Removal of Ge Islands in Al-Induced Layer-exchanged Ge Thin Films on Glass Substrates by Selective Etching Technique

Al-induced layer-exchanged Ge (ALILE-Ge) combined with epitaxy is a promising way to fabricate advanced electronic optical devices on foreign substrates as well as Si large-scale integrated circuits. The presence of Ge islands on the surface of the ALILE-Ge seed layer was a problem because the islands deteriorated the crystal quality of the epitaxial layer. This paper gives a solution to this problem: the Ge islands were selectively etched by H2O2 treatment. The ALILE-Ge seed layer was protected by using the oxidized Al membrane, prepared between Ge and Al, as an etching mask. By initially preparing the thick Ge layer and then removing the excess Ge islands, we improved the coverage of the ALILE-Ge seed layer on the substrate. The resulting ALILE-Ge provided a high (111) orientation fraction (96%) and large grains (> 100 μm). This Ge layer appears promising for use in seed layers for epitaxial Ge, group III-V compound semiconductors, and other advanced materials.

Recently, layer exchange between metal and Ge layers was achieved by using diffusion-control layers between the Ge and metal layers and optimizing the annealing temperature [29][30][31][32][33][34][35].In addition, large-grained, (111)-oriented Ge layers on plastics were demonstrated [36,37].Using the ALILE-Ge as an epitaxial seed for a vapor-grown Ge layer is a promising way to fabricate a high-quality Ge on insulator.According to secondary ion mass spectrometry, the ALILE-Ge layer contains 0.5% Al atoms [33].Considering the Al diffusion coefficient in Ge (10 -16 cm 2 /s at 550 °C), the Al contamination in the epitaxially grown layer may not be dominant [38].On the other hand, the resulting ALILE-Ge had small-grained Ge islands stacked on the high-quality Ge bottom layers.This is a serious problem when the ALILE layer is used for a seed layer.We suppressed the Ge islands by controlling the thickness ratio of Ge to Al [33,34].However, we found a trade-off problem between the island suppression and the good coverage of the bottom Ge layer on the substrate.In this paper, we present a new way to remove Ge islands with keeping the good coverage of the bottom Ge layer.

Experimental Procedure
Al layers (thickness: 50 nm) were first prepared onto quartz glass (SiO2) substrates (RF power: 100 W, base pressure: ~3.0 × 10 -4 Pa, deposition rete: 0.65 nm/s), and then exposed to air for 5 min to form native AlOx layers as diffusion limitting layers.After that, a-Ge layers (thickness: 50 nm, 100 nm, and 150 nm) were prepared (RF power: 50 W, base pressure: ~3.0 × 10 -4 Pa, deposition rete: 0.50 nm/s).All depositions were carried out at room temperature using a radio-frequency magnetron sputtering method.Those samples were annealed at 350 °C for 100 h in a N2 ambient to induce layer exchange between Ge and Al.
The island removal procedure is schematically shown in Fig. 1.Initially, surface oxide layers are removed by using diluted HF solutions (1.5%, 5 sec).After that, the Ge islands are removed by using H2O2 solutions (50%) for 30 min for 50-nm-thick Ge, 1 h for 100-nm-thick Ge, and 1.5 h for 150-nm-thick Ge.Here, we expect that the AlOx interlayers work as etching masks and protect the bottom-Ge layers.Finally, Al and AlOx layers are etched by HF treatment (1.5%, 1 min).The surface morphology of the samples was observed by using scanning electron microscopy (SEM).The crystal orientation and grain size of the resulting Ge layers were characterized by electron backscatter diffraction (EBSD) analysis.

Results and Discussion
The surface morphologies of the samples are summarized in Fig. 2. The Ge islands can be seen in the surfaces of the as-annealed samples in Figs.2(a)-2(c).The area of the Ge islands increases with increasing the initial Ge thickness.This is natural because the eventual thickness of the bottom Ge layer is equal to the initial Al thickness (50 nm) leaving the excess Ge on the upper layer as the Ge island [33,35].If the layers exchanged completely, the surface area ratio of Ge and Al should be 0:1, 1:1, and 2:1 in Figs.2(a)-2(c), respectively, considering the volume preservation of Ge and Al.However, the current results show that the occupation of Ge is larger than the expected values.This behavior is because Al pushed up to the top layer reacts with a-Ge, forming Ge crystals at the top layer.Alternatively, the bottom-Ge has spaces as seen in Figs.2(g  AlOx layers, and bared the entire bottom Ge layers without leaving any trace of the Ge islands.This result proves that the AlOx interlayer worked as the etching mask, and protected the bottom Ge layer from the H2O2 solutions.For the 50-nm-thick Ge sample, the Ge coverage on the substrate is insufficient as representatively shown in Fig. 2(g), while the 100-nm and 150-nm thick Ge samples provide the relatively good Ge coverage as shown in Figs.2(h) and 2(i).This result indicates that the initial a-Ge thicker than Al is essential for the good coverage of Ge because the initial a-Ge layer is partially consumed as the source of the Ge islands.
Figure 3 shows the EBSD images of the bared bottom Ge layers.Figs.3(a)-3(c) indicate that the highly (111)-oriented Ge layers owing to the annihilation of the small-grained, randomly-oriented Ge islands [33,34].We note that the wide areas with random orientation in Fig. 3  The EBSD analysis was used to derive the area-fractions of the (111) orientation and the average grain size from the EBSD maps shown in Fig. 3.The results are shown in Fig. 4. Here, the bared SiO2 regions, shown as wide random-oriented regions in Fig. 3(a), are subtracted from the calculation.Figs.4(a)-4(c) indicate that all samples have the high (111) fractions of no less than 90%; the (111) fraction slightly decreases with increasing the Ge thickness.The grain size also decreases with increasing the Ge thickness, as shown in Figs.4(d)-4(f).These behaviors are explained as follows.The thicker Ge likely provides the higher diffusion rate of Ge to Al, and then leads to the higher nucleation rate resulting in the smaller grains.The high diffusion rate also decreases the (111) fraction, because the preferential (111) orientation in ALILE-Ge is owing to the slow growth under thermal equilibrium conditions.Considering both the Ge coverage and crystal quality, the 100-nm-thick Ge sample seems to be the most favorable among the samples.Thus, we can conclude that the selective etching technique established in this work is effective for achieving the highly (111)-oriented, large-grained Ge layer on glass with high coverage.

Conclusion
We proposed a way to fabricate a high-quality Ge on glass based on ALILE combined with a newly established wet etching process.The low-quality Ge islands formed in ALILE were selectively etched by the H2O2 treatment, while the high-quality bottom Ge was protected by the AlOx etching mask.As a result, the sample with 100-nm-thick Ge and 50-nm-thick Al allowed for the highly (111)-oriented, large-grained Ge layer covering the almost entire surface of the glass substrate.The island-annihilated Ge layer will be useful as an ideal epitaxial seed layer for various functional materials to fabricate advanced devices on glass.
)-2(i).Figs.2(d)-2(f) indicate that the H2O2 treatment effectively removed the Ge islands, and suggest that the AlOx-coated bottom Ge layers are partially bared.As shown in Figs.2(g)-2(i), the HF treatment removed Al and

Fig. 1 .
Fig. 1.Schematic of the Ge island removal in AIC.
Figure3shows the EBSD images of the bared bottom Ge layers.Figs.3(a)-3(c) indicate that the highly (111)-oriented Ge layers owing to the annihilation of the small-grained, randomly-oriented Ge islands[33,34].We note that the wide areas with random orientation in Fig.3(a) correspond to the bared SiO2, while the randomly-oriented parts in Figs.3(b) and 3(c) almost correspond to Ge.The grain sizes can be estimated from the TD maps in Figs.3(d)-3(f): large grains (~100 μm) are observed for all samples.

Fig. 3 .
Fig. 3. EBSD images in (a)-(c) ND and (d)-(f) TD of the Ge layers for the samples with the initial Ge thickness of 50 nm, 100 nm, and 150 nm.The ND and TD maps correspond to the same region.Coloration indicates crystal orientation, according to the legend.The black solid lines in the TD maps indicate random grain boundaries.

Fig. 4 .
Fig. 4. Distribution histograms of (a)-(c) (111) orientation fractions and (d)-(f) average grain sizes of the resulting Ge layers of the samples with the initial Ge thickness of 50 nm, 100 nm, and 150 nm.The integrated values of the area fraction from 0° to 15° are shown in (a)-(c); the average grain sizes are shown in (d)-(f).Both the crystal orientation and the grain size were obtained from the EBSD maps shown in Fig. 3.