Electronic characteristics of Au/Al_xGa_1-xN structures grown with various x values

We have studied the growth characteristics of Al_xGa_1-xN/GaN and electronic properties of Au/Al_xGa_1-xN structures grown by the MOCVD variation of x. The Al_xGa_1-xN layers were grown on undoped GaN/sapphire (0 0 0 1) epitaxial layers in a horizontal MOCVD reactor at a reduced pressure of 300 Torr. Al composition, x in Al_xGa_1-xN/GaN determined by DCXRD are 0.12, 0.15, 0.19 and 0.2, respectively. Nevertheless, enough feeding of TMA (trimethylaluminum) source during the growth at 300 Torr and 1070 °C, the mole fraction x in Al_xGa_1-xN did not increase any more above 0.2. The FWHMs of the DCXRD for (0 0 0 2) diffraction from Al_0.12Ga_0.88N, Al_0.15Ga_0.85N, Al_0.19Ga_0.81N and Al_0.2Ga_0.8N are 450, 469, 502 and 474 arcsec, respectively. While the carrier concentration of Al_xGa_1-xN layers increases, the mobility decreases with the increase of Al mole fraction x. After fabricating Au/Al_xGa_1-xN Schottky diodes, electronic properties of the structures were evaluated by I-V measurement as a function of x. The breakdown voltages of Au/Al_xGa_1-xN diodes at reverse bias mode were in the range of 2.5-15 V. And, the turn on voltages at forward bias mode were in the range of 0.5-5.0 V with the variation of x from 0.12 to 0.19. While Au/Al_xGa_1-xN diodes having low mole fraction of Al showed relatively good rectifying behaviors, the samples having high x exhibited very leaky I-V behaviors. The current density-voltage (J-V) characteristics at forward bias mode were also evaluated as a function of carrier concentrations which are directly related to x. From each curve of ln (J) vs. V, I yielded the effective barrier heights (Φ_B) ranging from 0.6 to 0.78 V, and the ideal factor (η) is in the range of 2.7-3.5. The barrier heights obtained in this work are close to each other, but the ideal factors are slightly different.