Telluride/selenide and telluride/sulphide superlattices
This chapter discusses telluride/selenide and telluride/sulphide superlattices. The most widely studied telluride/selenide structures are ZnTe/ZnSe and ZnSe/BeTe superlattices (SL). Both have a type-II band offset. Graded SLs are frequently used in the p-doped contact region of LEDs and LDs to achieve an efficient hole injection. Both BeTe and ZnSe have lattice constants close to the one of GaAs. The alloy BexZn1-xSe1-yTey is lattice matched to GaAs for x = 0.084 and y = 0.095. A high Be content in an optically active layer without loosing the lattice matching to GaAs is achieved by ZnSe/BeTe SL with a ZnSe layer thickness of about twice the BeTe one. The type-II valence-band offset is found to vary nearly linearly between 0.54 and 1.14 eV when going from ZnTe to ZnSe substrates. Above-barrier states are studied at room temperature in type-II CdSe/ZnTe SLs by photoreflectance, electroreflectance, and photoconductivity. Tellurium is an isoelectronic trap in ZnSe with high PL emission efficiency for trapped excitons. Accordingly, intense emission related to isoelectronic traps of Te atoms (2.64 eV) and of Te clusters (2.48 eV) is observed in ZnSe/ZnTe SLs on GaAs with fractional layers of ZnTe. A large blue shift of the type-II PL of some 500 meV accompanied by a drop of PL lifetime by two orders of magnitude is observed in ZnSe/BeTe SLs under high excitation. A moderate blue shift of some 20 meV is found in ZnSe/ZnTe SLs under high-excitation conditions.