External quantum efficiencies η as high as 7.2% have been obtained for gallium phosphide red light emitting diodes. The p-n junctions were prepared using a p-on-n liquid phase epitaxy process in which the dopant levels significantly differ from those previously reported. Doping profiles for these junctions are compared with earlier n-on-p structures (η = 1–2%) and it is suggested that the observed high efficiencies result from (1) more efficient electron injection, (2) increased O concentration in the p-type layers which may result in a higher concentration of Zn–O complexes, and/or (3) fewer free holes in the p region contributing to nonradiative recombination.
The effect of a mixed crystal transition zone (grading) on the perfection of crystals grown by epitaxial deposition onto substrates of differing lattice parameter and expansion coefficient is considered. The number of misfit dislocations required to compensate lattice mismatch at the growth temperature and magnitude of bending stresses which develop during subsequent cooling have been computed as a function of the width of the transition zone δ. These results have been applied to the case of GaP grown onto GaAs seeds. Using a simple model for lattice displacements, it is predicted that grading does not reduce the number of misfit dislocations but merely distributes them over the width δ so that the density is reduced in proportion to δ−1. The bending stresses in the grown layer are shown to be highly dependent on the width of the transition zone and thickness of the grown layer. Moreover, for a given seed thickness and zone width there is an optimum thickness for the growth layer for which the bending stress is smallest.
The origins of dislocations in GaP grown by liquid phase epitaxy (LPE) and their effects upon green electroluminescence (EL) efficiency have been investigated by chemical etch pitting and scanning electron microscopy. Under normal growth conditions, the dislocation etch pit density (Pn) and configuration in the epitaxial layers are controlled by, and are approximately the same as, that in the liquid encapsulated Czochralski substrate. All dislocations which are revealed by chemical etching in the p LPE layer cause a localized reduction in luminescence. The dislocations are shown to behave as regions of rapid (nonradiative) recombination. Typically, reductions of a factor of -2 in green EL efficiency occur when Pn for the LPE layers is in the -2-5 X 10 5 cm-2 range, corresponding to an average separation of about two to four diffusion lengths in this material. The experimental results are supported by a simple theoretical model for the effects of dislocations on EL efficiency. These considerations explain why dislocations in GaP LPE layers for red light-emitting diodes (LED's), in contrast to green LED's, typically have no significant effect on EL efficiency since diffusion lengths are much shorter in the red LED's. PACS numbers: 78.60.F, 85.6O.J, 61.70.M [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 128.82.252.58 On: Tue
A series of variously doped
normalGaP
liquid phase epitaxy (LPE) layers were analyzed to establish the distribution of Zn,O and residual impurities contributing to the net impurity gradient previously observed in the p‐layer of high efficiency
normalGaP
LPE diodes. From these experiments we conclude that the net impurity gradient is primarily a consequence of a decreasing Zn concentration along the growth direction, and, to a lesser degree, an increasing residual donor concentration. The distribution of intentional and unintentional impurities was found to be independent of substrate doping level.The effect of cooling rate on the various impurity distributions was also studied, covering the range 0.5°–18°C/min. Residual impurity levels and associated gradients were significantly reduced by decreasing the cooling rate which indicates that residual impurity incorporation is kinetically controlled, probably by slow diffusion of these impurities in front of the growing LPE layer. Similarly, the O level appears to be decreased by reduced cooling rate. In contrast, Zn doping was found to be independent of cooling rate which suggests that its incorporation was essentially under equilibrium conditions. Assuming this to be the case, we have shown that the observed Zn distribution is consistent with the temperature dependence of the Zn solid solubility. Using an equilibrium description for Zn incorporation, we have computed the near‐junction Zn and O concentrations in the previously reported high efficiency diodes to be
6.7×1017 cm−3 normaland 2.7×1017 cm−3
, respectively. The corresponding level of residual impurities yields a net donor concentration of
≲3×1016 cm−3
.
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