Analysis of the Carrier Transport Mechanism in AlxGa1-xN/GaN Schottky Barrier Diodes

Abstract

The carrier transport mechanism of AlxGa1-¬xN/GaN Schottky barrier diodes (SBDs) is analysed.In the first part, the dominant carrier transport mechanism of AlxGa1-xN/GaN SBDs with increasing Al mole fraction x (x = 0.2, 0.3, 0.4, and 0.5) is investigated. The Schottky barrier height (SBH) linearly increases with the work function of Schottky metal with the slope factor of ~0.5 irrespective of x. The SBH, the ideality factor, and the reverse leakage current, however, show an increasing deviation from the predicted values given by the thermionic-emission theory as x increases. The XPS analysis reveals enhanced predominant incorporation of oxygen donors and an increase in the energy band bending at the surface of AlGaN with increasing x, indicating that the carrier transport by tunnelling through the thin, heavily doped Schottky barrier becomes dominant.In the second part, the dominant carrier transport mechanism of CF4 plasma-treated AlGaN/GaN SBDs at various RF powers is investigated. After plasma treatment the reverse leakage current is reduced possibly due to the incorporated fluorine atoms into AlGaN layer, as confirmed by synchrotron radiation photo-emission spectroscopy. The reverse leakage current in the sample without plasma treatment is not sensitive to temperature, indicating tunneling current is dominant. On the other hand, the reverse leakage current in the plasma-treated sampls at RF power of 100 W increases exponentially with temperature, suggesting a thermally activated transport mechanism is involved. The SBHs and ideality factors estimated from temperature-dependent forward current characteristics reveal that thermionic emission is not the dominant mechanism. The activation energy estimated from the Arrhenius plot is approximately 0.6 eV, suggesting that the emission of carriers from fluorine-related deep-level stats into the conducting dislocation states is the most likely dominant carrier transport mechanism.