Deep donors and acceptors in β-Ga2O3 crystals: Determination of the Fe2+/3+ level by a noncontact method

Electron paramagnetic resonance (EPR), infrared absorption, and thermoluminescence (TL) are used to determine the Fe2+/3+ level in Fe-doped β-Ga2O3 crystals. With these noncontact spectroscopy methods, a value of 0.84 ± 0.05 eV below the conduction band is obtained for this level. Our results clearly establish that the E2 level observed in deep level transient spectroscopy (DLTS) experiments is due to the thermal release of electrons from Fe2+ ions. The crystals used in this investigation were grown by the Czochralski method and contained large concentrations of Fe acceptors and Ir donors, and trace amounts of Cr donors. Exposing a crystal at room temperature to 325, 375, or 405 nm laser light converts neutral Fe3+ acceptors to their singly ionized Fe2+ charge state and, at the same time, converts a similar number of neutral Ir3+ donors to the Ir4+ charge state. The Fe3+ EPR spectrum slowly recovers after the light is removed, as electrons are thermally released from Fe2+ ions to the conduction band. Most of these released electrons recombine nonradiatively with holes at the deep Ir4+ donors. Using a general-order kinetics model, the analysis of isothermal recovery curves for the Fe3+ EPR signal taken between 296 and 310 K gives the activation energy for the decay of the photoinduced Fe2+ ions. A TL peak, with emitted light having wavelengths longer than 500 nm, occurs near 349 K when a few of the electrons released from Fe2+ ions recombine radiatively with holes at Ir4+ and Cr4+ donors. Photoluminescence and EPR verify the presence of Cr3+ ions.