Structural and electronic characteristics of Fe-doped β-Ga2O3 single crystals and the annealing effects

As capture traps, Fe impurities were intentionally incorporated into beta-gallium oxide (β-Ga2O3) crystals to compensate for unintentional n-type conductivity for applications of semi-insulated single-crystal substrates with high-performances. The systematic investigations are performed to comprehend the influence of the Fe addition on the structural, optical, electronic properties of the β-Ga2O3 crystal, and the annealing effect by using a combination of multi-disciplinary techniques. The Fe-doped β-Ga2O3 crystal exhibits a good single-crystal phase and a high optical transmittance from 400 nm to 2000 nm from the measurements of high-resolution X-ray diffraction and optical transmission spectroscopies. Raman scattering spectra revealed that the high-frequency phonon modes which belong to the stretching and bending of tetrahedron were significantly inhibited by the Fe addition to the β-Ga2O3 crystal. EPR results discovered that the presence of Fe3+ ions preferentially in the octahedral over the tetrahedral sites of the monoclinic structure. After an annealing treatment, the crystalline quality was improved and the oxygen vacancies were reduced. The absorption edge redshifted and the transmittance decreased slightly. In particular, it was discovered that the position of the Fermi level is deviated towards the valence band and the total number of spins of Fe3+ ions was halved from 5.32 × 1014 to 2.86 × 1014 spins/mm3. The annealing treatment not only improved the crystal quality, but also activated irons trap centers and further decreased the conductivity.