Papers by Author: Valeria Puglisi

Abstract: In this paper, the evolution of the electrical behaviour of GaN and AlGaN materials after high-temperature annealing and thermal oxidation is discussed. In particular, annealing above 1100°C, required for electrical activation of implanted species, increases the surface state density, reducing the metal/GaN Schottky barriers and increasing the leakage current. On the other hand, the thermal oxidation at 900°C of AlGaN/GaN heterostructures showed the formation of a thin oxide layer, which can be able to passivate surface defects and/or can serve as inter-device isolation. However, a decrease of the sheet carrier density in the two dimensional electron gas (2DEG) was observed when the material is subjected to such high thermal budgets. The results are discussed considering the possible optimizations and applications to GaN-devices technology.

Abstract: Gallium nitride (GaN) and related alloys (AlxGa1-xN) are promising semiconductors for high-frequency and high-power devices applications. In particular, the growth of AlGaN/GaN heterostructures on SiC substrates, due to the high thermal conductivity of SiC, can lead to significant improvement in the device performances. An important issue in the fabrication of AlGaN/GaN devices is the influence of the required thermal budget (700-900°C) on the properties of Ohmic contacts and device isolation. In this work, the influence of thermal annealing on the fabrication of AlGaN/GaN devices was studied. Ti/Al/Ni/Au multilayers led to an Ohmic behavior with a specific resistance c in the 10-5 cm2 range upon annealing between 750°C and 850°C. The electrical behavior of Ohmic contacts was correlated with the evolution of the formed phases and with the temperature behavior of c. The stability of the inter-device isolation obtained by nitrogen-implantation during annealing at these temperatures was also monitored.

Abstract: The recent improvement of GaN material quality launched new perspective for its application in power devices. However, ion-implanted guard-ring edge terminations, necessary to improve the breakdown voltage, are not well developed as in SiC technology. Indeed, the effects of high-temperature annealing, required for the electrical activation of the implanted species in GaN, on the electrical behaviour of Schottky contact was not reported. In this work, the influence of high temperature annealing (1150-1200°C) on the surface morphology of GaN and on the electrical behaviour of Schottky contact was studied. Although the morphology of GaN surface did not substantially change after annealing, a worsening of the electrical behaviour of Schottky contact was observed. This latter was ascribed to the formation of a high density of interface states after annealing.

Abstract: In this work, the electrical properties of Pt/GaN Schottky contacts were studied. The temperature dependence of the barrier height and ideality factor, and the low experimental value of the Richardson’s constant, were discussed considering the formation of an inhomogenous Schottky barrier. Local current-voltage measurements on Pt/GaN contact, performed with a conductive atomic force microscope, demonstrated a Gaussian distribution of the local barrier height values and allowed to monitor the degree of inhomogeneity of the barrier. The presence of defects, terminating on the bare GaN surface, was correlated with the electrical behavior of the inhomogeneous barrier.