Abstract
The external quantum efficiency of zinc-diffused hemispherically-shaped Ga1-xAlxAs light-emitting diodes has been investigated as a function of temperature, peak emission energy, and current. External quantum efficiencies as high as 13.5% and 52% have been achieved at 296°K and 77°K, respectively. An analysis of the temperature dependence of the spectral characteristics (emission spectral shape, peak emission energy, and spectral half-width) shows that the improved efficiencies at room temperature result from reduced internal bulk absorption due to a composition gradient in the emitter. Values as small as 3.4 have been observed for the ratio of the 77°K efficiency to the 296°K efficiency. The decrease of the external quantum efficiency at high peak emission energies (short wavelengths) agreed with the decrease theoretically predicted from the composition dependence of the direct and indirect energy gaps of Ga1-xAlxAs. A decrease of external quantum efficiency at low currents was shown to be due to a nonradiative parallel component of current.
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