The liquidus isotherms of the In‐Ga‐As system were determined at 600° and 700°C by the improved seed dissolution technique. From the results of the liquidus isotherms and the lattice‐constant measurements, the substrate orientation dependence of solution compositions to grow lattice‐matched
In0.53Ga0.47normalAs
layers on the (100), (111)A, and (111)B
normalInP
substrates by liquid phase epitaxy (LPE) was exactly obtained in the temperature range 600°–790°C. The solution compositions of Ga for the (111)A and (111)B faces are always larger than those for the (100) face over the entire temperature range 600°–790°C. The solution compositions of As for the (111)A and (111)B faces are always smaller than those for the (100) face over the entire temperature range 600°–790°C. In order to study effects of the surface free energy on the In‐Ga‐As phase diagram which was determined by using the LPE method, the In‐Ga‐As phase diagram was calculated, for the first time, by adding the surface free energy to the chemical free energy of the
In1−xGaxnormalAs
solid phase. According to the calculated phase diagram, the composition of Ga in the solution to obtain the (111) oriented
In0.53Ga0.47normalAs
is smaller than the composition of Ga in the solution to obtain the (100)‐oriented
In0.53Ga0.47normalAs
. This tendency of the calculated results is contrary to the experimental results. That is to say, the difference in the surface free energy is not a main reason for the substrate orientation dependence of the In‐Ga‐As phase diagram.