Scanning Electron Microscope Characterization of GaP Red-Emitting Diodes

Accurate measurements of the near-junction properties of three different red-emitting GaP p-n junction structures have been made using the scanning electron microscope (SEM). The structures consisted of similar Zn, O-doped p-type liquid-phase-epitaxial (LPE) layers grown on different Te-doped n-type substrates, which include solution-grown (SG) crystals, liquid-encapsulated Czochralski (LEC) crystals, and LPE layers grown on LEC crystals. These junctions have yielded diodes with electroluminescent efficiencies as high as 6.0, 2.0, and 4.5%, respectively. These measurements, which include minority-carrier diffusion lengths, the spatial distributions of luminescent efficiency, and the detailed properties of the space-charge region, have provided a self-consistent picture of the factors which determine electroluminescent efficiency. In this study, all of the junctions were determined to be n+-p structures which exhibit uniform red luminescent efficiencies in the p region near the junction. The differences between the structures were found to lie in the relative contributions of nonradiative recombination centers, which tend to reduce both luminescent efficiency in the p region and injection efficiency into the p region. These centers were found to be most prominent for the LEC single LPE junctions, to a lesser degree for the LEC double LPE junctions due to a buffering effect of the additional n layer, and least of all in the SG single LPE junctions. For these 6.0% SG single LPE diodes, the injection efficiencies and luminescent efficiencies were determined to be 50% and 20%, respectively, allowing for 60% external coupling efficiencies. These values are consistent with those obtained from diode electrical and electroluminescent measurements, and agree with photoluminescent measurements in the p region.