The surface modification/oxidation of GaAs substrates in electrolytes has been investigated from the viewpoint of bio-sensing devices using AlGaAs/GaAs FET structures, where the cultivation of T2 cells directly on a bare gate-surface is intended for a high sensitivity to cell-activities such as adhesion or cell-cell interaction. The stability of GaAs surface (gate surface of FET) in DMEM solution (electrolyte for cell cultivation) is essential and must be elucidated in order to realize such cell-sensor devices, especially when gate-bias voltage is applied through the electrolyte.

When n+ GaAs(001) substrate was immersed in deionized pure water (37^oC) for 24h under 17 μW VIS-light, the oxide layer of 70-80 nm in thickness was formed on the surface. For the immersion in the dark, however, the thickness was as small as 10-20 nm, indicating that the photo-excited electron-hole pairs cause the surface oxidation. Moreover, we have found that NaH2PO4 is effective to suppress the photo-induced oxidation of GaAs. The oxide thickness of 6-8 nm was obtained by 24h immersion in 0.8 mM NaH₂PO₄ solution, even under VIS-light. The suppression mechanism we consider is that the mid gap states of GaAs surface are modulated by H₂PO₄ fragments, and as a result, the surface-band bending prevents the photo-excited holes from diffusing to the surface.

The application of gate-bias voltage (Vgs) through DMEM solution enhances the degradation of bare-gate-FET performance. Based on the above findings, we have succeeded in suppressing the degradation adequately by shielding the gate area from VIS light, but not by NaH₂PO₄. The decrease in Ids (Vgs = 0.5 V) was 14% for 10h operation in the dark, while it was 98% under VIS light.

This study clearly shows that the cells can be cultivated on bare GaAs surfaces in DMEM solution (containing 0.8 mM NaH₂PO₄), and adequate FET performance is maintained in the dark for cell-activity measurements.

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