Brighter LEDs with Quantum Wells

Light-emitting diodes made of indium gallium nitride provide better lumines­cence efficiency than many of the other materials used to create blue and green LEDs. But a big challenge of working with InGaN is its known dislocation density defects that make it difficult to understand its emission properties. Now, researchers in China report an InGaN LED structure with high lumi­nescence efficiency and what is believed to be the first direct observation of transition carriers between different loca­lization states within InGaN. The loca­lization states were confirmed by temperature-dependent photo­luminescence and excitation power-dependent photo­luminescence.

LEDs made of indium gallium nitride provide better luminescence efficiency than many of the other materials used to create blue and green LEDs. This figure shows the transition process of carriers between different localization states with increasing temperatures. (Source: Y. Li / AIP)

Localization states theory is commonly used to explain the high lumi­nescence efficiency gained via the large number of dis­locations within InGaN materials. Loca­lization states are the energy minima states believed to exist within the InGaN quantum well region (discrete energy values), but a direct observation of loca­lization states was elusive until now. “Based primarily on indium content fluc­tuations, we explored the energy minima that remain within the InGaN quantum well region,” said Yangfeng Li, a post­doctoral fellow at the Hong Kong University of Science and Tech­nology. “Such energy minima will capture the charge carriers – electrons and holes – and prevent them from being captured by defects. This means that the emission effi­ciency is less affected by the large number of defects.”

The group’s direct observation of loca­lization states is an important discovery for the future of LEDs, because it verifies their existence, which was a long-standing open scientific question. “Segre­gation of indium may be one of the reasons causing loca­lization states,” said Li. “Due to the existence of loca­lization states, the charge carriers will mainly be captured in the localization states rather than by non­radiative recom­bination defects. This improves the high lumines­cence efficiency of light-emitting devices.”

Based on the group’s electro­luminescence spectra, “the InGaN sample with stronger localization states provides more than a twofold enhancement of the light-output at the same current-injection conditions as samples of weaker loca­lization states,” Li said. The researchers’ work can serve as a reference about the emission properties of InGaN materials for use in manu­facturing LEDs and laser diodes.

They plan to continue to explore gallium nitride-related materials and devices “not only to gain a better under­standing of their loca­lizations but also the properties of InGaN quantum dots, which are semi­conductor particles with potential appli­cations in solar cells and elec­tronics,” Li said. “We hope that other researchers will also conduct in-depth theo­retical studies of loca­lization states.” (Source: AIP)

Reference: Y. Li et al.: Visualizing carrier transitions between localization states in a InGaN yellow–green light-emitting-diode structure, J. Appl. Phys. 126, 095705 (2019); DOI: 10.1063/1.5100989

Link: Key Laboratory for Renewable Energy, Beijing Key Laboratory for New Energy Materials and Devices, Chinese Academy of Sciences, Beijing, China

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