No abstract

High-electron-mobility transistors (HEMTs) based on AlGaN/GaN heterostructures were successfully tested as chemically sensing devices. Exposing the unprotected polar GaN surface in the gate area of a HEMT to liquids of different polarity, milliampere changes in the source-drain current could be detected. These sensing effects are likely to arise from chemical interactions with a sheet of ionic charge on the free GaN surface which compensates the electronic charge of a two-dimensional electron gas at a subsurface AlGaN/GaN interface.

In the present article recent results concerning sensor applications of AlGaN layers and AlGaN/GaN heterostructures are summarized. The piezoresistive effect in piezoelectric AlGaN layers is investigated and the dependence of the piezoresistive gauge factor on the Al content is attributed to the influence of strain induced piezoelectric fields. An enhancement of this effect is observed in AlGaN/GaN heterostructures, resulting in high longitudinal gauge factors.The response of gas sensitive Pt:GaN Schottky diodes to hydrogen and hydrogen containing gases is analyzed up to temperatures of 600°C and employed to realize gas sensitive field effect transistors which are demonstrated to operate up to 400°C. In addition, ion sensitive field effect transistors (ISFETs) have also been fabricated on the basis of AlGaN/GaN heterostructures. The GaN surface shows a high pH sensitivity which is attributed to the presence of a thin native metal oxide layer on the surface.

We review the influence of GaN crystal polarity on various properties of epitaxial films and electronic devices. GaN films grown on sapphire by MOCVD or HVPE usually exhibit Ga-face polarity. N-face polarity is obtained either on the backside of such layers after removal from the substrate, or by turning the crystal polarity in MBE growth via a thin AlN buffer layer. In addition to rather obvious differences in their structural and morphological features, Ga-and N-face samples differ also in their electronic properties. Thus, different Schottky barrier heights are observed for both polarities, the position and detailed properties of spontaneously formed twodimensional electron gases vary with polarity, and the adsorption of gases and ions also show an influence of the two different surfaces. A particular interesting possibility is the growth of lateral polarity heterostructures with predetermined macroscopic domains of different polarity separated by inversion domain boundaries. These structures make use of the crystal polarity as a new degree of freedom for the investigation of electronic properties of III-nitrides and for novel devices. Introduction The fact that the III-nitrides InN, GaN, and AlN are essentially ionic solids with a strong charge transfer between the very electronegative nitrogen atoms and the less electronegative metal atoms (In, Ga, or Al) has important consequences for many physical properties of this material system.The thermodynamically stable wurtzite structure of III-nitrides has a polar axis parallel to the c-direction of the crystal lattice. Deviations of the real atomic charge distributions from the point charge model of the ideal wurtzite lattice give rise to a macroscopic spontaneous polarization P of the III-nitrides, which can reach values up to 0.1 C/m 2 . Electrostatically, such macroscopic lattice polarizations are equivalent to twodimensional fixed lattice charge densities s ¼ P with values between 10 13 and 10 14 e/cm 2 located at the two surfaces of a sample (about 1-10% of a monolayer) [1,2]. A direct consequence of this large macroscopic polarization and the corresponding fixed surface charges is the appearence of large internal electric fields,

The wetting behaviour of GaN surfaces with N-face and Ga-face polarity and the influence of different surface treatments is studied by measuring the wetting angle of highly purified water by microscopic imaging. We found that wet thermal oxidation of the surface leads to a decreased wetting angle indicating an improved wetting behaviour. The presence of Al in AlN or AlGaN leads to a further reduction of the wetting angle, which is attributed to the presence of Al 2 O 3 on the surface. In addition the comparison of Ga-and N-face material revealed a lower wetting angle for all N-face samples. XPS analysis showed the enhanced formation of native oxide on the surface with N-face polarity.