Evaluation of Gallium Nitride for Active Microwave Devices.
Final rept. 1 Jan 75-30 Sep 80,
UNIVERSITY OF SOUTHERN CALIFORNIA LOS ANGELES ELECTRONIC SCIENCES LAB
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The predicted figure of merit for GaN as a transit-time limited microwave power amplifier material is significantly greater than that of Si or GaAs because of the small electron mass, the large optical phonon energies and the large bandgap of GaN. To confirm these predictions by measuring the saturated drift velocity and the pair-production thresholds, it was necessary to prepare uncompensated n-type single crystals with carrier densities below about 10 to the 17th powercc. Because the melting point of GaN is extremely high, 2,000C, with a corresponding equilibrium N2 pressure of 40,000 atm, it was necessary to use a chemical vapor deposition method wherein GaCl was allowed to react with NH3, a more active source of N than N2 and kinetically stable with respect to N2 at temperatures below 1100C to grow single crystal GaN epitaxially on sapphire substrates. However, their surface morphology was non-planar, and they displayed Ga occlusions, microcracks and voids, the latter two appearing only in the thicker layers. Very slow growth eliminated all these problems except the non-planarity. Epitaxial growth of planar, crystallographically sound crystals was finally achieved on basal plane substrates. All the crystals grown were n-type with carrier densities of 10 to the 18th to the 20th powercc. These observed densities were more or less independent of all crystal growth parameters, including replacing the Ga halide reactant source with a Ga organometal source.
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