Electroabsorption in a GaAs n-i-p-i structure with selective contacts fabricated by Focused Ion Molecular Beam Epitaxy

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Optical modulators fabricated using doping superlattice structures have been of interest recently. To enable the optical absorption coefficient of these structures to be modulated by an external electrical bias, selective ohmic contacts to the p and n-type layers are essential. This was first demonstrated by Doehler et al. (Appl. Phys. Lett. 49 (12), 1986), using the technique of shadow masking with Molecular Beam Epitaxial (MBE) growth. They successfully fabricated selective contacts to a GaAs n-i-p-i superlattice, which showed good p-n junction characteristics and strong electroluminescence at room temperature. In this paper, we describe the use of the Focused Ion Molecular Beam Epitaxial (FIMBE) method to fabricate similar structures in a single-step process with minimal ex-situ processing. Using dopant ion beams of Si and Be from a Au Si Be liquid metal ion source, 2mm doped squares were patterned during MBE growth with a 0.5mm lateral overlap. After ex-situ wet etching for mesa definition and evaporation of AuBe and AuGeNi for the p and n-type contacts respectively, the electrical characteristics were measured at room temperature. A turn-on voltage of 0.6V and a reverse saturation current of 10nA was measured in the dark at room temperature. Optical transmission experiments showed a peak change in absorption for 900nm wavelength light when an AC bias from −2V to +2V around a DC bias of 1V was applied to the ohmic contacts.