High-Speed Growth of Epitaxial Rutile GeO2 Thin Film on (001) TiO2 Under Highly Oxygen-Rich Condition and Its Structural Analysis

Abstract

Recently, rutile germanium dioxide (r-GeO2) has been proposed as a novel ultra-wide bandgap (UWBG) semiconductor with ambipolar dopability, high carrier mobilities, and a higher thermal conductivity than Ga2O3, in addition to its wide bandgap (4.68 eV). These excellent properties indicate that r-GeO2 can be utilized as high-performance bipolar devices with high breakdown voltage. On the other hand, although r-GeO2 is most stable phase at the ambient condition, fabrication of its thin film is reported to be quite difficult, owing to the existence of energetically deep meta-stable phases, namely, amorphous and quartz phase, and high vapor pressure of r-GeO2. In this study, we accomplished fabrication of (001)-oriented r-GeO2 thin films on (001) rutile titanium dioxide (r-TiO2) substrate with high growth rate (≧1 mm/h) by mist chemical vapor deposition (mist CVD). Moreover, we investigated the structural properties of r-GeO2 film by X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques.First of all, bis [2-carboxyethylgermanium (IV)] sesquioxide (C6H10Ge2O7) as a precursor. To analyze the decomposition process of C6H10Ge2O7 and optimize the growth conditions, Thermogravimetry-Differential Thermal Analysis (TG-DTA) was conducted in aqueous vapor. By the TG-DTA, GeO2 was found to be synthesized from C6H10Ge2O7 at 553-783 ℃ in aqueous vapor. Therefore, we determined the growth temperature of r-GeO2 thin film to be 550-850 ℃. As shown in XRD 2q/w scan profile (Fig. 1), (001)-oriented r-GeO2 thin film was grown on (001) r-TiO2 substrate at growth temperature (T g) of 700-775 ℃. The growth rates of all the r-GeO2 films were found to be 1.2-1.7 mm/h, which were much higher than those grown by molecular beam epitaxy (MBE) (10 nm/h). Moreover, XRD F scan revealed that r-GeO2 thin film was epitaxially grown without rotational domains.It is considered that the higher growth rate by mist CVD compared to that by MBE is attributed to the lower desorption rate of GeO in mist CVD due to its highly oxygen-rich condition. In Fig. 2, we show the schematics of GeO desorption from the surface in (a) MBE and (b) mist CVD, inspired by Wang et al. In mist CVD, oxygen partial pressure is much higher (~760 Torr) than that in MBE (1×10- 6 Torr), resulting lower vapor pressure of GeO and lower desorption rate in mist CVD system. Higher oxygen partial pressure also suppresses oxygen vacancies in the film. Moreover, in mist CVD, oxygen was continuously supplied by water in the reaction area. Therefore, such highly oxygen-rich condition as seen in mist CVD is considered to contribute to high temperature growth and high growth rate.Cross-sectional TEM image and Selected Area Electron Diffraction (SAED) patterns at r-TiO2 substrate and the r-GeO2/r-TiO2 interface viewed along the <110> zone axis are shown in Fig. 3 (a), (b), and (c), respectively. The r-GeO2 thin film for the TEM observation was grown at lower growth rate (50 nm/h) for higher crystallinity. In Fig. 3 (b), clear diffraction spots of r-TiO2 substrate were visible. On the other hand, in Fig. 3 (c), clear diffraction spots of r-GeO2 thin film were observed as well as those of r-TiO2 substrate at the interface. From the diffraction spots, we calculated lattice parameters of the r-GeO2 thin film to be a=4.5 Å and c=2.7 Å. This indicates that the film suffers from significant tensile stress along the in-plane direction, and in-plane strain value was ~2.3%. Thus, in order to release the strain and achieve higher crystalline quality, using a buffer layer or other substrates with less lattice mismatches to r-GeO2 may be needed.We believe that these results greatly contribute to the future development of r-GeO2 applied for power-electronics devices and exploration of novel materials that are difficult to synthesize under normal growth conditions.This work was supported, in part, by JSPS KAKENHI under Grant No. 21H01811. Figure 1