Standard effusion cells comprise a single radiative filament wrapped uniformly around the crucible. Consequently, the ends are not thermally symmetrical (especially when the shutter is open) and the temperature is inhomogeneous within the crucible along its central axis. This thermal gradient, which can reach 150° or more between the bottom and top is not a major problem when high vapor pressure materials such as Sn or sublimating materials such as As are evaporated.
For materials which require higher evaporation temperatures, however (i.e. Ga & In), such a temperature gradient introduces a risk of condensation at the top of the crucible. A "spitting" phenomenon of the melted materials can then occur, triggering the formation of morphological defects in the deposited layer.
Double hot zone cells re-establish uniformity of temperature all along the crucible, even when the shutter is open, by preferentially heating the filament located close to the top of the cell.
Experimental results from tens of different laboratories illustrate the efficiency of such double hot zone cells, especially for Gallium and Indium. According to associated publications, the use of double hot zone cells acts to reduce the oval defect density (ODD) by a factor of between 3 and 20 compared with conventional cells.
Recommended materials: The Double Zone cell is suitable for low vapor pressure materials (700 to 1200°C for main elements of a classical compound for a growth speed around 1µm/h). The "Cold Lip" model is more suitable for Aluminum.
ADDON DOUBLE ZONE CELL CHARACTERISTICS
Two heating zones with independent temperature control and power supply.
Compatibility with all common power supplies.
Wide range of crucible capacities.
Two double thermocouples.
Usable without modification on most MBE systems.
Optional integrated / cooling shutter.
Compatibility: This cell is available in a range of models fully compatible with existing systems manufactured by Varian, VG, EPI, Perkin-Elmer, Ulvac, Anelva and Riber. Others versions are also available on request (please provide us with details of the flange size, connecting tube and in-vacuum length).
Thermocouples: A double thermocouple is installed in each zone. C Type (W/Re 5/26%) is standard. Other types are also available.
Power systems and temperature controllers: Two independent power units are necessary for double zone cells. Most existing power units are compatible. ADDON models are recommended if new power units are needed (see specific documentation).
ADDON CELL CONCEPT AND STABILITY OF TEMPERATURE FOR ANY SHUTTER POSITION:
The top of the ADDON double zone cell is particularly well shielded, to decrease the thermal loss at the lip of crucibles transparent to I.R. radiation. The temperature of the cell thus remains steady, independent of whether the shutter is the open or closed. Figure B illustrates the behavior of the pressure reading on the flux gauge (Beam Equivalent Pressure) with, or without, control of the upper heater.
REDUCTION OF NITRIDE CREATION IN GROWTH OF GaN:
The use of a flux of either Nitrogen or a compound containing Nitrogen during growth of GaN can trigger a process of nitride build up at the mouth of the gallium crucible. It is possible, however, to get rid of these nitrides if the temperature at the top of the crucible is increased to about 1000/1100°C.
ADDON can supply double zone cells adapted for this special requirement.
An integrated shutter mechanism can be fitted on most of the cells described here.
Integrated cooling circuit:
If necessary a water cooling circuit can be integrated on most if the cells described here.
VG compatible cells include the cooling circuit as a standard feature.
||Two, independently controllable.
|Maximum Operating Temperature:
||1 400 °C
|Maximum Outgassing Temperature:
||1 600 °C
||better than +/- 0.1 °C
||Two Double C type (W Re 5/26%)
|MINIMUM MOUNTING FLANGE
25 35 40 58 35CF / 70mm / 2.75"
60 70 85 125 150 63CF / 114 mm / 4.5"
||Crucible O.D plus 0.2 mm
||200 to 400 mm. Please contact us for other values.
|Temperatures / Power Consumption:
|Power Supply Connectors:
||System or ADDON
||-Other types of thermocouples
-Integrated cooling circuit