Optical Modulation by Controlling the Charge State of Deep Impurity Levels

Abstract

<p>Measurements of thallium and indium doped Silicon-On-Insulator rib waveguidesshow optical absorption at a wavelength of 1550nm, dependent on the charge stateof the associated deep-level. Therefore, it is possible to use this effect to modulatewaveguide transmission by means of local depletion and/or injection of free-carriersto change deep-level occupancy. A one-dimensional model based on the generationand recombination process described by the modified Shockley-Read-Hall (SRH)mechanism was developed using MATLABc programming language in order to computethe optical absorption of a 1550nm wavelength as a function of the density ofneutrally-charged thallium or indium centers. This numerical model is in reasonableagreement with the experimental data for samples co-doped with low and mediumphosphorus concentrations. The values of optical absorption cross-section calculatedfor thallium are 2.9×10−17 ± 0.25cm2 and 3.2×10−17 ± 0.12cm2 for ion implantationdoses of 7.4×10−13cm−2 and 1.2×10−14cm−2, respectively. Also described is the thedesign, fabrication and characterization of an optical modulator using a four-terminalp+pnn+ diode on an indium-doped Silicon-On-Insulator rib waveguide. Modulationby controlling the charge state of deep impurity levels in silicon was thus demonstrated.Modulation bandwidth in the 2-10MHz regime was measured and the depthof modulation is approximately 0.48dB/V in forward bias and 0.25dB/V in reversebias. This is the first report of the implementation of an optical silicon-waveguidemodulator based on a periodically interleaved pn-junction configuration. In addition,the influence of indium, as a dopant in silicon (utilizing the Impurity PhotovoltaicEffect), as a means to increase the efficiency of a thin film silicon solar cell wasinvestigated using the same samples. Under certain doping conditions and geometricalconfigurations, a cell efficiency greater than 24% was measured —a somewhatremarkable result for these silicon thin films of 2.5μm</p>