Finite element simulation of CdZnTe Bridgman growth

A.M. MARTÍNEZ; M. R. ROSENBERGER; A.B. TRIGUBÓ; C.E. SCHVEZOV; N.E. WALSÖE DE RECA

San Luis, Pcia de San Luis - Argentina

Congreso; III Reunión de la Asociación Argentina de Cristalografía (Miembro de la Internacional Union of Crystallography); 2007

Asociación Argentina de Cristalografía

The main goal of this work is to grow single crystalline Cd(1-y)ZnyTe (0≤y≤0,1) ingots by the Bridgman method with good structural and electrical quality to be used in different devices. Also as substrates for epitaxial film growth. These single crystals are used in the production of x and γ ray detectors and also as substrates in the epitaxial growth of capable materials for the detection of IR radiation: Hg (1-X) Cd (X) Te (MCT), Hg (1-X-Y) Cd (X) Zn (Y) Te (MCZT). The Bridgman method consists on the fractional solidification of the previously melted alloy when going through a temperature gradient. In this case the furnace is clamped at a certain height while the ampoule slowly descends at a constant rate by means of a synchronous motor.   The mechanical properties are deeply related to the material microstructure, for such a reason the simulation of the solidification is made by Numerical Methods (Finite Elements) to determine the parameters that control the process and in this way to get a better structural quality of the single crystalline semiconductor. These methods allow diminish the number of experimental tests and consequently the costs and time procedures. The ingot characterization results compared to the simulation predictions allows settle the growth conditions as much as the simulation parameters. The geometrical symmetry in the experimental growth set up allows an axial symmetry in a bidimensional model. This considerably reduces the number of nodes in the mesh and consequently the necessary ram memory for the calculation as much as the calculation time.  The process is modelled keeping in mind different stages to reduce even more the calculation time. The low growth rate allows quasi stationary states in all stages to be considered. In the first stage it is considered a vertical Bridgman furnace in which there is a double wall quartz ampoule in vacuum that descends with a speed of 3.32 mm/h. The temperature profile is obtained on the wall of the internal quartz ampoule and this information is used in the second stage. In the latter mentioned stage the CdZnTe material is in the internal quartz ampoule with a solid-liquid interface that can be observed in the simulation results by means of the phase change isotherm (1365 K). This model is called single block since initially there is only a liquid phase. With the course of time the solid phase nucleates and it is in contact with the liquid phase. Finally there is totally a solid phase in the quartz ampoule. The simulation is carried out following the movement of the interface until reaching the complete solidification of the semiconductor. The form and position of the solid-liquid interface is analyzed and compared in the cases which convection is or is not considered.