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. The jet has been used with NH_3 to grow GaN on (0001) sapphire and (001) MgO. Growth rates exceeding 0.3 um/hr at substrate temperatures of 780 degree C and background pressures of less than 10^(-8) torr have been obtained. At comparable incident flux densities the background pressures are between 4 and 10 times lower than those achieved using a dosing tube pointed directly at the sample. Incident NH_3 fluxes corresponding to beam equivalent pressures of 5x10^(-5) torr are typical. At high nozzle pressures the flux begins to roll off because of scattering with background gases between the nozzle and skimmer. NH_3 condensation on the skimmer and nozzle is inhibited using electrical heaters. However, dissociation of NH_3 on the exposed nozzle filaments limits the maximum nozzle temperature to 400 degree C. The dissociated NH_3 leaves the jet as N_2 and H_2 and adversely affects the jet performance.

This work was partially supported by the ONR (N00014-93-1-0241) and AFOSR (F49620-93-1-0080).