Optical net gain measurement on Al_0.07Ga_0.93N/GaN multi-quantum wells

We report net gain measurements at room temperature in Al_0.07Ga_0.93N/GaN 10-period multi-quantum well layers emitting at 367 nm, using the variable stripe length method. The separate confinement heterostructure was designed targeting electron-beam pumped lasing at 10 kV. The highest net gain value was 131 cm^-1, obtained at the maximum pumping power density of the experimental setup (743 kW/cm²). The net gain threshold was attained at 218 kW/cm² using 193 nm optical pumping. From these experiments, we predict an electron-beam-pumped lasing threshold of 370 kW/cm² at room temperature, which is compatible with the use of compact cathodes (e.g. carbon nanotubes). In some areas of the sample, we observed an anomalous amplification of the photoluminescence intensity that occurs for long stripe lengths (superior to 400 µm) and high pumping power (superior to 550 kW/cm²), leading to an overestimation of the net gain value. We attribute such a phenomenon to the optical feedback provided by the reflection from cracks, which were created during the epitaxial growth due to the strong lattice mismatch between different layers.