  Performing deep level transient spectroscopy (DLTS) on Schottky diodes, we
investigated defect levels below the conduction band minima (Ec) in Czochralski
(CZ) grown unintentionally-doped (UID) and vertical gradient freeze (VGF)-grown
Zr-doped beta-Ga2O3 crystals. In UID crystals with an electron concentration of
10^17 cm-3, we observe levels at 0.18 eV and 0.46 eV in addition to the
previously reported 0.86 (E2) and 1.03 eV (E3) levels. For 10^18 cm-3 Zr-doped
Ga2O3, signatures at 0.30 eV (E15) and 0.71 eV (E16) are present. For the
highest Zr doping of 5*10^18 cm-3, we observe only one signature at 0.59 eV.
Electric field-enhanced emission rates are demonstrated via increasing the
reverse bias during measurement. The 0.86 eV signature in the UID sample
displays phonon-assisted tunneling enhanced thermal emission and is consistent
with the widely reported E2 (FeGa) defect. The 0.71 eV (E16) signature in the
lower-Zr-doped crystal also exhibits phonon-assisted tunneling emission
enhancement. Taking into account that the high doping in the Zr-doped diodes
also increases the electric field, we propose that the 0.59 eV signature in the
highest Zr-doped sample likely corresponds to the 0.71 eV signature in
lower-doped samples. Our analysis highlights the importance of testing for and
reporting on field-enhanced emission especially the electric field present
during DLTS and other characterization experiments on beta-Ga2O3 along with the
standard emission energy, cross-section, and lambda-corrected trap density.
This is important because of the intended use of beta-Ga2O3 in high-field
devices and the many orders of magnitude of possible doping.
