Zachary Lindsey1 Matthew Rhoades1 Renato Camata1

1, University of Alabama at Birmingham, Birmingham, Alabama, United States

Transition metal (TM)-doped II-VI semiconductors are promising materials for mid-infrared (mid-IR) LEDs and laser sources. When a II-VI semiconductor such as ZnSe is doped with TM ions such as Cr2+, the resulting broadband emission in the 2-3 µm spectral range (due to emission from the dopant) creates the potential for tunable lasing in the mid-IR. A significant challenge in obtaining the electroluminescent (EL) structure needed for a laser diode based on this system is the high resistivity of the mid-IR active layer. This is because the dopant species that provide for mid-IR emission also make the host material (ZnSe) insulating. Cr2+:ZnSe films with strong photoluminescence exhibit resistivity as high as 1.5×1010 Ω*cm. On the other hand, samples co-doped with aluminum, which drastically reduces the resistivity to 100-150 Ω*cm, show virtually no luminescence. Finding a compromise between acceptable conductivity (for electrical pumping) and robust luminescence (for optical functionality) is essential for achieving reliable EL and lasing under electrical excitation. In this work we study the conductivity of Cr2+:ZnSe thin films of various thicknesses (100-600 nm) grown by pulsed laser deposition (PLD) on degenerately-doped (100) GaAs substrates at a temperature of 450°C under vacuum (base pressure ~10-7 torr) using the focused beam of a KrF excimer laser with a fluence of 1.5 J/cm2. A 300-nm layer of ZnS0.1Se0.9 (n-type), was also deposited by PLD on top of the Cr2+:ZnSe thin films as a prototype waveguiding material of lower refractive index. Future structures will feature thicker ZnS0.1Se0.9 layers above and below the active layer for effective optical confinement. All structures are capped with metallic contacts and annealed at 300°C in 5% H2/N2 in an effort to passivate defect states in the various metal/semiconductor and semiconductor/semiconductor interfaces. Thin films and structures were analyzed via x-ray diffraction to determine whether films were single crystal, textured with epitaxial orientation, or polycrystalline. In the case of Cr2+:ZnSe thin films below the critical thickness of ~150 nm, the entire structure is epitaxial and lattice matched to the GaAs substrate, resulting in the best conductivity characteristics. The density of extended defects is higher in thicker films, leading to poorer conductivity. Electrochemical impedance spectroscopy measurements yielded resistivity within the range 4.5×105 – 1.6×106 Ω*cm for all samples. These values can be almost entirely attributed to the resistivity of the mid-IR active layer. This level of resistivity offers the possibility of the high current densities needed in the active layer for electrical excitation. We describe our efforts in co-doping of this same active Cr2+:ZnSe layer with copper (Cu), coupled with p-type, Cu-doped ZnS0.1Se0.9 top layers to further tune the resistivity of Cr2+:ZnSe into a range amenable for efficient electrically excited laser sources.