The Dual Filament Source uses two independent heaters to control the temperature variation along the length of the crucible. The bottom filament is required for thorough source and crucible outgassing, and to achieve higher growth rates.
The Dual Filament Source is used in “cold-lip” mode with aluminum only. The lip region is deliberately kept cooler than the rest of the crucible to prevent aluminum creep. Aluminum flux uniformity may be optimized by adjusting the crucible temperature gradient.
The Dual Filament Source for MBE is available in a range of crucible sizes, including direct replacements for most commercial MBE systems. A standard Dual Filament Source with conical crucible may be preferable in cases where a smaller charge capacity is acceptable and fast ramping of source temperature is critical.
Data from dozens of facilities have shown that the Dual Filament Source significantly lowers oval defect densities. Defect densities are reduced by a factor of 5-10 when a “cold-lip” Single Filament Source is replaced with a “hot-lip” Dual Filament Source.
When used for gallium and indium evaporation, hot-lip sources reduce the formation of oval defects at the growth surface, leading to improved device performance. The most widely accepted theory about these source-related morphological defects is that they are caused by “spitting” from the melt, which occurs when droplets of condensation form at the crucible orifice and then roll back into the hot melt.1
The figure provided illustrates how hot-lip heating prevents condensation and thereby eliminates spitting. Impurities in the melt, such as gallium oxide, may also cause oval defects. In this case, eliminating condensation is also beneficial since the droplets may otherwise gather impurities from the surrounding environment. When used for aluminum, the Dual Filament Source allows sufficient heating at the orifice to provide good flux uniformity while providing a “cold lip” to prevent aluminum creep that can damage the source heat shielding and heater filaments. The Dual Filament Source, operated in “cold-lip” mode, provides enhanced aluminum source reliability, together with great performance.
1. D.G. Schlom, et al, J.Vac. Sci. Technol. B 7, 296 (1989).