Surface modification/oxidation of GaAs in electrolytes for cell-cultivating bio-sensing devices

Kazunari Ozasa 1,2Shigeyuki NEMOTO 1Masahiko HARA 1Mizuo MAEDA 1

1. Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako-Shi, Saitama 351-0198, Japan
2. CREST-JST, 3-1-6-5 Shibuya, Tokyo 150-0002, Japan


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 (37oC) 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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Presentation: poster at E-MRS Fall Meeting 2005, Symposium F, by Kazunari Ozasa
See On-line Journal of E-MRS Fall Meeting 2005

Submitted: 2005-05-27 10:15
Revised:   2009-06-07 00:44
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