研究目的

To demonstrate truly bulk ZnGa2O4 single crystals of high structural perfection obtained directly from the melt for the ?rst time, which enable substrate preparation for epitaxial growth and devices. Another objective is to provide basic structural, chemical, electrical, and optical characterization of the melt-grown ZnGa2O4 single crystals, which may contribute to better understanding of the material properties, in particular its electrical properties.

研究成果

We demonstrated the growth and characterization of truly bulk ZnGa2O4 single crystals obtained directly from the melt. A high melting point of ZnGa2O4 of 1900 ± 20 ?C leads to its thermal decomposition in the liquid phase and to intensive incongruent evaporation resulted in a composition shift. Optimized growth conditions resulted in stoichiometric or near-stoichiometric single crystals with a normal spinel structure. ZnGa2O4 maintains a single crystalline structure with the Ga/Zn ratio up to about 2.17, above which an admixture of β-Ga2O3 is formed. The obtained single crystals were of good structural quality characterized by the FWHM of the rocking curve of (100)-oriented wafers even below 25 arcsec. ZnGa2O4 single crystals could be either electrical insulators or n-type semiconductors with the resistivity, free electron concentration, and Hall mobility of 0.002–0.1 ?cm, 3 × 1018–9 × 1019 cm?3, and 40–107 cm2 V?1 s?1, respectively. The semiconducting state could be switched into electrically insulating state by annealing in the presence of oxygen at temperatures ≥700 ?C for at least several hours. Transmittance spectra show a steep absorption edge at 275 nm, while the optical bandgap concluded from the absorption coef?cient is direct with a value of about 4.6 eV. Melt-grown ZnGa2O4 single crystals are thermally stable up to 1100 and 700 ?C upon annealing in oxidizing and reducing atmospheres, respectively. Being cubic and having no easy cleavage planes, ZnGa2O4 crystals enable easy wafer fabrication not only for electronic and optoelectronic applications but also can serve as good lattice matched substrates for epitaxial growth of magnetic ferrite spinel ?lms.

研究不足

The high melting point of ZnGa2O4 of 1900 ± 20 ?C leads to its thermal decomposition in the liquid phase and to intensive incongruent evaporation resulted in a composition shift. The volume of ZnGa2O4 single crystals from one growth experiment is about 8 cm3, much smaller than that of Ga2O3 obtained by the Czochralski method (about 160 cm3).