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Journal of the Optical Society of America B

Photorefractive effect in InP:Fe dominated by holes at room temperature: influence of the indirect transitions

Cafer Özkul, S. Jamet, Philippe Gravey, K. Turki, Georges E. Bremond

Published Sep 1, 1994• 4 citations
Publisher

Abstract

Iron-ion concentrations have been determined from measurements of the photoinduced-grating erasure times. We checked the accuracy of this method by performing two-wave mixing experiments at λ = 1.32 μm without an applied field. After verifying the fit of the extended single-defect model with indirect transitions and the two-wave mixing gain data at λ = 1.06 μm, we conclude that the indirect transitions involving the excited state of Fe2+ can induce a reduction of the photorefractive effect in InP:Fe when the dominant photocarriers are holes at room temperature.

DOI
10.1364/josab.11.001668

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P. U. Halter, Jean Claude Fabre, Gérald Roosen

For understanding and optimization of the photo refractive gain it is important to know the basic parameters of photo-ionization and recombination processes. In the nanosecond regime the pulse energies required for maximum coupling gain in semiconductors easily reach the limit for saturation of the deep centers. especially if one of the recombination times is larger than the pulse duration. In that case, induced absorption (or bleaching) will appear due to a redistribution of the population of the different levels of the deep center and counterbalance photorefractive amplification. The present simultaneous study of the kinetics of the induced absorption and the photocurrent in an InP:Fe sample reveals a number of interesting parameters like electron and hole recombination times, ionization cross sections and Fe2+/Fe3+ concentrations.

Photorefractive Effects, Materials, and Devices•Jan 1, 1990•0 citations
Iron-ion concentrations have been determined from measurements of the photoinduced-grating erasure times. We checked the accuracy of this method by performing two-wave mixing experiments at λ = 1.32 μm without an applied field. After verifying the fit of the extended single-defect model with indirect transitions and the two-wave mixing gain data at λ = 1.06 μm, we conclude that the indirect transitions involving the excited state of Fe2+ can induce a reduction of the photorefractive effect in InP:Fe when the dominant photocarriers are holes at room temperature.
A comparative study of the temperature dependence of the two-wave mixing gain is performed at three wavelengths (i.e., 1.06, 1.32, and 1.54 µm) for the fringe spacing values Λ=1.6 µm and 5 µm. With the latter the gain sign changes at low temperature for λ=1.06 and λ=1.54 µm, but not for λ=1.32 µm. The gain measurements are performed both with and without an external electric field. The theoretical model must fit all the experiments. In our case the energy level of the second defect is found to be near the valence band. We demonstrate that the agreement between the theory and the experiments performed at room temperature is improved by incorporating the indirect transitions (involving the excited state of iron) into the two-defect model. The result of this approach is presented as the extended two-defect model.
We have observed two-beam coupling gain (0.273 cm−1) that exceeds absorption (0.145 cm−1) in InP:Fe without applying an external field at λ=1.32 μm. Measurements of the gain coefficient as a function of grating spacing fit the photorefractive theory for simultaneous electron-hole transport, with the effective density of traps NE = (3.86 ± 1.1)NE=(3.86±1.1)× 1015 cm−3 and ξ, the factor that accounts for competition between electrons and holes, equaling −0.729±0.026. Absorption measurements, combined with the photoionization cross sections of electrons and holes, confirm these results. The agreement between theory and experiment suggests how to choose impurity concentrations that optimize net gain.
We show that even if there is no direct hole–electron competition in InP:Fe the gain is lower than expected. We attribute this reduction of the gain to a thermally induced hole–electron competition that involves the excited state of Fe2+ (5T2). We present a new model for the photorefractive effect in InP:Fe, which we use to interpret our beam-coupling results. Some material parameters of the InP:Fe sample and particularly of the excited state are deduced. The same energy-transfer measurements conducted in GaAs give us the ratio of hole and electron photoionization cross sections.
For understanding and optimization of the photo refractive gain it is important to know the basic parameters of photo-ionization and recombination processes. In the nanosecond regime the pulse energies required for maximum coupling gain in semiconductors easily reach the limit for saturation of the deep centers. especially if one of the recombination times is larger than the pulse duration. In that case, induced absorption (or bleaching) will appear due to a redistribution of the population of the different levels of the deep center and counterbalance photorefractive amplification. The present simultaneous study of the kinetics of the induced absorption and the photocurrent in an InP:Fe sample reveals a number of interesting parameters like electron and hole recombination times, ionization cross sections and Fe2+/Fe3+ concentrations.