University of Victoria, 2008

Techniques used

Zone refining

zone refiner picture and schematic drawingIn zone melting processes, molten zones are passed through the length of an ingot. When zone melting is applied to a mixture of several constituents, the composition of the solid created at a freezing interface will be different than the composition of the liquid melt. In a typical zone refining process, several zones are passed through the ingot. During each pass, contaminants are rejected by the freezing interface, accumulate in the molten zone, and are carried toward the end of the ingot. The result is an ingot with one especially pure end and one heavily contaminated end.

The crystal growth laboratory operates a 3-zone zone refining apparatus that is designed to simulate the system used commercially by 5N Plus, Inc. The laboratory is working with 5N Plus to improve their zone refining processes for tellurium and cadmium metal through numberical modelling and experimentation.

Experiments last year focussed on the removal of selenium from tellurium ingots. Currently, experiments are aiming to improve the stability of molten zones in cadmium by varying the temperature gradient in the ingot.

Travelling Heater Method (THM)

The travelling heater method is a solution growth technique for growing and synthesizing binary and ternary compound semiconductors. In THM synthesis of cadmium telluride, a saturated solution of cadium in tellurium is held molten by a narrow heater. As the heater moves upward, a 50% molar feed source is dissolved at the melting interface and 50% molar compound is deposited at the freezing interface.

With this process, a constant, controlled, but slow growth is achieved. The quality of grown crystals in THM is very sensitive to the relative movement of the temperature profile that determines the growth rate. THM has a number of advantages over melt techniques, namely less thermal stresses, growth of ternary alloys, and uniform crystal composition.

Currently, polycrystaline cadmium telluride is being synthesized and grown from a tellurium melt in 1-inch ampoules. The effect of magnetic fields on mass transport within the solution zone is being studied experimentally by testing the stability of the growth interface at a range of heater speeds.

schematic drawing of thm

Liquid Phase Electroepitaxy (LPEE)

Growth is achieved by passing an electric current though the crucible. The growth mechanisms of LPEE are:

Electromigration: sustains the growth process, and the growth rate is proportional to the applied electric current density and also the applied magnetic field intensity.

Peltier Cooling (PC)/Heating (PH):PC initiates the growth process, and PH dissolves the needed material into the solution.

schematic drawing of lpee
  • In LPEE, crystalline layers from a dilute metallic solution are deposited onto a substrate of similar composition. When the solution (for instance a Te-rich Cd-Zn-Te solution) is brought in contact with the solid (single crystal CdZnTe) substrate, growth is initiated and sustained at a constant furnace temperature by passing an electric current through the growth cell. Two mechanisms are responsible for growth: i) electromigration of Cd, Te, Zn species towards the substrate, caused by the applied electric current, and ii) Peltier cooling at the growth interface, causing crystallization on the substrate by supercooling the solution in the vicinity of the substrate, and Peltier heating at the dissolution interface, supplies the required material to the solution by dissolving the source material constantly. In addition, the effects such as the natural convection due to thermal and solutal gradients in the solution, the Joule heating in the source and substrate due to the passage of the electric current, the applied magnetic field, and possible finite mass transport rates at the interface also play a significant role in the process.
  • LPEE has a number of advantages over other techniques. For instance: The growth process and the composition uniformity can be controlled better (LPEE provides the best compositional uniformity compared with the other bulk growth techniques),
  • A ternary growth can begin with a binary substrate (whenever the lattice mismatch is at an acceptable level),
  • The source can constantly supply the required materials to the solution to grow very thick crystals, and
  • Due to low temperature gradients in the solution (at most within a few ?C) the grown crystals have less thermal stresses.