Adjustment of heater power distribution during VGF growth of GaAs crystals
The Bridgman technique is widely applied to grow high quality III-V semiconductors such as GaAs, InP, etc. The vertical gradient freeze method (VGF) is a modification of the conventional Bridgman method without moving the ampoule or heaters. In this case, the crystallization process is provided by changing the heat supply in the electrodynamic gradient multi-zone furnace. Optimization of heat transfer in the furnace, and the melt and gas flows is the key point in growing quality crystals.
The following physical phenomena affect crystallization in VGF growth:
- heat transfer by radiation and diffusion in a particular hot zone;
- melt convection and convective heat exchange under the crystallization front, including free surface phenomena and magnetic field effects;
- gas convection and heat exchange at the gas/crystal interface (encapsulant flow during liquid encapsulated crystal growth);
- latent heat release on the crystallization front.
Schematic view of the VGF setup for growing 6 inch diameter GaAs crystals
Computed evolution of temperature distribution in the reactor
Optimization of process recipe is a powerful approach to increase yield and improve crystal quality. In the presented example, optimization of the process has been performed without hardware modifications – for each of the three heaters, we suggested individual dependence of the power as a function of time. Optimized recipe allowed us to achieve more convex melt/crystal interface shape in the beginning of cylindrical part of the crystal and significantly reduce probability of multi-crystalline material growth. Lower values of temperature gradients reduce dislocation density in the crystal and probability of crystal cracking.
Original heater power evolution during the growth process
Optimized heater power evolution during the growth process
One can see a noticeable effect of the heater power optimization on the shape of the crystallization front, as well as significant drop in temperature gradients at the melt-crystal interface (right), which allows us to hope for a considerable improvement of the crystal quality.
Heater power optimization is not the only way to tackle this problem. Other ways to decrease the temperature gradients and dislocation density during VGF growth include:
- heat transfer by radiation and diffusion in a particular hot zone;
- special design of heaters and increasing the number of independent heaters;
- heat shields around the “seed” part of the crystal;
- temperature engineering of the bottom part of the ampoule;
- design of the side and upper insulations.
Evolution of the crystallization front before (black) and after the optimization (red). The correlation between the crystal shape and the temperature gradients along the front
Modeling of dopant distribution
GaAs crystals are doped with different elements to achieve the required electric conductivity for semi-conducting or semi-insulating wafers. Control of segregation of doping materials is important to increase a high yield of GaAs wafers with electric conductivity values within the specification limits. Modeling of melt flow coupled with segregation helps to obtain distribution of dopant, for example Si or Zn in the crystal.
Si doping distribution
Zn doping distribution