Abstract: An epitaxial semiconductor crystal plate or wafer capable of attaining increased reliability with enhanced luminance, a manufacturing method thereof as well as a light-emitting diode (LED). It has been found that epitaxial wafers with enhanced illuminance and increased yield of manufacture can be fabricated by specifically arranging a double-heterostructure epitaxial wafer such that the interface between its p-type clad layer 2 and p-type GaAlAs active layer 3 and that between an n-type GaAlAs clad layer 4 and p-type GaAlAs active layer 3 measure 1.times.10.sup.17 cm.sup.-3 or less in oxygen concentration. Also, in order to cause the oxygen concentration near the p-type GaAlAs active layer 3 in layers of the epitaxial wafer to be less than or equal to 1.times.10.sup.17 cm.sup.-3, it may be preferable that a nondoped GaAs polycrystal for use as a preselected original material in liquid-phase epitaxial growth be less than or equal to 1.times.10.sup.16 cm.sup.-3 or thereabout.