Lasers & Sources

Better understanding of zinc oxide crystals could lead to more-efficient short-wavelength LEDs

Dec. 7, 2020

Spectroscopic analysis of ZnO crystals grown by the hydrothermal method probes the semiconducting crystals to detect defects and impurities.

John Wallace

An IQE curve is obtained by omnidirectional photoluminescence (ODP) spectroscopy (left). Y axis is IQE percentage, lower X axis is excitation light power density Pcw (W cm-2), and upper X axis is excitation rate G (sec-1 cm-2). Separated light-emitting rate (WR) and non-light-emitting rate (WNR) are shown at right, where Y axis is the inverse of the rate (ns), lower X axis is excitation light power density (nJ/cm2), and upper X axis is excited carrier concentration (cm-2).

Zinc oxide (ZnO) is an attractive material among direct-bandgap semiconductors. Due to its large bandgap energy and large exciton binding energy, the material has light-emitting properties as well as the ability to sustain a large electric field that enables it to power electronic devices because of its large bandgap energy and large exciton binding energy.

A recent study done by Kazunobu Kojima and Shigefusa Chichibu of Tohoku University (Sendai, Japan) has measured the internal quantum efficiency (IQE) of ZnO crystals for both light-emitting and non-light-emitting processes. In high-quality ZnO crystals, nonradiative recombination centers (NRCs) are an important phenomenon affecting near-band-edge (NBE) emission. NRCs act as undesired energy dissipation channels and reduce the IQE of the NBE emission.

To understand whether the light-emitting process or the non-light-emitting process was more important in determining the behavior of IQE, Kojima and his colleagues measured the IQE values of a ZnO crystal grown by the hydrothermal method. To do so, they employed a technique created by Kojima and fellow researchers known as omnidirectional photoluminescence (ODP) spectroscopy—a nondestructive method for probing semiconducting crystals with light to detect defects and impurities.

The IQE characteristics in ZnO crystals were examined under photopumping conditions. IQE values indicated a constant behavior for weak photopumping conditions and a monotonic increase for strong excitation. Because a significant decrease was observed for the non-light-emitting process with photopumping, the origin of the IQE increase was revealed to be dominated by the deceleration of the non-light-emitting process due to the saturation of NRCs.

"Obtaining a quantitative breakdown of IQE from both processes allows us to better design semiconductors to improve IQE," said Kojima, lead author of the study.

The result could help lead to very efficient short-wavelength LEDs.

Source: http://www.tohoku.ac.jp/en/press/accelerate_decelerate_light_emitting_zno.html

REFERENCE:

1. Kazunobu Kojima and Shigefusa F. Chichibu, Applied Physics Express (2020); https://doi.org/10.35848/1882-0786/abcd73.

About the Author

John Wallace | Senior Technical Editor (1998-2022)

John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.