Double Zone Cells: 

Please contact us to find the best suitable model for your needs: webrequest@episerve.de

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 it’s 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.

* Reduction in morphologie defects
* Improved flux stability when shutter is opened (low flux burst)
* Best option for evaporating Ga & In
* Decrease nitride creation in GaN growth
* Operating temperatures between 700 & 1200°C

 

Double zone cells - 200 ccm - with water cooling

Uniformity

Thickness uniformity of a GaAsAl 1,1µm layer deposited with a 700 cc Double Zone cell (Ga) and a 200 ccHot Zone / Cold Zone Aluminium cell onto a 3x4" multiwafer platen.

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.

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).

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 Nitrides 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.

Flux burst

Shutter-induced flux bursts observed with a Double Zone cell and a Single Zone cell of same capacity (40 cc)

Specifications
Filament: Two independently filaments
Maximum 0perating temperature: 1 400 °C
Maximum outgassing temperature: 1 600 °C
Temperature stability: better than +/- 0.1 °C
Thermocouples: Two double C type (W Re 5/26%)
Minimum mounting flange: for crucible capacity 25 35 40 58 ccm 35CF / 70mm / 2.75", for crucible capacity 60 70 85 125 150 ccm 63CF / 114 mm / 4.5", higher capacities on request
In-vacuum diameter: Crucible O.D plus 0.2 mm
In-vacuum length: 200 to 400 mm. (other on request)
Temperatures / power consumption: See curves
Power supply connectors: System or ADDON via universal UNIPLUG

Options Other types of thermocouples, integrated cooling circuit, integrated shutter, inserts