Electroabsorption in a GaAs n-i-p-i structure with selective contacts fabricated by focused ion molecular beam epitaxy

Optical modulators fabricated using doping superlattice structures have bee n of interest recently. To enable the optical absorption coefficient of the se structures to be modulated by an external electrical bias, selective ohm ic contacts to the p and n-type layers are essential. This was first demons trated by Doehler et al. (Appl. Phys. Lett. 49 (12), 1986), using the techn ique of shadow masking with Molecular Beam Epitaxial (MBE) growth. They suc cessfully fabricated selective contacts to a GaAs n-i-p-i superlattice, whi ch 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 bea ms 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-sit u wet etching for mesa definition and evaporation of AuBe and AuGeNi for th e p and n-type contacts respectively, the electrical characteristics were m easured at room temperature. A turn-on voltage of 0.6V and a reverse satura tion current of 10nA was measured in the dark at room temperature. Optical transmission experiments showed a peak change in absorption for 900nm wavel ength light when an AC bias From -2V to +2V around a DC bias of 1V was appl ied to the ohmic contacts.