Free expansion jet sources can produce directed, energetic beams of gas molecules at high fluxes. A jet which reduces the gas load on the MBE growth chamber and which can be mounted into a standard source port is presented. It incorporates differential pumping via a liquid nitrogen cryopump coil and skimmer. Its utility is compared to a simple dosing tube placed near and pointing directly at the sample. Since differential pumping, which keeps the total gas load on the chamber low, requires considerable space, it is relatively simpler to place the tube closer to the sample than the jet source. In our jet design the nozzle is heated to increase the beam energy and the skimmer is heated to inhibit condensation. The skimmer and differential pumping also isolate the hot surfaces from the growth chamber. It can be operated for up to eight hours without regeneration and supplies the ammonia with up to four times the translational velocity of a (non heated) leak. At an incident ammonia flux of 1E15 cm^(-2)s(-1) the background pressures are about 20 times lower than those achieved using the leak pointed directly at the sample. At higher flux densities (by increasing the nozzle pressure) the beam is attenuated by the scattering with background gases. Dissociation of ammonia on the exposed nozzle filaments limits the maximum nozzle temperature to about 400 degree C and the maximum ammonia velocity to 1.8 km/s. The dissociated ammonia leaves the jet as N2 and H2 and adversely affects the Jet performance.

Using the supersonic jet, GaN growth rates exceeding 0.3 um/h could be obtained at substrate temperatures of 880 degree C and at background pressures of 1E-8 torr. A number of films were grown under Ga rich conditions. Preliminary results suggest that the films grown at 800 degree C grew more slowly than those at 880 degree C with the sticking coefficient for nitrogen increasing significantly. Under the same conditions of flux and substrate temperature, use of the jet reduces the sticking coefficient of nitrogen. As the ammonia velocity was increased the sticking coefficient for nitrogen was further reduced. When using the jet, films grown with a high gallium to nitrogen ratio exhibited larger surface roughness.