Photoelectronic Properties of ZnSe Crystals

The photoelectronic properties of $p$-type ZnSe: Cu and of $n$-type self-activated ZnSe(SA) have been investigated in single crystals. Results for ZnSe: Cu may be described by a multivalent-copper-impurity model in which ${\mathrm{Cu}}^{+}$ and ${\mathrm{Cu}}^{+2\mathrm{}}$ ions substituting on the Zn sublattice are responsible for the red and green luminescence bands, respectively. The green luminescence emission (2.34 eV) is from the conduction band to the empty ${\mathrm{Cu}}_{\mathrm{Zn}}^{\mathrm{x}}$ center. One of the red emission bands (1.97 eV) is from the conduction band to the empty ${\mathrm{Cu}}_{\mathrm{Zn}}$′ center; a second red band (1.95 eV) is from a shallow level near the conduction band to the empty ${\mathrm{Cu}}_{\mathrm{Zn}}$′ center. The ${\mathrm{Cu}}_{\mathrm{Zn}}$′ center is also the major sensitizing center for $n$-type photoconductivity in $p$-type ZnSe: Cu, and it is the dominant acceptor center controlling $p$-type conductivity. Optical absorption in the infrared is due to an internal transition within the ${\mathrm{Cu}}_{\mathrm{Zn}}^{\mathrm{x}}$, which has an (Ar) $3d^9$ electronic configuration, plus a transition from the valence band to the empty ${\mathrm{Cu}}_{\mathrm{Zn}}$′ center. Several annealing and impurity-incorporation experiments performed on ZnSe: Cu support this model for the Cu luminescence centers. The sensitizing center for $n$-type photoconductivity in $n$-type ZnSe(SA) is not the same as that associated with ${\mathrm{Cu}}_{\mathrm{Zn}}$′ in $p$-type ZnSe: Cu; it lies closer to the valence band and has a capture cross section for electrons ${10}^{-3\mathrm{}}$ that of the ${\mathrm{Cu}}_{\mathrm{Zn}}$′ sensitizing center.