Optically Detected Defect Levels in Ga₂O₃

Point defects and their charge transition levels in Ga₂O₃ are critical to its success in high power electronics. When investigating defects or transition levels, however, often little information is known about the defect itself; rather, identification is made by monitoring carriers excited to or from the defect. Time-dependent photo-induced electron paramagnetic resonance (photo-EPR) offers the opposite approach: little is known about the carriers, but the identity of the defect is clear. Basically, the technique is an optical absorption measurement performed on a specific, known defect. The process consists of 1) identifying an EPR spectrum with a specific defect structure; 2) monitoring the amplitude of the EPR during illumination with selected wavelengths, and 3) Fitting the time-dependent data with a series of rate equations representing the type of transition. An optical cross section spectrum, σ(E), obtained from the data is compared to one which includes the defect level (E_D) as well as defect-lattice interaction or relaxation energy as parameters.<br/>Photo-EPR is performed with a conventional 10 GHz EPR spectrometer and a series of LEDs from 0.7 to 4.4 eV. Samples studied include bulk Fe- or Mg-doped Ga₂O₃ grown by the Czochralski or floating zone methods. Samples were examined with X-ray diffraction to determine the orientation of the crystal faces and secondary ion mass spectrometry to determine the concentration of the typical impurities, Si, Fe, and Ir. The concentration of Fe and Mg in the intentionally doped samples were 5-7x10¹⁸ cm^-3 and 6-30x10¹⁸ cm^-3, respectively. All samples were insulating.<br/>The defect levels for Fe^2+/3+and Mg^-/0 were directly probed using the photo-EPR method. Data obtained from both Mg-doped and Fe-doped samples yielded a defect level of 0.6 eV below the conduction band edge for the 2+ to 3+ transition of Fe. As an optical technique, we are also able to extract the lattice relaxation energy, which for this transition is 0.7 eV. The transition level is similar to that determined by several other groups and the relaxation energy confirms the value calculated from density functional theory [1,2]. Theoretical calculations for the defect level for neutral to negative transition of Mg was reported to be ~ 1 – 1.5 eV above the valence band maximum (VBM) with a lattice relaxation of 1.1 eV [3-5]. Recent experimental measurements, however, place the level only 0.6 eV above VBM [6]. Our results, which agree with both the defect level and relaxation energy predicted by theory, are 1.35 eV above VBM (E_d) and 1.1 eV (relaxation energy). The Fe and Mg-related process represent direct defect-to-band transitions. In addition to these, the talk will highlight the role of other point defects such as Ir and the much-discussed V_Ga, both of which are observable with EPR.<br/>The work was supported by the National Science Foundation under DMR-1904325<br/>1. M. E. Ingebrigtsen et al, Appl. Phys. Lett. <b>112</b>, 042104 (2018).<br/>2. C. A. Lenyk et al , J. Appl. Phys. 126, 245701 (2019).<br/>3. Q. D. Ho et al, J. Appl. Phys. <b>124</b>, 145702 (2018).<br/>5. A. Kyrtsos et al, Appl. Phys. Lett. <b>112</b>, 032108 (2018).<br/>5. J. L. Lyons, Semicond. Sci. Technol. <b>33</b>, 05LT02 (2018).<br/>6. C. A. Lenyk et al, Appl. Phys. Lett. <b>116</b>, 142101 (2020).