AlGaN/GaN heterostructures for enhancement mode transistors

Today the continuous increase of electric power demand is in our society a global concern. Hence, the reduction of the energy consumption has become the main task of modern power electronics. In this context, wide band semiconductors (WBG), such as gallium nitride (GaN) and related alloys, have outstanding physical properties that can enable to overcome the limitations of Silicon, in terms of operating power, frequency and temperature of the devices. An interesting aspects related to GaN materials is the possibility to grow AlGaN/GaN heterostructures, in which a two dimensional electron gas (2DEG) is formed at the heterojunction. Basing on the presence of the 2DEG, AlGaN/GaN heterostructures are particularly interesting for the fabrication of high electron mobility transistors (HEMTs). One of the most challenging aspects on this field is the development of enhancement mode AlGan/GaN HEMT. This devices would offer a simplified circuitry, in combination with favourable operating conditions for device safety. Hence, this thesis is entitled AlGaN/GaN heterostructures for enhancement mode transistors . The aim of this work was to clarify the mechanisms ruling the electronic transport at some relevant interfaces in AlGaN/GaN devices, after surface modification processes used in normally-off technologies. The thesis is divided in 6 chapters. In the first two chapters, the properties of GaN and related AlGaN alloys are described, explaining the formation of the 2DEG and the working principle of HEMT devices. In chapter 3, a nanoscale characterization of modified AlGaN surfaces is presented in order to deplete the 2DEG. Two different approaches have been studied, i.e., the use of a fluorine plasma treatment and the use of a local oxidation process. Even though a depletion of the 2DEG is possible, several reliability concerns need to be investigated before a practical application to devices can be envisaged. Among the possible approaches for enhancement mode transistors using AlGaN/GaN heterostructures, the use of a p-GaN gate contact seems to be the most interesting one. Hence, chapter 4 reports a detailed investigation on the formation of Ohmic contact to p-GaN. The evolution of a Au/Ni bilayer, annealed at different temperatures and in two different atmospheres (Ar or N2/O2) was considered. The electrical measurements of the contacts annealed under different conditions demonstrated a reduction of the specific contact resistance in oxidizing atmosphere. Structural characterizations of the metal layer associated with nanoscale electrical measurements, allowed to give a possible scenario on the Ohmic contact formation mechanisms. Finally, the temperature dependence of the specific contact resistance allowed the extraction of the metal/p-GaN barrier. The fabrication and characterization of AlGaN/GaN transistors with the use of a p-GaN cap layer under the gate contact is presented in chapter 5. The electrical characterization of p-GaN/AlGaN/GaN transistors demonstrated a significant positive shift of the threshold voltage (Vth) with respect to devices without p-GaN gate. A normally-off behaviour of the devices (Vth= +1.4 V) was obtained upon a reduction of the barrier layer thickness and Al concentration. Finally, a preliminary study on the use of nickel oxide (NiO) as a dielectric below the Schottky gate contact in AlGaN/GaN heterostructures is reported in the last chapter. First, a structural and morphological investigation of the NiO layers grown by MOCVD showed continuous epitaxial film. The electrical measurements on devices allowed to extract a value of the dielectric constant for the grown NiO very close to the theoretical one, and a reduction of the leakage current in HEMT structures integrating such a dielectric. The experimental results of this thesis are summarized in the conclusive section, that also briefly describes the remaining open issues and the possible continuation of this research activity