6.1 Jet Theory and Design Equations - Introduction

In a conventional MBE system gas effuses from a source cavity or "oven" through a hole into a chamber maintained at high vacuum by continuous pumping. The gas in the oven is at a density sufficiently low so that the molecules undergo no collisions in passing through the hole. The particles will continue on their straight path until they strike a surface. Therefore emerging particles can be easily defined to a beam by a collimator. Such sources are limited to applications where only low intensity beams are needed as well as particles with low translational velocity. Source ovens have a maximum operating temperature of about 3000°K which corresponds to an upper limit in mean kinetic energy of the beam of 0.5 eV per molecule.

For kinetic energies above 3 eV, methods of beam generation include charge-exchange neutralization, laser vaporization of solids, laser-sustained plasmas, and negative ion photodetachment. For kinetic energies below 3 eV sources have been constructed using furnace tubes, arc. discharge, and electrodeless rf or microwave discharges (Ref. 18). Furnace tubes usually are operated at pressures less than 1 torr, giving a rather low forward intensity and a broad velocity distribution.

Beams formed by charge-exchange neutralization of electrostatically accelerated ions have essentially no upper limit to their energy but because of space-charge effects cannot provide the intensity necessary for many experiments at energies below 10 eV. Also, ion damage is a main concern in growing high quality single crystalline material. Consequently it is desirable to have a high intensity source not using ions operating between 0.5 eV and 10 eV. In just this range occur the gas phase collision processes important in many chemical reactions and in high temperature transport phenomena (Ref. 20).

Supersonic nozzle sources (jets) characteristically produce beams of greater intensity and smaller translational velocity dispersion than effusive sources (Ref. 19). The translational energy of nozzle beams can also be varied over a very wide range by heating the gas used in the gas reservoir before the expansion occurs. Due to the fact that no ions are involved the issue of ion damage is avoided. Supersonic beams therefore offer several clear advantages over simple effusive sources for controlled MBE experiments and make kinetic energies between 0.5 and 10 eV accessible in combination with the seeding technique (Ref. 20).

The purpose of this chapter is to introduce the reader to the operating principles of supersonic jet sources in a (R)MBE environment as well as to summarize important design and characterization equations used in subsequent chapters, whose origin are covered in the appendix.

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