Free expansion jet sources can produce energetic beams of gas molecules at high fluxes. Two jets are presented as nitrogen sources for the growth of GaN and compared to the use of NH_3 supplied from a conventional leak.

A small jet for use with NH_3 was mounted into a Varian Gen II source port. The jet reduced the gas load on the growth chamber compared to a leak valve and was used to investigate the adsorption of NH_3 during GaN growth. This unseeded jet incorporates differential pumping via a liquid nitrogen cryopump coil and inhibits condensation on the skimmer and nozzle using resistive heaters. Desorption mass spectroscopy (DMS) and film thickness measurements were used to determine the growth rate vs the incident translational energy. For the DMS measurements, the H_2 evolved during growth was taken to be the growth rate. Under ammonia limited growth conditions we find that the GaN growth rate decreases as the kinetic energy of the incident ammonia increases. The activation energy of the reduction in growth rate with increasing kinetic energy of the ammonia is 0.09 +- 0.02 eV, which suggests a reduction in the sticking coefficient of physisorbed NH_3.

A high temperature jet was used to investigate the dissociative chemisorption of nitrogen on GaN. A Ga terminated GaN surface was exposed to an energetic nitrogen beam over a range of nozzle temperatures. The surface was then reacted with Ga and the number of Ga atoms that reacted determined from DMS. Exposure of the GaN surface to 2% molecular nitrogen seeded in hydrogen at a nozzle temperature of 2600 K, corresponding to a nitrogen energy of about 6.7 eV, resulted in significant nitradation of the Ga terminated GaN, suggesting that it is possible to dissociatively chemisorb nitrogen on a GaN surface. These results are supported by Auger analysis performed on sapphire which was nitrided using the above conditions.

The DMS spectra of GaN surfaces that had been nitrided with the supersonic jet are different than those that have been nitrided only with NH_3. Both spectra indicate that a surface coverage of Ga must be built up before nitradation occurs. The coverage required using the nitrogen jet is significantly larger.

* Partially Supported by the Office of Naval Research