The total annual energy consumption in the United States for lighting is approximately 800 Terawatt-hours and costs $80 billion to the public. The energy consumed for lighting throughout the world entails to greenhouse gas emission equivalent to 70% of the emissions from all the cars in the world. A novel solution to lighting with higher efficiency will drastically reduce the energy consumption and help greenhouse gas emissions to be lowered. Novel green light emitting diodes are the key components of an affordable, durable and environmentally benign lighting solution that can perform at superior energy conversion efficiency.

The current lighting technologies are compared in Table 1. Compared to light bulbs and fluorescent tubes, light emitting diodes (LEDs) achieve superior energy efficiency with a long lifetime. Lower ownership cost and no toxicity of the LEDs make them both affordable and environmentally benign lighting solution.

Table 1: Comparison of Lighting Sources

LEDs are semiconductors in which the light emission comes from a crystalline layer called active layer, sandwiched between an n-type and a p-type layer. When a voltage is applied between the layers, electrons and holes injected into the active layer recombine and the energy is released in the form of light. The wavelength of the emitted light (λ) is determined by the bandgap energy of the active layer. Therefore, proper engineering of the material composition in the active layer is essential to achieve the intended wavelength of the light emission.

The human eye is sensitive only to light in the visible spectrum, ranging from violet (λ~400 nm) through red (λ~700 nm). However, the human eye is most sensitive to green (λ~555 nm) and green light strongly affects the human perception to the quality of white light. Although ultrabright and efficient blue InGaN-based LEDs are readily available (that enabled blue-ray technology), the performance of green LEDs is still far from adequate for lighting use as seen in Figure 1. This “green gap” prevents the generation of high performance white LEDs based on color mixing shown in Figure 2(a).

Light-emitting diodes based on In_xGa_1-xN alloy are currently the most promising candidates for fulfilling the green gap. InGaN is a direct wide bandgap semiconductor with an emission that can span the entire visible spectrum via increasing the indium content (x) of In_xGa_1-xN. Basically, the alloy is precision engineered for light emission at a target wavelength. However, the higher indium content required in the active layers for green emission causes leakage problems. In particular, high indium content of the In_xGa_1-xN enabling green light emission becomes unstable at elevated substrate temperature (T_s). Figure 1(a) shows that the indium leaks out of the active layers in the InGaN/GaN Multi-Quantum Wells (MQW) once annealed at GaN growth temperatures. Conventionally, a GaN layer is grown on top of an InGaN MQW active layer to complete the LED. This GaN layer is grown at significantly higher T_s than the InGaN MQW active layer in order to obtain high structural quality. This leads, however, to indium leaking out of the active layers (as shown in Figure 1(a)), which reduces the LED efficiency and spectral quality (Figure 1(b)). It is essential to prevent thermal-induced indium diffusion in order to obtain InGaN-based green LEDs with superior performance.

Figure 1: (a) High Resolution Transmission Electron Microscope (HR-TEM) Lattice Parameter Mapping image of pre- and post-annealed InGaN active layers showing thermally-induced indium non-uniformity in InGaN, (b) The efficacy of the InGaN and InGaAlP active layers with respect to emission wavelength showing the significant efficacy drop in green regime (so called “the green gap”) that precludeshigh performance white LEDs.

ZnO, being a wide bandgap material that has a low toxicity and the same crystal structure as GaN, makes a good candidate for integration in InGaN devices. Through use of Pulsed Laser Deposition (PLD) for ZnO growth, the ultimate growth step could be performed at significantly lower Ts than is typically required for GaN growth. In our work, ZnO layer was substituted for GaN layer in an (In)GaN-based green LED. The top layer was thus ZnO rather than GaN that prevented indium leaking out of the active layer. Our innovative process for novel green light emitting diodes is illustrated in Figure 2(b) and experimental results are available in our recent publications. 

High crystallographic quality of the final hybrid LED structure and the integrity of the MQWs were confirmed by X-Ray Diffraction. The fabricated LEDs showed rectifying I/V characteristics with a turn-on voltage of 2.5 V and a discrete green electroluminescence emission peaked at around 510 nm, which was readily visible to the naked eye. Our innovative solution solves the indium leakage problem and enables pure green LEDs.

Figure 2: (a) Generation of white lighting via color-mixing of red, green and blue LEDs, (b) Hybrid green LED structure preventing the indium from leaking out of the high indium content green emitting active layer, (c) The growth techniques used to realize the hybrid green LED: Metalorganic Chemical Vapor Deposition (MOCVD) growth for GaN, InGaN active layer and AlGaInN current blocking layer (CBL), PLD growth for atop ZnO layer.

Employment of ZnO atop of green emitting high indium content InGaN LEDs brings up a new era of designs as it solves a major problem of conventional green LEDs: it prevents the much-needed indium for green emission from leaking out. Our innovative ZnO – InGaN hybridization approach enables higher spectral quality and efficient green LEDs that fulfills the green gap and enables white LEDs based on color-mixing (the most energy efficient method of white light generation) as well as other commercial applications such as TVs and novel display systems.