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Photorefractive Effects, Materials, and Devices

Influence of the Excited State of Fe2+ on Photorefractive Effect in InP

Philippe Delaye, P. U. Halter, Gérald Roosen

Published Jan 1, 1990• 0 citations
Publisher

Abstract

Although the excited state of Fe2+ is well known in InP:Fe (figure 1), its influence on photorefractive effect has never been taken into account. We report here a study of this influence and show that the presence of this state induces in the photorefractive effect a new hole-electron competition. This theory well explains the photorefractive coupling gain results obtained on a crystal without any classical hole-electron competition(αn«αp).

DOI
10.1364/pmed.1990.pd11

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arXiv:0909.1217•Physical Review A•Sep 25, 2009•4 citations
Although the excited state of Fe2+ is well known in InP:Fe (figure 1), its influence on photorefractive effect has never been taken into account. We report here a study of this influence and show that the presence of this state induces in the photorefractive effect a new hole-electron competition. This theory well explains the photorefractive coupling gain results obtained on a crystal without any classical hole-electron competition(αn«αp).
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.
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.
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.
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.
This paper extends the existing theory of two-carrier photorefractivity resonance, which is generally applied to iron doped indium phosphide (InP:Fe), to the case of low nonharmonic illumination. The space charge field profile is computed, and the variations of its amplitude, width and position are determined as functions of the background intensity. The effect of photorefractive resonance on these quantities is evidenced, contributing to the understanding of published experimental results in InP:Fe.