For high vapor pressure or sublimating materials such as As, P, Te, Sb, Se, S and others, SVT Associates has engineered a series of Valved Sources to provide highly controlled deposition fluxes from large capacity reservoirs. Traditional evaporation sources rely on thermal stability and control to determine the deposition rate. With high vapor pressure or sublimating materials, small fluctuations in temperature or surface area can create a large fluctuation in deposition rate. The temperature fluctuations can be seen during ramping sequences or during shutter transients (due to shutter actuation). The SVTA Valved Evaporation Sources have the advantage of integrating a variable mechanical valve to limit the conductance from the evaporation (sublimation) reservoir to the growth chamber. A constant temperature may be maintained and the valve position is used to adjust the flux allowing for quickly responding, stable and reproducible deposition rate.
In the Valved Cracker Source, a high temperature region is implemented around the flux exit poriton of the source to thermally dissociate larger molecules to smaller, more reactive species. This is particularly useful in III-V MBE Systems or II-VI MBE Systems, where molecules (such as the As4 tetramer) are cracked into smaller species (such as the As2 dimer). Epitaxially grown layers often prefer these smaller species, increasing material incorporation efficiency and promoting proper stoichiometry of the crystalline films.
