AlGaN/GaN-basierte-pH-Sensoren für biochemische Anwendungen

In this work AlGaN/GaN heterostructures for sensing applications are developed and investigated. These structures are promising for pH-sensing in the food industry or with further optimization as biosensors for the detection of DNA-hybridisation, biofilms or cell reactions. The first part of this work concerns the search for a biocompatible and chemical stable passivation to achieve long term stability and a low signal-drift for pH-measurements. Therefore, SixNy-SiO2-multilayer, sputtered SixNy-doublelayer and diamond-passivations are employed that can reduce the signal drift to 0.002 pH/h. For industrial applications in the food industry an important criteria is the stability in clean-in-place procedures. These warm, highly acidic or basic solutions are a challenge for the stability of the passivations. SixNy passivations show a good resistance against basic NaOH solution but an insufficient coverage of the metallic contacts enable etching so that they last only 30 cleaning cycles. Only the diamond passivation offers sufficient coverage that withstands 40 cycles. Furthermore the manufacture technology for the surface sensitive pH-sensors has been improved as they are very sensitive to technological effects that influence and alter the 2DEG confined in the AlGaN/GaN-structures and providing the conductive channel of the sensor. Hence the influences of different commonly used plasma processes for etching and for resist removal were investigated. It turned out that the different plasma treatments affect the mobility and the carrier density of the 2DEG. To minimise these effects several heterostructures, SixNy coatings and temperature dependent annealing were systematically evaluated and improved. To realise bio-sensory applications the pH-sensor was adapted to the respective method. For DNA detection the sensor surface was photochemically functionalized with TFAAD molecules that are covalently bound and permit the reusability of the device. To examine cell-reactions HEK-cells were cultivated on the sensor surface chemically activated using Ionomycine. Finally a bio film was detected as the sensor resistance decreased due to the growth of algae that was stimulated by UV radiation on the sensor surface.