the back surface of a-Ge thin film watched through the transparent SiO 2 substrate.Nomarski optical micrographs suggest that Ge atoms shift to the back surface of Al thin film by lateral growing, and finally cover the whole substrate during annealing time.Figure 3(d) and Figure 3(e) show EDX spectra achieved from a-Ge thin film surface before and after annealing time for 30 h, respectively.The voltage was 2.7 keV for the selective detection zone of elements near a-Ge thin film surface.These EDX spectra show that the surface thin film changed from Ge to Al during whole annealing time.Figure 3(f) show the appearance of a sharp peak in XRD curve after annealing.The sharp peak near 27˚ relates to (111)-orientation planes of crystal Ge, and any other sharp peaks are not observed in the measured zone.These results show that Ge thin film crystallized and oriented to preferential (111)-orientation planes through layer exchange.The (111)-orientation planes ratio would be estimated precisely by EBSD measurement.We have affirmed a-Ge thin film has been accomplished on SiO 2 substrate through Al-induced crystallization process.The EBSD measurement can characterize the (111)-orientation planes of poly-Ge thin film which depending on the T a and t air statistically during Al induced crystallization.The lower Ta and longer t air demanded the longer annealing time to finish Al induced crystallization.Al induced crystallization of a-Si thin films had also the same tendency.[9] This tendency can be illustrated as follows: The lower T a reduces the reaction rate of Al and Ge atoms by the Arrhenius law [10]; and the longer t air thickens the AlO x diffusion controlinterlayer, and decreases the diffusion rate of Al and Ge atoms [11].The (111)-orientation planes of a-Ge thin film also depend on both T a and t air during Al induced crystallization, and the (111)-orientation planesbecome dominant with decreasing T a and increasing t air .This action is also the same as the a-Si thin film during Al induced crystallization [12].The reason for this behavior can be explained as follows: The Ge nuclei happen on the surface of SiO 2 substrate because the thickness of Ge and Al thin films is 50 nm [13], respectively, and (111)-orientation planes have the lowest interfacial energy in the diamond structure [14].The lower T a and longer t air provide the lower reactive rate and lower diffusion rate of Ge and Al atoms.These reasons make it no chance to generate the other orientation planes with high interfacial energies, so resulting in the preferential (111)-orientation planes.
The EBSD measurement analysis results are shown in Figure 4. Here, the definition of the (111)-orientation planes ratio contain those planes which tilted within 10 from the exact (111)-orientation planes.Figure 4 clearly shows that the (111)-orientation planes ratio increase with increasing t air and decreasing T a .
We noticed that the (111)-orientation planes ratio reach over 90% by combining the lower T a (325˚C) and longer t air (30 min).In the [15] report for Al induced crystallization of a-Ge thin film, the (111)-orientation planes fraction was limited to 68%.The main reason is the higher annealing temperature T a (410˚C).
In addition, the EBSD measurement analysis found the average grain diameter is 12 µm for these thin films.These grain diameter average values are the highest level in the previous reports about poly-Ge thin film on SiO 2 substrates in lower annealing temperature process [16].
EDX analysis was also performed and the analysis results were shown in   The crystal (111)-orientation planes of poly-Ge for thin films A, B, C and D were characterized by the XRD rocking curve of (111)-orientation Ge reflection planes and summarized in Figure 7.All thin films have a sharp peak around 13.7˚ indicating preferential (111)-orientation planes.The (111)-orientation planes are interpreted from the viewpoint of the minimal interfacial energy between Ge thin film and SiO 2 substrate [21].The FWHM values of the sharp peak of XRD rocking curve are plotted in the insert figure, which indicates that the lower annealing temperature provides weaker (111)-orientation planes but larger FWHM values.This behavior is likely due to the unstable thermal equilibrium condition during the lower annealing temperature.
The crystal (111)-orientation planes and average the grain size of poly-Ge for thin films A, B, C and D were characterized using EBSD measurement.Figures 8(a)-(d) show the crystal (111)-orientation planes in the normal direction (ND).For the sample A, poly-Ge thin film has highly (111)-orientation planes.Figure 8(e)-(h) show the crystal other orientation plane in the transverse direction (TD) and indicate that the average grain size of poly-Ge thin film decrease with the annealing temperature decreasing.This result suggests that the a-Ge insertion layer and the GeO x diffusion control interlayer increase the nucleation frequency and promote poly-Ge thin film crystallization.Nevertheless, Figure 8(h) indicates that the sample D has larger grain with approximately 10 μm even under the 180˚C annealing temperature.
The (111)-orientation planes ratio and the average grain size of poly-Ge thin film were calculated using EBSD measurement.Figure 9(a) and Figure 9(b) show the typical (111)-orientation planes ratio and the average grain size of thin film D. Figure 9(a) indicates that poly-Ge thin film principally consists of planes with tilting within 10˚ of the exact (111)-orientation planes.This result agrees with XRD rocking curve in Figure 7.The whole (111)-orientation planes ratio was defined as the integrated values of the area ratios of exact (111)-orientation

Conclusion
In summary, the T a and t air strongly influenced the crystal (111)-orientation planes of a-Ge thin film during Al induce crystallization.The ratio of (111)orientation planes increases with t air increasing and T a decreasing.By combining the lower T a (325˚C) and longer t air (30 min), the ratio of (111)-orientation planes reaches over 90%.At the same time, the lower temperature Al induce crystallization of a-Ge thin film can be achieved the larger grain size and

S
. G. Dong et al.DOI: 10.4236/msce.2018.6200323 Journal of Materials Science and Chemical Engineering et al. have improved the (111)-orientation planes ratio and the average grains size of poly-Ge thin film significantly by forming the diffusion controlinterlayer (AlO x ) between Ge and Al thin film during Al-induced crystallization[1].However, it is still difficult to achieve the larger grainspoly-Ge thin film on SiO 2 substrate below the softening substrate temperature (<200˚C).M. Kurosawa et al. recently studied Al-induced crystallization of the a-Ge thin film on SiO 2 substrate, and they have acquired the preferentially (111)-orientation planes (~68%) poly-Ge thin film by decreasing the thickness of Al and Ge thin film to 50 nm [6].Hu et al. have achieved the (111)-orientation planespoly-Ge (~70%) thin film on SiO 2 substrate by using the GeO x diffusion control interlayer structure during Al-induced crystallization [7].From Al-induced crystallization of the a-Ge thin film in our first experiments, we have gained (111)-orientation planes (~90%) poly-Ge thin film grown on SiO 2 substrate by lowing the annealing temperature to 325˚C and forming AlO x diffusion control interlayer at the same time.In our second experiments, we investigate Al-induced crystallization of the poly-Ge thin film by lowering the crystallization temperature, which is depended on the layer exchange growth mechanism.The larger grains and (111)-orientation planes poly-Ge thin film on SiO 2 substrate can be achieved by Al-induced crystallization technique at temperatures as low as 180˚C.We can also control the crystal orientation of poly-Ge thin film to (111)-orientation planesby regulating the annealing temperature during Al-induced crystallization, or the thickness of Al and Si thin film and the diffusion control interlayer (AlO x or GeO x ) between Si and Al thin film [8].
Figure 1.Schematic structure of poly-Ge thin film preparation procedure in our first experiments.

Figure 2 .
Figure 2. Schematic structures of the poly-Ge thin film preparation procedure in our second experiments.

Figure 3 .
Figure 3. Ge atoms shift to the back surface (a), grow (b) and cover the whole substrate (c) during the annealing.These EDX spectra explain that the surface thin film changed from Ge to Al before (d) and after the whole annealing (e).The sharp peak near 27˚ appeared after annealing (f).

Figure 4 .
Figure 4.The (111)-orientation planes fraction reach over 90% by combining the lower T a (325˚C) and longer t air (30 min).

Fig- ure 5 .
These data proved the layer exchange of Ge and Al thin film has been finished and the uniform formation of poly-Ge thin film on SiO 2 substrate.The observed area did not contain any other kind of defects except (111)-orientation defects.Since (111)-orientation defects are related with the weakest bond in the diamond structure[17], and no other defects grown on the poly-Ge thin film surface.Consequently, Al induce crystallization of a-Ge thin film is useful as an epitaxial layer for those advanced flexible substrate materials.In our second experiments, the thin films A, B, C, and D were annealed at 300˚C, 275˚C, 225˚C, and 180˚C, respectively.Nomarski optical microscopy suggested the completion of the layer exchange for all thin films after annealing time for 100 h.In addition, Raman spectra in Figure6(a) show the largest sharp peak at around 292 cm −1 for all thin films after annealing.These results indicate the crystallization of a-Ge thin film is better, although the sharp peak shift to the lower wave number compared to the crystal Ge actual sharp peak (~300 cm −1 )[18].These larger wave number shifts are not entirely understood, but they are possibly due to the residual Al atoms (~0.5%) in Ge thin film.The insert figure in Figure6(a) shows the full width at half maximum (FWHM) of Raman spectra sharp peak as a function of the annealing temperature.The FWHM is independent on the annealing temperature, whereas the FWHM increases with the annealing temperature decreasing for the solid phase crystallization of a-Ge thin films[19].This result suggests that Al induced crystallization process is Journal of Materials Science and Chemical Engineering

Figure 5 .Figure 6 .
Figure 5.The layer exchange of Ge and Al thin film and the uniform formation of poly-Ge thin film on SiO 2 substrate.

Figure 6 (
Figure 6(b) shows the Arrhenius plots of the crystallization finish time for thin films A, B, C and D. Here, the crystallization finish time is defined as the time of the layer exchange completion.The resulting poly-Ge thin film region covers more than 95% area of SiO 2 substrate for all thin films.As seen in Figure 6(b), the a-Ge insertion layer and the GeO x diffusion control interlayer work effectively for the lower crystallization temperature.In particular, thin film D, combining the a-Ge insertion layer and the GeO x diffusion control interlayer, significantly reduces the crystallization temperature to 180˚C.The mechanism of the crystallization temperature reduction is explained as follows: during Al induced crystallization, Ge atoms diffuse from top a-Ge thin film to Al thin film through GeO x diffusion control interlayer.The Ge nucleation occurs when Ge atoms concentration in Al thin film is supersaturated.After that, the lateral growth of Ge crystals propagates due to the continuous supply of Ge atoms from top a-Ge thin film through GeO x the diffusion control interlayer.Based on these mechanisms, a-Ge insertion layer works for initial Ge atoms doped in Al thin film, which facilitates Ge atoms concentration supersaturation, but the GeO x diffusion control interlayer promotes Ge atoms diffusion compared to the conventional AlO x diffusion control interlayer.

Figure 7 .
Figure 7. XRD rocking curves of (111)-orientation Ge reflection planes of thin films A, B, C and D. FWHM values for each thin film as a function of crystallization temperature are summarized in the insert figure.

Figure 8 .
Figure 8. EBSD images of poly-Ge for the samples A, B, C and D: (a)-(d) ND and (e)-(h) TD relative to substrate, where the ND and TD figures correspond to the same zone.The other colors indicate the crystal other orientation planes.

Figure 9 .
Figure 9. Histogram distributions of the (111)-orientation planes ratio (a) and the average grain size (b) of poly-Ge for sample D.

(
111)-orientation planes of poly-Ge thin film on SiO 2 substrate.There are mainly two growth promotion techniques: a) The initial Ge atoms are doped in Al thin film by inserting 1 nm thickness a-Ge thin film below Al thin film; b) The diffusion is enhanced by substituting GeO x for AlO x as the diffusion control in-

Table 1 .
Poly-Ge thin film prepared in our second experiments.