In the frame of this research area we study the electric properties of gallium nitride (GaN) plain layers and GaN based structures. Especially, we deal with high electron mobility transistor (HEMT) structures. In cooperation with the LABONIT technology group of our department we perform characterization of the grown layers by resistivity and Hall effect measurements, usually in van der Pauw configuration. Obtained results are used as a feedback for technology group; at the same time the results allow us to assess contributions of various parts of the structure (active layers, buffers, interfaces, surfaces) to the transport. The systems we are dealing with in detail are the GaN-based HEMT structures. We develop and optimize various methods of the characterization for different structure types (e.g. so-called normally-on and normally-off structures or HEMTs prepared on different).
Experimental techniques used are not only the above van der Pauw DC measurements but also the AC (capacitance) measurements, especially those employing mercury probe, which enables us quick characterization and comparison of HEMT wafers and their parts. This way, we can establish, in some structures, quantitative parameters of conductive channels and conditions for turning the channels on or off. Magnetotransport measurement of conductive channels are performed at low (~1 K) temperatures, as well. This way we can prove or disprove the 2D nature of the electron gas involved, and determine its parameters. In this field, we concentrate on further development of the magnetocapacitance methods already applied, in our group, for studying 2D systems in AlGaAs structures where we demonstrated topological phase transition induced by magnetic field.