A laser scanning confocal microscope （LSCM） and a field-emission scanning electron microscope （FE- SEM） were used to study the defects in CdMnTe crystals, such as twin boundaries, Te inclusions, and dislocations. Twin boundaries were usually decorated with Te inclusions, which could induce dislocations. The optical, elec- trical properties and detector performance of CdMnTe crystals with twins and free of twins were compared. The results showed that the wafers with a high density of twins usually had lower average IR transmittance and poorer crystalline quality. Besides, the energy spectra indicated that twin boundaries in a CdMnTe detector had a negative effect on detector performance; the values of both the energy resolution and （μτ）e were nearly half of those for a single crystal detector.
