Photoluminescent Saturation in GaP (Zn, O)

This paper describes a room-temperature study of the red emission from GaP (Zn, O) as a function of 4880- and 5145-Å photoexcitation intensity, and was initiated to determine the importance of nonradiative bulk recombination centers shunting the radiative path through the Zn–O complexes. The technique used is the investigation of the saturation behavior of the emitted red light as a function of excitation intensity. An analysis is given in which the functional dependence of the red-light emission on such parameters as the excitation absorption coefficient, diffusion length, surface recombination velocity, and excitation beam diameter is considered. The diffusion lengths and surface recombination velocities were obtained by means of low-intensity excitation spectral measurements. Diffusion lengths covering the range 0.5–5.0 μ were measured and surface recombination veloclties from 5 to 100 times the diffusion velocity were extracted. The samples saturated at intensities ranging from 3×1019 photons/sec cm2 to 2×1021 photons/sec cm2. This wide range was correlated with the correspondingly large ranges of diffusion lengths and minority carrier lifetimes. From the analysis and experimental results, it was concluded that (i) for photoexcitation with beam diameters many times greater than the diffusion length the emitted red light will saturate at high excitation intensities, independent of the presence of nonradiative bulk centers, because of nonradiative recombination at the surface. The analysis demonstrated that this conclusion is true even when the excitation penetration depth is ten times greater than the diffusion length. (ii) When beam diameters of the same order as the diffusion length are utilized, diffusion parallel to the surface is important, and it is possible to extract information on the relative importance of bulk nonradiative centers. (iii) Numerical values measuring the significance of the nonradiative bulk recombination centers could not be obtained from the saturation measurements, but the saturation measurements and diffusion length measurements yielded values for the capture cross section of conduction-band electrons into a Zn–O complex σc. The values fall in the range 1.5–4.5×10−16 cm2.