Bulk Single Crystals and Physical Properties of Rutile GeO<sub>2</sub> for High‐Power Electronics and Deep‐Ultraviolet Optoelectronics
Zbigniew Galazka, Roberts Blukis, Andreas Fiedler, Saud Bin Anooz, Jijun Zhang, M. Albrecht, T. Remmele, Tobias Schulz, Detlef Klimm, Mike Pietsch, Albert Kwasniewski, Andrea Dittmar, Steffen Ganschow, Uta Juda, Karoline Stolze, Manuela Suendermann, Thomas Schroeder, Matthias Bickermann
Abstract
The top‐seeded solution growth for rutile GeO 2 single crystals using alkali carbonates or fluorides as flux is applied. Structural data of obtained single crystals confirm the rutile phase with a = b = 4.3966 Å and c = 2.8612 Å. The crystals with diameter of 5–15 mm are either undoped or intentionally doped with Sb 5+ , Sn 4+ , Al 3+ , Ga 3+ , and F − ions. It is found that Sb 5+ is a very efficient n‐type donor enabling free electron concentration even above 10 20 cm −3 ; thus, Sb‐doped GeO 2 is a potential substrate for vertical power devices. In contrast, crystals doped with Al and Ga do not show p‐type conductivity suggested by the theory. The onset of the absorption occurs at 5.0 and 5.5 eV perpendicular and parallel to the c‐axis, respectively. Rutile GeO 2 shows a very intense photoluminescence peaking at 420 nm (blue) and 520 nm (green). Raman spectra show narrow lines, in particular at high phonon energy ( B 1g , 170 cm −1 ). Prepared wafers show FWHM values of rocking curves below 30 arcsec and polishing is achieved down to RMS roughness of 0.15 nm. Transmission electron microscopy images do not show point or extended structural defects with uniform Sb distribution.