Paper EM-WeM13
Improvement of AlGaN/GaN HEMT and GaN Schottky Contact Device Performance by Reduction of Epitaxial Film Dislocation Density

Wednesday, October 17, 2007, 12:00 pm, Room 612

The electrical characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) and GaN Schottky contacts were correlated with dislocations and other material defects. AlGaN/GaN heterostructures and GaN epitaxial films were grown using conventional MOCVD and pendeo-epitaxy (PE), a lateral growth technique that reduces the dislocation density of the epitaxial films by 2-3 orders of magnitude. Current-voltage (I-V) and capacitance-voltage (C-V) measurements were conducted to determine the quality of the Ni gate (Schottky) contacts to both the conventional and PE films. The Schottky contacts to the PE material all displayed a single, homogeneous Schottky barrier height evidenced by the linearity of the log I-vs-V plot over 4-5 orders of magnitude. Conversely, the Schottky contacts to the conventional material displayed an inhomogeneous Schottky barrier height, with a characteristic "knee" at low voltage indicating the presence of a low Schottky barrier height. The average ideality factor increased from 1.71 for the PE material to 2.29 for the conventionally grown GaN. The average reverse leakage current increased from 7.5x10^-4 A for the PE GaN to = 4.0x10^-3 A for the conventionally grown GaN. The electrical properties were then correlated with improved material quality as determined by several microscopy techniques. The conventional GaN epitaxial films were found to have an RMS surface roughness twice as large as that of the PE film. Similarly, cathodoluminescence revealed that the near band edge intensity of the PE films was almost an order of magnitude higher than the conventionally grown material, indicating the presence of fewer defects in the PE material. Devices fabricated on the AlGaN/GaN heterostructure also displayed variations in electrical properties. Variations in the ideality factor, Schottky barrier height, and reverse leakage current density were 1.60-2.60, Φ_B=0.60-0.95 eV, and J=1x10^-4-1x10¹ A/cm², respectively. These variations correlated with a variation in local etch pit density directly under the gate contact as determined by SEM. For devices with high leakage-current density, the etch-pit density was found to be twice as high as that of devices with low leakage current density. Determining the relationship between the electrical characteristics and materials defects will facilitate the fabrication of high-power and high-frequency devices with improved performance and reliability.