Constant photocurrent method to probe the sub-bandgap absorption in wide bandgap semiconductor films: the case of α-Ga2O3

The optical absorption coefficient is one of the fundamental properties of semiconductors and is critical to the development of optical devices. Herein, a revival of the constant photocurrent method is presented to measure sub-bandgap absorption in wide bandgap semiconductor films. The method involves maintaining a constant photocurrent by continually adjusting the impinging photon flux across the energy spectrum. Under such conditions, the reciprocal of the photon flux for uniformly absorbed light is proportional to the absorption coefficient. This method is applied to α-Ga ₂O ₃ and reveals that it can access the absorption coefficient from 1 × 10 ⁵ cm ⁻¹ at the band edge (5.3 eV) to 0.8 cm ⁻¹ close to mid-bandgap (2.7 eV). Changes in the steepness of the absorption curve in the sub-bandgap region are in excellent agreement with defect states of α-Ga ₂O ₃ reported by deep level transient spectroscopy, indicating that the technique shows promise as a probe of energetically distributed defect states in thin film wide bandgap semiconductors.