Nitride based heterostructures with Ga- and N-polarity for sensing applications

Abstract

In this paper, a first comparative study on the sensing behavior of AlGaN/GaN and AlInN/GaN heterostructures is presented, especially with respect to the impact of crystal polarity. Ga-polar samples were grown completely by metal-organic vapor-phase epitaxy (MOVPE), whereas N-polar structures were realized by a combination of molecular-beam epitaxy and MOVPE. The epitaxial structures were characterized by high resolution X-ray diffraction, scanning electron microscopy, atomic force microscopy and transmission electron microscopy, particularly with regard to strain, surface and interface roughness. Sheet carrier density and mobility of the 2-dimensional charge carrier gas were determined by Hall measurements. The heterostructures were applied as open-gate sensors and their sensing behavior was analyzed by exposure to different polar liquids. Generally, the device sensitivity is dominated by the sheet carrier density indicating a direct correlation between localized carriers at the interface and surface. A superior carrier concentration in N-polar samples enhanced their response to polar fluids. Due to a higher sheet carrier density, AlInN-based devices are more sensitive to polar liquids compared to conventional AlGaN/GaN heterostructures.