Abstract: 3 inch in diameter Si-doped GaAs single crystals have been grown by high pressure LEC technology. Si concentration is greater than 1×10¹⁸/cm ³ and EPD is less than 1×10⁴/cm ². It has been found that there was slag resulting in non-crystallizability on solid-liquid interface when PBN crucible or B₂O₃ with much water was used and the necessary dopant weight was 4 times more than theoretical doping amount. The results were explained.