Design and Characterization of a UHV Arcjet Nitrogen Source
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ABSTRACT A UHV-compatible nitrogen arcjet suitable for the growth of III-nitrides by molecular beam epitaxy is described and characterized. The arcjet operates at powers between lOW and 300W (the highest power used for these studies); typical nitrogen flows range between 5sccm and 100sccm. Optical emission spectra show the presence of activated atomic (N*) and molecular (N2*) nitrogen. A collisional radiative equilibrium model has been employed to provide insight into the excitation state of the active nitrogen. These results indicate that the arcjet is capable of supplying atomic nitrogen fluxes consistent with growth rates on the order of several monolayers per second. Langmuir probe measurements conducted near the position of the sample holder in the MBE chamber show the charged particle flux density is very low. The arcjet operates over a large powerpressure parameter space, and properties of the arc can be systematically "tuned" to provide a source suitable for selected-energy-epitaxy. INTRODUCTION The fundamental problem to be overcome in III-N epitaxy is the requirement for a nitrogen source that supplies an activated form of nitrogen. Due to the metastable nature of the growth process of the nitrides a source of activated nitrogen must be employed. 1. RF plasma excitation, electron cyclotron resonance, and microwave plasma sources have been used to generate atomic nitrogen from N 2 2 3,4. Though these classes of activated nitrogen sources have been employed to grow a wide variety of III-N structures and devices, they have a number of limitations. These include, limited growth rates, and significant ion content in the activated nitrogen beam 5. A second class of nitrogen sources that have been employed for the growth of III-N in a molecular beam epitaxy environment are supersonic jets6 . Investigators working with supersonic jets have often heated the jet nozzle in an effort to excite or crack the molecules forming the beam. This approach is fundamentally limited by the temperature obtained by resistive heating of the nozzle and by the nature of the flow within the nozzle which limits the thermal exchange between the gas and the constrictor wall 7 . The arcjet is a further development of the resistive heating approach in that an electron column or arc is generated down the centerline of the nozzle, and is therefore in intimate contact with the high pressure gas flow within the nozzle constrictor. The electron arc is essentially guided by the laminar gas flow. At the exit of the nozzle, the pressure changes dramatically, and the electron arc expands out, in a mushroom-like shape, and anchors on the exit wall forming an arc foot. The gas flow is radiatively and conductively heated through the constrictor zone as well as during the passage through the arc foot. In our implementation, we have selected materials,
325 Mat. Res. Soc. Symp. Proc. Vol. 482 01998 Materials Research Society
constrictor and nozzle geometries, pressure and power levels that minimize ion yield and maximize the atomic nitrogen flux.
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