Dynamics Of The Electroabsorption In MSM-Modulators based on Low Temperature Grown GaAs

Abstract

We report on carrier and field dynamics in low temperature grown GaAs (LT-GaAs) layers by differential transmission. LT-GaAs is grown by molecular beam epitaxy under standard growth conditions except a lower substrate temperature of 200°C to 300°C instead of the usual 600°C. This leads to an incorporatioln of up to 1.5% excess arstmic in the crystal, mainly as A h and Asi, accompanied by V G ~ . These defects cause some outstanding properties of the material, for instance an extremely short carrier lifetime. Subsequent annealing at temperatures between 450°C and 900°C results in high resistivity and raised breakdown fields. All properties of LT-GaAs depend strongly on both the growth and the annealing temperature. The combination of short carrier lifetimes and high breakdown fields makes LT-GaAs a very interesting material for ultrafast electro-optical modulators. In particular, they are suitable for switching high optical power due to low (dark currents and small photo currents, which result in low electrical energy dissipation. IDC measurements on metal-semiconductor-metal (MSM)-modulators on 1 pm thick LT-(A1)GaAs layers yield remarkalble transmission changes ofup to 1:1.8. In this paper we focus on the dynamics of electroabsorption. Lifetimes of photogenerated carriers in LT-GaAs are well investigated without electric fields. However, the behavior under the influence of an electric field is essential for the development of electronic devices of any kind. We performed pump and probe experiments on LT-GaAs MSIM-modulators with lOOfs optical pulses from a mode-locked Ti:sapphire laser with spectral width of 8 nm. Taking advantage of an epitaxial lift-off technique we separated the LT-GaAs layers from the substrates and attached them to a thin sheet of glass. The field is applied via interdigitated finger contacts (MSM-contacts). We detected the transmission changes of th~e probe beam which are induced by the pump beam as a function of the delay between the two pulses. Measurements with and without applied voltage are compared'.