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The above curves show the dissociation efficiency of molecular hydrogen cracked on an heated tungsten surface.
The efficiency is strongly dependent of the H2 pressure, that is the flow of molecules for a given source conductance, the flows used here will give system vacuum pressures of 10-6 to 10-5 mbar with the usual pumping speed of standard MBE systems ( pumping speed of several 100 l/min). The best working conditions should take the cracking efficiency / flow rate dependance in consideration. The following table shows some atomic H flow data at usual working conditions. |
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| This source generates a flux of atomic hydrogen through high temperature thermal cracking of molecular hydrogen (or deuterium) with a tungsten filament. A flux of atomic hydrogen has been shown to convert carbon and oxygen based contaminants on substrate surfaces into volatile species which evaporate readily at relatively low temperatures. Hence an atomic hydrogen source is very useful for in-situ low temperature cleaning of substrates. It has also been suggested that growth rate in Ga-rich deposition of GaN is considerably enhanced under a flux of atomic hydrogen. An atomic hydrogen source represents an ideal solution for in-situ low temperature surface preparation, substrate cleaning and selective MBE. The minimum vacuum flange required is a DN35CF (2" / 70mm). All models are equipped with a water cooling system to regulate the flange temperature . OPTIONS: On request, the source can be equipped with a thermocouple which allows regulation of the temperature in the cracking zone. The thermocouple does not give a readout of the absolute temperature of the filament because it is too high. It can, however, be used for calibration. Also as an option the source can be furnished with a second gas entry for use with gases which do not need to be cracked. In this case the W filament can be supplied with 4 Amps so as to avoid condensation in the supply tubes for this gas. If this source is used in a system without cryopanels (LN2 or water) it is a good idea to install a water cooled panel around the cracking zone. The minimum flange size for this option is DN60CF.
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