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INTRODUCTIONElectronic devices based on the III-nitride group of materials (GaN, AlN, and InN) have recently shown great promise for high-frequency/high-power applications. For the most part, these devices are based on low Al-composition, AlGaN/GaN heterostructure field-effect transistors (HFETs). Metal-organic vapor-phase epitaxy (MOCVD) based AlGaN/ GaN FETs grown on sapphire substrates have shown room-temperature two-dimensional electron gas (2DEG) mobilities in excess of 1600 cm 2 /Vs, 1 while mobilities over 2000 cm 2 /Vs have been obtained using 6H-SiC substrates. 2 In addition, such devices have achieved power performance up to 9.1 W/mm at a frequency of 8.2 GHz. 3 However, these devices all use Schottky contacts as the gate barrier, which have not yet proven to be stable at high temperatures. In contrast, metal-insulator-semiconductor (MIS)-based FETs can use hightemperature stable insulators at the interface. The use of a gate insulator also allows the gate to be placed much closer to the channel, possibly leading to higher intrinsic transconductance. AlN, with its relatively high dielectric constant (8.5) and wide bandgap (6.2 eV), has the potential to be an excellent choice for the gate dielectric in 5 The authors presented a first report on AlN/GaN MISFETs with promising electrical characteristics and very low interface-state density, proving the possibility of a high-quality AlN/GaN interface. 6,7 However, from the growth standpoint, it is difficult to grow high-quality AlGaN layers with high Al composition. 8 The problems of lattice-mismatch (2.47% for AlN on GaN), prereactions between TMAl and NH 3 , and three-dimensional (3-D) growth must be overcome for effective high-quality AlN/GaNbased MISFET devices. Smorchkova et al. 9 have recently shown results for molecular-beam-epitaxygrown AlN/GaN structures with thin AlN barriers. In this paper, we present a systematic study of the impact of MOVPE growth parameters on the AlN/GaN material and the heterostructure formed between them. In addition, we have recently presented results using cathodoluminescence microanalysis to prove a correlation between 2DEG formation, interface scattering, and recombination processes at the AlN/GaN interface. 10 EXPERIMENTALThe GaN and AlN layers were grown by low-pressure (60-110 torr) MOVPE on c-plane (0001) sapphire substrates. The reactor was a custom-built, horizontal quartz-tube reactor connected to a modi-2.-Currently with IQE plc Head Office, St. Mellons, Cardiff CF3 0EG, United Kingdom Low-pressure, metal-organic vapor-phase epitaxy (MOVPE) was used to grow AlN/GaN metal-insulator-semiconductor (MIS) heterostructures with AlN thickness between 3 nm and 30 nm. The Hall mobility was found to decrease with increasing AlN thickness, with optimal mobility measured at 5-nm AlN. By decreasing the ammonia flow during AlN growth (lower V/III ratio), surface and interface quality were greatly improved with a corresponding improvement in electrical properties. For the optimal V/III ratio, room-temperature (RT) mobility and...