AlxIn1-xP LEDs with II-VI Cladding Layers for Efficient Red and Amber Emission

Realizing solid state lighting systems with high efficiency and enhanced functionality will require the adoption of color-mixing (cm-LEDs) architectures combining highly efficient red, green, blue and amber light emitting diodes. Already, the hybrid architectures (hy-LEDs) pair phosphor-converted wide bandgap nitride LEDs with (AlxGa1-x)0.51In0.49P red LEDs to improve the emission spectrum and reduce losses in the infrared. However, the efficiencies of (AlxGa1- x)0.51In0.49P red and amber LEDs are still too low to truly enable these approaches. The DOE Solid-State Lighting R&D Plan highlights red and amber LED improvement as a long-term research goal. Excessive electron leakage out of the cladding layers, in particular, leads to strong thermal droop and red LED efficiencies of ~25% at normal operating temperatures. The typical device architecture for red and amber LEDs is based on indirect bandgap (AlyGa1-y)0.51In0.49P cladding layers used to confine injected electrons and holes within a lower, direct bandgap (AlxGa1- x)0.51In0.49P light-emitting active layer (x < y). The challenge with this approach is that the electron confinement is limited and decreases as the emission wavelength is shortened. Hence, thermal droop and overall device efficiency are very poor for amber LEDs. Innovative solutions to this problem must be sought through the use of new semiconductor materials and structures that will increase electron confinement. We plan to develop AlxIn1-xP LEDs with a II-VI semiconductor electron cladding layer to greatly increase the carrier confinement. Such an approach has the potential to improve red and amber LED efficiency, temperature stability, and light extraction. AlxIn1-xP active layers provide high direct bandgap energies needed to reach amber wavelengths without detrimental internal losses. ZnSe-based alloys are attractive candidates for the electron cladding layer because they can be engineered to provide high conduction band energies relative to the AlxIn1-xP active layer while maintaining roughly the same lattice constant. The lower index of refraction of the II-VI cladding layer will also aid light extraction. While extended defects have largely precluded the use of II-VI semiconductors as the light emitting layer in LEDs and lasers, the effects of these defects on device performance can be mitigated by limiting the use of II-VI materials to the thin, heavily doped cladding layer. Proposed research will focus on optimizing the quality of the AlxIn1-xP active layer and the heteropolar growth of the III-V/II-VI interface, tuning the II-VI cladding layer composition, and device design to maximize electron confinement and LED efficiency. Significant improvement in the performance of red and amber LEDs through this new approach is expected to advance the capabilities of next generation solid-state lighting technologies.