This Small Business Innovative Research (SBIR) Phase I project will conduct feasibility studies for development of light emitting diodes (LEDs) that emit in the green spectral region. A need exists for efficient green emitting LEDs for use as a green source used singly and also for use in LED lamps that utilize direct color-mixing of three or more colors, e.g., red, green and blue, to achieve white. Green LED sources are typically made from indium gallium nitride (InGaN) semiconductor material; however, such LEDs suffer from a decrease in efficiency as operating current is increased. The research objective is to develop a green LED with high quantum efficiency at high current values by reducing the observed efficiency droop at operating current values. This research objective will be achieved by forming a hybrid LED comprised of a p-type zinc oxide (ZnO) semiconductor layer deposited on the quantum well region of an InGaN wafer during the wafer fabrication process. The p-type ZnO layer will provide additional hole carriers and thereby increase quantum efficiency. The anticipated technical results will be an increased efficiency of at least 20% for the hybrid LED device.
The broader impact/commercial potential of this project will enhance technical and scientific understanding of the mechanisms by which a layer of p-type ZnO semiconductor material with high hole concentration and deposited during wafer growth in close proximity to the active layer region comprised of InGaN quantum wells can increase significantly the quantum efficiency of the hybrid LED device fabricated from such wafers. A potential societal impact is energy savings for the U.S. by employing higher efficiency electrical lighting and thereby decreasing electrical power demand and bright color displays. The potential commercial impact of the project is availability of a green LED with sufficiently high efficiency to be utilized in combination with blue and red LEDs to achieve a commercially viable white LED lamp by direct color-mixing, without use of phosphors to achieve green light from blue. A direct color mixing approach will eliminate inefficiencies associated with use of down-converting phosphors. These direct color-mixed LED lamps will be cost effective and possess spectral qualities desirable to consumers that will help speed market entry. The market sector impacted includes semiconductor chip manufacturing, white light lamps for residential and commercial use, and color displays.
High-power Green LED chips were fabricated and assembled with different packages such as a lamp type (called 5mm cone) and 5050 SMD (Surface Mount Device). The optical power and EQE (External Quantum Efficiency) were measured. The test results show that the EQE is around 40%, which is the highest one has ever reported to the best of our knowledge. The output power for 5mm cone-type LED is 17 mW at 20 mA. The result was confirmed by different groups at non-profit and profit entities such as NIST (National Institute of Standards and Technology). Under the support of the NSF SBIR grant (NSF Award Number: IIP-1012256), we have accomplished a number of major objects towards the development of high-power Green LEDs for general lighting and full-color display applications, including the output power enhancement by using p-ZnO as a hole injection layer, and demonstration of fully packaged p-ZnO/GaN LEDs. Based on these achievements, we demonstrated the feasibility of fabricating warm-white solid-state lamps made with these enhanced G-LEDs and the other B-/R-LEDs would be comparable in cost and efficiency with existing conventional lamps. Using p-type ZnO for additional hole injection is the key to bring extremely efficient high-power Green p-ZnO/GaN LEDs to the market. The effort in the Phase I was designed to provide foundational information and address tasks necessary to initiate production of high-power Green LEDs in the Phase II works. Fabrication of optimized hybrid Green p-ZnO/GaN LEDs will be performed in future Phase II efforts with high Wall-plug efficacy (Lm/W) of > 130 lm at 1W operation. With strong test results and technological proof-of-concept, we firmly believe that high-power p-ZnO/GaN LEDs will be worldwidely used in future enerations of lighting bulbs and new high-tech products.
