Arsenic Valved Cracker Cell

The DCA VCP series valved crackers have a large heated tank (crucible) for the arsenic charge, a high temperature cracker stage and a conductance control valve between the two. The tank is heated to produce As4 which is then directed to the substrate either as As₄ or As₂ depending on the cracker stage temperature. The control valve regulates the amount of arsenic entering the growth chamber. Both the cracker stage and the arsenic tank temperature are regulated with PID temperature controllers.

The control valve allows precise flux control over wide flux range. The data shows beam equivalent pressure (BEP) measured with an ion gauge beam flux monitor using different control valve positions. This particular example is for 320 ^oC tank temperature. The control valve allows linear flux control from 3x10^-8 Torr to 4x10^-6 Torr range for this tank temperature setting. Higher maximum flux control ranges can be set using different tank temperatures. A tank temperature of 380 ^oC gives linear flux control range up to 3x10^-5 Torr BEP.

The DCA arsenic cracker has been designed to overcome the common problems with conventional valved crackers - problems with tank opening after prolonged use and valve clogging causing flux instabilities. The arsenic tank has a vacuum tight flange with a sealing mechanism which prevents the tank and the tank lid from sticking together. The arsenic tank heating profile has been carefully optimized to guarantee that the control valve is always at a higher temperature than the tank. This prevents arsenic condensation to the valve mechanism. The valve itself is a module which can be easily changed for different flux control ranges depending on the application.

The DCA VCP series arsenic valved cracker cells are available for MBE systems from most MBE manufacturers. The high temperature cracker stage can be provided with a water cooling shroud depending on the cryopanel design of the MBE growth chamber. The valved crackers are available with tank volumes of 250 cc, 500 cc, 1000 cc, 2000 cc, 5000 cc and 9000 cc. The 250 cc and 500 cc models are ideal for the R450 MBE system while the 1000 cc and 2000 cc models are recommended for the P600 MBE system.

The same cracker can be used with slight modification for selenium, sulphur and tellurium evaporation.

A motorized valve controller is available as an option.