pH-dependent photocorrosion of GaAs/AlGaAs quantum well microstructures

Semiconductor microstructures comprising stacks of GaAs/AlGaAs layers have found attractive applications for photocorrosion-based detection of electrically charged molecules immobilized in the vicinity of their surfaces. We have investigated sensitivity of the photocorrosion of GaAs/AlGaAs microstructures with a stack of 30 GaAs quantum well (QW) layers to pH of aqueous solutions ranging between 2.2 and 11.2. The effect was studied by measuring QW emission for bare and (3-mercaptopropyl)-trimethoxysilane (MPTMS) coated microstructures. It has been determined that in highly both acidic (pH 2.2) and alkaline (pH 11.2) solutions the uncoated microstructures photocorrode at relatively high rates (∼0.83 nm/min), while in moderate pH (7–9) solutions the photocorrosion proceeds at rates reduced to ∼0.33 nm/min, suggesting that some oxides accumulate on the in situ revealed surfaces of GaAs and Al0.35Ga0.65As. The photocorrosion at a moderate-to-high pH 10.2 revealed the formation of a series of well-defined PL maxima each time the photocorrosion front passes from the GaAs to the Al0.35Ga0.65As surface. For MPTMS functionalized samples, a series of similar origin, well-defined PL maxima have been observed in a more alkaline solution characterized by pH of 11.2. A delayed position of the first PL maximum illustrates the passivation function of the MPTMS layer, although its depolymerization is observed in a highly alkaline environment.