Density functional theory calculations are used to study the molecular and dissociative adsorption of water on the (-201) β-Ga O surface. The effect of adsorption of different water-like species on the geometry, binding energies, vibrational spectra and the electronic structure of the surface are discussed. The study shows that although the hydrogen evolution reaction requires a small amount of energy to become energetically favourable, the over potential for activating the oxygen evolution reaction is quite high. The results of our calculations provide insight as to why a high voltage is required in experiments to activate the water-splitting reaction, whereas previous studies of gallium oxide predicted very low activation energies for other energetically more favourable facets. Application of this work to studies of GaN-based chemical sensors with gallium oxide surfaces shows that it is possible to select the gate bias so that the sensors are not influenced by water-splitting reactions. It was also found that in the region where water splitting does not occur, the surface can exist in two states, that is, water or hydroxyl terminated.
The charging mechanism of the interface between an AlGaN/GaN based open-gate ion-sensitive field-effect transistor and electrolyte is studied theoretically. Density functional theory calculations are performed to obtain the energy minimum structure of the surface oxide and electrolyte interface. Thermodynamics based relations are employed to obtain the double layer parameters. An analytical model is applied to study the carrier density modulation of the AlGaN/GaN heterostructure by the influence of the surface charge.
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