Reflectance-based optically detected resonance studies of neutral and negatively charged donors in ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ quantum wells

We report a reflectance-based optically detected resonance (ODR) study of neutral and negatively charged donors in ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ quantum-well structures. The intensity of the $e_1{h}_1\mathrm{}\left(1s\right)\mathrm{}$ excitonic reflectance feature was modulated by resonant absorption of a monochromatic far-infrared (FIR) laser beam. An externally applied magnetic field was used to bring the internal transitions among various impurity states in resonance with the FIR photon energy. Predicted, but not previously reported, internal transitions of negatively charged donors were observed and were found to be in good agreement with theoretical calculations. This study establishes that the reflectance-based ODR technique is a sensitive tool for the investigation of donor states (both neutral and negatively charged) in semiconductor quantum wells.