Impurity Photoeffect in GaAs p-n Junctions
An investigation was made of the photo-emf spectra of GaAs p—n junctions at ∼90 and 293°K in the photon energy range ħω ≥ 0.46 eV , using modulated or constant illumination and various values of the reverse bias. T wo types of sample were investigated: those prepared by the diffusion of sulfur in p-type gallium arsenide and those prepared by the diffusion of zinc in n-type crystals. The photo-emf spectra obtained using modulated illumination at T ≈ 90°K indicated the presence of impurities in the samples prepared by the diffusion of sulfur; these impurities had energy levels at 0.49, 0.54, 0.59, 0.65, 0.73, 0.90, and 1.05 eV. These levels were compared with the known levels of impurities usually encountered in gallium arsenide (such as copper, oxygen, and iron) and with lattice defect levels. In a certain part of the spec - trum, the photosensitivity decreased with time, which was attributed to the liberation of carriers from impurity levels. It was found that the steady-state photo-emf appeared only at photon energies ħω ⋟ 0.5 Eg and it was due to the single-stage optical excitation of minority carriers in the space-charge layer. The transient photo-emf depended weakly on the reverse bias across the p-n junction but the steady-state effect was increased strongly by such a bias.
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