Spectral Measurement Of The Breakdown Limit Of Beta-Ga2o3 And Tunnel Ionization Of Self-Trapped Excitons And Holes

Owing to its strong ionic character coupled with a light electron effective mass, /3-Ga2O3 is an unusual semiconductor where large electric fields (approximately 1-6 MV/cm) can be applied while still maintaining a dominant excitonic absorption peak below its ultrawide band gap (Eg similar to 4.6-4.99 eV). This provides a rare opportunity in the solid state to examine exciton and carrier self-trapping dynamics in the strongfield limit at steady state. Under sub-band-gap photon excitation, we observe a field-induced redshift of the spectral photocurrent peak associated with exciton absorption and a thresholdlike increase in peak amplitude at high field associated with self-trapped hole ionization. The field-dependent spectral response is quantitatively fitted with an exciton-modified Franz-Keldysh effect model, which includes the electricfield-dependent exciton-binding energy due to the quadratic Stark effect. Saturation of the spectral redshift with reverse bias is observed exactly at the onset of dielectric breakdown, providing a spectral means to detect and quantify the local electric field and dielectric breakdown behavior in /3-Ga2O3. Additionally, the field-dependent responsivity provides an insight into the photocurrent-production pathway, revealing the photocurrent contributions of self-trapped excitons (STXs) and self-trapped holes (STHs) in /3-Ga2O3. Photocurrent and p-type transport in /3-Ga2O3 are quantitatively explained by field-dependent tunnel ionization of excitons and self-trapped holes. We employ a quantum-mechanical model of the field-dependent tunnel ionization of STXs and STHs in /3-Ga2O3 to model the nonlinear field dependence of the photocurrent amplitude. Fitting to the data, we estimate an effective mass of valence-band holes (18.8m0) and an ultrafast self-trapping time of holes (0.045 fs). This indicates that minority-hole transport in /3-Ga2O3 can only arise through tunnel ionization of STHs under strong fields.