Kinetics of Red Luminescence in GaP

Abstract

A model is presented and analyzed which describes the recombination kinetics of red luminescence in GaP. The need for the model arose with the recent observation of a substantially different bias variation of red electroluminescent intensity, in GaP diodes (Zn–O doped p-layers) produced by the liquid-phase epitaxial (LPE) process. It was previously observed that at low bias the intensity varied as exp (qV/kT), and at high bias it varied as exp (qV/2kT). The LPE diodes show the same low-level bias variation but at high biases the intensity varies as V (nonexponentially). Previous models for the recombination kinetics described the observed bias variations in terms of saturation of radiative recombination centers by electrons injected into the p-type bulk (Nelson) and in the space charge region (Morgan) of the diodes. In the present model, which is essentially an extension of Nelson's, a parallel, nonsaturable route is added so that the electron lifetime can remain finite even though the radiative centers saturate. It is shown that when the parallel route dominates the removal of excess electrons, the model describes the recently observed saturation characteristic. On the other hand, when the parallel route is negligible, the previously reported bias variation is obtained. Experimental results are presented for room temperature steady-state electroluminescence over a wide range of injection levels in optimally Zn–O doped GaP LPE diodes and compared with the predictions of the model. Under the assumption that the p-type material adjacent to the junction has the same properties as the bulk substrate, the following results were obtained: (a) At maximum electroluminescent efficiency about 80% of the injected electrons decay via the parallel nonproductive route; (2) the electron radiative recombination center (Zn–O complex) density is in the range 0.5–1.0×1016 cm−3; (3) the electron capture cross section of the center is in the range 3.3–6.7×10−16 cm2; and (4) the excess electron density required to half fill the centers is 0.75–1.5×1015 cm−3. Preliminary results of photoluminescence experiments are in agreement with the predictions of the model and the numerical values presented above.