Comparative study on residual strain and thermal stress in 4-in GaAs crystals grown by LEC and VB techniques

Abstract

Residual strains of 4-in diameter GaAs crystals grown by liquid encapsulated Czochralski (LEC) and vertical Bridgman (VB) techniques have been measured by scanning infrared polariscope (SIRP). It was found that the residual strain of the substrate cut at the shoulder portion of a LEC-grown crystal was remarkably larger than that cut at the middle portion of the crystal. It was also found that the residual strains of the substrates cut at the shoulder and middle portions of a VB-grown crystal were both smaller than those of the LEC-grown crystal, and showed the similar values and profiles with each other. Von Mises stress and excess shear stress according to the Jordan's criterion were calculated to understand these experimental results. It was found that large thermal stress was caused during the LEC growth especially in the early stage, while thermal stresses caused during the VB growth were considerably smaller than those caused during the LEC growth from the shoulder to the middle. These experimental and analytical results suggest that the difference in the residual strain along the growth axis between LEC and VB crystals is due to the difference of the thermal stress caused during the crystal growth between LEC and VB techniques.